Method for separating ammonia

Abstract

To provide a method of separating ammonia gas, in which, under high-temperature and high-pressure conditions, ammonia gas is separated from a mixed gas composed of multiple components including hydrogen gas, nitrogen gas, and ammonia gas by permeating the zeolite membrane with high permeability.SOLUTION: The present invention provides a method in which a mixed gas including hydrogen, nitrogen, and ammonia is fed to a zeolite membrane to separate ammonia, wherein the pressure of the mixed gas fed to the zeolite membrane is in a range of 0.8 MPaA or more to 20 MPaA or less, the separation temperature is from 100°C or higher to 500°C or lower, the ammonia / nitrogen permeability coefficient ratio of the zeolite membrane is 100 or more, and the ammonia / hydrogen permeability coefficient ratio is 70 or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for separating ammonia gas from a mixed gas containing multiple components including ammonia gas, hydrogen gas, and nitrogen gas by selectively allowing the ammonia gas to permeate through a zeolite membrane under high temperature and high pressure conditions. [Background technology]

[0002] BACKGROUND ART In recent years, membrane separation and concentration methods using membranes such as polymer membranes and zeolite membranes have been proposed as methods for separating gas mixtures. While polymer membranes have the advantage of being easily processable into, for example, flat membranes and hollow fiber membranes, they have the technical problems of swelling easily and having low heat resistance. Furthermore, polymer membranes are easily deformed by pressure, which reduces their separation performance, making them impractical for separating ammonia gas under high-temperature and high-pressure conditions, which is one of the problems of the present invention.

[0003] In contrast, zeolite membranes have regular sub-nanometer pores and function as molecular sieves, so they are not only able to selectively allow specific molecules to pass through, but are also expected to be highly durable separation membranes that can separate and concentrate molecules over a wider temperature range than polymer membranes.

[0004] BACKGROUND ART In recent years, membrane separation methods for separating ammonia gas from gases containing hydrogen gas and nitrogen gas are expected to be applied to ammonia production processes, for example (Non-Patent Document 1). The ammonia production reaction is an equilibrium reaction, and thermodynamically, high-pressure, low-temperature conditions are considered favorable. However, to ensure the catalytic reaction rate, high-temperature, high-pressure production conditions are generally required. Furthermore, because unreacted hydrogen gas and nitrogen gas coexist with ammonia gas in the produced mixed gas, the process of recovering the product ammonia gas from the produced mixed gas requires cooling the mixed gas to approximately -20°C to -5°C to condense and separate the ammonia (Non-Patent Documents 2 and 3). At temperatures and pressures at commercial plant levels, the concentration of ammonia gas contained in the produced mixed gas is low. Therefore, in the ammonia cooling and separation process from the produced mixed gas, it is necessary to cool the ammonia, including the large amount of unreacted gas, which is characteristically energy-intensive.

[0005] In order to avoid such an energy-intensive process, a process has been proposed in which the cooling condensation separation method used in the purification process is replaced with a separation method using an inorganic membrane to efficiently recover high-concentration ammonia gas (Patent Documents 1 and 2).

[0006] Furthermore, in recent years, research has been progressing on reducing the energy required for the ammonia synthesis process by lowering the temperature and pressure of the catalytic reaction through the development of highly active catalysts. However, because a reduction in pressure increases the load on the ammonia cooling and separation process, there is a need to develop high-performance membranes as a more efficient ammonia separation method (Non-Patent Document 1).

[0007] Furthermore, Patent Document 3 reports a reaction separation type ammonia production process in which a membrane is installed in a reactor and ammonia gas is produced while the produced ammonia gas is simultaneously recovered. This process is expected to improve the conversion rate of the raw material gas to ammonia gas and reduce the amount of recovered gas recycled to the reactor during production.

[0008] An example of such a separation method that can be applied to the ammonia production process is a method in which a zeolite membrane is used to selectively allow ammonia gas to permeate from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas.

[0009] Patent Document 2 proposes an efficient ammonia separation method in which a specific zeolite having an eight-membered oxygen ring is used to separate ammonia gas from a mixed gas of ammonia gas and hydrogen gas and / or nitrogen gas.

[0010] Furthermore, Non-Patent Document 4 describes ammonia separation using a silicalite membrane, reporting that ammonia selectively permeates at low temperatures below 100° C. Similarly, Non-Patent Document 5 reports that a membrane made of silicalite in nanosheet form has extremely high ammonia permeability.

[0011] Furthermore, Patent Document 4 proposes a zeolite membrane that has high separation selectivity even under high temperature conditions of about 100 to 325°C. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-247654 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-058433 [Patent Document 3] Japanese Patent Publication No. 2022-61320 [Patent Document 4] Japanese Patent Application Publication No. 2022-95749 [Non-patent literature]

[0013] [Non-Patent Document 1] ACS Sustainable Chem. Eng. 2023, 11, 9880-9899 [Non-patent document 2] Chemical Society of Japan, 6th Edition, Chemical Handbook, Applied Chemistry, Vol. I, Maruzen Co., Ltd. (2003), p. 581 [Non-patent document 3] Chemical Engineering Society, Chemical Process Collection 1st Edition, Tokyo Kagaku Dojin, p153 [Non-patent document 4] AIChE Journal June 2006 Vol. 52, No. 6, 2055-2065 [Non-patent document 5] Chem. Commun., 2021, 57, 580-582 [Non-patent document 6] Special Steel Club, Special Steel (2012), Vol. 61 No. 3, P. 18 [Non-Patent Document 7] Sumitomo Electric, Technical Review (2012), January issue, No. 180, Development of Ammonia Detoxification Equipment, p. 93 Summary of the Invention [Problem to be solved by the invention]

[0014] However, there is a problem that the method of selectively permeating hydrogen gas and / or nitrogen gas from a mixed gas containing hydrogen gas, nitrogen gas, and ammonia gas does not provide an economical process. For example, the technology of Patent Document 1 not only requires a complicated process of separating ammonia gas from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas in at least two stages, but also requires a step of recovering ammonia from both the mixed gas permeated in the first stage and the non-permeated mixed gas, making the process complicated.

[0015] In contrast, the zeolite membranes that selectively allow ammonia to permeate, as described in Patent Document 2 and Non-Patent Documents 4 and 5, are capable of single-stage separation, allowing for a simple process configuration. However, in the method of separating ammonia by molecular sieving using the pore size of zeolite proposed in Patent Document 2, the permeance ratio (ideal separation factor) of ammonia gas to nitrogen gas is about 14, which is not sufficient in separation performance and leaves room for improvement. Furthermore, Non-Patent Document 4 reports that when the supply gas pressure is 10 Bar and the temperature range is 20 to 100°C, the ammonia permeability coefficient tends to increase as the temperature increases; however, when the temperature is increased to 120°C, the ammonia permeability coefficient decreases and the separation selectivity also decreases. This indicates that ammonia and the acid sites of the zeolite have a strong adsorption interaction in the low temperature range of 10 Bar and below 100°C, but the adsorption interaction weakens above 100°C, resulting in a decrease in separation selectivity. Furthermore, Non-Patent Document 5 also reports that when the supply gas pressure is 3 Bar and the temperature range is below 100°C, the separation selectivity tends to decrease as the temperature increases. Both results are separation performance results in the low temperature range below 100°C, and it cannot be said that practical membrane performance is obtained in the high temperature range above 100°C.

[0016] To solve these problems, Patent Document 4 reports that by using a zeolite membrane with specific physical properties, it is possible to highly selectively permeate ammonia gas from a mixed gas of hydrogen gas, nitrogen gas, and ammonia gas as a feed gas at a temperature range of 100 to 325°C under a pressure of 0.4 MPaA. For example, in Table 8 (performance evaluation at 325°C), the ammonia permeability was 1.1 x 10 -7 mol / (m 2 s Pa), ammonia / nitrogen permeability ratio of 230, ammonia / hydrogen permeability ratio of 34), and Table 13 (325°C evaluation results, ammonia permeability of 2.8 × 10 -8 mol / (m 2The confirmed membrane performance is described in Patent Document 4, where the pressure is 300 psi (1000 psi), the ammonia / nitrogen permeability ratio is 368, and the ammonia / hydrogen permeability ratio is 69. However, these results are the result of evaluating membrane separation performance under conditions of extremely low pressure compared to processes that operate under high-pressure conditions, such as an ammonia production process. In particular, when ammonia is separated by a separation mechanism of adsorption / desorption onto zeolite as described in the examples of the present application, membrane performance such as ammonia permeability, ammonia / nitrogen permeability ratio, and ammonia / hydrogen permeability ratio generally tends to decrease as the pressure is increased. Considering this, the membrane performance described in Patent Document 4 has room for improvement in performance under high-pressure conditions.

[0017] The present invention has been made in view of the above-mentioned conventional circumstances, and an object of the present invention is to provide a method for separating ammonia gas from a mixed gas consisting of multiple components including hydrogen gas, nitrogen gas, and ammonia gas by allowing the ammonia gas to permeate through a zeolite membrane with high permeability under high temperature and high pressure conditions. [Means for solving the problem]

[0018] In order to solve the above problems, the present inventors have further investigated the separation of ammonia using a zeolite membrane and have found that ammonia can be separated by adjusting the separation conditions of the pressure and temperature of the mixed gas, and the ammonia / nitrogen permeability coefficient ratio and the ammonia / hydrogen permeability coefficient ratio of the zeolite membrane within specific ranges. Typically, membrane separation uses a pressure difference as a driving force, so from this perspective, it is desirable to have as high a feed gas pressure as possible. However, in separations using the adsorption / desorption mechanism on zeolite as in the present invention, ammonia, which is basic, adsorbs to the zeolite adsorption sites, which have acidic properties. Therefore, the higher the feed gas pressure, the stronger the adsorption force at the zeolite adsorption sites tends to be. Therefore, when the feed gas pressure is high, the rate at which ammonia is adsorbed and desorbed by zeolite slows, resulting in a decrease in ammonia permeability. On the other hand, for hydrogen gas and nitrogen gas, which have low adsorption properties, increasing the feed gas pressure has little effect on adsorption to zeolite and only a limited effect on permeability. Therefore, it has generally been thought that increasing the feed gas pressure not only reduces ammonia permeability, but also significantly reduces the ammonia / nitrogen permeability ratio and the ammonia / hydrogen permeability ratio. However, in the separation method of the present invention, it was found that there are preferred ranges of pressure and temperature as factors for adjusting the adsorption interaction between ammonia and zeolite adsorption sites, and it was found that ammonia can be separated without reducing the ammonia permeability, leading to the completion of the present invention.

[0019] The embodiments of the present invention have been achieved based on these findings and provide the following.

[0020] [1] A method for separating ammonia by supplying a mixed gas containing hydrogen, nitrogen, and ammonia to a zeolite membrane, The pressure of the mixed gas supplied to the zeolite membrane is 0.8 MPaA or more and 20 MPaA or less, and the separation temperature is 100°C or more and 500°C or less, A method for separating ammonia, wherein the zeolite membrane has an ammonia / nitrogen permeability coefficient ratio of 100 or more and an ammonia / hydrogen permeability coefficient ratio of 70 or more. [2] The method for separating ammonia according to [1], characterized in that the linear velocity of the mixed gas in the separation device equipped with the zeolite membrane is 0.5 cm / sec or more and 10 m / sec or less. [3] The method for separating ammonia according to [1] or [2], wherein the ammonia gas concentration in the mixed gas is less than 50% by volume. [4] The method for separating ammonia according to any one of [1] to [3], wherein the zeolite membrane is an aluminosilicate zeolite membrane. [5] The ammonia permeability of the zeolite membrane is 8.5 × 10 -8 mol / (m 2 The method for separating ammonia according to any one of [1] to [4], characterized in that the pressure is 0.005 s·Pa or higher. [6] The framework density of the zeolite constituting the zeolite membrane is 14.3 T / nm 3 The method for separating ammonia according to any one of [1] to [5], characterized in that: [7] The method for separating ammonia according to any one of [1] to [6], wherein the structure of the zeolite constituting the zeolite membrane is MFI or RHO. [Effects of the Invention]

[0021] According to an embodiment of the present invention, ammonia gas can be separated by passing the ammonia gas through a zeolite membrane with high permeability even under high pressure conditions, such as 0.8 MPaA or higher, and high temperature conditions, such as 100° C. or higher. As a result, the membrane area required for separation can be reduced, enabling ammonia separation at low cost using small-scale facilities. As a specific example of application of the ammonia separation method of the present invention, ammonia separation can be used in the following processes. (1) Ammonia production process (2) Reaction separation type ammonia production plant (3) Ammonia decomposition process (4) Separation of ammonia from exhaust gas containing ammonia from nitriding furnaces, etc.

[0022] (1) Ammonia production process In ammonia production processes such as the Haber-Bosch process, when recovering ammonia from a mixed gas consisting of multiple components including ammonia gas recovered from a reactor, hydrogen gas, and nitrogen gas, this method can separate ammonia gas more efficiently than conventional cooling condensation separation methods, thereby reducing the cooling energy required for ammonia condensation.

[0023] (2) Reaction separation type ammonia production plant According to the ammonia separation method of the present invention, a reaction separation type ammonia producer can also be designed in which a zeolite membrane is installed in a reactor and ammonia gas is produced under high temperature and high pressure conditions while the produced ammonia gas is simultaneously collected.

[0024] (3) Ammonia decomposition process Since one ammonia molecule contains 1.5 hydrogen molecules, it is expected to play a role as a hydrogen carrier. In this case, at the site of hydrogen use, ammonia is decomposed through a decomposition reaction to obtain hydrogen gas. However, since the ammonia decomposition reaction is an equilibrium reaction, the reactor outlet gas contains hydrogen gas, nitrogen gas, and ammonia gas. Therefore, by applying the ammonia separation method of the present invention to the outlet of the decomposition reactor, it becomes possible to realize a hydrogen production process in which unreacted ammonia gas is recovered and reused.

[0025] (4) Separation of ammonia from exhaust gas containing ammonia from nitriding furnaces, etc. In equipment that uses ammonia gas as a raw material, such as a nitriding furnace, the ammonia contained in the exhaust gas is generally decomposed into nitrogen gas and hydrogen gas in a decomposition furnace and detoxified before being disposed of (Non-Patent Document 6). Other treatment methods include absorbing ammonia gas into water, as in a scrubber, and disposing of the resulting ammonia water, or neutralizing it with sulfuric acid and disposing of it as ammonium sulfate (Non-Patent Document 7). However, these methods dispose of ammonia in a different form, making it impossible to reuse. By using the ammonia separation of the present invention, ammonia can be recovered from exhaust gas and reused, thereby improving the cost of the ammonia raw material and the cost of waste disposal. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is a schematic diagram showing the configuration of an apparatus used in an ammonia gas separation test in the Examples. [Figure 2] 1 is a graph showing the measurement results of the permeability coefficient ratio as a function of the gas linear velocity of a mixed gas in an example. DETAILED DESCRIPTION OF THE INVENTION

[0027] The following describes in more detail the embodiments of the present invention. However, the description of the constituent elements described below is an example of an embodiment of the present invention, and the present invention is not limited to these contents, and can be implemented in various modifications within the scope of its gist. In this specification, zeolite refers to a zeolite as defined by the International Zeolite Association (IZA). Its structure is characterized by X-ray diffraction data. In this specification, a "porous support-zeolite membrane composite in which a zeolite membrane is formed on a porous support" may be referred to as a "zeolite membrane composite" or a "membrane composite." In addition, a "porous support" may be simply referred to as a "support." In this specification, "hydrogen gas," "nitrogen gas," and "ammonia gas" may be simply referred to as "hydrogen," "nitrogen," and "ammonia," respectively. Meanwhile, ammonia separation in this invention refers to obtaining a mixed gas containing a higher concentration of ammonia gas from a mixed gas containing ammonia gas.

[0028] An embodiment of the ammonia separation method of the present invention is a method of separating ammonia by supplying a mixed gas containing hydrogen, nitrogen, and ammonia to a zeolite membrane, characterized in that the pressure of the mixed gas supplied to the zeolite membrane is 0.8 MPaA or more and 20 MPaA or less, the separation temperature is 100°C or more and 500°C or less, and the zeolite membrane has an ammonia / nitrogen permeability coefficient ratio of 100 or more and an ammonia / hydrogen permeability coefficient ratio of 70 or more. Another embodiment of the method for separating ammonia of the present invention is characterized in that a mixed gas consisting of ammonia and multiple components including hydrogen and / or nitrogen is brought into contact with a specific zeolite membrane, and ammonia is selectively permeated and separated from the mixed gas. Details are explained below.

[0029] <Ammonia manufacturing method> The ammonia separation method according to this embodiment can be effectively used to efficiently separate ammonia from a mixed gas containing at least ammonia, hydrogen, and nitrogen, and therefore is effective when used in combination with an ammonia production method that produces such a mixed gas. That is, in addition to an ammonia production method that includes a first step of producing ammonia from hydrogen and nitrogen and a second step of separating the ammonia produced in the first step by an ammonia separation method described below, and in which ammonia is separated from the ammonia produced in the first step in the second step, a preferred embodiment of the present invention also includes an ammonia production method in which the first and second steps proceed in a single reactor. Proceeding the first and second steps in a single reactor means that the first and second steps proceed simultaneously. In other words, in one embodiment of the present invention, ammonia gas is produced from hydrogen gas and nitrogen gas in a vessel, and ammonia can be efficiently produced in the vessel while ammonia is separated from a mixed gas containing the produced ammonia gas. There are no particular limitations on the industrial production of ammonia, but the Haber-Bosch process is one example. This process essentially uses iron oxide as a catalyst. Nitrogen and hydrogen gas react over the catalyst at high temperatures and pressures of 300–500°C and 10–40 MPaA to produce ammonia. The ammonia produced in the reactor outlet gas is cooled, condensed, and separated for recovery as a product, while unreacted nitrogen and hydrogen gas are separated and recycled as feed gas. In the 1980s, a Ru-based supported catalyst was developed as an improvement over the Haber-Bosch process, enabling ammonia production under lower pressures. A process combining this with the Haber-Bosch process has also been commercialized, but the basic production process has remained unchanged for over 100 years. Thus, industrial ammonia production catalysts are generally broadly divided into iron-based and Ru-based catalysts. The theoretical molar ratio of hydrogen to nitrogen (H / N) is ideal for ammonia production. However, because Ru-based catalysts are prone to catalyst poisoning by hydrogen, production conditions with a lower molar ratio are preferred. Considering this point, although not particularly limited, the ammonia production catalytic process to be combined with the ammonia separation technology of the present invention is preferably a process using a Ru-based catalyst, since this approaches the preferred volume ratio of hydrogen gas / nitrogen gas contained in the feed gas in ammonia separation, which will be described later, and this combination makes it possible to reduce the amount of hydrogen permeating during separation of the produced ammonia.

[0030] <Method for decomposing ammonia> The ammonia decomposition method according to this embodiment can be effectively used when efficiently separating ammonia from a mixed gas containing at least ammonia, hydrogen, and nitrogen, and therefore is effective when used in combination with a production method for obtaining hydrogen by decomposing ammonia from which such a mixed gas is obtained. That is, this method includes a first step of producing hydrogen and nitrogen from ammonia, and a second step of separating unreacted ammonia that did not react in the first step by an ammonia separation method described below, and recycles the ammonia recovered in the second step to the first step.

[0031] <Method for separating ammonia> An embodiment of the method for separating ammonia of the present invention is characterized in that a zeolite membrane is used, and a mixed gas composed of a plurality of components including ammonia, hydrogen, and nitrogen is brought into contact with the zeolite membrane, thereby selectively allowing ammonia to permeate and be separated from the mixed gas. The method for separating ammonia of the present invention is characterized in that a mixed gas consisting of ammonia and a plurality of components including hydrogen and / or nitrogen is brought into contact with the zeolite membrane using a specific zeolite membrane, and ammonia is selectively permeated and separated from the mixed gas. As described above, according to the present invention, ammonia gas is produced from hydrogen gas and nitrogen gas in a reactor, and the produced ammonia gas is allowed to permeate through a zeolite membrane in the reactor, thereby enabling efficient production and recovery of ammonia.

[0032] In an embodiment of the present invention, the adsorption of ammonia onto zeolite is utilized, and ammonia separation is based on the hopping mechanism of ammonia within the zeolite pores. Therefore, it is preferable to control the ammonia gas concentration in a mixed gas (feed gas) containing hydrogen gas, nitrogen gas, and ammonia gas to a specific level or higher. This concentration is preferably 1.0 vol% or higher as the ammonia gas concentration in the feed gas. This is because the ammonia adsorbed onto the zeolite is in an adsorption equilibrium relationship with ammonia gas in the gas phase, and the adsorption capacity of ammonia onto the zeolite is highly dependent on the ammonia gas concentration in the feed gas. Among the above, the ammonia gas concentration in the feed gas is preferably 1.0 vol% or higher, more preferably 6.0 vol% or higher, even more preferably 8.0 vol% or higher, particularly preferably 11.0 vol% or higher, particularly preferably 15.0 vol% or higher, and most preferably 18.0 vol% or higher. On the other hand, since the lower the ammonia gas concentration in the feed gas, the more the adsorbed ammonia can reduce the interference with the permeation of ammonia in the feed gas, the ammonia gas concentration in the feed gas is usually preferably less than 50% by volume, preferably 40% by volume or less, more preferably 35% by volume or less, even more preferably 30% by volume or less, particularly preferably 25% by volume or less, and especially preferably 20% by volume or less. Note that the concentration of ammonia in the feed gas is determined by sampling the feed gas, analyzing its components, and using the molar fraction of ammonia as the volume %. The volume % of other gases is also determined by the molar fraction as the volume %.

[0033] In the present invention, the ammonia separation temperature is one of the important factors for improving the ammonia permeability of a zeolite membrane, and is 100°C or higher and 500°C or lower. As mentioned above, under high-pressure conditions, the adsorption-desorption equilibrium of ammonia at the acid sites of a zeolite membrane is generally significantly biased toward adsorption, resulting in a decrease in ammonia permeability. However, within the above temperature range, the acidity of the acid sites is weakened, and the adsorption reaction rate is reduced, making it possible to achieve high permeability even under high-pressure conditions. The temperature range of 100°C or higher and 500°C or lower is a temperature range in which the ammonia production reaction, which produces a mixed gas, proceeds favorably and is therefore considered to be sufficiently high. Furthermore, the separation temperature significantly affects not only the permeability but also the long-term durability of the zeolite membrane used and the production energy balance of the entire process using the ammonia separation membrane. From these perspectives, in the present invention, the temperature during ammonia separation is preferably 450°C or lower, more preferably 400°C or lower, particularly preferably 350°C or lower, particularly preferably 300°C or lower, and most preferably 250°C or lower. On the other hand, from the viewpoint of the adsorption interaction between ammonia and zeolite acid sites, the higher the temperature, the weaker the adsorption and the better the ammonia permeability. Therefore, the separation temperature is more preferably 120°C or higher, particularly preferably 150°C or higher, particularly preferably 180°C or higher, and most preferably 200°C or higher. When ammonia is separated under these preferred temperature conditions, the desorption rate of ammonia adsorbed in the zeolite pores improves, and as a result, the ammonia permeation rate through the zeolite membrane improves. Note that the separation temperature refers to the temperature of the atmosphere in which the zeolite membrane is installed, and specifically refers to the temperature inside a module equipped with a zeolite membrane.

[0034] In the present invention, the higher the pressure of the gas (mixed gas) supplied, the greater the partial pressure difference that drives membrane separation, thereby increasing the amount of ammonia permeating per unit area. Therefore, the pressure of the mixed gas is typically 0.8 MPaA or higher, preferably 1.5 MPaA or higher, more preferably 2.5 MPaA or higher, even more preferably 3.5 MPaA or higher, particularly preferably 3.7 MPaA or higher, particularly preferably 4.0 MPaA or higher, and most preferably 5.0 MPaA or higher. On the other hand, a low pressure makes it easier for ammonia adsorbed on the zeolite acid sites to desorb from the acid sites, increasing the ammonia permeability. From this perspective, the pressure of the mixed gas is typically 20 MPaA or lower, preferably 18 MPaA or lower, more preferably 15 MPaA or lower, even more preferably 12 MPaA or lower, particularly preferably 10 MPaA or lower, particularly preferably 9.0 MPaA or lower, and most preferably 8.1 MPaA or lower. The pressure range of 0.8 MPaA or more and 20 MPaA or less is a temperature range in which the ammonia production reaction that produces the mixed gas proceeds favorably, and therefore can be said to be a sufficiently high pressure.

[0035] The pressure on the permeation side is not particularly limited as long as it is lower than the pressure of the gas on the supply side, but is, for example, 5 MPaA or less, preferably 3 MPaA or less, more preferably 1 MPaA or less, even more preferably 0.5 MPaA or less, and most preferably 0.1 MPaA or less, and may be reduced to a pressure below atmospheric pressure in some cases. When separating ammonia until the concentration of the feed gas becomes low, a low pressure on the permeation side is preferred, and reducing the pressure to below atmospheric pressure makes it possible to separate ammonia until the ammonia gas concentration in the feed gas becomes even lower.

[0036] The differential pressure between the feed-side gas and the permeation-side gas requires a higher feed-side pressure as the differential pressure increases, but since this reduces the effect of the adsorption interaction between ammonia and zeolite acid sites, it is usually 20 MPa or less, preferably 18 MPa or less, more preferably 15 MPa or less, even more preferably 12 MPa or less, particularly preferably 10 MPa or less, especially preferably 9.0 MPa or less, and most preferably 8.5 MPa or less. Furthermore, since the differential pressure is the driving force for membrane separation, a larger differential pressure is better in order to reduce the membrane area, and is usually 0.5 MPa or more, preferably 1.0 MPa or more, more preferably 3.0 MPa or more, even more preferably 3.6 MPa or more, especially preferably 5.0 MPa or more, especially preferably 6.0 MPa or more, and most preferably 8.0 MPa or more.

[0037] Here, the differential pressure refers to the difference between the pressure on the supply side and the pressure on the permeation side of the gas. Furthermore, pressure expressed in units of [PaA] refers to absolute pressure unless otherwise specified, and pressure expressed in units of [Pa] refers to the difference between absolute pressures or the difference between gauge pressures unless otherwise specified.

[0038] In the separation method, the zeolite membrane has an ammonia / nitrogen (NH3 / N2) permeability coefficient ratio of 100 or more. By setting the NH3 / N2 permeability coefficient ratio within this range, only ammonia can be permeated and separated from a mixed gas containing nitrogen and ammonia with high permeability. From the viewpoint of further improving the selective permeability in separation, the NH3 / N2 permeability coefficient ratio is usually 100 or more, preferably 200 or more, more preferably 500 or more, particularly preferably 1000 or more, particularly preferably 3000 or more, particularly preferably 6000 or more, and most preferably 15000 or more. There is no particular upper limit for the NH3 / N2 permeability coefficient ratio, and it is usually 1,000,000 or less.

[0039] In the separation method, the zeolite membrane has an ammonia / hydrogen (NH3 / H2) permeability coefficient ratio of 70 or more. By setting the NH3 / H2 permeability coefficient ratio within this range, ammonia can be permeated and separated from a mixed gas containing hydrogen and ammonia at a high permeability. From the viewpoint of further improving the selective permeability in separation, the NH3 / H2 permeability coefficient ratio is preferably 120 or more, more preferably 1500 or more, particularly preferably 2500 or more, particularly preferably 3500 or more, particularly preferably 4300 or more, and most preferably 5000 or more. There is no particular upper limit for the NH3 / H2 permeability coefficient ratio, and it is usually 1,000,000 or less.

[0040] The permeability ratio described above refers to the ratio of the permeances (transmittances) of two gases under the operating conditions of a separation method according to one embodiment of the present invention. Specifically, the ammonia / nitrogen permeability ratio means ammonia permeance / nitrogen permeance, and the ammonia / hydrogen permeability ratio means ammonia permeance / hydrogen permeance. Permeance (also called "permeability") is the amount of a permeating substance divided by the product of the membrane area, time, and the partial pressure difference between the supply side and permeation side of the permeating substance, and is expressed in units of [mol / (m 2 s·Pa). In an embodiment in which a zeolite membrane composite in which a zeolite membrane is formed on a porous support is mounted in a membrane module, the permeance value measured using the membrane composite is used. The permeance can be measured, for example, using the method described in the Examples below.

[0041] In the separation method, the zeolite membrane allows ammonia gas to pass through with high permeability. In the separation method, the ammonia permeability of the zeolite membrane is typically 6.0 × 10 -8 or more, preferably 8.5 x 10 -8 More preferably, 9.0 × 10 -8 or more, more preferably 1.0 × 10 -7 More preferably, 1.2 × 10 -7 More preferably, 1.5 × 10 -7or more, most preferably 2.0 × 10 -7 The upper limit is not particularly limited, and is usually 3.0 × 10 -4 The following is the result.

[0042] In the separation method, the nitrogen permeability of the zeolite membrane is usually 5.0 × 10 -10 Less than or equal to 1.0 × 10 -10 Less than or equal to 5.0 × 10 -11 Below, particularly preferably 1.0 × 10 -11 Below, most preferably 1.0 × 10 -11 Ideally, the permeance is 0, but in practice it is 1×10 -10 ~1×10 -14 It may be on the order of about this.

[0043] In the separation method, the hydrogen permeability of the zeolite membrane is usually 5.0 × 10 -10 Less than or equal to 1.0 × 10 -10 Less than or equal to 5.0 × 10 -11 Below, particularly preferably 1.0 × 10 -11 Below, most preferably 5.0 × 10 -12 Ideally, the permeance is 0, but in practice it is 1×10 -10 ~1×10 -14 It may be on the order of about this.

[0044] The permeability coefficient ratio can be adjusted to the above range by appropriately setting various conditions in the separation method, as long as the mixed gas pressure and separation temperature satisfy the above ranges. Examples of the conditions to be set include, but are not limited to, the linear velocity of the mixed gas, the zeolite membrane, the mixed gas composition (composition ratio of hydrogen gas to nitrogen gas, ammonia concentration), the pressure on the permeation side, and whether or not a sweep gas is used on the permeation side.

[0045] The linear velocity of the mixed gas (feed gas) should be such that it is possible to compensate for the decrease in the permeating gas and to mix the feed gas so that the concentration of the less permeable gas in the immediate vicinity of the membrane matches the concentration in the entire gas. From the viewpoint of further improving the NH3 / H2 permeability coefficient ratio and the NH3 / N2 permeability coefficient ratio, the linear velocity of the gas is preferably 0.5 cm / sec or more, more preferably 0.7 cm / sec or more, particularly preferably 0.8 cm / sec or more, particularly preferably 0.9 cm / sec or more, and most preferably 1.0 cm / sec, although it depends on the tube diameter of the zeolite membrane and the separation performance of the membrane. Although there is no particular upper limit, if the amount of gas supplied is small, vibrations will occur in the membrane module, which can prevent damage to the membrane, so the linear gas velocity is usually 10 m / sec or less, preferably 5 m / sec or less, more preferably 1 m / sec or less, even more preferably 0.8 m / sec or less, particularly preferably 0.5 m / sec or less, especially preferably 0.3 m / sec or less, and most preferably 0.1 m / sec or less. In this specification, the linear gas velocity refers to the linear gas velocity on the supply side within the membrane module in which the zeolite membrane is mounted.

[0046] The gas composition of the feed gas (mixed gas) is not particularly limited, but the volume ratio of hydrogen gas to nitrogen gas contained in the feed gas is usually 3.0 or less, preferably 2.9 or less, more preferably 2.8 or less, even more preferably 2.5 or less, particularly preferably 2.3 or less, particularly preferably 2.0 or less, and most preferably 1.8 or less. By adjusting the volume ratio to this value, the amount of hydrogen permeated during ammonia separation is reduced, improving ammonia separation selectivity. For these reasons, it is preferable to combine this embodiment with a Ru-based ammonia production catalyst process in which the volume ratio of hydrogen gas to nitrogen gas in the feed gas is reduced. On the other hand, the lower limit of the hydrogen gas to nitrogen gas volume ratio is not particularly limited, but the lower the ratio, the better the ammonia separation selectivity. Therefore, the lower the volume ratio, the more improved ... Here, the stated upper and lower limits are valid within the range of significant figures, i.e., an upper limit of 3 or less means 2.5 or more and less than 3.5, while 0.2 or more means 0.15 or more and less than 0.25, and 1.0 or more means 0.95 or more and less than 1.05.

[0047] In the method for separating ammonia from a mixed gas of the present invention, a sweep gas is not usually used. A sweep gas is a gas supplied to the permeation side of the separation membrane, rather than a gas introduced into the feed gas side before separation and permeation. In other words, a sweep gas is a gas supplied separately from the feed gas before separation and permeation, and a gas different from the feed gas is passed through the permeation side, and the gas that permeates the membrane is recovered. By introducing a sweep gas into the permeation side of the membrane, the ammonia concentration in the membrane can be diluted and the partial pressure difference, which is the driving force for the membrane, can be increased. However, under conditions where the feed gas pressure is high, as in the present invention, the partial pressure difference is large enough even without using a sweep gas. If a sweep gas is used, the ammonia gas that has permeated the membrane and become concentrated is diluted, which can make it difficult to reuse the recovered ammonia. On the other hand, in the case where the ammonia concentration on the non-permeated side is reduced, the partial pressure difference is increased using a sweep gas, which allows ammonia to permeate the membrane more efficiently and recovers a gas with a lower ammonia concentration than the feed gas. Therefore, when it is desired to reduce the ammonia concentration in the non-permeated side gas, a sweep gas may be used. When a sweep gas is used in the present invention, for example, gas 9 supplied from line 12 shown in Fig. 1 is used as the sweep gas. The pressure of the sweep gas is usually atmospheric pressure, but is not particularly limited to atmospheric pressure and is preferably 2 MPaA or less, more preferably 1 MPaA or less, and even more preferably 0.5 MPaA or less, with the lower limit being preferably 0.05 MPaA or more, more preferably 0.10 MPaA or more, and even more preferably 0.15 MPaA or more. In some cases, the sweep gas may be used at a reduced pressure.

[0048] The linear velocity of the sweep gas is not particularly limited, but is usually 0.5 mm / sec or more, preferably 1 mm / sec or more, and there is no upper limit, but it is usually 1 m / sec or less, preferably 0.5 m / sec or less.

[0049] The apparatus used for gas separation is not particularly limited, but a zeolite membrane is usually used as a membrane module (hereinafter, "zeolite membrane and / or separation apparatus using a zeolite membrane" may be simply referred to as "membrane module"). The membrane module may be, for example, an apparatus as schematically shown in FIG. 1, or a membrane module exemplified in "Gas Separation and Purification Technology" (Toray Research Center, Inc., 2007, p. 22, etc.). In the membrane module, the zeolite membrane may be used in the form of a zeolite membrane composite in which the zeolite membrane is formed on a porous support. The operation of separating a mixed gas in the apparatus of FIG. 1 will be explained in the Examples section.

[0050] When performing membrane separation of ammonia from a mixed gas, membrane modules may be used in multiple stages. In this case, the gas to be separated may be supplied to the first membrane module, and the non-permeated gas that does not permeate the membrane may be further supplied to the second membrane module, or the permeated gas may be supplied to the second membrane module. The former method can further increase the concentration of less permeable components in the non-permeated side, while the latter method can further increase the concentration of more permeable components in the permeated gas. A combination of these methods can also be used. When separation is performed using membrane modules provided in multiple stages, the pressure of the feed gas may be adjusted by a pressure booster or the like as necessary when the gas is supplied to the membrane module in the subsequent stage.

[0051] Furthermore, when using multiple membrane modules, membranes with different performance characteristics can be installed in each stage. Generally, membranes with high permeability have low separation performance, while membranes with high separation performance tend to have low permeability. Therefore, when processing gas components to be separated or concentrated to a desired concentration, membranes with high permeability require a smaller membrane area, but low-permeability components also easily permeate to the permeate side, resulting in a low concentration of highly permeable components in the permeate side gas. Conversely, membranes with high separation performance tend to limit the permeation of low-permeability components to the permeate side, resulting in a high concentration of highly permeable components in the permeate side gas, but require a larger membrane area. Separation using a single type of membrane makes it difficult to control the relationship between the required membrane area and the permeation and non-permeation amounts of the target gas for concentration or separation. However, using membranes with different performance characteristics makes this easier. By selecting the optimal membrane area and the permeation and non-permeation amounts of the target gas for concentration or separation, based on the membrane cost and the price of the gas to be separated and recovered, membranes can be installed to maximize overall benefits.

[0052] For example, if ammonia cannot be sufficiently separated with one membrane separation stage, the non-permeate gas can be separated using several more membrane stages. Also, if the membrane's ammonia / hydrogen separation is not sufficient with one membrane separation stage and a large amount of hydrogen is contained along with ammonia on the permeate side, the permeate gas can be separated using a membrane with high ammonia and hydrogen separation performance.

[0053] The zeolite membrane used in the present invention preferably has excellent chemical resistance, oxidation resistance, heat stability, and pressure resistance, and also exhibits high ammonia permeability and separation performance, and has excellent durability.

[0054] The term "high permeability" used here means a sufficient throughput, for example, the permeance [mol / (m 2 ·s·Pa)] (ammonia permeability) is typically 4.0 × 10 when ammonia is passed through at a temperature of 200°C and a differential pressure of 8 MPa. -8 or more, preferably 5.0 x 10 -8 More preferably, 7.0 × 10 -8 or more, more preferably 1.0 × 10 -7 More preferably, 1.2 × 10 -7 More preferably, 1.5 × 10 -7 or more, most preferably 2.0 × 10 -7 The upper limit is not particularly limited, and is usually 3.0 × 10 -4 The following is the result.

[0055] In addition, the permeance of the nitrogen component of the zeolite membrane used in the present invention [mol / (m 2 ·s·Pa)] (nitrogen permeability) is usually 1.0×10 when nitrogen is passed through at a temperature of 200°C and a differential pressure of 8 MPa. -9 Less than 5.0 × 10 -10 or less, more preferably 1.0 × 10 -10 Below, particularly preferably 5.0 × 10 -11 Below, most preferably 1.0 × 10 -11 Ideally, the permeance is 0, but in practice it is 1×10 -10 ~1×10 -14 It may be on the order of about this.

[0056] In addition, the hydrogen component permeance [mol / (m 2·s·Pa)] (hydrogen permeability) is usually 1.0×10 when hydrogen is permeated at a temperature of 200°C and a differential pressure of 8 MPa. -9 Less than 5.0 × 10 -10 or less, more preferably 1.0 × 10 -10 Below, particularly preferably 5.0 × 10 -11 Below, most preferably 1.0 × 10 -11 Ideally, the permeance is 0, but in practice it is 1×10 -10 ~1×10 -14 It may be on the order of about this.

[0057] Here, permeance (also called "permeability") is the amount of permeating substance divided by the product of the membrane area, time, and the partial pressure difference between the supply side and permeation side of the permeating substance, and its unit is [mol / (m 2 In an embodiment in which a zeolite membrane composite in which a zeolite membrane is formed on a porous support is mounted in a membrane module, the permeance value measured using the membrane composite is used.

[0058] (Zeolite) In the present invention, the zeolite constituting the zeolite membrane is preferably an aluminosilicate. The aluminosilicate is composed mainly of oxides of Si and Al, and may contain other elements as long as the effects of the present invention are not impaired. The cationic species contained in the zeolite of the present invention are preferably cationic species that are easily coordinated to the ion exchange sites of the zeolite, such as cationic species selected from the group consisting of elements of Groups 1, 2, 8, 9, 10, 11, and 12 of the periodic table, NH4 + and two or more of these cationic species, more preferably cationic species selected from the group of elements of Groups 1 and 2 of the periodic table, NH4 + and two or more cationic species thereof.

[0059] The SiO2 / Al2O3 molar ratio of the aluminosilicate is not particularly limited, but is usually 6 or more, preferably 7 or more, more preferably 8 or more, even more preferably 10 or more, particularly preferably 11 or more, particularly preferably 12 or more, and most preferably 13 or more. On the other hand, it is usually 500 or less, preferably 100 or less, more preferably 75 or less, even more preferably 50 or less, even more preferably 40 or less, and most preferably 30 or less. Using a zeolite with an SiO2 / Al2O3 molar ratio in this specific range is preferable because it improves the denseness of the zeolite membrane and its durability, such as chemical reactivity resistance and heat resistance. Furthermore, from the perspective of the separation performance of ammonia permeation from a mixed gas containing multiple components including ammonia, hydrogen, and nitrogen, it is preferable to use a zeolite containing a larger amount of Al, because the acid sites of the Al element serve as ammonia adsorption sites, as described above. Using a zeolite with the above SiO2 / Al2O3 molar ratio allows for high ammonia permeability and high selectivity separation.

[0060] The structure of the zeolite used in the present invention, when expressed in terms of the codes specified by the International Zeolite Association (IZA), includes, for example, ABW, ACO, AEI, AEN, AFI, AFT, AFX, ANA, ATN, ATT, ATV, AWO, AWW, BIK, CHA, DDR, DFT, EAB, EPI, ERI, ESV, GIS, GOO, ITE, JBW, KFI, LEV, LTA, MER, MON, MTF, OWE, PAU, PHI, RHO, RTE, RWR, SAS, SAT, SAV, SIV, TSC, UFI, VNI, YUG, AEL, AFO, AHT, DAC, FER, HEU, IMF, ITH, MEL, MFS, MWW, OBW, RRO, SFG, STI, SZR, TER, TON, TUN, WEI, MFI, MON, PAU, PHI, MOR, and FAU.

[0061] Among them, the framework density is 19.0T / nm 3The following zeolites are preferred, more preferably AEI, AFX, CHA, DDR, ERI, LEV, LTA, RHO, MOR, MFI, FAU, even more preferably AEI, AFX, CHA, DDR, LTA, RHO, MOR, MFI, FAU, particularly preferably CHA, LTA, RHO, MOR, MFI, or FAU, particularly preferably CHA, RHO, or MFI, and most preferably RHO or MFI. By using a zeolite with a low framework density, when permeating components other than ammonia are present in a mixed gas containing ammonia, the resistance to permeation of these permeating components can be reduced, making it easier to increase the amount of ammonia permeated. The framework density of the zeolite membrane used in the present invention is 19.0 T / nm 3 The following zeolites are preferred, more preferably AFX, CHA, DDR, ERI, LTA, RHO, MOR, MFI, and FAU, even more preferably CHA, DDR, LTA, RHO, MOR, MFI, and FAU, particularly preferably CHA, RHO, or MFI, and most preferably RHO and MFI. On the other hand, the higher the framework density, the better, typically 13.0T / nm 3 or more, preferably 13.5T / nm 3 More preferably, 14.0 T / nm or more 3 More preferably, 14.3 T / nm 3 More than 15.0 T / nm, particularly preferably 15.0 T / nm 3 More preferably, 16.0 T / nm or more 3 Above 17.0T / nm, most preferably 17.0T / nm 3 Ammonia moves within the zeolite pores from the high-pressure side to the low-pressure side while hopping between adsorption points in the zeolite, so a higher framework density means that ammonia is more strongly attracted to the adsorption points on the low-pressure side, thereby increasing the rate at which ammonia moves within the membrane.

[0062] Here, the framework density (unit: T / nm 3 ) is the unit volume of zeolite (1 nm 3It means the number of T atoms (atoms other than oxygen that make up the zeolite framework) present per 1000 sq m (1000 sq m), and this value is determined by the structure of the zeolite. The relationship between framework density and zeolite structure is shown in ATLAS OF ZEOLITE FRAMEWORK TYPES Sixth Revised Edition 2007 ELSEVIER.

[0063] The zeolite membrane used in the ammonia separation method of the present invention has a small effective pore size when the ions in the zeolite are exchanged with monovalent ions having a large ionic radius, whereas when the ions are exchanged with monovalent ions having a small ionic radius, the effective pore size approaches the pore size inherent to the zeolite structure. Additionally, the permeation rate can be controlled by controlling the molecular adsorption performance.

[0064] The effective pore size of zeolite can also be reduced by silylation treatment. For example, by silylating the terminal silanols on the outer surface of a zeolite membrane and then laminating a silylated layer, the effective pore size of the pores facing the outer surface of the zeolite can be reduced.

[0065] The separation function of the zeolite membrane used in the present invention is not particularly limited, but is manifested by controlling the surface properties of the zeolite to control the affinity and adsorption of gas molecules to the zeolite membrane. That is, by controlling the polarity of the zeolite, the adsorption of ammonia to the zeolite can be controlled, making it easier for ammonia to pass through.

[0066] By substituting Al atoms for Si atoms in the zeolite framework, the polarity can be increased, allowing highly polar gas molecules such as ammonia to be actively adsorbed and permeated through the zeolite pores. It is also possible to control the polarity of the resulting zeolite by adding other atom sources besides Al, such as Ga, Fe, B, Ti, Zr, Sn, and Zn, to the aqueous reaction mixture for hydrothermal synthesis.

[0067] (Zeolite membrane) The zeolite membrane in the present invention refers to a membranous material made of zeolite, preferably formed by crystallizing zeolite on the surface of a porous support. The membrane may contain, as necessary, inorganic binders such as silica and alumina, organic substances such as polymers, or silylating agents for modifying the zeolite surface, in addition to zeolite. The preferred zeolite contained in the zeolite membrane used in the present invention is as described above, but the zeolite contained in the zeolite membrane may be one type or multiple types. Furthermore, the zeolite membrane may contain zeolites that tend to form in a mixed phase, such as ANA, GIS, and MER, or amorphous components other than crystals.

[0068] The zeolite membrane used in the present invention may be a zeolite membrane characterized in that the molar ratio of nitrogen element to Al element determined by X-ray photoelectron spectroscopy is 0.01 or more and 4 or less. The zeolite membrane preferably has a surface in which the molar ratio of nitrogen atoms to Al atoms, as determined by X-ray photoelectron spectroscopy (XPS), falls within a specific range. Here, the "surface of the zeolite membrane" in this specification refers to the surface of the zeolite membrane on the side where a mixed gas consisting of multiple components including ammonia and hydrogen and / or nitrogen is supplied for ammonia separation. When the zeolite membrane composite is used in the form of a membrane formed on a porous support, it refers to the side that is not in contact with the porous support. In this specification, the molar ratio of nitrogen atoms to Al atoms contained in the zeolite membrane is a value determined by X-ray photoelectron spectroscopy (XPS) under the following measurement conditions:

[0069] (Measurement conditions) X-ray source during measurement: Monochromated Al-Kα ray, output 16kV-34W Background determination in quantitative calculations: Shirley's method

[0070] The zeolite membrane used in the present invention has a nitrogen atom content on the zeolite membrane surface, as determined by the XPS measurement, relative to the Al atoms on the zeolite membrane surface, expressed as a molar ratio of typically 0.01 or more, preferably 0.05 or more, more preferably 0.10 or more, even more preferably 0.20 or more, particularly preferably 0.30 or more, and particularly preferably 0.50 or more. The upper limit is not particularly limited because it depends on the structure of the nitrogen-containing cation species in the zeolite membrane and the amount of nitrate ions remaining after nitration treatment of the zeolite membrane, if necessary. However, it is typically 4 or less, preferably 3 or less, and more preferably 1 or less. The use of a zeolite having a surface composition with such a specific nitrogen atom / Al atomic ratio can improve the denseness and durability of the zeolite membrane, such as chemical reactivity resistance and heat resistance, and can separate ammonia with high permeability and high selectivity from a mixed gas consisting of ammonia and multiple components including hydrogen and / or nitrogen. The upper and lower limits are defined within the range of significant figures. That is, the upper limit of 4 or less means less than 4.5, while 0.01 or more means 0.005 or more.

[0071] The nitrogen atoms contained in the zeolite membrane used in the present invention are substituted by ammonium ions (NH4 + nitrogen atoms derived from cationic species protonated from organic amines having 1 to 20 carbon atoms, such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, ethylenediamine, dimethylethylenediamine, tetramethylethylenediamine, diethylenetriamine, triethylenetetraamine, aniline, methylaniline, benzylamine, methylbenzylamine, hexamethylenediamine, N,N-diisopropylethylamine, N,N,N-trimethyl-1-adamantanamine, pyridine, and piperidine; nitrogen atoms derived from an organic template when a nitrogen-containing organic template (structure-directing agent) is used in producing a zeolite membrane; and nitrogen atoms derived from nitrate ions remaining after the nitrate treatment of a zeolite membrane, which is performed as needed, as described below.

[0072] Controlling the content of Si atoms on the zeolite membrane surface and, if necessary, controlling the content of nitrogen atoms on the zeolite membrane surface, as determined by X-ray photoelectron spectroscopy (XPS), to a specific range tends to significantly improve the separation selectivity when separating ammonia from a mixed gas composed of multiple components contained on the zeolite membrane surface. Therefore, it is preferable to allow nitrogen atoms to coexist on the zeolite membrane surface and appropriately control their content. When nitrogen atoms are present on the zeolite membrane surface as necessary, the content of the nitrogen atoms relative to the Al atoms on the zeolite membrane surface is typically 0.01 or more, preferably 0.05 or more, more preferably 0.10 or more, even more preferably 0.20 or more, particularly preferably 0.30 or more, and particularly preferably 0.50 or more. The upper limit is not particularly limited because it depends on the structure of the nitrogen-containing cation species in the zeolite contained in the zeolite membrane and the amount of nitrate ions remaining when the zeolite membrane is subjected to nitrate treatment as necessary, but is typically 4 or less, preferably 3 or less, and more preferably 1 or less. The use of a zeolite having a surface composition with such a specific nitrogen atom / Al atomic ratio is preferable because it can improve the denseness of the zeolite membrane and its durability, such as chemical reactivity resistance and heat resistance, and also enables highly selective separation of ammonia from a mixed gas consisting of ammonia and multiple components including hydrogen and / or nitrogen. The upper and lower limits described above are valid within the range of significant figures. That is, an upper limit of 4 or less means less than 4.5, while a value of 0.01 or more means 0.005 or more.

[0073] In this embodiment, furthermore, controlling the content of alkali metal atoms contained in the zeolite membrane surface, as determined by XPS measurement, within a specific range tends to improve ammonia permeability during ammonia separation from a mixed gas containing ammonia and multiple components including hydrogen and / or nitrogen. Therefore, controlling the content as needed is one of the preferred embodiments. Examples of alkali metal atoms present on the zeolite membrane surface as needed include Li, Na, K, Rb, Cs, and atoms of two or more of these metals. Among these, Li, Na, and Cs are preferred, with Na being more preferred because of its excellent ammonia separation performance and being a commonly used alkali metal. These alkali metal atoms exist in the form of cations as ion pairs at the Al sites in the zeolite constituting the zeolite membrane. They are typically introduced into the zeolite by ion exchange treatment of the synthesized zeolite membrane, as described below. When alkali metal atoms are present on the zeolite membrane surface as needed, the content of the alkali metal atoms relative to the Al atoms on the zeolite membrane surface is, in molar ratio, 0.01 or more, preferably 0.02 or more, more preferably 0.03 or more, even more preferably 0.04 or more, and particularly preferably 0.05 or more, with the upper limit usually being 0.10 molar equivalents or less, preferably 0.070 molar equivalents or less, more preferably 0.065 molar equivalents or less, even more preferably 0.060 molar equivalents or less, and particularly preferably 0.055 molar equivalents or less. Controlling the alkali metal atom content within the above range tends to improve ammonia permeability while maintaining high ammonia separation selectivity, which is preferable.

[0074] The zeolite membrane of the present invention may be a zeolite membrane containing zeolite, and the molar ratio of alkali metal element to Al element determined by X-ray photoelectron spectroscopy may be 0.01 or more and 0.070 or less. The zeolite membrane of the present invention preferably has a surface in which the molar ratio of alkali metal atoms to Al atoms, as determined by X-ray photoelectron spectroscopy (XPS), falls within a specific range. In this specification, the molar ratio of alkali metal atoms to Al atoms contained in the zeolite membrane is a value determined by X-ray photoelectron spectroscopy (XPS) under the following measurement conditions:

[0075] In this embodiment, examples of alkali metal atoms contained in the zeolite membrane surface determined by the above-mentioned XPS measurement include Li, Na, K, Rb, Cs, and two or more of these metal atoms. Among these, Li, Na, and Cs are preferred, with Na being more preferred because it has excellent ammonia separation performance and is a commonly used alkali metal. These alkali metal atoms exist in the form of cations as ion pairs at the Al sites in the zeolite constituting the zeolite membrane, and are usually introduced into the zeolite by ion exchange treatment of the synthesized zeolite membrane, as described below.

[0076] In this embodiment, it is important to control the content of alkali metal atoms contained in the zeolite membrane surface as determined by the XPS measurement described above. The content, in terms of molar ratio relative to Al atoms on the zeolite membrane surface, is 0.01 or more, preferably 0.02 or more, more preferably 0.03 or more, even more preferably 0.04 or more, and particularly preferably 0.05 or more, with the upper limit usually being 0.10 molar equivalents or less, preferably 0.070 molar equivalents or less, more preferably 0.065 molar equivalents or less, even more preferably 0.060 molar equivalents or less, and particularly preferably 0.055 molar equivalents or less. By controlling the alkali metal element content within the above range, it is possible to improve ammonia permeability while maintaining high ammonia separation selectivity.

[0077] The zeolite membrane used in the present invention may be a zeolite membrane composite for ammonia separation comprising a porous support and a zeolite membrane containing zeolite on the surface of the porous support, and the rate of change in the thermal expansion coefficient of the zeolite at 300°C and at 400°C relative to the thermal expansion coefficient at 30°C may be within a specific range. Specifically, the rate of change in the thermal expansion coefficient of the zeolite at 300°C relative to the thermal expansion coefficient at 30°C may be within ±0.25%, and the rate of change in the thermal expansion coefficient at 400°C relative to the thermal expansion coefficient at 30°C may be within ±0.35%. The thermal expansion coefficient that defines the zeolite of this embodiment is a numerical value calculated under the following conditions. In this specification, when the numerical value of the thermal expansion coefficient is a positive number, it indicates that the zeolite has expanded, and when it is a negative number, it indicates that the zeolite has contracted.

[0078] (Method for measuring the rate of change in thermal expansion coefficient) In the present invention, the rate of change in the thermal expansion coefficient of a zeolite at a predetermined temperature relative to the thermal expansion coefficient at 30°C can be calculated by the following formula (1) using the crystallite constant measured at 30°C and the predetermined temperature by a temperature-programmed XRD measurement method under the following conditions.

[0079] (Temperature-raised XRD measurement equipment specifications)

[0080] [Table 1]

[0081] (Measurement conditions)

[0082] [Table 2] Measurement atmosphere: air Temperature rise condition: 20℃ / min Measurement method: XRD measurement is carried out after holding at the measurement temperature for 5 minutes. The measurement data is subjected to fixed slit correction using a variable slit.

[0083] Rate of change of thermal expansion coefficient = (crystal lattice constant measured at a given temperature) ÷ (crystal lattice constant measured at 30°C) - 1 (1)

[0084] The rate of change in the thermal expansion coefficient of the zeolite used in the present invention at 300°C relative to the thermal expansion coefficient at 30°C is, in absolute value, 0.25% or less, preferably 0.20% or less, more preferably 0.15% or less, particularly preferably 0.10% or less, and most preferably 0.05% or less. That is, the rate of change in the thermal expansion coefficient of the zeolite at 30°C relative to the thermal expansion coefficient at 300°C is within ±0.25%, preferably within ±0.20%, more preferably within ±0.15%, particularly preferably within ±0.10%, and most preferably within ±0.05%. On the other hand, the rate of change in the thermal expansion coefficient of zeolite at 400°C relative to the thermal expansion coefficient at 30°C is, in absolute value, 0.35% or less, preferably 0.30% or less, more preferably 0.25% or less, particularly preferably 0.20% or less, particularly preferably 0.15% or less, and most preferably 0.10% or less. That is, the rate of change in the thermal expansion coefficient of zeolite at 400°C relative to the thermal expansion coefficient at 30°C is within ±0.35%, preferably within ±0.30%, more preferably within ±0.25%, particularly preferably within ±0.20%, particularly preferably within ±0.15%, and most preferably within ±0.10%.

[0085] Although the reason for this is not yet clear and is not particularly limited, it is thought that even if the zeolite thermally contracts or expands during the temperature rise process, the zeolite moves appropriately on the support, does not generate cracks, and forms a dense zeolite membrane composite that exhibits high separation performance suitable for high-temperature conditions. Therefore, to stably separate ammonia under high-temperature conditions, a zeolite that exhibits nonlinear thermal expansion / contraction behavior during the temperature rise process may be used. The zeolite used in the present invention is not particularly limited, but known examples include RHO (DR Corbin et al. J. Am. Chem. Soc. 112, 4821-4830), MFI, AFI, and DDR (Park S. H. et al. Stud. Surf. Sci. Catal. 1997, 105, 1989-1994).

[0086] Furthermore, the ratio of the rate of change in thermal expansion coefficient at 300°C to the rate of change in thermal expansion coefficient at 30°C of the zeolite of this embodiment at 400°C to the rate of change in thermal expansion coefficient at 30°C is typically 120% or less, preferably 115% or less, more preferably 110% or less, particularly preferably 105% or less, and most preferably 103% or less, in absolute value. A zeolite composite membrane formed on a porous support using a zeolite exhibiting such a specific ratio of rate of change in thermal expansion coefficient between specific temperatures can suppress the occurrence of grain boundary cracks due to local thermal expansion (contraction) of the zeolite, even when non-uniform heat generation occurs in a reactor at the beginning of the ammonia production reaction, for example, and therefore can stably and efficiently separate ammonia to the permeation side at high permeability.

[0087] The zeolite composite membrane of this embodiment is preferably prepared by a process of attaching a zeolite having a thermal expansion coefficient change rate within a specific range to a porous support as seed crystals during membrane synthesis, which often enables stable and highly selective ammonia separation even under high-temperature conditions. The absolute value of the change in thermal expansion coefficient of the zeolite used as seed crystals in the preparation of such a zeolite composite membrane is 0.25% or less, preferably 0.20% or less, more preferably 0.15% or less, particularly preferably 0.10% or less, and most preferably 0.05% or less, relative to the thermal expansion coefficient at 30°C. On the other hand, the absolute value of the change in thermal expansion coefficient at 400°C relative to the thermal expansion coefficient at 30°C is usually 0.30% or less, preferably 0.25% or less, more preferably 0.20% or less, particularly preferably 0.15% or less, and most preferably 0.10% or less.

[0088] The rate of change in the thermal expansion coefficient of a zeolite at a specific temperature, which is a feature of this embodiment, can be controlled by appropriately selecting the cation species of the zeolite used, as described below. For example, the relationship between the cation species and the thermal expansion coefficient of RHO zeolite is known to vary depending on the cation species contained in the zeolite, as described in Chemical Communications, 2000, pp. 2221-2222. Therefore, in order to obtain a zeolite membrane that can stably separate ammonia with high selectivity even under the high-temperature conditions of this embodiment, it is particularly important to select a specific cation species from among the RHO zeolites. Meanwhile, with regard to the thermal expansion coefficient of the MFI zeolite described in the examples of this embodiment, a zeolite membrane exhibiting the characteristics of this embodiment can be produced by appropriately selecting the cation species in the zeolite, as in the case of the RHO zeolite described above. The cationic species contained in the zeolite of this embodiment are preferably cationic species that are easily coordinated to the ion exchange sites of the zeolite, and examples thereof include cationic species selected from the group consisting of elements of Groups 1, 2, 8, 9, 10, 11, and 12 of the periodic table, NH4 +and two or more of these cationic species, more preferably cationic species selected from the group of elements of Groups 1 and 2 of the periodic table, NH4 + and two or more cationic species thereof.

[0089] The zeolite used in this embodiment is preferably an aluminosilicate. The SiO2 / Al2O3 molar ratio of the aluminosilicate is not particularly limited, but is usually 6 or more, preferably 7 or more, more preferably 8 or more, even more preferably 10 or more, particularly preferably 11 or more, especially preferably 12 or more, and most preferably 13 or more. The upper limit is usually such that the amount of Al is approximately the same as an impurity, and the SiO2 / Al2O3 molar ratio is usually 500 or less, preferably 100 or less, more preferably 75 or less, even more preferably 50 or less, especially preferably 40 or less, and most preferably 30 or less. Using a zeolite with an SiO2 / Al2O3 molar ratio in this specific range can improve the density of the zeolite membrane as well as its durability, such as chemical reactivity resistance and heat resistance. Furthermore, from the viewpoint of separation performance for permeating ammonia from a gas mixture consisting of multiple components including ammonia and hydrogen and / or nitrogen, it is preferable to use a zeolite containing a specific amount of Al, because the acid sites of the Al element serve as ammonia adsorption sites, as described above. By using a zeolite having the above SiO2 / Al2O3 molar ratio, ammonia can be separated with high selectivity and high permeability. The SiO2 / Al2O3 molar ratio of the zeolite can be adjusted by the reaction conditions of the hydrothermal synthesis, which will be described later.

[0090] The thickness of the zeolite membrane used in the present invention is not particularly limited, but is usually 0.01 μm or more, preferably 0.1 μm or more, more preferably 0.3 μm or more, even more preferably 0.7 μm or more, even more preferably 1.0 μm or more, and particularly preferably 1.5 μm or more. It is also usually 100 μm or less, preferably 60 μm or less, more preferably 20 μm or less, even more preferably 15 μm or less, even more preferably 10 μm or less, and particularly preferably 5 μm or less. When the zeolite membrane thickness is equal to or greater than the above-mentioned lower limit, defects tend to be less likely to occur and separation performance tends to be improved. When the zeolite membrane thickness is equal to or less than the above-mentioned upper limit, permeation performance tends to be improved. Furthermore, in the high-temperature region, cracks are less likely to occur in the zeolite membrane due to temperature rise, which tends to suppress a decrease in separation selectivity at high temperatures.

[0091] The average primary particle size of the zeolite forming the zeolite membrane is not particularly limited, but is usually 5 nm or more, preferably 10 nm or more, more preferably 20 nm or more, and even more preferably 30 nm or more, with the upper limit being equal to or less than the membrane thickness. If the average primary particle size of the zeolite is equal to or greater than the above-mentioned lower limit, the zeolite grain boundaries can be made small, thereby achieving good separation selectivity. Therefore, it is most preferable that the average primary particle size of the zeolite is the same as the thickness of the zeolite membrane. In this case, the zeolite grain boundaries can be made the smallest. Zeolite membranes obtained by hydrothermal synthesis, which will be described later, are preferred because the zeolite particle size and membrane thickness may be the same. In the present invention, the average primary particle diameter is determined by measuring the primary particle diameters of 30 or more arbitrarily selected particles when observing the surface or fracture surface of the zeolite membrane of the present invention with a scanning electron microscope, and then calculating the average value.

[0092] The shape of the zeolite membrane is not particularly limited, and any shape can be adopted, such as tubular, hollow fiber, monolith, honeycomb, etc. The size of the zeolite membrane is also not particularly limited, and it is formed, for example, as a zeolite membrane composite formed on a porous support having the size described below.

[0093] (Porous support) In the present invention, the zeolite membrane is preferably formed on the surface of a porous support, etc. Preferably, the zeolite is crystallized in the form of a membrane on the porous support.

[0094] The porous support used in the present invention preferably has chemical stability sufficient to allow zeolite to crystallize into a membrane on its surface. Suitable porous supports include gas-permeable porous polymers such as polysulfone, cellulose acetate, aromatic polyamide, vinylidene fluoride, polyethersulfone, polyacrylonitrile, polyethylene, polypropylene, polytetrafluoroethylene, and polyimide; sintered ceramics such as silica, α-alumina, γ-alumina, mullite, zirconia, titania, yttria, silicon nitride, and silicon carbide; sintered metals and mesh-like molded bodies such as iron, bronze, and stainless steel; and inorganic porous bodies such as glass and carbon molded bodies. Among these, inorganic porous supports such as sintered ceramics, sintered metals, glass, and carbon molded bodies are preferred for use in ammonia separation at high temperatures because of their excellent mechanical strength, deformation resistance, thermal stability, and reactivity resistance at high temperatures. The inorganic porous support is preferably a sintered ceramic, a solid material whose basic or majority components are composed of inorganic non-metallic substances.

[0095] As described above, preferred ceramic sintered bodies include ceramic sintered bodies containing α-alumina, γ-alumina, silica, mullite, zirconia, titania, yttria, silicon nitride, silicon carbide, etc., but these may be sintered bodies of a single material or a mixture of two or more materials sintered together. A part of the surface of these ceramic sintered bodies may be converted into zeolite during zeolite membrane synthesis, which increases the adhesion between the porous support and the zeolite membrane, thereby improving the durability of the zeolite membrane composite. In particular, inorganic porous supports containing at least one of alumina, silica, and mullite are more preferred because they can be easily partially zeoliteized, resulting in a stronger bond between the inorganic porous support and the zeolite, making it easier to form a dense zeolite membrane with high separation performance.

[0096] The porous support used in the present invention preferably has, on its surface (hereinafter also referred to as "porous support surface"), the action of crystallizing the zeolite formed on the porous support.

[0097] The pore size of the porous support surface is preferably controlled. The average pore size of the porous support near the surface is usually 0.02 μm or more, preferably 0.05 μm or more, more preferably 0.1 μm or more, more preferably 0.15 μm or more, even more preferably 0.5 μm or more, particularly preferably 0.7 μm or more, most preferably 1.0 μm or more, and usually 20 μm or less, preferably 10 μm or less, more preferably 5 μm or less, and particularly preferably 2 μm or less. By using a porous support having a pore size in this range, a dense zeolite membrane that improves ammonia separation selectivity can be formed. The surface of the porous support is preferably smooth, and the surface may be polished with a file or the like, if necessary.

[0098] The pore size of the porous support used in the present invention in the portion other than the vicinity of the surface of the porous support is not limited and does not need to be particularly controlled, but the porosity of the other portion is usually 20% or more, more preferably 30% or more, and usually 60% or less, preferably 50% or less. The porosity of the portion other than the vicinity of the surface of the porous support affects the permeation flow rate when separating gas or liquid, and when the porosity is above the above-mentioned lower limit, the permeate tends to diffuse easily, while when it is below the above-mentioned upper limit, it tends to be easier to prevent a decrease in the strength of the porous support. In addition, as a method of controlling the permeation flow rate, a porous support formed by combining porous bodies with different porosities in layers may be used.

[0099] The shape of the porous support used in the present invention is not limited as long as it can effectively separate mixed gases or liquid mixtures, and specific examples include flat, tubular, cylindrical, honeycomb-shaped structures with numerous through-holes, and monolithic structures. The size of the porous support is optional and may be appropriately selected and adjusted so as to obtain the desired zeolite membrane composite. Among these, a tubular shape of the porous support may be preferable.

[0100] The length of the tubular porous support is not particularly limited, but is usually 2 cm or more, preferably 4 cm or more, more preferably 5 cm or more, particularly preferably 8 cm or more, particularly preferably 10 cm or more, and most preferably 40 cm or more, and is usually 200 cm or less, preferably 150 cm or less, more preferably 130 cm or less, even more preferably 120 cm or less, and particularly preferably 100 cm or less. When the length of the porous support is equal to or greater than the above lower limit, the amount of mixed gas separated per support can be increased, thereby reducing equipment costs. On the other hand, when the length is equal to or less than the above upper limit, the production of the zeolite membrane composite can be simplified, and further, problems such as fragility due to vibration during use can be prevented.

[0101] The inner diameter of the tubular porous support is usually 0.1 cm or more, preferably 0.2 cm or more, more preferably 0.3 cm or more, particularly preferably 0.4 cm or more, and usually 2 cm or less, preferably 1.5 cm or less, more preferably 1.2 cm or less, and particularly preferably 1.0 cm or less. The outer diameter is usually 0.2 cm or more, preferably 0.3 cm or more, more preferably 0.6 cm or more, and particularly preferably 1.0 cm or more, and usually 2.5 cm or less, preferably 1.7 cm or less, and more preferably 1.3 cm or less. The wall thickness of the tubular porous support is usually 0.1 mm or more, preferably 0.3 mm or more, more preferably 0.5 mm or more, even more preferably 0.7 mm or more, even more preferably 1.0 mm or more, and particularly preferably 1.2 mm or more, and usually 4 mm or less, preferably 3 mm or less, and more preferably 2 mm or less. If the inner diameter, outer diameter, and wall thickness of the tubular porous support are each above the above lower limit, the strength of the support can be improved and it can be made less likely to break. Furthermore, if the inner diameter and outer diameter of the tubular support are each equal to or less than the above upper limit, the size of the equipment required for ammonia separation can be reduced, which can be economically advantageous. Furthermore, if the wall thickness of the tubular support is equal to or less than the above upper limit, the permeation performance tends to be improved.

[0102] (Zeolite membrane composite) In the present invention, the zeolite membrane is preferably used as a zeolite membrane composite comprising at least zeolite and a support. In the present invention, the zeolite membrane composite is a composite in which the above-mentioned zeolite is fixed in the form of a membrane, preferably in a crystallized state, to the surface of the above-mentioned porous support or the like, and in some cases, it is preferable that a part of the zeolite is fixed to the inside of the support. The zeolite membrane composite is preferably, for example, one in which zeolite is crystallized into a membrane on the surface of a porous support by hydrothermal synthesis.

[0103] The position of the zeolite membrane on the porous support is not particularly limited. When a tubular support is used, the zeolite membrane may be formed on the outer surface, the inner surface, or even on both surfaces depending on the application system. The zeolite membrane may be formed by laminating it on the surface of the support, or may be crystallized so as to fill the pores in the surface layer of the support. In this case, it is important that the crystallized membrane layer does not have cracks or continuous micropores, and forming a so-called dense membrane is preferable in terms of improving separation performance.

[0104] Furthermore, there are no particular limitations on the zeolite and support that constitute the zeolite membrane composite, and it is preferable to use any combination of the above-mentioned zeolites and supports. Among these, particularly preferred combinations of zeolite and porous support include an MFI type zeolite-porous alumina support, an RHO type zeolite-porous alumina support, a DDR type zeolite-porous alumina support, an AFI type zeolite-porous alumina support, a CHA type zeolite-porous alumina support, and an AEI type zeolite-porous alumina support, and preferably a CHA type zeolite-porous alumina support, an MFI type zeolite-porous alumina support, and an RHO type zeolite-porous alumina support, and more preferably an MFI type zeolite-porous alumina support and an RHO type zeolite-porous alumina support. In one embodiment of the present invention, the zeolite-porous alumina support is preferably an MFI type zeolite-porous alumina support or an RHO type zeolite-porous alumina support, and more preferably an RHO type zeolite-porous alumina support.

[0105] <Method of manufacturing zeolite membrane composite> The method for forming the zeolite membrane composite that can be used in the present invention is not particularly limited as long as it is a method that can form the above-mentioned zeolite membrane on a porous support, and can be produced by any known method. For example, any of the following methods can be used: (1) a method of crystallizing zeolite into a membrane on a support, (2) a method of fixing zeolite to a support with an inorganic binder or an organic binder, (3) a method of fixing a polymer in which zeolite is dispersed to a support, and (4) a method of impregnating a support with a zeolite slurry and, in some cases, suctioning the zeolite to fix it to the support.

[0106] Among these, a method of crystallizing zeolite in a membrane form on a porous support is particularly preferred. Although there are no particular limitations on the crystallization method, a method of placing the support in a reaction mixture for hydrothermal synthesis used to produce zeolite (hereinafter, this may be referred to as an "aqueous reaction mixture") and directly carrying out hydrothermal synthesis to crystallize zeolite on the surface of the support is preferred. In this case, the zeolite membrane composite can be produced, for example, by placing an aqueous reaction mixture homogenized by adjusting the composition in a heat-resistant and pressure-resistant container such as an autoclave containing a porous support therein, sealing the container, and heating the container for a certain period of time.

[0107] The aqueous reaction mixture contains a Si atom source, an Al atom source, an alkali source, and water, and further contains an organic template (structure directing agent) as needed. To facilitate a deeper understanding of the method for producing a zeolite membrane composite, the methods for producing an RHO-type zeolite membrane composite and an MFI-type zeolite membrane composite will be described in detail below as representative examples. However, the zeolite membrane and the method for producing the same of the present invention are not limited thereto.

[0108] (RHO type zeolite membrane) The RHO-type zeolite used in the present invention refers to a zeolite having a RHO structure in the code that defines the structure of zeolites determined by the International Zeolite Association (IZA). The RHO-type zeolite has a structure characterized by having a three-dimensional pore composed of an 8-membered oxygen ring with a diameter of 3.6×3.6 Å, and its structure is characterized by X-ray diffraction data. The framework density of the RHO-type zeolite used in the present invention is 14.1 T / 1000 Å. The framework density means the number of atoms other than oxygen that constitute the framework per 1000 Å of the zeolite, and this value is determined by the structure of the zeolite. The relationship between the framework density and the structure of the zeolite is shown in ATLAS OF ZEOLITE FRAMEWORK TYPES Fifth Revised Edition 2007 ELSEVIER. 3 The framework density means the number of atoms other than oxygen that constitute the framework per 1000 Å of the zeolite, and this value is determined by the structure of the zeolite. The relationship between the framework density and the structure of the zeolite is shown in ATLAS OF ZEOLITE FRAMEWORK TYPES Fifth Revised Edition 2007 ELSEVIER.

[0109] (MFI-type zeolite membrane) The MFI-type zeolite used in the present invention refers to a zeolite having an MFI structure in the code that defines the structure of zeolites determined by the International Zeolite Association (IZA). The MFI-type zeolite has a structure characterized by having a three-dimensional pore composed of a 10-membered oxygen ring with a diameter of 5.1×5.5 Å or 5.3×5.6 Å, and its structure is characterized by X-ray diffraction data. The framework density of the MFI-type zeolite used in the present invention is 17.9 T / 1000 Å. The framework density means the number of atoms other than oxygen that constitute the framework per 1000 Å of the zeolite, and this value is determined by the structure of the zeolite. The relationship between the framework density and the structure of the zeolite is shown in ATLAS OF ZEOLITE FRAMEWORK TYPES Fifth Revised Edition 2007 ELSEVIER. 3 The framework density means the number of atoms other than oxygen that constitute the framework per 1000 Å of the zeolite, and this value is determined by the structure of the zeolite. The relationship between the framework density and the structure of the zeolite is shown in ATLAS OF ZEOLITE FRAMEWORK TYPES Fifth Revised Edition 2007 ELSEVIER.

[0110] <Method for manufacturing RHO-type zeolite membrane>

[0111] (silicon atom source) The silicon (Si) atom source used in the aqueous reaction mixture is not particularly limited, and examples thereof include aluminosilicate zeolite, fumed silica, colloidal silica, amorphous silica, sodium silicate, methyl silicate, ethyl silicate, silicon alkoxides such as trimethylethoxysilane, tetraethyl orthosilicate, and aluminosilicate gel, and preferred examples include aluminosilicate zeolite, fumed silica, colloidal silica, amorphous silica, sodium silicate, methyl silicate, ethyl silicate, silicon alkoxide, and aluminosilicate gel. These may be used alone or in combination of two or more.

[0112] The Si atom source is used so that the amounts of other raw materials used relative to the Si atom source fall within the preferred ranges described above or below.

[0113] (Aluminum atom source) The aluminum (Al) atom source used in the production of the porous support-RHO type zeolite membrane composite is not particularly limited, and examples thereof include aluminosilicate zeolite, amorphous aluminum hydroxide, aluminum hydroxide having a gibbsite structure, aluminum hydroxide having a bayerite structure, aluminum nitrate, aluminum sulfate, aluminum oxide, sodium aluminate, boehmite, pseudo-boehmite, aluminum alkoxide, and aluminosilicate gel. Aluminosilicate zeolite, amorphous aluminum hydroxide, sodium aluminate, boehmite, pseudo-boehmite, aluminum alkoxide, and aluminosilicate gel are preferred, and aluminosilicate zeolite, amorphous aluminum hydroxide, sodium aluminate, and aluminosilicate gel are particularly preferred. These may be used alone or in combination of two or more.

[0114] The aluminosilicate zeolite may be used alone or in combination of two or more. When an aluminosilicate zeolite is used as the Al atom source, it is preferable that the aluminosilicate zeolite accounts for 50 mass% or more, particularly 70 to 100 mass%, and especially 90 to 100 mass% of the total Al atom source. When an aluminosilicate zeolite is used as the Si atom source, it is preferable that the aluminosilicate zeolite accounts for 50 mass% or more, particularly 70 to 100 mass%, and especially 90 to 100 mass% of the total Si atom source. When the proportion of the aluminosilicate zeolite is within this range, the Si atom / Al atom molar ratio of the RHO-type zeolite membrane is high, resulting in a zeolite membrane with excellent acid resistance and water resistance and a wide range of applications.

[0115] The preferred range of the amount of Al atom source (including the above-mentioned aluminosilicate zeolite and other Al atom sources) used relative to the silicon (Si atoms) contained in the raw material mixture other than the seed crystals (Al atom / Si atom ratio) is usually 0.01 or more, preferably 0.02 or more, more preferably 0.04 or more, and even more preferably 0.06 or more, and usually 1.0 or less, preferably 0.5 or less, more preferably 0.2 or less, and even more preferably 0.1 or less. Controlling the amount used within this range makes it easier to control the contents of nitrogen atoms and alkali metal elements in the zeolite within the preferred ranges of the present invention. Furthermore, the Al atom / Si atom ratio can be increased by reducing the amount of silicon atom source used relative to the aluminum atom source, while the ratio can be decreased by increasing the amount of silicon atom source used relative to the aluminum atom source.

[0116] In the zeolite membrane used in the present invention, if the Al atom / Si atomic ratio exceeds 1.0, the water resistance and acid resistance of the obtained RHO type zeolite membrane may be low, and the applications of the zeolite membrane may be limited. If the Al atom / Si atomic ratio is less than 0.01, it may be difficult to obtain an RHO type zeolite membrane.

[0117] The aqueous reaction mixture may contain, in addition to the silicon atom source and the aluminum atom source, other atom sources such as gallium (Ga), iron (Fe), boron (B), titanium (Ti), zirconium (Zr), tin (Sn), and zinc (Zn).

[0118] The type of alkali used as the alkali source is not particularly limited, and alkali metal hydroxides and alkaline earth metal hydroxides can be used.

[0119] The metal species of these metal hydroxides are usually sodium (Na), potassium (K), lithium (Li), rubidium (Rb), cesium (Cs), calcium (Ca), magnesium (Mg), strontium (Sr), and barium (Ba), preferably Na, K, and Cs, and more preferably Na and Cs. Two or more metal species of the metal oxides may be used in combination, and specifically, it is preferable to use Na and Cs in combination. Specific examples of metal hydroxides that can be used include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide, and cesium hydroxide; and alkaline earth metal hydroxides such as calcium hydroxide, magnesium hydroxide, strontium hydroxide, and barium hydroxide.

[0120] Additionally, hydroxide ions, which are counter anions of the organic template described below, can be used as the alkali source for the aqueous reaction mixture.

[0121] In the crystallization of the zeolite according to the present invention, an organic template (structure-directing agent) is not necessarily required. However, by using a type of organic template corresponding to each structure, the ratio of silicon atoms to aluminum atoms in the crystallized zeolite increases, thereby improving the crystallinity, and therefore it is preferable to use an organic template.

[0122] The organic template may be of any type as long as it can form the desired zeolite membrane, and one type of template may be used, or two or more types may be used in combination.

[0123] The type of organic template suitable for the reaction varies depending on the zeolite structure to be synthesized, and an organic template that can obtain the desired zeolite structure may be used. Specifically, for example, in the case of the RHO structure, 18-crown-6-ether or the like may be used.

[0124] When the organic template is a cation, it is accompanied by an anion that is not detrimental to the formation of the zeolite. Representative of such anions are Cl. - , Br - , I - These include halogen ions, hydroxide ions, acetates, sulfates, and carboxylates, etc. Among these, hydroxide ions are particularly preferred, and in the case of hydroxide ions, they function as an alkali source as described above.

[0125] The molar ratio of the Si atom source to the organic template in the aqueous reaction mixture (organic template / SiO ratio) is typically 0.005 or greater, preferably 0.01 or greater, more preferably 0.02 or greater, even more preferably 0.05 or greater, particularly preferably 0.08 or greater, and most preferably 0.1 or greater, and typically 1 or less, preferably 0.5 or less, more preferably 0.4 or less, even more preferably 0.35 or less, particularly preferably 0.30 or less, and most preferably 0.25 or less. When the organic template / SiO ratio of the aqueous reaction mixture is within this range, a dense zeolite membrane can be formed, and a zeolite with excellent acid resistance and reduced Al atom desorption can be obtained. Furthermore, under these conditions, a particularly dense and acid-resistant RHO-type aluminosilicate zeolite can be formed.

[0126] The use of an appropriate amount of alkali metal atom source facilitates the coordination of the organic structure-directing agent (described below) with aluminum in a suitable state, thereby facilitating the formation of a crystal structure. The molar ratio (R / Si atom) of the alkali metal atom source (R) to the silicon (Si atom) contained in the raw material mixture for hydrothermal synthesis other than the seed crystal is usually 0.1 or more, preferably 0.15 or more, more preferably 0.20 or more, even more preferably 0.25 or more, particularly preferably 0.30 or more, and particularly preferably 0.35 or more, and is usually 2.0 or less, preferably 1.5 or less, more preferably 1.0 or less, even more preferably 0.8 or less, particularly preferably 0.6 or less, and most preferably 0.5 or less.

[0127] If the molar ratio of the alkali metal atom source to silicon (R / Si atoms) is greater than the above upper limit, the produced zeolite tends to dissolve, which may result in failure to obtain zeolite or a significantly reduced yield. If the R / Si atoms is less than the above lower limit, the raw material Al atom source and Si atom source do not dissolve sufficiently, which may result in failure to obtain a uniform raw material mixture for hydrothermal synthesis, making it difficult to produce RHO-type zeolite.

[0128] (amount of water) The amount of water in the raw material mixture for hydrothermal synthesis, expressed as a molar ratio to the silicon (Si atoms) contained in the raw material mixture other than the seed crystals, is usually 10 or more, preferably 20 or more, more preferably 30 or more, even more preferably 40 or more, and particularly preferably 50 or more, and is usually 200 moles or less, preferably 150 or less, more preferably 100 or less, even more preferably 80 or less, and particularly preferably 60 or less. If the molar ratio is greater than the upper limit, the reaction mixture may be too dilute, making it difficult to form a defect-free, dense membrane. If the molar ratio is less than 10, the reaction mixture may be too concentrated, making it easy for spontaneous nuclei to form, which may inhibit the growth of RHO zeolite from the support and make it difficult to form a dense membrane.

[0129] (seed crystal) In the present invention, seed crystals may be used as one component of the raw material (raw material compound) for producing "zeolite." During hydrothermal synthesis, it is not necessary to have seed crystals present in the reaction system, but the presence of seed crystals can promote the crystallization of zeolite on the porous support. The method for having seed crystals present in the reaction system is not particularly limited, and methods such as adding seed crystals to the aqueous reaction mixture, as in the synthesis of powdered zeolite, or attaching seed crystals to the support can be used. In the present invention, however, it is preferable to attach seed crystals to the support. Attaching seed crystals to the support in advance makes it easier to produce a dense zeolite membrane with high separation performance.

[0130] The seed crystals to be used may be of any type as long as they are zeolite that promotes crystallization, but for efficient crystallization, they preferably have the same crystal type as the zeolite membrane to be formed. For example, when forming a zeolite membrane of RHO-type aluminosilicate, it is preferable to use seed crystals of RHO-type zeolite.

[0131] The particle size of the seed crystals is preferably close to the pore size of the support, and may be crushed as necessary. The particle size is usually 20 nm or more, preferably 50 nm or more, more preferably 100 nm or more, even more preferably 0.15 μm or more, particularly preferably 0.5 μm or more, and most preferably 0.7 μm or more, and is usually 5 μm or less, preferably 3 μm or less, more preferably 2 μm or less, and particularly preferably 1.5 μm or less. Depending on the pore size of the support, a smaller particle size of the seed crystal may be desirable, and the seed crystal may be crushed as necessary. The particle size of the seed crystal is usually 5 nm or more, preferably 10 nm or more, and more preferably 20 nm or more, and usually 5 μm or less, preferably 3 μm or less, and more preferably 2 μm or less.

[0132] The method for attaching seed crystals to a support is not particularly limited. For example, a dip method in which seed crystals are dispersed in a solvent such as water and the support is immersed in the dispersion to attach the seed crystals to the surface; a suction method in which seed crystals are dispersed in a solvent such as water and the support, with one end sealed, is immersed in the dispersion and then suctioned from the other end to firmly attach the seed crystals to the support surface; or a method in which seed crystals are mixed with a solvent such as water to form a slurry and then applied to the support. The dip and suction methods are desirable for controlling the amount of seed crystal attachment and producing zeolite membranes with good reproducibility, while the suction and suction methods in which seed crystals are applied in a slurry state are desirable for adhering the seed crystals to the support. Furthermore, for the purpose of adhering the seed crystals to the support and / or removing excess seed crystals, rubbing and pressing the support to which the seed crystals are attached with a finger wearing a latex glove is also preferably performed after the dip or suction method.

[0133] The solvent in which the seed crystals are dispersed is not particularly limited, but water or an alkaline aqueous solution is particularly preferred. The type of alkaline aqueous solution is not particularly limited, but a sodium hydroxide aqueous solution or a potassium hydroxide aqueous solution is preferred. These alkaline species may also be mixed. The alkali concentration of the alkaline aqueous solution is not particularly limited, but is usually 0.0001 mol% or more, preferably 0.0002 mol% or more, more preferably 0.001 mol% or more, and even more preferably 0.002 mol% or more. The alkali concentration is also usually 1 mol% or less, preferably 0.8 mol% or less, more preferably 0.5 mol% or less, and even more preferably 0.2 mol% or less.

[0134] The solvent in which the seed crystals are dispersed is not particularly limited, but water is particularly preferred. The amount of seed crystals to be dispersed is not particularly limited, and is usually 0.05% by mass or more, preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, particularly preferably 2% by mass or more, and most preferably 3.0% by mass or more, based on the total weight of the dispersion. Furthermore, the amount is usually 20% by mass or less, preferably 10% by mass or less, more preferably 5% by mass or less, and even more preferably 4% by mass or less.

[0135] If the amount of dispersed seed crystals is too small, the amount of seed crystals attached to the support will be small, which may result in areas on the support where zeolite is not produced during hydrothermal synthesis, resulting in a defective membrane. On the other hand, for example, the amount of seed crystals attached to the porous support by the dipping method becomes almost constant when the amount of seed crystals in the dispersion exceeds a certain level, so if the amount of seed crystals in the dispersion is too large, a lot of seed crystals will be wasted, which is disadvantageous in terms of cost.

[0136] It is desirable to attach seed crystals to a support by dipping, suction, or by applying a slurry, and then dry the resulting mixture before forming a zeolite membrane. The drying temperature is usually 50°C or higher, preferably 80°C or higher, and more preferably 100°C or higher, and usually 200°C or lower, preferably 180°C or lower, and more preferably 150°C or lower. The drying time is not problematic as long as the mixture is sufficiently dried, but is usually 10 minutes or longer, preferably 30 minutes or longer. There is no particular upper limit, but from an economical viewpoint, it is usually 5 hours or shorter.

[0137] After drying, the support to which the seed crystals are attached is preferably rubbed and pressed with fingers or the like wearing latex gloves, in order to adhere the seed crystals to the support and / or to remove excess seed crystals.

[0138] The amount of seed crystals to be attached in advance to the porous support is not particularly limited, and the amount is determined by the number of crystals per 1 m of the membrane-forming surface of the porous support. 2 The mass per unit area is usually 0.1 g or more, preferably 0.3 g or more, more preferably 0.5 g or more, even more preferably 0.80 g or more, and most preferably 1.0 g or more, and usually 100 g or less, preferably 50 g or less, more preferably 10 g or less, even more preferably 8 g or less, and most preferably 5 g or less.

[0139] If the amount of seed crystals attached is less than the above lower limit, crystals tend to be difficult to form, resulting in insufficient or uneven membrane growth. Furthermore, if the amount of seed crystals exceeds the above upper limit, the seed crystals may increase the surface irregularities, or seed crystals that fall off the support may facilitate spontaneous nucleation, inhibiting membrane growth on the support. In either case, it tends to be difficult to produce a dense zeolite membrane.

[0140] When forming a zeolite membrane on a porous support by hydrothermal synthesis, there is no particular limitation on the method for immobilizing the support, and any form such as vertical placement, horizontal placement, etc. In this case, the zeolite membrane may be formed by a static method, or may be formed while stirring the aqueous reaction mixture.

[0141] Hydrothermal synthesis is carried out by placing the support carrying the seed crystals as described above and the prepared hydrothermal synthesis mixture or an aqueous gel obtained by aging the mixture in a pressure-resistant container, and maintaining the mixture at a predetermined temperature under self-generated pressure or under a gas pressure that does not inhibit crystallization, while stirring, rotating or rocking the container, or leaving it stationary. Hydrothermal synthesis in a stationary state is desirable because it does not inhibit crystal growth from the seed crystals on the support.

[0142] The reaction temperature when forming a zeolite membrane by hydrothermal synthesis is not particularly limited, and may be any temperature suitable for obtaining a membrane with the desired zeolite structure. The reaction temperature is usually 100°C or higher, preferably 110°C or higher, more preferably 120°C or higher, particularly preferably 130°C or higher, particularly preferably 140°C or higher, and most preferably 150°C or higher, and usually 200°C or lower, preferably 190°C or lower, more preferably 180°C or lower, and even more preferably 170°C or lower. If the reaction temperature is too low, the zeolite may be difficult to crystallize. If the reaction temperature is too high, a type of zeolite different from the desired zeolite may be easily produced.

[0143] The heating (reaction) time when forming a zeolite membrane by hydrothermal synthesis is not particularly limited, and may be any time suitable for obtaining a membrane with the desired zeolite structure. It is usually 3 hours or more, preferably 8 hours or more, more preferably 12 hours or more, and particularly preferably 15 hours or more, and usually 10 days or less, preferably 5 days or less, more preferably 3 days or less, even more preferably 2 days or less, and particularly preferably 1.5 days or less. If the reaction time is too short, the zeolite may be difficult to crystallize. If the reaction time is too long, a type of zeolite different from the desired zeolite may be easily produced.

[0144] The pressure during hydrothermal synthesis is not particularly limited, and the autogenous pressure that occurs when an aqueous reaction mixture placed in a sealed vessel is heated to the above-mentioned temperature range is sufficient. If necessary, an inert gas such as nitrogen may be added.

[0145] The density of the zeolite membrane can be improved by repeating the hydrothermal synthesis multiple times. When the hydrothermal synthesis is repeated multiple times, the zeolite membrane composite obtained in the first hydrothermal synthesis is washed with water, dried by heating, and then immersed again in a newly prepared aqueous reaction mixture for hydrothermal synthesis. The zeolite membrane composite obtained after the first hydrothermal synthesis does not necessarily need to be washed with water or dried, but washing with water and drying can maintain the intended composition of the aqueous reaction mixture. When the synthesis is performed multiple times, the number of synthesis times is usually two or more and usually ten or less, preferably five or less, and more preferably three or less. The water washing may be repeated once or multiple times.

[0146] The zeolite membrane composite obtained by hydrothermal synthesis is washed with water, then heat-treated, and dried. Here, heat treatment means drying the zeolite membrane composite by applying heat, and also means removing an organic template, if used, by calcining the organic template.

[0147] When the purpose of the heat treatment is drying, the temperature is usually 50° C. or higher, preferably 80° C. or higher, more preferably 100° C. or higher, and usually 200° C. or lower, preferably 150° C. or lower. When the purpose of the heat treatment is to remove the organic template by calcination, the temperature is usually 250° C. or higher, preferably 300° C. or higher, more preferably 350° C. or higher, even more preferably 400° C. or higher, and usually 800° C. or lower, preferably 600° C. or lower, even more preferably 550° C. or lower, particularly preferably 500° C. or lower.

[0148] When the purpose is to remove the organic template by calcination, if the heat treatment temperature is too low, the proportion of the organic template remaining tends to be high, reducing the number of pores in the zeolite and potentially reducing the amount of permeation when used to separate ammonia. If the heat treatment temperature is too high, the difference in the thermal expansion coefficients between the support and the zeolite becomes large, which may lead to the likelihood of cracks occurring in the zeolite membrane, causing the zeolite membrane to lose its denseness and resulting in reduced separation performance.

[0149] The heat treatment time is not particularly limited as long as it is sufficient to sufficiently dry the zeolite membrane and remove the organic template by calcination, and is preferably 0.5 hours or more, more preferably 1 hour or more, for the purpose of drying, and is preferably 1 hour or more, more preferably 5 hours or more, for the purpose of removing the organic template by calcination, although this varies depending on the temperature increase rate and temperature decrease rate. The upper limit of the heating time is not particularly limited, and is usually 200 hours or less, preferably 150 hours or less, more preferably 100 hours or less.

[0150] The heat treatment for calcining the template may be carried out in an air atmosphere, but may also be carried out in an atmosphere containing an inert gas such as nitrogen or oxygen.

[0151] When the hydrothermal synthesis is carried out in the presence of an organic template, it is appropriate to wash the obtained zeolite membrane composite with water and then remove the organic template by, for example, a heat treatment or extraction, preferably by the above-mentioned heat treatment, i.e., calcination.

[0152] When performing a heat treatment for the purpose of firing and removing an organic template, the heating rate should be made as slow as possible in order to prevent cracks from occurring in the zeolite membrane due to the difference in the thermal expansion coefficients of the porous support and the zeolite. The heating rate is usually 5 °C / min or less, preferably 2 °C / min or less, more preferably 1 °C / min or less, still more preferably 0.5 °C / min or less, and particularly preferably 0.3 °C / min or less. The lower limit of the heating rate is usually 0.1 °C / min or more in consideration of workability.

[0153] Also, in the heat treatment for the purpose of firing and removing an organic template, it is necessary to control the cooling rate after the heat treatment in order to avoid cracks from occurring in the zeolite membrane. Similar to the heating rate, the slower the cooling rate, the more desirable it is. The cooling rate is usually 5 °C / min or less, preferably 2 °C / min or less, more preferably 1 °C / min or less, still more preferably 0.5 °C / min or less, and particularly preferably 0.3 °C / min or less. The lower limit of the cooling rate is usually 0.1 °C / min or more in consideration of workability. <Method for Producing MFI-Type Zeolite Membrane>

[0154] (Silicon Atom Source) Examples of the silicon (Si) atom source used in the aqueous reaction mixture include aluminosilicate zeolite, fumed silica, colloidal silica, amorphous silica, sodium silicate, methyl silicate, ethyl silicate, silicon alkoxides such as trimethylethoxysilane, tetraethyl orthosilicate, aluminosilicate gel, etc. Preferably, fumed silica, colloidal silica, amorphous silica, sodium silicate, methyl silicate, ethyl silicate, silicon alkoxide, aluminosilicate gel are mentioned. These may be used alone or in combination of two or more.

[0155] The Si atom source is used such that the usage amounts of other raw materials with respect to the Si atom source are respectively within the preferred ranges described above or below.

[0156] (Aluminum Atom Source) The aluminum (Al) atom source used in producing the porous support-MFI zeolite membrane composite is not particularly limited, and examples thereof include aluminosilicate zeolite, amorphous aluminum hydroxide, aluminum hydroxide having a gibbsite structure, aluminum hydroxide having a bayerite structure, aluminum nitrate, aluminum sulfate, aluminum oxide, sodium aluminate, boehmite, pseudo-boehmite, aluminum alkoxide, and aluminosilicate gel. Amorphous aluminum hydroxide, sodium aluminate, boehmite, pseudo-boehmite, aluminum alkoxide, and aluminosilicate gel are preferred, and amorphous aluminum hydroxide, sodium aluminate, and aluminosilicate gel are particularly preferred. These may be used alone or in combination of two or more.

[0157] The preferred range of the amount of aluminum atom source (including the aluminosilicate zeolite and other aluminum atom sources) used relative to the silicon (Si atoms) contained in the raw material mixture other than the seed crystals (Al atom / Si atom ratio) is, in terms of molar ratio, usually 0.001 or more, preferably 0.005 or more, more preferably 0.01 or more, even more preferably 0.015 or more, and most preferably 0.02 or more, and usually 1.0 or less, preferably 0.5 or less, more preferably 0.4 or less, even more preferably 0.35 or less, particularly preferably 0.3 or less, and most preferably 0.25 or less. Controlling the amount used within this range makes it easier to control the contents of nitrogen atoms and alkali metal elements in the zeolite within the preferred ranges of the present invention. To increase the Al atom / Si atom ratio, the amount of silicon atom source used relative to the aluminum atom source can be reduced, while to decrease the ratio, the amount of silicon atom source used relative to the aluminum atom source can be increased.

[0158] The aqueous reaction mixture may contain, in addition to the Si atom source and the Al atom source, other atom sources such as Ga, Fe, B, Ti, Zr, Sn, and Zn.

[0159] The type of alkali used as the alkali source is not particularly limited, and alkali metal hydroxides and alkaline earth metal hydroxides can be used.

[0160] Specific examples of metal hydroxides that can be used include alkali metal hydroxides such as sodium hydroxide, potassium hydroxide, lithium hydroxide, rubidium hydroxide, and cesium hydroxide; and alkaline earth metal hydroxides such as calcium hydroxide, magnesium hydroxide, strontium hydroxide, and barium hydroxide.

[0161] Additionally, hydroxide ions, which are counter anions of the organic template described below, can be used as the alkali source for the aqueous reaction mixture.

[0162] In the crystallization of the zeolite according to the present invention, an organic template is not necessarily required. However, by using a type of organic template (structure-directing agent) corresponding to each structure, the ratio of silicon atoms to aluminum atoms in the crystallized zeolite increases, thereby improving the crystallinity, and therefore it is preferable to use an organic template.

[0163] The organic template may be of any type as long as it can form the desired zeolite membrane, and one type of template may be used, or two or more types may be used in combination.

[0164] The type of organic template suitable for the reaction varies depending on the zeolite structure to be synthesized, and an organic template that can obtain the desired zeolite structure may be used. Specifically, for example, tetrapropylammonium hydroxide may be used for the MFI structure.

[0165] When the organic template is a cation, it is accompanied by an anion that is not detrimental to the formation of the zeolite. Representative of such anions are Cl. - , Br - , I -These include halogen ions, hydroxide ions, acetates, sulfates, and carboxylates, etc. Among these, hydroxide ions are particularly preferred, and in the case of hydroxide ions, they function as an alkali source as described above.

[0166] The molar ratio of the Si atom source to the organic template in the aqueous reaction mixture (organic template / SiO ratio) is typically 0.005 or greater, preferably 0.01 or greater, more preferably 0.02 or greater, particularly preferably 0.05 or greater, and particularly preferably 0.1 or greater, and typically 1 or less, preferably 0.5 or less, more preferably 0.3 or less, particularly preferably 0.25 or less, and particularly preferably 0.2 or less. When the organic template / SiO ratio of the aqueous reaction mixture is within this range, a dense zeolite membrane can be formed, and a zeolite with excellent acid resistance and low Al desorption can be obtained. Furthermore, under these conditions, a particularly dense and acid-resistant MFI-type aluminosilicate zeolite can be formed.

[0167] The use of an appropriate amount of alkali metal atom source facilitates the coordination of the organic structure-directing agent (described below) with aluminum in a suitable state, thereby facilitating the formation of a crystal structure. The molar ratio R / Si of the alkali metal atom source (R) to the silicon (Si) contained in the mixture of raw materials for hydrothermal synthesis other than the seed crystals is usually 0.01 or more, preferably 0.02 or more, more preferably 0.03 or more, even more preferably 0.04 or more, and particularly preferably 0.05 or more, and is usually 1.0 or less, preferably 0.6 or less, more preferably 0.4 or less, even more preferably 0.2 or less, and particularly preferably 0.1 or less.

[0168] If the molar ratio of the alkali metal atom source to silicon (R / Si) is greater than the upper limit, the produced zeolite is likely to dissolve, which may result in failure to obtain zeolite or a significantly reduced yield. If R / Si is less than the lower limit, the Al atom source and Si atom source as raw materials may not dissolve sufficiently, which may result in failure to obtain a uniform raw material mixture for hydrothermal synthesis, making it difficult to produce MFI zeolite.

[0169] (amount of water) The amount of water in the raw material mixture for hydrothermal synthesis, expressed as a molar ratio to the silicon (Si) contained in the raw material mixture other than the seed crystals, is usually 10 or more, preferably 20 or more, more preferably 40 or more, even more preferably 50 or more, particularly preferably 60 or more, and most preferably 70 or more, and is usually 500 moles or less, preferably 300 or less, more preferably 200 or less, even more preferably 150 or less, and particularly preferably 100 or less. If the molar ratio is greater than the upper limit, the reaction mixture may be too dilute, making it difficult to form a defect-free, dense membrane. If the molar ratio is less than 10, the reaction mixture may be too concentrated, making it easier for spontaneous nuclei to form, which may inhibit the growth of MFI zeolite from the support and make it difficult to form a dense membrane.

[0170] (seed crystal) In the present invention, seed crystals may be used as one component of the raw material (raw material compound) for producing "zeolite." During hydrothermal synthesis, it is not necessary to have seed crystals present in the reaction system, but the presence of seed crystals can promote the crystallization of zeolite on the porous support. The method for having seed crystals present in the reaction system is not particularly limited, and methods such as adding seed crystals to the aqueous reaction mixture, as in the synthesis of powdered zeolite, or attaching seed crystals to the support can be used. In the present invention, however, it is preferable to attach seed crystals to the support. Attaching seed crystals to the support in advance makes it easier to produce a dense zeolite membrane with high separation performance.

[0171] The seed crystals to be used may be of any type as long as they are zeolite that promotes crystallization, but for efficient crystallization, they preferably have the same crystal type as the zeolite membrane to be formed. For example, when forming a zeolite membrane of MFI type aluminosilicate, it is preferable to use seed crystals of MFI type zeolite.

[0172] The particle size of the seed crystals is desirably close to the pore size of the support, and they may be crushed before use as necessary. The particle size is usually 1 nm or more, preferably 10 nm or more, more preferably 50 nm or more, even more preferably 0.1 μm or more, particularly preferably 0.5 μm or more, especially preferably 0.7 μm or more, and most preferably 1 μm or more, and is usually 5 μm or less, preferably 3 μm or less, more preferably 2 μm or less, most preferably 1.5 μm or less, and particularly preferably 1.2 μm or less. Depending on the pore size of the support, a smaller particle size of the seed crystal may be desirable, and the seed crystal may be crushed as necessary. The particle size of the seed crystal is usually 0.5 nm or more, preferably 1 nm or more, and more preferably 2 nm or more, and usually 5 μm or less, preferably 3 μm or less, and more preferably 2 μm or less.

[0173] The method for attaching seed crystals to a support is not particularly limited. For example, a dip method in which seed crystals are dispersed in a solvent such as water and the support is immersed in the dispersion to attach the seed crystals to the surface; a suction method in which seed crystals are dispersed in a solvent such as water and the support, with one end sealed, is immersed in the dispersion and then suctioned from the other end to firmly attach the seed crystals to the support surface; or a method in which seed crystals are mixed with a solvent such as water to form a slurry and then applied to the support. The dip and suction methods are desirable for controlling the amount of seed crystal attachment and producing zeolite membranes with good reproducibility, while the suction and suction methods in which seed crystals are applied in a slurry state are desirable for adhering the seed crystals to the support. Furthermore, for the purpose of adhering the seed crystals to the support and / or removing excess seed crystals, rubbing and pressing the support to which the seed crystals are attached with a finger wearing a latex glove is also preferably performed after the dip or suction method.

[0174] The solvent in which the seed crystals are dispersed is not particularly limited, but water or an alkaline aqueous solution is particularly preferred. The type of alkaline aqueous solution is not particularly limited, but a sodium hydroxide aqueous solution or a potassium hydroxide aqueous solution is preferred. These alkaline species may also be mixed. The alkali concentration of the alkaline aqueous solution is not particularly limited, but is usually 0.0001 mol% or more, preferably 0.0002 mol% or more, more preferably 0.001 mol% or more, and even more preferably 0.002 mol% or more. The alkali concentration is also usually 1 mol% or less, preferably 0.8 mol% or less, more preferably 0.5 mol% or less, and even more preferably 0.2 mol% or less.

[0175] The solvent in which the seed crystals are dispersed is not particularly limited, but water is particularly preferred. The amount of seed crystals to be dispersed is not particularly limited, and is usually 0.05% by mass or more, preferably 0.1% by mass or more, more preferably 0.12% by mass or more, even more preferably 0.15% by mass or more, particularly preferably 0.18% by mass or more, and most preferably 0.2% by mass or more, based on the total weight of the dispersion. The amount is also usually 20% by mass or less, preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 3% by mass or less, particularly preferably 1% by mass or less, and most preferably 0.5% by mass or less.

[0176] If the amount of dispersed seed crystals is too small, the amount of seed crystals attached to the support will be small, which may result in areas on the support where zeolite is not produced during hydrothermal synthesis, resulting in a defective membrane. On the other hand, for example, the amount of seed crystals attached to the porous support by the dipping method becomes almost constant when the amount of seed crystals in the dispersion exceeds a certain level, so if the amount of seed crystals in the dispersion is too large, a lot of seed crystals will be wasted, which is disadvantageous in terms of cost.

[0177] It is desirable to attach seed crystals to a support by dipping, suction, or by applying a slurry, and then dry the resulting mixture before forming a zeolite membrane. The drying temperature is usually 50°C or higher, preferably 80°C or higher, and more preferably 100°C or higher, and usually 200°C or lower, preferably 180°C or lower, and more preferably 150°C or lower. The drying time is not problematic as long as the mixture is sufficiently dried, but is usually 10 minutes or longer, preferably 30 minutes or longer. There is no particular upper limit, but from an economical viewpoint, it is usually 5 hours or shorter.

[0178] After drying, the support to which the seed crystals are attached is preferably rubbed and pressed with fingers or the like wearing latex gloves, in order to adhere the seed crystals to the support and / or to remove excess seed crystals.

[0179] The amount of seed crystals to be attached in advance to the porous support is not particularly limited, and the amount is determined by the number of crystals per 1 m of the membrane-forming surface of the porous support. 2 The mass per unit area is usually 0.1 g or more, preferably 0.3 g or more, more preferably 0.5 g or more, even more preferably 0.80 g or more, and most preferably 1.0 g or more, and usually 50 g or less, preferably 20 g or less, more preferably 10 g or less, even more preferably 5 g or less, and most preferably 3 g or less.

[0180] If the amount of seed crystals attached is less than the above lower limit, crystals tend to be difficult to form, resulting in insufficient or uneven membrane growth. Furthermore, if the amount of seed crystals exceeds the above upper limit, the seed crystals may increase the surface irregularities, or seed crystals that fall off the support may facilitate spontaneous nucleation, inhibiting membrane growth on the support. In either case, it tends to be difficult to produce a dense zeolite membrane.

[0181] When forming a zeolite membrane on a porous support by hydrothermal synthesis, there is no particular limitation on the method for immobilizing the support, and any form such as vertical placement, horizontal placement, etc. In this case, the zeolite membrane may be formed by a static method, or may be formed while stirring the aqueous reaction mixture.

[0182] Hydrothermal synthesis is carried out by placing the support carrying the seed crystals as described above and the prepared hydrothermal synthesis mixture or an aqueous gel obtained by aging the mixture in a pressure-resistant container, and maintaining the mixture at a predetermined temperature under self-generated pressure or under a gas pressure that does not inhibit crystallization, while stirring, rotating or rocking the container, or leaving it stationary. Hydrothermal synthesis in a stationary state is desirable because it does not inhibit crystal growth from the seed crystals on the support.

[0183] The reaction temperature when forming a zeolite membrane by hydrothermal synthesis is not particularly limited, as long as it is a temperature suitable for obtaining a membrane with the desired zeolite structure, but is usually 100°C or higher, preferably 120°C or higher, more preferably 140°C or higher, particularly preferably 150°C or higher, particularly preferably 160°C or higher, and most preferably 170°C or higher, and is usually 200°C or lower, preferably 190°C or lower, more preferably 185°C or lower, and particularly preferably 180°C or lower. If the reaction temperature is too low, the zeolite may be difficult to crystallize. On the other hand, if the reaction temperature is too high, a type of zeolite different from the desired zeolite may be easily produced.

[0184] The heating (reaction) time when forming a zeolite membrane by hydrothermal synthesis is not particularly limited, and may be any time suitable for obtaining a membrane with the desired zeolite structure. It is usually 3 hours or more, preferably 5 hours or more, more preferably 10 hours or more, particularly preferably 13 hours or more, and particularly preferably 15 hours or more, and usually 10 days or less, preferably 5 days or less, more preferably 3 days or less, even more preferably 2 days or less, and particularly preferably 1 day or less. If the reaction time is too short, the zeolite may be difficult to crystallize. If the reaction time is too long, a type of zeolite different from the desired zeolite may be easily produced.

[0185] The pressure during hydrothermal synthesis is not particularly limited, and the autogenous pressure that occurs when an aqueous reaction mixture placed in a sealed vessel is heated to the above-mentioned temperature range is sufficient. If necessary, an inert gas such as nitrogen may be added.

[0186] The zeolite membrane composite obtained by hydrothermal synthesis is washed with water, then heat-treated, and dried. Here, heat treatment means drying the zeolite membrane composite by applying heat, and also means removing an organic template, if used, by calcining the organic template.

[0187] When the purpose of the heat treatment is drying, the temperature is usually 50° C. or higher, preferably 80° C. or higher, more preferably 100° C. or higher, and usually 200° C. or lower, preferably 150° C. or lower. When the purpose of the heat treatment is to remove the organic template by calcination, the temperature is usually 350° C. or higher, preferably 400° C. or higher, more preferably 450° C. or higher, even more preferably 500° C. or higher, and usually 900° C. or lower, preferably 800° C. or lower, even more preferably 700° C. or lower, and particularly preferably 600° C. or lower.

[0188] If the purpose is to remove the organic template by calcination, a too low heat treatment temperature tends to increase the proportion of the organic template remaining, reducing the number of pores in the zeolite and potentially reducing the amount of permeation when used to separate ammonia. If the heat treatment temperature is too high, the difference in thermal expansion coefficient between the support and the zeolite becomes large, which may lead to the likelihood of cracking in the zeolite membrane, causing the zeolite membrane to lose its density and reduce its separation performance. When tetrapropylammonium hydroxide is used as the organic template, the content of nitrogen atoms in the zeolite can be controlled by adjusting the heat treatment temperature.

[0189] The heat treatment time is not particularly limited as long as it is sufficient to sufficiently dry the zeolite membrane and remove the organic template by calcination, and is preferably 0.5 hours or more, more preferably 1 hour or more, for the purpose of drying, and is preferably 1 hour or more, more preferably 5 hours or more, for the purpose of removing the organic template by calcination, although this varies depending on the temperature increase rate and temperature decrease rate. The upper limit of the heating time is not particularly limited, and is usually 200 hours or less, preferably 150 hours or less, more preferably 100 hours or less.

[0190] The heat treatment for calcining the template may be carried out in an air atmosphere, but may also be carried out in an atmosphere containing an inert gas such as nitrogen or oxygen.

[0191] When the hydrothermal synthesis is carried out in the presence of an organic template, it is appropriate to wash the obtained zeolite membrane composite with water and then remove the organic template by, for example, a heat treatment or extraction, preferably by the above-mentioned heat treatment, i.e., calcination.

[0192] The heating rate during the heat treatment for calcining and removing the organic template is desirably as slow as possible to prevent cracks from forming in the zeolite membrane due to the difference in thermal expansion coefficient between the porous support and the zeolite. The heating rate is usually 5°C / min or less, preferably 2°C / min or less, more preferably 1°C / min or less, particularly preferably 0.5°C / min or less, and most preferably 0.3°C / min or less. The lower limit of the heating rate is usually 0.1°C / min or more, taking workability into consideration.

[0193] In addition, in the heat treatment for calcining and removing the organic template, the temperature drop rate after the heat treatment must also be controlled to avoid cracking of the zeolite membrane. As with the temperature increase rate, the slower the temperature drop rate, the more desirable it is. The temperature drop rate is usually 5°C / min or less, preferably 2°C / min or less, more preferably 1°C / min or less, particularly preferably 0.5°C / min or less, and most preferably 0.3°C / min or less. The lower limit of the temperature drop rate is usually 0.1°C / min or more, taking workability into consideration.

[0194] (ion exchange) The synthesized zeolite membrane may be ion-exchanged as needed. The thermal expansion characteristics and thermal stability of ammonia separation of zeolite are significantly affected by the cation species in the zeolite, so this ion exchange is an important control method. Furthermore, the ammonia permeability and / or separation performance of the zeolite membrane may be improved depending on the cation species used. That is, the cation species used in the present invention are appropriately selected taking into account the ammonia permeability and separation performance while maintaining the thermal expansion characteristics and thermal stability of ammonia separation of the zeolite.

[0195] When a zeolite membrane is synthesized using an organic template, ion exchange is usually performed after removing the organic template. In the present invention, the ions to be exchanged are NH4 + or any of the cationic species obtained by protonating an organic amine having 1 to 20 carbon atoms, such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, ethylenediamine, dimethylethylenediamine, tetramethylethylenediamine, diethylenetriamine, triethylenetetraamine, aniline, methylaniline, benzylamine, methylbenzylamine, hexamethylenediamine, N,N-diisopropylethylamine, N,N,N-trimethyl-1-adamantanamine, pyridine, and piperidine, and other cationic species are also preferred. + , K. + , Li + , Rb + , Cs + Alkali metal ions such as Ca 2+ , Mg 2+ , Sr 2+ , Ba 2+ and ions of transition metals such as Fe, Cu, Zn, Ga, and La may coexist. Among these, protons, NH4 + , Na + , Li + , Cs +, Fe ions, Ga ions, and La ions are preferred. A plurality of these ions may be mixed in the zeolite, and the method of mixing the above ions is preferably adopted in order to balance the thermal expansion characteristics and ammonia permeability of the zeolite. By controlling the cation species and their amounts to be ion-exchanged in this way, it becomes possible to control the ammonia affinity of the zeolite and the effective pore size within the zeolite pores, thereby increasing the ammonia separation selectivity and improving the ammonia permeation rate. Among these, the ion species that increase the ammonia separation selectivity are NH4 + Cationic species obtained by protonating organic amines having 1 to 20 carbon atoms, such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, ethylenediamine, dimethylethylenediamine, tetramethylethylenediamine, diethylenetriamine, triethylenetetraamine, aniline, methylaniline, benzylamine, methylbenzylamine, hexamethylenediamine, N,N-diisopropylethylamine, N,N,N-trimethyl-1-adamantanamine, pyridine, and piperidine, are preferred, and among these, NH4 + For the above reasons, cationic species in which small molecular amines such as amines having 1 to 6 carbon atoms are protonated are more preferred, and among these, NH4 + On the other hand, the ion species that improve the ammonia permeation rate are protons, Na + , Li + , Cs + , Fe ions, Ga ions, and La ions are preferred, and Na + , Li + , Cs + The ion is particularly preferred, Na + In the present invention, the molar ratio of nitrogen atoms to Al atoms in the zeolite membrane can be controlled by adjusting the amount of ions exchanged, which require nitrogen-containing ion species.

[0196] In the present invention, Na is contained in the zeolite. +When Na ions are contained, the content thereof is, in molar ratio relative to the Al atoms in the zeolite, usually 0.01 or more, preferably 0.02 or more, more preferably 0.03 or more, even more preferably 0.04 or more, and particularly preferably 0.05 or more. The upper limit is not particularly limited, but is usually 0.10 molar equivalents or less, preferably 0.070 molar equivalents or less, more preferably 0.065 molar equivalents or less, even more preferably 0.060 molar equivalents or less, and particularly preferably 0.055 molar equivalents or less. + By using a zeolite with a zeolite / Al atomic ratio, ammonia can be separated with high permeability from a mixed gas consisting of multiple components including ammonia and hydrogen and / or nitrogen.

[0197] Ion exchange can be performed by treating the calcined zeolite membrane (e.g., when an organic template is used) with nitrates, sulfates, phosphates, organic acid salts, hydroxides, and halogen salts of Cl and Br of the cations to be ion-exchanged, or with an acid such as hydrochloric acid, typically at room temperature to 100°C, followed by water washing or hot water washing at 40°C to 100°C. The solvent used for the ion exchange treatment can be water or an organic solvent as long as it dissolves the salt to be ion-exchanged. The concentration of the salt to be treated is typically 10 mol / L or less, with a lower limit of 0.1 mol / L or more, preferably 0.5 mol / L or more, and more preferably 1 mol / L or more. These treatment conditions can be appropriately set depending on the salt and solvent used. When using an acid such as hydrochloric acid, the acid concentration is typically 5 mol / L or less, as the acid destroys the crystalline structure of the zeolite, and the temperature and time can be appropriately set. Furthermore, since the ion exchange rate increases when the ion exchange treatment is repeated, the number of times the ion exchange treatment is performed is not particularly limited, and the treatment may be repeated until the desired effect is achieved. Furthermore, since the ion-exchanged zeolite membrane may be calcined at 200 to 500°C as necessary to remove the residue from the ion exchange treatment, if any residue from the ion exchange treatment raw materials remains in the zeolite pores after the ion exchange treatment, this will hinder gas permeability.

[0198] (Nitrate treatment) In some embodiments of the zeolite composite membrane used in the present invention, it is preferable to use a nitrate treatment in combination as a method for adjusting the nitrogen atom content in the zeolite membrane. Therefore, the nitrate treatment will be described below.

[0199] In the zeolite membrane composite used in the present invention, the synthesized zeolite membrane may be subjected to a nitrate treatment if necessary. The nitrate treatment may be performed while the zeolite membrane composite contains an organic template or after the organic template has been removed by calcination. The nitrate treatment may be performed by immersing the zeolite membrane composite in a solution containing a nitrate. This may be preferable because the nitrate can seal fine defects present on the membrane surface. Furthermore, when nitrate is present in the zeolite pores, it has the effect of improving the affinity of the zeolite membrane with ammonia, making it a suitable method for improving ammonia permeability. The solvent used for the nitrate treatment may be water or an organic solvent as long as the salt dissolves. The nitrate used is not limited, but examples include magnesium nitrate, calcium nitrate, barium nitrate, aluminum nitrate, gallium nitrate, indium nitrate, iron nitrate, cobalt nitrate, nickel nitrate, copper nitrate, and zinc nitrate. These may be used alone or in combination. Among these, magnesium nitrate, calcium nitrate, barium nitrate, aluminum nitrate, gallium nitrate, and indium nitrate are preferred, with magnesium nitrate, calcium nitrate, barium nitrate, and aluminum nitrate being more preferred, and aluminum nitrate being particularly preferred because it has a significant effect of sealing fine defects present on the surface of the zeolite membrane, thereby improving ammonia separation performance. The nitrate concentration is usually 10 mol / L or less, with a lower limit of 0.1 mol / L or more, preferably 0.5 mol / L or more, and more preferably 1 mol / L or more. The treatment temperature is usually from room temperature to 150°C or less, and the treatment may be carried out for approximately 10 minutes to 48 hours. These treatment conditions may be appropriately set depending on the type of nitrate and solvent used. The zeolite membrane after the nitrate treatment may be washed with water, and by repeating the water washing, the nitrogen atom content of the zeolite membrane can be adjusted to a preferred range.

[0200] (Aluminum salt treatment) In the zeolite membrane composite used in the present invention, the synthesized zeolite membrane may be subjected to an aluminum salt treatment, if necessary. The aluminum salt treatment may be performed while the zeolite membrane composite contains an organic template or after the organic template has been removed by calcination. The aluminum salt treatment is performed, for example, by immersing the zeolite membrane composite in a solution containing an aluminum salt. This may result in the aluminum salt sealing fine defects present on the membrane surface. Furthermore, when aluminum salt is present in zeolite pores, it has the effect of attracting ammonia, making it a suitable method for improving ammonia permeability. The solvent used for the aluminum salt treatment may be water or an organic solvent as long as the salt dissolves. There is no limitation on the aluminum salt used, but examples include aluminum nitrate, aluminum sulfate, aluminum chloride, aluminum phosphate, aluminum acetate, aluminum carbonate, and aluminum hydroxide. These may be used alone or in combination of two or more. The concentration of the aluminum salt is typically 10 mol / L or less, with a lower limit of 0.1 mol / L or more, preferably 0.5 mol / L or more, and more preferably 1 mol / L or more. The treatment temperature is typically between room temperature and 150°C or less, and the treatment may be carried out for approximately 10 minutes to 48 hours. These treatment conditions may be appropriately set depending on the aluminum salt and solvent type used. The zeolite membrane after aluminum salt treatment may be washed with water, and the Al atom content of the zeolite membrane can be adjusted by repeated washing with water. To increase the Si atom / Al atom ratio of the present invention, it is preferable to reduce the concentration or treatment amount of the aluminum salt used for treatment or increase the number of water washes after aluminum salt treatment. Conversely, to decrease the ratio, it is preferable to increase the concentration or treatment amount of the aluminum salt used for treatment or reduce the number of water washes after aluminum salt treatment.

[0201] (Silylation treatment) In the zeolite membrane composite used in the present invention, the synthesized zeolite membrane may be subjected to a silylation treatment, if necessary. The silylation treatment is performed by immersing the zeolite membrane composite, for example, in a solution containing a Si compound. This modifies the zeolite membrane surface with the Si compound, thereby imparting specific physicochemical properties. For example, by reliably forming a layer rich in Si—OH on the zeolite membrane surface, the polarity of the membrane surface can be improved, thereby improving the separation performance of polar molecules. Furthermore, modifying the zeolite membrane surface with a Si compound can sometimes have the effect of sealing fine defects present on the membrane surface. Furthermore, the pore size of the zeolite can be controlled by the silylation treatment, and this treatment can also be suitably used to improve the ammonia separation selectivity.

[0202] The solvent used for the silylation treatment may be water or an organic solvent. The solution may also be acidic or basic, in which case the silylation reaction is catalyzed by the acid or base. There are no limitations on the silylating agent used, but alkoxysilanes are preferred. The treatment temperature is usually from room temperature to 150°C or less, and the treatment may be carried out for approximately 10 minutes to 30 hours. These treatment conditions may be appropriately set depending on the silylating agent and solvent used.

[0203] In the zeolite membrane composite used in the present invention, the content of nitrogen atoms contained in the zeolite membrane surface can be controlled by, as described above, adjusting the Al atom / Si atom ratio of the zeolite by selecting a nitrogen atom-containing cation species in the zeolite contained in the zeolite membrane, adjusting the content of nitrogen atoms by adjusting the amount of ion exchange by ion exchange, using a nitrogen atom-containing organic template (structure-directing agent) when producing the zeolite membrane as needed and adjusting the amount of the organic template added or the heating temperature and heating time when the organic template is removed by baking, treating the zeolite membrane with nitrate, adjusting the number of times the nitric acid-treated zeolite membrane is washed with water, or by an appropriate combination of these methods. As described above, the content of Al atoms contained in the zeolite membrane surface of the zeolite membrane composite used in the present invention can be controlled by adjusting the Al atom / Si atom ratio in the zeolite contained in the zeolite membrane, treating the zeolite membrane with an aluminum salt, adjusting the number of times the aluminum salt-treated zeolite membrane is washed with water, or by an appropriate combination of these methods.

[0204] The zeolite composite membrane thus produced has excellent properties and can be suitably used as a membrane separation means for separating ammonia from a mixed gas in the present invention. [Example]

[0205] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples as long as it does not depart from the gist of the invention. Note that the values ​​of various production conditions and evaluation results in the following examples represent preferred upper or lower limit values ​​in the embodiments of the present invention, and a preferred range may be defined by combining the upper or lower limit values ​​with the values ​​in the following examples or values ​​between the examples. In the following, "RHO-type silicate zeolite" will be simply referred to as "RHO-type zeolite," and "MFI-type silicate zeolite" will be simply referred to as "MFI-type zeolite."

[0206] [Separation performance measurement] The separation performance of the zeolite membrane composite was measured as follows.

[0207] (1) Ammonia separation test An ammonia separation test was performed using the apparatus shown schematically in Figure 1 as follows. In Figure 1, a cylindrical zeolite membrane composite 1 is housed in a stainless steel pressure vessel 2 and installed in a thermostatic chamber (not shown). The thermostatic chamber is equipped with a temperature control device to allow temperature adjustment of the sample gas. The tip of the support of the cylindrical zeolite membrane composite 1 is sealed with a sealing part (circular end pin) 3. The other end is connected to a joint 4, which is in turn connected to the pressure vessel 2. A pipe (permeation gas recovery pipe) 11 for discharging permeation gas 8 is connected to the inside of the cylindrical zeolite membrane composite via the joint 4, and the permeation gas recovery pipe 11 extends to the outside of the pressure vessel 2. A pipe (sweep gas supply pipe) 12 for supplying sweep gas 9 via the permeation gas recovery pipe 11 is inserted into the zeolite membrane composite 1. Furthermore, a pressure gauge 5 that measures the pressure on the supply side of the sample gas and a back pressure valve 6 that adjusts the pressure on the supply side by controlling the discharge of non-permeable gas 10 from the pressure vessel 2 are connected to any point leading to the pressure vessel 2. Each connection is airtight. In the apparatus shown in Figure 1, the flow rate of a mixed gas containing ammonia gas (NH3), nitrogen gas (N2), and hydrogen (H2) was adjusted as the feed gas and supplied between a pressure-resistant container 2 and a zeolite membrane composite 1. The back-pressure valve 6 was adjusted to maintain a constant pressure difference between the gas on the feed side (feed gas) 7 and the permeated gas 8 that had permeated through the membrane. The exhaust gas 8 discharged from a pipe 11 was analyzed by a micro gas chromatograph, and the concentration and flow rate of the permeated gas were calculated. The linear velocity of the gas in the pressure vessel 2 was calculated by dividing the flow rate of the mixed gas by the value obtained by subtracting the cross-sectional area of ​​the zeolite membrane composite (the area when the zeolite membrane composite 1 is cut perpendicularly to the direction from the joint 4 to the plugging portion 3) from the cross-sectional area of ​​the pressure vessel 2 (the area when the pressure vessel 2 is cut perpendicularly to the direction from the joint 4 to the plugging portion 3).

[0208] [SiO2 / Al2O3 molar ratio measurement] The SiO2 / Al2O3 molar ratio of the zeolite membrane described in the Production Examples was measured under the following conditions. (EDS mapping analysis conditions) Detector: XFlash 6 / 60 (Bruker) Measurement software: Esprit 2.3 (Bruker) Measurement conditions: Measurement magnification: 1200x or 3500x (observation field: 107 x 80 μm or 37 x 27 μm) Image resolution = 512 x 384 pixels Acceleration voltage = 6.0 kV WD (working distance) = 10 ± 1 mm Spot Intensity = 50 Pulse throughput = 60 kcps (per detector element. Four elements are used for analysis) Accumulation time = 180 seconds (EDS Object Analysis Conditions) Detector: XFlash 6 / 60 (Bruker) Measurement software: Esprit 2.3 (Bruker) Measurement conditions: Measurement magnification: 6000x (observation field: 21 x 16 μm) Image resolution = 512 x 384 pixels Acceleration voltage = 6.0 kV WD (working distance) = 10 ± 1 mm Spot Intensity = 50 Pulse throughput = 60kcps Accumulation time = 180 seconds (Object analysis conditions) Analysis software: Esprit 2.3 (Bruker) Method: Object Analysis Map display settings: EDS calibration: Calibrated to meet 10Kcps at an accelerating voltage of 15kV and Cu-Kα radiation using a copper standard. ·Method: Quantitative analysis (Phi-Rho-Z method) (SiO2 / Al2O3 molar ratio calculation formula) SiO2 / Al2O3 molar ratio (SAR) = (Si atomic concentration) / {(Al atomic concentration) / 2}

[0209] In the ammonia separation test, in order to remove components such as moisture and air from the pressure vessel 2, the sample gas used was used for drying and exhausting at a temperature higher than the measurement temperature, and then the sample gas temperature and the differential pressure between the supply gas 7 side and the permeated gas 8 side of the zeolite membrane composite were kept constant. After the permeated gas flow rate was stabilized, the flow rate of the sample gas (permeated gas) 8 that had permeated the zeolite membrane composite 1 was measured. From the measured values, the gas permeance [mol / (m 2 The pressure difference between the supply side and permeation side of the feed gas 7 (pressure difference) was used as the pressure when calculating the permeance. In the case of a mixed gas, the partial pressure difference was used. The ratio of the permeance of each gas was determined by calculating the permeance of each gas and then calculating the ratio.

[0210] [Production Example A1: Production of MFI-type zeolite membrane composite 1] (Raw material mixture for hydrothermal synthesis) A raw material mixture for hydrothermal synthesis was prepared by the following method. 0.75 g of sodium aluminate (containing 62.2% by mass of Al2O3) was added to a mixture of 4.4 g of NaOH (granular, manufactured by Wako Co., Ltd.) and 266 g of water, and the mixture was stirred at room temperature. 34 g of colloidal silica (Snowtec-40, manufactured by Nissan Chemical Co., Ltd.) was added, and the mixture was stirred at 50°C for 4 hours to prepare a raw material mixture for hydrothermal reaction. The composition (molar ratio) of this raw material mixture for reaction was SiO2 / Al2O3 / NaOH / H2O = 1.0 / 0.02 / 0.54 / 70.

[0211] (Porous support) The porous support was an 8 cm alumina tube (outer diameter 12 mm, inner diameter 9 mm, tubular) manufactured by Iwao Porcelain Industries Co., Ltd., which had been washed by flowing demineralized water and then dried.

[0212] (Seed crystal dispersion) MFI type zeolite was ground in a mortar and prepared, and seed crystals were dispersed in the ground zeolite so that the seed crystal concentration was 0.2 mass %, to prepare a seed crystal dispersion.

[0213] (Production of membrane complex) One side of the porous support was sealed, and the other side was vacuumed using a vacuum pump. The porous support was then immersed in the above-mentioned seed crystal dispersion for 5 seconds, and then dried at 100°C for at least 2 hours to attach the seed crystals to the porous support. The seed crystals attached to the support surface were then fixed to the support surface using a rubbing method, and unnecessary seed crystals were removed. Three porous supports with attached seed crystals were prepared using this method. The mass of the seed crystals attached by this procedure was approximately 0.004 to 0.005 g. The three porous supports with attached seed crystals were immersed vertically in a Teflon® inner tube containing the above-mentioned hydrothermal synthesis raw material mixture. The autoclave was then sealed and heated in a thermostatic chamber under autogenous pressure for 3 hours from room temperature to 160°C, 2 hours from 160°C to 180°C, and 15 hours. After cooling for a predetermined time, the porous support-zeolite membrane composite was removed from the reaction mixture, washed with demineralized water, and dried at 100°C for at least 2 hours to obtain MFI zeolite membrane composite 1. The mass of the MFI zeolite crystallized on the porous support was 0.09 to 0.10 g. The air permeability of the calcined zeolite membrane composite was 0.0 to 0.1 ml / min. The zeolite membrane was irradiated with an electron beam from the fracture surface of the obtained zeolite membrane composite, and the SiO2 / Al2O3 molar ratio was measured to be 19.

[0214] [Production Example A2: Production of MFI-type zeolite membrane composite 2] (Raw material mixture for hydrothermal synthesis) A raw material mixture for hydrothermal synthesis was prepared by the following method. 0.15g of sodium aluminate (containing 62.2% by mass of Al2O3) was added to a mixture of 13.65g of a 50 wt% NaOH aqueous solution and 101g of water, and the mixture was stirred at room temperature for 10 minutes. 32.3g of colloidal silica (Nissan Chemical Industries, Snowtec-40) was added to the mixture, and the mixture was stirred at 50°C for 5 hours to prepare a raw material mixture for the hydrothermal reaction. The composition (molar ratio) of this raw material mixture for the reaction was SiO2 / Al2O3 / NaOH / H2O = 3.05 / 0.013 / 0.193 / 100, and SiO2 / Al2O3 = 239.

[0215] (Seed crystal dispersion) ZSM5 zeolite (HSZ-800 series 822H0A manufactured by Tosoh Corporation) was ground in a mortar and prepared, and seed crystals were dispersed in the ground zeolite so that the concentration of the seed crystals was about 0.4% by mass, to prepare a seed crystal dispersion. (Production of membrane complex) A porous support that had been treated in the same manner as in Production Example A1 was immersed in the above-mentioned seed crystal dispersion for 1 minute, then dried at 70° C. for 1 hour, and then immersed again in the seed crystal dispersion for 1 minute and then dried at 70° C. for 1 hour to attach seed crystals to the support. The mass of the attached seed crystals was approximately 0.0016 g.

[0216] The three supports with the seed crystals attached were each immersed vertically in a Teflon (registered trademark) inner tube (200 ml) containing the above-mentioned mixture of raw materials for hydrothermal synthesis. The autoclave was then sealed and heated at 180°C for 30 hours in a static state under autogenous pressure. After the predetermined time had elapsed and the mixture was allowed to cool, the support-zeolite membrane composite was removed from the reaction mixture, washed, and then dried at 100°C for 3 hours to obtain MFI zeolite membrane composite 2. The mass of the MFI zeolite crystallized on the support was 0.26 to 0.28 g. The air permeability of the membrane composite after calcination was 0.0 to 0.1 ml / min.

[0217] [Production Example A3: Production of RHO-type zeolite membrane composite] (mixture for hydrothermal synthesis) The following raw material mixture was prepared for hydrothermal synthesis. 6.8 g of 18-crown-6-ether (Tokyo Chemical Industry Co., Ltd.), 2.1 g of NaOH (Kishida Chemical Co., Ltd.), and 4.2 g of CsOH·HO (Mitsuwa Chemical Co., Ltd.) were dissolved in 125.9 g of water and stirred at 80 °C for 3 hours to obtain a crown ether-alkali aqueous solution. The crown ether-alkali aqueous solution was then added dropwise to 8.9 g of Y-type (FAU) zeolite (SAR = 30, Zeolyst CBV720) and 0.2 g of aluminum hydroxide (AlO 53.5 wt%, Aldrich) to prepare a hydrothermal synthesis raw material mixture. The gel composition (molar ratio) of the resulting hydrothermal synthesis raw material mixture was SiO / AlO / NaOH / CsOH / HO / 18-crown-6-ether = 1 / 0.040 / 0.36 / 0.18 / 50 / 0.18.

[0218] (Support) The porous support was an alumina tube (outer diameter 12 mm, pore size 0.15 μm, manufactured by Iwao Porcelain Industries Co., Ltd.) cut to a length of 80 mm, washed with water, and then dried.

[0219] (Seed crystal dispersion) RHO-type zeolite was ground in a mortar and prepared, and seed crystals were dispersed in the ground zeolite so that the concentration of the seed crystals was about 0.5% by mass, to prepare a seed crystal dispersion.

[0220] (Production of membrane complex) One side of the porous support was sealed, and the other side was vacuumed using a vacuum pump. The porous support was then immersed in the seed crystal dispersion for 1 minute, then removed to the atmosphere and vacuumed for 5 minutes. The seed crystals attached to the support surface were then fixed to the surface by rubbing, and unnecessary seed crystals were removed. Using this method, three porous supports with attached seed crystals were prepared. The mass of the seed crystals attached by this procedure was approximately 0.01 to 0.02 g.

[0221] Next, the support with the attached seed crystals was immersed vertically in a Teflon (registered trademark) inner tube containing a mixture of raw materials for hydrothermal synthesis, the autoclave was sealed, and it was heated at 180°C for 24 hours under autogenous pressure.

[0222] After a predetermined time had passed and the mixture was allowed to cool, the support-zeolite membrane composite was removed from the autoclave, washed with demineralized water, and dried at 100°C for at least 2 hours. After drying, the air permeability in the as-made state was 0.0 to 0.01 ml / min. Next, to remove the template, the resulting membrane composite was calcined at 300°C for 24 hours to obtain an RHO-type zeolite membrane composite. The weight of the RHO-type zeolite crystallized on the support was calculated by subtracting the weight of the calcined zeolite membrane composite from the weight of the support, giving a weight of 0.26 g.

[0223] Next, the RHO-type zeolite membrane composite after template removal was placed in a Teflon (registered trademark) inner tube (200 ml) containing 150 g of 1 M ammonium nitrate aqueous solution. The autoclave was sealed and heated at 100°C for 1 hour in a static state under autogenous pressure.

[0224] After a predetermined time had passed and the composite RHO-type zeolite membrane was allowed to cool, it was removed from the aqueous solution and washed with ion-exchanged water at 100°C for 1 hour. The zeolite membrane was irradiated with an electron beam from the fracture surface of the resulting composite zeolite membrane, and the SiO2 / Al2O3 molar ratio was measured to be 12.

[0225] Next, the above treatment with 1M ammonium nitrate water was repeated five times, and then the mixture was dried at 100°C for more than 4 hours and then treated with NH4 + The RHO-type zeolite membrane composite was obtained.

[0226] NH4 + The RHO-type zeolite membrane composite was placed in a Teflon (registered trademark) inner tube (200 ml) containing 150 g of a 1 M aqueous aluminum nitrate solution. The autoclave was sealed and heated at 100°C for 1 hour in a static state under autogenous pressure.

[0227] After a predetermined time has passed and the RHO-type zeolite membrane composite is left to cool, it is taken out of the aqueous solution, washed with water, and then dried at 100°C for 4 hours or more. + A RHO-type zeolite membrane composite was obtained. For ammonia separation evaluation, Al-treated NH4+ The RHO-type zeolite membrane composite was used.

[0228] [Examples 1 to 14, Comparative Examples 1 to 3] <Evaluation of membrane separation performance> Ammonia separation tests from ammonia / hydrogen / nitrogen mixed gas were carried out using the MFI zeolite membrane composite 1, MFI zeolite membrane composite 2, and RHO zeolite membrane composite, using the apparatus shown in Figure 1.

[0229] The conditions used for the separation evaluation in Examples 1 to 14 and Comparative Examples 1 to 3 are as follows, and the specific conditions are shown in Table 3. Supply gas pressure: 0.4MPaA~8.1MPaA Permeation gas pressure: 0.1 MPaA (atmospheric pressure) Separation temperature: 30℃~250℃ Gas linear velocity: 0.3 to 45.6 cm / sec Ammonia concentration of gas supplied to zeolite membrane: 5 to 18% by volume Effective cross-sectional area: 0.10~1.91cm 2 The effective cross-sectional area refers to the cross-sectional area through which the gas supplied to the membrane in the membrane module passes. In this separation evaluation, no sweep gas was used.

[0230] [Table 3]

[0231] From the gas compositions and permeation amounts of the obtained permeable and non-permeable gases, the ammonia permeance (NH3 permeance), hydrogen permeance (H2 permeance), nitrogen permeance (N2 permeance), ammonia / hydrogen permeance ratio (NH3 / H2 permeability coefficient ratio), and ammonia / nitrogen permeance ratio (NH3 / N2 permeability coefficient ratio) were calculated, and the results are shown in Table 3. Figure 2 also shows the measurement results of the permeability coefficient ratio as a function of the gas linear velocity of the mixed gas.

[0232] As shown in Table 3, by setting the mixed gas pressure to 0.8 MPaA or more and 20 MPaA or less, the separation temperature to 100°C or more and 500°C or less, and by setting the ammonia / nitrogen permeability coefficient ratio of the zeolite membrane to 100 or more and the ammonia / hydrogen permeability coefficient ratio to 70 or more, ammonia gas permeated the zeolite membrane at a higher permeability than in the comparative example in which these conditions were not met.

[0233] Furthermore, as shown in Figure 2, we found that increasing the linear gas velocity to 0.5 cm / sec or higher is effective in improving ammonia permeability. This is believed to be because, although the ammonia concentration on the non-permeated side momentarily decreases when ammonia permeates the zeolite membrane, increasing the linear gas velocity increases the ammonia supply velocity, suppressing ammonia concentration polarization on the non-permeated side and thereby improving permeability. Surprisingly, we also found that increasing the linear gas velocity significantly improves the ammonia / nitrogen permeability ratio and the ammonia / hydrogen permeability ratio. This is thought to be because supplying ammonia near the zeolite membrane at a linear velocity above a certain level makes the ammonia supply velocity faster than the ammonia permeating the zeolite membrane, thereby attracting ammonia to the acid sites on the surface of the zeolite membrane and locally increasing the ammonia concentration near the zeolite membrane surface. Indeed, Examples 3 to 6 in Table 3 show that the higher the ammonia concentration of the feed gas, the higher the ammonia / nitrogen permeability ratio and the ammonia / hydrogen permeability ratio.

[0234] Furthermore, as shown in Table 3, in Comparative Example 2, the pressure of the supply gas was low relative to the temperature of 200°C, so the adsorption of ammonia to the zeolite was weak, and it can be seen that the separation method of the present invention using the separation mechanism of adsorption / desorption to zeolite does not satisfy practical performance.

[0235] Comparative Example 3 shows the measurement results of a separation test at 30°C. Because the separation temperature is low, ammonia is strongly adsorbed to the zeolite acid sites, and the desorption rate is slow, resulting in an ammonia permeability of 0.9 × 10 -8 mol / (m2 s·Pa). As a result, the ammonia / nitrogen permeability coefficient ratio and the ammonia / hydrogen permeability coefficient ratio were also low at 3. This shows that controlling the separation temperature within a specific range is effective in achieving highly selective separation of ammonia even under high-pressure conditions such as those of the present invention. [Explanation of symbols]

[0236] 1 Zeolite membrane composite 2. Pressure vessel 3 Sealing part at the tip of the support 4. Junction between the zeolite membrane composite and the permeate gas recovery tube 5. Pressure gauge 6 Back pressure valve 7. Supply gas (sample gas) 8 Permeable Gas 9 Sweep Gas 10 Non-permeable gas 11 Permeation gas recovery tube 12 Sweep gas supply pipe

Claims

1. A method for separating ammonia by supplying a mixed gas containing hydrogen, nitrogen, and ammonia to a zeolite membrane, the pressure of the mixed gas supplied to the zeolite membrane is 0.8 MPaA or more and 20 MPaA or less, and the separation temperature is 100°C or more and 500°C or less; The method for separating ammonia, wherein the zeolite membrane has an ammonia / nitrogen permeability coefficient ratio of 100 or more and an ammonia / hydrogen permeability coefficient ratio of 70 or more.

2. 2. The method for separating ammonia according to claim 1, wherein the linear velocity of the mixed gas in the separation device equipped with the zeolite membrane is 0.5 cm / sec or more and 10 m / sec or less.

3. 3. The method for separating ammonia according to claim 1, wherein the ammonia gas concentration in the mixed gas is less than 50% by volume.

4. 3. The method for separating ammonia according to claim 1, wherein the zeolite membrane is an aluminosilicate zeolite membrane.

5. The ammonia permeability of the zeolite membrane is 8.5×10 －８ mol / (m ２ 3. The method for separating ammonia according to claim 1 or 2, wherein the pressure is 1.0 MPa (0.05 MPa) or more.

6. The framework density of the zeolite constituting the zeolite membrane is 14.3 T / nm ３ 3. The method for separating ammonia according to claim 1 or 2, wherein the above-mentioned

7. 3. The method for separating ammonia according to claim 1, wherein the structure of the zeolite constituting the zeolite membrane is MFI or RHO.

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