CONTINUOUS ZEOLITH SYNTHESIS PROCESS ZSM-5

Abstract

CONTINUOUS ZSM-5 ZEOLITH SYNTHESIS PROCESS The present invention relates to a continuous synthesis process for ZSM-5 type zeolite crystals, comprising continuously feeding a tubular reactor with a synthesis medium comprising a silica source, an alumina source, and seeds; rapidly heating the synthesis medium to a temperature between 100°C and 300°C; crystallizing it at a temperature at least equal to or higher than the temperature of the preceding step; and continuously recovering the ZSM-5 type zeolite crystals. Fig. 1

Description

Title of the invention: CONTINUOUS SYNTHESIS PROCESS FOR ZSM-5 ZEOLITH

[0001] The present invention relates to the field of zeolites and more particularly to the field of synthetic zeolites, and more particularly to the preparation of synthetic zeolites, in particular zeolites with a high silicon content, and especially to the continuous preparation of synthetic zeolites with a high silicon content, with high levels of purity and crystallinity.

[0002] Zeolites are crystalline aluminosilicates in which the proportion of silicon and aluminum, often denoted as the Si / Al atomic ratio, is highly variable. Furthermore, several crystal structures are possible for the same Si / Al ratio, and several zeolites with different Si / Al ratios can exhibit the same crystal structure. The synthesis methods are very diverse and vary in their ease of implementation and execution, depending on the target Si / Al ratio and the desired crystal structure.

[0003] In particular, the zeolite known as ZSM-5, which is an MFI zeolite, is a microporous crystalline aluminosilicate involved in various industrial applications such as adsorption, catalysis, separation, and ion exchange. Industry therefore requires relatively large quantities of this synthetically produced ZSM-5 zeolite.

[0004] The ZSM-5 zeolite synthesis routes known today suffer, however, from many disadvantages, among which we can mention the use of organic structuring agents, relatively long synthesis times, for example from a few tens of hours to a few days, as well as the use of autoclaves, since the synthesis most often requires high temperatures and therefore pressure.

[0005] In addition, conventional industrial syntheses of ZSM-5 are most often carried out with large-scale installations, generally with heating of the synthesis gel and / or reaction medium, by steam injection and / or by double jacket, which requires high energy expenditure and often leads to problems of production regularity.

[0006] Syntheses of ZSM-5 with organic structuring agent are described for example in documents US7244409 and AU2014413311. These syntheses require on the one hand the presence of organic structuring agent in the reaction medium and on the other hand the destruction, generally by calcination, after synthesis of the zeolite, of this organic structuring agent.

[0007] The use of organic structuring agents is therefore harmful to the environment. Furthermore, the use of structuring agents entails high manufacturing costs and an additional industrial step for the disposal of said organic structuring agent.

[0008] To overcome these drawbacks, other publications mention syntheses without organic structuring agents, such as publication US2013144100, which describes a synthesis of large-crystal ZSM-5 zeolites. However, in this description, the process uses gluconic acid or its salts as the aluminum complexing agent(s). Furthermore, the synthesis time (approximately 2 to 100 hours) remains too long for efficient, economical, and profitable industrial syntheses.

[0009] US patent 5240892 describes batch syntheses lasting several hours, where the crystallization step is carried out in an autoclave. US patent 6261534 also describes a process for synthesizing ZMS-5 zeolite, without a structuring agent but in the presence of two sources of metal and non-metal oxides with a molar ratio greater than 12. Furthermore, the synthesis times are long, for example, exceeding 24 hours.

[0010] Among the processes carried out continuously in tubular reactors, we may mention in particular international application WO2017216236 and international application WO2018167414 which both exemplify only syntheses of zeolites with a low Si / Al ratio (chabazite, zeolites A and X).

[0011] The main objective of the present invention is therefore to propose a new method for preparing ZSM-5 zeolite that overcomes the aforementioned drawbacks. In particular, one objective of the present invention is to provide a readily industrializable, economical, and efficient process for synthesizing ZSM-5 zeolite. Another objective of the invention is to provide a readily industrializable, economical, and efficient process for synthesizing ZSM-5 zeolite with short synthesis times. Yet another objective is to provide a readily industrializable, economical, and efficient process for synthesizing ZSM-5 zeolite with short synthesis times, the resulting zeolite being in the form of crystals of a size compatible with industrial applications, and in particular sizes larger than nanometers.

[0012] The Applicant has now discovered that the aforementioned objectives can be achieved in whole or at least in part through the invention which is now detailed in the following description.

[0013] Thus, a first object of the invention consists of a continuous synthesis process for ZSM-5 zeolite that does not require the use of an organic structuring agent. The process of the invention also allows for syntheses that can be performed without a step of maturation, and with relatively short crystallization times, generally less than 5 hours, even less than 4 hours and, more generally, less than 3 hours. The process according to the present invention is perfectly suited for the continuous synthesis of ZSM-5 crystals.

[0014] In the present invention, and unless otherwise indicated, all ranges of values ​​introduced by the expressions "from ... to ..." or "between .... and ...." or other similar expressions intended to frame two values, are understood to include the limits.

[0015] More particularly, the present invention relates to the continuous synthesis process for ZSM-5 type zeolite crystals, said process comprising at least the following steps a) to d): a) continuous feeding of a tubular reactor with a synthesis medium comprising a silica source, an alumina source and seeds; b) heating the synthesis medium for a period equivalent to the residence time in at most 1 / 3, preferably 1 / 4, preferably 1 / 5 of the total length of the tubular reactor, up to a value between 100°C and 300°C; c) crystallization at a temperature at least equal to or greater than the temperature of the previous step; d) continuous recovery of ZSM-5 type zeolite crystals, with a size between 0.2 pm and 20.0 pm, preferably between 0.2 pm and 10.0 pm, better still between 0.3 pm and 7.0 pm, advantageously between 0.3 pm and 5.0 pm.

[0016] The ZSM-5 type zeolite crystals obtained by the process of the invention defined above generally have a size between 0.2 pm and 20.0 pm, preferably between 0.2 pm and 10.0 pm, even better between 0.3 pm and 7.0 pm, advantageously between 0.3 pm and 5.0 pm.

[0017] In the process of the invention, the reactor is a tubular reactor, optionally but preferably equipped with one or more stirring systems selected from mechanical stirring and oscillatory stirring systems, and also combinations of one or more mechanical stirring systems with one or more oscillatory stirring systems. However, for the process of the present invention, only one type of stirring system, either mechanical or oscillatory, is preferred.

[0018] Preferably, the method of the present invention comprises only one agitation system, either mechanical or oscillatory. According to a preferred embodiment, the method of the invention comprises a single mechanical agitation system. According to another preferred embodiment, the method of the invention comprises a single agitation system generated by an oscillatory motion.

[0019] The means of agitation can be of any type well known to a person skilled in the art, and for example, but not limited to, when the reactor is a reactor tubular reactor adapted for continuous operation, can be fitted with restrictions (such as rings, baffles and others), can be equipped with one or more stirring systems (stirring shaft fitted with several stirring impellers, cascade of agitators distributed along the reactor), one or more oscillating or pulsating systems (allowing a back-and-forth movement of the reaction medium to be generated by means of, for example, piston, membrane, head-to-tail pumps), and others, as well as two or more of these techniques combined.

[0020] In a preferred embodiment of the invention, the process is implemented in a tubular reactor with restrictions and equipped with a system for imparting pulsations to the fluid circulating in the reactor, as described for example in NiTech's US20090304890 application.

[0021] In a preferred embodiment of the process of the invention, the tubular reactor makes it possible to ensure a continuous flow in a straight line, possibly with one or more curves. The tubular reactor generally and most often has a constant internal diameter that can vary considerably, and preferably between 1 mm and 1000 mm, preferably between 1 mm and 800 mm, preferably again between 1 mm and 500 mm, for example between 3 mm and 400 mm.

[0022] The total length of the tubular reactor can also vary in large proportions and is generally between 0.5 m and 100 m, preferably between 0.8 m and 80 m, and preferably between 1 m and 70 m.

[0023] The reactor volume is to be adapted according to the zeolite production requirements. Typically, it can vary between 0.04 m3 and 10 m3, preferably between 0.05 m3 and 5 m3, with a length / diameter aspect ratio typically greater than 100, preferably greater than 140, and preferably even greater than 180. Depending on the geometry of the reactor, the flow rate can typically vary between 0.02 m3 h 1 and 20 m3 h *.

[0024] The reactor used for the process of the present invention further comprises at least one heating system covering at least a portion of the reactor's length, and optionally, a thermal insulation system covering all or part of the reactor's length. The reactor may also include one or more ultrasonic sources to promote crystallization and / or the formation of well-defined crystals, i.e., crystals with few or no aggregates.

[0025] Said at least one heating system may be of any type well known to those skilled in the art, and may, for example, be chosen from steam injection, double jacket, the addition of a microwave source, and a combination of one or more of the aforementioned methods. The heating system must allow for a rapid temperature increase up to the crystallization temperature, as will be described later.

[0026] The synthesis medium is prepared continuously by mixing a silica source and from an alumina source. This synthesis medium is prepared by mixing the components of said synthesis medium by any means well known to those skilled in the art and more particularly by means of a mixer, for example and preferably a rotor / stator type shear mixer.

[0027] By source of silica is meant any source well known to a person skilled in the art and in particular a solution, preferably aqueous, of silicate, in particular of silicate or orthosilicate of alkali or alkaline earth metal, for example sodium, or colloidal silica or tetraethyl orthosilicate, the latter not being preferred.

[0028] By alumina source, we mean any alumina source well known to those skilled in the art and in particular a solution, preferably aqueous, of aluminium sulfate, aluminium nitrate, aluminate, in particular aluminate of alkali or alkaline earth metal, for example sodium.

[0029] In a preferred embodiment, the synthesis medium comprises: - a source of silica which is an aqueous solution of silicate or orthosilicate of an alkali or alkaline earth metal, for example and preferably sodium, or colloidal silica, and - an alumina source which is an aqueous solution of aluminum sulfate, aluminum nitrate, aluminate, in particular aluminate of alkali or alkaline earth metal, for example sodium.

[0030] By seeds, we mean any source of seeds well known to those skilled in the art and in particular a nucleating solution, or zeolite crystals of type MFI (ZSM-5 or Silicalite-1) or of type MEL, possibly previously ground or cryo-ground preferably to a submicron size.

[0031] The seeds are introduced continuously, either mixed with the silica source and / or the alumina source, or after the introduction of the silica and alumina sources. The introduction of the seeds is preferably carried out upstream of the crystallization step. The weight percentage of the seeds relative to the total weight of the synthesis medium is generally and most often between 0.5% and 20%, preferably between 1% and 10%.

[0032] The SiO2 / Al2O3 molar ratio in the synthesis medium, before the introduction of the seeds, is generally between 16 and 400, preferably between 16 and 350, and preferably between 20 and 300, inclusive. The H2O / SiO2 molar ratio is between 1 and 100, preferably between 3 and 90, and preferably between 5 and 70, inclusive. The Na2O / SiO2 molar ratio is between 0.01 and 0.9, preferably between 0.01 and 0.7, and preferably between 0.01 and 0.5, inclusive.

[0033] According to yet another preferred embodiment, the synthetic medium, before the introduction of the seeds, presents: - a SiO2 / Al2O3 molar ratio between 16 and 400, preferably between 16 and 350, preferably between 20 and 300, including terminals, - a molar ratio of H2O / SiO2 between 1 and 100, preferably between 3 and 90 and preferably between 5 and 70 inclusive, and - a Na2O / SiO2 molar ratio is between 0.01 and 0.9, preferably between 0.01 and 0.7, preferably between 0.01 and 0.5, inclusive.

[0034] In one embodiment, the synthesis medium used in the process of the invention may include one or more auxiliary agents, such as, for example, one or more organic solvents, advantageously chosen from water-soluble solvents and, for example, those chosen from alcohols, advantageously an alcohol chosen from propanol, butanol, pentanol, hexanol, preferably butanol.

[0035] The heating step can be carried out by any means known to those skilled in the art, provided that the synthesis medium rapidly reaches the desired temperature, typically between 100°C and 300°C. Rapid heating can be simply expressed as the equivalent residence time in at most 1 / 3, preferably 1 / 4, and preferably 1 / 5 of the total length of the tubular reactor. This fraction of length can, in certain cases, and if desired, be as high as 1 / 10 of the total length of the tubular reactor. Heating can be carried out by any method well known to those skilled in the art, and for example by steam injection, by a double jacket, or by adding a microwave source, or by a combination of one or more of the aforementioned methods.

[0036] The process of the present invention is characterized in particular by the fact that, and this is the main object of the present invention, the heating of the synthesis medium up to the crystallization temperature is carried out very rapidly.

[0037] Indeed, it has been discovered in a completely surprising way that this rapid heating allows in particular the obtaining of zeolite crystals of high purity, without impurities, or at least with only a few traces of impurities.

[0038] Thus, as previously stated, the reaction medium is continuously fed into the tubular reactor and is immediately heated in a heating zone corresponding to a duration equivalent to at most 1 / 3, preferably 1 / 4, and preferably 1 / 5, or even up to 1 / 10, of the total length of the tubular reactor. Following this heating zone, the reaction medium, at a temperature of 100°C to 300°C, continues its progress through the tubular reactor where it crystallizes to form the desired ZSM-5 zeolite crystals.

[0039] According to the present invention, the crystallization step is carried out at high temperatures and under pressure, the pressure being at least equal to the autogenous pressure. Advantageously, the crystallization step is carried out at a temperature ranging from 100°C to 300°C, preferably from 150°C to 220°C, more preferably from 170°C to 210°C, and most preferably from 180°C to 210°C.

[0040] The duration of the crystallization step can vary greatly and is generally between a few minutes and several hours, most often for a duration ranging from 30 minutes to 5 hours, preferably from 30 minutes to 3 hours, preferably still from 1 hour to 2.5 hours.

[0041] It must be understood that thanks to the rapid heating of the synthesis medium for a period equivalent to the residence time in at most 1 / 3, preferably 1 / 4 and preferably 1 / 5 of the total length of the tubular reactor, up to a value between 100°C and 300°C, as indicated above, the crystallization time is for its part equivalent to at least 2 / 3, preferably 3 / 4 and preferably 4 / 5 of the total length of the tubular reactor.

[0042] As previously stated, the flow rate in the tubular reactor can vary in large proportions and is generally and typically between 0.02 m3 h 1 and 20 m3 h1, depending on the geometry of the reactor, the desired synthesis rates, the different types of equipment used for mixing the starting solutions, for bringing the crystallization temperature up, and other things.

[0043] Another further advantage of the process of the invention, a direct consequence in particular of the rapid heating up to the crystallization temperature, is realized by very short synthesis times of ZSM-5 crystals, especially when compared to the industrial synthesis times available today in the prior art.

[0044] Furthermore, the synthesis process of the present invention is a continuous process, which represents a significant advantage compared to conventional industrial synthesis processes, which generally require large-scale installations and often produce batches that are not very homogeneous in terms of product quality. The continuous process according to the present invention thus offers numerous advantages, as indicated above, to which can be added the reduction in the size of the installations, the decrease in energy consumption, and the improvement in the consistency of production quality.

[0045] The continuous process according to the present invention ensures homogeneous mixing of the reaction medium, particularly during crystallization, which allows for the simple and efficient production of crystals with homogeneous particle size and morphology. Thus, the process of the invention continuously generates ZSM-5 type zeolite crystals with a Si / Al ratio of 10 to 60, preferably 10 to 50, and preferably 12 to 40.

[0046] According to one embodiment, the crystals obtained by the process of the present invention have a size greater than 0.2 pm, and preferably greater than 0.3 pm, inclusive of the terminals, and, most often, a size between 0.2 pm and 20.0 pm, preferably between 0.2 pm and 10.0 pm, even better between 0.3 pm and 7.0 pm, advantageously between 0.3 pm and 5.0 pm.

[0047] The number average size of the zeolite crystals is estimated by scanning electron microscopy (SEM). To estimate the size of the zeolite crystals in the samples, a series of images is taken at a magnification of at least 5000x. The length of at least 200 crystals is then measured using dedicated software, for example, Smile View from LoGraMi. The accuracy is on the order of 3%. The ZSM-5 crystals are pure; this purity is verified by the absence of parasitic phases identified by XRD.

[0048] Thus, the crystals obtained according to the process of the present invention are generally and most often characterized by a Dubinin volume equal to or greater than 0.10 g.cm³, preferably equal to or greater than 0.13 g.cm³, preferably equal to or greater than 0.14 g.cm³. The Dubinin (or microporous Vmi) volume is determined in a conventional manner known to those skilled in the art, in particular from the measurement of the adsorption isotherm of a gas at its liquefaction temperature, for example nitrogen, argon, oxygen, and others. Preferably, nitrogen is used.

[0049] Prior to this adsorption measurement, the zeolite crystals of the invention are degassed between 300°C and 450°C for a period of 9 to 16 hours under vacuum (P < 6.7 x 10⁴ Pa). For example, for a zeolite with an MFI structure such as ZSM-5, the nitrogen adsorption isotherm at 77 K is then measured on a Micromeritics ASAP 2020 type instrument, taking at least 35 measurement points at relative pressures with a P / Po ratio between 0.002 and 1. The microporous volume is determined according to the Dubinin and Raduskevitch equation from the isotherm obtained, applying ISO 15901-3:2007. The microporous volume thus evaluated is expressed in cm³ of liquid adsorbent per gram of anhydrous adsorbent. The measurement uncertainty is ± 0.003 g / cm³.

[0050] As previously stated, the process of the present invention makes it possible, in particular, to eliminate the need for an organic structuring agent. Besides the advantage of simplifying implementation, the absence of a structuring agent significantly reduces the environmental impact, as these organic agents are very often toxic. Another advantage is that the process without a structuring agent avoids an additional step of removing the organic structuring agent, thus reducing the production costs of zeolite.

[0051] Without wishing to be bound by theory, it appears that the process of the present invention, in which the reaction medium is rapidly brought to the crystallization temperature, facilitates obtaining a well-crystallized, homogeneous and impurity-free material characterized by crystals having a relative crystallinity, measured according to ASTM D5758, of between 95% and 140%, preferably between 95% and 135%, most preferably between 95% and 130%.

[0052] The invention is now illustrated by means of the following examples, which in no way limit the invention, the scope of which is defined by the claims annexed to this description.

[0053] Figures 1, 2 and 3 represent the X-ray diffractograms of the crystals obtained respectively in examples 1, 2 and 3. Example 1 (according to the invention):

[0054] Continuous synthesis of ZSM-5, with seed addition

[0055] The continuous synthesis of ZSM-5 zeolite consists of feeding a tubular reactor with the silicate, aluminate, and seed solutions. A sodium silicate solution with a composition of 6.9 Na₂O - 93 SiO₂ - 951 H₂O is prepared. A sodium aluminate solution with a composition of 1.4 Na₂O - 1 Al₂O₃ - 311 H₂O is prepared. The seeds consist of ZSM-5 crystals (Alfa AESAR, CAS 1318-02-1) at a concentration of 2% by weight relative to the weight of the synthesis medium.

[0056] The synthesis medium is therefore prepared by simultaneously feeding the chamber of the inline shear mixer using two pumps: the flow rate of the aluminate solution is 100 g / min and that of the silicate is 450 g / min. The seeds are added just before the inlet to the tubular reactor. The synthesis medium is heated in the tubular reactor by means of a double jacket to reach the crystallization temperature of 200°C over a length equivalent to 1 / 6 of the total length of the tubular reactor. The feed rate is fixed to ensure a total residence time in the tubular reactor of 120 minutes. At the end of this synthesis, a pure ZSM-5 zeolite, i.e., exhibiting a diffractogram strictly characteristic of an MFI type zeolite, is obtained (see X-ray diffractogram, [Fig. 1]), and has a Dubinin volume of 0.14 g.cm³ Example 2 (Comparative):

[0057] Continuous synthesis of ZSM-5, without seed addition

[0058] In this example of continuous synthesis of ZSM-5 zeolite, a reactor is fed tubular with a sodium silicate solution of composition 6.9 Na2O - 93 SiO2 -951 H2O and a sodium aluminate solution of composition 1.4 Na2O - 1 Al2O3 -311 H2O.

[0059] The synthesis medium is prepared continuously using a rotor / stator shear mixer by simultaneously mixing the aluminate solution and the silicate solution. The synthesis medium is thus prepared by simultaneously feeding the chamber of the Silverson in-line shear mixer using two peristaltic pumps: the flow rate of the aluminate solution is 100 g min⁻¹ and that of the silicate solution is 450 g min⁻¹. The synthesis medium is heated in the tubular reactor by means of a double jacket to reach the crystallization temperature of 200°C over a length equivalent to 1 / 6 of the total length of the tubular reactor. The feed rate is fixed to guarantee a residence time in the tubular reactor of 120 minutes.

[0060] The X-ray diffractogram ([Fig.2]) of the product obtained from this synthesis shows that in the absence of seeds an amorphous product is obtained. Example 3 (comparative):

[0061] Batch synthesis of ZSM-5, with seed addition

[0062] The batch synthesis of ZSM-5 zeolite consists of introducing into a batch reactor a sodium silicate solution, a sodium aluminate solution and seeds.

[0063] A silicate solution with the composition 6.9 Na₂O 93 SiO₂ 951 H₂O is prepared. An aluminate solution with the composition 1.4 Na₂O 1Al₁₂O₃ 311 H₂O is prepared. The seeds consist of ZSM-5 crystals (Alfa AESAR, CAS 1318-02-1) at a rate of 2% by weight relative to the weight of the synthesis medium.

[0064] The synthesis medium is prepared by mixing the sodium silicate solution, the sodium aluminate solution, and then the seeds in the batch reactor. The reactor is then heated to 200°C by a double jacket. The residence time in the reactor is 2 hours.

[0065] The X-ray diffractogram ([Fig.3]) of the product obtained from this synthesis shows the presence of ZSM-5 of structural type MFI but also the presence of another parasitic phase, a zeolite of structural type MOR.

Claims

Claims

1. Process for the continuous synthesis of ZSM-5 type zeolite crystals, said process comprising at least the following steps a) to d): a) continuously feeding a tubular reactor with a synthesis medium comprising a silica source, an alumina source and seeds; b) heating the synthesis medium for a duration equivalent to the residence time in at most 1 / 3, preferably 1 / 4, more preferably 1 / 5 of the total length of the tubular reactor, up to a value between 100°C and 300°C; c) crystallization at a temperature at least equal to or higher than the temperature of the previous step; d) continuous recovery of the ZSM-5 type zeolite crystals.

2. A method according to claim 1, wherein said tubular reactor is provided with one or more stirring systems selected from mechanical stirring and oscillation stirring system, as well as combinations of one or more mechanical stirring systems with one or more oscillation stirring systems.

3. A method according to claim 1 or claim 2, wherein said tubular reactor has a constant internal diameter of between 1 mm and 1000 mm, preferably between 1 mm and 800 mm, more preferably between 1 mm and 500 mm, for example between 3 mm and 400 mm.

4. Method according to any one of the preceding claims, in which the seeds are chosen from nucleating solution and crystals of zeolites of the MFI or MEL type.

5. Method according to any one of the preceding claims, in which the percentage by weight of the seeds relative to the total weight of the synthetic medium is between 0.5% and 20%, preferably between 1% and 10%.

6. A method according to any one of the preceding claims, wherein the synthesis medium comprises: - a source of silica which is an aqueous solution of alkali or alkaline earth metal silicate or orthosilicate, or colloidal silica, and - a source of alumina which is an aqueous solution of aluminum sulfate, aluminum nitrate, aluminate, in particular alkali or alkaline earth metal aluminate.

7. Method according to claim 6 in which the synthesis medium, before introduction of the seeds, has: - a SiO2 / Al2O3 molar ratio of between 16 and 400, preferably between 16 and 350, preferably between 20 and 300, limits included, - a H2O / SiO2 molar ratio of between 1 and 100, preferably between 3 and 90 and preferably between 5 and 70, limits included, and - a Na2O / SiO2 molar ratio of between 0.01 and 0.9, preferably between 0.01 and 0.7, preferably between 0.01 and 0.5, limits included.

8. A process according to any preceding claim, wherein the crystallization step is carried out at a temperature ranging from 100°C to 300°C, preferably from 150°C to 220°C, more preferably from 170°C to 210°C, and most preferably from 180°C to 210°C.

9. A method according to any one of the preceding claims, wherein the duration of the crystallization step is between 30 minutes and 5 hours, preferably 30 minutes and 3 hours, more preferably 1 hour and 2.5 hours.

10. A method according to any preceding claim, wherein the flow rate is between 0.02 m3 h 1 and 20 m3 h1.

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