Method for producing formic acid, and catalyst for producing formic acid

Abstract

To provide a method for producing formic acid in which the degradation of catalytic activity is suppressed and the yield of formic acid is increased when the catalyst used in the formic acid reaction is recovered and reused in the formic acid reaction. [Solution] A method for producing formic acid, comprising: a preparation step of a catalyst for formic acid production comprising a Pd-based catalyst in which a palladium component is supported on a catalyst support, and a Co-based catalyst comprising a cobalt component and an aluminum component in an amount such that the molar ratio (Co / Al) is 1.0 or more and 7.0 or less; a first formic acid reaction step of producing formic acid by reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing the formic acid production catalyst; a catalyst separation step of separating the formic acid production catalyst from the reaction product after the first formic acid reaction step; a catalyst regeneration step of washing and regenerating the separated formic acid production catalyst to obtain a regenerated formic acid production catalyst; and a second formic acid reaction step of producing formic acid by reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing the regenerated formic acid production catalyst.

Description

[Technical Field]

[0001] This disclosure relates to a method for producing formic acid and a catalyst for producing formic acid. [Background technology]

[0002] In recent years, concern about global warming has increased, and the Conference of the Parties (COP) to the United Nations Framework Convention on Climate Change, which discusses international frameworks for reducing greenhouse gas emissions, aims to keep the rise in average temperature since the pre-industrial era well below 2°C as a common long-term global goal, aiming to suppress peak emissions as early as possible and to reduce them rapidly in accordance with the latest science. The COP21 Paris Agreement states that all countries should strive to formulate and submit long-term low-emission greenhouse gas development strategies. The European Green Deal is also moving to strengthen policies through legislation, such as carbon neutrality by 2050 and raising reduction targets at the midpoint. In Japan, the government has also declared carbon neutrality by 2050. In response to these developments, the development of countermeasures technologies for reducing carbon dioxide is being actively pursued in various places. As one countermeasure technology, several attempts have been proposed to convert emitted carbon dioxide into useful substances, but a large amount of energy is required to convert carbon dioxide into another substance, and the development of effective catalysts to accelerate the reaction is desired.

[0003] Furthermore, in order for a technology to contribute to carbon dioxide reduction, it is necessary to produce a useful substance that is in high demand. Formic acid (HCOOH) is a useful substance that can be produced using carbon dioxide (CO2) and hydrogen (H2) as raw materials, and is positioned as a technology for reducing carbon dioxide.

[0004] The following documents describe techniques for producing formic acid by chemical reaction. For example, Patent Document 1 discloses a method for producing formic acid characterized by reacting carbon dioxide and hydrogen in a supercritical state in the presence of metallic ruthenium supported on a carrier and a basic organic compound. Furthermore, Patent Document 2 discloses a synthesis method for synthesizing HCOOR (where R is H or Na) in the presence of a reducing agent, a catalyst, a polar solvent, and a base.

[0005] Furthermore, Patent Document 3 discloses a method for producing formic acid by hydrogenating carbon dioxide at a pressure of 0.2 to 30 MPa and a temperature of 100°C in the presence of a catalyst containing a tertiary amine (I), a diamine (II), a polar solvent, and gold. Furthermore, Patent Document 4 discloses a method for producing a hydrogen carrier material for formic acid by hydrogenating carbon dioxide using a hydrogenation catalyst in which hydrogen-dissociable metal atoms are supported as single atoms on the surface of a layered double hydroxide. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2001-288137 [Patent Document 2] International Publication No. 2016 / 024293 [Patent Document 3] International Publication No. 2013 / 186156 [Patent Document 4] Japanese Patent Publication No. 2018-103158 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The inventors have found that when producing formic acid by reacting carbon dioxide and hydrogen, it is effective to use a catalyst for formic acid production that includes a Pd-based catalyst, in which a palladium component is supported on a catalyst support, and a Co-based catalyst, in which a cobalt component is present. However, when this formic acid production catalyst is reused in the reaction after being used for formic acid, its catalytic activity sometimes decreases significantly. Since the formic acid production catalyst contains expensive and rare metals such as Pd and Co, it is desirable to recover and reuse it after use in the formic acid reaction, and therefore, it is required that the degradation of activity be suppressed even in the catalyst recovered after the reaction.

[0008] Therefore, the present disclosure aims to provide a method for producing formic acid in which the degradation of catalytic activity is suppressed and the yield of formic acid can be increased when the catalyst used in the formic acid reaction is recovered and reused in the formic acid reaction, and a catalyst for producing formic acid in which the degradation of catalytic activity is suppressed when it is recovered after being used in the formic acid reaction and reused in the formic acid reaction. [Means for solving the problem]

[0009] The present inventors have discovered that by adding an appropriate amount of Al to a Co-based catalyst, the catalytic activity does not deteriorate even when reused after being used in the formic acid reaction, leading to this disclosure. Specifically, they have discovered that by using a formic acid production catalyst in which the molar ratio of cobalt and aluminum elements (Co / Al) contained in the Co-based catalyst is 1.0 or more and 7.0 or less, the formic acid production catalyst used in the first formic acid reaction step to produce formic acid by reacting carbon dioxide and hydrogen can be separated, recovered, and then washed and regenerated, allowing it to be used again as a formic acid production catalyst in the second formic acid reaction step to produce formic acid, leading to this disclosure.

[0010] In other words, the method for producing formic acid and the catalyst for producing formic acid related to this disclosure are as follows.

[0011] <1> A step to prepare a catalyst for formic acid production comprising a Pd-based catalyst in which a palladium component is supported on a catalyst support, and a Co-based catalyst containing a cobalt component and an aluminum component in an amount such that the molar ratio of cobalt element to aluminum element (Co / Al) is between 1.0 and 7.0, A first formic acid reaction step involves reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing a catalyst for formic acid production to produce formic acid, A catalyst separation step for separating the formic acid production catalyst from the reaction product after the first formic acid reaction step, and a catalyst regeneration step for washing and regenerating the formic acid production catalyst separated in the catalyst separation step to obtain a regenerated formic acid production catalyst, A second formic acid reaction step involves reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing the aforementioned catalyst for regenerated formic acid to produce formic acid, A method for producing formic acid containing [the specified ingredient]. <2> The Pd-based catalyst is a catalyst in which the palladium component and the silver component are supported on a catalyst support. <1> The method for producing formic acid as described above. <3> The catalyst support is a catalyst support whose main component is titanium. <1> or <2> The method for producing formic acid as described above. <4> The catalyst separation step includes a first separation step of separating unreacted gaseous components, including carbon dioxide and hydrogen, from the reaction product after the first formic acid reaction step to obtain a solid-liquid component comprising a liquid component containing formic acid and the solvent and a solid component containing the formic acid production catalyst, and a second separation step of separating the solid component from the solid-liquid component. The unreacted carbon dioxide and hydrogen separated in the first separation step are used as the carbon dioxide and hydrogen in the second formic acid reaction step. <1> ~ <3> A method for producing formic acid as described in any one of the items. <5> The second separation step is followed by a solvent separation step of separating the solvent from the liquid component after the solid component has been separated in the second separation step. The solvent separated in the solvent separation step is used as the solvent in the second formic acid reaction step. <4> The method for producing formic acid as described above. <6> A Pd-based catalyst in which a palladium component and a silver component are supported on a catalyst carrier mainly composed of titanium, and a Co-based catalyst containing a cobalt component and an aluminum component in an amount such that the molar ratio (Co / Al) of cobalt element to aluminum element is 1.0 or more and 7.0 or less, for producing formic acid.

Advantages of the Invention

[0012] According to the present disclosure, when recovering the formic acid production catalyst used in the reaction of formic acid and using it again in the reaction of formic acid, a method for producing formic acid in which deterioration of the catalyst activity is suppressed and the yield of formic acid can be increased, and a formic acid production catalyst in which deterioration of the catalyst activity is suppressed when it is recovered after being used in the reaction of formic acid and used again in the reaction of formic acid can be provided.

Brief Description of the Drawings

[0013] [Figure 1] It is a flowchart showing the flow of the method for producing formic acid according to an embodiment of the present disclosure. [Figure 2] It is a graph showing the XRD results of the formic acid production catalyst after carrying out the formic acid reaction step in Examples.

Modes for Carrying Out the Invention

[0014] An embodiment which is an example of the present disclosure will be described. These descriptions and examples are illustrative of the embodiments and do not limit the scope of the invention. In this specification, a numerical range represented by "~" means a range including these numerical values as the lower limit value and the upper limit value when "more than" and "less than" are not attached to the numerical values described before and after "~". Also, a numerical range when "more than" or "less than" is attached to the numerical values described before and after "~" means a range not including these numerical values as the lower limit value or the upper limit value. In the numerical ranges described stepwise in this specification, the upper limit of one stepwise numerical range may be replaced with the upper limit of another stepwise numerical range, or with the values ​​shown in the examples. Similarly, the lower limit of one stepwise numerical range may be replaced with the lower limit of another stepwise numerical range, or with the values ​​shown in the examples. Furthermore, unless otherwise specified, the percentage (%) used for content refers to "mass%". A percentage of "0" indicates that the component is optional and does not need to be included.

[0015] Each component may contain multiple types of the relevant substance. When referring to the amount of each component in a composition, if there are multiple substances corresponding to each component in the composition, unless otherwise specified, it refers to the total amount of those multiple substances present in the composition. The term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended function is achieved.

[0016] <Method for producing formic acid> A method for producing formic acid according to the embodiment of this disclosure includes the steps of: preparing a catalyst for formic acid production comprising a Pd-based catalyst in which a palladium component is supported on a catalyst support, and a Co-based catalyst containing a cobalt component and an aluminum component in an amount such that the molar ratio of cobalt element to aluminum element (Co / Al) is 1.0 or more and 7.0 or less; a first formic acid reaction step in which carbon dioxide and hydrogen are reacted under pressurized conditions in a non-basic solvent containing the catalyst for formic acid production to produce formic acid; a catalyst separation step in which the catalyst for formic acid production is separated from the reaction product after the first formic acid reaction step; a catalyst regeneration step in which the catalyst for formic acid production separated in the catalyst separation step is washed and regenerated to obtain a regenerated catalyst for formic acid production; and a second formic acid reaction step in which carbon dioxide and hydrogen are reacted under pressurized conditions in a non-basic solvent containing the regenerated catalyst for formic acid production to produce formic acid.

[0017] When producing formic acid by reacting carbon dioxide and hydrogen, it is effective to use a formic acid production catalyst containing the above-mentioned Pd-based catalyst and the above-mentioned Co-based catalyst as catalysts. However, since the above-mentioned formic acid production catalyst contains expensive and rare metals such as Pd and Co, it is desirable to recover and reuse it rather than discard it after the formic acid reaction. On the other hand, when the formic acid production catalyst was used again after being used in the formic acid reaction, its catalytic activity sometimes decreased significantly. In particular, while the formic acid production catalyst showed excellent formic acid productivity at a reaction temperature of about 100°C (specifically, in the range of 40°C to 120°C, more preferably in the range of 70°C to 110°C), its catalytic activity sometimes decreased significantly when reused after being used in the formic acid reaction at this reaction temperature range. Therefore, the present inventors considered using a formic acid production catalyst in which aluminum is added to the Co-based catalyst, and the molar ratio of cobalt element to aluminum element (Co / Al) is 1.0 to 7.0, and recovering and reusing it in the production of formic acid, which led to this disclosure.

[0018] Specifically, we found that by using a formic acid production catalyst containing a Co-based catalyst with Al added in a molar ratio of Co within the above range, and by separating and recovering the catalyst used in the first formic acid reaction step to produce formic acid by reacting it with carbon dioxide and hydrogen, and then washing and regenerating it, it can be used again as a formic acid production catalyst in the second formic acid reaction step to produce formic acid by reacting it with carbon dioxide and hydrogen.

[0019] The following describes in detail each step of the method for producing formic acid according to the embodiment of this disclosure.

[0020] A method for producing formic acid according to the embodiments of this disclosure includes the following steps. (0) A process to prepare a catalyst for formic acid production comprising a Pd-based catalyst in which a palladium component is supported on a catalyst support, and a Co-based catalyst containing a cobalt component and an aluminum component in an amount such that the molar ratio of cobalt element to aluminum element (Co / Al) is 1.0 or more and 7.0 or less. (1) The first formic acid reaction step involves reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing a catalyst for formic acid production to produce formic acid. (2) Catalyst separation step for separating the catalyst for formic acid production from the reaction product after the first formic acid reaction step. (4) Catalyst regeneration process to obtain a regenerated formic acid production catalyst by washing and regenerating the formic acid production catalyst separated in the catalyst separation process. (5) Second formic acid reaction step, in which carbon dioxide and hydrogen are reacted under pressurized conditions in a non-basic solvent containing a catalyst for regenerated formic acid to produce formic acid.

[0021] Furthermore, (2) the catalyst separation step preferably includes the following steps. (2-1) First separation step: Separating the gaseous components containing unreacted carbon dioxide and hydrogen from the reaction product after the first formic acid reaction step to obtain a solid-liquid component containing a liquid component containing formic acid and a solvent, and a solid component containing a catalyst for formic acid production. (2-2) Second separation step to separate the solid component from the solid-liquid component.

[0022] Furthermore, (2-2) after the second separation step, it is preferable to include the following steps. (3) Solvent separation step, which separates the solvent from the liquid component after the solid component has been separated in the second separation step.

[0023] Furthermore, (2-1) it is preferable to use the unreacted carbon dioxide and hydrogen separated in the first separation step as carbon dioxide and hydrogen in the second formic acid reaction step. Also, (3) it is preferable to use the solvent separated in the solvent separation step as the solvent in the second formic acid reaction step.

[0024] In other words, in the method for producing formic acid according to the embodiment of this disclosure, it is preferable, as an example, to produce formic acid according to the flow chart shown in Figure 1. As shown in Figure 1, first, a formic acid reaction step (1) is performed to produce formic acid by reacting carbon dioxide (CO2) and hydrogen (H2) in a solvent containing a catalyst for formic acid production. Next, a catalyst separation step (2) is performed to separate the catalyst for formic acid production from the reaction product after the 1st formic acid reaction step. In the catalyst separation step (2), first, a first separation step (2-1) is performed to separate the gaseous components containing unreacted carbon dioxide and hydrogen from the reaction product after the 1st formic acid reaction step, thereby obtaining a solid-liquid component containing a liquid component containing formic acid and solvent, and a solid component containing the catalyst for formic acid production. Next, a second separation step (2-2) is performed to separate the solid component containing the catalyst for formic acid production from the solid-liquid component, thereby obtaining a liquid component containing formic acid and solvent. Next, a solvent separation step (3) is performed to separate the solvent from the liquid component after the solid component has been separated in the second separation step. Through the solvent separation step (3), the product, formic acid, is obtained. Furthermore, the formic acid production catalyst separated in (2-2) the second separation step is washed and regenerated to obtain a regenerated formic acid production catalyst (4) after the catalyst regeneration step, it is used in (5) the formic acid reaction step (second formic acid reaction step) to produce formic acid by reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing the regenerated formic acid production catalyst. Furthermore, the unreacted carbon dioxide and hydrogen separated in (2-1) the first separation step, and the solvent separated in (3) the solvent separation step, are also used as carbon dioxide and hydrogen, and as the solvent, in (5) the second formic acid reaction step.

[0025] (0) Process of preparing a catalyst for formic acid production First, prepare the catalyst for formic acid production. The catalyst used in the first formic acid reaction step is a formic acid production catalyst comprising a Pd-based catalyst in which a palladium component is supported on a catalyst support, and a Co-based catalyst containing cobalt and aluminum components in amounts such that the molar ratio of cobalt to aluminum (Co / Al) is between 1.0 and 7.0.

[0026] ·Pd-based catalyst Pd-based catalysts have a palladium component supported on a catalyst support. Preferably, the Pd-based catalyst has a silver component supported on the catalyst support in addition to the palladium component.

[0027] Here, in the catalyst manufacturing process, the palladium and silver components exist mainly as oxides when calcined (unreduced), but mainly as metallics when reduced. Furthermore, depending on the manufacturing conditions, usage conditions, and storage conditions, metals and oxides may be mixed and their proportions may change. In the catalyst described in this disclosure, even if the palladium and silver components exist as oxides, they are reduced to metals during the reaction by the reducing atmosphere and perform the necessary catalytic function, so they do not need to exist only in a metallic state. Note that trace amounts of raw materials (precursors) may remain in the catalyst.

[0028] Furthermore, the Pd-based catalyst may also be a catalyst in which, in addition to palladium and silver components, manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), ruthenium (Ru), and gold (Au) components are supported on the catalyst support.

[0029] Pd-based catalysts have a specific surface area of ​​10 m². 2 / g or more 1000m 2 It is preferable that the amount be less than or equal to 50m 2 / g or more 500m 2 It is more preferable to have a value of less than / g, and 100m 2 / g or more 200m 2 A value of less than or equal to / g is even more preferable. Specific surface area of ​​10 m² 2 A concentration of 1000 m² or more ensures a sufficient reaction field and yields high catalytic activity. 2 Maintaining a value of less than / g allows for high catalyst strength. The specific surface area of ​​the catalyst can be measured by the BET method using gas adsorption.

[0030] The shape of the Pd-based catalyst is not particularly limited, but it is preferably in the form of a powder or a molded body formed by the aggregation of powders.

[0031] The palladium component is supported on the catalyst support.

[0032] As the catalyst support, a catalyst support mainly composed of titanium (hereinafter simply referred to as "Ti catalyst support") is preferred. By using a Ti catalyst support, the active metal can be dispersed, increasing the catalyst surface area which is the reaction field, and thereby improving catalytic activity.

[0033] A titanium-based catalyst support (Ti catalyst support) is one in which the titanium oxide content is 70% by mass or more relative to the catalyst support, and may also contain impurities other than titanium that are present in the manufacturing process of the catalyst support. This content ratio is the mass of titanium oxide relative to the total mass of impurities and titanium oxide. The titanium oxide content in the Ti catalyst support shall be measured by ICP-AES after pretreatment such as acid decomposition or alkali melting. Furthermore, to analyze the catalyst and determine the titanium oxide content in the titanium-based catalyst support, it is effective to analyze the cross-section of the catalyst particles using a scanning electron microscope (SEM-EDX) or a transmission electron microscope (TEM-EDX). Although there is some variability due to the localized analysis of the field of view in SEM and TEM, the titanium oxide content in the Ti catalyst support shall be determined by using the average of 10 component analysis values.

[0034] Titanium-based catalyst supports can be manufactured using conventional methods such as the sulfuric acid method and the chlorine method. Commercially available products may also be used. Both anatase and rutile crystal forms can be used, and the form is not limited to powder.

[0035] In addition to the Ti catalyst support, other catalyst supports such as MgO, SiO2, NPS (N-doped Polymer-Silica composite), Al2O3, and Nb2O3 can also be used.

[0036] A Pd-based catalyst can be obtained, for example, by supporting a palladium component (preferably a silver component) on a catalyst support (preferably a Ti catalyst support), and generating an active metal by reduction of the palladium component supported on the catalyst support.

[0037] Methods for loading the palladium component onto the catalyst support include conventional impregnation methods, incipient wetness methods, precipitation methods, and ion exchange methods, with impregnation methods being preferred because they allow for easy control of the loading amount.

[0038] The reaction conditions for the support are not particularly limited, but for example, it can be obtained by adding the Pd compound raw material to an aqueous solution in which the catalyst support is dispersed, stirring at room temperature (e.g., 20°C) (e.g., stirring for 1 hour), then removing water under reduced pressure and drying (e.g., drying for 24 hours).

[0039] The reduction reaction conditions are not particularly limited, but a Pd-based catalyst can be obtained by reducing the above-mentioned dried catalyst in an aqueous solution with a reducing agent and washing the resulting solid several times with pure water.

[0040] The Pd compound (preferably an Ag compound) used as the raw material (precursor) is not particularly limited as long as it is soluble in a solvent, but it is preferable to use a water-soluble compound that can be used in aqueous solution during the loading operation, from the viewpoint of reducing manufacturing costs and improving the safety of the manufacturing work environment.

[0041] ·Co-based catalyst Co-based catalysts have a spinel crystalline phase. The valence of Co contained in the Co-based catalyst can be confirmed by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS).

[0042] Co-based catalysts can be obtained, for example, by the citric acid method. While they can also be obtained by conventional methods such as coprecipitation or homogeneous precipitation, the citric acid method or hydrothermal synthesis method is preferable from the viewpoint of forming a uniform crystal structure.

[0043] For example, a Co-based catalyst can be prepared by a process that involves heating and removing moisture using a citric acid method, and then calcining the resulting dried material. The firing temperature is preferably between 200°C and 1200°C. A firing temperature of 200°C or higher allows for good sintering and high catalytic activity. On the other hand, a firing temperature of 1200°C or lower suppresses excessive sintering, ensuring a sufficient specific surface area of ​​the catalyst and resulting in high catalytic activity.

[0044] While there are no particular restrictions on the Co compound used as a raw material for preparing the Co-based catalyst, as long as it is soluble in a solvent, it is preferable to use a water-soluble compound that can be prepared in an aqueous solution, from the viewpoint of reducing manufacturing costs and improving the safety of the manufacturing work environment. An example of a Co compound is Co(NO3)2·6H2O.

[0045] Co-based catalysts have a molar ratio (Co / Al) of cobalt to aluminum of 1.0 to 7.0. The reason for the degradation of catalytic activity in this formic acid production catalyst after use in the formic acid reaction is presumed to be the conversion of a portion of the Co-based catalyst into inactive CoCO3. Furthermore, experiments have revealed that this conversion of CoCO3 occurs after the catalyst is first reduced to Co or CoO under the reaction atmosphere.

[0046] Adding Al in an amount such that the molar ratio of Al (Co / Al) is 7.0 or less is thought to prevent degradation of catalytic activity by suppressing the formation of CoCO3, thereby improving the reduction resistance. On the other hand, adding excessive Al (i.e., adding an amount of Al such that the molar ratio (Co / Al) is less than 1.0) will reduce the active sites of the Co-based catalyst. Therefore, the molar ratio of cobalt and aluminum (Co / Al) should be controlled within an appropriate range of 1.0 to 7.0. From the above viewpoint, a molar ratio (Co / Al) of 2.0 to 7.0 is preferable, and a molar ratio of 3.5 to 7.0 is more preferable.

[0047] Here, we will explain a method for adding aluminum to a Co-based catalyst. First, an Al compound containing aluminum is added to the Co compound, which is the raw material for the Co-based catalyst. Then, the water is removed by heating using the citric acid method, and the dried material is calcined to prepare a Co-based catalyst with added Al. Examples of Al compounds include Al(NO3)3·6H2O, Al(i-OPr)3, and AlCl3.

[0048] • Production of catalysts for formic acid A catalyst for formic acid production can be manufactured by performing the steps of obtaining a Pd-based catalyst and obtaining a Co-based catalyst, respectively. The Pd-based catalyst and Co-based catalyst obtained in each step may be added to the solvent and mixed in the solvent during formic acid production, or the Pd-based catalyst and Co-based catalyst may be mixed in advance before use in formic acid production. Alternatively, the Pd-based catalyst and Co-based catalyst may be mixed in the solvent before formic acid production, or they may be added during the reaction between carbon dioxide and hydrogen. The Pd-based catalyst and Co-based catalyst may be added to the solvent simultaneously or sequentially, and the order of addition does not matter.

[0049] The catalyst for formic acid production is not limited to the form composed solely of the Pd-based catalyst and Co-based catalyst described above. It may also contain impurities introduced during the manufacturing process, components that do not inhibit the formic acid production effect, etc. In this case, it is preferable that the catalyst for formic acid production contains a total of 90% by mass or more of the Pd-based catalyst and Co-based catalyst described above.

[0050] (1) First formic acid reaction step In the first formic acid reaction step, formic acid is produced by reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing a catalyst for formic acid production.

[0051] ·Raw materials In the first formic acid reaction step, carbon dioxide (CO2) and hydrogen (H2) are used as the raw material gases for formic acid. The pressure ratio of carbon dioxide to hydrogen (CO2 / H2) is preferably between 0.2 and 5, and more preferably between 0.5 and 2. A pressure ratio of 0.2 or higher ensures a sufficient amount of hydrogen, allowing the hydrogenation reaction to proceed easily. On the other hand, a pressure ratio of 5 or lower maintains high catalytic activity.

[0052] ·solvent As non-basic solvents, examples include alcohol-based solvents such as methanol, ether-based solvents such as tetrahydrofuran (THF), acetone, and water. Water (especially pure water) is preferred as a solvent because it does not form formate salts and has a low environmental impact.

[0053] The amount of solvent relative to the amount of formic acid production catalyst is not particularly limited, but the mass ratio of the formic acid production catalyst to the solvent (mass of formic acid production catalyst / mass of solvent) is preferably in the range of 1 to 10. Furthermore, it is acceptable if the solvent contains substances other than the formic acid production catalyst, as long as they do not inhibit the formic acid production effect.

[0054] The solvent used in the first formic acid reaction step and the solvent used in the second formic acid reaction step described later may be the same or different, but it is preferable that they be the same solvent from the viewpoint of being able to reuse the solvent. Furthermore, the solvent used in the second formic acid reaction step and the washing solution used in the catalyst regeneration step described later may be the same liquid or different liquids, but it is preferable that they be the same liquid from the viewpoint of being able to tolerate residual washing solution. Therefore, it is more preferable that the solvent used in the first formic acid reaction step, the solvent used in the second formic acid reaction step, and the washing solution used in the catalyst regeneration step are the same liquid.

[0055] Reaction conditions The reaction temperature in the production of formic acid is not particularly limited, but it is preferably between 40°C and 120°C, and more preferably between 70°C and 110°C. A reaction temperature of 40°C or higher yields high catalytic activity. On the other hand, a reaction temperature of 120°C or lower increases the solubility of carbon dioxide in non-basic solvents. From the viewpoint of high catalytic activity, a reaction temperature of 100°C is most preferable. Even when using this reaction temperature, the catalyst regeneration ability is ensured by using the aforementioned formic acid production catalyst, and even if the catalyst used in the formic acid reaction is recovered and reused in the formic acid reaction, the deterioration of catalytic activity is suppressed and the yield of formic acid is increased.

[0056] The reaction pressure in the production of formic acid is not particularly limited, but is preferably between 0.1 MPa and 10 MPa, and more preferably between 1 MPa and 5 MPa. A pressure above 0.1 MPa allows for high catalytic activity. A pressure of 10 MPa or less prevents the plant's pressure resistance design from being set too high, thus controlling equipment costs.

[0057] Furthermore, while both homogeneous and heterogeneous catalysts have been developed for the synthesis of formic acid from CO2, when considering industrialization, homogeneous catalysts result in a uniform mixing of reactants and catalyst, making separation of the product and catalyst difficult. In contrast, heterogeneous catalysts, including the formic acid production catalyst in the embodiments of this disclosure, allow for easy separation of the product and catalyst by physical separation, which is advantageous for industrialization.

[0058] (2) Catalyst separation process In the catalyst separation step, the catalyst for formic acid production is separated from the reaction product after the first formic acid reaction step. In the first formic acid reaction step, the only compound produced by the reaction of carbon dioxide and hydrogen is formic acid. Therefore, the reaction product consists of, for example, unreacted carbon dioxide and hydrogen as gaseous components, the solvent and reaction product (i.e., formic acid) as liquid components, and the catalyst for formic acid production (i.e., Pd-based catalyst and Co-based catalyst) as solid components.

[0059] The catalytic separation step preferably includes (2-1) a first separation step and (2-2) a second separation step.

[0060] (2-1) First separation process (gas-solid-liquid separation process) In the first separation step (gas-solid-liquid separation step), unreacted gaseous components containing carbon dioxide and hydrogen are separated from the reaction product after the first formic acid reaction step to obtain a solid-liquid component containing a liquid component containing formic acid and a solvent, and a solid component containing a catalyst for formic acid production.

[0061] Furthermore, the unreacted carbon dioxide and hydrogen gases separated in the first separation step can be reused as raw material gases (i.e., carbon dioxide and hydrogen) in the second formic acid reaction step.

[0062] One method for separating unreacted gaseous components, including carbon dioxide and hydrogen, from the reaction product after the first formic acid reaction step is to release the pressure (for example, return to atmospheric pressure) on the solvent that was reacted under pressurized conditions in the formic acid reaction step. By releasing the pressure after the first formic acid reaction step has been carried out under pressurized conditions, the gaseous components and solid-liquid components can be separated. From the viewpoint of efficiently separating gas components, the pressure is preferably 0.1 MPa or more and 0.5 MPa or less, and more preferably 0.1 MPa (i.e., atmospheric pressure).

[0063] The temperature in the first separation step (gas-solid-liquid separation step) is not particularly limited, but from the viewpoint of improving separation efficiency, it is preferable to perform the first separation step at a temperature lower than the boiling points of the liquid components, formic acid and the solvent.

[0064] It is possible that some of the liquid components (formic acid and solvent) that could not be removed may be present in the separated gaseous components. However, even if the separated gaseous components (unreacted carbon dioxide and hydrogen) are reused as raw material gases in the second formic acid reaction step, the impact on the reaction will be small if the amount of liquid components present is small. Therefore, gaseous components that contain some liquid components (formic acid and solvent) can be used directly in the second formic acid reaction step.

[0065] (2-2) Second separation process (solid-liquid separation process) In the second separation step (solid-liquid separation step), the solid component containing the catalyst for formic acid production is separated from the solid-liquid component obtained in the first separation step to obtain a liquid component containing formic acid and a solvent.

[0066] The formic acid production catalyst, separated in the second separation step as a solid component, can then be reused as a formic acid production catalyst in the second formic acid reaction step after undergoing a catalyst regeneration step, as described later.

[0067] As a method for separating the solid component containing the catalyst for formic acid production from the solid-liquid component obtained in the first separation step, known solid-liquid separation methods such as filtration, centrifugation, and separation into supernatant and precipitate using a sedimentation tank can be used.

[0068] (3) Solvent separation process Furthermore, it is preferable to have a solvent separation step (3) after the second separation step, in which the solvent is separated from the liquid component after the solid component has been separated in the second separation step. In the solvent separation step, the solvent is separated from the liquid component after the solid component has been separated in the second separation step. This yields the desired product, formic acid.

[0069] Furthermore, the solvent separated in the solvent separation step can be reused as a non-basic solvent in the second formic acid reaction step.

[0070] Methods for separating the solvent from the liquid component containing formic acid and the solvent obtained in the second separation step include, for example, separation by distillation, solidification and solid-liquid separation, and separation by difference in specific gravity.

[0071] The separation method by distillation utilizes the fact that formic acid has a boiling point of 101°C and separates the formic acid from the solvent using a distillation apparatus. If the solvent contained in the liquid component has a lower boiling point than formic acid, then by this distillation separation, the solvent can be recovered as a gas and the formic acid as a liquid. Examples of solvents with lower boiling points than formic acid include acetone, methanol, and ethanol. If the solvent contained in the liquid component has a higher boiling point than formic acid, then by this distillation separation, the solvent can be recovered as a liquid and the formic acid as a gas. Examples of solvents with higher boiling points than formic acid include 1-hexanol and triglycerides. The distillation conditions vary depending on the solvent used, and it is preferable to set the optimal conditions as appropriate.

[0072] The method of solidifying and separating the solid-liquid is based on the fact that formic acid has a melting point of 8°C. It involves cooling the substance to a temperature below its melting point, allowing it to solidify, and then separating the solid-liquid substance using a known solid-liquid separation method. If the solvent in the liquid component has a lower melting point than formic acid, this method allows for the recovery of the solvent as a liquid and the formic acid as a solid. If the solvent in the liquid component has a higher melting point than formic acid, this method allows for the recovery of the solvent as a solid and the formic acid as a liquid. Known solid-liquid separation methods include, for example, filtration, centrifugation, and sedimentation tanks to separate the supernatant from the precipitate.

[0073] One method of separation based on the difference in specific gravity is used when there is a large difference in specific gravity between the solvent and formic acid, and the two substances are not miscible.

[0074] Furthermore, if the generated formic acid is used as a solution (e.g., an aqueous solution), and the solvent in the liquid component obtained in the second separation step is not reused as the solvent in the second formic acid reaction step, then a solvent separation step may not be necessary. In this case, the liquid component containing the formic acid and solvent obtained in the second separation step is used as is as the formic acid solution.

[0075] (4) Catalyst regeneration process In the catalyst regeneration process, the formic acid production catalyst separated in (2) the catalyst separation process is washed and regenerated to obtain a regenerated formic acid production catalyst.

[0076] The formic acid production catalyst obtained in the catalyst separation process has solvent and the product, formic acid, attached to it. Therefore, from the viewpoint of improving catalytic activity, the formic acid production catalyst obtained in the catalyst separation process is washed to remove the formic acid and other residues.

[0077] One method of washing involves adding the obtained formic acid production catalyst to a liquid and washing it by stirring. The liquid used for washing in the catalyst regeneration process is not particularly limited, but examples include the non-basic liquids listed as solvents in the first formic acid reaction step. The liquid used for washing is preferably a polar solvent from the viewpoint of removing formic acid and other substances, and among these, water (especially pure water) is preferred.

[0078] The solvent used in the second formic acid reaction step and the cleaning solution used in the catalyst regeneration step described later may be the same liquid or different liquids, but it is preferable that they be the same liquid from the viewpoint of being able to tolerate residual cleaning solution. Furthermore, it is more preferable that the solvent used in the first formic acid reaction step, the solvent used in the second formic acid reaction step, and the washing solution used in the catalyst regeneration step are the same liquid.

[0079] (5) Second formic acid reaction step In the second formic acid reaction step, formic acid is produced by reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing the formic acid production catalyst obtained in the catalyst regeneration step (i.e., the regenerated formic acid production catalyst). Furthermore, it is preferable not only to reuse the formic acid production catalyst (regenerated formic acid production catalyst) used in the first formic acid reaction step, but also to use (i.e., reuse) the unreacted carbon dioxide and hydrogen separated in (2-1) the first separation step, and the solvent separated in (3) the solvent separation step, in the second formic acid reaction step. This makes it possible to increase the utilization efficiency of carbon dioxide and hydrogen, and the utilization efficiency of the solvent.

[0080] The reaction method and conditions in the second formic acid reaction step are the same as those in the first formic acid reaction step, and therefore a detailed explanation is omitted here. From the viewpoint of high catalytic activity, a reaction temperature of 100°C is most preferable. Even when using this reaction temperature, catalyst regeneration is ensured by using the aforementioned formic acid production catalyst. In other words, even if the catalyst used in the formic acid reaction is recovered and reused, the degradation of catalytic activity is suppressed, and the yield of formic acid is increased.

[0081] Furthermore, the formic acid production catalyst (regenerated formic acid production catalyst) used in the second formic acid reaction step can be subjected to the aforementioned (2) catalyst separation step and (4) catalyst regeneration step to obtain another regenerated formic acid production catalyst, which can then be used again in the formic acid reaction step (i.e., the third formic acid reaction step). Moreover, even in the third formic acid reaction step and beyond, the formic acid production catalyst used can be reused in subsequent formic acid reaction steps. In the formic acid production method according to the embodiment of this disclosure, the formic acid production catalyst can be regenerated any number of times, and the deterioration of catalytic activity during reuse is suppressed. Similarly, the unreacted carbon dioxide, hydrogen, and solvent after the second formic acid reaction step can be separated and reused in the formic acid reaction step (i.e., the third formic acid reaction step), and furthermore, the unreacted carbon dioxide, hydrogen, and solvent can be reused in subsequent formic acid reaction steps from this third formic acid reaction step onward. [Examples]

[0082] Hereinafter, the present disclosure will be described in more detail with reference to examples, but the present disclosure is not limited to these examples.

[0083] (Production of Pd-based catalyst body: catalyst carrier mainly composed of titanium) Palladium component (tetraamminepalladium(II) chloride monohydrate, manufactured by Aldrich, product number: 323438) and silver component (silver nitrate, manufactured by Nacalai Tesque, product number: 31018-72) were added to an aqueous solution in which a commercially available catalyst carrier mainly composed of titanium (manufactured by Nippon Aerosil Co., Ltd., product number: titanium dioxide P25, content of titanium oxide in the catalyst carrier: not less than 70% by mass) was dispersed, stirred at room temperature (20 °C) for 1 hour, then water was removed under reduced pressure, and dried for 24 hours. Next, the dried solid was reduced in an aqueous solution using a reducing agent (NaBH4), and the obtained solid was washed several times with pure water to obtain a Pd-based catalyst body (PdAg / TiO2). In the obtained Pd-based catalyst body, the molar ratio of palladium to silver (Pd / Ag) was 1.0, the mass ratio of palladium to the catalyst carrier (Pd / catalyst carrier) was 0.01, and the specific surface area was 100 m 2 / g.

[0084] (Production of Co-based catalyst body) The amount of the aqueous citric acid solution was fixed and prepared so that the molar ratio of cobalt element to aluminum element (Co / Al) became a predetermined value. For example, when Co / Al = 6.5, 3.4 g of Co(NO3)2·6H2O, 0.68 g of Al(NO3)3·6H2O were mixed with 9.0 mL of 1.5 M aqueous citric acid solution, and dried at 80 °C for 3 hours. Next, the dried solid was calcined at 800 °C for 5 hours to obtain a Co-based catalyst body. When analyzed by XRD, it had a spinel crystal phase.

[0085] (Catalyst) A Pd-based catalyst (PdAg / TiO2) and a Co-based catalyst were used at a ratio of 1:1 (mass ratio) of 30 mg each per 15 mL of solvent (water). The molar ratio of cobalt to aluminum (Co / Al) was 6.5.

[0086] <Example 1> • First formic acid reaction step The first formic acid production was carried out using carbon dioxide at 2 MPa and hydrogen at 2 MPa (total pressure 4 MPa) as reaction gases, with the reaction temperature, total pressure, and reaction time as described in Table 1, using water as the solvent and a Pd-based catalyst (PdAg / TiO2) and a Co-based catalyst (Co / Al=6.5) as catalysts.

[0087] • Second formic acid reaction step The solvent that had undergone the first formic acid reaction step was subjected to (2) a catalyst separation step (specifically (2-1) the first separation step and (2-2) the second separation step), as well as (3) a solvent separation step and (4) a catalyst regeneration step to obtain a catalyst for regenerated formic acid production. Unreacted carbon dioxide and hydrogen were obtained in (2-1) the first separation step, and the solvent was further separated in (3) the solvent separation step. The obtained formic acid production catalyst, unreacted carbon dioxide and hydrogen, and solvent were reused, and a second formic acid production was carried out under the reaction temperature, total pressure, and reaction time conditions shown in Table 1.

[0088] • Third and fourth formic acid reaction steps From the solvent after the second formic acid reaction step, the catalyst for regenerated formic acid production, unreacted carbon dioxide and hydrogen, and the solvent were obtained using the same method as in the "second formic acid reaction step" described above. Using this recycled formic acid production catalyst, unreacted carbon dioxide and hydrogen, and solvent, the third formic acid production was carried out under the reaction temperature, total pressure, and reaction time conditions shown in Table 1. Furthermore, from the solvent that had undergone the third formic acid reaction step, the catalyst for regenerated formic acid production, unreacted carbon dioxide and hydrogen, and the solvent were obtained using the same method as in the "second formic acid reaction step" described above. Using this regenerated formic acid production catalyst, unreacted carbon dioxide and hydrogen, and solvent, the fourth formic acid production was carried out under the reaction temperature, total pressure, and reaction time conditions shown in Table 1.

[0089] <Example 2> The experiment was conducted in the same manner as in Example 1, except that a Co-based catalyst with a Co / Al ratio of 2.0 was used.

[0090] <Comparative Example 1> The experiment was conducted in the same manner as in Example 1, except that an Al-free Co-based catalyst was used.

[0091] Table 1 shows the amount of formic acid produced in the first to fourth formic acid reaction steps, and the total amount of formic acid produced in each of the first to fourth steps.

[0092] [Table 1]

[0093] As shown in Table 1, in both Example 1 and Example 2, the catalyst for formic acid production, unreacted carbon dioxide and hydrogen, and solvent could be reused in each of the second to fourth formic acid reaction steps, and formic acid could be produced in each of the second to fourth formic acid reaction steps.

[0094] On the other hand, while Comparative Example 1 showed excellent formic acid production in the first trial, its activity deteriorated significantly in the second to fourth trials, resulting in poor reusability.

[0095] Next, the amount of formic acid produced in the first formic acid reaction step was carried out using Co-based catalysts with different Co / Al ratios, and the XRD results of the post-reaction catalyst are shown in Table 2, and Figure 2.

[0096] [Table 2]

[0097] As shown in Table 2, if the amount of Al added is excessive, the effect of the Co-based catalyst is lost (Co / Al = 0.5). Also, as shown in Figure 2, if the amount of Al added is insufficient, CoCO3 is formed, and the effect of preventing a decrease in catalytic activity is lost (Co / Al = 15.5). Therefore, it was found that it is important to control the Co / Al ratio within an appropriate range.

Claims

1. A step to prepare a catalyst for formic acid production comprising a Pd-based catalyst in which a palladium component is supported on a catalyst support, and a Co-based catalyst containing a cobalt component and an aluminum component in an amount such that the molar ratio of cobalt element to aluminum element (Co / Al) is 1.0 or more and 7.0 or less, A first formic acid reaction step involves reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing the formic acid production catalyst to produce formic acid, A catalyst separation step is performed to separate the catalyst for formic acid production from the reaction product after the first formic acid reaction step, A catalyst regeneration step is performed to wash and regenerate the formic acid production catalyst separated in the catalyst separation step to obtain a regenerated formic acid production catalyst, A second formic acid reaction step involves reacting carbon dioxide and hydrogen under pressurized conditions in a non-basic solvent containing the aforementioned catalyst for regenerated formic acid to produce formic acid, A method for producing formic acid containing [the specified ingredient].

2. The method for producing formic acid according to claim 1, wherein the Pd-based catalyst is a catalyst in which the palladium component and the silver component are supported on a catalyst carrier.

3. The method for producing formic acid according to claim 1, wherein the catalyst support is a catalyst support mainly composed of titanium.

4. The catalyst separation step includes a first separation step of separating unreacted gaseous components, including carbon dioxide and hydrogen, from the reaction product after the first formic acid reaction step to obtain a solid-liquid component comprising a liquid component containing formic acid and the solvent and a solid component containing the formic acid production catalyst, and a second separation step of separating the solid component from the solid-liquid component. The method for producing formic acid according to claim 1, wherein the unreacted carbon dioxide and hydrogen separated in the first separation step are used as the carbon dioxide and hydrogen in the second formic acid reaction step.

5. The second separation step is followed by a solvent separation step of separating the solvent from the liquid component after the solid component has been separated in the second separation step. The method for producing formic acid according to claim 4, wherein the solvent separated in the solvent separation step is used as the solvent in the second formic acid reaction step.

6. A catalyst for formic acid production comprising a Pd-based catalyst in which palladium and silver components are supported on a catalyst support mainly composed of titanium, and a Co-based catalyst containing cobalt and aluminum components in an amount such that the molar ratio of cobalt to aluminum (Co / Al) is 1.0 or more and 7.0 or less.

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