WO2014202855A1 - Method of producing formic acid - Google Patents

Abstract

The invention concerns a method of producing formic acid from carbon dioxide and water by electroreduction of the CO₂ in the gaseous phase. The method uses electrochemical cells of which the cathode consists of a conductive porous solid and an active layer containing dispersed metal nanoparticles and catalysing the reaction selectively to formic acid. The resulting formic acid is then separated from the residual CO₂ by crystallization, the CO₂ being recycled to the electrochemical cell.

Description

 PROCESS FOR PRODUCING FORMIC ACID

FIELD OF THE INVENTION Increasing the amount of carbon dioxide in the atmosphere, due in particular to the burning of fossil fuels, is now recognized as one of the major factors of global warming. Many solutions have been proposed for capturing the carbon dioxide contained in combustion fumes, especially for sequestration. However, the final sequestration of carbon dioxide presents many technical difficulties and generates a lot of concern because of the risks involved, especially those related to the sudden release into the atmosphere of large quantities of carbon dioxide stored for example under the sea or in aquifers, as has happened in the past with naturally occurring carbon dioxide. A complementary solution to the sequestration of captured carbon dioxide is to valorize it, converting it into a valuable chemical asset, that is to say a chemical used by the industry.

The present invention relates to a novel process for producing formic acid from CO ₂ . This process is electrochemical, that is, it uses electric current to make formic acid and oxygen from CO ₂ and water. This process is much simpler than current formic acid production processes, so it could be competitive if the price of electricity is not too high. This method could advantageously be used to store electrical energy produced intermittently (solar, wind) or surplus (production peaks).

EXAMINATION OF THE PRIOR ART

The electrochemical conversion of C0 ₂ was discovered in the 19th century (Royer, MECRHebd.Seances Acad.Sci.Paris 1870, T70, 731-732). It was proposed in the 1980s, for example in US Patent 4,673,473, or in the article by Cook, MacDuff and Sammels (J. Electrochem Soc., 1988, 135 (6), 1470-1471), in particular for the production of hydrocarbons.

In the 2000s, with concerns about C0 ₂ , new proposals were made.

At the scale of an industrial electrochemical process, one of the difficulties is to obtain a sufficient current density and a good selectivity towards the desired product. In fact, current density largely determines the economic viability of the process. US Patent 2012/0228147 reports for this purpose the use of a massive metal cathode (indium, lead, tin, cadmium or bismuth), and an aqueous electrolyte, containing heterocyclic aromatic amines such as 4- hydroxy pyridine which act as a homogeneous catalyst. The process described in the patent cited has certain drawbacks: first of all it is necessary to maintain the pH of the aqueous solution between 4.3 and 5.5 pH range within which a majority of the acid formic is in its basic form.

The material balance then shows that we consume a large amount of a base, which is unfavorable. On the other hand the cited patent mentions the need to maintain the concentration of formic acid below 500 ppm, which makes the separation of this product almost impossible, the liquid-vapor equilibria C0 ₂ / H ₂ O / formic acid being strongly unfavorable in this case. US Pat. No. 2008/223727 discloses a process for the electrochemical conversion of carbon dioxide by continuously feeding a biphasic liquid-gas mixture containing CO ₂ and an aqueous electrolyte. In this case the selectivity to formic acid is obtained only for pH greater than or equal to 7.

As for the aforementioned patent (US 2012/0228147), this pH constraint is a brake on the development of this type of process. Indeed formic acid has a pKa = 3.75, which means that the formic acid is obtained in its basic form and thus a basic stoichiometric amount is consumed. The material balance is therefore unfavorable.

On the other hand, formic acid presents with water an azeotrope which greatly complicates its extraction. This is the problem of the traditional method of manufacturing formic acid which requires a succession of three distillation columns to be able to separate 95% formic acid.

The US2012 / 0171583 patent uses a fuel cell by injecting hydrogen and CO ₂ as fuels, and claims the synthesis of methanol and propanol from CO ₂ by means of an aromatic polyamine catalyst deposited on an electrode. gas diffusion.

In this case, the hydrogen brings energy, dissociates at the anode (oxidation) and allows the reduction of C0 ₂ . It is the combination of platinum particles and an aromatic polyamine which is claimed to allow a selectivity of the reduction of CO ₂ to alcohols (methanol, propanol).

It is known in the field of catalytic hydrogenation processes C0 _2j that conversion to alcohols can be achieved at relatively high temperatures, typically above 220 ° C to promote the reaction (Razali et al. Ren. Sust Ener, Rev. 16 (2012) 4951). The catalysts employed and the temperature conditions do not allow in any case the synthesis of formic acid. Indeed, this compound degrades thermally at temperatures above about 170 ° C.

The novelty of the present invention lies in the generation of formic acid by an electrolytic route in the CO ₂ compartment directly in vapor form (i.e. the formic acid is generated at a partial pressure lower than the saturation vapor pressure of formic acid at operating temperature).

This generation of formic acid directly in the vapor state allows separation facilitated by condensation (in solid or liquid form) of formic acid from the gas stream. In doing so, the C0 ₂ comes into direct contact with the electrons via the porous electrode, on the metal particles that are the centers of the reaction and with the protons via a solid polymer electrolyte. The protons involved in the reduction of C0 ₂ come directly from water molecules introduced at the anode.

In contrast, the catalytic hydrogenation processes of C0 ₂ which necessarily use hydrogen having a low carbon footprint, and therefore generally derived from the electrolysis of water or as a by-product of the electrochemical chlor-alkali process, use necessarily two steps.

The first step is electrolysis for the hydrogen generation, its storage under pressure, then in a second stage, a catalytic conversion, in a second process, with specific conditions which generally are not adapted with the conditions of a intermittent process (such as wind power generation or solar generation).

The process according to the present invention offers the advantage of using only water, C0 ₂ and electricity for the generation of formic acid by CO ₂ reduction. The use of high surface area porous electrodes, coupled with the use of metal particles catalyzing the selective formation of formic acid, offers the possibility of producing formic acid under high current density conditions and facilitated separation. . SUMMARY DESCRIPTION OF THE FIGURES

Figure 1 shows a formic acid production module showing the anode portion and the cathode portion, the two parts being separated by a porous membrane allowing the passage of H + ions.

FIG. 2 is a diagram of the process according to the invention in a first variant showing the recovery circuit of formic acid by means of a liquid formic acid loop. Figure 3 is a diagram of the process according to the invention in a second variant showing the recycling of residual CO 2 to the electrolytic cell. SUMMARY DESCRIPTION OF THE INVENTION

The present invention can be defined as an electrochemical process for producing formic acid from CO 2 and water vapor introduced to the cathode and water vapor introduced to the anode, using one or more electrochemical cells. in parallel or stacked, said method using electrochemical cells whose cathode is composed of a porous conductive solid and an active layer containing dispersed metal nanoparticles and catalyzing the reaction selectively towards formic acid according to the reaction: C0 ₂ + H ₂ 0 → HCOOH + ½ 0 ₂

at the anode: the water vapor is decomposed at the anode with oxygen and protons H ⁺ H ₂ 0 → ½ 0 ₂ + 2 H ⁺ + 2e and,

at the cathode: the carbon dioxide is reduced directly in the vapor phase under the effect of an electrochemical potential in contact with a porous cathode and with a solid polymer electrolyte, C0 ₂ + 2H ⁺ + 2e → HCOOH

the formic acid produced is then separated from the residual CO ₂ preferentially by crystallization,

method in which the surface density of the current is greater than 0.1 A / cm 2 and preferably between 0.2 A / cm 2 and 2 A / cm 2, and the potential difference between the anode and the cathode is less than 4 volts . According to a preferred characteristic of the electrochemical process for the production of formic acid according to the invention, the catalyst used at the cathode of the electrolytic cell consists of metal particles selected from those which have a selectivity towards formic acid, ie indium, tin, lead, bismuth, cadmium, thallium alone or as a mixture. Preferably, the catalyst used at the cathode of the electrolytic cell is selected from the following metals: indium, tin, lead, bismuth.

According to another preferred characteristic of the electrochemical process for the production of formic acid according to the invention, the temperature measured at the level of the electrochemical cell is between 20 ° C. and 160 ° C.

According to another preferred characteristic of the electrochemical process for producing formic acid according to the invention, the pressure in the electrolytic compartment is between 1 and 60 bar, and preferably between 1 and 10 bar.

According to another preferred characteristic of the electrochemical process for producing formic acid according to the invention, the water vapor introduced into the anode is entrained by an inert carrier gas with respect to electro-oxidation.

According to another preferred characteristic of the electrochemical process for the production of formic acid according to the invention, the CO 2 introduced to the cathode of the electrolytic cell is driven by an inert carrier gas with respect to electro-reduction. According to another characteristic of the electrochemical process for producing formic acid according to the invention, a liquid formic acid loop circulating at a moderate temperature of 25 ° C. to 50 ° C. allows the elimination of the formic acid crystals formed in the exchanger 19a or 19b, the liquid formic acid from the storage 22, being sucked by the pump 24 through the conduit 23, and then reheated in the heat exchanger 25 with tempered water, or by any other heating means, before being sent through the conduit 26 in the exchanger 19 a or b, wherein the circulation of refrigerant (20 a or b) has been stopped.

Finally, in a preferred variant of the process according to the present invention, the residual CO 2 obtained after exchanger 19a or 19b is recompressed in a compressor and reintroduced with CO 2 at the cathode compartment constituting the charge of the process. DETAILED DESCRIPTION OF THE INVENTION

The method will be better understood from FIGS. 1, 2 and 3 described below. Description of Figure 1

FIG. 1 represents an electrochemical cell used in the process according to the invention. A current is applied between an anode composed of elements 2, 3 and 4 and a cathode composed of elements 6, 7 and 8. The elements 2 and 6 are conductive plates, possibly bipolar in the case of stacking of several electrochemical cells. , responsible for distributing the gases and collecting the electric current. They are conventionally encountered in electrolysers (for example for the electrolysis of water) or fuel cells. These conductive plates are machined or stamped so as to distribute the gases in conduits (9, 10) formed in these plates.

The elements 3 and 7 are diffusion layers of porous conductive material (typically made of carbon fibers), and elements 4 and 8 are the active catalytic layers deposited either on the diffusion layers or on the membrane 5. This membrane 5 is a solid polymer electrolyte allowing the transfer of H ⁺ ions between the anode and the cathode.

The active layer 8 of the cathode is composed of a conductive porous material (such as porous carbon) containing nanoparticles of metal. The metal particles for the cathode used for the invention are chosen from those which have a selectivity towards formic acid such as indium, tin, lead, bismuth, cadmium, thallium, alone or as a mixture .

Anode side is sent into the cavity 9 of the water, liquid or preferably in the form of water vapor arriving through the conduit 11. When it is desired to work with steam water, the water may be possibly entrained by a gas inert to electrolysis such as nitrogen, so as to have steam water at temperatures below the vaporization temperature of the water at the operating pressure. On the cathode side, CO ₂ gas is sent into the cavity 10 via line 13 at a pressure of between 1 and 60 bar absolute and at a temperature of between 20 ° C. and 160 ° C.

 On the anodic side, the water, under the effect of the current, decomposes into oxygen and H + protons. Oxygen is evolved via line 12, optionally mixed with steam and optionally with an inert carrier gas such as nitrogen, coming from line 11.

The protons H ⁺ under the effect of the current, migrate through the solid polymer electrolyte membrane, from the anode compartment to the cathode compartment, and preferentially react on the metal sites of the cathode with the CO ₂ to give formic acid which leaves the cell mixed with the CO ₂ and any other gases contained in the CO 2 arriving via the conduit 14.

In a manner known to those skilled in the art of electrolysis, several electrochemical cells may be used in parallel or in stacking, to obtain the desired capacity of formic acid. The applied current is optimized so as to guarantee high current densities while limiting the cell voltage to a value less than or equal to 4V, and preferably less than or equal to 3V. In this way, secondary reactions, such as, for example, the generation of hydrogen, are avoided to a minimum.

The surface density of current is generally between 0.1 and 2A / cm 2, and preferably greater than 0.2 A / cm 2, and more preferably greater than 0.4 A / cm 2. Description of Figure 2

FIG. 2 represents a first flow diagram of formic acid production process according to the invention. Water or water vapor, optionally mixed with an inert carrier gas, is introduced via line 11 into the electrochemical cell (or the electrochemical cells in parallel or stacked) denoted 1. Outlet 12 contains oxygen (or an oxygen-water mixture and optionally an inert gas, which can be separated by condensation of water).

The CO ₂ is fed through the conduit 15 into the heat exchanger 16 where it is preheated by steam or any other means before being sent via the conduit 13 to the cell or cells 1.

At the outlet of the electrochemical cell (s) 1, a mixture C0 ₂ and formic acid, and any other gases contained in the CO 2 arriving via the conduit 13, and possibly some traces of hydrogen, CO, and water are brought by the conduit 14 to the heat exchanger 17, wherein the mixture is cooled and condensed partially with the aid of cooling water, or air.

The partially condensed mixture is then sent through line 18 to the heat exchanger 19a or 19b, where it is cooled to around 5 ° C by propane or another compound or refrigerant mixture circulating in line 20a or b . At this temperature, the formic acid crystallizes and is deposited on the walls of the exchanger.

The residual gas, which essentially contains C0 ₂ , can be discharged via line 21, preferably to a flare, to eliminate any traces of hydrogen, CO, methane or formic acid contained in said residual gas. .

When the pressure drop is too high in the exchanger 19a (respectively 19b), due to the deposition of formic acid crystals, the flow of the conduit 18 is directed to the other exchanger 19b (respectively 19a), which has In the meantime, the crystallized formic acid which it contains has been freed. The two exchangers 19a and 19b therefore operate alternately, one being in the formic acid crystal deposition position when the other is in the crystal removal position).

The elimination of the formic acid crystals is obtained by circulation of liquid formic acid from the storage 22, sucked by the pump 24 through the conduit 23, and then reheated in the heat exchanger 25 with water temperate, or by any other means of heating, before being sent via line 26 into exchanger 19a or b, in which the circulation of refrigerant (20a or b) has been stopped.

 Formic acid circulated at moderate temperature (25 to 50 ° C.) makes it possible to melt the crystallized formic acid deposited in exchanger 19a or 19b and to return it to storage 22 via line 27.

Description of Figure 3

FIG. 3 represents a second process flow diagram of formic acid production according to the invention in which residual CO 2 emitted by line 21 is recompressed in a compressor 29 to be reintroduced into the input stream 15 at the level of the cell Electrochemical 1. This variant is an improvement of the basic version corresponding to Figure 2 which allows not reliberate CO2 to the atmosphere. Water or water vapor is introduced via line 11 into the electrochemical cell (or electrochemical cells in parallel) 1.

Outlet 12 contains oxygen (or an oxygen-water mixture and optionally an inert gas, which can be separated by condensation of water). The CO ₂ is fed through the conduit 15 into the heat exchanger 16 in which it is preheated by steam, or any other means, before being sent through the conduit 13 to the cell or cells 1.

At the outlet of the electrochemical cell (s) 1, a mixture C0 ₂ and formic acid is fed through line 14 to the heat exchanger 17, in which the mixture is cooled and condensed partially with the aid of cooling water, or air.

The partially condensed mixture is then sent through line 18 to the heat exchanger 19a or 19b, where it is cooled to about 5 ° C by propane or another compound or refrigerant mixture, circulating in line 20a. or b.

At this temperature, the formic acid crystallizes and is deposited on the walls of the exchanger 19a or 19b. When the pressure drop is too high in the exchanger because of the deposits of formic acid crystals, the flow of the conduit 18 is directed to the other exchanger 19b or 19a, which has been cleared in the meantime of the crystallized formic acid it contains, by circulation of liquid formic acid. This liquid formic acid comes from the storage 22, is sucked by the pump 24 through the conduit 23, and is then heated in the heat exchanger 25 with tempered water or by another heating means, before to be sent via line 26 into exchanger 19a or b, in which the circulation of refrigerant (20a or b) has been stopped. The formic acid circulated at moderate temperature (25 to 50 ° C.) makes it possible to melt crystallized formic acid on the walls of exchanger 19b or 19a, and to return it to storage via line 27.

The residual gas, which contains for the most part C0 ₂ , and some traces of formic acid (and possibly a carrier gas), is sent via line 21 to the compressor 29, mixed with the C0 ₂ feed brought by the conduit 28.

At the outlet of the compressor 29, the gas is returned via the duct 15 to the heat exchanger 16.

EXAMPLE ACCORDING TO THE INVENTION

In this example, the faradic efficiency is 80%, the current density is 0.4 A / cm and the potential difference between the electrodes is 3V.

The formic acid produced is 10,000 tons / year (1.25 ton / h)

 The oxygen produced is 3,470 tons / year, ie 300 Nm / h

The amount of C0 ₂ consumed is 9,560 tonnes / year (1.2 tonnes / h)

 The quantity of water consumed is 3910 tonnes / year (0.49 tonnes / h)

The electrode area required is 455 m ²

The consumption of utilities is as follows:

 Electricity: kWh / h

Cell 1 5460 Compressor 29 82

Pump 24 1

Cold unit 58 Total 5601 kWh / h

The water consumption is as follows:

Low pressure water vapor (ton / h)

 16 x 0.4 exchanger

Total: 0.4 tonnes / h

Cooling water: m / h

 Water at 25 ° C returned to 35 ° C is assumed. Part of the water (15.3 m) is cooled from 25 to 20 ° C in the exchanger 25 to heat the formic acid storage which will allow to melt the crystallized formic acid in the exchanger 19 or b

Cold group 25 m / h

 3

 Exchanger 17 27 m / h

Total 52 m ³ / h

This example demonstrates the economic feasibility of the process according to the invention, provided that the cost of electricity remains in the current values.

Claims

1) Electrochemical process for producing formic acid from CO 2 introduced at the cathode and water vapor introduced to the anode, using one or more electrochemical cells in parallel or stacked, said method using electrochemical cells of which the cathode is composed of a conductive porous solid and an active layer containing dispersed metal nanoparticles and catalyzing the reaction selectively to formic acid according to the reaction: C0 ₂ + H ₂ 0 → HCOOH + ½ 0 ₂ at the anode: the water vapor is decomposed into oxygen and H ⁺ protons according to H ₂ 0 → ½ 0 ₂ + 2 H ⁺ + 2e at the cathode: the carbon dioxide is reduced directly in the vapor phase under the effect of an electrochemical potential in contact with a porous cathode and with a solid polymer electrolyte according to: C0 ₂ + 2H ⁺ + 2e → HCOOH the formic acid produced is then separated from the residual CO ₂ , preferentially by crystallization,  method in which the surface density of the current is greater than 0.1 A / cm 2 and preferably between 0.2 A / cm 2 and 2 A / cm 2, and the potential difference between the anode and the cathode is less than 4 volts. 2) An electrochemical process for the production of formic acid according to claim 1, wherein the catalyst used at the cathode of the electrolytic cell consists of metal particles selected from those which have a selectivity towards formic acid is indium, tin, lead, bismuth, cadmium, thallium alone or as a mixture. 3) electrochemical process for producing formic acid according to claim 1, wherein the catalyst used at the cathode of the electrolytic cell is selected from the following metals: indium, tin, lead, bismuth. An electrochemical process for producing formic acid according to claim 1, wherein the temperature measured at the electrolytic cell is between 20 ° C and 160 ° C. 5) An electrochemical process for producing formic acid according to claim 1, wherein the pressure in the electrolytic compartment is between 1 and 60 bar, and preferably between 1 and 10 bar. 6) electrochemical process for producing formic acid according to claim 1, wherein the water vapor introduced to the anode is entrained by a carrier gas inert with respect to the electro-oxidation. 7) electrochemical process for producing formic acid according to claim 1, wherein the CO 2 introduced to the cathode of the electrolytic cell is driven by an inert carrier gas with respect to electro-reduction. 8) An electrochemical process for the production of formic acid according to claim 1, wherein the following sequence of steps is carried out:  water or water vapor, optionally mixed with an inert carrier gas, is introduced via line 11 into the electrochemical cell (or electrochemical cells in parallel or stacked) denoted 1,  the outlet 12 contains oxygen (or an oxygen-water mixture and optionally an inert gas, which can be separated by condensation of the water), the CO ₂ is fed through the conduit 15 into the heat exchanger 16 in which it is preheated by steam or any other means, before being sent through the conduit 13 to the cell or cells 1 , at the outlet of the electrochemical cell (s) 1, a mixture C0 ₂ and formic acid, and any other gases contained in the CO 2 are fed via line 14 to the heat exchanger 17, in which the mixture is cooled and condensed partially using cooling water, or air, - The partially condensed mixture at the outlet of the exchanger (17) is then sent through line 18 to the heat exchanger 19a or 19b, in which it is cooled around 5 ° C with propane (or another compound or refrigerant mixture) flowing in the conduit 20 a or b,  the formic acid crystallizes and is deposited on the walls of the exchanger 19a or 19b (which function alternately in crystallization or in circulation to melt the crystals formed), the residual gas, which essentially contains CO ₂ , is discharged through line 21, preferably towards a flare, to eliminate any traces of hydrogen, CO, methane or formic acid contained in said residual gas; ,  when the pressure drop is too high in the exchanger 19a (respectively 19b), due to the deposition of formic acid crystals, the flow of the duct 18 is directed towards the other exchanger 19b (respectively 19a), which has been cleared in the meantime of the crystallized formic acid which it contains,  the elimination of the formic acid crystals is obtained by circulation of liquid formic acid coming from the storage 22, sucked by the pump 24 through the conduit 23, and then reheated in the heat exchanger 25 with the aid of tempered water, (or by any other means of heating), before being sent through the conduit 26 in the exchanger 19 a or b, in which the circulation of refrigerant (20 a or b) has been stopped.  the formic acid circulates at a moderate temperature (25 to 50 ° C.) and melts the crystallized formic acid deposited in the exchanger 19a or 19b and returns it to the storage 22 via the conduit 27. 9) An electrochemical process for producing formic acid according to claim 8, wherein the residual CO 2 obtained after exchanger 19a or 19b is recompressed in a compressor 29 and reintroduced with CO2 at the cathode compartment constituting the process feed.