WO2018127852A1

WO2018127852A1 - Carbon monoxide, hydrogenm sulfur dioxide and elemental sulfur production from carbon dioxide reduction by hydrogen sulfide

Info

Publication number: WO2018127852A1
Application number: PCT/IB2018/050113
Authority: WO
Inventors: Lawrence D'souza
Original assignee: SABIC Global Technologies BV
Current assignee: SABIC Global Technologies BV
Priority date: 2017-01-09
Filing date: 2018-01-08
Publication date: 2018-07-12
Anticipated expiration: 2019-07-09

Abstract

A method of catalytically producing carbon monoxide (CO), hydrogen (H₂), sulfur dioxide (SO₂), and elemental sulfur (S) directly from carbon dioxide (CO₂) and hydrogen sulfide (H₂S) is disclosed. The method includes (a) obtaining a reaction mixture comprising CO₂(g) and H₂S(g), and (b) contacting the reaction mixture with a catalyst under conditions sufficient to produce a product stream comprising CO(g), H₂(g), SO₂(g), and S₂(g) from CO₂(g) and H₂S(g).

Description

CARBON MONOXIDE, HYDROGENM SULFUR DIOXIDE AND ELEMENTAL SULFUR PRODUCTION FROM CARBON DIOXIDE REDUCTION BY HYDROGEN SULFIDE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U. S. Provisional Patent Application No. 62/443,983 filed January 9, 2017, which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

[0002] The invention concerns a method for catalytically producing carbon monoxide (CO), hydrogen (H₂), sulfur dioxide (S0₂), and elemental sulfur (S) directly from carbon dioxide (C0₂) and hydrogen sulfide (H₂S).

B. Description of Related Art

[0003] Carbon dioxide is a relatively stable and non-reactive molecule when compared with carbon monoxide. Carbon monoxide is more interesting in this respect, as it can be used to produce several downstream chemical products. For instance, synthesis or "syngas" (which includes carbon monoxide and hydrogen gases) is oftentimes used to produce chemicals such as methanol, tert-buty! methyl ether, ammonia, fertilizers, 2-ethyl hexanol, formaldehyde, acetic acid, and 1-4 butane diol.

[0004] Syngas can be produced by common methods such as methane steam reforming technology as shown in reaction equation (1), partial oxidation of methane as shown in reaction (2), or dry reforming of methane as shown in reaction (3).

CH₄ + H₂0 ^CO + 3 H₂ AH298 = 206 kJ (1).

CH₄ + 0.5O₂ - CO + 2H₂ AH₂98K = - 38 kJ/mol (2).

CH₄ + C0₂ 2CO + 2H₂ ΔΗ₂₉₈κ = 247 kJ (3).

While the reactions in equations (1) and (2) do not utilize carbon dioxide, equation (3) does. Commercialization attempts of the dry reforming of methane have suffered due to high energy consumption, catalyst deactivation, and applicability of the syngas composition produced in this reaction. Equation (4) illustrates catalyst deactivation event due to carbonization (i.e., formation of carbon (C)).

CH₄ + 2C0₂ -> C + 2CO + 2H₂0 (4). [0005] Other attempts to convert carbon dioxide into carbon monoxide include the catalytic reduction of carbon dioxide using hydrogen as shown in equation (5).

CO2+ H₂ ^CO + H2O AH= 41.8 KJ/mol (5).

The process of equation (5), which is also known as a reverse water gas shift reaction, is mildly endothermic and takes place at temperatures at about 450 °C. However, commercialization of this process suffers from hydrogen availability. In particular, hydrogen is relatively expensive to produce and isolate. Thus, the present costs and sources of hydrogen are not favorable on a commercial scale to convert CO2 to CO per equation (5).

[0006] There have also been attempts to react hydrogen sulfide with carbon dioxide, the results of which have been inefficient and oftentimes failed to produce desired products. For example, U.S. Patent No. 4,432,960 to Herrington et al. discloses a reaction between carbon dioxide and hydrogen sulfide to produce carbonyl sulfide and water. The produced carbonyl sulfide is then reacted with oxygen to produce carbon monoxide and sulfur dioxide. A problem with this approach is the need for multiple reactions to obtain the desired carbon monoxide and sulfur dioxide products (sulfur dioxide can be used to produce sulfuric acid, which can be used to produce further desired chemical products). This ultimately results in a more complex and cost inefficient process. In U.S. Patent No. 4,999,178 to Bowman, provides theoretical studies of reacting carbon dioxide with hydrogen sulfide through a thermochemical route to produce a stream that includes elemental sulfur, CO2, CO, H2 and water, or a stream that includes COS, H2, or a stream that includes CS2 and CO2. In U.S. Patent No. 5,346,679 to Osaki et al., an attempt is made to react hydrogen with carbon dioxide in the presence of metal sulfide catalyst to show that reactant gas contaminated with hydrogen sulfide did not affect the catalyst activity for the conversion of CO2 to CO. Recently in U.S. Patent Application Pub. No. 2016/0185596 to Manenti et al., an attempt is made to produce syngas from carbon dioxide and hydrogen sulfide. This process, however, relies on the use of oxygen in the reaction mixture to help drive the reaction, which can increase the cost and complexity of the process as well as introduce issues of producing unwanted by-products through oxidation reactions.

[0007] A recent attempt has also been made to produce carbon monoxide from carbon dioxide using elemental sulfur. For example, International Patent Application Pub. No. WO 2016/061205 to D'Souza et al. discloses this process as equation (6).

2CO2 + S 2CO + SO2 ΔΗ = -8 kJ/mol (6). While this process is successful, one potential drawback is the need for elemental sulfur, which can be relatively costly to obtain. By way of illustration, elemental sulfur is commercially produced by the Claus process in petroleum refineries, where hydrogen sulfide is converted into elemental sulfur at around 850 °C in the presence of oxygen, which is shown in equation (7).

2 H₂S + 3 0₂→2 SO2 + 2 H2O AH = -518 kJ mol^_1 (7).

The SO2 then reacts with hydrogen sulfide to produce elemental sulfur and H2O as shown in the equation (8).

The overall equation by combining equations (7) and (8) is shown in equation (9).

8 H2S + 5 O2→ SO2 + 7/2 S2 + 8 H2O (9).

The process illustrated in equation (9) is purely thermal and takes place without the use of a catalyst. The Claus process then continues with a catalytically driven reaction taking place over activated alumina or titania and increases the sulfur yield as shown in the equation (10). 2 H₂S + S0₂→3 S + 2 H2O AH = -1165.6 kJ mol^"1 (10).

The commercial prices associated with obtaining element sulfur can be relatively high in view of the above-described Claus process and other logistics. Therefore, its use in chemical reactions such as that described in WO 2016/061205 could potentially be considered cost- ineffective for industrial scale applications. SUMMARY OF THE INVENTION

[0008] A solution to the problems associated with the production of carbon monoxide (CO) from carbon dioxide (CO2) has been discovered. In particular, the solution resides in the ability to catalytically reduce CO2 with hydrogen sulfide (H2S) in the presence of a catalyst to produce a product stream that includes CO, hydrogen (H2), sulfur dioxide (SO2), and elemental sulfur (S), preferably S2. In addition to the production of CO and H2, which can be used as syngas to produce a variety of downstream chemicals, the production of SO2 and elemental sulfur S can also be advantageously used to produce additional downstream chemicals. Notably, these products can be obtained directly from the catalytically driven reaction of CO2 with H2S rather than using multiple reactions and without the need for the presence of elemental sulfur (e.g., S2), hydrogen (H2), oxygen (O2), and/or methane (CH4) in the reaction mixture, thereby providing for a more elegant and cost-effective process when compared with at least the aforementioned processes used to convert CO2 to CO. The following equation (11) provides an illustration of a reaction process of the present invention.

2C0₂(g) + 3H₂S(g) ^ 2CO(g) + 3H₂ (g) +S0₂(g) + S₂(g) (11).

In a particular aspect of the present invention, the product stream can further include water (H2O) and carbonyl sulfide (COS) as illustrated in equation (12).

3C0₂(g) + 4H₂S(g) ^ 2CO(g) + 3H₂ (g) +S0₂(g) + S₂(g) + H₂0(g) + COS(g) (12).

Still further, the product stream can also include unreacted C0₂(g) and H₂S(g).

[0009] In one aspect of the present invention there is disclosed a method of catalytically producing carbon monoxide (CO), hydrogen (H₂), sulfur dioxide (SO2), and elemental sulfur (S) directly from carbon dioxide (CO2) and hydrogen sulfide (H2S). The method can include (a) obtaining a reaction mixture that includes C0₂(g) and H₂S(g) and (b) contacting the reaction mixture with a catalyst under conditions sufficient to produce a product stream that includes CO(g), H₂(g), S0₂(g), and S₂(g) from C0₂(g) and H₂S(g). The reaction mixture can be a single feed stream that includes a combination of C0₂(g) and H₂S(g) or can be separate feed streams that contact the catalyst. In certain aspects, the reaction mixture can be free of/not include any one of, any combination of, or preferably all of H₂(g), 0₂(g), methane gas CH4(g), and/or elemental sulfur gas (preferably S₂(g)). The produced product stream can further include H₂0(g) and (COS(g)). The product stream can further include unreacted C0₂(g) and H₂S(g). In a particular embodiment, the product stream consists essentially of or consists of CO(g), S₂(g), S0₂(g), COS(g), H₂(g), C0₂(g), H₂0(g), and H₂S(g). The product stream, in some instances, does not include carbon disulfide gas (CS₂(g)). The reaction mixture can include a C0₂(g):H₂S(g) molar ratio of 1 : 10 to 10: 1, preferably 1 : 1 to 5: 1, or more preferably 3 : 1 to 1 :3. In some particular instances, the reaction mixture can include a C0₂(g): H₂S(g) molar ratio of about 1 : 1, about 2: 1, or about 5: 1. The reaction temperature in step (b) can be at least 800 °C, preferably 800 °C to 3000 °C, more preferably 900 °C to 2000 °C, or most preferably 1000 °C to 1500 °C. In one preferred instance, the reaction temperature in step (b) can be 1000 °C to 1100 °C, or about 1050 °C. The reaction can be performed under a pressure of 1 to 25 bar and/or a gas hourly space velocity (GHSV) of 500 to 100,000 h^"1. The catalyst used in the reaction can include a metal sulfide, a metal carbide, a metal nitride, olivine, a pyrochlore, a perovskite, a metal oxide, a spinel, a lanthanide, a lanthanide oxide, or any combination thereof. The catalyst can include at least one, two, or three, or more metals, metal oxides, metal carbides, or metal sulfides having different crystal forms or the same crystal form. The metal, metal oxide, metal sulfide, metal carbide, or metal nitride can include a Group IIA, IIIA, IV A, VA, VIA, VIIA, IB, IIB, IIIB, IVB, VB, VIB, or VIII metal, or any combination thereof. In one instance of the present invention, the catalyst is preferably a metal carbide, preferably silicon carbide (SiC). In certain instances, the catalyst can include a lanthanide or lanthanide oxide selected from La, Ce, Dy, Tm, Yb, Lu, Ce0₂, Dy₂0₃, Tm₂0₃, Yb₂0₃, Lu₂0₃, or La₂0₃, or any combination thereof. The catalyst can be a bulk catalyst or a supported catalyst. In instances where the catalyst is a support catalyst, the support can be a metal oxide support, a metal sulfide support, a metal carbide support, a metal nitride support, a metal phosphate support, an olivine support, a perovskite support, a spinel support, a columbite support, or any combination of supports thereof.

[0010] In one instance of the present invention, the catalyst can be a metal carbide, preferably silicon carbide (SiC). The temperature of the reaction can be 800 °C to 1200 °C, preferably 900 °C to 1150 °C, more preferably 950 °C to 1100 °C, or even more preferably 1000 °C to 1100 °C, or about 1050 °C. The reaction can be carried out at atmospheric pressure or under a selected pressure (e.g., 1 to 25 bar), preferably atmospheric pressure. The gas hourly space velocity (GHSV) of the reaction can be from 500 to 100,000 h^"1. In some other instances, the GHSV can be less than 500 h^"1, such 1 to 400 h^"1, 1 to 100 h^"1, 1 to 20 h^"1, or 5 to 15 h^"1, or about 7 h^"1. The reaction mixture can include a C0₂(g):H₂S(g) molar ratio of 1 : 10 to 10: 1, preferably 1 : 1 to 5: 1, more preferably 3 : 1 to 1 :3, or even more preferably about 1 : 1. [0011] In some particular instances of the present invention, the method can further include isolating and/or converting the produced CO(g) into syngas or can be subjected to a water gas shift reaction to produce additional H₂(g). In other aspects, the produced S0₂(g) can be isolated and/or converted to S0₃(g), and the S0₃(g) can be subsequently converted to sulfuric acid. In particularly preferred instances, the produced S0₂(g) can be isolated by a condensation process. In instances where the product stream includes CO(g), S₂(g), S0₂(g), H₂(g), H₂0(g), and COS(g), these individual products can be isolated from the product stream. By way of example, the produced H₂(g) and COS(g) can each be isolated by a membrane separation process, the produced H₂(g) can be isolated by a pressure swing adsorption process, and/or the produced S₂(g) can be separated by condensation by lowering the temperature of the product stream to less than 110 °C. In a particularly preferred instance where the product stream include CO(g), S₂(g), S0₂(g), COS(g), H₂(g), C0₂(g), H₂0(g), and H₂S(g), the product stream can be processed in the following manner: (i) the product stream can be cooled to a temperature of less than 110 °C to condense and separate S₂(g) from the product stream; (ii) the cooled product stream can then be heated and fed into a water gas shift reactor to produce additional H2 gas from a water gas shift reaction resulting in a hydrogen enriched product stream; (iii) the hydrogen enriched product stream can be fed into a pressure swing adsorption unit to separate H₂(g) from the hydrogen enriched product stream to produce a hydrogen depleted product stream; (iv) C0₂(g) can be separated from the hydrogen depleted product stream by an amine adsorption process and optionally reused in step (a); (v) the produced stream from step (iv) can be cooled to less than -10 °C to condense and separate S0₂(g) from the product stream; and (vi) the remaining COS(g) and H₂S(g) can be separated from one another, preferably by a cryogenic distillation process, and the H₂S(g) can optionally be reused in step (a) and the COS(g) can optionally oxidized with 0₂(g) to produce C0₂(g), S0₂(g), and heat, wherein the heat is used to assist in driving the reaction in step (b) and/or to pre-heat the reaction mixture in step (a) before performing step (b).

[0012] In yet another embodiment of the present invention, there is disclosed a reaction mixture that can include carbon dioxide gas (C0₂(g)) and hydrogen sulfide gas (H₂S(g)). The reaction mixture can consist essentially of, or consist of, C0₂(g) and H₂S(g). The molar ratio of C0₂(g):H₂S(g) can be 1 : 10 to 10: 1, preferably 1 : 1 to 5: 1, or more preferably 3: 1 to 1 :3. In some particular instances, the molar ratio of C0₂(g): H₂S(g) can be about 1 : 1, about 2: 1, or about 5: 1. In certain embodiments, hydrogen gas, oxygen gas, methane gas, water, and/or elemental sulfur gas, or any combination thereof, or all thereof, are not present in the reaction mixture.

[0013] A further embodiment of the present invention includes a product stream that can include CO(g), H₂(g), S0₂(g), and S₂(g). The produced product stream can further include H₂0(g) and (COS(g)). The product stream can further include unreacted C0₂(g) and H₂S(g). In a particular embodiment, the product stream consists essentially of, or consists of, CO(g), S₂(g), S0₂(g), COS(g), H₂(g), C0₂(g), H₂0(g), and H₂S(g). The product stream, in some instances, does not include carbon disulfide gas (CS₂(g)).

[0014] Also disclosed in the context of the present invention is a system for producing carbon monoxide (CO) and sulfur dioxide (S0₂). The system can include an inlet or a first inlet and a second inlet, a reactor, and an outlet in fluid communication. The inlet can be for a feed that can include a carbon dioxide gas (C0₂(g)) and hydrogen sulfide gas (H₂S(g)) or a first inlet for a feed that can include C0₂(g) and a second inlet for a feed that can include H₂S(g). The reactor can include a reaction zone in fluid communication with the inlet, the outlet or both. In some instances, a continuous flow reactor, for example, a plug-flow reactor or a fluidized reactor can be used. The outlet can be in fluid communication with the reaction zone to remove a product stream of the present invention, such a product stream can include CO(g), H₂(g), S0₂(g), and elemental sulfur gas, preferably S₂(g). The reaction zone can further include a catalyst capable of catalyzing the conversion of C0₂(g) and H₂S(g) into a product stream of the present invention. The system can further include an apparatus capable of separating the individual products from the product mixture. Non-limiting examples of separation apparatuses includes a condenser apparatus capable of condensing the produced S0₂(g) to S0₂(1) and separating the S0₂(1) from the produced CO(g) and COS(g), a membrane apparatus capable of separating CO(g) from COS(g), and a scrubber apparatus capable of separating CO(g) from trace amounts of COS(g) and S0₂(g). The condenser apparatus can be downstream from the reactor, the membrane apparatus can be downstream from the condenser apparatus, and the scrubber apparatus can be downstream from the membrane apparatus. Another non- limiting example of separation apparatus system includes a combination of a condenser apparatus capable of condensing the produced S0₂(g) to S0₂(1) and separating the S0₂(1) from the produced CO(g) and COS(g), a cryogenic distillation apparatus capable of condensing the COS(g) to COS(l), and separating the COS(l) from the CO(g), and a scrubber apparatus capable of separating CO(g) from trace amounts of COS(g) and S0₂(g). The condenser apparatus can be downstream from the reactor, and the cryogenic distillation apparatus is downstream from the condenser apparatus, and the scrubber apparatus is downstream from the cryogenic distillation apparatus. The system can include an outlet in fluid communication with a COS/CO separation system (e.g., the membrane system) and configured to produce COS from a COS/CO stream. The system can also include a COS inlet in fluid communication with the outlet of the separation system and the reactor.

[0015] In yet another aspect of the present invention there is disclosed a system that can be used with the processes disclosed in the present invention. The system can include (a) a first reaction zone configured to produce heat from an exothermic reaction of a first reaction mixture and a first product stream; (b) a second reaction zone that includes a gaseous reaction mixture of C0₂ and hydrogen sulfide and configured to receive the produced heat from the first reaction zone in an amount sufficient to heat the gaseous reaction mixture and produce a second product stream that can include a product stream of the present invention, such as a product stream that includes CO(g), H₂(g), S0₂(g), and S₂(g) and optionally H₂0(g), (COS(g)), C0₂(g) and H₂S(g); (c) a first outlet in fluid communication with the first reaction zone and configured to remove the first product stream from the first reaction zone; and (d) a second outlet in fluid communication with the second reaction zone and configured to remove the second product stream from the second reaction zone. The exothermic first reaction mixture can include COS and an oxygen source O2 and the first product stream can include CO2 and SO2 generated by the combustion of the COS. The oxygen source can be air, oxygen enriched air, and/or oxygen gas. In some embodiments, the second reaction zone encompasses the first reaction zone. Such a configuration can form a concentric reactor and the annulus of the concentric reactor can be the first reaction zone. Heat generated from the first reaction zone can be transferred from the first reaction zone to the second reaction zone in an amount sufficient to drive the carbon dioxide and hydrogen sulfide reaction in the presence of a catalyst. In certain embodiments, the first product stream absorbs heat from the exothermic reaction, and the system further includes a heat exchanging unit in fluid communication with the first outlet and the second reaction zone and configured to exchange heat between the heated first product stream and a gaseous reaction feed stream and provide the heated gaseous reaction feed stream to the second reaction zone, where the gaseous feed stream comprises CO2 and hydrogen sulfide. The heat transferred to the gaseous reaction feed stream is sufficient to drive the carbon dioxide and hydrogen sulfide reaction. The temperature of the produced heat can be at least 250 °C or 250 °C to 2500 °C, preferably 900 °C to 2300 °C, most preferably 1000 °C to 2200 °C. In some embodiments, the first reaction outlet is configured to provide the produced heat to another system, preferably a power generating system. In certain embodiments, the second reaction zone includes a catalyst (e.g., a bulk metal catalyst or a supported catalyst) capable of catalyzing the reaction of CO2 and hydrogen sulfide to produce the second product stream. The catalyst can any one of those described throughout the present application.

[0016] In still another aspect of the present invention there is disclosed an integrated system that can be used with the processes disclosed in the present invention. The system can include a first reaction zone having a first gaseous reactant stream/mixture of the present invention that can include CO2 and gaseous hydrogen sulfide. The first reaction zone can produce a first product stream of the present invention that includes CO(g), H2(g), S02(g), and S2(g), and optionally H20(g), (COS(g)), C02(g) and H2S(g). The system can also include a second reaction zone having a second gaseous reactant stream that can include carbonyl sulfide (COS), the second reaction zone can produce a second product stream that can include CO and S2. At least a portion of the COS in the second reactant stream may be from the first product stream. The system may further include a third reaction zone having a third gaseous reactant stream comprising CS2 along with one or more of CO2, O2, or SO2. [0017] Also disclosed in the context of the present invention are embodiments 1-20. Embodiment 1 is a method of catalytically producing carbon monoxide (CO), hydrogen (H₂), sulfur dioxide (S0₂), and elemental sulfur (S) directly from carbon dioxide (CO2) and hydrogen sulfide (H₂S), the method comprising: (a) obtaining a reaction mixture comprising C0₂(g) and H₂S(g); and (b) contacting the reaction mixture with a catalyst under conditions sufficient to produce a product stream comprising CO(g), H₂(g), S0₂(g), and S₂(g) from C0₂(g) and H₂S(g). Embodiment 2 is the method of embodiment 1, wherein the reaction mixture is free of H₂(g), oxygen gas 0₂(g), and optionally methane gas CH₄(g). Embodiment 3 is the method of any one of embodiments 1 to 2, wherein the product stream further comprises H₂0(g) and carbonyl sulfide gas (COS(g)). Embodiment 4 is the method of any one of embodiments 1 to 3, wherein the product stream further comprises C0₂(g) and H₂S(g). Embodiment 5 is the method of embodiment 4, wherein the product stream consists essentially of or consists of CO(g), S₂(g), S0₂(g), COS(g), H₂(g), C0₂(g), H₂0(g), and H₂S(g). Embodiment 6 is the method of any one of embodiments 1 to 5, wherein the reaction mixture comprises a C0₂(g):H₂S(g) molar ratio of 1 : 10 to 10: 1, preferably 1 : 1 to 5: 1, or more preferably 3 : 1 to 1 :3. Embodiment 7 is the method of embodiment 6, wherein the reaction mixture comprises a C0₂(g): H₂S(g) molar ratio of about 1 : 1, about 2: 1, or about 5: 1. Embodiment 8 is the method of any one of embodiments 1 to 7, wherein the product stream does not include carbon disulfide gas (CS₂(g)). Embodiment 9 is the method of any one of embodiments 1 to 8, wherein the reaction temperature in step (b) is at least 800 °C, preferably 800 °C to 3000 °C, more preferably 900 °C to 2000 °C, or most preferably 1000 °C to 1500 °C or 1000 °C to 1100 °C. Embodiment 10 is the method of any one of embodiments 1 to 9, wherein the reaction pressure is 1 to 25 bar. Embodiment 11 is the method of any one of embodiments 1 to 10, wherein a gas hourly space velocity (GHSV) of 500 to 100,000 h^"1 is used in step (b). Embodiment 12 is the method of any one of embodiments 1 to 11, wherein the catalyst comprises a metal, a metal oxide, a metal sulfide, a metal carbide, a metal nitride, olivine, a pyrochlore, a perovskite, a metal oxide, a spinel, a lanthanide, a lanthanide oxide, or any combination thereof. Embodiment 13 is the method of embodiment 12, wherein the catalyst comprises at least one, two, or three, or more metals, metal oxides, metal carbides, or metal sulfides in a different or the same crystal form. Embodiment 14 is the method of any one of embodiments 12 to 13, wherein the metal, metal oxide, metal sulfide, metal carbide, or metal nitride includes a Group IIA, IIIA, IV A, VA, VIA, VIIA, IB, IIB, IIIB, IVB, VB, VIB, or VIII metal, or any combination thereof, or wherein the catalyst comprises a lanthanide or lanthanide oxide selected from La, Ce, Dy, Tm, Yb, Lu, Ce0₂, Dy₂0₃, TimCb, Yb₂Q₃, Lu₂0₃, or La₂Q₃, or any combination thereof. Embodiment 15 is the method of embodiment 12, wherein the catalyst is a metal carbide, preferably silicon carbide (SiC). Embodiment 16 is the method of any one of embodiments 1 to 15, wherein the catalyst is a bulk catalyst or a supported catalyst and wherein the support is a metal oxide, a metal sulfide, a metal carbide, a metal nitride, a metal phosphate, olivine, a perovskite, a spinel, a columbite, or any combination thereof. Embodiment 17 is the method of any one of embodiments 1 to 16, wherein: (i) the produced CO(g) is isolated and/or converted into syngas, or subjected to water gas shift reaction to convert to H₂(g); and/or (ii) the produced S0₂(g) is isolated and/or converted to SCb(g), and the SCb(g) is subsequently converted to sulfuric acid. Embodiment 18 is the method of any one of embodiments 1 to 17, wherein the product stream comprises CO(g), S₂(g), S0₂(g), COS(g), H₂(g), H₂0(g), and the produced S0₂(g) is isolated by a condensation process. Embodiment 19 is the method of any one of embodiments 1 to 18, wherein the product stream comprises CO(g), S₂(g), S0₂(g), H₂(g), H₂0(g), and COS(g), and: (i) the produced H₂(g) and COS(g) are each isolated by a membrane separation process; (ii) the produced H₂(g) is isolated by a pressure swing adsorption process; and/or (iii) the produced S₂(g) is separated by condensation by lowering the temperature of the product stream to less than 110 °C. Embodiment 20 is the method of any one of embodiments 1 to 19, wherein the product stream comprises CO(g), S₂(g), S0₂(g), COS(g), H₂(g), C0₂(g), H₂0(g), and H₂S(g), and wherein: (i) the product stream is cooled to a temperature of less than 110 °C to condense and separate S₂(g) from the product stream; (ii) the cooled product stream is heated and fed into a water gas shift reactor to produce additional H₂ gas from a water gas shift reaction resulting in a hydrogen enriched product stream; (iii) the hydrogen enriched product stream is fed into a pressure swing adsorption unit to separate H₂(g) from the hydrogen enriched product stream to produce a hydrogen depleted product stream; (iv) C0₂(g) is separated from the hydrogen depleted product stream by an amine adsorption process and optionally reused in step (a); (v) the produce stream from step (iv) is cooled to less than -10 °C to condense and separate S0₂(g) from the product stream; and (vi) the remaining COS(g) and H₂S(g) are separated from one another, preferably by a cryogenic distillation process, and the H₂S(g) is optionally reused in step (a) and the COS(g) is optionally oxidized with 0₂(g) to produce C0₂(g), S0₂(g), and heat, wherein the heat is used to assist in driving the reaction in step (b) and/or to pre-heat the reaction mixture in step (a) before performing step (b).

[0018] The following includes definitions of various terms and phrases used throughout this specification. [0019] The term "catalyst" means a substance, which alters the rate of a chemical reaction. "Catalytic" or "catalytic active" means having the properties of a catalyst.

[0020] The phrase "bulk catalyst" includes a catalyst that is not supported by an inert carrier or inert support. [0021] The phrase "supported catalyst" includes a catalyst that is supported by an inert carrier or inert material. The term "inert" means a substance, which does not participate in any chemical reaction described throughout the specification.

[0022] The terms "about" or "approximately" are defined as being close to as understood by one of ordinary skill in the art. In one non-limiting embodiment, the terms are defined to be within 10%, preferably within 5%, more preferably within 1%, and most preferably within 0.5%.

[0023] The term "substantially" and its variations are defined to include ranges within 10%, within 5%, within 1%, or within 0.5%.

[0024] The terms "wt.%", "vol.%", or "mol.%" refers to a weight percentage of a component, a volume percentage of a component, or molar percentage of a component, respectively, based on the total weight, the total volume of material, or total moles, that includes the component. In a non-limiting example, 10 grams of component in 100 grams of the material is 10 wt.%) of component.

[0025] The terms "inhibiting" or "reducing" or "preventing" or "avoiding" or any variation of these terms, when used in the claims and/or the specification, includes any measurable decrease or complete inhibition to achieve a desired result.

[0026] The term "effective," as that term is used in the specification and/or claims, means adequate to accomplish a desired, expected, or intended result.

[0027] The use of the words "a" or "an" when used in conjunction with any of the terms "comprising," "including," "containing," or "having" in the claims, or the specification, may mean "one," but it is also consistent with the meaning of "one or more," "at least one," and "one or more than one."

[0028] The words "comprising" (and any form of comprising, such as "comprise" and "comprises"), "having" (and any form of having, such as "have" and "has"), "including" (and any form of including, such as "includes" and "include") or "containing" (and any form of containing, such as "contains" and "contain") are inclusive or open-ended and do not exclude additional, unrecited elements or method steps.

[0029] The methods of the present invention can "comprise," "consist essentially of," or "consist of particular ingredients, components, compositions, etc. disclosed throughout the specification. With respect to the transitional phase "consisting essentially of," in one non- limiting aspect, a basic and novel characteristic of the methods of the present invention is the ability to produce a product stream comprising CO(g), H₂(g), S0₂(g), and S₂(g) from a reaction mixture that includes C0₂(g) and H₂S(g). This can be done without using any of 0₂(g), H₂(g), CH₄(g), H₂0(g), and/or S₂(g) in the reaction mixture. [0030] Other objects, features and advantages of the present invention will become apparent from the following figures, detailed description, and examples. It should be understood, however, that the figures, detailed description, and examples, while indicating specific embodiments of the invention, are given by way of illustration only and are not meant to be limiting. Additionally, it is contemplated that changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] FIG. 1 is an illustration of various products that can be produced from syngas.

[0032] FIG. 2 is a schematic of a reactor system of the present invention. [0033] FIG. 3 is a schematic of reactor system in combination with a separation system of the present invention.

[0034] FIG. 4 are plots of the equilibrium composition of different gaseous products of the present invention with a feed composition of 1 kmol C0₂ (g) and 1 kmol H₂S(g).

[0035] FIG. 5 are plots of the equilibrium composition of different gaseous products of the present invention with a feed composition of 1 kmol C0₂ (g) and 2 kmol H₂S(g).

[0036] FIG. 6 are plots of the equilibrium composition of different gaseous products of the present invention with a feed composition of 2 kmol C0₂ (g) and 1 kmol H₂S(g).

[0037] FIG. 7 are plots of the equilibrium composition of different gaseous products of the present invention with a feed composition of 5 kmol C0₂ (g) and 1 kmol H₂S(g). [0038] FIG. 8 are plots of the equilibrium composition of different gaseous products of the present invention with a feed composition of 5 kmol CO2 (g) and 5 kmol H₂S(g).

[0039] FIG. 9 are plots of the thermochemical equilibrium composition for CO2 and H2S as a function of temperature. [0040] FIG. 10 is a reaction product composition for a H2S+CO2 reaction at different temperatures over an SiC catalyst.

DETAILED DESCRIPTION OF THE INVENTION

[0041] The present invention provides a solution to the current problems associated with converting carbon dioxide to carbon monoxide. The solution resides in reacting gaseous hydrogen sulfide with carbon dioxide in the presence of a catalyst to produce CO(g), H₂(g), S0₂(g), and S₂(g), which is represented by equation (11) shown above. The reaction can be tuned via the reaction temperature, amounts of reactants used, and/or the catalyst used to obtain a desired product stream profile. For instance, other reaction products that can be produced during the reaction include COS(g) and H₂0(g). Unreacted C0₂(g) and H₂S(g) can also be present in the product stream. Notably, this reaction can be performed in a single reaction step and without the presence of 0₂(g), H₂(g), CH₄(g), H₂0(g), and/or S₂(g) in the reaction stream/mixture (reaction stream and reaction mixture can be used interchangeably throughout this specification). Each of the products produced by the reaction of the present invention can be further processed into desired chemicals. By way of example, the produced carbon monoxide can be converted to syngas by converting part of the carbon monoxide to into hydrogen gas by the water gas shift reaction. Syngas can be used in a variety of processes to produce desired chemicals, examples of which are provided in FIG. 1. The produced SO2 can be converted into SO3 and then sulfuric acid and ultimately ammonium sulfate fertilizers. Similarly, COS(g) and S₂(g) can be converted into valuable commercial products or used as reactants to produce more carbon monoxide. These and other non-limiting aspects of the present invention are discussed in further detail in the following sections.

A. Reaction Feed

[0042] The reactant mixture or feed in the context of the present invention can include a gaseous mixture that includes, but is not limited to, hydrogen sulfide gas (H₂S(g)) and carbon dioxide gas (C0₂(g)). Alternatively, the H₂S(g) and C0₂(g) feeds can be introduced separately and mixed in a reactor. A benefit of using hydrogen sulfide as a starting material is that it is abundant and relatively inexpensive to obtain as compared to hydrogen gas. Hydrogen sulfide can be obtained from various sources. In one non-limiting instance, the hydrogen sulfide can be obtained from a waste or recycle gas stream (e.g., from a plant on the same site, such as a petroleum refinery plant). A benefit of recycling hydrogen sulfide as a starting material in the process of the invention is that it can reduce the amount of hydrogen sulfide emitted to the atmosphere, as hydrogen sulfide can result in acid rain or harm to the environment. Carbon dioxide used in the present invention can be obtained from various sources. In one non-limiting instance, the carbon dioxide can be obtained from a waste or recycle gas stream (e.g. from a plant on the same site, like for example from ammonia synthesis) or after recovering the carbon dioxide from a gas stream. A benefit of recycling such carbon dioxide as a starting material in the process of the invention is that it can reduce the amount of carbon dioxide emitted to the atmosphere (e.g., from a chemical production site).

[0043] In some instances, the reactant mixture may further contain other gases, preferably other gases that do not negatively affect the reaction. Examples of such other gases include nitrogen or argon. In some aspects of the invention, the reactant gas stream is substantially devoid of other reactant gas such as hydrocarbon gases, oxygen gas, hydrogen gas, water or any combination thereof. Hydrocarbon gases include, but are not limited to, Ci to Cs hydrocarbon gases, such as methane, ethylene, ethane, propane, propylene, butane, butylene, isobutene, pentane and pentene. In a particular aspect of the invention the gaseous feed contains 0.1 wt.% or less, or 0.0001 wt.% to 0.1 wt.% of combined other reactant gas. In the reactant mixture, a molar ratio of C0₂(g) to H₂S(g) can range from 1 : 10 to 10: 1, 3 : 1 to 1 :3, 2:8 to 8:2, 3 :7 to 7:3 or 3 : 1 to 1 :3, or about 1 : 1, about 2: 1, about 5 : 1 and any range or value there between. Ratios lower than 1 : 10 and higher than 10: 1 are also contemplated in the context of the present invention. Ultimately, the ratio can be varied to produce a desired reaction product profile. B. Reaction Products

[0044] The products made from the reduction of carbon dioxide with hydrogen sulfide in the gas phase can be varied by adjusting the molar ratio of C0₂(g) to H₂S(g), the reaction conditions, or both. The major products produced from the reaction of carbon dioxide and hydrogen sulfide is carbon monoxide, sulfur dioxide, hydrogen, and elemental sulfur (S₂(g)) as shown in reaction equation (1 1). Another product that can be produced by the reaction is COS as shown in equation (12). In some aspects of the invention, the distribution of products in the product stream (for example, COS(g), S0₂, C0₂, CO and S0₂) can be controlled by adjusting the ratio of carbon dioxide to sulfur from 1 : 10 to 10: 1 and 3 : 1 to 1 :3 and the temperature of the reaction.

1. COS Formation

[0045] Without wishing to be bound by theory, it is believed that, as shown in equation (13), carbon dioxide initially reacts with hydrogen sulfide to form carbonyl sulfide (COS), water, CO, S2 and H2.

3C0₂(g) + 4H₂S(g) ^ COS(g) + H₂0(g) + 2CO(g) + 3H₂ (g) + S₂(g) (13).

In some aspect of the invention, the amount of COS(g) produced can be adjusted by varying the temperature of the reaction. At a temperature at about 800 °C, the product stream contains little to substantially no COS.

2. CO, SO2, S2, and H₂ Formation

[0046] Without wishing to be bound by theory, it is believed that the carbonyl sulfide in equation (13) reacts with carbon dioxide to form SO2 and CO as shown in equation (14) or thermally decompose to S2 and CO as shown in equation 15. When the CO2 to H2S molar ratio is 1 :5 or greater, the formation of sulfur dioxide can be suppressed. The produced CO can react with the produced water to form CO2 and H2 as shown in equation (16).

COS(g) + 2C0₂(g) ^ S0₂(g) + 3CO(g) (14).

COS(g) ^ S₂(g) + CO(g) (15).

CO(g) + H₂0(g) ^ C0₂(g) + H₂(g) (16). [0047] In some aspects of the invention, CO(g), H2(g), S02(g), and S2(g) are produced at temperatures between 800 °C and 3000 °C, 900 °C to 2000 °C, or 1500 °C to 1700 °C, or at least, equal to, or between any two of 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, 1500 °C, 2000 °C, 2500 °C, and 3000 °C, , with a preferred temperature of between 1000 and 1600 °C. A CO2 to FhS molar ratio can be 1 : 10 to 10: 1, 3 : 1 to 1 :3, 1 : 1, about 2: 1, or about 5: 1. The ratio of CO(g) to S02(g) in the product mixture can range from 20: 1 to 120: 1, or 50: 1 to 110: 1, or at least, equal to or between any two of 20: 1, 30: 1, 40: 1, 50:, 60: 1, 70: 1, 80: 1, 90: 1, 100: 1, 110: 1, and 120: 1, or about 51 : 1, about 87: 1, or about 111 : 1. The temperature of the reaction and/or CO2/H2S ratio can be adjusted to produce a desired CO/SO2 ratio or a desired CO/H2 ratio. For example, if a high CO/SO2 is desired, a temperature of 800 °C at a CO2/H2S ratio of 1 : 1 or a temperature of at least 1080 °C at a CO2/H2S ratio of 1 :2. On the other hand, if a high CO/H2 ratio is desired, a CO2/H2S ratio of 5: 1 and temperature of 1100 to 1200 °C can be used. The equilibrium ratios of CO(g) to S0₂(g) at 921 °C, 1080 °C, and 1184 °C are summarized in

Table 1.

Table 1

C. Process

[0048] The reaction of carbon dioxide and hydrogen sulfide can be performed at conditions to produce a product stream that includes carbon monoxide, sulfur dioxide, hydrogen, and optionally carbonyl sulfide. Non-limiting examples of the process for the reduction of carbon dioxide to carbon monoxide in the presence of hydrogen sulfide are illustrated with reference to the Figures.

1. Process for the Conversion of C0₂ and H₂S to H₂

[0049] FIG. 2 is a schematic of reactor system 200 of the present invention. The reactors used for the present invention can be fixed-bed reactors, stacked bed reactors, fluidized bed reactors, slurry or ebullating bed reactors, spray reactors, or plug flow reactor. In some embodiments, a fixed bed reactor is used. The reactors can be manufactured from chemical (e.g., corrosion) resistant material (e.g., sulfur, H₂S, and/or carbon dioxide). A non-limiting example of such material is stainless steel. The system 200 can include gaseous reactant feed stream 202 and a reactor 204. In some embodiments, the gaseous reactant feed is a mixture of C0₂ and H2S. CO2 and H2S can be obtained from various sources. In one non-limiting instance, the CO2 can be obtained from a waste or recycle gas stream (e.g., from a plant on the same site such as from ammonia synthesis, or a reverse water gas shift reaction) or after recovering the carbon dioxide from a gas stream. A benefit of recycling carbon dioxide as a starting material in the process of the invention is that it can reduce the amount of carbon dioxide emitted to the atmosphere (e.g., from a chemical production site). H₂S can be obtained from a plant on the same site such as a refinery process.

[0050] Gaseous reactant feed stream 202 can be in fluid communication with the reactor 204 via an inlet 206 on the reactor. Flow of gaseous reactant feed stream 202 can be regulated to control the amount of the gaseous reactant feed entering the reactor 204. Gaseous reactant feed 202 can include CO2, H2S, and inert gas (e.g., N2 and/or Ar) in a CO2 to H2S volume ratio of 1 : 10 to 10: 1, 3 : 1 to 1 :3, or 5: 1 to 1 :5 or any range or value there between. In some embodiments, the reactant stream is free of H₂(g), oxygen gas 02(g), and optionally methane gas CH₄(g). As shown, the reactant gas feed stream 202 is feeding into single inlet 206, however, it should be understood that the number of inlets and/or separate feed sources can be adjusted to reactor sizes and/or configurations. For example, CO2 and H2S can be feed to reactor 204 via two inlets. The reactor 204 can include reaction zone 208 having the catalyst capable of catalyzing the reaction of CO2 and H2S. The reactor can include various automated and/or manual controllers, valves, heat exchangers, gauges, etc. necessary for the operation of the reactor. The reactor can have the necessary insulation and/or heat exchangers to heat or cool the reactor as necessary. The amounts of the reactant feed and the catalyst used can be modified as desired to achieve a given amount of product produced by the system 200. Reactor 204 can include outlet 210 in fluid communication with reaction zone 208 and configured to remove product stream 212 that includes CO(g), H2(g), S0₂(g), and S₂(g) from the reaction zone.

[0051] In reaction zone 208, gaseous reactant feed stream 202 can be contacted with the catalyst of the present invention to produce product stream 212. Reaction zone 208 can further include the reactant feed and the product stream. Conditions sufficient for the reaction of H2S and CO2 in reactor 204 to CO(g), H₂(g), S0₂(g), and S₂(g) from C0₂(g) and H₂S(g) include temperature, time, space velocity, and pressure. The temperature range for the reaction can range from about 800 °C to 3000 °C, more preferably 900 °C to 2000 °C, or most preferably 1000 °C to 1500 °C, 1100 °C to 1200 °C and all ranges there between. In one preferred instance, the reaction temperature can be 1000 °C to 1100 °C, or about 1050 °C. The gas hourly space velocity (GHSV) for the hydrogenation reaction can range from about 500 h^"1 to 100,000 h^"1, 1000 h^"1 to 50,000 h^"1, 3000 h^ to 30,000 h^"1, 5000 h^"1 to 10,000 h^"1 or any range or value there between. The average pressure for the conversion of CO2 can range from about 0.1 MPa to about 2.5 MPa, about 0.5 MPa to about 1.0 MPa and all pressures there between. The upper limit on pressure can be determined by the reactor used. The conditions for reaction can be varied based on the type of the reactor.

[0052] The products produced can include CO(g), H2(g), SC"2(g), and S2(g) and optionally COS(g), and/or water. In some aspects, the catalyst can be included in the product stream. Both inlet 206 and outlet 210 can be opened and closed as desired. The CO2 conversion can be at least 50% about 50% to 80%>, or about 60%>. Product stream 212 can exit reactor 204 via outlet 210 and be stored, further processed, or transported to other processing units for further use {e.g., see FIG. 1). In a non-limiting example, product stream 212 can be separated into a sulfur stream and a product gas stream {e.g., a stream containing CO(g), H₂(g), and S0₂(g), and optionally unreacted C0₂ and H₂S). Any unreacted reactant gas {e.g., C0₂ and H₂S) can be separated, and recycled to the reactor {e.g., included in the reactant feed) to further maximize the overall conversion of C0₂ to CO and H₂, thereby increasing the efficiency and commercial value of the C0₂/H₂S to CO and H₂ process of the present invention.

2. Product Recovery Systems [0053] In some aspects of the process, the components of the gaseous product mixture can be separated into sulfur, sulfur dioxide, carbonyl sulfide, carbon monoxide or combinations thereof using known separation technology methods. In some embodiments, thermal-based separation systems {e.g., condensation, distillation, and the like) can be used to remove each component and produce a pure stream of CO. Other forms of separation, such as chemi- and phy si -sorption systems can also be used to remove particular components. In some embodiments, the product stream after removal of sulfur can be heated to convert the produced CO to C0₂ and H₂.

[0054] Referring to FIG. 3, a schematic system 300 that includes the reaction system 200 in combination with a product recovery system. Gaseous product stream 212 can exit reactor 204 and enter condensing unit 302 at a temperature at or near the reaction temperature, and can include CO(g), H₂(g), S0₂(g), and S₂(g) and optionally COS(g), unreacted C0₂(g), unreacted H₂S(g), and/or water. In condensing unit, hot gaseous product stream 212 can be cooled to a temperature sufficient to condense sulfur gas from the gaseous stream and form sulfur- containing stream 304 and gaseous product stream 306. For example, to a temperature less than 110 °C. Sulfur gas (S₂(g)) in the context of the present invention can include, but is not limited to, all allotropes of sulfur {i.e., S« where n=\ to∞). Non-limiting examples of sulfur allotropes include S, S₂, S₄, S₆, and Ss. Sulfur-containing stream 304 can exit condensing unit 302 and be stored, transported, or used in other processing units. Gaseous crude product stream 306 can include CO(g), H₂(g), S0₂(g), and optionally COS(g), unreacted C0₂(g), unreacted H₂ S(g) and/ or water.

[0055] Gaseous product stream 306 can exit condensing unit 304 and enter water-gas shift unit 308. Steam 310 can also enter water-gas shift unit 308. In water-gas shift unit 308, CO and steam (H2O) can react to form CO2 and H2 as shown in reaction equation 16 to produce water-gas shift product stream 312 that has increased amounts H2 and, if CO2 is present, increased amount of CO2 as compared to the same products in gaseous crude product stream 306. Water-gas shift product stream 312 can include C02(g), H2(g), S02(g), and optionally COS(g) and unreacted H2S. Water-shift reaction temperatures can be 200 °C to 450 °C, 300 to 400 °C, or any range or value there between. In some embodiments, an additional catalyst is used promote the water-gas shift reaction. Non-limiting examples of water-gas shift catalysts include CuO/ZnO/AhCb or Fe/Cr/Mg oxide catalyst, or the like.

[0056] Water-gas shift product stream 312 can exit water-gas shift unit 308 and enter H2 separation unit 314. In H2 separation unit 314, H2 stream 316 can be separated from water-gas shift product stream 312 to produce crude product stream 318 that includes C02(g), S02(g), and optionally COS(g) and unreacted H2S. H2 separation unit 314 can be any separation unit capable of separating H2 from other gases. By way of example, H2 separation 314 can be a membrane unit or a series of membrane units, a pressure swing adsorption unit or the like. H2 stream 316 can be stored, transported, or used in other processing units. Crude product stream 318 can exit H2 separation unit 314 and enter CO2 separation unit 320. In CO2 separation unit 320, CO2 stream 322 can be separated from crude product stream 318 to produce crude product stream 324 that includes S02(g), and optionally COS(g) and unreacted H2S. CO2 separation unit 320 can be any separation unit capable of separating CO2 from other gases. By way example, CO2 separation unit 320 can be an amine adsorption unit. Separated CO2 stream can exit CO2 separation unit 320 and be combined with reactant feed stream 202. Crude product stream 324 can exit CO2 separation unit 320 and enter SO2 separation unit 326. In SO2 separation unit 326, SO2 can be separated from COS and unreacted H2S. In some embodiments, no COS or H2S is present so separation unit 326 is not necessary. SO2 separation unit 326 can be any separation unit capable of separating SO2 from COS and H2S. By way of example, SO2 separation unit 326 is a condensing unit that is capable of cooling the gas mixture below -10 °C, at which point the SO2 is liquefied and produces liquid SO2 stream 328 and COS/H2S stream 330. Liquid SO2 stream 328 can exit SO2 separation unit 326 and be store, transported, or used in other processing units. By way of example, the sulfur dioxide produced using the method of the invention can be converted to SO3, which can be further processed into sulfuric acid and ammonium sulfate as shown in the equations (17) through (20).

SO3 + H2SO4 H2S2O7 (18). H2S2O7 + H2O 2H2SO4 (19).

2 H3 + H2SO4 ( H₄)₂S04 (20).

[0057] COS/H2S stream 330 can exit SO2 separation unit 326 and enter H2S separation unit 332. In H2S separation unit 332, H2S can be separated from COS to form H2S stream 334 and COS stream 336. H2S separation unit 332 can be any separation unit capable of separating H2S from COS. By way of example, H2S separation unit 332 is a cryogenic distillation unit. H2S stream 332 can exit H2S separation unit 332 and be combined with CO2 stream 324 and/or combined with reactant gas stream 202. As shown, CO2 stream 324 and H2S stream 334 are combined as one stream and feed to reactant gas stream 202. It should be understood that each stream can be separately fed to reactor 204 and/or gas stream 202. COS stream 336 can be stored, transported or used in other processing units. In some embodiments, the COS can be oxidized using O2 to produce CO2 and SO2 as shown in equation (21).

COS +1.5 0₂ = C0₂ + SO2 ΔΗ773 K = -549.7 kJ/mol (21)

[0058] The heat generated from this reaction can be utilized to drive the endothermic CO2 and H2S reaction, which can require high temperature and/or the recovered heat can be used to preheat the feed before entering into reactor 204. By way of example, the carbonyl sulfide produced using the method of the invention can be used in the production of thiocarbamates. Thiocarbamates can be used in commercial herbicide formulations. The method of the invention provides an advantage over commercially prepared COS, which is synthesized by treatment of potassium thiocyanide and sulfuric acid as shown in equation (22).

KSCN + 2 H2SO4 + H2O→ KHSO4 + H4HSO4 + COS (22)

[0059] The conventional treatment produces potassium bisulfate and ammonium bisulfate which needs to be separated, which is a difficult and time consuming process. The method of the invention provides an efficient and economic method solution to the production of COS. D. Catalysts

[0060] The metals that can be used in the context of the present invention to create bulk metal oxides or supported catalysts include a metal from Columns 2-6 and 9-12 of the Periodic Table or compound thereof, or lanthanide or compound thereof, or any combination thereof.

[0061] The metals or metal compounds can be purchased from any chemical supplier such as Sigma-Aldrich, Alfa-Aeaser, Strem, etc. Non-limiting examples of Column 2 metals (alkaline-earth metals) and Column 2 metal compounds include Mg, MgO, Ca, CaO, Ba, BaO, Sr, SrO or any combinations thereof. Non-limiting examples of Column 11 metals and Column 11 metal compounds include Cu and CuO. Column 3 metals and Column 3 metal compounds include, but are not limited to, Sc, SC2O3, the lanthanides or lanthanide compounds, or any combination thereof. Lanthanides that can be used in the context of the present invention to create lanthanide oxides can include La, Ce, Dy, Tm, Yb, Lu, or combinations of such lanthanides. Non-limiting examples of lanthanide oxides include Ce0₂, Dy₂0₃, Tm₂0₃, Yb₂0₃, Lu₂0₃, or La₂0₃, or any combination thereof. Lanthanide oxides can be produced by methods known in the art such as by high temperature (e.g., >500 °C) decomposition of lanthanide salts or by precipitation of salts into respective hydroxides followed by calcination to the oxide form. Column 4 metals and Column 4 metal compounds include, but are not limited to, Zr and Zr02. Column 6 metals and Column 6 metal compounds include, but are not limited to, Cr, Cr₂0₃, Mo, Mo₂0₃, or any combination thereof. Columns 9-11 metals and metal compounds include, but are not limited to, Ru, Ru0₂, Os, Os0₂, Co, Co₂0₃, Rh, Rh₂0₃, Ir, lr₂0₃, Ni, Ni₂0₃, Pd, Pd₂0₃, Pt, Pt₂0₃, or combinations thereof. The catalytic material can include catalytic metals such as Sc, Zr, Mo, Cr, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, La, Ce, Dy, Tm, Yb, or Lu or any combination thereof. Base metals can include Mg, Ca, Ba, or any combination thereof. Non- limiting examples of metal oxides, Sc₂0₃, Zr0₂, Mo₂0₃, Cr₂0₃, MoO, Ru0₂, Os0₂, Co₂0₃, Rh₂0₃, lr₂0₃, Ni₂0₃, Pd₂0₃, Pt₂0₃, La₂0₃, Ce0₂, Dy₂0₃, Tm₂0₃, Yb₂0₃, Lu₂0₃ MgO, CaO, BaO, or CuO. Metal oxides can be of spinels (general formula: M₃0₄), olivine (general formula: M2S1O4) or perovskites (general formula: M¹M²0₃), classes or any other crystal structure like sphalerite, antifluorite, wurtzite, rutile, etc.

[0062] Catalysts or catalyst support may also include metal sulfides, carbides, nitrides, or phosphates. Non-limiting examples of metal sulfides include ZnS, FeS, MnS, MnS₂, PtS, PtS₂, S, N1S2, Ni₃S₂, CoS, C0S2, Co₂S₃, M0S2, Mo₂S₃, PbS, PbS₂, A1₂S₃, ReS₂, Re₂Sv, IrS₂, CaS, L12S, MgS, Na₂S, Na₂S₂, Sc₂S₃, Y₂S₃, S1S2, SrS, TaS₂, VS2, V₂S₃, NbS₂, ZeS₂, Ws₂, WS₃, La₂S₃, Ce₂S₃ CeS, TiS, Ti2S3, Bi2S3, CdS, Cu₂S, Ga₂S, Ga₂S₃, InS, In₂S₃, PbS, Sb₂S₃, Sb₂S₅, Se₄S4, Se₂S6, SnS, SnS₂, or the like. Non-limiting examples of metal carbides include SiC, M02C, TiC, CrC, WC, OsC and VC or the like. Non-limiting examples of metal nitrides such as M02N, TiN, VN, WN and CrN or any metal nitrides. A non-limiting example of a metal phosphate is Ni-Mo-P. In some preferred instances, the catalyst can be a metal carbide, and preferably silicon carbide (SiC). EXAMPLES

[0063] The present invention will be described in greater detail by way of specific examples. The following examples are offered for illustrative purposes only, and are not intended to limit the invention in any manner. Those of skill in the art will readily recognize a variety of noncritical parameters which can be changed or modified to yield essentially the same results.

Example 1

(Equilibrium Calculations of Reactions)

[0064] Multiphase equilibrium composition calculation was done by using HSC Chemistry 7.1 software (Outotec Oyi, Espoo, Finland). The parameters used in the calculations were ratios of gaseous carbon dioxide to gaseous FhS ranging of 1 : 1, 1 :2, 2: 1, 5: 1 and 1 :5 at temperatures between 500-1500 °C. FIGS. 4-8 are graphs of the calculated equilibrium composition obtained by treating different ratios of CO2 with gaseous FhS at temperatures from 500-1500 °C. FIG. 4 are plots of the equilibrium composition of different gaseous species with a feed composition of 1 kmol CO2 (g) and 1 kmol FhS(g). FIG. 5 are plots of the equilibrium composition of different gaseous species with a feed composition of 1 kmol CO2 (g) and 2 kmol FhS(g). FIG. 6 are plots of the equilibrium composition of different gaseous species with a feed composition of 2 kmol CO2 (g) and 1 kmol FhS(g). FIG. 7 are plots of the equilibrium composition of different gaseous species with a feed composition of 5 kmol CO2 (g) and 1 kmol FhS(g). FIG. 8 are plots of the equilibrium composition of different gaseous species with a feed composition of 1 kmol CO2 (g) and 5 kmol FhS(g). The calculated results demonstrate that gaseous H2S reacts with CO2 to form equilibrium mixture of SO2, CO, Fh, FhO, and COS in different amounts at different temperature. Table 2 lists the CO2 and FhS conversion at different temperatures with different feed compositions derived from the FIGS. 4-8 data. Table 3 lists the concentration of CO, COS, SO2, and Fh in the product gases at equilibrium conversion at different temperatures with different feed compositions derived from the FIGS. 4-8 data. Table 2

Table 3

[0065] From the data from FIGS. 4-8 and Table 2, it was determined that the conversion varied with feed composition. About 61% CO2 conversion was achieved at 1185 °C with feed composition at a CO2 to H2S of 1 :2. Increasing the CO2 partial pressure in the feed resulted in increased H2S conversion and vice versa as shown in Table 2. High CO2 or H2S conversion was achieved by increasing the partial pressure of either of CO2 or H2S at the expense of high separation and recycle cost. For example, about 83% H2S conversion was achieved at 1185 °C with a CO2 to H2O feed ratio of 1 :5. At this ratio more than 50% unconverted CO2 needed to recovered and recycled, which contributed to a high cost of production.

[0066] Referring to Table 3, it was determined that with a CO2 to H2S ratio of 1 :5, up to 31%) (CO+H2) yield was achieved at 1184 °C. An increase the CO+H2 yield will be achieved by extracting H20(g) formed by some technique, one such technique could be using ceramic membrane reactor. By continuously removing H2O (g) formed one can increase the amount of CO2 conversion and CO yield. [0067] The overall balanced equation for reactants and products at equilibrium condition at 1100 °C can be written as shown in equation (23). FIG. 9 are plots of the thermochemical equilibrium composition for reaction (10) as a function of temperature. The dH of the reaction is 66 kJ/mol; and dG is 13 kJ/mol and dS= 39 J/K.

0.4 CO(g) + 0.1 H₂(g) + 0.4 H₂0(g) + 0.05 COS(g) + 0.7 C0₂(g) + 0.4 H₂S(g) + 0.3 S₂(g) (23)

Example 2

(Reaction of H₂S and C0₂ With Metal Carbide Catalyst)

[0068] H₂S and C0₂ decomposition was carried out on silicon carbide (SiC) material at high temperatures (850 °C, 950 °C, and 1050 °C) and atmospheric pressure. 50 ml/min (10.5 vol.% H₂S, 10.5 vol.% CO2 + 79 vol.% Ar) was passed through the quartz reactor containing 500 mg of SiC catalyst. The final GHSV was equal to 7 ml h^"1 mg^"1. Micro Gas chromatography from Agilent was used to measure the reaction products. [0069] Table 4 gives the amount of CO2 and H₂S converted at 850 °C, 950 °C, and 1050 °C. The conversion values are close to that of the thermodynamic equilibrium values calculated using HSC software.

Table 4

(Percentage conversion of H₂S and C0₂ at different temperature and atmospheric pressure over SiC catalyst)

[0070] FIG. 10 gives the product composition at different temperatures. The product distribution shows a close match to equilibrium composition calculated using HSC software. However, metallic sulfur was not detected in the reaction product. Also, trace amount of COS was detected which is beyond the quantifiable limits. Moreover, no CS2 was detected in the product gases.

Prophetic Example

(Reaction of H₂S and C0₂)

[0071] The following non-limiting prophetic example can be used to perform additional tests in the context of the present invention. Typically, reactants CO2 and H2S will be fed into the reactor (See, FIGS. 1 and 2) where a catalyst capable of catalyzing the CO2 and H2S reaction to produce mixture of gases is placed in a fixed bed fashion. The reaction will take place at around 900-1200 °C, so the product gases will be cooled to about <110 °C to condense S2(g) into elemental sulfur. The remaining gases will be fed into a water gas shift reactor, where the gases will be mixed with steam to produce hydrogen gas. The hydrogen rich mixture gases will be fed into the pressure swing adsorption unit where hydrogen will be selectively separated from the rest of the gases. CO2 will be separated by well-known amine adsorption technology. The SO2 will be separated by cooling the gas mixture below -10 °C at which point it converts to liquid. The remaining gas mixture will be separated by cryogenic distillation or any other available technique to separate COS and H2S. The separated H2S and CO2 can be recycled back into the reaction reactor along with make-up CO2 and H2S gases. The COS will be oxidized using O2 to produce CO2 and SO2. The heat generated in this reaction can be utilized to drive the main reaction. Table 5 lists select physical properties of the processing components.

Table 5

Claims

1. A method of catalytically producing carbon monoxide (CO), hydrogen (H2), sulfur dioxide (SO2), and elemental sulfur (S) directly from carbon dioxide (CO2) and hydrogen sulfide (H2S), the method comprising:

(a) obtaining a reaction mixture comprising CC"2(g) and H2S(g); and

(b) contacting the reaction mixture with a catalyst under conditions sufficient to produce a product stream comprising CO(g), H2(g), SC"2(g), and S2(g) from C0₂(g) and H₂S(g).

2. The method of claim 1, wherein the reaction mixture is free of H2(g), oxygen gas 02(g), and optionally methane gas CH₄(g).

3. The method of any one of claims 1 to 2, wherein the product stream further comprises H20(g) and carbonyl sulfide gas (COS(g)).

4. The method of claim 3, wherein the product stream further comprises C02(g) and H₂S(g).

5. The method of claim 4, wherein the product stream consists essentially of or consists of CO(g), S₂(g), S0₂(g), COS(g), H₂(g), C0₂(g), H₂0(g), and H₂S(g).

6. The method of claim 1, wherein the reaction mixture comprises a C02(g):H2S(g) molar ratio of 1 : 10 to 10: 1, preferably 1 : 1 to 5: 1, or more preferably 3 : 1 to 1 :3.

7. The method of claim 6, wherein the reaction mixture comprises a C02(g): H2S(g) molar ratio of about 1 : 1, about 2: 1, or about 5: 1.

8. The method of claim 1, wherein the product stream does not include carbon disulfide gas (CS₂(g)).

9. The method of claim 1, wherein the reaction temperature in step (b) is at least 800 °C, preferably 800 °C to 3000 °C, more preferably 900 °C to 2000 °C, or most preferably 1000 °C to 1500 °C or 1000 °C to 1100 °C.

10. The method of claim 1, wherein the reaction pressure is 1 to 25 bar.

11. The method of claim 1, wherein a gas hourly space velocity (GHSV) of 500 to 100,000 h^"1 is used in step (b).

12. The method of claim 1, wherein the catalyst comprises a metal, a metal oxide, a metal sulfide, a metal carbide, a metal nitride, olivine, a pyrochlore, a perovskite, a metal oxide, a spinel, a lanthanide, a lanthanide oxide, or any combination thereof.

13. The method of claim 12, wherein the catalyst comprises at least one, two, or three, or more metals, metal oxides, metal carbides, or metal sulfides in a different or the same crystal form.

14. The method of claim 12, wherein the metal, metal oxide, metal sulfide, metal carbide, or metal nitride includes a Group IIA, IIIA, IVA, VA, VIA, VIIA, IB, IIB, IIIB, IVB, VB, VIB, or VIII metal, or any combination thereof, or wherein the catalyst comprises a lanthanide or lanthanide oxide selected from La, Ce, Dy, Tm, Yb, Lu, Ce0₂, Dy₂0₃, Tm₂0₃, Yb₂0₃, Lu₂0₃, or La₂0₃, or any combination thereof.

15. The method of claim 12, wherein the catalyst is a metal carbide, preferably silicon carbide (SiC).

16. The method of claim 1, wherein the catalyst is a bulk catalyst or a supported catalyst and wherein the support is a metal oxide, a metal sulfide, a metal carbide, a metal nitride, a metal phosphate, olivine, a perovskite, a spinel, a columbite, or any combination thereof.

17. The method of claim 1, wherein:

(i) the produced CO(g) is isolated and/or converted into syngas, or subjected to water gas shift reaction to convert to H₂(g); and/or

(ii) the produced S0₂(g) is isolated and/or converted to S0₃(g), and the S0₃(g) is subsequently converted to sulfuric acid.

18. The method of claim 1, wherein the product stream comprises CO(g), S₂(g), S0₂(g), COS(g), H₂(g), H₂0(g), and the produced S0₂(g) is isolated by a condensation process.

19. The method of claim 1, wherein the product stream comprises CO(g), S₂(g), S0₂(g), H₂(g), H₂0(g), and COS(g), and:

(i) the produced H₂(g) and COS(g) are each isolated by a membrane separation process;

(ii) the produced H₂(g) is isolated by a pressure swing adsorption process; and/or

(iii) the produced S₂(g) is separated by condensation by lowering the temperature of the product stream to less than 110 °C. The method of claim 1, wherein the product stream comprises CO(g), S₂(g), S0₂(g), COS(g), H₂(g), C0₂(g), H₂0(g), and H₂S(g), and wherein:

(i) the product stream is cooled to a temperature of less than 110 °C to condense and separate S₂(g) from the product stream;

(ii) the cooled product stream is heated and fed into a water gas shift reactor to produce additional H₂ gas from a water gas shift reaction resulting in a hydrogen enriched product stream;

(iii) the hydrogen enriched product stream is fed into a pressure swing adsorption unit to separate H₂(g) from the hydrogen enriched product stream to produce a hydrogen depleted product stream;

(iv) C0₂(g) is separated from the hydrogen depleted product stream by an amine adsorption process and optionally reused in step (a);

(v) the produce stream from step (iv) is cooled to less than -10 °C to condense and separate S0₂(g) from the product stream; and

(vi) the remaining COS(g) and H₂S(g) are separated from one another, preferably by a cryogenic distillation process, and the H₂S(g) is optionally reused in step (a) and the COS(g) is optionally oxidized with 0₂(g) to produce C0₂(g), S0₂(g), and heat, wherein the heat is used to assist in driving the reaction in step (b) and/or to pre-heat the reaction mixture in step (a) before performing step (b).