CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Application No. 62/279,318, filed January 15, 2016. The contents of the referenced application are incorporated into the present application by reference.

FIELD

[0002] The disclosed subject matter relates to methods for producing syngas from carbon dioxide.

BACKGROUND

[0003] Syngas, also known as synthesis gas, is primarily a mixture of carbon monoxide (CO) and hydrogen (H ₂ ), but can also contain carbon dioxide (C0 ₂ ) and/or water (H ₂ 0). Syngas can be a feedstock for producing higher hydrocarbons, such as fuels. Syngas can also be used to produce various chemicals, including olefins, methanol, ethylene glycol, and aldehydes. In these processes, the composition of the syngas, and particularly the stoichiometric ratio of H ₂ and C0 ₂ in the syngas, can be important in determining which materials are produced.

[0004] Although syngas can be produced from hydrocarbons, such as natural gas, increased concern over the environmental impact of carbon dioxide emissions has generated interest in techniques for converting carbon dioxide into syngas. Certain methods for producing syngas from carbon dioxide are known in the art. For example, U.S. Patent Publication No. 2010/0190874 discloses a method for generating syngas from hydrogen and carbon dioxide, which includes contacting the feedstream with a manganese oxide catalyst containing an additional metal oxide. European Patent Publication No. EP2788117 discloses a catalyst for use in the hydrogenation of carbon dioxide to produce syngas at temperatures from 400°C to 600°C. The catalyst is a supported manganese oxide catalyst containing an auxiliary metal. International Patent Publication No. WO2015066117 discloses a method for generating syngas over a manganese oxide catalyst that can further include another metal oxide, a support, and/or an auxiliary metal.

[0005] However, there remains a need for improved methods for producing syngas from carbon dioxide. The present disclosure addresses these and other needs.

SUMMARY OF THE DISCLOSED SUBJECT MATTER

[0006] The disclosed subject matter provides novel methods for producing syngas from carbon dioxide.

[0007] In certain embodiments, an exemplary method of producing syngas from carbon dioxide includes contacting a feedstream comprising hydrogen and carbon dioxide with a supported metal oxide catalyst including at least one auxiliary metal to produce a product stream including syngas having a molar ratio of hydrogen to carbon monoxide (H ₂ :CO) of less than 2: 1. The feedstream can have a molar ratio of carbon dioxide to hydrogen (C0 ₂ :H ₂ ) of about 1 : 1 to about 2: 1.

[0008] The metal oxide catalyst can include a metal selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ru, Rh, Pd, Ag, Cd, Pt, Au, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and combinations thereof. The metal oxide catalyst can include a support, for example, alumina (A1 ₂ 0 ₃ ), silica (Si0 ₂ ), titania (Ti0 ₂ ), zirconia (Zr0 ₂ ), chromium (III) oxide (Cr ₂ 0 ₃ ), magnesia (MgO), cerium (IV) oxide (Ce0 ₂ ), and combinations thereof. In certain embodiments, the metal oxide catalyst can include Mn/Al ₂ 0 ₃ and/or Ce/Al ₂ 0 ₃ . The metal oxide catalyst can include an auxiliary metal, such as Li, Be, Na, Mg, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Sr, Ru, Rh, Cs, Ba, Pd, Ag, Cd, Pt, Au, and combinations thereof. In certain embodiments, the auxiliary metal is Cu. In certain embodiments, the catalyst includes Cu-Mn/Al ₂ 0 ₃ and/or Cu-Ce/Al ₂ 0 ₃ .

[0009] The method can further include hydrogenating the carbon dioxide in the feedstream to form carbon monoxide and water. In certain embodiments, the hydrogenation reaction can be performed at temperatures ranging from about 600°C to about 700°C.

[0010] In certain embodiments, the syngas in the product stream can have a molar ratio of hydrogen to carbon monoxide (H2:CO) of about 1 : 1.

[0011] In certain embodiments, the method can further include separating carbon dioxide and water from the product stream. The produced syngas can be used in an oxo synthesis reaction or to produce monoethylene glycol.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 depicts a method for producing syngas from carbon dioxide according to one exemplary embodiment of the disclosed subject matter.

DETAILED DESCRIPTION

[0013] The presently disclosed subject matter provides novel methods for producing syngas from carbon dioxide.

[0014] For the purpose of illustration and not limitation, FIG. 1 is a schematic representation of a method for the hydrogenation of carbon dioxide to form syngas according to a non-limiting embodiment of the disclosed subject matter. The method 100 can include contacting a feedstream containing carbon dioxide and hydrogen with a supported metal oxide catalyst having at least one auxiliary metal 101.

[0015] The feedstream can undergo a hydrogenation reaction to form a product stream 102. Particularly, a product stream containing syngas can be produced by the hydrogenation of carbon dioxide. For example, carbon dioxide (C0 ₂ ) and hydrogen (H ₂ ) in the feedstream can react to form carbon monoxide (CO) and water (H ₂ 0) in a reverse water gas shift reaction. The reverse water gas shift reaction is illustrated by:

C0 ₂ + H _{2 <} → CO + H ₂ 0 (Formula 1)

[0016] The reverse water gas shift reaction is equilibrium-driven, and can be performed under conditions resulting in only partial conversion of C0 ₂ and H ₂ . Thus, the hydrogenation of carbon dioxide by the reverse water gas shift reaction can result in a product stream containing C0 ₂ and H ₂ , as well as CO and H ₂ 0. The ratio of H ₂ and CO in the product stream can be manipulated by varying process conditions, e.g., reaction conditions, catalyst type or amount, or the ratio of C0 ₂ to H ₂ in the feedstream.

[0017] According to the disclosed methods, the feedstream can contain C0 ₂ and H ₂ . "Feedstream" as used herein can refer to a single feedstream or multiple feedstreams, which can be combined before or during the hydrogenation reaction. For example, the feedstream can be a single mixture of H ₂ or C0 ₂ . Alternatively or additionally, multiple feedstreams containing H ₂ and/or C0 ₂ can be provided. The C0 ₂ in the feedstream can originate from various sources. For example, the C0 ₂ can be sourced from other chemical processes, e.g., as a waste product, or unconverted C0 ₂ can be recovered from the product stream and recycled to the feedstream. The H ₂ in the feedstream can also originate from various sources, for example from gaseous streams from other chemical processes.

[0018] H ₂ and C0 ₂ can be provided in a specific ratio in the feedstream. For example, the molar ratio of C0 ₂ and H ₂ (C0 ₂ :H ₂ ) in the feedstream can range from about 0.5: 1 to 5: 1, e.g., about 0.5: 1, 0.6: 1, 0.7: 1, 0.8: 1, 0.9: 1, 1 : 1, 1.2: 1. 1.4: 1, 1.6: 1, 1 :8: 1, 2: 1, 3 : 1, 4: 1, or 5: 1. In certain embodiments, the feedstream can contain C0 ₂ and H ₂ in a molar ratio of about 1 : 1 to 2: 1.

[0019] As used herein, the term "about" or "approximately" means within an acceptable error range for the particular value as determined by one of ordinary skill in the art, which will depend in part on how the value is measured or determined, i.e., the limitations of the measurement system. For example, "about" can mean a range of up to 20%, up to 10%, up to 5%), and or up to 1% of a given value.

[0020] In certain embodiments, the feedstream can be provided at atmospheric pressure. Alternatively, the feedstream can be pressurized, e.g., to from about 0 bar to about 15 bar.

[0021] The catalyst for use in the presently disclosed methods can be any catalyst suitable for the hydrogenation of C0 ₂ to form CO and H ₂ 0. For example, the catalyst can be a supported metal oxide catalyst. In certain embodiments, the catalyst can include an auxiliary metal.

[0022] For example, the catalyst can include a metal oxide or a mixed metal oxide. The catalyst can contain a variety of metals, including transition metals and rare earth metals. For example, the catalyst can contain Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ru, Rh, Pd, Ag, Cd, Pt, Au, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and/or combinations thereof. In particular embodiments, the catalyst contains a Mn oxide and/or a Ce oxide.

[0023] The catalyst for use in the disclosed methods can include metal having high reduction potential. During the hydrogenation of C0 ₂ , coke deposits (i.e., carbonaceous deposits) can form on the catalysts and reduce catalyst stability. For example, a side reaction of the hydrogenation of C0 ₂ is the Boudouard reaction, which is illustrated in Formula 2:

2CO _< → CC-2 + C (Formula 2)

[0024] Particularly, the Boudouard reaction can occur more frequently with higher molar ratios of C0 ₂ and H ₂ (C0 ₂ :H ₂ ) in the feedstream. The Boudouard reaction is a redox reaction in which CO is reduced to form C0 ₂ and oxidized to form carbon, i.e., coke deposits, which can coat the catalyst. The catalyst can be regenerated by oxidizing the coke deposits to form CO. Although C0 ₂ can be used to regenerate the catalyst, a metal having high reduction potential, i.e., a strong oxidizing agent, can be used to oxidize coke deposits.

[0025] For example, the catalyst can include an auxiliary metal having high reduction potential, such as Li, Be, Na, Mg, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Rb, Sr, Ru, Rh, Cs, Ba, Pd, Ag, Cd, Pt, Au, and/or combinations thereof. In particular embodiments, the auxiliary metal is Cu.

[0026] The catalyst can further include a support material. For example, the support material can be alumina (AI2O3), silica (Si0 ₂ ), titania (Ti0 ₂ ), zirconia (Zr0 ₂ ), chromium (III) oxide (Cr ₂ 0 ₃ ), magnesia (MgO), cerium (IV) oxide (Ce0 ₂ ) and/or combinations thereof. In certain embodiments, the support material is alumina. In particular embodiments, the catalyst can include Cu-Mn/Al ₂ 0 ₃ and/or Cu-Ce/Al ₂ 0 ₃ .

[0027] The catalysts of the presently disclosed subject matter can be prepared using any suitable method known in the art. For example, the catalysts can prepared by coprecipitation of the support and metal(s), or the support can be impregnated using a metal salt.

[0028] In certain embodiments, the catalyst can be loaded into a reactor for the hydrogenation reaction. For example, the catalyst can be within a fixed bed reactor. The dimensions and structure of the reactor can vary depending on the capacity of the reactor. The capacity of the reactor unit can be determined by the reaction rate, the stoichiometric quantities of the reactants and/or the feed flow rate. The reactor can be operated under adiabatic or isothermal conditions.

[0029] The contact time for contacting the feedstream with the catalyst can depend on a number of factors including, but not limited to, the temperature, the pressure, the amount of catalyst, and the flowrate of reactants, i.e., C0 ₂ and H ₂ , in the feedstream. In certain embodiments, the feedstream can contact the catalyst for from about 1 second to about 10 minutes.

[0030] As noted previously, the reaction temperature can be a factor in determining the composition of the product stream. In certain embodiments, the reaction can be maintained at a temperature from about 500°C to about 800°C, or from about 600°C to about 700°C. For example, the reaction can be maintained at a temperature from about 600°C to about 650°C. In certain embodiments, the reaction can be carried out at a temperature of about 600°C, about 620°C, or about 640°C.

[0031] Syngas having various molar ratios of H ₂ to CO (H ₂ :CO) can be useful for different applications. For example, syngas having a high H ₂ to CO ratio, e.g., greater than about 4: 1, can be used for methanol synthesis. Syngas having a molar ratio of H ₂ to CO of about 2: 1 can be suitable for olefins synthesis. Syngas having a molar ratio of H ₂ to CO of less than about 2: 1 can be suitable for oxo synthesis or the production of monoethylene glycol. Although it is possible to adjust the H ₂ to CO ratio in syngas, e.g., by a subsequent reverse water gas shift reaction or by mixing the syngas with another gaseous stream, it can be desirable to generate a product stream from the hydrogenation reaction which contains a suitable H ₂ to CO ratio for a downstream use.

[0032] Methods according to the presently disclosed subject matter can produce a product stream containing syngas with various molar ratios of H ₂ to CO 103. For example, the product stream can contain syngas having a molar ratio of H ₂ to CO of less than 3 : 1, less than 2.8: 1, less than 2.6: 1, less than 2.4: 1, less than 2.2: 1, or less than 2: 1. In certain embodiments, the syngas can have a molar ratio of H ₂ to CO of about 1.5: 1, 1.4: 1, 1.3 : 1, 1.2: 1, 1.1 : 1, or 1 : 1. In particular embodiments, the syngas has a molar ratio of H ₂ to CO of about 1 : 1.

[0033] In certain embodiments, C0 ₂ is only partially converted to CO. For example, C0 ₂ conversion can be from about 15% to about 50%, from about 20% to about 40%, or from about 23% to about 30%. In certain embodiments, unconverted C0 ₂ in the product stream can be recovered and recycled to the feedstream. By way of example, C0 ₂ can be separated by an acid gas removal process. H ₂ 0 can be separated by condensation, i.e., by cooling the product stream.

[0034] The product stream produced by the presently disclosed methods can include CO, H ₂ , C0 ₂ and/or H ₂ 0. In certain embodiments, a method can include separating at least some C0 ₂ and/or H ₂ 0 from the CO and H ₂ in the product stream to produce purified syngas.

[0035] The syngas produced by the presently disclosed methods can be suitable for use in oxo synthesis reactions, which can alternatively be termed hydroformylation reactions. Alternatively or additionally, the syngas produced by the presently disclosed methods can be suitable for the production of monoethylene glycol.

[0036] The methods of the presently disclosed subject matter provide advantages over certain existing technologies for producing syngas from carbon dioxide. Exemplary advantages include the hydrogenation of carbon dioxide with improved catalyst stability and the production of syngas with improved composition. Particularly, as demonstrated in the examples below, the methods disclosed herein provide stable catalysts for the hydrogenation of carbon dioxide to form syngas having a molar ratio of H ₂ to CO of about 1 : 1, which can be suitable for downstream oxo synthesis and/or the production of monoethylene glycol.

[0037] The following examples provide methods of producing syngas from carbon dioxide in accordance with the disclosed subject matter. However, the following examples are merely illustrative of the presently disclosed subject matter and should not be considered as a limitation in any way.

Example 1: Hydrogenation of CO2 over Cu-Mn/AhOi catalyst

[0038] In this Example, carbon dioxide (C0 ₂ ) was hydrogenated in the presence of hydrogen (H ₂ ) to produce syngas. The hydrogenation of carbon dioxide (C0 ₂ ) was performed at a temperature of 600°C in the presence of a Cu/Mn-Al ₂ 03 catalyst. The catalyst loading was 8 mL. The flow rate of hydrogen (H ₂ ) was 14.56 cc/min and the flow rate of C0 ₂ was 15.44 cc/min. Table 1 displays the composition of the syngas after two and four days on stream, as well as the conversion of C0 ₂ and the H ₂ to CO ratio of the syngas. Table 1. Composition of Example 1 syngas.

[0039] After two and four days on stream, the Cu/Mn-Al ₂ 0 ₃ catalyst was stable and achieved conversion of C0 ₂ of about 39%. Additionally, the hydrogenation reaction produced syngas with a ratio of H ₂ :CO of about 2: 1.

Example 2: Hydrogenation of CO2 over Cu-Mn/AhOj catalyst

[0040] In this Example, a hydrogenation reaction was performed as in Example 1, but with different flow rates of C0 ₂ and H ₂ . The hydrogenation of C0 ₂ was performed at a temperature of 600°C in the presence of a Cu/Mn-Al ₂ 0 ₃ catalyst. The catalyst loading was 8 mL. The flow rate of H ₂ was 12 cc/min and the flow rate of C0 ₂ was 18 cc/min. Table 2 displays the composition of the syngas after two and four days on stream, as well as the conversion of C0 ₂ and the H ₂ to CO ratio of the syngas.

Table 2. Composition of Example 2 syngas.

[0041] After two and four days on stream, the Cu/Mn-Al ₂ 0 ₃ catalyst was stable and achieved conversion of C0 ₂ of about 33%. Additionally, the hydrogenation reaction produced syngas with a ratio of H ₂ : CO of about 1.4: 1.

Example 3: Hydrogenation of CO? over Cu-Mn/AhO _j catalyst

[0042] In this Example, a hydrogenation reaction was performed with the catalyst of Examples 1 and 2, but at a higher temperature and with different flow rates of C0 ₂ and H ₂ . The hydrogenation of C0 ₂ was performed at a temperature of 640°C in the presence of a Cu/Mn-Al ₂ 0 ₃ catalyst. The catalyst loading was 8 mL. The flow rate of H ₂ was 10 cc/min and the flow rate of C0 ₂ was 20 cc/min. Table 3 displays the composition of the syngas after two and four days on stream, as well as the conversion of C0 ₂ and the H ₂ to CO ratio of the syngas.

Table 3. Composition of Example 3 syngas.

[0043] After two and four days on stream, the Cu/Mn-Al ₂ 03 catalyst was stable and achieved conversion of C0 ₂ of about 30%. Additionally, the hydrogenation reaction produced syngas with a ratio of H ₂ :CO of about 1.2: 1.

Example 4: Hydrogenation of CO2 over Cu-Ce/AhO _j catalyst

[0044] In this Example, C0 ₂ was hydrogenated in the presence of H ₂ to produce syngas. The hydrogenation of C0 ₂ was performed at a temperature of 600°C in the presence of a Cu/Ce-Al ₂ 0 ₃ catalyst. The catalyst loading was 6 mL. The flow rate of H ₂ was 10 cc/min and the flow rate of C0 ₂ was 20 cc/min. Table 4 displays the composition of the syngas after two and four days on stream, as well as the conversion of C0 ₂ and the H ₂ to CO ratio of the syngas.

Table 4. Composition of Example 4 syngas.

[0045] After two and four days on stream, the Cu/Ce-Al ₂ 0 ₃ catalyst was stable and achieved conversion of C0 ₂ of about 28%. Additionally, the hydrogenation reaction produced syngas with a ratio of H ₂ :CO of about 1.5: 1. Example 5: Hydrogenation of CO2 over Cu-Ce/AhO _j catalyst

[0046] In this Example, a hydrogenation reaction was performed as in Example 4, but at a higher temperature. The hydrogenation of C0 ₂ was performed at a temperature of 620°C in the presence of a Cu/Ce-Al ₂ 03 catalyst. The catalyst loading was 6 mL. The flow rate of H ₂ was 10 cc/min and the flow rate of C0 ₂ was 20 cc/min. Table 5 displays the composition of the syngas after two and four days on stream, as well as the conversion of C0 ₂ and the H ₂ to CO ratio of the syngas.

Table 5. Composition of Example 5 syngas.

[0047] After two and four days on stream, the Cu/Ce-Al ₂ 03 catalyst was stable and achieved conversion of CO ₂ about 28%. Additionally, the hydrogenation reaction produced syngas with a ratio of H ₂ :CO of about 1.3 : 1.

Example 6: Hydrogenation of CO? over Cu-Ce/AhO _j catalyst

[0048] In this Example, a hydrogenation reaction was performed as in Examples 4 and 5, but at a higher temperature. The hydrogenation of CO ₂ was performed at a temperature of 640°C in the presence of a Cu/Ce-Al ₂ 03 catalyst. The catalyst loading was 6 mL. The flow rate of H ₂ was 10 cc/min and the flow rate of CO ₂ was 20 cc/min. Table 6 displays the composition of the syngas after two and four days on stream, as well as the conversion of CO ₂ and the H ₂ to CO ratio of the syngas. Table 6. Composition of Example 6 syngas.

[0049] After two and four days on stream, the Cu/Ce-Al ₂ 0 ₃ catalyst was stable and achieved conversion of C0 ₂ of about 30%. Additionally, the hydrogenation reaction produced syngas with a ratio of H ₂ :CO of about 1.3 : 1.

Example 7: Hydrogenation of CO2 over Cu-Ce/AhOa catalyst

[0050] In this Example, a hydrogenation reaction was performed as in Examples 6, but with different flow rates of C0 ₂ and H ₂ . The hydrogenation of C0 ₂ was performed at a temperature of 640°C in the presence of a Cu/Ce-Al ₂ 0 ₃ catalyst. The catalyst loading was 6 mL. The flow rate of H ₂ was 7.5 cc/min and the flow rate of C0 ₂ was 15 cc/min. Table 7 displays the composition of the syngas after two and four days on stream, as well as the conversion of C0 ₂ and the H ₂ to CO ratio of the syngas.

Table 7. Composition of Example 7 syngas.

[0051] After two and four days on stream, the Cu/Ce-Al ₂ 0 ₃ catalyst was stable and achieved conversion of C0 ₂ of about 23%. Additionally, the hydrogenation reaction produced syngas with a ratio of H ₂ : CO of about 1 : 1.

* * *

[0052] In addition to the various embodiments depicted and claimed, the disclosed subject matter is also directed to other embodiments having other combinations of the features disclosed and claimed herein. As such, the particular features presented herein can be combined with each other in other manners within the scope of the disclosed subject matter such that the disclosed subject matter includes any suitable combination of the features disclosed herein. The foregoing description of specific embodiments of the disclosed subject matter has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosed subject matter to those embodiments disclosed.

[0053] It will be apparent to those skilled in the art that various modifications and variations can be made in the systems and methods of the disclosed subject matter without departing from the spirit or scope of the disclosed subject matter. Thus, it is intended that the disclosed subject matter include modifications and variations that are within the scope of the appended claims and their equivalents.