CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of U.S. Provisional Application No. 62/255,968, filed November 16, 2015. The contents of the referenced application are incorporated into the present application by reference.

FIELD

[0002] The presently disclosed subject matter relates to methods and catalysts for conversion of carbon dioxide (C0 ₂ ) into synthesis gas (syngas) with simultaneous production of olefins and methanol.

BACKGROUND

[0003] Synthesis gas (also known as syngas) is a mixture of carbon monoxide (CO) and hydrogen (H ₂ ). Syngas can be prepared by reacting C0 ₂ with H ₂ . This process can be described as hydrogenation of C0 ₂ . C0 ₂ and H ₂ can react to form carbon monoxide (CO) and water (H ₂ 0) through a reverse water gas shift (RWGS) reaction. The RWGS reaction is endothermic and can be described by the following equation:

(1) C0 ₂ + H ₂ → CO + H ₂ 0

The RWGS reaction is reversible; the reverse reaction (from CO and H ₂ 0 to C0 ₂ and H ₂ ) is known as the water gas shift reaction. The RWGS reaction can be conducted under conditions that provide partial conversion of C0 ₂ and H ₂ , thereby creating an overall product mixture that includes C0 ₂ , H ₂ , CO, and H ₂ 0.

[0004] Syngas is a versatile mixture that can be used to prepare light olefins, methanol, acetic acid, aldehydes, and many other important industrial chemicals. For example, methanol synthesis from syngas can be described by Formula 1 in equilibrium with the following equations: (2) CO + 2H ₂ <→ CH ₃ OH

[0005] However, the efficiency of the preparation of different chemicals, for example, methanol versus light olefins, from syngas can depend on the composition of the syngas. Syngas containing H ₂ and CO in a molar ratio (H ₂ :CO) of about 2: 1 can be useful for olefin synthesis while a ratio of more than 4.5: 1 is useful for methanol synthesis. Different catalysts are often required to produce different industrial chemicals from the same syngas mixture.

[0006] Thus, there remains a need in the art for new methods and catalysts for conversion of C0 ₂ into syngas products compatible with both olefin and methanol synthesis, improved catalyst stability, and improved overall economy.

SUMMARY OF THE DISCLOSED SUBJECT MATTER

[0007] The presently disclosed subject matter provides a method of preparing syngas, which can include providing a reaction chamber that comprises a catalyst which can include Zn, Cu, and Zr and feeding a reaction mixture including H ₂ and C0 ₂ to the reaction chamber. The method can further include contacting H ₂ and C0 ₂ with the catalyst at a reaction temperature of about 500 to about 900 °C to provide a product mixture that includes H ₂ , CO, H ₂ 0, and unreacted C0 ₂ . The method can also include separating at least a portion of H ₂ 0 from the product mixture to provide syngas and separating at least a portion of the unreacted C0 ₂ , to provide a second C0 ₂ stream. The method can include subjecting syngas to a Fischer- Tropsch synthesis (FT) reaction to provide light olefins and subjecting the second C0 ₂ stream to methanol synthesis in the presence of a catalyst comprising Zn, Cu, and Zr to provide methanol.

[0008] In certain embodiments, the catalyst is Zn-Cu-Zr-O.

[0009] In certain embodiments, the reaction mixture comprises H ₂ and C0 ₂ in a molar ratio (H ₂ :C0 ₂ ) of about from about 3 : 1 to about 1 : 1. [0010] In certain embodiments, the reaction temperature of the syngas reaction is from about 600 °C to about 800 °C. In certain embodiments, the reaction temperature is about 600 °C, about 650 °C, about 680 °C, or about 800 °C.

[0011] In certain embodiments, the reaction temperature of methanol synthesis is greater than about 200 °C. In certain embodiments, the reaction temperature is about 250 °C or 240 °C.

[0012] In certain embodiments, the syngas product mixture comprises less than about 30% C0 ₂ , by mole or less than about 12% C0 ₂ , by mole.

[0013] In certain embodiments, the CO ₂ conversion of methanol synthesis is from about 10%) to about 15%> by mol. In certain embodiments, the CO ₂ conversion about 10%> or about 14.3% by mol.

[0014] In certain embodiments, the methanol selectivity is from about 35% to about 45% by mol. In certain embodiments, the methanol selectivity is about 36.4% or about 44.1%> by mol.

[0015] In certain embodiments, FT synthesis and methanol synthesis proceed simultaneously.

[0016] In certain embodiments, pressure of the syngas reaction is from about 1 to about 25 bar.

[0017] In certain embodiments, the pressure of the methanol synthesis reaction is from about 50 to about 55 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 is a schematic representation of one exemplary process of the presently disclosed subject matter. DETAILED DESCRIPTION

[0019] There remains a need in the art for new methods and catalysts for preparing olefins and methanol from syngas. The presently disclosed subject matter provides novel methods of converting C0 ₂ and H ₂ into syngas with a product mixture compatible with both olefin synthesis and methanol synthesis. The presently disclosed subject matter also provides improved methods of preparing light olefins and methanol. The presently disclosed subject matter includes the surprising discovery that Zinc-Copper-Zirconium-Oxide (Zn-Cu-Zr-O) catalysts can be utilized both for preparation of syngas and synthesis of methanol. Such catalysts can exhibit long-term stability.

[0020] 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. Reactors and Reaction Chambers

[0021] The methods of the present disclosure can involve fixed bed isothermal or adiabatic reactors suitable for reactions of gaseous reactants and reagents catalyzed by solid catalysts. The reactor can be constructed of any suitable materials capable of holding high temperatures, for example from about 200°C to about 800°C. Non-limiting examples of such materials can include metals, alloys (including steel), glasses, ceramics or glass lined metals, and coated metals. The reactor can also include a reaction vessel enclosing a reaction chamber.

[0022] The dimensions of the reaction vessel and reaction chamber are variable and can depend on the production capacity, feed volume, and catalyst. The geometries of the reactor can be adjustable in various ways known to one of ordinary skill in the art.

[0023] In certain embodiments, reaction conditions within the reaction chamber can be isothermal. That is, hydrogenation of C0 ₂ can be conducted under isothermal conditions. In certain alternative embodiments, a temperature gradient can be established within the reaction chamber. For example, hydrogenation of C0 ₂ can be conducted across a temperature gradient using an adiabatic reactor.

[0024] The pressure within the reaction chamber can be varied, as is known in the art. In certain embodiments, the pressure within the reaction chamber can be atmospheric pressure, i.e., about 1 bar. In certain embodiments, the pressure within the reaction chamber can be from about 1 bar to about 60 bar. In certain embodiments, the pressure within the reaction chamber can be from about 20 bar to about 55 bar. In certain embodiments, the pressure within the reaction chamber can be from about 50 bar to about 55 bar. In certain embodiments, the pressure within the reaction chamber can be about 25 bar.

Catalysts

[0025] Catalysts suitable for use in conjunction with the presently disclosed matter can be catalysts capable of catalyzing RWGS reactions, i.e., hydrogenation of C0 ₂ and capable of catalyzing methanol synthesis. In certain embodiments, the catalyst can be a solid catalyst, e.g., a solid-supported catalyst. The catalyst can be a metal oxide or mixed metal oxide. In certain embodiments, the catalyst can be located in a fixed packed bed, i.e., a catalyst fixed bed. In certain embodiments, the catalyst can include solid pellets, granules, plates, tablets, or rings.

[0026] In certain embodiments, the catalyst can include one or more transition metals. The catalyst can include zinc (Zn), copper (Cu), and/or zirconium (Zr). In certain embodiments, the catalyst can include Zn, Cu, Zr and O. In certain embodiments, the catalyst can include Zn, Cu, and Zr in a molar ratio of about 10: 1 : 1 to about 1 : 1 : 10, about 1 : 10: 1 to about 10: 10: 1, or about 1 : 10: 10 (Zn:Cu:Zr). [0027] In certain embodiments, the catalyst can include a solid support. That is, the catalyst can be solid-supported. By way of non-limiting example, the solid support can constitute about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, or 95% of the total weight of the catalyst.

[0028] In certain embodiments, the catalyst can include one or more additional metals in addition to Zn, Cu, and Zr. The additional metal(s) can include lanthanum (La), calcium (Ca), potassium (K), tungsten (W), and/or aluminum (Al). In certain embodiments, the additional metal(s) can be present in an amount between about 1% and 25%, relative to the total weight of the catalyst. For example, the catalyst can include about 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 18%, 20%, 22%, or 25% of the additional metal(s), by weight. The remainder of the catalyst can be oxygen (i.e., the oxygen present in a metal oxide) and solid support (e.g., A1 ₂ 0 ₃ ).

[0029] The catalysts of the presently disclosed subject matter can be prepared according to various techniques known in the art. For example, metal oxide catalysts suitable for use in RWGS reactions can be prepared from various metal nitrates, metal halides, metal salts of organic acids, metal hydroxides, metal carbonates, metal oxyhalides, metal sulfates, and the like. In certain embodiments, a transition metal oxide (e.g., a Zn, Cu or Zr oxide, or a mixed Zn/Cu/Zr oxide) can be precipitated along with a solid support (e.g., A1 ₂ 0 ₃ ). In certain embodiments, and as exemplified in the Examples below, catalysts can be prepared by precipitation of metal nitrates.

Reaction Mixtures

[0030] The presently disclosed subject matter provides methods of converting mixtures of H ₂ and C0 ₂ into syngas via the reverse water gas shift (RWGS) reaction. A mixture of H ₂ and C0 ₂ can be termed a "reaction mixture." The mixture of H ₂ and C0 ₂ can alternatively be termed a "feed mixture" or "feed gas." [0031] The C0 ₂ in the reaction mixture can be derived from various sources. In certain embodiments, the C0 ₂ can be a waste product from an industrial process. In certain embodiments, CO ₂ that remains unreacted in the RWGS reaction can be recovered and utilized in the methanol synthesis reaction.

[0032] Reaction mixtures suitable for use with the presently disclosed methods can include various proportions of H ₂ and CO ₂ . In certain embodiments, the reaction mixture can include H ₂ and CO ₂ in a molar ratio (H ₂ :C0 ₂ ) between about 5:1 and about 1:2, e.g., about 5:1, 4:1, 3:1, 2.8:1, 2.6:1, 2.5:1, 2.4:1, 2.3:1, 2.2:1, 2.1:1, 2:1, 1.9:1, 1.8:1, 1.7:1, 1.6:1, 1.5:1, 1.4:1, 1.3:1, 1.2:1, 1.1:1, 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, or 1:2. The reaction mixture can include H ₂ and CO ₂ in a molar ratio (H ₂ :C0 ₂ ) of about 3 : 1 to about 1:1. In certain embodiments, the reaction mixture can include H ₂ and CO ₂ in a molar ratio (H ₂ :C0 ₂ ) of about 2.86:1, 2.95:1, 1.6:1 or 1.5:1.

Methods of Preparing Syngas, Light Olefins and Methanol

[0033] The methods of the presently disclosed subject matter include methods of preparing syngas. In one embodiment, an exemplary method can include providing a reaction chamber, as described above. The reaction chamber can include a catalyst, as described above. The method can further include feeding a reaction mixture, as described above, to the reaction chamber. The method can additionally include contacting H ₂ and CO ₂ (present in the reaction mixture) with the catalyst at a reaction temperature from about 200 °C to about 800 °C, thereby inducing a RWGS reaction to provide a product mixture that includes H ₂ and CO. The product mixture can further include H ₂ 0 (a product of the RWGS reaction, as shown in Equation 1) and unreacted CO ₂ . The method can further include separating the unreacted CC^for synthesis of methanol.

[0034] CO ₂ and H ₂ can be fed into the reaction chambers at various flow rates. The flow rate can be varied, as is known in the art. In certain embodiments, the flow rate can be from about 1 to about 500 cc/min. In certain embodiments, the flow rate can be from about 10 to about 125 cc/min. In certain embodiments, the flow rate can be about 11.2, 16.6, 18.8, 25, 30.8, 32, 42, or 124 cc/min.

[0035] The reaction temperature can be understood to be the temperature within the reaction chamber, i.e., for hydrogenation or methanol synthesis. The reaction temperature can influence the RWGS reaction, including conversion of C0 ₂ and H ₂ , the ratio of H ₂ :CO in the product mixture, and the overall yield. In certain embodiments, the RWGS reaction temperature can be greater than 600 °C, e.g., greater than about 610 °C, 620 °C, 630 °C, 640 °C, 650 °C, 660 °C, 680 °C, or 690 °C. In certain embodiments, the RWGS reaction temperature can be from about 900 °C and about 600 °C. In certain embodiments, the reaction temperature can be about 650 °C. In certain embodiments, the reaction temperature can be about 600 °C. In certain embodiments, the reaction temperature can be about 680 °C. In certain embodiments, the reaction temperature can be about 800 °C. In certain embodiments, the methanol synthesis reaction temperature can be greater than 200 °C, e.g., greater than about 210 °C, 220 °C, 230 °C, 240 °C, or 250 °C. In certain embodiments, the reaction temperature can be about 250 °C. In certain embodiments, the reaction temperature can be about 240 °C.

[0036] The RWGS can proceed with partial conversion of C0 ₂ and H ₂ , thus providing a product mixture that includes CO, H ₂ 0, C0 ₂ , and H ₂ . In certain embodiments, the RWGS reaction can be performed from about 50% to about 70% conversion of C0 ₂ . In certain embodiments, the RWGS reaction can be performed to greater than about 70% conversion of C0 ₂ . In certain embodiments, the RWGS reaction can be performed to about 49.5%, or about 67.5%) conversion of C0 ₂ . In certain embodiments, the RWGS reaction can be performed to about 49.5%, 50.6%, 57.6%, 58.1%, 60.8%, 64.8%, 65.7%, 67.4% or 67.5% conversion of C0 ₂ . Adjustment of the degree of conversion of C0 ₂ and H ₂ as well as adjustment of the ratio of C0 ₂ and H ₂ in the reaction mixture can therefore influence the ratio of H ₂ and CO in the syngas product formed. For example, use of a higher molar ratio of H ₂ :C0 ₂ in the reaction mixture can increase the molar ratio of H ₂ :CO in the product mixture.

[0037] In certain embodiments, the product mixture can include H ₂ and CO in a molar ratio (H ₂ :CO) of about 0.5: 1 to about 5: 1. In certain embodiments, the product mixture can include H ₂ and CO in a molar ratio (H ₂ :CO) of about 1 : 1 to about 3 : 1, e.g., about 1 : 1, 1.1 : 1, 1.2: 1, 1.3 : 1, 1.4: 1, 1.5: 1, 1.6: 1, 1.7: 1, 1.8: 1, 1.9: 1, 2: 1, 2.1 : 1, 2.2: 1, 2.3 : 1, 2.4: 1, 2.5: 1, 2.6: 1, 2.7: 1, 2.8: 1, 2.9: 1, or 3 : 1. In certain embodiments, the product mixture can include H ₂ and CO in a molar ratio (H ₂ :CO) of from about 1.5: 1 to about 3 : 1, from about 2: 1 to about 3 : 1, or about 2.5: 1. As noted above, the molar ratio (H ₂ :CO) of the product mixture can be influenced by the molar ratio (H ₂ :C0 ₂ ) of the reaction mixture.

[0038] In certain embodiments, the RWGS can be performed to relatively high conversion. That is, the amount of C0 ₂ present in the product mixture can be relatively low. In certain embodiments, the product mixture can include less than about 30% C0 ₂ , by mole or less than about 12%) C0 ₂ , by mole. For example, the product mixture can include about 29%>, 28%>, 27%, 21%, 20%, 19%, 18%, 17%, 16%, 15%, 14%, 13%, 12%, 11%, 10%, 9%, 8% by mole. In certain embodiments, the product mixture can include about 25.4% C0 ₂ by mole. In certain embodiments, the product mixture can include about 9.80% C0 ₂ by mole.

[0039] In certain embodiments, the methods of the presently disclosed subject matter can include separating at least a portion of C0 ₂ from the product mixture, to provide purified syngas. C0 ₂ can be separated by various techniques known in the art. In certain embodiments, C0 ₂ can be removed from the product mixture and contributed to the methanol formation reaction mixture, thereby recycling C0 ₂ and improving overall economy of the process. [0040] The presently disclosed subject matter also provides methods of preparing light olefins. In one embodiment, an exemplary method of preparing light olefins can include conducting a RWGS reaction to convert C0 ₂ and H ₂ into a product mixture that includes H ₂ , CO, C0 ₂ , and H ₂ 0, as described above. The method can additionally include separating at least a portion of C0 ₂ and/or H ₂ 0 from the product mixture, to provide purified syngas. The method can further include subjecting purified syngas to a Fischer-Tropsch synthesis (FT) reaction to provide light olefins.

[0041] The presently disclosed subject matter also provides methods of preparing methanol. In one embodiment, an exemplary method of preparing methanol can include conducting a RWGS reaction to convert C0 ₂ and H ₂ into a product mixture that includes H ₂ , CO, C0 ₂ , and H ₂ 0, as described above. The method can additionally include separating at least a portion of C0 ₂ from the product mixture. The method can further include subjecting the separated C0 ₂ to a methanol reaction simultaneous with the FT reaction. The method can include using the same catalyst for both the RWGS and methanol synthesis reactions.

[0042] In certain embodiments, the methanol reaction can proceed to high methanol selectivity. In certain embodiments, the methanol selectivity is from about 1 to about 50% mol. In certain embodiments, the methanol selectivity is from about 10 to about 45% mol. In certain embodiments, the methanol selectivity is about 36.4 or about 44.1% mol. In certain embodiments, the methanol reaction can result in high CO selectivity. In certain embodiments, the CO selectivity is from about 1 to about 70% mol. In certain embodiments, the CO selectivity is from about 50 to about 65% mol. In certain embodiments, the CO selectivity is about 63.6 or about 55.9% mol. In certain embodiments, C0 ₂ conversion of the methanol reaction is from about 1 to about 20% mol. In certain embodiments, C0 ₂ conversion of the methanol reaction is from about 10 to about 15% mol. In certain embodiments, the methanol selectivity is about 10 or about 14.3% mol. [0043] For the purpose of illustration and not limitation, FIG. 1 is a schematic representation of an exemplary process 100 according to the disclosed subject matter. In certain embodiments, a RWGS reaction can be integrated with a FT reaction and methanol synthesis. As shown in FIG. 1, a reaction mixture 101 can form syngas 103 in the presence of a catalyst 102. The process can include separation 104 of unreacted C0 ₂ from the syngas product mixture 103. Unreacted C0 ₂ can be combined 107 with additional H ₂ 110 and reacted in the presence of a catalyst 108 to produce methanol 109. Simultaneously, syngas 103 can undergo FT synthesis 105, to produce light olefins 106. In certain embodiments, the catalyst 102 and 108 are the same catalyst.

[0044] The methods of the presently disclosed subject matter can have advantages over other techniques for preparation of syngas and preparation of light olefins. The presently disclosed subject matter includes the surprising discovery that catalysts containing Zr, Cu, and Zn can be used to promote both RWGS reactions and methanol synthesis.

[0045] As demonstrated in the Examples, the methods of the presently disclosed subject matter can provide syngas containing H ₂ and CO in a molar ratio suitable for use in FT reactions. Moreover, the methods of the presently disclosed subject matter can prepare methanol simultaneously with the FT reaction, with the same catalyst used to prepare syngas. Additional advantages of the presently disclosed subject matter can include improved energy efficiency and overall economy.

EXAMPLES

EXAMPLE 1 - Hvdrogenation at 650 °C

[0046] In this example, C0 ₂ undergoes hydrogenation at 650 °C.

[0047] C0 ₂ at a flow rate of 11.2 cc/minute and H ₂ at a flow rate of 32 cc/min are reacted in a quartz reactor with a catalyst loading of 8mL Zn-Cu-Zr-0 at a temperature of 650 °C. The results are summarized in Table 1. Table 1. Hydrogenation reaction products in mol %

EXAMPLE 2 - Hydrogenation at 600 °C

[0048] In this example, C0 ₂ undergoes hydrogenation at 600 °C.

[0049] C0 ₂ at a flow rate of 11.2 cc/minute and H ₂ at a flow rate of 32 cc/min are reacted in a quartz reactor with a catalyst loading of 8mL Zn-Cu-Zr-0 at a temperature of 600 °C. The results are summarized in Table 2.

Table 2. Hydrogenation reaction products in mol % EXAMPLE 3 - Hydrogenation at 680 °C

[0050] In this example, C0 ₂ undergoes hydrogenation at 680 °C.

[0051] C0 ₂ at a flow rate of 18.8 cc/minute and H ₂ at a flow rate of 30.8 cc/min are reacted in a quartz reactor with a catalyst loading of 8mL Zn-Cu-Zr-0 at a temperature of 680 °C. The results are summarized in Table 3. Table 3. Hydrogenation reaction products in mol % EXAMPLE 4 - Hydrogenation at 680 °C

[0052] In this example, C0 ₂ undergoes hydrogenation at 680 °C.

[0053] C0 ₂ at a flow rate of 16.6 cc/minute and H ₂ at a flow rate of 25 cc/min are reacted in a quartz reactor with a catalyst loading of 8mL Zn-Cu-Zr-0 at a temperature of 680 °C. The results are summarized in Table 4.

Table 4. Hydrogenation reaction products in mol %

EXAMPLE 5 - Hydrogenation at 800 °C

[0054] In this example, C0 ₂ undergoes hydrogenation at 800 °C.

[0055] C0 ₂ at a flow rate of 16.6 cc/minute and H ₂ at a flow rate of 25 cc/min are reacted in a quartz reactor with a catalyst loading of 8mL Zn-Cu-Zr-0 at a temperature of 800 °C. The results are summarized in Table 5.

Table 5. Hydrogenation reaction products in mol % EXAMPLE 6 - Methanol synthesis

[0056] In this example, methanol is synthesized.

[0057] Unreacted C0 ₂ from a RWGS reaction is reacted at a flow rate of 42 cc/minute with H ₂ at a flow rate of 124 cc/min in the presence of a Zn-Cu-Zr-0 catalyst at a temperature of 250 °C and a pressure of 750psi. The reaction products include C0 ₂ , H ₂ , CH ₃ OH and H ₂ 0. The results are summarized in Table 6. Table 6. Methanol synthesis products in mol %

EXAMPLE 7 - Methanol synthesis at 240 °C

[0058] In this example, methanol is synthesized.

[0059] Unreacted C0 ₂ from a RWGS reaction is reacted at a flow rate of 42 cc/minute with H ₂ at a flow rate of 124 cc/min in the presence of a Zn-Cu-Zr-0 catalyst at a temperature of 240 °C and a pressure of 750psi. The reaction products include C0 ₂ , H ₂ , CH3OH and H ₂ 0. The results are summarized in Table 7.

Table 7. Methanol synthesis products in mol %

[0060] The only byproduct of this reaction was CO. The CO can be recycled within the reaction to undergo conventional methanol synthesis as summarized in Formula 2, discussed above.

* * * [0061] Although the presently disclosed subject matter and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosed subject matter as defined by the appended claims. Moreover, the scope of the disclosed subject matter is not intended to be limited to the particular embodiments described in the specification. Accordingly, the appended claims are intended to include within their scope such alternatives.