Alkali metal doped molybdenum carbide supported on gamma-alumina for selective CO2 hydrogenation into CO

Abstract

A supported heterogeneous catalyst material for catalyzing the reverse water-gas shift (RWGS) reaction for the selective formation of CO using an alkali metal-doped molybdenum carbide on a gamma alumina support (A-Mo2C/γ-Al2O3, A=K, Na, Li). The A-Mo2C/γ-Al2O3 catalyst is synthesized by co-impregnation of molybdemun and alkali metal precursors onto a γ-Al2O3 support. It is then carburized to form the A-Mo2C/γ-Al2O3.

Description

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to CO₂ hydrogenation reactions to selectively produce CO via the reverse water-gas shift (RWGS) reaction for down-stream hydrocarbon synthesis.

Description of the Prior Art

The high concentration of CO₂ in seawater, ca. 100 mg L⁻¹, represents a significant opportunity to extract and use this CO₂ as a C₁ feedstock for synthetic fuels. Through an existing process patented by the U.S. Navy (U.S. Pat. No. 9,303,323), CO₂ and H₂ can be concurrently extracted from seawater and used as reactants for direct Fischer-Tropsch from CO₂ (CO₂-FT) to produce valuable oxygenates, specialty chemicals and intermediate hydrocarbons (C₂-C₆) for synthetic fuel. (Wang et al., Chem. Soc. Rev. 40, 3703-3727 (2011) and Centi et al., Today, 148, 191-205 (2009)). If the energy input is nuclear or renewable, the entire process can be considered CO₂-neutral. (Willauer et al., J. Renew. and Sustain. Energ., 4, 033111 (2012)).

The most commonly used catalysts for CO₂-FT are slight variations of Fe and Co-based Fischer-Tropsch (FT) catalysts, which show promise, but are not specifically designed for the CO₂ reactant. (Kaiser et al., Chem-Ing-Tech, 85, 489-499 (2013), Chakrabarti et al., Ind. Eng. Chem. Res., 54, 1189-1196 (2015), and Dorner et al., Energ. Environ. Sci., 3, 884-890 (2010)). The current optimal catalyst, K—Mn—Fe/Al₂O₃, achieves a CO₂ conversion of 41.4% and a selectivity towards C₂-C₅+ hydrocarbons of 62.4% at a gas hourly space velocity (GHSV) of 0.0015 L g⁻¹ s⁻¹, but the mechanism is poorly understood, making catalyst improvements challenging. (Dorner et al., Appl. Catal. A-Gen., 373, 112-121 (2010)). There is some consensus that an Fe carbide formed during the reaction is the catalytically active phase (Lee et al., J. Mol. Catal. A-Chem., 301, 98-105 (2009)); however, reports also state that Fe catalysts are poisoned by water, an unavoidable byproduct, negatively influencing catalytic activity and product selectivity. (Riedel et al., Appl. Catal. A-Gen., 186, 201-213 (1999) and Willauer et al., J. CO2 Util., 3-4, 56-64 (2013)). Conversely, Co-based catalysts are water tolerant (Schulz et al., in Studies in Surface Science and Catalysis, Vol. 107 (Eds.: dePontes et al.), Elsevier, pp. 193-200 (1997)) and modifying an Fe catalyst with Co improves catalytic performance and selectivity towards C₂+ hydrocarbon products. (Satthawong et al., Catal. Today, 251, 34-40 (2015) and Satthawong et al., Top. Catal., 57, 588-594 (2014)). Improvements have also been made to Fe-based catalysts by adding Cu, which enhances CO₂-FT activity and selectivity. (Satthawong et al., Top. Catal., 57, 588-594 (2014)).

Although there are promising catalysts for CO₂-FT, the structure-property relationships that control activity and selectivity to intermediate hydrocarbons are not well studied. (Porosoff et al., Energ. Environ. Sci., 9, 62-73 (2016)). Furthermore, because of the complexity of CO₂-FT, the alternative route of feeding CO produced from reverse water-gas shift (RWGS) into a FT reactor must also be considered. For industrial RWGS, operating temperatures are very high, typically at or above 600° C. at 2.8 MPa, over ZnO/Al₂O₃ and ZnO/Cr₂O₃ catalysts. Because methane (CH₄) is thermodynamically favored below 600° C., these catalysts require high temperatures to selectively produce CO, which also results in substantial deactivation. (Joo et al., Ind. Eng. Chem. Res., 38, 1808-1812 (1999) and Park et al., Journal of Chemical Engineering, 17, 719-722 (2000)). To make fuel synthesis from CO₂ viable, a low-cost and stable RWGS catalyst is first required, which can achieve high selectivity to CO over a wide range of conversion and operating temperatures.

Recently, Pt-based catalysts have been investigated for RWGS (Kattel et al., Angew. Chem. Int. Edit., 128, 8100-8105 (2016) and Porosoff et al., J. Catal., 301, 30-37 (2013)), but they are expensive, and thus, unviable for an industrial scale CO₂ conversion process. As an alternative, transition metal carbides (TMCs) are low-cost, with similar electronic properties to precious metals. (Levy et al., Science, 181, 547-549 (1973) and Porosoff et al., Chem. Comm., 51, 6988-6991 (2015)). Density functional theory (DFT) calculations over the TMC, molybdenum carbide (Mo₂C) demonstrate that Mo-terminated Mo₂C has many properties similar to transition metals including Ru, Fe, Co and Ni catalysts, all of which are active for CO₂ conversion. (Medford et al., J. Catal., 290, 108-117 (2012)). DFT calculations by Shi et al. further illustrate that CO₂ dissociation (CO₂→CO+O) is more favorable than CO₂ hydrogenation (CO₂+H→HCOO or COOH) over Mo₂C, suggesting high CO selectivity. (Shi et al., Appl. Catal. A-Gen., 524, 223-236 (2016)). Reactor experiments over unsupported-Mo₂C powder catalysts for RWGS at 300° C. and 0.11 MPa show 8.7% conversion and 93.9% selectivity towards CO (Porosoff et al., Angew. Chem. Int. Edit., 53, 6705-6709 (2014)), confirming the DFT calculations. Another study over Mo₂C nanowires also reports high activity and CO selectivity at 600° C. (Gao et al., Catal. Comm., 84, 147-150 (2016)). The high intrinsic activity of Mo₂C originates from CO₂ binding in a bent configuration, leading to spontaneous breakage of a C═O bond, leaving CO and O bound to the surface. (Posada-Perez et al., Phys. Chem. Chem. Phys., 16, 14912-14921 (2014)). The CO can desorb from the surface, while the oxy-carbide (O—Mo₂C) is restored to the active carbide through hydrogenation. (Porosoff et al., Angew. Chem. Int. Edit., 53, 6705-6709 (2014)).

Mo₂C can also be modified with metal nanoparticles (Cu, Co, Ni), which influence the product selectivity, leading to MeOH with Cu (Posada-Perez et al., Catal. Sci. Technol., 6, 6766-6777 (2016)), C₂+ hydrocarbons with Co and CH₄ with Ni. (Griboval-Constant et al., Appl. Catal. A-Gen., 260, 35-45 (2004) and Xu et al., Catal. Lett., 145, 1365-1373 (2015)). Because modifying Mo₂C with a metal promoter can further tune the selectivity between MeOH, C₂+ hydrocarbons or CH₄, it may be possible to modify Mo₂C to selectively produce even more CO across a wide range of conversions and temperatures. Experimental and theoretical studies suggest that potassium (K) promoters increase the binding energy, and therefore, reactivity of CO₂, thereby promoting C═O bond scission and formation of CO. (Solymosi et al., Catal. Lett., 66, 227-230 (2000) and Pistonesi et al., Catal. Today, 181, 102-107 (2012)).

Molybdenum carbide has been employed as a catalyst for CO₂ hydrogenation as a pure material, supported on γ-Al₂O₃ and when modified with various metals (Co, Ni, Fe). It has been used as an alternative to precious metals for many catalytic reactions, and more recently has been applied to CO₂ hydrogenation. CO₂ hydrogenation over these previous catalysts is comparable to the current invention; however, the selectivity and yield to CO is significantly lower.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a class of catalysts for CO₂ hydrogenation via the RWGS reaction to selectively produce CO for down-stream hydrocarbon synthesis. Alkali metal-doped molybdenum carbide, supported on gamma alumina (A-Mo₂C/γ-Al₂O₃, A=K, Na, Li), is synthesized by co-impregnation of (NH₄)6Mo₇O₂₄.4H₂O and A-NO₃ precursors (A=K, Na, Li) onto a γ-Al₂O₃ support. The A-Mo/γ-Al₂O₃ catalyst is then carburized to form the A-Mo₂C/γ-Al₂O₃.

Alkali metal-promoted molybdenum carbide supported on gamma alumina is a low-cost, stable and highly selective catalyst for RWGS over a wide range of conversion. These findings are supported by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations.

These and other features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description, appended claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the synthesis procedure for alkali metal doped molybdenum carbide supported on gamma alumina.

FIG. 2A is a low magnification scanning electron microscopy (SEM) image of K—Mo₂C/γ-Al₂O₃. FIG. 2B is a high magnification SEM image of K—Mo₂C/γ-Al₂O₃.

FIG. 3 is a schematic of a reactor set-up for CO₂ hydrogenation.

FIG. 4A is a plot of CO₂ conversion versus time for the Mo₂C and A-Mo₂C (A=K, Na, Li) supported on γ-Al₂O₃. FIG. 4B is a plot of production of CO and CH₄ versus time for Na—Mo₂C/γ-Al₂O₃, Li—Mo₂C/γ-Al₂O₃ and Mo₂C/γ-Al₂O₃.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a supported heterogeneous catalyst material for catalyzing the RWGS reaction for the selective formation of CO. The catalyst has a support material of γ-Al₂O₃ and an active material of alkali-metal doped molybdenum carbide. The alkali-metal component of the active material may comprise one or more alkali-metal precursors in elemental form or in the form of oxides, with the metals being K, Na, Li, or any combination thereof. The molybdenum component of the active material may comprise one or more molybdenum precursors in the form of carbides, oxycarbides, oxides, elemental molybdenum, or any combination thereof.

FIG. 1 shows the synthesis procedure for alkali metal doped molybdenum carbide supported on gamma alumina. Alkali metal-doped molybdenum carbide, supported on gamma alumina (A-Mo₂C/γ-Al₂O₃, A=K, Na, Li) was synthesized by co-impregnation of (NH₄)6Mo₇O₂₄.4H₂O and A-NO₃ precursors (A=K, Na, Li) onto a γ-Al₂O₃ support by the evaporation deposition method. In brief, the precursors were dissolved in deionized water at the concentrations required to obtain molar ratios of 1/4/15 A/Mo/γ-Al₂O₃, which translates to 2% potassium (K), 1.2% sodium (Na), 0.4% lithium (Li) and 20.8% Mo loading on the γ-Al₂O₃ support. Aqueous solutions of the metal precursors were added to a beaker of γ-Al₂O₃ and dried overnight under stirring at 60° C., then calcined in air overnight at 350° C.

The A-Mo/γ-Al₂O₃ catalyst was then carburized in a 21% CH₄ in H₂ mixture at 600° C. for 2.5 hours to form the A-Mo₂C/γ-Al₂O₃. After the first 1.5 hour, the CH₄ was shut off and the carbide was cooled to room temperature in H₂. At room temperature, the catalyst was passivated in 1% O₂ in N₂ for several hours. FIG. 2A shows a low magnification scanning electron microscopy (SEM) image of K—Mo₂C/γ-Al₂O₃, and FIG. 2B shows a high magnification SEM image of K—Mo₂C/γ-Al₂O₃.

CO₂ hydrogenation via the RWGS reaction is performed while flowing carbon dioxide, hydrogen gas, or any combination thereof over the A-Mo₂C/γ-Al₂O₃ catalyst material. FIG. 3 shows a schematic of a reactor set-up for CO₂ hydrogenation. In the CO₂ hydrogenation experiment, 500 mg of A-Mo₂C/γ-Al₂O₃ was loaded into a ¼ in stainless steel reactor and reduced under 50 sccm H₂ at 50 psig for 2.5 h at 300° C. After reduction, the reactor was isolated and the bypass pressurized to 290 psig with 6.3 sccm CO₂, 18.9 sccm H₂ and 5.0 sccm N₂, for a H₂:CO₂ ratio of 3:1. At 290 psig, concentration of the reactants in the bypass was recorded as a baseline and gases were flowed into the reactor. Reactions were run for 22 h at 300° C. and concentrations of reactants and products were measured by an inline gas chromatograph.

Table 1 shows a summary of performance of Mo₂C and A-Mo₂C (A=K, Na, Li) supported on γ-Al₂O₃ for CO₂ hydrogenation. FIG. 4A shows a plot of CO₂ conversion versus time for the Mo₂C and A-Mo₂C (A=K, Na, Li) supported on γ-Al₂O₃, and FIG. 4B shows a plot of production of CO and CH₄ versus time for Na—Mo₂C/γ-Al₂O₃, Li—Mo₂C/γ-Al₂O₃ and Mo₂C/γ-Al₂O₃. The CO₂ hydrogenation via the RWGS reaction can achieve a CO yield of 12% or greater and a CO selectivity of 90% or greater.

TABLE 1
Catalyst	Conversion/%	CO Selectivity/%	CO Yield/%
Mo2C/γ-Al2O3	19.9	73.5	14.6
K—Mo2C/γ-Al2O3	17.2	95.9	16.5
Na—Mo2C/γ-Al2O3	19.6	86.3	16.9
Li—Mo2C/γ-Al2O3	19.8	62.1	12.3

The increased CO yield from doping a Mo₂C/γ-Al₂O₃ catalyst with alkali metals offers an improved route for CO production from CO₂. The best currently available catalysts can only achieve a CO yield and selectivity of 14.6% and 75% at 300° C., respectively, while K—Mo₂C/γ-Al₂O₃ reaches a CO yield and selectivity of 16.5% and 96%, respectively. Selectively producing CO from CO₂ enables a facile route to synthesize synthetic hydrocarbons from CO₂ through down-stream Fischer-Tropsch.

Na—Mo₂C/γ-Al₂O₃ reaches a similar CO yield to K—Mo₂C/γ-Al₂O₃, while Li—Mo₂C/γ-Al₂O₃ shows a lower selectivity to CO than Mo₂C/γ-Al₂O₃. Maintaining the same A:Mo weight ratio in Li—Mo₂C/γ-Al₂O₃ results in a significantly lower weight fraction of Li because of the lower atomic weight of Li relative to Na and K. It is possible this lower amount of dopant results in the lower CO selectivity for Li—Mo₂C/γ-Al₂O₃. The Li:Mo and Na:Mo ratios can be further optimized.

The addition of K to catalysts as a promoter has not yet been recorded with a Mo₂C-based catalyst for CO₂ hydrogenation. Furthermore, doping Mo₂C-based catalysts with Li and Na has not been attempted in literature for CO₂ hydrogenation. By doping Mo₂C/γ-Al₂O₃ with alkali metals, CO selectivity substantially increases for K and Na, which is likely caused by attenuation of the electronic properties of the Mo₂C phase. These electronic effects are only present when Mo₂C is doped with a small amount of alkali metal, thereby attenuating the CO binding energy and preventing further hydrogenation into CH₄ or other hydrocarbons.

A-Mo₂C/γ-Al₂O₃ (A=K, Na, Li) was also tested at other temperatures (250-1000° C.), other alkali metal loadings (0.1-15%), other Mo loadings (1-70%), carburization temperatures (400-1000° C.) on other supports (SiO₂, TiO₂, ZrO₂), gas compositions (CO₂:H₂=1:1, 1:2, 1:3) and pressures (0-350 psig). Higher temperature improves conversion for K—Mo₂C/γ-Al₂O₃ to 28.6%, without the expense of CO selectivity (94.8%). Increasing K loading to 5% increases CO selectivity to 99.4% at the expense of conversion (3.8%). Higher Mo loading lowers conversion to 6.6% and raises selectivity slightly to 97.8%.

The exact optimal metal loading and A:Mo (A=K, Na, Li) ratio on the γ-Al₂O₃ support can be further optimized based on this finding of such high CO selectivity, especially over Na—Mo₂C/γ-Al₂O₃ and K—Mo₂C/γ-Al₂O₃.

Example

In this example, kinetic experiments and characterization tools were combined with DFT calculations to probe the catalytic properties of K-promoted Mo₂C and understand the reaction mechanisms of CO₂ dissociation. Flow reactor results indicate that K—Mo₂C/γ-Al₂O₃ is a highly active and stable RWGS catalyst exhibiting high selectivity towards CO over a range of operating conditions, with the presence of K promoting CO₂ dissociation to CO. These findings were supported by X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) measurements and DFT calculations.

To experimentally determine the effect of K addition on Mo₂C-based supported catalysts, K—Mo₂C/γ-Al₂O₃ and the corresponding Mo₂C, Mo and K—Mo control catalysts, all supported on γ-Al₂O₃, were synthesized through an evaporation-deposition procedure. XRD measurements over the reduced catalysts indicate that each of the synthesized catalysts contain a combination of MoO₂, β-Mo₂C and metallic Mo. Each of these phases was assigned to the synthesized catalysts by comparing the XRD spectra with the standard database for specific bulk Mo phases. XRD measurements of the Mo-based catalysts indicated that Mo₂C/γ-Al₂O₃ and 2 wt % K—Mo₂C/γ-Al₂O₃ contained a mixture of β-Mo₂C and MoO₂ supported on γ-Al₂O₃. All supported Mo-based catalysts exhibited large peaks at 45.8° and 66.6°, from the γ-Al₂O₃ support, and no identifiable peaks for MoO₃ were present in any of the samples. Closer inspection of the XRD spectra revealed the presence of a phase assigned to metallic Mo at 40.5°, 58.7° and 73.7° on the K—Mo₂C/γ-Al₂O₃ and K—Mo/γ-Al₂O₃ catalysts. These peaks were not present in Mo₂C/γ-Al₂O₃, suggesting that the addition of K promotes the formation of a metallic Mo phase.

SEM images with EDS mapping of the reduced catalysts were used to better identify the structure of K—Mo₂C/γ-Al₂O₃. Overall, the morphology and particle size of the catalysts appeared to be similar, with the SEM image of Mo₂C/γ-Al₂O₃ found in the SI. The EDS maps, however, showed that the distribution of Mo over each catalyst was notably different. The EDS map of the Mo₂C/γ-Al₂O₃ catalyst, found in the SI, indicated that molybdenum was evenly distributed over the γ-Al₂O₃ support. On K—Mo₂C/γ-Al₂O₃, there was both (1) a large degree of segregation between Mo and Al-rich areas and (2) K being preferentially found in the Mo-rich areas, which suggests K directly affects the electronic properties of the active Mo₂C phase.

Regardless of the differences in catalyst particle size and morphology, there was no significant difference in catalytic activity between the two samples. The conversion of Mo₂C/γ-Al₂O₃ and K—Mo₂C/γ-Al₂O₃ was similar. Although the activity of the two catalysts was comparable, the addition of 2 wt % K to Mo₂C/γ-Al₂O₃ significantly improved the selectivity towards CO. There was a strong promotional effect from the addition of K, which led to high CO selectivity (˜95%) from 6 to 23% conversion, the thermodynamic maximum for RWGS at 300° C. with a 3:1 H₂:CO₂ mixture. Furthermore, the addition of the K promoter decreased the deactivation percentage from 11.7% to 7.3% after 68 h on stream, an improvement in catalytic stability.

The K loading was varied from 1-3 wt % to determine the effect of K on catalytic performance. The 1 wt % K—Mo₂C/γ-Al₂O₃ had a slightly higher CO yield than 2 wt % K—Mo₂C/γ-Al₂O₃, but with increased methane production, which wastes valuable H₂ and requires a separation step before FT. Furthermore, as K loading increased, there was a drop in catalytic activity, likely from the blocking of active sites. This relationship between K loading and CO yield was not linearly dependent on temperature. At the higher temperature, the 3 wt % K—Mo₂C/γ-Al₂O₃ achieved 40.5% conversion and 98.2% CO selectivity, which outperformed the 2 wt % K—Mo₂C/γ-Al₂O₃ and industrial ZnO/Al₂O₃ and ZnO/Cr₂O₃ catalysts. (Joo et al., Ind. Eng. Chem. Res., 38, 1808-1812 (1999)).

Uncarburized Mo/γ-Al₂O₃ and 2 wt % K—Mo/γ-Al₂O₃ catalysts were tested to clarify the role of metallic Mo identified in K—Mo₂C/γ-Al₂O₃ in the XRD measurements. The Mo/γ-Al₂O₃ and K—Mo/γ-Al₂O₃ control catalysts were reduced ex situ in pure H₂ at 600° C. prior to reaction to form metallic Mo. The pre-reduction step ensured the high activity and CO selectivity of the Mo₂C-based catalysts originated from the Mo carbide phase, and not metallic Mo. The Mo carbides, synthesized with CH₄, were more active than the corresponding uncarburized catalysts, indicating that the carburization step was necessary for high catalytic activity and that the metallic Mo phase in K—Mo₂C/γ-Al₂O₃ was not solely responsible for the high performance.

By modifying Mo₂C/γ-Al₂O₃ with a K promoter, the CO selectivity and yield increased significantly, and approached the maximum thermodynamic yield for RWGS, under the appropriate reaction conditions. Addition of K also improved the catalyst stability, with only 7.3% deactivation after 68 h on stream. Catalyst characterization by SEM with EDS clearly showed that K is preferably found in Mo-rich regions, while Mo is more evenly distributed in Mo₂C/γ-Al₂O₃. Furthermore, K—Mo₂C/γ-Al₂O₃ maintained the Mo in a reduced and active state as evidenced by XPS measurements. These experimental results are supported by DFT calculations, which showed enhanced CO₂ adsorption and reduced CO₂ dissociation barriers on the K-promoted, compared to the pristine, Mo-terminated β-Mo₂C(001) surfaces. Notably, the DFT calculations predicted a 2.8 kcal mol⁻¹ lower activation barrier for CO formation upon K addition, which is in excellent agreement with the experimentally measured difference of 2.6 kcal mol⁻¹. These findings show that K—Mo₂C/γ-Al₂O₃ is a highly selective catalyst for producing CO from CO₂ and has the potential to be used as a commercial RWGS catalyst.

The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles “a,” “an,” “the,” or “said,” is not to be construed as limiting the element to the singular.

Claims

1. A supported heterogeneous catalyst material for catalyzing the reverse water-gas shift (RWGS) reaction for the selective formation of CO, comprising: a support material comprising γ-Al2O3; andan active material comprising alkali-metal doped molybdenum carbide, wherein the alkali-metal component of the active material comprises one or more alkali-metal precursors in elemental form or in the form of oxides, said metal being Na.

2. A supported heterogeneous catalyst material for catalyzing the reverse water-gas shift (RWGS) reaction for the selective formation of CO, comprising: a support material comprising γ-Al2O3; andan active material comprising alkali-metal doped molybdenum carbide, wherein the alkali-metal component of the active material comprises one or more alkali-metal precursors in elemental form or in the form of oxides, said metal being Li.