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-Mo.sub.2C/γ-Al.sub.2O.sub.3, A=K, Na, Li). The A-Mo.sub.2C/γ-Al.sub.2O.sub.3 catalyst is synthesized by co-impregnation of molybdemun and alkali metal precursors onto a γ-Al.sub.2O.sub.3 support. It is then carburized to form the A-Mo.sub.2C/γ-Al.sub.2O.sub.3.

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-Al.sub.2O.sub.3; and an active material comprising alkali-metal doped molybdenum carbide.

2. The catalyst material of claim 1, 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 metals being selected from the group consisting of K, Na, Li, or any combination thereof.

3. The catalyst material of claim 1, wherein the molybdenum component of the active material comprises one or more molybdenum precursors in the form of carbides, oxycarbides, oxides, elemental molybdenum, or any combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 shows the synthesis procedure for alkali metal doped molybdenum carbide supported on gamma alumina.

[0013] FIG. 2A is a low magnification scanning electron microscopy (SEM) image of K—Mo.sub.2C/γ-Al.sub.2O.sub.3. FIG. 2B is a high magnification SEM image of K—Mo.sub.2C/γ-Al.sub.2O.sub.3.

[0014] FIG. 3 is a schematic of a reactor set-up for CO.sub.2 hydrogenation.

[0015] FIG. 4A is a plot of CO.sub.2 conversion versus time for the Mo.sub.2C and A-Mo.sub.2C (A=K, Na, Li) supported on γ-Al.sub.2O.sub.3. FIG. 4B is a plot of production of CO and CH.sub.4 versus time for Na—Mo.sub.2C/γ-Al.sub.2O.sub.3, Li—Mo.sub.2C/γ-Al.sub.2O.sub.3 and Mo.sub.2C/γ-Al.sub.2O.sub.3.

DETAILED DESCRIPTION OF THE INVENTION

[0016] 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-Al.sub.2O.sub.3 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.

[0017] 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-Mo.sub.2C/γ-Al.sub.2O.sub.3, A=K, Na, Li) was synthesized by co-impregnation of (NH.sub.4)6Mo.sub.7O.sub.24.4H.sub.2O and A-NO.sub.3 precursors (A=K, Na, Li) onto a γ-Al.sub.2O.sub.3 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-Al.sub.2O.sub.3, which translates to 2% potassium (K), 1.2% sodium (Na), 0.4% lithium (Li) and 20.8% Mo loading on the γ-Al.sub.2O.sub.3 support. Aqueous solutions of the metal precursors were added to a beaker of γ-Al.sub.2O.sub.3 and dried overnight under stirring at 60° C., then calcined in air overnight at 350° C.

[0018] The A-Mo/γ-Al.sub.2O.sub.3 catalyst was then carburized in a 21% CH.sub.4 in H.sub.2 mixture at 600° C. for 2.5 hours to form the A-Mo.sub.2C/γ-Al.sub.2O.sub.3. After the first 1.5 hour, the CH.sub.4 was shut off and the carbide was cooled to room temperature in H.sub.2. At room temperature, the catalyst was passivated in 1% O.sub.2 in N.sub.2 for several hours. FIG. 2A shows a low magnification scanning electron microscopy (SEM) image of K—Mo.sub.2C/γ-Al.sub.2O.sub.3, and FIG. 2B shows a high magnification SEM image of K—Mo.sub.2C/γ-Al.sub.2O.sub.3.

[0019] CO.sub.2 hydrogenation via the RWGS reaction is performed while flowing carbon dioxide, hydrogen gas, or any combination thereof over the A-Mo.sub.2C/γ-Al.sub.2O.sub.3 catalyst material. FIG. 3 shows a schematic of a reactor set-up for CO.sub.2 hydrogenation. In the CO.sub.2 hydrogenation experiment, 500 mg of A-Mo.sub.2C/γ-Al.sub.2O.sub.3 was loaded into a ¼ in stainless steel reactor and reduced under 50 sccm H.sub.2 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 CO.sub.2, 18.9 sccm H.sub.2 and 5.0 sccm N.sub.2, for a H.sub.2:CO.sub.2 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.

[0020] Table 1 shows a summary of performance of Mo.sub.2C and A-Mo.sub.2C (A=K, Na, Li) supported on γ-Al.sub.2O.sub.3 for CO.sub.2 hydrogenation. FIG. 4A shows a plot of CO.sub.2 conversion versus time for the Mo.sub.2C and A-Mo.sub.2C (A=K, Na, Li) supported on γ-Al.sub.2O.sub.3, and FIG. 4B shows a plot of production of CO and CH.sub.4 versus time for Na—Mo.sub.2C/γ-Al.sub.2O.sub.3, Li—Mo.sub.2C/γ-Al.sub.2O.sub.3 and Mo.sub.2C/γ-Al.sub.2O.sub.3. The CO.sub.2 hydrogenation via the RWGS reaction can achieve a CO yield of 12% or greater and a CO selectivity of 90% or greater.

TABLE-US-00001 TABLE 1 Catalyst Conversion/% CO Selectivity/% CO Yield/% Mo.sub.2C/γ-Al.sub.2O.sub.3 19.9 73.5 14.6 K—Mo.sub.2C/γ-Al.sub.2O.sub.3 17.2 95.9 16.5 Na—Mo.sub.2C/γ-Al.sub.2O.sub.3 19.6 86.3 16.9 Li—Mo.sub.2C/γ-Al.sub.2O.sub.3 19.8 62.1 12.3

[0021] The increased CO yield from doping a Mo.sub.2C/γ-Al.sub.2O.sub.3 catalyst with alkali metals offers an improved route for CO production from CO.sub.2. The best currently available catalysts can only achieve a CO yield and selectivity of 14.6% and 75% at 300° C., respectively, while K—Mo.sub.2C/γ-Al.sub.2O.sub.3 reaches a CO yield and selectivity of 16.5% and 96%, respectively. Selectively producing CO from CO.sub.2 enables a facile route to synthesize synthetic hydrocarbons from CO.sub.2 through down-stream Fischer-Tropsch.

[0022] Na—Mo.sub.2C/γ-Al.sub.2O.sub.3 reaches a similar CO yield to K—Mo.sub.2C/γ-Al.sub.2O.sub.3, while Li—Mo.sub.2C/γ-Al.sub.2O.sub.3 shows a lower selectivity to CO than Mo.sub.2C/γ-Al.sub.2O.sub.3. Maintaining the same A:Mo weight ratio in Li—Mo.sub.2C/γ-Al.sub.2O.sub.3 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—Mo.sub.2C/γ-Al.sub.2O.sub.3. The Li:Mo and Na:Mo ratios can be further optimized.

[0023] The addition of K to catalysts as a promoter has not yet been recorded with a Mo.sub.2C-based catalyst for CO.sub.2 hydrogenation. Furthermore, doping Mo.sub.2C-based catalysts with Li and Na has not been attempted in literature for CO.sub.2 hydrogenation. By doping Mo.sub.2C/γ-Al.sub.2O.sub.3 with alkali metals, CO selectivity substantially increases for K and Na, which is likely caused by attenuation of the electronic properties of the Mo.sub.2C phase. These electronic effects are only present when Mo.sub.2C is doped with a small amount of alkali metal, thereby attenuating the CO binding energy and preventing further hydrogenation into CH.sub.4 or other hydrocarbons.

[0024] A-Mo.sub.2C/γ-Al.sub.2O.sub.3 (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 (SiO.sub.2, TiO.sub.2, ZrO.sub.2), gas compositions (CO.sub.2:H.sub.2=1:1, 1:2, 1:3) and pressures (0-350 psig). Higher temperature improves conversion for K—Mo.sub.2C/γ-Al.sub.2O.sub.3 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%.

[0025] The exact optimal metal loading and A:Mo (A=K, Na, Li) ratio on the γ-Al.sub.2O.sub.3 support can be further optimized based on this finding of such high CO selectivity, especially over Na—Mo.sub.2C/γ-Al.sub.2O.sub.3 and K—Mo.sub.2C/γ-Al.sub.2O.sub.3.

Example

[0026] In this example, kinetic experiments and characterization tools were combined with DFT calculations to probe the catalytic properties of K-promoted Mo.sub.2C and understand the reaction mechanisms of CO.sub.2 dissociation. Flow reactor results indicate that K—Mo.sub.2C/γ-Al.sub.2O.sub.3 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 CO.sub.2 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.

[0027] To experimentally determine the effect of K addition on Mo.sub.2C-based supported catalysts, K—Mo.sub.2C/γ-Al.sub.2O.sub.3 and the corresponding Mo.sub.2C, Mo and K—Mo control catalysts, all supported on γ-Al.sub.2O.sub.3, were synthesized through an evaporation-deposition procedure. XRD measurements over the reduced catalysts indicate that each of the synthesized catalysts contain a combination of MoO.sub.2, β-Mo.sub.2C 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

[0028] Mo phases. XRD measurements of the Mo-based catalysts indicated that Mo.sub.2C/γ-Al.sub.2O.sub.3 and 2 wt % K—Mo.sub.2C/γ-Al.sub.2O.sub.3 contained a mixture of β-Mo.sub.2C and MoO.sub.2 supported on γ-Al.sub.2O.sub.3. All supported Mo-based catalysts exhibited large peaks at 45.8° and 66.6°, from the γ-Al.sub.2O.sub.3 support, and no identifiable peaks for MoO.sub.3 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—Mo.sub.2C/γ-Al.sub.2O.sub.3 and K—Mo/γ-Al.sub.2O.sub.3 catalysts. These peaks were not present in Mo.sub.2C/γ-Al.sub.2O.sub.3, suggesting that the addition of K promotes the formation of a metallic Mo phase.

[0029] SEM images with EDS mapping of the reduced catalysts were used to better identify the structure of K—Mo.sub.2C/γ-Al.sub.2O.sub.3. Overall, the morphology and particle size of the catalysts appeared to be similar, with the SEM image of Mo.sub.2C/γ-Al.sub.2O.sub.3 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 Mo.sub.2C/γ-Al.sub.2O.sub.3 catalyst, found in the SI, indicated that molybdenum was evenly distributed over the γ-Al.sub.2O.sub.3 support. On K—Mo.sub.2C/γ-Al.sub.2O.sub.3, 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 Mo.sub.2C phase.

[0030] 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 Mo.sub.2C/γ-Al.sub.2O.sub.3 and K—Mo.sub.2C/γ-Al.sub.2O.sub.3 was similar. Although the activity of the two catalysts was comparable, the addition of 2 wt % K to Mo.sub.2C/γ-Al.sub.2O.sub.3 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 H.sub.2:CO.sub.2 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.

[0031] The K loading was varied from 1-3 wt % to determine the effect of K on catalytic performance. The 1 wt % K—Mo.sub.2C/γ-Al.sub.2O.sub.3 had a slightly higher CO yield than 2 wt % K—Mo.sub.2C/γ-Al.sub.2O.sub.3, but with increased methane production, which wastes valuable H.sub.2 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—Mo.sub.2C/γ-Al.sub.2O.sub.3 achieved 40.5% conversion and 98.2% CO selectivity, which outperformed the 2 wt % K—Mo.sub.2C/γ-Al.sub.2O.sub.3 and industrial ZnO/Al.sub.2O.sub.3 and ZnO/Cr.sub.2O.sub.3 catalysts. (Joo et al., Ind. Eng. Chem. Res., 38, 1808-1812 (1999)).

[0032] Uncarburized Mo/γ-Al.sub.2O.sub.3 and 2 wt % K—Mo/γ-Al.sub.2O.sub.3 catalysts were tested to clarify the role of metallic Mo identified in K—Mo.sub.2C/γ-Al.sub.2O.sub.3 in the XRD measurements. The Mo/γ-Al.sub.2O.sub.3 and K—Mo/γ-Al.sub.2O.sub.3 control catalysts were reduced ex situ in pure H.sub.2 at 600° C. prior to reaction to form metallic Mo. The pre-reduction step ensured the high activity and CO selectivity of the Mo.sub.2C-based catalysts originated from the Mo carbide phase, and not metallic Mo. The Mo carbides, synthesized with CH.sub.4, 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—Mo.sub.2C/γ-Al.sub.2O.sub.3 was not solely responsible for the high performance.

[0033] By modifying Mo.sub.2C/γ-Al.sub.2O.sub.3 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 Mo.sub.2C/γ-Al.sub.2O.sub.3. Furthermore, K—Mo.sub.2C/γ-Al.sub.2O.sub.3 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 CO.sub.2 adsorption and reduced CO.sub.2 dissociation barriers on the K-promoted, compared to the pristine, Mo-terminated β-Mo.sub.2C(001) surfaces. Notably, the DFT calculations predicted a 2.8 kcal mol.sup.−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.sup.−1. These findings show that K—Mo.sub.2C/γ-Al.sub.2O.sub.3 is a highly selective catalyst for producing CO from CO.sub.2 and has the potential to be used as a commercial RWGS catalyst.

[0034] 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.