UNSUPPORTED ALKALI METAL-PROMOTED MOLYBDENUM CARBIDE CATALYSTS

Abstract

An unsupported bulk alkali-promoted molybdenum carbide (A-Mo.sub.2C) catalyst for use in the reverse water-gas shift (RWGS) reaction, and a method of making the same, is presented. In embodiments, a method for forming an unsupported bulk molybdenum carbide (Mo.sub.2C) catalyst promoted with an alkali earth metal includes: generating phase pure molybdenum trioxide (MoO.sub.3) by calcining a molybdate precursor salt; producing non-passivated Mo.sub.2C from carburization of the phase pure MoO.sub.3; passivating the non-passivated Mo.sub.2C to form passivated Mo.sub.2C; and producing an active unsupported alkali metal (A) promoted Mo.sub.2C (A-Mo.sub.2C) catalyst from the passivated Mo.sub.2C and an alkali metal carbonate (A-CO.sub.3).

Claims

1. A method of forming an unsupported bulk molybdenum carbide (Mo.sub.2C) catalyst promoted with an alkali earth metal, the method comprising: generating phase pure molybdenum trioxide (MoO.sub.3) by calcining a molybdate precursor salt; producing non-passivated Mo.sub.2C from carburization of the phase pure MoO.sub.3; passivating the non-passivated Mo.sub.2C to form passivated Mo.sub.2C; and producing an active unsupported alkali metal (A) promoted Mo.sub.2C (A-Mo.sub.2C) catalyst from the passivated Mo.sub.2C and an alkali metal carbonate (A-CO.sub.3).

2. The method of claim 1, wherein the molybdate precursor salt is selected from the group consisting of: ammonium molybdate tetrahydrate ((NH.sub.4).sub.6Mo.sub.7O.sub.24.Math.4H.sub.2O), sodium molybdate (Na.sub.2MoO.sub.4), and molybdenum chloride (MoCl.sub.5).

3. The method of claim 1, wherein the molybdate precursor salt is calcined at a temperature of greater or equal to 600 C. for 12 hours.

4. The method of claim 1, wherein a minimum gas hourly space velocity for the carburization is greater or equal to 4.510.sup.3 L kg.sup.1 hr.sup.1.

5. The method of claim 1, wherein the carburization occurs for at least 4 hours.

6. The method of claim 1, wherein producing the non-passivated Mo.sub.2C comprises carburizing the phase pure MoO.sub.3 in a flow of methane gas (CH.sub.4) and hydrogen gas (H.sub.2) for at least 4 hours at a temperature of greater or equal to 600 C.

7. The method of claim 1, wherein the alkali metal (A) is selected from the group consisting of: lithium (Li), sodium (Na), potassium (K), and combinations thereof.

8. The method of claim 1, wherein the passivating the non-passivated Mo.sub.2C comprises exposing the non-passivated Mo.sub.2C to a flow of dioxygen (O.sub.2) and nitrogen gas (N.sub.2) for 24 hours.

9. The method of claim 8, wherein the flow occurs at a total gas hourly space velocity (GHSV) of at least 150 L kg.sup.1 h.sup.1.

10. The method of claim 1, wherein a quantity of the A.sub.2CO.sub.3 applied is proportional to a total surface area of the passivated Mo.sub.2C and comprises between 110.sup.5 mols A per square meter (mol A m.sup.2) and 2.510.sup.6 mol A m.sup.2.

11. The method of claim 1, wherein producing the A-Mo.sub.2C catalyst comprises heating a mixture of the passivated Mo.sub.2C and an aqueous solution of the alkali metal carbonate (A.sub.2CO.sub.3) at a temperature of 80 C. until evaporation occurs.

12. The method of claim 1, further comprising drying the unsupported A-Mo.sub.2C catalyst to produce a bulk dry powder of the unsupported A-Mo.sub.2C catalyst.

13. The method of claim 1, wherein the unsupported A-Mo.sub.2C catalyst is selected from the group consisting of: lithium-promoted molybdenum carbide (LiMo.sub.2C), sodium-promoted molybdenum carbide (NaMo.sub.2C), and potassium-promoted molybdenum carbide (KMo.sub.2C).

14. The method of claim 1, wherein a molar ratio of the alkali metal (A) to molybdenum (Mo) content in the unsupported A-Mo.sub.2C catalyst is between 1:2 and 1:8.

15. The method of claim 14, wherein the molar ratio of the alkali metal (A) to molybdenum (Mo) content in the A-Mo.sub.2C catalyst is 1:4.

16. An active, low temperature Reverse Water-Gas Shift (RWGS) catalyst comprising an unsupported alkali metal (A) promoted molybdenum carbide (A-Mo.sub.2C) material.

17. The active RWGS catalyst of claim 16, wherein the alkali metal (A) is selected from the group consisting of: lithium (Li), sodium (Na), potassium (K), and combinations thereof.

18. The active RWGS catalyst of claim 16, wherein a molar ratio of the alkali metal (A) to molybdenum (Mo) content in the A-Mo.sub.2C catalyst is between 1:2 and 1:8.

19. The active RWGS catalyst of claim 16, wherein the RWGS catalyst achieves a CO yield and CO.sub.2 conversion of 38.6% or greater during RWGS at weight hourly space velocities of 3.610.sup.5 L kg.sup.1 hr.sup.1 or greater at a temperature of 450 C.

20. The active RWGS catalyst of claim 16, wherein the RWGS catalyst achieves a CO yield of 13.5% or greater and a CO.sub.2 conversion of 13.7% or greater at weight hourly space velocities of 1810.sup.4 L kg.sup.1 hr.sup.1 or greater at a temperature of 300 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Aspects of the present invention are described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention.

[0013] FIG. 1 is a flowchart of an exemplary process for generating alkali metal promoted Mo.sub.2C in the absence of a catalytic support for use in the RWGS reaction, in accordance with embodiments of the invention.

[0014] FIG. 2 is a plot of X-ray diffraction (XRD) patterns for products at various stages of the synthesis process of FIG. 1, in accordance with embodiments of the invention.

[0015] FIG. 3 depicts a reactor system 300 utilized to evaluate a variety of unsupported catalysts for use in a low temperature RWGS reaction.

[0016] FIG. 4 shows a plot of experimental CO.sub.2 conversions and CO yield versus weight hourly space velocity (WHSV) (L kg.sup.1 h.sup.1) for unsupported and supported catalysts at 300 C.

[0017] FIG. 5A shows a plot 500A of CO.sub.2 conversion and CO yield for both unsupported and supported catalysts as a function of the reagent flowrate at a reactor temperature of 450 C.

[0018] FIG. 5B shows a plot 500B of CO.sub.2 conversions and CO yield as a function of weight hourly space velocity (WHSV) for various loadings of potassium on bulk, unsupported Mo.sub.2C catalysts at 450 C.

DETAILED DESCRIPTION

[0019] Aspects of the present invention relate generally to catalysts for the Reverse Water Gas Shift (RWGS) reaction and, more particularly, to bulk, unsupported, alkali-promoted Mo.sub.2C catalysts.

[0020] Embodiments of the invention provide a new process for generating alkali (A) promoted molybdenum carbide (Mo.sub.2C) in the absence of a catalytic support, for use as a catalyst in the RWGS reaction. In implementations, the alkali metal is selected from lithium (Li), Sodium (Na) and potassium (K). Unsupported Mo.sub.2C catalysts of the present invention include alkali metal dopants for selective hydrogenation of carbon dioxide (CO.sub.2) into carbon monoxide (CO). In aspects of the invention, Mo.sub.2C is synthesized before the alkali metal is incorporated.

[0021] FIG. 1 is a flowchart 100 of an exemplary process for generating alkali metal promoted Mo.sub.2C in the absence of a catalytic support for use in the RWGS reaction, in accordance with embodiments of the invention.

[0022] At 101, phase pure MoO.sub.3 (molybdenum trioxide) is generated from a molybdate precursor salt, such as (NH.sub.4).sub.6Mo.sub.7O.sub.24.Math.4H.sub.2O (ammonium molybdate tetrahydrate), Na.sub.2MoO.sub.4 (sodium molybdate), or MoCl.sub.5 (molybdenum chloride). In implementations, (NH.sub.4).sub.6Mo.sub.7O.sub.24.Math.4H.sub.2O is calcined in a controlled and isolated oven environment (e.g., in a muffle furnace) at a temperature of 600 C. for 12 hours to generate phase pure MoO.sub.3 (molybdenum trioxide).

[0023] At 102, non-passivated molybdenum carbide (Mo.sub.2C) is produced from MoO.sub.3. In implementations, the MoO.sub.3 generated at step 101 is carburized in a flowing gas blend of 20% CH.sub.4 (methane) with a H.sub.2 (hydrogen) balance for 4 hours at a temperature of 600 C., using a weight hourly space velocity of 4.510.sup.3 liters per kilogram per hour (L kg.sup.1 h.sup.1), then cooled to room temperature under the same flowing gas blend to generate the non-passivated Mo.sub.2C. In implementations, higher flowrates or longer carburization times may be applied without detriment to the resulting catalyst.

[0024] At 103, the non-passivated Mo.sub.2C is passivated using a blend of oxygen (O.sub.2) and nitrogen (N.sub.2). In implementations, following carburization and before introduction to ambient atmosphere, the non-passivated MO.sub.2C material produced at step 102 is passivated at room temperature using a flowing blend of 1% O.sub.2 (dioxygen) with a N.sub.2 (nitrogen gas) balance for 24 hours at a total gas hourly space velocity (GHSV) of 150 L kg.sup.1 h.sup.1. The resulting passivated bulk Mo.sub.2C is air stable and is phase pure as confirmed by powder X-ray diffraction (XRD). See the discussion of FIG. 2 below. The term bulk when applied to a catalyst herein, refers to a catalyst where the catalytically active substance makes up the entire material-meaning there isn't a separate support material.

[0025] At 104, an alkali metal promoted MO.sub.2C catalyst product (A-Mo.sub.2C) is produced from the passivated Mo.sub.2C. In implementations, the passivated (and unsupported) Mo.sub.2C is added to a solution of A.sub.2CO.sub.3 (where A is an alkali metal) in deionized water. The quantity of A.sub.2CO.sub.3 applied must be proportional to the total surface area of the unsupported Mo.sub.2C, and may be up to 110.sup.5 mols of potassium (K) per square meter ([mols K] m.sup.2) but no less than 2.510.sup.6 [mols K] m.sup.2. The solution mixture of MO.sub.2C and A.sub.2CO.sub.3 is then stirred at 80 C. until complete evaporation occurs.

[0026] At 105, the resulting alkali promoted molybdenum carbide (A-Mo.sub.2C) powder from step 104 is dried, thereby producing a bulk dry powder of unsupported A-Mo.sub.2C. In implementations, the A-Mo.sub.2C powder is dried in a muffle furnace at 120 C. for 8 hours to produce the dry unsupported A-Mo.sub.2C. In implementations, the product is LiMo.sub.2C, NaMo.sub.2C, or KMo.sub.2C.

Experimental Results

[0027] FIG. 2 is a plot 200 of X-ray diffraction (XRD) patterns for products at various stages of the synthesis process of FIG. 1. FIG. 2 shows a plot of X-ray intensity as a function of the diffraction angle (2-theta). The process is shown going from bottom to top, starting at MoO.sub.3 at 201, through Mo.sub.2C at 202, and ending with alkali metal promoted Mo.sub.2C (KMo.sub.2C) at 203. An Mo.sub.2C reference pattern is also depicted at 204. Characterization by X-ray diffraction indicates the synthesized Mo.sub.2C at 202 matches the Mo.sub.2C reference pattern at 204, indicating a phase pure material. Additionally, there is no change to the crystal phase of the resulting KMo.sub.2C at 203 as compared to the Mo.sub.2C at 202.

[0028] The specific surface areas obtained through nitrogen physisorption and Brunauer-Emmett-Teller (BET) analysis for all of the initial combinations of Mo, K, and -Al.sub.2O.sub.3 are tabulated in Table 1 below. In general, BET analysis is a technique used to measure the specific surface area of materials, primarily by studying gas adsorption. BET is commonly performed using nitrogen physisorption, which is a technique used to characterize the surface area and porosity of materials by measuring the amount of nitrogen gas adsorbed onto a material's surface at a specific temperature, typically 77 K.

[0029] The RWGS activity for various catalysts was tested at 300 C., 19.7 atm, 3:1 H.sub.2 to CO.sub.2 feed stream, and at a GHSV of 3.610.sup.3 L kg.sup.1 h.sup.1. The measured activity, selectivity, and overall CO yield for the RWGS reactions are also included in Table 1. The RWGS reaction is thermodynamically limited and therefore, the maximum theoretical CO yield under the conditions tested in Table 1 is 23%.

TABLE-US-00001 TABLE 1 The Mo phase after carburization as analyzed by X-ray diffraction, surface area determined by BET, and the RWGS activity for various catalysts tested at 300 C., 19.7 bar and a 3:1 molar ratio of H.sub.2 to CO.sub.2. Various GHSV's are reported for the supported and unsupported catalysts. For all catalysts, the Mo starting material in the first column is (NH.sub.4).sub.6Mo.sub.7O.sub.244H.sub.2O. CO.sub.2 CO CO Mo Phase Conv. Sel. Yield Starting Material Following BET Surface GHSV (%) (%) (%) (Molar Ratios) Carburization Area (m.sup.2 g.sup.1) (L kg.sup.1h.sup.1) 300 C., 3:1 H.sub.2:CO.sub.2 Prior Lit. Method Metallic Mo 1.0 3.6 10.sup.3 0.0 N.A N.A. Bulk KMo (1/4).sup.10, 15 Bulk Mo Mo.sub.2C 13.7 3.6 10.sup.3 32.6 8.2 2.7 Current Invention Mo.sub.2C 10.9 3.6 10.sup.3 20.5 46.3 9.5 Bulk KMo (1/4) Current Invention Mo.sub.2C 10.9 9.0 10.sup.3 20.9 75.0 15.7 Bulk KMo (1/4) Current Invention Mo.sub.2C 10.9 1.8 10.sup.4 19.9 86.3 17.2 Bulk KMo (1/4) Current Invention Mo.sub.2C 10.9 4.5 10.sup.4 19.0 93.4 17.7 Bulk KMo (1/4) Current Invention Mo.sub.2C 10.9 9.0 10.sup.4 16.6 97.4 16.2 Bulk KMo (1/4) Current Invention Mo.sub.2C 10.9 1.8 10.sup.5 13.8 98.0 13.5 Bulk KMo (1/4) KMo@-Al.sub.2O.sub.3 Mo.sub.2C 142 3.6 10.sup.3 20.6 90.6 18.7 (1/4/15) KMo@-Al.sub.2O.sub.3 Mo.sub.2C 142 1.8 10.sup.4 14.1 97.2 13.7 (1/4/15) KMo@-Al.sub.2O.sub.3 Mo.sub.2C 142 6.6 10.sup.4 2.1 95.7 2.0 (1/4/15) KMo@-Al.sub.2O.sub.3 Mo.sub.2C 142 1.3 10.sup.5 1.2 99.2 1.2 (1/4/15) Mo@-Al.sub.2O.sub.3 (4/15) Mo.sub.2C 154 3.6 10.sup.3 18.7 73.1 13.7

[0030] With reference to Table 1, the potassium (K) promoter clearly improved the performance of the Mo.sub.2C for the RWGS reaction when supported on the -Al.sub.2O.sub.3, with both the CO.sub.2 conversion and CO selectivity measured to be higher than for the unpromoted Mo.sub.2C/-Al.sub.2O.sub.3 catalyst. In the absence of -Al.sub.2O.sub.3 and K promoters, the unsupported Mo.sub.2C possessed a surface area of 13.7 m.sup.2 g.sup.1 and exhibited the highest CO.sub.2 conversions described in Table 1. However, the products of CO.sub.2 hydrogenation were dominated by complete hydrogenation to CH.sub.4, with CO making up only 8.2% of the total products detected. The inherently high activity of the unsupported Mo.sub.2C suggests that it may serve as an effective RWGS catalyst, if CO formation can be selectively promoted using dopants such as K or other alkali metals. When an alkali metal is co-deposited with the molybdenum precursor in the absence of -Al.sub.2O.sub.3 and prior to calcination to obtain MoO.sub.3, metallic molybdenum results. The resulting metallic Mo is completely unreactive for CO.sub.2 hydrogenation under the conditions tested.

[0031] FIG. 3 depicts a reactor system 300 utilized to evaluate a variety of unsupported catalysts for use in a low temperature RWGS reaction. The system 300 includes N.sub.2, H.sub.2 and CO.sub.2 gas sources 301-303, which feed gas to an enclosed catalyst bed 306. An ice bath cold trap 308 is placed downstream of the catalyst bed 306 and is configured to remove water from gas generated within the catalyst bed 306 before the gas is sent to a gas chromatograph (GC) detector at 310 for analysis.

[0032] Selective hydrogenation of CO.sub.2 to CO through the RWGS reaction was performed using the reactor system 300 of FIG. 3. Catalyst materials were pelletized under a force of 2 tons for a total of 10 minutes and then passed through a sieve to obtain catalyst grains between 200 and 350 microns (m) in diameter. The catalyst materials were then mixed with silicon carbide (SIC), an inert material for the low temperature RWGS, to a total mass of 500 milligrams (mg) before being loaded into a diameter stainless steel reactor 307 (housing catalyst bed 306). The ratio of KMo.sub.2C to SiC was varied in order to test a wide range of GHSVs at similar reagent flow rates. Upon loading the reactor, the catalyst bed was pretreated at 300 C. with H.sub.2 gas at a pressure of 19.7 atmospheres (atm) and a GHSV of 610.sup.3 L kg.sup.1 h.sup.1 for 2.5 hours. The catalyst bed was then exposed to a flowing blend of H.sub.2 to CO.sub.2 at a 3:1 molar ratio. The flowing blend also contained a small flowrate of N.sub.2 (16% by volume) which was used as an internal standard for in-line gas chromatography (GC) analysis. The total pressure in the reactor under experimental conditions was 19.7 atm while the reactor temperature was varied between 30 and 450 C. In all experiments, the reactors were allowed to equilibrate for 4 hours to achieve steady-state conversions, verified by GC analysis, before the concentration of reactant and product gasses were recorded.

[0033] FIG. 4 shows a plot 400 of experimental CO.sub.2 conversions and CO yield versus weight hourly space velocity (WHSV) (L kg.sup.1 h.sup.1) for unsupported and supported catalysts at 300 C. . . . FIG. 4 plots the CO.sub.2 conversion and CO selectivity for both the unsupported bulk KMo.sub.2C and the supported KMo.sub.2C/Al.sub.2O.sub.3 catalysts as a function of reagent flow rate at 300 C. and 19.7 atm. A dashed line is shown representing the thermodynamic limit for CO.sub.2 conversion, which is 23% at 300 C. The data in FIG. 4 is tabulated in Table 1, above.

[0034] The total CO.sub.2 conversion was higher for the unsupported KMo.sub.2C sample under all conditions tested. At GHSVs of 910.sup.3 L kg.sup.1 h.sup.1 or less, greater CO yields were obtained for the KMo.sub.2C/-Al.sub.2O.sub.3 sample, as the majority of CO.sub.2 hydrogenation products for the unsupported catalyst consisted of undesirable CH.sub.4. However, as the reagent flowrate was increased above 910.sup.3 L kg.sup.1 h.sup.1, the unsupported KMo.sub.2C catalyst significantly outperforms the supported KMo.sub.2C/-Al.sub.2O.sub.3 catalyst. For instance, the unsupported catalyst is shown to have a CO yield of 16.2% at a GHSV of 9.010.sup.4 L kg.sup.1 h.sup.1 whereas the supported catalyst, at a lower GHSV of 6.610.sup.4 L kg.sup.1 h.sup.1, had a CO yield of just 2.0%. This is likely due to the lower overall surface area and microporosity of the unsupported KMo.sub.2C which reduces the overall impact of mass transfer limitations, allowing the catalyst to operate at steady state conditions under much higher reagent flow rates.

TABLE-US-00002 TABLE 2 Catalyst surface area calculated by BET analysis and RWGS activity at select space velocities for a variety of bulk Mo.sub.2C catalysts that are promoted by varying amounts of potassium. The reaction conditions were 450 C. and a 3:1 ratio of H.sub.2 to CO.sub.2. Starting BET CO.sub.2 CO CO Material Surface GHSV Conv. Sel. Yield (Molar Area (L kg.sup.1 (%) (%) (%) Ratios) (m.sup.2 g.sup.1) h.sup.1) 450 C., 3:1 H.sub.2:CO.sub.2 KMo@- 142 1.3 10.sup.5 26.2 99.9 26.1 Al.sub.2O.sub.3 (1/4/15) Bulk KMo 10.9 1.8 10.sup.5 41.3 91.5 37.8 (1/4) Bulk KMo 11.2 1.8 10.sup.5 37.1 83 30.8 (1/8) Bulk KMo 10.5 1.8 10.sup.5 35.9 98.2 35.3 (1/2)

[0035] FIG. 5A shows a plot 500A of CO.sub.2 conversion and CO yield for both unsupported and supported catalysts as a function of the reagent flowrate at a reactor temperature of 450 C. FIG. 5A is presented in order to better demonstrate the temperature range of catalyst performance, as well as the excellent reactor throughput. At the elevated temperature of 450 C., the differences in the reactor throughput are pronounced, with the unsupported K-MO.sub.2C catalyst achieving CO yields of 38.6% at a total GHSV of 3.610.sup.5 L kg.sup.1 h.sup.1, roughly 88% of the thermodynamic limit under these conditions. For reference, under the same temperatures and pressures, the supported KMo.sub.2C/-Al.sub.2O.sub.3 catalyst achieves a CO selectivity of only 33% at a reduced GHSV of 6.610.sup.4.

[0036] FIG. 5B shows a plot 500B of CO.sub.2 conversions and CO yield as a function of weight hourly space velocity (WHSV) for various loadings of potassium (K) on bulk unsupported Mo.sub.2C catalysts at 450 C. To determine the influence of the alkali promoter, Mo.sub.2C catalysts were prepared with varying loadings of K. FIG. 5B plots the CO.sub.2 conversions and CO selectivity at 450 C. and 19.7 atm for three unsupported KMo.sub.2C catalyst with various K loadings of 2.510.sup.6, 5.010.sup.6, and 1.010.sup.5 mol K m.sup.2. These are labelled by their respective molar ratios relative to Mo: 1:8, 1:4, and 1:2, respectively. Catalyst performance is clearly optimal at K to Mo ratios of 1:4, as this loading provided the greatest CO.sub.2 conversion and highest CO selectivity. Additionally, this catalyst exhibited a CO selectivity of 100% at high GHSV's. For the catalysts with the 1:8 K to Mo ratio, the CO.sub.2 conversion was significantly less when compared to the catalyst with 1:4 ratio. Additionally, the CO selectivity was also reduced, showing 83% and 93% at space velocities of 1.810.sup.5 and 3.610.sup.5 L kg.sup.1 h.sup.1, respectively. Finally, the catalyst that was loaded with twice the amount of K showed little difference between the observed CO.sub.2 and CO selectivity as a function of GHSV, with CO.sub.2 conversions of 36% and a CO selectivity of 98%. The thermodynamic limit for CO.sub.2 conversion, which is 43.8% at 450 C. is also depicted.

[0037] Based on the above, embodiments of the invention provide a new method of preparing alkali metal promoted metal carbide catalysts without the need for metal oxide supports for the low temperature reverse water gas shift reaction. These catalysts maintain the high activity observed for supported carbide catalysts; however, the unsupported catalysts described herein show improved activity at high GHSVs when compared to existing supported catalysts.

[0038] The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the embodiments described. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.