(606d) Early Transition Metal Carbide and Nitride Catalysts for Fischer-Tropsch Synthesis: Investigating Activity, Selectivity, and Stability

Dwindling crude oil resources and increased awareness of the impact of carbon emissions encourage the development of processes and materials to convert renewable resources into liquid transportation fuels and chemical feedstocks. The production of liquid hydrocarbon fuels from biomass-derived synthesis gas via Fischer-Tropsch synthesis (FTS) has emerged as a promising route, producing fuels with very low aromaticity and virtually zero sulfur content. In order to improve the feasibility of this process, FTS catalysts with enhanced activities and selectivities towards valuable products such as C₂-C₄ hydrocarbons, gasoline, and diesel need to be demonstrated. Moreover, these catalysts need to be stable under FTS conditions, i.e. resistant to coking.

Early transition metal carbides and nitrides are a promising class of materials for use as FTS catalysts [1, 2]. Previous efforts have shown that Mo₂C produces mainly light hydrocarbons [3] and has stable activities for H₂:CO ratios ranging from 1:1 to 3:1 [2]. These materials also possess properties similar to those of platinum-group metals [4, 5], and are sulfur tolerant [6]. Moreover, transition metal carbides and nitrides can be synthesized with high surface areas ranging up to 200 m²/g [7]. The objectives of this work are to determine the FTS activities and selectivities of Group V and VI carbides and nitrides as well as investigate their structural and catalytic stability under FTS conditions.

Experimental

A series of high surface area Group V and VI transition metal carbides and nitrides (Mo₂C, Mo₂N, VC, VN, NbC, NbN, W₂C, and W₂N) was synthesized via temperature programmed reaction of the parent metal oxide with either 15% CH₄/H₂ or NH₃. Phase analysis of these materials was performed using X-ray diffraction (XRD). The surface areas were determined via N₂ physisorption at 77K. CO uptakes were determined via pulsed chemisorptions at room temperature and atmospheric pressure. Surface morphologies of these materials were investigated using a field emission gun scanning electron microscope (SEM) equipped with an x-ray energy dispersive spectroscopy detector. X-ray photoelectron spectroscopy (XPS) was employed to investigate changes in surface composition due to catalyst pretreatment and exposure to FTS conditions.

The FTS performance characteristics were evaluated using 100-200 mg of catalyst supported on a quartz wool plug in a stainless steel U-tube reactor enclosed in an electric furnace. Transition metal carbide and nitride catalysts were pretreated prior to reaction with 15% CH₄/H₂ and NH₃ respectively. Pretreatment temperatures for each catalyst were selected based on temperature programmed reduction experiments. The reactions were carried out under the following conditions: 2:1 H₂:CO, 200-250°C, 25 bar, and GHSV = 14000 ? 44000 h^-1. Effluent gas compositions were analyzed using a Varian gas chromatograph equipped with thermal conductivity and flame ionization detectors. Two commercial catalysts (Fe/SiO₂ and Co/Al₂O₃) were used to benchmark performance. Spent catalysts were characterized using XRD, SEM, and XPS to determine structural stability under FTS conditions.

Results and Discussion

All of the carbide and nitride catalysts were active for FTS, however, the V, Nb, and W carbides and nitrides were much less active than the Mo₂C and Mo₂N catalysts. Gravimetric CO consumption rates for the Mo₂C and Mo₂N catalysts are compared to those for the Fe and Co commercial catalysts in Fig. 1. For Mo₂C and Mo₂N, approximately 21%, 11%, and 20% of the CO was converted to CH₄, C₂, and C₃+ hydrocarbons respectively, compared to 44%, 4%, and 42% for the Co catalyst. Approximately 40% of the CO was converted into CO₂ over Mo₂C and Mo₂N, presumably due to their high water gas shift activities, and ~6% was converted to methanol. The Fe catalyst showed very similar selectivities to the Mo₂C and Mo₂N catalysts. The Co catalyst did not appear to be as active for the water gas shift and did not form methanol. Some degree of water gas shift activity could be useful for adjusting the H₂:CO ratio of biomass-derived synthesis gas (H₂:CO = 0.8 ? 1.2) to a more usable range.

To compare the selectivity of the various catalysts, the product distributions were fit to the Anderson-Schulz-Flory model. The α values for the catalysts were as follows: 0.62 for Co/Al₂O₃, 0.52 for Fe/SiO₂, and 0.43 for Mo₂C and Mo₂N. This result indicates that the Mo₂C and Mo₂N catalysts favored shorter chain hydrocarbons as compared to the commercial catalysts. Moreover, the Fe and Co commercial catalysts produced a higher olefin-to-paraffin molar ratio for a given carbon number than did Mo₂C and Mo₂N. Mo₂C and Mo₂N also showed good structural and catalytic stability under FTS conditions.

Conclusions

Early transition metal carbides and nitrides have been demonstrated to be active and selective catalysts for Fischer-Tropsch synthesis. In addition, these materials are stable under FTS conditions. Carbide-based catalysts may be a suitable catalyst for biomass-derived syngas conversion due, in part, to their water-gas shift activities. Nevertheless, the Mo₂C catalyst favored short chain hydrocarbons compared to the commercial catalysts. Discovering ways to adjust the selectivity of Mo₂C towards longer chain hydrocarbons is a focus of future research.

References

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