Abstract
CO hydrogenation over alumina-supported Mo carbide catalyst synthesized by temperature-programmed carburization of the impregnated MoO3 utilizing H2/C3H8 feed has been investigated. Samples were prepared following a 23-factorial design with temperature, hydrocarbon composition and reaction time as the carburization factors. Catalysts were characterized using N2 physisorption, NH3 temperature-programmed desorption and solid total organic carbon (TOC) analysis. Solid-state carburization kinetics was studied in a thermogravimetric analyzer at different heating rates. Statistical analysis revealed that carburization temperature was a strong determinant of key physicochemical properties (BET, pore volume, acid site strength and concentration). Increasing the H2:C3H8 ratio in the carburizing gas reduced the acid site strength and concentration but improved overall hydrocarbon synthesis rate, olefin-to-paraffin ratio and specific activity. All catalysts possessed Lewis acid centers. The carburizing temperature was also a statistically significant factor in the determination of the FT reaction metrics. A two-stage process involving the transformation of Mo oxide to the MoC1−x carbide via an intermediate oxycarbide phase was observed. The solid-state kinetics was captured by a contracting volume model and Arrhenius treatment implicated the occurrence of a ‘compensation effect’ for the carbothermal synthesis. Although the rate-composition envelope exhibited qualitatively similar features for the entire Mo carbide class, the optimum feed composition for maximum hydrocarbon formation rate varied between 0.67 and 0.75 depending on the catalyst preparation conditions and the carbon number. The olefin-to-paraffin ratio has a reciprocal relationship to the H2 partial pressure but experienced an exponential decay with carbon number. Catalyst with 15wt.% Mo loading produced the highest hydrocarbon synthesis rate and chain growth probability.
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