Abstract
This study has evaluated the performance of molybdenum carbide as catalyst for Fischer-Tropsch synthesis. The effect of different promoter oxide groups (alkali, alkaline-earth, transition and rare-earth metals) and various semiconductor oxide supports (Al2O3, TiO2, SiO2 and ZrO2) was examined. Runs were conducted in a computer-controlled fixed-bed reactor at different feed compositions and reaction temperatures. Both α-MoC1-x and β-MoC1-x (0≤x<1) phases were formed during carburization run with a mixture of H2/C3H8 on promoted and unpromoted MoC1-x with no detectable MoO3 phase as measured in X-ray diffraction suggesting the complete conversion of MoO3 precursor to final carbide phase. Temperature-programmed carburization showed that the transformation from MoO3 to the MoC1-x phase was a 2-step process involving the formation of an intermediate oxycarbide form. The existence of a compensation effect and isokinetic relationship for oxycarbide and carbide phase formation over promoted and unpromoted catalysts suggested that solid-state carburization was governed by a topotactic mechanism. The optimal carburization rate was observed at H2/C3H8=5:1 and TiO2 support exhibited the highest carbide formation rate. MoC1-x catalysts possessed weak and strong acid as well as basic centers. Although H2 and CO chemisorbed on MoC1-x surface, CO adsorption was stronger than H2 chemisorption due to higher CO uptake and heat of desorption. Promoter addition enhanced strong basic site concentration and CO uptake. Carburization with 5H2/1C3H8 at 973 K for 2 h appeared to be optimal preparation condition for olefin selectivity. TiO2 support was the best support for maximum CO consumption rate whilst Al2O3 gave the highest olefin-to-paraffin ratio. Promoted MoC1-x catalysts gave improved chain growth probability and CO consumption rate with optimal H2 mole fraction of 0.67-0.75. Fischer-Tropsch activity and chain growth factor increased with K loading and attained a maximum at 3wt%K. A quantitative relationship between activation energy and feed composition as well as carbon number was obtained over MoC1-x catalyst. Fischer-Tropsch activity was stable with time-on-stream for 120 h. A mechanism involving molecular CO chemisorption with gas phase H2 attack to yield enolic species was used to develop a kinetic model that adequately captured individual hydrocarbon species and the behavior of olefin-to-paraffin ratio with composition.