Fischer–Tropsch synthesis: Effect of promoter type on alumina-supported Mo carbide catalysts

Abstract

Molybdenum carbide catalysts promoted with K, Na, Co, and Ce have been synthesized by temperature-programmed carburization using a mixture of H 2/C 3H 8 and evaluated for the Fischer–Tropsch synthesis. Both α- and β-Mo carbide catalysts were produced during carburization via the formation of oxycarbide intermediate phase. The two-stage Mo carbide synthesis appeared to be characterized by a compensation effect and isokinetic relationship for both promoted and undoped catalysts suggesting that the solid state transformation mechanism was similar across the promoters. The catalysts possessed relatively high BET area (177–210 m 2 g cat −1) with finely dispersed Mo carbide particles (ca. 10 nm) on the alumina support. The presence of weak and strong acid–base sites was also confirmed by NH 3 and CO 2-TPD runs. CO hydrogenation rate increased with H 2 composition reaching optimal activity at y H 2 between 0.67 and 0.75. FT activity improved with promoter addition in the order; K–MoC 1− x /Al 2O 3 > Na–MoC 1− x /Al 2O 3 > Ce–MoC 1− x /Al 2O 3 > Co–MoC 1− x /Al 2O 3 > MoC 1− x /Al 2O 3 while chain growth probability varied with feed composition and was enhanced by all promoters. The reaction rate data were adequately described by an Eley–Rideal model.