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

Molybdenum carbide catalysts promoted with K, Na, Co, and Ce have been synthesized by temperature-programmed carburization using a mixture of H2/C3H8 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 m2 gcat−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 NH3 and CO2-TPD runs. CO hydrogenation rate increased with H2 composition reaching optimal activity at yH2yH2 between 0.67 and 0.75. FT activity improved with promoter addition in the order; K–MoC1−x/Al2O3 > Na–MoC1−x/Al2O3 > Ce–MoC1−x/Al2O3 > Co–MoC1−x/Al2O3 > MoC1−x/Al2O3 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.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (69 K)Download as PowerPoint slideHighlights► Mo carbide catalyst promoted with K, Na, Co, and Ce all exhibited similar solid-state two-stage conversion process. ► Fischer–Tropsch activity improved in the order; K > Na > Ce > Co > unpromoted MoC1−x/Al2O3. ► The oxycarbide phase seems to possess the active site for FT reaction. ► Similar FTS mechanism prevailed on all catalysts as implicated by compensation effect. ► The reaction rate data could be represented by an Eley–Rideal kinetic model involving gas phase H2 attack of chemisorbed CO.