Effect of a High Thermal Capacitance Core–Shell Structure on Co-Ru/SiO₂ Catalyst for Low Temperature Fischer–Tropsch Synthesis

Abstract

Commercial cylindrical mesoporous silica pellets (3 mm diameter by 3–6 mm length) were modified by coring the pellets and inserting a 1 mm diameter copper wire along the long axis of the pellet, to give a pseudo core–shell support. While there were negligible differences in the thermal conductance of the two supports, the volumetric thermal capacitance of the core–shell support was 4.1 times greater than the unmodified silica. Fischer–Tropsch synthesis (FTS) catalysts comprised of 16 wt% Co and 1.5 wt% Ru immobilized on the native pellets (control catalyst, CT) or on the core–shell support (CS-Cu catalyst) were prepared, placed in a tubular packed-bed reactor and reduced with H₂ at 400 °C. The catalysts were conditioned for FTS (255 °C; 10 atm; H₂/CO = 2; GSV 510 h⁻¹) by cooling to 150 °C, changing to a syngas atmosphere, and slowly ramping to the run temperature of 255 °C over 8 h. Measurements of the catalyst bed temperature and furnace temperature during the activation and run time revealed frequent and large temperature spikes (∆T ~ 70 °C) in the CT bed, especially in the first 12 h of operation. In comparison, runs using the CS-Cu catalyst experienced fewer and less substantive temperature spikes (∆T ~ 30 °C). From the thermal data and the FTS productivity data, it was clear that the CT catalyst experienced a substantially greater degree to deactivation due to the thermal spikes than the CS-Cu catalysts. At similar conversions, the CS-Cu showed 50% greater productivity (g_product/g_Co^–h) and a small but reproducible improvement in C₅₊ selectivity (52–55 wt%). Notably, the CS-Cu catalyst gave an appreciably smaller amount of the olefinic product (3 vs 15%). The thermal capacitance of the CS-Cu clearly moderates the negative consequences of local exotherms in the catalyst bed, especially during the activation phase of the FTS run.