Theoretical and Experimental Studies on the Promoting Effect of Boron on Cobalt Catalyst Used for Fischer-Tropsch Synthesis (FTS)

Abstract

Supported cobalt catalysts exhibit favorable activity and selectivity for Fischer-Tropsch Synthesis (FTS), but deactivate slowly. To explore deactivation by carbon deposition, the stability of various forms of deposited carbon was evaluated using Density Functional Theory (DFT). A surface carbide and graphene islands were calculated to be thermodynamically stable. Two forms of deposited carbon were also distinguished experimentally after 200 hours of FTS. Based on this mechanistic insight, boron was proposed as a promoter to enhance the stability of cobalt catalysts. DFT calculations indicate that boron and carbon display similar binding preferences, and boron could selectively block the deposition of resilient carbon deposits. To evaluate the theoretical predictions, supported 20 wt% cobalt catalysts were promoted with 0.5 wt% boron and tested under realistic FTS conditions in a micro fixed bed reactor. Boron promotion was found to reduce the deactivation rate six-fold, without affecting selectivity and activity.