PREPARATION OF RUTHENIUM PROMOTED COBALT CATALYSTS BY AUTOCOMBUSTION METHOD FOR FISCHER-TROPSCH SYNTHESIS

Abstract

The Fischer–Tropsch synthesis (FTS) catalysts with high reduction levels were synthesized through an autocombustion method using citric acid (CA) as a reductant and nitrate ions (N) as oxidants. The as-synthesized catalysts were used directly in FTS reaction without further reduction. The effects of the promoter, reductant types and citric acid contents on the catalyst structures and FTS performance were systematically studied. Results indicated that the introduction of a small amount of promoter improved the catalyst reducibility during the autocombustion process, and significantly enhanced the FTS activity. The CO conversion of the catalyst increased rapidly from 0.8 to 41.4 % after Ru promotion for Co catalysts. The citric acid contents (molar ratio of citric acid to nitrates: CA/N) in the precursor also played an important role in controlling the structures and FTS performance of the catalysts. For the RuCo catalysts, when the CA/N molar ratio was increased, the metal reduction level increased and the Co crystalline size decreased but the activity of the catalyst first increased and then decreased while metal reduction level and the activity of the iron-based catalysts were decreased. An excessive amount of the reductant could result in more residual carbon species and decrease the activity of the catalyst. Compared with the iron-based catalyst prepared by the conventional co-precipitation method (CN), the CO conversion of the autocombustion catalyst with CA/N = 0.1 was higher than that of the catalyst CN. For different types of reductants (at the same molar ratio of reductants to nitrates = 0.3), the catalyst prepared by citric acid exhibited the highest activity whereas the catalyst synthesized by oxalic acid showed the lowest methane selectivity. The FTS catalysts prepared by the autocombustion method, which omits the complex and high-energy consumption reduction process, can be used directly for highly efficient FTS and thus will be more promising in the future.