Effect of Mn on Co/HMS-Mn and Co/SiO2-Mn catalysts for the Fischer-Tropsch reaction

Abstract

The structural changes of cobalt deposited on SiO2 and Hexagonal Mesoporous Silica (HMS) supports were studied when Mn was used as a promoter and added to the catalyst during the synthesis of the supports by the sol-gel method. This synthetic method was used to promote the contact between the highly dispersed Mn in the supports and the Co added by incipient impregnation. The characterization results reveal that the presence of Mn promoted the dispersion of deposited cobalt oxide particles, causing a “wetting-like” effect of the cobalt oxide on the support surface, which led to a decrease of the average particle size of cobalt oxide from 13 to 10 nm in absence and presence of Mn respectively. The characterization results indicate that the Co2+ species react with Mn ions to form a likely Mn-Co spinel. Additionally, it was found that the promotion effects of Mn on the catalytic activity at atmospheric pressure for the Fischer-Tropsch synthetic reaction on both studied supports were significantly different because of the low metal loading used (<5 wt% of Co and 0.8 wt% of Mn) and the differences in their specific surface areas and texture. In the case of the catalysts using SiO2 as the support, the addition of Mn resulted in an enhancement of the CO consumption, whereas for the case of catalysts with HMS as the support, its presence was counterproductive, possibly because there was limited contact between the Mn and Co and because a fraction of the Mn was incorporated into the HMS framework. Furthermore, the catalytic tests at high pressure (20 bar) indicates that the catalysts were not fully selective toward hydrocarbon production, indicating that there are secondary reactions (e.g., water gas-shift reaction) due to non-reduced Co-oxide species. Additionally, at these operating conditions, the CO consumption rate increased proportionally as function of the Mn loading in the catalysts, whereas the selectivity toward olefins declined.