Higher alcohols from syngas using a K/Ni/MoS_2 catalyst: Trace sulfur in the product and effect of H_2S-containing feed

摘要

Two types of experiments have been performed related to the higher alcohol synthesis from syngas over a K-Ni-MoS_2 catalyst which beforehand has been operated for 1000 h on stream in sulfur-free syngas. In the first experimental part, sulfur-free syngas was used as feed and the condensed liquid product was found to contain 67 ppmw sulfur, while the sulfur concentration in the gas was 19 ppmv. The gas phase was found to contain mainly COS and H_2S, while the liquid phase contained methanethiol (13.8 ppmw S), ethanethiol (10.6 ppmw S), dimethyl sulfide (21.3 ppmw S), ethyl methyl sulfide (12.2 ppmw S), unidentified sulfur compounds (7.9 ppmw S) together with some dissolved COS (0.5 ppmw S) and H_2S (1.2 ppmw S). In the second experimental part, the effect of feeding syngas containing 170 ppm H_2S compared to a sulfur-free syngas was studied, while all products were carefully monitored online. The presence of H_2S in the syngas was found to increase CO conversion, but the largest change was found in product selectivity. The hydrocarbon selectivity greatly increased at the expense of alcohol selectivity, while the alcohol distribution shifted towards longer alcohols (increased C_2+OH/MeOH ratio). From product yields it became clear that most of the increased CO conversion with H_2S in the feed was due to increased methane formation (and CO_2 formation due to the water-gas shift reaction). The presence of H_2S in the feed greatly increased the concentration of all sulfur compounds. Together with COS, formation of thiols (methanethiol and ethanethiol) was especially favored by the presence of H_2S. The thioether concentration also increased, however, to a much lower extent.