Effects of H 2S and process conditions in the synthesis of mixed alcohols from syngas over alkali promoted cobalt-molybdenum sulfide

Abstract

The present work is an investigation of how the process conditions influence the synthesis of mixed alcohols from syngas over a K 2CO 3/Co/MoS 2/C catalyst. The emphasis in the investigations is upon the effects of H 2S in the syngas feed. However the effects of the temperature and of the partial pressures of H 2 and CO are also investigated. With or without H 2S in the feed the pre-sulfided catalyst requires an initiation period to reach a stabilized behavior, but the duration of this period depends upon the H 2S level. Operation with a feed containing more than 103 ppmv H 2S leads to a fairly rapid stabilization of the product distribution and ensures that higher alcohols are the dominant reaction products. With less than 57 ppmv H 2S in the feed the stabilization of the product distribution is much slower, and methanol is the dominant product. An investigation of the reaction kinetics indicates a high CO coverage and low hydrogen coverage. Hydrogen sulfide in the syngas feed generally promotes chain growth for both alcohols and hydrocarbons, but lowers the alcohol selectivity by enhancing the hydrocarbon formation. The highest alcohol productivity reached in these investigations was 0.276 g/g cat./h, and this was achieved at 350 °C, 100 bar, GHSV = 5244 h −1, Feed: 49.9 vol% H 2, 50.1 vol% CO. Finally it is found that sulfur fed to the reactor as H 2S is incorporated into the condensed alcohol product, and the incorporation of sulfur species into the product continues for some time after H 2S has been removed from the feed. When the catalyst is operated with an S-free syngas feed, the amount of sulfur in the condensed liquid product decreases over time, but after 35 h of operation with an S-free syngas the alcohol product still contains 340 ppmw of sulfur. Thiols appear to be the dominant sulfur compounds in the product.