Insight into structure-reactivity relationships and reaction pathways for higher alcohol synthesis from syngas over potassium promoted molybdenum sulfide supported catalysts

Abstract

As the world’s oil reserves decline, the need for petroleum-free routes for the production of lower olefins, the building blocks of the chemical industry, is ever growing. A promising route to meet this demand is the use of the versatile chemical feedstock, syngas, for the production of higher alcohols that can be later converted into olefins. This synthesis route has been the focus of many investigations with different families of catalysts. A particularly promising family is potassium promoted MoS2 based catalysts due to its resistance to sulfur poisoning, less severe coke deposition, and high selectivity towards ethanol. However, limited understanding of structure-reactivity relationships, reaction pathways, and active sites with this family of catalysts is available due to the complexity of the catalyst and reaction mechanism(s), as well as the extreme reaction conditions needed for syngas conversion to higher alcohols. In this thesis, a comprehensive investigation on the effect of supported K/MoS2 catalysts structure (with two distinct carbon and MgAl oxide supports) on reactivity was performed, while introducing a new catalyst composition with desirable reactivity. The correlations determined between the selectivity and the MoS2 layer stacking (HC selectivity ~ single MoS2 layer, C3+OH selectivity ~ double MoS2 layers) provided the basis for the elucidation of reaction pathways. Using methanol, ethanol, and ethylene co-feed experiments, preliminary insight into the reaction pathways over K/MoS2 supported catalysts was gained from changes in product distributions. A K/bulk-MoS2 catalyst was used as a control catalyst in co-feed experiments to investigate the role of the support in the reaction pathway(s). Based on similar normalized productivities of C3+OH for ethanol and ethylene co-feed experiments, it was proposed that alcohol formation proceeded primarily via the same acyl intermediate as olefin carbonylation. 13C2-ethanol and 13C2-ethylene co-feed experiments confirmed this hypothesis, as similar carbon enrichment in C2+OH was observed, where only the terminal carbons were enriched. While CO insertion is the primary pathway to higher alcohols over K/MoS2 supported catalysts, 13C2-ethanol co-feed conclusively show evidence that ethanol self-coupling to form 13C4-1-butanol species occurs to a small extent over the MgAl oxide supported catalyst, as a secondary pathway.