Reaction path analysis for 1-butanol dehydration in H-ZSM-5 zeolite: Ab initio and microkinetic modeling

Abstract

Dispersion corrected periodic density functional theory (DFT) is used to construct a microkinetic model for 1-butanol dehydration in H-ZSM-5. The latter is applied to determine the effect of reaction conditions on dehydration rates, product selectivity and dominant reaction pathway. The consecutive reaction scheme of 1-butanol dehydration to ether followed by ether decomposition offers lower energy barriers as compared to the direct conversion of 1-butanol to 1-butene. The direct dehydration of 1-butanol to 1-butene occurs via an E2 (anti) elimination at low 1-butanol partial pressure and shifts to a 1-butanol assisted 1,2-syn-elimination with increasing 1-butanol partial pressure. The ether formation reaction proceeds via an SN2-type nucleophilic substitution mechanism, involving substitution of the –OH2 group of the protonated alcohol by 1-butanol, while ether decomposition predominantly occurs via a 1,2-syn-elimination mechanism. The effect of reaction conditions viz. reaction temperature, site time, 1-butanol and water partial pressure is studied. The reaction conditions govern the coverage of key surface species which in turn has a significant role in determining the dominant reaction mechanism and product selectivity. Under industrially relevant conditions, the presence of water has no significant effect on 1-butanol conversion and product selectivity. A higher reaction temperature, higher site time and lower 1-butanol partial pressure favor a higher 1-butene selectivity.