Besides being a renewable energy source, the catalytic conversion of bio-alcohols can serve as a sustainable means for the production of high value chemicals. Butenes produced by dehydration of butanol could serve as a building block for several essential compounds such as fuels and polymers. Nevertheless, the selective conversion of the feed is the key to cost effectiveness and success of these processes. An in-depth understanding of the underlying reaction mechanism is necessary for the selection and design of an appropriate catalyst. Alcohol dehydration on Brĝnsted acid sites can occur via intramolecular and intermolecular reaction pathways [1,2,3]. Moreover, 1-butene produced by dehydration of 1-butanol can undergo double bond and skeletal isomerization to form cis/trans-2-butene and isobutene respectively.
In this study, we present a first principles based microkinetic model to study the conversion of 1-butanol to di-1-butyl ether and butene isomers in H-ZSM-5 and H-ZSM-22 zeolite (Figure 1). Dispersion-corrected periodic density functional theory (DFT-D2) is used to elucidate the underlying reaction mechanism and to construct the microkinetic model, which in turn allows to gain insights into the dominant reaction pathway and the effect of reaction conditions on reaction rates and product selectivity. The adsorbed 1-butanol molecule can undergo a direct dehydration reaction producing 1-butene via several mechanisms (i.e. E1, syn-elimination, anti-elimination, butoxide-mediated dehydration, butanol-assisted syn-elimination) or react in a sequential manner to yield di-1-butyl ether (via SN1 or SN2 substitution reactions) which can further decompose to 1-butene and 1-butanol (via syn- or anti-elimination). The double bond isomerization (via butoxide-mediated stepwise or concerted mechanisms) and the skeletal isomerization (via a cyclic transition state) of 1-butene produced from the dehydration reaction is also investigated.
The reaction energetics clearly favor the intermolecular dehydration of butanol to ether via the SN2 reaction mechanism followed by ether decomposition via syn-elimination. The calculated activation barriers of 92 and 140 kJ/mol for ether formation and decomposition in H-ZSM-5 are in close agreement with the literature-reported values . A comprehensive investigation of the effect of reaction conditions, viz. reaction temperature, site time,1-butanol and water partial pressure, on the reaction rates and product selectivity is performed using reaction path analysis. The microkinetic simulation results were able to capture the experimentally observed trends for the dehydration of 1-butanol in H-ZSM-5 . They also reveal the crucial role of reaction conditions in determining the key surface species, dominant reaction mechanism and pathway. These insights on the change in dominant mechanisms allow us to reconcile the conflicting observations reported at different operating conditions. Finally, the difference in performance of the investigated zeolites in butene skeletal isomerization reaction is attributed to a better stabilization of the transition state structures in H-ZSM-22 over H-ZSM-5. Such insights on the effect of zeolite topology on the reaction rates and product selectivity can guide us towards a rational catalyst design.
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Figure 1: Reaction scheme for conversion of 1-butanol to di-1-butyl ether and butene isomers