First principles kinetic study on the effect of zeolite framework on 1-butanol dehydration

Understanding the role of zeolite topology in defining its catalytic performance is of prime importance for the development of catalytic processes. Herein, a first-principles-based microkinetic study of 1-butanol dehydration is used to illustrate the effect of different zeolites (i.e., H-FAU, H-ZSM-5, H-ZSM-22, and H-FER) on the dehydration activity and product selectivity. Under identical reaction conditions, microkinetic simulations show significant variation in dehydration rates and butene/ether selectivity profiles within the different zeolites. H-ZSM-5 has the highest catalytic activity, whereas H-FAU and H-FER exhibit a higher butene selectivity. In the large pore H-FAU, the weaker dispersive stabilization of the dimer makes the butene formation by monomolecular direct dehydration via a concerted anti elimination compete with di-n-butyl ether formation. In H-FER, steric constraints due to partial confinement of the protonated di-n-butyl ether in the 8-MR channel decrease its stability, favoring its further decomposition to butene via a concerted syn elimination of butanol. On the other hand, the higher ether selectivity in H-ZSM-5 and H-ZSM-22 is rationalized on the basis of a higher stability for adsorbed ether and a higher activation barrier for ether decomposition. In addition to the effect of the zeolite framework, this study further highlights the pivotal role of the reaction conditions in determining the most abundant reaction intermediate, dominant reaction paths, and underlying reaction mechanisms. In general, for all four zeolites, an increase in reaction temperature and a decrease in butanol feed partial pressure favors direct dehydration of butanol to butene (via butanol monomer). However, a decrease in reaction temperature and increase in butanol feed partial pressure favors dimer-mediated dibutyl ether formation. An increase in conversion favors direct dehydration and dibutyl ether decomposition to butene.

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