Dehydrocondensation of alcohols to form ethers over mesoporous SBA-15 catalyst

Abstract

A propylsulfonic acid-derivatized mesoporous SBA-15 catalyst was examined for activity and selectivity in the dehydrocondensation of methanol and isobutanol to form ethers, principally methyl isobutyl ether (MIBE) and dimethyl ether (DME), and dehydration of isobutanol to olefins, at elevated temperatures and pressures. It was shown that the catalyst maintained activity for > 1600 h , and the Brønsted acid sites corresponding to 1 meq/g of catalyst were stable under these reaction conditions until they were deliberately in situ poisoned by addition of pyridine. X-ray photoelectron spectroscopy (XPS) revealed that the pyridine was selectively and strongly adsorbed on the sulfonic acid groups. Kinetic studies demonstrated that MIBE and DME were formed by surface-catalyzed S N2 reactions that follow Langmuir–Hinshelwood kinetics involving competitive adsorption of the alcohols, while isobutene formation utilizes a vacant Brønsted acid site adjacent to the adsorbed isobutanol molecule. Isobutene formation rate exhibited a distinct maximum as a function of the alcohol pressure. This kinetic behavior is similar to that observed with Nafion-H and other catalysts. Because of the high apparent activation energy of isobutene synthesis compared with the much lower apparent activation energies of the ethers, lower reaction temperatures along with higher reactant pressures favor the formation of ethers over olefins. Theoretical modeling of the reaction pathway located stationary points including the transition states (TS) with barrier energies for all reactions involved, and showed that for ether synthesis one alcohol was adsorbed on a sulfonic acid site (I) via a donor H bond while the second alcohol was adsorbed on a proximal sulfonic acid site (II) via an acceptor H bond. In the intermediate, methyl transfer from the adsorbed methanol to the isobutanol oxygen occurs to form MIBE. At the same time, isobutanol donates its proton to a sulfonic acid group (II) whose oxygen acts as a base, and the methanol OH group accepts a proton from a sulfonic acid group (I) and is split off as water. To balance the surface charge, the original proton on sulfonic acid group (II) is transferred to sulfonic acid group (I), and MIBE is desorbed. The flexibility of the TS involving the concerted movement of all three protons is essential for the connectivity of the TS with the reactants and products and the umbrella inversion of the methyl group during its transfer. The theory has thus not only yielded a picture that is fully consistent with observed isotope flows, sp 3 carbon inversion, and temperature/pressure-dependent selectivity, but also revealed unprecedented detail of the concerted proton transfer and the role of acid–base interactions in this dual-site catalysis.