(656c) Acylation of Furans in Acidic Zeolites – a DFT Study

The acylation of furans is a potentially valuable process for sustainable conversion of biomass-derived furans to aromatics such as para-methyl styrene, which is widely used in making plastics. The conventional Friedel-Crafts acylation is not environmentally friendly, as it uses Lewis acid catalysts like AlCl₃, which is corrosive, toxic and produces a large volume of acidic liquid quench during removal. Acidic zeolites have been found to be alternative heterogeneous catalysts for acylation because they offer good shape selectivity and are easy to regenerate. In this study, we use density-functional theory to investigate the acylation of methylfuran (MF) by acetic anhydride on two solid acids, the Brønsted-acidic Al-BEA and the Lewis-acidic Sn-BEA. In Al-BEA, we find classic Brønsted-acidic catalytic pathways for aromatic electrophilic substitution whereby the acetic anhyrdride accepts the acidic proton and dissociates to form the acylium cation that subsequently electrophilically substitutes a hydrogen in the MF ring. In Sn-BEA, the nature and structure of the active site changes, depending on whether the Sn metal center is hydroxylated or not. The unhydrolyzed (“closed”) site behaves in a purely Lewis-acidic fashion whereby the acylium cation attacks the aromatic ring of MF while the acetoxy anion is stabilized by coordination to the metal center. Remarkably, we find that the hydrolyzed (“open”) active site has a mixed Lewis acid-Brønsted acid character, the latter by virtue of the silanol group (SiOH) in the vicinity of the hydroxylated metal center. Contrary to expectations, we find that the catalytic pathways that actively involve the SiOH of the “open” site are Brønsted-acidic in character and favored over other pathways in the hydrolyzed and unhydrolyzed Sn-BEA sites. We present full energy profiles of all the calculated reaction pathways and find that the rate-limiting step of the most favorable pathway in Al-BEA and Sn-BEA is the proton back donation from the acylated MF to the zeolite framework, which is consistent with the available data from kinetic isotope effect studies.