Tandem Aromatization of Oxygenated Furans by Framework Zinc In Zeolites. A Computational Study

We have performed electronic structure calculations to characterize the active site of the zeolite CIT-6 (isomorphically substituted Zn-Beta) and to study the Diels–Alder dehydrative aromatization of methyl-5-(methoxymethyl)-furoate (MMFC) and of the dimethyl ester of 2,5-furan-dicarboxylic acid (DMFDC) with ethylene. Three types of active sites have been investigated: a site where the framework charge was balanced by two protons (Z0); a site with an H+ and a Li+ as counter-cations (Z1); and an active site with two Li+ counter-cations (Z2). Using NBO and Bader analysis, we conclude that Zn incorporation into the zeolite framework is not through covalent bonding to framework oxygen atoms but rather is through ionic bonding. Despite the ionic character of the active site and the generally strong Lewis acidic nature of Zn­(II) cations, we find no catalysis of the Diels–Alder reaction for the two furans tested. On the other hand, the dehydration of the Diels–Alder cycloadduct can be either Brønsted or Lewis acid catalyzed depending on the active site type. The Z0-type sites are found to be more active than the Z1- and Z2-type sites. Furthermore, the Z0-type sites exhibit both Lewis and Brønsted acid characters with similar catalytic activities. In full agreement with experiment, we find that the conversion of MMFC to (4-methoxymethyl) benzenecarboxylate via Diels–Alder dehydrative aromatization is easier than the conversion of DMFDC to dimethyl terephthalate as the two electron withdrawing groups in DMFDC stabilize the corresponding cycloadduct against dehydration.