Tautomerization of Phenol at the External Lewis Acid Sites of Scandium-, Iron- and Gallium-Substituted Zeolite MFI

We have employed density functional theory calculations to analyze the possible tautomerization of phenol mediated by three different Lewis acid sites at the external (010) surface of zeolite MFI. A silicon atom of the silanol group was substituted by Sc, Fe, and Ga metal atoms, which adopted a formal charge of 3+. This substituted silanol was dehydrated in order to form three-coordinated Lewis acid sites. The tautomerization of phenol involves the adsorption of the molecule on the Lewis site, the dissociation of the phenolic O–H bond and the transfer of the proton to the zeolite framework. This proton is transferred from the zeolite to the C atom at ortho positions to the phenolic O atom, thus generating the tautomer. The acidity of the substituted Lewis sites follows a strength order of Ga < Fe < Sc, where the Sc substitution provides the lowest energy barriers: 33 kJ/mol for the dissociation of the O–H bond, and 32 kJ/mol for the formation of the C–H bond, calculated with the GGA functional PBE including Grimme’s dispersion corrections. We observed that the GGA functional PBE produces binding energies and energy barriers with a difference of less than 13 kJ/mol compared to the meta-GGA TPSS and rev-TPSS and the hybrid-GGA HSE06.