Density functional theory study of the zeolite-mediated tautomerization of phenol and catechol

Because the structure of lignin consists mostly of inter-linked phenolic monomers, its conversion into more valuable chemicals may benefit from isomerization processes that alter the electronic structure of the aromatic rings. The tautomerization of phenolic-type compounds changes the hybridization from sp2 to sp3 of the carbon atom at the ortho position, which disables the aromaticity and facilitates the subsequent hydrogenation process. Here, we have performed a Density Functional Theory study of the tautomerization of phenol and catechol at the external surface of zeolite MFI. The tautomerization starts with the adsorption of the molecule on three-coordinated Lewis acid sites, followed by the dissociation of the phenolic hydroxyl group, with the transfer of the proton to the zeolite framework. The rotation of the deprotonated molecule enables a more favourable orientation for the back-transfer of the proton to the carbon atom at the ortho position. The energy barriers of the process are smaller than 55 kJ/mol, suggesting that this transformation is easily accessible under standard reaction conditions.