Catalytic methylation of phenol on MgO - Surface chemistry and mechanism

Reactions of phenol and methanol catalyzed by MgO have been explored by kinetic measurements and in situ IR spectroscopy combined with computational studies of sorbed molecules. On MgO, methanol partly transforms to formaldehyde above 250 °C. Adsorbed phenol forms phenolate species, and the energetically preferred mode of adsorption leads to an almost orthogonal orientation of the aromatic ring with respect to the catalyst surface. All molecules involved adsorb preferably at the corner sites of MgO (three-co-ordinated Mg atoms). The main reaction products are anisole and o-cresol, the latter dominating above 300 °C. At very low conversions, salicylic aldehyde is observed as primary reaction product, being rapidly transformed to o-cresol. It is only observed during the initial accumulation of adsorbed species on the catalyst surface, but not under steady-state conditions on a fully covered catalyst surface. Therefore, o-cresol formation starts with the reaction between phenol and formaldehyde to salicylic alcohol, which in turn is rapidly transformed to salicylic aldehyde and subsequently to o-cresol. Salicylic aldehyde may also form via the bimolecular disproportionation of salicylic alcohol to o-cresol and aldehyde. The parallel reaction to o-cresol, not involving the formation of salicylic aldehyde as intermediate, proceeds via the reduction of salicylic alcohol to o-cresol by formaldehyde. The identified mechanism may open new synthetic approaches for the production of functionalized phenol derivatives and, even more importantly, for the defunctionalization of substituted phenols potentially available at large scale from deconstructed lignin.