Solvent-determined mechanistic pathways in zeolite-H-BEA-catalysed phenol alkylation

Abstract

Alkylation of phenolics is of great importance in synthetic chemistry and the valorization of lignocellulosic-biomass-derived streams. Here, we unravel how alkylating reactants and solvents significantly alter the reaction pathways of zeolite-catalysed alkylation of phenol in the liquid phase. The carbenium ion formed from the dehydration of cyclohexanol or from the adsorption and protonation of cyclohexene acts as the electrophile, inducing carbon–carbon bond formation. Cyclohexanol at Bronsted acid sites (BAS) forms hydrogen-bonded monomers and protonated dimers in apolar solvents. The dimer appears to generate a much lower concentration of carbenium ions compared with the monomer. Higher alkylation rates in apolar solvents than in water are caused by the energetically more-favourable carbenium ion formation from either alcohol or olefin on non-hydrated zeolite BAS than on hydronium ions produced by BAS in pores filled with water. Zeolite-catalysed alkylations of phenolic compounds offer unique possibilities for the valorization of renewable aromatics into substituted arenes. Now, a mechanistic study reveals that the course of the reaction can be dramatically altered by changing the polarity of the solvent, which affects the nature of surface species and the pathway for the generation of the alkylating electrophile.