Mechanism of Phenol Alkylation in Zeolite H-BEA Using in Situ Solid-State NMR Spectroscopy

The reaction mechanism of solid-acid-catalyzed phenol alkylation with cyclohexanol and cyclohexene in the apolar solvent decalin has been studied using in situ ¹³C MAS NMR spectroscopy. Phenol alkylation with cyclohexanol sets in only after a majority of cyclohexanol is dehydrated to cyclohexene. As phenol and cyclohexanol show similar adsorption strength, this strict reaction sequence is not caused by the limited access of phenol to cyclohexanol, but is due to the absence of a reactive electrophile as long as a significant fraction of cyclohexanol is present. ¹³C isotope labeling demonstrates that the reactive electrophile, the cyclohexyl carbenium ion, is directly formed in a protonation step when cyclohexene is the coreactant. In the presence of cyclohexanol, its protonated dimers at Brønsted acid sites hinder the adsorption of cyclohexene and the formation of a carbenium ion. Thus, it is demonstrated that protonated cyclohexanol dimers dehydrate without the formation of a carbenium ion, which would otherwise have contributed to the alkylation in the kinetically relevant step. Isotope scrambling shows that intramolecular rearrangement of cyclohexyl phenyl ether does not significantly contribute to alkylation at the aromatic ring.