PNNL

Abstract

The alkylation of phenol with cyclohexanol/cyclohexene on acidic zeolites is investigated under relatively mild conditions (= 160 °C) in decalin. The reaction is catalyzed primarily by Brønsted acid sites (BAS). Moderately strong BAS and spacious microporous environments (i.e., large-pore acidic zeolites HBEA and HY) are important criteria for effective phenol alkylation to form C-C coupling products, while very strong BAS appear to be responsible for rapid catalyst deactivation. Turnover frequencies of phenol alkylation with both cyclohexanol and cyclohexene are higher when BAS are confined in HBEA and HY zeolite pores than in solids without molecularly sized pore constraints. Product distribution is adjusted by the acidity and structure of zeolites, as well as the ratios of reactants. HBEA favors the formation of mono-alkylation products, while HY catalyzes the reaction with enhanced formation of di-alkylates. The alkylation of mixed aromatic oxygenates with cyclic alcohols shows largely similar kinetic characteristics to phenol-cyclohexanol alkylation. Dehydration of alcohols always dominates over the alkylation at the onset of the reaction. A dominant fraction of cyclohexanol/substituted cyclohexanols in HBEA and HY pores exists as protonated dimers at prevalent operating conditions. Carbenium ions, the direct electrophile for aromatic oxygenates, are produced mainly from the adsorption and protonation of olefins. They also form from alcohol monomers, hydrogen-bonded to the BAS at low alcohol concentrations (= 0.1 M), but not from protonated alcohol dimers. Therefore, for a tandem operation with these oxygenates (alcohol dehydration followed by phenols-olefins alkylation) in apolar liquids, the alcohol concentration must be kept low to avoid the formation of unreactive surface species and mitigate their inhibitory effects on olefin adsorption and protonation.