Alkylation of phenol with cyclohexene over solid acids: insight in selectivity of O- versus C-alkylation

Abstract

Alkylation of phenol with cyclohexene with acid catalysts leads to the formation of both O- and C-alkylated products, which are all useful in a variety of industries. The O-alkylated product cyclohexyl phenyl ether is a valuable perfume and can also serve as a precursor to diphenyl ether, a very important bulk chemical. The efficacy of various acid catalysts such as sulphated zirconia, sulphonic acid treated hexagonal mesoporous silica (SO₃-HMS), 20% (w/w) dodecatungstophospheric acid (DTP) supported on K-10 clay, 20% (w/w) cesium salt of DTP (Cs_2.5H_0.5PW₁₂O₄₀) supported on K-10 clay (Cs-DTP/K-10) and 20% (w/w) DTP/HMS was studied to improve the selectivity to cyclohexyl phenyl ether. A mixture of 2-cyclohexylphenol, 4-cyclohexylphenol and cyclohexyl phenyl ether was obtained with different selectivities. However, 20% (w/w) DTP/K-10 clay was the most active and selective catalyst for O-alkylation in the range of 45–70 °C at atmospheric pressure. The selectivity to O- versus C-alkylation is strongly dependent on temperature, and at lower temperatures, the selectivity to cyclohexyl phenyl ether increases. The best operating temperature is 60 °C. A mathematical model is built to interpret the kinetic data and develop a mechanism.