Disproportionation and Transalkylation of Phenol and Dimethylphenol on Zeolite Catalysts

Abstract

Substantial amounts of phenolic compounds are present in medium-low temperature coal tar (MLCT), of which dimethylphenol (DMP) is not as valuable to be utilized as the more abundant cresol due to its complex composition and difficulty in isolation. In this study, the disproportionation and transalkylation reactions of MLCT-related model compound, i.e., 2,6-DMP, in phenol over zeolite catalysts are investigated using a fixed-bed reactor for sustainable new option to utilize MLCT-derived phenolic mixtures. Reactivity is promoted at high temperatures and associated with zeolite acidity and pore structure. Since disproportionation and transalkylation reactions have certain spatial requirements, the micropores of MFI-type zeolite may cause spatial barriers that make it difficult to carry out the reactions. MCM-22, characterized by MWW-type zeolite, maximized the conversion of 2,6-DMP due to its strong Bronsted acidity and large mesopore volume. FAU-type zeolite HY with a 3D large 12-ring through-channel shows relatively small spatial confinement and certain molecular sieving ability, which enables bimolecular reactions while allowing cresols to flow out of the pores efficiently to obtain the highest cresol selectivity. The addition of phenol significantly inhibits the spontaneous disproportionation of 2,6-DMP to tricresol. Besides, o-cresol dominate the cresol products, suggesting that the selectivity of o-cresol is kinetically controlled.