Tertiary butylation of phenol on Cu_1-xCo_xFe₂O₄: catalysis and structure-activity correlation

Abstract

A systematic study of catalytic tertiary butylation of phenol was carried out with isobutene as a function of temperature, feed composition, time on stream, space velocity, and catalyst composition on Cu_1-xCo_xFe₂O₄ (x=0 to 1) system. Tertiary butylation of phenol gives three products, namely, 2-tert-butyl phenol, 4-tert-butyl phenol, and 2,4-di-tert-butyl phenol. The phenol conversion and selectivity of these products depend on the reaction parameters. A good correlation was found between the activity, in terms of phenol conversion and various product selectivities for this reaction, and the acid-base properties of the catalysts. High activity is achieved with x=0.5 composition, illustrating the importance of a 1:1 combination of Cu and Co and the necessity for optimum concentrations of acid-base centers for this reaction. A reaction mechanism involving the interaction of phenoxide from phenol and the tert-butyl cation from isobutene on Cu_1-xCo_xFe₂O₄ is proposed. X-ray photoelectron spectroscopy and X-ray induced Auger electron spectroscopic analysis of fresh and spent catalysts revealed a partial reduction of metal ions due to reaction. Valence band studies clearly revealed an increase in the overlap of metal ion 3d bands from fresh to spent catalysts as reflected from a large decrease in the energy gap between them. The better catalytic results observed with x=0.5 are attributed to an optimum distribution of Cu species with heteroatom neighbors, maximum overlap between the Cu and Co 3d bands, and intermediate acid-base character.