Studies in surface science and catalysis vol:84 issue:Part A-C pages:1457-1464
The effect of solvent polarity on reaction rate and selectivity in the cyclohexylation of phenol over an ultrastable Y zeolite is analyzed in terms of its effect on the adsorption of reagents and products on the one hand and on the surface reaction on the other hand. It is shown that in absence of diffusional limitations, variations of activity and selectivity with solvent polarity are determined by the equilibrium distribution of reagents and products between the bulk solution and the cavities of the zeolites, the latter acting as micro-reactors. Polar solvents, as nitrobenzene and nitropropane strongly adsorb on the catalyst and suppress catalytic activity. Due to the high polarity of the solvent, the primary products are expelled from the catalyst, suppressing consecutive reactions. In apolar solvents, the concentration of phenol on the catalyst increases at the expense of cyclohexene, the latter becoming more stabilized in the bulk solution. This also results in decreased rates of primary and secondary reactions. At intermediate solvent polarity both reagents adsorb on the catalyst, resulting in the highest reaction rate.
ZEOLITES AND RELATED MICROPOROUS MATERIALS: STATE OF THE ART 1994