Phenol oxidation by nitrous oxide: The role of zeolite catalyst acidity

Abstract

Gas-phase phenol oxidation using nitrous oxide on zeolite catalysts is of great interest as an alternative method of dihydroxybenzene (DHB) synthesis. Catalysts for conducting this reaction with high selectivity in the sum of three isomers of DHB (85–90%) were discovered earlier. The aim of this work is to optimize these catalysts to regulate the regioselectivity of the reaction. The effect of thermal steam treatment (TST) on zeolite catalytic properties is studied. Using NMR and IR spectroscopy, an increase in the TST temperature in the range of 550–750°C is shown to be accompanied by 5-fold reduction in the concentration of Bronsted acidic sites in zeolite with a simultaneous 2to 3-fold increase in the number of a-sites responsible for the acidity in oxidation by nitrous oxide. Varying the TST temperature is found to be powerful tool for influencing the reaction selectivity in individual DHB isomers. The reduction in zeolite acidity changes the isomer ratio in favor of catechol. The reasons for this phenomenon and its relation to catalyst deactivation are discussed.