Roles of alkali-metal added to Cu-NaZSM-5 catalysts in the oxidation of benzyl alcohol

Abstract

Alkali-metal addition to Cu-NaZSM-5 zeolite promotes oxidation activity (mainly partial oxidation rather than complete oxidation) using benzyl alcohol oxidation as a model reaction. The alkali-metal added to Cu-NaZSM-5 is present as the corresponding oxide, such as Na₂O. The percentage of Cu ion exchanged and the added Na/Cu atomic ratio in Na-added Cu-NaZSM-5 are important factors governing the oxidation activity. The added alkali-metal is suggested to be located in the neighbourhood of the Cu species as ligands, rather than covering the Cu species in Cu-NaZSM-5 zeolite. The influence of added Na on the adsorption and redox properties of Cu-NaZSM-5 zeolite were investigated to shed light on the role of the added alkali-metal during oxidation by means of ethanol and CO adsorptions, O₂ uptake, TPD of ethanol and temperature-programmed reaction, in addition to the results of the oxidation reaction. The amounts of O₂ uptake by the prereduced Na-added Cu-NaZSM-5 were considerably larger than those by the corresponding Cu-NaZSM-5 without alkali-metal. The added alkali-metal is considered to help accommodate the adsorbed oxygen species, which is dissociated and is responsible for the partial oxidation of benzyl alcohol. The added alkali-metal also allows the Cu^I species to be more easily oxidized and to adsorb greater amounts of reactant alcohol; this explains the promotion effect of the added alkali-metal observed in the gas-phase catalytic oxidation of benzyl alcohol.