Catalytic oxidation of toluene over CuNaHY zeolites: Coke formation and removal

Abstract

Catalytic deep oxidation of toluene at low concentration in air, has been studied over CuNaHY zeolites with different copper contents. The reaction has been investigated in the course of time-on-stream, at temperatures between 150 and 500°C. The effect of copper content on the conversion of toluene into CO2 and on the formation of carbonaceous deposits (coke) retained inside the zeolite pores has been analyzed as a function of the reaction temperature. The analysis of the coke extracted from the catalysts showed that it is mainly composed of aromatic hydrocarbons and oxygenated aromatic compounds, the latter being predominant at low temperature and preferentially formed on the catalysts containing more copper. A bifunctional mechanism was proposed to explain the formation of coke over CuNaHY catalysts. Based on the differences of the nature of coke formed on NaHY and CuNaHY catalysts, and on a discussion about the coke formation and location, it was possible to conclude that copper sites are involved in both toluene and coke oxidation reactions. The increase of copper content from 1 to 8wt.% promotes the oxidation of toluene and facilitates the removal of coke formed in the course of reaction.