Catalytic oxidation of volatile organic compounds 1. Oxidation of xylene over a 0.2 wt% Pd/HFAU(17) catalyst

Abstract

The transformation of o-xylene in low concentration (1700 ppmv) in air over 0.2 wt% Pd/HFAU catalyst with framework and total Si/Al ratios of 17 was investigated in a flow reactor at temperatures between 150°C and 320°C. At short time-on-stream (TOS), whatever the temperature, no xylene appears at the reactor outlet. Below 250°C, this total conversion is due only to the retention of heavy reaction products inside the zeolite pores (“coke”), whereas above this temperature it is due partly to this retention, partly to the oxidation of xylene into CO 2. At higher temperatures, the conversion of xylene into CO 2 increases with TOS. At 290°C, it becomes complete for TOS>30 min. The amount of coke passes through a maximum at a reaction temperature of 230°C. The composition of coke at TOS=60 min was determined at various temperatures. At 150°C, coke consists mainly of (i) aromatic hydrocarbons (65 wt%): methyldiphenylmethane, methylfluorene, methylanthracene, and phenanthrene compounds, and of (ii) oxygenated aromatic compounds, mainly with hydroxy groups. At 200°C, oxygenated compounds become predominant whereas above 250°C coke is mainly constituted by very polyaromatic compounds resulting from the transformation of oxygenated compounds. Mechanisms involving successively protonic and palladium sites (bifunctional catalysis) are proposed to explain the formation of the main coke components.