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

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;It temperatures between 150 degrees C and 320 degrees C. At short time-on-stream (TOS), whatever the temperature, no xylene appears at the reactor outlet. Below 250 degrees 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 CO2. At higher temperatures, the conversion of xylene into CO2 increases with TOS. At 290 degrees C, it becomes complete for TOS>30 min. The amount of coke passes through a maximum at a reaction temperature of 230 degrees C. The composition of coke at TOS=60 min was determined at various temperatures. At 150 degrees 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 degrees C, oxygenated compounds become predominant whereas above 250 degrees 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. (C) 1999 Elsevier Science B.V. All rights reserved.