Catalytic oxidation of volatile organic compounds (VOCs). Oxidation of o-xylene over Pd and Pt/HFAU catalysts

The transformation of o-xylene in low concentration (1 700 ppmv) into air w as investigated over Pd and Pt/HFAU catalysts (framework Si/Al ratio equal to 17 and 100). Whatever the catalyst, o-xylene oxidation into CO2 and H2O is accompanied by the retention within the zeolite pores of heavy compounds ('coke'). The relative significance of these reactions depends on the oper ating conditions (temperature, time-on-stream) and on the catalyst characte ristics (Pd or PI, Si/Al ratio). Over Pt and Pd/HFAU(17), time-on-stream ha s a positive effect on the xylene oxidation apparently related to the reduc ibility of Pd and Pt species during the reaction. The higher activity of Pt /HFAU catalysts can be attributed to its greater number of active species ( especially Pt-o). Those active species can be more rapidly formed than Pd-o by auto reduction during the calcination of Pt precursor. Whatever the met al, the higher the Si/Al ratio of the support, the faster the xylene oxidat ion and the lower the coke formation. This can be related to the higher pro portion of reduced species (Pd-o and Pt-o) formed on the more dealuminated catalyst but also to the hydrophobicity of the support. Indeed, the hydroph obicity of the zeolite play a positive role in the oxidation activity in pr esence of steam; the higher the Si/Al ratio of the zeolite, the faster the o-xylene oxidation. Thus a catalyst with a low platinum content supported o n a hydrophobic zeolite (0.10 Pt/HFAU(100)) allows to oxidising totally o-x ylene at 210 degreesC in presence of steam. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.