Screening of Pd and Ni supported on sol-gel derived oxides for dichloromethane hydrodechlorination

Several alumina, titania, zirconia and silica xerogels modified with approximately 0.05 wt.% Pd or 0.05 wt.% Ni were synthesized by cogellation and conventional impregnation method using Pd and Ni acetylacetonate as metal precursors. Thermolysis of supported complexes was conducted in static air at 300 and 350degreesC for Pd and Ni loaded samples, respectively. The resulting catalysts were characterized by XRD, XRF, BET, TPR, and tested in the vapor phase hydrodechlorination of dichloromethane (DCM) with molecular hydrogen. Prior to reaction, catalysts were reduced at 300degreesC in 5% H-2/He. The synthetic mixture consisted of 1200 ppm CH2Cl2 1.2% H-2 and balance He at a space velocity of 0.005 g min ml(-1). Palladium supported catalysts were more active than those containing nickel, i.e. DCM conversions at 200degreesC over Pd(I)-Al2O3 almost doubled those obtained over Ni(I)-Al2O3 and no intermediate reaction products were observed over Pd supported catalysts whereas CH3Cl was formed over nickel catalysts. Comparison of the specific activities of selected catalysts expressed as TOF, indicate that the observed differences in activity are related to the nature of the support and the accessibility of the metal function to the fluid phase, even though textural properties also appear to play a role. The most active catalyst, palladium impregnated on sol-gel titania (Pd(I)-TiO2) had an initial specific activity of 0.36 s(-1) but, it strongly deactivated after a 48 h exposure to 1200 ppm DCM or by thermolysis of the Pd precursor in static air at 600degreesC. Although palladium impregnated in sol-gel alumina exhibited lower activity it was much more stable than Pd(I)-TiO2. (C) 2004 Elsevier B.V. All rights reserved.