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HYDEODECLORINATION OF CCI2F2 USING PD SUPPORTED ON Y-AL2O3 CATALYSTS
Citation:
Sridara,**CS, K. S. Rama Rao, AND E SahleDemessie*. HYDEODECLORINATION OF CCI2F2 USING PD SUPPORTED ON Y-AL2O3 CATALYSTS. Presented at 228th American Chemical Society National Mtg, Philadelphia, PA, August 22 - 26, 2004.
Impact/Purpose:
To inform the public
Description:
Chlorofluorocarbons (CFCs), along with other chlorine- and bromine-containing compounds, have been implicated in the accelerated depletion of ozone in the Earth's stratosphere. The use of chlorofluorocarbons (CFCs) has been proscribed worldwide since they are known to be responsible for stratospheric ozone depletion. Research groups have been working for effective conversion of the CFCs in to useful compounds [1-4]. Selective hydrodechlorination of CFCs has been identified as a useful process for their transformation as it yields either hydrochlorofluorocarbons (HCFCs) or hydrofluorocarbons (HFCs) that have low or almost zero ozone depletion potential (ODP) value. Hydrochlorofluorocarbons (HCFCs) still contain chlorine atoms, but the presence of hydrogen makes them reactive with chemical species in the troposphere. This greatly reduces the prospects of the chlorine reaching the stratosphere, as chlorine will be removed by chemical processes in the lower atmosphere. Hydrofluorocarbons (HFCs), potential replacements for CFCs that contain no chlorine, have been evaluated for potential effects of fluorine compounds on ozone destruction. Selective hydrodechlorination of CCl2F2 (CFC-12) yields CH2F2 (HFC-32), which itself is a refrigerant and has zero ozone depletion value. In the selective hydrogenolysis of CCl2F2, Pd supported on -Al2O3, active carbon, graphite, AlF3, MgF2 and TiO2, ZrO2, TiF3, ZrF4 were used. Palladium black was also used as a catalyst.
In this study, alumina supported palladium catalysts are prepared by wet impregnation technique with varying Pd loading. The catalysts after drying are calcined at 400oC for 4h in hydrogen flow to obtain Pd in poorly dispersed state. These catalysts are tested for their activity and selectivity in the hydrogenolysis of CCl2F2 to CH2F2. Using modification of conventional temperature program reduction set-up, it has been possible to distinguish between halide reduction and carbon reduction and based on these results mechanism of reduction of Pd/Al2O3 (both fresh and spent forms of catalysts) is elucidated.
Low dispersed Pd/ -Al2O3 catalysts are obtained by H2 reduction of the catalysts at 400oC. Increase in particle size is found to increase the CCl2F2 hydrogenolysis activity and CH2F2 selectivity. On Pd/ -Al2O3 catalysts CH4 selectivity was found to be more at reaction temperature of more than 240 oC and at such temperatures they were also found to deactivate due mainly to coke formation. While -PdHx formation was observed on Pd/ -Al2O3 catalysts, poorly dispersed Pd catalysts are found to help attain steady state faster in the hydrogenolysis of CCl2F2. Transmission electron microscope revealed that there is a strong redespersion of palladium is taking place during the reaction. Studies on the effect of palladium loading on the activity and selectivity in the conversion of CCl2F2 to CH2F2 and CH4 have shown that 8wt% Pd on - Al2O3 is optimum in giving maximum CH2F2 yields. The hydrodechlorination activity correlates with particle size from TEM, CO-uptake, X-ray diffraction and temperature programmed reduction data. The results show that Pd particle size has influence on the CCl2F2 hydrogenolysis activity and selectivity to CH2F2 and CH4.