CATALYTIC HYDRODECHLORINATION: POLLUTION ABATEMENT METHODOLOGY AND SERENDIPITOUS ROUTE TO ORDERED CARBON GROWTH
Dr. Mark A. Keane
Department of Chemical and Materials Engineering
University of Kentucky
Friday, February 23, 2001 3:00pm
Ben Bandy Conference Center
Center for Applied Energy Research
Chlorinated aromatics are well established as major sources of air pollution. At present, disposal of chlorinated waste takes place mainly in hazardous waste landfills or by incineration. Disposal via landfills will soon be completely prohibited while incineration can result in the generation of highly toxic polychlorinated dibenzofurans and dibenzodioxins. Non-thermal technologies which include direct adsorption on activated carbon, solvent extraction and microbiological degradations do not constitute exhaustive solutions and only offer a means of concentration. Moreover, if the extracted materials are mixtures of chlorinated isomers, these are not, without some difficulty, recovered for reuse. Catalytic hydrodehalogenation represents an alternative and innovative approach whereby the hazardous material is transformed into recyclable products in a closed system with no toxic emissions. Moreover, mixed isomers arising from an uncontrolled chlorination step can be converted back to the single parent raw material precursor from which they originated, in short a unique process of chemical desynthesis.
This presentation will focus on the gas phase hydrodechlorination of a range of mono- and poly-chlorinated aromatics using Ni/SiO2 catalysts. The role of catalyst preparation/composition in determining dechlorination efficiency is considered and catalyst stability is discussed. The suitability of several kinetic expressions, based on mechanistic considerations, to represent the experimental data are assessed. Model refinement has taken into account reaction on non-uniform surfaces with the possible involvement of spillover hydrogen. Process optimization in the treatment of a polychlorinated feedstock is addressed in terms of a complete dechlorination to raw material or a partial (selective) dechlorination to a target isomer.
Catalyst deactivation can be attributed to the formation of an unreactive surface NiCl2 species. The latter effect was probed by contacting a freshly activated Ni/SiO2 catalyst directly with the pertinent hydrogen halide and monitoring the subsequent catalytic activity. A dramatic loss of activity resulted which was unexpectedly accompanied by the growth of filamentous carbon from the surface of the catalyst. The growth of such carbonaceous materials is certainly well established either by arc discharge/hydrocarbon decomposition methodologies but each route typically requires temperatures in excess of 723 K. In this case appreciable carbon filament growth was achieved at 553 K, i.e. at least 150 K lower than typically required. This presentation will include some preliminary characterization results for the filamentous carbon growth that has been generated in this serendipitous manner.