Hydrodechlorination of tetrachloroethylene over modified red mud:: deactivation studies and kinetics

In previous works, sulphided red mud (a by-product in the production of alumina by the Bayer process) was shown to be active as catalyst for the hydrodechlorination of tetrachloroethylene, although it presented low stability at reaction conditions. In this work, the performance of red mud modified according to different methods proposed in the literature has been studied. The activation methods tested consist essentially in dissolving the red mud in an acid solution (HCl or a mixture HCl/H3PO4), followed by precipitation with ammonia and calcining at 500 degreesC. Catalysts prepared by these methods have been found to present better activity and resistance to deactivation than unmodified red mud. Fresh and used samples of the catalysts tested were characterised by nitrogen adsorption, X-ray diffraction, scanning electron microscopy, X-ray dispersion spectroscopy, temperature programmed oxidation, NH3 temperature programmed desorption and thermogravimetric analysis. Results lead to think than in the case of modified red mud, deactivation is related with fouling phenomena, whereas in the case of unmodified red mud, the poisoning by HCl is the main deactivation cause. Finally, the kinetic of the hydrodechlorination of tetrachlorethylene over the two modified red mud was studied, both with and without addition of CS2 in the reaction feed. In all the cases a kinetic model based on the Langmuir-Hinselwood postulates, considering chemisorption of both TTCE and H-2 (associatively adsorption) in analogous active sites, provided a good fit to the experimental data. (C) 2001 Elsevier Science B.V. All rights reserved.