The catalytic transformation of chlorofluorocarbons in hydrogen on metal-based catalysts supported on inorganic fluorides

The use of metal halides as carriers for supported metal catalysts allows to obtain stable and selective materials for the hydrodechlorination of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) to hydrofluorocarbons (HFCs). The most used catalyst is Pd/AlF₃, but unexpected selectivities have been obtained with Pd on “ZrF₄” oxyfluoride materials or KMgF₃ perovskite-like structure. From a survey of the kinetics, mechanism and surface complexes occurring in the transformation of CFC on Pd, explanations of the beneficial use of metal fluorides as carriers are provided. It is proposed that the good hydrodechlorination selectivity observed on Pd/fluoride comes from an electronic modification of Pd by substoichiometric fluoride species, e.g. AlF_x (x<3), in decoration onto the metal particles. The electron withdrawing effect of these species decreases the disponibility of Pd d electrons and favors the desorption of pallado-fluorocarbenes, e.g. =CF₂, CF₃–CF=, etc., to yield HFC compounds. It is also demonstrated that the dilution of the Pd surface by the decorating AlF_x species decreases the probability of occurrence of surface complexes exchanging multiple bonds with Pd, e.g. CF₃–C=, and leading to deeply hydrogenated compounds. Several methods, alloying, coprecipitation, etc. allow to prepare Pd/fluoride with enhanced interaction between Pd and these substoichiometric species.