Hydrodechlorination of Tetrachloromethane over Palladium Catalysts Supported on Mixed MgF(2)-MgO Carriers

Pd/MgO, Pd/MgF(2) and Pd/MgO-MgF(2) catalysts were investigated in the reaction of CCl(4) hydrodechlorination. All the catalysts deactivated in time on stream, but the degree of deactivation varied from catalyst to catalyst. The MgF(2)-supported palladium with relatively large metal particles appeared the best catalyst, characterized by good activity and selectivity to C(2)-C(5) hydrocarbons. Investigation of post-reaction catalyst samples allowed to find several details associated with the working state of hydrodechlorination catalysts. The role of support acidity was quite complex. On the one hand, a definite, although not very high Lewis acidity of MgF(2) is beneficial for shaping high activity of palladium catalysts. The MgO-MgF(2) support characterized by stronger Lewis acidity than MgF(2) contributes to very good catalytic activity for a relatively long reaction period (~5 h) but subsequent neutralization of stronger acid centers (by coking) eliminates them from the catalyst. On the other hand, the role of acidity evolution, which takes place when basic supports (like MgO) are chlorided during HdCl reactions, is difficult to assess because different events associated with distribution of chlorided support species, leading to partial or even full blocking of the surface of palladium, which plays the role of active component in HdCl reactions.