Abstract
Pd/MgO, Pd/MgF2 and Pd/MgO-MgF2 catalysts were investigated in the reaction of CCl4 hydrodechlorination. All the catalysts deactivated in time on stream, but the degree of deactivation varied from catalyst to catalyst. The MgF2-supported palladium with relatively large metal particles appeared the best catalyst, characterized by good activity and selectivity to C2-C5 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 MgF2 is beneficial for shaping high activity of palladium catalysts. The MgO-MgF2 support characterized by stronger Lewis acidity than MgF2 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.
Highlights
Catalytic hydrodehalogenation (HdCl) of harmful organic compounds, such chloromethanes, CFCs or chlorobenzenes on supported metal catalysts, appear a useful method for transforming those detrimental compounds into valuable chemicals [1,2]
Characterization of prepared MgO-MgF2 carriers was reported in earlier publications, the “sol-gel”
For the “sol-gel” series, the content of magnesium oxide had a considerable effect on the surface area of the MgO-MgF2 systems, i.e., the surface area of MgO-MgF2 was seven times larger than that of pure magnesium fluoride and still larger than that of pure MgO
Summary
Catalytic hydrodehalogenation (HdCl) of harmful organic compounds, such chloromethanes, CFCs or chlorobenzenes on supported metal catalysts, appear a useful method for transforming those detrimental compounds into valuable chemicals [1,2]. Studied in the reaction of hydrodechlorination of 1,1-dichlorotetrafluoroethane, Pd catalysts supported on AlF3 and MgF2 (characterized by very dissimilar acidities) did not show large differences in their catalytic behavior [7]. This does not seem to function for highly acidic supports which, in addition, produce polymeric species, which in turn would be spilt over onto the metal surface [18] In this respect, advantages of MgO as appropriate support of the hydrodechlorination catalyst are presented by several authors [20,21,22]. By increasing the fluorine content of the magnesium oxide fluoride, the Lewis acidity increases whereas the basicity decreases [30] These sites can be tuned over a wide range of MgO-MgF2 composition giving access to optimized catalytic activity and selectivity of these phases as was found for Michael additions [31]. MgO by MgCl2 should result in an increase in the strength of Lewis acid sites on the surface of the catalyst
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