Catalytic Hydrodehalogenative coupling of Dichlorodifluoromethane on Supported Platinum and Palladium Bimetallic Catalysts

摘要

Ethylene and propylene are very important chemicals used as feedstocks in modern days. Halocarbons can be a promising starting material to form C-C coupling products. The biggest challenge of forming coupling products from chlorofluorocarbons (CFCs) is to improve the coupling pathways by suppressing other paths of reactions that produce less attractive products.The kinetics results of the reaction of CF2Cl2 and H2 catalyzed by PtCu/C catalysts, presented in Chapter 3, showed the effect of Cu content on the selectivity pattern. The observation was that the coupling selectivity increased with the increasing Cu content. The observation led to the investigation of the role of Cu sites in the coupling product formation. Chapter 4 discusses the results of introducing CO in the reaction mixture. The maximum coupling selectivity was increased from 55% to 69%. The catalyst was also exposed to water for better mixing of the precursors. The water-exposed catalyst showed an overall coupling selectivity of ~82% and the performance was very stable. It was suggested that water exposure of the fresh catalyst increased bimetallic particle formation. As a result the number and size of Pt particles decreased. Both the results point toward fact that Pt or bimetallic sites may be responsible for C1 product formation and Cu sites are responsible for coupling products formation.The performances of the palladium bimetallic catalysts were discussed in chapter 5. Monometallic Pd produced 75% hydrocarbon oligomerization products (C2 to C5) and monometallic Cu, Ag, Co and Fe/C were inactive under the reaction condition. For Pd-Ag/C catalyst, there might be a very significant amount of bimetallic particle formation and Ag segregates hugely to the surface of the bimetallic particles. The hydrodehalogenation and coupling reactions are occurring over the Ag sites. For Pd-Cu there will be a moderate surface segregation of Cu to the surface and there will be just enough Pd surface atoms to dissociate hydrogen to Cu sites for dehalogenation and coupling. For Pd-Co/C, the function of Co is just to dilute the Pd ensemble. For Pd-Fe, Fe acts as Pd site blocking and Pd ensemble size reducing element. Two of the bimetallic catalysts (Pd-Cu and Pd-Co) were very stable though their activities were lower than monometallic Pd.