(297g) Development of Palladium Oxide Catalysts Supported On Nanoparticle Oxides for C-H Activation and C-C Coupling Reactions

A number of palladium oxide catalysts supported on various nanoparticle and conventional oxides were prepared and tested for activity in the C-H activation and C-C coupling of 4-methylpyridine.  This is an environmentally friendly, atom economic reaction since waste formation is minimized at the source.  However, it is a slow reaction with limited yields. Our research has therefore focused on improving these catalysts to obtain higher product yields. 

PdO has been shown to be important for the preparation of an active catalyst in this reaction, possibly due to facilitated H abstraction.  In search of catalysts that can outperform the commonly used commercial Pd/C catalysts, a number of different oxides were used as supports for PdO.  PdO/n-Al₂O₃ (nanoparticle alumina support) and PdO/p-TiO₂ (porous titania support) were shown to be the most active catalysts in the oxidative coupling of 4-methylpyridine, significantly outperforming the commercial Pd/C catalyst.  However, other nanoparticle supports, such as nanoparticle zirconia (n-ZrO₂) and zinc oxide (n-ZnO), can also give reasonable yields despite rather low support surface areas (35-60 m²/g).  Therefore, active catalysts can be prepared by depositing PdO onto very high surface area supports (≥500 m²/g), or onto supports that will result in strong palladium-support interactions. 

Catalyst characterizations reveal that there is no simple correlation between the Pd surface areas (measured after reduction of surface PdO) and the catalytic activities for these reactions.  The results indicate that this is a complex reaction system and a high PdO surface area is important but not sufficient to yield a highly active catalyst.  The complexity of the reaction system is even more evident when adding a third oxide to the system (PdO/MeO_x/support, where MeO_x is the added metal oxide).  The best catalyst reported to date for this reaction is PdO/ZrO₂/n-Al₂O₃ prepared using the coprecipitation method.