Catalyst Properties - Decarboxylative Condensations

While several studies have compared various metal oxides for decarboxylative condensation reactions of acids and aldehydes, most were limited in scope due to the low conversions and/or partial pressures employed. In particular, the detrimental effects of water, often present in the feeds and always a product of the reactions, have usually been ignored. Regenerability of spent catalysts has also been given scant attention. 

In previous work using Ce02 and other rare-earth oxide (REO) catalysts, ketonization selectivity was found to be low compared to that of aldolization, cracking or reductive coupling reactions. However, in recent work it has been determined that many supported REOs are selective, long-lived decarboxylative condensation catalysts at high conversions, especially when acetic acid is one reactant.  

Among the more common oxides outside the transition metal oxides, it was found that Ce02- or Zr02-containing catalysts demonstrate the best overall 

Ce02 crystallizes in the fluorite (fee) structure with octahedral coordination of the cations. This structure is maintained upon reduction to at least CeOu and sometimes lower. Metastable phases of varying compositions, all with defective fluorite structures, have been observed, suggesting that the diffusion of oxygen vacancies into the bulk is rapid at reduction temperatures this is a reason why the surface and bulk reduction peaks in the TPR spectrum overlap. The (defective) fluorite structure is retained even to high degrees of reduction in CeOx/ZrOx.  

Mixed Ce/REO s (e.g., La, Y, Hf) and Ce/Zr mixed oxides are often more easily reduced than Ce02 itself. These ions are either similar in size to Ce or smaller, providing for increased oxygen mobility. However, the (defective) fluorite structure is retained during the reduction unless the doping exceeds several %.i38,i4o,i5o p j. degree of reduction vs. temperature, some of the differences in the literature are undoubtedly due to differences in the intimacy of mixing between the oxides. 

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