Desirable Catalyst Properties in Selective Oxidation Reactions

Heterogeneous oxide-catalyzed selective oxidation reactions can be classified into dehydrogenation and dehydrogenation with oxygen insertion. The oxide properties that are Important In each of the steps in these reactions are discussed. The breaking of the C-H bonds in alkanes is facilitated by weakly adsorbed oxygen. The C-H bond breaking of alkenes is enhanced by strongly basic surface lattice oxygen and cations that are soft acid and undergo redox readily. Desorption of alkenes and dienes is enhanced by cations that are hard acid. The selective CO bond formation Is controlled by the number and the ease of removal of the available lattice oxygen, while the combustion reaction can be minimized by shortening the residence time of the surface intermediates, weakening the adsorption of the desired products and minimizing the amount of weakly adsorbed oxygen or the density of combustion sites. The function of a promoter Is to enhance the rate of the rateand selectivity-determining steps. Thus the desirable influence of a promoter on the solid depends on the nature of the critical step.