OPTIMUM DESIGN OF ZEOLITE/SILICA-ALUMINA CATALYSTS

An analysis of the rate of formation of the desired product and of the selectivity has been carried out for composite catalysts wherein small particles of zeolite are distributed nonuniformly within a large particle of the more porous silica-alumina. Two nonuniform distributions have been analyzed, namely, (i) the composite-center distribution, where a uniform dilute region of zeolite in silica-alumina is surrounded by a silica-alumina matrix, and (ii) the composite-surface distribution, where a uniform dilute region is located around the pure silica-alumina matrix. Two reaction schemes, A→B→C and A→C→D, which fairly approximate the catalytic cracking process over the Zeolite/silica-alumina catalyst have been examined. The reaction scheme A→B→C is employed for the composite-center distribution, while the reaction scheme A→C→D is considered for the composite-surface distribution, because the overall rate of formation of the desired product can be thus optimized. There are conditions under which the selectivity can also be optimized

It is shown that for both nonuniform distributions, the rate of formation of the desired component can exhibit, for appropriate values of the parameters, a maximum with respect to both the overall volume fraction of silica-alumina and the ratio of the radius of the core region to that of the catalyst pellet. Finally, the analysis shows that the rate of formation of the desired compound and the selectivity can be greater for the nonuniform distributions than the uniform distribution when the two contain the same amount of zeolite.