SYNTHESIS OF ZSM-5 WITH VARIABLE CRYSTALLITE SIZE AND ITS INFLUENCE ON PHYSICOCHEMICAL PROPERTIES

By varying period and temperature of the aging and crystallization, products with different crystallite sizes ranging from 0.3 to 4.0-mu-m were obtained. The crystallization period was found considerably reduced when potassium fluoride and/or seed cyrstals (up to 10 wt.%) were added to the synthesis gel. A prolonged crystallization period at constant temperature was found to be operative in increasing the crystallite size on account of Ostwald ripening. Equilibrium sorption capacity of water in these crystallites (0.3-4.0-mu-m) was almost constant. This indicated a nearly constant SiO2/Al2O3 ratio in the product. There was a marginal decrease in the n-hexane sorption capacity and an appreciable decrease in the cyclohexane sorption capacity from 6.45 to 3.31 wt.% with an increase in crystallite size from 0.3 to 4.0-mu-m, due to the tailing effect. The effect of crystallite size on the product distribution in methanol conversion and xylene-forming reactions is discussed. Increase of crystallite size was found to influence the olefin selectivity in the methanol conversion reaction. The para-xylene selectivity in the toluene methylation and toluene disproportionation reactions was attributed to a longer diffusional path and inhomogeneous distribution of aluminum in case of the larger crystallites.