Synthesis of ZSM-5 with variable crystallite size and its influence on physicochemical properties

Abstract

Zeolite ZSM-5 (SiO2/Al2O3 = 80) was synthesized in the temperature range 353–453 K from the gel having oxide mole composition: 4.38 (TPA)2:27.6 Na2O:Al2O3:86 SiO2:3286 H2O By varying period and temperature of the aging and crystallization, products with different crystallite sizes ranging from 0.3 to 4.0 μm 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 μm) 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 μm, 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.