Synthesis of ZSM-5 from Ethanol-Containing Systems. Influence of the Gel Composition

Abstract

ZSM-5 zeolite has been synthesized using ethanol as a templating agent. The effects of composition on the physicochemical properties of the zeolite (Al content, crystal and particle sizes) and on the kinetics of the crystallization have been studied. The dilution of the starting gel with water leads to a decrease of both the nucleation and crystal growth rates, whereas the increase of the ethanol/SiO[sub 2] molar ratio favors both processes in agreement with the templating role of the alcohol. The variation of the SiO[sub 2]Al[sub 2]O[sub 3] ratio causes drastic changes in the crystallization: the presence of a high proportion of Al in the gel delays the nucleation period but accelerates the crystal growth step. A certain contribution of the autocatalytic mechanism to the nucleation is present at the lowest SiO[sub 2]/Al[sub 2]O[sub 3] ratio investigated, the autocatalytic nuclei being probably formed around Al species in the gel phase. Thus, pure ZSM-5 cannot be obtained at high SiO[sub 2]/Al[sub 2]O[sub 3] ratios. The enhancement of the basicity leads to a decrease of the nucleation period, whereas a maximum of the crystal growth rate is observed for the intermediate OH[sup [minus]]/SiO[sub 2] ratio of 0.12 due to the competition between crystallizationmore » and crystal redissolution processes at high OH[sup [minus]] concentrations. The Al content of the zeolite is only changed by variation of the SiO[sub 2]/Al[sub 2]O[sub 3] ratio in the gel, its proportion being higher in the zeolite phase.« less