The role of alkali cations in the synthesis of Ga-ZSM-5 in fluoride medium

Abstract

Up to three galliums per unit cell were introduced into the framework of MFI, using fluorine-containing media. Initial gels with a composition of 10SiO 2– xMF– yGa(NO 3) 3–1.25TPABr–330H 2O (where M=NH 4, Na, K and Cs, x=9 and 18, y=0.1 and 0.3, and TPABr=tetrapropylammonium bromide) were heated in hydrothermal conditions at 170°C. The NH + 4-containing system is the most effective in the incorporation of gallium into the MFI framework, whereas the Na- and Cs-containing systems were the least effective. The crystallization patterns and crystal morphologies are also influenced by the fluoride salt added in the reaction mixture. The kinetic data show that the alkali cations exert both electrostatic and specific effects in the stabilization of the first nuclei and the stabilization of tiny crystals under development. The influence of the cations can be different either on the induction rate or on the crystal growth rate.