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

Up to three galliums per unit cell were introduced into the framework of MF I, using fluorine-containing media. Initial gels with a composition of 10Si O(2)-xMF-yGa(NO3)(3)-1.25TPABr-330H(2)O (where M =NH4, Na, K and Cs, x=9 an d 18, y=0.1 and 0.3, and TPABr=tetrapropylammonium bromide) were heated in hydrothermal conditions at 170 degrees C. The NH4+-containing system is the most effective in the incorporation of gallium into the MFI framework, whe reas the Na- and Cs-containing systems were the least effective. The crysta llization patterns and crystal morphologies are also influenced by the fluo ride salt added in the reaction mixture. The kinetic data show that the alk ali cations exert both electrostatic and specific effects in the stabilizat ion of the first nuclei and the stabilization of tiny crystals under develo pment. The influence of the cations can be different either on the inductio n rate or on the crystal growth rate. (C) 1999 Elsevier Science B.V. All ri ghts reserved.