The influence of alkali cations on the synthesis of ZSM-5 in fluoride medium

The role of NH4+, Na+, K+ and Cs+ cations was investigated in the synthesis of ZSM-5 using fluoride as mineralizing agent. The amounts of aluminum and alkali cations were varied in gels of initial composition 10SiO(2)-xMF-yAl (OH)(3)-1.25TPABr-330H(2)O with M=NH4, Na, K and Cs, x = 9, 15 and 24, y = 0.16, 0.5 and 1.0 and TPABr = tetrapropylammonium bromide. The potassium-co ntaining system is the most effective one with respect to incorporation of aluminum into the zeolite framework, while the ammonium-containing system i s the least effective one. Crystallization patterns and crystal morphologie s are strongly influenced by the nature of the fluoride salt added to the r eaction mixture. The kinetic data show that the cations not only influence the reaction rates by their electrostatic interaction, but also due to thei r specific interaction with the fluoride anion in various siliceous and alu minosiliceous complexes. (C) 1999 Elsevier Science B.V. All rights reserved .