The crystallization kinetics, the composition of the various phases and the physical state of the different components are examined during the crystallization of ZSM-5 zeolite via the combined physico-chemical techniques of PIGE, EDX, atomic absorption, X.P.S., thermal analysis (TG-DTA), 29Si, 27A1, I3C, 'Li and 23Na n.m.r. A nucleation process on the gel/liquid interface and crystal growth through liquid phase transportation is evidenced. In our synthesis conditions, rather large amounts of SiOR defect groups (R = H, M or tetrapropylammonium) are present in the final crystalline phases, even at high Al contents [2-3 Al per unit cell (u.c.)]. 'Liand 23Na-n.m.r. data indicate that in the beginning of the synthesis the small alkali cations are incorporated into the zeolitic channels as hydrated counterions to (Si-0-AI)-negative charges and later as less hydrated counterions to SiO-defect groups.High silica zeolites (Si/Al > 10) are usually formed from neutral or basic hydrogels containing sources of silica and alumina, alkali or ammonium ions and organic molecules such as (alky1)amines or quaternary ammonium compounds. 1-For pH values usually higher than 10, the high OH-concentration enhances the dissolution rate of the amorphous gel and consequently the crystal growth ate.^,^The presence or absence of particular cations in the gel may influence the course of zeolite formation ~ignificant1y.l.~ Organic species present in the gel are responsible for the formation of high silica zeolites by templating' and/or space-fi1ling6-' effects. Indeed, organic bases have been found to favour strongly the formation of particular doublering silicate anions which constitute secondary building units for different ~e o l i t e s . ~~~-" . As space-fillers, the organic cations stabilize the zeolite 1attice.'*'l2 ZSM-5 zeolites can also be formed in organic-free systems. In this case, the structuredirecting role of the inorganic cations was emphasized.6* 13-17 In the absence of organic cations, however, only sodium is able to direct the structure of ZSM-5 in rather selected conditions, namely for low Na,O/SiO, and high Si02/Al,03 ratios. l8 Quite recently, however, evidence was presented for the formation of ZSM-5 from a pure potassic Large crystals of ZSM-5 can be prepared at low pH (3-10) in a fluoride-containing 11 and elevated m e d i ~m . ' ~. ~~ NH4-ZSM-5 is also obtained by using NH,OH at pH = pressure.21* 22
