Characterization of crystalline and amorphous phases during the synthesis of (TPA, M)-ZSM-5 zeolites (M = Li, Na, K)

Abstract

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

Zeolite growth by synergy between solution-mediated and solid-phase transformations

“…These ions quickly dissolve the solid gel matrix to produce soluble T-O À , T-OH species. Then, the soluble T-O À , T-OH species may regroup around the hydrated cations (Na + , TPA + ) to form the structure-directing agent containing T-O (SDA-T-O) species [52][53][54][55][56][57] which can undergo aggregation to form <3 nm primary sub-colloidal particles. 25,28,57,58 The primary sub-colloidal particles aggregate and densify into condensed aggregates (equilibrated gel).…”

“…47 According to this mechanism, the gel is dissolved into the solution and the zeolite product crystallises from the resulting solution. 2,47,53,55,56,60 However, the homogeneous nucleation in the liquid phase can be difficult to occur due to the supersaturation which is considerably lower than the critical supersaturation needed for the formation of zeolite nuclei in the liquid phase. 61 In addition, the results of some studies of zeolite nucleation represent the direct evidence on the existence of most zeolite nuclei in the gel matrix.…”

Factors That Control Zeolite L Crystal Size

“…A large variety of physical methods have been used to shed light on the reactions that lead to zeolite nucleation. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] Nevertheless, there is not a universal mechanism that could describe the intimate steps that lead to zeolite formation in different zeolite systems. This is most probably due to the fact that zeolite formation in different systems follows different routes.…”

Detection of Antigen Protein by Using Fluorescence Cross‐Correlation Spectroscopy and Quantum‐Dot‐Labeled Antibodies

“…The crystallization pathway may vary for different crystallization systems, which complicates further the interpretation of experimental data. A large variety of physical methods have been used to shed light on the reactions that lead to zeolite nucleation 14–32. Nevertheless, there is not a universal mechanism that could describe the intimate steps that lead to zeolite formation in different zeolite systems.…”