The hydrothermal synthesis of zeolites-Precursors, intermediates and reation mechanism

Microporous and Mesoporous Materials 82 (2005) 1–78
www.elsevier.com/locate/micromeso

Review

The hydrothermal synthesis of zeolites: Precursors,
intermediates and reaction mechanism
Colin S. Cundy
a

b

a,*

, Paul A. Cox

b

Centre for Microporous Materials, School of Chemistry, University of Manchester, P.O. Box 88, Sackville Street,
Manchester M60 1QD, United Kingdom
School of Pharmacy and Biomedical Sciences, University of Portsmouth, St Michael’s Building, White Swan Road,
Portsmouth PO1 2DT, United Kingdom
Received 5 November 2004; accepted 11 February 2005
Available online 8 April 2005

Abstract
An account is presented of the mechanistic aspects of hydrothermal zeolite synthesis. The introduction provides a historical and
experimental perspective and is followed by a summary of proposed mechanisms and associated modelling studies. The central section of the review contains a description of the most probable mechanistic pathways in zeolite formation. In this, the reaction stages
of the induction period, nucleation and crystal growth are examined in chronological sequence. Finally, particular aspects of the
synthesis process such as the constitution of growth species, template–framework interactions and the nature of zeolite solubility
are treated in more detail.

Emphasis is placed upon the chemical basis of zeolite synthesis. Fundamental to this are the TAOAT bond-making and bondbreaking reactions which establish the equilibration between solid and solution components. The consequent generation of order,
driven by energy differences and strongly moderated by kinetic limitations, is essentially one of continuous evolution. However, the
discreet step of nucleation provides a discontinuity in which isolated regions of local order are superceded by the establishment of a
periodic crystal lattice, capable of propagation. Crystal growth occurs through an in-situ, localised construction process from small,
mobile species ordered by the participating cations.
The process of hydrothermal zeolite synthesis can be most adequately explained by a mechanism based upon the solution–mediation model, whether or not there is a visible liquid phase. The common presence of mobile species emphasises the overall similarity
of zeolite synthesis reactions so that the need to distinguish any separate ‘‘gel rearrangement’’ or ‘‘solid-phase transformation’’
mechanism becomes unnecessary.
Ó 2005 Elsevier Inc. All rights reserved.
Keywords: Hydrothermal synthesis; Nucleation; Crystal growth; Modelling; Mechanism

Contents

1.

*

Part I: Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. History of hydrothermal zeolite synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Corresponding author. Tel.: +44 161 200 4512; fax: +44 161 200 4559.
E-mail address: (C.S. Cundy).

1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2005.02.016


2

C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

2.
3.

4.

5.
6.

7.

8.

9.

10.

1.2. Scope and structure of this review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Experimental observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Summary of proposed mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Richard Barrer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. The early work of Breck and Flanigen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3. KerrÕs recirculation experiment and the work of Ciric. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Studies at Leningrad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Overview—1959 to 1971 and beyond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.6. Introduction of organic templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.7. Chang and Bell and after . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Modelling the processes of zeolite synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Mathematical models of synthesis reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

4.2. Molecular modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.1. Modelling zeolite-template pairs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2.2. Cluster calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Part II: Synthesis mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
The induction period . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
The evolution of order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. The nature of the amorphous material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Primary and secondary amorphous phases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.2.1. The Montpellier study. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.2.2. Related investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.3. Further evidence for pre-crystalline order from synthesis studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.2. General considerations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.3. Determination of zeolite nucleation patterns from measurements on the resulting crystals . . . . . . . . . . . . . . . . . 16
7.3.1. Studies under isothermal conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.3.2. Ageing studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.4. The use of seed crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
7.5. Autocatalytic nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.6. Nucleation in zeolite systems—The nature of the reaction sol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.7. Nucleation in zeolite systems—Homogeneous or heterogeneous? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
7.8. Nucleation in zeolite systems—Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.9. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1. Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.2. Experimental observations—Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
8.3. Experimental observations—Studies of macrocrystalline systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.4. Experimental observations—Studies of nanocrystalline systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8.5. Size-dependent growth of nanocrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

8.6. Growth models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.7. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Part III: Key topics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
The nature of growth species and the role of aggregation processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.1. Growth from soluble, pre-fabricated units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
9.2. Growth from simple species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.3. Mineralising agents other than hydroxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.4. Growth from particles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.4.1. Particle aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.4.2. Chemical and physical consequences of an aggregation mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Solid state transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. Hydrothermal synthesis in the presence of a liquid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
10.2. Hydrothermal synthesis in the apparent absence of a liquid phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40


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11.

12.

13.

14.

15.

16.

17.

18.

19.

10.3. Non-aqueous syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.4. The role of water in apparent solid state transformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.5. Solid state transformations at high temperatures and pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10.6. Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ageing effects in zeolite synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.1. Ageing as a means to control product phase purity and crystal size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.2. Rationalisation of ageing effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.3. Detailed analyses of ageing-related effects in silicalite synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X-ray amorphous zeolites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.1. XRD evidence for ‘‘X-ray amorphous zeolites’’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.2. IR evidence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.3. Evidence from other physical measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.4. Evidence from catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.5. Evidence from the synthesis process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.6. Other amorphous materials related to zeolites—zeolite degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12.7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Template–framework interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.1. Geometric matching. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.2. Template classification and versatility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.3. Structure blocking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.4. Variations induced by heteroatoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
13.5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Solubility and supersaturation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.1. Zeolite solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.2. Zeolite solubility as a function of base concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.3. Supersaturation in relation to zeolite crystal growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.4. Thermodynamic vs. kinetic factors in zeolite synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Zeolite dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.1. The kinetics of dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.2. Morphological and compositional changes on dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
15.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Metastability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.1. Precursors, intermediates and co-products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.2. Layer structures as transients and precursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.3. Conversion of one zeolite into another . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.4. Ostwald ripening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16.5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Optimisation of zeolite syntheses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.1. Comparisons of zeolite synthesis reaction rates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2. Procedures for improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2.1. Reaction optimisation (and its limitations). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2.2. Addition of seed crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2.3. Additives and ‘‘promoters’’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.2.4. Microwave synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
17.3. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Relationship of zeolite synthesis mechanism to that of other porous materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.1. Zeolites and clathrate hydrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.2. Zeotypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.3. Microporous vs. mesoporous structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. A chemical model for the crystal growth of zeolite molecular sieves . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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52

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Part I: Background
1. Introduction
1.1. History of hydrothermal zeolite synthesis
The history of man-made zeolites can be traced back
to the claimed laboratory preparation of levynite by St
Claire Deville in 1862 [1]. However, zeolite synthesis as
we know it today had its origins in the work of Richard
Barrer and Robert Milton, commencing in the late
1940s. Barrer (principally at Imperial College, London)
began his work by investigating the conversion of
known mineral phases under the action of strong salt
solutions at fairly high temperatures (%170–270 °C).
Among the products, species P and Q [2–4] (isostructural variants) displayed unique characteristics and represented the first synthetic zeolite unknown as a natural
mineral. These materials were later found to have the
KFI structure [5] determined subsequently for zeolite
ZK-5 [6,7]. Robert Milton (in the Linde Division of
the Union Carbide Corporation, Tonawanda, New
York) pioneered the use of more reactive starting materials (freshly precipitated aluminosilicate gels), enabling
reactions to be carried out under milder conditions and
leading to the discovery of zeolites A [8] and X [9]. By
1953, Milton and his colleagues had synthesised 20 zeolites, including 14 unknown as natural minerals [10].
Following the foundations laid in the 1950s, the next

decade saw many significant developments. Earlier work
on zeolite synthesis had utilised only inorganic reaction
components but in 1961 the range of reactants was expanded to include quaternary ammonium cations [11–
13]. The introduction of organic constituents was to
have a major impact upon zeolite synthesis and the
key step followed quite rapidly with the disclosure in
1967 of the first high-silica phase, zeolite beta [14], whilst
the archetypal high-silica zeolite, ZSM-5, was discovered in 1972 [15].
There has subsequently been a large rise in the number of known synthetic zeolites [16] and also the discovery of new families of zeolite-like or zeolite-related
materials [17]. The latter ‘‘zeotypes’’ may be represented
by the microporous alumino- and gallo-phosphates (AlPOs and GaPOs) [18–20] and titanosilicates (such as
ETS-10) [21–23]. Such materials display great compositional diversity and frequently have frameworks unknown for zeolites. This increased structural flexibility
has its origins in the available spectrum of heteroelement
atomic radii, bond lengths and bond angles, and in the
emergence of coordination numbers greater than four.
Even greater divergence from the norm of microporous
aluminosilicates is seen in a major new class of zeoliterelated phases discovered in the early 1990s. Mesoporous materials, synthesised with the aid of surfactant
molecules and typified by the M41S [24,25] and SBA

[26,27] series, have periodic structures with far larger
˚ ) but are not conventionally
pore sizes (up to %200 A
crystalline [28–30].
Investigative work aimed at gaining an understanding
of the synthesis process has its origins in the 1960s.
These studies have continued up to the present day,
spurred on at various points by discoveries of new materials, advances in synthetic techniques, innovations in
theoretical modelling methods and, especially, by the
development of new techniques for the investigation of
reaction mechanisms and the characterisation of products. It is the purpose of the present Review to offer,

for the case of zeolites, an account of such exploratory
and background work.
1.2. Scope and structure of this review
In an earlier survey [31], a summary was given of the
main discoveries and advances in thinking in the field of
zeolite synthesis from the 1940s up to 2002. That account was principally concerned with the pattern of
discovery and the consequent progression of ideas.
Discussion of the mechanism of zeolite synthesis was
limited to this evolutionary context. This present review
attempts to expand this critical argument and to describe in detail the most probable steps by which amorphous aluminosilicate reagents are converted to
crystalline molecular sieves. In addition to summarising
earlier proposals, it will be necessary to bring forward
some further ideas which are perhaps new in the current
context. This should clarify the link between nucleation
and crystal growth by considering the chemical steps
which are common to both. Fortunately, when this is
done the picture becomes simpler rather than more complex and the need to make any differentiation between,
for example, ‘‘solution-mediated growth’’ and ‘‘gel rearrangement’’ finally disappears. The text is confined to
hydrothermal methods of synthesis and concentrates
on aluminosilicate zeolites, mentioning alternative zeotypes or other porous materials only when this is necessary to illustrate or broaden the main argument.
However, it seems very probable that the main features
observed for zeolites will also be found in the synthesis
of closely related materials, modified in some cases
through the differences in composition, structure, polarity and solution chemistry.
The sections of this survey fall into three groups. In
Part I and following the above brief historical introduction, the experimental observations associated with a
typical hydrothermal zeolite synthesis are outlined and
the various interpretations which have been advanced
to explain them summarised (Sections 1–3). Section 4 reviews work on modelling the processes of zeolite synthesis. Part II represents an attempt to put forward a
detailed and self-consistent view of the most probable

mechanistic pathways in zeolite formation, presented


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

within the overall chronology of the hydrothermal synthesis reaction, i.e. the induction period and the nature
of the amorphous material (Sections 5 and 6), the mechanism by which zeolite crystals are nucleated (Section 7)
and the mechanism of zeolite crystal growth (Section 8).
In Part III (Sections 9–18), some key problems and
questions associated with zeolite synthesis are addressed
in more detail, leading, finally (Section 19), to some
overall conclusions.
For further general information on the subject of zeolite synthesis, the reader is referred to the standard textbooks [17,32–35] and recent reviews [31,36–43].

2. Experimental observations
A typical hydrothermal zeolite synthesis can be described in briefest terms as follows:
1. Amorphous reactants containing silica and alumina
are mixed together with a cation source, usually in
a basic (high pH) medium.
2. The aqueous reaction mixture is heated, often (for
reaction temperatures above 100 °C) in a sealed
autoclave.
3. For some time after raising to synthesis temperature,
the reactants remain amorphous.
4. After the above ‘‘induction period’’, crystalline zeolite product can be detected.
5. Gradually, essentially all amorphous material is
replaced by an approximately equal mass of zeolite
crystals (which are recovered by filtration, washing
and drying).
This is illustrated schematically in Fig. 1. The elements (Si, Al) which will make up the microporous

5

framework are imported in an oxide form. These oxidic
and usually amorphous precursors contain SiAO and
AlAO bonds. During the hydrothermal reaction in the
presence of a ‘‘mineralising’’ agent (most commonly an
alkali metal hydroxide), the crystalline zeolite product
(e.g. zeolite A) containing SiAOAAl linkages is created.
Since the bond type of the product is very similar to that
present in the precursor oxides, no great enthalpy
change would be anticipated. In fact, the overall free energy change for a zeolite synthesis reaction is usually
quite small, so that the outcome is most frequently
kinetically controlled [31,43–46].
Kinetic control is a pervading influence throughout
zeolite synthesis, where the desired product is frequently
metastable. Much of the know-how in this industrially
important area centres around choice of the exact conditions for product optimisation, so that the required
material can be prepared reproducibly and to the same
specification [47]. Such considerations will often influence the choice of starting reagents. Whilst these may include the simple oxides or hydroxides mentioned above
(e.g. precipitated silica or alumina trihydrate), it is also
very common for the reagents to represent some degree
of pre-combination, as for example in sodium silicate
solution or solid sodium aluminate. These materials
may represent advantages in cost or ease of processing
but may also offer optimum routes to particular materials, since flexibility in the choice of reagents enables
equilibria to be approached from different directions.
This may offer kinetic benefits, such as the preferred
nucleation of one phase over another in situations where
mixtures may otherwise co-crystallise. A good general

appreciation of the field of practical zeolite synthesis
can be obtained from the handbook issued by the Synthesis Commission of the International Zeolite Association (IZA) [48].

Fig. 1. Hydrothermal zeolite synthesis. The starting materials (SiAO and AlAO bonds) are converted by an aqueous mineralising medium (OHÀ
and/or FÀ) into the crystalline product (SiAOAAl bonds) whose microporosity is defined by the crystal structure.


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3. Summary of proposed mechanisms

3.2. The early work of Breck and Flanigen

The brief summary given above (Section 2) outlines
the transformation of an amorphous, aqueous
aluminosilicate gel under the action of heat into a crystalline zeolite product. In the present section, an overview is presented of the main suggestions which have
been put forward to explain these experimental observations. The historical aspects of this topic have been discussed more fully in our earlier account [31] and some of
the main mechanistic proposals are summarised in Table 1. The details of the synthesis mechanism form the
subject of Part II (Sections 5–8).

In 1960, Flanigen and Breck reported [51,52] a study
in which XRD measurements were employed to follow
the crystallisation with time of zeolite Na-A (at
100 °C) and Na-X (at 50 and 100 °C). They showed
the now-familiar S-shaped growth curves and described
an induction period followed by a sudden rapid growth.
The morphological changes observed [53] were interpreted as a successive ordering of the gel as crystallisation proceeds, leading to a conclusion that crystal
growth takes place predominantly in the solid phase.

Their conclusions may be summarised as follows
[31,51,52]:

3.1. Richard Barrer

1. Extensive heterogeneous nucleation occurs during
formation of the highly supersaturated gels.
2. The nuclei do not necessarily represent a unit cell but
may consist of more preliminary building units of
polyhedra (e.g. the hexagonal prism) as suggested
by Barrer et al. [49].
3. During the induction period, the nuclei develop to a
critical size and then grow rapidly to small and uniform sized crystals.
4. Growth of the crystal proceeds through a type of
polymerisation and depolymerisation process (breaking and remaking Si,AlAOASi,Al bonds), catalysed
by excess hydroxyl ion and involving both the solid
and liquid phases (although the solid phase appears
to play the predominant role).

The first consideration of synthesis mechanism was
that given by Barrer, Baynham, Bultitude and Meier
in 1959. The discussion section of a paper [49] in which
a wide variety of alumino-, gallo- and germano-silicates
were synthesised begins as follows:
‘‘The formation of diverse kinds of structural framework leads to questions as to the mechanism of growth.
The phases are often obtained reproducibly in yields
nearing 100% and the free-energy balance between the
many possible aluminosilicate nuclei must be delicate.
The development of elaborate and continued space patterns by progressive additions of single (Al,Si)O4 tetrahedra is difficult to imagine, particularly in the case of very
open zeolite structures. The formation of these frameworks is, however, much more easily visualised if in the

aqueous crystallising magma there are secondary building units in the form of rings of tetrahedra or polyhedra.
These may pack in various simple coordinations to yield
different aluminosilicates.’’
Examples of some possible ions were then tabulated:
rings of 3–6 tetrahedra, the double-4-ring, the double-6ring (or 3 4-rings). It was further pointed out that such
units could give rise to more complex ones, such as the
linking of six 4-ring anions to give the nosean-sodalite
cubo-octahedral unit. Barrer returned to this theme in
a later review, considering that the growth of aluminosilicate crystals from alkaline media was unlikely to proceed by the capture of single monomeric silicate and
aluminate tetrahedral ions TOnÀ
4 since ‘‘in the elaborate
porous crystalline structures of the zeolites, for instance,
it would seem difficult for the lattice to persist in its very
open pattern when rapidly adding such small units’’ [50].
He felt that ‘‘a plausible process would be the accretion
in simple coordination of polygonal or polyhedral anions by condensation polymerisation’’, giving as examples the 4-ring, 6-ring, cube and hexagonal prism and
the formation of the ‘‘crankshaft’’ double chain (found
in feldspars and the phillipsite-harmotome zeolites) by
linkage of 4-rings.

In a subsequent review [54], Breck described zeolite
formation in the following terms: the gel structure is
depolymerised by hydroxide ions; rearrangement of
the aluminosilicate and silicate anions present in the
hydrous gel is brought about by the hydrated cation
species present; tetrahedra re-group about hydrated sodium ions to form the basic polyhedral units (24-hedra);
these then link to form the massive, ordered crystal
structure of the zeolite.
3.3. Kerr’s recirculation experiment and the work of Ciric
A paper published by George Kerr in 1966 describes

[55] an experiment carried out to test the hypothesis that
[56] ‘‘a zeolite could be formed via dissolution of gel by
sodium hydroxide solvent followed by deposition of zeolite crystals from gel-derived species in solution.’’ In the
experiment, a sodium hydroxide solution at 100 °C was
circulated through two filters, the first of which contained a specially prepared amorphous sodium aluminosilicate, whilst the second held crystals of zeolite Na-A.
When the experiment was terminated after about 4 h,
nearly all of the amorphous solid had been dissolved
and the zeolite sample (estimated to be essentially
100% zeolite A by water sorption) had approximately


Table 1
Summary of principal proposals for zeolite synthesis mechanism, 1959–2004
Principal system studied

Main features of mechanism

Barrer [49,50]

Various low-silica
phases

Condensation polymerisation
of polygonal and polyhedral anions

Flanigen and Breck [51–54]

Na-A, Na-X

Linkage of polyhedra (formed by

M+-assisted arrangement of anions):
crystal growth mainly in the solid phase

Kerr [55,56]

Na-A

Crystal growth from solution species

Schematic summary

fast

Amorphous solid ! soluble speciesðSÞ
slow

ðSÞ þ nucleiðor zeolite crystalsÞ À! zeolite A

Zhdanov [57]

Na-A, Na-X

Solid M liquid solubility equilibrium,
nuclei from condensation reactions,
crystal growth from solution

Amorphous solid phase
Accumulation of zeolite crystals

Derouane, Detremmerie,

Gabelica and
Blom [58–62]

Na,TPA-ZSM-5

Synthesis ‘‘A’’: liquid phase ion
transportation. Synthesis ‘‘B’’:
solid hydrogel phase transformation

Chang and Bell [63]

Na,TPA-Si-ZSM-5

Embryonic clathrate TPA-silicate units,
ordered into nuclei through OHÀ-mediated
SiAOASi cleavage/recombination

Burkett and Davis [64–66]

TPA-Si-ZSM-5

Pre-organised inorganic–organic composites,
nucleation through aggregation, crystal
growth layer-by-layer

Leuven Group [67–73]

TPA-Si-ZSM-5

Oligomers ! precursor ‘‘trimer’’ (33 Si)

! ·12 ! ‘‘nanoslabs’’, growth by
aggregation

Liquid phase

Formation of nuclei

C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

Author(s) [Ref.]

7


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C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

Some striking advances in thinking and technique
were reported by Zhdanov at the Second International
Zeolite Conference in 1970 [57]. Measurements on crystal linear growth rates for zeolite A showed directly for
the first time the effect of temperature in increasing
growth rate and that the crystals grew at a near-constant
rate over the majority of the synthesis period. From this
latter observation and the product crystal size distribution, Zhdanov was also able to deduce the nucleation
rate profile over the course of the reaction. These considerations, together with measurements of chemical
changes in the solution phase of the reaction mixture
and detailed consideration of such phenomena as the
induction period and seeding effects, led to a more chemically detailed picture of zeolite crystallisation. In this
view, the solid and liquid phases are connected by the

solubility equilibrium. Condensation reactions give rise
to ‘‘primary aluminosilicate blocks (4- and 6-membered
rings)’’ and crystal nuclei. Crystal growth occurs from
solution until dissolution of the amorphous phase is
complete. Analytical data supported the proposition
that the composition of the crystals depended on that
of the liquid phase from which they crystallised.

polyhedral building units. Similar precursor units were
envisaged by Flanigen and Breck, although their thinking was focused largely upon the solid phase. In this
view, the initial, random aluminosilicate gel structure
was dis-assembled into its constituent tetrahedra by
the action of OHÀ ions and new, oligomeric polyhedral
units were formed through the ordering influence of the
cations. Crystal growth proceeded by an OHÀ-catalysed
polymerisation and depolymerisation process, involving
predominantly the solid phase but with some contribution from solution species. The studies of Zhdanov
and of Kerr provided a more solution-oriented perspective. The original amorphous gel was seen as a dynamic
entity, in equilibrium (or coming to equilibrium) with
the liquid phase. Dissolving under the action of heat
and base, the gel released active soluble species into
the solution from which nuclei formed and grew, from
solution, into crystals, although the detailed nature of
the migrating units was unspecified.
During the later 1970s, the significance of the solution
phase in zeolite synthesis was to become increasingly
apparent, as demonstrated by two Raman spectroscopic
studies on the formation of zeolite A. Observing no
changes with time other than the appearance of crystalline product, McNicol et al. concluded that crystallisation occurred within the solid phase of the gel [75,76].
However, by using a combination of chemical analyses,

Raman spectroscopy, XRD, sorption and particle size
measurements, Angell and Flank [77] reached the opposite conclusion. They demonstrated that the mechanism
involved formation and subsequent dissolution of an
amorphous aluminosilicate intermediate, with solution
transport from the gel to the growth surface of the crystallite. This view was reinforced by two further synthetic
studies. Culfaz and Sand examined crystallisation rates
for mordenite, zeolite X and zeolite A [78]. From considerations of rate limitations by diffusion and seed crystal
surface area, they deduced that crystal growth in these
cases occurred from solution. Kacirek and Lechert [79]
used detailed kinetic studies on seeded faujasite syntheses to develop further the solution growth model, concluding that the rate-determining step was the
connection of silicate species to the surface of the crystal. They also pointed out that, under their conditions,
the solution phase would contain essentially only monomers and dimers during the crystallisation of zeolite X,
with higher oligomers (perhaps up to Si20) present in
the synthesis of the more siliceous Y-types.

3.5. Overview—1959 to 1971 and beyond

3.6. Introduction of organic templates

It may be useful at this point to summarise the main
opinions expressed up to the year 1971. Barrer had concluded that zeolite crystallisation was a solution-mediated process, the structure being formed by the
condensation polymerisation of anionic polygonal or

In 1961, two groups of workers disclosed the effect of
introducing quaternary ammonium cations into zeolite
synthesis. Barrer and Denny described amine-associated
routes to zeolites A and X [11] whilst Kerr and Kokotailo published [12,13] data on a tetramethylammonium

doubled in mass. From these (and other [55]) observations, the mechanism was perceived to be that of rapid
dissolution of the amorphous solid to yield soluble species. The rate-determining step was then the combination of these soluble species with nuclei or zeolite

crystals to yield the zeolitic product.
Working for the same company (Mobil) but not in
the same laboratories, Julius Ciric presented in 1968
the most detailed study of zeolite synthesis published
at that date [74]. Kinetic curves were determined from
water sorption and chemical analyses were carried out
on reaction filtrates. In addition, data were obtained
by particle counter, optical microscopy and BET surface
area methods. The work adds much to the ideas set out
in the Kerr report [55], pointing to a solution-mediated
growth mechanism modified by the presence of the gel
phase (so that transport of growth species to crystals
embedded in gel is restricted by diffusion through the
gel). In addition, Ciric pointed out that his kinetic results were consistent with BarrerÕs ideas of anionic
blocks [49,50] as well as with the Flanigen–Breck view
[51] on the catalysis by OHÀ ions.
3.4. Studies at Leningrad


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(TMA) silica-rich version of zeolite A named ZK-4 (Si/
Al up to 1.7). At first, no new structures resulted from
this pioneering work, but in 1967 Wadlinger, Kerr and
Rosinski reported [14] the discovery of the first high-silica zeolite, zeolite beta (5 < Si/Al < 100), made using the
tetraethylammonium cation. ZSM-5 followed in 1972
[15], the original syntheses being based on a tetrapropylammonium (TPA)–sodium mixture. All these materials
were formed as crystalline products containing the
encapsulated organic cations, leading to the idea of
‘‘templated’’ synthesis with the organics acting as structure-directing agents (SDAs). In terms of the mechanistic alternatives discussed above (Section 3.5), the

introduction of these organic reactants provided new
possibilities for probing the chemistry of the synthesis
reaction. Investigating the synthesis of zeolite A and
other aluminous zeolites, McNicol et al. were able to detect the clathration of TMA units by a shift in the
754 cmÀ1 Raman band, supported by results from
Eu3+ phosphorescence spectroscopy [76]. They found
no evidence for cage-like building blocks in either solution or solid before the onset of crystallisation.
In time, the attractive idea of a strong ‘‘lock and key’’
relationship between framework and template would
come to dominate much of the thinking on synthesis
mechanism and two reviews published in the early
1980s focused attention on this area of host-guest science [80,81]. However, a scheme [58] introduced by Derouane and co-workers at about the same time was aimed
mainly at explaining experimental observations and concentrated on the inorganic gel chemistry [58–62]. Based
on investigations using a wide variety of techniques,
they proposed two pathways for ZSM-5 formation.
The use of Al-rich ingredients and polymeric silica was
pictured as generating a small number of nuclei which
grew by a liquid phase ion transportation process to
yield large ZSM-5 single crystals (synthesis A). This aspect of the suggested reaction mechanism therefore
bears many resemblances to the solution-mediated
scheme of Zhdanov (Section 3.4). For syntheses of type
B (typified by high Si/Al ratios and the use of ‘‘monomeric’’ Na silicate), the results were interpreted in terms
of numerous nuclei which rapidly yielded very small
ZSM-5 microcrystallites directly within the hydrogel
in a process described as a solid hydrogel phase
transformation.
The first suggestion as to how the presence of an organic template molecule might modify the physical
chemistry of the synthesis medium was put forward by
Flanigen and co-workers [82,83]. It was proposed that
the crystallisation mechanism of siliceous zeotypes involves clathration of the hydrophobic organic cation

in a manner analogous to the formation of crystalline
water clathrates of alkylammonium salts. Thus, under
synthesis conditions, the silica tetrahedra assemble into
a framework in place of the hydrogen-bonded water

9

‘‘lattice’’ of the water clathrate and surround the hydrophobic organic guest molecules. In this way, the structural chemistry of water below room temperature is
translated to that of silica near 200 °C. This concept
was developed and extended in a landmark paper by
Chang and Bell [63].
3.7. Chang and Bell and after
The work of Chang and Bell [63] was based upon
studies of the formation of ZSM-5 from Al-free precursor gels at 90–95 °C using XRD, 29Si MAS NMR spectroscopy and ion exchange. The NMR results suggested
that major changes in gel structure occur during the
early stages of reaction. This was confirmed by the demonstration of ion sieve effects indicating that, in the
tetrapropylammonium (TPA) system, embryonic structures with Si/TPA = 20–24 are formed rapidly upon
heating. These first-formed units may resemble ZSM-5
channel intersections (4 per unit cell of 96 tetrahedral
atoms), each containing essentially one TPA+ cation,
and thus provide a possible mechanism for ZSM-5
nucleation. In this scheme, the hydrophobic effect and
the isomorphism between water and silicate structure
lead to (i) formation of a clathrate-like water structure
around the template, and then (ii) conversion of the
clathrate-like hydrate to a clathrate-like silicate by isomorphous substitution of silicate for water in the embryonic units. Such units are initially randomly connected
but in time become ordered (‘‘annealed’’) through repeated cleavage and recombination of siloxane bonds,
mediated by hydroxide ion. Thus, nucleation occurs
through progressive ordering of these entities into the final crystal structure. This dynamic-assembly argument
is very reminiscent of that originally put forward by

Flanigen and Breck for an inorganic system [31,51] (Section 3.2).
The principal concepts advanced by Chang and Bell
[63] have been extended in a series of papers in which
Burkett and Davis [64–66] examine the role of TPA as
structure-directing agent in silicalite synthesis, primarily by MAS NMR spectroscopy. 1HA29Si CP MAS
NMR results provide direct evidence for the existence
of pre-organised inorganic–organic composite structures in which the TPA molecules take up a conformation similar to that adopted in the zeolite product. The
initial formation of the inorganic–organic composite is
initiated by overlap of the hydrophobic hydration
spheres of the inorganic and organic components. Subsequent release of ordered water enables favourable
van der Waals interactions to be established. Nucleation is then brought about through aggregation of
these composite species. Crystal growth occurs through
diffusion of the same species to the surface of the growing crystallites to give a layer-by-layer growth
mechanism.


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Broadly similar ideas have also been developed in
what has become a very extensive study by a team at
Leuven. Using a wide variety of experimental techniques, the work has concentrated on a detailed characterisation of the MFI precursor material originally
described by Schoeman [84,85]. The first papers in the
series [67–71] identified constituent ‘‘nanoslabs’’ having
dimensions 1.3 · 4.0 · 4.0 nm with nine intersections
per particle, each of these containing a TPA cation.
Aggregation of such nanoslabs leads to larger particles
measuring up to 15.6 · 8 · 8 nm and ultimately to the
crystalline colloidal MFI-type material which forms

the final product of the synthesis. More recently
[72,73], specific silicate oligomers (particularly a pentacyclic dodecamer) were identified as intermediates in
nanoslab evolution. However, the elaborately detailed
interpretations adopted in these studies are the subject
of increasing criticism [86–88].
Some of the principal ideas from the period described
in Sections 3.1–3.7 are summarised in Table 1.

4. Modelling the processes of zeolite synthesis
As computing capability has mushroomed, modelling
methods have become an increasingly important adjunct
to experimental studies. It is convenient to consider
‘‘reaction models’’ and ‘‘molecular models’’ under separate headings, although it is hoped that in due course the
two branches of the subject will grow together.
4.1. Mathematical models of synthesis reactions
Reaction models as used by chemists and engineers
are of two basic types: (i) those using a kinetic approach
and (ii) those founded on a thermodynamic approach.
Those in the first category [89] range from simple empirical correlations to complex computer programs. Of the
more complex treatments, the most important are those
based on particle numbers, such as the population balance model [90,91] developed extensively by Thompson
and co-workers and built upon the basic equation (for a
well-mixed reactor)

For example, predictions of crystal size and size distribution can be developed to reflect changes in nucleation
and growth behaviour brought about by gel ageing (Section 11.2).
The only significant themodynamics-based model of
zeolite synthesis to have emerged is the equilibrium
model of Lowe [92]. This was initially developed to provide insight into the pH changes which occur in the
course of high-silica zeolite syntheses [93]. The model

considers the zeolite synthesis process as a series of
pseudo-equilibria:
amorphous solid $ solution species $ crystalline zeolite
— progress of reaction !

At the start, amorphous solid is in equilibrium with
solution species. This initial equilibrium is then maintained while product crystals grow from the supersaturated solution. Finally, when all the amorphous
precursor has been consumed, the crystalline zeolite
equilibrates with its mother liquor. LoweÕs original
sketch of this process is reproduced in Fig. 2.
This simple analysis enables the solution chemistry,
and in particular the effects of solubility and pH, to be
understood at a fundamental level [92]. Computer modelling of the pH function provides a good simulation of
the types of pH curve observed experimentally [94]. The
most notable feature is the sharp rise in pH which occurs
when all of the solid gel phase has been consumed and
control of the solubility is transferred to the crystalline
product. The difference between initial and final pH values is directly related to the difference in solubility between zeolite product and gel precursor, providing a
measure of the strength of the templating effect for a series of organic additives [95]. The most effective template
gives the most stable (least soluble) product and hence
the largest pH rise. Perhaps the key relationship is the

on
on
n
þQ
¼À ;
ot
oL
s

where n is a number density function (characterising the
crystal size distribution at any time), t is time, L is crystal length, Q is the crystal linear growth rate and s is residence time. Further relationships set boundary
conditions and the material balance. Solutions for the
resulting cohort of equations can be developed to provide simulations covering a wide variety of conditions.
In this way, hypothetical reactions can readily be explored to assess the effect of changing reaction variables
and introducing other components such as seed crystals.

Fig. 2. Conceptual basis for the Lowe equilibrium model [92] of the
zeolite synthesis process (B.M. LoweÕs original sketch). Control of
solubility passes from the initial equilibrium between amorphous solid
and solution species to the final equilibration between the crystalline
zeolite product and its mother liquor.


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

11

ratio of precursor and product solubility constants (Ks),
since this is directly related to the solution supersaturation (Section 14) and hence the driving force for the
crystallisation reaction (DG):


K s;gel
DG ¼ ÀRT ln
:
K s;zeolite
4.2. Molecular modelling
In terms of zeolite synthesis, molecular modelling
methods have provided insight into (i) the determination

of the location and energies of the templating agents occluded within zeolite structures during synthesis and (ii)
the detailed investigation of small framework fragments.
4.2.1. Modelling zeolite-template pairs
The key advantage modelling methods offer over
experimental techniques is that they permit investigation
of the energetics of template–framework interactions.
However, experimental studies, most notably X-ray diffraction, have yielded crucial data on template location,
enabling modelling methods to be validated. Modelling
studies of zeolite-template interactions have usually focused on the use of molecular mechanics calculations.
This basic methodology has been employed in several
different ways to investigate the relationship between
template molecules and the zeolite product. Moini
et al. [96] optimised the geometry of the template molecules using the molecular mechanics approach prior to
docking them ‘‘by eye’’ into the framework structure.
In this way, they demonstrated the excellent void filling
properties exhibited by EU-1 (EUO) templates, as
shown here in an alternative representation (Fig. 3).
This type of technique was also successfully employed
by Schmitt and Kennedy to derive new templates for
ZSM-18 (MEI) based on their geometric match with
the framework [97].
The molecular mechanics approach has been enhanced by the addition of a Monte Carlo algorithm to
dock the guest molecule inside the framework prior to
the application of an energy minimisation or simulated
annealing routine. The application of these techniques
has shown convincingly that the relative non-bonded energy between the template and framework can be used
as a measure of the efficacy with which a selected template can form a particular product. Shen and co-workers [98] have demonstrated the importance of including
the Gibbs Free Energy in order to fully rationalise the
synthesis of ZSM-11 by TBA ions. The addition of a
simulated annealing protocol to the Monte Carlo routine is particularly useful when several templates are

optimised within the simulation box. Stevens et al. [99]
successfully used this method to show that a template
which can be used to synthesise different products under
different synthesis conditions has a similar binding en-

Fig. 3. The energy minimised location for dibenzyldimethylammonium ions shown relative to the EU-1 channel system.

ergy in all its products, and that levels of void filling
are also similar in each case.
Zones et al. have made effective use of modelling
methods to aid their search for bulky molecules that
are too large, or have the wrong geometries, to synthesise common default products. This has resulted in the
discovery of several new structures [100]. An automated
‘‘De Novo’’ approach for tailored template design has
been developed by Lewis and co-workers [101]. In this
method, a template molecule is ‘‘grown’’ computationally inside the zeolite host in order to maximise the
non-bonded interactions between template and the surrounding lattice. This has resulted in the successful design of a new template for DAF-5 (a CoAPO material)
[102].
4.2.2. Cluster calculations
The properties of small silica fragments critical to the
synthesis process are difficult to study via experimental
methods. In this respect, cluster calculations, usually
based on quantum mechanical approaches, have proved
highly valuable as a tool to probe the detailed structure,
geometry and reactions for a wide range of fundamental
silica fragments likely to be of importance in the synthesis process (see for example Refs. [103,104]). Pereira
et al. [105,106] have developed these procedures to


12

C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

examine the interaction between solvated zeolite fragments and template molecules. The results from this
study demonstrate that the organic molecule plays a
key role in stabilizing clusters of framework material,
preserving their structure under the influence of water.
Methods for investigating nucleation processes have recently been extended by the work of Wu and Deem
[107]. A Monte Carlo procedure has been developed in
order to model a silicate solution on the atomic scale
in the absence of a template molecule. This study has
yielded an insight into some of the fundamental processes associated with nucleation such as estimation of
the nucleation barrier and the critical cluster size.

In Sections 5–8, suggestions for the most probable
mechanistic pathways in zeolite formation are described
in sequence: induction period, nucleation, crystal
growth. In order to balance coherence with conciseness,
some of the background material necessary to support
the main arguments is noted only briefly. Fuller accounts of such subjects as the nature of growth species
and the role of aggregation processes are given later,
in the ‘‘key topics’’ part of this review (Sections 9–18).

individual reactants; after it, the reaction mixture contains a myriad of small zeolite crystals, already formed
and in the course of growing larger. However, the
growth process once underway displays no discontinuity, so that the mechanism of further growth is essentially that already established in the early stages. This
point is reinforced by the analysis of the induction period made by Subotic´ and co-workers [109], where it is
shown that the activation energy determined from s is
essentially that of the entire crystallisation process. A
further modelling study demonstrates the importance

of including the lag time (tr) in analyses of zeolite crystallisation and confirms the importance of gel/solution
rearrangements to this element of the induction period
[110]. It is also shown that size-dependent crystal solubility (the Kelvin effect) is not a significant contributor
to the crystal growth-rate function (see also Sections
8.5 and 16.4).
In Section 6, we consider further the significance of tr
and the nature of the equilibration processes which
transform the amorphous phase before zeolite product
appears. These prove to be of prime importance in setting the stage for the unique event of nucleation (Section
7). It will then be demonstrated (Section 8) that such
processes also provide the link connecting the chemistry
of nucleation with that of crystal growth.

5. The induction period

6. The evolution of order

The induction period is the time (t) between the notional start of the reaction and the point at which crystalline product is first observed. It will therefore depend
on the moment chosen for setting t = 0 (often taken as
the time at which the reactants reach the working temperature) and upon the method of analysis used to detect the product (most usually X-ray diffraction). For
precipitation reactions, classical nucleation theory
[108] divides this period (s) into a number of subunits:

6.1. The nature of the amorphous material

Part II: Synthesis mechanism

s ¼ tr þ tn þ tg ;
tr is referred to as the relaxation time and is said to be
the time required ‘‘for the system to achieve a quasisteady-state distribution of molecular clusters’’. In zeolite terms, this can be equated with the equilibration

reactions taking place on mixing the reagents and allowing them to reach reaction temperature, during which
period the observable distribution of silicate and aluminate ions (and other species) is established (see Section
6.2). The subunits tn (the time for the formation of a stable nucleus) and tg (the time for the nucleus to grow to a
detectable size) are directly translatable into zeolite
chemistry.
It will be apparent from the above that the induction
period encompasses all the significant events of zeolite
formation. Before this time, there existed only the

It is often convenient to treat the amorphous phase as
a constant quantity which, apart from depletion, remains essentially unchanged throughout the synthesis.
Such an approach is usually adopted for the purposes
of reaction modelling [89–92] (Section 4). However,
the dynamic nature of this material was recognised in
the earliest mechanistic studies (Section 3). Flanigen
and Breck envisaged [51,52] a transformation through
polymerisation and depolymerisation catalysed by excess hydroxyl ion, whilst Zhdanov [57] and Kerr [55]
saw the initial amorphous gel as coming to equilibrium
with the liquid phase and releasing active soluble species
into the solution, thus changing with time. As will be
shown, this transformation in the nature of the amorphous phase during the early stages of the reaction is
very significant.
6.2. Primary and secondary amorphous phases
At the point where the synthesis reactants are initially
mixed together, a visible gel is frequently formed. This
will be referred to as the primary amorphous phase. In
some cases (‘‘clear solution’’ syntheses), this primary
phase is colloidal, and thus invisible to the naked eye,

C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

SiO2 source

Al2O3 source

M+ OH –

Q+ OH–

partly-reacted, heterogeneous
non-equilibrium mixture:
solution + solid
↑

PRIMARY AMORPHOUS PHASE
time

temperature

– H2O
SixOy(OH)z

Si(OH)4
EQUILIBRATED
SOLUTION

OH–

OH–

– H2O
ionised monomers

ionised polymers

Al(OH)4–
colloidal and gel
silicates

colloidal and gel
aluminosilicates
↑

↑

SECONDARY AMORPHOUS PHASE
Fig. 4. Equilibration of the starting mixture to establish a partly
ordered intermediate (secondary amorphous phase) and a characteristic distribution of solution species.

Solution

Reactants

but its function and behaviour are essentially the same
(see Section 7.6). The primary amorphous phase
represents the initial and immediate product from the
reactants and is a non-equilibrium and probably heterogeneous product containing (for example) (a) precipitated amorphous aluminosilicates, (b) coagulated silica
and alumina precipitated from starting materials destabilised by the change in pH and increase in salt content
and (c) unchanged reactants. The pH of such a mixture

is not usually a useful characteristic measurement, since
it depends on particular circumstances and will change
with age.
After some time, either on standing, or—more rapidly—on heating at reaction temperature, the above
mixture undergoes changes due to the equilibration
reactions which occur (Section 7.8) and is converted
into a pseudo-steady-state intermediate, the secondary
amorphous phase. Concurrently, the relationship between the solid and solution phases approaches an
equilibrium and a characteristic distribution of silicate
and aluminosilicate anions is established (Fig. 4). A
pH measurement will now provide a useful reference
point from which, in high-silica zeolite synthesis, the
progress of the reaction can be monitored by recording
subsequent changes [92,93]. In the final stage of the
synthesis reaction (usually at elevated temperature for
a prolonged period), the secondary amorphous phase
is converted into the crystalline zeolite product (Fig.
5).
The concept of an equilibrated intermediate phase
is clearly expressed in ZhdanovÕs representation of
the synthesis process [57] (Section 3.4) and is also implied in the type-A and type-B schemes of Derouane
et al. [58] (Section 3.6—see also Table 1). In order
to show that such equilibration has occurred, the balance of solution species and the partition of components between the solid phase and solution can be
probed by a variety of analytical and spectroscopic
techniques (e.g. Refs. [61,111–116]). The structuring
action of cations is apparent from their role in the
organisation of the solid phase [117], as described
below. In some cases, there are also strong interactions
with solution species—as demonstrated, for example,
by the correlation between the occurrence of the cubic

octamer [Si8O20]8À and the presence of the TMA cation [118].
Several groups of authors have commented upon the
existence of the secondary amorphous phase. Angell and
Flank [77] report (for zeolite A synthesis) that ‘‘. . . the
initially formed sodium aluminosilicate gel is converted
via solution transport to an apparently amorphous aluminosilicate intermediate. It is this latter material which
is converted to crystalline zeolite via dissolution by the
basic medium.’’ Fahlke et al. in 1987 observed (for zeolite Y) an immediate precipitation of a silica-rich primary gel, followed by its slow dissolution and then the

13

nucleation
Initial
gel

random

(a)

Equilibrated
gel

increasingly ordered

(b)

growth

Crystalline
product

(c)

Fig. 5. The evolution of order, from the primary amorphous phase (a)
through the secondary amorphous phase (b) to the crystalline product
(c).

precipitation of a secondary gel having a similar Si/Al
ratio to that of the zeolite product [119]. However, a
study by the Montpellier group in 1993 [117] is particularly informative and merits a more detailed description.


14

C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

6.2.1. The Montpellier study
This investigation [117] is the clearest demonstration
yet published of the multi-stage nature of the zeolite synthesis reaction as outlined in Fig. 5. In this work, Nicolle
et al. prepared a series of Na, TEA-aluminosilicate gels
of varying composition (TEA = tetraethylammonium)
and heated samples of these to 150 °C (i) for a relatively
short period and (ii) for a longer time. The solid products from each reaction period were isolated and analysed. The actual heating times varied with the
reactivity of the particular composition but were aimed
at achieving in the first instance (i) an equilibrated gel,
and in the second case (ii), a crystalline zeolite product.
The four compositions chosen (A–D) crystallised in the
second stage to (A) zeolite beta, (B) mordenite, (C)
ZSM-12 and (D) ZSM-5. However, the most significant
findings came from the analysis of the initial, equilibrated products.

X-ray diffraction revealed only broad features indicative of local ordering but no crystal lines. All samples
showed aluminium in tetrahedral coordination only
(27Al MAS NMR) and no zeolitic bands in their infrared
spectra. Chemical analysis demonstrated that the hydrothermal treatment had greatly modified the composition
of the solid phase (i.e. compared to that before heating),
with TEA+ replacing Na+ as the preferred cation. The
TEA content was nearly constant, corresponding to
one organic cation per 16–20 T-atoms, whatever the
method of preparation or the Si/Al ratio. Other, specific
tests were carried out on selected samples. Equilibrated
samples A and B had (after calcination) cation exchange
capacities in close agreement with their aluminium contents (%85 mmol NHþ
4 /100 g). For equilibrated samples
(A)–(C), calcination generated a micropore system having about 60% of the capacity shown by the zeolite beta
crystallised from composition A. Amorphous sample B
showed the same micropore volume for cyclohexane as
for nitrogen but was impermeable to trimethylbenzene.
The authors concluded that the amorphous, equilibrated samples differed from the gels initially precipitated at room temperature and had been formed by
dissolution and reprecipitation. These secondary products were silicoaluminates sharing several properties
with high-silica zeolites. In both cases, clusters of silica
or alumina tetrahedra had been organised (templated)
around a bulky molecule, whose decomposition left a
micropore system. The essential difference between the
two classes of solids was represented by the dispersion
of the cluster geometry. Zeolites featured well-defined
and repeatable site geometries whereas the equilibrated,
amorphous products presented irregular and hence aperiodical, local organisation. From data discussed elsewhere (Sections 6.3 and 12) it is clear that these are
general observations and not, for example, associated
only with the synthesis of zeolites templated by quaternary ammonium ions. Similar phenomena occur during

the preparation of aluminous (inorganic) zeolites
[77,119].
6.2.2. Related investigations
There appears at first sight to be a discrepancy between the observations of the Montpellier group [117]
and those described in a detailed investigation of zeolite
beta synthesis by other workers. On the basis of thermal
analysis and charge-balance data, Perez-Pariente et al.
[120] concluded that almost no TEA was associated with
the amorphous solid. However, in later work, Camblor
and Perez-Pariente reported the presence of TEA cations associated with the aluminate sites in pre-crystalline
samples [121], concluding nevertheless that the overall
crystallisation pattern (by a solution transport mechanism) was the same in both cases. The most obvious differences between the two studies were the use of
tetraethyl silicate and a low crystallisation temperature
(100 °C) in the earlier work, whereas the later investigation employed amorphous silica (Aerosil) and a reaction
temperature of 135 °C. However, a more significant disparity lies probably not with these variables but in the
cation balance. The 1991 study centred around synthesis
compositions which are fairly typical of routine zeolite
beta preparations, namely
x½yK; ð1 À yÞNaŠ2 O 12:5ðTEAÞ2 O Al2 O3 50SiO2 750H2 O

where x was varied from 0 to 4.5 and y from 0 to 1, with
OH/SiO2 constant at 0.56. The range of compositions
used for the 1987 study was generally similar—with
the exception of the gel used for the analysis of the
amorphous intermediate (B2 at 12 h), which was
7:8Na2 O 0:11K2 O 1:5ðTEAÞ2 O Al2 O3 30SiO2 360H2 O

The very high M/TEA ratio will have a major effect
upon the partition of cations between the liquid and
gel phases, so that the low TEA content of the isolable

solid phase is perhaps not unexpected.
6.3. Further evidence for pre-crystalline order from
synthesis studies
Tsuruta et al. studied the initial product in the synthesis of zeolite A from concentrated solutions in the
presence of an anionic surfactant [122]. Using X-ray
and electron diffraction, they found that the amorphous
aluminosilicate isolated under mild reaction conditions
(60 °C, 15 min) possessed a short-range order of Si
and Al atoms similar to that in crystalline zeolite A.
Electron diffraction, thermal analysis and FTIR spectroscopy were used by Subotic´ and co-workers in a
study of the structural properties of X-ray amorphous
sodium and potassium aluminosilicate gels [123,124].
The gels were found to contain structurally ordered regions, or particles of a partly crystalline phase inside


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

an amorphous matrix. Subsequent hydrothermal treatment in 2 M NaOH at 80 °C yielded zeolites A and X.
Walton and OÕHare [125] examined amorphous gallium
silicates isolated in the early stages of the hydrothermal
synthesis of Ga-hydroxysodalite by means of EXAFS
and XANES spectroscopy. Both Ga and Si K-edge EXAFS indicated some degree of medium-range order (i.e.
beyond the first coordination shell), indicative of an extended network of alternating Ga(OSi)4 and Si(OGa)4
units resembling the structure of the crystalline product.
A related multi-nuclear MAS NMR and neutron diffraction study of amorphous zeolite A precursors by Yang
et al. [126] detected changes in medium range order prior
to crystallisation as Al (accompanied by Na+ cations)
was incorporated into the silicate network. However,
both spectroscopic and diffraction techniques indicated
that there were no well-defined structural units (e.g.

SBUs) present before the formation of the zeolite crystals. Rather equivocal results were obtained from a
129
Xe NMR study of samples taken during the course
of various zeolite syntheses [127]. Evidence of gel structuring (large cavities) was found for Na,TPA-ZSM-5
and Na-Y but not for Na,TEA-ZSM-20. However, only
in the latter case did the NMR measurement show any
appreciable microcrystallite formation in the absence
of XRD crystallinity.
There have been several other investigations of the precrystalline state of MFI-type synthesis compositions. In
the work of Burkett and Davis, 1HA29Si CP MAS
NMR spectroscopy demonstrated that the TPA molecules and silicate species were in close proximity in the
heated gel before the characteristic XRD and IR fingerprints of crystalline silicalite became apparent [64]. No
such correspondence was found in the unheated gel. This
result was supported by an in-situ multinuclear NMR
study at 80 °C [128]. Hydrogen bonds between the organic
and water-clathrated molecules are progressively replaced by hydrophobic interactions between the organic
and silica species. Similar evidence of TPA occlusion
comes from SAXS [129] and SANS [130,131] measurements. Several groups of workers have reported FTIR
bands at 550–560 cmÀ1 in isolated material which is Xray amorphous [84,130–134], these being interpreted as
indicative of the presence of material having the MFI
structure. However, some caution is necessary in this
analysis since such an absorption is not unique to ZSM5. It is advisable to compare the bands which should appear at both 550 and 450 cmÀ1: for a well-crystallised
ZSM-5, the 550:450 intensity ratio should be %0.7, a figure which decreases with decreasing crystallinity [135].

15

(primary amorphous phase). However, over a period of
time (tr), and especially on heating, silicate and aluminosilicate equilibration reactions will occur, leading to a
re-distribution of species and a repartition of reaction
components between the solid and liquid phases as they

approach equilibrium with one another. Cations play a
structuring role in the organisation of the solid phase,
which is generated as a new or reconstructed material
having a similar chemical composition to that of the
eventual zeolite product but lacking in long-range, periodical organisation and hence amorphous to X-rays (secondary amorphous phase). However, elements of local
order will be present and will impart to the amorphous
intermediate some of the chemical and physical properties associated with the final, crystalline, ‘‘isomeric’’ zeolite. This is shown diagrammatically in Fig. 5 and is
discussed further when the topic of ‘‘X-ray amorphous
zeolites’’ is considered (Section 12). In Section 7, this
semi-organised solid phase is shown to be pivotal in
the transforming step of zeolite nucleation.
In the above discussion, a firm distinction is made between primary and secondary amorphous phases in order
to emphasise the importance of the changes taking place.
However, the situation in some zeolite syntheses may be
less clear cut, since the different stages in the process may
overlap, e.g. nucleation may begin whilst the bulk of the
reaction mixture is still far from attaining a steady state
(this is more likely to occur in high temperature syntheses). This ‘‘relative kinetics’’ effect is an important factor
in the dependence of the outcome of a synthesis reaction
upon the nature and mode of mixing of the reactants.
Certainly, in cases where no intermediate samples are
taken for physico-chemical characterisation, the researcher may be unaware that the progression from reactants to product is anything other than a continuum.
The extent to which the secondary amorphous phase
can be identified or isolated will also depend upon the
nature of the synthesis mixture. In some cases, partition
of the reactants between the liquid and solid phases may
be such that little or no material may be identifiable by
conventional methods of solid–liquid separation, so that
colloid chemical techniques may be necessary to observe
it (see Sections 7.6 and 7.7).

Finally, it should be noted that in Fig. 5 gross heterogeneity (e.g. the presence of some unchanged starting components) in the primary amorphous phase has been
ignored for the sake of simplicity and the material illustrated as a single homogeneous phase of random structure.

7. Nucleation
6.4. Summary
7.1. Introduction
When first prepared, the reaction mixture for a zeolite
synthesis consists of a non-equilibrium combination of
components—heterogeneous and at best partly reacted

From the above (Section 6), it is clear that the equilibration which allows the conversion of primary to


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

secondary amorphous phases (Fig. 5 and Section 7.8)
forms an extremely important part of the zeolite synthesis process. By this means, the initial gel reacts with
solution species to establish (or approach) a pseudosteady-state distribution of liquid phase species, predominantly ionic but with a wide range of charge/mass
ratios (Fig. 4 and e.g. Refs. [111–116]). The changes in
the amorphous phase involve an increase in structural
ordering but without the establishment of the periodic zeolite lattice itself. For this, a discreet nucleation event has
to occur. In this step, a statistical selection of the reconstructed areas reach a critical nuclear size and degree of
order such that a periodic structure is able to propagate,
i.e. crystal growth can begin.
In the following treatment, nucleation is first considered from a classical standpoint (Section 7.2), after
which experimental methods for the determination of
nucleation patterns are described (Section 7.3). Next,
brief reviews are given on the topics of seeding (Section
7.4) and autocatalytic nucleation (Section 7.5). Finally,
the special features of zeolites are examined, leading to

a consideration of the underlying mechanism (Sections
7.6–7.8).
7.2. General considerations
Amongst the components (tr, tn, tg) comprising the
induction period (s) (Section 5), tn is the time taken to
form a stable nucleus. Fig. 6 illustrates the classical concept of a critical nuclear size [108]. Beyond this point
(where nuclear radius (r) = rc), sufficient matter has
come together in an ordered way for the cohesive energy
(DGv) to outweigh the energy expended in creating a surface dividing the nucleus from the continuum (DGs).
This enables a stable unit to be formed: a viable nucleus,
capable of growth. For zeolite systems, a critical size of
1–8 unit cells has been estimated by Thompson and
Dyer [90]. However, the chemical and physical processes
leading up to critical nucleus formation are governed by
the structure being formed and by the experimental conditions. If these conditions change, then rc will also vary,
as can be seen from the general form of the expression
for r below [108]:
r¼

2rv
;
kT ln S

where r is the interfacial tension (surface energy per
unit area), v is the molecular volume and S is the supersaturation ratio. A notional nucleation rate (J) can be
defined as the rate of production of units having radius
r P rc. A simple measure of J is often taken by assuming J / 1/s [31,78]. However, since s is a compound
term, the danger in this procedure is apparent, unless
tr and tg are known to be small compared with tn
[109,110].

∆GS
+ ve

Free energy (∆G)

16

∆Gc =
0

rc

4
πσrc2
3
r

- ve
∆GV

∆G

Fig. 6. The energetics of nucleation, illustrating the concept of a
critical nucleus of radius rc; beyond this size, the net energy gain from
the resultant (DG) of cohesive (DGv) and surface (DGs) terms is
favourable to growth (after Mullin [108]).

It seems reasonable to suppose that the construction
of the nucleus of a zeolite crystal may involve a more

complex assembly process than is necessary for simpler
substances whose unit cells are smaller and contain far
fewer atoms, although there should be no difference in
basic principles. One possible special feature in the formation of microporous crystals has however been identified by Pope, who has pointed out that the energetics
of zeolite nucleation may be greatly modified from that
of dense phases because of the presence of the large
internal surface area [136].
The nomenclature for the most usually recognised
types of nucleation can be summarised as follows
[108]: primary nucleation may be homogeneous (from
solution) or heterogeneous (induced by foreign particles); secondary nucleation (induced by crystals) may
be considered as a special case of heterogeneous nucleation in which the nucleating particles are crystals of
the same phase. Good general accounts of nucleation
in zeolite systems (along with further references) will
be found in reviews by Barrer [137] and Thompson
[37,91].

7.3. Determination of zeolite nucleation patterns from
measurements on the resulting crystals
Obtaining direct experimental data on nucleation
processes is very difficult due to the extremely small fraction of the total mass involved and the problems of distinguishing this from surrounding reactants of very
similar nature and composition. Consequently, much
of the information available is at best that of early
growth behaviour rather than of nucleation itself. Perhaps only the recent cryo-TEM [129] and HRTEM studies [134,138,139] (Section 7.7) genuinely fall outside this
category and can be considered as direct evidence of


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

17

nucleation phenomena. However, much useful data can
be obtained from ‘‘circumstantial evidence’’, i.e. measurements of the appearance, growth and size distribution of the crystals resulting from the earlier, hidden
patterns of nucleation.
7.3.1. Studies under isothermal conditions
It is still fairly common practice to estimate nucleation rates in zeolite synthesis from the reciprocal of
the induction times [78], despite the acknowledged pitfalls (Section 7.2) [31]. This is largely because of the difficulty of making more appropriate and accurate
measurements, particularly if temperature variation is
to be included. An improvement on this can be found
in the procedure of Zhdanov and Samulevich, which enables the calculation of isothermal nucleation rate profiles from determinations of growth rate and crystal
size distribution [140,141]. Originally implemented in
analyses of zeolite Na-A [57] and Na-X [140] crystallisation, the method has subsequently been applied to other
zeolite systems, including silicalite [142,143]. If it is supposed that all the crystals in a batch have the same
(known) growth rate behaviour, the total growth time
of each crystal can be calculated. Assuming also that
the nucleation point for each crystal can be obtained
by linear extrapolation to zero time, the nucleation profile for the whole batch can be determined, as illustrated
in Fig. 7. (Although Zhdanov and Samulevich were the
first to apply the above type of analytical procedure to
zeolite synthesis, Giaya and Thompson, whilst researching their own numerical technique for determining the
crystal size distribution function [144], discovered that
the approach had been discussed in a wider context
much earlier by Bransom and Dunning [145].)
7.3.2. Ageing studies
Although the types of investigation mentioned above
yield useful data, far more detailed information on
nucleation behaviour can be obtained from ageing studies. In these cases, a reaction mixture is allowed to age at
one temperature and is then crystallised at a second
(usually higher) temperature. By correlating changes
(principally of size) in the product crystals with the

length of the ageing period, aspects of the nucleation
behaviour can be deduced. In particular, the evolution
of order (Section 6) in the zeolite synthesis mixture
(equivalent to the concept of accumulation of protonuclei) can be determined quantitatively. A detailed account of ageing processes is given in Section 11.

Fig. 7. Determination of nucleation rate profiles by the method of
Zhdanov and Samulevich [140] as applied to silicalite synthesis [142].
From final product crystal size distribution (a), crystal linear growth
rate (b) allows calculation of nucleation rate as a function of reaction
time (c). Data consistency is shown by comparison of calculated
mass% growth rate ((d), curve) with experimental values ((d), marked
points). Note that the length axis in (a) becomes a time axis in (b)–(d).

7.4. The use of seed crystals
It is a common and very useful practice to add seed
crystals to zeolite reaction mixtures and excellent accounts of this technique are available [37,91,146]. The

two main effects anticipated are (i) a reduction in the
synthesis time and (ii) a ‘‘direction’’ of the synthesis
towards a desired phase with consequent reduction in


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impurities. It is also possible to exert control over product crystal size distribution [147–150]. The bases for
these valuable influences are worthy of brief
consideration.
The basic action of seed crystals (when added effectively) is to provide surface area on which the required

product can grow. This removes the necessity for such
surface to be self-generated by primary nucleation and
thus reduces the induction time (s), since the tn component of s is eliminated (see Section 7.2). In some cases
(equivalent to the traditional scratching with a glass
rod in organic chemistry), almost any intervention
may produce the desired effect. However, in others there
is a definite element of surface recognition by which one
phase can be given a kinetic advantage whilst potential
competitor products are relatively unresponsive.
If seed crystals are added to a synthesis mixture, they
may behave in a number of ways, i.e. the crystals may
(a) remain inert, (b) dissolve, (c) act as pure seeds, in
that mass is deposited upon them and they grow, or
(d) give rise to secondary nuclei and hence a new crop
of crystals. In general, it is necessary to provide sufficient
surface area to achieve an effect, so that it is rare for a
very small quantity of large crystals to cause any significant change in the normal course of the reaction. Such
crystals can usually be found at the end of the synthesis,
either intact (i.e. case (a)) or showing signs of attack
(case (b)). If the added material is unstable in the synthesis medium, it will normally dissolve and only influence
the course of the synthesis if added in sufficient quantity
to alter significantly the reaction stoichiometry (limit of
case (b)).
The balance between seed growth (c) and secondary
nucleation (d) depends on the nature of the system,
the quantity of material added and the degree of agitation. If the surface area of seed crystals present is sufficient to absorb most of the available flux of growth
species and thus prevent the effective solution supersaturation from reaching high levels, then most of the
growth in the system will take place on the seed crystals,
whose size and growth rate can be closely controlled and
predicted by kinetic modelling [79,89–91,147,148]. Very

small (colloidal) zeolite crystals are, as would be expected, particularly effective [149–152]. As the quantity

Table 2
Calculated and observed product crystal sizes in seeded silicalite
synthesis [152]
Seeding level (%)

5
10
20
Seed size 43 nm.

Product crystal size (nm)
Calculated

Observed (±1 nm)

111
89
74

98
83
70

(or, more correctly, surface area) of added seed material
is reduced, then the natural supersaturation and consequent self-nucleation will no longer be suppressed and
the product crystal size will deviate more and more from
that predicted on the basis of linear growth on seed crystals only (Table 2).
Secondary nucleation certainly occurs in zeolite systems but has been the subject of few detailed studies

[37,91]. If also assisted by agitation, then some form of
collision breeding [37,91,108,153] is likely to be operative on a macro- or micro-scale. From a practical point
of view, secondary nucleation has been observed in syntheses which for various reasons (sometimes unknown)
are very reluctant to self-nucleate even though the compositions are believed to be near the optimum. Possibly
‘‘active’’ impurities [37,154] are absent, or nucleation/
growth poisons are inadvertently present [155,156]. In
such cases, the use of seed crystals can be very effective
since the response to secondary nucleation is a geometric
function of the quantity added. This is demonstrated in
the synthesis of siliceous forms of EUO-type zeolite
(EU-1, ZSM-50) [16] (Fig. 8). The reactions [157] were
run at 150 °C for three days using the systems
10Na2O : xAl2O3 : 60SiO2 : Template : 3000H2O, where
either x = 0, Template = 13 dibenzyldimethylammonium chloride (DMDBACl), or x = 0.05, Template = 10
hexamethonium bromide (HEXBr2). In the absence of
seed crystals, reactions gave either very low yields or
products heavily contaminated with impurity phases
(mainly EU-2 and cristobalite). With DMDBA template, crystallisation was greatly assisted by the inclusion
of seed crystals (5 mass% based on silica). Large ‘‘parent’’ and small ‘‘daughter’’ crystals are clearly distinguishable. The greater available surface area provided
by calcined seed makes this a more effective nucleant
(Fig. 8b) than the uncalcined material (Fig. 8a), resulting in a larger number of smaller product crystals. The
spawning of daughter crystals from a partly dissolving
calcined seed crystal is shown in Fig. 8c. A similar effect
was found for the HEX preparation of EU-1, where the
product crystal size from calcined seed (Fig. 8d) was
again smaller (1 lm) than that from as-made material
(2 lm, not shown). The micrographs reveal a further
interesting point. The same batch of Na,DBDMAEUO seed was used in all cases but it is clearly the synthesis reaction template which controls the product
morphology.
A further aspect of seeding has been demonstrated by

Bronic´ and co-workers. The syntheses of zeolites Y and
P [158] and of zeolite A [159] have been investigated
using a 2-compartment reactor in which seed crystals
and reactant gel were separated by a submicron membrane. Crystal growth occurred in both compartments,
in some cases giving different zeolite phases (Y on seed
side, P on gel side). However, it was found that the seed
crystals modify the crystallisation process only if they


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

19

Fig. 8. Seed crystals (large) seen amongst the induced secondary crystal population in siliceous EUO-type zeolite synthesis. Templates were: (a–c)
DMDBA, (d) HEX. Seed crystals were: (a) as-made, (b–d) calcined. Note that the micrographs vary in scale. See text for discussion.

are in physical contact with the gel. Thus it can be
deduced that growth species in the liquid phase travel
freely through the membrane, whereas nucleation is controlled more locally. These studies are thus reminiscent
of the Kerr recirculation experiment [55,56] (Section
3.3).
7.5. Autocatalytic nucleation
One of the sources of nuclei proposed in the classic
paper by Zhdanov [57] was that some germs lie dormant
in the amorphous phase until activated by release into
the solution through gel dissolution. Since the rate of
gel dissolution must increase with the rate of consumption of growth species by the increasing cumulative crystal surface area, the process was seen as ‘‘autocatalytic’’.
This approach has been elaborated and modelled by

Subotic´ and co-workers [143,160–165]. However, it has

been pointed out [166] that the nucleation periods predicted by the model are unrealistically long. The idea
was therefore modified [167] by postulating that the dormant nuclei were located preferentially near the periphery of the gel particles and therefore became activated
much earlier in the crystal growth/gel dissolution process. A subsequent study by Falamaki et al. [168] on
ZSM-5 crystallisation using this modified approach gave
excellent agreement between the model and experiment.
However, there appears to be no chemical justification
for the assumption and any self-consistent set of model
parameters which generated nuclei as a suitably limited
function of synthesis time would presumably produce an
equivalent result. This is demonstrated by the work of
Nikolakis et al. [169] who provide an alternative to the
hypothesis of nuclei release from the dissolving gel by


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C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

treating gel dissolution and nucleation as interfacial phenomena controlled by the gel–solution boundary. Their
model for the gel microstructure gives rise to a maximum in the temporal surface area function which corresponds to the observed early maximum in the nucleation
rate.
7.6. Nucleation in zeolite systems—The nature of the
reaction sol
In Sections 7.3–7.5, nucleation has been considered
largely from an experimental standpoint. The mechanistic basis for these phenomena will now be discussed. As
a first step, it is necessary to expand upon the consideration of the synthesis reaction mixture introduced in the
original summary of experimental observations (Section
2). In a typical situation (from which there will be variants), there is likely to be present (i) amorphous material, (ii) a solution phase and (iii) one or more
crystalline phases. It is relatively easy to identify component (iii), even if the crystals are of colloidal dimensions.
The distinction between constituents (i) and (ii) is much

less clear cut and has been the source of much
misunderstanding.
The simplest case is that in which most of the amorphous material is visibly solid in nature, as in traditional
syntheses of aluminous zeolites such as A or X. At the
low Si/Al ratio and high pH of these preparations,
nearly all the Si,Al-derived nutrients are present as a
solid gel phase, with the balance existing as low molecular weight species (monomers, oligomers) in a strongly
alkaline (Na+OHÀ) solution phase [170]. Separation of
solid and liquid phases by normal laboratory methods
(filtration, centrifugation) will achieve a reasonably
effective division of the reaction mixture into a liquid
phase containing (largely) true solution species
[111,171] and a solid phase from which the proportion
of crystalline to amorphous material can be determined
by XRD measurement.
At higher Si/Al ratios and lower pH, the situation becomes complicated by an increasing proportion of high
molecular weight silicate and aluminosilicate components. The distinction between solution-related polymers and colloidal material is a problem which lies at
the heart of the understanding of silicate chemistry
[171–174]. However, the sequence of polymeric species
Si1, Si2, Si3 ,. . ., Sin cannot be regarded as a continuum,
since at some point the larger components behave as if
they are particulate and phase-separated, resembling
an invisible precipitate [171–173]. Thus, whereas one
part of the silicate and aluminosilicate loading of the
liquid phase behaves as true solution species, a second,
colloidal fraction resembles a separate amorphous
phase. This latter is essentially equivalent, in terms of
energetics, to the solid, visible, amorphous gel but is
invisible to optical detection and inseparable from the

Fig. 9. The colloid problem: the liquid phase resulting from conventional solid–liquid separation is likely to contain both true solution
species and colloidal amorphous material.

liquid phase by filtration or (standard) centrifugation.
The result of this is that separation of solid and liquid
phases in syntheses of this type does not remove amorphous material from the liquid phase (Fig. 9). Indeed,
in most so-called ‘‘clear solution’’ syntheses (see e.g.
Refs. [130,142,175–177]), all the amorphous feedstock
is stored by the system in this way, forming the nutrient
which provides the driving force for the crystallisation
of the zeolite product. Thus, any consideration of nucleation and crystal growth in hydrothermal zeolite synthesis must take into account the role of amorphous
components at all levels of subdivision.
7.7. Nucleation in zeolite systems—Homogeneous or
heterogeneous?
In zeolite synthesis, there is evidence for the participation of both primary and secondary (crystal-catalysed) nucleation, although the former is believed to be
the most prevalent [37,91]. Even so, there is often disagreement on whether the predominant mechanism is
homogeneous or heterogeneous. For example, a recent
study combining both theoretical and experimental
work on the nucleation of zeolite A concluded that
nucleation was homogeneous except in the case of seeding with certain high surface-area titanias [178]. Related
comparisons of calculated and observed behaviour for
zeolites A and Y by Bronic´ and Subotic´ found that the
contribution of homogeneous nucleation was negligible
[179]. However, it can be seen from the above discussion
of the nature of the liquid phase (Section 7.6) that there
is a more subtle ambiguity within the primary classification as to whether an apparently homogeneous nucleation event may be genuinely so—or in reality
heterogeneous—depending on whether or not the colloid phase is involved.


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

A study on (initially) clear solution synthesis of aluminous zeolites by Aiello et al. [180] showed that heterogeneous nucleation appeared to occur on gel lamellae
which separated at an early stage from the solution
phase. A similar phenomenon was later observed in
the clear solution crystallisation of silicalite, where the
first visible crystals were seen to nucleate on traces of
amorphous material (‘‘gel rafts’’) which could be seen
using optical microscopy [142]. The authors of the latter
paper suggested that ‘‘the heterogeneous process directly
observable from the small amount of gel precipitated
early in the reaction. . . may be indicative of a more general heterogeneous process involving the much smaller,
invisible, colloidal gel particles which represent the principal reservoir of silica in the system.’’ The presence of
these particles was at that time inferred from indirect
measurements. More recently, this has been confirmed
by the application of improved scattering techniques
[31,85,181] and electron microscopy (see below).
For syntheses in which a visible gel phase is present
(probably the most familiar case), examination of the
reaction mixture under an optical microscope usually reveals gel particles of around 1000–50,000 nm (1–50 lm)
in size. These dimensions are system-dependent and also
liable to change through processes of agglomeration or
agitation-induced fission. However, crystallisation in

21

such systems seems always to be associated with the
gel phase, so that the product crystals, when observed
in mid-reaction, are found to be intimately dispersed
within a matrix of amorphous material. From observations on dilute reaction mixtures [142,180], it is reasonable to assume that these crystals have indeed
nucleated heterogeneously within the gel phase. Such a

situation is credible since nutrient concentration gradients will be greatest at the solid–solution interface and
the surface of the amorphous phase will provide sites
at which the free energy change necessary for nucleus
formation is lower than would be the case for homogeneous nucleation (equivalent to a reduced number of degrees of freedom).
A more complex type of heterogeneous nucleation
has been observed by Zandbergen [182]. A Na,TPA-silicalite synthesis mixture was heated quiescently at 65 °C
and the solid phase which began to form after two days
was examined from time to time by HREM. After four
days, two types of morphologies were observed within
the solids: (A) large (2 lm) amorphous, irregularly
shaped particles, and (B) agglomerates of small, partly
crystalline particles (0.1 lm). The ratio B/A + B and
the fraction of crystalline particles in B both increased
with reaction time. The d-spacings detected in B
suggested the initial formation of a dense phase. Large

Fig. 10. Direct observation of zeolite Y nucleation on colloidal gel particles by HRTEM [139]: (a) amorphous particles in freshly prepared
aluminosilicate solution; (b–d) development of crystallinity upon hydrothermal treatment at 100 °C after (b) 28 h, (c) 48 h and (d) 75 h. (Reproduced
from Ref. [139] with permission.)


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C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

d-values in the ED patterns were observed only after
25 days and only in the vicinity of the dense crystalline
phase, after which the presence of the MFI phase increased rapidly. Very small zeolite crystals (<10 nm)
were rare, indicating very rapid growth once the critical
size had been reached.

Clear solution synthesis is a special case of gel synthesis in which the amorphous particles remain of colloidal
or sub-colloidal dimensions (650 nm), i.e. invisible by
optical examination. However, the considerations of
crystal nucleation and growth are essentially the same.
This has recently been beautifully demonstrated using
electron microscopy for the clear solution syntheses of
zeolites A [138], Y [139] and Si-MFI [134] by Mintova
and co-workers (Fig. 10). Using high-resolution transmission electron microscopy (HRTEM) in conjunction
with in situ DLS, X-ray diffraction and other techniques, Mintova et al. have imaged the development of
crystalline structure within amorphous gel particles of
nanometre dimensions, i.e. the process depicted schematically in Fig. 5. Single zeolite A crystals were observed [138] to nucleate in amorphous gel particles of
40–80 nm in size within three days at room temperature.
The embedded zeolite A nanocrystals grew at room temperature, consuming the gel particles and forming a colloidal suspension of 40–80 nm crystals. A broadly
similar picture was found for the zeolite Y [139] and
Si-MFI [134] phases.
It is worth noting that the only case in which a ‘‘clear
solution’’ synthesis does not involve some amorphous
material is that in which all components are in true solution and crystallisation is driven by differences in solubility only (see Section 14.3). This is very rare and can
yield only small amounts of product—a very dilute system of moderate Si/Al ratio could perhaps provide an
example.
A further comment can here be made concerning the
interpretation of the phenomenological and practical
differences observed between the ‘‘A’’ and ‘‘B’’ type
pentasil syntheses of Gabelica and co-workers [58–62]
(Section 3.6). Dissimilarities in reagent sources and
reaction composition lead to changes in the partition
of reaction components between solution, macroscopic
gel and colloid phases. These in turn give rise to the
observed differences, particularly in nucleation behaviour. In case ‘‘A’’, nucleation occurs mainly on colloidal particles in the liquid phase. In ‘‘B’’, the lower base
level, lower silica/water ratio and higher Na/TPA ratio

lead to a much higher proportion of amorphous solids
and a very high rate of nucleation within the solid gel
phase.
In summary, it is concluded that (i) the most common
process of zeolite nucleation relies on a primary nucleation mechanism and (ii) the most probable primary
nucleation mode is heterogeneous and centred upon
the amorphous phase of the reaction mixture (which

for most ‘‘clear solution’’ syntheses is colloidal in
nature).
7.8. Nucleation in zeolite systems—Mechanism
This section considers the implications of the equilibration reactions which lead to the formation of the
semi-ordered secondary amorphous phase and how
related processes then give rise to zeolite nuclei. In
Fig. 5, the primary amorphous phase was illustrated
diagrammatically as a glass-like network having no
elements of order. Where there is a driving force
for zeolite formation, i.e. negative DG in the
equation


K s;gel
DG ¼ ÀRT ln
ðfrom Section 4.1Þ;
K s;zeolite
then there will be an overall tendency for this phase to
become more ordered, as represented in the figure by
the appearance of grouped hexagons for the secondary
amorphous phase (equilibrated gel) and a hexagonal
network to signify the periodic structure of the crystalline product. (Note that, although the hexagon does represent a 6-T-atom ring, there is no chemical or structural

significance in the choice of this specific unit: it is simply
a convenient geometric shape with which to represent
the development of an ordered region. It does not imply
any particular role or importance for 6-rings in general.)
The chemical route by which such progressive ordering is brought about is that highlighted by Flanigen and
Breck [51,52] and by Chang and Bell [63], namely the
breaking and remaking of Si,AlAOASi,Al (TAOAT)
bonds catalysed by hydroxyl ion, the related condensation reaction also playing a part:
TAOH þ À OAT
TAOAT þ OHÀ
TAOH þ HOAT
TAOAT þ H2 O
However, the anions do not work in isolation and a crucial role in the ordering process is played by the cations
present in the synthesis system. These will act to attract
around themselves energetically favourable coordination spheres of oxy-species and in so doing will generate
certain preferred geometries. In this way, cation-dependent elements similar to those present in the eventual
zeolite product will gradually be assembled. The notion
of cation-assisted ordering goes back to the ideas of
Breck [54] (Section 3.2) and was more recently generalised in terms of ordering through minimisation of the
potential energy of molecular assemblies by Brunner
[183]. The structuring of silicate and aluminosilicate
units through electrostatic and (for organo-cations)
van der Waals forces as proposed by Flanigen [82,83],
Chang and Bell [63] and Burkett and Davis [64–66]
has also been discussed earlier (Sections 3.6 and 3.7).
The following argument builds on these ideas to suggest


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

a specific mechanism for the construction of the zeolite
lattice.
In Fig. 11, a cation is envisaged as migrating to a temporary site in a developing structure at which the coordination geometry is suitable but not ideal. Whilst in
place, the cation mediates the acquisition of new T-units
from solution (through the condensation reactions given
above), guiding them into a more favourable coordination geometry and in so doing generating a periodically
regular local structure. In due course, the cation may
have sufficient of its original hydration shell replaced
by lattice oxygen donors to become a fixture at the
newly created site. Otherwise (and depending upon the
charge balance), the cation may migrate to function sim-

Fig. 11. The basic mechanism for the cation-mediated assembly of
ordered regions: (a) nomenclature and symbolism; (b) details of in-situ
construction process by addition of solution units to a surface site. The
same mechanism can be applied to zeolite crystal growth. See text for
full description.

23

ilarly elsewhere. Statistically, some areas of the overall
structure will be more ordered than others and in due
course, particular ‘‘islands of order’’ will be established
randomly throughout the now semi-ordered network
(as (b) in Fig. 5). These constitute the ‘‘proto-nuclei’’
discussed later on (Section 11). The whole system is dynamic and at any one moment such islands are being
created, up-graded or (by dissolution) destroyed. However, the overall trend will be towards an increasing degree of order. Eventually, some areas of the structure
become sufficiently ordered that a periodic lattice can
propagate, i.e. nucleation has occurred. This discontinuity takes the form of a topological rearrangement akin
to an isomerisation. Such a transformation can be

viewed as a first order phase transition [184] and corresponds to the achievement of critical radius described in
Section 7.2. From this point, the kinetics of accretion
sufficiently outweigh the kinetics of dissolution to result
in net growth. Thus, the reversible equilibration reactions characteristic of the tr component of the induction
period (s) are superceded by the initial net growth represented by tg. However, the underlying chemistry remains
the same and the reactions still maintain a significant reverse component.
Zeolite nucleation is therefore a discreet event which
could be defined as ‘‘a phase transition whereby a critical volume of a semi-ordered gel network is transformed
into a structure which is sufficiently well ordered to form
a viable growth centre from which the crystal lattice can
propagate.’’ Two comments may be added. The critical
volume refers to the requirement that the germ structure
needs to be of a certain minimum size, corresponding to
the critical radius (rc) of Section 7.2. Following this, it
may be surmised that the potential nucleus does not
have to be perfect in its structure – rather, it needs to
be ‘‘good enough’’ to function as a nucleus under the
prevailing circumstances. This reflects the condition that
rc is not a fixed quantity but varies, depending on the
values of other key variables such as temperature and
supersaturation. Just as the critical size is situationdependent, then a variable ‘‘perfection index’’ can also
be envisaged, the necessary degree of precise periodicity
modulating with the growth environment. Thus, for
example, a rather defective potential nucleus might remain dormant (or dissolve) under quasi-equilibrium
conditions but may grow into a crystal under a high
supersaturation driving force. An analogy for the gradual evolution of such a nucleus would be the cutting of a
new key for a tumbler (‘‘Yale’’-type) lock. Initially, there
is no chance that the key will function. However, as the
machining progresses, the new key will gradually become more perfect and at some definite point will perform the discreet action of opening the lock.
Comparison of the new key with an original master

key may reveal errors and imperfections but within a
certain tolerance it will perform the required function


24

C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

in the existing circumstances. It is ‘‘good enough’’. However, if circumstances change (e.g. the lock becomes hot
and expands), it is possible that the ‘‘imperfect’’ key may
no longer open it.
7.9. Summary

upon the method of detection—most commonly a combination of visual inspection or microscopy with X-ray
diffraction. Thereafter, crystal growth can be monitored
by the same techniques and the resulting S-shaped
growth curve of bulk crystallinity against time is by
far the most commonly reported measurement of zeolite
crystallisation kinetics.

As can be seen from the above, the overall process of
zeolite nucleation is a summation of a complex chain of
events. This is necessarily so since it encompasses the entirety of the transformation from an initially random
structure to the beginnings of a regular, periodic crystal
lattice. However, it is suggested that the basic steps are
quite simple, namely: (i) mixing of the reactants to give
a non-equilibrated, inhomogeneous starting material
(primary amorphous phase), (ii) equilibration to form
a semi-organised precursor (secondary amorphous
phase) containing ‘‘islands of order’’ or ‘‘proto-nuclei’’,

(iii) the establishment of sufficient regular structure
within a statistical distribution of ordered sites to enable
such structure to propagate (the nucleation step itself),
and (iv) the beginnings of crystal growth on the established nuclei. The reversible condensation reactions
which constantly make and break TAOAT bonds in
the dynamic reaction medium of hydrothermal synthesis
provide the chemical mechanism by which all these
changes are accomplished. The accumulation of T-units
(e.g. T(OH)3OÀ) from solution is mediated by the associated cations, which provide structural organisation for
the assembling architecture.

8. Crystal growth
In Sections 5–7, the pathway of zeolite synthesis has
been followed from the assembling of reaction components through to the inception of crystal growth. Crucial
intermediate stages can be identified: the development of
local order and the enabling discontinuity of the nucleation step. Thus, nearly all the main features of zeolite
formation have already been described. However, the
remaining stage—the growth of a nascent microcrystal
into a material entity—is of great significance, not least
because it is this final step which we most commonly
monitor in the laboratory or synthesis plant and which
gives us the tangible zeolite products, sorbents and catalysts with which we are familiar. The present section
discusses the experimental observations on this process
and their mechanistic basis.
8.1. Experimental methods
In a typical zeolite synthesis, the first substantive evidence for a successful reaction is the appearance of crystals of the product. As noted earlier (Section 5), this
signal for the end of the induction period is dependent

Fig. 12. A cautionary tale in particle size analysis. The monodisperse
but anisotropic silicalite crystals (a) have caused particle counters

operating on conductivity (b) (Coulter Multisizer) and diffraction (c)
(Malvern 3300 Laser Particle Sizer) to report a multimodal distribution based on the angularity of the crystals and their hydrodynamic
behaviour in suspension.


C.S. Cundy, P.A. Cox / Microporous and Mesoporous Materials 82 (2005) 1–78

More closely related to the growth mechanism itself is
the crystal linear growth rate and to determine this some
form of temporal size measurement is necessary. Unfortunately, the crystals of synthetic zeolites are usually too
small (<1–20 lm) and the experimental conditions too
severe for the application of many of the traditional
crystal growth measurement techniques [108,153,185].
Nevertheless, data can be obtained, usually from samples taken during the course of small-scale experiments,
through the use of particle counters relying on (for
example) light scattering, conductivity across an orifice,
or diffraction effects. However, unless the product crystal size range is narrow, the result will be some form of
size distribution, which must be very carefully interpreted. Problems are likely to arise the more the sample
deviates from a spherical shape (Fig. 12). In the example
illustrated [186], the instruments have provided results
showing multimodal distributions, based on the angularity of the crystals and their hydrodynamic behaviour
in suspension. (When analysed by the measurement of
SEM micrographs, the sample in fact showed a more
uniform size distribution (±3%) than the glass microsphere standard supplied for calibration purposes.)
More directly, the sizes of individual crystals can be
measured by optical or electron microscopy. These
methods were used in the first determinations of linear
growth rates (for zeolites A and X) by Zhdanov and
co-workers [57,140] and were later applied to ZSM-5
synthesis by Nastro and Sand [187]. In these cases, measurements were carried out on syntheses sampled or terminated at a series of different reaction times. The first

in-situ growth rate determinations were made by Lowe
and associates [94,188], who used optical microscopy
to study the growth behaviour of ZSM-5 and other
high-silica zeolites by direct observation of reaction mixtures contained in glass capillaries. This work was extended by Sano, Iwasaki and co-workers using a
specially constructed cell in which a sample of ZSM-5
reaction mixture maintained at temperatures up to
170 °C could be directly observed using an optical reflection microscope [189,190]. The procedure was later further refined by the addition of an interferometric
technique which allowed the detailed observation of
growth behaviour on all three faces of silicalite crystals
[191].
The above methods permit the growth kinetics of zeolite crystals in one, two or (in favourable circumstances)
three dimensions to be studied as a function of reaction
conditions. In this way, inferences can be indirectly
drawn concerning the manner in which the crystal is
being assembled from the components available in the
synthesis mixture (Section 8.2). Most recently, data from
atomic force microscopy (AFM) and high-resolution
TEM (HRTEM) have provided more detailed evidence
of the surface construction process [192]. AFM is one
of a new family of scanning probe microscopies

25

[193,194]. In this technique, a mechanical probe with a
tip of atomic dimensions is rastered over the surface of
the sample. The derived signals can be processed into
images which have a lateral resolution of %20 nm in
the principal (x, y) plane of the specimen but which are
sensitive to features 61 nm in size in the vertical (z)
dimension. The results provide information on zeolite

crystal surfaces of astonishing detail (see below). In a
further new development, Cr-sputtered SEM images
showing unprecedented resolution of similar surface
topography have recently been reported [195,196].
8.2. Experimental observations—Introduction
The majority of reports on zeolite crystallisation
kinetics are based upon bulk crystallinity measurements,
usually as determined by X-ray diffraction. In these
cases, it is not possible to estimate crystal growth rates
in the absence of further information on nucleation
behaviour or crystal size distributions. However, there
are now in the literature a significant number of investigations of zeolite synthesis in which crystal linear
growth rates have been reported (Table 3). From this
information, it is possible to draw some conclusions
on the nature of zeolite crystal growth in relation to that
of other ionic, or partially ionic, substances.
The linear growth rates (0.5Dl/Dt) of zeolite crystals
vary from about 0.1 lm hÀ1 for zeolites A and X to
around 0.02 lm hÀ1 for ZSM-5 (for near-optimum compositions at %90 °C), whilst the values for some zeolites
such as EU-1 may be an order of magnitude lower [188].
These growth rates are 2–4 orders of magnitude smaller
than typical values for simple ionic salts (such as alums
or alkali halides) or simple molecular compounds (e.g.
sucrose) [208,209]. This large difference reflects the nature of the zeolite synthesis reaction, in which a largely
covalent, polymeric structure is being assembled piece
by piece as TAOAT bonds are formed one after another. This is clearly a more complex and intricate process than the packing of relatively simple units in ionic
or molecular crystals where bonding is electrostatic or
van der Waals and relatively non-directional.
The plots of crystal size against time typically show a
constant linear growth rate for most of the reaction with

a final tailing-off due to nutrient depletion, as illustrated
in Fig. 7b and also later in Section 11.3 (Fig. 22a). From
this and also from the magnitude of the activation energies (%45–90 kJ molÀ1), it is very probable that the rate
controlling process is the surface integration step of
crystal growth itself [33,37,188]. If the reactions were
generally dissolution-rate limited (as found for unreactive silica sources [210,211]), the linear growth plots
would be intrinsically curved as the reagent sources were
called upon to provide ever increasing nutrient fluxes at
diminishing surface areas. The activation energies are
typical of those for making and breaking TAO bonds