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

Microporous and Mesoporous Materials 82 (2005) 178 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 Michaels Building, White Swan Road, Portsmouth PO1 2DT, United Kingdom Received November 2004; accepted 11 February 2005 Available online 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, templateframework 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 dierences 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However, the discreet step of nucleation provides a discontinuity in which isolated regions of local order are superceded by the establishment of a periodic crystal lattice, capable of propagation Crystal growth occurs through an in-situ, localised construction process from small,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 solutionmediation 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 ể 2005 Elsevier Inc All rights reserved Keywords: Hydrothermal synthesis; Nucleation; Crystal growth; Modelling; Mechanism Contents * Part I: Background Introduction 1.1 History of hydrothermal zeolite synthesis Corresponding author Tel.: +44 161 200 4512; fax: +44 161 200 4559 E-mail address: colin.cundy@manchester.ac.uk (C.S Cundy) 1387-1811/$ - see front matter ể 2005 Elsevier Inc All rights reserved doi:10.1016/j.micromeso.2005.02.016 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 10 1.2 Scope and structure of this review Experimental observations Summary of proposed mechanisms 3.1 Richard Barrer 3.2 The early work of Breck and Flanigen 3.3 Kerrếs recirculation experiment and the work of Ciric 3.4 Studies at Leningrad 3.5 Overview1959 to 1971 and beyond 3.6 Introduction of organic templates 3.7 Chang and Bell and after 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 systemsThe nature of the reaction sol 20 7.7 Nucleation in zeolite systemsHomogeneous or heterogeneous? 20 7.8 Nucleation in zeolite systemsMechanism 22 7.9 Summary 24 Crystal growth 24 8.1 Experimental methods 24 8.2 Experimental observationsIntroduction 25 8.3 Experimental observationsStudies of macrocrystalline systems 26 8.4 Experimental observationsStudies 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 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 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 zeoliteszeolite degradation 12.7 Conclusions Templateframework 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 41 41 42 42 43 43 44 44 46 47 47 47 48 48 48 49 49 50 50 50 51 51 52 52 52 53 53 54 54 54 55 56 57 57 57 58 59 60 61 61 61 63 63 63 63 63 64 64 64 65 66 67 67 69 69 70 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 Part I: Background 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 (%170270 C) Among the products, species P and Q [24] (isostructural variants) displayed unique characteristics and represented the rst 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 signicant 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 rst 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) [1820] and titanosilicates (such as ETS-10) [2123] Such materials display great compositional diversity and frequently have frameworks unknown for zeolites This increased structural exibility 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 typied 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 [2830] 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 oer, 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 eld 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 dierentiation between, for example, solution-mediated growth and gel rearrangement nally disappears The text is conned 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, modied in some cases through the dierences 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 13) Section 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) 178 within the overall chronology of the hydrothermal synthesis reaction, i.e the induction period and the nature of the amorphous material (Sections 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 918), some key problems and questions associated with zeolite synthesis are addressed in more detail, leading, nally (Section 19), to some overall conclusions For further general information on the subject of zeolite synthesis, the reader is referred to the standard textbooks [17,3235] and recent reviews [31,3643] Experimental observations A typical hydrothermal zeolite synthesis can be described in briefest terms as follows: Amorphous reactants containing silica and alumina are mixed together with a cation source, usually in a basic (high pH) medium The aqueous reaction mixture is heated, often (for reaction temperatures above 100 C) in a sealed autoclave For some time after raising to synthesis temperature, the reactants remain amorphous After the above induction period, crystalline zeolite product can be detected Gradually, essentially all amorphous material is replaced by an approximately equal mass of zeolite crystals (which are recovered by ltration, washing and drying) This is illustrated schematically in Fig The elements (Si, Al) which will make up the microporous 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 [31,4346] Kinetic control is a pervading inuence 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 specication [47] Such considerations will often inuence 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 oer optimum routes to particular materials, since exibility in the choice of reagents enables equilibria to be approached from dierent directions This may oer kinetic benets, such as the preferred nucleation of one phase over another in situations where mixtures may otherwise co-crystallise A good general appreciation of the eld of practical zeolite synthesis can be obtained from the handbook issued by the Synthesis Commission of the International Zeolite Association (IZA) [48] Fig Hydrothermal zeolite synthesis The starting materials (SiAO and AlAO bonds) are converted by an aqueous mineralising medium (OH and/or F) into the crystalline product (SiAOAAl bonds) whose microporosity is dened by the crystal structure C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 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 The details of the synthesis mechanism form the subject of Part II (Sections 58) 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 Extensive heterogeneous nucleation occurs during formation of the highly supersaturated gels The nuclei 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] During the induction period, the nuclei develop to a critical size and then grow rapidly to small and uniform sized crystals 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 rst 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 dicult 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 dierent aluminosilicates. Examples of some possible ions were then tabulated: rings of 36 tetrahedra, the double-4-ring, the double-6ring (or 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 TOn since in the elaborate porous crystalline structures of the zeolites, for instance, it would seem dicult 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 Kerrs 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 lters, the rst 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 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 Summary of principal proposals for zeolite synthesis mechanism, 19592004 Principal system studied Main features of mechanism Barrer [49,50] Various low-silica phases Condensation polymerisation of polygonal and polyhedral anions Flanigen and Breck [5154] 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 speciesSị slow Sị ỵ nucleior zeolite crystalsị ! 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 [5862] 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 OH-mediated SiAOASi cleavage/recombination Burkett and Davis [6466] TPA-Si-ZSM-5 Pre-organised inorganicorganic composites, nucleation through aggregation, crystal growth layer-by-layer Leuven Group [6773] TPA-Si-ZSM-5 Oligomers ! precursor trimer (33 Si) ! ã12 ! nanoslabs, growth by aggregation Liquid phase Formation of nuclei C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 Author(s) [Ref.] C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 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 rst time the eect 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 prole 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 eects, 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 OH ions and new, oligomeric polyhedral units were formed through the ordering inuence of the cations Crystal growth proceeded by an OH-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 unspecied During the later 1970s, the signicance 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 diusion 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 Overview1959 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 eect 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 ltrates 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 modied by the presence of the gel phase (so that transport of growth species to crystals embedded in gel is restricted by diusion through the gel) In addition, Ciric pointed out that his kinetic results were consistent with Barrerếs ideas of anionic blocks [49,50] as well as with the FlanigenBreck view [51] on the catalysis by OH ions 3.4 Studies at Leningrad C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 (TMA) silica-rich version of zeolite A named ZK-4 (Si/ Al up to 1.7) At rst, no new structures resulted from this pioneering work, but in 1967 Wadlinger, Kerr and Rosinski reported [14] the discovery of the rst 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 cm1 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 [5862] 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 (typied 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 rst 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 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 9095 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 conrmed by the demonstration of ion sieve eects indicating that, in the tetrapropylammonium (TPA) system, embryonic structures with Si/TPA = 2024 are formed rapidly upon heating These rst-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 eect 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 nal 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 [6466] 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 inorganicorganic composite structures in which the TPA molecules take up a conformation similar to that adopted in the zeolite product The initial formation of the inorganicorganic 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 diusion of the same species to the surface of the growing crystallites to give a layer-by-layer growth mechanism 10 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 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 rst papers in the series [6771] identied constituent nanoslabs having dimensions 1.3 ã 4.0 ã 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 ã ã nm and ultimately to the crystalline colloidal MFI-type material which forms the nal product of the synthesis More recently [72,73], specic silicate oligomers (particularly a pentacyclic dodecamer) were identied as intermediates in nanoslab evolution However, the elaborately detailed interpretations adopted in these studies are the subject of increasing criticism [8688] Some of the principal ideas from the period described in Sections 3.13.7 are summarised in Table 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 rst 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 reect changes in nucleation and growth behaviour brought about by gel ageing (Section 11.2) The only signicant 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 Loweếs original sketch of this process is reproduced in Fig This simple analysis enables the solution chemistry, and in particular the eects 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 dierence between initial and nal pH values is directly related to the dierence in solubility between zeolite product and gel precursor, providing a measure of the strength of the templating eect for a series of organic additives [95] The most eective template gives the most stable (least soluble) product and hence the largest pH rise Perhaps the key relationship is the on on n ỵQ ẳ ; 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 eect of changing reaction variables and introducing other components such as seed crystals Fig Conceptual basis for the Lowe equilibrium model [92] of the zeolite synthesis process (B.M Loweếs original sketch) Control of solubility passes from the initial equilibrium between amorphous solid and solution species to the nal equilibration between the crystalline zeolite product and its mother liquor 64 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 of microwave dielectric heating [483487] The benets to zeolite synthesis have been discussed in a review [305] In conventional (thermal) heating, reactions may be strongly inuenced by the rate of heat transfer through the walls of the containing vessel and the resulting thermal gradients across the reaction volume These limitations are modied when a dierent form of energy input is applied The most apparent advantage of microwave heating is the signicant shortening of reaction time which is found in virtually every case The factors underlying the reduced timescale have been analysed [151,305] in terms of the contributions from (i) thermal lag, (ii) the induction period and (iii) crystal growth In summary, the major accelerations derive from (i) and (ii) by virtue of the rapid heating rate to the working temperature and from non-equilibrium (e.g dierential heating) contributions to equilibration reactions and the nucleation process Under pseudo-steady state conditions, there is no dierence between crystal linear growth rates for comparable thermal- and microwaveheated preparations [150,152] A further potential advantage of microwave heating is that of selectivity, in cases where some component of the system is selectively sensitive to microwave energy input, or at least to heating rate Such a situation might arise in a heterogeneous reaction mixture containing components of dierent dielectric properties, or through dierences in the rival rates of nucleation and growth of competing phases Probable instances of the former are provided by the synergistic eect of microwave heating and nanocrystal seeding in MFI synthesis [151] and by the failure of zeolite formation in a dry system through immobilisation of key reagents through dierential heating [304] Phase selectivity in zeolite Y synthesis has been demonstrated by the suppression of undesired co-products in microwave-mediated preparations [488], even under conditions (150 C) where zeolite P would normally dominate in a thermal synthesis [489] Microwave heating thus oers an alternative to conventional thermal methods in zeolite synthesis It is often more convenient and may oer practical advantages (e.g in the synthesis of surface lms [490]) A significant reduction in overall synthesis time is usually observed and there may be the possibility of selectivity in heat input or phase purity However, there is no evidence for special (i.e athermal) microwave eects 17.3 Summary In balancing the respective merits of dierent zeolite syntheses, it is important that realistic and sucient criteria are chosen for the comparisons Some form of quantitative reaction prole and product size measurement are essential Changes in reaction parameters should normally permit some measure of optimisation, although the scope for change may be limited by phase purity requirements Shortening of synthesis times can often be achieved through seeding procedures and by the use of microwave heating However, any claims for chemical promoters should be viewed with caution 18 Relationship of zeolite synthesis mechanism to that of other porous materials In order to keep this survey within reasonable bounds, the subject matter has been restricted to the eld of aluminosilicate zeolites and their related zeolitic silica polymorphs However, it is necessary to place this work in perspective by making some reference to the synthesis of other porous materials The most signicant distinctions relate to the dierent assembly mechanisms and are reected in dierences in rates of formation It has already been pointed out (Section 8.2) that the crystal linear growth rates of zeolites are several orders of magnitude lower than those of simple inorganic salts In the latter case, the building blocks of the crystal are already present in solution (e.g Na+ and Cl ions for common salt), so that the process of crystal assembly is relatively facile, depending on a sequence of (i) migration to the growth site, (ii) desolvation and (iii) surface integration Depending on the circumstances, any one of these (fairly comparable) steps might be rate limiting Although the same general considerations apply to zeolites, there is a major dierence in step (iii), which involves the making of covalent bonds as the key operation in the in-situ construction of a polymeric inorganic structure This is a relatively slow process and is usually rate controlling, the main exceptions deriving from the use of unreactive starting materials (dissolution limitation) [210] or the controlled addition of nutrients (supply limitation) [148,186] 18.1 Zeolites and clathrate hydrates The extremes of mechanism and rate noted above are exemplied in the contrast between the syntheses of members of the zeolite family [48] and those of the water-soluble, complex-cation silicate hydrates [491] The former require hydrothermal syntheses at elevated temperature for hours or days, whereas the latter can be rapidly crystallised from aqueous solutions under ambient conditions An interesting comparison is that between the A-type zeolite ZK-4 [12,13] and the tetramethylammonium octasilicate hydrate [Me4N]8[Si8O20] ặ 65H2O [492] The synthesis mixtures will in both cases contain a distribution of species, including both the TMA cation and the cubic octasilicate anion [116,493,494] Both materials can be considered as being constructed entirely from D4R silicate or aluminosilicate rings associated with TMA C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 cations (and additional sodium cations in the case of ZK-4) However, in the case of the zeolite, the D4R rings are bound covalently in an innite three-dimensional network with the TMA cations locked into sodalite cages The silicate hydrate is also a host-guest compound but a far more labile one The host structure consists of a three-dimensional heteronetwork made up of cubic octasilicate anions and water molecules linked via hydrogen bonds, the guest TMA cations being sited within large, irregular cage-like voids This polyhydrate melts incongruently at 72 C to form a liquid phase and a second crystalline heteronetwork clathrate, the host structure of which possesses the topology of the zeolite structure type AST This therefore provides an even more direct comparison between two materials having similar topologies but dierent bonding regimes, namely the fragile ionic and H-bonded network of the AST-type clathrate hydrate and the robust, covalent framework of the isostructural AST-zeolite Related double-four-ring units have recently formed the basis of a synthesis strategy discussed by Villaescusa et al [495] Using a variety of quaternary cations (Q+), it is found that compounds of the type Q+[Ge8O12(OH)8F] are formed under mild conditions Within every cube resides a uoride ion, giving each germanate unit its negative charge At higher temperatures and longer reaction times, three-dimensional networks are formed Since these products, some with very open frameworks (Fig 31), contain complete or ring-opened D4R units linked together in various ways, the authors have adopted the premise of pre-nucleation building units (PNBUs) [496,497], suggesting that the materials have been formed by condensation of such units preexisting in the solution The respective merits of this hypothesis and possible alternative explanations have been discussed earlier (Section 9) and are revisited below (Section 18.2) The eect of Ge-substitution in altering relative stabilities and thus opening pathways to new structures has been noted earlier (Section 13.4) Outside the realm of zeotypes and in a rather dierent system, cubic Si8O20 building blocks have been crosslinked into a porous vanadosilicate solid matrix by reacting the metallated (Bu3Sn)8Si8O20 with VOCl3 [498] In this case, there is no doubt about the direct participation of the D4R precursor However, the chemistry of this non-aqueous route is completely dierent from that of the hydrothermal synthesis, being in essence an alkoxide-transfer process brought about by a specic organotin reagent In a similar context, it is worth noting the existence of some structural parallels between zeolites and certain coordination polymers Although far removed in composition and chemical properties, some self-assembled metal complexes share a common topology with microporous aluminosilicates such as zeolite A and sodalite [499,500] 65 Fig 31 The D4R unit and one of the interrupted germanate zeolite frameworks containing structurally related components [495]: (a) the Ge8O12(OH)8F anion, (b) layer B of the STAG-1 structure (in the a, b plane) showing one of each of the two SBUs 18.2 Zeotypes The many dierent ideas on synthesis mechanism expressed for zeolites (Section 3) have also been applied to the synthesis of zeotype materials However, as mentioned above (Section 18.1), there has been far greater emphasis on the view of the crystallisation process as an assembly of pre-fabricated units [496,497] Complex models of zeotype formation by linkage of one- to three-dimensional precursor units have been proposed [316,501,502] Based largely on NMR evidence of solution species and the commonality of similar structural units in the products, a recent review of metallophosphate formation discusses a synthesis mechanism in which signicant elements of the structure exist in solution as identiable building blocks before being clipped together to from the precipitated product [503] However, there seems no reason at present to reject the alternative possibility (Section 9.1) that much of this chemistry goes on at the liquidsolid interfacial growth points rather than (as is implied) independently of the growing crystal A suggestion for the detailed crystallisation mechanism of AlPO4-21 [504] could again be reinterpreted in this manner It is undeniable, however, that many zeotype systems behave very dierently from their zeolite counterparts 66 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 One of the most striking demonstrations of this concerns the framework zinc phosphates, which can be crystallised very rapidly under very mild conditions [505509], suggesting a more nearly ionic assembly route For example, the sodalite analogue Na6(ZnPO4)6 ặ 8H2O can be made from a mixture of ZnO, H3PO4, NaOH and water in 24 h at room temperature [506], whilst Na,TMA-ZnPO4-faujasite can be synthesised in 30 at C [509] There is also evidence that some AlPO-type products may be much more soluble in their mother liquors than is the case for zeolites, in some cases exhibiting a retrograde temperature solubility which causes a marked reduction in yield unless the product is ltered hot [510] Even further along the polarity scale are the inorganicorganic hybrid materials [511] such as the complex metal carboxylates In an intriguing recent report [512], a single reaction mixture produced ve dierent cobalt succinates, the structure obtained depending only upon the reaction temperature As the temperature was increased from 60 to 250 C, the products became less hydrated and more dense, the structures changing from chains to sheets to frameworks across the series It seems unlikely that any completely new concepts will be necessary to explain the formation patterns of zeotypes However, just as the range of behaviour observed for zeolites requires a exible (but coherent) mechanistic scheme, this spectrum will need to be further extended in order to allow for the dierences in composition, structure, polarity and solution chemistry found in zeotype synthesis It should also be noted that the structures of many of the as-made zeolite-related materials synthesised under mild conditions not survive calcination to remove the organic component They thus have no microporosity and should be regarded as inorganicorganic complexes having zeolite-like topologies rather than true zeotypes 18.3 Microporous vs mesoporous structures The classic synthesis of mesoporous materials employs cationic surfactant molecules to generate the characteristic structure of the products For example, RMe3 N+ cations (R = CnH2n+1, n = 816) are used in the synthesis of the MCM-41 and MCM-48 members of the M41S family [24,25] Initially, two possible formation pathways were proposed by the Mobil group [25] In the rst, silicate species interact with a pre-existing liquid crystal phase to form the composite mesophase The alternative model is based on the cooperative interaction of inorganic species with surfactant micelles in solution to generate the ordered composite structure Since most syntheses are carried out at surfactant concentrations well below those necessary for the formation of liquid crystalline phases [513], the second route is the more probable and has been developed into a more gen- eral cooperative assembly model by Stucky and coworkers [514,515] This emphasises the role of specic interactions between surfactant headgroups and inorganic species and identies three key factors: (i) multidentate binding of silicate oligomers to the cationic surfactant, (ii) preferential silicate polymerisation in the interface region and (iii) charge density matching between the surfactant and the silicate It is interesting to compare the basic stages in the synthesis of microporous and mesoporous materials and to note the dierences in the kinetic limitations Firouzi et al have pointed out the importance in mesophase synthesis of separating the eects of self-assembly from the kinetics of silicate polymerisation [514] For MCM-41, the spontaneous hexagonal ordering of micellar rods on encapsulation by silicate ions was demonstrated by 14 N NMR [513] and EPR [516], whilst the latter also indicated the hardening of the inorganic phase in a second step as the silicate ions polymerised at the interface In an in-situ ATR FTIR study of MCM-41 formation by Holmes et al [517], slow dissolution of the silica source highlighted the relative rates of subsequent reactions Under these conditions, the cooperative assembly of the embryonic MCM-41 structure occurred rapidly, with further crosslinking through condensation reactions being clearly identied as the slow step in the main synthesis sequence These observations allow some parallels to be drawn between the syntheses of two types of ordered, porous silica polymorphs such as (for example) the zeolitic silicalite and the mesoporous MCM-41 (Fig 32) The initial silica dissolution step (1) is not normally rate-limiting in zeolite synthesis, although it may become so for very unreactive sources of silica [210] However, in MCM-41 synthesis, even the dissolution of a reactive silica source such as fumed silica may control the initial reaction rate since the formation of the initial mesophase complex (IM) is so rapid (step 2M) [517] This complex (IM) possesses the overall hexagonal symmetry of the nal hydrothermal product (IIM) but is less crosslinked (i.e higher Q3/Q4 ratio) and less well ordered In a further step (3M) under hydrothermal conditions, the condensation reactions necessary to crosslink the structure are slowly brought about as IM is converted into IIM The nal crosslinking is completed as the surfactant template is lost on calcination (step 4M) The zeolite equivalent of the MCM-41 precursor (IM), complex IZ, is formed to a greater or lesser extent, depending on the reaction conditions, and is the secondary amorphous phase discussed earlier (Section 6.2) In contrast to the preparation of the mesophase, the hydrothermal crystallisation step (3Z) is usually the ratelimiting reaction in zeolite synthesis (Section 8.2) Compounds I and II are in both cases related by their relative degrees of order For the zeolite, complex IZ is likely to have roughly the same composition as the C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 SiO2 source step OH Silicate species in solution 2Z Organic-inorganic OOOooooOrganic I complex IZ 3Z hydrothermal Crystalline organicinorganic complex II Z 4Z calcination Q+ S+ 2M Initial mesophase complex IM hydrothermal 3M Crosslinked mesophase structure II M calcination 67 ions and cations which already exist in the synthesis solution It seems that aluminosilicate zeolites fall largely into the rst category whilst at least some of the aluminophosphates display a decidedly more polar character Such a wide spectrum of reactivity suggests dierences also in mechanism Thus, while the concept of pre-nucleation building units probably does not apply to the situation of zeolite synthesis, it may prove to be more relevant to the assembly mode of some zeotypes A more fundamental dierence exists between the construction mode of zeolite-family members and that of mesoporous materials This is largely a case of synthesis being the mirror of structure since the mesoporous materials are non-crystalline Zeolites not develop their most characteristic properties until the crystal has been formed and this is usually a slow process Conversely, mesoporous materials (or at least their porelled precursors) derive their characteristic periodic structure from the assembly of a surfactant-silicate (or other surfactant-oxide) complex Since this does not initially involve the formation of covalent bonds, it can occur very rapidly 4M 19 Summary and conclusions Zeolite (crystalline silica polymorph) III Z Ordered mesoporous silica III M Fig 32 Schematic comparison between the synthesis of a crystalline zeolitic polymorph (e.g silicalite) (Z) and that of an ordered mesoporous silica (e.g MCM-48) (M), using a quaternary ammonium template (Q+) or surfactant (S+) respectively Reaction steps are shown in Arabic numerals and reaction intermediates by Roman numerals derived zeolite (IIZ), but has only local order Material IIZ is a crystalline compound with fully developed longrange periodicity The mesoporous intermediates IM and IIM dier to a much smaller extent, sharing similar composition and status as ordered amorphous solids The extent of ordering increases in the sequence IM < IIM < IIIM as the degree of silicate condensation becomes greater For the zeolite, the sequence is IZ ( IIZ % IIIZ Compounds IIZ and IIIZ are both crystalline and dier essentially only in composition The objective of this review has been to provide an insight into the hidden machinery underlying the overt experimental procedures of zeolite synthesis As with most practical enterprises, it is not necessary to understand the machinery in order to operate the process but such expediency is unlikely to provide a future for the technology! No attempt will be made in this concluding section to provide a complete digest of all that has gone before Rather, a few of the most salient points will be revisited and their importance summarised The overall requirements for the synthesis process to operate can be outlined in a simple scheme (Fig 33) This highlights the fundamental importance of mobility, the generation of which is the basic function of the socalled mineralising component in zeolite synthesis Such a component, most usually (but not exclusively) AX anions 18.4 Summary The syntheses of zeolite-related crystalline compounds can be considered as being on a continuous scale between the extremes of quartz-like and salt-like materials In the rst case there is the slow, in-situ assembly under autoclave conditions of a largely covalent polymeric structure from simpler building blocks In the latter situation, there is a more rapid precipitation under mild conditions of an essentially ionic structure from an- REACTANTS A attack by X crystallisation, release of X ZEOLITE PRODUCT X Fig 33 Simplied scheme illustrating the cyclic role of the mineraliser X in zeolite synthesis (where X may be, for example, OH or F) In many cases, some X anions or their derivatives (e.g AlOHị ) will be retained in the product 68 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 hydroxide ion, converts the reactants into mobile and reactive forms, thus enabling the synthesis to proceed Acting in some ways as a catalyst, much of the mineraliser is released at the end of the cycle Recent work has provided new perspectives on some of the components within this overall scheme The induction period, by nature and denition the least visibly active stage of the synthesis process, paradoxically has been shown to incorporate all the main steps in zeolite formation, since it covers the transition between totally amorphous material and the establishment of crystallinity Also, the rate limiting step for layer-by-layer crystal growth in zeolites has for some time been identied as the chemical reaction in which growth units are integrated into the crystal surface Experimental and modelling studies have now allowed a further dissection of the growth mechanism, revealing the importance of the surface nucleation step: this provides the dominant rate limitation within the overall growth process, the lateral spreading of a nucleated layer being relatively rapid It appears increasingly probable that the mobile units responsible for structural evolution and crystal growth are small, mobile solution species, with aggregation processes signicant only at very small particle sizes The most likely growth mechanism functions through an in-situ, localised construction process in which the building units are ordered and assembled at the growth site by the participating cations By acting as coordination centres for water molecules, silicate anions and other polar units, these provide much of the blueprint for the spatial architecture of the developing structure (Fig 11) The overall process at a growth point on a crystal surface is in some ways analogous to that of weaving or knitting The dierent building materials (separate lines of thread) are taken in, assembled (woven) according to a denite plan (fabric pattern) into the nished article (cloth) which issues from the assembly point (loom) as a continuous network Whilst the myriad examples of synthesis reactions display a broad spectrum of diversity, it can be helpful to consider the similarities rather than the dierences The apparent complexity should not be allowed to obscure the pervading inuence of a small number of key reactions which enable the entire process to function Predominant amongst these are the TAOAT bond-making and bond-breaking reactions: TAOH ỵ OAT TAOAT ỵ OH TAOH ỵ HOAT TAOAT ỵ H2 O These reactions establish the equilibration between solid and solution components which is fundamental to the ordering process of zeolite synthesis (Fig 5) This process is essentially one of continuous evolution, driven by energy dierences and powerfully moderated by kinetic limitations The single discontinuity is found in the discreet step of nucleation, at which point the periodic crystal lattice is energetically established and subsequently able to propagate freely Much of the published discussion on the mechanism of zeolite synthesis has been concerned with location, and in particular as to whether key events occur in the solid phase or in solution It is hoped that the evidence and reasoning presented in this survey have demonstrated that this is a sterile and unnecessary argument Once the overall equilibration mechanism is accepted, it can be seen that the diering individual situations are all variations upon a theme, the exact situation depending upon the experimental circumstances, and in particular upon reaction composition and concentration Thus, the two extremes of solution-mediated and gel rearrangement mechanism mentioned at various points (e.g Sections 3.6 and 10) and depicted by Caullet and Guth [38,518] may be replaced by the more realistic and generally applicable scheme shown in Fig 34 This mechanism should apply universally to all hydrothermal zeolite syntheses, with a minimum of modication (for example, in the case where the starting material is itself crystalline, as in a zeolite-to-zeolite transformation) In earlier sections, we have described the progress in understanding which is being brought about by developments in both theoretical and experimental techniques Notable practical advances include the proliferation of new families of materials and consequent discovery of novel types of frameworks Amongst instrumental methods, the potential of HRTEM and AFM is outstanding for the elucidation of structural and surface detail The continuing development of environmental cells will transform microscopy into a technique for the investigation of synthesis mechanism Computational approaches should more and more facilitate the investigation of experimental situations which would be dicult or impossible to realise in the laboratory Overall, it is hoped to have shown in this survey that, although the extent of knowledge in the realm of hydrothermal zeolite synthesis is far from complete, a sound scientic basis for most of the mechanistic details is already well established To conclude, we speculate on a possible new approach to the understanding of zeolite formation Starting from the biological viewpoint of the very precise transcription mechanism for protein manufacture within a living cell, the question arises as to the nature of the equivalent (but far more diuse) coding for a particular phase within the realm of zeolite synthesis In a recent review, Corma and Davis [43] point out that all the ễintelligenceế must be contained in the starting reagents to allow them to self-assemble properly in high yield. Such intelligence takes the form not only of the chemical compounds but also their physical form The encapsulated energetic and kinetic programme also C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 69 Fig 34 A generalised mechanism for zeolite synthesis A fragment or domain of amorphous material (a) equilibrates with solution species (anions and cations) to develop elements of local order (b) In due course, the equilibration process leads to an area of sucient order for a periodic structure to become establishedi.e nucleation has occurred (c) The same equilibration reactions (TAOAT bond-making and bond-breaking) then allow the nascent crystal to grow and the amorphous areas to dissolve (d) The self-assembly process is mediated by the associated solvated cations, which act as coordination centres (templates) for the construction of the framework (central insert) These transformations most usually take place via a bulk solution phase, but may occur within a solvated layer at the surface of a dry solid (apparent solid-phase transformation) has external input from variables such as temperature and degree of mixing These larger instruments will eventually bear upon the detailed mechanism of the assembly process itself through the medium of ionic and intermolecular forces Amongst this complexity lies the critical information Some individual correlations are readily demonstratedfor example in the work of Zones et al [519] who show in a pyramid plot that there is an inverse relationship between the size and complexity of an organo-cation template and the range of zeolites that it will synthesise However, this is only one aspect of a much larger problem A solution may lie nearer to the emerging discipline of cheminformatics rather than to the traditional approach of the chemist Chemistry in providing partial support They are also indebted to the authors of cited papers for their permission to reproduce original diagrams Thanks are due also to R.W Thompson for preprints of forthcoming work and to the family of the late Dr B.M Lowe for allowing the use of unpublished material Appendix A A chemical model for the crystal growth of zeolite molecular sieves B.M Lowe (University of Edinburgh), C.M Lowe A summary of the paper presented at the 13th Annual Meeting of the British Zeolite Association, Chislehurst, July, 1990 Acknowledgements The authors would like to acknowledge the helpful contributions of J.R Agger and colleagues in improving the manuscript and of the UMIST Department of The late Dr Barrie Lowe was prevented by illness from publishing or extending this preliminary computational study It was carried out using one of the earliest types of personal computer available in the UK, the 70 C.S Cundy, P.A Cox / Microporous and Mesoporous Materials 82 (2005) 178 BBC home computer A summary is given here since the present authors are aware of no subsequent work which similarly attempts to describe zeolite crystal growth in chemically signicant terms Regrettably, no copy of the original source code has survived and this account has been compiled from the material presented in the 1990 lecture The computer program started by initiating the growth of a predetermined two-dimensional layer, e.g a hexagonal network or a layer from a known zeolite structure A growth unit such as monomer, dimer, or cyclic pentamer was then chosen and the layer allowed to grow from the designated unit, selecting growth sites either at random from all possible sites (unsecured growth) or at random from sites which gave the maximum bonding to the existing growth layer (secured growth) Growth layer T-atoms attached to only one or two others were allowed to move or become detached After a sucient number of unit additions (usually 3000), growth was stopped and the results (displayed as a network) analysed Growth layer T-atoms attached to three others were dened as crystalline (number NX) and T-atoms with fewer lattice bonds as amorphous (number NA) The function k ẳ N A =N X ị1=2 was found to be characteristic of the growth layer type chosen (linearly proportional to the framework density) and always greater for unsecured growth Particularly interesting was the defect analysis The number of defects increased with the size and complexity of the chosen growth unit and was always much larger for unsecured growth Growing the structure with a mixture of a complex unit (e.g cyclic pentamer) and monomer substantially reduced the number of defects All defects could be healed by further growth with monomer only The conclusions (given here in full) were: The growth layer of a molecular sieve has a signicant amount of amorphous character even when monomeric growth units are used Monomer is a suitable growth unit for all framework structures Larger growth units are more likely to produce material with defects The more defects, the less likely the crystal is to grow Larger units may attach to the growth layer in 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