Zeotypes synthesis

This short review of zeolite and zeotype synthesis is written for those who are relatively new to the field. It aims to present an overall introduction to some fundamental aspects of the subject and to indicate where further information can be found. An account of experimental practice is followed by a summary of mathematical modelling procedures. Observations from crystallisation studies then introduce basic principles of the synthesis process. 

A typical hydrothermal zeolite or zeotype synthesis can be described as follows ... 

Selected reviews and surveys relating to zeolite and zeotype synthesis... 

Fig. 2. Zeolite or zeotype synthesis crystal linear growth plot (solid line) and corresponding bulk growth curve (broken line). The crystallinity curve has here been calculated (by cubing the linear growth values).

Fig. 4. Zeolite and zeotype synthesis order is established ((a)-(d)) within a fragment of the solid phase through interaction with solution species in a cation-mediated self-assembly process (inset (c)).

However, whilst chemical differences between the different classes of microporous materials clearly do exist, it is not yet clear to what extent these affect the mechanism of synthesis. Possibly the products do derive directly from PNBUs but there seems no reason at present to reject the alternative possibility that much of this chemistry goes on at the liquid-solid interfacial growth points rather than (as is implied) independently of the growing crystal. Tt 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 flexible (but coherent) mechanistic scheme, this spectrum will need to be further extended in order to allow for the differences in composition, structure, polarity and solution chemistry found in zeotype synthesis. 

Overall Milton s concept of hydrothermal crystallization of reactive gels has been followed with various additions and modifications for most of the molecular sieve, zeolite, and zeotype materials synthesis since the late 1940s. 

Gonradsson, T., Dadachov, M.S., and Zou, X.D. (2000) Synthesis and structure of (Me3N)6[Ge32064](H20)4.5, a thermally stable novel zeotype with 3D interconnected 12-ring channels. Micropor. Mesopor. Mat., 41, 183-191. 

Zeotype and Mesoporous Solids Chemistry Design, Synthesis and Catalytic Properties... 

Chapter 1 is a general overview of zeolite, zeotype and mesoporous solids chemistry, including their design, synthesis and general catalytic properties. Chapter 2 deals with the problems and pitfalls that may occur in the applications of zeolites and other microporous and mesoporous solids to fine chemical synthesis. The remaining chapters deal with specific applications of these catalysts to fine chemical synthesis. 

Similarly, zeotype molecular sieves are synthesized by mixing the basic ingredients with the organic template, e.g. aluminophosphates are prepared from alumina and phosphoric acid. Other main group or transition elements can be incorporated into the framework by adding them to the initial sol-gel. Alternatively, different elements can be introduced by post-synthesis modification (see later), e.g. by dealumination followed by insertion of the new elements into the framework position [31]. 

The synthesis of the first members of a new family of aluminophosphate molecular sieves (the ALPOs) was disclosed by Union Carbide scientists in the early 1980s (Wilson et al., 1982a,b). The zeotype frameworks of the ALPO structures can be pictured as alternating [AlOJ and [P02]+ units and so are electrically neutral with both Al and P occupying adjoining T-sites. 

The use of organic molecules in the synthesis of zeotype solids is an especially interesting preparative method for extended inorganic solid materials. Organic molecules cannot survive the harsh conditions of the classical high-temperature route involving reaction of the components in the solid state. Structure-directed synthesis thus belongs to the "soft chemistry" routes for the preparation of solid-state compounds [12, 13]. 

Similar to the porosils, the dense, thermodynamically stable Si02 modification a-quartz is also prepared under hydrothermal conditions. However, in the industrial process for the production of quartz, the temperatures are rather high (around 400°C). In this process, NaOH is added as a mineralizer to the aqueous solution to promote dissolution of the silica precursor. The reaction mixtures for the preparation of porosils and other zeotype materials also generally contain a mineralizer, but the reaction conditions are much milder. Synthesis temperatures are below 200°C, typically between 140 and 180°C. Some zeolites can even be prepared from aqueous solutions under reflux at normal pressure. These mild synthesis conditions provide the kinetic control necessary to form metastable products [5-9]. 

The synthesis of the / NA/AlP04-5 and that of the cadmium sulfide/zeolite Y composites are typical of the preparation methods used to generate zeotype-based host-guest materials. These usually involve multi-step syntheses. Typical reaction sequences are ... 

The use of organometallic molecules as SDAs is promising due to several other interesting properties. Bein [40] has recently reviewed the chemistry and applications of zeotype/organometallic composites that were prepared by conventional post-synthesis modifications. In most cases, the results apply to directly synthesized composite structures as well. For example, organometallic molecules can serve as valuable precursors for catalytic systems that are immobilized inside the zeotype framework and thus profit from the shape-selectivity that makes zeolites themselves important catalytic systems. 

The first organometallic SDA that was successfully used for the crystallization of zeotypes was the cobalticinium cation. After an early patent report, Shepelev and Balkus and our group independently reported the synthesis of cobalticinium nonasils [41, 42]. In our syntheses, we observed in addition the formation of octadecasil, [42] dodecasil IH [42] and ZSM-48 [47]. The conditions under which these different products are formed are summarized in Table 1. 

In addition to the results on porosils given above, the cobalticinium cation has also been applied to the synthesis of microporous solids of other host compositions. Balkus et al. have described their results for the synthesis of open-framework aluminophosphates, [54] and Kallus et al. have determined the structure of a cobalticinium-containing gallium phosphate. This compound, however, is not a true zeotype since it does not possess a three-dimensional framework structure [55]. 

Z. Lin, J. Zhang, and G. Yang, Synthesis and Structure of KBGe206 The First Chiral Zeotype Borogermanate with 7-Ring Channels. Inorg. Chem., 2003, 42, 1797-1799. 

Descriptions of the syntheses of zeolites and zeotypes are very widely disseminated in the journal and patent literature. Searches carried out on, for example, zeolite A or ZSM-5 would reveal hundreds of examples. In general, it is often instructive to look both at some of the original synthesis procedures (often as patent examples) and also at some of the most recently published methods. For all materials, the extent of information increases with the length of time since first disclosure. This means that the amount of data on (for example) recently-discovered zeotype phases may be quite limited. A very useful source for experimental methods and reliable synthetic procedures for zeolites and the more common zeotypes can be found in the handbook issued by the Synthesis Commission of the International Zeolite Association (IZA) [51]. 

STUDIES OF ZEOLITE AND ZEOTYPE CRYSTALLISATION A STEP-BY-STEP DESCRIPTION OF THE SYNTHESIS PROCESS... 

Successful syntheses of a wide variety of zeolite and zeotype materials have been developed over the last 50 years. Much is known about the way in which these structures are formed, particularly from recent detailed studies using advanced techniques of microscopy (HRTEM and AFM). However, a number of issues surrounding the mechanism of synthesis remain incompletely resolved. This is partly due to experimental difficulty but is also a reflection of the fact that not all systems arc the same. Nevertheless, in the foregoing account an attempt has been made to review our existing knowledge in terms of basic similarities between one reaction regime and another. In this way, it is hoped to establish some overall principles which, with appropriate modification, may be found to be generally applicable, or at least to provide a framework for further analysis. 

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