Zeolitic silica polymorphs

The first discovery of a zeolite was recorded in 1756 (1). Since that time numerous natural and synthetic zeolites, silica polymorphs, and aluminophosphate-based molecular sieves have been reported. The largest ring in these materials consists of 12 tetrahedral (12 T) atoms. This boundary has been in existence for over 180 years since the first zeolite to contain 12 T-atom rings, gmelinite, was discovered in 1807 (1). Recently, we have... 

Ostwald s step rule holds that a thermodynamically unstable mineral reacts over time to form a sequence of progressively more stable minerals (e.g., Morse and Casey, 1988 Steefel and Van Cappellen, 1990 Nordeng and Sibley, 1994). The step rule is observed to operate, especially at low temperature, in a number of min-eralogic systems, including the carbonates, silica polymorphs, iron and manganese oxides, iron sulfides, phosphates, clay minerals, and zeolites. 

ITQ-4, which has the formula Si32064.(C 4I I20NF)216, is another relatively new 12-MR ID channel-type zeolite that forms a pure silica polymorph (50-52). The structure of ITQ-4, which has been assigned the structure code IFR, is shown in Fig. 8. This view along the 001 direction reveals the distorted 12-MR pores that are... 

Many of the zeolites described in Section II. A form all silica polymorphs, so it is reasonable to expect the isoelectronic germanium to form related structures. However, until recently there were relatively few examples of open framework germinates. By using an organocation as a template, a series of Ge02 molecular... 

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

Note that dense silica polymorphs or silica-rich zeolites (ZSM-5, KZ-2, ZSM-22) are formed at the expense of metastable ZSM-48 for long reactions times in monoamine and diamine bearing systems (see above), indicating that excess of Al in the initial hydrogel does not play a particular role in the formation of these phases. In contrast, a higher Al content in the... 

Whereas most effort has been directed toward the zeolite ZSM-5 and its all-silica polymorph silicalite, other frameworks such as zeolite A, faujas-ite, and ferrierite have also been considered. This theoretical interest mirrors the commercial relevance of these zeolites. 

Xenon has been considered as the diffusing species in simulations of microporous frameworks other than faujasite (10-12, 21). Pickett et al. (10) considered the silicalite framework, the all-silica polymorph of ZSM-5. Once again, the framework was assumed to be rigid and a 6-12 Lennard-Jones potential was used to describe the interactions between Xe and zeolite oxygen atoms and interactions between Xe atoms. The potential parameters were slightly different from those used by Yashonath for migration of Xe in NaY zeolite (13). In total, 32 Xe atoms were distributed randomly over 8 unit cells of silicalite at the beginning of the simulations and calculations were made for a run time of 300 ps at temperatures from 77 to 450 K. At 298 K, the diffusion coefficient was calculated to be 1.86 X 10 9 m2/s. This... 

Microporous materials are typified by natural and synthetic zeolites that are crystalline 3D aluminosilicates with open channels or cages. Synthetic and structural concepts of zeolites have to a large extent shaped the development of microporous materials during the past 50 years. For example, the use of organic structure-directing agents in the synthesis of high-silica zeolites and their all-silica polymorphs contributed to... 

The replacement of framework anions (i.e. with chalcogens (e.g. S ) represents a more recent approach for generating microporous materials. The efforts to make microporous chalcogenides began with germanium or tin sulfides. However, germanium or tin sulfides do not form microporous materials similar to all-silica polymorphs of zeolites. It was later found that the incorporation of low-valent cations such as Mn + into the Ge-S composition helped to generate 3D frameworks. 

Silicalite-1 is the totally siliceous form of the zeolite MFl (ZSM-5), another silica polymorph. It belongs to the P2iln = Clh monoclinic space group (n. 14) with Z = 96. It transforms into an orthorhombic structure, belonging to the Pnma = Dlt space group (n. 62) between 350 and 363 K. Substituted silicalites such as ZSM-5 zeolite (see below) and Ti-silicalite adopt the orthorhombic structure even at room temperature, the transition temperature being strongly shifted to lower temperatures. 

As a consequence of its composition, a zeolite framework may be charged or neutral. From the examples above, any silica polymorph and any aluminophosphate would be neutral. An aluminosilicate would carry a negative charge most zeolites have negative frameworks. In recent years, some zeolites have been synthesized whose frameworks carry a positive charge. 

The channels and cavities of zeolites with neutral frameworks may be empty. Certainly no ions are needed to balance the charge of the framework. The most common examples are zeolites of composition Si02 (the silica polymorphs) and AIPO4 (the alpo s). These zeolites... 

Although it is possible to synthesize zeolites with positive frameworks, for example by replacing Si " " with in a silica polymorph, far less work has been done to explore such systems. None of the zeolites found in nature are of this type. 

Astala, Auerbach, and Monson277 used plane wave LDA calculations to examine structural characteristics of five all-silica zeolites SOD, LTA, CHA, MOR, and MFI. The first four of these materials can be examined in calculations with 36-72 atoms in a periodic unit cell, but the MFI structure is considerably larger and requires 288 atoms in a single unit cell. Astala et al. argue that LDA calculations give more accurate results for the structure and energies of silica polymorphs than GGA calculations. Demuth et al. used LDA and GGA calculations to examine the properties of Al substitutions in morde-nite both in a protonated form and in the presence of Na cations.278... 

The history of molecular sieve synthesis encompasses some 40 years and includes the preparation of aluminosilicate zeolites U), phosphorus-substituted aluminosilicates (2 3), and the microporous silica polymorphs (4). More recently, the compositional and... 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

Except for water, silica is the most extensively studied MX2 compound. One of the challenges in studying silica is its complex set of structures. Silica has several common polymorphs under different conditions of temperature [1] and pressure [4], as seen in Figs. 2 and 3. For instance, cristobalite is the crystalline silica polymorph at atmospheric pressure above 1,470°C. It is built on an fee lattice with 24 ions per unit cell. This structure is, in fact, the simplest form of silica. In addition to five polymorphs (quartz, coesite, stishovite, cristobalite, tridymite) that have thermodynamic stability fields, a large and increasing number of metastable polymorphs have been synthesized. These include vitreous silica, clathrasils, and zeolites [2], Except for stishovite, all these structures are based on frameworks of... 

Powder X-ray profiles of silicalite-1 prepared by various research workers, during their preparation of molecular sieves by isomorphously substituting the T element by titanium, tin, zirconium, vanadium, chromium, molybdenum, differ in their crystallinity (80-95%) and their unit cell volume values (Table 1). It is obvious from the Table that whenever a metal ion is being substituted, a metal free all silica polymorph should also be prepared for deciphering the unit cell expansion. The variation in the unit cell volume obtained by various workers is an indication of the inherent problems in synthesising zeolites and molecular sieves with repeatable metal substitution in the framework. 

---