Clathrate Compounds of Silica

Home Search Collections Journals About Contact us My IOPscience Clathrate compounds of silica This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 103203 (http://iopscience.iop.org/0953-8984/26/10/103203) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 159.149.103.9 This content was downloaded on 19/01/2015 at 15:24 Please note that terms and conditions apply. Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 103203 (18pp) doi:10.1088/0953-8984/26/10/103203 Topical Review Clathrate compounds of silica Koichi Momma National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki, 305-0005, Japan E-mail: [email protected] Received 29 October 2013, revised 11 December 2013 Accepted for publication 19 December 2013 Published 19 February 2014 Abstract A review on silica clathrate compounds, which are variants of pure silica zeolites with relatively small voids, is presented. Zeolites have found many uses in industrial and domestic settings as materials for catalysis, separations, adsorption, ion exchange, drug delivery, and other applications. Zeolites with pure silica frameworks have attracted particular interest because of their high thermal stability, well-characterized framework structures, and simple chemical compositions. Recent advances in new synthetic routes have extended the structural diversity of pure silica zeolite frameworks. Thermochemical analyses and computational simulations have provided a basis for applications of these materials and the syntheses of new types of pure silica zeolites. High-pressure and high-temperature experiments have also revealed diverse responses of these framework structures to pressure, temperature, and various guest species. This paper summarizes the framework topologies, synthetic processes, energetics, physical properties, and some applications of silica clathrate compounds. Keywords: clathrate, clathrasil, zeolite, silica (Some figures may appear in colour only in the online journal) Contents 5. Applications 11 5.1. Gas storage 11 1. Introduction1 5.2. Membranes for gas separation 12 2. Structural topology2 6. Summary 12 2.1. Zeolites and zeolite-like materials2 Acknowledgment 12 12 2.2. Clathrasils containing [5 ] cage2 References 13 2.3. Sodalite–cancrinite series3 2.4. Other clathrasils and silica zeolites (zeosils)3 1. Introduction 2.5. Periodic building unit4 3. Synthetic processes and occurrences in nature5 Silica clathrate compounds (or clathrasils) are zeolite-like materials constructed of pure silica framework structures that 3.1. Synthesis5 possess small cage-like voids. Clathrasils were first discovered 3.2. Natural minerals6 in the 19th century as the rare natural mineral melanophlogite 4. Physical properties7 [1], although its crystal structure was not elucidated until 1965 4.1. Thermodynamic properties7 [2]. High-silica zeolites were first synthesized in the early 4.2. Structural disorder9 1960s at Mobil, Inc., and since the first report of the synthetic high-silica clathrate compound ZSM-39 in 1981 [3], clathrasils 4.3. Behaviors under high pressures and high tem- have been extensively studied in the context of zeolite science peratures 10 [4–11]. The properties of zeolites are closely linked to their 0953-8984/14/103203C18$33.00 1 c 2014 IOP Publishing Ltd Printed in the UK J. Phys.: Condens. Matter 26 (2014) 103203 Topical Review framework structures and chemical compositions. Whereas tetrahedral framework allows for ion exchange and reversible aluminosilicate zeolites are hydrophilic electrostatically neu- dehydration [22]. The commission of the International Min- tral frameworks of pure silica zeolites are highly hydrophobic eralogical Association (IMA) defined a zeolite mineral as and stable, even after releasing guest molecules at high temper- a crystalline substance, including non-aluminosilicates, with atures. The development of zeolites with higher Si=Al ratios is a structure that is characterized by a framework of linked desired not only for their improved thermal stability but also tetrahedra, each consisting of four O atoms surrounding a to optimize their catalytic activity, as synthetic Al-rich zeolites cation [23]. This framework contains open cavities in the form exhibit such high catalytic activity levels that they often of channels and cages, which are usually occupied by H2O transform feed stocks to coke, bypassing valuable intermediate molecules and extra-framework cations that are commonly products [12]. exchangeable. A simple criterion for distinguishing zeolites Clathrasils have also attracted interest as analogue mate- and zeolite-like materials from denser tectosilicates is based rials of other classes of clathrate compounds, e.g., clathrate on the framework density (FD), which is the number of hydrates, which are probably the best-known example of tetrahedrally-coordinated atoms per 1 nm−3 [24]. For all clathrate compounds. Structural analogy exists between hy- unique and confirmed framework topologies with FD < 21, drogen bonds linking oxygen atoms in clathrate hydrates and framework type codes (FTC) consisting of three capital letters oxygens linking tetrahedrally-coordinated atoms (T-atoms) (in boldface type) are assigned by the Structure Commission of in aluminosilicates [2]. Three structural types of clathrate the International Zeolite Association (IZA). As of July 2013, hydrates, structures I [13], II [14], and H [15], are found in 213 unique topologies are listed in the IZA database (see also nature [16, 17], and all of the three isotypic materials are syn- the Atlas of Zeolite Framework Types [21, 25]). thesized in the clathrasil families [2,3,6]. Clathrate hydrates Porous tectosilicates are subdivided into two classes are prospective candidates for hydrogen and geological CO2 depending on the sizes of the void ‘windows’ [12, 26]. storage [18, 19]. Clathrasils are also candidates for permanent Clathrasils and clathralites have cage-like voids with small CO2 storage, and their thermochemistry is of special interest in windows, typically six-membered or smaller rings, into which this context [20]. Although zeolites have a rather high density void-filling guest species cannot pass through. Zeosils and for applications in on-board hydrogen storage, well-known zeolites have channel-like voids with ‘windows’ large enough crystal structure and easy ion exchange make zeolites ideal for guest species to pass through. materials for systematic studies [18]. The diffusion of various molecules in clathrasils is extensively studied for practical 12 applications in gas separation membranes. 2.2. Clathrasils containing [5 ] cage The present review first summarizes the framework MEP is the framework topology of the rare silica mineral topologies of clathrasils based on their cage similarities. A melanophlogite, which is historically recognized as the first description of different framework topologies on the basis clathrasil [2]. Melanophlogite is isotypic with the cubic sI of a common periodic building unit is also introduced for gas hydrate. The unit cell contains two T512U cages and six some representative clathrasils. In section3, the synthetic T51262U cages (figure1). Based on mass spectrometry analyses processes and occurrences of clathrasil minerals in natural [4] and the Raman spectra [27, 28] of natural melanophlogite, environments are discussed. One of the fundamental concepts CH4,N2, CO2, and H2S were reported as guest species in common to a variant synthesis route is the chemical activity the cages. Noble gases can also be encapsulated within the reduction of reaction sources. The roles of template molecules cages [4, 29]. The ideal symmetry of the MEP framework in the crystallization of particular zeolite topologies are also is Pm3Nn [5], but its actual symmetry depends on the guest discussed. In section4, thermodynamic stabilities, structural species and is reduced with decreasing temperatures. A sample properties, and behaviors under high temperature or high from a locality in Italy [27] and synthetic crystals with pressure are discussed. The virtually rigid nature of the SiO 4 CH C N C CO or with Kr C N as guest molecules [4] tetrahedra generates unique characteristics of pure silica 4 2 2 2 were reported as cubic at room temperature. Some other natural zeolites, such as structural disorder and negative thermal samples transformed to tetragonal P4 =nbc with a- and b-axes expansion. Clathrasils undergo diverse structural changes at 2 doubled [30] at temperatures ranging from 40 to 65 ◦C[5]. elevated temperatures and pressures, including reversible or With an exception of MEP, the framework topologies non-reversible amorphization and phase transitions, depending of all other clathrasils with T512U cages are based on the on the pressure media and the presence or absence of guest molecules. Finally, general applications of silica clathrates are ‘dodecasil layer’, which comprises a polytypic series of do- briefly reviewed in section5, followed by a summary of the decasils and deca-dodecasils. Dodecasil-1H (DOH)[6] is the review in section6. simplest member of the dodecasil series and is constructed from hexagonal layers of face-sharing T512U pentagonal do- decahedra cages. The layers are stacked in AA sequences and 2. Structural topology from additional T435663U and T51268U cages. The framework topology of DOH is identical to that of the hexagonal sH gas 2.1. Zeolites and zeolite-like

Load more