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Clathrate compounds of silica

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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

0953-8984/14/103203+18$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 [512] cages and six some representative clathrasils. In section3, the synthetic [51262] 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 Pm3¯n [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 + N + CO or with Kr + 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 [512] 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 [512] pentagonal do- decahedra cages. The layers are stacked in AA sequences and 2. Structural topology from additional [435663] and [51268] cages. The framework topology of DOH is identical to that of the hexagonal sH gas 2.1. Zeolites and zeolite-like materials hydrate [15]. Dodecasil-3C (MTN)[7, 31, 32], also known as Zeolites and zeolite-like materials do not comprise of an ZSM-39 [3], holdstite [33] or CF-4 [34], is another member easily definable family of crystalline solids [21]. Histori- of the dodecasil series and is constructed from ABC ABC cally, a zeolite was defined as an aluminosilicate whose open stacking of the dodecasil layers. Dodecasil-3C is isotypic

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Figure 1. The cage types and framework structures of clathrasils containing [512] cage. with the cubic sII gas hydrate [3]. The framework consists of face-sharing [512] and additional [51264] cages. At high temperatures, the space group of dodecasil-3C is the highest possible symmetry of the topology Fd3¯m [35], but its actual symmetry depends on the temperature and guest species. The reported symmetries of the room temperature phases are Fd3¯ for spherical guest species (Kr, Xe, CH4, and N(CH3)3)[7], I 41/a for guest molecules THF [32] and t-butylamine [31], ¯ I 42d for the guest molecule pyridine [36], and F222 or F2dd Figure 2. Cages constituting the frameworks of some of the for the guest molecule pyrrolidine [31]. sodalite–cancrinite series structures. (a) [4665] cage, (b) [4668] The deca-dodecasil framework comprises a dodecasil cage, (c) [46611] cage, and (d) [46617] cage. layer interconnected through additional six-membered rings (T6 rings). Deca-dodecasil-3R [10] is the only confirmed 2.3. Sodalite–cancrinite series member of this polytypic series and was assigned the code The sodalite–cancrinite series minerals are a group of alumi- DDR. Another reported polytype, deca-dodecasil-3H (DD3H), nosilicates with polytypic framework structures. The following has been synthesized only in minor amounts [37, 38]. The DDR minerals in this series have cage-like voids with windows framework is derived via stacking the layers in an ABC ABC smaller than the T6-ring: afghanite (AFG), farneseite (FAR), sequence and the DD3H framework is derived via stacking franzinite (FRA), giuseppettite (GIU), liottite (LIO), bystrite the layers in an AAB AAB sequence. In the DDR structure, (LOS), marinellite (MAR), sodalite (SOD), and tounkite-like the 19-hedra [435126183], are interconnected through eight- mineral (TOL). Cages constituting these topologies are shown membered rings, forming a two-dimensional pore system in figure2. Synthetic Si-sodalite (SOD) is the only previously between the dodecasil layers. This polytypic series has no known pure silica variant among these topologies. Si-sodalite current counterpart in clathrate hydrate chemistry [38]. was also the first microporous silicate synthesized from an MEP, DOH, MTN, DDR and DD3H are the only topolo- organic solvent system [41]. The SOD framework is one of [ 12] gies known for clathrasils containing 5 cages. Disordered the simplest tetrahedral frameworks with only [4668] cubo- structures have not been found, although oriented intergrowth octahedra cages. between DOH and DDR [11] and between DOH and MTN [39] has been observed. In the clathrate hydrate family, how- 2.4. Other clathrasils and silica zeolites (zeosils) ever, there is a complex clathrate hydrate of choline hydrox- ide tetra-n-propylammonium fluoride (C16H99.66FN2O32.33), Table1 summarizes the currently known pure silica materials having an extremely long c-dimension (∼9 nm) [40]. While of zeolite-type structures, representing approximately 20% of the large guest molecule is located in large supercages formed the known zeolite topologies. Among these topologies, frame- by four [512] and/or [435663] cages, an ideal stacking sequence work structures of octadecasil (AST)[42], nonasil (NON)[9], can be written as CABBCAABC CABBCAABC. RUB-10 (RUT)[43], and sigma-2 (SGT)[44] are constructed

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Table 1. Pure silica zeolite type structures.

Pure silica material FTC Space groupa FD Reference SSZ-24 AFI P6/mcc 16.9 [45] Octadecasil AST Fm3¯m 15.8 [42] SSZ-55 ATS Cmcm 16.1 [46] Zeolite beta *BEA P4122 15.3 [47] ITQ-14 BEC P42/mmc 15.1 [48] CIT-5 CFI I mma 16.8 [49, 50] Chabazite CHA R3¯m 15.1 [51, 52] Deca-dodecasil-3R DDR R3¯m 17.9 [10] Dodecasil-1H DOH P6/mmm 17.0 [6] UTD-1 DON Cmcm 17.1 [53] UE-1 EUO Cmme 17.1 [54] Dealuminated zeolite-Y FAU Fd3¯m 13.3 [55] Ferrierite FER I mmm 17.6 [52, 56, 57] GUS-1 GON Cmmm 17.7 [58] SSZ-42, ITQ-4 IFR C2/m 17.2 [59–63] ITQ-32 IHW Cmce 18.5 [64] ITQ-7 ISV P42/mmc 15.0 [65] ITQ-3 ITE Cmcm 15.7 [66] ITQ-13 ITH Amm2 17.4 [67] ITQ-12 ITW C2/m 17.7 [68, 69] ITQ-24 IWR Cmmm 15.6 [70] ITQ-29 LTA Pm3¯m 14.2 [71] ZSM-11 MEL I 4¯m2 17.4 [72, 73] Melanophlogite MEP Pm3¯n 17.9 [5, 28, 30] Silicalite, ZSM-5 MFI Pnma 18.4 [74, 75] ZSM-48 *MRE I mma 19.7 [11, 76, 77] MCM-35 MTF C2/m 20.7 [78] ZSM-39, dodecasil-3C, CF4 MTN Fd3¯m 17.2 [3,7, 32, 34] ZSM-23 MTT Pmmn 18.2 [79] ZSM-12 MTW C2/m 18.2 [80, 81] ITQ-1, MCM-22, SSZ-25 MWW P6/mmm 15.9 [82] Nonasil NON Fmmm 17.6 [9, 83] RUB-41 RRO P2/c 17.9 [84] RUB-3 RTE C2/m 17.2 [85] RUB-10 RUT C2/m 18.1 [86] RUB-24 RWR I 41/amd 19.2 [87] SSZ-73 SAS I 4/mmm 14.9 [88] Sigma-2 SGT I 41/amd 17.5 [44] Sodalite SOD I m3¯m 16.7 [89] SSZ-35, ITQ-9, MU-26 STF C2/m 16.9 [62, 90] SSZ-23 STT P21/n 17.0 [91, 92] SSZ-74 -SVR Cc 17.2 [93] Theta-1 TON Cmcm 18.1 [94]

a Space group represents the highest symmetry of the topology. only of cage-like small voids. Pure silica zeolite feasibilities [5186283] and [5156173] in DD3H and [51262] in MEP. T30 and energetics are discussed in more detail in section 4.1. units can be connected into two types of layers, PerBU1 and PerBU2. In PerBU1, the T30-units form a pseudo-hexagonal 2.5. Periodic building unit layer (figure3(b)). In PerBU2, the T30-units are arranged in an orthogonal layer (figure3(c)). The ‘extended’ PerBU1 and Although the crystal structures of clathrasils are well described PerBU2 are obtained when additional T6-rings are connected on the basis of cages, another depiction of the structures to them. based on periodic building units (PerBUs) highlights different Neighboring PerBU1s can be connected via oxygen aspects of their structural similarities and provides interesting bridges along the normal to the layers in three ways: (1) AB insight into their crystal growth process [38]. In the descrip- (or BC or CA) sequence with +(2/3a + 1/3b) shift, (2) AC (or tion based on cages, [512] is the common cage in DDR, CB or BA) sequence with −(2/3a + 1/3b) shift, and (3) AA DD3H, DOH, MEP, and MTN. However, a set of ‘half-cages’ (or BB or CC) sequence with no lateral shift (figures3(d) and consisting of 30 T atoms (figure3(a)) is the fundamental (e)). The DOH and MTN framework structures are obtained unit of the PerBUs common to these five structures. These via AA and ABC stacking of the PerBU1, respectively. The ‘half-cages’, or 12-ring double cups, hereafter called T30 units, extended PerBU1s can also be connected in three ways: (1) AB are essentially the bottoms of the cages, in which the structure- (or BC or CA), (2) AC (or CB or BA), and (3) AA (or BB or CC) directing template molecules are trapped, i.e., the [51268] sequences. The DDR structure is obtained via ABC stacking in DOH, [51264] in MTN, [435126183] in DDR and DD3H, of the extended PerBU1, and the DD3H structure is obtained

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Figure 3. Components of the PerBUs of dodecasil series and their stacking sequence [38]. (a) T30 unit composed of 30 T atoms, (b) hexagonal PerBU1, (c) orthogonal PerBU2, (d) the AB-type stacking of PerBU1, (e) the AA-type stacking of PerBU1, and (f) the ‘extended’ PerBU2. The structures were redrawn with VESTA [95]. through AAB stacking. The only previously known connection Table 2. Typical SDAs for four clathrasil types (after [38]). mode of orthogonal PerBU is that of ‘extended’ PerBU2s with FTC Template housing cage Template molecule no lateral shift (figure3(f)). The resulting framework structure MEP [51262] Methylamine: CH N is that of cubic MEP. 5 MTN [51264] Methylamine: CH N The fact that these clathrasils are constructed from the 5 Trimethylamine: N(CH ) same T30 unit, which is the cage region where the structure- 3 3 Pyrrolidine: (CH ) NH directing agents (SDA) are trapped, indicates that such half- 2 4 Piperidine: (CH ) NH open cups provide a site for interaction with SDA. The T30 2 5 Tetrahydrofuran: C H O unit also generates new speculations concerning precursors 4 8 DOH [51268] Piperidine: (CH ) NH in solution. Although many intermediate sequences, in which 2 5 1-aminoadamantane: C H the stacking sequence has a long periodicity or repeats in a 10 16 DDR [435126183] 1-aminoadamantane: C H disordered manner, are theoretically possible, no such ‘inter- 10 16 mediate’ materials have been reported. Therefore, the size and shape of the organic guest molecule represent the structure- dramatically reduces the surface area and thus enables the directing force, leading to the specific framework structure maintenance of low supersaturation in hydrothermal systems. types and preventing stacking disorder from occurring [38]. Silicalite-1 (MFI) with sizes of approximately 3 mm was syn- This structure-directing influence of the clathrasil templates thesized from a piece of quartz glass. Similarly, large crystals is different from what is known of zeolites with channel-type of ANA-, JBW-, CAN-, and SOD-type aluminosilicates were voids, such as zeolites beta (*BEA) and NU-86, which are produced with ceramic boats and quartz glass [96, 97]. Gao only known to occur as disordered frameworks. et al have also reported a similar bulk material dissolution method for the synthesis of a Si-MFI-type zeolite, using a 3. Synthetic processes and occurrences in nature monocrystalline silicon slice as the silicon source to form millimeter-sized crystals [98]. Giant crystals of dodecasil-3C 3.1. Synthesis (MTN) of up to 1 cm have also been prepared with a combi- Zeolites and clathrasils are generally synthesized hydrother- nation of silica rods and fumed silica [99]. mally with water as the solvent in a sealed autoclave at For crystallization of clathrasils, guest species play an in- temperatures typically ca. 150–250 ◦C. The crystallization of dispensable role. The cage-like framework structures cannot be zeolites and the resultant crystal structures are sensitive to the constructed without guest species. Large template molecules, precursor composition, the use of SDA (also called ‘template’), which fit into large cages, play the structure-directing role the specific route of precursor preparation, and the synthesis controlling the topology of framework to crystallize. Typical temperature and duration. Crystal sizes are controlled by the SDAs for four types of clathrasils are listed in table2. Because rates of nucleation and crystal growth, which are strongly the presence of large template molecules serving as SDA is dependent on the supersaturation of the solution. a precondition for seed formation, the formation of larger More reactive silicon sources, for example, will generate cages around SDA is considered an elementary step during a supersaturated solution very rapidly and thus the formation crystallization [38]. Small gaseous molecules, such as CH4, of many nucleation sites. Rather than using powders, gels, N2,O2,H2S, CO2, and Ar, fit into small cages and can or solutions, the use of bulk materials for the silicon source stabilize framework structures. These small gases are referred

5 J. Phys.: Condens. Matter 26 (2014) 103203 Topical Review to as ‘help gases’. For the formation of clathrate hydrates ionic liquids as both solvents and templates [108–110]. This with relatively large guest species such as 2,2-dimethylpentane procedure, which was termed ionothermal synthesis [108], and aminoadamantane, the presence of small ‘help gases’ would be a potential route for syntheses of pure silica zeolites. is essential [15, 100]. For clathrasils, however, ‘help gases’ Xu et al reported the conversion of dry aluminosilicate gel appear to play only a minor role in the stability of clathrasil to MFI zeolite via contact with vapors of water and volatile frameworks because Si–O–Si bonds of clathrasil frameworks amines [111]. The method, designated ‘vapor-phase transport’ are much more rigid than the O–H··· O hydrogen bonds in (VPT) [112], has many applications in synthesizing various clathrate hydrates. Gunawardane et al [11] synthesized five types of zeolites [111–114]. Thin films of pure silica zeolites clathrasils of MEP-, MTN-, DOH-, DDR-, and NON-types with LTA, CHA and ITW topologies have also been obtained from aqueous silica solutions under hydrothermal conditions, by the VPT method using a fluoride mineralizing agent [115]. in the presence of their characteristic guest molecules and Microwave-assisted synthesis has the advantages of remark- in the absence of atmospheric gases. In the presence of air ably reducing synthesis time, inhibiting the formation of im- purity phases, and controlling the crystal morphology, orien- during the syntheses, the presence of N2, CO2, and Ar in the products was detected by mass spectrometry analyses, whereas tation, and particle-size distributions on zeolite membranes [116]. These advantages are due in part to the thermal effects in the absence of ‘help gases’ during the syntheses, the small of microwave heating as compared with conventional heating; cages of the products were found to be empty. No appreciable i.e., microwave heating can lead to much higher heating rates differences in the crystallization rates of dodecasil-1H (DOH) and volumetric heating without heat diffusion effects. By and dodecasil-3C were observed with air as a ‘help gas’ contrast, even though the energy of a microwave photon is and in the absence of atmospheric gases. In the case of far too small to break chemical bonds, numerous experimental melanoplilogites (MEP) and nonasils (NON), much longer observations on reaction rate enhancement in a microwave periods were required for the initial crystallization in the field could not be solely interpreted by the thermal effects absence of atmospheric gases. Air also facilitated crystal- of the microwaves, and the effects of non-equilibrium energy lization of dodecasil-3C in some conditions; in the absence fluctuations or drift motions of matters are suggested to be the of air, ZSM-48 (*MRE) appeared as the major phase along ‘specific microwave effect’. Excellent reviews regarding the with dodecasil-3C. Therefore, although the interaction energy microwave-assisted synthesis of inorganic materials, including between the ‘help gas’ and the host structure in clathrasils is zeolites, have been previously published [116–118]. small, this factor becomes significant at the stability limit of different framework types. 3.2. Natural minerals Some ‘mineralizers’ are known to promote the crystal- lization of zeolites and often serve a vital role for synthesizing Three types of clathrasils occur in nature: melanophlog- large crystals. One of the best-known mineralizers is the flu- ite (MEP), chibaite (MTN), and the DOH-type mineral. oride ion, which is particularly effective in the crystallization These clathrasil minerals crystallize from low-temperature of clathrasils and high-silica zeolites. The addition of fluoride hydrothermal solutions occurring in the diagenesis process of ions to reaction mixtures results in the formation of silicon sedimentary rocks. Melanophlogite was first discovered from complex fluorides in solution. These species are hydrolyzed sedimentary sulfur deposits in Sicily, Italy [1]. In Chvaletice, more slowly and therefore favor crystal growth rather than Bohemia, melanophlogite occurs in a metamorphosed sedi- nucleation [101]. Fluorine is sometimes enclosed in a small mentary pyrite-rhodochrosite deposit from the Algonkian age cage, and when it is present in the structure, it is often linked [119]. Melanophlogite was also found at an active submarine to a four-membered ring [42, 102]. vent in the Cascadia margin [120], where there is a large area of gas hydrate deposits [17]. Chibaite and the DOH-type mineral Tertiary alkanolamines such as triethanolamine (TEA) are were found in tuffaceous sedimentary rocks deposited near known to promote the crystallization of large aluminosilicate the plate margin by the Paleo–Izu arc and the triple junction zeolites by chelating alumimun [103, 104], leading to a of the Pacific, Philippine Sea and North America plates [39]. reduced release rate of aluminium nutrient [101]. Pyrocatechol What is common in these geological settings is the presence was reported to have a similar effect in the synthesis of of small gasses such as CH4, CO2, and H2S. These molecules Si-sodalite (SOD): chelating silicon to form the intermediate weigh up to ∼10 wt% that of natural clathrasil minerals [4, silicon–pyrocatechol complex, inducing the slow release of 39], serving as templates that are necessary for cage-like crystal nutrients and limiting nucleation [105]. These additives framework structures to crystallize. are therefore referred to as ‘nucleation suppressing agents’. Based on the association with other silica minerals and Syntheses of silica zeolites using non-aqueous solutions, their relation in the diagenesis process, the formation temper- vapor-phase transport (VPT) and microwave heating have atures of natural clathrasil minerals are estimated to be much also achieved great results. Si-sodalite was the first zeo- lower than typical synthesis conditions. Melanophlogite often lite synthesized from an organic solvent (ethylene glycol or occurs with amorphous silica (opal-A), cristobalite (opal-CT) 1-propanol) [41]. Various types of zeolites, including pure and chalcedony [28, 119, 121]. Chibaite and the DOH-type silica MTN, MER, MFI, aluminosilicates MFI and FER, mineral are closely associated with opal-A and are rarely have been synthesized in organic solvents without additional in contact with quartz [39]. Opal-A almost completely con- water [106, 107]. These results suggest that water is not verts to opal-CT when burial temperatures reach approxi- an essential medium for zeolite syntheses. Aluminosilicates mately 35–50 ◦C, and quartz forms extensively when tem- and aluminophosphates zeolites can also be synthesized using peratures reach ∼80 ◦C[122, 123]. Opal-CT has been argued

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Figure 4. Quartz pseudomorphs after chibaite (white crystals) and Figure 5. Formation enthalpies for silica zeolites, Ge and Al melanophlogite (semi-transparent cubic crystals) found at Arakawa, phosphate zeotypes, and mesoporous silica: green symbols, dense Minami-Boso City, Chiba Prefecture, Japan [39]. FOV: 6 cm. and zeolitic silica phases; blue symbols, aluminophosphates; orange symbols, Ge-zeolites; and magenta symbols, cubic (stars) and to form at much lower temperatures, at least as low as 17– hexagonal (diamonds) mesoporous silica. Reprinted from [126] with 21 ◦C[124]. These associated minerals indicate that natural the permission of the American Chemical Society. clathrasil minerals occurred at temperatures of at least as low as −1 ∼80 ◦C[39]. (6.8–14.4 kJ mol above quartz). The excess enthalpies of The growth process of natural clathrasil minerals and their the silica zeolites with respect to α-quartz are comparable ∼ −1 alteration process during diagenesis are of special interest to that of silica glass ( 9 kJ mol above quartz [129]). Because amorphous silicas derived from gels are higher in because they are potential materials for geological CO2 storage energy than fused glass by 0–10 kJ mol−1, silica zeolites and [20, 39]. CO2-containing clathrasil minerals can store green- house gases in geological settings without the requirement amorphous silicas occupy an overlapping range of energies [126]. Figure5 shows a trend of formation enthalpies among of metal ions, which are necessary when trapping CO2 as zeolitic silicas, mesoporous silicas, and aluminophosphate carbonates. Compared with CO2 hydrates, clathrasils are more resistant to high pressure and high temperature in geolog- [126]. On a large energy scale, the formation enthalpy relative ical settings. However, because of the higher solubility of to α-quartz increases with increasing molar volume but seems −1 clathrasils with respect to quartz, initially occurring clathrasil to reach an upper limit at values near 30 kJ mol . On crystals are often dissolved and recrystallized to quartz during a finer scale, however, the energy does not scale with any subsequent diagenesis [39, 125]. At low temperatures (well be- simple correlation to molar volume. This variation in energy low 100 ◦C), these dissolution and recrystallization processes suggests that complex interactions among the different factors occur simultaneously and very slowly at the micrometer or sub- of the framework structure affect the framework stability. The micrometer scale while preserving the original clathrasil crys- presence of strained rings is one factor that can contribute to the tal shape. These quartz ‘crystals’ that possess clathrasil shapes excess energy. The formation enthalpy for MEI, for example, ∼ −1 are referred to as quartz ‘pseudomorphs’ after clathrasils. is 3 kJ mol higher than that of EMT, which has a similar Quartz pseudomorphs after chibaite and melanophlogite are molar volume. The former framework has three-membered shown in figure4. rings, whereas the latter does not. By contrast, FAU has enthalpy and molar volume values similar to those of MEI but does not have three-membered rings. 4. Physical properties Bushuev and Sastre [130] conducted a comprehensive computational study on the energetics of pure silica zeolites 4.1. Thermodynamic properties that included all of the topologies in the IZA database Zeolitic pure silica materials are metastable with respect to at that time. The authors’ calculation closely reproduced α-quartz, which is the thermodynamically stable phase at experimental enthalpies of pure silica zeolites with respect ambient conditions. Experimentally, the entropy differences to quartz. Based on statistical distributions of thermodynamic between the different crystalline siliceous polymorphs and properties, the upper energetic limit for synthesized pure silica −1 α-quartz are known to be small and span a narrow range [126]. zeolites was proposed to be ca. 16 kJ mol SiO2 above α- Petrovic et al [127] measured the formation enthalpy of the quartz, and the densities of excess energy of pure silica zeolites high-silica zeolites ZSM-5 (MFI), ZSM-11 (MEL), ZSM-12 lie in the region where 1E/V < 0.5 kJ cm−3. Contributions (MTW), and SSZ-24 (AFI) and of cubic and hexagonal fauja- of van der Waals and three-body O–Si–O interactions to the site (FAU), determining that the range of energies in these silica excess energies were found to be insignificant (figure6). By zeolites is quite narrow (7–14 kJ mol−1 above quartz). Piccione contrast, the three-body Si–O–Si and electrostatic interactions et al [128] further extended this work to a larger range of ma- serve more significant roles in the stability of zeolites. The van terials (AST, BEA, CFI, CHA, IFR, ISV, ITE, MEL, MFI, der Waals, three-body O–Si–O and Si–O–Si interactions seem MWW, and STT) and obtained a similar range of energies to be independent of molar volume because of the short-range

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Figure 6. van der Waals, electrostatic, and O–Si–O and Si–O–Si three-body interactions in pure silica zeolites with respect to quartz. Reprinted from [130] with the permission of the American Chemical Society. nature of these interactions. On the other hand, the excess 400 K were reported by Boerio-Goates et al [132] in detail. energies from long-range electrostatic interactions tend to The heat capacities of three of the polymorphs (FAU, MFI, increase with molar volumes, and this tendency qualitatively and MTT) are greater than that of quartz over the entire explains the large-scale trend of formation enthalpy versus temperature region of study, while that of *BEA drops below molar volumes shown in figure5. that of quartz for T > 240 K. In addition, the excess heat Piccione et al [131] reported the entropies of pure silica capacity of all four polymorphs relative to quartz is greater than zeolites of *BEA-, FAU-, MFI-, and MTT-types using heat that exhibited by amorphous silica for T < 200 K. Because capacity measurements obtained from adiabatic calorimetry. amorphous forms of a substance have higher heat capacities at The entropies of the transition to quartz at 298.15 K span low temperatures than their crystalline counterparts, this result a very narrow range of 3.2–4.2 J K−1 mol−1 above quartz, is unexpected and remains unsolved. despite the factor of 2 differences in the molar volumes Similar values of enthalpy, entropy, and Gibbs free energy of FAU and quartz. Thus, the entropy contribution to the relative to α-quartz were reported for guest-free melanophlog- −1 −1 −1 overall thermodynamics of SiO2 phase transformations is ite: 1H 9.5 ± 0.5 kJ mol [20], 1S = 6.7 J mol K and much smaller than the enthalpy contribution. Only for dense 1G = 7.5 kJ mol−1 [133], respectively. The heat capacities silica polymorphs do the entropies vary linearly with molar and entropies of guest-containing melanophlogites are larger volume for the phases, whereas for silica zeolites, they reach an than those of guest-free samples, reflecting the presence of −1 −1 approximately constant value of 45.1 ± 0.7 J K mol . The enclathrated molecules, such as CH4,N2, and CO2. Some small entropy differences of the different silica frameworks difficulties remain in determining the heat capacity and entropy rxn rxn reflect the strong tetrahedral bonding in the framework. The of reactions at 298 K (1CP and 1S ) that are accom- heat capacities of these four silica zeolites ranging from 14 to panied by the enclathration of gaseous molecules because

8 J. Phys.: Condens. Matter 26 (2014) 103203 Topical Review

Figure 7. The Si–O bond length (in Å) distribution for pure silica Figure 8. A plot demonstrating average Si–O distances (Å) against zeolite structures collected at temperatures ranging from −100 to average atomic displacements of oxygen atoms in the structure. 25 ◦C. Reprinted from [90] with the permission of Chemistry Only the results of structure refinements from single-crystal data ◦ Materials. collected between −100 and 25 C are compiled here. The bond lengths generally decrease with increasing atomic displacements. the concentrations and distributions of various molecules in Reprinted from [90] with the permission of Chemistry Materials. the natural melanophlogite samples analyzed are not quan- h i titatively known. By using cage occupancies based on pre- where d is the Si–O distance and CN(O) is the coordination vious studies in addition to thermodynamic properties for number of oxygen atoms. However, there are apparently very short bond lengths. The mean Si–O bond lengths of ideal gaseous CH4,N2, and CO2, Geiger et al [133] ob- tained preliminary values of 1Crxn = 80.1 J mol−1 K−1 and clathrasils such as MEP, MTN, and DOH are typically P shorter than the mean values of 1.62 Å reported for silica 1Srxn = −642.2 J mol−1 K−1 for a sample from Mt. Hamil- frameworks [136]. A correlation exists between bond length ton, USA, and 1Crxn = 202.2 J mol−1 K−1 and 1Srxn = P and the atomic displacement parameters of oxygen sites − −1 −1 802.2 J mol K for a sample from Sicily, Italy at 298 K (figure8)[90]. The bond length generally decreases with and 1 bar. The formation enthalpies for the guest-containing increasing atomic displacements. A similar phenomenon is samples remain unknown, and therefore their Gibbs free ener- known to occur in dense framework silicates with increasing gies cannot be determined. temperature, where the apparent Si–O bond lengths decrease and the Si–O–Si angles increase with increasing temperature 4.2. Structural disorder [137, 138]. A molecular dynamics simulation demonstrated that time-averaged hSi–Oi distances continuously increase Inter-atomic forces that operate within SiO4 tetrahedra are with increasing temperature, and the apparent decrease in the much stronger than forces that act between these tetrahedra. distances between the mean hSii and hOi positions is due to Therefore, SiO4 tetrahedra in framework silicates behave more the vibration of tetrahedra with a rigid-unit mode [138]. By an like rigid units, which rotate and translate without distortion identical mechanism, the correlation of bond length and atomic of the tetrahedra. The rigid-unit mode (RUM) model well displacement shown in figure8 indicates the static or dynamic describes structural phase transitions in framework silicates disorder of oxygen atoms because of the rotational tilting [134]. The essential feature in the RUM model is a vibrational of tetrahedra. The phenomenon is schematically explained in mode that can propagate in a framework structure without figure9. distorting the tetrahedra. Inter-tetrahedral forces are weaker Figure 10 presents the Si–O–Si angle distributions for than intra-tetrahedral forces and Si–O–Si angles are rather pure silica zeolites [90]. The range of values from 133.6(4) flexible, but inter-tetrahedral forces play significant roles to 180(1)◦ reflects their flexibility. The histogram shows in framework configurations. For example, the α–β phase a peak at 148◦ with a shoulder at 153◦. These values fit transition of quartz involves a rigid-unit mode that preserves quite well with the values from 39 bond angles in well- the Si–O–Si bond angle. The very solid nature of SiO4 defined silica polymorphs [136], which give a maximum at tetrahedra and the rather large dependence of framework approximately 147◦ and a shoulder at approximately 157◦. Ab energy on flexible Si–O–Si angles involve static or dynamic initio calculations on both molecules and extended systems disorders in the tetrahedral framework. have suggested that Si–O–Si bond angles <135◦ destabilize A histogram of Si–O bond lengths in pure silica zeolite zeolitic structures [139]. Liebau suggested that most of the structures is shown in figure7[90]. The mean value of ∼1.60 Å 180◦ Si–O-Si angles reported in silicates are caused by corresponds to the histogram peak and compares well with the unresolved static disorder [136]. The thermal parameters of predicted value of 1.609 Å by Brown et al [135] using the oxygen atoms with Si–O–Si angles of 180◦ are mostly larger following equation: than average for silicate structures. A single crystal x-ray diffraction study of dodecasil-1H d = 1.579 + 0.015hCN(O)i using a synchrotron x-ray source revealed that all of the O sites

9 J. Phys.: Condens. Matter 26 (2014) 103203 Topical Review

Figure 10. Si–O–Si angle distributions for pure silica zeolites. Reprinted from [90] with the permission of Chemistry Materials.

frameworks are electrostatically neutral and highly hydropho- bic, having only weak van der Waals interactions between their guest species. Upon heating, small guest molecules can Figure 9. Schematic illustration of the actual and refined Si–O be removed and guest-free materials can be obtained. For distances when SiO4 tetrahedra vibrate according to a rigid-unit mode. When an O atom vibrates rotationally around its mean example, CO2,N2, and CH4 molecules in melanophlogite position ‘A’ while keeping the Si–O distance constant, O atom will were largely removed after heating at 923 K for 7 h [29], have doughnut-shaped distribution. On the other hand, in the and guest-free melanophlogite was obtained after heating at conventional structure refinements based on harmonic 1223 K for 6 h [20]. However, the complete removal of approximation of vibrations, the refined position of the O atom will be ‘A’. Then the apparent Si–O distance (Si–A) is always shorter larger guest molecules is difficult, if not impossible, because than the actual Si–O distance (represented by Si–B), and the former of the relatively small windows of the cages and lack of becomes even shorter as the vibration increases. channels. Tetramethylammonium in dodecasil-3C was not completely removed after heating at 1473 K in air for 9 h are split [140]. In the refinement of the averaged structural [141]. A pyridine guest was not removed after rapidly heating model with O atoms placed at their mean positions, a mean dodecasil-3C to 973 K [36]. During the heat treatment of Si–O distance and a Si–O–Si angle of 1.568 Å and 170.5◦, clathrasils, parts of the organic molecules often decompose and respectively, were obtained with a relatively large mean Ueq(O) 2 remain in the structure, changing the crystals’ color to black. value of 0.065 Å . Such a small Si–O distance and a large Pyrrolidine-containing dodecasil-3C was treated at 1173 K for Si–O–Si angle are consequences of symmetry constraints. 48 h to remove the template; however, it is not clear whether When the O sites were shifted from special positions and each this was completely successful, as the crystals were reported was split into 2 to 4 sites, a more reasonable geometry was to have turned black during the procedure [35]. In a study obtained (mean Si–O distance, 1.596 Å; Si–O–Si angle, 155◦; of piperidine-containing dodecasil-3C, 101 days of heating at mean U (O), 0.038 Å2), although the mean Si–O distance eq 1173 K was required to obtain a guest-free material [142]. resulted from the constraint of the Si–O distance to 1.60(3) Å The negative thermal expansions of pure silica zeo- during the refinement process. The authors suggested that the lites MFI, DOH, DDR, and MTN and of aluminophos- true local structure of dodecasil-1H is P1 because the split phate AFI were first reported by Park et al [142]. All of some of the O sites cannot be related to each other by as-synthesized materials showed positive volume expan- P6/mmm symmetry operations, and the structure perturbation sion coefficients in the temperature range of 120–298 K. is non-periodic because no indication for a superstructure was By contrast, calcined guest-free materials showed negative found. A similar static disorder was reported for dodecasil-3C volume expansion at high temperatures. Strong negative [35]. thermal expansions were reported for a number of pure silica zeolites, such as MWW [143], ITE [143], STT [143], CHA 4.3. Behaviors under high pressures and high temperatures [144], IFR [63, 144], ISV [145], and STF [145]. Negative Clathrasils and zeosils are more stable than aluminosilicate thermal expansion is not as unusual as was once thought zeolites at high temperatures. The framework structures of alu- and is generally caused by structural effects, including the minosilicate zeolites generally collapse on heating <∼900 K, cooperative motions of the rigid-unit modes and transverse accompanied by loss of molecular water or the reactions vibrations of two coordinated oxygen atoms [146]. between framework atoms and extra-framework cations. The Upon compression and/or heating, zeolites collapse to framework structures of clathrasils and zeosils, however, are denser amorphous phases at temperatures well below their stable at >∼900 K. Rigid SiO2 frameworks require no extra- melting points. In the dense silica polymorphs quartz and framework atoms for stabilizing framework structures. These coesite, pressure-induced order–disorder transitions occur at

10 J. Phys.: Condens. Matter 26 (2014) 103203 Topical Review 20–35 GPa and 300 K [147–149]. Thermally- and pressure- 6 GPa. In the pressure-decreasing cycle, a sudden decrease induced amorphization of aluminosilicate zeolites are equiva- of the unit cell volume to its original compression curve was lent processes induced by mechanical instability [150]. It was observed. This anomalous volume increase by compression also shown that a zeolite may present two distinct amorphous was attributed to the intrusion of additional helium into the phases: a low-pressure (∼2 GPa) low-density amorphous cage. Various other experimental and computational studies phase and a high-pressure (∼6 GPa) high-density amor- demonstrate that intrusion of guest species into zeolite frame- phous phase [150, 151]. A pressure-induced amorphization works increases their bulk modulus, prevents amorphization of clathrasils were first reported by Tse et al [152] and Tse and from occurring up to higher pressure, and facilitate reversible Klug [153] in dodecasil-3C (MTN) and deca-dodecasil-3R transition from amorphous to crystalline phases after de- (DDR). These authors used NaCl as a pressure-transmitting compression [155–158]. A high-pressure cubic-to-tetragonal medium, compressing both guest-containing and guest-free phase transition of the cubic phase of melanophlogite was samples. Dodecasil-3C transformed to an amorphous mate- reported by Tribaudino et al [159]. The sample contained rial at ∼6 GPa, and the transition was reversible when a only methane as a guest molecule and had cubic symmetry at guest molecule (tetrahydrofuran, THF) existed in the cages, room temperature. A methanol:ethanol:water (16:3:1, MEW) whereas the transition was irreversible when the cages were mixture and silicon-oil (Si-oil) were used as the pressure empty. Transformation of deca-dodecasil-3R to a (probably) media. The phase transition from cubic to tetragonal was disordered phase was observed with infrared spectra changes observed at 1.14 GPa in both MEW and Si-oil runs. The at ∼4 GPa for the guest-free sample and at ∼7 GPa for tetragonal phase was stable up to the highest pressure (6 GPa) the guest (aminoadamantane)—containing sample. Through in the run with MEW, but it switched back to cubic symmetry experiments and theoretical molecular dynamics calculations, at 3.12 GPa in Si-oil. This difference is probably due to the presence of undeformable units, such as guests, was the non-hydrostatic character of Si-oil at P > 0.9 GPa. The determined to be essential for the reversible process. A similar high-pressure transition was found to be very similar to the pressure-induced amorphization was reported by Xu et al [154] thermally induced cubic-to-tetragonal transition observed by for guest-free melanophlogite at ∼8 GPa. The authors directly Nakagawa et al [160]. compressed the sample with a diamond anvil cell without a pressure medium. The pressure-induced amorphous phase 5. Applications is an intermediate, metastable state. When the amorphous phase at 8.03 GPa was heated up to 1473 K, diffraction 5.1. Gas storage peaks characteristic of coesite, which is the thermodynam- ically stable phase at these pressure conditions, started to Two promising applications of clathrasils are gas storage and appear at 873 K. Yagi et al [29] also reported similar but use as membranes for gas separations. Because of the small more complicated amorphization behaviors of melanophlogite windows of their frameworks, only small gases can diffuse fast at high pressure. They compressed natural melanophlogite enough in clathrasil frameworks for practical applications. The samples containing CO2 as a main guest molecule and relatively high densities of aluminosilicates make them less preheated guest-free samples up to 25 GPa using various practical for on-board hydrogen storage, and yet they remain pressure media at room temperature. When methane or an attractive to scientists for their potential and well-characterized alcohol–water mixture were used as pressure-transmitting framework structures, analogous to some other candidates of media, or when direct compression was applied, unheated hydrogen storage, such as clathrate hydrates and metal-organic melanophlogite irreversibly amorphized at approximately frameworks (MOFs) [161–165]. 17 GPa, in which the volume was decreased to approximately From molecular dynamics simulations, a window of 70% of its original volume. No structural transition was clathrasil cages larger than six-membered rings is permeable 12 observed up to that pressure. Preheated melanophlogite, to H2 [166]. In both volume and window size, the [5 ] cage however, was much more compressible, amorphizing only at is very important for the purpose of trapping H2 at ambient approximately 3 GPa when the volume was decreased to 80%. conditions. The H2-containing DDR-type clathrasil, DD3R, This behavior changed completely when helium was used as was synthesized at 453 K in the presence of 50 bar of gaseous 12 the pressure-transmitting medium. The unheated sample was hydrogen, and 0.8 ± 0.1 H2 molecules per [5 ] cage of much less compressible and neither a phase transition nor DD3R were determined by means of prompt gamma activation amorphization was observed up to approximately 25 GPa. analysis (PGAA) and 1H MAS NMR spectroscopy [165]. The Preheated samples exhibited the same compression curve hydrogen-storage capacity (0.2 wt%) should be improved by up to approximately 17 GPa of that observed for unheated increasing the concentration of [512] cages in the framework 12 samples using a helium pressure medium. However, when topology, although the H2 in the [5 ] cages was released only the pressure exceeded approximately 17 GPa, isostructural above 1100 K. For improving the hydrogen-storage capacity transition occurred with a sudden increase in unit cell volume of zeolites, a large volume of micropores and a suitable by 10%. The two isostructural phases with different volumes diameter near the kinetic diameter of a hydrogen molecule are coexisted in a narrow pressure range of less than 1 GPa, and important [164]. Classical molecular mechanics simulations above 18 GPa, the high-pressure phase was compressed in a on hydrogen-storage capacities of 12 pure silica zeolites has normal manner up to 24 GPa. This sudden increase in volume shown that flexible non-pentasils RHO, FAU, KFI, LTA is a reversible process with a large hysteresis of approximately and CHA display the highest maximal capacities, ranging from

11 J. Phys.: Condens. Matter 26 (2014) 103203 Topical Review

2.86 to 2.65 mass%, which correlates well with experimental investigated the self-diffusion of H2 in pure silica MTN results obtained at low temperatures [162]. For FAU-, MFI- and SOD by means of molecular dynamics simulations. The and MOR-type zeolites with similar Al concentration, hydro- H2 diffusion rate in MTN was found to be considerably lower gen adsorption capacity, isosteric heat of adsorption, surface than that in SOD. Although the flexibilities of the MTN coverage, and micropore occupancy were reported to increase and SOD frameworks do not differ greatly and their activation in the order of FAU< MFI< MOR [163]. Irrespective of energies are roughly equal, it was suggested that a large the framework structure, the adsorption capacity, heat of ad- cage volume leads to a lower diffusion rate because of a sorption, surface coverage, and micropore occupancy of these smaller chance of finding hydrogen molecules near the cage’s three zeolites increased with increasing Al content of zeolites. six-membered rings. These results suggest that the ideal zeolite for an efficient The removal and recovery of CO2 from flue and natural hydrogen storage material is one with a high electrostatic field gases are of great interest because CO2 is the main component and narrow pores without intersections. At low H2 pressure, of greenhouse gases. The hydrophobicity of high-silica zeo- hydrogen adsorption capacities were reported to correlate lites offers an advantage of retaining their adsorption capacity with the isosteric heat of adsorption [167], which reflects in water. The all-silica DDR has high selectivity of adsorption the interaction energy between hydrogen and host materials. for CO2 over CH4 [180, 181]. Molecular simulations on pure The heat of adsorption of the above-mentioned FAU-, MFI- silica zeolites have indicated that CHA and DDR yield the −1 and MOR-type zeolites is roughly in the order of 6–7 kJ mol best permeation selectivities among 12 zeolite topologies for when the adsorbed amount is 40 mL g−1, and when adsorbed the separation of CO2 and CH4 [182]. MFI, LTA, and DDR ex- amount is smaller, it ranges up to 11.7 kJ mol−1 [163]. These hibited high separation performances of CO2/N2 and CH4/N2, values are comparable to other candidates of hydrogen storages comparable to those of MOFs [183]. DDR was also reported such as MOFs [167, 168] and carbon materials [169]. to be a very effective molecular sieve for the separation or purification of propane–propene mixtures [184]. 5.2. Membranes for gas separation

The permeation and separation of gas mixtures through ze- 6. Summary olitic membranes are controlled via balancing the effects of adsorption and diffusion in micropores. For example, Dong Among the 213 known topologies of zeolites, 43 topologies are et al [170] studied the gas permeation selectivity between recognized as pure silica zeolites (zeosils). The present review hydrogen and light hydrocarbons (C1–C4) with all-silica MFI focused on clathrasils, a variant class of pure silica zeolites ◦ zeolite (silicalite) membranes at 25–500 C and feed pressures that possess cage-like small voids. Although a number of ◦ of 0.1–0.4 MPa. At temperatures <100 C, the membrane hypothetical clathrasil dodecasil topologies exist, disordered showed excellent separation properties, prohibiting the per- structures have yet to be discovered. The sizes and shapes of the meation of hydrogen and increasing the permeance of hydro- structure-directing template molecules control which topology carbons with increasing molecular masses. This behavior is of clathrasils grow. Template molecules that fit into the larger due to the increasing adsorption strength of the component cages in a framework topology serve as SDAs, whereas the in the hydrophobic silicalite with increasing molecular mass, small gaseous molecules that fit into the smaller cages play and the permeation is primarily determined by preferential rather minor roles in the crystallization of clathrasils. Because ◦ adsorption at these temperatures. At 500 C, the membrane the frameworks of clathrasils are electrostatically neutral and becomes permselective for hydrogen and smaller hydrocar- hydrophobic, only weak interactions between guest species bons because diffusion in the pores controls the permeance and host frameworks occur. However, such weak van der and the preferential adsorption effect is negligible at such a Waals interactions, as well as the rigid-unit mode of SiO4 high temperature. tetrahedra, serve vital roles on symmetry lowering, the long- Hydrogen production from fossil fuels involves the sep- range ordering and/or disordering of the framework structures. aration of H2 from small molecules, such as CH4, CO2, CO, The apparently short Si–O distances and large Si–O–Si angles H2O, and H2S at high temperatures, at which permeation is often reported for clathrasil structures are due to the static controlled by diffusion, and zeolitic membranes can selectively disorder of rigid SiO tetrahedra. Further studies investigating permeate and separate H from these small molecules [171]. 4 2 the details of guest–host interactions, local configurations of Clathrasils and pure silica zeolites have the advantage of high the frameworks, and long-range ordering and/or disordering thermal stability, whereas zeolites with low Si/Al ratios are are desired to elucidate the fundamental mechanisms behind generally unsuitable for operating at high temperatures and the diverse responses of clathrasil frameworks to guest species, moist atmospheres. The MFI- and DDR-types of high-silica temperature, pressure, and pressure media. zeolites have been extensively studied for H2 separation [170– 177]. Clathrasils with six-membered rings as their largest openings, such as SOD, MTN, and DOH, are also considered Acknowledgment as promising candidates for hydrogen storage and purification. For the diffusion of H2 to occur through six-membered rings, The author thanks Dr Kenji Sakurai of the National Institute framework flexibility is very important because it allows the for Materials Science, Japan, for providing the inspiration and ring to adapt to the hydrogen passage [178]. Berg et al [179] motivation for this paper.

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