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Clathrate compounds of silica

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2014 J. Phys.: Condens. Matter 26 103203

(http://iopscience.iop.org/0953-8984/26/10/103203)

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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 silicaKoichi Momma

National Museum of Nature and Science, 4-1-1 Amakubo, Tsukuba, Ibaraki, 305-0005, Japan

E-mail: k-momma@kahaku.go.jp

Received 29 October 2013, revised 11 December 2013Accepted for publication 19 December 2013Published 19 February 2014

AbstractA review on silica clathrate compounds, which are variants of pure silica zeolites withrelatively small voids, is presented. Zeolites have found many uses in industrial and domesticsettings as materials for catalysis, separations, adsorption, ion exchange, drug delivery, andother applications. Zeolites with pure silica frameworks have attracted particular interestbecause of their high thermal stability, well-characterized framework structures, and simplechemical compositions. Recent advances in new synthetic routes have extended the structuraldiversity of pure silica zeolite frameworks. Thermochemical analyses and computationalsimulations have provided a basis for applications of these materials and the syntheses of newtypes of pure silica zeolites. High-pressure and high-temperature experiments have alsorevealed diverse responses of these framework structures to pressure, temperature, and variousguest 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

1. Introduction 1

2. Structural topology 2

2.1. Zeolites and zeolite-like materials 22.2. Clathrasils containing [512] cage 22.3. Sodalite–cancrinite series 32.4. Other clathrasils and silica zeolites (zeosils) 32.5. Periodic building unit 4

3. Synthetic processes and occurrences in nature 5

3.1. Synthesis 53.2. Natural minerals 6

4. Physical properties 7

4.1. Thermodynamic properties 74.2. Structural disorder 94.3. Behaviors under high pressures and high tem-

peratures 10

5. Applications 115.1. Gas storage 115.2. Membranes for gas separation 12

6. Summary 12Acknowledgment 12References 13

1. Introduction

Silica clathrate compounds (or clathrasils) are zeolite-likematerials constructed of pure silica framework structures thatpossess small cage-like voids. Clathrasils were first discoveredin the 19th century as the rare natural mineral melanophlogite[1], although its crystal structure was not elucidated until 1965[2]. High-silica zeolites were first synthesized in the early1960s at Mobil, Inc., and since the first report of the synthetichigh-silica clathrate compound ZSM-39 in 1981 [3], clathrasilshave been extensively studied in the context of zeolite science[4–11]. The properties of zeolites are closely linked to their

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

framework structures and chemical compositions. Whereasaluminosilicate zeolites are hydrophilic electrostatically neu-tral frameworks of pure silica zeolites are highly hydrophobicand stable, even after releasing guest molecules at high temper-atures. The development of zeolites with higher Si/Al ratios isdesired not only for their improved thermal stability but alsoto optimize their catalytic activity, as synthetic Al-rich zeolitesexhibit such high catalytic activity levels that they oftentransform feed stocks to coke, bypassing valuable intermediateproducts [12].

Clathrasils have also attracted interest as analogue mate-rials of other classes of clathrate compounds, e.g., clathratehydrates, which are probably the best-known example ofclathrate compounds. Structural analogy exists between hy-drogen bonds linking oxygen atoms in clathrate hydrates andoxygens linking tetrahedrally-coordinated atoms (T-atoms)in aluminosilicates [2]. Three structural types of clathratehydrates, structures I [13], II [14], and H [15], are found innature [16, 17], and all of the three isotypic materials are syn-thesized in the clathrasil families [2, 3, 6]. Clathrate hydratesare prospective candidates for hydrogen and geological CO2storage [18, 19]. Clathrasils are also candidates for permanentCO2 storage, and their thermochemistry is of special interest inthis context [20]. Although zeolites have a rather high densityfor applications in on-board hydrogen storage, well-knowncrystal structure and easy ion exchange make zeolites idealmaterials for systematic studies [18]. The diffusion of variousmolecules in clathrasils is extensively studied for practicalapplications in gas separation membranes.

The present review first summarizes the frameworktopologies of clathrasils based on their cage similarities. Adescription of different framework topologies on the basisof a common periodic building unit is also introduced forsome representative clathrasils. In section 3, the syntheticprocesses and occurrences of clathrasil minerals in naturalenvironments are discussed. One of the fundamental conceptscommon to a variant synthesis route is the chemical activityreduction of reaction sources. The roles of template moleculesin the crystallization of particular zeolite topologies are alsodiscussed. In section 4, thermodynamic stabilities, structuralproperties, and behaviors under high temperature or highpressure are discussed. The virtually rigid nature of the SiO4tetrahedra generates unique characteristics of pure silicazeolites, such as structural disorder and negative thermalexpansion. Clathrasils undergo diverse structural changes atelevated temperatures and pressures, including reversible ornon-reversible amorphization and phase transitions, dependingon the pressure media and the presence or absence of guestmolecules. Finally, general applications of silica clathrates arebriefly reviewed in section 5, followed by a summary of thereview in section 6.

2. Structural topology

2.1. Zeolites and zeolite-like materials

Zeolites and zeolite-like materials do not comprise of aneasily definable family of crystalline solids [21]. Histori-cally, a zeolite was defined as an aluminosilicate whose open

tetrahedral framework allows for ion exchange and reversibledehydration [22]. The commission of the International Min-eralogical Association (IMA) defined a zeolite mineral asa crystalline substance, including non-aluminosilicates, witha structure that is characterized by a framework of linkedtetrahedra, each consisting of four O atoms surrounding acation [23]. This framework contains open cavities in the formof channels and cages, which are usually occupied by H2Omolecules and extra-framework cations that are commonlyexchangeable. A simple criterion for distinguishing zeolitesand zeolite-like materials from denser tectosilicates is basedon the framework density (FD), which is the number oftetrahedrally-coordinated atoms per 1 nm−3 [24]. For allunique and confirmed framework topologies with FD< 21,framework type codes (FTC) consisting of three capital letters(in boldface type) are assigned by the Structure Commission ofthe International Zeolite Association (IZA). As of July 2013,213 unique topologies are listed in the IZA database (see alsothe Atlas of Zeolite Framework Types [21, 25]).

Porous tectosilicates are subdivided into two classesdepending on the sizes of the void ‘windows’ [12, 26].Clathrasils and clathralites have cage-like voids with smallwindows, typically six-membered or smaller rings, into whichvoid-filling guest species cannot pass through. Zeosils andzeolites have channel-like voids with ‘windows’ large enoughfor guest species to pass through.

2.2. Clathrasils containing [512] cage

MEP is the framework topology of the rare silica mineralmelanophlogite, which is historically recognized as the firstclathrasil [2]. Melanophlogite is isotypic with the cubic sIgas hydrate. The unit cell contains two [512

] cages and six[51262

] cages (figure 1). Based on mass spectrometry analyses[4] and the Raman spectra [27, 28] of natural melanophlogite,CH4, N2, CO2, and H2S were reported as guest species inthe cages. Noble gases can also be encapsulated within thecages [4, 29]. The ideal symmetry of the MEP frameworkis Pm3n [5], but its actual symmetry depends on the guestspecies and is reduced with decreasing temperatures. A samplefrom a locality in Italy [27] and synthetic crystals withCH4 + N2 + CO2 or with Kr+ N2 as guest molecules [4]were reported as cubic at room temperature. Some other naturalsamples transformed to tetragonal P42/nbc with a- and b-axesdoubled [30] at temperatures ranging from 40 to 65 ◦C [5].

With an exception of MEP, the framework topologiesof all other clathrasils with [512

] cages are based on the‘dodecasil layer’, which comprises a polytypic series of do-decasils and deca-dodecasils. Dodecasil-1H (DOH) [6] is thesimplest member of the dodecasil series and is constructedfrom hexagonal layers of face-sharing [512

] pentagonal do-decahedra cages. The layers are stacked in AA sequences andfrom additional [435663

] and [51268] cages. The framework

topology of DOH is identical to that of the hexagonal sH gashydrate [15]. Dodecasil-3C (MTN) [7, 31, 32], also known asZSM-39 [3], holdstite [33] or CF-4 [34], is another memberof the dodecasil series and is constructed from ABC ABCstacking of the dodecasil layers. Dodecasil-3C is isotypic

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

Figure 1. The cage types and framework structures of clathrasils containing [512] cage.

with the cubic sII gas hydrate [3]. The framework consistsof face-sharing [512

] and additional [51264] cages. At high

temperatures, the space group of dodecasil-3C is the highestpossible symmetry of the topology Fd3m [35], but its actualsymmetry depends on the temperature and guest species. Thereported symmetries of the room temperature phases are Fd3for 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 F2ddfor the guest molecule pyrrolidine [31].

The deca-dodecasil framework comprises a dodecasillayer interconnected through additional six-membered rings(T6 rings). Deca-dodecasil-3R [10] is the only confirmedmember of this polytypic series and was assigned the codeDDR. Another reported polytype, deca-dodecasil-3H (DD3H),has been synthesized only in minor amounts [37, 38]. The DDRframework is derived via stacking the layers in an ABC ABCsequence and the DD3H framework is derived via stackingthe layers in an AAB AAB sequence. In the DDR structure,the 19-hedra [435126183

], are interconnected through eight-membered rings, forming a two-dimensional pore systembetween the dodecasil layers. This polytypic series has nocurrent counterpart in clathrate hydrate chemistry [38].

MEP, DOH, MTN, DDR and DD3H are the only topolo-gies known for clathrasils containing [512

] cages. Disorderedstructures have not been found, although oriented intergrowthbetween DOH and DDR [11] and between DOH and MTN[39] has been observed. In the clathrate hydrate family, how-ever, there is a complex clathrate hydrate of choline hydrox-ide tetra-n-propylammonium fluoride (C16H99.66FN2O32.33),having an extremely long c-dimension (∼9 nm) [40]. Whilethe large guest molecule is located in large supercages formedby four [512

] and/or [435663] cages, an ideal stacking sequence

can be written as CABBCAABC CABBCAABC.

Figure 2. Cages constituting the frameworks of some of thesodalite–cancrinite series structures. (a) [4665

] cage, (b) [4668]

cage, (c) [46611] cage, and (d) [46617

] cage.

2.3. Sodalite–cancrinite series

The sodalite–cancrinite series minerals are a group of alumi-nosilicates with polytypic framework structures. The followingminerals in this series have cage-like voids with windowssmaller than the T6-ring: afghanite (AFG), farneseite (FAR),franzinite (FRA), giuseppettite (GIU), liottite (LIO), bystrite(LOS), marinellite (MAR), sodalite (SOD), and tounkite-likemineral (TOL). Cages constituting these topologies are shownin figure2. Synthetic Si-sodalite (SOD) is the only previouslyknown pure silica variant among these topologies. Si-sodalitewas also the first microporous silicate synthesized from anorganic solvent system [41]. The SOD framework is one ofthe simplest tetrahedral frameworks with only [4668

] cubo-octahedra cages.

2.4. Other clathrasils and silica zeolites (zeosils)

Table 1 summarizes the currently known pure silica materialsof zeolite-type structures, representing approximately 20% ofthe known zeolite topologies. Among these topologies, frame-work structures of octadecasil (AST) [42], nonasil (NON) [9],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 Fm3m 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 R3m 15.1 [51, 52]Deca-dodecasil-3R DDR R3m 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 Fd3m 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 Pm3m 14.2 [71]ZSM-11 MEL I 4m2 17.4 [72, 73]Melanophlogite MEP Pm3n 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 Fd3m 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 m3m 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 feasibilitiesand energetics are discussed in more detail in section 4.1.

2.5. Periodic building unit

Although the crystal structures of clathrasils are well describedon the basis of cages, another depiction of the structuresbased on periodic building units (PerBUs) highlights differentaspects of their structural similarities and provides interestinginsight into their crystal growth process [38]. In the descrip-tion based on cages, [512

] is the common cage in DDR,DD3H, DOH, MEP, and MTN. However, a set of ‘half-cages’consisting of 30 T atoms (figure 3(a)) is the fundamentalunit of the PerBUs common to these five structures. These‘half-cages’, or 12-ring double cups, hereafter called T30 units,are essentially the bottoms of the cages, in which the structure-directing template molecules are trapped, i.e., the [51268

]

in DOH, [51264] in MTN, [435126183

] in DDR and DD3H,

[5186283] and [5156173

] in DD3H and [51262] in MEP. T30

units can be connected into two types of layers, PerBU1 andPerBU2. In PerBU1, the T30-units form a pseudo-hexagonallayer (figure 3(b)). In PerBU2, the T30-units are arranged inan orthogonal layer (figure 3(c)). The ‘extended’ PerBU1 andPerBU2 are obtained when additional T6-rings are connectedto them.

Neighboring PerBU1s can be connected via oxygenbridges along the normal to the layers in three ways: (1) AB(or BC or CA) sequence with+(2/3a+ 1/3b) shift, (2) AC (orCB or BA) sequence with −(2/3a+ 1/3b) shift, and (3) AA(or BB or CC) sequence with no lateral shift (figures 3(d) and(e)). The DOH and MTN framework structures are obtainedvia AA and ABC stacking of the PerBU1, respectively. Theextended PerBU1s can also be connected in three ways: (1) AB(or BC or CA), (2) AC (or CB or BA), and (3) AA (or BB or CC)sequences. The DDR structure is obtained via ABC stackingof the extended PerBU1, and the DD3H structure is obtained

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

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 connectionmode of orthogonal PerBU is that of ‘extended’ PerBU2s withno lateral shift (figure 3(f)). The resulting framework structureis that of cubic MEP.

The fact that these clathrasils are constructed from thesame T30 unit, which is the cage region where the structure-directing agents (SDA) are trapped, indicates that such half-open cups provide a site for interaction with SDA. The T30unit also generates new speculations concerning precursorsin solution. Although many intermediate sequences, in whichthe stacking sequence has a long periodicity or repeats in adisordered manner, are theoretically possible, no such ‘inter-mediate’ materials have been reported. Therefore, the size andshape of the organic guest molecule represent the structure-directing force, leading to the specific framework structuretypes and preventing stacking disorder from occurring [38].This structure-directing influence of the clathrasil templatesis different from what is known of zeolites with channel-typevoids, such as zeolites beta (*BEA) and NU-86, which areonly known to occur as disordered frameworks.

3. Synthetic processes and occurrences in nature

3.1. Synthesis

Zeolites and clathrasils are generally synthesized hydrother-mally with water as the solvent in a sealed autoclave attemperatures typically ca. 150–250 ◦C. The crystallization ofzeolites and the resultant crystal structures are sensitive to theprecursor composition, the use of SDA (also called ‘template’),the specific route of precursor preparation, and the synthesistemperature and duration. Crystal sizes are controlled by therates of nucleation and crystal growth, which are stronglydependent on the supersaturation of the solution.

More reactive silicon sources, for example, will generatea supersaturated solution very rapidly and thus the formationof many nucleation sites. Rather than using powders, gels,or solutions, the use of bulk materials for the silicon source

Table 2. Typical SDAs for four clathrasil types (after [38]).

FTC Template housing cage Template molecule

MEP [51262] Methylamine: CH5N

MTN [51264] Methylamine: CH5N

Trimethylamine: N(CH3)3Pyrrolidine: (CH2)4NHPiperidine: (CH2)5NHTetrahydrofuran: C4H8O

DOH [51268] Piperidine: (CH2)5NH

1-aminoadamantane: C10H16DDR [435126183

] 1-aminoadamantane: C10H16

dramatically reduces the surface area and thus enables themaintenance of low supersaturation in hydrothermal systems.Silicalite-1 (MFI) with sizes of approximately 3 mm was syn-thesized from a piece of quartz glass. Similarly, large crystalsof ANA-, JBW-, CAN-, and SOD-type aluminosilicates wereproduced with ceramic boats and quartz glass [96, 97]. Gaoet al have also reported a similar bulk material dissolutionmethod for the synthesis of a Si-MFI-type zeolite, using amonocrystalline silicon slice as the silicon source to formmillimeter-sized crystals [98]. Giant crystals of dodecasil-3C(MTN) of up to 1 cm have also been prepared with a combi-nation of silica rods and fumed silica [99].

For crystallization of clathrasils, guest species play an in-dispensable role. The cage-like framework structures cannot beconstructed without guest species. Large template molecules,which fit into large cages, play the structure-directing rolecontrolling the topology of framework to crystallize. TypicalSDAs for four types of clathrasils are listed in table 2. Becausethe presence of large template molecules serving as SDA isa precondition for seed formation, the formation of largercages around SDA is considered an elementary step duringcrystallization [38]. Small gaseous molecules, such as CH4,N2, O2, H2S, CO2, and Ar, fit into small cages and canstabilize framework structures. These small gases are referred

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

to as ‘help gases’. For the formation of clathrate hydrateswith relatively large guest species such as 2,2-dimethylpentaneand aminoadamantane, the presence of small ‘help gases’is essential [15, 100]. For clathrasils, however, ‘help gases’appear to play only a minor role in the stability of clathrasilframeworks because Si–O–Si bonds of clathrasil frameworksare much more rigid than the O–H· · ·O hydrogen bonds inclathrate hydrates. Gunawardane et al [11] synthesized fiveclathrasils of MEP-, MTN-, DOH-, DDR-, and NON-typesfrom aqueous silica solutions under hydrothermal conditions,in the presence of their characteristic guest molecules andin the absence of atmospheric gases. In the presence of airduring the syntheses, the presence of N2, CO2, and Ar in theproducts was detected by mass spectrometry analyses, whereasin the absence of ‘help gases’ during the syntheses, the smallcages of the products were found to be empty. No appreciabledifferences in the crystallization rates of dodecasil-1H (DOH)and dodecasil-3C were observed with air as a ‘help gas’and in the absence of atmospheric gases. In the case ofmelanoplilogites (MEP) and nonasils (NON), much longerperiods were required for the initial crystallization in theabsence of atmospheric gases. Air also facilitated crystal-lization of dodecasil-3C in some conditions; in the absenceof air, ZSM-48 (*MRE) appeared as the major phase alongwith dodecasil-3C. Therefore, although the interaction energybetween the ‘help gas’ and the host structure in clathrasils issmall, this factor becomes significant at the stability limit ofdifferent framework types.

Some ‘mineralizers’ are known to promote the crystal-lization of zeolites and often serve a vital role for synthesizinglarge crystals. One of the best-known mineralizers is the flu-oride ion, which is particularly effective in the crystallizationof clathrasils and high-silica zeolites. The addition of fluorideions to reaction mixtures results in the formation of siliconcomplex fluorides in solution. These species are hydrolyzedmore slowly and therefore favor crystal growth rather thannucleation [101]. Fluorine is sometimes enclosed in a smallcage, and when it is present in the structure, it is often linkedto a four-membered ring [42, 102].

Tertiary alkanolamines such as triethanolamine (TEA) areknown to promote the crystallization of large aluminosilicatezeolites by chelating alumimun [103, 104], leading to areduced release rate of aluminium nutrient [101]. Pyrocatecholwas reported to have a similar effect in the synthesis ofSi-sodalite (SOD): chelating silicon to form the intermediatesilicon–pyrocatechol complex, inducing the slow release ofcrystal nutrients and limiting nucleation [105]. These additivesare therefore referred to as ‘nucleation suppressing agents’.

Syntheses of silica zeolites using non-aqueous solutions,vapor-phase transport (VPT) and microwave heating havealso achieved great results. Si-sodalite was the first zeo-lite synthesized from an organic solvent (ethylene glycol or1-propanol) [41]. Various types of zeolites, including puresilica MTN, MER, MFI, aluminosilicates MFI and FER,have been synthesized in organic solvents without additionalwater [106, 107]. These results suggest that water is notan essential medium for zeolite syntheses. Aluminosilicatesand aluminophosphates zeolites can also be synthesized using

ionic liquids as both solvents and templates [108–110]. Thisprocedure, which was termed ionothermal synthesis [108],would be a potential route for syntheses of pure silica zeolites.Xu et al reported the conversion of dry aluminosilicate gelto MFI zeolite via contact with vapors of water and volatileamines [111]. The method, designated ‘vapor-phase transport’(VPT) [112], has many applications in synthesizing varioustypes of zeolites [111–114]. Thin films of pure silica zeoliteswith LTA, CHA and ITW topologies have also been obtainedby the VPT method using a fluoride mineralizing agent [115].Microwave-assisted synthesis has the advantages of remark-ably reducing synthesis time, inhibiting the formation of im-purity phases, and controlling the crystal morphology, orien-tation, and particle-size distributions on zeolite membranes[116]. These advantages are due in part to the thermal effectsof microwave heating as compared with conventional heating;i.e., microwave heating can lead to much higher heating ratesand volumetric heating without heat diffusion effects. Bycontrast, even though the energy of a microwave photon isfar too small to break chemical bonds, numerous experimentalobservations on reaction rate enhancement in a microwavefield could not be solely interpreted by the thermal effectsof the microwaves, and the effects of non-equilibrium energyfluctuations or drift motions of matters are suggested to be the‘specific microwave effect’. Excellent reviews regarding themicrowave-assisted synthesis of inorganic materials, includingzeolites, have been previously published [116–118].

3.2. Natural minerals

Three types of clathrasils occur in nature: melanophlog-ite (MEP), chibaite (MTN), and the DOH-type mineral.These clathrasil minerals crystallize from low-temperaturehydrothermal solutions occurring in the diagenesis process ofsedimentary rocks. Melanophlogite was first discovered fromsedimentary sulfur deposits in Sicily, Italy [1]. In Chvaletice,Bohemia, melanophlogite occurs in a metamorphosed sedi-mentary pyrite-rhodochrosite deposit from the Algonkian age[119]. Melanophlogite was also found at an active submarinevent in the Cascadia margin [120], where there is a large area ofgas hydrate deposits [17]. Chibaite and the DOH-type mineralwere found in tuffaceous sedimentary rocks deposited nearthe plate margin by the Paleo–Izu arc and the triple junctionof the Pacific, Philippine Sea and North America plates [39].What is common in these geological settings is the presenceof small gasses such as CH4, CO2, and H2S. These moleculesweigh up to ∼10 wt% that of natural clathrasil minerals [4,39], serving as templates that are necessary for cage-likeframework structures to crystallize.

Based on the association with other silica minerals andtheir relation in the diagenesis process, the formation temper-atures of natural clathrasil minerals are estimated to be muchlower than typical synthesis conditions. Melanophlogite oftenoccurs with amorphous silica (opal-A), cristobalite (opal-CT)and chalcedony [28, 119, 121]. Chibaite and the DOH-typemineral are closely associated with opal-A and are rarelyin contact with quartz [39]. Opal-A almost completely con-verts to opal-CT when burial temperatures reach approxi-mately 35–50 ◦C, and quartz forms extensively when tem-peratures reach ∼80 ◦C [122, 123]. Opal-CT has been argued

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

Figure 4. Quartz pseudomorphs after chibaite (white crystals) andmelanophlogite (semi-transparent cubic crystals) found at Arakawa,Minami-Boso City, Chiba Prefecture, Japan [39]. FOV: 6 cm.

to form at much lower temperatures, at least as low as 17–21 ◦C [124]. These associated minerals indicate that naturalclathrasil minerals occurred at temperatures of at least as low as∼80 ◦C [39].

The growth process of natural clathrasil minerals and theiralteration process during diagenesis are of special interestbecause they are potential materials for geological CO2 storage[20, 39]. CO2-containing clathrasil minerals can store green-house gases in geological settings without the requirementof metal ions, which are necessary when trapping CO2 ascarbonates. Compared with CO2 hydrates, clathrasils are moreresistant to high pressure and high temperature in geolog-ical settings. However, because of the higher solubility ofclathrasils with respect to quartz, initially occurring clathrasilcrystals are often dissolved and recrystallized to quartz duringsubsequent diagenesis [39, 125]. At low temperatures (well be-low 100 ◦C), these dissolution and recrystallization processesoccur simultaneously and very slowly at the micrometer or sub-micrometer scale while preserving the original clathrasil crys-tal shape. These quartz ‘crystals’ that possess clathrasil shapesare referred to as quartz ‘pseudomorphs’ after clathrasils.Quartz pseudomorphs after chibaite and melanophlogite areshown in figure 4.

4. Physical properties

4.1. Thermodynamic properties

Zeolitic pure silica materials are metastable with respect toα-quartz, which is the thermodynamically stable phase atambient conditions. Experimentally, the entropy differencesbetween the different crystalline siliceous polymorphs andα-quartz are known to be small and span a narrow range [126].Petrovic et al [127] measured the formation enthalpy of thehigh-silica zeolites ZSM-5 (MFI), ZSM-11 (MEL), ZSM-12(MTW), and SSZ-24 (AFI) and of cubic and hexagonal fauja-site (FAU), determining that the range of energies in these silicazeolites is quite narrow (7–14 kJ mol−1 above quartz). Piccioneet al [128] further extended this work to a larger range of ma-terials (AST, BEA, CFI, CHA, IFR, ISV, ITE, MEL, MFI,MWW, and STT) and obtained a similar range of energies

Figure 5. Formation enthalpies for silica zeolites, Ge and Alphosphate zeotypes, and mesoporous silica: green symbols, denseand zeolitic silica phases; blue symbols, aluminophosphates; orangesymbols, Ge-zeolites; and magenta symbols, cubic (stars) andhexagonal (diamonds) mesoporous silica. Reprinted from [126] withthe permission of the American Chemical Society.

(6.8–14.4 kJ mol−1 above quartz). The excess enthalpies ofthe silica zeolites with respect to α-quartz are comparableto that of silica glass (∼9 kJ mol−1 above quartz [129]).Because amorphous silicas derived from gels are higher inenergy than fused glass by 0–10 kJ mol−1, silica zeolites andamorphous silicas occupy an overlapping range of energies[126]. Figure 5 shows a trend of formation enthalpies amongzeolitic silicas, mesoporous silicas, and aluminophosphate[126]. On a large energy scale, the formation enthalpy relativeto α-quartz increases with increasing molar volume but seemsto reach an upper limit at values near 30 kJ mol−1. Ona finer scale, however, the energy does not scale with anysimple correlation to molar volume. This variation in energysuggests that complex interactions among the different factorsof the framework structure affect the framework stability. Thepresence of strained rings is one factor that can contribute to theexcess energy. The formation enthalpy for MEI, for example,is ∼3 kJ mol−1 higher than that of EMT, which has a similarmolar volume. The former framework has three-memberedrings, whereas the latter does not. By contrast, FAU hasenthalpy and molar volume values similar to those of MEIbut does not have three-membered rings.

Bushuev and Sastre [130] conducted a comprehensivecomputational study on the energetics of pure silica zeolitesthat included all of the topologies in the IZA databaseat that time. The authors’ calculation closely reproducedexperimental enthalpies of pure silica zeolites with respectto quartz. Based on statistical distributions of thermodynamicproperties, the upper energetic limit for synthesized pure silicazeolites was proposed to be ca. 16 kJ mol−1 SiO2 above α-quartz, and the densities of excess energy of pure silica zeoliteslie in the region where 1E/V < 0.5 kJ cm−3. Contributionsof van der Waals and three-body O–Si–O interactions to theexcess energies were found to be insignificant (figure 6). Bycontrast, the three-body Si–O–Si and electrostatic interactionsserve more significant roles in the stability of zeolites. The vander Waals, three-body O–Si–O and Si–O–Si interactions seemto 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 excessenergies from long-range electrostatic interactions tend toincrease with molar volumes, and this tendency qualitativelyexplains the large-scale trend of formation enthalpy versusmolar volumes shown in figure 5.

Piccione et al [131] reported the entropies of pure silicazeolites of *BEA-, FAU-, MFI-, and MTT-types using heatcapacity measurements obtained from adiabatic calorimetry.The entropies of the transition to quartz at 298.15 K spana very narrow range of 3.2–4.2 J K−1 mol−1 above quartz,despite the factor of 2 differences in the molar volumesof FAU and quartz. Thus, the entropy contribution to theoverall thermodynamics of SiO2 phase transformations ismuch smaller than the enthalpy contribution. Only for densesilica polymorphs do the entropies vary linearly with molarvolume for the phases, whereas for silica zeolites, they reach anapproximately constant value of 45.1± 0.7 J K−1 mol−1. Thesmall entropy differences of the different silica frameworksreflect the strong tetrahedral bonding in the framework. Theheat capacities of these four silica zeolites ranging from 14 to

400 K were reported by Boerio-Goates et al [132] in detail.The heat capacities of three of the polymorphs (FAU, MFI,and MTT) are greater than that of quartz over the entiretemperature region of study, while that of *BEA drops belowthat of quartz for T > 240 K. In addition, the excess heatcapacity of all four polymorphs relative to quartz is greater thanthat exhibited by amorphous silica for T < 200 K. Becauseamorphous forms of a substance have higher heat capacities atlow temperatures than their crystalline counterparts, this resultis unexpected and remains unsolved.

Similar values of enthalpy, entropy, and Gibbs free energyrelative to α-quartz were reported for guest-free melanophlog-ite:1H 9.5± 0.5 kJ mol−1 [20],1S = 6.7 J mol−1 K−1 and1G = 7.5 kJ mol−1 [133], respectively. The heat capacitiesand entropies of guest-containing melanophlogites are largerthan those of guest-free samples, reflecting the presence ofenclathrated molecules, such as CH4, N2, and CO2. Somedifficulties remain in determining the heat capacity and entropyof reactions at 298 K (1C rxn

P and 1Srxn) that are accom-panied by the enclathration of gaseous molecules because

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Figure 7. The Si–O bond length (in Å) distribution for pure silicazeolite structures collected at temperatures ranging from −100 to25 ◦C. Reprinted from [90] with the permission of ChemistryMaterials.

the concentrations and distributions of various molecules inthe natural melanophlogite samples analyzed are not quan-titatively known. By using cage occupancies based on pre-vious studies in addition to thermodynamic properties forideal gaseous CH4, N2, and CO2, Geiger et al [133] ob-tained preliminary values of 1C rxn

P = 80.1 J mol−1 K−1 and1Srxn

=−642.2 J mol−1 K−1 for a sample from Mt. Hamil-ton, USA, and 1C rxn

P = 202.2 J mol−1 K−1 and 1Srxn=

−802.2 J mol−1 K−1 for a sample from Sicily, Italy at 298 Kand 1 bar. The formation enthalpies for the guest-containingsamples remain unknown, and therefore their Gibbs free ener-gies cannot be determined.

4.2. Structural disorder

Inter-atomic forces that operate within SiO4 tetrahedra aremuch stronger than forces that act between these tetrahedra.Therefore, SiO4 tetrahedra in framework silicates behave morelike rigid units, which rotate and translate without distortionof the tetrahedra. The rigid-unit mode (RUM) model welldescribes structural phase transitions in framework silicates[134]. The essential feature in the RUM model is a vibrationalmode that can propagate in a framework structure withoutdistorting the tetrahedra. Inter-tetrahedral forces are weakerthan intra-tetrahedral forces and Si–O–Si angles are ratherflexible, but inter-tetrahedral forces play significant rolesin framework configurations. For example, the α–β phasetransition of quartz involves a rigid-unit mode that preservesthe Si–O–Si bond angle. The very solid nature of SiO4tetrahedra and the rather large dependence of frameworkenergy on flexible Si–O–Si angles involve static or dynamicdisorders in the tetrahedral framework.

A histogram of Si–O bond lengths in pure silica zeolitestructures is shown in figure 7 [90]. The mean value of∼1.60 Åcorresponds to the histogram peak and compares well with thepredicted value of 1.609 Å by Brown et al [135] using thefollowing equation:

d = 1.579+ 0.015〈CN(O)〉

Figure 8. A plot demonstrating average Si–O distances (Å) againstaverage atomic displacements of oxygen atoms in the structure.Only the results of structure refinements from single-crystal datacollected between −100 and 25 ◦C are compiled here. The bondlengths generally decrease with increasing atomic displacements.Reprinted from [90] with the permission of Chemistry Materials.

where d is the Si–O distance and 〈CN(O)〉 is the coordinationnumber of oxygen atoms. However, there are apparentlyvery short bond lengths. The mean Si–O bond lengths ofclathrasils such as MEP, MTN, and DOH are typicallyshorter than the mean values of 1.62 Å reported for silicaframeworks [136]. A correlation exists between bond lengthand the atomic displacement parameters of oxygen sites(figure 8) [90]. The bond length generally decreases withincreasing atomic displacements. A similar phenomenon isknown to occur in dense framework silicates with increasingtemperature, where the apparent Si–O bond lengths decreaseand the Si–O–Si angles increase with increasing temperature[137, 138]. A molecular dynamics simulation demonstratedthat time-averaged 〈Si–O〉 distances continuously increasewith increasing temperature, and the apparent decrease in thedistances between the mean 〈Si〉 and 〈O〉 positions is due tothe vibration of tetrahedra with a rigid-unit mode [138]. By anidentical mechanism, the correlation of bond length and atomicdisplacement shown in figure 8 indicates the static or dynamicdisorder of oxygen atoms because of the rotational tiltingof tetrahedra. The phenomenon is schematically explained infigure 9.

Figure 10 presents the Si–O–Si angle distributions forpure silica zeolites [90]. The range of values from 133.6(4)to 180(1)◦ reflects their flexibility. The histogram showsa peak at 148◦ with a shoulder at 153◦. These values fitquite well with the values from 39 bond angles in well-defined silica polymorphs [136], which give a maximum atapproximately 147◦ and a shoulder at approximately 157◦. Abinitio calculations on both molecules and extended systemshave suggested that Si–O–Si bond angles <135◦ destabilizezeolitic structures [139]. Liebau suggested that most of the180◦ Si–O-Si angles reported in silicates are caused byunresolved static disorder [136]. The thermal parameters ofoxygen atoms with Si–O–Si angles of 180◦ are mostly largerthan average for silicate structures.

A single crystal x-ray diffraction study of dodecasil-1Husing a synchrotron x-ray source revealed that all of the O sites

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Figure 9. Schematic illustration of the actual and refined Si–Odistances when SiO4 tetrahedra vibrate according to a rigid-unitmode. When an O atom vibrates rotationally around its meanposition ‘A’ while keeping the Si–O distance constant, O atom willhave doughnut-shaped distribution. On the other hand, in theconventional structure refinements based on harmonicapproximation of vibrations, the refined position of the O atom willbe ‘A’. Then the apparent Si–O distance (Si–A) is always shorterthan the actual Si–O distance (represented by Si–B), and the formerbecomes even shorter as the vibration increases.

are split [140]. In the refinement of the averaged structuralmodel with O atoms placed at their mean positions, a meanSi–O distance and a Si–O–Si angle of 1.568 Å and 170.5◦,respectively, were obtained with a relatively large mean Ueq(O)value of 0.065 Å2. Such a small Si–O distance and a largeSi–O–Si angle are consequences of symmetry constraints.When the O sites were shifted from special positions and eachwas split into 2 to 4 sites, a more reasonable geometry wasobtained (mean Si–O distance, 1.596 Å; Si–O–Si angle, 155◦;mean Ueq(O), 0.038 Å2), although the mean Si–O distanceresulted from the constraint of the Si–O distance to 1.60(3) Åduring the refinement process. The authors suggested that thetrue local structure of dodecasil-1H is P1 because the splitof some of the O sites cannot be related to each other byP6/mmm symmetry operations, and the structure perturbationis non-periodic because no indication for a superstructure wasfound. A similar static disorder was reported for dodecasil-3C[35].

4.3. Behaviors under high pressures and high temperatures

Clathrasils and zeosils are more stable than aluminosilicatezeolites at high temperatures. The framework structures of alu-minosilicate zeolites generally collapse on heating <∼900 K,accompanied by loss of molecular water or the reactionsbetween framework atoms and extra-framework cations. Theframework structures of clathrasils and zeosils, however, arestable at >∼900 K. Rigid SiO2 frameworks require no extra-framework atoms for stabilizing framework structures. These

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 betweentheir guest species. Upon heating, small guest molecules canbe removed and guest-free materials can be obtained. Forexample, CO2, N2, and CH4 molecules in melanophlogitewere largely removed after heating at 923 K for 7 h [29],and guest-free melanophlogite was obtained after heating at1223 K for 6 h [20]. However, the complete removal oflarger guest molecules is difficult, if not impossible, becauseof the relatively small windows of the cages and lack ofchannels. Tetramethylammonium in dodecasil-3C was notcompletely removed after heating at 1473 K in air for 9 h[141]. A pyridine guest was not removed after rapidly heatingdodecasil-3C to 973 K [36]. During the heat treatment ofclathrasils, parts of the organic molecules often decompose andremain in the structure, changing the crystals’ color to black.Pyrrolidine-containing dodecasil-3C was treated at 1173 K for48 h to remove the template; however, it is not clear whetherthis was completely successful, as the crystals were reportedto have turned black during the procedure [35]. In a studyof piperidine-containing dodecasil-3C, 101 days of heating at1173 K was required to obtain a guest-free material [142].

The negative thermal expansions of pure silica zeo-lites MFI, DOH, DDR, and MTN and of aluminophos-phate AFI were first reported by Park et al [142]. Allas-synthesized materials showed positive volume expan-sion coefficients in the temperature range of 120–298 K.By contrast, calcined guest-free materials showed negativevolume expansion at high temperatures. Strong negativethermal expansions were reported for a number of pure silicazeolites, such as MWW [143], ITE [143], STT [143], CHA[144], IFR [63, 144], ISV [145], and STF [145]. Negativethermal expansion is not as unusual as was once thoughtand is generally caused by structural effects, including thecooperative motions of the rigid-unit modes and transversevibrations of two coordinated oxygen atoms [146].

Upon compression and/or heating, zeolites collapse todenser amorphous phases at temperatures well below theirmelting points. In the dense silica polymorphs quartz andcoesite, pressure-induced order–disorder transitions occur at

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20–35 GPa and 300 K [147–149]. Thermally- and pressure-induced amorphization of aluminosilicate zeolites are equiva-lent processes induced by mechanical instability [150]. It wasalso shown that a zeolite may present two distinct amorphousphases: a low-pressure (∼2 GPa) low-density amorphousphase and a high-pressure (∼6 GPa) high-density amor-phous phase [150, 151]. A pressure-induced amorphizationof clathrasils were first reported by Tse et al [152] and Tse andKlug [153] in dodecasil-3C (MTN) and deca-dodecasil-3R(DDR). These authors used NaCl as a pressure-transmittingmedium, compressing both guest-containing and guest-freesamples. Dodecasil-3C transformed to an amorphous mate-rial at ∼6 GPa, and the transition was reversible when aguest molecule (tetrahydrofuran, THF) existed in the cages,whereas the transition was irreversible when the cages wereempty. Transformation of deca-dodecasil-3R to a (probably)disordered phase was observed with infrared spectra changesat ∼4 GPa for the guest-free sample and at ∼7 GPa forthe guest (aminoadamantane)—containing sample. Throughexperiments and theoretical molecular dynamics calculations,the presence of undeformable units, such as guests, wasdetermined to be essential for the reversible process. A similarpressure-induced amorphization was reported by Xu et al [154]for guest-free melanophlogite at∼8 GPa. The authors directlycompressed the sample with a diamond anvil cell withouta pressure medium. The pressure-induced amorphous phaseis an intermediate, metastable state. When the amorphousphase at 8.03 GPa was heated up to 1473 K, diffractionpeaks characteristic of coesite, which is the thermodynam-ically stable phase at these pressure conditions, started toappear at 873 K. Yagi et al [29] also reported similar butmore complicated amorphization behaviors of melanophlogiteat high pressure. They compressed natural melanophlogitesamples containing CO2 as a main guest molecule andpreheated guest-free samples up to 25 GPa using variouspressure media at room temperature. When methane or analcohol–water mixture were used as pressure-transmittingmedia, or when direct compression was applied, unheatedmelanophlogite irreversibly amorphized at approximately17 GPa, in which the volume was decreased to approximately70% of its original volume. No structural transition wasobserved up to that pressure. Preheated melanophlogite,however, was much more compressible, amorphizing only atapproximately 3 GPa when the volume was decreased to 80%.This behavior changed completely when helium was used asthe pressure-transmitting medium. The unheated sample wasmuch less compressible and neither a phase transition noramorphization was observed up to approximately 25 GPa.Preheated samples exhibited the same compression curveup to approximately 17 GPa of that observed for unheatedsamples using a helium pressure medium. However, whenthe pressure exceeded approximately 17 GPa, isostructuraltransition occurred with a sudden increase in unit cell volumeby 10%. The two isostructural phases with different volumescoexisted in a narrow pressure range of less than 1 GPa, andabove 18 GPa, the high-pressure phase was compressed in anormal manner up to 24 GPa. This sudden increase in volumeis a reversible process with a large hysteresis of approximately

6 GPa. In the pressure-decreasing cycle, a sudden decreaseof the unit cell volume to its original compression curve wasobserved. This anomalous volume increase by compressionwas attributed to the intrusion of additional helium into thecage. Various other experimental and computational studiesdemonstrate that intrusion of guest species into zeolite frame-works increases their bulk modulus, prevents amorphizationfrom occurring up to higher pressure, and facilitate reversibletransition from amorphous to crystalline phases after de-compression [155–158]. A high-pressure cubic-to-tetragonalphase transition of the cubic phase of melanophlogite wasreported by Tribaudino et al [159]. The sample containedonly methane as a guest molecule and had cubic symmetry atroom temperature. A methanol:ethanol:water (16:3:1, MEW)mixture and silicon-oil (Si-oil) were used as the pressuremedia. The phase transition from cubic to tetragonal wasobserved at 1.14 GPa in both MEW and Si-oil runs. Thetetragonal phase was stable up to the highest pressure (6 GPa)in the run with MEW, but it switched back to cubic symmetryat 3.12 GPa in Si-oil. This difference is probably due tothe non-hydrostatic character of Si-oil at P > 0.9 GPa. Thehigh-pressure transition was found to be very similar to thethermally induced cubic-to-tetragonal transition observed byNakagawa et al [160].

5. Applications

5.1. Gas storage

Two promising applications of clathrasils are gas storage anduse as membranes for gas separations. Because of the smallwindows of their frameworks, only small gases can diffuse fastenough in clathrasil frameworks for practical applications. Therelatively high densities of aluminosilicates make them lesspractical for on-board hydrogen storage, and yet they remainattractive to scientists for their potential and well-characterizedframework structures, analogous to some other candidates ofhydrogen storage, such as clathrate hydrates and metal-organicframeworks (MOFs) [161–165].

From molecular dynamics simulations, a window ofclathrasil cages larger than six-membered rings is permeableto H2 [166]. In both volume and window size, the [512

] cageis very important for the purpose of trapping H2 at ambientconditions. The H2-containing DDR-type clathrasil, DD3R,was synthesized at 453 K in the presence of 50 bar of gaseoushydrogen, and 0.8 ± 0.1 H2 molecules per [512

] cage ofDD3R were determined by means of prompt gamma activationanalysis (PGAA) and 1H MAS NMR spectroscopy [165]. Thehydrogen-storage capacity (0.2 wt%) should be improved byincreasing the concentration of [512

] cages in the frameworktopology, although the H2 in the [512

] cages was released onlyabove 1100 K. For improving the hydrogen-storage capacityof zeolites, a large volume of micropores and a suitablediameter near the kinetic diameter of a hydrogen molecule areimportant [164]. Classical molecular mechanics simulationson hydrogen-storage capacities of 12 pure silica zeolites hasshown that flexible non-pentasils RHO, FAU, KFI, LTAand CHA display the highest maximal capacities, ranging from

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2.86 to 2.65 mass%, which correlates well with experimentalresults obtained at low temperatures [162]. For FAU-, MFI-and MOR-type zeolites with similar Al concentration, hydro-gen adsorption capacity, isosteric heat of adsorption, surfacecoverage, and micropore occupancy were reported to increasein the order of FAU< MFI< MOR [163]. Irrespective ofthe framework structure, the adsorption capacity, heat of ad-sorption, surface coverage, and micropore occupancy of thesethree zeolites increased with increasing Al content of zeolites.These results suggest that the ideal zeolite for an efficienthydrogen storage material is one with a high electrostatic fieldand narrow pores without intersections. At low H2 pressure,hydrogen adsorption capacities were reported to correlatewith the isosteric heat of adsorption [167], which reflectsthe interaction energy between hydrogen and host materials.The heat of adsorption of the above-mentioned FAU-, MFI-and MOR-type zeolites is roughly in the order of 6–7 kJ mol−1

when the adsorbed amount is 40 mL g−1, and when adsorbedamount is smaller, it ranges up to 11.7 kJ mol−1 [163]. Thesevalues are comparable to other candidates of hydrogen storagessuch as MOFs [167, 168] and carbon materials [169].

5.2. Membranes for gas separation

The permeation and separation of gas mixtures through ze-olitic membranes are controlled via balancing the effects ofadsorption and diffusion in micropores. For example, Donget al [170] studied the gas permeation selectivity betweenhydrogen and light hydrocarbons (C1–C4) with all-silica MFIzeolite (silicalite) membranes at 25–500 ◦C and feed pressuresof 0.1–0.4 MPa. At temperatures <100 ◦C, the membraneshowed excellent separation properties, prohibiting the per-meation of hydrogen and increasing the permeance of hydro-carbons with increasing molecular masses. This behavior isdue to the increasing adsorption strength of the componentin the hydrophobic silicalite with increasing molecular mass,and the permeation is primarily determined by preferentialadsorption at these temperatures. At 500 ◦C, the membranebecomes permselective for hydrogen and smaller hydrocar-bons because diffusion in the pores controls the permeanceand the preferential adsorption effect is negligible at such ahigh temperature.

Hydrogen production from fossil fuels involves the sep-aration of H2 from small molecules, such as CH4, CO2, CO,H2O, and H2S at high temperatures, at which permeation iscontrolled by diffusion, and zeolitic membranes can selectivelypermeate and separate H2 from these small molecules [171].Clathrasils and pure silica zeolites have the advantage of highthermal stability, whereas zeolites with low Si/Al ratios aregenerally unsuitable for operating at high temperatures andmoist atmospheres. The MFI- and DDR-types of high-silicazeolites have been extensively studied for H2 separation [170–177]. Clathrasils with six-membered rings as their largestopenings, such as SOD, MTN, and DOH, are also consideredas promising candidates for hydrogen storage and purification.For the diffusion of H2 to occur through six-membered rings,framework flexibility is very important because it allows thering to adapt to the hydrogen passage [178]. Berg et al [179]

investigated the self-diffusion of H2 in pure silica MTNand SOD by means of molecular dynamics simulations. TheH2 diffusion rate in MTN was found to be considerably lowerthan that in SOD. Although the flexibilities of the MTNand SOD frameworks do not differ greatly and their activationenergies are roughly equal, it was suggested that a largecage volume leads to a lower diffusion rate because of asmaller chance of finding hydrogen molecules near the cage’ssix-membered rings.

The removal and recovery of CO2 from flue and naturalgases are of great interest because CO2 is the main componentof greenhouse gases. The hydrophobicity of high-silica zeo-lites offers an advantage of retaining their adsorption capacityin water. The all-silica DDR has high selectivity of adsorptionfor CO2 over CH4 [180, 181]. Molecular simulations on puresilica zeolites have indicated that CHA and DDR yield thebest permeation selectivities among 12 zeolite topologies forthe separation of CO2 and CH4 [182]. MFI, LTA, and DDR ex-hibited high separation performances of CO2/N2 and CH4/N2,comparable to those of MOFs [183]. DDR was also reportedto be a very effective molecular sieve for the separation orpurification of propane–propene mixtures [184].

6. Summary

Among the 213 known topologies of zeolites, 43 topologies arerecognized as pure silica zeolites (zeosils). The present reviewfocused on clathrasils, a variant class of pure silica zeolitesthat possess cage-like small voids. Although a number ofhypothetical clathrasil dodecasil topologies exist, disorderedstructures have yet to be discovered. The sizes and shapes of thestructure-directing template molecules control which topologyof clathrasils grow. Template molecules that fit into the largercages in a framework topology serve as SDAs, whereas thesmall gaseous molecules that fit into the smaller cages playrather minor roles in the crystallization of clathrasils. Becausethe frameworks of clathrasils are electrostatically neutral andhydrophobic, only weak interactions between guest speciesand host frameworks occur. However, such weak van derWaals interactions, as well as the rigid-unit mode of SiO4tetrahedra, serve vital roles on symmetry lowering, the long-range ordering and/or disordering of the framework structures.The apparently short Si–O distances and large Si–O–Si anglesoften reported for clathrasil structures are due to the staticdisorder of rigid SiO4 tetrahedra. Further studies investigatingthe details of guest–host interactions, local configurations ofthe frameworks, and long-range ordering and/or disorderingare desired to elucidate the fundamental mechanisms behindthe diverse responses of clathrasil frameworks to guest species,temperature, pressure, and pressure media.

Acknowledgment

The author thanks Dr Kenji Sakurai of the National Institutefor Materials Science, Japan, for providing the inspiration andmotivation for this paper.

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