Thermodynamic properties of silicalite SiO2

Johnson, Gerald K.; Tasker, Ian R.; Howell, David; Smith, J.V.

doi:10.1016/0021-9614(87)90068-1

Cited by 35 publications

(48 citation statements)

References 23 publications

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“…The atomic positions of framework atoms and sorbed molecules, the synthesis of a wide range of Al/Si compositions, the removal and insertion of Al in secondary processes, and the infra-red characterization of hydroxyls, etc., are documented strongly (12). Silicalite is thermally unstable with respect to quartz, cristobalite, and tridymite, but more stable than silica glass or high pressure coesite and stishovite (21).…”

mentioning

confidence: 99%

Biochemical evolution. I. Polymerization on internal, organophilic silica surfaces of dealuminated zeolites and feldspars

Smith

1998

Proc. Natl. Acad. Sci. U.S.A.

117

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Catalysis at mineral surfaces might generate replicating biopolymers from simple chemicals supplied by meteorites, volcanic gases, and photochemical gas reactions. Many ideas are implausible in detail because the proposed mineral surfaces strongly prefer water and other ionic species to organic ones. The molecular sieve silicalite (Union Carbide; ‫؍‬ Al-free Mobil ZSM-5 zeolite) has a three-dimensional, 10-ring channel system whose electrically neutral Si-O surface strongly adsorbs organic species over water. Three -O-Si tetrahedral bonds lie in the surface, and the fourth Si-O points inwards. In contrast, the outward Si-OH of simple quartz and feldspar crystals generates their ionic organophobicity. The ZSM-5-type zeolite mutinaite occurs in Antarctica with boggsite and tschernichite (Al-analog of Mobil Beta). Archean mutinaite might have become de-aluminated toward silicalite during hot/cold/wet/dry cycles. Catalytic activity of silicalite increases linearly with Al-OH substitution for Si, and Al atoms tend to avoid each other. Adjacent organophilic and catalytic Al-OH regions in nanometer channels might have scavenged organic species for catalytic assembly into specific polymers protected from prompt photochemical destruction. Polymer migration along weathered silicic surfaces of micrometer-wide channels of feldspars might have led to assembly of replicating catalytic biomolecules and perhaps primitive cellular organisms. Silica-rich volcanic glasses should have been abundant on the early Earth, ready for crystallization into zeolites and feldspars, as in present continental basins. Abundant chert from weakly metamorphosed Archaean rocks might retain microscopic clues to the proposed mineral adsorbent/catalysts. Other framework silicas are possible, including ones with laevo/dextro one-dimensional channels. Organic molecules, transition-metal ions, and P occur inside modern feldspars.

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mentioning

confidence: 99%

Biochemical evolution. I. Polymerization on internal, organophilic silica surfaces of dealuminated zeolites and feldspars

Smith

1998

Proc. Natl. Acad. Sci. U.S.A.

117

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“…Experimental evidence shows that MFI is unstable with respect to quartz or cristobalite below 1000 K. 3 Our results suggest MFI becomes stable above 1400 K. New experimental information above 1000 K is needed to test our prediction regarding the thermodynamic stability of MFI. Our model clearly lacks details such as solvent and charge, which may be crucial for a qualitative understanding of zeolite formation and stability.…”

Section: Discussionmentioning

confidence: 93%

“…In this picture, the use of a template or structure-directing agent ͑SDA͒ in zeolite synthesis is a means of trapping the silica into an open-framework structure with channels and cavities. It is worthwhile to ask whether this view is generally correct, considering that thermochemistry measurements of zeolite formation find that zeolite instability decreases with temperature, 3,4 suggesting that under certain circumstances zeolites might be thermodynamically stable relative to dense phases such as quartz. We address this question by applying molecular simulations of a model of silica to examine whether all-silica zeolite frameworks SOD, LTA, MFI, and FAU exhibit regimes of thermodynamic stability in the silica phase diagram.…”

Section: Introductionmentioning

confidence: 99%

“…We address this question by applying molecular simulations of a model of silica to examine whether all-silica zeolite frameworks SOD, LTA, MFI, and FAU exhibit regimes of thermodynamic stability in the silica phase diagram. The thermodynamic stability of pure silica zeolites has been studied by experimental calorimetry [3][4][5][6][7] and molecular modeling. [8][9][10][11][12][13][14][15] Most modeling studies report potential energy optimizations, while most experiments measure lattice enthalpies.…”

Section: Introductionmentioning

confidence: 99%

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Further studies of a simple atomistic model of silica: Thermodynamic stability of zeolite frameworks as silica polymorphs

Ford

Auerbach

Monson

2007

The Journal of Chemical Physics

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We have applied our previously reported model of silica based on low coordination and strong association ͓J. Chem. Phys. 121, 8415 ͑2004͔͒, to the calculation of phase stability of zeolite frameworks SOD, LTA, MFI, and FAU as silica polymorphs. We applied the method of Frenkel and Ladd for calculating free energies of these solids. Our model predicts that the MFI framework structure has a regime of thermodynamic stability at low pressures and above ϳ1400 K, relative to dense phases such as quartz. In contrast, our calculations predict that the less dense frameworks SOD, LTA, and FAU exhibit no regime of thermodynamic stability. We have also used our model to investigate whether templating extends the MFI regime of thermodynamic stability to lower temperatures, by considering templates with hard-sphere repulsions and mean-field attractions to silica. Within the assumptions of our model, we find that quartz remains the thermodynamically stable polymorph at zeolite synthesis temperatures ͑ϳ400 K͒ unless unphysically large template-silica attractions are assumed. These predictions suggest that some zeolites such as MFI may have regimes of thermodynamic stability even without template stabilization.

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“…Johnson et al provide thermodynamic properties for Reaction 1 at temperatures of 298.15 and 1000 K (28)(29)(30)(31). The values at 298.15 K will be used as a lower bound for the feasibility of the reaction, since the change in Gibbs function is less at 1000 K (typical synthesis conditions involve temperatures of 400-500 K).…”

Section: Thermodynamic Analysis Of Pure-silica Zsm-5 Synthesismentioning

confidence: 99%

Synthesis of Porous Silicates

Helmkamp¹

1995

Annual Review of Materials Research

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The issues of importance and future concern in the synthesis of porous silicates and porous materials that contain a large fraction of silica, e.g. zeolites and other crystalline molecular sieves, are reviewed. The thermodynamics of zeolite synthesis are discussed, including a detailed thermodynamic analysis of the synthesis of pure-silica ZSM-5. The kinetics of porous silicate synthesis are reviewed, with particular emphasis on the control of porous structure formation through the use of organic structuredirecting agents. Ordered mesoporous materials are discussed in the context of distinguishing their features from zeolites in order to describe further the unique properties of each class of material. Finally, several unresolved issues in the understanding of the synthesis process are outlined, the resolutions of which would aid in the synthesis of porous silicates by design.

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Thermodynamic properties of silicalite SiO2

Cited by 35 publications

References 23 publications

Biochemical evolution. I. Polymerization on internal, organophilic silica surfaces of dealuminated zeolites and feldspars

Biochemical evolution. I. Polymerization on internal, organophilic silica surfaces of dealuminated zeolites and feldspars

Further studies of a simple atomistic model of silica: Thermodynamic stability of zeolite frameworks as silica polymorphs

Synthesis of Porous Silicates

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