SiO-.cntdot. .cntdot. .cntdot.HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites

Koller, Hubert; Lobo, Raúl F.; Burkett, Sandra L.; Davis, Mark E.

doi:10.1021/j100033a036

Cited by 248 publications

(273 citation statements)

References 10 publications

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“…It is well-known that, as-prepared, ZSM-5 contains defects in roughly equivalent or greater number than the number of Brønsted acid sites. 38,39 Our results show that defects located on Si can lower the PA by about 12 kcal/mol, but only if the defect is immediately adjacent to the Brønsted acid site. A defect associated with Al can have a similar effect on the PA.…”

Section: Theoretical Approachmentioning

confidence: 71%

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5

1997

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Density functional theory has been used to determine the effects of local composition and structure on the proton affinity (PA) of H-ZSM-5. These calculations were performed with clusters ranging in size from 34 to 41 atoms. For isolated Al sites, the associated PA is 329 kcal/mol. Introduction of a defect in the form of a silanol group immediately adjacent to such a Brønsted acid site reduces the PA to 316 kcal/mol. For an isolated structure containing two Al atoms in next-nearest-neighbor T sites, the value of PA lies between 330 and 298 kcal/mol, depending on the location of the two charge-compensating cations, and is not affected by the presence of a silanol group on the Si atom situated between the two Al atoms. However, the presence of a trigonally coordinated Si atom immediately adjacent to the Brønsted acid site reduces the PA dramatically to 281 kcal/mol, but the presence of trigonally coordinated Al in the vicinity of a Brønsted acid site has no effect on the value of PA. However, the presence of an extraframework Lewis acid center in the form of AldO + will lower the PA for a proton associated with one of the Al atoms in a structure containing two next-nearest-neighbor Al atoms, to 305 kcal/mol. Defects situated on Al atoms have PAs lying between 338 and 320 kcal/mol.

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Section: Theoretical Approachmentioning

confidence: 71%

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5

1997

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“…The majority of these studies involve samples that have been processed or calcined, with only a few examples of studies on materials where the organic materials used for templates are still present. [22,23] In the high-speed 1 H MAS NMR of calcined molecular sieves the isolated or single hydroxyl species (SiOH), the geminal hydroxyl (Si(OH) 2 ) and the hydrogen bonded hydroxyl species (SiOH ÁÁÁ HOSi) have been assigned to the d ¼ þ1.9 ppm, þ2.4 ppm, and þ3.0 ppm resonances respectively. [24] It is well established that increasing hydrogen bond strength results in an increasing 1 H NMR chemical shift.…”

Section: Resultsmentioning

confidence: 99%

Investigation of Templated Mesoporous Silicate Thin Films Using High Speed, Solid‐State ¹H MAS and Double Quantum NMR Spectroscopy

Alam

Fan²

2003

Macro Chemistry & Physics

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Two‐dimensional DQ 1H MAS NMR has been used the investigate the local structure of a surfactant‐templated silicate thin film prepared from adding 4% polyoxyethylene(10) cetyl ether to an acidic TEOS silica sol. A close spatial contact between the surfactant and the silicate present in these materials could be demonstrated, while the high sensitivity of the NMR experiments allowed systems with limited amounts of material to be investigated. The detected inorganic‐organic interactions in these materials provide additional information into the chemical processes occurring during the self‐assembly process and the formation of meso‐structured materials.

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“…Not much is known about the details of the interface between the silica nanoparticles and the organic cations, but if the crystal structures of cubic silsesquioxanes hydrates are used as a guide, [48][49][50] one would expect a strongly bound layer of water molecules between the silanol/siloxi groups and the TPA cations. [51] It is also hard to imagine that concerted changes in the structure of the silica nanoparticles to form zeolite-like structures are feasible as this would entail simultaneous and coordinated breaking and forming of multiple Si-O-Si bonds in these highly cross-linked particles.…”

Section: Structure Direction In Zeolite Synthesismentioning

confidence: 99%

Silica Self‐Assembly and Synthesis of Microporous and Mesoporous Silicates

Rimer

Fedeyko

Vlachos

et al. 2006

Chemistry A European J

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131

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The microstructure of silica in basic aqueous solutions containing organic cations and prepared from monomeric precursors is reviewed and interpreted within the context of classical ideas of self-assembly of molecular aggregates. The solution properties can be understood by using the pseudo-phase separation approach coupled to the acid-base chemistry of silanol groups and the Poisson-Boltzmann equation. The silica nanoparticles frequently observed in these systems have a core-shell structure with silica in the core and the organic cations at the shell. Individual particles are observed when the forces between particles are repulsive-as is the case for small cations such as tetramethylammonium or tetrapropylammonium-and extended structures such as M41S materials are formed when the forces are attractive--as is the case for surfactants such as cetyltrimethylammonium. These ideas are useful to understand the evolution of zeolite synthesis gels from nucleation to crystal growth. Although at room temperature the silica and the organic cations are segregated, upon heating the organic cations are embedded within the particles. This transformation signals the onset of structure direction whereby the size and geometry of the organic cation induce changes in the structure of silica that may lead to zeolite nuclei.

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SiO-.cntdot. .cntdot. .cntdot.HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites

Cited by 248 publications

References 10 publications

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5

Investigation of Templated Mesoporous Silicate Thin Films Using High Speed, Solid‐State ¹H MAS and Double Quantum NMR Spectroscopy

Silica Self‐Assembly and Synthesis of Microporous and Mesoporous Silicates

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SiO-.cntdot. .cntdot. .cntdot.HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites

Cited by 248 publications

References 10 publications

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5

Investigation of Templated Mesoporous Silicate Thin Films Using High Speed, Solid‐State 1H MAS and Double Quantum NMR Spectroscopy

Silica Self‐Assembly and Synthesis of Microporous and Mesoporous Silicates

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Product

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Investigation of Templated Mesoporous Silicate Thin Films Using High Speed, Solid‐State ¹H MAS and Double Quantum NMR Spectroscopy