On the Microporous Nature of Mesoporous Molecular Sieves

Göltner, Christine G.; Smarsly, Bernd M.; Berton, Beate; Antonietti, Markus

doi:10.1021/cm0010755

Cited by 140 publications

(134 citation statements)

References 31 publications

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“…These results are in reasonable agreement with the findings of Göltner et al and provide a further indication of the microporous character of silicas obtained from the templating of lyotropic phases of PEO containing block copolymers. 29 Second, the experimental data in Table 2 also show certain trends concerning the degree of microporosity as a function of the relative block lengths. For instance, the contribution of micropores to the overall specific surface area is substantially lower for the C 12 E 30 silica compared to the C 8 E 4 silica despite the larger PEO block.…”

Section: It Followsmentioning

confidence: 87%

“…However, in a recent work it was already figured out that just a part of the PEO chains are located in the silica walls leading to additional microporosity. 29 In this picture, the PEO chains that are not situated in the silica wall will, instead, contribute to the mesopore volume ("three phase model"). Obviously, a deeper knowledge of the distribution of PEO is essential to get more insights into the nanocasting mechanism and to learn about amphiphilic organization during the nanocasting.…”

Section: Investigation Of the Porositymentioning

confidence: 99%

“…The raw data can also be obtained from the study of Göltner et al 29 The micropore volume created by one EO unit in the nanocasting procedure was found to be approximately ≈2 × 10 -23 cm 3 . With this value and the micropore volumes in Table 2, the data in Table 3 are obtained.…”

Section: It Followsmentioning

confidence: 99%

“…However, concerning the distribution of PEO within the organic-inorganic hybrid material, a different model has already been developed. 29 A part of the PEO chains may form a separate "phase" between the hydrophobic block and the silica phase, which was therefore termed the "three-phase model". Within the scope of this model, it was assumed that this PEO phase contributes to the mesoporosity.…”

Section: Introductionmentioning

confidence: 99%

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Preparation of Porous Silica Materials via Sol−Gel Nanocasting of Nonionic Surfactants: A Mechanistic Study on the Self-Aggregation of Amphiphiles for the Precise Prediction of the Mesopore Size

2001

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Sol-gel nanocasting is used to imprint the soft-matter structures of lyotropic phases of nonionic n-alkylpoly(ethylene oxide) amphiphiles ("C x E y ") into solid porous silica. Small angle X-ray scattering (SAXS), nitrogen sorption, and transmission electron microscopy (TEM) are used to investigate the dependence of the porosity on the block lengths or the block volumes, respectively. It is found that the size of the mesopores is a function of the lengths/volumes of both the alkyl chain (N A ) and the PEO block (N B ). Moreover, the materials contain a substantial degree of additional microporosity. A quantitative model is developed that relates the amphiphile organization during the nanocasting to the size of the mesopores and the microporosity. In particular, it turns out that depending on the number of EO units a fraction of the PEO chains contributes to the mesoporosity, while a significant portion leads to additional micropores. This model provides a quantitative description of the distribution of the hydrophobic and hydrophilic blocks within the lyotropic phase itself. Our findings indicate that the interface areas b 2 of single surfactant chains are a function of the block lengths, which can be described by a scaling law b 2 ∝ N A 0 16 N B 0 4 . Mixtures of chemically equivalent amphiphiles with different block ratios are studied in further detail. It is seen that every pore size between the size originating from the "parent" templates can be adjusted simply by mixing various amounts of two surfactants, proving that true mixed phases act as a template for the silica pores.

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Section: It Followsmentioning

confidence: 87%

Section: Investigation Of the Porositymentioning

confidence: 99%

Section: It Followsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Preparation of Porous Silica Materials via Sol−Gel Nanocasting of Nonionic Surfactants: A Mechanistic Study on the Self-Aggregation of Amphiphiles for the Precise Prediction of the Mesopore Size

2001

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“…[51][52][53] This method allows the determination of the so-called chord-length distribution (CLD) function, which is an accurate measure for pore size, pore shape, and in case of bimodalities, also pore size distribution. The CLD extrapolated to vanishing lengths also gives a measure for the 'angularity' of the wall, i.e.…”

Section: Analysis Of Porous Materials: Problems and Developmentsmentioning

confidence: 99%

Porous materials via nanocasting procedures: innovative materials and learning about soft-matter organization

2002

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Nanocasting, the 3D-transformation of self-assembled organic nanostructures into hollow inorganic replicas under preservation of fine structural details has recently turned out to be a versatile tool, both for the synthesis of porous media with new pore topology as well as for the characterization of the assembled structures themselves. This review gives a review on recent work describing the potential and restrictions of nanocasting using surfactants, polymers, colloids as well as supramolecular tectons as porogens.

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From Cyclodextrin Assemblies to Porous Materials by Silica Templating

et al. 2001

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Since the discovery by Beck, Vartuli et al. [1,2] that ordered mesoporous silicas, such as MCM-41, can be obtained by surfactant templating, the development of new porous materials and porogens as well as the characterization of the resulting structures has become one of the most promising fields in modern materials chemistry. [3±5] Meanwhile, these observations have been extended to a number of techniques by which pore morphology, pore size, and wall material can be varied over a broad range. [6±14] One of the modifications of the original process relevant here is the ™nanocasting∫ introduced by Attard, Glyde, and Gˆltner. [15] This is a high-concentration synthesis in which the solidified inorganic compound is a copy of the original phase structure, that is its structure is predetermined by the selection of the template phase. They used lyotropic liquid-crystalline phases, derived from surfactants or-later-block copolymers, [15±17] as templates and obtained the material either as a homogeneous film or as a monolithic object. In addition, this technique also allows the structural characterization of fragile phase structures and transition states of polymers or surfactants by analyzing the well-contrasted solid replica instead of the original soft organic phases. [16,18] We extend this approach to supramolecular aggregates of nonpolar cyclodextrins (CDs) in aqueous solution as templates. Our aim is to exploit the use of CDs to generate porous materials, focussing on the fine details of the pore structure and the self-organization of this template. The self-assembly of cyclodextrins has been assumed previously, [19±23] but their organization could-to the best of our knowledge-not be determined in detail.CDs are cyclic oligosaccharides consisting of covalently linked glucose units (6 units a-CD, 7 units b-CD, 8-units g-CD); they are characterized by a hydrophilic exterior and a hydrophobic interior [24] the size of which is in the range of 1.5 ± 2 nm. The relatively large number of hydroxy groups per unit area should ideally mediate compatibility with an oxidic inorganic matrix through hydrogen bonding. In fact, this has been proven by three reports in which CDs were employed at low concentrations in a sol ± gel process to incorporate functional units such as dyes into a dense and nonporous silica. [25±27] So far, CD-containing solutions have been difficult to analyze by standard techniques, thus information about the assembly of CDs in water is still rather limited. We employ nanocasting to investigate the CD assemblies as templates for the silica pore in detail, and we also investigate the driving force for the self-assembly of the cyclodextrin molecules to ordered aggregates.In a solution of a hydrophilic sugar, such as a cyclodextrin, one would assume that it is moleculary dispersed in the aqueous phase. Nanocasting by a sol ± gel process should then result in a porous material in which each pore exhibits the size and shape of a CD molecule, and in which the pores are not mutually ordered. Depending on ...

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On the Microporous Nature of Mesoporous Molecular Sieves

Cited by 140 publications

References 31 publications

Preparation of Porous Silica Materials via Sol−Gel Nanocasting of Nonionic Surfactants: A Mechanistic Study on the Self-Aggregation of Amphiphiles for the Precise Prediction of the Mesopore Size

Preparation of Porous Silica Materials via Sol−Gel Nanocasting of Nonionic Surfactants: A Mechanistic Study on the Self-Aggregation of Amphiphiles for the Precise Prediction of the Mesopore Size

Porous materials via nanocasting procedures: innovative materials and learning about soft-matter organization

From Cyclodextrin Assemblies to Porous Materials by Silica Templating

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