Sol-gel strategies for controlled porosity inorganic materials

Brinker, C. Jeffrey; Sehgal, Rakesh; Hietala, Susan L.; Deshpande, Ravindra; Smith, Douglas M.; Loy, Douglas A.; Ashley, Carol S.

doi:10.1016/0376-7388(93)e0129-8

Cited by 259 publications

(144 citation statements)

References 27 publications

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“…If the concentration of the polymer is high, underestimation of R g will result because As can be seen from the equation, smaller fractal dimensions of entanglement. Near the percolation threshold the estimaencourage interpenetration (16).…”

Section: Introductionmentioning

confidence: 99%

Sol–Gel Synthesis and Characterization of Microporous Silica Membranes I: SAXS Study on the Growth of Polymeric Structures

Nair

Elferink

Keizer

et al. 1996

Journal of Colloid and Interface Science

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series (11) will present the synthesis chemistry of silica sols Silica-based microporous membranes can be successfully pro-to make unsupported membranes with predictable pore size/ cessed by sol-gel techniques. However tailor making of such mem-porosity. This article aims to give a detailed account of small branes is still not possible because of the unavailability of system-angle X-ray scattering of silica sols and further calculations atic studies on membrane oriented sol processing and characteriza-of fractal dimensions and monomer/polymer sizes. Fractal alcohol condensation r GSi{OR / HO{SiG S GSi{O{SiG / ROH R alcoholysis INTRODUCTION[2] Microporous silica membranes are attractive materials for gas separation and membrane reactors. The synthesis and water condensation r GSi{OH / HO{SiG S GSi{O{SiG / H 2 O R hydrolysis characterization of such membranes and their precursor materials have been dealt with by many authors for the past few[3] years. Authors like Iler (1) and Scherer (2) have detailed descriptions of the whole sol-gel chemistry of silica and Yoldas (3) and Klein (4) have elaborated the sol-gel syn-Microporous structures are generally made available by acid thesis with emphasis to the wet gel characteristics. A number catalysis of these reactions. Hydrolysis will then take place of articles were also published (5-9) about typical sol-through a fast protonation of the alkoxide, followed by congel silica membranes with porosity/permeation/separation densation to siloxane bond. By controlling the synthesis conbehavior. Sol-gel silica binary systems for membrane syn-ditions and reactant concentrations the sol-gel morphology thesis were also dealt with (10). Even with all these studies, can be changed (2). This is essentially achieved by the there are no selection rules available for tailor making micro-control of rates of hydrolysis and condensation by changing porous membranes. the amount of water or catalyst used. The structure of the It is the aim of the present authors to present a more resulting polymers can change from more or less linear to detailed account of the sol-gel synthesis of silica along with weakly branched polymers. The morphology of the resulting characteristics of the resulting membranes, in a series of gels depends on the structure of these polymers and to a articles of which this is the first. The second part of this better extent the efficiency of packing.For the formation of microporous membranes, the ability of the polymers to interpenetrate must be considerable. Short 1 To whom correspondence should be addressed at Advanced Polymer branched linear polymers are the best in this regard. HowLaboratory, Japan High Polymer Center, Yanagibashi 2-22-13, Taito-ku, Tokyo 111, Japan.ever, the probability is higher to form a dense structure when From the fractal range, where the intensity has a power three, where the object is Euclidean. As the fractal dimension law relation, the dimension D f can be calculated as the negadecreases the structure will look sparser, giving essentially ...

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Section: Introductionmentioning

confidence: 99%

Sol–Gel Synthesis and Characterization of Microporous Silica Membranes I: SAXS Study on the Growth of Polymeric Structures

Nair

Elferink

Keizer

et al. 1996

Journal of Colloid and Interface Science

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“…Because the lowest reaction rate for hydrolysis occurs at pH = 7 [29], a lot of silica networks are still unreacted, so the electrophilic tendency of condensation terminates, and the alcohol condensation mechanism becomes favourable [30,31]. This condition gives rise to more locally branched networks, leading to narrow pore sizes and small pore volumes, thus giving rise to dense network and then low transparency.…”

Section: Discussionmentioning

confidence: 99%

“…This may be explained by the fact that silica has its isoelectrical point (IEP), the point at which the zeta potential is equal to zero, at approximately pH = 4, or 4.5, to be exact. At the IEP, the reaction rate for condensation is at a minimum; hence, the water condensation mechanism becomes favourable to the alcohol condensation mechanism [30]. This condition may give rise to less branchy networks and lead to large pore sizes, resulting in higher adsorption quantities at higher relative pressures.…”

Section: Discussionmentioning

confidence: 99%

Tuned optical transmittance in single-step-derived silica aerogels through pH-controlled microstructure

et al. 2017

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We have systematically studied the relationship between synthesis pH and morphological and optical properties of silica aerogels. We have determined through SEM and BET that there is a systematic correlation between the pH of the initial silica solution and the surface area, porosity, and pore size of the resulting aerogel. We find that optical transmittance, particularly its wavelength dependence (dispersion), is strongly governed by the microstructure and, therefore, synthesis pH. We have determined that the microstructure of the aerogels fall into three broad categories: monostructural, which is characterized by repeating elongated microstructural features; fractal, which shows a distinctive structure that is emulated on multiple length scales; and isotropic, which is characterized by having no distinct features in its microstructure. Simply by controlling the pH of the synthesis environment, we can tune the optical properties of silica aerogels through pH controlled the microstructural modification. We have found that pH = 1-5 gives high dispersion, pH = 6-7 results in low transmittance and low dispersion, pH = 8 shows the highest transmittance with the lowest dispersion, and pH = 9-10 transitions back to lower transmittance and higher dispersion.

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“…During the first 120 min of reaction the IR peaks, corresponding to the BTESE ethoxy groups (SiOEt) as well as the one representative for (free) water decreased in intensity, while the intensity of the Si-OH vibration (corresponding to the hydrolysis product) increased simultaneously. After around 2 h of reaction, the free water line intensity increased slightly, which can be ascribed as a water condensation reaction where water is released as a by-product [9]. This FTIR analysis might not provide accurate information, but it gives an indication that hydrolysis of BTESE at 20°C proceeds in 2 h. The combination of the lower amount of water in the reaction mixture and of already (partly) hydrolysis of BTESE after 2 h of reaction might sufficiently prevent the Nb-or Taalkoxide precursors from fast hydrolysis and enhance the reaction of the metal ion with the BTESE hybrid silica precursor.…”

Section: Doped Btese Sol Characterizationmentioning

confidence: 99%

“…The application of microporous silica membranes for hydrogen purification has been subject of study for a long time [4]. These membranes are either prepared by a chemical vapor deposition (CVD) technique [6,7] or by using sol-gel methods [8][9][10]. Although silica membranes, fabricated by sol-gel methods, show great promise in gas separation, the technical challenges such as hydrothermal stability are still a point of concern [11].…”

Section: Introductionmentioning

confidence: 99%

Doped microporous hybrid silica membranes for gas separation

Qureshi

Besselink

Elshof

et al. 2015

J Sol-Gel Sci Technol

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Hybrid silica (i.e., bis-triethoxysilylethane: BTESE) membranes doped with B, Ta or Nb were made through a sol-gel process. Triethyl borate, tantalum (V) ethoxide (TPE) and niobium (V) ethoxide (NPE) were selected as doping precursors. The doping concentration was optimized to produce sols, suitable for membrane fabrication. Thermal stability, structural analysis, crosssectional micrographs and single gas permeation experiments were performed on these membranes, and results are compared with an undoped BTESE membrane. It was observed that the synthesized doped BTESE materials and membranes resulted into a more open (and, in one occurrence, SF 6 permeable) pore microstructure, showing high permeances of larger gas molecules, while having a crosssectional thickness comparable to undoped BTESE membranes. Graphical Abstract& Louis Winnubst a.j.a.winnubst@utwente.nl;

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Sol-gel strategies for controlled porosity inorganic materials

Cited by 259 publications

References 27 publications

Sol–Gel Synthesis and Characterization of Microporous Silica Membranes I: SAXS Study on the Growth of Polymeric Structures

Sol–Gel Synthesis and Characterization of Microporous Silica Membranes I: SAXS Study on the Growth of Polymeric Structures

Tuned optical transmittance in single-step-derived silica aerogels through pH-controlled microstructure

Doped microporous hybrid silica membranes for gas separation

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