Acid-catalyzed Sol-gel Process Research Articles

Aggregation Behavior of Alkoxide-Derived Silica in Sol-Gel Process in Presence of Poly(ethylene oxide)

Growth behavior of silica in an acid catalyzed sol-gel process from silicon alkoxide in the presence of poly(ethylene oxide), PEO, was investigated by in situ small angle X-ray scattering, SAXS, and 29Si NMR measurements. The results of SAXS, that aggregation and gel formation behaviors of silica were affected by the presence of PEO, suggested a strong attractive interaction between silica oligomers and PEO. A possible reaction scheme of silica in the presence of PEO is as follows; (1) PEO and small silica oligomers coexist in the solution without specific interaction just after hydrolysis of the silicon alkoxide. (2) With the progress of condensation, a ramified aggregated complex between PEO and silica oligomers is formed, which is characterized by larger apparent value of radius of gyration and smaller fractal dimension than in the PEO-free system. (3) After gelation, the fractal dimension of scatterers remains to be smaller than that in the PEO-free system, because PEO associated with the silica network inhibits aggregation within the gel networks. Furthermore, PEO inhibits the condensation in the aging and in the drying process, leading to less strongly crosslinked dry gel. A temporal maximum in the time evolution of Rg was observed for the samples separated into two phases with their characteristic domain size being larger than several micrometers. This is considered to be a phenomenon related to increase and divergence of correlation length near and at the critical point.

Microstructural Analysis of the Effects of Poly(ethylene glycol) on an Acid Catalyzed Sol-Gel Derived Ceramic Material

Ceramic materials have been derived from an acid catalyzed sol-gel process. The addition of different molecular weights and concentrations of polyethylene glycol (PEG) to the sol mixture modifies the phase behaviour of the sol-gel process. The resulting gel is burned at 973 K to make porous ceramic materials. Nitrogen adsorption-desorption isotherms are used to assess the effects of PEG on the internal structure of the burned ceramic material. These isotherms indicate an extensive pore network exists consisting of micropores and mesopores. In the micropore region of the isotherms, the αS-plot analysis reveals changes in specific primary micropore volumes, specific total pore volumes, specific external surface areas and specific SPE surface area when PEG is added in the sol-gel process. The average pore width and the overall mesopore size distribution curves shift to higher pore size values and ranges on addition of PEG to the sol-gel mixture. The presence of PEG during the sol-gel process leads to an apparent narrowing of the micropore size distribution. The results of this work clearly indicate that the molecular weight and the concentration of a polymer, such as PEG, influences the eventual internal structure of a ceramic after burning.

Matrix effects on coupled chemical reactions in solid-state voltammetry: disproportionation of uranium(V) in a silica xerogel

Abstract The reduction of UO22+ in silica prepared by an acid-catalyzed sol-gel process was studied by cyclic voltammetry in the absence of a contacting liquid phase. The silica was prepared to have residual water on the order of 20% by weight. The UV that was generated electrochemically from UO22+ disproportionated rapidly in the xerogel matrix at pH values where it was stable on the time scale of the experiment in aqueous solution. The voltammetry was not altered when the silica was templated with Triton X-114 to increase the pore diameter; hence, electrostatic interaction between the anionic sites on the xerogel backbone and the cationic electrolysis products was postulated as the cause of the enhanced disproportionation rate. Comparison with the voltammetry of UO22+ in Nafion® supported the hypothesis of UIV formation by disproportionation of the UV. Doping the xerogel with La3+, which has a high affinity for anionic sites, blocked the formation of UIV; a quasi-reversible one-electron reduction of UO22+ was observed, thereby providing a positive test of the hypothesis.

AMM-MnxSi-Catalyzed Selective Oxidation of Toluene

Amorphous, microporous mixed oxides, based on silica with isolated Mn centers in the amorphous silica matrix, were prepared by means of an acid catalyzed sol–gel process. The materials have a narrow micropore distribution with a pore diameter of 0.8 nm. XANES investigations of the materials on the Mn absorption edge provide evidence that isolated octahedrally coordinated MnIIsites predominate. Further characterizations include XRD, sorption measurements, and light microscopy. The good activity of the new catalysts for the selective oxidation of toluene to benzaldehyde is attributed to the homogeneously distributed manganese in the shape selective environment of the micropores.

17O-NMR of Sol-Gel Processes of TEOS and TMOS

Water consumption and formation in the acid catalyzed sol-gel processing of TEOS and TMOS can be followed using 17O-NMR. By using 17O-enriched water, insight into the hydrolysis and condensation in the acid step of the sol-gel process can be obtained. It is found that, after initially strong consumption of water due to hydrolysis, a steady state water concentration results. This amount of water increases upon dilution of the reaction mixture with alcohol. For a hydrolyzed TMOS-methanol-water system lower water contents are found than in a comparable TEOS-ethanol system. Addition of ethanol to a hydrolyzed TMOS methanol system enhances the condensation and a higher water concentration is found.

Spectrofluorimetric studies of water content in sol—gel processing of TMOS

Measurements of water during acid-catalyzed sol—gel processing TMOS have been carried out. The compositions of the initial sols correspond to TMOS:H 2O:MeOH=1: n:3, where n = 1, 2, 4, 6, 8, 15 and 25. Pyranine, a sensitive water photoprobe enables monitoring fast water content changes. Initial decrease of water content due to hydrolysis of TMOS was fast for sols corresponding to ( n ≥ 15). A minimum was reached in about 1 min. For n ≤ 2 these changes were much slower. Subsequent increase of water was caused by condensation reactions. Changes of water content and mass which occur more rapidly after pin-holing the seals of cuvettes containing sols—gels are caused by evaporation of pore liquid. The results of the numerical fits of these changes suggest that this liquid consists mainly of H 2O and MeOH.

Structural evolution of a formamide modified sol—Spectroscopic study

We investigated the influence of formamide on the acid-catalyzed sol-gel process at 40°C, by Fourier transformed infrared spectroscopy. Two silica sols were studied: one containing formamide and one without. Following the time evolution of both the Si−O(H) stretching vibration at around 950 cm−1 and the Si−O(−Si) vibration between 1080 and 1200 cm−1, we were able to describe the structural evolution of each sol. We thus observed, in the case of the formamide containing sol, a peculiar two-step structural evolution which eventually resulted in a highly interconnected gel.

Amorphous Microporous Titania–Silica Mixed Oxides: Preparation, Characterization, and Catalytic Redox Properties

Microporous titania–silica mixed oxides with a narrow monomodal pore size distribution at pore diameters of 0.7 nm with highly dispersed titanium in the silica matrix have been obtained by a simple acid-catalyzed sol–gel process in the absence of chelating agents or the prehydrolysis techniques. A mixture of titanium(IV)alkoxide and tetraethoxysilane (TEOS) has been hydrolysed in alcoholic solution with aqueous hydrochloric acid followed by calcination, resulting in amorphous, microporous mixed oxides. There is no limitation on the chemical composition, which covers the whole range from microporous silica to microporous titania. The structural and chemical properties of the materials as a function of preparation parameters (such as acid, titania, or water content, respectively, nature of alcohol, gelation temperature, drying conditions, and titanium source) have been studied by means of physisorption (Ar and N2), X-ray powder diffraction, spectroscopic techniques, high resolution TEM (in combination with EDX and electron diffraction) and catalytic test reactions (epoxidation of olefins, selective oxidations of saturated hydrocarbons). The high Ti dispersion was negatively affected by changes in the Ti source and/or the alcohol used for the sol–gel process. Increasing BET surface areas were detected as function of the Si/Ti ratio at an optimum in acid concentration. The Si-excess materials stayed X-ray amorphous up to 1173 K. FTIR studies after pyridine treatment showed the materials having weak acidity only. With increasing Ti content an increase in Ti–O–Ti connectivity is observed by DRIFT spectroscopy, resulting in a decrease in epoxidation activity. The glasses show not only catalytic activity for selective oxidation reactions with TBHP comparable with that of other titania containing materials, but size selective epoxidations of olefins are interpreted as shape selectivity resulting from their distinct microporosity. Competitive adsorption experiments of water and octane suggest the hydrophilicity of the amorphous oxides to be the major difference in comparison to their zeolitic analogues.

Study of the action of formamide on the evolution of a sol by pH measurements and Fourier transformed infra-red spectroscopy

Abstract pH measurements and Fourier transformed infrared spectroscopy (FTIR) were used to investigate the influence of formamide on the acid-catalyzed sol-gel process at 40°C. Two sols were studied: one containing formamide and the other not. Formamide, by reacting with the catalyzing protons was found to increase the sol pH, thus modifying the condensation reaction conditions. FTIR studies showed that in the presence of formamide condensation first, preferentially, leads to small oligomers of less than six Si atoms. These oligomers then condense and thus increases the degree of cross-linking of the gel network. The structure of the silica thus formed evolves towards the structure of vitreous silica. In the sol without formamide, condensation leads to long linear chains with little cross-linking. The gel will only have the interconnected structure of vitreous silica after calcination.

Amorphous microporous mixed oxides as selective redox catalysts

Amorphous microporous homogeneously mixed oxides based on Ti or V in silica can be prepared by an acid catalyzed solgel process. The materials have been characterized by TEM, UV, IR and sorption methods. The glasses can be used for the selective epoxidation of alkenes. The catalytic properties are comparable to those of the well known crystalline Ti-containing zeolites TS-1, Ti-MCM-41 and Ti-Beta. Amorphous microporous homogeneously mixed metal oxides seem to be promising new catalytic materials supplementing the well studied selective zeolites.

Sol-gel polymerization: analysis of molecular mechanisms and the effect of hydrogen

Abstract The polymerization of silicic acid molecules (H4SiO4) was simulated using classical molecular dynamics with multibody potentials. The mechanisms involved in the condensation process were analyzed. Although a condensation mechanism was observed in the simulations which corresponds to the mechanism commonly assumed, additional mechanisms not previously presented were also observed. The advancement of polymerization was found to depend on the non-bridging oxygens' coordination with hydrogen, the number of bridging oxygens around five-coordinated silicons and atomic momentum. The results show consistency with the chemistry described for the acid catalyzed sol-gel process where hydrogen acts as the catalyst and chains form before rings.

Physical Properties of Dried Na2O-SiO2 Monoliths

ABSTRACTDried monolithic gels of compositions XNa2O-(1-X)SiO2 (with X from 0 mol% to 7 mol%) were obtained by acid catalyzed sol-gel processing using tetramethylorthosilicate and NaNO3. Optically clear monoliths in the range of 20 to 90 mm diameter by 3 to 20 mm thickness were routinely produced by controlling the processing schedule. Complete drying in air without cracking was achieved at 180°C from 10–14 days. The bulk density and optical properties of the gels, characterized by UV/VIS/NIR transmission, IR reflection and index of refraction, showed a compositional dependence on Na2O similar to melt glasses of the same Na2O content. However, the true density and microhardness decreased with Na2O content in contrast to melt glasses.