Authors:Rakesh Kumar, Anil Kumar, and Ashok Khanna
trichloride for LAB synthesis, yet this remains as the inevitable alkylating agent. Several research groups have synthesized solid superacid catalyst by reacting AlCl 3 vapor with silica or alumina and these solid superacids show Hammett acidity function, Ho
Authors:Nabila Guindy, S. El Enien, F. El Hosiny, and S. El Gamal
The hydration of two calcium hydroxide— silica fume mixtures was studied at 25°C, these are Mix I and Mix II with molar lime/silica
ratios of 1 and 1.7, respectively. The free lime, free silica and chemically combined water contents were determined at various
time of hydration from which the molar CaO/SiO2 ratios of the formed calcium silicate hydrate, C−S−H, were calculated. The results indicated that hydration takes place in
six steps where C−S−H (I) is formed at early stages of hydration, for Mix I, While for Mix II formation of C−S−H (I) and C−S−H
(II) were detected by X-ray diffraction analysis and differential thermal analysis.
Thermal stability of silica-polyvinylsiloxane systems with different ratio of hydrophilic and hydrophobic components and its
forms with adsorbed dyes were studied using thermochemical method. It was shown that the maximum of endoeffect relating to
dehydroxylation of sorbents surface is observed at 325C. The maximum temperature of the second endoeffect depends on the
organosilica sorbents composition and increases with a decrease of hydrophilic and hydrophobic sites ratio. It was found that
the values of the summary thermal effect lowers upon the raise of the quantity of silanol groups. It was shown that thermal
stability of composition materials is higher in comparison with initial organosilica sorbents.
Authors:K. Venkatesan, N. Sati Sasidharan, and P. Wattal
The amphoteric nature of hydrous silica-titania gel and its sorption behaviour towards cesium were studied. In NaNO3 solution, the point of zero change (pHpzc) of the gel was found to be 4.22 by pH-titration. The fraction of protonated, deprotonated and neutral surface hydroxyl groups as a function of pH have been computed. Sorption of cesium increased with the increase of pH, reached maximum at a pH of 7 followed by a plateau. Significant uptake of cesium was observed even when the pH was less than pHpzc. Nearly 70% sorption was observed at pHpzc. The free energy of specific adsorption was found to be –18.7 kJ·mol–1.
Authors:L. Zane Miller, James J. Rutowski, Jonathan A. Binns, Guillermo Orts-Gil, D. Tyler McQuade, and Jeremy L. Steinbacher
We present a rapid approach for forming monodisperse silica microcapsules decorated with metal oxide nanoparticles; the silica–metal oxide composites have a hierarchical architecture and a range of compositions. The details of the method were defined using titania precursors. Silica capsules containing low concentrations of titania (<1 wt. %) were produced via an interfacial reaction using a simple mesofluidic T-junction droplet generator. Increasing the titania content of the capsules was achieved using two related, flow-based postsynthetic approaches. In the first approach, a precursor solution containing titanium alkoxides was flowed through a packed-bed of capsules. The second approach provided the highest concentration of titania (3.5 wt. %) and was achieved by evaporating titanium precursor solutions onto a capsule packed-bed using air flow to accelerate evaporation. Decorated capsules, regardless of the method, were annealed to improve the titania crystallinity and analyzed by optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (PXRD), and Fourier transform infrared (FT-IR) spectroscopy. The photocatalytic properties were then compared to a commercial nanoparticulate titania, which the microcapsule-supported titania outperformed in terms of rate of degradation of an organic dye and recyclability. Finally, the generality of the flow-based surface decoration procedures was demonstrated by synthesizing several composite transition metal oxide–silica microparticle materials.
Fluorinated silica gels at various fluorine content were prepared via sol-gel by hydrolysis of 3,3,3-trifluoropropyltrimethoxysilane
and tetraethoxysilane mixtures. The gels, of nominal stoichiometry Si(CH2CH2CF3)XO(2-X/2)(X=0.1-1), were characterized by FT-IR, X-ray photoelectron spectroscopy (XPS) and N2 adsorption analysis. The thermal stability of the fluorinated samples was investigated by coupling thermogravimetric measurements
with mass spectrometric and gas chromatographic analyses of the evolved gaseous species. The chemical reactions occurring
in the gel matrices during heating were siloxane chain rearrangements involving condensation between residual hydroxyl and
ethoxyl groups in the 100-350C temperature range, whereas the thermal decomposition of the fluoroalkyl groups were observed
at higher temperatures (450-600C). The release of the fluoroalkyl moieties also involved C-F/Si-O bond exchanges inside the
siloxane chains, with gas-phase evolution of different fluorinated silicon units.
Thermogravimetric analysis of silica gel has shown that the loss in weight between 30° and 910°C can be quantitatively explained
on the basis of water being lost from three distinct and different populations of sites on the silica gel surface. The results
indicate that the site energies of the three different populations are randomly distributed and, consequently, the resulting
weight loss steps from each population can be described by the integral of a simple normal distribution with temperature.
The calculated weight loss obtained by assuming three different site-groups having randomly distributed adsorption energies
is, within experimental error, coincident with the experimental data. It is also shown that the water evolved from the second
population of sites originates from strongly bound water and may also contain water generated by the condensation of (geminal)
silanol groups contained in the overlapping and neighbouring population.
Two mixtures of pesticides have been separated by two-dimensional thin-layer chromatography with adsorbent gradients of the type silica-wettable with water octadecyl silica or silica-cyanopropyl. The pesticides were identified by
values in both chromatographic systems and by comparison of UV spectra.
Silica-titania gels containing 25, 50 and 75 mol% TiO2 were prepared and the sorption of alkali and alkaline earth metal ions by these materials has been studied. Distribution coefficient values for Cs+ and Sr2+ ions were found to pass through broad maxima as a function of TiO2 content except in the case of Sr under alkaline conditions where there was a continuous increase. Capacity values also increased with TiO2 content and samples containing 50% (for K+ and Cs+), 75% (for Li+ and Na+) and 25%/50% (for Ca2+, Sr2+ and Ba2+) TiO2 exhibited maximum capacities. However, unlike with alkali metals, capacities of a given sorbent for the three alkaline earth ions were almost the same. Large capacities obtained for the latter ions seem to indicate a mineral-forming reaction with 25% and 50% materials. On the other hand, the 25% TiO2 gel seems to sorb Sr at trace level by an ion exchange mechanism. Coupled with its Cs sorption capability, this material may find potential use in large scale decontamination of low level waste solutions.
Authors:K. Draoui, R. Denoyel, M. Chgoura, and J. Rouquerol
The adsorption of the paraquat molecule on silica, kaolinite, illite and montmorillonite was studied with regard to both the determination of adsorption isotherms and the microcalorimetric measurement of adsorption enthalpies. Surface concentration at saturation can be related to the neutralization of the surface charge. Part of the observed variation in adsorption enthalpies from one mineral to another can be attributed to the degree of fitting of the adsorptive molecule to the spatial repartition of surface charges. A particularly interesting system is that involving montmorillonite, where there is evidence of the possibility of neutralization, by the same paraquat molecule, of two sites relating to different platelets.