Silica Xerogel Samples Research Articles

Imidazolium-based ionic liquid silica xerogel as catalyst to transform CO2 into cyclic carbonate

In this work silica xerogel samples containing different imidazolium ILs (EMIM-CF3SO3, EMIM-MSO3, BMIM-Cl, MBMIM-TF2N, BMIM-TF2N and EMIM-TF2N) were synthesized, characterized and used as catalyst in cyclic carbonate synthesis. ILs mass percentage was varied from 5% to 25%. The effect fostered by the ILs mass variation in the synthesis of silica xerogel was observed both in the materials characterization as well as in the performance of these materials as solid catalysts in the cyclic carbonate synthesis from the cycloaddition reaction of CO2 with propylene epoxide. The obtained silica xerogel samples (SXs) were characterized by FTIR, RAMAN, TGA, SEM and TEM. The selectivity of the cycloaddition reaction was analyzed by GC and 1H and 13C NMR. The best results were obtained for SX-EMIM MSO3 (20% of IL) and SX-BMIM Cl (15% of IL) with propylene carbonate yields of 91.4% and 83.4% and selectivities >99% and 97.4% respectively.

A systematic study of the hydration and drying process of silica xerogels using Cu(II) EPR spectroscopy

The influence of the hydration and drying process on the line shape and signal intensity of the electron paramagnetic resonance spectra recorded from Cu(II) ions present in silica xerogels calcined at various temperatures was investigated. The experimental Cu(II) electron paramagnetic resonance spectra were found to consist of a superimposition of three individual subspectra (Γ1, Γ2 and Φ), which reflect different local environments in which the Cu(II) ions were located. The results demonstrate that: (i) Within experimental error, the spin Hamiltonian parameters of each individual subspectrum remain, in the course of the experiments, identical. (ii) The hydration process changed the relative contribution from the individual subspectra (Γ1, Γ2 and Φsignificantly, and increased the overall electron paramagnetic resonance signal intensity by a factor of more than ten, as compared with the non-hydrated silica xerogels. (iii) On re-drying the hydrated silica xerogel samples, the original line shape and original signal intensity values were restored. Thus, measurement of the relative contributions of the individual subspectra can be used as a sensitive method with which to monitor the hydration/drying process in silica xerogels. As a caveat, we conclude that the influence of the hydration/drying process should be taken into account in the interpretation of Cu(II) electron paramagnetic resonance spectra of calcined silica xerogel samples, which provides the real novelty of the present report. Relative contribution of subspectra (Γ1, Γ2 and Φ) to Cu(II) EPR spectra of silica xerogels calcined at the temperatures quoted and then hydrated for 15 min and for 3 days.

Effect of gamma rays on the structural and optical properties of a new hybrid monolith silica sucrose xerogel

A new hybrid monolithic silica xerogel containing sucrose has been synthesized via the sol–gel method. The resulting sucrose silica xerogels were exposed to gamma radiation with a dose in the range of 10–100kGy. The effect of gamma rays on the physical and structural properties of sucrose silica xerogels was investigated by Fourier transform infrared spectroscopy (FTIR) and UV visible spectroscopy. The results of FTIR spectroscopy show that the irradiation process causes changes in the network's structure by inducing defects. The UV visible spectra present three main band defects; E′ center, Non-Bridging Oxygen Hole center (NBOHC) and carbonyl radicals in irradiated sucrose silica xerogel samples. Additionally, the results show that the values of optical band gap energy depend on irradiation. Therefore, generated defects suggest that the behavior of sucrose silica xerogels was modified from an insulator (Eg=5.82eV) to a semiconductor (Eg=3.16eV) under gamma irradiation.

Structural and optical properties of Eu3+/Gd3+ ions in silica xerogels and powders obtained by sol–gel method

The xerogels and sol–gel powders with different quantitative composition have been studied based on excitation and emission measurements as well as luminescence decay analysis. The structure of prepared silica sol–gel materials was examined using FT-IR spectroscopy technique. The photoluminescence spectra of Eu3+ ions were registered upon two different excitation wavelengths related to direct excitation of Eu3+ ions (λexc = 393 nm) and indirect excitation through energy transfer process from Gd3+ to Eu3+ (λexc = 273 nm). Upon direct excitation of Eu3+ (7F0 → 5L6 transition) the characteristic emission bands assigned to the 5D0 → 7FJ(0–4) electronic transitions were observed. Also, it was found that enhanced luminescence of Eu3+ is a result of change the excitation parameter through energy transfer phenomenon (8S7/2 → 6IJ transition of Gd3+) and occurred in both xerogels and powder samples. In this way, a UV photon absorbed by Gd3+ is converted into visible light emitted by Eu3+ ions. Obtained results clearly indicated that high concentration of Gd3+ ions in powders led to particularly strong enhance the emission in red spectral range and allowed for more than 12-times prolongation of luminescence lifetime for the 5D0 excited state of Eu3+ compared to silica xerogel samples. The values of energy transfer efficiency were also estimated based on luminescence decay times of the 6P7/2 state of Gd3+ ions.

Partial crystallization in exotic phase stishovite of amorphous silica xerogels induced by incorporation of chlorophyll species

Using the Rietveld refinement, we analyze the structural evolution under thermal treatment of silica xerogel samples with incorporation of extract from frozen spinach leaves, prepared by sol–gel method with low molar ratio (R) of water to TEOS of R = 1, and R = 5. Partial crystallization of silica xerogel in the form of stishovite has been obtained at atmospheric pressure conditions and temperatures of 200 °C, they both, very lower than those specified by the phase diagram, in samples containing chlorophyll aggregates dispersed in amorphous silica. The results show that the incorporation of spinach extracts leave together with low water contents induce devitrification of the silica matrix in exotic phase stishovite.

B12-induced formation of stishovite in sol–gel produced amorphous SiO2 matrices

Composites containing vitamin B12 (cyanocobalamin) dispersed in amorphous silica xerogel were studied structurally as a function of annealing temperature. Silica xerogel samples were prepared by the sol–gel method using an ethanol:H2O:TEOS molar ratio of 4:11.6:1 and loaded with cyanocobalamin. We found that the structure of the cobalamin is unaltered, although decoordination of the benzimidazole nucleobase of B12, whereas the amorphous quartz structure of the matrix is maintained under heat-treatment without low-cristobalite phase transformation, typically of this kind of materials. We found in our samples partial crystallization of the glass matrix in form of stishovite obtained at very lower pressure than those specified by the phase diagram, and temperatures about 400 °C due to the presence of vitamin B12. The presence of stishovite is corroborated by the Rietveld refinement method.

Rapid Sintering of Silica Xerogel Ceramic Derived from Sago Waste Ash Using Sub-millimeter Wave Heating with a 300 GHz CW Gyrotron

In this paper, we present and discuss experimental results from a microwave sintering of a silica-glass ceramic, produced from a silica xerogel extracted from a sago waste ash. As a radiation source for the microwave heating a sub-millimeter wave gyrotron (Gyrotron FU CW I) with an output frequency of 300 GHz has been used. The powders of silica xerogel have been dry pressed and then sintered at temperatures ranging from 300°C to 1500°C. The influence of the sintering temperature on the technological properties such as porosity and bulk density was studied in detail. Furthermore, X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy have been used in order to study the structure of the produced silica glass-ceramics. It has been found that the silica xerogel crystallizes at a temperature of 800°C, which is about 200°C lower than the one observed in the conventional process. The silica xerogel samples sintered by their irradiation with a sub-millimeter wave at 900°C for 18 minutes are fully crystallized into a silica glass-ceramic with a density of about 2.2 g/cm3 and cristobalite as a major crystalline phase. The results obtained in this study allow one to conclude that the microwave sintering with sub-millimeter waves is an appropriate technological process for production of silica glass-ceramics from a silica xerogel and is characterized with such advantages as shorter times of the thermal cycle, lower sintering temperatures and higher quality of the final product.

Coesite Formation at Ambient Pressure and Low Temperatures

Partial crystallization of silica xerogel in the form of coesite has been obtained at low-pressure conditions and temperatures of∼565∘C, in samples containing chlorophyll aggregates dispersed in amorphous silica. Silica xerogel samples were prepared by the sol-gel method using an ethanol:H2O:TEOS molar ratio of 4:11.6:1 and loaded with extracts from frozen spinach leaves. The silica xerogel microstructure of the powders was studied as a function of annealing temperature. It was found that partial crystallization of the glass matrix in the form of coestite was obtained at lower pressure than those specified by the phase diagram. Chlorophyll aggregates were added to the starting solutions which, upon thermal treatments, form small colloidal particles in the glass matrix. The presence of coesite is corroborated by the Rietveld refinement method.

Structural Effects of Heat-Treated Silica Xerogel Induced by Incorporation of Chlorophyll Species

Composites containing chlorophyll aggregates dispersed in amorphous silica are of interest because of their optical attractive properties. The silica powders added with chlorophyll species, prepared by the sol-gel method, were studied using X-ray diffraction, IR spectroscopy and differential temperature analysis. Silica xerogel samples were prepared using an ethanol/H2O/TEOS molar ratio of 4:11.6:1 and loaded with extracts from frozen spinach leaves. The silica xerogel microstructure of the powders was studied as a function of the annealing temperature. We found in our samples partial crystallization of the glass matrix in form of tridymite and cristobalite phases and quenching centers or nonfluorescing aggregates due to denaturation of photosystem promoted by chlorophyll decomposition after400∘C.

Rietveld refinement of amorphous SiO 2 prepared via sol–gel method

Using the Rietveld refinement, we analyze the structural evolution under thermal treatment of silica xerogel samples prepared by the sol–gel method with molar ratio ( R) of water to TEOS of R = 5 and R = 11.66. We refine the structure of compounds using the Maud program and we found the unit cell parameters and atomic positions of the refined silica amorphous atoms for whole samples. The results show us that the amorphous structure is quartz-like or low-cristobalite-like, depending on the molar ratio values and the heat-treatment of the samples. For R = 5 different quartz structures are obtained, whereas for R = 11.66 we obtain close structures in which a transformation of quartz-like amorphous to low-cristobalite amorphous phase occurs at about 600 °C. These results give some scopes to explain the partial crystallization of the silica at relatively low temperatures obtained when metallic species are present.

Structural Study of Silica Xerogel Composites Containing Pd Aggregates

Composites containing Pd aggregates dispersed in amorphous silica are of interest from both a fundamental and applied point of view because of their attractive catalytic properties. The silica powders added with palladium, prepared by the sol-gel method, were studied using X-ray diffraction, and IR- and UV-Vis-spectrophotometry. Silica xerogel samples were prepared using a ethanol/H2O/TEOS molar ratio of 4:11.6:1 and loaded with sodium tetrachloropalladate. The silica xerogel microstructure of the powders was studied as a function of annealing temperature. Attention was paid to the evolution of the glass matrix as well as the palladium aggregates in the SiO2 matrix. We found in our samples partial crystallization of the glass matrix in form of quartz and cristobalite phases with palladium oxide and metallic palladium phase at 1000∘C. The Rietveld refinement method was used in order to determine the percentage of the phase contents.

Annealing Behavior of Silica Gel Powders Modified with Silver Crystalline Aggregates

The annealing behaviour of silica powders added with silver, prepared by the sol-gel method, was studied using X-ray diffraction. Partial crystallization of amorphous SiO2 samples as low as 600°C has been observed. For that, silver needed to be added to the precursor solution in such a way that it formed aggregates. Silica xerogel samples were prepared using a molar ratio ethanol/H2O/TEOS of 4:11.6:1 and loaded with silver in three different ways: in the form of silver nitrate, silver chloride, or chemically synthesised silver fine particles. The microstructure of the silica xerogel powders was studied as a function of annealing temperature. Attention was paid to the evolution of the glass matrix as well as the silver aggregates in the SiO2 matrix. Partial crystallization of the glass matrix was achieved at temperatures much lower than those specified by the phase diagram, independently of preparation method of the silver aggregates.

Influence of the synthesis conditions on the photoluminescence of silica gels

The photoluminescence spectra of silica xerogel samples synthesized with ethanol as solvent and xerogel where the ethanol was exchanged by water before drying are reported. In addition, the photoluminescence spectrum of a silica cryogel synthesized with tert-butanol as solvent was investigated. The samples were modified by formamide. Bands at 2.00, 2.20, 2.32 and 2.46 eV were identified. In the photoluminescence spectra of all samples. The band at 2.00 eV is caused by the presence of silane, and the band at 2.20 eV is connected with the nonstoichiometric composition of silica. The photoluminescence band at 2.32 eV was found to originate from the organic groups of the solvent. The origin of this band are E? defect centers, which is a prominenet paramagnetic defect in conventional a ? SiO2.

Influence of the synthesis conditions on the photoluminescence of silica gels

The photoluminescence spectra of silica xerogel samples synthesized with ethanol as solvent and xerogel where the ethanol was exchanged by water before drying are reported. In addition, the photoluminescence spectrum of a silica cryogel synthesized with tert-butanol as solvent was investigated. The samples were modified by formamide. Bands at 2.00, 2.20, 2.32 and 2.46 eV were identified. In the photoluminescence spectra of all samples. The band at 2.00 eV is caused by the presence of silane, and the band at 2.20 eV is connected with the nonstoichiometric composition of silica. The photoluminescence band at 2.32 eV was found to originate from the organic groups of the solvent. The origin of this band are E’ defect centers, which is a prominenet paramagnetic defect in conventional a – SiO2.

Fluorosensor action of bis-9-anthryl derivatives immobilised in silica xerogel

Abstract Silica xerogel samples with entrapped series of three fluorescent chemosensors of the Ant-R-Ant type, where Ant is an anthryl group and R is a receptor (spacer) with donor atoms, were prepared by the sol–gel process as chemical recognition phases. The absorption spectra characteristic of alkylanthracene are nearly the same for the free and complexed chemosensors in solution and those encapsulated in the silica xerogels. The fluorescence emission of the samples is specifically enhanced owing to the coordination reaction, with Cu II ions in particular. Emission decays can be described by a triple exponential; the lifetimes at room temperature are short. EPR spectra of Cu II ions complexed by chemisorption with the fluorosensors immobilised in silica have a fairly well resolved hyperfine structure and show the tetragonal–octahedral environment of the central ion. The recognition phase with fluorosensor 1 of the type Ant-NOON-Ant (where N and O are donor atoms) in silica can undergo cycles chemisorption–desorption many times.

Kinetics of densification of porous silica gels: a structural and textural study

The present work is devoted to the structural analysis of silica xerogel samples containing controlled pore sizes ranging from 25 to 270 Å. It focuses on the kinetics of densification processes. Porous xerogels were treated in air at the fixed temperature of 1050 °C for periods varying from 10 min to 3 h and were allowed to cool down in open air. They were characterized by density and nitrogen adsorption–desorption measurements, as well as by Raman spectroscopy. The structural studies were based on the evolution of the intensities and profiles of certain characteristic bands in the Raman spectra. Among these are the two defects bands, D 1 and D 2 associated with the vibrations of four- and three-membered silica rings, respectively, and the ν(Si–O–Si) band centered at 430 cm −1 . Analyses allowed a characterization of the structural changes occurring with densification of the amorphous matrix. In addition it was found that all modifications are brought to a halt after a characteristic time which depends on the initial porosity. It is important to note that the final microscopic structure is the same for all samples regardless of the initial pore size.

Structure and dynamics of hydrogenated silica xerogel by Raman and Brillouin scattering

Structure and dynamics of silica xerogel samples are compared with melt quenched amorphous silica by means of infrared, Raman and Brillouin spectroscopes. In particular the attention was concentrated on a family of hydrogenated sol-gel systems whose nominal stoichiometric formula is Si 2O 3H 2, Si 8O 13H 6, Si 4O 7H 2, and Si 8O 15H 2. Thermal treatment of the samples were made at 500°C and at 800°C for 24 h. Samples were cooled at a rate of 0.3°C/min. Densities were measured by pycnometer technique. The densification of the sol-gel network occurs at about 500°C depending on the samples. At about 800°C the structure and the dynamics together with physical properties, such as sound velocities and elastic constants, become indistinguishable from those of glasses derived from melting.