SiO2 Xerogels Research Articles

SELECTION OF SUITABLE CATALYSTS FOR PRODUCTION OF NEW NANOCOMPOSITE MATERIALS

The article is devoted to the principles of hydrolytic polycondensation of tetramethoxysilane (TMOS) in an alkaline environment. Monolithic samples of SiO2 particles and xerogels were synthesized as a result of hydrolytic polycondensation of tetramethoxysilane in alkaline medium. Amine catalysts - pyridine derivatives were used to carry out the process. As a result, samples of different sizes were synthesized depending on the concentration of substances taken as catalysts. Smaller diameter particles were obtained at lower amine concentrations and larger diameter particles at higher concentrations. As the pKa value of the catalysts used in the synthesis of xerogels increases, certain increases and decreases in the properties of the resulting particles have been observed. Thus, with an increase in the pKa value, the density and hardness of the samples decreased, the percentage of SiO2 increased, and at the same time, the gelation time in the sol-gel system decreased. Images of the synthesized particles were obtained using a scanning electron microscope (SEM). Keywords: sol-gel, hydrolysis, polycondensation, tetramethoxysilane, xerogel, pKa, catalyst, SEM.

Origin of the Springback Effect in Ambient-Pressure-Dried Silica Aerogels: The Effect of Surface Silylation

Ambient pressure drying (APD) can prospectively reduce the costs of aerogel fabrication and processing. APD relies solely on preventing shrinkage or making it reversible. The latter, i.e., the aerogel re-expansion after drying (so-called springback effect-SBE), needs to be controlled for reproducible aerogel fabrication by APD. This can be achieved by an appropriate surface functionalization of aerogel materials (e.g., SiO2). This work addresses the fabrication of monolithic SiO2 aerogels and xerogels by APD. The effect of several silylation agents, i.e., trimethylchlorosilane, triethylchlorosilane, and hexamethyldisilazane on the SBE is studied in detail, applying several complementary experimental techniques, allowing the evaluation of the macroscopic and microscopic morphology as well as the composition of SiO2 aerogels. Here, we show that some physical properties, e.g., the bulk density, the macroscopic structure, and pore sizes/volumes, were significantly affected by the re-expansion. However, silylation did not necessarily lead to full re-expansion. Therefore, similarities in the molecular composition could not be equated to similarities in the SBE. The influences of steric hindrance and reactivity are discussed. The impact of silylation is crucial in tailoring the SBE and, as a result, the APD of monolithic aerogels.

Synthesis of Mesoporous Silica Xerogel from Geothermal Sludge using Sulfuric Acid as Gelation Agent

A large amount of sludge is produced by the geothermal brine at the Dieng Geothermal power plant, exceeding 165 tons per month. This sludge is generally not utilized, except for use in landfills. The precipitate (sludge) is primarily composed of silica. The aim of this research is to synthesis mesoporous silica (SiO2) xerogel from geothermal sludge (GS) and to investigate the effects of pH as an effort to elevate the economic value of sludge through alkaline extraction followed by acidification. SiO2 xerogel was prepared by extracting the GS to become sodium silicate (Na2SiO3) assisted by a base NaOH and precipitated using H2SO4 as a gelation agent. The FTIR analysis of the SiO2 xerogel showed a group of silanol (Si-OH) and siloxane (Si-O-Si). The XRD analysis indicated that SiO2 xerogel was amorphous. Furthermore, it was observed from nitrogen absorption-desorption using BET (Breneur-Emmet-Teller) method test that decreased pH tends to the specific surface area increase, and the pore size becomes decrease. The largest specific surface area observed at SiO2 xerogel prepared at pH of 5.5 reached 400.10 m2/g with a pore size of 4.5 nm. The pore sized for all cases was in the range of 4 ~12 nm, indicating that the SiO2 xerogels were mesoporous. Pore size of the as-prepared silica affected the thermal stability property of the sample.

Synthesis of Pd(0) Nanoparticles in TiO2–SiO2 Xerogel

Pd/TiO2–SiO2 xerogels containing 0.15–0.75 wt % palladium are prepared from Pd(NO3)2 · 2H2O solutions in a mixture of tetrabutoxytitanium (TBT) and tetraethoxysilane (TEOS) in a desiccator under acetic acid–water vapors in the batch mode at 20°С followed by drying and calcination of the material at 850°С. IR spectroscopy showed that the presence of Pd(NO3)2 · 2H2O has no effect on the amount of newly formed Ti–O–Si bonds in the double oxide TiO2–SiO2 over the range of salt concentrations studied. X-ray powder diffraction showed that palladium present in the xerogel calcined at 850°С is in the form of Pd(0) nanoparticles. The electron-microscopic morphology study showed that the items prepared (Pd(0) nanoparticles) had near-spherical shapes; their sizes fell in the range 20–230 nm. A means for varying Pd(0) nanoparticle size in the material is to vary the palladium(II) salt concentration at the dissolution stage.

Synthesis of TiO2 and TiO2—SiO2 xerogels by hydrolysis of tetrabutoxytitanium and tetraethoxysilane in acetic acid atmosphere

The TiO2 and TiO2—SiO2 xerogels were prepared by hydrolyzing tetrabutoxytitanium or its mixture with tetraethoxysilane. The synthesis was conducted in an acetic acid—water atmosphere under stationary conditions followed by calcinations. The final TiO2 and TiO2—SiO2 xerogels represented 3D globular pore structure with a narrow globule size distribution. At hydrolysis of terabutoxytitanium, acetic acid vapors favored formation of TiO2 xerogel with a higher surface area compared to the material produced without acidic catalyst.

Preparation and composition analysis of catalysts supported by CuO-CoO-MnO/SiO2 nanocomposite aerogels

Using tetraethyl orthosilicate (TEOS) as Si source, and aqueous solution of Cu, Co and Mn acetates as the precursors, CuO-CoO-MnO/SiO2 nanocomposite aerogels were prepared through different methods, such as sol-gel process, CO2 supercritical drying (SCD) technique, and ethanol SCD technique, respectively. Catalysts supported by the nanocomposite aerogels were prepared by impregnation method. They were used in the synthesis of diphenyl carbonate (DPC). The microstructures of the nanocomposite aerogels and the catalysts were observed with transmission electron microscopy (TEM). The specific surface area, pore size and pore size distribution of the nanocomposite aerogels were determined by the Brunauer–Emmett–Teller (BET) method. The compositions of the nanocomposite aerogels and the catalysts were analyzed by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The results show that nanocomposite aerogel prepared by ethanol SCD technique shows lower transition metals loss compared with that through CO2 SCD technique. The mole fraction of active transition metals in the nanocomposite aerogel prepared through ethanol SCD technique is 13.77 mol%, and that in the ultrafine particles of the nanocomposite aerogel is 67.65%. The mole fraction of active transition metals decreased to some extent after it was used as a catalyst carrier in the synthesis of DPC. The yield of DPC catalyzed by the novel supported catalyst is 26.31 mass%, which is much higher than other porous carriers, such as SiO2 xerogels, SiO2 aerogels, CuO/SiO2 composite xerogels, CuO/SiO2 composite aerogels and CuO-CoO-MnO/SiO2 composite xerogels synthesized in this study.

Physicochemical properties of TiO2–SiO2 xerogel modified by hydrogen peroxide

How the specific surface area and the amorphous-to-crystalline titania phase ratio in TiO2–SiO2 (14 mol % TiO2) xerogels change during the fivefold repeated cycles comprising the hydrogen peroxide treatment of the xerogel followed by drying and calcining of the binary material, was traced by the BET method, X-ray powder diffraction, and IR spectroscopy.

Synthesis of mesoporous SiO2 xerogel/chitosan mixed-matrix membranes for butanol dehydration

Novel mesoporous SiO2 xerogel/chitosan (CS) mixed-matrix membranes (MMMs) were successfully prepared by incorporating sol–gel synthesized SiO2 xerogels into glutaraldehyde (GA) cross-linked CS membranes. The effects of the SiO2 doping amount and the feed temperature on the performance of the separation of butanol and water was also investigated in this study. The 0.25wt% SiO2 xerogel/CS MMMs possessed the best pervaporation performance and the largest PSI value of 1102kg/m2h with a permeate flux of 736g/m2h (Permeability of 3.25×10−5gmm−2h −1Pa−1) and a separation factor of 1498 at a feed temperature of 50°C. This PSI value is not only comparable but also exceeds that of other membranes reported in the literature. The separation factor and the flux of the SiO2 xerogel/CS MMMs in this study clearly exceed the upper limit of the previously reported MMMs. As a result, the as-prepared novel SiO2 xerogel/CS MMMs showed an improved performance during the butanol and water separation process, which raises the possibility of future pervaporation applications using this novel SiO2 xerogel/CS MMMs.

A simple method to ameliorate hierarchical porous structures of SiO2 xerogels through adjusting water contents

In this contribution, hierarchical micro-/meso-porous SiO2 xerogels were successfully prepared through a Stöber methodology coupled with following drying process. The SiO2 xerogels were consisted of nanoparticles of 20–40nm in size with different contents of water. Fourier transform infrared spectroscopy proved that SiOC2H5 and SiOH groups could be formed in SiO2 xerogels. Further analyses declared that the amount of the SiOC2H5 groups decreased while the concentration of SiOH was firstly increased and then suffered a decline with improving contents of water. Besides, the SiO2 xerogels was endowed with controllable micro-/meso-porous structure. Furthermore, the formation mechanism of the micro-/meso-porous SiO2 xerogels was tentatively put forward. As a consequence, SiO2 xerogels with controllable hierarchical micro-/meso-porous structure could act as the smart material for huge development in catalytic fields.

Synthesis of ZrO2–SiO2 and ZrO2–SiO2–Cu(II) xerogels by joint hydrolysis in an aqueous ammonia atmosphere

ZrO2–SiO2 xerogels have been synthesized through hydrolysis of a mixture of tetrabutoxyzirconium and tetraethoxysilane in a desiccator in a vapor of a 15% aqueous NH3 atmosphere. ZrO2–SiO2–Cu(II) xerogels were synthethized analogously through joint hydrolysis of a mixture of the organometallic precursors and copper(II) chloride. The effect of synthesis conditions on the physical and chemical properties of the resulting material has been studied.

Fluorescence and Textural Characterization of Ortho-Amine Tetraphenylporphyrin Covalently Bonded to Organo-Modified Silica Xerogels.

Most of the studies performed with porphyrins involve these species functionalized with peripheral substituents lying on the same macrocyclic molecular plane. The main objective of this work deals with the successful preservation and optimization of the fluorescence of a uncommonly used porphyrin species, i.e. tetrakis-(ortho-amino-phenyl)-porphyrin; a molecule with substituents localized not only at one but at both sides of its molecular plane. In cases like this, it must be stressed that fluorescence can only be partially preserved; nevertheless, intense fluorescence can still be reached by following a twofold functionalization strategy involving: (i) the bonding of substituted macrocycles to the pore walls of (ii) organo-modified silica monoliths synthesized by the sol-gel method. The analysis of both absorption and emission UV spectra evidenced a radiation energy transfer taking place between the porphyrin and the host silica matrix. Our results showed that the adequate displaying of the optical properties of macrocyclic species trapped in SiO2 xerogels depend on the polarity existing inside the pores, a property which can be tuned up through the adequate selection of organic groups used to modify the surface of the pore cavities. Additionally, the pore widths attained in the final xerogels can vary depending on the identity of the organic groups attached to the network. All these facts finally demonstrated that, even if using inefficient surface functionalization species, such as ortho-substituted tetraphenylporphyrins, it is still possible to modulate the pore shape, pore size, and physicochemical environment created around the trapped macrocycles. The most important aspect related to this research deals with the fact that the developed methodology offers a real possibility of controlling both the textural and morphological characteristics of a new kind of hybrid porous materials and to optimize the physicochemical properties of diverse active molecules trapped inside the pores of these materials.

СИНТЕЗ И СРАВНИТЕЛЬНЫЙ АНАЛИЗ ПОРИСТЫХ БИНАРНЫХ ОКСИДОВ ZrO2 – SiO2, СИНТЕЗИРОВАННЫХ НА ОСНОВЕ ХЛОРОКИСИ ЦИРКОНИЯ (IV) И ТЕТРАБУТОКСИЦИРКОНИЯ С ИСПОЛЬЗОВАНИЕМ В КАЧЕСТВЕ МАТРИЦЫ ЦЕЛЛЮЛОЗЫ

The technique of synthesis of porous binary xerogels ZrO2 – SiO­ with use as precursors of zirconium (IV) oxychloride and tetrabutoxyzirconium was developed. Hydrolysis of theprecursorsis carried outin a desiccatorin the atmosphereNH3 + H2Oin steady state conditionsinthe absence of organicsolvents. The technique of synthesis of binary material based onzirconium (IV) oxychloride includes the following stages: impregnation of cellulose plates with aqueous solutions of zirconium (IV) oxychloride;hydrolysis of the salt in the atmosphere NH3 + H2O followed by drying the cellulose oxide composite; impregnation of composite withtetraethoxysilane and a stage of burning out the cellulose constituent. In the case where the tetrabutoxyzirconium acts as precursor, impregnation ofcelluloseis carried out with premixedtetrabutoxyzirconium and tetraethoxysilane. The developed technique allows to obtain ZrO2 – SiO2 xerogels with ZrO2 content up to 50 wt.%. The influence of theprecursorsZrO2 to theporosity andspecific surface areaof binary material was studied. Conditions that provide the formation of Zr–O–Si groups in ZrO2 – SiO2 xerogels were determined.

Ultraviolet and visible up-conversion luminescence of Er3+/Yb3+ co-doped CaF2 nanocrystals in sol–gel derived glass-ceramics

Glass-ceramics containing Er3+/Yb3+ codoped CaF2 nanocrystals have been prepared by using the controlled crystallization above 650°C of the (Yb3+/Er3+) codoped CaF2–SiO2 xerogels. X-ray diffraction and scanning electron microscopy data have revealed the formation of CaF2 nanocrystals of about 16nm size. For pumping at 973nm, this glass-ceramics shows, besides green and red up-conversion luminescence, violet (at ∼408nm) and near-ultraviolet emission (at ∼380nm). The mechanisms responsible for the upconversion luminescence are presented and discussed. The fluorescence lifetimes of the metastable levels responsible for violet, green and red luminescence are measured and their quantum efficiencies are estimated.

Influence of Ti–O–Si hetero-linkages in the photocatalytic degradation of Rhodamine B

The influence of Ti–O–Si hetero-linkages in the degradation of Rhodamine B (RhB) dye over TiO2–SiO2 xerogels is exemplified by XPS analysis. We demonstrate a relationship between the percentage surface content of Ti–O–Si and the rate of photocatalytic degradation of RhB. Our detailed surface investigation revealed that the overall degradation of RhB is enhanced due to the high surface percentage content of Ti–O–Si species, high crystallinity of titania phase, and its effective dispersion on a high surface area porous silica support.

Structural characterization of amorphous and nanostructured bismuth silicate xerogels

Bismuth silicate xBi2O3·(1 − x)SiO2 xerogels, 0 ≤ x ≤ 0.5, doped with gadolinium have been synthesized by sol–gel method. The study aims to evidence by X-ray diffraction (XRD), electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) the structural changes in amorphous Bi2O3–SiO2 xerogels, function of Bi2O3 content and heat treatment. As was proved by XRD, nanostructured samples were obtained after 30 min heat treatment at 400 °C of the as-prepared samples with x > 0, in the mainly amorphous matrices nanocrystals of Bi5.6Si0.5O9.4 phase being developed. The dimension of the nanocrystallites are function of the bismuth content, and vary from few nanometers to 25 nm for samples with x = 0.14 and x = 0.5 respectively. The local order around silicon, reflected by 29Si MAS NMR spectra, is less dependent on both composition and thermal history of the samples, showing that an amorphous silicon rich phase is present in all samples. The changes in the local order around Gd3+ ions, as reflected by the EPR spectra dependence on composition and especially on the heat treatment, support the assumption that a part of these paramagnetic ions is incorporated in the bismuth rich phase.

Synthesis and Characterization of Al<sub>2</sub>O<sub>3</sub>-SiC Nano Composite by Sol-Gel Method and the Effect of TiO<sub>2</sub> on Sintering

In the present article, α-Al2O3 nano powder was synthesized by a simple aqueous sol–gel method by using AlCl3 as precursor. It was shown that the gel calcined at 1000°C, 1100 °C and 1200 °C resulted in the formation of a crystalline α-Al2O3 nano powder. In continue TEOS and saccharose was used to prepare SiO2 xerogels containing carbon nano particles. The conversion of the gel to β-SiC nano powders was accomplished by carbothermal reduction at 1500°C for 1 h in argon atmosphere. In second pace alumina matrix composites with nano particles of 5 vol% SiC were prepared with the addition of TiO2 as sintering aid and densified by pressureless sintering method at 1600°C and 1630°C for 2 h in nitrogen atmosphere. Green pellets were obtained by uniaxial pressing of 137 MPa. Maximum density (97.3%) was achieved at 1630°C. Vickers hardness was 16.5 GPa after sintering at 1630°C. Scanning electron microscopy revealed that the SiC particles were well distributed throughout the composite matrix. The precursors and the resultant powders were characterized by X-ray diffraction (XRD), thermal analysis (STA) and scanning electron microscopy (SEM).

Silica xerogels as pharmaceutical drug carriers

This review focuses on silica xerogels obtained by the sol-gel method and their application as drug delivery systems. SiO2 xerogels are potential biomaterials to be used as matrix materials for the extended and controlled release of different kinds of biologically active agents administered by various routes. The article includes some representative examples that describe the encapsulation of bioactive molecules and model compounds inside a silica matrix produced by the conventional sol-gel method or by ultrasound hydrolysis. The drug release rate from xerogels could be modified by adjusting several parameters, such as the type of precursor, the concentration of the catalyst and drying temperature. In vitro and in vivo studies have shown the efficacy and biodegradability of these composites. The potential application of silica xerogels as drug carrier systems is critically analyzed and discussed.

Sol-gel synthesis of compact nanohybrid structures based on silica gels

Our recent achievements in the synthesis of monolithic SiO2 xerogels (nonfractured upon drying), as well as hybrid composite nanomaterials and silica glasses based on these xerogels, are considered briefly. A number of new promising methods are developed for preparing organic-inorganic nanohybrids, inorganic nanohybrids, and xerogels doped with transition metal and lanthanide complexes that are chemically bound to the silica matrix and exhibit interesting and technically valuable photophysical, photochemical, and electrochemical properties.

SiO2–TiO2 xerogels for tailoring the release of brilliant blue FCF

Xerogels consisting of SiO2 and TiO2 were explored for controlled release of brilliant blue FCF (BBF). Both SiO2 and SiO2–TiO2 xerogels were prepared by way of sol–gel processing, and the BBF release behavior was compared. SiO2–TiO2 xerogels with varying TiO2 content were also studied and the BBF release behavior was determined for each SiO2–TiO2 xerogel. It was found that the release of BBF from SiO2 xerogels can be increased by the addition of TiO2 content, and the amount and rate of BBF released from the SiO2–TiO2 xerogels can be changed by modifying the amount of TiO2 included during the preparation of the xerogels, where the SiO2–TiO2 xerogels with a higher content of TiO2 released a higher fraction of BBF in water media when compared to the release from SiO2–TiO2 xerogels with lower amounts of TiO2. The experimental results have to be explained by a combination of porous structure, in situ dissolution–condensation during the BBF elution and the change of surface chemistry of the xerogel network with the addition of TiO2.

Preparation and luminescence properties of Eu2+ in SiO2 xerogels

Al-doped and B, Al co-doped SiO2 xerogels with Eu2+ ions were prepared only by sol–gel reaction in air without reducing heat-treatment or post-doping. The luminescence characteristics and mechanism of europium doping SiO2 xerogels were studied as a function of the concentration of Al, B, the europium concentration and the host composition. The emission spectra of the Al-doped and B, Al co-doped samples all show an efficient emission broad band in the blue violet range. The blue emission of the Al-doped sample was centered at 437nm, whereas the B, Al co-doped xerogel emission maximum shifted to 423nm and the intensity became weaker. Concentration quenching effect occurred in both the Al-doped and B, Al co-doped samples, which probably is the result of the transfer of the excitation energy from Eu2+ ions to defects. The highest Eu2+ emission intensity was observed for samples with the Si(OC2H5)4:C2H5OH:H2O molar ratio of 1:2:4.