Silica Wet Gels Research Articles

Ambient-Dried Silica Aerogel Powders Derived from Coal Gangue by Using One-Pot Method.

In this paper, we report a new and convenient method for the synthesis of insulating aerogel by recycling solid waste coal gangue, which can reduce the industrial production cost of silica aerogels and realize high value-added utilization of solid waste. Sodium silicate was prepared from a cheap industrial waste coal gangue as the precursor for silica aerogels, which was used for silica wet gel preparation by a one pot method; this method of solvent exchange/surface modification was carried out quickly by mechanical stirring process, and the wet gels derived from coal gangue were dried under ambient pressure condition. A high surface area (~748 m2/g) nanostructured aerogel with a 3D open porous microstructure was synthesized, which exhibits a low density (~0.18 g/cm3) and a superior thermal insulation performance (~0.033 W·m−1·K−1). More significantly, the synthetic yield of silica aerogel powder by recycling coal gangue can reach 92%.

Hydrophobic silica aerogels synthesized in ambient conditions by preserving the pore structure via two-step silylation

Surface modification, which is simply about the end-capping of the reactive silanol groups located in silica wet gel to satisfy hydrophobic behavior, has great importance in the production of silica aerogels in ambient conditions. In this study, silica aerogels were synthesized with a sol-gel method via ambient pressure drying. A two-step surface modification was performed on wet gels by using various silanes with different contents (6%, 10% or 20% by vol in n-hexane) to control the extent of irreversible shrinkage during drying. Mono-functional (TMCS) and three-functional silanes (MTMS, MTES and MEMO) were selected for this purpose. The impact of the type and amount of the silylating agents on the microstructure, pore characteristics and hygroscopic nature of resulting aerogels were identified by conducting FTIR, SEM and BET analyses and contact angle measurements. According to characterization results, MTES exhibits a competitive performance compared to classically used TMCS, as the silica aerogels modified with 10% of MTES was obtained in a well-developed with mesoporous structure with very high specific surface area (SBET = 964m2/g) and high hydrophobicity (Θ = 137°). On the other hand, organically functionalized silane MEMO and MTMS, also display noteworthy results (monolithical structure with highly developed porous network and high degree of hydrophobicity) at the low contents (at 6% and 10%). These outcomes are crucial as they may encourage the future attempts on introducing three-functional silanes during surface modification, as these silanes yield high silylation performance and causes the synthesis of well-qualified silica aerogel in ambient conditions.

Effects of processing conditions on porosity features of polyurea cross-linked silica aerogels synthesized by ambient pressure drying

Lightweight porous polyurea-modified silica aerogels have been synthesized through an ambient pressure drying technique. The silica wet gels were prepared by copolymerization and gelation of tetraethoxysilane (TEOS) and 3-aminopropyltriethoxylsilane (APTES) during which amine functional groups were introduced into the skeletal framework. The amine sites then became anchors for cross-linking the silica particles of preformed skeleton by attachment of hexamethylene diisocyanate (HDI), resulting in a conformal polyurea coating on the surface of silica aerogels. The effects of APTES/TEOS volume ratio and HDI concentrations on the evolution of microstructure and pore characteristics of silica aerogels were investigated in detail. The various APTES/TEOS volume ratio did not resulted in a significant change of the density, while pores size in the network structure became larger when the APTES/TEOS volume ratio increased from 10% to 30%. The amount of polyurea cross-linked into the aerogels appeared to vary regularly with the increase of HDI concentration from 0 to 20%. More polyurea was attained when silica wet gels were immersed into the solution of a larger HDI concentration, which rendered the interparticle neck region of silica aerogels wider. The resulting conformal coatings of polyurea reinforced the inorganic silica networks while inevitably occupied some voids at the same time. The investigation provides data for the synthesis of tailor made polyurea-modified silica aerogels under ambient pressure drying method.

Experimental deconvolution of depressurization from capillary shrinkage during drying of silica wet-gels with SCF CO2 why aerogels shrink?

Silica aerogels are prepared by drying wet-gels under conditions that eliminate surface tension forces, typically by exchanging the pore-filling solvent with liquid or supercritical fluid (SCF) CO2 that is vented off like a gas. Thereby, silica wet-gels should not shrink during drying, but they do. According to the literature, most shrinkage (~71%) happens during depressurization of the autoclave. Here, based on prior literature, and working with wet-gels obtained via base-catalyzed gelation of tetramethylorthosilicate (TMOS), the basic hypothesis was that depressurization shrinkage takes place at the primary/secondary particle level. For this to happen there has to be available space to accommodate merging secondary particles, and a driving force. Secondary particles are mass fractals (by SAXS) and their empty space can accommodate primary particles from neighboring assemblies. The driving force was assumed to be H-bonding developing between surface silanols as soon as all fluids are removed from the pores. That hypothesis was put to test by replacing gelation solvents with nonhydrogen bonding toluene or xylene. Indeed, while the total drying shrinkage of toluene- or xylene-filled wet-gels was equal to that observed with aerogels obtained from acetone-filled wet-gels (~8–9%), the major part of that shrinkage (~74%) was transferred to the wet-gel stage. The remaining shrinkage (~26%) was assigned to interfacial tension forces between the pore-filling solvent and liquid or SCF CO2. Having transferred the major part of drying shrinkage to the wet-gel stage has technological implications, because it is easier to manipulate gels at that stage. Furthermore, our results underline that optimization of the drying process should take into account the fact that drying of silica wet-gels into aerogels is a two-stage moving boundary problem.

Synthesis and Characterization of Copper-Nanoparticle-Containing Silica Aerogel Prepared via Rapid Supercritical Extraction for Applications in Three-Way Catalysis

ABSTRACTCopper-alumina and copper-silica aerogels formed by impregnation of a copper(II) salt into an alumina or silica wet gel before supercritical extraction have been found to contain copper in multiple oxidation states: Cu0, Cu+1 and Cu+2. These aerogels are effective at catalyzing the reduction of NO and the oxidation of HCs and CO under conditions similar to those found in automotive three way catalysts. In this work we have developed a preparation method incorporating Cu0, Cu+1 and Cu+2 nanoparticles directly into silica aerogels. Nanoparticles in the form of (a) Cu0 nanorods (100 nm diameter, 10-20 μm length); (b) Cu+1 nanoparticles (350 nm diameter); and (c) Cu+2 nanoparticles (25-55 nm diameter) were added (0.5-15% by weight) to separate precursor mixtures consisting of tetramethyl orthosilicate, methanol, water and ammonia. These precursor mixtures were then processed using a rapid supercritical extraction (RSCE) method to form aerogels. The resulting aerogels show evidence of nanoparticles dispersed throughout the silica aerogel structure. Addition of Cu+1 and Cu+2 nanoparticles decreases the surface area of the aerogels significantly. X-Ray diffraction shows that regardless of initial oxidation state of the nanoparticles, crystalline Cu0 is detected after RSCE processing to 290 °C. Following heat treatment at 700 °C, crystalline Cu+2 is detected. The copper containing silica aerogels are found to be catalytically active with light-off temperatures (50% conversion) for NO and CO at 400 °C in three-way catalytic applications.

The relationship between the silica wet-gels relative optical transmittance at 500 nm and structural features of silica aerogels

Abstract Aerogels are usually synthesized from supercritical drying of wet-gels. High quality aerogels are usually characterized with high specific surface area (measured by BET method here) and high degree of polymerization of silicate (characterized by 29 Si NMR here). The quality of the aerogels depends highly on the quality of the corresponding wet-gels because the skeletal structure in the wet-gels changes only slightly in the supercritical drying process. Here we present a simple method to predict the quality of the desired silica aerogels before the supercritical drying. We found that the relative optical transmittance of the silica wet-gels at 500 nm had good linear correlation with the specific surface area and the degree of silicate polymerization in the corresponding aerogels. We can assess the quality and screen out the low-quality wet-gels from supercritical drying operation, and greatly reduce the cost for aerogel production.

Hydrophobic silica aerogel derived from wheat husk ash by ambient pressure drying

Silica aerogels from wheat husk ash (WHA) were prepared via a sol–gel process by ambient pressure drying. Silica was extracted from WHA by NaOH solution to form sodium silicate, which was used as precursor for aerogels. Silica wet gels were synthesized by resin-exchange-alkali-catalysis of the sodium silicate solution, followed by solvent exchange with ethanol (EtOH) and hexane in turn. Consequently, a mixture of trimethylchlorosilane, EtOH and hexane was used for surface modification of the wet gels in order to obtain hydrophobic silica aerogels. The density, pore structure, hydrophobic property and thermal insulation property of the obtained silica aerogels were investigated in detail. The results show that the formation of silica aerogels can be successfully realized at a SiO2/H2O weight ratio varying from 0.065 to 0.167. Silica aerogels possess a desirable pore structure with a surface area ranging from 513 ± 5 to 587 ± 6 m2/g, a pore volume from 2.3 ± 0.3 to 4.0 ± 0.1 cm3/g and a pore size from 9 ± 2 to 15 ± 1 nm, an outstanding hydrophobic property with a water contact angle of 147 ± 0.1° and a distinguished thermal insulation property with a low thermal conductivity ranging from 0.009 ± 0.0001 to 0.012 ± 0.0002 W/(m·K).

Effect of surface modification on physical properties of silica aerogels derived from fly ash acid sludge

The silica aerogels were synthesized using fly ash acid sludge via ambient pressure drying. The silica in the fly ash acid sludge was extracted using a NaOH solution to form the sodium silicate solution. Silica wet gels were formed by sol–gel process of the obtained sodium silicate solution. To prevent the condensation and shrinkage of the wet gel structure during drying, the surfaces of wet gel were modified using trimethylchlorosilane (TMCS) via solvent exchange and surface modification. The effects of surface modification on the textural and physical properties of the silica aerogels were investigated. The properties of the synthesized silica aerogels were analyzed with SEM, BJH, FT-IR, DTA-TG and contact angle measurement. The results indicated that the silica gel modified using TMCS could undergo reversible shrinkage during ambient pressure drying and thereby preserved mesoporous structure. The modified aerogel had much lower density (0.085 g/cm3), higher specific surface area (700 m2/g) and larger pore volume (3.29 cc/g) than those of the unmodified aerogel. The contact angle measurements indicated that the modified aerogel had a hydrophobic property with contact angle of 140°.

Ionic liquids as dynamic templating agents for sol–gel silica systems: synergistic anion and cation effect on the silica structured growth

This work presents a detailed investigation about the influence of a complementary set of imidazolium ionic liquids (IL: [C4MIm][BF4]–[C10MIm][BF4]–[C7O3MIm][BF4]–[C4MIm][NTf2]–[C10MIm][NTf2]–[C7O3MIm][NTf2]) in the preparation of hydrolytic sol–gel silica wet gels and xerogels, providing insights into their hierarchical structures, morphologies of primary particles and aggregation domains. The reported IL decreased dramatically the gelation time (up to ~500 times), evolving the systems to transparent or opalescent wet gels with distinct consistencies, or silica powders. Characterization of the obtained silica xerogels was performed by transmission and scanning electron microscopy, atomic force microscopy, X-ray diffraction, small-angle X-ray scattering and thermogravimetric analysis. In general, the IL allowed controlling the silica particle size, color, compactness, structures and morphology. The xerogels showed homogenous and ordered structures (spherical or triangular) of different sizes and with distinct microporosities, depending on the IL applied. These results highlight the key role of IL in the sol–gel silica synthesis with controlled properties, which can be further tuned for specific applications by changing small process parameters.

Mass and surface fractal in supercritical dried silica aerogels prepared with additions of sodium dodecyl sulfate.

Silica wet gels were prepared from hydrolysis of tetraethoxysilane (TEOS) with additions of sodium dodecyl sulfate (SDS). The surfactant was removed after gelation. Wet gels exhibited mass-fractal structure with mass-fractal dimension D (typically around 2.25) in a length scale extending from a characteristic size ξ (typically about 10 nm) of the mass-fractal domains to a characteristic size a0 (typically between 0.3 and 0.4 nm) of the primary particles building up the fractal domains. ξ increased while D and a0 diminished slightly as the SDS quantity increased. Aerogels with typical specific surface of 1000 m(2)/g and density of 0.20 g/cm(3) were obtained by supercritical drying of the wet gels after washing with ethanol and n-hexane. The pore volume and the mean pore size increased with the increase of the SDS quantity. The aerogels presented most of the mass-fractal characteristics of the original wet gels at large length scales and exhibited at a higher resolution level at about 0.7 nm a crossover to a mass-surface fractal structure, with apparent mass-fractal dimension Dm ∼ 2.4 and surface-fractal dimension Ds ∼ 2.6, as inferred from small-angle X-ray scattering (SAXS) and nitrogen adsorption data.

Structure of Hydrophobic Ambient-Pressure-Dried Aerogels Prepared by Sonohydrolysis of Tetraethoxysilane with Additions of N,N-Dimethylformamide

Silica wet gels with the same silica content were prepared by the sonohydrolysis of tetraethoxysilane (TEOS) with additions of dimethylformamide (DMF). DMF plays a role in the overall hydrolysis/gelification/aging step of the sol-gel process, providing more consolidated wet gels with larger syneresis degrees and densities. The structure of the as-obtained wet gels can be interpreted as being built up of mass-fractal domains with fractal dimension D = 2.2 and radius of gyration decreasing from about 14 to 12 nm with increasing quantity of DMF. Monolithic hydrophobic aerogels were prepared after washing of the wet gels with isopropyl alcohol (IPA), silylation with trimethylchrorosilane (TMCS), and ambient-pressure drying (APD). The specific surface area of the APD aerogels was found to be about 900 m(2)/g, and the mean silica particle size was about 2.0 nm, approximately independent of the DMF quantity, whereas the porosity decreased slightly with increasing amount of DMF, fairly accompanying the behavior of the radius of gyration of the precursor wet gels. The mass-fractal characteristics were preserved in the APD aerogels, but the radius of gyration of the mass-fractal domains was reduced to values between 2.8 to 4.0 nm, with the values decreasing slightly with the DMF quantity, and the fractal domains developed a surface-mass-fractal structure with the overall washing/silylation/APD treatment. The structural characteristics of the APD aerogels as determined by SAXS were found to be in notable agreement with those inferred from nitrogen adsorption.

Effect of various structure directing agents on the physicochemical properties of the silica aerogels prepared at an ambient pressure

We studied the effects of various surfactants on the textural properties (BET surface area, pore size, and pore volume) of the silica aerogels prepared at an ambient pressure. A simple surface modification of silica gel prepared at an ambient pressure through hydrolysis and polycondensation of TEOS as a silica precursor was conducted using various structure directing agents. The treatment was found to induce a significant difference in the porosity of the silica aerogel. Highly porous silica aerogels with bimodal porous structures were prepared by modifying the surface of the silica wet-gel (alcogel) with trimethylchlorosilane (TMCS) in order to preserve its porosity. The samples were analyzed by small-angle X-ray scattering and nitrogen adsorption. In this work, a possible new type of highly porous hydrophobic silica aerogel with a bimodal porous structure is presented. A hydrophilic extremely porous (high surface area and large pore volume) silica aerogel was obtained by heating the as-synthesized hydrophobic silica aerogel at 400°C for 1h. There was a significant effect of structure directing agent on the textural properties, such as specific surface area, pore size distribution and cumulative pore volume of the silica aerogels.

Fabrication of spherical silica aerogel/magnetite nanocomposite particles

Fe3O4/SiO2 nanocomposite particles were synthesized from a hydrophobic Fe3O4 suspension, prepared by precipitating Fe (II) and Fe (III) chlorides and using a hydrophobic SiO2 wet gel from a silicic acid solution derived from a sodium silicate (water glass) solution. Fe3O4 nanoparticles having a crystallite size of ~10nm embedded in SiO2 aerogel can be successfully fabricated by mixing the Fe3O4 suspension and silica wet gel in n-hexane. It was clearly observed that the Fe3O4 nanoparticles were well crystallized and homogeneously dispersed in the SiO2 aerogel. The Fe3O4/SiO2 nanocomposite particles exhibited superparamagentic behavior. The saturation magnetization was determined to 34.5, 18.2, and 8.3emu/g for 90wt% Fe3O4, 70wt% Fe3O4, and 50wt% Fe3O4/SiO2 nanocomposite particles, respectively.

Fabrication of Spherical Silica Aerogel Granules from Water Glass by Ambient Pressure Drying

Spherical silica aerogel granules were fabricated from water glass. A silicic acid solution was mixed with n‐hexane and a surfactant, which was used as a stabilizer, to produce silicic acid droplets in n‐hexane. Ammonia was used to gelate the silicic acid droplets and the silica hydrogel droplets were subsequently surface‐modified using a TMCS (trimethylchlorosilane)/n‐hexane solution followed by solvent exchange from water to n‐hexane. Silica wet gel droplets were dried at 80°C under ambient pressure. A perfect spherical silica granule between 230 and 123 μm in diameter was obtained and its size can be controlled by surfactant content. On the other hand, the surfactant had little effect on the particle size distribution and its uniformity lies between 0.332 and 0.454. The tapping density, pore volume, and BET surface area of the silica aerogel granule were ~0.08 g/cm3, 3.0 cm3/g, and 700 m2/g, respectively. FT‐IR spectrum analysis showed that the surface of the silica granules was modified by Si–CH3 groups, which allowed the aerogel to be hydrophobic.

Cost-effective synthesis of silica aerogels from fly ash via ambient pressure drying

Silica aerogels were synthesized from the industrial fly ash by ambient pressure drying method. The process consists of two stages, preparation of sodium silicate solution from fly ash by hydrothermal reaction with sodium hydroxide, and synthesis of porous silica aerogels from the obtained sodium silicate solution. Silica wet gels were formed by vitriol-catalysis or resin-exchange-alkali-catalysis of the obtained sodium silicate solution. The trimethylchlorosilane(TMCS)/ethanol(EtOH)/hexane mixed solution was used for solvent exchange/surface modification of the wet gel so as to obtain porous silica aerogels via ambient pressure drying. The results indicated that the synthesized silica aerogels were lightweight and hydrophobic. The BET specific surface area, pore volume and average pore diameter were 362.2–907.9 m 2 g − 1 , 0.738–4.875 cm 3 g − 1 , and 7.69–24.09 nm respectively. Particularly, the synthesized silica aerogels by resin-exchange-alkali-catalysis method showed uniform mesoporous structure, and had much higher specific surface area (907.9 m 2 g − 1 ) and pore volume (4.875 cm 3 g − 1 ) than that of by vitriol-catalysis process.

Effect of different trimethyl silylating agents on the hydrophobic and physical properties of silica aerogels

An essential feature of the ambient pressure dried aerogel manufacturing process is the end-capping of the reactive silanol groups in the silica wet gel. In this report, we have presented the effect of two different trimethyl silylating agents viz. trimethylchlorosilane (TMCS) and hexamethyldisilazane (HMDZ) on the hydrophobic and physical properties of ambient pressure dried silica aerogels. The hydrogels were prepared by sol–gel processing of sodium silicate precursor (Na 2SiO 3) in the presence of acetic acid catalyst followed by vapour passing treatment and different solvent exchanging steps. The silylating agent in hexane was used for end-capping of the silanols present on the silica surface of the gel. To study silylation behavior silylating agent/Na 2SiO 3 molar ratio was varied from 2.4 to 5.6. The aerogels have been characterized by density, % of volume shrinkage, porosity, % of optical transmission, Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Thermo-gravimetric and Differential Thermal Analysis (TG-DTA) and contact angle measurements.

Optimization of instantaneous solvent exchange/surface modification process for ambient synthesis of monolithic silica aerogels

The instantaneous solvent exchange/surface modification (ISE/SM) process for the ambient synthesis of crack-free silica aerogel monoliths with a high production yield was optimized. Monolithic forms of silica wet gels were obtained from aqueous colloidal silica sols prepared via the ion exchange of sodium silicate solutions. Crack-free silica aerogel monoliths were synthesized via an ISE/SM process using isopropyl alcohol/trimethylchlorosilane as a modification agent and n-hexane as a main solvent, followed by ambient drying. The optimum process conditions of the ISE/SM process were investigated by clarifying the reaction mechanism and phenomena. Most effective ranges of process variables on the ISE/SM stage were determined as 0.2500–0.3567 of TMCS/H 2O (pore water) in molar ratio and 15–30 of n-hexane/TMCS in volumetric ratio, with a reaction temperature below 283 K. Crack-free silica aerogel monoliths synthesized via these conditions had a well-developed mesoporous structure and excellent properties (bulk density of 0.12–0.14 g/cm 3, specific surface area of 724 m 2/g), and a high yield (nearly 80%).

Structural Modifications in Silica Sonogels Prepared with Additions of Poly(vinyl alcohol)

Silica wet gels were prepared from acid sonohydrolysis of tetraethoxysilane (TEOS) and additions of poly(vinyl alcohol) (PVA)−water solution. Aerogels were obtained from supercritical CO2 extraction. The samples were studied by thermal gravimetric (TG) analysis, small-angle X-ray scattering (SAXS), and nitrogen adsorption. The structure of wet gels can be described as a mass fractal with dimension D equal to 2.0 on the whole length scale experimentally probed by SAXS, from ∼0.3 to ∼15 nm. Pure and low-PVA-addition wet gels exhibit an upper cutoff accounting for a finite characteristic length ξ of the mass fractal structure. Additions of PVA increase ξ without modifying D, which was attributed to a steric effect of the polymer in the structure. The pore volume fraction of the aerogels diminishes typically about 11% with respect to that of the wet gels, although nitrogen adsorption could be underestimating some porosity. The pore size distribution of the aerogels is shifted toward the mesopore region with t...

Fabrication of the Monolithic Silica Aerogels Using Sodium Silicate and its Network Strengthening

Silica wet gels were prepared from water glass (29 wt.% SiO2) by using Amberlite as a ion exchange resin. After washing in distilled water, the wet gels were further aged in a solution of TEOS/EtOH to strengthen the 3-dimensional network structure. With the increasing TEOS content in aging solution, BET surface area and porosity of the ambient dried silica aerogel were decreased, and the average pore diameter was decreased from 30nm to 10nm. Also, higher density and compressive strength were obtained in case of higher TEOS content. This is due to the precipitation of SiO2 nano particle by TEOS. Hence, TEOS addition plays an important role in both strengthening and increasing stiffness of silica wet gel network. By adding 30 vol.% TEOS, a crack-free monolithic silica aerogel tiles were obtained and its density, compressive strength, and thermal conductivity were shown 0.232 g/cm3, 7.3 MPa, and 0.029 W/mK, respectively.

Strengthening of Water Glass Based Aerogel by TEOS

In order to prevent the irreversible collapse of the silica aerogel backbone upon the drying of the liquid phase of silica wet gel derived from water glass based silicic acid, we tried to strengthen the aerogel back bone by aging silica wet gels in water and TEOS/ethanol solutions. Although aging of silica wet gels in water has been shown to grow the neck between silica particles of the aerogel backbone, it is hard to obtain a crack-free aerogel monolith. On the other hand, the mechanical stability of silica aerogels was improved significantly by aging the wet gel in TEOS/ethanol solutions with different TEOS content.