Zirconia aerogels and xerogels: Influence of solvent and acid on structural properties

High-surface-area zirconia aerogels with meso- to macroporosity have been prepared by an acid-catalyzed alkoxide-sol-gel route with tetrabutoxyzirconium(IV) and subsequent high-temperature supercritical drying at 578 K. The effect of solvent (ethanol, propanol, butanol, t-amylalcohol), amount of nitric acid, calcination temperature, and drying method was studied by nitrogen physisorption, X-ray diffraction, Fourier transform Raman and diffuse reflectance infrared Fourier transform spectrosopy, scanning electron microscopy, thermal analysis, and temperature-programmed desorption of NH₃. After calcination in air at 573 or 773 K, the aerogels possess specific surface areas of up to 270 or 180 m² · g^–1, respectively. The use of ethanol as solvent resulted in the highest specific surface areas and pore volumes (up to 1.5 cm³ · g^–1) among all samples studied, whereas bulky t-amylalcohol caused a shift of the maxima of the broad pore size distributions from 30 to 70 nm. With the corresponding xerogels, prepared via the same wet-chemical procedure but evaporatively dried at ambient temperature, butanol resulted in a maximum at 3 nm and t-amylalcohol in a bimodal pore size distribution with maxima at 3 and 15 nm. The variation of the acid-to-alkoxide ratio in the range 0.08–0.12 at a hydrolysis level of 4 did not significantly influence the structural properties of aerogels and related xerogels. In contrast to the aerogels, the xerogels had significantly lower specific surface areas and prominent microporosity. All uncalcined aerogels contained crystalline ZrO₂, whereas the corresponding uncalcined xerogels were X-ray amorphous and crystallized only during calcination at 573 K. Both aerogels and xerogels possessed Brønsted-type and Lewis-type acid sites. With the xerogels, the density of acid sites on the surface was significantly lower. This behaviour is attributed to the higher amounts of organic residues which persisted in and on the xerogels up to 773 K and thus blocked the acid sites partially.     

有机-无机杂化柔性硅气凝胶的制备与表征

以甲基三甲氧基硅烷（MTMS）和四乙氧基硅烷（TEOS）为混合硅源、甲醇为溶剂,通过酸碱两步催化溶胶-凝胶法制备湿凝胶,经超临界流体干燥得到块状二氧化硅气凝胶。用扫描电镜、氮气吸附脱附测试以及热重分析等手段对气凝胶的微观形貌、比表面积、孔径分布、弯曲性、压缩性、热稳定性等进行研究,结果表明：MTMS/TEOS比例会影响气凝胶的微观结构、弯曲和压缩性以及热稳定性,以MTMS/TEOS=8/1制得的气凝胶密度为0.11 g·cm^-3、孔隙率为94.2%、比表面积为693.3 m²·g^-1、最大弯曲角可达92°、最大压缩比例可达41.2%、压缩回弹率为100%。     

The influence of preparation method on the physicochemical properties of titania–silica aerogels

A series of titania–silica aerogels with different compositions were prepared using four different preparation methods. The preparation steps were followed by a high temperature supercritical drying (HTSCD). It was found that application of ethanol and 2-propanol as the solvents for the titania precursor (with or without modification with acetyloacetone) resulted in materials with BET surface area up to 990 m²/g, total pore volume up to 5.6 cm³/g and density as low as 0.041 g cm⁻³. Obtained aerogels were mesoporous materials with the average pore diameter in the range 11–27 nm. It was stated that application of the I method (prehydrolysis) resulted in aerogels with higher average pore diameter than other preparation methods while aerogels with the lowest average pore diameter were obtained using the so-called IV, impregnation method. Anatase form of titania was found in all prepared samples. The prepared aerogels were being applied as catalysts in photodegradation of salicylic acid solution in water. The obtained results suggest a much higher catalytic efficiency of titania, which is present in aerogel than it is in the case of commercial P25 Degussa titanium dioxide.     

Enhanced thermal shrinkage behavior of phenolic-derived carbon aerogel-reinforced by HNTs with superior compressive strength performance

A novel carbon/m-HNTs composite aerogel was synthesized by introducing the modified halloysite nanotubes (m-HNTs) into phenolic (PR) aerogels through chemical grafting, followed with carbonization treatment. In order to explore the best proportion of HNTs to phenolic, the micromorphology of PR/m-HNTs were investigated by SEM before carbonization, confirming 10 wt% of m-HNTs is most beneficial to the porous network of aerogels. The interaction between PR and HNTs was studied by FTIR spectra, and microstructure evolution of the target product-carbon/m-HNTs composite aerogel were illustrated by SEM and TEM techniques. SEM patterns indicated that the carbon/m-HNTs aerogels maintain a stable porous structure at 1000 °C (carbonization temperature), while a ~20 nm carbon layer was formed around m-HNTs generating an integral unit through TEM analysis. Specific surface area and pore size distribution of composite aerogels were analyzed based on mercury intrusion porosimetry and N₂ adsorption–desorption method, the obtained results stayed around 500 m²g^?1 and 1.00 cm³g^?1 (pore volume) without significant discrepancy, compared with pure aerogel, showing the uniformity of pore size. The weight loss rate (26.76%) decreased greatly compared with pure aerogel, at the same time, the best volumetric shrinkage rate was only 30.83%, contributed by the existence of HNTs supporting the neighbor structure to avoid over-shrinking. The highest compressive strength reached to 4.43 MPa, while the data of pure aerogel was only 1.52 MPa, demonstrating the excellent mechanical property of carbon/m-HNTs aerogels.     

Titania aerogels prepared by low-temperature supercritical drying. Influence of extraction conditions

Titania aerogels with meso- to macroporosity and high specific surface area were prepared by varying the conditions of semicontinuous extraction of methanolic titania gels with CO₂. The conditions varied were extraction temperature, extraction duration, and CO₂ in liquid or supercritical state. The resulting titania aerogels were characterised by means of nitrogen physisorption, X-ray diffraction, thermal analysis and transmission electron microscopy. All uncalcined aerogels contained significant amounts of organic residues (12–14 wt% elemental carbon), and remained X-ray amorphous during calcination in air up to 673 K. Thermoanalytical studies showed that crystallization generally occurred in the range 730–745 K. The variation of the extraction temperature at either constant density or pressure of CO₂, the use of either liquid or supercritical CO₂, and the duration of extraction greatly influenced surface area, pore size distribution, and pore volume. The highest specific surface area (623 m² g^–1) and nitrogen pore volume (4.0 cm³ g^–1) were obtained, if the density of supercritical CO₂ corresponded to that of methanol at the lowest temperature applied (313 K). The studies indicate that textural properties can be varied over a wide range by choosing appropriate extraction conditions.     

An investigation of the performance of catalytic aerogel filters

Gas permeable, photoactive and crack-free titania–silica aerogels of high titanium content (i.e., up to Ti/Si = 1) were prepared by two-steps acid–base catalyzed method involving an acid-catalyzed prehydrolysis of silicon alkoxide followed by a base-catalyzed hydrolysis/condensation reactions with a chelated titania precursor. The prepared titania–silica aerogels displayed good mechanical strength (>30 kN m⁻²), large surface area (>550 m²/g), mesoporous structure (8–11 nm) and good gas permeation. The porous aerogels trap and filter airborne particulates and the titania–silica aerogel have a fair performance for aerosol (65%) and bioaerosol (94%) filtrations. The photoactive anatase nano-TiO₂ crystallized within the aerogel displays an order of magnitude higher reaction rate for UVA photooxidation of trichloroethylene compared to commercial Degussa P25 TiO₂. The bactericidal activity of the titania–silica aerogel for Bacillus subtilis cells under UVA was also six orders of magnitude better.     

Effects of calcining conditions of kaolinite on pore structures of mesoporous materials prepared by the selective leaching of calcined kaolinite

This paper describes the effects of calcining conditions of kaolinite on pore structures of the porous materials obtained from the selective leaching of calcined kaolinite using KOH solution. Mesoporous -Al₂O₃ was the predominant crystalline phase in the samples calcined in the temperature range between 950°C and 1050°C for 24 h. The mean specific surface area of these samples was approximately 250 m² · g^–1 and the mean total pore volume was approximately 0.8 ml · g^–1. The pore size distribution curves of these samples showed a sharp peak at around 2–3 nm pore radius. This peak was sharper for the sample calcined at 1000°C for 24 h. On the other hand, the pore sizes of the sample calcined at 1100°C for 24 h increased abruptly to 10–20 nm and this change corresponded to the formation of mullite in the sample. The pore sizes of the samples calcined at 1100°C varied with calcining time. The specific surface area and total pore volume decreased, the longer the calcining time of the samples, and this was correlated with an increase in the amount of mullite in the samples.     

Aerogel of nitrate glycerol ether cellulose based on phase separation in acetone/ethanol mixed solvents system

Nitrate glycerol ether cellulose (NGEC) alcogels are formed in the ternary NGEC/acetone/ethanol system. NGEC aerogels are prepared from NGEC alcogels after solvent exchange and drying under supercritical CO₂ (scCO₂). The aerogels are prepared with various densities and porosities, relating directly to the initial ethanol content. NGEC aerogels had surface areas of up to 183 m² g^?1 and large mesopore volumes with a combination of large macropore volumes and a wide range of mesopore sizes. The aerogels with larger pore size distribution range, average pore diameter, and mesopore and macropore volume were obtained from system with higher ethanol content. The aerogels were further characterized by X‐ray diffraction, Brunauer–Emmett–Teller analysis, electron microscopy, and thermogravimetric analysis. The results showed that the NGEC aerogels clearly retained the crystalline structure from NGEC. Compared with NGEC powders, the thermal decomposition of NGEC aerogel is accelerated and this process becomes more acute. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41405.     

Surface Modification of Silica Aerogels Dried with 2-Methyl-1-Propanol in the Sub-Critical Pressure

Silica aerogels were made by sol-gel techniques using industrial silicon derivatives (polyethoxydisiloxanes, E-40), followed by drying under sub-critical pressure with iso-butanol. The shrinkage (linear), specific surface area, S_BET, SEM, TEM and the pore size distribution of the silica aerogels were investigated. The results show that the shrinkage (linear) is below 5%, diameter of the silica particles is about 6 nm and the pore size of the silica aerogels is 10 nm. The specific surface area of the silica aerogel is 559.2 m²/g. IR and NMR techniques were used to determine the organic groups on the silica matrix, GC/MS was also introduced to analyse the composition of the recycled iso-butanol. The surface modification and the reactions of iso-butanol to the silica aerogel are also discussed.     

Hydrothermal Synthesis of Novel Smectite-Like Mesoporous Materials

Hydrothermal reaction of slurries made from Si-Mg hydrous oxide and water yielded trioctahedral smectite-like mesoporous materials (SMMs). The OH anion had a marked effect on the properties of SMMs under hydrothermal condition whereas Na cation did not influence SMMs properties. With the increase in synthesis pH at 200°C, pore volume, specific surface area and average pore diameter decreased whereas methylene blue adsorption increased in the pH range of 8.5 to 11.4. Therefore, the SMMs having different layer charges could be synthesized. Both micropore and macropore formation rates decreased with increasing synthesis temperature in the range of 125–200°C and the formation of latter increased over 200°C. As a result, the SMMs synthesized at 200°C from slurries of pH 9.0–9.3 revealed the maximum mesopore formation rate: 0.98 and gave a narrow pore size distribution curve of 3–5 nm. The SMMs were characterized by specific surface areas of 243–679 m² g^-1, pore volumes of 0.20–0.48 cm³ g^-1, average pore diameters of 2.2–5.4 nm and methylene blue adsorption values of 0.16–0.96 meq · g^-1.     

High surface area methyltriethoxysilane-derived aerogels by ambient pressure drying

In this paper we report the synthesis of methyltriethoxysilane (MTES) based aerogels by non-supercritical/ambient pressure drying. The alcogels have been aged in different concentrations of silane precursor solutions before drying and aerogels with low density and high porosity were obtained. The 60% vol silane aged aerogel shows a surface area of 416 m²/g with a pore volume of 0.99 cm³/g and a maximum surface area of 727 m²/g was obtained for 80% vol silane aged aerogel. The non-silane aged sample possess a surface area of 471 m²/g with a total pore volume of 0.83 cm³/g. The aerogels show broad pore-size distribution. The FT-IR studies reveal the retention of Si–C bond in the network and the formation of a hydrophobic gel. The ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS-NMR) studies were also employed to characterize the local environment around the silicon atoms and to obtain information on the condensation degree of the gel network. By varying the hydrolysis pH, highly flexible aerogels have also been successfully prepared. The porosity studies on the flexible aerogels are also presented here.     

New carbon materials with high porosity in the 1-7 nm range obtained by chemical activation with phosphoric acid of resorcinol-formaldehyde aerogels

Chemical activation of resorcinol-formaldehyde aerogels with phosphoric acid results in materials containing both intra-particle microporosity (pore volume ∼0.18 cm³g⁻¹ and mean-pore-width ∼1 nm) and inter-particle micro/mesoporosity. The latter forms as a result of partial collapse of the mesopore structure of the organic aerogel and can be controlled by varying the phosphoric acid/organic aerogel ratio. Increasing this ratio leads to higher pore volume and size and it was possible to obtain micro/mesopore volumes as high as 1.23 cm³g⁻¹ with pore widths up to ∼7 nm. Over 90% of these pores were accessible even after blocking all of the ultramicroporosity by pre-adsorption of n-nonane.     

Cu-doped ZrO2 aerogel: A novel catalyst for CO hydrogenation to CH3OH

Sol-gel derived Cu/ZrO₂ aerogels have been prepared and then their potential for CO hydrogenation towards methanol has been explored. These aerogels had very high surface areas (i.e. up to 250 m² g^-1) even after reduction in H₂ at 573 K. X-ray diffraction and transmission electron microscopy showed that fresh aerogels were amorphous and composed of clusters of particles below 5 nm in size. A fraction of these primary particles grew to about 10 nm after 24 h of the catalysed reaction. The fresh aerogel was very active and stable in this methanol synthesis reaction. A relatively strong interaction of the support with the Cu is indicated by TPR and this appears to correlate well with the high activity shown. Certainly, the high activity was not simply attributable to the high surface area of such aerogel samples.     

Large size and low density SiOC aerogel monolith prepared from triethoxyvinylsilane/tetraethoxysilane

Crack-free silicon oxycarbide (SiOC) aerogel monolith was fabricated by pyrolysis of precursor aerogel prepared from triethoxyvinylsilane/tetraethoxysilane (VTES/TEOS) using sol-gel process and ambient drying. Effects of different precursors, the amount of base catalyst (NH₄OH) and the heating rate during pyrolysis on the properties such as monolithicity, bulk density, surface area and pore size distribution of aerogels were investigated. The results show that the crack-free SiOC aerogel can be easily obtained from VTES/TEOS as compared to that of methyltriethoxysilanes/tetraethoxysilane (MTES/TEOS) and phenyltriethoxysilanes/tetraethoxysilane (PhTES/TEOS) precursors. The influence of heating rate during pyrolysis process on shrinkage rate, ceramic yield and surface area of the SiOC aerogels could be ignored, while the variation in the amount of NH₄OH exerted a strong impact on the properties of SiOC aerogels. Increasing the amount of NH₄OH resulted in the decrease of bulk density and surface area of SiOC aerogels from 0.335 g/cm³ and 488 m²/g to 0.265 g/cm³ and 365 m²/g. The resultant SiOC aerogels exhibit high compressive strength (1.45–3.17 MPa). ²⁹Si MAS NMR spectra revealed the retention of Si-C bond in the SiOC aerogels after pyrolysis at 1000 °C. The present work demonstrates VTES/TEOS is a promising co-precursors to easily and low cost synthesize large size SiOC aerogel monolith.     

Mesoporous H3PW12O40-silica composite: Efficient and reusable solid acid catalyst for the synthesis of diphenolic acid from levulinic acid

Mesoporous H₃PW₁₂O₄₀-silica composite catalysts with controllable H₃PW₁₂O₄₀ loadings (4.0–65.1%) were prepared by a direct sol–gel–hydrothermal technique in the presence of triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer. Powder X-ray diffraction (XRD) patterns and nitrogen sorption analysis indicate the formation of well-defined mesoporous materials. With H₃PW₁₂O₄₀ loading lower than 20%, the materials exhibit larger BET surface area (604.5–753.0 m² g⁻¹), larger and well-distributed pore size (6.1–8.6 nm), larger pore volume (0.75–1.2 cm³ g⁻¹), and highly dispersed Keggin unit throughout the materials. Raman scattering spectroscopy studies confirm that the primary Keggin structure remained intact after formation of the composites. As a novel kind of reusable solid acid catalyst, as-prepared H₃PW₁₂O₄₀-silica composite was applied for the synthesis of diphenolic acid (DPA) from biomass platform molecule, levulinic acid (LA), under solvent-free condition, and remarkably high catalytic activity and stability were observed.     

Monolithic zinc oxide aerogel with the building block of nanoscale crystalline particle

In the present paper, cost-effective zinc chloride has been utilized to synthesize zinc-based aerogel using the epoxide addition sol–gel process. After supercritical drying of wet gels with CO₂, zinc-based aerogels have been prepared. With the subsequently thermal treatment in selected atmospheres and designed heating program, zinc oxide aerogel of high porosity, large specific area (125 m² g^?1), narrow particle size distribution and excellent hexagonal structure was obtained. The microstructures and compositions of zinc oxide aerogels obtained in different atmospheres were characterized. The results showed that decreasing the ratio between oxygen and nitrogen contributed to the maintenance of monolithic aerogels and desirable microstructure. FTIR, XRD, and TG were applied to analyze the chemical and physical changes of the thermal treatment process. Those analyses are able to provide a reference for preparing other metal oxide aerogels. It should be a promising method to design new metal oxide aerogels with attractive nanoarchitectures.     

In situ production of spherical aerogel microparticles

Due to their high surface area, low density, open pore structure and excellent insulation properties aerogels are intensively investigated since the past decades for a diverse range of applications. The current methods of silica aerogel production by supercritical extraction produce monolithic aerogels, where the sol is aged in molds and dried by extraction with supercritical CO₂. Aerogels in the form of spherical microparticles would be beneficial for many applications, for instance, drug delivery for respiratory route; or as insulating materials. However, because of aerogel's mechanical properties, it is difficult, rather impossible, to obtain spherical microparticles by milling or crushing of the monolithic aerogels. This work presents a new method to produce biocompatible spherical aerogel microparticles using an emulsion technique (in situ production) followed by supercritical extraction of the resulted dispersion (gel-oil). Water in oil emulsion was produced by mixing the sol (dispersed phase) with a vegetable oil (continuous phase) followed by the gelation of the dispersed phase. The size distribution of the final gel particles was found to be influenced by agitation, surfactant concentration and sol:oil volume ratios. The gel-oil dispersion was subsequently extracted with supercritical CO₂, Silica aerogel spherical microparticles with a surface area of 1100 m²g⁻¹, pore volume of 3.5 cm³/g and different mean particle diameters ranging from 200 μm to a few millimeters were produced using the presented method.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

Impact of supercritical drying and heat treatment on physical properties of titania/silica aerogel monolithic and its applications

TiO₂–SiO₂ monolithic aerogels were homogeneously prepared using sol–gel method. Critical point of drying of TiO₂–SiO₂ gels with ethanol was studied for 30, 60, 90 and 120 min. Subsequently, the gels were dried with supercritical ethanol, resulting in amorphous aerogels that crystallized following heat treatment at 550 °C from 1 to 5 h. The TiO₂–SiO₂ aerogels were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements. The molar ratio of SiO₂:TiO₂ was 6 and the synthetic strategy revealed that TiO₂–SiO₂ aerogel, had a surface area 868 m²/g, particle size 40 nm, density 0.17 g/cm³ and 80% porosity. The finding indicated that from economic point of view, TiO₂–SiO₂ gel should be supercritical dried for 30 min and heat-treated for 5 h. The TiO₂–SiO₂ aerogel monoliths photocatalyst synthesized using sol–gel method provided insight into the characteristics that make a photocatalyst material well-suited for photodegradation of phenol and cyanide in an industrial waste stream containing Cl⁻, S²⁻ and NH₄⁺. Interestingly, after multiple reuse cycles (i.e. ≥7), photodegradation systems with regenerated photocatalyst showed a slightly decreasing of photoactivity 2–4%. The overall kinetics of photodegradation of either phenol or cyanide using TiO₂–SiO₂ aerogel photocatalyst was found to be of first order.     

High surface area SiC/silicon oxycarbide glasses prepared from phenyltrimethoxysilane-tetramethoxysilane gels

Phenyltrimethoxysilane (PhTMS) was hydrolyzed or cohydrolyzed with tetramethoxysilane (TMOS) to make aerogels and xerogels. Porous SiC/silicon oxycarbide glasses were prepared by further pyrolyzing these gels in inert atmosphere up to 1500°C. The pore structure and chemical nature were studied by nitrogen and water sorption measurement, chemical analysis, X-ray diffraction and scanning electron microscopy. It has been found that the addition of PhTMS into TMOS gels decreased the surface area and porosity of TMOS gels, but enhanced their hydrophobicity and thermal stability. Pyrolyzing 25 mole% PhTMS-75 mole% TMOS aerogel in argon resulted in porous silicon oxycarbide glass which has a surface area of 581 m²/g at 1000°C. Pyrolyzing pure PhTMS gels at 1400°–1500°C produced porous SiC/C/silicon oxycarbide composites having surface areas in the range of 400–500 m²/g.Also with the Department of Agronomy.