Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase,photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.     

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.     

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.     

Zirconia aerogels: Effect of acid used in the sol-gel process on morphological properties

Zirconia aerogels have been prepared from butanolic zirconium(IV) tetra-n-butoxide diluted in ethanol via stoichiometric hydrolysis with water in ethanol. Nitric acid or acetic acid were used to modify the sol-gel process. After calcination in air at 573 K, the aerogel prepared with nitric acid possesses a specific surface area of 240 m² · g^–1 and a unimodal pore size distribution with a maximum at ca. 24 nm, whereas the use of acetic acid results in an aerogel with specific surface area of 228 m² · g^–1 and bimodal pore size distribution with maxima at 3 and 65 nm. The crystalline fractions of both aerogels are predominantly tetragonal with a small contribution of monoclinic ZrO₂.     

Super/subcritical fluid extractions for preparation of the crystalline titania

Various extraction techniques which use fluids in super/subcritical state (CO₂, H₂O, CH₃OH) were used for the preparation of crystalline TiO₂ from surfactant-mediated titania organogels (the amorphous reaction intermediate products of surfactant-mediated sol-gel synthesis). Crystallinity, purity and texture of TiO₂ powders (titania aerogels) were studied in detail for different treatment modes of titania organogels. Suitable combinations of extraction solvents and extraction temperature and pressure were considered for synthesis of TiO₂ powder crystalline phase (anatase) with high specific surface area and purity (carbon content < 0.1 wt.%). Optimum results were achieved by combined extraction procedures—subcritical water extraction and supercritical fluid extraction or pressurised liquid extraction. Moreover, the possible tuning of the TiO₂ powder pore-size distribution by regulation of temperature at constant pressure for the combined subcritical water extraction and pressurised liquid extraction was found.     

A route to highly porous grinding wheels by selective extraction of pore inducers with dense carbon dioxide

A novel route for the production of highly porous vitrified grinding wheels was developed via selective extraction of pore inducers with dense CO₂. The extraction was performed with liquid and supercritical CO₂ (scCO₂) at temperatures ranging from 295 to 338 K, pressures from 8.8 to 27.6 MPa and flow rates of 3.4×10⁻⁵ and 7.5×10⁻⁵ kg s⁻¹ CO₂. The extraction rate was a strong function of temperature, flow rate, and flow direction, while unaffected by particle size of the pore inducer and pressure. The extraction had no detrimental effect on the green wheel’s microstructure. Grinding tests were performed on the CO₂ extracted pore induced wheels and results were compared to those from a conventionally manufactured pore induced grinding wheel. The extracted grinding wheels performed similarly to the conventional wheels. At high metal removal rates, the extracted wheels with large pore sizes outperformed the wheels with smaller particle sizes as well as the conventional wheel. This may be due to the larger pore sizes increasing lubrication at the surface and increasing the wheel strength.     

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.     

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

Titania–silica aerogels with different titania content were prepared. Four preparation methods differing mainly in approach to precursors hydrolysis were applied, while only three of them allowed total hydrolysis of silica precursor before titania precursor was added. The preparation of mixed products of titania and silica hydrolysis precursors containing gels was followed by high temperature supercritical drying (HTSCD) and thermal treatment at 500 °C. Obtained mixed oxides in form of aerogels were characterized by BET surface areas up to 1000 m²/g, mesopore volumes up to 1.6 cm³/g and bulk densities as low as 0.04 g/cm³. Even 18 h lasting aging did not allow to produce narrow diameter range mesoporous materials, their broad pore diameter distributions resulted in average pore sizes varying from 10 to nearly 30 nm. XRD measurements proved the presence of anatase crystalline form of titania, while silica was present in amorphous form. SEM studies indicated presence of isolated titania particles on titania–silica surface while joint hydrolysis method was applied. Titania–silica aerogels obtained by the simultaneous hydrolysis of precursors and the impregnation method showed high photocatalytic activity in degradation of salicylic acid in water. Activities of these mesoporous photocatalysts were higher than commercial P25 Degussa TiO₂. Comparison of activity of pure TiO₂ (P25 Degussa) and aerogels indicates higher utilization of titania present in mesoporous mixed oxides.     

Drying processes for preparation of titania aerogel using supercritical carbon dioxide

Supercritical carbon dioxide drying was performed for the preparation of titania aerogels from sol–gel routes. The conditions of supercritical carbon dioxide drying were 313–323 K and 7.8–15.5 MPa. The solvents in titania wet gels obtained from the sol–gel routes were replaced by acetone. The titania aerogels obtained from supercritical carbon dioxide drying form needle-like structures. In supercritical carbon dioxide drying, the extraction rates of acetone from the wet gels were measured by using an on-line Fourier transform infrared spectroscope. It was found that the titania aerogels with lower cohesion were induced from the formations of homogenous phase for carbon dioxide + acetone system and the lower extraction rates of acetone. Furthermore, titania films were prepared by the depositions of the titania aerogels on ITO-coated PET substrates. The needle-like aerogels with lower cohesion derive the titania film with high surface area.     

Preparation and characterization of polyhedral oligomeric silsesquioxane–titania aerogels

The novel polyhedral oligomeric silsesquioxane (POSS)–titania aerogels which contain different contents of titania were successfully prepared by the sol–gel process and subsequently supercritical drying with carbon dioxide. All the aerogels are monolithic and the densities of those aerogels are low. The FTIR spectra of the aerogels showed the resulting POSS–TiO₂ composite aerogels had homogeneous Si–O–Ti bonds. The microstructure, surface composition and thermal stability were measured by FESEM, XPS and TGA. With the increasing of titania contents, the aggregated particles of the aerogels in the microstructure got larger and larger. The texture of the aerogels was measured by XRD and nitrogen adsorption/desorption and showed that they were amorphous and had high surface area (>500 m²/g).     

Hydrothermal synthesis and characterization of copper containing crystalline silicate mesoporous materials from gel mixture

Novel copper-containing crystalline silicate mesoporous materials (SCMM) have been synthesized by the hydrothermal treatment of slurries of silicon–magnesium–copper hydroxide precipitates along with quaternary ammonium salt. X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirmed a house-of-cards type structure consists of very thin platy silicates. Nitrogen adsorption–desorption isotherms of calcined material show that it has a high surface area (550 m² g⁻¹) and porosity properties. Pore characteristics are similar to that of MCM-41 and FSM-16, and fine-tuning of the pore size was achieved easily by modulating the synthesis temperature. Identification and the location of copper species in Cu-SCMM were done by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR), respectively. ESR data of air-dried Cu-SCMM consist of clearly defined g_||=2.34, A_||=140×10⁻⁴ cm⁻¹ and g=2.08 at room temperature and g_||=2.34, A_||=160×10⁻⁴ cm⁻¹ and g=2.10 at 77 K. The resulting material exhibited superior catalytic activity towards the hydrogenation of α–β unsaturated aldehyde in supercritical carbon dioxide.     

Preparation of controlled porosity carbon aerogels for energy storage in rechargeable lithium oxygen batteries

Porous carbon aerogels are prepared by polycondensation of resorcinol and formaldehyde catalyzed by sodium carbonate followed by carbonization of the resultant aerogels in an inert atmosphere. Pore structure of carbon aerogels is adjusted by changing the molar ratio of resorcinol to catalyst during gel preparation and also pyrolysis under Ar and activation under CO₂ atmosphere at different temperatures. The prepared carbons are used as active materials in fabrication of composite carbon electrodes. The electrochemical performance of the electrodes has been tested in a Li/O₂ cell. Through the galvanostatic charge/discharge measurements, it is found that the cell performance (i.e. discharge capacity and discharge voltage) depends on the morphology of carbon and a combined effect of pore volume, pore size and surface area of carbon affects the storage capacity. A Li/O₂ cell using the carbon with the largest pore volume (2.195 cm³/g) and a wide pore size (14.23 nm) showed a specific capacity of 1290 mA h g⁻¹.     

Drying of resorcinol-formaldehyde gels with CO2 medium

The effect of using real supercritical conditions in the CO₂ drying process on the structure and texture of resorcinol-formaldehyde networks is investigated by low temperature nitrogen adsorption, scanning electron microscopy and by small and wide angle X-ray scattering. If supercritical conditions are maintained throughout the whole extraction process the resulting networks exhibit much more developed porosity. The surface area of the supercritically dried gel, in excess of 500 m²/g, is more than twice that of the sample dried with liquid CO₂. Pore volumes are also significantly higher in all pore classes. In the supercritical region the applied pressure strongly affects the porosity, while the effect of temperature is limited. Drying time also influences the total pore volume of the samples, but not the mesopore and micropore volumes. The volume filling character of the molecular adsorption process in this system is illustrated by the difference in surface areas measured by small angle X-ray scattering and that by nitrogen adsorption.     

Supercritical CO2 Extraction of Chlorophyll a from Spirulina platensis with a Static Modifier

Supercritical CO₂ extraction with a static modifier was applied to extract chlorophyll a from Spirulina platensis. The effects of the process were investigated by single‐factor and response surface analysis experiments. The optimal process parameters for supercritical CO₂ extraction were determined to be: ethanol/water as the modifier, 40 vol.‐% water content in the modifier, 21.2 mL modifier volume, 1 h static soaking time, 2 h dynamic extraction time, 48.7 MPa extraction pressure, 326.4 K extraction temperature, and 10 g min^–1 CO₂ flow rate. The optimized chlorophyll a extraction yield was 6.84 mg g^–1. A comparison of the experimental results suggested that the yield of chlorophyll a by supercritical CO₂ extraction with modifier was higher than that obtained by conventional solvent extraction.     

Synthesis and characterization of monolithic carbon/silicon carbide composite aerogels

Resorcinol–formaldehyde/silica composite (RF/SiO₂) aerogels were synthesized using sol–gel process followed by supercritical CO₂ drying. Monolithic carbon/silicon carbide composite (C/SiC) aerogels were formed from RF/SiO₂ aerogels after carbothermal reduction. X-ray diffraction and transmission electron microscopy demonstrate that β-SiC was obtained after carbothermal reduction. Scanning electron microscopy and nitrogen adsorption/desorption reveal that the as-prepared C/SiC aerogels are typical mesoporous materials. The pore structural properties were measured by nitrogen adsorption/desorption analysis. The resulting C/SiC aerogels possess a BET surface area of 564 m²/g, a porosity of 95.1 % and a pore volume of 2.59 cm³/g. The mass fraction of SiC in C/SiC aerogels is 31 %.     

Matrimid Aerogels by Temperature‐Controlled,Solution‐Based Crosslinking

This paper reports on a temperature‐controlled, solution‐based method to prepare diamine crosslinked Matrimid aerogels. Addition of a diamine to a preheated polymer solution resulted in a well‐dispersed solution, allowing formation of a homogeneous gel upon cooling. The gels (studied by FTIR and AFM) were dried by solvent extraction with supercritical CO₂. The resulting aerogels showed surface areas of approximately 150 m² · g^?1 and porosities of 0.66–0.69 mL · g^?1 with polymer domains and pore sizes of tens of nanometers. A room temperature‐prepared, inhomogeneous aerogel gave approximately 250 m² · g^?1 and 0.31 mL · g^?1 with meso‐ and micropores. SEM images of the aerogels show similar surface features as AFM images of the Matrimid solvent gels.

     

Effects of drying conditions on physicochemical properties of epoxide sol−gel derived α-Fe2O3 and NiO: A comparison between xerogels and aerogels

A simple and easily operated supercritical CO₂ dryer was designed and manufactured with the aim of producing high-surface-area mesoporous α-Fe₂O₃ (hematite) and NiO aerogels. The gels were synthesized by a sol?gel method with the aid of propylene oxide (PO), as the gelation agent, and then dried and calcined at different conditions. The effects of drying and calcination conditions on the physicochemical properties of the final aerogels were investigated using X-ray diffraction (XRD), N₂ adsorption-desorption, Fourier-transformed infrared spectroscopy (FT-IR), and field emission scanning electron microscopy (FE-SEM) analyses. It was demonstrated that α-Fe₂O₃ and NiO aerogels with high surface areas and mesoporosities could be successfully synthesized using the home-made supercritical CO₂ dryer. Supercritical drying of the gels resulted in α-Fe₂O₃ (186 m²/g) and NiO (178 m²/g) aerogels with 186% and 34% higher surface areas, respectively, than xerogels obtained via simple drying at 80°C using a laboratory oven. In addition, the results showed that supercritical CO₂ drying could enhance preservation of the porous network of the oxide nanostructures at high calcination temperatures via suppression of sintering phenomenon. Calcination of α-Fe₂O₃ and NiO aerogels at 600°C yielded 225% and 53% higher surface areas than the corresponding xerogel, confirming the significance of drying step in the sol?gel method. Asphaltene adsorption from a model oil with asphaltene concentration of 3000 ppm indicated that the aerogels possessed higher adsorption capacities for the bulky asphaltene molecules than xerogels calcined at the same temperature of 600°C, which was due to their enhanced textural properties.     

Adsorption kinetics and thermodynamics of azo-dye Orange II onto highly porous titania aerogel

We report herein a kinetic and thermodynamic study of the adsorption of azo-dye Orange II from aqueous solutions onto titania aerogels. Aerogels structure was confirmed by FTIR and N₂ adsorption revealed their specific surface area (500 m²/g), pore volume (2.86 cm³/g) and pore size (mean 13.9 nm). Adsorption tests were conducted in batch reactors under various conditions where the effect of pH, temperature, contact time, dye concentration, and adsorbent dose were studied. Experiments performed at pH 2 show the optimal adsorption due to the best surface charge interactions. The temperature shows a weak influence with a decrease in the adsorption uptake as the temperature increases. Adsorption kinetics is shown to be very fast and follows a pseudo second-order indicating the coexistence of chemisorption and physisorption with the intra-particle diffusion being the rate controlling step. The experimental data fit perfectly with Sips isotherms and reveal the ability of titania aerogel to adsorb 420 mg of Orange II per gram of adsorbent at the optimal conditions. The thermodynamic study reveals the activation energy (42.1 kJ mol⁻¹) and the changes in Gibbs free energy (1.2 kJ mol⁻¹), enthalpy (−16.4 kJ mol⁻¹), and entropy (−58 J mol⁻¹ K⁻¹). The entire regeneration of the titania aerogel adsorption sites at pH 11 and 30 °C shows a total recovery of the dye and the efficient reusability and the economic interest of these adsorbing materials for environmental purposes.     

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.     

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.