High surface area alumina-supported nickel (II) oxide aerogels using epoxide addition method

Nickel (II) oxide aerogels with an amorphous alumina support were synthesized by the expoxide addition method. The monoliths were obtained by adding propylene oxide to an alcoholic solution of hydrated metal nitrate salts. The wet gels were dried by supercritical extraction to produce porous monolithic aerogels. The as-synthesized aerogels were amorphous containing aluminum and nickel hydroxides. Annealing of the as-synthesized aerogels at 400 °C yields crystalline nickel oxide materials which retain the high surface areas (>160 m²/g) and porosities of the original aerogels. The resultant aerogel materials were characterized using powder X-ray diffraction, thermo-gravimetric analysis, scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray analysis, and nitrogen adsorption/desorption analysis.     

Silica–titania aerogel monoliths with large pore volume and surface area by ambient pressure drying

Ambient pressure drying has been carried out for the synthesis of silica–titania aerogel monoliths. The prepared aerogels show densities in the range 0.34–0.38 g/cm³. The surface area and pore volume of these mixed oxide aerogels are comparable to those of the supercritically dried ones. The surface area for 5wt% titania aerogel has been found to be as high as 685 m²/g with a pore volume of 2.34 cm³/g and the 10wt% titania aerogel has a surface area of 620 m²/g with a pore volume of 2.36 cm³/g. Some gels were also made hydrophobic by a surface treatment with methyltrimethoxysilane and trimethylchlorosilane. The surface modified aerogels possess high surface areas in the range of 540–640 m²/g, and are thermally stable in terms of retaining hydrophobicity up to a temperature of 520 °C. The pore size distribution of the aerogels clearly indicates the preservation of the aerogel structure. High Resolution Transmission Electron microscopy has been employed to characterise the aerogels and Fourier Transform infrared spectroscopy to study the effect of titania addition to silica and the surface modification. X-ray diffraction patterns were recorded to verify the molecular homogeneity of the aerogel.     

Synthesis of transparent silica aerogels using tetraalkylammonium fluoride catalysts

Tetraalkylammonium fluoride salts have been employed as catalysts for the synthesis of silica aerogels by a two-step, sol–gel method. Aerogel materials were characterized by N₂ physisorption and SEM. The effect of the type of catalyst on the optical transparency of obtained aerogels has been examined. It has been found that such compounds allow the synthesis of silica aerogels with the highest optical transmittance ever reported for such materials. The optimal catalysts are tetrabutyl and tetraoctyl ammonium fluoride, with which aerogels with transparency as high as 96% and extinction coefficient as low as 3.5 m⁻¹ can be prepared.     

Polyurea based aerogel for a high performance thermal insulation material

Less fragile lightweight nanostructured polyurea based organic aerogels were prepared via a simple sol–gel processing and supercritical drying method. The uniform polyurea wet gels were first prepared at room temperature and atmospheric pressure by reacting different isocyanates with polyamines using a tertiary amine (triethylamine) catalyst. Gelation kinetics, uniformity of wet gel, and properties of aerogel products were significantly affected by both target density (i.e., solid content) and equivalent weight (EW) ratio of the isocyanate resin and polyamine hardener. A supercritical carbon dioxide (CO₂) drying method was used to extract solvent from wet polyurea gels to afford nanoporous aerogels. The thermal conductivity values of polyurea based aerogel were measured at pressures from ambient to 0.075 torr and at temperatures from room temperature to −120 °C under a pressure of 8 torr. The polyurea based aerogel samples demonstrated high porosities, low thermal conductivity values, hydrophobicity properties, relatively high thermal decomposition temperature (~270 °C) and low degassing property and were less dusty than silica aerogels. We found that the low thermal conductivities of polyurea based aerogels were associated with their small pore sizes. These polyurea based aerogels are very promising candidates for cryogenic insulation applications and as a thermal insulation component of spacesuits.     

An innovative CaSiO<Subscript>3</Subscript> dielectric material from eggshells by sol–gel process

Calcium silicate (CaSiO₃), wollastonite, with a molar ratio of CaO:SiO₂ of 1:1, was synthesized by a sol–gel process and sintered at 1,100°C for 1 h. The synthesis of calcium silicate was carried out using chicken eggshells as the starting material possessing several advantages such as low cost, high purity, and less moisture sensitivity, when compared with those obtained from metal alkoxide precursors via the sol–gel process. The CaSiO₃ samples have the triclinic or anorthic phase formations and good electrical properties. The dielectric constant and electrical conductivity are 62.59 ± 0.44 and 8.0052 × 10⁻⁴ (Ω.m)⁻¹, respectively, at 25°C and 1 MHz. The transmission electron microscopy (TEM) images of the samples show a good dispersion and uniform particles with an average diameter of about 0.5 nm. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM), and Simultaneous thermal analysis (STA) were used to verify the synthesis.     

RF-aerogels catalysed by ammonium carbonate

The application of ammonium carbonate (AC) as catalyst for the preparation of RF-aerogels leads to organic aerogels without metallic impurities in contrast to conventional catalysts like sodium carbonate. To synthesize the AC catalyzed RF aerogels we varied the catalyst and formaldehyde concentration in wide ranges. The nanostructure varies accordingly over an order of magnitude. The particle sizes in the dry aerogel network, determined by scanning electron microscopy, are in the range of 0.15–4 μm. The aerogel densities are in the range of 370–420 kg m⁻³. The specific surface measured by nitrogen adsorption (BET) varies from 0.5 to 13 m² g⁻¹ which equals a specific surface area from 0.7 to 20 μm⁻¹. Thermogravimetry is employed to study the drying process, annealing reactions and decomposition of the aerogel into a carbon aerogel.     

Synthesis of a low-density tantalum oxide tile-like aerogel monolithic

A tile-like Ta₂O₅ aerogel monolithic was prepared by sol–gel method. Tantalum ethoxide and tantalum pentachloride were used as precursors, ethanol as the solvent, and propylene oxide as the gelation agent. Field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods were used to characterize the as-prepared aerogel. The combined results of these techniques indicated the presence of tantalum pentachloride and propylene oxide leading to the high porosity, high surface area, and low density of the Ta₂O₅ aerogel monolithic.     

Gold modified cadmium sulfide aerogels

Monolithic aerogels composed of cadmium sulfide nanoparticles partially modified with metallic gold (CdS-Au) are reported. The semiconductor–metal nanoparticles are synthesized using an inverse micelle media of Bis-(2-ethylhexyl)sulfosuccinate sodium salt (AOT) in heptane, followed by capping with 4-fluorothiophenol and precipitation with triethylamine. The nanoparticles are then dispersed in acetone and gel formation is achieved using nanoparticle condensation strategy. The resultant CdS-Au aerogel materials are mesoporous, with an interconnected network of semiconductor–metal nanoparticles. A detailed microstructure analysis of the semiconductor–metal aerogels via transmission electron microscopy indicates that the final gold concentration significantly impacts the semiconductor–metal aerogel morphology and porosity.     

New synthesis of pure CexZr1?xO2 mixed oxides (0?≤?x?≤?1) by an epoxide sol–gel method

Pure ceria-zirconia mixed oxides Ce _x Zr_1−x O₂ with high specific surface area were synthesized with a new epoxyde driven sol–gel route and characterized by thermal analysis, X-ray diffraction studies and transmission electron microscopy. This sol–gel method is cheap and uses only a few steps. The Ce _x Zr_1−x O₂ mixed oxides were obtained in the range of 0 ≤ x ≤ 1 (except for x = 0.8) and crystallised at 350 °C after decomposition of the gels. This temperature is very low in comparison with the other methods. The studies of the influence of different synthesis parameters (concentration of the sol and decomposition temperature) allowed us to determine the conditions to obtain the best homogeneity in the gel to avoid the formation of a mixture of phases instead of mixed oxides. This approach leads to the synthesis of oxide with specific surface area above 100 m² g⁻¹. The elaboration of an ambigel could increase this value up to 195 m² g⁻¹ for x = 0.5. This sol–gel synthesis offers new perspectives for these oxides in several applications. Generally, these oxides are difficult to obtain pure in large range of composition at low-temperature and with high specific surface area by other methods.     

Synthesis of MWCNTs doped sodium silicate based aerogels by ambient pressure drying

The successful incorporation of multiwalled carbon nanotubes (MWCNTs) into silica aerogels prepared by sol–gel method is reported herein. Pure silica aerogels prepared using sodium silicate precursor by ambient pressure drying are so fragile that they cannot be used easily. MWCNTs were used as reinforcements to improve the mechanical properties of silica aerogels. Results show that inserting small amounts of MWCNTs in the gels causes enhanced dimensional stability of silica aerogels. The silica aerogels were prepared by doping MWCNTs in silica matrix before gelation. The influence of MWCNTs on some microstructural aspects of silica matrix has been studied using nitrogen adsorption–desorption isotherms. From SEM study it is confirmed that the silica particles get capped on the surface of MWCNTs suggesting an enhanced toughness. Further, FTIR, Raman, EDAX, thermal conductivity and hydrophobicity studies of these doped aerogels were carried out. By addition of MWCNTs, silica aerogels were formed with 706 m²/g BET and 1,200 m²/g Langmuir surface areas and 149^o contact angle. Low density (0.052 g/cc) and low thermal conductivity (0.067 W/m K) MWCNTs doped silica aerogels were obtained for the molar ratio of Na₂SiO₃::H₂O::MWCNTs::citric acid::TMCS at 1::146.67::2.5 × 10⁻³::0.54::9.46 respectively with improved mechanical strength.     

Photo catalytic oxidation of TNT using TiO<Subscript>2</Subscript>-SiO<Subscript>2</Subscript> nano-composite aerogel catalyst prepared using sol–gel process

The degradation of nitro aromatics like trinitrotoluene (TNT) released in the waste water from explosive process plants is the serious problem due to toxic and explosive nature of TNT. The poor response of TNT to biodegradation enhanced the gravity of the problem. We have demonstrated that high specific surface area TiO₂–SiO₂ nano-composite aerogel is promising photo catalyst in successful treating of TNT contaminated aqueous solution. The TiO₂–SiO₂ composite aerogel with nominal content of 20 and 50% TiO₂, used as catalyst, were prepared by co-precursor sol–gel method using titanium isopropaxide and tetramethylorthosilicate as source of titania and silica, respectively. The XRD studies confirmed formation of anatase phase of crystalline TiO₂ with nano sized crystallites. The TiO₂–SiO₂ aerogel showed specific surface area of 1,107 and 485 m²/g for the aerogels containing 20 and 50% TiO₂, respectively. The 100 ppm TNT solution was treated, in 700 ml capacity reaction vessel, using H₂O₂ oxidizer and TiO₂–SiO₂ aerogel catalyst in presence of UV light (8 W UV lamp). Using TiO₂–SiO₂ (50/50) aerogel with surface area of 485 m²/g, we succeeded to reduce the TOC to 1 ppm within 3.5 h where as using TiO₂/SiO₂ (20/80) aerogel with surface area of 1,107 m²/g, the TOC was reduced to about only 7 ppm in the same time. It revealed that the combination of high TiO₂ content and high specific surface area is an important factor to achieve effective and faster degradation of TNT for complete mineralization.     

Synthesis and characterization of silica xerogels obtained via fast sol–gel process

The synthesis and physical properties of high surface area silica xerogels obtained by a two-step sol–gel process in the absence of supercritical conditions are reported. The hydrolysis and condensation reactions were followed by infrared spectroscopy. The increment in the bands corresponding to silanol and hydroxyl groups suggests that the hydrolysis reaction was complete during the first 30 min. The effect on surface area and global reaction time under various reaction conditions, such as type of alkaline catalyst and solvents, water–monomer and solvent–monomer molar ratios, was also studied. The obtained results suggest that surface area was increased using 3-aminopropyltriethoxysilane as catalyst. The use of isopropyl alcohol as solvent promotes the reduction of the capillary stress, giving a well-structured xerogel. As a conclusion, with H₂O/i-PrOH/TEOS in a molar ratio of 10:4:1, it was possible to obtain silica xerogels with surface areas about 1,240 m²/g. Such surface areas are comparable with those obtained under supercritical conditions (aerogels), and higher than those xerogels conventionally obtained under normal condition (500–800 m²/g).     

Subcritically dried RF-aerogels catalysed by hydrochloric acid

Hydrochloric acid (HCl) used as catalyst for the preparation of RF-aerogels leads to organic aerogels in very short gelation times. The gelation time can be varied from a few seconds to minutes. The wet gels can be dried under ambient conditions. By variation of the sol composition or catalyst concentration the microstructure of the dry gels can be modified. The aerogel densities are in the range of 210–410 kg/m³. The particle sizes, determined by scanning electron microscopy (SEM), are in the range of 700–1,500 nm. The particles look essentially spherical and their size spectrum can be close to monodisperse. The specific surface is measured by nitrogen adsorption (BET). Thermogravimetry (TGA) is employed to study the drying process, annealing reactions and decomposition of the aerogel into a carbon aerogel.     

Optically transparent silica aerogels based on sodium silicate by a two step sol–gel process and ambient pressure drying

Interest in improving the optical transmission of sodium silicate-based aerogels by ambient pressure drying led to the synthesis of aerogels using a two-step sol–gel process. To produce optically transparent silica aerogel granules, NH₄F (1 M) and HCl (4 M) were used as hydrolyzing and condensation catalysts, respectively. The silica aerogels were characterized by their bulk density, porosity (%), contact angle and thermal conductivity. Optical transmission of as synthesized aerogels was studied by comparing the photos of aerogel granules. Scanning electron microscopic study showed the presence of fractal structures in these aerogels. The degree of transparency in two step sol–gel process-based aerogels is higher than the conventional single step aerogels. The N₂ adsorption–desorption analysis depicts that the two step sol–gel based aerogels have large surface areas. Optically transparent silica aerogels with a low density of ∼0.125 g/cc, low thermal conductivity of ∼0.128 W/mK and higher Brunauer, Emmett, and Teller surface area of ∼425 m²/g were obtained by using NH₄F (1 M), HCl (4 M), and a molar ratio of Na₂SiO₃::H₂O::trimethylchlorosilane of 1::146.67::9.46. The aerogels retained their hydrophobicity up to 500 °C.     

Effect of post-treatment (gel aging) on the properties of methyltrimethoxysilane based silica aerogels prepared by two-step sol–gel process

The properties of silica aerogels are highly dependent on the post-treatment steps like gel washing, gel aging and gel drying. The experimental results of the studies on one of the post-treatment steps i.e. gel aging effect on the physical and microstructural properties of methyltrimethoxysilane (MTMS) based silica aerogels, are reported. These hybrid aerogels were prepared by two step sol–gel process followed by supercritical drying. The molar ratio of MeOH/MTMS (M) was varied from 7 to 35 by keeping the H₂O/MTMS (W) molar ratio constant at 4. The as prepared alcogels of different molar ratios were aged from 0 to 5 days. It was observed that 2 days of gel aging period is the optimum gel aging period for good quality aerogels in terms of low density, less volume shrinkage and high porosity. The well tailored network matrix with low density (0.04 g/cm³), less volume shrinkage (4.5%), low thermal conductivity (0.05 W/mK) and high porosity (98.84 %) was obtained for 2 days of gel aging period of M = 35. Further, the gelation time varied from 8 to 1 h depending on the M values. The gelation time was being more for lesser M values. The aerogels were characterized by bulk density, porosity, volume shrinkage, thermal conductivity, Scanning Electron Microscopy and the Fourier Transform Infrared spectroscopy.     

Rapid synthesis of the low thermal expansion phase of Al<Subscript>2</Subscript>Mo<Subscript>3</Subscript>O<Subscript>12</Subscript> via a sol–gel method using polyvinyl alcohol

Aluminum molybdate was successfully synthesized using a simplified PVA assisted sol–gel method resulting in highly crystalline, monophasic (monoclinic P2₁/a) samples. These materials could readily be obtained at temperatures of 600 and 700 °C after calcining for as little as 15–20 min. Scanning electron microscopy and X-ray powder diffraction indicated that even the sample calcined at 600 °C for 20 min was free of impurities and composed of submicron sized particles (~300 nm). Transmission electron microscopy was used to confirm the monophasic character and submicron dimensions of the as-prepared powders. In addition to producing high quality samples, it was also observed that the metal to PVA ratio used during this simplified synthesis, could be used as a control parameter for tailoring the particle sizes of the final product.     

Catalytic Applications of Aerogels

This paper is a review of studies on aerogel synthesis and its application for catalytic uses, which have been conducted for the past decade at the Clean Technology Research Center, Korea Institute of Science and Technology. Various organic and inorganic aerogels employing sol–gel modifications exhibited catalytically favorable textural and chemical properties such as extremely high porosity, high surface area, well-developed mesoporosity, good thermal stability, and high dispersion, which eventually led to excellent performance in catalytic reactions. Reactions observed in these studies include hydrogenation, photodecomposition, selective oxidation, complete oxidation, ammoxidation, reforming, and electrooxidation. The specific catalytic behaviors can be explained in terms of the strong active sites-support interactions, high thermal stability, and extremely high dispersion.     

Ni-based xero- and aerogels as catalysts for nitroxidation processes

Porous nanocomposites made out of nickel dispersed on silica or alumina matrices were prepared as prospective catalysts for the nitroxidation of hydrocarbons in the form of aerogel or xerogel by adopting either a supercritical or a conventional gel drying procedure. The structural and textural features of the materials were investigated by X-ray diffraction, transmission electron microscopy and N₂ physisorption and combined to the acid/base and reducibility data as deduced by adsorption microcalorimetry and temperature programmed reduction (TPR) profiles. The alumina-based samples are made out of nanocrystalline nickel aluminate and are mesoporous, although the aerogel has larger pore volumes and surface area than the xerogel. On the other hand, in the silica-based samples nickel oxide nanocrystals are dispersed on amorphous silica, the size of the nanocrystals being around 5 nm in the microporous xerogel and 14 nm in the mainly mesoporous aerogel. TPR data point out that the alumina-based samples have similar reducibility, whereas significant differences were observed in the silica-supported composites, the NiO–SiO₂ aerogel exhibiting improved reducibility at low temperature. The NO-catalyst interaction was monitored by temperature programmed NO reaction coupled to mass spectrometry and preliminary tests on the use of the NiO–SiO₂ xerogel and aerogel nanocomposites for the catalytic nitroxidation of 1-methyl-naphthalene to 1-naphthonitrile were obtained in a fixed-bed continuous-flow reactor. The data indicate that the aerogel exhibits larger selectivity than the corresponding xerogel, pointing out the importance of tuning the sol–gel parameters in the design of porous composite materials for catalytic applications.     

Hierarchical microstructure and formative mechanism of low-density molybdena-based aerogel derived from MoCl<Subscript>5</Subscript>

Low density 150 kg/m³ molybdena-based aerogel was prepared by using MoCl₅ as precursor, polyacrylic acid (PAA) as additive and propylene oxide as gelation accelerator via the dispersed inorganic sol–gel method, followed by carbon dioxide supercritical fluid drying. Characterizations of the composition indicate that the as-synthesized aerogel is composed of molybdenum oxide and PAA derivatives; electron microscopy photographs show the hierarchical microstructure of the aerogel, including both spherical secondary particles and root-like primary fibers. Based on the analysis of coordination state and special morphology, it is suggested that the additive PAA not only guides the growth of the primary fibers but improves the crosslinkage between the secondary particles to form a robust skeleton.     

Bithiazole-bridged polysilsesquioxane and its metal complexes: synthesis and magnetic properties

A novel alkoxysilylated derivative based on 2,2′-diamino-4,4′-bithiazole (DABTH) was firstly synthesized. The corresponding polysilsesquioxane (PBSIBTH) and its metal complexes (PBSIBTH-Eu³⁺, PBSIBTH-Tb³⁺ and PBSIBTH-Ni²⁺) were also obtained via sol–gel method, respectively. The morphology of their xerogels was investigated by scanning electron microscopy means. The magnetic measurements of these polymer complexes show all obtained solid materials feature soft ferromagnet properties at low temperature. The metal ions in polymer complexes have a significant influence on both microstructure and magnetic properties.