The ultrahighly nanoporous aerogel is recognized as a state of matter rather than as a functional material, because of its qualitative differences in bulk properties, transitional density and enthalpy between liquid and gas, and diverse chemical compositions. In this review, the characteristics, classification, history and preparation of the aerogel were introduced. More attention was paid to the sol-gel method for preparing different kinds of aerogels, given its important role on bridging the synthetic parameters with the properties. At last, preparation of a novel single-component aerogel, design of a composite aerogel and industrial application of the aerogel were regarded as the research tendency of the aerogel state in the near future.

The study of heat resistance of alumina aerogels has drawn great attention because of their high-temperature thermal insulation and catalyst applications. However, the main problem for the synthesis of heat-resistant alumina aerogels is their limited heat resistance and the drastic decrease of specific surface area upon heat treatment. Herein, trimethylethoxysilane (TMEO)-modified alumina aerogels are prepared by introduction of TMEO during sol–gel and supercritical fluid drying (SCFD) process. The introduced TMEO not only restricts the condensation of surface hydroxyl groups during aging and drying but also produces small silica particles on the alumina surface at high temperatures which inhibits the crystal growth upon heat treatment. Hence, the heat resistance of alumina aerogels is significantly enhanced after TMEO modification. The optimized TMEO-modified alumina aerogel has no cracks and little shrinkage during high-temperature SCFD, and shows no shrinkage and a high specific surface area of 147 m² g⁻¹ after heat treatment at 1200 °C. The TMEO-modified alumina aerogel possesses enhanced adsorption performance for gentian violet after firing at 1200 °C and shows low thermal conductivities of 0.13 and 0.18 W m⁻¹ K⁻¹ at 800 and 1000 °C, respectively. This may significantly contribute to their high-temperature applications such as thermal insulation, adsorption, catalysts, catalyst supports, etc.

Carbon aerogel (CA) has a rich porous structure, in which micropores and mesopores provide a huge specific surface area to form electric double layers. This property can be applied to the application of capacitive deionization (CDI). The adsorption effect of CA electrode on Cu²⁺ in an aqueous solution was explored for solving heavy metal water pollution. The CAs were synthesized by a sol-gel process using an atmospheric drying method. The structure of CAs was characterized by scanning in an electron microscope (SEM) and nitrogen adsorption/desorption techniques. The adsorption system was built using Cu²⁺ solution as the simulation of heavy metal pollution solution. The control variate method was used to investigate the effect of the anion species in copper solution, the molar ratio of resorcinol to catalyst (R/C) of CA, and the applied voltage and concentration of copper ion on the adsorption results.

The trans-resveratrol (RSV)-loaded silica aerogel (RLSA) was prepared by the sol-gel method, adding the drug during the aging process, solvent replacement and freeze drying. A series of characterizations showed that RSV stays in the silica aerogel in two ways. First, RSV precipitates due to minimal solubility in water during the solvent replacement process. After freeze drying, the solvent evaporates and the RSV recrystallizes. It can be seen from scanning electron microscope (SEM) and transmission electron microscope (TEM) images that the recrystallized RSV with micron-sized long rod-shaped is integrated with the dense silica network skeleton. Second, from small-angle X-ray scattering (SAXS) results, a portion of the RSV molecules is not crystallized and the size is extremely small. This can be attached to the primary and secondary particles of silica to enhance its network structure and inhibit shrinkage, which is why the volume and pore size of RLSA is larger. In addition, the diffusion of RSV in silica alcogel was studied by a one-dimensional model. The apparent diffusion coefficients of inward diffusion, outward diffusion and internal diffusion were calculated by fitting the time- and position-dependent concentration data. It was found that the outward diffusion coefficient (5.25 × 10⁻¹⁰ m²/s) is larger than the inward (2.93 × 10⁻¹⁰ m²/s), which is probably due to the interface effect. The diffusion coefficients obtained for different concentrations in the same process (inward diffusion) are found to be different. This suggests that the apparent diffusion coefficient obtained is affected by molecular adsorption.