Haixun Xu

Synthesis and thermal insulation performance of silica aerogel from recycled coal gangue by means of ambient pressure drying

Silica aerogel materials are well recognized for their superinsulation performance and are regarded as one of the hot candidates to revolutionize building insulation. To date, high production cost related to exorbitant precursors as well as cumbrous multi-step hydrophobization process has often narrowed the field of applications. In this work, granular silica aerogel materials were synthesized by extracting SiO2 from recycled rich silicon coal gangue, followed by one-step hydrophobization and ambient pressure drying. Lightweight (about 0.16 g/cm3) and nanostructural aerogels were obtained through this route. They exhibit a 3D open porous microstructure with around 600 cm2/g surface area and 20 nm of the average pore diameter, thermal conductivity of 4-5 mm packed granules is 20-25 mW/(m·K), which was proved by both guarded hot plate and hot-wire transient methods. This study offers a new facile route for the synthesis of silica aerogel from recycled solid waste coal gangue and suggests a method, which may lead to a cost reduction in terms of industrial production.

A Novel Environmental Route to Ambient Pressure Dried Thermal Insulating Silica Aerogel via Recycled Coal Gangue

Coal gangue, one of the main hazardous emissions of purifying coal from coalmine industry, is rich in silica and alumina. However, the recycling of the waste is normally restricted by less efficient techniques and low attractive output; the utilization of such waste is still staying lower than 15%. In this work, the silica aerogel materials were synthesized by using a precursor extracted from recycled silicon-rich coal gangue, followed by a single-step surface silylation and ambient pressure drying. A low density (~0.19 g/cm3) nanostructured aerogel with a 3D open porous microstructure and high surface area (~690 m2/g) was synthesized, which presents a superior thermal insulation performance (~26.5 mW·m−1·K−1 of a plane packed of 4-5 mm granules which was confirmed by transient hot-wire method). This study offers a new facile route to the synthesis of insulating aerogel material by recycling solid waste coal gangue and presents a potential cost reduction of industrial production of silica aerogels.

Structural characteristics and photocatalytic activity of ambient pressure dried SiO2/TiO2 aerogel composites by one-step solvent exchange/surface modification

An ambient pressure synthesis of SiO2/TiO2 binary aerogel was prepared through the low-cost precursors of titanium tetrachloride (TiCl4) and sodium silicate (Na2O·nSiO2). After gelation, solvent exchange and surface modification were performed simultaneously and the modified gel was finally dried under ambient pressure. Microstructural analyses by transmission electron microscope (TEM) indicate that fabricated SiO2/TiO2 aerogel composite shows similar sponge-like nanostructure as silica aerogel, and the Brunauer–Emmett–Teller (BET) analysis shows that the specific surface area of the composite reaches 605 m2/g, and the average pore size is 9.7 nm. Such binary aerogel exhibits significant photocatalytic performance in this paper for treating model pollutant of methyl orange (MO), and the decolorizing efficiency of MO is detected as 84.9% after 210 mins exposure to UV light irradiation. Degraded gel suspends in the water so as to separate from solution for reuse, and after 4 times recycling, 70% degradation efficiency can be easily reached when composite catalyzed system is exposed for 210 mins under UV irradiation.

Influence of drying methods on fractal geometric characteristics of mesoporous silica aerogels

Chemical modification/ambient drying method and freeze drying method were introduced to research the synthesis of mesoporous silica aerogels. By analyzing N2 gas adsorption/desorption isotherms, the fractal geometric characteristics of gels were focused. The overall surface fractal dimensions were determined by analyzing N2 gas adsorption branch and a Frenkel-Halsey-Hill (FHH) equation was empolyed to determine surface fractal dimension Df. It is found that, during ambient drying process, VTMCS/VWetgel ratio plays a crucial role in the changes of geometric feature, the key point is 50%, when the ratio is lower, and surface roughness increases with the ratio, when it exceeds 50%, the surface is almost unaffected by the modification. While freeze drying always tends to get larger Df, freeze drying process could cause a rough surface of the gels. Compared with traditional porosity and specific surface area analyses, fractal geometry may be expected to be favorable for mesoporous structural analyses of materials.

Microwave-assisted hydrothermal synthesis of carbon materials with tunable microstructure

A facile microwave-assisted hydrothermal route has been developed for a synthesis of versatile carbon materials. The monosaccharide fructose aqueous solution was adopted as the starting material, and the pH of the solution was adjusted to be in acidic (pH 4), neutral (pH 7) and basic (pH 10.5) conditions. The pH buffered fructose solutions were treated at different temperatures by a microwave-assisted hydrothermal technique. As-prepared carbon materials displayed pH and temperature dependent multi-morphologies (porous, spherical or core-shell), which were determined by transmission and scanning electron microscopic analyses (TEM and SEM). And the hypothesis of dehydration mechanism of hydrothermal synthesis was analyzed by ultraviolet extinction and Fourier transform infrared spectroscopy. It was found that as compared with normal hydrothermal synthesis, microwave assistance could efficiently increase the production yield and improve the spherical geometry of the carbon particles in neutral condition. By changing the pH of the system, acidic pH induces aggregation of the spheres, while basic pH produces more trends toward core-shell or sponge-like porous structure. The study opens a novel route to the production of polytropic carbon materials and suggests a potential niche market established from the green synthesis.