One-Pot Sol-Gel Synthesis of Highly Insulative Hybrid P(AAm-CO-AAc)-Silica Aerogels with Improved Mechanical and Thermal Properties

Silica aerogels and their derivatives have outstanding thermal properties with exceptional values in the thermal insulation industry. However, their brittle nature restricts their large-scale commercialization. Thus, enhancing their mechanical strength without affecting their thermal insulating properties is essential. Therefore, for the first time, highly thermally stable poly(acrylamide-co-acrylic acid) partial sodium salt is used as a reinforcing polymer to synthesize hybrid P(AAm-CO-AAc)-silica aerogels via epoxy ring-opening polymerization in the present study. Functional groups in P(AAm-CO-AAc) partial sodium salts, such as CONH2 and COOH, acted as nucleophiles for the epoxy ring-opening reaction with (3-glycidyloxypropyl)trimethoxysilane, which resulted in a seven-fold enhancement in mechanical strength compared to that of pristine silica aerogel while maintaining thermal conductivity at less than 30.6 mW/mK and porosity of more than 93.68%. Moreover, the hybrid P(AAm-CO-AAc)-silica aerogel demonstrated improved thermal stability up to 343 °C, owing to the synergetic effect between the P(AAm-CO-AAc) and the silica aerogel, corresponding to the thermal stability and strong covalent bonding among them. These excellent results illustrate that this new synthetic approach for producing hybrid P(AAm-CO-AAc)-silica aerogels is useful for enhancing the mechanical strength of pristine silica aerogel without impairing its thermal insulating property and shows potential as an industrial heat insulation material.

Synthesis of silica aerogel films in liquid molds

By virtue of their low density and thermal conductivity, aerogels constitute attractive thermal insulators. Of those, aerogel films are best suited for thermal insulation in microsystems. Processes for the synthesis of aerogel films with thicknesses smaller than 2 µm or thicker than 1 mm are well established. However, for microsystems films in the range of a few microns and up to several hundred microns would be beneficial. To circumvent the present limitations, we describe a liquid mold made of two immiscible liquids, used here to produce aerogel films thicker than 2 µm in a single molding step. Following gelation and aging, the gels were removed from the liquids and dried using supercritical carbon dioxide. In contrast to spin/dip coating, liquid molding avoids solvent evaporation from the gel's outer surface during gelation and aging, films are free-standing and have smooth surfaces. The choice of liquids determines the aerogel film thickness. As a proof of concept, 130 µm thick homogeneous and high porosity (>90%) silica aerogel films were synthesized in a liquid mold with fluorine oil and octanol. The resemblance of the liquid mold approach to the float glass technique offers the prospect of mass production of large sheets of aerogel films.

Spherical Attapulgite/Silica Aerogels Fabricated via Different Drying Methods with Excellent Adsorption Performance

Dye wastewater has caused great harm to the environment, which is an urgent problem to be solved. As typical three-dimensional porous materials, aerogels have attracted great interest in dye wastewater treatment. In this work, spherical attapulgite/silica (ATP/SiO₂) gels were initially prepared by easily scalable sol-gel dripping methods and then dried to aerogels with three drying techniques, namely, supercritical CO₂ drying (SCD), freeze-drying (FD), and ambient pressure drying (APD). The effect of the drying techniques and heat-treated temperature on the physical characteristic, morphological properties, microstructure, and chemical structure of the spherical ATP/SiO₂ aerogels were investigated. The macroscopic morphology of the spherical ATP/SiO₂ aerogels was homogeneous and integrated without local cracking. The average pore diameter and specific surface area of the spherical ATP/SiO₂ aerogels prepared by the three drying techniques were in the range of 6.8-8.6 nm and 218.5-267.4 m²/g, respectively. The heat treatment temperature had a significant effect on the pore structure and the wetting properties of the aerogels. The 600 °C heat-treated aerogels were subjected to adsorption tests in methylene blue (MB) solution (60 mg/g, 100 mL), which exhibited a great adsorption capacity of 102.50 mg/g. Therefore, the resulting spherical ATP/SiO₂ aerogels possessed multipath preparation and exhibited an efficient adsorption performance, with the potential to be applied as an adsorbent for dye wastewater.

Development of Regular Hydrophobic Silica Aerogel Microspheres for Efficient Oil Adsorption

The objective of this research was to develop new hydrophobic silica aerogel microspheres (HSAMs) with water glass and hexmethyldisilazane for oil adsorption. The effects of the hexmethyldisilazane concentration and drying method on the structure and organic liquid adsorption capacity were investigated. The hexmethyldisilazane concentration of the modification solution did not influence the microstructure and pore structure in a noteworthy manner, which depended more on the drying method. Vacuum drying led to more volume shrinkage of the silica gel microsphere (SGM) than supercritical CO₂ drying, thus resulting in a larger apparent density, lower pore volume, narrower pore size distribution, and more compact network. Owing to the large pore volume and pore size, the HSAMs synthesized via supercritical CO₂ drying had a larger organic liquid adsorption capacity. The adsorption capacities of the HSAMs with pore volumes of 4.04-6.44 cm³/g for colza oil, vacuum pump oil, and hexane are up to 18.3, 18.9, and 11.8 g/g, respectively, higher than for their state-of-the-art counterparts. The new sorbent preparation method is facile, cost-effective, safe, and ecofriendly, and the resulting HSAMs are exceptional in capacity, stability, and regenerability.

Auto-Continuous Synthesis of Robust and Hydrophobic Silica Aerogel Microspheres from Low-Cost Aqueous Sodium Silicate for Fast Dynamic Organics Removal

An efficient auto-continuous globing process was developed with a self-built apparatus to synthesize pure silica aerogel microspheres (PSAMs) using sodium silicate as a precursor and water as a solvent. A hydrophobic silica aerogel microsphere (HSAM) was obtained by methyl grafting. A reinforced silica aerogel microsphere (RSAM) was prepared by polymer cross-linking on the framework of the silica gel. The pH value of the reaction system and the temperature of the coagulating bath were critical to form perfect SAMs with a diameter of 3.0 ± 0.2 mm. The grafted methyl groups are thermally stable up to 400 °C. Polymer cross-linking increased the strength significantly, owing to the polymer coating on the framework of silica aerogel. The pore volumes of HSAM (6.44 cm³/g) and RSAM (3.17 cm³/g) were much higher than their state-of-the-art counterparts. Their specific surface areas were also at a high level. The HSAM and RSAM showed high organic sorption capacities, i.e., 17.9 g/g of pump oil, 11.8 g/g of hexane, and 22.2 mg/g of 10 mg/L methyl orange. The novel preparation method was facile, cost-effective, safe, and eco-friendly, and the resulting SAM sorbents were exceptional in capacity, dynamics, regenerability, and stability.

Structural variations and generation of binding sites in Fe-loaded ZSM-5 and silica under the effect of UV-irradiation and their role in enhanced BTEX abatement from gas streams

Here, we have investigated the effect of UV-pretreatment on the physicochemical properties of an adsorbent. The UV-irradiated Fe-incorporated ZSM-5 (UV-FZ5) showed structural and chemical changes arising due to UV-cleaving of framework bonds resulting in a decreased crystallinity and change in the local environment of Fe species. More visible modifications were observed for UV-irradiated iron-containing silica (UV-Fe/AS) where silica network reconfiguration, increased hydroxyl density, and change in the Fe coordination were estimated. A 0.5-81.2% increase in BTEX adsorption was recorded for UV-irradiated adsorbents. These hiked performances were attributed to the increased pore size, increased hydroxyl density, and formation of newer isolated Fe³⁺ framework species. For FZ5, adsorption occurred via size-selective diffusion followed by hydrogen bonding and cation-pi interaction, whereas, for Fe/AS, diffusion was followed by cation-pi interactions. Moreover, adsorbents retained their adsorption capacity for multiple cycles and were found economically suitable for treating VOCs-contaminated air.

Flexible and Adaptable Fuel Cell Pack with High Energy Density Realized by a Bifunctional Catalyst

A proton exchange membrane fuel cell (PEMFC) system with a hydrogen generator could have higher energy density than flexible batteries and supercapacitors and is possible to meet the urgent demand of flexible electronics. However, a flexible PEMFC pack is still not available due to the absence of a flexible hydrogen generator. To solve this problem, we successfully invented a flexible and adaptable hydrogen generator, which was realized by a new bifunctional aerogel catalyst with the abilities of both storing and producing hydrogen. The flexible hydrogen generator can produce hydrogen at room temperature when the device is inverting, bending, and rotating. By combining this flexible hydrogen generator and the unique flexible PEMFC stack of our group, we originally made a highly flexible and adaptable fuel cell pack with a high theoretical energy density (up to 722 Wh·kg^-1) and current achieved energy density (135.9 Wh·kg^-1). Such a PEMFC pack is highly promising to meet the high demand of flexible electronics.

Synthetic Polymer Aerogels in Particulate Form

Aerogels have been defined as solid colloidal or polymeric networks of nanoparticles that are expanded throughout their entire volume by a gas. They have high surface areas, low thermal conductivities, low dielectric constants, and high acoustic attenuation, all of which are very attractive properties for applications that range from thermal and acoustic insulation to dielectrics to drug delivery. However, one of the most important impediments to that potential has been that most efforts have been concentrated on monolithic aerogels, which are prone to defects and their production requires long and costly processing. An alternative approach is to consider manufacturing aerogels in particulate form. Recognizing that need, the European Commission funded “NanoHybrids”, a 3.5 years project under the Horizon 2020 framework with 12 industrial and academic partners aiming at aerogel particles from bio- and synthetic polymers. Biopolymer aerogels in particulate form have been reviewed recently. This mini-review focuses on the emerging field of particulate aerogels from synthetic polymers. That category includes mostly polyurea aerogels, but also some isolated cases of polyimide and phenolic resin aerogels. Particulate aerogels covered include powders, micro granules and spherical millimeter-size beads. For the benefit of the reader, in addition to the literature, some new results from our laboratory concerning polyurea particle aerogels are also included.

The effect of graphene-nanoplatelets on gelation and structural integrity of a polyvinyltrimethoxysilane-based aerogel

Aerogels suffer greatly from poor mechanical properties resulting from their particulate structure. They also experience noticeable pore shrinkage during drying due to their low structural integrity. These shortfalls limit their broad application. To enhance the mechanical properties and improve the structural integrity of silica-based aerogels, graphene nanoplatelets (GnPs), as a nanofiller, were embedded into the solution of polymerized vinyltrimethoxysilane (VTMS) to prepare P-VTMS-based silica/GnP (PE-b-Si/GnP) hybrid aerogel monoliths based on sol–gel synthesis and supercritical drying. The inclusion of GnPs in our polymer-based silica aerogel processes reinforced the nanostructure and suppressed PE-b-Si nanopore shrinkage during supercritical drying, thus acting as an effective anti-shrinkage nanofiller. Accordingly, the GnPs significantly contributed to the PE-b-Si solution's uniform gelation and to the change of the hydrophilic nature to a hydrophobic one even with 1 wt% addition. In this study, the influence of the GnP content on the sol–gel process, structure, and physical properties of PE-based silica aerogels is studied.

Aerogels suffer greatly from poor mechanical properties resulting from their particulate structure.

Hydrophilic and Hydrophobic Mesoporous Silica Derived from Rice Husk Ash as a Potential Drug Carrier

This work describes the preparation of mesoporous silica by the green reaction of rice husk ash (RHA) with glycerol, followed by the modification and the potential use as a drug carrier. The reaction was carried out at 215 °C for 2 h. The solution was further hydrolyzed with deionized water and aged for various times (24, 48, 120, 360, 528 and 672 h) before calcinations at 500 °C for 24 h. Further treatment of prepared mesoporous silica was performed using trimethylmethoxysilane (TMMS) to obtain hydrophobic Mesoporous silica. For all synthesized silicas, silica contents were as high as 95 wt %, whereas organic residues were less than 3 wt %. RHA-glycerol showed the highest specific surface area with smallest pore diameter (205.70 m²/g, 7.46 nm) when aged for 48 h. The optimal hydrolysis-ageing period of 120 h resulted in 500.7 m²/g specific surface area, 0.655 cm³/g pore volume and 5.23 nm pore diameter. The surface modification of RHA-glycerol occurred through the reaction with TMMS as confirmed by FTIR (Fourier-transform infrared spectroscopy). Ibuprofen was selected as a model drug for the adsorption experiments. The adsorption under supercritical CO₂ was carried out at isothermal temperature of 40 °C and 100 bar; % ibuprofen loading of TMMS modified mesoporous silica (TMMS-g-MS) was 6 times less than that of mesoporous silica aged for 24 h (MS-24h) due to the hydrophobic nature of modified mesoporous silica, not surface and pore characteristics. The release kinetics of ibuprofen-loaded mesoporous silicas were also investigated in vitro. The release rate of ibuprofen-loaded MS-24h was much faster than that of ibuprofen-loaded TMMS-g-MS, but comparable to the crystalline ibuprofen. The slower release rate was attributed to the diffusion control and the stability of hydrophobic nature of modified silica. This would allow the design of a controlled release drug delivery system.

Comparative study of modified/non-modified aluminum and silica aerogels for anionic dye adsorption performance

Developing effective and low-cost adsorbents is of great significance for controlling water contamination.

Organic–inorganic hybridization for the synthesis of robust in situ hydrophobic polypropylsilsesquioxane aerogels with fast oil absorption properties

In situ hydrophobic polypropylsilsesquioxane aerogels (PSAs) were successfully synthesized via an organic–inorganic hybridization method by a sol–gel process, in which propyltriethoxysilane (PTES) and tetraethylorthosilicate (TEOS) were used as co-precursors. ²⁹Si NMR and FTIR analyses indicated the high degree of condensation of the precursors and proved the attachment of propyl (–C₃H₇) groups in PSAs, respectively. By means of incorporating propyl groups, both mechanical robustness and in situ hydrophobicity were obtained. Meanwhile, the mechanical strength, contact angle and density obviously increased with the increase in propyl groups. Under optimized conditions, as-prepared PSA could endure up to a 70% maximum linear compression with few cracks. Benefiting from the robust structure and in situ hydrophobicity, PSAs showed high absorption capacities (8–10 times that of its own weight) and fast absorption properties (<20 s) for a wide range of organic solvents and could be reused at least 5 times.

In situ hydrophobic and mechanically robust polypropylsilsesquioxane aerogels (PSAs) were successfully synthesized via an organic–inorganic hybridization method by a sol–gel process.

Novel hydrophilic and hydrophobic amorphous silica: Characterization and adsorption of aqueous phase organic compounds

Very few studies have investigated the adsorption performance of hydrophobic and hydrophilic silicas with dissolved organics in water, which is a required final step during produced water treatment. The cost of functionalization also hinders the use of hydrophobic materials as sorbents. Novel hydrophilic silicas, prepared at low temperature and ambient pressure, were characterised by SEM, FTIR and BET analysis, and studied for the adsorption of aqueous phase organic compounds at concentrations below their solubility limits. Adsorption capacities were found to be up to 264 mg/g for benzene and 78.8 mg/g for toluene. Direct comparison is made with the analogous hydrophobic version of one of the silica materials, demonstrating comparable uptakes for benzene concentrations lower than 50 mg/L. This finding supports the hypothesis that, at very low aqueous phase organic concentrations, hydrophobicization has no discernible effect on access of the pollutants to the internal porosity of the material.

Preparation of transparent monolithic methylsilsesquioxane (MSQ) aerogels via ambient pressure drying

Transparent monolithic MSQ aerogels were prepared by an ambient pressure drying method. The obtained hydrophobic aerogels showed good stability to various aggressive environments.