One-pot synthesis of rationally-designed flexible, robust, and hydrophobic ambient-dried molecularly-bridged silica aerogels with efficient and versatile oil/water separation applications

Silica aerogels (SAs) are attractive porous materials due to their unique properties: continuous interconnected porous network, lightweight, high porosity, large specific surface area (SSA), low density, low thermal conductivity, high optical transmission, and low sound velocities. However, their commercialization is hindered by poor mechanical properties (brittleness, low strength, poor dimensional stability), high environmental sensitivity (hydrophilic, hygroscopic nature), and energy-intensive synthesis (supercritical drying). Efforts to improve SAs have included adjusting preparation conditions (pH, temperature, solvents, precursors) and minimizing the degree of crosslinking (DOC) while increasing the degree of freedom (DOF) using tri- and di-alkoxy silane precursors. Other strategies involve morphological adjustments (pore size, skeletal ratio) and incorporating SAs into flexible substrates or reinforcing with fillers. However, these methods have shortcomings. The molecular bridging concept offers a promising alternative, introducing flexible molecular spacers into the silica backbone to improve flexibility. The thiol-ene click reaction is a particularly effective method for synthesizing the bis-silane monomers needed for molecular bridging, offering simplicity, speed, and environmental friendliness. This study designs and synthesizes novel superflexible and hydrophobic BSAs using new functionalized molecular bridges via thiol-ene click reactions, aiming to improve flexibility, porosity, density, shrinkage, pore size, and SSA compared to existing flexible SAs. Two new symmetric bis-silane precursors are synthesized, and ambient pressure drying (APD) is used to improve the environmental impact and cost of the synthesis process.

The literature extensively covers the limitations of traditional silica aerogels and various approaches to enhance their mechanical properties and hydrophobicity. Studies have explored altering synthesis parameters, using different precursors, and incorporating additives. Molecular bridging has emerged as a key strategy, utilizing various coupling reactions like Schiff's base, Heck reaction, and thiol-isocyanate reactions to create flexible linkages. The thiol-ene click reaction is highlighted for its advantages in synthesizing bis-silane monomers due to its selectivity, mild conditions, and green nature, ultimately leading to flexible thioether bonds. Previous research on molecularly-bridged silica aerogels demonstrates improved flexibility, but there's a need for cost-effective and straightforward processes to achieve tailored properties. This research builds upon existing knowledge by introducing novel bis-silane precursors and employing ambient pressure drying for a more efficient and environmentally friendly approach.

Two bridged silane precursors, Sipr-C4 and Sipr-cyh, were synthesized via a UV-initiated thiol-ene click reaction using (3-mercaptopropyl)methyldimethoxy-silane (MPMDMS) as the thiol silane precursor and 1,4-butanediol divinyl ether (BDVE) or 1,4-cyclohexanedimethanol divinyl ether (CDVE) as the crosslinkers. The reaction also included a photoinitiator (2,2-dimethoxy-2-phenylacetophenone (DMPA)). These precursors were then used in a one-pot acid-base catalyzed sol-gel process followed by washing and ambient pressure drying (APD) to produce the corresponding BSAs, BSA-C4 and BSA-cyH. The synthesized materials were characterized using FTIR, solid-state 13C and 29Si MAS NMR spectroscopy, and XPS to analyze chemical structure and surface composition. SEM, nitrogen adsorption-desorption isotherm analysis, and pycnometry were used to examine morphology and microstructure (porosity, particle size, pore diameter, SSA). Mechanical properties were evaluated through compression and tensile tests, and wettability was assessed using static water contact angle (WCA) measurements. Oil sorption kinetics, saturation sorption capacity, and oil/water separation performance (static and continuous modes) were also investigated. The recyclability of the materials was evaluated through manual squeezing and vacuum-drying.

The synthesized BSAs exhibited a combination of desirable properties. FTIR, NMR, and XPS analyses confirmed the successful thiol-ene reaction and the formation of the silica network. The aerogels displayed ultra-low density (0.098-0.12 g/cm³), high porosity (90.3-91.7%), and large specific surface areas (77.31-117.64 m²/g). SEM images revealed a highly interconnected 3D porous network with micro-sized spheres. The BSAs demonstrated exceptional mechanical properties, including high compressibility (up to 99% strain for BSA-cyH), excellent stretchability (up to 47% elongation at break for BSA-cyH), and high resilience, withstanding 200 cyclic fatigue tests at 80% compressive strain. The aerogels were superoleophilic and highly hydrophobic (WCA up to 146.5°). Oil sorption kinetics showed fast uptake for low-viscosity solvents and slower uptake for high-viscosity oils. The saturation sorption capacity was high for various solvents and oils, increasing with solvent density. The BSAs exhibited excellent recyclability, maintaining high oil sorption capacity after 10 consecutive squeezing cycles. Oil/water separation tests demonstrated effective separation under static and dynamic conditions, including under simulated harsh conditions (waves, whirlpools). Continuous oil/water separation using vacuum-assisted pumping showed ultra-high oil removal fluxes (up to 10392 L/m²/h). Finally, the BSAs demonstrated excellent demulsification capabilities, achieving high separation efficiency (>99.8%) for oil-in-water emulsions.

The superior properties of the BSAs are attributed to the synergistic effects of several factors: the flexible molecular bridges providing structural flexibility and stress absorption; the abundant methyl groups reducing DOC and increasing DOF, contributing to hydrophobicity and resilience; the thick inter-particle connections ensuring structural integrity; and the large porosity and pore size facilitating oil uptake and separation. The difference in performance between BSA-C4 and BSA-cyH is attributed to the different molecular bridges, with the cyclic structure of BSA-cyH providing greater flexibility. The results demonstrate the effectiveness of the rational design approach in tailoring the properties of silica aerogels for specific applications. The one-pot synthesis method and ambient pressure drying offer a significant advantage in terms of cost-effectiveness and environmental impact. The excellent performance under harsh conditions highlights the potential of these materials for real-world applications.

This study successfully synthesized novel flexible and hydrophobic bridged silica aerogels via a facile one-pot process using a thiol-ene click reaction and ambient pressure drying. The obtained materials exhibited superior mechanical properties, high hydrophobicity and superoleophilicity, leading to excellent oil sorption and oil/water separation capabilities under various conditions. The environmentally friendly synthesis method and remarkable recyclability make these materials highly promising for large-scale oil spill cleanup and oily wastewater treatment. Future research could explore further modifications to the molecular bridges to fine-tune properties and explore applications beyond oil/water separation.

The study primarily focused on toluene and other organic solvents. Further investigation is needed to evaluate the performance with a broader range of oils and under more diverse environmental conditions. While the manual squeezing method is efficient for recycling, some oil residue remains in the aerogel after each cycle. Long-term stability studies under extreme conditions over an extended period of time are also warranted. Although the mechanical resilience under the testing conditions is excellent, the behavior of the aerogels at a large-scale is yet to be assessed in real conditions.