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Nanoporous organosilica membranes are successfully coated on porous alumina tubes and tested for desalination via membrane distillation. The membranes produced pure water (up to 13 kg m(-2) h(-1)) across an extreme range of salt concentrations (10-150 g L(-1) NaCl) at moderate temperatures (≤60 °C) without exhibiting the characteristic flux decay of competing materials.

An unconventional nanoporous organosilica membrane has been tested in a vacuum membrane distillation (MD) process for water desalination. We propose a modified approach to understand the transport mechanism of water molecules through the nanopores of this membrane. The modified approach stems from the fact that the membrane has a hydrophilic surface (contact angle < 90°) and so capillary pressure, which draws liquid water into the nanopore, must be considered when establishing the mathematical model. However, increased friction arising from the dramatic increase in shear viscosity of water in nanoconfined spaces balances the capillary flow against the evaporative mass transport to avoid pore wetting. Notably, the liquid/vapour interface is no longer formed at the pore entrance as with a conventional hydrophobic membrane, but rather exists deeper in the pore channel as a consequence of capillary pressure. This was backed by experimental observations (no pore wetting) and SEM evidence which showed salt nucleation and growth existed only on the membrane surface, and did not infiltrate the membrane support layers. The impacts of pore size, membrane thickness, substrate thickness, concentration polarization, porosity, and contact angle on water flux and pore intrusion depth were tested using the model. Pore size was the most influential parameter with an > 80% increase in permeation flux if the pore size increased from 2 to 3 nm at 60 °C. However, pore wetting is expected if d p > 3.4 nm, particularly at low temperatures where the slower evaporation rate promoted greater pore intrusion. Concentration polarization was shown to be negligible which agreed well with experimentally observed water fluxes which remained relatively constant despite feed salinity increasing from 0 to 150 g L-1. Lastly, the membrane hydrophilicity was found to impact on water flux and pore intrusion in a complex relationship with pore size. Ultimately, hydrophilic pores less than 3 nm in diameter offer a good combination of good water flux and minimal water intrusion suggesting that ordered mesoporous organosilica membranes have potential in MD applications.

A direct synthesis method is introduced to prepare mesoporous carbon-silica nanocomposite (CSN) membranes for water-treatment applications. Unlike the intricate and expensive nanocasting method, this triconstituent co-assembly method is a one-pot synthesis method using Pluronic F127 as templating agent with a hybrid organic-inorganic matrix formed by tetraethylorthosilicate (TEOS), resorcinol and formaldehyde. The silica content is varied in the polymer solution to investigate the material properties, stability of the nanocomposite mesostructured and membrane performance in vacuum membrane distillation (VMD). The CSN materials are carbonised under nitrogen at temperatures of 600-900 °C without any significant lattice shrinkage, demonstrating excellent stability. They possess a highly ordered pore structure with moderate BET surface area (430-550 m 2 g -1 ) and narrow pore size distribution at around 5.5-7.6 nm. Based on the FTIR and NMR analyses, there is no covalent bond between the carbon and silica networks, but the carbon compound was found to affect the condensation degree of the silica. Raising the temperature from 700 to 900 °C leads to further condensation of the carbon network, which in turn releases hydroxyl or water groups that can attack adjacent siloxane bonds. The CSN membranes performed well in VMD with water permeation flux up to 12 L m -2 h -1 and salt rejection > 99 %. This work shows that a different strategy of modifying silica-based membrane can be successfully applied for the desalination of saline waters through VMD.