High performance ES40-derived silica membranes for desalination

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Access to potable water has become one of the top 10 global problems facing our contemporary society. Desalination of vast amounts of seawater is a promising measure to ameliorate this stress. Numerous thermal and membrane processes have been employed to desalt water, of which reverse osmosis (RO) using polymeric membranes is the most mature technology and has been regarded as the benchmark in current desalination technologies. However, due to the high pressure requirement for RO operation, other membrane processes, such as membrane distillation and pervaporation have been developed as the alternatives to further reduce the energy cost. Furthermore, due to the lack of chemical/thermal stability and high tendency to scaling and fouling of polymeric membranes, novel inorganic membranes, in particular silica derived membranes, are emerging as potential candidates for desalination. The majority of published works on silica membranes utilizes tetraethyl orthosilicate (TEOS) as the silica precursor and interlayers are generally required to reduce the roughness of the porous substrates, followed by 4 to 6 thin-film coatings and calcinations with heating rates of 1–2 oC min-1 to form defect free silica membranes, which consume 10–14 days with extra cost. Therefore, the major target of this research is to produce interlayer-free silica membranes fabricated by rapid thermal processing (RTP) for desalination application. Ethyl silicate 40 (ES40) is used as precursor to prepare silica membranes in this work owing to their superior thermal stability as compared to the TEOS analogous product. Firstly, the sol-gel processing of ES40 was investigated in this thesis. At different temperatures and H2O/Si ratios, ES40 underwent various degrees of phase separation behaviour and sol-gel reactions. It was found that the hydrolysis and condensation reactions decreased with decreasing reaction temperature. Dense silica structures were obtained using low H2O/Si ratios, whereas higher porosity was produced from a phase-separated sol-gel system with high H2O/Si ratios. This tailoring process facilitated further condensation reactions and crosslinking of silica chains, leading to stronger silica matrices which were more resistant to densification. The second part of this work is focused on improvement of the hydrostability of ES40-derived silica membrane materials by adjusting the reactant ratios of sol-gel synthesis process, as the low resistance of silica membranes to water vapor attack, which induce enlargement of the pore size followed by failure in salt rejection, can be detrimental to desalination performance. Investigations were carried out on the effects of reactant ratios on the microstructure of the silica matrices and their hydrothermal stability under harsh conditions (550 °C, 75 mol% vapour). The most hydrothermally stable matrix was obtained by decreasing the ethanol ratio whilst increasing water and acid ratios. The improved hydrothermal stability was attributed to the further transition of I silanol to siloxane groups on one hand and the formation of a more openly branched silica network on the other. Further work was conducted on the preparation of ES40 derived interlayer-free silica membranes using RTP on the optimized reactant ratios as determined from above investigations. RTP involves a rapid heating rate and a short period of high temperature exposure up to 1 hour. This thesis shows for the first time the preparation of interlayer-free silica membranes by RTP. A major advantage of this novel preparation method is that the final membranes were coated with 2 silica layers only and were prepared in less than 3 hours, a considerable and significant reduction from 10-14 days for TEOS silica membranes. A comparison of the structure properties of silica materials produced by RTP and conventional thermal processing (CTP) found that the silica produced from RTP has more siloxane bridges and larger porosity, which can potentially deliver the corresponding membranes enhanced hydrostability and water fluxes. The preparation feasibility of membranes was dependent on the pH of the sol-gel. The best membranes were prepared with ES40 sols with pH 4, which adhered well to substrates during the RTP calcination step. Water fluxes reached 17.8 kg m-2 h-1 for seawater (NaCl 3.5 wt%) at 60 °C whilst reaching salt rejections 95–99%. The long term stability tests showed stable water flux and salt rejection until ~300 hours. This is a major improvement from TEOS silica membranes which tend to fail within a few hours. In order to further improve the hydrostability, the effect of H2O/Si, EtOH/Si and pH of the sol-gel solutions on silica xerogel properties and membrane performance was carried out. The prepared membranes gave good water flux (7 kg m-2 h-1) and excellent salt rejection (>99%) for seawater desalination. In addition, long term testing at at room temperature for 3.5 wt% saline solutions showed that the ES40 membrane was stable for 800 hours, whilst delivering 99% salt rejection. These results strongly suggested that high quality interlayer-free membranes were successfully prepared using RTP from ES40 precursor for the first time, delivering performance well above the current state of art.