Add time:07/20/2019         Source:sciencedirect.com

This work demonstrates for the first time the preparation of interlayer-free silica membranes by the Rapid Thermal Processing (RTP) method at 630 °C using Ethyl silicate (cas 11099-06-2) (ES40) as a silica precursor. RTP resulted in water being retained inside the xerogel pores at higher temperatures, thus favouring condensation reactions and the formation of siloxane bridges. As a result, RTP ES40 xerogels were stronger than their conventional thermal processing (CTP) counterparts and also had higher pore volumes and surface areas. Nevertheless, RTP ES40 xerogels were characterised by a tri-modal pore size distribution. The majority of pores were in the microporous domain with a minor fraction of mesopores. Successful preparation of membranes without a conventional interlayer was dependent on the pH of the sol-gel, as sols with a pH 4 or 6 were able to penetrate into the α-alumina substrate allowing strong adhesion to the support and formed a defect free top layer. The best interlayer-free RTP silica membrane was prepared with a pH 4 sol reaching high water fluxes of 17.8 kg m−2 h−1 at 60 °C and high salt rejection (>99%) for desalination of 3.5 wt% NaCl synthetic seawater. The dilution of the sol with ethanol also played an important role in the membrane stability. Membranes prepared from a sol of pH 4 with EtOH:Si ratio 255:4 were stable for 120 h, but a reduction in this ratio to 200:4 increased the stability to 300 h. This improvement was attributed to thicker films derived from more viscous sols.

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