Rapid thermal treatment of interlayer-free ethyl silicate 40 derived membranes for desalination

This work demonstrates for the first time the preparation of interlayer-free silica membranes by the Rapid Thermal Processing (RTP) method at 630 degrees C using ethyl silicate (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 alpha-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 degrees 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. (C) 2016 Elsevier B.V. All rights reserved