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

Abstract

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 (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 17.8kgm−2h−1 at 60°C and high salt rejection (>99%) for desalination of 3.5wt% 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 120h, but a reduction in this ratio to 200:4 increased the stability to 300h. This improvement was attributed to thicker films derived from more viscous sols.