Synthesis of organically bridged trialkoxysilanes bearing acetoxymethyl groups and applications to reverse osmosis membranes

New bridged trialkoxysilanes bearing acetoxymethyl groups were synthesized by double hydrosilylation of 1,6-diacetoxy-2,4-butadiyne, using two equivalents of triethoxysilane and a metal catalyst. With a Ru catalyst, the reaction proceeded via anti-addition to provide BTES-Ac-a as a single isomer, while a similar reaction with a Pt or Rh catalyst provided an isomeric mixture of syn-adducts BTES-Ac-b. Reverse osmosis (RO) silica membranes were prepared by the sol–gel process with BTES-Ac-a and BTES-Ac-b and the membranes were examined with respect to water desalination using a 2000 ppm NaCl aqueous solution. NaCl rejection of the membranes increased to reach 96% at the early stage of the RO experiments. However, the rejection decreased gradually to 85% after 70 and 200 h for BTES-Ac-a and BTES-Ac-b, respectively, due to hydrolytic decomposition of the silica network during the experiments. In contrast, a membrane prepared from copolymerization of BTES-Ac-a with ethane-bridged bistrialkoxysilane (BTES-E1) showed improved stability towards hydrolysis with stable NaCl rejection of 96% with higher water permeance (3.5 × 10−13 m3 m−2 s−1 Pa−1) than that of a membrane prepared by homopolymerization of BTES-E1 (2.7 × 10−14 m3 m−2 s−1 Pa−1) reported previously.