New Insights into the Microstructure-Separation Properties of Organosilica Membranes with Ethane, Ethylene, and Acetylene Bridges

Microporous organosilica membranes with ethane, ethylene, and acetylene bridges have been developed and the extensive microstructural characterization has been discussed in relation with separation properties of the membrane. The organosilica network structure and the membrane performances can be controlled by adjusting the flexibility, size, and electronic structure of the bridging groups. A relatively narrow size distribution was obtained for the novel acetylene-bridged sol by optimizing the sol synthesis. Incorporation of larger rigid bridges into organosilica networks resulted in a looser microstructure of the membrane, which was quantitatively evaluated by N2 sorption and positron annihilation lifetime (PAL) measurements. Molecular weight cutoff (MWCO) measurements indicated that the acetylene-bridged membrane had a larger effective separation pore size than ethane- and ethylene-bridged membranes, leading to a relatively low NaCl rejection in reverse osmosis. In quantum chemical calculations, a more open pore structure and increased polarization was observed for the acetylene-bridged networks, which led to a significant improvement in water permeability. The present study will offer new insight into design of high-performance molecular separation membranes.