Synthesis and characterization of silica membranes exhibiting an ordered mesoporosity. Control of the porous texture and effect on the membrane permeability

Hexagonal mesoporous silica membranes were prepared by the sol-gel route us ing a silicon alkoxide as silica precursor and alkyltrimethylammonium bromi des to form the templating liquid crystal mesophase. These 2D mesoporous st ructures consist of cylindrical pores whose axes are preferentially aligned parallel to the surface of the substrate due to interfacial interactions a t both air-layer and layer-substrate interfaces. In order to increase the p ore accessibility and to improve the permeability of the hexagonal porous l ayers, the starting solutions were seeded with silica nanoparticles. This s eeding promotes a heterogeneous nucleation of the templating mesophase by c reating additional interfaces inside the hybrid gelling solution. X-ray dif fraction techniques and transmission electron microscopy demonstrated clear ly that the preferential ordering of the cylindrical pores parallel to the surface can be destroyed by nanoparticle seeding. Nitrogen permeation measu rements were carried out on unseeded and seeded hexagonal silica membranes. As expected from the previous static characterizations, the membrane perme ability is strongly increased ( x 10) in the case of seeded layers. The mea sured permeabilities were discussed from the expected model of gas transpor t in mesoporous materials. (C) 2001 Elsevier Science B.V. All rights reserv ed.