Sol–Gel‐Derived Silica Membranes

Abstract

Abstract In this article, sol–gel‐derived silica membranes are reviewed in terms of the control of silica network size and hydrothermal stability, which are the most important issues for the practical, industrial application of amorphous silica membranes. For control of the silica network size, “organic template” and “spacer” methods have been introduced. Bridged alkoxides with organic groups between two Si atoms can be used for the control of silica network size via the “spacer” method. The pore size distribution, as determined by single‐gas permeation and normalized Knudsen‐based permeance (NKP), suggests the following order for average pore sizes owing to differences in the minimum units of the silica precursor: bis(triethoxysilyl) ethane (BTESE)‐derived silica (Si−C−C−Si unit) > bis(triethoxysilyl) methane (BTESM)‐derived silica (Si−C−Si unit) > tetraethoxysilane (TEOS)‐derived silica (≡Si−O). For hydrothermal stability of amorphous silica membranes at high temperatures, metal‐doped silica membranes have been introduced, particularly Ni‐ and Co‐doped silica membranes that have superior hydrothermal stability and hydrogen permeation performance. For example, Ni‐doped silica membranes showed a H 2 permeance above 1.0 × 10 −7 mol/(m 2 s Pa) with a H 2 /N 2 permselectivity of more than several hundred after exposure to a steamed atmosphere (500 °C, partial pressure of steam: 400 kPa). Doped metals may exist as metal ions, as covalently bound compounds, or as tiny crystals dispersed in amorphous silica networks, and one possible mechanism by which they prevent the densification of silica networks is that they increase in stability under hydrothermal conditions. Also, in this article, we summarize recent progress in metal‐doped silica membranes, such as Nb−SiO 2 and Pd−SiO 2 , which have shown unique gas permeation properties.