Synthesis of Hydrogen Permselective SiO₂, TiO₂, Al₂O₃, and B₂O₃ Membranes from the Chloride Precursors

Hydrogen-permselective membranes were synthesized by chemical vapor deposition of SiO₂, TiO₂, Al₂O₃, and B₂O₃ layers within the pores of Vycor tubes. The deposition involved reaction of SiCl₄, etc., with water at 100-800 °C depending on the chloride and the reaction geometry. Permselective SiO₂ layers could be formed in either the one-sided or the two-sided (opposing reactants) geometry, while deposition of TiO₂ and Al₂O₃ layers was achieved only in the two-sided geometry. The permeation coefficients at 450 °C were 0.3 and 0.1 cm³/(cm²·min·atm) for SiO₂ membranes produced in the one-sided and two-sided geometries, respectively. The H₂:N₂ permeation ratios were 1000-5000. The TiO₂ and Al₂O₃ membranes had somewhat lower permeation coefficients and H₂:N₂ ratios. The membranes were characterized by scanning electron microscopy (SEM) and electron microprobe analysis (EMA). A reaction mechanism is suggested involving heterogeneous reactions of chloride and water molecules with hydroxyl and chloride groups in the growing deposit layer.