Synthesis of size-controlled boehmite sols: application in high-performance hydrogen-selective ceramic membranes

A hydrogen-selective silica–tantala composite membrane was prepared by chemical vapor deposition (CVD) of tetraethyl orthosilicate (TEOS) and tantalum ethoxide (TaEO) at 650 °C. The membrane composite consisted of a thin (ca. 15 nm) silica–tantala toplayer, a graded γ-alumina interlayer, and a porous zirconia tubular support. The graded γ-alumina interlayer was deposited via a sol–gel process using progressively smaller mono-dispersed boehmite (AlOOH) sols, with the boehmite sol nanoparticle size optimized by adjusting the solution pH and aging time. The resultant ceramic membrane exhibited high H2 selectivities over CH4, N2, and O2 of 16 000, 5000, and 640, respectively, with an H2 permeance of 1.4 × 10−7 mol m−2 s−1 Pa−1 at 650 °C. Exposure to 16 mol% steam at 650 °C for 200 h caused an H2 permeance decline of 25%, and simultaneous H2 selectivity increase over CH4, N2 and O2 to 71 900, 42 600 and 2460, respectively. The hydrothermal stability and the final H2 selectivities are the highest ever reported for silica-based membranes. The high performance is attributed to the defect-free deposition of the silica–tantala layer due to the optimization of the graded γ-alumina interlayer. Optimization of boehmite sol particle size and stability is crucial for state-of-the-art membranes, with these results also applicable in different fields.