Seeding-free synthesis of dense zeolite FAU membranes on 3-aminopropyltriethoxysilane-functionalized alumina supports

A seeding-free synthesis strategy was developed for the preparation of dense and phase-pure zeolite FAU membranes using 3-aminopropyltriethoxysilane (APTES) as covalent linker between the FAU layer and the alumina support. APTES acts as molecular linker for anchoring the FAU precursors onto the support surface to promote the nucleation and the formation of a thin, well intergrown FAU membrane. Additionally, at the pH of zeolite FAU crystallization (about 11), the zeta potential of the alumina support is changed by APTES-treatment from strongly negative to neutral, which will be helpful to reduce the electrostatic rejection between the negatively charged precursor species and the alumina support. The SEM and XRD characterizations indicate that a relative thin but dense and phase-pure FAU zeolite membrane with a thickness of about 2.5 μm can be formed on the α-Al2O3 support after crystallization at 75 °C for 24 h, and no cracks, pinholes or other defects are visible in the membrane layer. The zeolite FAU membranes prepared on APTES-modified Al2O3 supports display higher separation selectivities than those prepared on non-modified Al2O3 support. For binary mixtures at 100 °C and 1 bar, the mixed gas separation factors of H2/CO2, H2/N2, H2/CH4 and H2/C3H8, are found to be 6.5, 6.0, 4.0 and 11.1, respectively, which are higher than the corresponding Knudsen coefficients. Further, relative high H2 permeances of about 4.0 × 10−7 mol m−2 s−1 Pa−1 can be obtained through the FAU membrane due to the thin layer and the relative wide pores of 0.74 nm.