Metal-induced microporous aminosilica creates a highly permeable gas-separation membrane

Abstract

Hybrid microporous aminosilica membranes have been successfully synthesized via doping with Ag-, Cu- and Ni- into dense bis[3-(trimethoxysilyl)propyl] amine (BTPA) membranes, which creates micropores via the crosslinking between donor pairs of electrons in the amine moiety and electron acceptors in the empty “d” orbital of the transition metal. The formation of micropores within the coordinated covalently bonded compound was investigated via Ultraviolet-Visible spectroscopy (UV-Vis), Fourier Transform Infrared spectroscopy (FT-IR), X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and the isotherms of N₂ and CO₂ sorption. Values for the surface area and pore volume of the metal-doped BTPA were both expanded by increasing the metal coordination affinity to 214 m² g⁻¹ and 0.185 cm³ g⁻¹, respectively. The effect of metal doping on the membrane separation performance was evaluated using a single-gas permeation system and the activation energy of permeance was measured. Gas permeation was increased following the doping process due to the formation of a microporouse structure on the order of Ni-BTPA > Cu-BTPA > Ag-BTPA > BTPA, which corresponds to higher affinity for metal coordination. Permeation behavior was dominated by the molecular sieving effect that showed a high level of H₂ permeance at 4.45 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with a H₂/SF₆ permeance ratio that reached 15 500, which is indicative of a defect-free membrane. By comparison with Cu-BTPA (14.8 kJ mol⁻¹), Ag-BTPA (19.5 kJ mol⁻¹), and BTPA (31.1 kJ mol⁻¹) membranes, the Ni-BTPA membrane showed the lowest value for CO₂ activation energy (8.9 kJ mol⁻¹), which can be ascribed to the microporosity that was formed by a higher coordination interaction. The nickel-doped BTPA achieved high levels of both permeance of N₂ and selectivity for N₂/SF₆ at 3.75 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ and 1900, respectively.