Structural transformation of the nickel coordination-induced subnanoporosity of aminosilica membranes for methanol-selective, high-flux pervaporation

A novel strategy to modify the dense framework from different types of amine-functionalized organosilica membranes has been successfully applied via nickel-coordination to induce the formation of microporous membranes. Prior to the nickel-coordination reaction, aminosilica sols were prepared via hydrolysis of several types of amine precursors: bis [3-(trimethoxysilyl) propyl] amine (BTPA), trimethoxy [3-(methyl amino) propyl] silane (MAPTS), and 3-aminopropyl triethoxy silane (APTES). The optimal nickel/amine mole ratio was established within a range from 0 to 0.50 mol mol⁻¹, and calcinations of the membranes were performed at 250, 300, and 350 °C. We found that nickel doping restructured the aminosilica network via a coordination bond, which then increased both the rigidity of the organic chain and the surface area of the resultant materials on the order of nickel-doped bis [3-(trimethoxysilyl) propyl] amine (Ni-BTPA) > nickel-doped 3-aminopropyl triethoxy silane (Ni-APTES) > nickel-doped trimethoxy [3-(methyl amino) propyl] silane (Ni-MAPTS). Spectroscopy characterization studies such as Ultraviolet–visible (UV–vis), Fourier Transform Infrared (FT-IR), and X-ray Diffraction (XRD) along with micropore analysis of N₂ sorption isotherms revealed that the formation of a coordinated network could be sterically hindered by the existence of non-hydrolyzable methyl groups on the pendant chain. The prepared composite membranes were utilized for the pervaporation of various types of organic mixtures for a composition ratio of methanol/solvents of 10/90 wt% at 50 °C. All nickel-composite membranes showed high flux and outstanding performance for the pervaporation of methanol/dimethyl carbonate (MeOH/DMC) and methanol/toluene (MeOH/Tol) mixtures. Ni-BTPA membranes with bridge-structured secondary-amine ligands recorded values for the flux that reached as high as 1.42 kg m⁻² h⁻¹ with an optimum separation factor for MeOH/Tol of 5,000.