Tailoring the Separation Behavior of Hybrid Organosilica Membranes by Adjusting the Structure of the Organic Bridging Group

Abstract

Hybrid organically linked silica is a highly promising class of materials for the application in energy‐efficient molecular separation membranes. Its high stability allows operation under aggressive working conditions. Herein is reported the tailoring of the separation performance of these hybrid silica membranes by adjusting the size, flexibility, shape, and electronic structure of the organic bridging group. A single generic procedure is applied to synthesize nanoporous membranes from bridged silsesquioxane precursors with different reactivities. Membranes with short alkylene (CH2 and C2H4) bridging groups show high H2/N2 permeance ratios, related to differences in molecular size. The highest CO2/H2 permeance ratios, related to the affinity of adsorption in the material, are obtained for longer (C8H16) alkylene and aryl bridges. Materials with long flexible alkylene bridges have a hydrophobic surface and show strongly temperature‐dependent molecular transport as well as a high n‐butanol flux in a pervaporation process, which is indicative of organic polymerlike properties. The versatility of the bridging group offers an extensive toolbox to tune the nanostructure and the affinity of hybrid silica membranes and by doing so to optimize the performance towards specific separation challenges. This provides excellent prospects for industrial applications such as carbon capture and biofuel production.

Structure–property tuning in hydrothermally stable sol–gel-processed hybrid organosilica molecular sieving membranes

“…In DLCA, the approach of particles is governed by Brownian transport while the reaction rate is much higher than the rate of diffusion. A SAXS study on sols with -CH 2 -, -C 2 H 4 -, -C 8 H 16 -, -p-C 6 H 4 -and -p-C 6 H 4 -p-C 6 H 4 -bridging groups showed that although the reactivity of the precursors varied, the values of D i were similar (D i = 1.50-1.60) when the radii of gyration were similar (R g = 1.5-2.3 nm; hydrodynamic diameter 6 nm) [12]. Defect-free membranes with molecular sieving properties could be formed from all five sols.…”

“…7 [13]. The permeability of water decreased gradually with increasing value of x. Aromatic bridges like p-phenylene and di-p-phenylene bridges (CH x :Si = 6, 12) seem to be less effective in promoting the flux of n-butanol compared with alkyl bridges with similar CH x :Si ratios [12].…”

“…Combinations of different precursors might allow the formation of microporous membranes with small pore size and hydrophobic character, but this is still a largely unexplored area of research. A water/n-butanol pervaporation study on hybrid organosilica membranes made from precursors with -C n H 2n -(n = 1, 2, 8) and -(p-C 6 H 4 ) m -(m = 1, 2) bridges showed that the n-butanol flux was larger for the systems with n = 8 and m = 2 [12], while the water flux remained relatively constant. It suggests that while the permeability of less hydrophilic fluids like n-butanol is promoted by a higher CH x :Si ratio, the accompanying enlargement of the pore size seems to be playing an equally significant role.…”

“…To avoid a wide distribution of sizes in the sol, the hydrolysis is often carried out in two steps so that water-lean conditions are maintained throughout the process [24]. It is noted that in later studies hybrid organosilica membranes were mostly prepared from a single type of alkoxide precursor such as BTESE or bis(triethoxysilyl)methane (BTESM) [12,25,26], although condensation of monomers into relatively inert cyclic dimers that do not contribute to gel formation may have led to a decreased yield in these cases [22,27].…”

“…Bridged silsesquioxanes that have already been reported for hybrid organosilica membranes contain aliphatic or aromatic bridging groups [12][13][14]. The precursors referred to in this article are a,x-bis(triethoxysilyl)-R compounds, where R is an alkylene -C n H 2n -(n = 1, 2, 3, 6, 8, or 10), ethenylene (-C 2 H 2 -), ethynylene (-C:C-) or an arylene Fig.…”