Evaluation of hybrid silica sols for stable microporous membranes using high-throughput screening

Abstract

Microporous membranes are a promising option for energy-efficient molecular separations. Longterm hydrothermal stability of the membrane material is of prime importance for several industrial processes. Here, a short overview of silica-based membrane materials and their hydrothermal stability is presented. Following this, the development of a series of organic-inorganic hybrid silica sols is described, based on a,x-bis(triethoxysilyl)-precursors with bridging methane, ethane, propane, and benzene groups. High-throughput screening was used to scan a range of sol parameters, followed by membrane preparation from the most promising sols. These organicinorganic hybrid silica (HybSi Ò ) membranes were used in dewatering of lower alcohols by pervaporation. Separation factors up to 200 were found for ethanol/water mixtures, and up to 23 for methanol/water mixtures. Modest permselectivity values for hydrogen over nitrogen were found, ranging up to 20.7 for the shortest bridging group. It was concluded that the length of the organic bridge has a clear effect on the pore size distribution and the selectivity of the membrane.

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

“…Longer bridge lengths [12] and stiffer bridges [14] lead to larger pores and therefore to lower permselectivities, although long flexible chains such as -C 8 H 16 -appear to collapse, leading to loss of permeability. On the other hand, BTESM-derived membranes with permselectivities of 15-21 for H 2 /N 2 and 7-9 for H 2 /CH 4 [28] perform only marginally better than BTESE-derived membranes in H 2 separation even though their bridge length is shorter. They show higher permselectivity in the separation of Apart from compositional differences, the sol-gel fabrication process itself also has a significant influence on the final properties of the membrane.…”

“…These seemingly conflicting results show that not only pore size, but also factors like pore connectivity and physicochemical interactions between permeating species and the membrane matrix are important to consider. Even higher water fluxes and higher separation factors are obtained when the -C 2 H 4 -bridge (BTESE) is replaced by -CH 2 -(BTESM) [25,28]. The pore size of this system is between that of BTESE and BTESE-MTES [25].…”

“…Comparable sols with an average size of 4.9 nm yielded defect-free membranes [19], but sols with an average hydrodynamic diameter of 13 nm formed thick films with visible macroscopic cracks after drying. High-throughput screening approaches have been employed to optimize the colloid size of various bridged precursors with methylene, ethylene, p-phenylene and di-p-phenylene bridging groups [28]. Hydrolysis ratio, acid ratio and reaction temperature were varied systematically.…”

“…Not many membranes can separate this mixture effectively under high flux conditions. In comparison, BTESE-MTES and BTESE show no appreciable selectivity for methanol-water, and BTESE-MTES only has low selectivity for ethanol-water (a H 2 O=EtOH = 15) [28]. BTESM has separation factors[150 for ethanol, i-propanol and n-butanol and shows higher water fluxes under otherwise similar conditions [28].…”

“…Very high separation factors H 2 /N 2 [ 4000 have been reported for silica in the 1990s [4], but the permselectivity values are much lower for MTES-derived methylated silica [15]. The gas separation selectivity of hybrid organosilica membranes is also rather moderate [28,50]. The low intrinsic permselectivity has been ascribed to the relatively open pore structure of -C 2 H 4 -and -C 2 H 2 -bridged silicas [42,64], and that hypothesis is supported by molecular dynamics simulations of the pore size distribution of BTESE hybrid organosilica and silica [80,81], see Fig.…”