Development of Silica-Derived Molecular Sieving Membranes Via Controlling Microporous Structure

Silica is one of the most attractive materials for hydrogen separation because of its amorphous structures, through which small molecules such as helium and hydrogen can permeate, in a wide temperature range. At the end of the 1980s, sol-gel derived amorphous silica membranes were found to have excellent separation performance for hydrogen or helium at high temperatures.[1] Accordingly, many research groups have focused on the development of amorphous silica membranes for gas separation and pervaporation.[1,2]
However, two critical issues remain to be solved in the practical application of amorphous silica membranes in the separation and purification process: the hydrothermal stability of amorphous silica networks, and the difficulty of controlling the silica network size. In the present study, pore size controllability via “spacer” method and fluorine doping was introduced based on the results of gas permeation properties.

Pore size control and gas permeation properties of organosilica membranes
The selection of Si precursors made it possible to control the network size of silica structure.[3-7] Organosilica membranes were fabricated using bridged organoalkoxysilanes (bis(triethoxysilyl)methane (BTESM), bis(triethoxysilyl)ethane (BTESE), bis(triethoxysilyl)propane (BTESP), bis(trimethoxysilyl)hexane (BTMSH), bis(triethoxysilyl)benzene (BTESB), and bis(triethoxysilyl)octane (BTESO)) to produce highly permeable molecular sieving membranes. Organosilica membranes showed H2/N2 and H2/CH4 permeance ratios that ranged from 10-150. H2 selectivity was decreased with an increase in the carbon number between 2 Si atoms, indicating that the network pore size could be successfully controlled even by utilizing the flexible organic bridges (Si-C3-Si, Si-C6-Si, and Si-C8-Si).

Pore size control and improved stability via fluorine doping
The utilization of triethoxyfluorosilane (TEFS) containing Si-F bonds, the addition of NH4F and alkali fluorides, and HF-catalyzed reactions can be accepted as a common method to eliminate the Si-OH density. The incorporation of Si-F bonds in an amorphous structure affected the Si-O-Si bond angle, and this change in the SiO2 structure made it possible to form looser and more-uniform structures.[8-10] Highly permeable CO2 separation membranes (CO2 permeance: 4.1 x 10-7 mol m-2 s-1 Pa-1, CO2/CH4 permeance ratio: 300 at 35 oC) were successfully developed via fluorine doping of the SiO2 matrix.[8] Recently, a molecular sieving membrane was fabricated using TEFS, and the densification of the network structure was apparently suppressed by comparison with a TEOS-derived structure, due to the presence of fewer Si-OH groups.[10]

References
[1] Ockwig, N. W., Nenoff, T. M., Chem. Rev., 107, 4078 (2007). [2] Lin, Y. S. et al., Sep. Purif. Method., 31, 229 (2002). [3] Kanezashi, M. et al., J. Am. Chem. Soc., 131, 414 (2009). [4] Kanezashi, M. et al., Ind. Eng. Chem. Res., 51, 944 (2012). [5] Kanezashi, M. et al., AIChE J., 58, 1733 (2012). [6] Kanezashi, M. et al., J. Membr. Sci., 415-416, 478 (2012). [7] Kanezashi, M. et al., AIChE J., 63, 4491 (2017). [8] Kanezashi, M. et al., ChemNanoMat, 2, 264 (2016). [9] Kanezashi, M. et al., ACS Appl. Mater. Interfaces, 9, 24625 (2017). [10] Kanezashi, M. et al., J. Membr. Sci., 549, 111 (2018).