Orchestrating pore structure of hybrid silica membranes for molecular separations

Abstract

Sol-gel derived microporous silica membranes have shown unprecedented hydrogen gas selectivities in dry atmospheres; however their performance is reduced in humid conditions due insufficient material stability. To deal with this issue, one of the strategies is to incorporate hydrocarbons (either as a bridge or at terminals) in the silica matrix. However, the possibility of utilizing bridged-hydrocarbons in silica system i.e. hybrid silica, has not been studied in detail for molecular separations, which was the main focus of this research. The synthesis protocol of sol-gel derived microporous hybrid silica [1,2-bis(triethoxysilyls)ethane (BTESE)] membranes was developed and the effect of dip-sol rheology on the selective layer thickness and membrane performance was studied in detail. The understanding of sol synthesis and membrane fabrication was further investigated by reducing the acid contents (by a factor 15) during sol synthesis in order to enhance the performance of BTESE membranes. The newly developed BTESE membranes were found nitrogen impermeable, i.e. the pore size and structure of these membranes was smaller than nitrogen gas (below 0.36 nm). Doping of elements and metal ions in hybrid silica matrix was performed with the aim to further improve the stability and permselectivity (permeance and selectivity) of these membranes. It was observed that element doping in BTESE matrix resulted in enhanced fluxes of all the observed gases (He, H2, CO2, N2, CH4, and SF6) with no improvement in the hydrogen permselectivities when compared with undoped BTESE membranes. That triggered us to develop another hybrid silica precursor (N,N,N`,N`- (tetrakis-(3-(triethoxysilyl)propyl) malonamide [TTPMA]; in co-operation with IMS-UT), in which metals and ions can be easily and homogenously dispersed, resulting in membranes with enhanced hydrogen separation index. Two ions Ce and Ni were doped in the TTPMA matrix, and like BTESE, molecular sieving phenomena was observed in Ce-TTPMA membranes while Ni-TTPMA membranes were also found nitrogen impermeable confirming a more constricted pore microstructure of these membranes. An attempt to develop a mesoporous hybrid silica structure as an alternative to traditional mesoporous γ-Al2O3, which degrades fast at low pH systems, was made and it was figured out that the hybrid sol was penetrated inside the macroporous α-Al2O3 membrane supports. Either an increase in size of hybrid silica sol particles up to about 40 nm, or the use of a viscosity modifier can be a way to prevent sol infiltration and developing a smooth mesoporous hybrid silica surface on macroporous alumina supports.