Molecular Sieving Silica (MSS) membranes are novel amorphous microporous membranes. Conventionally MSS membranes have been synthesised by sol-gel processes employing a single-step catalysed hydrolysis of tetraethylorthosilicate in ethanol solution and using HNO3 or HCl as a catalyst. For weakly branched systems prepared with low H20/Si molar ratio (r<5) by the single-step catalysed hydrolysis sol-gel process, there is the tendency for structures to interpenetrate forming micropore apertures of molecular size. Furthermore, there is a strong relation between weakly branched systems with 29Si NMR uncondensed species (Q2 and Q3) consistent with fractal dimension df<2. By the same token, a two-step catalysed hydrolysis with an initial r=1 followed after 90 minutes by an additional r~3 further enhanced the production of weakly branched structures.
If the concept of two-step catalysed hydrolysis for bulk gels can be translated to membrane film technology, pore size reduction can be potentially achieved resulting in improved membrane separation factor. Therefore, the most important novel features of this work are the development of MSS membranes for gas separation based on the two-step catalysed hydrolysis sol-gel process, in which greater pore size tailorability and superior gas separation performance are achieved over conventional single-step processed membranes. Three fundamental measures are used to test the contributions of this work, as compared with the single-step membrane films:
Based on 29Si NMR studies, the two-step xerogels consistently had higher Q3 and Q2 contributions compared with the single-step process. These results suggest that the condensation reactions were inhibited in the double-step catalysis hydrolysis sol-gel process, an important factor in the production of gels of weakly branched systems.
Based on typical permeance data, the double-step membranes have much higher permselectivities for He/C02 (29.6-336) than single-step membranes (1.3-6.7) while CO2/CH4 (1.8-36) permselectivities are much lower (5.5 to 130). These results strongly suggest that the single-step sol-gel preparation method has a good probability of controlling the pore radius of the xerogel structure at the region of 3.4Å while the two-step sol-gel preparation method shows an excellent pore tuning for pore sizes at about 3Å.
The energy of mobility (Em) for He permeance (10.9-17.2 kJ.mol-1) for the two-step membranes are one and half to over four fold higher than the single step membranes (3.6-10.6 kJ.mol-1). The energy of mobility of CO2 permeance also follows the same trend. As the energy of mobility is lower for larger pores than small pores, the membranes prepared with the two-step sol-gel process consistently showed higher energy of mobility and therefore a smaller pore radius than the single-step sol-gel membranes.
The membranes films are cast on α-Al2O3 supports and a membrane process development method is fully described including the use of organic template agents in intermediate film layers as substrate bases for weakly branching film coating. Furthermore an extensive surface characterisation of the materials are undertaken including spectroscopy analysis such as 29Si NMR, FTIR, TGA XPS and SEM. The study of the sorption characteristics of the materials for H2, CO2, N2, O2 and CH4 are obtained using volumetric and gravimetric methods.
The testing of membranes for single gas permeation is fully supported by applying a mathematical model which takes into consideration the adsorption characteristics of the silica materials. The model for the permeation of single gas in a dynamic system is based on microporous diffusion which is the limiting permeation step. To compliment this work, a model and analysis of the permeation of binary mixtures in a steady state system (Wicke-Kallenbach) is also developed. In similar fashion to permselectivites of single gases, the selectivity of 50/50 binary mixtures resulted in very high separation for H2/CH4 (17-250) and H2/CO2 (10-27). It was found that the flux of binary mixtures is dominated by the membrane pore size distribution and differences in adsorption and mobility. Using a Wicke-Kallenbach set-up, the permeation of binary mixtures increased with the partial pressure complying with a natural logarithm function rather than linear as it is reported in the literature for silicalite membranes. The two-step membranes compare reasonably well or even better than silica and silicalite membranes reported in the literature in terms of permeation, separation of gases and activation of energy.