42086-AC5
Synthesis and Diffusion Properties of Ordered Nanoporous Silica Membranes with Controlled Macroscopic Morphology
Our research effort in the past year was directed towards characterization and improvement of ordered mesoporous silica membranes that were synthesized by a counter-diffusion self-assembly (CDSA) method. In this method a porous polycarbonate support is placed at the interface separating a silica source and water phase (water, surfactant and acid). Water, surfactant and silica precursors counter-diffuse into the pores of the support, where the silica precursor undergoes hydrolysis and condensation. Fibrous silica containing a large number of straight or helically ordered mesopores (~3nm) align within the pores.
The CDSA growth of ordered mesoporous silica plugs within the pores of hydrophilic and hydrophobic track-etched polycarbonate supports was studied. Three methods (A, B and C) were used to grow these membranes. Method B was developed to accommodate for evaporation induced self assembly (EISA) and Method C to not only account for EISA but also allow for the alignment of the support at the interface of the two phases. The membranes were characterized by XRD, SEM and oxygen gas permeation. Nitrogen sorption studies where also conducted to analyze pore structure, surface area and pore volume of the silica plugs. Since a very small sample was available per membrane (~0.001g), it was necessary to collect the plugs from 9-12 membranes of each type to conduct these studies. It was determined from TGA measurements that the polycarbonate support degrades at 575°C, so the membranes were heated to 650°C to ensure complete removal of the polycarbonate support and to calcine the silica plugs.
Method A resulted in uniform plugging of all pores, as evidenced by SEM micrographs, and confirmed by a two order decrease in permeation (1.40-6mol/m2∙Pa∙s). XRD and Nitrogen sorption studies on hydrophobic and hydrophilic plugs did not show the presence of any ordered mesopores. This is because the arrangement of micelles is hindered as evaporation of the solvent is blocked due to the presence of a column of the silica source. Therefore hydrolysis and condensation of the silica source takes place in the absence of an ordered template resulting in a disordered structure.SEM micrographs of membranes synthesized by Method B revealed that good plugging can achieved in hydrophobic supports. The permeance of the as-synthesized hydrophobic membrane was 5.37∙10-6mol/m2∙Pa∙s, indicating that most of the pores in the support were plugged. The corresponding XRD patterns showed the presence of three reflection peaks, indexed as (100), (200), and (110) planes, typical of hexagonal pore symmetry. The interplanar spacing (d100) and lattice parameter (a0) were calculated to be 4.05nm and 4.68nm respectively. A typical Type–IV nitrogen sorption isotherm was observed for the silica plugs, which is characteristic of mesoporous materials with well-aligned channels. A narrow pore size distribution with mean BJH diameter of 2.7nm was obtained from the desorption branch, which was similar to that obtained from the adsorption branch, which is indicative of uniform cylindrical pore shapes. The plugs have a BET surface area close to 400m2/g and pore volume of 0.26cm3/g. These values are lower than expected, but can be explained by the fact that a very small sample mass (~5.0µg) was used for characterization, leading to errors in calculation. The hydrophilic plugs also showed similar results, with a pore size, surface area, and pore volume of 2.6nm, 836m2/g, and 0.59m2/g respectively. Some membranes synthesized by this method had particles deposited on the supports and no plugging was seen, which is believed to occur due to improper placement of the support at the interface.
Membranes synthesized by Method C were well plugged. SEM images revealed that 80-90% of the pores were plugged in hydrophobic supports. Permeance of the membranes was 3.7∙10-6mol/m2∙Pa∙s, indicating the synthesis of good quality membranes. Three reflection peaks in the XRD pattern, indexed as (100), (200), and (110), verified hexagonal ordering, and the interplanar spacing and lattice parameter were calculated to be 4.14 and 4.78nm respectively. A typical Type-IV isotherm was obtained from the silica plugs and a narrow pore size distribution with mean BJH diameter of 2.5nm was obtained. The surface area and pore volume where found to be 990m2/g and 0.60cm2/g respectively. Results of the study have shown that ordered mesoporous silica membranes can be synthesized with the CDSA approach. The major factors that affect membrane quality are the transport of precursors, alignment of the support and evaporation induced self assembly. When all there of these factors are taken into account, as proven by Method C, good quality membranes can be synthesized.
