Niobia-silica and silica membranes for gas separation
Prof. Dr. Ing. D.H.A. Blank
Dr. Ir. J.E. ten Elshof
This thesis describes the development of ceramic membranes suitable for hydrogen separation and CO2 recovery from gaseous streams. The research work was focused on the three different parts of which gas‑selective ceramic membranes are composed, i.e., the microporous gas‑selective silica layer, the mesoporous interlayer and the macroporous support.
Very high retention of CO2 was achieved by introducing Nb5+ ions in the microporous silica framework of the silica top layer. Niobia-silica membranes were fabricated by coating an asymmetric γ-alumina disk with a niobia-silica polymeric sol prepared from metal alkoxide precursors. The sol had a Nb : Si molar ratio equal to 0.33. The film was fired at 500 °C. Generally, the permeance of the gases in this membrane decreased steadily with molecular size. Helium and hydrogen showed thermally activated transport through the membrane. The permeance of carbon dioxide deviated strongly from the general trend and was more than 5 times lower than the permeability of SF6. The interaction of CO2 with the pores was studied by ATR-FTIR on a thin film, and is probably due to a relatively strong interaction between Nb-bound hydroxyl groups and CO2. The promising results obtained with this membrane initiated further optimization of its microstructure.
Small angle X-ray scattering (SAXS) analysis of different niobia-silica sols showed that the fractal dimension (Df) of the sol particles increased as a function of time, to remain constant at an upper value equal to 1.9, when tetraethyl orthosilicate (TEOS) is used as silica source. The gyration radius (Rg) of these sols grew proportionally to t0.5. For this reason a diffusion limited growth mechanism was proposed. Dilution and moderate reaction temperatures can be employed to slow down the reaction rate in order to control the size of the particles more precisely. Sols with higher Nb:Si molar ratios showed a higher degree of condensation. This is due to the higher reactivity of the niobia precursors compared to TEOS. Higher growth rates were measured for sols with higher water contents. The concentration of acid can also promote the growth rate of polymeric particles, especially in the early stages of synthesis when a reaction limited growth mechanism cannot be excluded.
Disk membranes were prepared from two of the sols studied by SAXS, and the resuting permeabilities were compared with that of the first niobia-silica membrane. This indicated that less developed sols have a tendency to penetrate into the pores of the support, which yields membranes with a high resistance even to small gases like helium and hydrogen. A strong decrease of helium pearmeance was also observed when the Nb : Si ratio in the sol was increased from 0.33 to 0.8. This is probably due to the higher density of the material with the highest Nb loading.
A tubular niobia-silica membrane was prepared by coating the outer part of an alumina support. Despite a certain amount of defects on the membrane surface, the hydrogen and helium ideal selectivities of the tubular membrane towards other gases were higher than the selectivity of the support. Hence, although the coating procedure could be optimized further, there are indications that Nb/Si oxide membranes could find applications in real separation modules.
The hydrothermal stability of niobia-silica membranes was investigated and compared with the stability of pure silica membranes by exposing them to 0.56 bar of steam for 70 h at 150 and 200 °C. Single gas permeation experiments were performed before and after these treatments. The results showed that both membranes were densified by steam exposure. However, the decrease of helium and hydrogen permeability was less marked for niobia-silica than for pure silica.
The second part of research was focused on the development of novel mesoporous interlayers, with the aim of improving the overall hydrogen permeance of this membrane component. It was shown that a surfactant-containing polymeric sol of silica nanoparticles, could be coated on an α-alumina support with 80-90 nm larges pores, simply by varying its rheological properties. The deposition method allowed the preparation of a templated silica membrane directly on macroporous α-alumina in a facile way. Some penetration of silica particles into the α‑alumina support was encountered, and they prepared layers were equally permeable to hydrogen as γ-alumina. It was shown that doping of the silica phase with about 2-3 mol% Zr increased the hyrothermal stability of the mesoporous silica phase considerably.
In the final part of the thesis a method is presented to employ silicon nitride microsieves with hexagonally ordered 500-1000 nm wide perforations as highly permeable support for silica-based membranes. Due to the high perforation density and the low effective thickness of the sieve ((1 (m), the resistance of this type of support to gas flow is negligible. The challenge was to cover the perforations with a thin self-standing templated mesoporous silica film. A transfer technique was developed to transfer the preformed silica film onto the microsieve. A sacrificial polymeric layer was used to cover the inside of the perforations and ensure a smooth support surface for the silica film. No penetration of the silica film into the perforations was observed. The method allows to overcome the conventional stacked‑layers architecture of ceramic membranes, which is also common in gas selective membranes. This novel method can in principle be used for all kinds of materials and can be exploited in a large number of applications: sensing, microreactors, microfluidic devices, etc. The thin layers that were prepared according to this method do not permit selective gas separation yet, due to the presence of a small number of defects. However, the mesoporous nature of the film, and the absence of a high concentration of macroscopic defects was demonstrated by selective liquid-phase ion transport experiments.