High-selectivity, high-flux silica membranes for gas separation
Prof. Dr. Ir. H. Verweij
This thesis describes the processing of silica membranes, and the use of clean‑room techniques, which has resulted in silica membranes without detectable mesoscopic defects and significantly improved transport properties. The results signify an important step toward the industrial application of these membranes such as purification of H2 and natural gas as well as the selective removal of CO2 and air cleaning processes.
The ability of amorphous microporous silica membranes with pore diameters < 2 nm to pass small molecules while blocking larger ones has been known for years. State-of-the-art microporous silica membranes consist of a thin silica layer on top of a supported mesoporous (2 nm<pore Ø<50 nm) g‑Al2O3 membrane, that provides mechanical strength. The key problem, limiting industrial application of silica membranes was the poor reproducibility of the fabrication process which results in large fluctuations in performance and often poor separation properties. Improving the membrane properties by lowering of defect size and density is currently one of the greatest challenges in inorganic membrane preparation and a subject of the present study.
Coherent and strong porous a‑Al2O3 membrane supports were used on which a g‑Al2O3 layer with 4 nm pores in diameter and ~4 mm in thickness was deposited by dip-coating the supports in a boehmite sol followed by drying and calcining at 600°C. The g‑Al2O3 membranes, in turn, are used as a substrate for microporous silica membranes, prepared by dip-coating in a polymeric silica sol, followed by drying and calcining at 400°C or 600°C (further referred to as Si(400) and Si(600) membranes). The silica sol is prepared by acid-catalysed hydrolysis and condensation of tetra-ethyl-ortho-silicate (TEOS). Synthesis of Boehmite is done in a clean room with class 1000 conditions. The dipping process is carried out in a flow cupboard in a clean air stream (class 100 conditions). The preparation and characterisation of Si(400) and Si(600) membranes is described in chapter 2 and 3.
Well‑controlled processing of every membrane fabrication step, clean process liquids and the use of a clean‑room is very important in membrane preparation. If the membrane preparation is done with extreme care but not performed in a clean‑room the presence of large defects and hence non‑reproducible membranes are essentially inevitable. One defect with a diameter > 100 nm per cm2 of membrane surface will lead to substantial deterioration of all selectivities and especially those involving CH4 and CO2. The use of a clean-room reduces the average concentration of particles < 0.5 mm in our normal laboratory air from 18 million/m3 to less than 100/m3 in the flow cupboard where the membranes are prepared. Without clean room conditions, the number of defects caused by particles from the air is estimated to be at least 5 defects of roughly 0.5mm in diameter per cm2 of membrane surface. This number reduces to <<1 when our clean‑room conditions are deployed.
Morphological characterisation of the membranes reveal a very thin silica layer of ~30 nm on top of the g‑Al2O3 support layer. Membrane transport properties are reproducible within 25%, and the membranes can be made in large quantities with rejection ratios <15%. The permeance of the membranes is in most cases independent of pressure, P, and shows a slight increase with temperature, except for CO2. The CO2 flux is nearly constant with T and appears to decrease slightly at higher temperatures. In CO2 and to a lesser extent in CH4 transport, adsorption processes may play an important role. The adsorption of CO2 and CH4 on silica membrane material can be best fitted with the Langmuir‑Freundlich isotherm because of the weak adsorbent‑adsorbate interactions. Adsorption studies are described in chapter 4.
The estimated pore size via size exclusion based on kinetic diameters results in a Si(400) membrane pore diameter between 3.8 Å and 5.5Å. The permeance and Fa values obtained compare favorably with reported results. For the Si(400) membranes, the H2 permeance at 200°C is ~2´10-6 mol/m2×s×Pa with Fa>500 for H2/CH4 separation and a Fa = 7 for H2/CO2 and for CO2/CH4 Fa = 75. The Si(400) membranes show promise as air separation membranes as well with, the Fa = 4 for O2/N2.
The Si(600) membranes have an Fa = 70 for H2/CO2 with an H2 permeance of 6´10-7 (mol/m2×s×Pa) at 200°C. The smaller Si(600) permeance is a result of densification of the structure and a smaller pore size. The large increase of Fa for H2/CO2 from 7 with Si(400) to about 70 can also be attributed to a decrease in the amount of terminal hydroxyl groups at the internal surface of the silica because higher calcination temperatures lead to lower hydroxyl concentrations. A decrease of the amount of hydroxyls makes the material more hydrophobic, which may result in a lower (surface) occupation and hence, a lower CO2 permeance. Methane does not permeate at all through the Si(600) membranes. Estimation of the pore size results in a Si(600) pore diameter between 3.6 Å and 3.8 Å which is smaller than that estimated for Si(400) membranes.
Since the ultimate goal of the research described in this thesis is the design of membranes which can recover H2 in a coal‑heated power plant it is also important that the membranes can withstand these conditions. Thus, besides the performance of the membranes, the stability under coal gasification atmospheres is of great importance. These atmospheres contain a considerable amount of water vapour and operate at a temperature of about 300‑400°C. The study on membrane stability is discussed in chapter 6. It is shown that the thermal stability of Si(600) membranes is better than that of Si(400). The membranes remained their microporous structure for more than 240 hours of testing at 300°C. This was concluded from the fact that permselectivities exceeded the Knudsen separation numbers. The hydrothermal stability tests showed a complete break down of both Si(400) and Si(600) membranes after a short time (40 hours) or a long time (900 hours) depending on the water vapour pressure and temperature at which the membranes were exposed. It is found that the water vapour pressure is of larger importance on membrane deterioration than temperature. The higher the water vapour pressure the faster the break down and the larger the holes finally formed in the membrane. The membrane break down seems to be a chain reaction. As soon as larger molecules start to permeate, the total break down proceeds rather fast compared to the exposure time before these larger molecules start to permeate.
In an attempt to decrease the interaction of water vapour with silica membranes a novel, more hydrophobic silica membrane is made, as described in chapter 5. These membranes, indicated as MeSi(400), are 10 times more hydrophobic than the Si(400) membranes. The hydrophobicity is due to the addition of methyl‑tri‑ethoxy‑silane (MTES) during silica sol preparation. The MeSi(400) membranes show very high fluxes for gasses of small molecular size (H2, He, CO2, O2, N2 and CH4) and a good permselectivity (20-50) for these gasses with respect to SF6 and larger alkanes like C3H8 and n‑C4H10. The hydrophobic character of the membranes and the high fluxes for small gasses enables, contrary to Si(400) and Si(600) membranes, the possibility for application in humid process streams. These features offer also the possibility of technically feasible air purification via molecular sieving.