Preparation of microporous silica membranes for gas separation

Microporous silica membranes for hydrogen separation were prepared on a γ-alumina coated α-alumina tube by sol-gel method. The reactants of sol-gel chemistry were tetraethoxysilane (TEOS) and methacryloxypropyl-trimethoxysilane (MOTMS). The silane coupling agent, MOTMS, was added as a template in order to control the pore structure to the silicon alkoxide, TEOS. In particular, the microporous membranes were prepared by changing the molar ratio of MOTMS with respect to other substances, and their pore characteristics were analyzed. Then, the effects of thermal treatment on the micropore structure of the resulting silica membranes were investigated. The pore size of the silica membrane prepared after calcination at 400–700 ‡C was in the range of 0.6–0.7 nm. In addition, permeation rates through the membranes were measured in the range of 100–300 dgC using H₂, CO₂, N₂, CH₄, C₂H₆, C₃H₆ and SF₆. The membrane calcined at 600 ‡C showed a H₂ permeance of 2×10^-7-7×10^-7 molm^-2s^-1Pa^-1 at permeation temperature 300 ‡C, and the separation factors for equimolar gas mixtures were 11 and 36 for a H₂/CO₂ mixture and 54 and 132 for a H₂/CH₄ mixture at permeation temperatures of 100 ‡C and 300 ‡C, respectively.     

sol-gel法制备微孔SiO2分离膜的研究进展

微孔SiO2分离膜在气体分离、催化、膜反应器等领域有着巨大的应用潜力。本文首先简要介绍了获得微孔的两种途径，然后重点对sol—gel法制备微孔SiO2分离膜的研究进展进行综述。     

Gas separation using sol-gel derived microporous zirconia membranes with high hydrothermal stability☆

A microporous zirconia membrane with hydrogen permeance about 5 × 10?8 mol·m?2·s?1·Pa?1, H2/CO2 permselectivity of ca. 14, and excellent hydrothermal stability under steam pressure of 100 kPa was fabricated via polymeric sol–gel process. The effect of calcination temperature on single gas permeance of sol–gel derived zirconia membranes was investigated. Zirconia membranes calcined at 350 °C and 400 °C showed similar single gas permeance, with permselectivities of hydrogen towards other gases, such as oxygen, nitrogen, methane, and sulfur hexafluoride, around Knudsen values. A much lower CO2 permeance (3.7 × 10?9 mol·m?2·s?1·Pa?1) was observed due to the interaction between CO2 molecules and pore wall of membrane. Higher calcination tem-perature, 500 °C, led to the formation of mesoporous structure and, hence, the membrane lost its molecular siev-ing property towards hydrogen and carbon dioxide. The stability of zirconia membrane in the presence of hot steam was also investigated. Exposed to 100 kPa steam for 400 h, the membrane performance kept unchanged in comparison with freshly prepared one, with hydrogen and carbon dioxide permeances of 4.7 × 10?8 and~3 × 10?9 mol·m?2·s?1·Pa?1, respectively. Both H2 and CO2 permeances of the zirconia membrane de-creased with exposure time to 100 kPa steam. With a total exposure time of 1250 h, the membrane presented hydrogen permeance of 2.4 × 10?8 mol·m?2·s?1·Pa?1 and H2/CO2 permselectivity of 28, indicating that the membrane retains its microporous structure.     

Sol-gel synthesis of compact nanohybrid structures based on silica gels

Our recent achievements in the synthesis of monolithic SiO₂ xerogels (nonfractured upon drying), as well as hybrid composite nanomaterials and silica glasses based on these xerogels, are considered briefly. A number of new promising methods are developed for preparing organic-inorganic nanohybrids, inorganic nanohybrids, and xerogels doped with transition metal and lanthanide complexes that are chemically bound to the silica matrix and exhibit interesting and technically valuable photophysical, photochemical, and electrochemical properties.     

Ageing of microporous silica

A silica sol was frozen and then thawed yielding a suspension of plate-like silica which when dried gave a sample which was almost entirely composed of micropores. The silica aged when left to stand in the laboratory, but reached a constant value of activity after a period of about six months or less. No apparent change in microporosity occurred during the ageing process. Several selected samples were sintered at temperatures up to 800° at which point virtually all the adsorption capacity of the adsorbent was destroyed leaving adsorption to take place only on the external surface.     

Facile synthesis of hydrophobic microporous silica membranes and their resistance to humid atmosphere

The ethylene-modified silica membranes were prepared by the acid-catalyzed co-hydrolysis and condensation reaction of tetraethylorthosilicate (TEOS) and ethylenetriethoxysilane (TEVS) in ethanol and the final materials were characterized by scanning electron microscope (SEM), water contact angle measurement, solid-state ²⁹Si magic angle spinning nuclear magnetic resonance (²⁹Si MAS NMR), and N₂ adsorption. The modification leads to a transform from superhydrophilicity for the unmodified silica membranes to hydrophobicity for the modified materials. The ethylene-modified silica membranes are much less water sensitive than the unmodified materials because the hydrophobic ethylene groups replace a portion of the hydrophilic hydroxyl groups on the pore surface. The modified materials process a microporous structure with a narrow pore size distribution centered at 1.1 nm. Such a microporous structure can be stabilized after exposured to humid atmosphere for 450 h, in intense contrast to the collapse of the micropores in the unmodified silica membranes.     

Hydrothermal stability of fluorine-induced microporous silica membranes: Effect of steam treatment conditions

A fluorine-SiO₂ membrane was prepared using triethoxyfluorosilane (TEFS) as a Si precursor, and its hydrothermal stability was evaluated. The TEFS membrane calcined at 750°C had fewer Si-OH and Si-F groups in its network structure and showed H₂ permeance that was greater than 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ with H₂/N₂ and N₂/SF₆ permeance ratios of 10 and 210, respectively. This membrane performance was relatively stable under the temperature (< 500°C) used for steam treatment, regardless of the steam partial pressure (30, 90 kPa). On the other hand, when the steam treatment temperature was increased beyond 500°C, gas permeance decreased significantly and the membrane became highly selective for He and H₂ over smaller molecules (He/N₂: > 600, H₂/N₂: > 100). The relationship between the activation energy of H₂ and the permeance ratios (He/H₂, He/H₂O, H₂/H₂O) of a TEFS-derived membrane under steam treatment higher than 600°C resulted in a network pore size that approximated in conventional microporous SiO₂ membranes.     

Sol-gel derived alumina substrates

Sol-gel processing may provide an alternative to the current tape cast powder method for manufacturing alumina substrates. The two methods are compared and advantages/disadvantages are discussed. One potential disadvantage of the sol-gel derived alumina substrates has been their brittleness after drying. The use of glycerol improves the flexibility of the prefired dried gels, making it possible to fabricate a variety of shapes through punching, cutting and laminating.     

Poison resistant catalysis with microporous catalyst membranes

Amorphous microporous metal oxide membranes containing 1% Pt have been prepared by a modified sol-gel process. The pore diameters of these membranes range from 6 to 9 A and the membranes show a cutoff at MW 180 in liquid phase separation. The membranes are active hydrogenation catalysts and by separating the liquid phase from the hydrogen by the membrane, poison resistant hydrogenation has been achieved. This novel approach to the processing of three phase reactions combines the unique properties of nanofiltration membranes and platinum to obtain a better control over heterogeneous catalysis.     

聚丙烯微孔膜的研究进展

介绍了由熔融纺丝-冷却拉伸法和热致相分离法制备聚丙烯微孔膜的原理和过程。对当前膜研究领域的热点即膜的亲水化改性研究进行了重点评述,包括暂时性改性和永久性改性。阐述了各种改性方法的特点,着重讨论了亲水化改性对膜性能如膜的微孔孔径、孔隙率、透水量等的影响,强调膜表面亲水性的提高可改善膜的耐生物污染性。指出聚丙烯微孔膜的应用领域和发展前景。     

Gas permeation properties for organosilica membranes with different Si/C ratios and evaluation of microporous structures

Organosilica membranes were fabricated using bridged organoalkoxysilanes (bis(triethoxysilyl)methane (BTESM), bis(triethoxysilyl)ethane (BTESE), bis(triethoxysilyl)propane (BTESP), bis(trimethoxysilyl)hexane (BTMSH), bis(triethoxysilyl)benzene (BTESB), and bis(triethoxysilyl)octane (BTESO)) to produce highly permeable molecular sieving membranes. The effect of the organoalkoxysilanes on network pore size and microporous structure was evaluated by examining the molecular size and temperature dependence of gas permeance across a wide range of temperatures. Organosilica membranes showed H₂/N₂ and H₂/CH₄ permeance ratios that ranged from 10 to 150, corresponding to network pore size, and both H₂ selectivity decreased with an increase in the carbon number between 2 Si atoms. Organosilica membranes showed activated diffusion for He and H₂, and a slope of temperature dependence that increased approximate to the increase in the carbon number between 2 Si atoms. The relationship between activation energy and He/H₂ permeance ratio for SiO₂ and organosilica membranes suggested that the molecular sieving can dominate He and H₂ permeation properties via the rigid microporous structure, which was constructed by BTESM and BTESE. With increased in the carbon concentration in silica, polymer chain vibration in organic bridges, which is a kind of solution/diffusion mechanism, can dominate the permeation properties. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4491–4498, 2017     

Gas permeation properties of silica membranes with uniform pore sizes derived from polyhedral oligomeric silsesquioxane

The sol‐gel method was applied in the fabrication of homogenous polyhedral oligomeric silsesquioxane (HOMO‐POSS)‐derived silica membranes. Single gas permeation characteristics in a temperature range of 100–500^°C were examined to discuss the effect of silica precursor on amorphous silica networks. HOMO‐POSS‐derived membranes showed a CO₂ permeance of 1.1 × 10^?7 mol m^?2 s^?1 Pa^?1 with a CO₂/CH₄ permeance ratio of 131 at 100^°C, which is a superior CO₂/CH₄ separation performance by comparison with tetraethoxysilane (TEOS)‐derived silica membranes. Normalized Knudsen‐based permeance (NKP) was applied for quantitative evaluation of pore size. HOMO‐POSS‐derived membranes had loose amorphous silica structures compared to TEOS‐derived membranes and pore size was successfully tuned by changing the calcination temperatures. The activation energy for a HOMO‐POSS‐derived membrane fired at 550^°C with a uniform pore size of ～ 0.42 nm increased linearly with the ratio of the kinetic diameter of the gas molecule to the pore diameter, λ (=d_k/d_p), and showed a trend similar to that of DDR‐type zeolite membranes. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1733–1743, 2012     

Hydrogen storage performance of methyl-substituted mesoporous silica with tailored textural characteristics

Journal of Porous Materials - The present study reports a systematic analysis of morphology and hydrogen sorption capacity of mesoporous organic-inorganic silica prepared by varying the silica...     

In situ modification of microporous membranes

Under proper conditions, in situ polymerizations of acrylic acid monomer solutions were performed within the microporous structures of a hydrophobic membrane, Celgard ® 2500. poly(acrylic acid) modified pores have been characterized to be permanently wettable by aqueous solutions, to be capable of gelling water to withstand increased transmembrane pressure gradients, and also to possess a higher transmembrane electrical resistance. In cases of less concentrated acrylic acid monomer solutions, a cross-linker was employed to facilitate polymer permanency within the infrastructure of the hydrophobic membranes. © 1993 John Wiley & Sons, Inc.     

Pore size of microporous polymer membranes

Pore sizes of microporous polymer membranes were determined by the calculation based on the gas permeability of porous media. The gas permeability coefficient K (given by J = K Δp/l, where J is the steady-state gas flux, Δp is the pressure, difference, and l = the thickness of a membrane) for porous membrane can be given generally by where K₀ is the Knudsen permeability coefficient, η is the viscosity of the permeant gas, B₀ is the geometric factor of a membrane, and Δp? is the mean pressure of the gas on both sides of a membrane. From gas permeability measurements which yield the pressure dependence of gas permeability coefficient (expressed as above equation), the mean pore size of the porous membrane can be estimated as where M is the molecular weight of the permeant gas. The validity of this method was examined with various Millipore filters of which nominal pore sizes are known. It was confirmed that the method provided a simple and reliable means of estimating mean pore size of microporous membranes. The method was applied to investigate the influence of factors involved in preparation of microporous polysulfone membranes by coagulation procedure. It was found that the mean pore size of porous polysulfone membrane increases with (1) increasing with casting thickness, (2) increasing temperature of coagulation bath, and (3) decreasing concentration of polymer in casting solution (DMF as solvent). Water flux and water flux decline due to compaction are also examined as a faction of pore size, porosity, and the thickness of membranes.     

Sol-gel derived PbTio3-polymer piezoelectric composites

Amorphous lead titanate powder was prepared using a sol-gel process. The powder was fired to temperatures ranging from 500°C to 1200°C and soaked for various lengths of time. X-ray diffraction confirmed that the fired powders were tetragonal PbTiO₃. Surface area measurements indicated that the specific surface area of the powder decreased with increasing heat treatment. The powders were mixed with Eccogel polymer to make composites in which the ceramic phase constituted 60 percent by volume. Piezoelectric coefficients were found to be greatest for those composites containing powder fired to 1100°C and soaked for 1 hour. The d_hg_h value obtained for this heat treatment was 540×10^-15m²N^-1.