Silica Zirconia Membranes Research Articles

Silica-zirconia membrane supported on modified alumina for hydrogen production in steam methane reforming unit

A hydrogen-selective and hydrothermally stable membrane composed of silica-zirconia layer deposited on a modified alumina sub layer was successfully prepared. The composite membrane was used for hydrogen purification from synthesis gas in steam methane reforming process. Silica-zirconia layer was synthesized using the CVD method at 923K and atmospheric pressure while the alumina base layer was prepared via sol-gel procedure. DLS, XRD, SEM/EDX, and BET characterization techniques were used to prove that γ-alumina base and silica-zirconia layer are fabricated successfully. Using the composite membrane, hydrogen selectivity toward other gases has improved significantly. Moreover, H2/CH4, H2/CO, and H2/CO2 selectivity have been increased from 700, 350 and 70 in 5 h CVD synthesized membrane to 1600, 750 and 570 for 12 h CVD synthesized membrane respectively. The synthesized Silica-Zirconia membrane successfully altered gases permeability tendency order from H2 > CH4 > CO2 > CO to H2 > CO2 > CO > CH4 which leads to better separation of the product from methane feed. Finally, hydrothermal stability test demonstrated that permeability loss in silica-zirconia CVD coated membrane for H2 is 45.7% after 48 h, while for silica CVD coated on the modified alumina approaches 92.5%.

Permeation properties of silica-zirconia composite membranes supported on porous alumina substrates

A hydrogen-selective silica-zirconia composite membrane was prepared on a macroporous alumina support by chemical vapor deposition of tetraethylorthosilicate (TEOS) and zirconium (IV) tert-butoxide (ZTB) at 923K. The resulting membrane had a high H2 permeance of 3.8×10−7molm−2s−1Pa−1 with selectivities over CO2, N2 and CH4 of 1100, 1400 and 3700, respectively. Studies of the temperature dependence of the permeance of He, H2, and Ne demonstrated that the permeation mechanism was similar to that of dense silica membranes, involving solid-state diffusion with jumps of the permeating species between solubility sites. Parameters such as the site density, jump distance, and jump frequency were calculated and were physically plausible, and varied in reasonable manner with the mass and size of He, H2, and Ne. An alternative mechanism involving an activated gas translational mechanism was shown to fit the data but to give physically unrealistic parameters. The silica-zirconia membrane showed hydrothermal stability over a limited testing period of 48h. After exposure to 16mol% water vapor at 923K for 48h, a pure silica membrane showed a 68% decline with a H2 permeance of 4.5×10−8molm−2s−1Pa−1 and a H2 over N2 selectivity of 800, both of which continued to deteriorate. In comparison, a 10% ZrO2-SiO2 membrane showed a decline of 56% but to a level of 10−7molm−2s−1Pa−1 with a H2 over N2 selectivity of 5700. Importantly, the deterioration largely stabilized at that permeance level.

실리카-지르코니아 분리막 성능에 대한 다공성 지지체와 중간층의 영향

실리카-지르코니아 분리막 성능에 대한 다공성 지지체와 중간층의 영향

Fabrication of SiO2–ZrO2 (50/50) membranes on the porous stainless steel-tube support for pervaporation

The SiO2–ZrO2 membranes were fabricated with silica–zirconia sols on a porous stainless steel-tube support (O.D. = 10 mm, length = 20 mm, 316L SUS, Mott Corp. USA) by a dipping–rolling–freezing–fast drying (DRFF) and/or soaking–rolling–freezing–fast drying (SRFF) method. After coating of SiO2–ZrO2 sol, single gas permeation characteristics (He, H2, and N2) for SiO2–ZrO2 membranes were evaluated at room temperature, and the pervaporation experiments were performed at a specified feed isopropyl alcohol concentration of 90 wt% at 50°C with silica–zirconia membranes. The morphology of surface and the cross sections of the membrane were investigated with FE-SEM analysis. Judging from FE-SEM analysis, the surface of silica–zirconia membrane on the PSS tube support was denser, and the number of surface defects considerably reduced than DRFF method, and the additional modification via the SRFF significantly diminished surface defects, which could not be avoided during the DRFF step. The membrane the silica–zirconia membrane showed hydrogen permeability under 10−7 mol m−2 s−1 Pa−1 and low water selectivity in the range of 10–23. The silica–zirconia membranes with a smooth surface on the stainless steel tube supports, was quite well prepared by the DRFF and SRFF method in this study.

Preparation and pervaporation properties of silica–zirconia membranes

It is well known that the hydrothermal stability of silica is improved by adding zirconia, and porous silica–zirconia membranes (with 50 mol% zirconia) were prepared by sol–gel techniques. A homogeneous silica–zirconia composite sol, prepared by the hydrolysis and condensation reactions of tetraethoxysilane (TEOS) and zirconium n-butoxide (ZrB) in an ethanol solvent, gave uniform and defect-free membranes. Since it was necessary for the hydrolysis and condensation reaction rates of ZrB to be reduced because these reactions are faster than those of TEOS, the chemical reactivity of ZrB was modified with the chelating agent acetylacetone (acac). Silica–zirconia membranes were used for dehydration of isopropyl alcohol/water mixtures at 348 K by pervaporation to evaluate the effect of preparation conditions such as temperature, H +/(Si + Zr), H 2O/(Si + Zr) and acac/Zr molar ratios on membrane performance. The silica–zirconia membranes had the highest separation factor of 62.5 when they were prepared with H +/(Si + Zr), H 2O/(Si + Zr) and acac/Zr molar ratios 0.05, 4 and 0.6, respectively, and three coating cycles. The membranes were characterized by Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM).

Preparation of nanoporous silica–zirconia layers by in situ sol–gel method

Silica–zirconia membranes were prepared using a sol–gel process, never before used on this system. Thin layers of gel were grown on the outside of porous alumina support tubes using the permeation of water through pores of the tube to control the rate of hydrolysis with a reactive silica–zirconia alkoxide solution. A recipe was developed and optimum coating conditions were found to be the ratios of tetraethylorthosilicate (TEOS) to Zr propoxide to 1-propanol molar ratios (1:0·5:17), with a coating time of 300 s. The method allowed the formation of a membrane in a single step compared with repeated coating and firing. The coatings could be prepared adherent and without cracks when properly prepared.

Comparison of SiO 2-ZrO 2-50% and commercial SiO 2 membranes on the pervaporative dehydration of organic solvents

In this work several SiO 2-ZrO 2-50% tubular pervaporation (PV) membranes were prepared by the sol–gel and hot coating method. Their PV performance was investigated regarding the separation of water/isopropanol and water/acetone mixtures in terms of water flux and selectivity. The behaviour of these membranes was compared to the performance of two commercially available silica membranes (Pervatech BV, The Netherlands, and Pervap SMS, Sulzer, Germany) on the PV of water/isopropanol mixtures and a residual water/acetone mixture coming from the manufacture process of rubber antioxidants. An exponential dependence of the water flux with respect to the water activity in the feed was confirmed for both types of membranes. The value of the mass transfer parameter that describes the interaction of the permeating component, i.e. water, with the membrane has a very similar value for the commercial silica membranes and the prepared silica–zirconia membranes.

Thermal Stability of Silica-Zirconia Membranes

Abstract The thermal stability, phase transformation, surface morphology, pore size distribution and permeation of the defect-free silica-zirconia membrane were investigated by using X-ray diffraction (XRD), atomic force microscopy (AFM), gas adsorption analyzer (BET), and gas permeation apparatus, respectively. Using silica as the stabilizing agent, the defect-free membrane was much more stable than pure zirconia. The crystal transformation of zirconia in the silica-stabilized membrane could be prohibited by the interaction between silica and zirconia. ZrO 2 crystals were kept tetragonal below 900°C, the size of which did not change with temperature between 700°C and 900°C. It was further verified by the AFM observation, pore size analysis and permeation study. This thermal stability makes the silica-zirconia membrane a good choice as the intermediate layer for zeolite and Pd-based membranes.

Pervaporation Characteristics of Silica-Zirconia Membranes for Separation of Aqueous Organic Solutions

:Application of pervaporation membranes to practical separation and purification processes would be much more attractive if the separation performance (dux and selectivity) can be further improved. In addition to higher flux and selectivity, membrane stability at higher temperatures is quite important for efficient and economic separation in practical processes, considering that pervaporation at higher temperature preferably gives higher fluxes, resulting in smaller required membrane areas. In this respect, inorganic porous membranes could find a wider application in practical separation and purification processes.In this work, porous silica-zirconia membranes of pore size less than 1 nm were fabricated by improved sol-gel techniques to study the pervaporation characteristics of various aqueous organic solutions (isopropanol, I-propanol, acetone, tetrahydrofuran, and ethanol) at temperatures from 50 C to their normal boiling points. The water fluxes for these aqueous mixtures, with the exception of aqueous ethanol solution at the normal boiling point, exceeded 7kg·m−2 h−1 with separation factors larger than several hundreds at 10wt% water in the feed. The water-flux dependency on the feed concentration can be determined roughly by the vapor-liquid equilibrium; however, it is not always proportional to the partial vapor pressure of the feed at the separation temperature. The detailed separation characteristics are discussed based on a simple model of porous membranes proposed previously.

Pervaporation of Aqueous Organic Acid Solutions by Porous Ceramic Membranes

Porous silica, silica-zirconia and silica-titania membranes were prepared by the sol-gel techniques for separation of aqueous solutions of organic acids (acetic acid and propionic acid) by pervaporation at 50–100°C and at normal boiling points. Applying the hot coating procedures with the colloidal sols prepared in different concentrations (different particle sizes), a quite thin porous silica layer active for separation was formed on a porous α-alumina cylindrical substrate.A fresh porous silica membrane showed a long time-dependency of several hours in the pervaporation performance before reaching a steady state due to the formation of silanol groups on the pore surface. Because of its small thickness less than about 0.5 μm quite large water fluxes of about 300 mol·m–2·h–1 were observed with a separation factor higher than several hundreds at around 73 mol% of acetic acid (10 wt% water) at 100°C, for example. Stability tests of the porous ceramic membranes were performed in aqueous organic acid solutions to find that silica and silica-titania membranes were quite stable in an aqueous solution of organic acid of concentration higher than about 20 mol% acid. Silica-zirconia membranes, on the other hand, were not stable in aqueous organic acid solutions of high concentration. The membrane stability is discussed on the observed solubility of inorganic membrane materials. Furthermore, separation and permeation mechanisms were discussed based on a simple pore model for pervaporation proposed previously to give good coincidence between observed and simulated results.

Characterization of sol–gel derived membranes and zeolite membranes by nanopermporometry

Abstract Silica–zirconia membranes having pore sizes in the range 0.5–2 nm, determined using water as a condensable gas (vapor) by nanopermporometry, were prepared by the sol–gel process, and used in gas permeation experiments. The permeability ratio of He/N2 approached the Knudsen value (=2.6) for pore sizes larger than 2 nm. It then decreased with decreasing pore size, probably because of an enhanced contribution of surface diffusion in the small pore, and showed a minimum at an approximate pore size of 1 nm. It then increased to approximately 50 for a membrane having pore sizes of 0.3 nm. For the case of He/SF6, the curve appears to shift to a larger Kelvin diameter, probably because of the larger molecular size of SF6 as well as adsorption. The effects of non-condensable gases (He and N2) were examined using silica–zirconia membranes of 2 and 0.8 nm in pore size. The pore size distribution (PSD) curves measured by He and N2 were in good agreement with each other for membranes having a pore size as large as 2 nm. On the other hand, for the case of porous membranes having small pore sizes, PSD curves measured using He were shifted to a smaller pore size, compared with those measured by N2. This suggests the existence of micropores, which allowed the permeation of only He. Moreover, nanopermporometry was applied to MFI zeolite membranes to characterize selective (intracrystalline) and non-selective pores (intercrystalline) using hexane, and the data were in reasonable agreement with the observed separation performances.

Stability and performance of porous silica–zirconia composite membranes for pervaporation of aqueous organic solutions

Porous silica–zirconia membranes were fabricated by the sol–gel techniques to study their stability against water and the pervaporation performance of aqueous solutions of organic solvents. Zirconia (10–70 mol%) was added to silica to obtain silica–zirconia composite membranes by firing at 400–500 °C for pervaporation tests with organic solvent/water mixtures, such as iso-propyl alcohol (IPA)/water and tetrahydrofuran (THF)/water mixtures at their normal boiling points. The membrane coatings have been done effectively by the hot-coating methods proposed previously. Boiling water treatments introduced in the coating processes have made the membranes quite stable even in the high water concentration region of aqueous organic solutions at their normal boiling points. Zirconia contents larger than about 40 mol% have made the silica–zirconia membranes quite stable. The membranes of zirconia contents less than about 30 mol% were found not stable in a dilute aqueous solution of IPA. The membranes fabricated by the conventional dip-coating methods with slow drying were not stable against water because of the probable segregation of silica and/or silica-rich phases during drying. The membranes fired at lower temperature (400 °C) gave a higher water flux of around 500 mol m −2 h −1 (9 kg m −2 h −1) with a separation factor larger than 1500 at 10 wt.% of water in the boiling feed of IPA/water mixture, for example.

Porous Silica-Zirconia(50%) Membranes for Pervaporation of iso-Propyl Alcohol(IPA)/Water Mixtures.

Porous silica-zirconia (ZrO2: 50 mol%) membranes of pore size around 0.5 nm were prepared by the sol-gel methods for separation of IPA/water mixtures by pervaporation. The prepared membranes showed a long time dependency in separation performance for about 30 hours before their steady states were attained. The IPA flux decreased gradually down to less than 1 mol·m–2h–1, while the water flux was nearly constant (about 300 mol·m–2h–1) at 73 mol% of IPA (10 wt% of water) in the feed mixture at 75°C, resulting in a drastic increase of separation factor from around 40 to 2500. The membranes also showed a high stability even at high water concentration (60 mol% of water) at 75°C, where the water flux exceeded 800 mol·m–2h–1.The time dependency of the separation performance is probably due to the formation of -OH groups on the pore surface and the succeeding adsorption and/or the reaction of IPA molecules which can reduce the effective pore diameter.

Organic/inorganic Nanohybrid Membranes for Nanofiltration of Nonaqueous Solutions

ABSTRACTSilica/zirconia (SZ; Si/ Zr molar ratio = 9/1) membranes having pore sizes in the range of 1∼ 3 nm were prepared by the sol-gel process. Organic/ inorganic hybrid membranes were developed by modifying the surface of the silica-zirconia porous membranes via a gas-phase reaction with trimethylchlorosilane (TMCS) to give a surface that was modified with a monolayer of TMCS. Using nanopermporometry, it was found that the inner surface of membrane pores with diameters larger than several nms were successfully modified with TMCS. TMCS-modified membranes showed approximately the same permeability, Lp, irrespective of water concentration (10 and 100 ppm) in hexane. In contrast, an unmodified membrane, showed a decrease in Lp with increasing water concentration, which was pronounced at low permeation temperatures. This suggests that small amounts of water adsorbed to the inner surface of unmodified silica-zirconia membranes and blocked the permeation of hexane.

Nanofiltration in non-aqueous solutions by porous silica–zirconia membranes

Porous membranes having various average pore sizes, ranging from 1 to 4 nm, were prepared from silica–zirconia composite colloidal sols by sol–gel processes, and were used for nanofiltration (NF) experiments in non-aqueous solutions of ethanol and methanol. Silica–zirconia membranes, which were tested in pure alcohol solutions for the first time after the preparation of the membrane, showed a gradual decrease in flux for approximately 100 h and then reached a steady flux. When the feed, after reaching the steady flux with ethanol, was changed to another alcohol, steady flux was attained after only several hours. Ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol (PEG) of various molecular weights (PEG400, 600, 1000, and 2000) were nanofiltrated in methanol and ethanol solutions at 50°C. Rejections in non-aqueous solutions increased with applied pressure, which is similar to aqueous solutions. Control of pore size of silica–zirconia membranes showing molecular weight cut-offs in methanol solutions at 300, 600, 1000, and >1000, respectively, was possible by the appropriate choice of colloidal particle sizes. Rejection in methanol solution showed a tendency similar to that in ethanol solution, while rejection in methanol was slightly larger than in ethanol solutions. In addition, rejection in water was much smaller than in methanol solution. For example, the rejection of PEG600 in water and methanol was 0.03 and 0.74, respectively. These results suggest that solvent type plays an important role in determining rejection, as a result of the interaction with solvents and/or membrane surface.

Hydrothermal Stability and Performance of Silica-Zirconia Membranes for Hydrogen Separation in Hydrothermal Conditions.

Three types of silica-zirconia membranes of different zirconia content were prepared by the sol-gel method to test their stability and H2 separation performance in hydrothermal conditions. Some silica-zirconia membranes were prepared by “steam-firing” in which firing was carried out in steamed air in order to increase the hydrothermal stability, and then tested in hydrothermal conditions. The activation energy of H2 and He permeation increased with increasing zirconia content, suggesting that the networks of silica-zirconia matrix were densified with increasing zirconia content. In the hydrothermal conditions, the activation energy of H2 and He permeation also increased, indicating clearly that silica-zirconia membranes were also densified by water vapor at high temperature. The pores for N2, Co2 and CH4 permeation decreased with the increasing zirconia content and vanished in the hydrothermal conditions, leaving some pinholes. Some silica-zirconia membranes were prepared by firing in steamed air in order to increase the hydrothermal stability. This method appeared to be effective to increase hydrothermal stability of the silica-zirconia membrane.

Permeation of Liquids through Inorganic Nanofiltration Membranes

Inorganic nanofiltration membranes were prepared using silica–zirconia composite sols by the sol–gel method and applied for the permeation experiments of nonaqueous solutions. Pure solvent permeabilities of methanol, ethanol, and 1-propanol were measured at temperatures from 25 to 60°C. For membranes having pore sizes as large as 70 nm, the permeation mechanism of alcohols obeys the viscous flow mechanism irrespective of types of alcohols. On the other hand, the permeation mechanism through porous silica–zirconia membranes having pore diameters of 1 to 5 nm was found to be different from the viscous flow mechanism; small molecules showed larger permeabilities than large molecules. Activation energies of solvent permeability were found to be larger for large molecules and for membranes of smaller pore size.

Silica–zirconia membranes for nanofiltration

Inorganic nanofiltration membranes were fabricated from silica–zirconia composite colloidal sol (molar ratio Si/Zr=9/1) using a sol–gel process. Molecular weight cut-off (MWCO) was successfully controlled between 200 and 1000 Da by regulating the colloidal diameters of sol solutions in the final coating stage. The pure water permeabilities ranged from 0.15×10 −11 to 1.5×10 −11 m 3 m −2 s −1 Pa −1. Pore size and pore size distribution were estimated based on the dynamic method of humid air permeation, and found to be from 1.0 to 2.9 nm. The MWCO obtained from NF experiments using neutral organic solutes corresponds well with the pore diameters estimated from the dynamic permeation method. Silica–zirconia membranes were found to be stable in aqueous solution for periods in excess of four months.