Tomohisa Yoshioka

Design of Silica Networks for Development of Highly Permeable Hydrogen Separation Membranes with Hydrothermal Stability

A sol-gel method was applied for the development of highly permeable hydrogen separation membranes using bis(triethoxysilyl)ethane (BTESE) as a silica precursor. Hybrid silica membranes showed quite high hydrogen permeance (1 x 10(-5) mol m(-2) s(-1) Pa(-1)) with a high H(2)-to-SF(6) selectivity of 1000 because of loose organic-inorganic silica networks. Hybrid silica membranes were found to show high hydrothermal stability due to the presence of Si-C-C-Si bonds in silica networks.

Titania membranes for liquid phase separation: effect of surface charge on flux

Titania membranes were prepared by sol-gel processes. Molecular weight cut-offs (MWCO) were successfully controlled in the range from 500 to 1000 with pure water permeabilities of 0.6-1.5x10 -11 m 3 /m 2 .s.Pa. Nanofiltration experiments were carried out for various types of electrolytes (NaCl, Na 2 SO 4 , MgCl 2 , MgSO 4 ) using several membranes having different MWCOs (400, 500, >1000). Rejection showed the minimum values near the isoelectric point (IEP) which were determined by streaming potential measurements. Larger rejections were obtained for the case where electrolytes having divalent co-ions were nanofiltrated, while low rejection were observed for the case of electrolytes having divalent counterions. On the other hand, permeate volume fluxes were maximized near IEP and the fluxes were almost the same irrespective of types of electrolytes. However, permeate volume fluxes decreased at pH higher than IEP. The dependency was pronounced for the case of divalent counterions and smaller pore diameters, probably because of larger hydrodynamic resistance by ionically-adsorbed counter ions.

Development of Robust Organosilica Membranes for Reverse Osmosis

Hybrid organically bridged silica membranes have attracted considerable attention because of their high performances in a variety of applications. Development of robust reverse osmosis (RO) membranes to withstand aggressive operating conditions is still a major challenge. Here, a new type of microporous organosilica membrane has been developed and applied in reverse osmosis. Sol-gel derived organosilica RO membranes reject isopropanol with rejection higher than 95%, demonstrating superior molecular sieving ability for neutral solutes of low molecular weight. Due to the introduction of an inherently stable hybrid network structure, the membrane withstands higher temperatures in comparison with commercial polyamide RO membranes, and is resistant to water to at least 90 °C with no obvious changes in filtration performance. Furthermore, both an accelerated chlorine-resistance test and Fourier transform infrared analysis confirm excellent chlorine stability in this material, which demonstrates promise for a new generation of chlorine-resistant RO membrane materials.

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

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/N 2 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/SF 6 , the curve appears to shift to a larger Kelvin diameter, probably because of the larger molecular size of SF 6 , as well as adsorption. The effects of non-condensable gases (He and N 2 ) 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 N 2 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 N 2 . 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.

CATALYTIC MEMBRANE REACTION FOR METHANE STEAM REFORMING USING POROUS SILICA MEMBRANES

Catalytic membranes, which have hydrogen permselectivity over other gaseous molecules and catalytic activity for methane steam reforming, were prepared by 2 different procedures and applied to methane steam reforming at 450–500°C. Type A catalytic membranes were manufactured by the preparation of a hydrogen separation layer from silica-zirconia colloidal sols, followed by the application of a nickel catalyst coating. Type B catalytic membranes were prepared via the impregnation of a nickel catalyst inside the α-alumina porous substrates, followed by the application of a coating on the hydrogen separation layer. Hydrogen permselectivity over nitrogen was degraded by coating the catalyst layer, as in the Type A membranes, and in addition, methane conversion decreased with time probably because of catalyst sintering or carbon deposition. Type B catalytic membranes showed a steady conversion for a longer period than did Type A, and the permeability ratio of hydrogen to nitrogen was approximately 200; therefore, Type B was found to be the effective route to preparing catalytic membranes. Methane steam reforming through the use of catalytic membranes revealed that methane conversion beyond the equilibrium conversion levels could be achieved either by sweeping the permeate stream or by pressurizing the feed stream at 6 bar and not using sweeping gas.

Permeation Characteristics of Electrolytes and Neutral Solutes through Titania Nanofiltration Membranes at High Temperatures

Nanoporous titania membranes with controlled pore sizes ranging from 0.7 to 2.5 nm, which had molecular weight cutoffs (MWCO) ranging from 500 to 2000, were successfully prepared by sol-gel processing, and the transport characteristics were evaluated across a temperature range of 30-80 degrees C. With increasing temperature, the permeate flux increased 2- to 3-fold, depending on the pore size. The water permeation mechanism was found to be different from viscous flow and was explained by the state of the water (free water/bound water/nonfreezing water) inside confined pores. The rejection of neutral solutes such as raffinose, the separation mechanism of which is molecular sieving (steric hindrance), decreased with temperature whereas that of electrolytes (MgCl(2) and NaCl), the separation mechanism of which is the charge effect (Donnan exclusion), was approximately constant. The temperature dependence of neutral and electrolyte solutes was analyzed using the Spiegler-Kedem equation by combining the Arrhenius equations for diffusivity and viscosity, which we obtained DeltaE(m), the activation energy of diffusion, after eliminating the effect of viscosity. For large DeltaE(m), which corresponds to the rejection of neutral solutes on the basis of molecular sieving, rejection decreased with temperature but remained unchanged for small DeltaE(m), which corresponds to the rejection of electrolytes based on the charge effect.

Extremely thin Pd–silica mixed-matrix membranes with nano-dispersion for improved hydrogen permeability

Pd-silica mixed-matrix membranes with superior H(2) permeability and hydrothermal stability at high temperatures were successfully fabricated using a sol-gel method. The Pd-silica layer was quite thin (100-200 nm) and small Pd particles (several nm) dispersed well in an amorphous silica matrix.

Molecular dynamics studies on gas permeation properties through microporous silica membranes

Gas permeation mechanisms through a micropore of a vitreous silica (v-SiO 2 ) membrane were studied using a molecular dynamics (MD) simulation. Virtual v-SiO 2 membranes were prepared by the melt-quench methods using the modified Born-Mayer-Huggins pair potential and Stillinger-Waber three-body interactions. The particle-generating non-equilibrium MD technique was employed in order to simulate gas permeation phenomena, where permeating molecules were modeled as Lennard-Jones particles. This simulation method accommodates a change in the number of particles in a unit cell and, hence, an accurate simulation of the steady-state process of permeation can be achieved. The dependencies of permeance on temperature and pressure were discussed. For cylindrical pores of about 5 A in diameter, the calculated temperature dependencies of the permeance of He-like LJ (Lennard-Jones) particles were similar to those predicted by the normal Knudsen permeation mechanism, while, for CO 2 permeation, a temperature dependency larger than He and a significant deviation from the Knudsens could be observed. In the relatively high-temperature region (400-800 K), the simulated permeance of CO 2 was nearly independent of the upstream pressure, while at the temperature below 300 K, a pressure dependency of permeance was observed. Simulations of adsorption conducted on the same unit cell yielded a Henry-type isotherm at 400 K and a Langmuir-type isotherm at 260 K. These results indicate that gas-like permeation occurred in the higher-temperature region, where the permeation flux is proportional to the pressure drop across the pore. However, at lower temperatures, the transports of molecules as some type of adsorption phase might be dominant in such a small pore. A simple gas permeation model, considering the effect of the pore wall potential field and Langmuir type adsorption within a micropore explained those permeation properties of CO 2 well.

Tailoring the affinity of organosilica membranes by introducing polarizable ethenylene bridges and aqueous ozone modification.

Bis(triethoxysilyl)ethylene (BTESEthy) was used as a novel precursor to develop a microporous organosilica membrane via the sol-gel technique. Water sorption measurements confirmed that ethenylene-bridged BTESEthy networks had a higher affinity for water than that of ethane-bridged organosilica materials. High permeance of CO2 with high CO2/N2 selectivity was explained relative to the strong CO2 adsorption on the network with π-bond electrons. The introduction of polarizable and rigid ethenylene bridges in the network structure led to improved water permeability and high NaCl rejection (>98.5%) in reverse osmosis (RO). Moreover, the aqueous ozone modification promoted significant improvement in the water permeability of the membrane. After 60 min of ozone exposure, the water permeability reached 1.1 × 10(-12) m(3)/(m(2) s Pa), which is close to that of a commercial seawater RO membrane. Meanwhile, molecular weight cutoff measurements indicated a gradual increase in the effective pore size with ozone modification, which may present new options for fine-tuning of membrane pore sizes.

New Insights into the Microstructure-Separation Properties of Organosilica Membranes with Ethane, Ethylene, and Acetylene Bridges

Microporous organosilica membranes with ethane, ethylene, and acetylene bridges have been developed and the extensive microstructural characterization has been discussed in relation with separation properties of the membrane. The organosilica network structure and the membrane performances can be controlled by adjusting the flexibility, size, and electronic structure of the bridging groups. A relatively narrow size distribution was obtained for the novel acetylene-bridged sol by optimizing the sol synthesis. Incorporation of larger rigid bridges into organosilica networks resulted in a looser microstructure of the membrane, which was quantitatively evaluated by N2 sorption and positron annihilation lifetime (PAL) measurements. Molecular weight cutoff (MWCO) measurements indicated that the acetylene-bridged membrane had a larger effective separation pore size than ethane- and ethylene-bridged membranes, leading to a relatively low NaCl rejection in reverse osmosis. In quantum chemical calculations, a more open pore structure and increased polarization was observed for the acetylene-bridged networks, which led to a significant improvement in water permeability. The present study will offer new insight into design of high-performance molecular separation membranes.