Shigeharu Morooka

Pore structure of silica membranes formed by a sol-gel technique using tetraethoxysilane and alkyltriethoxysilanes

Porous silica membranes were formed by a sol–gel technique on a γ-alumina-coated α-alumina tube using sols which were prepared from tetraethoxysilane (TEOS) and octyl-, dodecyl- or octadecyltriethoxysilane. The alkyltriethoxysilanes were used as the template to control the pore size. The effect of the length of alkyl groups on pore structure, as well as the permeation properties of the silica membranes were then investigated. The molar ratio of water to total alkoxides in the sols, x, strongly influenced the pore structure. A silica membrane which was prepared with octadecyltriethoxysilane contained mesopores and cracks, but silica membranes prepared with octyl- and dodecyltriethoxysilanes were defect-free. Permeations to gasses with a variety of molecular sizes showed that the majority of micropores were in the 0.3–0.4xa0nm range for the silica membranes which had been calcined at 600°C. These micropores were basically formed by the decomposition of alkyl chains in the templates, while the mesopore structure was achieved during the gelation step.

Gas permeation and micropore structure of carbon molecular sieving membranes modified by oxidation

Abstract A BPDA-pp′ODA polyimide film was formed on the surface of a porous alumina support tube, which was then carbonized at an optimized temperature of 700°C. The membrane was further oxidized with either an O 2 –N 2 mixture or pure O 2 at 100–300°C. Oxidation at 300°C increased permeances, without damaging permselectivities, of the membrane. No difference in permeances between single and binary CO 2 –N 2 systems was observed. The carbon membranes possessed slit-shaped molecular sieving pores, in which molecules could pass one another by moving to the wider side of the slit. Permselectivity of the permeating molecules was determined by the narrow width of the slit-shaped micropores.

Separation of benzene/cyclohexane mixtures using polyurethane-silica hybrid membranes

Abstract Mixed sols were prepared by dissolving polyurethane (a 30xa0wt% solution in n -propanol, PU) and tetraethylorthosilicate (TEOS) in ethanol at PU:TEOS mass ratios of 1:2, 1:1, 2:1 and 3:1. Each of the sols was coated on a porous α-alumina support tube by the dipping method, and green membranes were heat-treated at 200°C for 1xa0h in an atmosphere of nitrogen. A PU membrane was also prepared with PU alone. The membranes were 5–6xa0μm thick. The polyurethane–silica membranes were swollen in benzene but only slightly in cyclohexane at room temperature. The degree of swelling in benzene decreased with increasing fractions of TEOS in the hybrid sols. The selectivity of benzene to cyclohexane was improved due to the suppression of swelling as a result of hybridization with TEOS. The total permeation flux and benzene/cyclohexane selectivity in the membrane prepared with a sol of PU:TEOS=1:1 were 3×10 −5 xa0kgxa0m −2 xa0s −1 and 19, respectively.

Microporous Inorganic Membranes for Gas Separation

Microporous inorganic membranes are potentially useful in gas separation in emerging areas such as catalytic reactors, gasification of coal, molten-carbonate and solid-electrolyte fuel cells, and water decomposition by thermochemical reactions. If the feed or product gases can be separated at elevated temperatures specific to each process, the energy required for purification could be greatly reduced. Advances in the development of inorganic membranes have been quite rapid in recent years. For example, in 1991 the reported CO 2 /N 2 selectivity at ambient temperature was less than 10, but by 1997 it had improved to approximately 100. The permeation rate and permselectivity of porous inorganic membranes are dependent on the microstructures of membrane/support composites such as pore size and distribution, porosity, tortuosity, and the affinity between permeating species and pore walls. Figure 1 shows the relationship between molecular weight and kinetic diameter (calculated from minimum equilibrium cross-sectional diameter) for a selected series of molecules. Hydrogen and helium are smaller and lighter than the others. Structural isomers such as n-C 4 H 10 and i-C 4 H 10 have the same mass but quite different sizes. Therefore, the control of micropores is of critical importance in these cases. However, the molecular masses and sizes of CO 2 and N 2 are not greatly different; thus the difference in affinity is important for separation of these molecules. In order to achieve effective separation of small-molecule gases, the membrane pores should be smaller than 2 nm. In the case of mesopores or macropores, gases permeate with low selectivities through these pores. In this article, preparation processes and permeation properties of porous inorganic membranes are reviewed, and permeation mechanisms are discussed.

Separation of hydrogen from steam using a SiC-based membrane formed by chemical vapor deposition of triisopropylsilane

Abstract A silicon carbide-based membrane was formed in the macropores of an α -alumina support tube by chemical vapor deposition of triisopropylsilane at 700–800°C with a forced cross-flow through the porous wall. The membrane permeated gases except H 2 O mainly by the Knudsen diffusion mechanism at permeation temperatures of 50–400°C. The H 2 /H 2 O selectivity was near or below unity because of the hydrophilic nature of the membrane. After a heat-treatment in Ar at 1000°C for 1xa0h, however, the membrane formed at a final evacuation pressure of 1xa0kPa exhibited a H 2 /H 2 O selectivity of 3–5, for a mixed feed of H 2 –H 2 O–HBr system, associated in a thermochemical water-splitting process. The H 2 permeance was (5–6)×10 −7 xa0molxa0m −2 xa0s −1 xa0Pa −1 at 50–400°C. The membrane maintained the H 2 /H 2 O selectivity for more than 100xa0h in the H 2 –H 2 O–HBr mixture at 400°C.

Applicability of palladium membrane for the separation of protium and deuterium

Separation of protium and deuterium using a palladium membrane was investigated under temperature and pressure conditions involving a phase transformation of the metal. The HD flux ratio for single-component systems showed a maximum of 7–8, when the membrane surfaces on the supply and permeate sides were in the β- and α-phases, respectively, for protium, and when both surfaces were in the α-phase for deuterium. The large difference in solubility of protium and deuterium in palladium was responsible for the high selectivity. For a mixture of H2 and D2, the HD ratio in palladium was expressed by an equation similar to Raoults law. The H and D fluxes were lower than those for single component systems, and the HD separation coefficient was 1.2–1.3.

Carbon dioxide separation from nitrogen using Y-type zeolite membranes

A polycrystalline Y-type zeolite membrane was formed by hydrothermal synthesis on the outer surface of a porous a-alumina support tube, which was polished with a finely powdered X-type zeolite for use as seeds. When an equimolar mixture of CO 2 and N 2 was fed into the feed side, the CO 2 permeance was nearly equal to that for the single-component system, and the N 2 permeance for the mixture was greatly decreased, especially at lower permeation temperatures. At 30°C, the permeance of CO 2 was higher than 10-7 mol.m 2 .s 1 .Pa 1 , and the permselectivity of CO 2 to N 2 was 20-100.