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A molecular sieve silica hybrid membrane was successfully prepared by using a counter diffusion chemical vapor deposition (CVD) method. Propyltrimethoxysilane (PrTMOS) was employed for a silica precursor. O3 was used as an oxidizer. Effects of deposition temperatures on permeation properties through the silica membranes were investigated. H2 (0.29 nm), N2 (0.36 nm) and SF6 (0.55 nm) permeances were measured at the deposited temperatures through the membranes. H2/N2 permeances ratios decreased sharply with increasing the deposition temperatures from 200°C to 400°C. In addition, H2 permeance through the membrane deposited at 400°C was very high (4.6 x 10-7 mol m-2 s-1 Pa-1) with a low H2/N2 permeances ratio (3.4). This shows that silica was not deposited at 400°C due to O3 decomposition at the high temperature deposition. On the other hand, N2/SF6 permeances ratios increased with increasing the deposition temperatures upto 320°C. The maximum value of N2/SF6 permeances ratio was 110 at 320°C deposition. This is much larger than that of Knudsen diffusion separation (2.3). According to the kinetic diameters of N2 and SF6, the pore sizes of the membrane were estimated at about 0.5 nm. These results show that the pore size of silica membranes can be controlled by changing the deposition temperatures. The decomposition reactions of propyl groups on silica hybrid materials were also investigated by TG and IR measurements. Propyl groups on silica hybrid materials decomposed at around 300°C and 400°C. Decomposition at around 300°C was C2H4 removal from the surface to remain methyl groups on the surface. Thus, we conclude that the high selective membrane deposited at 320°C was a silica hybrid membrane having methyl groups in the silica deposition. [ABSTRACT FROM AUTHOR]

Thin pure-silica chabazite (Si-CHA) membranes have been synthesized by using a secondary growth method on a porous silica substrate. A CO2 permeance of 2.62 × 10−6 mol m−2 s−1 Pa−1 with a CO2/CH4 permeance ratio of 62 was obtained through a Si-CHA membrane crystallized for 8 h using a parent gel of H2O/SiO2 ratio of 4.6. The CO2 permeance through the Si-CHA membrane on a porous silica substrate was twice as high as that through the membrane synthesized on a porous alumina substrate, which displayed a similar zeolite layer thickness. [ABSTRACT FROM AUTHOR]

Heat resistant hydrogen selective membranes are desired for use as membrane reactors in low-temperature hydrogen production via the steam reforming of hydrocarbons, which are usually operated over 1000 K. In addition, developing a multi-tubular type of membrane unit that can process more reactants is becoming more and more important in order to realize the practical use of membrane reactors. In this study, an all-ceramic module consisting of 6 silica membrane tubes with a comparatively large membrane area of around 0.04 m2 was fabricated by a counter-diffusion chemical vapor deposition technique. As a result, the H2/N2 ideal separation factor and the H2 permeance of the module were 1300 and 1.9 × 10-7 mol·m-2s-1Pa-1 at 873 K, respectively. In a 1000-hour thermal stability test for the silica membrane module, it was found that the H2 permeance initially decreased by about 30% and then became steady under ΔP = 0.95 MPa at 773 K. [ABSTRACT FROM AUTHOR]

Abstract: A silica membrane having an excellent H2/N2 permeance ratio (over 800) was prepared by the counter-diffusion chemical vapor deposition method. The H2/N2 permeance ratio was kept for 82 h under 76 kPa of steam at 773 K. The H2/H2O permeance ratio through the silica membrane was ca. 300 at 773 K. This is much higher than that through silica membranes prepared by a sol-gel method. The deposited silica layer was found in the γ-alumina layer of the substrate by TEM observations, indicating that the silica deposition is controlled by counter-diffusion of the precursors. Steam stability tests for porous γ-alumina substrates are also discussed. Pore size of the γ-alumina layer increased with an increasing steam treatment period at 773 K. The pore size of the γ-alumina layer was ca. 9 nm after 52 h of steam treatment. [Copyright &y& Elsevier]

Abstract: A stable silica membrane having excellent H2/N2 permeance ratio (over 1000) was prepared by the counter diffusion chemical vapor deposition method using tetramethyl orthosilicate (TMOS) and O2 as reactants at 873K. TMOS and O2 were provided in the opposing geometry of the substrates, and silica layer was deposited in the substrate pores. Apparent activation energies through the silica membranes increased with increasing deposition temperatures. The activation energy of H2 was ca. 20kJmol−1 through the membrane. H2 permeance at 873K permeation test was 1.5×10−7 molm−2 s−1 Pa−1. H2/N2 permeance ratio was kept for 21h under the typical steam-reforming conditions of methane for a membrane reactor (76kPa of steam at 773K). [Copyright &y& Elsevier]

The potential of the silica membrane reactors for use in the decomposition of hydrogen iodide (HI) was investigated by simulation with the aim of producing CO 2 -free hydrogen via the thermochemical water-splitting iodine-sulfur process. Simulation model validation was done using the data derived from an experimental membrane reactor. The simulated results showed good agreement with the experimental findings. The important process parameters determining the performance of the membrane reactor used for HI decomposition, namely, reaction temperature, total pressures on both the feed side and the permeate side, and HI feed flow rate were investigated theoritically by means of a simulation. It was found that the conversion of HI decomposition can be improved by up to four times (80%) or greater than the equilibrium conversion (20%) at 400 °C by employing a membrane reactor equipped with a tubular silica membrane. The features to design the membrane reactor module for HI decomposition of the thermochemical iodine-sulfur process were discussed under a wide range of operation conditions by evaluating the relationship between HI conversion and number of membrane tubes. • Potential of silica membrane reactors for HI decomposition was explored by simulation. • Limits of operability of the membrane reactor were investigated. • The model developed fits the experimental data very well. • The membrane reactor was found to have HI conversion limitations. • A compact demonstration membrane reactor can be designed for HI decomposition. [ABSTRACT FROM AUTHOR]

Abstract: Development of high performance hydrogen-selective inorganic membrane has become an important requisite for the production of hydrogen, an energy carrier that could cater the ongoing energy revolution. This paper describes a novel, hybrid method to process high performance hydrogen perm-selective membranes by the rapid CVD modification of a sol–gel silica layer. The initial N2 permeance values through a membrane were brought down by four orders of magnitude to 2.74×10−10 molm−2 s−1 Pa−1 within 5min of CVD, while maintaining the H2 permeance values as high as 6.43×10−7 molm−2 s−1 Pa−1 (H2/N2 =2300). It is likely that the reduction in the CVD zone thickness achieved by the presence of pre-existing sol–gel silica layer is the reason for the rapid modification of the pore structure to high performance membranes. [Copyright &y& Elsevier]

Hydrogen production from nuclear energy has attracted considerable interest as a clean energy solution to address the challenges of climate change and environmental sustainability. With respect to the large-scale and economical production of hydrogen using nuclear energy, the thermochemical water-splitting iodine-sulfur (IS) process is a promising method. The IS process uses sulfur and iodine compounds to decompose water into its elemental constituents, hydrogen and oxygen, by using three coupled chemical reactions: the Bunsen reaction; sulfuric acid decomposition; and hydrogen iodide (HI) decomposition. The decomposition of HI is the efficiency-determining step of the process. In this work, a membrane reactor with a silica membrane closed on one end was designed, and its potential for hydrogen production from HI decomposition was explored. In the reactor-module design, only one end of the membrane tube was fixed, while the closed-end of the tube was freely suspended to avoid thermal expansion effects. The closed-end silica membranes were prepared for the first time by a counter-diffusion chemical vapor deposition of hexyltrimethoxysilane. In application, HI conversion of greater than 0.60 was achieved at a decomposition temperature of 400 °C, which is three times greater than the equilibrium conversion (0.20). Thus, the membrane reactor with closed-end silica membrane was shown to produce a successful equilibrium shift in the production of hydrogen via HI decomposition in the thermochemical IS process. [Display omitted] • A membrane reactor equipped with a silica membrane closed on one end was designed. • Its potential for hydrogen production from HI decomposition was explored. • A closed-end silica membrane was prepared by counter-diffusion CVD method. • In application, HI conversion of greater than 0.60 was achieved at 400 °C. • Developed membrane reactor is applicable for hydrogen production via HI decomposition. [ABSTRACT FROM AUTHOR]

In this study, a corrosion-stable silica membrane was developed to be used in H 2 purification during the hydrogen iodide decomposition (2HI → H 2 + I 2), which is a new application of the silica membranes. From a practical perspective, the membrane separation length was enlarged up to 400 mm and one end of the membrane tubes was closed to avoid any thermal variation along the membrane length and sealing issues. The silica membranes consisted of a three-layer structure comprising a porous α -Al 2 O 3 ceramic support, an intermediate layer, and a top silica layer. The intermediate layer was composed of γ- Al 2 O 3 or silica, and the top silica layer that is H 2 selective was prepared via counter-diffusion chemical vapor deposition of a hexyltrimethoxysilane. To the best of our knowledge, this is the first report of 400-mm-long closed-end silica membranes supported on Si-formed α- Al 2 O 3 tubes produced via chemical vapor deposition method. A 400-mm-long closed-end membrane using a Si-formed α- Al 2 O 3 tube exhibited a higher H 2 /SF 6 selectivity of 1240 but lower H 2 permeance of 1.4 × 10−7 mol Pa−1 m−2 s−1 with compared with the membrane using a γ- Al 2 O 3 -formed α -Al 2 O 3 tube (907 and 5.6 × 10−7 mol Pa−1 m−2 s−1, respectively). The membrane using the Si-formed α -Al 2 O 3 tube was more stable in corrosive HI gas than a membrane with a γ- Al 2 O 3 -formed α -Al 2 O 3 tube after 300 h of stability tests. In conclusion, the developed silica membranes using the Si-formed α -Al 2 O 3 tubes seem suitable for membrane reactors that produce H 2 on large scale using HI decomposition in the thermochemical iodine–sulfur process. [Display omitted] • Corrosion-stable membrane was developed for H 2 production from HI decomposition. • Membrane length was enlarged up to 400 mm, and one end of the tubes was closed. • Membrane using Si-formed substrate remained stable in corrosive HI gas up to 300 h. • Developed membranes appear suitable for membrane reactor of HI decomposition. [ABSTRACT FROM AUTHOR]

A high-performance, H 2 -permselective silica membrane derived from hexyltrimethoxysilane (HTMOS) was developed for application in the thermochemical water-splitting iodine–sulfur process. Silica membranes, referred to here as HTMOS membranes, were prepared via counter-diffusion chemical vapor deposition on γ -alumina-coated α -alumina support tubes with outer diameters of 10 mm. Special attention was devoted to obtain high H 2 /HI selectivity, high H 2 permeance, and good stability in the presence of corrosive HI gas. The effects of the deposition conditions, temperature, and period were investigated. The HTMOS membrane prepared at 450 °C for 5 min exhibited high H 2 /HI selectivity (>175) with H 2 permeance on the order of 10 −7 mol Pa −1 m −2 s −1 . On the basis of stability experiments, it was found that the HTMOS membrane was stable upon HI exposure at a temperature of 400 °C for 11 h. [ABSTRACT FROM AUTHOR]

• High olefins selectivity was obtained through the two-layer membrane contactor. • The C 3 H 6 /C 3 H 8 selectivity was 236. • No need for further separation of C 3 H 6 and C 3 H 8. • The porous silica substrate was the key for the high selectivity. • The silica substrate improved the growth rate of the membrane. A MFI zeolite membrane was synthesized on a porous silica substrate. The defect-free MFI zeolite membrane was synthesized via a secondary growth method using seed crystals (Si/Al = 20) on a silica substrate in a parent gel of Si/Al = 100. The silica substrate prevented the dissolution of the seed crystals during the synthesis to promote the MFI zeolite membrane synthesis. The MFI zeolite membranes were employed for a contactor-type reactor for methanol-to-olefins (MTO) reactions to produce olefins, such as propylene. The methanol conversion was 99.9% with the selectivity of ethylene - pentene at 91.5% for the permeate side of the membrane contactor. The propylene selectivity was high at 48.5% with the C 3 H 6 /C 3 H 8 ratio at 236. [ABSTRACT FROM AUTHOR]