Counter-diffusion Chemical Vapor Deposition Research Articles

The Evaluation of Counter Diffusion CVD Silica Membrane Formation Process by In Situ Analysis of Diffusion Carrier Gas.

A new evaluation method for preparing silica membranes by counter diffusion chemical vapor deposition (CVD) was proposed. This is the first attempt to provide new insights, such as the decomposition products, membrane selectivity, and precursor reactivity. The permeation of the carrier gas used for supplying a silica precursor was quantified during the deposition reaction by using a mass spectrometer. Membrane formation processes were evaluated by the decrease of the permeation of the carrier gas derived from pore blocking of the silica deposition. The membrane formation processes were compared for each deposition condition and precursor, and the apparent silica deposition rates from the precursors such as tetramethoxysilane (TMOS), hexyltrimethoxysilane (HTMOS), or tetraethoxysilane (TEOS) were investigated by changing the deposition temperature at 400–600 °C. The apparent deposition rates increased with the deposition temperature. The apparent activation energies of the carrier gas through the TMOS, HTMOS, and TEOS derived membranes were 44.3, 49.4, and 71.0 kJ mol−1, respectively. The deposition reaction of the CVD silica membrane depends on the alkoxy group of the silica precursors.

Fabrication, permeation, and corrosion stability measurements of silica membranes for HI decomposition in the thermochemical iodine–sulfur process

In this study, a corrosion-stable silica membrane was developed to be used in H2 purification during the hydrogen iodide decomposition (2HI → H2 + I2), 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-Al2O3 ceramic support, an intermediate layer, and a top silica layer. The intermediate layer was composed of γ-Al2O3 or silica, and the top silica layer that is H2 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-Al2O3 tubes produced via chemical vapor deposition method. A 400-mm-long closed-end membrane using a Si-formed α-Al2O3 tube exhibited a higher H2/SF6 selectivity of 1240 but lower H2 permeance of 1.4 × 10−7 mol Pa−1 m−2 s−1 with compared with the membrane using a γ-Al2O3-formed α-Al2O3 tube (907 and 5.6 × 10−7 mol Pa−1 m−2 s−1, respectively). The membrane using the Si-formed α-Al2O3 tube was more stable in corrosive HI gas than a membrane with a γ-Al2O3-formed α-Al2O3 tube after 300 h of stability tests. In conclusion, the developed silica membranes using the Si-formed α-Al2O3 tubes seem suitable for membrane reactors that produce H2 on large scale using HI decomposition in the thermochemical iodine–sulfur process.

Development of a membrane reactor with a closed-end silica membrane for nuclear-heated hydrogen production

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.

Bench-Scale Membrane Reactor for Methylcyclohexane Dehydrogenation Using Silica Membrane Module.

Methylcyclohexane-toluene system is one of the most promising methods for hydrogen transport/storage. The methylcyclohexane dehydrogenation can be exceeded by the equilibrium conversion using membrane reactor. However, the modularization of the membrane reactor and manufacturing longer silica membranes than 100 mm are little developed. Herein, we have developed silica membrane with practical length by a counter-diffusion chemical vapor deposition method, and membrane reactor module bundled multiple silica membranes. The developed 500 mm-length silica membrane had high hydrogen permselective performance (H2 permeance > 1 × 10−6 mol m−2 s−1 Pa−1, H2/SF6 selectivity > 10,000). In addition, we successfully demonstrated effective methylcyclohexane dehydrogenation using a flange-type membrane reactor module, which was installed with 6 silica membranes. The results indicated that conversion of methylcyclohexane was around 85% at 573 K, whereas the equilibrium conversion was 42%.

Gas Permeation Property of Silicon Carbide Membranes Synthesized by Counter-Diffusion Chemical Vapor Deposition.

An amorphous silicon carbide (SiC) membrane was synthesized by counter-diffusion chemical vapor deposition (CDCVD) using silacyclobutane (SCB) at 788 K. The SiC membrane on a Ni-γ-alumina (Al2O3) α-coated Al2O3 porous support possessed a H2 permeance of 1.2 × 10−7 mol·m−2·s−1·Pa−1 and an excellent H2/CO2 selectivity of 2600 at 673 K. The intermittent action of H2 reaction gas supply and vacuum inside porous support was very effective to supply source gas inside mesoporous intermediate layer. A SiC active layer was formed inside the Ni-γ-Al2O3 intermediate layer. The thermal expansion coefficient mismatch between the SiC active layer and Ni-γ-Al2O3-coated α-Al2O3 porous support was eased by the low decomposition temperature of the SiC source and the membrane structure.

Comparison of experimental and simulation results on catalytic HI decomposition in a silica-based ceramic membrane reactor

In this study, the catalytic decomposition of hydrogen iodide was theoretically and experimentally investigated in a silica-based ceramic membrane reactor to assess the reactor's suitability for thermochemical hydrogen production. The silica membranes were fabricated by depositing a thin silica layer onto the surface of porous alumina ceramic support tubes via counter-diffusion chemical vapor deposition of hexyltrimethoxysilane. The performance of the silica-based ceramic membrane reactor was evaluated by exploring important operating parameters such as the flow rates of the hydrogen iodide feed and the nitrogen sweep gas. The influence of the flow rates on the hydrogen iodide decomposition conversion was investigated in the lower range of the investigated feed flow rates and in the higher range of the sweep-gas flow rates. The experimental data agreed with the simulation results reasonably well, and both highlighted the possibility of achieving a conversion greater than 0.70 at decomposition temperature of 400 °C. Therefore, the developed silica-based ceramic membrane reactor could enhance the total thermal efficiency of the thermochemical process.

Development of CVD Silica Membranes Having High Hydrogen Permeance and Steam Durability and a Membrane Reactor for a Water Gas Shift Reaction.

Water gas shift reaction of carbon monoxide (CO) with membrane reactors should be a promising method for hydrogen mass-production because of its high CO conversion, high hydrogen purity and low carbon dioxide emission. For developing such membrane reactors, we need hydrogen permselective membranes with high hydrogen permeance with order of 10−6 mol m−2 s−1 Pa−1 at 573 K and high steam durability. In this study, we have optimized the kind of substrates, precursors, vapor concentration, and chemical vapor deposition (CVD) time using the counter-diffusion CVD method for developing such membranes. The developed membrane prepared from hexamethyldisiloxane has a hydrogen permeance of 1.29 × 10−6 mol m−2 s−1 Pa−1 at 573 K and high steam durability. We also conducted water gas shift reactions with membrane reactors installed the developed silica membranes. The results indicated that reactions proceed efficiently with the conversion around 95–97%, hydrogen purity around 94%, and hydrogen recovery around 60% at space velocity (SV) 7000.

Silica-Based RO Membranes for Separation of Acidic Solution.

The development of acid separation membranes is important. Silica-based reverse osmosis (RO) membranes for sulfuric acid (H2SO4) solution separation were developed by using a counter diffusion chemical vapor deposition (CVD) method. Diphenyldimethoxysilane (DPhDMOS) was used as a silica precursor. The deposited membrane showed the H2SO4 rejection of 81% with a total flux of 5.8 kg m−2 h−1 from the 10−3 mol L−1 of H2SO4. The γ-alumina substrate was damaged by the permeation of the H2SO4 solution. In order to improve acid stability, the silica substrates were developed. The acid stability was checked by the gas permeation tests after immersing in 1 mol L−1 of the H2SO4 solution for 24 h. The N2 permeance decreased by 11% with the acid treatment through the silica substrate, while the permeance decreased to 94% through the γ-alumina substrate. The flux and the rejection through the DPhDMOS-derived membrane on the silica substrate were stable in the 70 wt % H2SO4 solution.

Hydrogen production tests by hydrogen iodide decomposition membrane reactor equipped with silica-based ceramics membrane

The decomposition of hydrogen iodide in the thermochemical water splitting iodine–sulfur process at an intermediate temperature (400 °C) using a catalytic membrane reactor was reported here, for the first time. The performance of a catalytic membrane reactor based on a hexyltrimethoxysilane-derived silica membranes (H2 permeance of 9.4 × 10−7 mol Pa−1 m−2 s−1 and H2/N2 selectivity of over 80.0.) was evaluated at 400 °C by varying the HI flow rates of 2.6, 4.7, 6.9, 8.4, and 9.7 mL min−1. The silica membranes were prepared by counter-diffusion chemical vapor deposition method on γ-alumina-coated α-alumina tubes. Hydrogen was successfully extracted from the membrane reactor using the silica membrane at 400 °C. A significant increase in HI conversion was achieved. The conversion achieved at an HI flow rate of 2.6 mL min−1 was approximately 0.60, which was greater than the equilibrium conversion in HI decomposition (0.22).

Preparation of an H2-permselective silica membrane for the separation of H2 from the hydrogen iodide decomposition reaction in the iodine–sulfur process

A high-performance, H2-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 H2/HI selectivity, high H2 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 H2/HI selectivity (>175) with H2 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.

Development of hydrogen-selective triphenylmethoxysilane-derived silica membranes with tailored pore size by chemical vapor deposition

Amorphous silica membranes were developed, based on an in silico molecular design, to exhibit excellent hydrogen-selective performance for separating hydrogen from mixtures containing larger organic molecules, such as methylcyclohexane and toluene. Triphenylmethoxysilane (TPMS) was synthesized and used as a novel precursor to prepare membranes by the counter-diffusion chemical vapor deposition method. Under the optimized bubbler temperature and counter-diffusion chemical vapor deposition reaction time, the fresh membranes showed high reproducibility and high hydrogen permeance in the order of 10−6molm−2s−1Pa−1 and high H2/SF6 ideal selectivity of over 12,000 at 573K. Moreover, the TPMS-derived membrane exhibited good stability after hydrogen regeneration, even after placement in a dehumidifier cabinet at room temperature for 90 days. Single gas permeation performance and normalized Knudsen-based permeance evaluation showed that the TPMS-derived membrane (three phenyl groups on the precursor) had a pore size of 0.486nm, and exhibited looser structures with larger pore size than those of a diphenyldimethoxysilane (DPDMS)-derived membrane (two phenyl groups on the precursor). These results suggest that the pore size of silica membranes can be tailored with various structured silica precursors containing phenyl groups.

High Hydrogen Permeance Silica Membranes Prepared by a Chemical Vapor Deposition Method

H2 permselective silica hybrid membranes were successfully prepared by using a counter diffusion chemical vapor deposition (CVD) method. Hexyltrimethoxysilane (HTMOS), phenyltrimethoxysilane (PhTMOS) or diphenyldimethoxysilane (DPhDMOS) were used as silica precursors. The oxidants were O3 or O2. These reactants were provided at the opposite side of the γ-alumina substrates, and the deposition occurred in the pores of the substrates. The HTMOS/O3 derived membrane deposited at 450°C showed the highest H2 permselectivity. The H2 permeace was 2.1×10-6 mol m-2 s-1 Pa-1 with the H2/SF6 permeance ratio of 5.9×106. H2 permeances through the HTMOS derived membranes increased with increasing the deposition temperatures. While the H2 permeance through the PhTMOS and DPhDMOS derived membranes decreased with increasing the deposition temperatures. The PhTMOS derived membrane prepared at 150°C showed the H2 permeance of 1.7×10-6 mol m-2 s-1 Pa-1 with the H2/SF6 permeance ratio of 13. The PhTMOS membrane prepared at 320°C showed the highest H2/SF6 permeance ratio of 1.8×104among the PhTMOS derived membranes. However, the H2/SF6 permeance ratio through the DPhDMOS membranes showed the different trend. Higher H2/SF6 permeance ratio was found through the DPhDMOS derived membranes deposited at 180°C and 360°C. The maximum H2/SF6 permeance ratio was 4.2×104 through the DPhDMOS membrane deposited at 180°C. The decomposition properties of organic groups on silica surface are investigated by using hydrolysis powders derived from the each silica precursor. The HTMOS powders showed O3 stability after the high temperature treatment. Thus, high H2 permselective membranes were prepared by the HTMOS at 450 °C.

High-temperature propylene/propane separation through silica hybrid membranes

C3H6-permselective silica hybrid membranes were prepared by using a counter-diffusion chemical vapor deposition (CVD) method. Propyltrimethoxysilane and hexyltrimethoxysilane (HTMOS) were used as silica precursors and O3 and O2 were used as oxidants. HTMOS was a suitable silica precursor for high-temperature CVD because of its high thermal stability. The N2/SF6 permeance ratio decreased from 64 to 12 as the deposition time increased from 5 to 90min. The maximum N2/SF6 permeance ratio was 2.2×105 through the membrane deposited at 450°C for 5min. The C3H6/C3H8 permeance ratio was 414 (C3H6 permeance 1.0×10−8molm−2s−1Pa−1) through this membrane.

Degradation mechanism of an H2-permselective amorphous silica membrane

Degradation mechanism of H2 permselectivity under atmosphere of steam/N2 = 3 at 773 K was investigated among amorphous silica membranes on γ-Al2O3-coated α-Al2O3 or Ni-doped γ-Al2O3-coated α-Al2O3 synthesized by counter diffusion chemical vapor deposition. Helium and H2 permeance drastically decreased during the first few hours of hydrothermal exposure. On the other hand, N2 permeance fluctuated during hydrothermal treatment in silica membrane on γ-Al2O3-coated α-Al2O3. The degree of N2 permeance change depended on the hydrothermal stability of the intermediate layer. Hydrogen permselectivity was affected by both the densification of amorphous silica and the sintering of the intermediate layer during hydrothermal exposure.

Development of Inorganic Silica Reverse Osmosis Membranes by Using a Counter-Diffusion Chemical Vapor Deposition Method

Silica hybrid membranes have been developed for use as reverse osmosis (RO) membranes by using a counter-diffusion chemical vapor deposition (CVD) method. A silica source (phenyltrimethoxysilane; PhTMOS) and O3 were provided at opposite sides of a porous alumina substrate at 300°C for 90 min. The RO permeation test was conducted for 100 mg L−1 NaCl at 3.0 MPa. The highest NaCl rejection was 94.2% for a total flux of 1.7 kg m−2 h−1. The module length is an important factor in obtaining highly selective RO membranes. The short module (6 cm) was better because of the higher O3 concentration in the module. The decomposition conditions of phenyl groups on the silica surface are discussed for the hydrolysis powder of PhTMOS. According to FT-IR measurements, phenyl groups remained on the silica surface after O3 treatment for 90 min at 300°C, whereas the number of silanol groups decreased by approximately 30% upon O3 treatment at 300°C. The high Na+ rejection can be explained by the reduction of the silanol groups in the membrane.

Hydrothermal stability of hydrogen permselective amorphous silica membrane synthesized by counter diffusion chemical vapor deposition method

Hydrothermal stability of hydrogen permselective amorphous silica membrane synthesized by counter diffusion chemical vapor deposition method

Water Gas Shift Reaction in a Membrane Reactor Using a High Hydrogen Permselective Silica Membrane

A membrane reactor (MR) for the water gas shift (WGS) reaction was developed by integrating a highly hydrogen permselective silica membrane. The membrane was prepared using an extended counter-diffusion chemical vapor deposition (CVD) method. A tetramethylorthosilicate (TMOS) silica source was fed from one side of the membrane support and oxygen gas fed from the other. The dense silica film was deposited on a porous support by pressurizing the side that TMOS is supplied. A high hydrogen permselective silica membrane was obtained by this method. A commercial Pt catalyst was used in the WGS reaction. Efficacy of the silica membrane toward the WGS reaction was investigated as a function of temperature (523-623 K), steam/carbon monoxide (S/C) ratio (1-3), differential pressure (0-100 kPa), and gas hourly space velocity (GHSV; 1800-5400 h−1). The CO conversion in the MR was higher than that for a fixed bed reactor (FBR) under all experimental conditions, and was also higher than the thermodynamic equilibrium conversion under almost all experimental conditions. This was due to the selective abstraction of hydrogen from the product stream by the silica membrane. At an S/C of 1.0, the CO conversion in the MR was superior to that in a FBR by 16.8%.

Preparation of Thin Li4SiO4 Membranes by Using a CVD Method

CVD (chemical vapor deposition) procedures were investigated by using a counter diffusion CVD method. The effects of silica precursors on the hydrogen permeation properties of the silica membranes were discussed. 5 types of silica alkoxides (tetramethylorthosilicate (TMOS), methyltrimethoxysilane (MTMOS), propyltrimethoxysilane (PTMOS), dimethyldimethoxysilane (DMDMOS), and trimethylmethoxysilane (TMMOS)) were employed as the silica precursors; they have different numbers of methyl groups. Hydrogen permeance through the DMDMOS membrane prepared at 500°C was 9.0 × 10–7mol m–2 s–1 Pa–1, and H2/N2 selectivity was 920. Activation energy of H2 permeation through the silica membrane prepared from TMOS was 10.5kJ mol-1 that was the maximum among the 5 types of the silica precursor. This indicates dense silica layer can be obtained from TMOS. Thus, TMOS was employed for the further Li4SiO4 preparation.High temperature CO2 permselective membranes were successfully prepared by using Li4SiO4 as a CO2 selective layer. Pinholes of the Li4SiO4 layer was filled by the CVD post treatment at 600°C. CO2/N2 permselectivity was 1.2 at the 600°C permeation test. The CO2 permeance ratio is higher than the Knudsen diffusion difference. Thus, this selectivity was explained by the CO2 selective adsorption on the Li4SiO4 layer.

Dehydrogenation of Methylcyclohexane To Produce High-Purity Hydrogen Using Membrane Reactors with Amorphous Silica Membranes

We developed a membrane reactor that can produce high-purity hydrogen in one step from methylcyclohexane. This membrane reactor combined a hydrogen-selective amorphous silica membrane prepared with dimethoxydiphenylsilane and oxygen and employing counter-diffusion chemical vapor deposition, and Pt/Al2O3 catalyst. The silica membrane showed excellent hydrogen permeance at 573 K of the order of 10−6 mol m−2 s−1 Pa−1 and high hydrogen/sulfur hexafluoride permselectivity of around 104. The membrane reactor exhibited equilibrium shifts as expected under reaction temperatures ranging from 473 to 553 K and reaction pressures ranging from 0.1 to 0.25 MPa, and these performances were successfully predicted using a simulation model, which was also developed in this study. Finally, we demonstrated that hydrogen with purity as high as 99.95% was produced from methylcyclohexane in the membrane reactor without using carrier gas or sweep gas.

Preparation of silica hybrid membranes for high temperature gas separation

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 × 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.