Y.S. Lin

Template-removal-associated microstructural development of porous-ceramic-supported MFI zeolite membranes

Defect-free thin MFI zeolite films were synthesized on porous α-alumina and yttria-doped zirconia (YZ) substrates by an in situ crystallization method using tetrapropylammonium hydroxide (TPAOH) as a template. The microstructure evolution of the supported zeolite films during calcination for template removal was studied by high-temperature X-ray diffraction and in situ gas permeance. Removal of the template from the zeolite films occurs at 350–500°C and is accompanied by a substantial shrinkage in the zeolite framework. After the template is removed from the zeolite at high temperatures, the zeolite crystals expand on cooling while the support shrinks. A compressive stress develops in the zeolite film during the cooling process when the zeolite crystallites are bound to the support after template removal. Without annealing prior to template removal, this stress induces cracks in the YZ-supported MFI films. High-quality MFI membranes were obtained on both α-alumina and YZ supports by using a suitable calcination temperature program. Removal of the template can either create or enlarge intercrystalline gaps as the zeolite crystallites decrease in size and draw away from adjacent crystallites. These gaps constitute the microporous non-zeolitic pores affecting the permselectivity of single-gas permeation for H2 and SF6. Both alumina- and YZ-supported MFI membranes show very good separation properties for hydrogen and n-butane mixtures, although the YZ-supported MFI films appear to have larger non-zeolitic pores than the alumina-supported membranes.

MICROPOROUS INORGANIC MEMBRANES

Recent development in microporous inorganic membranes represents a significant advance in materials for separation and chemical reaction applications. This paper provides an in-depth review of synthesis and properties of two groups (amorphous and crystalline) of microporous inorganic membranes. Amorphous microporous silica membranes can be prepared by the sol-gel and phase separation methods. Flat sheet, tubular and hollow fiber amorphous carbon membranes have been fabricated by various pyrolysis methods from polymer precursors. A large number of synthesis methods have been developed to prepare good quality polycrystalline zeolite membranes. Several techniques, including vapor and liquid approaches, are reviewed for pore structure modification to prepare microporous inorganic membranes from mesoporous inorganic membranes. Chemical, microstructural and permeation properties of these microporous membranes are summarized and compared among the several microporous membranes discussed in this paper. Theory for gas permeation through microporous membranes is also reviewed, with emphasis on comparison of theoretical with the experimental data. These inorganic microporous membranes offer excellent separation properties by the mechanisms of preferential adsorption, selective configurational diffusion or molecular sieving.

Synthesis and hydrogen permeation properties of ultrathin palladium-silver alloy membranes

Abstract The present work focuses on the synthesis and gas permeation properties of ceramic supported ultrathin palladium-silver alloy membranes. PdAg films with a thickness ranging from 250 to 500 nm are coated on the surface of 3 nm pore sol-gel derived γ-alumina support using an RF magnetron sputtering equipment. The coated PdAg membranes exhibit the same composition and phase structure as those of the PdAg foil used as the target in sputter deposition. The hydrogen to nitrogen separation factor of the ultrathin PdAg membrane is 5.7 at 250°C and increases with increasing temperature. Under proper preparation conditions, use of a pinhole-free γ-alumina support is the key to ensure the gas-tightness and high-selectivity of the coated PdAg membranes. A method is demonstrated for studying hydrogen permeation through ultrathin metallic films. Hydrogen permeation data at different hydrogen pressures and temperatures (100–250°C) are reported to examine the mechanism of hydrogen permeation through the ultrathin metallic membranes. The experimental results clearly indicate the dominant role of surface reactions for hydrogen permeation through ultrathin metallic films at low temperatures.

Kinetics of carbon dioxide sorption on potassium-doped lithium zirconate

Potassium-doped lithium zirconate (Li2ZrO3) sorbents with similar crystallite but different aggregate sizes were prepared by a solid-state reaction method from mixtures of Li2CO3, K2CO3, and ZrO2 of different particle sizes. Carbon dioxide sorption rate on the prepared Li2ZrO3 sorbents increases with decreasing sorbent aggregate size. It is the size of the aggregate, not the crystallite, of Li2ZrO3 that controls the sorption rate. Temperature effect on CO2 sorption is complex, depending on both kinetic and thermodynamic factors. A mathematical model based on the double-shell sorption mechanism was established for CO2 sorption kinetics and it can fit experimental data quite well. Above 500°C, the rate-limiting step of CO2 sorption is the diffusion of oxygen ions through the ZrO2 shell formed during the carbonation reaction. Oxygen ion conductivities in the ZrO2 shell were obtained by regression of the experimental CO2 uptake curves with the model and are consistent with the literature data.

A Comparative study on thermal and hydrothermal stability of alumina, titania and zirconia membranes

Abstract Thermal and hydrothermal stabilities of sol-gel derived γ-alumina, titania and zirconia membranes have been investigated using comprehensive experimental data on the pore and phase structure of these three membranes after heat treatment at high temperatures under air and steam/air atmospheres. For all three ceramic membranes heat treatment results in a decrease in the surface area and an increase in the pore size. The effect of sintering on the change of the pore structure of the three membranes decreases in the order: zirconia > titania > alumina. The presence of steam enhances the pore structure change of these three membranes at elevated temperatures, and the extent of the enhancing effects for the three membranes decreases in the same order. At temperatures higher than 900 (alumina), 600 (zirconia) and 450°C (titania), metastable to stable phase transformation occurs, causing a substantial change in the pore structure of all three membranes, for which the extent of the pore structure change decreases in the order: alumina > zirconia > titania. Presence of steam in the phase-transformation region also enhances the pore structure change, especially of zirconia and titania membranes. Doping drying control chemical additive alters the pore structure of these three ceramic membranes. These comprehensive experimental data are explained using the surface diffusion sintering mechanism, surface nucleation mechanism as well as the microstructure of the sol-gel synthesized ceramic materials.

Fabrication of ultrathin metallic membranes on ceramic supports by sputter deposition

The present work focuses on the synthesis of ultrathin palladium films (< 500 nm) grown on porous ceramic supports by the sputter deposition technique. The following two parameters were found most critical to the synthesis of the gas-tight metal-ceramic composite: substrate type (surface roughness) and the deposition temperature. Fairly gas-tight Pd films with good adhesion could be coated on sol-gel derived fine pore γ-alumina supports but not on coarse α-alumina supports. Poor adhesion between the coated film and the γ-alumina support was observed for films coated at room temperature, to a thickness of 300 nm or larger. Both coating temperature (80–600°C) and substrate type affect the grain size, nitrogen gas-tightness and the adhesion of the deposited metallic films. Characterization results show that 400°C is the optimum coating temperature. XRD and SEM data on these films show that the films are fairly crystalline, with a uniform and smooth surface morphology.

Template-free secondary growth synthesis of MFI type zeolite membranes

Abstract Good quality MFI type zeolite membranes were prepared on porous α-alumina supports by the secondary growth method using pure silica sol without organic template. X-ray photoelectron spectroscopy analysis of various MFI layers prepared by dip-coating and secondary growth indicates transfer of aluminum from the alumina support to the framework of the zeolite layer via two routes. Aluminum diffuses into the zeolite layer during calcination step at high temperature. It also dissolves in the synthesis solution and is incorporated into the zeolite framework during growth of the zeolite layer. Separation properties of an eight component hydrogen–hydrocarbon (C 1 –C 4 ) mixture were measured for the secondary-grown MFI zeolite membranes synthesized without template, and are compared with those of similar in situ synthesized MFI zeolite membranes prepared with template. The secondary-grown zeolite membrane is impermeable to propane, propylene, n -butane and isobutane, and its separation factor decreases with increasing carbon number of the permeating gases. The in situ crystallized zeolite membrane is impermeable only to isobutane and its separation factor increases with increasing carbon number of the permeating gases. These results are discussed in terms of template-removal associated intercrystalline pores in the zeolite layer and the corresponding permeation mechanism of the zeolite membranes prepared with and without template.

Fabrication of thin metallic membranes by MOCVD and sputtering

Abstract Thin (0.1–1.5 μm) Pd and Pd Ag alloy membranes have been prepared on porous ceramic substrates consisting of a macroporous α-Al2O3 disk coated with a sol-gel derived, mesoporous γ-Al2O3 top layer. Metallorganic chemical vapor deposition (MOCVD) and magnetron sputtering were used to coat the thin metallic membranes employing Pd(II) acetylacetonate and a 75% Pd-25% Ag alloy target, respectively. The gas transport properties of the thin metallic membranes were determined by multicomponent permeation experiments with He, H2 and Ar at 25–300°C and 1 atm total pressure. The H2 permeance and H2: He selectivity were in the range 1.0–2.0 × 10−7 mol m−2 s−1 Pa−1 and 30–200 at 300°C, respectively. The dependence of H2 permeation rates on membrane thickness and temperature suggest that surface reaction steps are rate-limiting for H2 transport through the thin, ceramic-supported metallic membranes made by MOCVD and sputtering.

Fabrication of a thin palladium membrane supported in a porous ceramic substrate by chemical vapor deposition

A thin, gas-tight palladium (Pd) membrane was prepared by the counter-diffusion chemical vapor deposition (CVD) process employing palladium chloride (PdCl2) vapor and H2 as Pd precursors. A disk-shaped, two-layer porous ceramic membrane consisting of a fine-pore γ-Al2O3 top layer and a coarse-pore α-Al2O3 substrate was used as Pd membrane support. A 0.5–1 μm thick metallic membrane was deposited in the γ-Al2O3 top layer very close to its surface, as verified by XRD and SEM with a backscattered electron detector. The most important parameters that affected the CVD process were reaction temperature, reactants concentrations and top layer quality. Deposition of Pd in the γ-Al2O3 top layer resulted in a 100- to 1000-fold reduction in He permeance of the porous substrate. The H2 permeation flux of these membranes was in the range 0.5–1.0 × 10−6 mol m−2 s−1 Pa−1 at 350–450°C. The H2 permeation data suggest that surface reaction steps are rate-limiting for H2 transport through such thin membranes in the temperature range studied.

Experimental studies on pore size change of porous ceramic membranes after modification

Experimental results on pore size change of a microfiltration (MF) -alumina membrane and an ultrafiltration (UF) γ-alumina membrane after modification by chemical vapor deposition (CVD) of solid oxides in the membrane pores are presented and explained using the results of a theoretical analysis. With an approx. 10-fold reduction in permeability, the average pore size of the MF membrane is found to increase after CVD modification, due to its relatively broader initial pore size distribution with a small amount of large pores and due to the particular CVD conditions (heterogeneous deposition mechanism) which give a pore narrowing rate independent of pore size. The effective pore size of the UF membrane appears to remain unchanged after modification with an approx. 50-fold reduction in permeability, as a result of the slit-shaped pores of the γ-alumina film and the particular modification conditions. The experimental and theoretical results suggest that, in order to reduce effectively the average pore size of a membrane by a modification process, the membrane should have a rather uniform pore size distribution, or the modification process should be conducted under conditions which give a pore narrowing rate proportional to the pore size.