Porous Α-alumina Substrates Research Articles

Control of Sequential MTO Reactions through an MFI-Type Zeolite Membrane Contactor.

A membrane for controlling methanol-to-olefin (MTO) reactions was developed, which featured an MFI-type zeolite membrane (Si/Al = 25) that was synthesized on a porous α-alumina substrate using a secondary growth method. Here, the H2/SF6 permeance ratios were between 150 and 450. The methanol conversion rate was 70% with 38% ethylene selectivity and 28% propylene selectivity as determined using a cross-flow membrane contactor. In order to improve the olefin selectivity of the membrane, the MFI zeolite layer (Si/Al = ∞) was coated on an MFI-type zeolite membrane (Si/Al = 25). Using this two-layered membrane system, the olefin selectivity value increased to 85%; this was 19% higher than the value obtained during the single-layer membrane system.

Defect-free high-silica CHA zeolite membranes with high selectivity for light gas separation

A systematic approach is described for the fabrication of defect-free high-silica zeolite membranes with CHA (SSZ-13) topology. Home-made hydrothermally-synthesized CHA seeds were coated on porous α-alumina substrates with a pore diameter of 80 nm and by means of a further hydrothermal treatment a zeolite membrane layer was formed. In order to obtain a thin and defect-free zeolite layer, the influence on the final microstructure of seed concentration during coating, coating method (rubbing, dip- or spin-coating) and crystal growth time was investigated. The template removal procedure was optimized to avoid the formation of cracks or defects. For an optimal thermal treatment, using a step-wise temperature increase to 500 °C, the membranes exhibit CO2/CH4 permselectivities of 25–30 with CO2 permeances of around 2 x 10−7 mol m−2 s−1 Pa−1 at 22 °C and 2 bar of pressure difference. O2 plasma pre-treatment prior to template removal increased the CO2/CH4 permselectivity to 176, while maintaining the same CO2 permeance values when no pre-treatment was used. The SF6 permeances, both at low (22 °C) and high (200 °C) temperatures, were below the detection limit (2 x 10−10 mol m−2 s−1 Pa−1), which in return results in very high N2/SF6 permselectivities of more than 700.

In-Plane Epitaxial Growth of Highly c-Oriented NH2 -MIL-125(Ti) Membranes with Superior H2 /CO2 Selectivity.

Preferred-orientation control has significant impact on the separation performance of MOF membranes. Under most conditions the preferred orientation of MOF membranes is dominated by the Van der Drift mechanism of evolutionary growth selection so that the obtained orientation may not be optimized for practical application. In this study, highly c-oriented NH2 -MIL-125 membranes were prepared on porous α-alumina substrates by combining oriented seeding and controlled in-plane epitaxial growth. Dynamic air-liquid interface-assisted self-assembly of c-oriented NH2 -MIL-125(Ti) seed monolayers, the use of layered TiS2 as the metal precursor, and single-mode microwave heating were crucial in ensuring the preferred c-orientation while simultaneously suppressing undesired twin growth. Owing to reduced grain boundary defects, the prepared c-oriented membranes showed an ideal H2 /CO2 selectivity of 24.8, which was 6.1 times higher than that of their randomly oriented counterparts under similar operating conditions.

DDR-type zeolite membrane synthesis, modification and gas permeation studies

DDR-type zeolite membrane was synthesized on porous α-alumina substrate by hydrothermal treatment of a ball-milled Sigmal-1 crystal seed layer in an aluminum-free precursor solution containing 1-Adamantylamine as the structure directing agent (SDA). The as-synthesized DDR zeolite membranes were defect-free but the supported zeolite layers were susceptible to crack development during the subsequent high-temperature SDA removal process. The cracks were effectively eliminated by the liquid phase chemical deposition method using tetramethoxysilane as the precursor for silica deposits. The modified membrane was extensively studied for H2, He, O2, N2, CO2, CH4, and i-C4H10 pure gas permeation and CO2/CH4 mixture separation. At 297K and 2-bar feed gas pressure, the membrane achieved a CO2/CH4 separation factor of ~92 for a feed containing 90% CO2, which decreased to 62 for a feed containing 10% CO2 with the CO2 permeance virtually unchanged at ~1.8×10−7mol/m2sPa regardless of the feed composition. It also exhibited an O2/N2 permselectivity of 1.8 at 297K. The gas permeation behaviors of the current aluminum-containing DDR type zeolite membrane are generally in good agreement with the findings in both experimental and theoretical studies on the pure-silica DDR membranes in recent literature.

In-situ aging microwave heating synthesis of LTA zeolite layer on mesoporous TiO2 coated porous alumina support

LTA zeolite layer was successfully grown on a superhydrophilic mesoporous titania layer coated onto porous α-alumina substrate. Mesoporous titania layer was formed as an intermediate bridge in the pore size variation between the macroporous α-alumina support and micro-porous LTA zeolite layer. In-situ aging microwave heating synthesis method was utilized to deposit the LTA zeolite layer. Mesoporous titania layer was pre-treated with UV photons and this was observed to have played a major role in improving the surface hydrophilicity of the substrate leading to formation of increased number of Ti–OH groups on the surface. This increase in Ti–OH groups enhanced the interaction between the synthesis gel and the substrate leading to strong attachment of the amorphous gel on the substrate, thus enhancing coverage of the LTA zeolite layer to almost the entire surface of the 1-inch (25.4mm) diameter membrane. LTA zeolite layer was developed via in-situ aged under microwave irradiation to study the effect of synthesis parameters such as in-situ aging time and synthesis time on the formation of the LTA zeolite layer. Optimized process parameters resulted in the formation of crack-free porous zeolite layer yielding a zeolite–titania–alumina multi-layer membrane with a gradient in porosity.

Zeolite Imidazolate Framework–Ionic Liquid Hybrid Membranes for Highly Selective CO2 Separation

Zeolitic imidazolate framework ZIF-69 membranes were grown on porous α-alumina substrates via seeded secondary growth and further functionalized by a CO2-selective tricyanomethanide anion/alkylmethylimidazolium cation-based ionic liquid (IL) to plug the gaps between the ZIF crystals yet leave the framework pores open for gas diffusion. In this configuration, ZIF intergrain boundaries and defects were repaired by a medium that exhibits high selectivity for CO2. As a result, the selectivity of the hybrid membrane was significantly higher than that of as-grown ZIF membranes and, because of the existence of the ZIF channels, the permeability was higher than that corresponding to bulk IL. Specifically, CO2 permeated 20 times faster than N2 through the intact ZIF pores and 65 times faster than through the bulk IL phase. The developed membranes at room temperature and under a 2 bar transmembrane pressure exhibited CO2 permeance of 5.6 × 10–11 and 3.7 × 10–11 mol m–2 s–1 Pa–1 and real CO2/N2 selectivities of 44 a...

Pore-fill-type Palladium–Porous Alumina Composite Membrane for Hydrogen Separation

Fabrication of thermally stable, low cost hydrogen separation membrane for pre-combustion process was studied. A novel composite membrane with Pd layer encapsulated in a porous α-alumina substrate (Pd/Al2O3(PF)) was successfully prepared. Pd was observed only in the porous substrate matrix at a depth of 10μm from the top surface. This membrane showed extremely high H2 permselectivity (αH2/N2 > 10,000) at 773K and good stability.

Synthesis of highly c-oriented ZIF-69 membranes by secondary growth and their gas permeation properties

A seeded growth procedure was successfully developed to synthesize highly c-oriented and well-intergrown zeolitic imidazolate framework-69 (ZIF-69) membranes on porous α-alumina substrates. The synthesis conditions were optimized both for seed preparation and for secondary growth. For seeding, a facile method was developed to prepare smaller and flat ZIF-69 microcrystals in order to make thin and c-oriented seed layers. While for secondary growth, a synthesis condition that favored the growth along the c-direction was chosen in order to form highly c-oriented ZIF-69 membranes after growth. As a result, the majority of ZIF-69 grains inside the membrane have their straight channels along the crystallographic c-axis aligned perpendicularly to the substrate surface. Such alignment was confirmed by both XRD and pole figure analysis. The mixture-gas separation studies that were carried out at room temperature and 1 atm gave separation factors of 6.3, 5.0, 4.6 for CO 2/N 2, CO 2/CO and CO 2/CH 4 respectively, and a permeance of ∼1.0 × 10 −7 mol m −2 s −1 Pa −1 for CO 2 in almost all mixtures. Both the separation factor and permeance were better than the performance of the ZIF-69 membranes prepared by the in situ solvothermal method due to improvement in the membrane microstructure by the seeded growth method.

Removal of BTX Compounds from Wastewaters Using Template Free MFI Zeolitic Membrane

MFI zeolite membranes were prepared on porous α-alumina substrates, using secondary growth of nano-seeded layers. The resulting membranes were characterized by means of Scanning Electron Microscopy (SEM), X-Ray Diffractometry (XRD), and pervaporation performance tests for separation of Benzene, Toluene and Xylene (BTX) mixture from contaminated water. The morphology, thickness, homogeneity, crystal preferential orientation and permeation properties of these membranes have been studied in relation to the seed layers. Successful separation of BTX mixture from water was performed by using the manufactured MFI zeolite membrane. The influence of temperature, feed concentration on the membrane separation efficiency systematically investigated. The total permeation fluxes were found to increase with increase in temperature and feed concentration. The separation factors increased with increasing feed concentration and decreased with increasing in temperature.

Preparation of Pore-fill-type Palladium–Porous Alumina Composite Membrane for Hydrogen Separation

Abstract A method for formation of the palladium layer in a thermally stable porous substrate was studied. A composite membrane with Pd layer encapsulated in a porous α-alumina substrate (Pd/Al2O3(PF)) was successfully prepared by a new seeding technique and electroless plating. From SEM–EDX, the Pd layer was observed only in the porous substrate at a depth of 10 µm from the top surface. This membrane showed extremely high H2 permselectivity (α H_2/N_2 > 10000) at 773 K and good stability.

Synthesis and characterization of ZIF-69 membranes and separation for CO2/CO mixture

Abstract Continuous and c- oriented ZIF-69 membranes were successfully synthesized on porous α-alumina substrates by an in situ solvothermal method. The membranes were characterized by XRD, SEM and single-gas permeation tests. The BET measurements on crystals taken from the same mother liquor that was used for membrane synthesis yield a Langmuir surface area of 1138 m 2 /g. The stability of the membrane towards heat and different solvents were studied. Single-gas permeation experiments through ZIF-69 membranes were carried out by a vacuum method at room temperature using H 2 , CH 4 , CO, CO 2 and SF 6 , respectively. The permeances were in the order of H 2 > CO 2 > CH 4 > CO > SF 6 . The separation of CO 2 /CO gas mixture was investigated by gas chromatograph (GC) and the permselectivity of CO 2 /CO was 3.5 ± 0.1 with CO 2 permeance of 3.6 ± 0.3 × 10 −8 mol m −2 s −1 Pa −1 at room temperature.

Preparation and characterization of V-K catalysts supported on porous α-alumina ceramic substrate for diesel soot oxidation

The V-K catalysts were produced on porous α-alumina substrate by a solution impregnation route and the compositions and catalytic activities for soot oxidation were studied by XRD, TG/DTG, DSC and TPR. According to the catalytic activity studies, the catalytic activity of the crystalline phases is in the order: KNO3+KVO3>K3V5O14+KVO3. The appearance of excessive KHCO3 phase will lead to the deterioration of catalytic activity when the catalysts contain higher KNO3 content. It is also found that when the K:V molar ratio is higher than 1:1, the prepared catalysts show a strong CO2 absorption characteristic and this behavior will become gradually significant with the increasing of K:V molar ratio. Considerable amount of absorbed CO2 are strongly bonded to the crystal lattice with onset desorption temperature of 200 °C.

Fabrication of MOF-5 membranes using microwave-induced rapid seeding and solvothermal secondary growth

Here we report the fabrication of continuous MOF-5 membranes via a secondary growth method. Densely packed MOF-5 seed layers on porous α-alumina substrates were rapidly prepared using microwave-induced thermal deposition (MITD) method. The MOF-5 seed layers were then solvothermally treated to grow into continuous MOF-5 membranes. The presence of a proton scavenging amine in precursor solutions during the secondary growth step was found to be critical to prevent the seed crystals from dissolving, yet enabling the growth of the MOF seed crystals into a continuous film. For the first time, preferentially oriented MOF-5 membranes were prepared by growing from oriented MOF-5 seed layers. Permeation results of MOF-5 membranes indicate Knudsen transport of small gas molecules, consistent with a previously reported result.

In situ high-temperature X-ray diffraction study of thin palladium/α-alumina composite membranes and their hydrogen permeation properties

The changes in the crystal structure of a thin palladium membrane (3–4 μm in thickness) supported on a porous α-alumina substrate were investigated by in situ high-temperature X-ray diffraction analysis at temperatures of 30–850 °C in static air atmosphere. The diffraction peaks of palladium and α-alumina shifted to a lower scanning angle (2 θ) with an increase in temperature, which was attributed to the thermal expansion of the crystal lattice. The significant enhancement in the intensity of the diffraction peak corresponding to palladium above 820 °C strongly indicated the occurrence of grain growth in palladium; a well-crystallized palladium phase was formed due to the decrease in the gaps between the grain boundaries. Hydrogen permeation tests were performed on unannealed and annealed membranes; annealing was conducted in argon atmosphere at 850 °C. The annealed membranes had considerably higher hydrogen permeation flux than the unannealed ones; this was probably due to an increase in hydrogen diffusivity in the palladium membrane layer.

Synthesis of continuous MOF-5 membranes on porous α-alumina substrates

The first continuous and well-intergrown MOF-5 membrane, evidenced from SEM imaging and X-ray diffraction (XRD), was successfully prepared on porous α-alumina substrate by in situ solvothermal synthesis. The BET measurements on crystals taken from the same mother liquor that was used for membrane synthesis yield a Langmuir surface area of 2259 m 2/g and a narrow pore size distribution centered at 1.56 nm. The permeation data for simple gases of H 2, CH 4, N 2, CO 2 and SF 6 show that the diffusion of simple gases through a MOF-5 membrane follows the Knudsen diffusion behavior.

Oriented Growth of ETS-4 Films Using the Method of Secondary Growth

Oriented growth of polycrystalline ETS-4 films on porous titania substrates was investigated using the method of secondary growth, utilizing synthesis mixtures with compositions of 3.6SiO2/1TiO2/5.5Na2O/xH2SO4/230.2H2O, where x = 4.4 or x = 3.6. Dense seed layers, obtained in situ by hydrothermal synthesis (x = 4.4), were partially (a, c)-out-of-plane oriented. Films were prepared in just one secondary growth step. Films developed by evolutionary, epitaxial growth of seed crystals and were highly (x = 4.4) b-out-of-plane oriented. Highly b-out-of-plane oriented ETS-4 films were also grown from dense, partially b-out-of-plane oriented seed layers on porous α-alumina substrates using identical synthesis mixtures. [Shattuck et al., Microporous Mesoporous Mater. 2006, 88, 56]. These findings suggest that the film orientation is dictated by the secondary growth conditions and is not determined by the seed layer orientation. This was attributed to the highly anisotropic growth rates of ETS-4 and the limited gro...

Hydrothermal synthesis of oriented ETS-4 films on porous α-alumina substrates

Preparation of ETS-4 films on porous α-alumina substrates was investigated using gel-forming synthesis mixtures (3.6SiO 2:1TiO 2:5.5Na 2O: xH 2SO 4:230.2H 2O, x = 4.4 or x = 3.6). The procedure involved two steps: direct in situ hydrothermal crystallization of seed layers, followed by their secondary growth. ETS-4 seed layers (thickness ∼ 3.0–4.0 μm) were obtained using gels with x = 4.4 when the autoclaves were rotated (72 RPM). The seed layers consisted of submicron plate-like crystals and were partially b-out-of-plane oriented. Films, obtained in the second step under static synthesis conditions, had a columnar microstructure and were highly ( x = 4.4) or less preferentially ( x = 3.6) oriented ( b-out-of-plane). This orientation is desirable for many applications of ETS-4 films. The observable dimensions of crystals in films increased more than twice from these in seed layers. Film thickness increased up to ∼10.0–11.0 μm. Incorporation of new crystals into the film was not detected. Thus, these films were obtained by direct, evolutionary growth of seed crystals. Secondary growth conditions that effectively decouple nucleation from growth of seeds, and provide the highest growth rate along the b-direction, were established for ETS-4.

Preparation of b-oriented MFI zeolite membranes on porous α-alumina substrates precoated with mesoporous silica sublayer

Preparation of b-oriented MFI zeolite membranes on porous α-alumina substrates precoated with mesoporous silica sublayer

In Situ Growth of Continuous b-Oriented MFI Zeolite Membranes on Porous α-Alumina Substrates Precoated with a Mesoporous Silica Sublayer

Continuous b-oriented MFI membranes were prepared on a mesoporous silica sublayer-coated porous α-alumina substrate by in situ crystallization. Single gas permeation of butane isomers through the MFI membrane was carried out as a function of temperature from 298 to 473 K. A decreased permeance of butane isomers on the b-oriented MFI membrane with increasing temperature is observed in this temperature range, which is attributed to a weak adsorption effect. Meanwhile, it was revealed that the b-oriented membrane can maintain a high ideal selectivity of n-butane/i-butane at elevated temperatures. On the contrary, a discrete membrane with a randomly oriented top layer was obtained on the uncoated α-alumina substrate. Further investigation on the evolution of the MFI membranes on the two kind substrates was performed with the focus on the understanding of the oriented growth of zeolite crystals. The surface properties of the substrates used played an important role in the formation of MFI membranes with differ...

Preparation of α-alumina-supported mesoporous bentonite membranes for reverse osmosis desalination of aqueous solutions

In this study, mesoporous bentonite clay membranes approximately 2 μm thick were prepared on porous α-alumina substrates by a sol–gel method. Nanosized clay particles were obtained from commercial Na-bentonite powders (Wyoming) by a process of sedimentation, washing, and freeze-drying. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption–desorption were employed for membrane characterization. It was found that the content of solids, concentration of polymer binder, and pH value of the clay colloidal suspension had critical influences on membrane formation during the dip-coating process. The membranes were tested for reverse osmosis separation of a 0.1 M NaCl solution. Both water permeability and Na + rejection rate of the supported membranes were comparable to those of the compacted thick membranes reported in the literature. However, due to the drastically reduced membrane thickness, water permeance and flux of the supported membranes were significantly higher than those of the compacted thick membranes. It was also observed that the calcination temperature played a critical role in determining structural stability in water and desalination performance of the clay membrane.