Porous α-Al2O3 Tubes Research Articles

Asymmetric Pore Windows in Pillar-Layered Metal-Organic Framework Membranes for H2/CO2 Separation.

In this study, a novel ultramicroporous pillar-layered Ni-LAP-NH2 [Ni2(l-asp)2(Pz-NH2)] (l-asp = l-aspartic acid, Pz-NH2 = aminopyrazine) membranes on porous α-Al2O3 tubes with high performance and good thermal stability was first fabricated using isostructural Ni-LAP[Ni2(l-asp)2(Pz)] (Pz = pyrazine) crystals as seeds. Utilizing the principle of reticular chemistry, here, we introduced the active amino side group into the Ni-LAP frameworks by replacing the pillar-layered ligand Pz with Pz -NH2 while maintaining the original Ni-LAP small pore size, and the amino side group induced a "steric hindrance" effect and the physical adsorption affinity, which synergistically delayed CO2 penetration. It was found that the preferential (111) orientation Ni-LAP-NH2 membrane (Z10) exhibited a high H2/CO2 separation performance with a separation factor of 41.7 and H2 permeance of 9.08 × 10-8 mol·m-2·s-1·Pa-1 under optimal conditions. These MOF materials demonstrated potential for industrial H2 purification due to their tunable pore structure and remarkable stability. Moreover, this strategy offers an effective approach to tailoring pillar-layered MOF membranes with targeted molecular sieving ability.

Fabrication of water-stable MOF-808 membrane for efficient salt/dye separation

Zirconium-based metal-organic framework (Zr-MOF) is considered as promising membrane candidate in terms of its tunable pore aperture, rich functionality, and excellent stability. In this study, we pioneered the fabrication of well-intergrown MOF-808 membrane via facile in situ solvothermal growth on porous α-Al2O3 tube. Owing to an appropriate hexagonal aperture window of 10.1 Å, prepared MOF-808 membrane possessed not only ultrahigh rejection rate for versatile dye molecules (e.g., 99.2% for Malachite green, 99.8% for Methyl blue, 99.6% for Congo red, and 99.3% for Calcein) with no loss in separation performance during 96 h of continuous operation, but also low rejection rate for simple salt (e.g., 6.3% for NaCl), rendering it suitable for efficient removal of various dyes from salt-containing wastewater. The resulting MOF-808 membrane achieved ultrahigh salt/dye separation factor of 287.3 with water permeance of 4.37 L m−2 h−1 bar−1. In addition, the MOF-808 membrane remained crack-free upon exposure to salt, acid, alkali solutions or hot water, indicating outstanding chemical stability, which held great promise for efficient dye extraction from textile wastewater.

Gas Permeation Properties of High-Silica CHA-Type Zeolite Membrane.

The polycrystalline CHA-type zeolite layer with Si/Al = 18 was formed on the porous α-Al2O3 tube in this study, and the gas permeation properties were determined using single-component H2, CO2, N2, CH4, n-C4H10, and SF6 at 303–473 K. The membrane showed permeation behavior, wherein the permeance reduced with the molecular size, attributed to the effect of molecular sieving. The separation performances were also determined using the equimolar mixtures of N2–SF6, CO2–N2, and CO2–CH4. As a result, the N2/SF6 and CO2/CH4 selectivities were as high as 710 and 240, respectively. However, the CO2/N2 selectivity was only 25. These results propose that the high-silica CHA-type zeolite membrane is suitable for the separation of CO2 from CH4 by the effect of molecular sieving.

Microwave synthesis of high-quality mordenite membrane by a two-stage varying heating-rate procedure

High performance mordenite membranes were successfully prepared on porous α-Al2O3 tubes by a novel two-stage varying heating-rate procedure under microwave assisted heating. The effect of heating rate on microstructure and performance of mordenite membrane was first time to be investigated. It was found that slowing down the heating rate at lower temperature range can improve the nucleation process, and further enhance compactness as well as performance of mordenite membrane. Crystallization time was also optimized for the preparation of high-quality mordenite membrane and it was reduced to only 75 min in this work. The as-synthesized M14 exhibits an excellent PV performance for dehydration of 90 wt% acetic acid/water mixture at 75 °C with a flux of 1.48 kg m−2 h−1 and a separation factor of larger than 10,000, which shows an excellent potential for dehydration of acetic acid aqueous solutions in industrial separation applications. Our work also provides a new idea for regulating microstructure and performance of zeolite membrane.

Co-based zeolitic imidazolate framework ZIF-9 membranes prepared on α-Al2O3 tubes through covalent modification for hydrogen separation

Hydrogen has been regarded as the most promising clean and renewable energy. Beside the production of the hydrogen, the separation of hydrogen is also an import issue before it can be used in fuel cells. Membrane-based separation technologies have gained considerable attentions due to its high efficiency and low energy consumption. Zeolite imidazolate framework (ZIF) membranes have drawn intense interest due to their zeolite-like properties such as permanent porosity, uniform pore size and exceptional thermal and chemical stability. It is rather challenged to prepare well-intergrown Co-based zeolitic imidazolate frameworks (ZIFs) membranes on porous α-Al2O3 tubes since Co-based ZIFs prefer to form crystals in the synthesis solution rather than grow as membrane layer on the support surface. In this work, we report the preparation of high-quality ZIF-9 membrane with high H2/CO2 selectivity and excellent thermal stability by using 3-aminopropyltriethoxysilane (APTES) as a covalent linker to modify the α-Al2O3 tube. Due to the formation of covalent bonds between APTES and ZIF-9, ZIF-9 nutrients are bound to the support surface, thus promoting the growth of dense and phase-pure ZIF-9 membrane with a thin thickness of about 4.0 μm. The gas separation performances of the ZIF-9 membrane were evaluated by single gas permeation and mixture gas separation of H2/CO2, H2/N2 and H2/CH4, respectively. The mixture separation factors of H2/CO2, H2/CH4, and H2/N2 of the ZIF-9 membrane are 21.5, 8.2 and 14.7, respectively, which by far exceeds corresponding Knudsen coefficients. Moreover, the as-prepared ZIF-9 membrane exhibits excellent stability at a relatively broad range of operating temperature, which is beneficial for the industrial application of hydrogen separation or further membrane reactor.

Dense and thin 13X membranes on porous α-Al2O3 tubes: preparation, structure and deep purification of oxygenated compounds from gaseous olefin flow.

The low contact efficiency, large mass transfer resistance and high operational cost of traditional 13X molecular sieve particle adsorbents (MSPs) have greatly limited their application in deep purification of trace oxygenated compounds from gaseous olefins. Herein, we successfully fabricated dense and thin 13X molecular sieve membranes on a porous α-Al2O3 tube (MSCMs) by a combination of 3-aminopropyl triethoxy silane (APTES) surface modification and vacuum pre-coating sol technology for purifying the above impurities. By a solid–solution transformation process, 13X molecular sieve membranes on MSCMs that were continuous and integral without any cracks, pinholes or other defects, and mainly composed of 1–1.5 μm regular 13X crystals with a thickness of 5–6 μm have been achieved. The purification performance of the MSPs, non-APTES functionalized MSCMs (nMSCMs) and MSCMs was evaluated by dynamic adsorption of N2 or C2H4 feed flow containing dimethyl ether, methanol and propanal impurities at room temperature. The results demonstrated that both the nMSCMs and MSCMs could deeply purify the trace amounts of the three oxygenated compounds to below 1 × 10−6 (mol mol−1) from gaseous olefins at an initial concentration of 20 × 10−6 (mol mol−1), exhibiting much more excellent purification performance than that of MSPs. In particular, the breakthrough times of MSCMs for dimethyl ether, methanol and propanal were 7 h, 32 h and 51 h in a N2 system, and 12.1 h, 53 h and 90 h in a C2H4 system. The cumulative adsorption amounts of MSCMs for dimethyl ether, methanol and propanal were 12.108 mg g−1, 35.812 mg g−1 and 103.129 mg g−1 in a N2 system, and 25.88 mg g−1, 94.19 mg g−1 and 256.26 mg g−1 in a C2H4 system, respectively. The regeneration experiment also indicated that the MSCMs had a more stable structure and a longer lifetime. The excellent purification performance of MSCMs could be attributed to the continuous 13X molecular sieve layers without non-adsorption interfacial voids. Hence, the MSCMs have great potential for future industrial application of trace oxygenated compound removal from gaseous olefins.

Synthesis of Potassium Ionic Sieve Membrane and its Selectivity to Potassium Ion in Seawater

Potassium ionic sieve membrane was synthesized on porous α-Al2O3 tube support by the hydrothermal synthesis. The zeolite membranes were characterized by means of XRD and SEM. And the single-gas permeability through the membranes and selectivity to K+, Na+, Ca2+, Mg2+ were measured. The results show that the ideal separation factor is 3.68, which is close to Knudsen diffusion ratio 3.74 for H2/N2; the separation factors of the potassium ionic sieve membrane are , , respectively, indicating its high separation selectivity to potassium ion.

Seeded synthesis of small polycrystalline NaY zeolite membrane using zeolite structure-directing agent and its pervaporation performance

Continuous and polycrystalline NaY zeolite membranes were synthesized successfully on porous α-Al2O3 tubes by zeolite structure-directing agent (ZSDA) method. This ZSDA method consists of two steps: preparation of NaY ZSDA and growth of NaY zeolite membranes with ZSDA by the secondary hydrothermal treatment. The synthesis parameters, such as aging time, aging temperature and the amount of ZSDA added, etc. were investigated. Scanning electron microscopy and X-ray diffraction technology were used to characterize the morphology and the crystal structure of the as-synthesized zeolite membranes, respectively. The results indicated that the growth of NaY zeolite membranes on the supports depended strongly on ZSDA, which could not only accelerate the formation of NaY zeolite membranes but also inhibit the transformation of NaY zeolite phase. It was also found that the prolonged aging time, the lower aging temperature and the increasing amount of ZSDA resulted in a crystal size diminution of the product, thus linking the aging process directly to the increasing number of formed nuclei. Compared with the larger crystal NaY zeolite membrane, the small crystal NaY zeolite membrane exhibited much higher separation selectivity. At 308 K, the highest separation selectivities achieved towards 1,3-propanediol/glycerol and 1,3-propanediol/glucose were 80 and 2,400, respectively.

Model development and validation of hydrogen transport through supported palladium membranes

Abstract Porous matrices are often used to provide structural support to thin Pd-based metallic membranes in H2 separation applications. Optimizing such composite membranes requires detailed understanding of all possible rate-controlling processes including surface and bulk processes in the metal and diffusion of gases through the porous media. In the work described in this paper, we fabricate a composite membrane by depositing a thin (∼5–6 μm) Pd film on a porous α-Al2O3 tube and then measure the H2 permeance of this composite membrane over a range of operating conditions. The rate-controlling processes for the H2 permeation are evaluated with a computational model which combines a detailed thermo-kinetic Pd–H2 interaction model and a porous media transport model. The Pd–H2 thermo-kinetic model is validated against literature data, and the porous media transport model is independently calibrated using experimental measurements. The combined composite membrane model gives good agreement with experiments over a large range of temperatures (250–450 °C) and H2 partial pressures (100–385 kPa). This validated model is then used to analyze the importance of design parameters such as Pd thickness and support micro-structure on H2 flux through the membrane. These parametric studies will also aid in assessing trade-offs between membrane structural robustness and overall performance.

Preparation and properties of TS-1 zeolite and film using Sil-1 nanoparticles as seeds

Titanium silicalite-1 (TS-1) zeolites and films supported on porous α-Al2O3 tubes were prepared using TS-1 and silicalite-1 (Sil-1) seeds by hydrothermal synthesis. The structure and catalytic properties of the TS-1 powders and films were characterized by SEM, XRD, FT-IR, UV–vis and styrene partial oxidation by hydrogen peroxide. The results showed that TS-1 prepared from Sil-1 seeds were as active as TS-1 from nucleated and TS-1 seeded syntheses, in spite of the concerns about nonuniformity and dilution caused by the use of pure silica Sil-1 seeds. The Ti atoms were successfully incorporated as tetra-coordinated titanium in the zeolite framework and the phenylacetaldehyde selectivity correlates well with the infrared band for Ti–O–Si of the TS-1. Reaction over TS-1 film indicates that reactions occurred at the topmost layer of the film due to the slow diffusion in the zeolite pores.

An in situ approach to synthesize pure phase FAU-type zeolite membranes: effect of aging and formation mechanism

FAU-type zeolite membranes were prepared on porous α-Al2O3 tubes by a two-stage in situ synthesis method. The synthesis was carried out in clear solutions without the aid of seeds or organic templates. The first stage synthesis was devoted to the in situ nucleation of FAU-type zeolite, aiming to induce an evenly covered zeolite layer on the support. In situ aging played an important role at this stage as it suppressed LTA phase and promoted the formation of a continuous zeolite layer. The second stage synthesis was performed in a dilute solution, where the pre-formed zeolite layer went on to grow and became compact. Scanning electron microscopy (SEM) observations show that the membranes were made up of well-shaped intergrown crystals, with the thickness around 7 μm. Pervaporation (a membrane-based separation process in which the liquid components diffuse through a membrane and are subsequently vaporized due to the low-pressure exerted on the permeate side of the membrane) measurements were carried out in a water/ethanol (10/90, w/w) mixture to evaluate the membrane performance. The structural and morphological evolutions of FAU-type zeolite membranes during synthesis were tracked down by X-ray diffraction and SEM. The formation mechanism was illuminated based on the experimental results.

Direct O2 Epoxidation of Propylene by the Membrane Reactor Loaded with Ag–Sr Catalyst

Abstract A novel membrane reactor process using porous α-Al2O3 tube loaded with silver–strontium catalyst realized direct O2 epoxidation of propylene with notable selectivity of propylene oxide. Segregated feed of propylene and O2 combined with facile permeation of reactants and products across the membrane was essentially important for the membrane epoxidation process.

The Photodecomposition of Acetaldehyde in Gas Phase Using Immobilized TiO2 on Porous α-Al2O3 Tube

AbstractsA study on the photocatalytic degradation of acetaldehyde using immobilized TiO

Selective Oxidation of CO in H2 by Permeation through Catalytically Active Zeolite Membranes.

When CO is removed from H2 by selective oxidation using a catalytically active membrane, the CO is oxidized during its permeation through the catalytic layer to the permeate side. Since the oxidation rate of CO is proportional to the inverse of its concentration, the rejection of CO using an H2-selective microporous layer which is overlaid on the zeolite layer is also effective. In this study, Y-type zeolite membranes with different thicknesses were prepared on the outer surface of porous α-Al2O3 tubes by hydrothermal synthesis. The membranes were modified to different extents by ion-exchange with Pt ions. Gas permeation properties of the membranes were then determined using single-component H2 and CO, as well as mixtures of H2, CO, O2 and Ar. Thus, the effects of the membrane thickness, the amount of Pt loading and the size of micropores on CO oxidation could be evaluated. For the Pt/NaY membrane prepared by hydrothermal synthesis for 12 h and ion-exchanged using a 2.0-mmol L–1 solution of [Pt(NH3)4]Cl2, the H2 permeance and the H2/CO separation factor were 1.1 × 10–6 mol m–2 Pa–1 s–1 and 1.9 at 473 K. The thickness of the zeolite layer had no detectable effect on CO oxidation. When the membrane was ion-exchanged with a 10.0-mmol L–1 solution of [Pt(NH3)4]Cl2, the CO/H2 mole fraction ratio on the permeate side was minimized to 0.007 at 493 K for a feed of H2:CO:O2:Ar = 4:2:1:93 on a molar basis. The membranes were further modified with SiO2 using dip-coating and chemical vapor deposition techniques. The latter technique led to a membrane, the outer surface of which was coated with an H2-selective microporous SiO2 layer. This resulted in a decrease in CO concentration in the zeolite layer. Thus the CO oxidation proceeded effectively in the catalytically active zeolite layer, and the CO/H2 mole fraction ratio on the permeate side was 0.01 at 523 K for a feed of H2:CO:O2:Ar = 4:2:1:93 on a molar basis.

Synthesis and characterization of Fe-MFI zeolite membrane on a porous α-Al2O3 tube

Fe-MFI and MFI membranes have been prepared by in-situ hydrothermal treatment on porous a-Al2O3 tubes. It has been shown by XRD, IR, SEM and single gas permeation measurements that the crack-free Fe-MFI zeolite membrane are formed and Fe species are incorporated into the framework of zeolite. The permeation rate of H2 and separation factor (H2/C3H8) for the synthesized Fe-MFI zeolite membrane are 3.0×10−6 mol s−1 m−2 Pa−1 and 25.8, respectively. About 15% conversion increase of ethylbenzene to styrene using the membrane as a reactor is achieved over the fixed-bed reactor. The reactivity of the Fe-MFI membrane is better than that of the Al-ZSM-5 membrane. The reason may be that the Fe-ZSM-5 preferentially adsorbs the ethylbenzene, which may be beneficial to the conversion of ethylbenzene, and the adsorption amount of styrene on Fe-ZSM-5 is much less than that on Al-ZSM-5, which may favorably prevent carbon deposition on the Fe-ZSM membrane and result in a higher conversion of ethylbenzene.

SiC Coating on Porous .GAMMA.-Al2O3 Using Alternative-Suply CVI Method.

SiC membranes were prepared by chemical vapor infiltration (CVI) onto γ-Al2O3-coated porous α-Al2O3 tubes. Source gases of SiH2Cl2 and C2H2 diluted with hydrogen were alternatively supplied to the porous tube during heating at 800-900°C, and evacuated after the cycles through the porous tube. According to this process, SiC could be infiltrated into porous alumina. The SiC infiltration was successfully controlled by repeated cycles of the alternating gas supply. The volume of gas, permeated into the porous tube, decreased with increasing number of infiltration cycles. The SiC membrane showed hydrogen parmeances of 1×10-8mol·m-2·s-1·Pa-1 with selectivity of 3.36 over nitrogen at 350°C.