Porous Α-alumina Research Articles

Highly concentrated multivariate ZIF-8 mixed-matrix hollow fiber membranes for CO2 separation: Scalable fabrication and process analysis

Here, we report on the fabrication of mixed matrix hollow fiber membranes (MMHFMs) for CO2 separation using highly-concentrated 6FDA-DAM/triple-ligand, amine-modulated zeolitic imidazolate framework-8 (TAZIF-8) (60/40 w/w) via a dip coating process. The application of a γ-alumina interlayer serves a dual purpose of reducing the infiltration of the 6FDA-/TAZIF-8 coating solution into the porous α-alumina support, while also providing a smooth surface for the support. Importantly, the co-solvent comprising N-methyl-2-pyrrolidone and dichloromethane further inhibits the formation of defects by maintaining an appropriate balance of volatility and polarity. This balanced combination is crucial in promoting phase separation and facilitating the dispersion of nanofillers within the polymer matrix, enabling the formation of a thin defect-free selective layer. The subsequent 40 wt% TAZIF-8 MMHFM attains superior CO2 separation performance to most existing MMHFMs (CO2 permeance of 648 GPU and CO2/N2 and CO2/CH4 selectivity of 39.0 and 34.2, respectively). Furthermore, it maintains excellent CO2 separation performance over 120 d owing to good interfacial interaction between 6FDA-DAM and TAZIF-8s. Last, but not least, our process analysis reveals that the introduction of the TAZIF-8 MMHFM into the first stage of a dual-stage membrane process can improve the total energy demand for post-combustion CO2 capture due to its excellent CO2/N2 selectivity. Our current study provides significant progress toward the industrial application of post-combustion CO2 capture.

Rapid synthesis of high-selective Al-rich beta zeolite membrane via an organic template-free route for pervaporation dehydration of water-n-butanol mixtures

The high-selective Al-rich beta zeolite membranes were rapidly fabricated on porous α-alumina supports via an organic template-free and fluoride-containing route. The growth process of Al-rich beta zeolite membrane transformed from high-silica beta seed crystals was investigated in details. With the help of NaF, the high-silica seed with a Si/Al ratio of ∼ 25 gradually dissolved into an amorphous layer, and then more AlO4 gathered on the amorphous layer and rearranged with SiO4 to form Al-rich beta crystals with a Si/Al ratio of ∼ 2.7. A continuous and compact membrane layer was obtained for a short crystallization time of 48 h by conventional heating. The synthesized Al-rich beta zeolite membrane with strong hydrophilicity was applied in the dehydration of organic solvents for the first time. The membrane exhibited a permeation flux of 2.84 kg m-2h−1 and a high separation factor of 17,900 for a 90 wt% n-BuOH/H2O mixture at 348 K. Even in a 95 wt% n-BuOH/H2O mixture, the membrane showed a high selectivity together with the water content in permeate of 99.93 wt%, indicating that the Al-rich beta zeolite membrane was a promising candidate for dehydration of organic solvents.

Alumina applied in bone regeneration: Porous α-alumina and transition alumina

Alumina is a polymorphic bioceramic that has been extensively investigated for application in bone regeneration. Dense α-alumina has been considered a suitable biomaterial for dental and orthopedic implants due to its superior mechanical properties. However, its use is limited due to its high inertia in a biological environment. Recent investigations have focused on its distinct phases and surface characteristics through the control of morphology, physical properties, and chemical composition to enhance bioactivity. This article presents a brief review of the developments in porous α-alumina and amorphous-γ-alumina transition. Most studies have shown that composites and alumina coated with bioactive materials, high surface area, and hydroxylated surface can significantly improve biological properties. Cellular responses such as fixation, growth, and proliferation, as well as biomineralization, are the main studies to validate improvements in bioactive properties.

Isothermal compressibility and isobaric thermal shrinkage of a porous α-alumina compact: thermodynamic calculations.

Two methods were proposed to calculate the thermodynamic parameters of porous ceramic compacts depending on their molar volume change with applied pressure and heating temperature, respectively. Molar volume of the porous α-alumina (α-Al2O3) compact was evaluated according to literature depending on both the applied pressure at room temperature and the heating temperature at atmospheric pressure. The isothermal compressibility coefficient, Gibbs energy, and work done on the compact by compression were calculated. The thermal shrinkage coefficient and activation energy as well as the change in enthalpy, entropy and Gibbs energy were calculated for partial sintering. The spontaneous nature of the treatments were discussed with respect to the obtained results.

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.

EPD method of seeding nano ZnO followed by CVD of organo-linker; a step by step method for synthesis of ZIF-8 thin layer on tubular α-alumina

In this work, a step by step approach consisting of electrophoretic deposition (EPD) followed by chemical vapor deposition (CVD) was employed to synthesize a uniform film of zeolitic imidazole framework-8 (ZIF-8) on the outer surface of the porous α-alumina tube. Upon providing conductivity to the alumina substrate, thin polypyrrole (Ppy) film was coated on its surface and the electrical conductivity of α-alumina was increased to 1 S/cm. Then, ZnO nanoparticles were deposited by the EPD method, using 50 V during 20 min, to supply localized nucleation sites for ZIF-8 layer formation. Thereafter, the outer surface of the ZnO seeded surface was exposed to the organo-linker (2- methylimidazole) vapors sublimated at 160 °C in an autoclave, at different contact times of 1, 3.5, and 7 h, respectively. The impact of the contact time was studied on the thickness and quality of the formed layers by characterization methods of XRD, HR-SEM, EDX, elemental mapping, FTIR, and AFM analyses. According to XRD and EDX analyses, the synthesized ZIF-8 layer during 3.5 h with the roughness of almost 200 nm possessed the highest uniformity, adhesion, and degree of conversion of parent ZnO to ZIF-8. Finally, the fabricated layer was examined to the permeation tests of pure gases of H2, CO2, N2, and CH4. High H2 permanence of up to 7.6*10−8 mol m−2. pa−1. s−1 and ideal selectivities of 5.24, 3.04, and 3.3 for H2/CO2, H2/N2, and H2/CH4, respectively, proved the potential of this tubular membrane for the H2 separation from the light gases, for the future studies.

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.

Highly porous α-alumina powders prepared with the self-assembly of an asymmetric PS-b-PEO diblock copolymer.

Highly porous and abundantly crystallized alumina (Al2O3) powders, can be prepared using an asymmetric polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer, which critically allowed solid-phase transition from γ- to α-Al2O3 in nanometer-scale as its high-temperature phase by suitable thermal treatment with a retention of PS-b-PEO templated porous structures.

Formation of porous α-alumina from ammonium aluminum carbonate hydroxide whiskers

Ammonium aluminum carbonate hydroxide (AACH) whiskers prepared by hydrothermal technique were employed as precursor material for development of porous alumina. After compaction of AACH whiskers at 8 bars, calcination was performed at 650 °C followed by sintering at different temperatures. The sintered samples were characterized by XRD, FTIR, SEM and mercury intrusion porosimetry. Mechanical strength was determined by compression testing. At sintering temperatures of 1200 °C to 1400 °C, the % age porosity was around 80%. At 1500 °C, the percentage porosity decreased to 71%. The as-prepared AACH consisted of bundles of whiskers with diameters as thick as 0.7 µm, while an individual whisker had a diameter of about 100 nm with an aspect ratio of about 33. A two-phase mixture consisting of θ- and α-alumina was obtained at 1100 °C, while at 1200 °C and above, single phase α-alumina was formed. θ-alumina retained the bundle-like morphology. However, transformation to α-alumina was accompanied by formation of bead-like morphology. These beads were joined together through necks/stems within the whiskers as well as across the parallel-lying whiskers. These necks grew at 1300 °C to form aggregates with smooth surfaces. At 1400 °C, these aggregates started joining with each other by neck formation and at 1500 °C, a three-dimensional network was formed. For sintering temperatures of up to 1400 °C, pores with sizes around 260 nm were very stable. At 1500 °C, significant pore growth took place along with an overall densification. Therefore, number of pores with sizes of around 260 nm decreased and those with sizes around 10 µm, 1 µm and 5 nm increased. The compression strength of samples sintered at 1100 °C to 1300 °C was in the range of 3.4–4.3 MPa. At 1400 °C, the strength increased to 5.2 MPa, while at 1500 °C, it jumped to 10.8 MPa due to the formation of three-dimensional network.

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.

Vapor separation of methanol-dimethyl carbonate mixture on SAPO-34 zeolite membrane

The separation of methanol-dimethyl carbonate (DMC) is an important but challenging process. Polymer membranes suffered from poor separation performance and stability. High performance SAPO-34 zeolite membrane was prepared on porous α-alumina supports with oil-bath heating method. Vapor separation of methanol-DMC mixtures on SAPO-34 membranes was systematically studied, including pressure drop, feed composition, temperature, flow rate, membrane degradation etc. SAPO-34 membranes demonstrated outstanding vapor separation performance, with methanol flux ~ 15 kg/(m2 h) and methanol/DMC separation factor ~ 1000 at 403 K and 0.6 MPa pressure drop for 90methanol:10DMCwt% feed. SAPO-34 membranes exhibited great stability in ~ 20 d continuous test. Methanol flux decreased substantially with time but it can be recovered to 80% of original value by a simple methanol rinse. Methanol/DMC separation factor exhibited significant increase with time, which can be attributed to the defect blocking by the chemical bonding of DMC in the defects, as verified by XPS and FT-IR analysis. Too much DMC adsorption on crystal surface hindered the free permeation of methanol. The methanol adsorption induced expansion of SAPO-34 crystals also helped shrinking defects effectively, which contributed to high separation factor. The addition of 1% water in the feed had little impact on separation performance. Compared with polymer membranes and other inorganic membranes, SAPO-34 membranes exhibited significantly improved flux and separation factor for the vapor separation of methanol-DMC mixture.

Synthesis of Silicalite Membrane with an Aluminum-Containing Surface for Controlled Modification of Zeolitic Pore Entries for Enhanced Gas Separation

The separation of small molecule gases by membrane technologies can help performance enhancement and process intensification for emerging advanced fossil energy systems with CO2 capture capacity. This paper reports the demonstration of controlled modification of zeolitic channel size for the MFI-type zeolite membranes to enhance the separation of small molecule gases such as O2 and N2. Pure-silica MFI-type zeolite membranes were synthesized on porous α-alumina disc substrates with and without an aluminum-containing thin skin on the outer surface of zeolite membrane. The membranes were subsequently modified by on-stream catalytic cracking deposition (CCD) of molecular silica to reduce the effective openings of the zeolitic channels. Such a pore modification caused the transition of gas permeation from the N2-selective gaseous diffusion mechanism in the pristine membrane to the O2-selective activated diffusion mechanism in the modified membrane. The experimental results indicated that the pore modification could be effectively limited within the aluminum-containing surface of the MFI zeolite membrane to minimize the mass transport resistance for O2 permeation while maintaining its selectivity. The implications of pore modification on the size-exclusion-enabled gas selectivity were discussed based on the kinetic molecular theory. In light of the theoretical analysis, experimental investigation was performed to further enhance the membrane separation selectivity by chemical liquid deposition of silica into the undesirable intercrystalline spaces.

Pure silica CHA-type zeolite membranes for dry and humidified CO2/CH4 mixtures separation

Dense and thin pure silica CHA-type zeolite (Si-CHA) membranes have been synthesized on porous α-alumina supports and investigated for CO2 permeation and CO2/CH4 separation performance under dry and humidified conditions. Si-CHA membranes with ultra-high permeance were greatly affected by performance suppression due to concentration polarization under high pressure and low feed-rate conditions. Under high feed-rate conditions at 298 K and a feed pressure of 0.1 MPaG, the CO2/CH4 selectivity was 130 with a CO2 permeance of 4.0 × 10–6 mol/(m2 s Pa) for an equimolar CO2/CH4 dry gas mixture. The presence of water vapor significantly reduced CO2 permeance, but the performance improved as feed pressure increased. The CO2/CH4 selectivities at high pressure with and without water vapor were essentially the same.

α-Alumina membrane having a hierarchical structure of straight macropores and mesopores inside the pore wall

We prepared a through-hole α-alumina membrane with a hierarchical porous structure from an anodic porous alumina membrane prepared using phosphoric acid electrolyte as the starting material. By heating the anodic porous alumina membrane to 1400°C, aluminum phosphate nanoparticles were segregated in the α-alumina matrix at the anion-incorporated outer layer and the high-purity alumina layer forming a cell boundary band sintered as a high-density α-alumina layer at the central core of the pore wall. When the heat-treated membrane was immersed in concentrated hydrochloric acid, a unique hierarchical porous α-alumina membrane structure was formed with straight macropores and mesopores inside the pore wall due to the dissolution of aluminum phosphate nanoparticles. The developed α-alumina membrane can be ultimately used as a multifunctional filter because of its unique hierarchical porous structure and extremely high chemical and thermal durability.

Synthesis and preliminary gas permeation studies of a tubular NaA zeolite membrane (NZM)

ABSTRACTDifferent from traditional seeded method, NaA zeolite membranes (NZMs) were prepared by in situ synthesis onto the inner side of porous α-alumina tubular supports in a hydrothermal synthesis reactor. The influences of pretreatment of porous tubular support, temperature, time, and synthetic cycle for the synthesis of the zeolite membranes were investigated. The operating conditions were optimized. Characterization of the membranes by scanning electron microscopy and X-ray diffraction showed that the crystalline materials on the inner surface of the porous α-alumina tubes were NaA-type zeolite. Single- and binary-gas permeation tests were conducted. Single-component permeabilities of hydrogen and nitrogen through the NZM changed slightly when the transmembrane pressure difference varied from 80 to 420 kPa. Its selectivity for H2 relative to N2 was about 5.3, which was greater than that of the Knudsen diffusion. The separation factors of binary gases H2/N2 and H2/CO2 at 473 K were 3.9 and 5.7, respectively, again exceeding the Knudsen diffusion level. The separation of binary gases suggests that the NaA-type zeolite membranes on α-alumina substrate were defect free and able to provide molecular sieving. The results demonstrate that the unseeded synthetic method presented in this work is successful and reliable.

Highly porous α-Al2O3 ceramics obtained by sintering atomic layer deposited inverse opals

Abstract A process for generating highly porous α-Al 2 O 3 ceramics has been developed. In this paper, a combination of self-assembly and atomic layer deposition is demonstrated as a means to fabricate inverse alumina opals, which have their structures transformed via sintering. The resulting highly porous structure is stable even after a 4 h dwell time at 1400 °C, in contrast to structures generated by conventional powder metallurgy, sol-gel or colloidal powder suspension infiltration methods. TEM analysis reveals that the structure consists of single grain domains of up to 3 µm, each containing a randomly interconnected network of alumina ligaments that share a common crystalline orientation, suggesting a different mechanism of grain boundary migration during sintering. These highly porous α-alumina ceramics are considered to be ideal for filtration or catalysis applications.

Preparation of novel functional MER zeolite membrane for potassium continuous extraction from seawater

Merlinoite(MER) zeolite membranes were synthesized on the outer surface of porous α-alumina tubular support by in-situ hydrothermal synthesis method without organic template. The synthetic MER zeolite membranes (sample 1, sample 2 and sample 3) were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX). The thickness of sample 2 without cracks is about 15 μm. The synthetic MER membrane which is mainly composed of Si, Al, O and K is typical merlinoite structure. The performance of sample 2 for potassium extraction in simulated seawater was researched by the membrane tube reactor. The continual ion-exchange reaction was carried out for 108 h at room temperature. The average exchange rate for K+ ion was 32.8% and the average rejection rates for Na+, Ca2+ and Mg2+ ions were 93.4, 98.4 and 98.2% respectively. So the MER zeolite membrane (sample 2) has preference selectivity for K+ ion and can be used as a potential inorganic membrane for potassium extraction from seawater.

Preparation and gas permeation properties on pure silica CHA-type zeolite membranes

Pure silica CHA-type zeolite (Si-CHA) membranes were synthesized by a hydrothermal secondary growth method on porous α-alumina supports. Using Si-CHA seed crystals as a crystalline nuclei allowed the formation of a dense Si-CHA layer. The Si-CHA membranes prepared in this study showed excellent gas permeance derived from their large pore volume and effective molecular sieve performance for gas separation. The Si-CHA membranes exhibited excellent H2 and CO2 permeance of 1.1×10−6 and 1.7×10−6 mol/m2sPa, respectively. The ideal selectivity of H2/CH4 and CO2/CH4 were 34 and 54, respectively. The stability test in the presence of water vapor on the Si-CHA membranes was performed and the Si-CHA membranes demonstrated good water vapor resistance, more so than the topologically analogous aluminosilicate zeolite membrane (SSZ-13). The Si-CHA membranes were unlikely to present pore blockages by water adsorption because of their hydrophobic pore systems.

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.

ZIF-8 membranes with improved reproducibility fabricated from sputter-coated ZnO/alumina supports

Zeolitic imidazolate framework-8 (ZIF-8) membrane has shown great potential for propylene/propane separation based on molecular sieving mechanism. Although diverse synthesis strategies were applied to prepare ZIF-8 membranes, it is still a challenge for reproducible fabrication of high-quality membranes. In this study, high-quality ZIF-8 membranes were prepared through hydrothermal synthesis under the partial self-conversion of sputter-coated ZnO layer on porous α-alumina supports. The reproducibility was significantly improved, compared with that from sol–gel coated ZnO layer, due to the highly controllable sputtering deposition of ZnO precursor. The relationship between the quality of as-synthesized membrane and amount of deposited ZnO was also determined. The effect of pressure drop in C3H6/C3H8 separation on separating performance was also examined.