NaA Membranes Research Articles

In-situ water separation enhanced methyl glycolate oxidation to methyl glyoxylate by catalytic membrane reactor

The selective oxidation of methyl glycolate by oxygen is a green route for the synthesis of methyl glyoxylate, which however faces great challenges like the consecutive hydrolysis reaction between methyl glyoxylate and by-product water. Herein, we fabricated an in-situ water separation catalytic membrane reactor (CMR) to suppress the hydrolysis of methyl glyoxylate, which can promptly and preferentially remove water from the reaction system by a hydrophilic NaA membrane. The MoO3@NaA CMR achieved a high methyl glyoxylate selectivity of 84.0 % at 260 °C and 2 bar, being 32.4 % higher than that of the conventional fixed-bed reactor (FBR), while the methyl glycolate conversion was almost the same for both modes. Moreover, the deactivation of the MoO3 catalyst caused by water in CMR was also greatly inhibited, exhibiting excellent stability as compared to FBR. The in-situ water separation strategy in this work contributed to a new concept to improve the selectivity of easily hydrolyzable products.

Preparation of ultra-high permeability dual-layer Al2O3 hollow fiber supports for zeolite membrane fabrication

Four-channel dual-layer Al2O3 hollow fiber with ultra-high permeability and mechanical strength was prepared by co-extrusion and co-sintering technique for the first time and used as support for NaA zeolite membranes fabrication. The macroporous inner layer of the Al2O3 hollow fiber, composed by large Al2O3 particle, gives supports the high permeability and mechanical strength, while the thin out layer, composed by small particles, provides an even and smooth surface for zeolite membrane growth. By using SiO2 as the sintering aid, the sintering temperature of the inner layer was reduced and match with that of the outer layer. The effects of SiO2 adding amount, sintering temperature, Al2O3 particle size and their content on the structure and properties of the prepared hollow fiber were investigated systematically. At a sintering temperature of 1520 °C, a dual-layer hollow fiber with surface pore size of 0.38 μm and fracture load of 24.52 N, exhibits pure water flux as high as 22.23 m3 m 2 h−1 bar−1. The NaA membrane prepared on this support achieved a flux of 14.14 kg m−2 h−1 and a separation factor >10,000 in pervaporation separation of EtOH/H2O solution (90 wt%, 75 °C). This work provides an efficient and low-cost way to prepare ceramic hollow fiber support for high-performance zeolite membrane fabrication.

NaA zeolite-clay composite membrane formulation and its use as cost-effective water softener

The present study describes the exchange of calcium and magnesium ions through an active layer of zeolite Na-A deposited on flat porous support made from clay. Microfiltration membrane's supports are prepared by uniaxial pressing dry mixture of clay, and 3% w/w activated carbon then sintered to 1000 °C. The zeolite Na-A (LTA) layer is deposited on the clay support using the hydrothermal method. An increase in the specific surface area and the pore volume and a decrease in pore diameter after deposition of the zeolite Na-A on the support is observed. SEM analysis of the zeolite Na-A shows well-defined cubic crystals with sizes between 1.3 and 2.4 μm. The efficiency of the zeolite membrane for the exchange of calcium and magnesium ions with the sodium (Na) within the zeolite-A cavities was evaluated by filtration tests of solutions containing chlorides salts of the two metals using a flow loop adjusted for dead-end filtration. The obtained results show that the Na+ ions of Zeolite-A were completely exchanged with Ca2+ and Mg2+ and that the maximum retention on the zeolite membrane reached 93.3% and 72.1% for Ca2+ and Mg2+ ions, respectively after 30 min of filtration. The electrochemical impedance spectroscopy (EIS) has shown an electron transfer increase of the membrane paste electrode exchanged with Ca2+, Mg2+ as single ions or as mixture, with a preference for Ca2+ compared to Mg2+, indicating that a combination of exchange and size exclusion mechanisms must be used to explain the membrane behavior. These properties make the zeolite Na-A membrane a suitable material that could be used effectively in the softening of hard water.

Effect of membrane performance variability with temperature and feed composition on pervaporation and vapor permeation system design for solvent drying.

The presence of water in organic solvents and biofuels can complicate their production and reuse because many hydrophilic solvents form difficult-to-separate mixtures with water (e.g., azeotropes). Pervaporation (PV) and vapor permeation (V⋅P) remove water from such mixtures via selective solution-diffusion transport through a membrane material. A recent article reviewed design factors that impact the effectiveness of PV/V⋅P solvent dehydration processes (J. Chem. Technol. Biotechnol 95: 495-512 (2020)). For the sake of simplicity, the earlier work assumed constant membrane permeabilities. The objective here is to explore the impact of variable permeabilities on predictions of PV/V⋅P system performance. A multiparameter expression relating permeability to process conditions was incorporated into the spreadsheet calculators from the previous work. Use of the expression was demonstrated with literature ethanol/water PV data for a NaA zeolite material and two poly (vinyl alcohol) (PVA) membranes. The variable permeabilities of the membranes yielded membrane area requirements that were 20-30% different from those calculated using permeances fixed at either end of the target water range. The impact of composition-dependent permeabilities was most pronounced on the fraction of ethanol transferred to the permeate for the NaA membrane. The inclusion of membrane permeabilities that vary with fluid composition and temperature noticeably altered predictions of the membrane area required to carry out water removal from ethanol by PV and of the transfer of ethanol to the permeate stream. Unless a PV/V⋅P process is expected to operate at a constant temperature and in a narrow concentration range, process performance estimates would be improved by inclusion of concentration- and temperature-dependent permeabilities or permeances.

Ethanol dehydration performance of three types of commercial-grade zeolite permselective membranes.

Many organic solvents form difficult-to-separate mixtures with water and have an affinity for water, making drying a potential reuse prerequisite. Pervaporation (PV) and vapor permeation (VP) membrane technologies hold promise for energy-efficient solvent drying. Several water-selective membrane materials are commercially available, but performance data is limited, particularly for two recently commercialized membrane materials: chabazite (CHA) and T-type zeolites. In this work, commercial-grade samples of CHA and T-type membranes, along with a NaA zeolite membrane, were evaluated for the removal of water from ethanol. The CHA sample had the highest initial PV water permeance (6820 GPU) and water permselectivity (3430) with 5 wt% water in ethanol at 50 °C. Initial NaA membrane performance was slightly lower (6060 GPU and 3260), while the T-type membrane had the lowest initial permeance and selectivity (4260 GPU and 1090). Performance declined over time, most notably for the NaA membrane, for which water permeance fell over 50% through 39 days of testing. The T-type membrane exhibited the steadiest PV water permeance, but the most variable ethanol permeance. The PV performance of the three membranes largely overlapped the predicted range for T-type membranes. That performance generally exceeds the anticipated ethanol drying performance of non-zeolitic PV membranes but is less than that predicted for NaA and CHA membranes. The present CHA membrane results, along with other recent reports, refine earlier predictions of the ethanol dehydration performance of that type of zeolite. The changing performance with time should be understood to properly design a solvent dehydration system.

Interzeolite Conversion-Synthesized Sub-1 μm NaA Zeolite Membrane for C2H2/C2H4 Selective Separation.

Zeolite membranes with reduced thickness and high continuity are of paramount importance for accelerating selective gas separation for resemblant molecules, and the synthesis of such membranes remains a grand challenge. Herein, we developed an interzeolite conversion synthesis approach to grow NaA zeolite membranes on NaX. The conversion of NaX into NaA proceeded via mild hydrothermal treatment of a dilute synthesis solution, preferentially forming a continuous polycrystalline NaA layer on the surface of NaX, which was precrystallized on a porous alumina support. The thickness of the NaA zeolite membrane was successfully controlled to the submicron scale (500 nm). The synthesized NaA membrane functioned as a selective separator for C2H2 and C2H4 gases. Taking the traditionally in situ grown membrane as a reference, the interzeolite-derived membrane exhibited a 3.5-fold separation factor and ∼4.0 times C2H2 permeance. This approach provides an alternative synthesis option for zeolite membranes with advanced properties, and high efficiency in terms of superior gas selectivity and permeability is promising in precise gas separation.

Tailor-Made Zeolitic Water Nanochannels for Liquid Fuel Production

Conversion of CO2 into valuable liquid fuels or other chemicals has been one of the attractive ways for effective utilization of CO2 produced by burning fossil fuels. However, this task is challenging due to the difficulties associated with the chemical inertness of CO2. In addition to research efforts devoted to the development of more active and selective catalysts for CO2 hydrogenation reaction, removing the by-product water from the system is proven beneficial for improving the reaction thermodynamics and kinetics toward desirable products. Therefore, highly water-selective membranes able to operate under reaction conditions like high temperature and pressure have long been pursued. In a recent study, Yu et al. reported a zeolite membrane with gas-impeding water-conduction nanochannels that selectively remove by-product water from CO2 dehydrogenation process, greatly enhancing CO2 conversion and methanol yield.

The study on the coupled process of column distillation and vapor permeation by NaA zeolite membrane for ethanol dehydration

A pilot was used to experimentally study the ethanol dehydration process, based on the coupled process of distillation and vapor permeation by NaA zeolite membrane, and the effects of the water content in the feed, reflux ratio, mass flow, the product purity of the distill and the operating stability were systematically analyzed. It was found that due to the placement of NaA membrane, the operation zone of the original column could be significantly extended in the coupling, which in return could reduce the column facility investment. Based on the interplay among the operation parameters, it was found that the water content of the distilled product from the column could be used as a critical parameter to balance the energy consumption of the column and the membrane facility investment, and a value of 20wt.% was recommended for the coupling. During the steady test of the coupling, the results indicated that the pilot plant could be continuously operated for 18h to dehydrate the feed (water/ethanol) from 60wt.% to 8wt.%, where the steady state was further validated in process simulation of Aspen plus. In summary, the parameters suitable for the stable operation of the distillation-vapor permeation coupling were investigated for the given condition. This research paves a foundation for the large-scale industrial design and application of the distillation-vapor permeation coupling.

Synthesis of chitosan-NaA membranes with non-uniform seeds on defective supports

It is of great significance to use cheaper macro-porous supports for NaA membranes. Herein, a chitosan modified strategy was designed to weaken the accurate matching between defective supports and seeds. Two types of defective α-alumina tubes were used as macro-porous supports. In order to further reduce the fabricating cost, non-uniform NaA seeds (0.3–3.2 μm) were prepared without the expensive structure-directing agent. By one simple dip coating process, the chitosan layer was coated on the surface of defective supports before applying seeds. Moreover, the binding strength of adsorbed crystals was qualitatively investigated by the ultrasonic treatment (35 kHz, 5 min). NaA membranes were subsequently prepared by the secondary crystallization process. The results indicated that the addition of a thin chitosan buffer layer really overcame the limitation of obvious defects (>10 μm). Meanwhile, as for the obvious cracks of 1.5% chitosan modified NaA membranes, the unbalanced hydrophilic swelling was offered as a feasible explanation. The optimized separation factor was 4000 with the flux of 1.32 kg m−2 h−1 (70 °C, 0.03 Pa). Overall, this efficient method can be treated as one potential strategy to prepare low-cost NaA membranes.

3D re-crosslinking of an acid-resistant layer on NaA tubular membrane for application in acidic feed

NaA membranes are recognized as one of the most promising materials for fabrication of high-performance pervaporation membrane for dehydration, which has been widely employed in dehydration industry. However, the chemical structure breakage of NaA in acidic conditions severely hampered the application of NaA membrane in purification of biomass fermentation. In order to overcome this bottleneck, we proposed construction of a protective acid-resistant skin on the 3D surface of NaA tube. The protective layer was prepared by self-assembly of sodium alginate (SA) and CaCl2, which formed the ‘egg-box’ network via crosslinking SA with Ca2+. The effect of cross-linking, SA concentration, layer numbers and operation pH on the membrane performance was investigated. When applied for pervaporation at acidic feed, the protective skin was partly broken, but it can be re-crosslinked again by simple ionic exchange to form a solid acid-resistant skin. After the re-crosslinking treatment, the performance of membrane can be recovered (the water content in the permeate and the flux were 99.6 wt% and 860 g/(m2.h), respectively) with operation condition at pH = 2. Notably, the membrane can be continuously operated in acid feed for 100 h and showed no obvious performance degradation.

Scale-up of NaA zeolite membranes using reusable stainless steel tubes for dehydration in an industrial plant

Large-scale NaA zeolite membranes in a length of 1000 mm were successfully synthesized for the first time on macroporous tubular stainless steel (MTSS) supports in this work. The influencing factors for membrane synthesis, such as pre-treatment, synthesis time, synthesis temperature, Na2O/SiO2 ratio, synthesis reproducibility, membrane stability and support reusability, were investigated systematically. The experimental results revealed that the high-flux NaA membranes prepared on the reusable MTSS supports exhibits a high total flux of 8.28 kg m−2 h−1 and a separation factor of 11,000 for the dehydration of a 90 wt% ethanol solution through pervaporation. The MTSS tubes can be used for at least 8 synthesis-rinse-synthesis circles, and the as-synthesized membranes still maintain high competitive pervaporation performance. Additionally, 93 pieces of membrane samples were assembled in an industrial plant that can produce approximate 99.8 wt% ethanol from 93 wt% ethanol mixtures with a product flow rate of about 60 kg h−1. Importantly, the membrane showed a high stability for running over a whole month, demonstrating that the MTSS-supported membranes have reliable stability for industrial applications.

Binderless zeolite NaX microspheres with enhanced CO2 adsorption selectivity

Abstract Zeolite NaX@NaA core-shell microspheres were prepared via a post-treatment secondary growth of zeolite NaA films on outer surface of binderless zeolite NaX microspheres. The obtained core-shell microspheres were composed of intergrown octahedral NaX particles inside, with particles size of ca. 500–750 nm, and continuous zeolite NaA films on the outer surface with the thickness of about 2 μm. Higher CO2 separation performance was observed for the core-shell microspheres comparing to the parental binderless zeolite NaX microspheres. The ideal separation factors of zeolite NaX@NaA core-shell microspheres for CO2/CH4 and CO2/N2 were 13 and 47, and the adsorption selectivities for the corresponding binary mixtures were 308 and 923, significantly higher than the binderless zeolite NaX microspheres of 9 and 19 as well as 264 and 735, respectively. After K+ ion exchanging, the core-shell zeolite microspheres have even higher adsorption selectivities of 326 and 1109 for CO2/CH4 and CO2/N2 binary mixtures. The crushing strength of the binderless zeolite NaX microspheres was increased from 0.46 MPa to 3.42 MPa after the secondary growth. In addition, the growth of zeolite A film was resultant from interzeolite conversion and the interzeolite conversion was investigated by the conversion of zeolite NaX to NaA crystals in NaA membrane synthesis gel.

Facial build-up of acid-resistance skin for high-stability zeolite NaA membrane

Zeolite NaA membranes have been widely used in dehydration of alcohols by pervaporation because of their excellent permeability and selectivity. However, the zeolite NaA membranes were criticized for the poor performance in acidic condition. Herein, we report a simple and effective method to enhance acid stability of zeolite NaA membranes. The key is a positively charged polyelectrolyte complex (PEC) was impregnated on the surface of zeolite NaA membranes. The PEC was synthesized from chitosan (CS) and sodium poly (vinyl sulfonate) (PVS). Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that the zeolite NaA membranes suffered from severe damage in both morphology and crystal structure after contact with acid, while the PEC/NaA composite membranes remained intact. Accordingly, the PEC/NaA composite membranes（1.5 wt% PEC in solution） exhibited excellent water permeability and selectivity in acid solution. For acidic 90 wt% ethanol/water (pH = 3) mixture separation, the permeate flux was 875 g/(m2 h), the water content in permeate was as high as 99.8 wt% at 40 °C. Meanwhile, the composite membranes showed a good acid stability for more than 200 h and this was attributed to the barrier effect of PEC layer, as well as the protonation ability of -NH2 groups in PEC. This method is expected to offer great opportunities for the dehydration of organics under acidic conditions.

Preparation of high‐performance zeolite NaA membranes in clear solution by adding SiO2 into Al2O3 hollow‐fiber precursor

Zeolite NaA membranes were prepared in a clear synthesis solution without the aid of nanoseeds. To improve the properties of the membranes formed in a clear solution, alumina hollow fibers were fabricated by adding silica powder to the conventional spinning slurry, resulting in hollow fibers with a mullite phase. Prior to the membrane synthesis, the hollow fibers were pretreated by dipping in an aged synthesis solution diluted with isopropanol. Dense zeolite NaA membranes on mullite‐containing alumina hollow fibers were successfully obtained at 100°C for 2 h without the aid of nanoseeds. The membranes have a good pervaporation performance with a high flux of 10.8 kg m−2 h−1 and a separation factor of over 10,000. The abundant mullite‐phase hydroxyl groups on the support surface promote the nucleation and growth of zeolite crystals on the support, resulting in dense membranes. © 2018 American Institute of Chemical Engineers AIChE J, 64: 2679–2688, 2018

Comparison of MFI-ZSM5 and NaA zeolite-type tubular membranes for the separation of water vapour from helium for tritium processes in future fusion reactors

Zeolite membranes have been increasingly used for gas separation applications, where the most studied are the MFI and NaA types. Typically, NaA membranes are used for dehydration applications whereas MFI are used for the separation of large/small molecules (e.g., CO2/H2). In the Tritium Laboratory Karlsruhe (TLK), both NaA and MFI-ZSM5 were selected for tritium processes where both molecular and oxidized tritiated species diluted in helium have to be processed in a continuous mode, as it is the case of the Tritium Extraction System of the Helium Cooled Pebble Bed breeding blanket. This paper presents and discusses the results obtained for the separation of a binary mixture of water vapour and helium, down to 0.2vol%, where hydrogen replaces its radioactive counterpart. These experiments were performed in the temperature range between 25 and 100°C. NaA demonstrates much better performances for feed concentrations ≥5vol%, eventually reaching separation factors higher by a factor of 100 than that of MFI-ZSM5. However, the MFI-ZSM5 membrane must be selected instead when moisture levels are ≤1vol%. In fact, at these conditions, the H2O/He separation factor for NaA barely exceeds 2, whereas it can be as high as 9.6 for MFI-ZSM5. Moreover, rather high permeances for H2O of around 1–2μmolm−1s−2Pa−1 were measured for both membranes. In the case of helium, the permeances were highly dependent on the temperature for high water contents, reaching values as low as 1×10−3μmolm−1s−2Pa−1 for the NaA membrane.

Preparation of highly stable porous SiC membrane supports with enhanced air purification performance by recycling NaA zeolite residue

NaA zeolite membrane which exhibits remarkable selectivity for dehydration of organic compounds has been widely applied in industrial scale. Typical preparation of NaA zeolite membrane involves the hydrothermal synthesis of the NaA zeolite and the formation of a zeolite membrane layer on a substrate. However, the preparation of NaA membrane generates large quantity of zeolite residues in the synthesis solution. In this paper, the NaA residues from the preparation of the NaA zeolite membrane was recovered and reused as fillers for the fabrication of porous SiC membrane support (PSCS). The effect of loading of NaA residue and sintering temperature on the porosity, pore size, gas permeability and bending strength of PSCS were investigated. The results showed that the addition of 8% of NaA residue in PSCS led to the decrease in sintering temperature from 2000 to 1200°C, as compared to sintering pure SiC powder. Also, the gas permeability and the bending strength were significantly increased to 1200m3 / (m2hkPa) and 25MPa, respectively compared to 600m3 / (m2hkPa) and 2.5MPa of the pristine PSCS without NaA residue. Furthermore, because of the high temperature reaction to form NaAlSi3O8, the PSCS loaded with NaA residue showed remarkable thermal shock resistance and resistance to acid and alkali corrosion. The successful use of NaA residue in the preparation of the PSCS provided a more economical process for the fabrication of PSCS using lower cost raw material and more thermally efficient sintering condition.

Application of NaA Membrane Reactor for Methanol Synthesis in CO2 Hydrogenation at Low Pressure

Abstract The process of CO2 hydrogenation into methanol has recently attracted more attention for reducing CO2, a main green-house gas in the atmosphere. In addition, methanol can be used as fuel or a basic chemical to satisfy the growing demand for energy in the world. In this study, the performance of NaA membrane reactor on the methanol yield and methanol selectivity obtained from the CO2 hydrogenation process was evaluated at different reaction conditions by varying the temperature, pressure, gas hourly space velocity (GHSV) and H2/CO2 ratio. The results show that the methanol yields and methanol selectivities obtained from the membrane reactor (MR) at all reaction conditions are higher than those from the traditional reactor (TR). The ratio of methanol yield in MR over methanol yield in TR are varied from 1.4 to 1.7 depending on the operating conditions. It is also observed that the use of membrane reactor is more efficient at low GHSV and the temperature in the range of 220–240°C.

Making of Nanopore NaA Zeolite Membranes for Saline Wastewater Treatment

Zeolite membranes have been studied extensively for more than fifteen years, mainly focusing on gas separation and liquid pervaporation processes. Recently, molecular dynamic simulation has shown that zeolite membranes are theoretically suitable for ion removal from aqueous solutions. This work proposes to use zeolite membranes for desalination of saline recycled wastewater for the possibility of avoiding the costly treatment needed for saline wastewater by Conventional approaches. NaA zeolite membrane indicates that it may be possible to simultaneously separate ions and dissolved organic compounds from water by pervaporation (PV) processes. NaA zeolite membranes composed of a continuous intergrowth of NaA zeolite crystals have been prepared hydro thermally on the surface of porous tubular supports. In this research, synthesis of zeolite NaA membrane was investigated. SEM and XRD analysis have been used to confirm zeolite NaA membrane formation. Membrane performance has been evaluated using an experimental pervaporation setup. Effects of operation condition (temperature, rate and pressure) on the membrane structure and performance have been investigated for NaA zeolite membranes grown onto seeded mullite supports.

Novel Organic-Dehydration Membranes Prepared from Zirconium Metal-Organic Frameworks

Membranes with outstanding performance that are applicable in harsh environments are needed to broaden the current range of organic dehydration applications using pervaporation. Here, well-intergrown UiO-66 metal-organic framework membranes fabricated on prestructured yttria-stabilized zirconia hollow fibers are reported via controlled solvothermal synthesis. On the basis of the adsorption–diffusion mechanism, the membranes provide a very high flux of up to ca. 6.0 kg m−2 h−1 and excellent separation factor (>45 000) for separating water from i-butanol (next-generation biofuel), furfural (promising biochemical), and tetrahydrofuran (typical organic). This performance, in terms of separation factor, is one to two orders of magnitude higher than that of commercially available polymeric and silica membranes with equivalent flux. It is comparable to the performance of commercial zeolite NaA membranes. Additionally, the membrane remains robust during a pervaporation stability test (≈300 h), including exposure to harsh environments (e.g., boiling benzene, boiling water, and sulfuric acid) where some commercial membranes (e.g., zeolite NaA membranes) cannot survive.

In situ impregnation−gelation−hydrothermal crystallization synthesis of hollow fiber zeolite NaA membrane

Chitosan-assisted in situ impregnation−gelation−hydrothermal (IGH) crystallization process has been developed for the preparation of hollow fiber zeolite NaA membranes. Firstly, chitosan-zeolite NaA composite hollow fibers were prepared successfully by assistance of a simple homemade tube-in-orifice spinneret. The composite hollow fibers were initially prepared by in situ impregnation–gelation–hydrothermal transformation of chitosan-silica hollow fibers in aluminate solution. Zeolite NaA membranes can be subsequently obtained on the outer surface of chitosan-zeolite NaA composite hollow fibers by in situ microwave hydrothermal treatment. The zeolite crystals in the composite hollow fibers serve as seeds for the growth of zeolite membrane. Moreover, the chitosan-silica hollow fibers were prepared by solidification of chitosan hollow fibers, which were formed in the tube-in-orifice spinneret from a chitosan-silica sol viscous aqueous solution, in the sodium hydroxide solution. Pervaporation for separation of 90 wt% ethanol aqueous solution was employed to examine the obtained membranes. The hollow fiber membranes showed high permeation flux and high stability.