MFI-type Zeolite Membranes Research Articles

Investigating the Physical and Operational Characteristics of Manufacturing Processes for MFI-Type Zeolite Membranes for Ethanol/Water Separation via Principal Component Analysis

In this study, Principal Component Analysis (PCA) was applied to discern the underlying trends for 31 distinct MFI (Mobil No. 5)-zeolite membranes of 11 textural, chemical, and operational factors related to manufacturing processes. Initially, a comprehensive PCA approach was employed for the entire dataset, revealing a moderate influence of the first two principal components (PCs), which collectively accounted for around 38% of the variance. Membrane samples exhibited close proximity, which prevented the formation of any clusters. To address this limitation, a subset acquisition strategy was followed, based on the findings of the PCA for the entire dataset. This resulted in an enhanced overall contribution and the revelation of diverse patterns among the membranes and the considered manufacturing factors (total variance between 55% and 77%). The segmentation of the data unveiled a robust correlation between silica (SiO2) concentration and pervaporation conditions. Additionally, a notable clustering of the chemical compositions of the preparation solutions underscored their significant influence on the operational efficacy of MFI zeolite membranes. On the other hand, an exclusive chemical composition of the preparation solution was noticed. This highlighted the high influence of the chemical composition on the operational efficiency of MFI zeolite membranes. The coupling of PCA with experimental results can provide a data-driven enhancement strategy for the manufacturing of MFI-type zeolite membranes used for ethanol/water separation.

Two-Dimensional MFI-Type Zeolite Flow Battery Membranes.

Vanadium flow battery (VFB) is one of the most reliable stationary electrochemical energy-storage technologies, and a membrane with high vanadium resistance and proton conductivity is essential for manufacturing high-performance VFBs. In this study, a two-dimensional (2D) MFI-type zeolite membrane was fabricated from zeolite nanosheet modules, which displayed excellent vanadium resistance (0.07 mmol L-1 h-1 ) and proton conductivity (0.16 S cm-1 ), yielding a coulombic efficiency of 93.9 %, a voltage efficiency of 87.6 %, and an energy efficiency of 82.3 % at 40 mA cm-2 . The self-discharge period of a VFB equipped with 2D MFI-type zeolite membrane increased up to 116.2 h, which was significantly longer than that of the commercial perfluorinated sulfonate membrane (45.9 h). Furthermore, the corresponding battery performance remained stable over 1000 cycles (>1500 h) at 80 mA cm-2 . These findings demonstrate that 2D MFI-type membranes are promising ion-conductive membranes applicable for stationary electrochemical energy-storage devices.

Low-temperature ozone treatment for p-xylene perm-selective MFI type zeolite membranes: Unprecedented revelation of performance-negating cracks larger than 10 nm in polycrystalline membrane structures

We report that low-temperature ozone calcination allowed MFI type polycrystalline zeolite membranes to maximize their p-/o-xylene separation factor (as high as ca. 2000) by suppressing defect formation. Conventional high-temperature calcination and rapid thermal processing, which provide poor and marked p-/o-xylene separation abilities, respectively, were used for comparison. The corresponding defect structures were quantitatively analyzed by image processing of fluorescence confocal optical microscopy images combined with membrane permeation modeling, revealing the main defects (grain boundary defects and cracks) and their tortuosity, porosity, and size. To the best of our knowledge, we, for the first time, demonstrated that in contrast to common belief, the minor portion of wider cracks instead of the major portion of narrower grain boundary defects determined the final permeation rates. Specifically, the MFI membrane prepared by high-temperature calcination contained many grain boundary defects (narrow; ca. 1 nm) and few cracks (wide; ca. 20 nm) that accounted for ca. 0.1% and 99.8%, respectively, of the slowly permeating o-xylene molar flux. In contrast, the ozone-treated MFI membranes, which only possessed grain boundary defects, achieved the high p-/o-xylene separation performance, underlining the need for the selective reduction in the number of wider cracks rather than ubiquitous, narrow grain boundary defects.

An Extrinsic-Pore-Containing Molecular Sieve Film: A Robust, High-Throughput Membrane Filter.

MFI type zeolites with 10-membered-ring pores (ca. 0.55 nm) have the ability to separate p-xylene (ca. 0.58 nm) from its bulkier isomers. Here, we introduced non-zeolitic micropores (ca. 0.6-1.5 nm) and mesopores (ca. 2-7 nm) to a conventional microporous MFI type zeolite membrane, yielding an unprecedented hierarchical membrane structure. The uniform, embedded non-zeolitic pores decreased defect formation considerably and facilitated molecular transport, resulting in high p-xylene perm-selectivity and molar flux. Specifically, compared to a conventional, crack network-containing MFI membranes of similar thickness (ca. 1 μm), the mesoporous MFI membranes showed almost double p-xylene permeance (ca. 1.6±0.4×10-7 mol m-2 s-1 Pa-1 ) and a high p-/o-xylene separation factor (ca. 53.8±7.3 vs. 3.5±0.5 in the conventional MFI membrane) at 225 °C. The embedded non-zeolitic pores allowed for decreasing the separation performance degradation, which was apparently related to coke formation.

An Extrinsic‐Pore‐Containing Molecular Sieve Film: A Robust, High‐Throughput Membrane Filter

AbstractMFI type zeolites with 10‐membered‐ring pores (ca. 0.55 nm) have the ability to separate p‐xylene (ca. 0.58 nm) from its bulkier isomers. Here, we introduced non‐zeolitic micropores (ca. 0.6–1.5 nm) and mesopores (ca. 2–7 nm) to a conventional microporous MFI type zeolite membrane, yielding an unprecedented hierarchical membrane structure. The uniform, embedded non‐zeolitic pores decreased defect formation considerably and facilitated molecular transport, resulting in high p‐xylene perm‐selectivity and molar flux. Specifically, compared to a conventional, crack network‐containing MFI membranes of similar thickness (ca. 1 μm), the mesoporous MFI membranes showed almost double p‐xylene permeance (ca. 1.6±0.4×10−7 mol m−2 s−1 Pa−1) and a high p‐/o‐xylene separation factor (ca. 53.8±7.3 vs. 3.5±0.5 in the conventional MFI membrane) at 225 °C. The embedded non‐zeolitic pores allowed for decreasing the separation performance degradation, which was apparently related to coke formation.

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.

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.

Diffusion behaviour of multivalent ions at low pH through a MFI-type zeolite membrane

Zeolite membranes have been widely examined for desalination. Work to date has shown effective performance in monovalent rich solutions, but the understanding of the unique ion-zeolite interactions for multivalent ion rich solutions, such as those found in acidic mining wastewaters, has not yet been studied. Filtration performance of MFI-type zeolite membrane was evaluated on a model multivalent ion, Fe3+, Al3+, Ca2+ and Mg2+, solution with total dissolved solids (TDS) of 97,000mgL−1 and pH2.03 at between 3MPa and 7MPa and 21°C to 70°C. At 7MPa and 21°C, rejection for Fe3+ was 97%, 80% for Al3+ and Mg2+, and 50% for Ca2+. This behaviour followed the rejection of ions with larger hydrated diameter, except for Al3+ which was attributed to its unique strong interaction with zeolites. However, an unusual trend of increasing rejection with increasing temperature was observed. Instead of activated transport which occurs with monovalent or dilute solutions, temperature accelerated the infiltration of multivalent ions into the zeolite structure to further block ions and synergistically increased rejection. Zeolite membranes exhibited unique effects in multivalent ion rich solution that could be further utilised in niche desalination applications or benefit other applications such as sensors.

Co3+ homogeneous mediator generation efficiency in a divided tubular electrochemical reactor with MFI-type zeolite membrane

Abstract In the present work a tubular electrochemical cell with MFI-type zeolite coated membrane (TZM) was engineered and evaluated using Co 2+ oxidation efficiency in high acid medium. Electrolysis of Co 2+ was carried out at room temperature (20 ± 3 °C) in 4 M H 2 SO 4 at anodic half-cell, and showed 42% Co 2+ oxidation with 0.3 V higher cell voltage than the commonly used Nafion324 membrane in planar arrangement. Further, the TZM membrane was robust when operated in the cell at different temperatures (10, 30 and 50 °C) as no noticeable change in surface morphology was observable by SEM analysis. Further testing showed Co 2+ oxidation efficiency decreased with temperature and increased with current density, suggesting stable operation of the TZM but structural change in cobalt precursor. The TZM membrane showed less resistance 0.91 Ω cm 2 (at 10 °C) than Nafion324, which is decreased with increasing temperature from 10 °C to 50 °C. A 72 h test of the TZM at slightly elevated current density (70 mA cm 2 ) showed increased oxidation efficiency of Co 2+ to 57% compared to the traditional planar cell with Nafion324 (43%). Therefore, the engineered tubular electrochemical reactor with TZM is robust for generation of homogeneous mediators towards environmental pollution removal.

A method for defect repair of MFI-type zeolite membranes by multivalent ion infiltration

MFI-type zeolite membranes are effective for molecular separations, however suffer from non-selective defects. Here we show a means of plugging defects by infiltrating with rigidly bound multivalent ions, and demonstrate improvement to performance as salt rejecting (water selective) desalination membranes. Ion uptake into the MFI-type zeolite powder during exposure to a pH 2 solution containing Fe3+, Al3+, Ca2+ and Mg2+ showed a strong uptake of Fe3+ and Al3+. N2 gas porosimetry showed an increase in micropore proportion, being evidence that adsorbed ions did not enter the intrinsic pores of MFI-type zeolites, instead occupying the larger microporous (grain boundaries) and mesopores. X-ray diffraction (XRD) showed shrinkage in the zeolite crystal, being evidence of loss of monovalent cations within the intrinsic pores supported by the ion uptake results. Zeolite membranes were infiltrated with the solution at 7 MPa. Acid and water leaching revealed Fe3+ was the most strongly incorporated. This was confirmed by energy-dispersive X-ray spectroscopy (EDS) mapping on the surface of the membrane. The practical effect of the defect repair method was demonstrated on four membranes, where salt rejections in a reverse osmosis experiment were consistently improved (e.g. salt rejection increased from 24% to 84%). Further work should consider benefits to other applications of zeolite membranes including gas separation and pervaporation.

A mathematical model for zeolite membrane module performance and its use for techno-economic evaluation of improved energy efficiency hybrid membrane-distillation processes for butane isomer separations

The aim of this work is to develop enabling tools that can assess the potential of zeolite membranes for industrial applications. The specific objectives are: (1) Develop a detailed mathematical model of a zeolite membrane separation process for accurate performance prediction under industrial conditions, and (2) Perform conceptual process design and techno-economic evaluation of the overall process for an application specific flowsheet. To this end, a detailed mathematical model, based on the real adsorption solution (RAS) theory and the Maxwell-Stefan formulation for transport was developed to describe permeation through a zeolite membrane. Effects like support resistances to transport, use of sweep gas and concentration polarization were included. The model is further applied to study butane isomer separation using a MFI-type zeolite membrane. A comparison of steady state flux and separation factor predicted by the model with the experimentally determined values suggests that the permeation in the real membranes is lower than that for the zeolite crystals. This lower permeance is attributed to the microstructural defects and suggests that there is a considerable scope of improvement in the performance of current state-of-the-art real MFI membranes. It is shown that up to a 10-fold increase in permeance through the membrane is practical, beyond which the external resistance starts to dominate. Furthermore, the detailed zeolite membrane model is integrated into a process level simulation, to evaluate the membrane performance in industrial settings, and a techno-economic analysis is also performed to this end. While single stage membrane process does not achieve the target purity and multi-stage membrane process requires prohibitively large area, a hybrid membrane/distillation process is found to be energy efficient and economically attractive.

Numerical analysis of H2/He gas separation experiments performed with a MFI-type tubular zeolite membrane

Zeolite membranes were identified to be used in the future fusion reactors to separate traces of tritiated species from large helium flows. Due to the lack of data in the literature, separation experiments with helium, hydrogen and water vapor have been performed in the Tritium Laboratory Karlsruhe (Germany). The obtained results shall be used to predict performances at reactor scale. Therefore, a simulation tool of a permeator has been developed to perform calculations ab-initio of its performance at several experimental conditions. In the present work, numerical results for the binary mixture hydrogen/helium at different concentrations are compared with the experimental results obtained with a zeolite MFI-ZSM5 tubular membrane, where separation factors below 2 were measured for different cuts. Very good agreement between experiments and simulations was observed. This agreement gives confidence to integrate this numerical tool into a membrane cascade simulator, in development in the TLK, toward the modeling of the zeolite membranes’ performances at reactor scale.

The influence of nanoseeds on the pervaporation performance of MFI-type zeolite membranes on hollow fibers

Thin MFI-type zeolite membranes are prepared on α-Al2O3 hollow fibers by secondary growth. Nanoseeds with different gel amounts have been prepared and the influences of silicalite-1 nanoseeds on membrane formation and separation performance of the membrane are investigated. Both the as-synthesized zeolite seeds and membranes are characterized by X-ray diffraction (XRD), dynamic light scattering (DLS) and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX). The separation properties of as-synthesized membranes are evaluated by pervaporation of 5 wt% ethanol aqueous solution at 60 °C. It is found that the gel amount in the nanoseeds is critical in obtaining continuous membranes with high separation performance. Membranes prepared via nanoseeds from ball-milled crystals show higher hydrophobicity and denser microstructure, and thus exhibit higher pervaporation performance than those prepared from as-synthesized nanoseeds.

Microstructure manipulation of MFI-type zeolite membranes on hollow fibers for ethanol–water separation

MFI-type zeolite membranes on hollow fibers are fabricated by a facile secondary growth method. The microstructure of the membranes is systematically manipulated by tuning the seed size, seed morphology and the structure-directing agent content. The as-synthesized MFI-type zeolite membranes are thoroughly characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, water contact angle, pervaporation and gas permeation tests. High quality thin membranes (~3μm) can be obtained on alumina hollow fibers by using appropriate seed size and morphology (e.g., 500nm coffin-shaped crystals), and certain low TPA+ content (e.g., TPA+/Si=0.05), achieving a separation factor as high as 58 with a high flux of 9.8kgm−2h−1 for pervaporation of a 5wt% ethanol aqueous solution at 60°C. A linear decrease relationship between the total flux and the separation factor is obtained. The high pervaporation performance of the membranes can be ascribed to their dense membrane layer and low concentration polarization effect owing to their particular microstructures.

TPAOH Template Removal from High-Silica ZSM-5 by Low-Temperature Hydrocracking

zeolite membranes, especially the mfi-type zeolite membranes, have attracted significant attention for decades because of their special properties. while organic templates such as tetrapropylammonium hydroxide (tpaoh) have typically been used for the synthesis of zsm-5 zeolite and zeolite membranes, the templates remain trapped in the as-synthesized zeolite crystals. a common method for removing organic templates and generating porous frameworks is calcination; however, during this process, the channel structure may be affected. in particular, for zsm-5 membranes, membrane defects may be produced and the separation efficiency therefore may decrease to some extent. in this study, the low-temperature hydrocracking of tpaoh in zsm-5 zeolite crystals was studied under h-2/n-2, while n-2 adsorption, thermogravimetric (tg) analysis, fourier transform infrared (ftir) spectroscopy, temperature-programmed desorption of ammonia (nh3-tpd), and raman spectroscopy were used to characterize zeolite samples. the results show that the organic template in the pores of zsm-5 can be effectively removed below 350 degrees c by low-temperature hydrocracking. characterization analyses by bet specific surface area, tg, ftir, and raman spectroscopy demonstrated that a reducing atmosphere containing h-2 was more conducive to template removal at low temperature than atmospheres of air or n-2. the degree of template removal increased with temperature increasing. the bet surface area of the crystal after hydrocracking at 280 degrees c reached 252 m(2).g(-1), although a small amount of organic residue remained. furthermore, after hydrocracking at 350 degrees c, the bet surface area reached 399 m(2).g(-1), and only trace amount of inorganic carbon residue remained. in addition, the introduction of hydrogen at low temperatures could prevent coke deposits on acid sites and thus zsm-5 zeolite crystals had greater numbers of acidic sites after low-temperature hydrocracking.

Modelling and optimization of critical parameters by hybrid RSM-GA for the separation of BSA using a tubular configured MFI-type zeolite microfiltration membrane

A tubular MFI-type zeolite membrane exhibited good rejection (82%) of BSA with a permeate flux of 4.63 × 10−5m s−1.

Application of robust MFI-type zeolite membrane for desalination of saline wastewater

Zeolites are potentially a more robust desalination alternative as they are chemically stable and possess the essential properties needed to reject ions. This work proposes to use zeolite membranes for desalination of saline recycled wastewater for the possibility of avoiding the costly pre-treatment needed for polymeric reverse osmosis membranes. The MFI-type zeolite membrane was developed on a tubular α-alumina substrate by a combined rubbing and secondary hydrothermal growth method. The prepared membrane was characterised by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and single gas (He or N2) permeation, and underwent desalination tests with saline wastewater under different conditions. When saline wastewater was fed at 7MPa to the zeolite membrane it showed a salt rejection of 80% based on electrical conductivity (EC) and a flux of 4Lm−2h−1. A254 (organics absorption at the wavelength of 254nm measured by a HACH DR5000 spectrophotometer) removal exceeded 90%. Slightly lower salt removal and A254 results were observed when operating at a lower pressure of 3MPa. During batch concentration runs on the saline wastewater, the EC of the feed water increased from the initial value of 1770µScm−1 to 3100µScm−1 over the 48h test time which indicates that a 43% water recovery was achieved. EC reduction remained >70% and flux was maintained at around 2Lm−2h−1 throughout the test period, indicating that the membrane resisted organic fouling. Chlorine stability studies showed that a long-term (7 days) strong hypochlorite clean did not significantly alter the flux or rejection confirming the chemical stability of zeolite membranes. Overall, the zeolite membrane showed excellent chemical resilience and produced a desalinated product suitable for reuse applications (e.g. irrigation, residential or industrial). Fluxes, however, need to be improved to be competitive with current polymeric membranes as do rejections for higher purity water applications.

Microstructural optimization of MFI-type zeolite membranes for ethanol–water separation

High quality pure-silica MFI-type zeolite membranes are successfully prepared by simply controlling the amount of structure directing agent (SDA), i.e., tetrapropylammonium (TPA+) in the synthesis solution for seeded growth. The effects of several synthesis parameters such as alkalinity (OH−/Si), TPA+ concentration (TPA+/Si), and crystallization time on the membrane pervaporation performance are investigated in detail. The synthesized MFI-type zeolite membranes are thoroughly characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, the water contact angle test, X-ray diffraction (XRD), the pervaporation test and gas permeation. The membrane microstructure is very sensitive to the TPA+/Si ratio of the synthesis solution. At a high TPA+/Si ratio (typically 0.17), parasitic twin crystals are intergrown in the zeolite layer, thus resulting in the formation of membrane defects during SDA removal treatment (calcination at 500 °C). When the membrane is prepared with a low TPA+/Si ratio (≤0.05), the appearance of twin crystals can be significantly suppressed and the synthesized dense MFI-type zeolite membranes exhibit pervaporation separation factors higher than 85 for 5 wt% ethanol–water mixtures at 60 °C. We demonstrate for the first time that controlling the SDA concentration in the precursor solution is beneficial to the elimination of membrane microstructural defects, showing a pathway to high quality MFI-type zeolite membranes.

Desalination of seawater ion complexes by MFI-type zeolite membranes: Temperature and long term stability

Ceramic membranes made from zeolites possess the nanoporous structure required for desalination of saline water including seawater. In this research, an α-Al2O3 supported MFI-type silicalite membrane was synthesised by the direct in-situ crystallisation method via a single hydrothermal treatment in an autoclave under autogenous pressure. Desalination performance of the prepared silicalite membrane was carried out with a seawater solution (0.3wt% TDS (total dissolved solids)) over a long period of around 180 days at a constant pressure of 700kPa at various temperatures. The prepared silicalite membrane achieved a high rejection (>93%) for all major seawater ions including Na+ (except for K+, 83%) at an applied pressure of 700kPa and room temperature (22°C), but showed a continuous decrease in ion rejection when increasing the temperature from 22°C and 90°C. Permeation flux of the zeolite membrane significantly increased with increasing in temperature. Upon closer observation of the major cations, size selective diffusion in the zeolite membrane was observed over the temperatures tested. Larger ions Ca2+ and Mg2+ were less responsive to temperature than smaller ions Na+ and K+. No changes in membrane structure were observed by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) after 180 days seawater exposure. However, energy-dispersive X-ray spectroscopy (EDS) mapping on the surface of the membrane revealed a small quantity of tightly bound divalent cations present in the structure, which appear to have penetrated the zeolite, accelerated by temperature. They were suspected to have altered the permstructure, explaining why original high rejections at room temperature were not reversed after heat exposure. The work has shown that zeolite membranes can desalinate seawater, but other unusual effects such as ion selective diffusion as a function of temperature indicate a unique property for desalination membrane materials.

Temperature and Pressure Effects of Desalination Using a MFI-Type Zeolite Membrane.

Zeolites are potentially a robust desalination alternative, as they are chemically stable and possess the essential properties needed to reject ions. Zeolite membranes could desalinate “challenging” waters, such as saline secondary effluent, without any substantial pre-treatment, due to the robust mechanical properties of ceramic membranes. A novel MFI-type zeolite membrane was developed on a tubular α-Al2O3 substrate by a combined rubbing and secondary hydrothermal growth method. The prepared membrane was characterised by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and single gas (He or N2) permeation and underwent desalination tests with NaCl solutions under different pressures (0.7 MPa and 7 MPa). The results showed that higher pressure resulted in higher Na+ rejection and permeate flux. The zeolite membrane achieved a good rejection of Na+ (~82%) for a NaCl feed solution with a TDS (total dissolved solids) of 3000 mg·L−1 at an applied pressure of 7 MPa and 21 °C. To explore the opportunity for high salinity and high temperature desalination, this membrane was also tested with high concentration NaCl solutions (up to TDS 90,000 mg·L−1) and at 90 °C. This is the first known work at such high salinities of NaCl. It was found that increasing the salinity of the feed solution decreased both Na+ rejection and flux. An increase in testing temperature resulted in an increase in permeate flux, but a decrease in ion rejection.