MFI Membranes Research Articles

Supported MFI zeolite membranes by cross flow filtration for water treatment

In the last years, intense research efforts were dedicated to the development of zeolite membranes in different separation processes.In this work, zeolite membranes were prepared and characterized by means of single gas and pure water permeation test. Moreover, the membranes performance in vacuum membrane distillation was also studied to determine their potentiality for desalination.Uncalcined defect free MFI (silicalite) membranes on different tubular supports were synthesized by using the secondary growth method coupled with the cross flow seeding procedure.Gas permeation tests showed that, using a support with pore size of 70nm and a zeolite slurry concentration of 0.032wt.%, uncalcined defect free MFI membranes were synthesized.To seal the defects formed after the calcination, two different curing treatments, new cross flow filtration with small crystals and dip coating with PDMS, were carried out.Very low defects concentration for the membranes cured with small crystals was evidenced in pure water permeation tests. In fact, permeance values in the nanofiltration range were obtained. Furthermore, these zeolite membranes were used in vacuum membrane distillation tests with promising results in terms of fluxes and salt rejections.

Preparation of MFI Membrane on Mesoporous-Layer-Modified Macroporous Al<sub>2</sub>O<sub>3</sub> Substrate by Secondary Growth Method and its Permeation Property

The compact MFI membrane supported on a mesoporous-layer-modified macroporous Al2O3 substrate has been prepared by the secondary growth method. SEM images indicate that the MFI membranes with the thickness of ca. 15 μm consist of highly intergrown crystals with remarkable integrity. The H2 permeance of MFI membranes on mesoporous/macroporous substrate is 8-18 times larger than that on the macroporous substrate, and further the permselectivity of H2/C3H8 maintains at the level of 20, which is 4 times higher than that of the corresponding Knudsen diffusion. More importantly, the real selectivity of the mixture of H2/C3H8 on these membranes is 2-4 times higher than that of the corresponding Knudsen diffusion. The method is attractive for the practical application.

Reprint of: An experimental study of micropore defects in MFI membranes

In the present work, two ultra-thin MFI membranes, prepared using hydroxide and fluoride ions as mineralizing agents, respectively, were carefully examined by permporometry. The amount of micropore defects, as determined by permporometry, differed significantly between the two different membranes. For the first time, it was demonstrated that the micropore defects determined by permporometry were most likely open grain boundaries. The results were verified by direct observation of the open grain boundaries by a state-of-the-art XHR-scanning electron microscopy instrument. In addition, the permporometry data were also consistent with permeation data using 1,3,5-trimethylbenzene (TMB) as a probe molecule, separation data using an equimolar mixture of n-hexane and TMB, and nitrogen adsorption data.

Improving separation performance of high-silica zeolite membranes by surface modification with triethoxyfluorosilane

Treatment with triethoxyfluorosilane (TEFS) enhances the hydrophobic properties of high-silica MFI and MEL membranes synthesized by a microwave-assisted secondary growth method. The modified membranes showed significant improvement in recovery of ethanol from ethanol/water mixtures by pervaporation. The TEFS treatment was also able to heal some intercrystalline defects in the zeolite membrane film. Additional physical–chemical characterization revealed that the external zeolite surface is selectively covered by the organosilane without blocking zeolite micropores. The grafted Si–F moieties are thermally and hydrothermally stable.

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

AbstractMembrane separation of CO2 from natural gas, biogas, synthesis gas, and flu gas is a simple and energy‐efficient alternative to other separation techniques. But results for CO2‐selective permeance have always been achieved by randomly oriented and thick zeolite membranes. Thin, oriented membranes have great potential to realize high‐flux and high‐selectivity separation of mixtures at low energy cost. We now report a facile method for preparing silica MFI membranes in fluoride media on a graded alumina support. In the resulting membrane straight channels are uniformly vertically aligned and the membrane has a thickness of 0.5 μm. The membrane showed a separation selectivity of 109 for CO2/H2 mixtures and a CO2 permeance of 51×10−7 mol m−2 s−1 Pa−1 at −35 °C, making it promising for practical CO2 separation from mixtures.

A Uniformly Oriented MFI Membrane for Improved CO2 Separation

Membrane separation of CO2 from natural gas, biogas, synthesis gas, and flu gas is a simple and energy-efficient alternative to other separation techniques. But results for CO2 -selective permeance have always been achieved by randomly oriented and thick zeolite membranes. Thin, oriented membranes have great potential to realize high-flux and high-selectivity separation of mixtures at low energy cost. We now report a facile method for preparing silica MFI membranes in fluoride media on a graded alumina support. In the resulting membrane straight channels are uniformly vertically aligned and the membrane has a thickness of 0.5 μm. The membrane showed a separation selectivity of 109 for CO2/H2 mixtures and a CO2 permeance of 51×10(-7) mol m(-2) s(-1) Pa(-1) at -35 °C, making it promising for practical CO2 separation from mixtures.

Desorption-controlled separation of natural gas alkanes by zeolite membranes

The performance of porous membranes is tremendously influenced by desorption, as alkane separations by a pressure stable MFI membrane revealed.

Maxwell–Stefan Modeling of Ethanol and Water Unary Pervaporation through a High-Silica MFI Zeolite Membrane

The pervaporative mass transfer of pure ethanol and water through a thin (0.5 μm) supported high-silica MFI membrane was studied experimentally at 30–70 °C, and modeled on the basis of the Maxwell–Stefan formalism. The temperature dependency of adsorption was described with the temperature dependency of pure component saturated vapor pressure. Two scenarios of coverage dependency, i.e., coverage-dependent and coverage-independent Maxwell–Stefan diffusivity, were applied in the modeling of the mass transfer through the zeolite film. In addition, the mass-transfer resistance of the support layers was taken into account. The derived unary models provided good representations of ethanol and water pervaporation flux. The study illustrates that pure component steady-state pervaporation flux measurements at different conditions offer a feasible basis for determining diffusion coefficients. Basically, pure component adsorption isotherms and derived diffusivities can be used in the modeling of pervaporative mass transfer of mixtures using zeolite membranes.

An experimental study of micropore defects in MFI membranes

In the present work, two ultra-thin MFI membranes, prepared using hydroxide and fluoride ions as mineralizing agents, respectively, were carefully examined by permporometry. The amount of micropore defects, as determined by permporometry, differed significantly between the two different membranes. For the first time, it was demonstrated that the micropore defects determined by permporometry were most likely open grain boundaries. The results were verified by direct observation of the open grain boundaries by a state-of-the-art XHR-scanning electron microscopy instrument. In addition, the permporometry data were also consistent with permeation data using 1,3,5-trimethylbenzene (TMB) as a probe molecule, separation data using an equimolar mixture of n-hexane and TMB, and nitrogen adsorption data.

Combination of membrane separation and gas condensation for advanced natural gas conditioning

Membrane separation and gas condensation are combined to reveal an advanced method for the separation of alkanes. First, the applicability of MFI membranes for alkane separation is principally demonstrated by means of realistic adsorption isotherms computed by configurational biased Monte Carlo (CBM) simulations. Next, dew point curves of mixtures comprising different ratios of n-butane (C4) and methane (C1) were calculated according to the thermodynamic methods of Soave–Redlich–Kwong (SRK) and Peng Robinson (PR). From that, isothermal phase boundaries in dependence on the composition of the gas mixture were derived and process parameters under which condensation of the alkane mixture occurs were predetermined. Experimentally, the separation performance of MFI membranes was recorded during separation of n-butane from methane. It was found that liquefied n-butane in the feed and a further liquefaction in the permeate enhance the separation selectivity of MFI zeolite membranes under sweeping conditions tremendously. At the dew point of the feed mixture a sudden rise of the separation factor α is observed. At a temperature of 258K a mixture with χC4=0.5 can be separated with a separation factor αC4/C1=174 due to liquefaction. Experiments without sweeping show a similar behaviour. When forming a two phase mixture in the feed an increase in overall condensation efficiency ηC4 is detected in the permeate. At 258K and pfeed=2bar and ppermeate=1bar 29.6% liquefied n-butane was isolated in the permeate from a mixture comprising χC4=0.5.

Ultrathin hydrophobic MFI membranes

Hydrophobic MFI membranes with a film thickness of less than 500nm and a very low amount of defects were grown on α-alumina supports in fluoride media at near neutral pH (denoted TPAF–MFI membrane). These membranes are at least 4 times thinner than previous membranes prepared in fluoride media at near neutral pH; moreover, corresponding separation performance is the first time reported in this paper. For comparison, MFI membranes with a film thickness of about 500nm were grown in hydroxide media at high pH (denoted TPAOH–MFI membrane). According to the permporometry results, the amount of defects in the TPAF–MFI membranes is about half of that in the TPAOH–MFI membranes, and most of the defects in both membranes are smaller than 1nm. The membranes were evaluated for separation of a feed of 5kPa n-butanol and 5kPa water, with 91kPa helium as balance gas and 101kPa helium as sweep gas. Both membranes displayed high permeance and the TPAF–MFI membrane was more selective to n-butanol than the TPAOH–MFI membrane. Interestingly, the total permeance and n-butanol/water separation factor increased to 11×10−7mol/(m2sPa) and 19, respectively, after activation of the TPAF–MFI membrane at 300°C in a helium flow saturated with n-butanol at 25°C. The TPAF–MFI membrane also displayed a CO2/H2 separation factor of 6 at 25°C and a CO2 permeance of 26×10−7mol/(m2sPa) for a feed of 450kPa CO2 and 450kPa H2 and 101kPa helium sweep gas.

Air separation at cryogenic temperature using MFI membranes

In the present work, zeolite membranes were for the first time evaluated for separations at cryogenic temperatures. MFI membranes were evaluated with a feed of synthetic air, at varying feed pressures between 1 and 5bar and a sweep stream of helium at a constant pressure of 1bar for temperatures in the range 77–110K. When the feed pressure was 1bar, the highest O2/N2 separation factor was 3.9, corresponding to a separation selectivity of 4.1, with an oxygen permeance of 6.7×10−7molm−2s−1Pa−1 at the optimum temperature 79K. This membrane performance is just above the upper bound in the 2008 Robeson selectivity–permeability plot for polymeric membranes. As the feed pressure increased, the maximum separation factors for O2/N2 decreased while the optimum separation temperatures increased. It is inferred that the separation is governed by condensation and effective transport of oxygen in the zeolite pores under these conditions. However, the adsorption of nitrogen, and thereby the transport of nitrogen, likely increases as the pressure is increased, which reduce the selectivity. To test this hypothesis, the feed was diluted with 33% helium and the total feed pressure was maintained at 1bar, corresponding to a partial pressure of air of 0.66bar. In this case, the maximum separation factor increased to 4.3, which supports the proposed separation mechanism.

Analyses on Diffusion Behaviors of Water, Na<sup>+</sup> and Cl<sup>-</sup> through the MFI Membranes by Molecular Simulations

To investigate the diffusion of water, Na+and Cl-in the MFI membrane system (Si/Al = 191), the grand canonical Monte Carlo (GCMC) method and the followed molecular dynamics (MD) simulations were used to predict the water absorption into aluminosilicate zeolite structure under various conditions of pressure and to determine the water and ions diffusion in the MFI membrane at different concentrations. The simulation results are consistent with the experimental data reported in the literature: as the separation proceeds, the salt ions concentration increase in the MFI membrane, and the diffusion coefficient of the water and salt ions decline.

Synthesis and separation properties of an α-alumina-supported high-silica MEL membrane

A thin high-silica MEL membrane was synthesized on a porous α-alumina hollow fiber support by a secondary growth approach. The membrane quality was evaluated by permporometry, single-gas permeation and butane isomer separation. Comparison of the pervaporation performance of MEL membranes with a MFI membrane of similar quality and thickness shows that MEL membranes offer a higher flux in ethanol pervaporation at nearly selectivity.

B]MFI membrane: Synthesis, physico-chemical properties and catalytic behavior in the double-bond isomerization of 1-butene

An MFI type borosilicate, [B]MFI (Si/B=56), membrane was synthesized on the external surface of tubular α-alumina support. The structure, morphology and acidity of the synthesized membrane were characterized by X-ray diffraction, SEM and FT-IR spectroscopy. A flow-through membrane reactor was constructed and the simultaneous catalytic and separation selectivity of the membrane was tested in the double-bond isomerization of 1-butene in the temperature range of 120–250°C and at 1-butene partial pressure of 18 and 50kPa. No product of skeletal isomerization was formed under these conditions. The catalytic selectivity is attributed to the low acid strength of the Brønsted sites generated by the framework boron. Equilibrium trans/cis-2-butenes ratio was obtained in the retentate stream (1.4–1.5), whereas, that ratio at the permeate stream was up to ∼2.2 at 250°C. The high trans/cis ratio was attributed to the permeation selectivity of the MFI membrane for trans-2-butene.

Fabrication of zeolite MFI membranes supported by α-Al2O3 hollow ceramic fifibers for CO2 separation

Abstract

Separation of CO2 from black liquor derived syngas using an MFI membrane

Membrane separation of CO2 from synthesis gas could be an energy efficient and simple alternative to other separation techniques. In this work, a membrane comprised of an about 0.7µm thick MFI film on a graded alumina support was used to separate CO2 from synthesis gas produced by pilot scale gasification of black liquor. The separation of CO2 from the synthesis gas was carried out at a feed pressure of 2.25MPa, a permeate pressure of 0.3MPa and room temperature. In the beginning of the experiment, when the H2S concentration in the feed was 0.5% and the concentration of water in the feed was 0.07%, a CO2/H2 separation factor of 10.4 and a CO2 flux of 67.0kgm−2h−1 were observed. However, as the H2S concentration in the feed to the membrane increased to 1.7%, the CO2/H2 separation factor and the CO2 flux decreased to 5 and 61.4kgm−2h−1, respectively. The results suggest that MFI membranes are promising candidates for the separation of CO2 from synthesis gas.

Preparation of zeolite MFI membranes on defective macroporous alumina supports by a novel wetting–rubbing seeding method: Role of wetting agent

High quality zeolite MFI membranes are successfully synthesized on inexpensive defective macroporous alumina tubes by the seeded growth method. A novel seeding method of wetting assisted rubbing (wetting–rubbing, WR) technique is developed and with this new WR method alumina supports are first wetted with a liquid agent followed by rubbing dry MFI seed crystals. Effects of two typical wetting agents, i.e., alcohol solvents (n-butanol) and water on the seed layer formation and membrane performance are investigated. Experimental data suggest that n-butanol is much more effective than water for preparation of a uniform and continuous MFI seed layer on the defective alumina support, which is attributed to the hydrophobic nature of MFI crystals and their better dispersion in alcohols. The zeolite membranes synthesized at 175°C for 4h on the well-seeded supports have a separation layer thickness of ∼3.5μm, and exhibit a high pervaporation performance with the ethanol/water separation factor of 62 and permeation flux of 1.82kgm−2h−1 for the 5:95 (wt%) ethanol/water mixture separation at 60°C. The WR seeding method may lead to reproducible preparation of zeolite membranes on defective low-cost supports.

On the performance of c-oriented MFI zeolite Membranes treated by rapid thermal processing

Rapid thermal processing (RTP) was shown to be effective in reducing defects, especially grain boundary defects in c-oriented MFI membranes (Choi et al., Science 325 (2009) 590-593). In this study, the xylene and butane separation performances of RTP-treated c-oriented MFI membranes, which were now synthesized with a shorter secondary growth time (from 2 d to 1 d), were investigated. These membranes achieved a good performance for separating both p-/o-xylene (∼88 separation factor (SF)) and n-/i-butane (∼30SF) isomers. In addition, fluorescence confocal optical microscopy was conducted to explore the defect structure by varying the contact time with the dye solution. This characterization revealed that the accessibility of dye molecules to grain boundary defects at the molecular level can be correlated with separation performance. Finally, the fluxes of xylene and butane isomers across RTP-treated c-oriented MFI membranes were deciphered by the Maxwell-Stefan formulation to obtain their diffusion coefficients. The diffusivities of the slow permeating component (i.e., o-xylene) were estimated to be lower than the literature values obtained by a zero length column method; this accounted for the high p-/o-xylene SF. We considered the combination of adsorbate-adsorbate and adsorbate-zeolite interactions to calculate the diffusivities of n-/i-butane isomers.

Maxwell–Stefan modeling of the separation of H2 and CO2 at high pressure in an MFI membrane

In the present study, a Maxwell–Stefan based model was developed for the separation of CO2 from H2 at high pressure in an MFI membrane. The usage of the Vignes interpolation formula for mixture surface diffusivities together with the IAST (ideal adsorbed solution theory) using bulk gas phase fugacities for mixture adsorption proved to be a feasible combination for this case. Both the effects of defects in the zeolite film and the mass transfer resistance caused by the support layers were studied and included in the model. Only pure component experimental data was used in the model building to predict the gas mixture permeation. The fitted diffusion parameters were in line with the literature values. The occupancy fraction dependence of CO2 surface diffusivity was utilized for the first time in the prediction of binary separation of H2/CO2 at high pressure on a real MFI membrane. Usage of an occupancy fraction dependence for CO2 surface diffusivity improved the model predictions. The adsorption parameter fitting for hydrogen based on the permeation measurements resulted in a feasible adsorption model, but should be used with caution. The model predicts binary separation measurement results relatively well. Both defects and support have a noticeable impact on the overall performance of the membrane.