Mesoporous Membranes Research Articles

Effect of Graphene and Fullerene Nanofillers on Controlling the Pore Size and Physicochemical Properties of Chitosan Nanocomposite Mesoporous Membranes

Chitosan (CS) nanocomposite mesoporous membranes were fabricated by mixing CS with graphene (G) and fullerene (F) nanofillers, and the diffusion properties through CS membranes were studied. In addition, in order to enhance the binding between the internal CS chains, physical cross‐linking of CS by sodium tripolyphosphate (TPP) was carried out. F and G with different weight percentages (0.1, 0.5, and 1 wt.%) were added on physically cross‐linked chitosan (CLCS) and non‐cross‐linked chitosan (NCLCS) membranes by wet mixing. Permeability and diffusion time of CLCS and NCLCS membranes at different temperatures were investigated. The results revealed that the pore size of all fabricated CS membranes is in the mesoporous range (i.e., 2–50 nm). Moreover, the addition of G and F nanofillers to CLCS and NCLCS solutions aided in controlling the CS membranes’ pore size and was found to enhance the barrier effect of the CS membranes either by blocking the internal pores or decreasing the pore size. These results illustrate the significant possibility of controlling the pore size of CS membranes by cross‐linking and more importantly the careful selection of nanofillers and their percentage within the CS membranes. Controlling the pore size of CS membranes is a fundamental factor in packaging applications and membrane technology.

Facile Fabrication, Structure, and Applications of Polyvinyl Chloride Mesoporous Membranes

Mesoporous polymer membranes are important in various separation processes and have become more crucial with increasing concerns in environment protection. Here, we report novel mesoporous membranes of polyvinyl chloride (PVC) nanofibers prepared via a modified freeze-extraction technique. The as-prepared nanofibers have an average diameter of ∼45 nm and are dispersed heterogeneously in an ethanol solution. The nanofiber formation is studied in detail. The resulting nanofiber dispersion is directly filtered on a macroporous support to fabricate mesoporous membranes. The as-fabricated membranes have a controllable thickness ranging from 360 nm to 1055 nm, and a high porosity up to 63% that is at least 5 times greater than that of most of the commercial ultrafiltration membranes. These membranes present an ultrahigh water flux up to 6612 L m–2 h–1 bar–1 and a good ferritin rejection during ultrafiltration. Moreover, these membranes show a fast and high-efficient absorption for Methylene Blue (MB). The newly developed mesoporous membranes have a great potential application in various separation processes.

Material properties and operating configurations of membrane reactors for propane dehydrogenation

A modeling‐based approach is presented to understand physically realistic and technologically interesting material properties and operating configurations of packed‐bed membrane reactors (PBMRs) for propane dehydrogenation (PDH). PBMRs composed of microporous or mesoporous membranes combined with a PDH catalyst are considered. The influence of reaction and membrane transport parameters, as well as operating parameters such as sweep flow and catalyst placement, are investigated to determine desired “operating windows” for isothermal and nonisothermal operation. Higher Damköhler (Da) and lower Péclet (Pe) numbers are generally helpful, but are much more beneficial with highly H2‐selective membranes rather than higher‐flux, lower‐selectivity membranes. H2‐selective membranes show a plateau region of conversion that can be overcome by a large sweep flow or countercurrent operation. The latter shows a complex trade‐off between kinetics and permeation, and is effective only in a limited window. H2‐selective PBMRs will greatly benefit from the fabrication of thin (∼1 µm or less) membranes. © 2014 American Institute of Chemical Engineers AIChE J, 61: 922–935, 2015

Impact of an ionic surfactant on the ion transfer behaviors at meso-liquid/liquid interface arrays

Abstract The influence of ionic surfactants, cetyltrimethylammonium bromide (CTAB), self-assembled within silica-nanochannels of a hybrid mesoporous silica membrane (HMSM) on simple ion transfer (IT) behaviors at the meso-water/1,2-dichloroethane (W/DCE) interface arrays supported by such a HMSM was investigated by voltammetry for the first time. Significantly, it is found that the CTAB in HMSM can dramatically enhance the peak-current responses corresponding to ITs of some anions and even lower their Gibbs transfer energies from W to DCE, which could be ascribed to an anion-exchange process between anions and the bromide of CTAB associated with partial ion-dehydration induced by the CTAB. This work will provide a new strategy to study anion transfer processes and improve the electroanalytical performance for anion detection at the liquid/liquid interface.

Aziridine-Functionalized Mesoporous Silica Membranes on Polymeric Hollow Fibers: Synthesis and Single-Component CO2 and N2 Permeation Properties

The synthesis of aziridine-functionalized mesoporous silica membranes on polymeric hollow fibers is described, and their single-component CO2 and N2 permeation properties are investigated under both dry and humid conditions to elucidate the unusual permeation mechanisms observed in these membranes. Hollow fiber-supported mesoporous silica membranes are amine-functionalized with aziridine to yield hyperbranched aminopolymers within the membrane pores. The effects of the hyperbranched polymers in the mesopores on gas transport properties are investigated by single-component gas permeation measurements. The hyperbranched aminosilica membrane shows counterintuitive N2-selective (over CO2) permeation during operation under dry conditions. Further characterization of the permeation behavior reveals the effects of strong adsorption of CO2 under dry permeation conditions, leading to reduced CO2 diffusivity because of CO2-induced amine cross-linking in the mesopores. On the other hand, the hyperbranched aminosilic...

The effects of fractality on hydrogen permeability across meso-porous membrane

A fractal theory employing a box-counting method was used to describe hydrogen gas diffusion into membrane pores in the meso-porosity regime. The diffusion of the gas into the membrane pore network confirmed the existence of fractal structure in the system. Two fractal identities to represent irregularity and roughness of pore surface and tortuosity of the membrane were obtained and analyzed. Their influences on hydrogen permeability were also evaluated. The fractal permeability model that reflects different hydrogen diffusion mechanisms was calculated and compared with that of the state of the art Kozeny–Carman equation.

Silylated Mesoporous Silica Membranes on Polymeric Hollow Fiber Supports: Synthesis and Permeation Properties

We report the synthesis and organic/water separation properties of mesoporous silica membranes, supported on low-cost and scalable polymeric (polyamide-imide) hollow fibers, and modified by trimethylsilylation with hexamethyldisilazane. Thin (∼1 μm) defect-free membranes are prepared, with high room-temperature gas permeances (e.g., 20,000 GPU for N2). The membrane morphology is characterized by multiple techniques, including SEM, TEM, XRD, and FT-ATR spectroscopy. Silylation leads to capping of the surface silanol groups in the mesopores with trimethylsilyl groups, and does not affect the integrity of the mesoporous silica structure and the underlying hollow fiber. The silylated membranes are evaluated for pervaporative separation of ethanol (EtOH), methylethyl ketone (MEK), ethyl acetate (EA), iso-butanol (i-BuOH), and n-butanol (n-BuOH) from their dilute (5 wt %) aqueous solutions. The membranes show separation factors in the range of 4-90 and high organic fluxes in the range of 0.18-2.15 kg m(-2) h(-1) at 303 K. The intrinsic selectivities (organic/water permeability ratios) of the silylated membranes at 303 K are 0.33 (EtOH/water), 0.5 (MEK/water), 0.25 (EA/water), 1.25 (i-BuOH/water), and 1.67 (n-BuOH/water) respectively, in comparison to 0.05, 0.015, 0.005, 0.08, and 0.14 for the unmodified membranes. The silylated membranes allow upgradation of water/organics feeds to permeate streams with considerably higher organics content. The selective and high-flux separation is attributed to both the organophilic nature of the modified mesopores and the large effective pore size. Comparison with other organics/water separation membranes reveals that the present membranes show promise due to high flux, use of scalable and low-cost supports, and good separation factors that can be further enhanced by tailoring the mesopore silylation chemistry.

Removal of hazardous volatile organic compounds from water by vacuum pervaporation with hydrophobic ceramic membranes

Hydrophobic alumina and titania micro and mesoporous ceramic membranes were prepared by grafting of C6F13C2H4Si(OEt)3 (C6) molecules and subsequently applied in a pervaporation (PV) process to the removal of hazardous organic solvents (MTBE, EtAc and BuOH) from binary aqueous solutions. The transport and separation properties of investigated membranes were discussed. Additionally it was found that membrane material and grafting time have an important impact of the pervaporative properties. All tested membranes were selective toward organics. Titania membrane was the most efficient and was characterized by the highest value of PSI and the highest value of permeate flux of organic compounds in water–EtAc (JEtAc=1.1kgh−1m−2; PSIEtAc=140kgh−1m−2) and water–MTBE (JMTBE=1.0kgh−1m−2; PSIMTBE=194kgh−1m−2) systems. This behavior of titania membrane was also discussed based on the Hansen solubility parameters. In presented work the evaluation of MTBE removal degree with titania modified membrane from water binary mixture in the pervaporation process was discussed.

One-step deposition of ultrafiltration SiC membranes on macroporous SiC supports

We fabricated nearly defect-free SiC membranes for potential ultrafiltration applications by conducting pyrolysis of allylhydrido polycarbosilane in the presence of submicron α-SiC particles. The SiC membranes were developed on commercial macroporous SiC supports by a low-temperature-process in which allylhydrido polycarbosilane acted to bond together crystalline α-SiC particles to form a porous layer. The suspensions of α-SiC powder and allylhydrido polycarbosilane in n-hexane or n-hexane/n-tetradecane were used for membrane fabrication by dip-coating. By using optimized n-hexane suspension with 5% w/w α-SiC powder and mass ratios of allylhydrido polycarbosilane to α-SiC powder of 0.6 and 0.8, we obtained nearly defect-free and uniform mesoporous membranes in a single coating procedure. No defects were found on the surface of these membranes by scanning electron microscopy. Moreover, during filtration tests, these membranes showed a water permeance of 0.05–0.06L (hm2bar)–1 and retention higher than 93% for polyethylene glycol (PEG) with a molecular mass of 100kDa. Retention for PEG with molecular masses of 1kDa, 8kDa and 35kDa was between 25% and 71%.

Topological Paths and Transient Morphologies during Formation of Mesoporous Block Copolymer Membranes

We systematically investigated the structure formation pathways and transient morphologies involved in the formation of mesoporous membranes by the self-assembly of block copolymers during nonsolvent-induced phase separation. Using AFM, SEM, and in situ synchrotron SAXS, we mapped the topological paths and characteristic transient structures into a ternary phase diagram. We focused on the stability region of an ordered pore phase which is relevant for the generation of integral asymmetric isoporous membranes. We could identify several characteristic morphologies, i.e., spinodal networks, sphere percolation networks, ordered pore structures, and disordered and ordered cylinder arrangements together with transient structures connecting their stability regions. With given evaporation rates for the pure solvents, we calculated the corresponding composition trajectories in the phase diagram to identify suitable experimental conditions in terms of initial polymer volume fraction, solvent composition, and immersion time to trap the desired pore structure.

Carbon dioxide permeation study through carbon hollow fiber membranes at pressures up to 55 bar

Two asymmetric carbon hollow fiber membranes were prepared and tested for CO2 permeance from low, atmospheric, to high pressures up to ∼55bar. Both membranes were produced via pyrolysis of polyimide hollow fiber membranes at 1050°C, the one in inert environment, while the other was activated with CO2 at the higher pyrolysis temperature. The carbon dioxide was chosen for the permeance performance of the studied membranes. A maximum in the curve of permeance versus equilibrium pressure is observed at 32bar for both studied carbon hollow fiber membranes. This phenomenon has close analogy to the case of mesoporous membranes. The result shows that the CO2 provides a maximum in the permeance values. This weakens considerably as the membrane ultra-microporosity is increased. The high pressure gas permeance study may be potentially useful for the identification of the optimal pressure and temperature conditions for efficient gas separations.

Modeling water flux and salt rejection of mesoporous γ-alumina and microporous organosilica membranes

The water and ion transport through a mesoporous γ-alumina membrane and a microporous organosilica membrane was simulated using the extended Nernst–Planck equation combined with models for Donnan, steric and dielectric interfacial exclusion mechanisms. Due to the surface charge within the pore, the electroviscous effect was introduced in the model. The modified model fits well the rejection and permeability data for both membranes. The organosilica membrane shows a higher selectivity compared to the γ-alumina membrane, but the permeate flux is lower. At low ionic strength the electroviscous effect lowers the water flux through the γ-alumina membrane. The electroviscous effect is negligible for the organosilica membrane because its absolute surface potential (~20mV) is low compared to the γ-alumina membrane (~60mV). The simulation shows that the electroviscous effect should be included for the membranes with high surface potential (>20mV) and a pore size below 2–5 times of the electroviscous double layer thickness.

PDMS grafting of mesoporous γ-alumina membranes for nanofiltration of organic solvents

In this paper grafting of mesoporous γ-alumina membranes with monovinyl terminated polydimethylsiloxane (PDMS), using 3-mercaptopropyltriethoxysilane (MPTES) as a linking agent, is described. The grafting performance of the organic moieties on γ-alumina powders was studied by FTIR. Contact angle measurements and solvent permeability tests were used to characterize the membrane properties. The results indicated that grafting reactions were successfully carried out. The toluene permeability of the membrane was reduced from 5.3 to 2.1L/m2hbar after grafting with the polymer. No degradation of the membrane material was observed after chemical stability tests in toluene for 6 days at room temperature and at elevated temperatures (up to 90°C).

The effect of sulfonic acid group content in pore-filled silica colloidal membranes on their proton conductivity and direct methanol fuel cell performance

We prepared mesoporous silica colloidal membranes pore-filled with polymer brushes with different degrees of sulfonation. In these membranes, the assembly of silica colloidal spheres serves as a rigid matrix containing a continuous network of interconnected mesopores and providing mechanical and thermal stability, non-swelling and water retaining properties, while sulfonic acid group-containing polymer brushes grown on the surface of silica provide proton conductivity. We studied the proton conductivity of these membranes as well as open circuit voltage and linear polarization of the fuel cells prepared using these membranes as a function of sulfonic acid group content in the pore-filling polymer brushes. We found a sigmoidal dependence of the proton conductivity on the amount of sulfonic acid groups. We showed that the proton conductivity of the membrane does not increase significantly after reaching ca. 75% sulfonic acid group content. The fuel cell performance, on the other hand, decreased after reaching ca. 65–70% sulfonic acid group content, which was attributed to the increased methanol permeability of the membranes at higher sulfonic acid group content.

Synthesis of stable Ti-containing mesoporous tubular membrane using silicalite-1 nanoparticles as seeds

Abstract Stable Ti-containing mesoporous membranes were successfully synthesized on different porous alumina tubes using nanosized Sil-1 particles as seeds by hydrothermal treatment, starting from sols having both CTABr and TPAOH structure directing agents. The samples were characterized by various techniques and tested in the gas permeation and hydroxylation reactions. The SEM images and XRD pattern detected a continuous membrane with well-ordered hexagonal mesoporous structure formed on the inner surface of ceramic tubes. Membranes prepared in the presence of zeolite nanocrystal seeds had zeolite structure units in the mesopore walls verified by FTIR, thus giving a better hydrothermal stability. The characterizations by EDX, XPS, FTIR and UV–Vis indicated that most of titanium in the growth solution was incorporated into the zeolite framework. Ti-containing mesoporous membranes prepared on the Sil-1 seeded support had larger proportions of tetrahedrally coordinated titanium, leading to better reaction activities and conversion rates. More Sil-1 nanoparticle seeds could promote more mesoporous zeolite to deposit on the support resulting in the formation of thicker mesoporous membrane. The gas permeation results confirmed that gas transport through the calcined mesoporous membrane was governed by Knudsen diffusion. Sil-1 seeding is a promising method to prepare high quality Ti-containing mesoporous membranes on porous ceramic tubes of different pore sizes.

Fabrication of mesoporous TiO2 membranes by a nanoparticle-modified polymeric sol process

A straightforward synthesis of crack-free mesoporous titania membrane on a macroporous support without intermediate layers by a nanoparticle-modified polymeric sol–gel process is reported. TiO2 nanoparticle (Degussa P25) was dispersed into the prepared sol to toughen the formed gel. This helped to avoid the cracking of membrane during the drying and early stage of sintering, particularly when the sol–gel transformation occurred on a substrate with an uneven surface. The results of X-ray diffraction and Brunauer–Emmett–Teller analyses show that the nanoparticle doping did not significantly affect the particle size of TiO2 nanocrystals; however, it slightly broadened the pore size distribution because it has a larger particle size compared to the prepared materials. The sols with and without P25 nanoparticle were subsequently dip-coated on a support with average pore size of 150nm, thus formed a mesoporous membrane layer after drying and sintering processes. An integrated, crack-free mesoporous TiO2 membrane layer was obtained by this method, while the membrane prepared with the original sol developed a few cracks. Further, the filtration experiment showed that the prepared membrane had a high flux, and the membrane with nanoparticle modification delivered better separation performance by rejection of dextran.

Paranematic-to-nematic ordering of a binary mixture of rodlike liquid crystals confined in cylindrical nanochannels.

We explore the optical birefringence of the nematic binary mixtures 6CB_{1-x}7CB_{x} (0 ≤ x ≤ 1) embedded into parallel-aligned nanochannels of mesoporous alumina and silica membranes for channel radii of 3.4 ≤ R ≤ 21.0 nm. The results are compared with the bulk behavior and analyzed with a Landau-de Gennes model. Depending on the channel radius the nematic ordering in the cylindrical nanochannels evolves either discontinuously (subcritical regime, nematic ordering field σ<1/2) or continuously (overcritical regime, σ>1/2), but in both cases with a characteristic paranematic precursor behavior. The strength of the ordering field, imposed by the channel walls, and the magnitude of quenched disorder varies linearly with the mole fraction x and scales inversely proportionally with R for channel radii larger than 4 nm. The critical pore radius, R_{c}, separating a continuous from a discontinuous paranematic-to-nematic evolution varies linearly with x and differs negligibly between the silica and alumina membranes. We find no hints of preferred adsorption of one species at the channels walls. By contrast, a linear variation of the nematic-to-paranematic transition point T_{PN} and of the nematic ordering field σ versus x suggests that the binary mixtures of cyanobiphenyls 6CB and 7CB keep their homogeneous bulk stoichiometry also in nanoconfinement, at least for channel diameters larger than ∼7 nm.

Molecular ordering of the discotic liquid crystal HAT6 confined in mesoporous solids

An optical polarimetry study of the orientational order of a discotic liquid crystal (DLC), 2,3,6,7,10,11-hexakis(hexyloxy)triphenylene (HAT6), confined into parallel-aligned cylindrical nanochannels of mesoporous alumina, silica, and silicon membranes as a function of temperature and channel radius (3.4–21.0nm) is presented. In contrast to the bulk state, the birefringence resulting from the confined discotic liquid crystal, exhibits a continuous temperature evolution and is positive indicating a dominating face-on (homeotropic) type molecular ordering with respect to the channel walls. Pronounced hysteresis effects observed in the experiment are traced to different nucleation sites of the low-temperature phase upon cooling and the high-temperature phase upon heating. Deviations from the Gibbs–Thomson scaling of the phase transition temperature for channel diameters below 10nm are traced to splay distortions of the confined liquid crystal.

Spatial Variation of Molecular Dynamics in the Nanoconfined Glass-Former Methanol

We report filling-fraction-dependent dielectric measurements on methanol confined in an array of parallel-aligned channels of 8 nm diameter and 100 μm length in a monolithic mesoporous silica membrane. For channel fillings up to 43%, the channel walls are covered with films of average thickness from 0 to 1 nm. Upon filling, the average relaxation time decreases by three orders of magnitude, and the apparent glass-transition temperature Tg decreases from 99 to 90 K. The local relaxation times τΔf of the molecules added in each adsorption step even change by 3.5 orders of magnitude. Analyzing the distribution of relaxation times, we show that added molecules alter the dynamics of those previously deposited. The speeding up of molecular dynamics upon filling hints to spatial heterogeneity and is in accordance with two scenarios, which are experimentally undistinguishable: (i) heterogeneity of the inner channel surface, where molecules are at first attached to strong adsorption sites and only subsequently to ...

“Brick-and-mortar” synthesis of free-standing mesoporous carbon nanocomposite membranes as supports of room temperature ionic liquids for CO2−N2 separation

Free-standing mesoporous carbon−graphitic carbon nanocomposite membranes with controllable pore size (7.3−11.3nm) were synthesized by the “brick-and-mortar” method, carbon black (CB) as “bricks” and soft-templated phenolic resin-based mesoporous carbon (MC) as the “mortar”. Immobilization of imidazolium-based room temperature ionic liquids ([Cnmim][Tf2N], n=2, 4, and 6) in the MC−CB membranes produced a series of supported ionic liquid membranes (SILMs) that are permselective for separation of CO2−N2 gas pair. Strong capillary forces resulting from the well-developed mesoporosity of the MC−CB membranes greatly enhanced the stability of the supported ionic liquids. This enabled the SILMs to operate under transmembrane pressures as high as 1000kPa without degrading their separation performance. This makes it possible to apply SILMs to high-pressure CO2 capture and separation processes, where high transmembrane pressure would greatly increase the permeate flux through the membranes.