Mixed Matrix Membranes For Gas Separation Research Articles

Morphological Study before and after Thermal Treatment of Polymer-Polymer Mixed-Matrix Membranes for Gas Separations.

A good integration of the polymer materials that form a mixed-matrix membrane (MMM) for gas separation is essential to reaching interesting permselective properties. In this work, a porous polymer network (PPN), obtained by combining triptycene and trifluoroacetophenone, has been used as a filler, which was blended with two o-hydroxypolyamides (HPAs) that act as polymer matrices. These polymer matrices have been thermally treated to induce a thermal rearrangement (TR) of the HPAs to polybenzoxazoles (β-TR-PBOs) through a solid-state reaction. For its structural study, various techniques have been proposed that allow us to undertake a morphological investigation into the integration of these materials. To access the internal structure of the MMMs, three different methods were used: a polishing process for the material surface, the partial dissolution of the polymer matrix, or argon plasma etching. The argon plasma technique has not only revealed its potential to visualize the internal structure of these materials; it has also been proven to allow for the transformation of their permselective properties. Force modulation and phase contrast in lift-mode techniques, along with the topographic images obtained via the tapping mode using a scanning probe microscope (SPM), have allowed us to study the distribution of the filler particles and the interaction of the polymer and the filler. The morphological information obtained via SPM, along with that of other more commonly used techniques (SEM, TGA, DSC, FTIR, WASX, gas adsorption, and permeability measurements), has allowed us to postulate the most probable structural configuration in this type of system.

Print-Assisted Layer - by - Layer Fabrication of Mixed-Matrix Membrane for Gas Separation

The process intensification principles of optimizing the driving forces/resistances at every scale and the spatial domain were investigated using a print-assisted technique. The investigation was done by using a LaserJet printer to fabricate structured mixed-matrix membranes for gas separation. The physico-chemical properties of the fabricated membranes were evaluated using FTIR, and XRD. The microstructure of the membrane was evaluated using Scanning Electron Microscope (SEM) and optical microscope. The separation performance was evaluated using constant-pressure gas permeation test. The capability of the membrane to separate a mixture of two gases were tested using CO2 and CH4. The results from optical microscope analysis showed that membrane fabricated using print-assisted method possessed a well-structured morphology which can enhance the predictability of their performance. The gas permeations results showed that the transport mechanism of the membranes changed from Knudsen to molecular sieve as the number of print layers was increased from 0 to 7 layers. Overall, the fabricated membrane showed some potentials for separating CO2/CH4 mixture.

Incorporating Carbon Nanotubes in Nanocomposite Mixed-Matrix Membranes for Gas Separation: A Review.

Carbon nanotube (CNT) is a prominent material for gas separation due to its inherent smoothness of walls, allowing rapid transport of gases compared to other inorganic fillers. It also possesses high mechanical strength, enabling membranes to operate at high pressure. Although it has superior properties compared to other inorganic fillers, preparation of CNTs into a polymer matrix remains challenging due to the strong van der Waals forces of CNTs, which lead to agglomeration of CNTs. To utilize the full potential of CNTs, proper dispersion of CNTs must be addressed. In this paper, methods to improve the dispersion of CNTs using functionalization methods were discussed. Fabrication techniques for CNT mixed-matrix membrane (MMM) nanocomposites and their impact on gas separation performance were compared. This paper also reviewed the applications and potential of CNT MMMs in gas separation.

Functionalized two-dimensional g-C3N4 nanosheets in PIM-1 mixed matrix membranes for gas separation

The virtuous of two-dimensional (2D) nanomaterials in high aspect ratio and tunable surface functionality have geared towards their implementation in mixed matrix membranes (MMMs) for advanced gas separation. In this study, the effects of functionalized 2D nanosheets in the MMMs for gas separation were investigated. The graphitic carbon nitride (g-C3N4) nanosheets were modified with various functional groups namely sulfuric acid group, aliphatic amino group, aromatic amino group, and sulfonic group via four different synthesis approaches. Polymers of intrinsic microporosity (PIMs, e.g., PIM-1) was selected as the continuous polymer phase due to its intrinsic high permeability and comparable selectivity towards different gas pair. The sulfonic acid functionalized g-C3N4 MMMs (e.g., PIM-1/g-C3N4-D) imparted high separation properties ascribed to the great CO2 affinity induced by the sulfonic acid groups. In particular, PIM-1/g-C3N4-D (99:1) MMM demonstrated promising CO2 separation performance, with CO2 permeability of 3740 Barrer and CO2/N2 selectivity of 19.8. Moreover, at 5 wt% loading of g-C3N4-D, the H2/N2 and O2/N2 separation of the MMMs had exceeded the 2008 Robeson upper bound, thanks to the periodic triangular ultramicropores in g-C3N4 that favor precise sieving of small gases. These results pave the way in using the developed membranes in practical H2 purification, air separation and CO2 capture.

Improved dispersion performance and interfacial compatibility of covalent-grafted MOFs in mixed-matrix membranes for gas separation

Mixed-matrix membranes (MMMs) have received much attention due to their processable advantages of polymer and high permeability and/or selectivity of porous metal-organic frameworks (MOFs) fillers. However, the interfacial defects caused by poor interaction between MOFs with polymers and the agglomeration phenomenon caused by uneven dispersion of MOFs are common problems in mixed-matrix membranes. Currently, the priming protocol is one of solutions to the above problems, but it cannot precisely regulate the dispersion of particles and the interfacial compatibility between two phases. Herein, covalent grafting of polyimide 6FDA-Durene onto the surface of UiO-66-NH2 can mitigate the aggregation of fillers inside the polymeric matrices and improve the interfacial interaction between two phases, thus significantly improving the CO2/CH4 separation performance on the as-synthesized MMMs. The explored gas transport mechanism indicated that the improved separation was due to the raise of solubility selectivity. Furthermore, the stronger covalent bond between fillers and polyimide than physical interaction of priming protocol also endows the improved anti-plasticization phenomenon for CO2/CH4 separation.

The Influence of Polymer Concentration and Formation Technique on Gas Transport and Gas Sorption Properties of Copolyetherimide-Based Composite Membranes Containing MIL-101 Filler

Composite mixed-matrix membranes for gas separation containing copolyetherimide Siltem® as the polymer matrix and metal-organic framework MIL-101 (10 wt %) as the active filler, were obtained using dry and wet-dry formation techniques. It has been found that the polymer concentration in the initial solution does not significantly affect the CO2 and CH4 permeability of the film membranes obtained by dry molding. The addition of MIL-101 increases the CO2/CH4 selectivity of the dry-formed membranes approximately by 2 times compared to the selectivity of the filler-free polymer membranes. The materials synthesized by the wet-dry formation possess increased permeability and inverted CO2/CH4 selectivity, which indicates a change in the gas transport mechanism. With the increase of polymer concentration, the selectivity of the membranes obtained by the wet-dry technique, increases significantly due to the formation of the dense selective layer.

Hydrothermal stable ZIF-67 nanosheets via morphology regulation strategy to construct mixed-matrix membrane for gas separation

Mixed matrix membranes (MMMs) based on metal-organic frameworks (MOFs) have proven to have high separation performance, which has great potential in separation applications. However, unsatisfactory hydrothermal stability is the Achilles' heel of MOFs in practical applications, which seriously hinders the development of MOF based MMMs. Under the advantage of crystal anisotropy, a morphology regulation strategy without post-treatment is presented for the first time to efficiently improve the stability of MOF fillers in aqueous solution. Contrary to the regular dodecahedron ZIF-67 crystals, the [211] oriented ZIF-67 nanosheets (NS) can maintain an ordered microporous framework integrally in the aqueous solution of membrane preparation. The exposed [211] crystal face exhibits enhanced stability due to a lower proportion of Co–N bond and higher surface atomic density. The flake morphology of ZIF-67 with economic incorporation (5.0 wt %) offers the MMMs significantly enhanced CO2 permeability of 139.4 barrer and CO2/N2 ideal selectivity of 73.2, improved by 50.7% and 76.0% compared with the pristine polymer membrane. This strategy provides a new idea for improving the hydrothermal stability of MOF materials and extends their separation applications under aqueous conditions.

Role of Filler Porosity and Filler/Polymer Interface Volume in Metal-Organic Framework/Polymer Mixed-Matrix Membranes for Gas Separation.

Metal-organic frameworks (MOFs) and inorganic fillers are frequently incorporated into mixed-matrix membranes (MMMs) to overcome the traditional trade-off in permeability ( P) and selectivity for pure organic polymer membranes. Therefore, it is of great interest to examine the influence of porous and nonporous fillers in MMMs with respect to the possible role of the polymer-filler interface, that is, the void volume. In this work, we compare the same MOF filler in a porous and nonporous state, so that artifacts from a different polymer-filler interface are excluded. MMMs with the porous MOF aluminum fumarate (Al-fum) and with a nonporous dimethyl sulfoxide solvent-filled aluminum fumarate (Al-fum(DMSO)), both with Matrimid as polymer, were prepared. Filler contents ranged from 4 to 24 wt %. Gas separation performances of both MMMs were studied by mixed gas measurements using a binary mixture of CO2/CH4 with gas permeation following the theoretical prediction by the Maxwell model for both porous and nonporous dispersed phase (filler). MMMs with the porous Al-fum filler showed increased CO2 and CH4 permeability with a moderate rise in selectivity upon increasing filler fraction. The MMMs with the nonporous Al-fum(DMSO) filler displayed a reduction in permeability while maintaining the selectivity of the neat polymer. A linear dependence of log P versus the reciprocal specific free fractional volume (sFFV) rules out a significant contribution from a void volume. The sFFV includes the free volume of the polymer and the MOF, but not the polymer-filler interface volume (so-called void volume). The sFFV for the MMM was calculated between 0.23 cm3/g for a 24 wt % Al-fum/Matrimid MMM and 0.12 cm3/g for a 24 wt % Al-fum(DMSO)/Matrimid MMM. The negligible effect of an interface volume is supported by a good matching of theoretical and experimental density of the Al-fum and Al-fum/(DMSO) MMMs which gave a specific void volume below 0.02 cm3/g, often even below 0.01 cm3/g.

Recent advances on mixed-matrix membranes for gas separation: Opportunities and engineering challenges

In the past decades, gas separation using polymeric membranes has received considerable attention and become one of the fastest growing research areas. However, existing polymeric membranes may not be able to keep up with the increasing separation needs for challenging gas mixtures such as N2/CH4 and light olefin/paraffin pairs on industrial scale due to their so-called permeability-selectivity bound. On the other hand, scaling-up issues poise huge challenges for highly permeable and highly selective inorganic membranes. Mixed-matrix membranes, composite membranes, provide an evolutionary solution to debottleneck the permeability-selectivity and scale-up issues currently faced by polymeric and inorganic membranes, respectively. Inorganic fillers in mixed-matrix membranes improve gas permeability and/or selectivity, outperforming polymeric membranes. Combined with relatively economical and simple scaling-up compared to inorganic membranes, mixed-matrix membranes could potentially be a next-generation membrane concept for gas separation applications. This review provides a brief summary on the recent progress in both flat sheet and hollow fiber mixed-matrix membranes with an emphasis on those made over the last five years. A separate section is dedicated to discussing engineering challenges transitioning from laboratory-scale to large-scale synthesis of mixed-matrix membranes. Finally, future prospects and perspectives in mixed-matrix membranes research are briefly outlined.

Origins and Evolution of Inorganic-Based and MOF-Based Mixed-Matrix Membranes for Gas Separations

Gas separation for industrial, energy, and environmental applications requires low energy consumption and small footprint technology to minimize operating and capital costs for the processing of large volumes of gases. Among the separation methods currently being used, like distillation, amine scrubbing, and pressure and temperature swing adsorption, membrane-based gas separation has the potential to meet these demands. The key component, the membrane, must then be engineered to allow for high gas flux, high selectivity, and chemical and mechanical stability at the operating conditions of feed composition, pressure, and temperature. Among the new type of membranes studied that show promising results are the inorganic-based and the metal-organic framework-based mixed-matrix membranes (MOF-MMMs). A MOF is a unique material that offers the possibility of tuning the porosity of a membrane by introducing diffusional channels and forming a compatible interface with the polymer. This review details the origins of these membranes and their evolution since the first inorganic/polymer and MOF/polymer MMMs were reported in the open literature. The most significant advancements made in terms of materials, properties, and testing conditions are described in a chronological fashion.

Particle-Size Effects on Gas Transport Properties of 6FDA-Durene/ZIF-71 Mixed Matrix Membranes

A series of 6FDA-Durene-ZIF71 mixed matrix membranes (MMMs) comprising three different ZIF-71 particle sizes (i.e., 30, 200, and 600 nm) have been fabricated to investigate the effects of particle size on the gas separation performance of MMMs. Compared to MMMs containing ZIF-71 particle size > 200 nm, ZIF-71 embedded MMMs with particle sizes of ≤ 200 nm exhibited significantly enhanced gas separation performance. The O2 and N2 permeability of MMMs comprising ZIF-71 with particle sizes ≤ 200 nm have higher permeability than the predicted ones using the Maxwell model. By considering the inefficiently packed polymer chain and the sieve-in-a-cage phenomena, the modified Maxwell model displayed comparable permeability over the experimental results suggesting the nonideal nature of MMMs. For the first time, it was found that 200 nm ZIF-71 generated the best MMM for gas separation, particularly for H2/CH4 systems. Therefore, a trade-off between ZIF-71 particle size vs gas separation performance must be taken in...

Tailoring Physical Aging in Super Glassy Polymers with Functionalized Porous Aromatic Frameworks for CO2 Capture

A series of chemically functionalized porous aromatic frameworks (PAFs) have been synthesized and deployed within mixed-matrix membranes for gas separation. This series of PAFs delivered for the first time simultaneous control of selective gas transport and physical aging within the membranes. New composites including native and metalated fullerenes were also prepared, and the composites exhibited exceptional increases in their porosity, which in turn resulted in ultrafast gas transport. CO2 permeability following PAF-1-Li6C60 infusion within poly(trimethylsilylpropyne) was as high as 50 600 Barrer, a 70% improvement. Remarkably, just 9% of this permeation rate diminished after 1 year of physical aging, compared to 74% in the native polymer. A series of characterization techniques revealed this phenomenon to be due to intercalation of polymer chains within the PAF pores, the strength of which is controlled by the levels of chemical functionalization and porosity. The membranes were exploited for gas separ...

Preparation and Characterization of C60‐Filled Ethyl Cellulose Mixed‐Matrix Membranes for Gas Separation of Propylene/Propane

AbstractMixed‐matrix membranes (MMMs) consisting of ethyl cellulose as continuous matrix and inorganic particle C60 as dispersed phase were prepared for propylene/propane separation. The impact of the C60 content on the separation properties of MMMs without and with ultraviolet cross‐linking was investigated. The increment of decomposition temperature and single glass temperature of ethyl cellulose/C60 MMMs indicates a strong interfacial interaction between polymer and fullerenes. After UV irradiation, the gas permeability coefficient of propylene and ideal separation factor of propylene/propane decreased, and new features appeared in scanning electron microscopy and atomic force microscopy images, testifying the photopolymerization reaction of C60 at a depth near to the surface. C60 could be acted as a possible replaced carrier for the separation of olefin/paraffin using membrane separation technology.

Surface Cross-Linking of ZIF-8/Polyimide Mixed Matrix Membranes (MMMs) for Gas Separation

The demand for cost-efficient separations requires membranes with high gas flux and high selectivity which opens the path for further improvements. Mixed matrix membranes (MMMs) made from 33.3 wt % ZIF-8 in 6FDA-durene were tested at 35 °C and 3.5 atm. At 33.3 wt % loading of ZIF-8, H2, N2, O2, and CH4 gas permeabilities increased approximately 400%. Cross-linking the surface of this MMM, by reacting with ethylenediamine vapor, yielded a 10-fold increase in H2/CO2, H2/N2, and H2/CH4 selectivities with respect to 6FDA-durene, preserving 55% of the H2 permeability of 6FDA-durene. The permselective properties of the cross-linked skin of the MMM fall above the most recent permeability–selectivity trade-off lines (2008 Robeson upper bounds) for H2/CO2, H2/N2, and H2/CH4 separations. To the best of our knowledge, this is the first example of a cross-linked ZIF/polymer MMM for gas separation.

Challenges and opportunities for mixed-matrix membranes for gas separation

Mixed-Matrix Membranes (MMMs) combining the benefits of both polymeric and inorganic materials have become a focus for the next-generation gas separation membranes. In this review, major challenges surrounding MMMs and the strategies to tackle these challenges are given in detail. The selection criteria of polymeric and inorganic materials are discussed in terms of their physical and chemical compatibility as well as large scale fabrication issues. Major models for separation performance prediction of MMMs are reviewed in terms of their interrelations and limitations. A discussion is provided regarding the future direction of MMMs.

Preparation and characterization of ITQ-29/polysulfone mixed-matrix membranes for gas separation: Effect of zeolite composition and crystal size

This work concerns the preparation and characterization of ITQ-29 zeolite crystals with high Si/Ge ratio (100–∞) and different particle size for obtaining mixed matrix membranes. The Si/Ge molar ratio and seeding content of the synthesis gel appeared to have an effect on the final crystal size, and particles of 2.5μm were obtained for pure silica composition with good crystallinity. These were introduced, at 4, 8 and 12wt% loadings, into a commercial polysulfone matrix to prepare mixed matrix membranes that showed promising results in the separation of H2/CH4 mixtures (highest H2 permeability 21.9 Barrer and a separation factor of 118 for the 4wt% ITQ-29/polysulfone membrane). The thermal treatment of the membranes and the type of solvent were also optimized to provide good interaction between the zeolite and the polymer, limiting aggregation of the particles in the matrix, and removing all residual solvent that hinders gas permeation performance.

Development of a Mechanistic Model for Mass Transfer in Sorption Selective Mixed-Matrix Membranes for Gas Separation

Development of a Mechanistic Model for Mass Transfer in Sorption Selective Mixed-Matrix Membranes for Gas Separation

Incorporation of Hybrid Crystalline Microporous Materials in Mixed-Matrix Membranes for Gas Separations

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Mixed-matrix membranes containing MOF-5 for gas separations

Metal–organic framework 5 (MOF-5) nanocrystals with a high surface area (3000 m 2/g) and high thermal stability (up to 400 °C) were synthesized and added to Matrimid ® to form mixed-matrix membranes for gas separations. Scanning electron microscopy (SEM) images of the membrane cross-sections revealed significant plastic deformation of the polymer matrix owing to the strong affinity between the MOF-5 and Matrimid ®. At 30% MOF-5 loading, the permeabilities of the gases tested increased 120% while the ideal selectivities remained constant compared to Matrimid ®. Residual gas analysis of permeates of gas blends with different mixture ratios revealed an increase in selectivity for CH 4.