Deterioration of metal–organic framework crystal structure during fabrication of poly(l‐lactic acid) mixed‐matrix membranes

Poly(l ‐lactic acid) (PLLA) and metal–organic framework (MOF) mixed‐matrix membranes were prepared by melt extrusion of PLLA with 5% (w/w) of either activated or water‐saturated Cu₃(BTC)₂ (Cu₃(C₉H₃O₆)₂(H₂O)₃·xH₂O, HKUST‐1). The morphology and the stability of injection‐molded samples were evaluated using thermogravimetric analysis, differential scanning calorimetry, gel permeation chromatography, X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The presence of activated and saturated MOF crystals increased the cold crystallization onset temperature as compared to neat PLLA. This can be attributed to the MOF crystals incorporated in the PLLA matrix, which decreased the mobility of PLLA and thus impeded the crystallization process. According to the XRD results, the activated MOF crystals were successfully incorporated into the PLLA matrix without altering the crystal structure of the MOF. Moreover, the findings from permeability and tensile tests as well as SEM imaging indicated good interfacial interactions between PLLA and activated MOF. However, during melt extrusion of PLLA with saturated MOF, water molecules from the saturated MOF altered the MOF crystal structure and contributed to the degradation of the PLLA polymer by reducing its molecular weight by around 21%. © 2013 Society of Chemical Industry     

Gas‐permeation performance of metal organic framework/polyimide mixed‐matrix membranes and additional explanation from the particle size angle

With MOFs of Cu₃(BTC)₂ and ZIF‐8 as the dispersed phases and four polyimides with CO₂ permeabilities ranging from 1.36 to 564 barrer as the continuous phase, the influence of metal organic frameworks on the gas‐separation properties of mixed‐matrix membranes (MMMs) was investigated. The results show that the gas permeabilities of all of the prepared MMMs greatly increased and even largely exceeded the predicted value of the Bruggeman model; for example, with the same Cu₃(BTC)₂ loading of 21.3 vol %, the O₂ permeability increase rate of our prepared Cu₃(BTC)₂/Matrimide 5218‐20 MMMs was 2.26 times, whereas that predicted by the Bruggeman model was only 1.05 times. In addition, when the gas permeability of the polymeric phase was far lower than the dispersed phase of ZIF‐8 or Cu₃(BTC)₂ compared with ZIF‐8, which had a particle size (R) around 150 nm, Cu₃(BTC)₂ of 5–15 µm showed a little better enhancing effect on the gas‐permeation performance of the MMMs. In addition to the properties of the dispersed and continuous phases, we speculated that the ratio between R of the dispersed phase to the membrane thickness (L) played an important role for MMMs; the larger R/L was, the greater the gas permeability of the MMMs was. This speculation was initially evidenced by the ZIF‐8/ODPA/TMPDA‐20 MMMs with different Ls. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45728.     

Enhanced gas separation properties of metal organic frameworks/polyetherimide mixed matrix membranes

Metal organic frameworks (MOFs) are supposed to be ideal additives for mixed matrix membranes (MMMs). In this article one kind of MOFs, Cu₃(BTC)₂, is synthesized, then directly incorporated into a model polymer (Ultem®1000) using N,N‐dimethylacetamide as solvent. Cu₃(BTC)₂ particles are uniformly dispersed and there are no interfacial defects in the prepared MMMs when Cu₃(BTC)₂ loading is not more than 35 wt %, seen in SEM images. Pure gas permeation tests show that gas permeability increases obviously with Cu₃(BTC)₂ loading increase, while ideal selectivities of CO₂/N₂ and CO₂/CH₄ are almost unchanged. For MMM with the best separation property, CO₂ permeability increases about 2.6 times and CO₂/N₂ selectivity remains almost unchanged. Results about gas diffusivity and solubility indicate that gas diffusivity and solubility make contribution to gas permeability increase at the same time but in different ways. Gas permeation properties of MMMs are well predicted by Maxwell or Bruggeman model. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40719.     

Fabrication of mixed‐matrix membranes with MOF‐derived porous carbon for CO2 separation

Mixed‐matrix membranes (MMMs) have shown great advantages but still face some challenges, such as the trade‐off between permeability and selectivity, stability, and the lack of efficient ways to enhance them simultaneously. Here, the fabrication of MMMs with metal‐organic frameworks derived porous carbons (MOF‐PCs) as fillers which exhibit selective‐facilitating CO₂ transport passage originating from interactions between fillers and CO₂ is showed. With the aid of the developed multicalcination method, MOF‐PCs with variable N‐contents were prepared and incorporated into PPO‐PEG matrix for the first time to prepare MMMs, which show excellent separation performance for CO₂/CH₄ mixture with a tunable separation performance by combining different N‐contents and surface areas of MOF‐PCs. Moreover, the developed MMMs have hydrothermal and chemical stability. This work not only presents a series of MMMs with both good separation properties and stability, it also provides useful information for guiding the fabrication of high performance MMMs for practical application. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3400–3409, 2018     

Effect of the ZIF‐8 Distribution in Mixed‐Matrix Membranes Based on Matrimid® 5218‐PEG on CO2 Separation

In theory, the combination of inorganic materials and polymers may provide a synergistic performance for mixed‐matrix membranes (MMMs); however, the filler dispersion into the MMMs is a crucial technical parameter for obtaining compelling MMMs. The effect of the filler distribution on the gas separation performance of the MMMs based on Matrimid®‐PEG 200 and ZIF‐8 nanoparticles is demonstrated. The MMMs were prepared by two different membrane preparation procedures, namely, the traditional method and non‐dried metal‐organic framework (MOF) method. In CO₂/CH₄ binary mixtures, the MMMs were tested under fixed conditions and characterized by various methods. Finally, regardless of the MMM preparation procedure, the incorporation of 30 wt % ZIF‐8 nanoparticles allowed to increase the CO₂ permeability in MMMs. The ZIF‐8 dispersion influenced significantly the separation factor.     

Effect of MIL‐53 (Al) MOF particles on the chain mobility and crystallization of poly(L‐lactic acid)

Polymer‐filler interactions significantly influence morphology, functionality, and various desirable properties of mixed matrix membranes (MMMs). In this study, chain mobility and crystallization of poly(l ‐lactic acid) (PLLA) MMM films prepared by solvent casting PLLA with 1, 5, 10, and 20% wt/wt of MIL‐53(Al) metal organic framework (MOF) were evaluated. The fabricated MMMs were characterized using differential scanning calorimetry, Fourier transform infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. Differential scanning calorimetry studies indicated that the addition of MOF particles in the PLLA matrix reduces the polymeric chain mobility, which affects the crystallization process. The percent crystallinity of neat PLLA was found to decrease from 4% in neat PLLA to completely amorphous structures in PLLA‐10% and PLLA‐20% MMMs, as observed in the second heating cycle. Fourier transform infrared spectroscopy data supports these observations. Thermogravimetric analysis results showed that PLLA‐MOF films are thermally less stable than neat PLLA suggesting that MOF particles act as a depolymerization catalyst for PLLA. Partial agglomeration of MOF particles was observed in the samples using scanning electron microscopy studies. This study indicates strong PLLA‐MIL‐53(Al) MOF interactions. In addition, this study also provides insight into the effect of MOF particles on the segmental mobility and morphology of PLLA‐MIL‐53 (Al) composite films. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45690.     

Matrix effect of mixed‐matrix membrane containing CO2‐selective MOFs

Facilitated mixed‐matrix membranes (MMMs) containing Cu‐metal organic frameworks (Cu‐MOFs) with high CO₂ selectivity on an asymmetric polysulfone support were fabricated and examined the effect of gas separation performance using different matrices. An amorphous poly(2‐ethyl‐2‐oxazoline) (POZ) and semicrystalline poly(amide‐6‐b‐ethylene oxide) (PEBAX^®MH 1657) block copolymer were chosen as the polymeric matrix and the effect of the matrix on CO₂ separation for MMMs containing Cu‐MOFs was investigated. The interaction of CO₂ in different matrix was investigated theoretically using the density functional theory method, and it was found that the amide segment in PEBAX would contribute more to the CO₂ solubility than ether segment. The morphological changes were investigated by differential scanning calorimetry, field emission scanning electron microscope and X‐ray diffractometer. The ideal selectivity of CO₂/N₂ was enhanced significantly with the addition of a Cu‐MOF, and the values are higher in the Cu‐MOF/PEBAX MMM compared with that in a POZ based asymmetric MMM. Improvement in the CO₂/N₂ selectivity of a Cu‐MOF/PEBAX MMM was achieved via facilitated transport by the CO₂‐selective Cu‐MOFs due to both their high adsorption selectivity of CO₂ over N₂ and the decreased crystallinity of PEBAX due to the presence of the Cu‐MOFs, which would provide a synergic effect on the CO₂ separation. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42853.     

Underlying mechanism of the hydrothermal instability of Cu<Subscript>3</Subscript>(BTC)<Subscript>2</Subscript> metal-organic framework

Water induced decomposition of Cu₃(BTC)₂ (BTC = benzene-1,3,5-tricarboxylate) metal-organic framework (MOF) was studied using dynamic water vapour adsorption. Small-angle X-ray scattering, Fourier transform infrared spectroscopy and differential scanning calorimetry analyses revealed that the underlying mechanism of Cu₃(BTC)₂ MOF decomposition under humid streams is the interpenetration of water molecules into Cu-BTC coordination to displace organic linkers (BTC) from Cu centres.     

Effect of graphene oxide on the behavior of poly(amide‐6‐b‐ethylene oxide)/graphene oxide mixed‐matrix membranes in the permeation process

Polyether‐block‐amide (Pebax)/graphene oxide (GO) mixed‐matrix membranes (MMMs) were prepared with a solution casting method, and their gas‐separation performance and mechanical properties were investigated. Compared with the pristine Pebax membrane, the crystallinity of the Pebax/GO MMMs showed a little increase. The incorporation of GO induced an increase in the elastic modulus, whereas the strain at break and tensile strength decreased. The apparent activation energies (E_p) of CO₂, N₂, H₂, and CH₄ permeation through the Pebax/GO MMMs increased because of the greater difficulty of polymer chain rotation. The E_p value of CO₂ changed from 16.5 kJ/mol of the pristine Pebax to 23.7 kJ/mol of the Pebax/GO MMMs with 3.85 vol % GO. Because of the impermeable nature of GO, the gas permeabilities of the Pebax/GO MMMs decreased remarkably with increasing GO content, in particular for the larger gases. The CO₂ permeability of the Pebax/GO MMMs with 3.85 vol % GO decreased by about 70% of that of the pristine Pebax membrane. Rather than the Maxwell model, the permeation properties of the Pebax/GO MMMs could be described successfully with the Lape model, which considered the influence of the geometrical shape and arrangement pattern of GO on the gas transport. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42624.     

Toughening of poly(l-lactic acid) with Cu3BTC2 metal organic framework crystals

Poly(l-lactic acid) (PLLA) and metal organic framework (MOF) composites were prepared by melt extrusion of PLLA with 5, 10 and 20% w/w of activated Cu₃(BTC)₂ MOF. The morphology and stability of injection-molded samples were evaluated using thermogravimetric analysis, differential scanning calorimetry (DSC), gel permeation chromatography, X-ray diffraction, and scanning electron microscopy (SEM). The composites showed improved toughness during the tensile tests as compared to the neat PLLA matrix. Toughness mechanism of the composites was studied using SEM and rheological studies. SEM images indicated that cavitation induced by debonding at the interface of PLLA and MOF particles during the uniaxial stress was primarily responsible for the improved toughness of the composites. The SEM images of the composites, the solid like plateau observed in the PLLA composites during the parallel plate rheology at low frequency, and the decrease in the cold crystallization enthalpy during the developed composites indicate potential for various applications, which include gas separation, energy and active packaging.     

Mixed matrix membranes of Pebax1657 loaded with iron benzene‐1,3,5‐tricarboxylate for gas separation

Mixed matrix membranes offer major advantages in gas separation processes due to desirable properties found in both organic and inorganic membranes. In this study, a novel mixed matrix membrane was prepared for such application by incorporating iron benzene‐1,3,5‐tricarboxylate (Fe‐BTC) into the poly(amide‐6‐b‐ethylene oxide) (Pebax1657) polymer. Membranes with various loadings of 5, 10, and 20 wt% Fe‐BTC in the polymer matrix were fabricated to investigate the effect of filler loading on the membrane performance. Membranes, prepared by solution‐casting were characterized by scanning electron microscopy, thermogravimetric analysis, Fourier transform infrared, X‐ray diffraction, and tensile test. Pure gas separation of CO₂, CH₄, and N₂ and ideal gas selectivity of CO₂/CH₄ and CO₂/N₂ were performed and permeation tests were carried out under 4, 8, and 12 bar pressures. Results show that adding Fe‐BTC into the Pebax1657 matrix improved both permeability and selectivity of the filled membranes. For instance, 10 wt% loading of Fe‐BTC into the Pebax1657 matrix led to CO₂ permeability increase of 49% as well as CO₂/CH₄ and CO₂/N₂ selectivities enhancements of about 36% and 16%, respectively. POLYM. COMPOS., 38:1363–1370, 2017. © 2015 Society of Plastics Engineers     

Efficient CO2 Separation Using a PIM-PI-Functionalized UiO-66 MOF Incorporated Mixed Matrix Membrane in a PIM-PI-1 Polymer

Metal–organic framework (MOF) incorporated mixed–matrix membranes (MMMs) attract great interest for gas separation applications because they overcome limitations faced by typical polymer membranes, including permeability–selectivity trade-off, aging effect, and plasticization phenomenon. However, optimal MOF–polymer interface compatibility is the key challenge in fabricating defect-free high-performance gas-separation MMMs. Here, a surface modification strategy of the UiO-66-NH₂ MOF using a covalently bound PIM-PI-oligomer is developed to engineer interface compatibility with the polymer that has an identical chemical structure (PIM-PI-1) in the MMMs. A series of MMMs are prepared with different loadings of homogeneously distributed PIM-PI-functionalized MOFs (PPM). Significant improvements in CO₂/N₂ and CO₂/CH₄ selectivity and permeability are achieved with these MMMs, ranging from 5 to 10 wt% of the PPM loadings. The MMM with 10 wt% loading (PPM-10@MMM) shows a CO₂ permeability of 3827.3 Barrer and a CO₂/N₂ and CO₂/CH₄ selectivity of 24 and 13.4, respectively. This surpasses the 2008 Robeson upper bound for CO₂/N₂ and is very close to the 2008 upper bound for CO₂/CH₄. The experimental results are further compared using Maxwell's equation for MMMs. The resulting MMMs show a plasticization resistance against CO₂ up to 25 atm pressure and anti-aging performance for 180 h.     

Fabrication of Homogenous Polymer‐Zeolite Nanocomposites as Mixed‐Matrix Membranes for Gas Separation

Interfacial void‐free mixed‐matrix membranes (MMMs) of polyimide (PI)/zeolite were developed using 13X and Linde type A nano‐zeolites and tested for gas separation purposes. Fabrication of a void‐free polymer‐zeolite interface was verified by the decreasing permeability developed by the MMMs for the examined gases, in comparison to the pure PI membrane. The molecular sieving effect introduced by zeolite 13X improved the CO₂/N₂ and CO₂/CH₄ selectivity of the MMMs. Separation tests indicated that the manufactured nanocomposite membrane with 30 % loading of 13X had the highest permselectivity for the gas pairs CO₂/CH₄ and CO₂/N₂ at the three examined feed pressures of 4, 8 and 12 atm.     

Metal Organic Frameworks as Solid Acid Catalysts for Acetalization of Aldehydes with Methanol

Room temperature acetalization of aldehydes with methanol has been carried out using metal organic frameworks (MOFs) as solid heterogeneous catalysts. Of the MOFs tested, a copper‐containing MOF [Cu₃(BTC)₂] (BTC=1,3,5‐benzenetricarboxylate) showed better catalytic activity than an iron‐containing MOF [Fe(BTC)] and an aluminium containing MOF [Al₂(BDC)₃] (BDC=1,4‐benzenedicarboxylate). The protocol was validated for a series of aromatic and aliphatic aldehydes and used to protect various aldehydes into commercially important acetals in good yields without the need of water removal. In addition, the reusability and heterogeneity of this catalytic system was demonstrated. The structural stability of MOF was further studied by characterization with powder X‐ray diffraction, Brunauer–Emmett–Teller surface area measurements and Fourier‐transformed infrared spectroscopic analysis of a deactivated catalyst used to convert a large amount of benzaldehyde. The performance of copper MOF as acetalization catalyst compares favourably with those of other conventional homogeneous and heterogeneous catalysts such as zinc chloride, zeolite and clay.     

Electrochemical synthesis,characterization and application of a microstructure Cu<Subscript>3</Subscript>(BTC)<Subscript>2</Subscript> metal organic framework for CO<Subscript>2</Subscript> and CH<Subscript>4</Subscript> separation

The electrochemical route is a promising and environmentally friendly technique for fabrication of metal organic frameworks (MOFs) due to mild synthesis condition, short time for crystal growth and ease of scale up. A microstructure Cu₃(BTC)₂ MOF was synthesized through electrochemical path and successfully employed for CO₂ and CH₄ adsorption. Characterization and structural investigation of the MOF was carried out by XRD, FE-SEM, TGA, FTIR and BET analyses. The highest amount of carbon dioxide and methane sorption was 26.89 and 6.63 wt%, respectively, at 298 K. The heat of adsorption for CO₂ decreased monotonically, while an opposite trend was observed for CH₄. The results also revealed that the selectivity of the developed MOF towards CO₂ over CH₄ enhanced with increase of pressure and composition of carbon dioxide component as predicted by the ideal adsorption solution theory (IAST). The regeneration of as-synthesized MOF was also studied in six consecutive cycles and no considerable reduction in CO₂ adsorption capacity was observed.     

Influence of copper oxide nanomaterials in a poly(ether sulfone) membrane for improved humic acid and oil–water separation

In this study, self‐synthesized copper(I) oxide (Cu₂O) nanoparticles were incorporated in poly(ether sulfone) (PES) mixed‐matrix membranes (MMMs) through the phase‐inversion method. A cubic arrangement and crystallite size of 28 nm was identified by transmission electron microscopy and X‐ray diffraction (XRD) for the as‐synthesized Cu₂O particles. The pristine PES membrane had a higher contact angle value of 88.50°, which was significantly reduced up to 50.10° for 1.5 wt % PES/Cu₂O MMMs. Moreover, XRD analysis of the Cu₂O‐incorporated PES membrane exhibited a new diffraction pattern at 36.46°. This ensured that the Cu₂O nanoparticles were distributed well in the PES matrix. Interestingly, the water permeability progressively improved up to 66.72 × 10^?9 m s^?1 kPa^?1 for 1.5 wt % PES/Cu₂O MMMs. Furthermore, the membrane performances were also evaluated with different feed solutions: (1) bovine serum albumin, (2) humic acid, and (3) oil–water. The enhanced rejection and lower flux reduction percentage were observed for hybrid membranes. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43873.     

Mixed matrix membranes containing polymer-embedded metal-organic framework microspheres

The controlling filler aggregation and strengthening interfacial interaction are of great scientific significance for mixed matrix membranes (MMMs). In this study, the polymer-embedded metal-organic framework (pMOF) microspheres (MSs) are designed by one-pot synthesis and employed as microfillers for improving separation performance of MMMs. Through adding polymer during solvothermal crystallization, the polymer chains are embedded into the MOF materials, and the morphologies of the MOFs are transformed from nanopaticles to polycrystalline MSs. Since the embedding of the identical polymer promotes the compatibility of polymeric matrixes and fillers, as well as the micrometer-sized porous MSs offer additionally superior and permanent transport pathways, the resulted MMMs display simultaneously enhanced selectivity and permeability for carbon capture. The CO₂/CH₄ selectivity and CO₂ permeability of the pMOF MMMs are achieved at 1.3 and 2.2 times as those of the pure polymeric membranes, and 1.5 and 1.2 times as those of the MOF MMMs, respectively.     

Improved gas transport properties of polyurethane–urea membranes through incorporating a cadmium-based metal organic framework

The increasing need for more efficient separation processes has motivated the development of polymer membranes that can provide fast and selective transport. In this work, cadmium-based metal–organic framework (MOF) nanoparticles and a polyurethane–urea (PUU) elastomer were synthesized. New mixed-matrix membranes (MMMs) were then fabricated from the nanoparticles and the PUU. SEM images verified that embedding the nanoparticles changes the morphology of the PUU and the nanoparticles disperse well in the PUU due to satisfactory compatibility of the polymer and nanoparticles. Fourier transform infrared spectroscopy and X-ray diffraction analysis confirmed the dispersion of the nanoparticles in the soft segment of the PUU. With increased temperature, gas permeabilities of the MMMs improved but their sieving ability deteriorated. An MMM incorporating 2.5 wt % of the MOF showed a CO₂ permeability of ~140 barrer and a CO₂/N₂ selectivity of ~30, which are 89 and 38% higher than those of the pristine membrane. Gas permeation tests showed that the higher CO₂/N₂ selectivity of the MMMs was due to improved solubility selectivity and the higher CO₂ permeability was a result of improved CO₂ diffusivity and solubility coefficients. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 137, 48704.     

Synthesis and properties of sulfonated biphenyl poly(ether sulfone) and its mixed‐matrix membranes containing carbon nanotubes for gas separation

We prepared mixed‐matrix membranes (MMMs) composed of carboxylated single‐walled carbon nanotubes (f‐SWCNTs) and a sulfonated biphenyl poly(ether sulfone) (S‐PPSU) polymer matrix. The thermal stability and properties of the pores of the S‐PPSU and f‐SWCNTs were characterized by thermogravimetric analysis and sorption isotherm curves, respectively; these showed that the surface and pore diameter decreased after the introduction of carboxyl groups to the single‐walled carbon nanotubes (SWCNTs), and the pore properties did not restore original values even when the f‐SWCNTs were preheated to 350 °C to remove carboxyl groups. The gas‐separation measurement showed that the MMMs comprised of the S‐PPSU and f‐SWCNTs possessed better gas‐separation properties than the ones composed of biphenyl poly(ether sulfone) and SWCNTs. The permeability for N₂, O₂, He, and CO₂ and the selectivity for O₂/N₂ and O₂/CO₂ were enhanced simultaneously because of the good dispersion of f‐SWCNTs and the improved interaction between the two phases. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44995.     

High speed spin coating in fabrication of Pebax 1657 based mixed matrix membrane filled with ultra-porous ZIF-8 particles for CO<Subscript>2</Subscript>/CH<Subscript>4</Subscript> separation

Modified ultra-porous ZIF-8 particles were used to prepare novel ZIF-8/Pebax 1657 mixed matrix membranes (MMMs) on PES support for separation of CO₂ from CH₄ using spin coating method. TEM and SEM were used to characterize modified ZIF-8 particles. SEM was also used to investigate the morphology of synthesized MMMs. The MMMs with thinner selective layer showed higher CO₂ permeability and lower CO₂/CH₄ selectivity in permeation tests compared to MMMs with thicker selective layer. The plasticization was recognized as the main reason for rise in CO₂ permeability and drop in CO₂/CH₄ selectivity of thinner MMMs. The gas sorption results showed that the high permeability of CO₂ in MMMs is mainly due to the high solubility of this gas in MMMs, leading to high CO₂/CH₄ solubility selectivity for MMMs. The fractional free volume and void volume fraction of MMMs increased as the thickness of membrane decreased. Applying higher mixed feed pressures and permeation tests temperatures resulted in increase in CO₂ permeability and decrease in CO₂/CH₄ selectivity. At highest testing temperature (60 °C), the CO₂ permeability of synthesized MMMs with thinner selective layer remarkably increased.