Mixed-Matrix Composite Membranes Based on UiO-66-Derived MOFs

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Applications of Polymer, Composite, and Coating Materials

Mixed-Matrix Composite Membranes Based on UiO-66-derived MOFs for CO2 Separation Hossein Molavi, and Akbar Shojaei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20869 • Publication Date (Web): 06 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Applied Materials & Interfaces

Mixed-Matrix Composite Membranes Based on UiO-66-derived MOFs for CO2 Separation Hossein Molavi1, Akbar Shojaei*1,2 1Institute

for Nanoscience and Nanotechnology (INST), Sharif University of Technology, P.O. Box 11155-8639, Tehran, Iran.

2Department

of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11155-9465, Tehran, Iran.

Corresponding author: Akbar Shojaei, Email: [email protected] Abstract We demonstrated a novel mixed matrix composite membrane (MMCM) based on acrylated polyurethane (APU) and UiO-66 nanoparticles to separate CO2/N2 mixture. UiO-66 and functionalized UiO-66 including NH2-UiO-66 and GMA-UiO-66 were loaded in APU/HEMA (APUH) matrix at variable concentrations between 3-30 wt%. APUH/ GMAUiO-66s MMCM exhibited strong adhesion with support layer of polyester/polysulfone (PS/PSF) which was not deteriorated after immersion in water for a long time (20 days). Incorporation of UiO-66 and its functionalized forms increased simultaneously permeability and CO2/N2 selectivity which was indeed superior in comparison with MMCMs reported previously. GMA-UiO-66 filled MMCM displayed a CO2 permeance of 14.5 Barrer and CO2/N2 selectivity of 53 at critical concentration (25 wt%). This attractive separation performance of APUH/UiO-66 offered an exciting platform for development of composite membrane in sustainable CO2 separations. Keywords: UV-curable acrylate polyurethane; Mixed-matrix composite membranes; Gas 1

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separation; Metal-organic framework; UiO-66. 1.

Introduction

Membrane technology is considered as an energy efficient and environment-friendly technique for the reduction of anthropological CO2 emissions, because of its prominent features such as low capital cost, easy scalability, simplicity of operation, compact system and reduced energy consumption in comparison to conventional amine-based CO2 scrubbing technologies.1,2 However, the main challenge associated with conventional polymeric membranes is an inherent trade-off between selectivity and permeability, which was first proposed by Robeson in 19913 and subsequently updated in 2008.4 To outstrip this behavior between permeability and selectivity, several kinds of membranes including composite membrane, facilitated transport membrane, thin film composite membrane, mixed-matrix membrane (MMM) and mixed-matrix composite membrane (MMCM) have been investigated extensively for CO2/N2 separations.5,6 Usually, MMMs consist of a polymer phase as continuous matrix and an inorganic phase as dispersed filler. In theory, these membranes possess the advantages of both polymer (facile processing, high performance and low cost) and inorganic filler (reasonable mechanical properties, favorable separation properties), so that, this type of membranes is susceptible to be exceeded the Robeson upper bound lines.7 Schematic of different forms of filled polymeric membranes are shown in scheme 1. As can be seen from case (a) of scheme 1, the filler particles are continuously dispersed in the whole cross section of conventional MMMs, while in MMCMs (case b in scheme 1) the filler particles are continuously dispersed only in the top selective layer. Actually, MMCM 2


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is made of a porous substrate which acts only as a mechanical support with thin dense layer which provides gas separation properties. These MMCMs provide considerable gas separation performance in comparison to conventional MMMs because of their outstanding properties such as low fabrication cost, easy processing, suitable mechanical properties, and rapid mass transfer because of lower gas trans-membrane resistance.8 Therefore, it can be expected that MMCMs can be effecinet system to achieve high permeance gas separation membranes. For instance, Fu et al.9 produced a defect-free, cross-linked and surface-confined thin film composite membrane for CO2 separation. The results demonstrated exceptional gas separation performance which approaches to the Robeson upper bound. Additionally, several recent studies have been carried out based on thin film composite membranes to improve their separation performance in gas and liquid phases.8,10-13

Scheme 1. Different types of mixed-matrix membranes, (a) conventional MMMs or symmetric membranes and (b) MMCMs or asymmetric membranes and SEM image of MMCM. It is to be noted that the major challenge in preparation of MMMs is unsuitable interfacial interactions between polymer matrix and nanoparticles, which causes many obstacles such as particle sedimentation and aggregation, defects and nonselective interfacial micro-voids, 3



and polymer chain rigidification leading to reduction of the gas separation performance of membranes and mechanical failure.14,15 Moreover, the main issue in fabricating MMCMs is improper interfacial adhesion between the mixed-matrix layer (selective layer) and the porous support layer which leads to detachment of the selective layer from the support layer because of poor physical adhesion and different swelling behaviors in exposure to humid stream. This later issue, swelling of membrane materials, is practically important; because the critical issue in CO2 separation is the presence of water in the feed stream.8 Therefore, in the present work, a novel high performance MMCM based on acrylateterminated polyurethane (APU) and UiO-66-derived MOF is presented for separation of CO2 from N2. This MMCM provides reasonable interfacial adhesion between the mixedmatrix layer as the selective layer and the support layer as well as excellent compatibly between filler nanoparticles and polymer chains. Additionally, UiO-66-derived MOFs were chosen as fillers because of their high water stability (more than 12 months),16 high thermal stability (up to 773K), good chemical resistance toward polar and organic solvents.17 A suitable polymeric membrane for CO2 separation should possess several characteristic features such as large CO2 permeability and great CO2 selectivity against other gases, longterm stability, high thermal and chemical stability, high resistance against plasticization effect of CO2 and cost effectiveness.18 Polyethylene glycol (PEG) is one of the most important CO2-philic polymeric membrane materials for separation of CO2 due to its inherent tendency toward CO2 molecules because of the strong quadrupole-dipole interaction between CO2 and ethylene oxide repeating unit2. However, the intrinsic crystallinity of PEG and its low CO2 permeability, particularly PEG with high molecular 4


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weights, restricts its capability to separate CO2 from other gases efficiently. Therefore, in order to reduce the crystallinity of PEG-based membranes several techniques have been proposed such as copolymerization, chemical crosslinking and physical blending with other amorphous polymers.2 Recently, we introduced APU diluted by suitable acrylate reactive diluent to obtain APU/acrylate-diluent (APUA) as efficient composite membrane materials for separation of mixture of gases.5 APUA exhibited many attractive characteristic features such as low viscosity and excellent processability, strong adhesion with support layer, capability to be cured with UV light at few seconds and good resistance to water swelling due to crosslinked structure which makes APUA as favorable candidate for practical composite membranes. However, neat APUA shows moderate permeability which should be improved as far as possible to make APUA much attractive alternative as membrane materials. In the present work we attempt to improve the gas separation performance of APUA by incorporation of UiO-66-derived MOFs while above mentioned benefits of APUA are retained. To do this, neat UiO-66 and functionalized UiO-66 with amine group (NH2-UiO66) and glycidyl methacrylate (GMA-UiO-66) are incorporated in APUA matrix to obtain MMM materials. Acrylated polyurethane can react with vinyl group of GMA-UiO-66 during the UV crosslinking of APUA. The overall view concerning the steps of reactions and chemical structures of the materials and the final MMCMs are depicted in scheme 2.

5



Scheme 2. Schematic representation of reaction steps for the synthesis of APU-g-GMAUiO-66 based membrane. 2.

Experimental

2.1. Materials Poly(ethylene glycol) (PEG) with molecular weight of 1000 g/mol was purchased from Merck, and dehydrated under vacuum oven at 80°C for 24 h before use. 2-hydroxyethyl methacrylate (HEMA) was purchased from Sigma-Aldrich, and dehydrated with a 4A molecular sieve. Toluene diisocyanate (TDI), 2-aminoterephthalic acid (2-ATA), dibutyltin dilaurate (DBTDL), chloroform, benzophenone, glycidyl methacrylate (GMA), dimethylformamide (DMF), terephthalic acid, tetrahydrofuran (THF), Nmethyldiethanolamine (MDEA), and zirconium (IV) chloride (ZrCl4) were obtained from Merck and Sigma–Aldrich, and used without further purification. Microporous polysulfone 6


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support was obtained from Sharif membrane technology center (SMTC). Carbon dioxide (CO2) and nitrogen (N2) were obtained from Roham Gas Co. (Tehran, Iran) with purities more than 99.8%. 2.1. Synthesis of MOFs UiO-66, NH2-UiO-66 and GMA-UiO-66 were synthesized based on the procedure reported in our previous studies,6,17,19-21 (see also supporting information). 2.2. Synthesis of acrylate polyurethane (APU) APU-1000 was synthesized by PEG: TDI: HEMA with mole ratio of 1:2:2 according to the two-step polymerization method described previously,5 (see also section S.3 in supporting information). 2.3. Preparation of mixed-matrix composite membranes (MMCMs) Desirable amount of MOF nanoparticles, i.e. UiO-66, NH2-UiO-66, and GMA-UiO-66, was dispersed in HEMA (70% of total used HEMA) through stirring vigorously for 12 h to achieve a homogenous mixture. Moreover, required amount of as-synthesized APU-1000 was also dissolved in HEMA (30% of total used HEMA) and stirred vigorously at slightly above ambient temperature until a clear and homogenous solution was obtained. The HEMA/MOF mixture and HEMA/APU mixture was stirred overnight and then mixed to obtain APU/HEMA/MOF composites. In these compositions, HEMA acts as reactive diluent for APU to reduce the viscosity and control the final crosslinking density of the samples. Therefore, the mixture of APU and HEMA reactive diluent (APU/HEMA) is abbreviated by APUH hereafter. The APU/HEMA ratio was kept to be 70/30 by weight in all APUHs; whereas, the content of MOFs was variable. Then MDEA as accelerator (2 wt 7



%) and benzophenone as initiator (2 wt %) were added to the APUH/MOF solution and stirred vigorously for 3 h. Finally, to remove air bubbles, the homogenous solution was degassed under vacuum and then cast on a flat-sheet microporous polysulfone support with a metal knife. The APUA selective layer was irradiated with a 1000 W-UV (365 nm) lamp for specified time.5 To comparison purposes, neat APUH composite membrane was prepared by the above mentioned procedure, without using MOFs. The average thickness of the prepared MMCMs measured by a micrometer caliper at five different pointes (160-180 μm) is found to be almost 20-40 μm. The thickness of selected MMCMs was also characterized by FESEM images (see Fig. S5). The composition of the mixture for preparing different MMCMs is given in Table 1. To investigate the influence of vinyl group of GMA in GMA-UiO-66 nanoparticles on the crosslinking reaction, two sets of samples containing GMA-UiO-66 with and without HEMA reactive diluent were also prepared. The samples with no reactive diluent were in fact the mixture of APU with GMA-UiO-66 at various loading which were abbreviated hereafter by w-MMCM. Meanwhile the neat APU without GMA-UiO-66 nanoparticles and no HEMA reactive diluent was abbreviated by w-APU.

8


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Table 1. Chemical composition of the selective layer of MMCM synthesized in this study. Sample code APUH w-APU MMCM-1 MMCM-2 MMCM-3 MMCM-4 MMCM-5 m-MMCM-1 m-MMCM-2 m-MMCM-3 m-MMCM-4 m-MMCM-5 aThe

Fillers UiO-66 UiO-66 UiO-66 UiO-66 UiO-66 NH2-UiO-66 NH2-UiO-66 NH2-UiO-66 NH2-UiO-66 NH2-UiO-66

Loading (%)a 3 6 10 12 15 3 6 10 12 15

Sample code

Fillers

Loading (%)a

m-MMCM-6

NH2-UiO-66

20

g-MMCM-1 g-MMCM-2 g-MMCM-3 g-MMCM-4 g-MMCM-5 g-MMCM-6 w-MMCM-1 w-MMCM-2 w-MMCM-3 w-MMCM-4

GMA-UiO-66 GMA-UiO-66 GMA-UiO-66 GMA-UiO-66 GMA-UiO-66 GMA-UiO-66 GMA-UiO-66 GMA-UiO-66 GMA-UiO-66 GMA-UiO-66

6 12 15 20 25 30 6 12 15 20

percent of MOF loading was determined using the following equation:22

MOF loading =

[

MMOF

] × 100

MMOF + MPolymer

where MMOF and MPolymer refer to the mass of MOF nanoparticles and polymer, respectively, in which MPolymer is equal to the mass of APUH. 2.4. Gas adsorption measurements The CO2 adsorption capacity of the MOFs was evaluated using an in-house experimental apparatus setup, which is shown schematically in Fig. S1 (see the supporting information) and the detailed experimental procedure was described elsewhere.17,19 2.5. Gas permeation measurement An in-house constant volume/variable pressure apparatus (see Fig. S2) was used to determine the single gas permeability (P), diffusion (D) and solubility (S) coefficients based on time-lag (θ) method using the following equations based on experimental details mentioned in supporting information as:6,22,23 V × L × T0

dp

(1)

P = A × Pf × P0 × T1 dt L2

D = 6θ

(2) 9



P

(3)

S=D

where P is the gas permeability presented in Barrer (1 Barrer = 10-10 cm3 (STP) cm cm-2 s-1 cmHg-1), V (cm3) is the downstream volume, L (cm) is the membrane thickness, A (cm2) is the effective membrane area, Pf (cmHg) is the upstream pressure, P0 and T0 are the pressure and temperature, respectively, at standard state which are equal to 76 cmHg and 273.15 K, T (K) is the operating temperature and dp/dt (cmHg/s) is the permeation rate.5,23 The ideal selectivity (α) for two gases was calculated as:5 Pi

Di

Si

αij = Pj = Dj × Sj

(5)

where Di/Dj is diffusivity selectivity while Si/Sj is solubility selectivity. 2.6. Characterization Attenuated total reflectance Fourier transform infrared (FTIR-ATR) spectra (Spectrum 100FT-IR Spectrometer, Perkin-Elmer) were used to characterize the synthesized MOFs and the prepared MMCMs. X-ray diffraction (XRD) (X-pertPhilips, pw 3040/60) analysis was used to record the crystallinity of synthesized MOFs and prepared MMCMs. The glass transition temperatures (Tg) of MMMs were measured by differential scanning calorimeter (DSC, TA Instruments Q100, USA) with temperature scanning rate of 5°C/min from -80°C to 150°C under N2 flow. Thermo gravimetric analyses (TGA) were carried out under N2 flow and a heating rate of 10°C/min using a Perkin-Elmer Pyris thermo gravimetric analyzer. Transmission electron microscope (Philips CM 120) with an accelerating voltage of 120 KV was used to investigate the dispersion of functionalized nanoparticles into polymer matrix. For TEM observation of MMCMs, the ultrathin slices of selective layer containing MOF nanoparticles with thicknesses of about 100 nm were cut by an 10


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ultramicrotome and transferred on a carbon-coated copper grid. Field emission scanning electron microscope (FESEM – MIRA3 TESCAN) equipped with energy dispersive X-ray spectroscopy (EDX) was used to characterize the morphologies of the synthesized MOFs, the cross section of prepared MMCMs, and elemental mapping of the MMCMs. 3.

Results and Discussion

3.1. Characterization of MOFs The prepared MOF nanoparticles were characterized by CO2 adsorption/desorption, BET, XRD, TGA, FESEM, ICP-OES, TEM, 1H NMR, mass spectroscopy, FTIR and elemental analysis, which their results were described in our previous studies.6,16,17,19-21 The XRD, BET and SEM analyses of UiO-66 indicated that the structure of this MOF was nearly intact after stirring in water for 12 months. Additionally, the results indicated that the structures of functionalized NH2-UiO-66 (GMA-UiO-66 and EDA-UiO-66) were preserved after functionalization with glycidyl methacrylate and ethylenediamine.6,17,19-21 3.2. Characterization of mixed-matrix composite membranes 3.2.1. FTIR-ATR analysis To determine the possible physical and chemical interactions between MOF nanoparticles and APUH matrix, the FTIR-ATR spectrum of pure APUH is compared with that of MMCMs containing different MOF nanoparticles in Fig. S3. According to Fig. S3, there are several characteristic peaks in FTIR-ATR spectra of different MMCMs. The strong and broad band at 3200-3500 cm-1 is assigned to the different -NH groups in urethane unit of APUH.5,24 The peaks appearing at 2850-2950 cm-1 are corresponding to aliphatic and aromatic C-H stretching vibration of APUH matrix, whereas the strong peaks at 1116 cm-1 11



and 1105 cm-1 are attributed to the stretching vibration of C-O in urethane unit and C-O-C in polyol groups, respectively.5,25 The characteristic absorption peak appeared around 1690-1750 cm-1 is related to the carbonyl groups in urethane unit, which provide the most useful information about microphase separation, so it is worthy to be investigated. As can be found from Fig. S3, the carbonyl stretching vibration of APUH matrix is shifted to lower frequencies by increasing the amount of MOF nanoparticles. This red shift might be due to formation of hydrogen bond between urethane groups of APUH and polar functional groups on the surface of MOF nanoparticles. Additionally, this hydrogen bond is obviously increased by increasing the polarity of functional groups of MOF nanoparticles, which is consistent with the results reported by other researchers.6,26 To determine quantitatively the effect of MOF nanoparticles on micro-phase separation of MMCMs, the spectral region of carbonyl groups in urethane unit (1640-1750 cm-1) was deconvoluted to two Gaussian peaks by Origin software (see Fig. S4), to distinguish two different modes of carbonyl (C=O) groups including free urethane carbonyl groups (at around 1730 cm-1) and urethane bonded carbonyl groups (at around 1702 cm-1). It is obvious that the intensity of free carbonyl group slowly decreased and shifted to the lower frequencies while the intensity of hydrogen bonded carbonyl group enhanced by raising the amount of MOFs loading, which demonstrates the promotion of hydrogen bonds between carbonyl groups and urethane -NH groups in the hard segments which causes more microphase separation between soft and hard segments. The affinity of MOF nanoparticles to be localized in soft segments is more than hard 12


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segments because of formation the hydrogen bonds between the polar groups on the surface of MOF nanoparticles and the ether groups of polyol.27 Therefore, in this case the available free ether groups of polyol to create hydrogen bonds with the urethane -NH groups decreases resulting in more hydrogen bond between urethane -NH groups and free carbonyl groups in hard segment. The similar results were also observed by Sadeghi et al.28 when using silica nanoparticles to prepare MMM based on conventional polyurethane. The (DPS) degree of phase separation between soft and hard segments is unique morphological characteristic which can dominate the final gas separation performance of MMCMs. Therefore, the DPS of different MMCMs is calculated using the following equation and the obtained results are summarized in Table 2:5 R

(6)

DPS = R + 1

where R is assigned to the carbonyl hydrogen bonding index and could be determined as follows:5 𝑅=

Abonded Afree

A1702

= A1730

(7)

in which A1702 and A1730 are the intensity of the bonded and free carbonyl groups, respectively. All of the prepared MMCMs were characterized three times with FTIR-ATR analysis to check reliability and reproducibility of the obtained results. Additionally, to better describe the changes in DPS, the error bars for R and DPS were added in Table 2. As can be found from Table 2, the DPS between hard and soft segment increases by increasing the content of MOFs irrespective of their functionality. This could be due to disruption of the hydrogen bond between the urethane –NH groups with ether groups of polyol in presence of MOF nanoparticles. It can be found that the DPS increases by 13



increasing the polar functionality of MOF nanoparticles which could be due to increasing the formation of hydrogen bonds between ether groups of polyol and –NH2 and –OH groups on the surface of modified MOFs. This phenomenon decreases the chances for the formation of hydrogen bonds between the hard and soft segment which results in more phase separation.28 Accordingly, higher degree of phase separation is relevant to w-MMCM-3 due to higher crosslinking density and lower hydrogen bonds between two phases caused by intensified steric hindrance in the presence of GMA-UiO-66 nanoparticles as crosslinking agent. This MOF with huge functionality prevents the formation of hydrogen bonds between the hard and soft segments. Thus, in this case the micro-phase separation is intensified which are in agreement with the results obtained from DSC analysis which would be discussed later in this article. Meanwhile, as there is no HEMA reactive diluent in w-MMCM-3, the content of hard segment would be lower which reduces the chances for the formation of hydrogen bond between the hard and soft segment resulting in more phase separation (DPS=0.72). The decrease in DPS at higher MOFs loading can be attributed to the agglomeration of MOF nanoparticles at such compositions, which lowers the hydrogen bonds formation between polar groups of MOF nanoparticles and ether groups of polyol. Therefore, this phenomenon provides more chances for the formation of hydrogen bond between the hard and soft segments which result in the decrease of micro-phase separations. The similar results were also reported by Alishiri et al. when using nanodiamond as filler to produce composite acrylated polyurthane.29

14


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Table 2. Microphase separation data obtained from FTIR-ATR analysis. Sample code w-APU APUH MMCM-1 MMCM-2 MMCM-3 MMCM-4 MMCM-5 m-MMCM-1 m-MMCM-2 m-MMCM-3 m-MMCM-4 m-MMCM-5

R 1.63 ± 0.22 1.27 ± 0.24 1.36 ± 0.12 1.45 ± 0.08 1.49 ± 0.14 1.60 ± 0.13 1.46 ± 0.24 1.42 ± 0.11 1.56 ± 0.16 1.68 ± 0.28 1.75 ± 0.21 1.99 ± 0.22

DPS 0.619 ± 0.023 0.559 ± 0.021 0.576 ± 0.018 0.592 ± 0.011 0.598 ± 0.020 0.615 ± 0.031 0.593 ± 0.028 0.587 ± 0.014 0.609 ± 0.019 0.627 ± 0.028 0.636 ± 0.013 0.666 ± 0.021

Sample code

R

DPS

m-MMCM-6

1.71 ± 0.26

0.631 ± 0.030

g-MMCM-1 g-MMCM-2 g-MMCM-3 g-MMCM-4 g-MMCM-5 g-MMCM-6 w-MMCM-1 w-MMCM-2 w-MMCM-3 w-MMCM-4

1.88 ± 0.14 1.98 ± 0.18 2.12 ± 0.07 2.33 ± 0.25 2.45 ± 0.23 2.20 ± 0.21 1.68 ± 0.14 1.91 ± 0.19 2.53 ± 0.23 2.16 ± 0.31

0.653 ± 0.027 0.664 ± 0.023 0.679 ± 0.012 0.699 ± 0.018 0.710 ± 0.031 0.688 ± 0.033 0.627 ± 0.018 0.656 ± 0.024 0.717 ± 0.031 0.684 ± 0.028

3.2.2. XRD analysis Fig. 1 displays the XRD patterns of mixed matrix membranes prepared in this study. For the sake of comparison, the XRD patterns of neat APUH, w-APU and MOF nanoparticles are also shown in Fig. 1. As can be inferred from Fig. 1, neat APUH membrane exhibits a broad peak at the range of 17-25° that is a characteristic feature for amorphous polymers;6,26 however, presence of small crystalline structure in APUH can also be possible.27,29 By incorporation of UiO-66s (both neat and modified), the position of XRD peak of APUH does not change; however, its intensity decreases by increasing the concentration of UiO66s. This suggests that UiO-66s do not promote the crystallinity of APUH, in spite of its influence on the enhancement of micro-phase separation between soft and hard segments, 15



as discussed above based on FTIR-ATR analysis. Basically, presence of MOF nanoparticles into APUH matrix disturbs the formation of crystalline region due to disturbing the arrangement of the nearby chains.5 As can be observed in Fig. 1, some characteristic peaks of MOF at around 7, 8 and 25° are also appeared in the APUH/MOF samples, particularly at higher concentrations of MOF. This suggests that MOF nanoparticles has retained their crystalline structure and distributed well in APUH matrix. Compared with MOF nanoparticles, the position of these new characteristic peaks of APUH/MOF shifts to lower degrees (higher d-spacing), and also increased by increasing the amount of polar functional groups on the surface of MOFs, which could be attributed to the penetration of APUH chins into the pores of porous MOFs because of good interfacial interaction between APUH matrix and modified MOF nanoparticles.

Fig. 1. XRD patterns of different MMCMs containing UiO-66 (a), NH2-UiO-66 (b), GMAUiO-66 (c), and GMA-UiO-66 with no HEMA (d).

16


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3.2.3. Morphological characteristics of APUH/MOFs Microstructure of MMCMs was investigated using FESEM micrographs. FESEM micrographs of the whole cross section of typical MMCMs (see Fig. S5) exhibit two distinctive layers including porous support layer which is coated with a dense selective layer.31 Accordingly, uniform interface with no detachment suggests good interfacial attraction between top mixed matrix selective layer and the bottom porous layer which might be attributed to the good wetting and formation of hydrogen bonds between APUH matrix and sulfone groups in polysulfone support layer. Such interface could improve the selectivity of CO2 against other gases. According to Fig. S5, the cross section of neat APUH membranes shows a fine, smooth, void free and defect-free surface which is characteristic of a dense selective APUH layer, while the cross section of MMCMs exhibits a rather rough morphology due to the presence of distributed MOFs in the APUH matrix. To further investigate the interfacial adhesion of selective dense layer to the porous support layer, FESEM analysis is carried out on the swollen g-MMCM-3 as a representative of all MMCMs and neat APUH membrane in water. The FESEM images (Fig. S6) indicate that no interfacial voids and delamination are observed between these two layers for g-MMCM3 after immersion in water for 10 days, while in the case of neat APUH membrane some voids at the interface are appeared after that period. This result could be due to presence of GMA-UiO-66 nanoparticles in APUH matrix and development of interfacial adhesion between two phases which restricts the swelling of APUH matrix. By increasing the soaking time up to 20 days, few defects are observed at the interface of g-MMCM-3 as well; however, the interfacial adhesion of two layers is not damaged severely and APUH 17



layer is still stacked to porous layer. This behavior suggests that this mixed-matrix composite membrane is one of the attractive candidates for CO2 separation in humid condition. The FESEM images of MMCMs containing different MOF nanoparticles show homogeneous dispersion of MOF nanoparticles into APUH matrix without any significant aggregations at low to moderate loadings (Figs. S7-S10). Such a good dispersion could be due to the hybrid (organic and inorganic) nature of MOF nanoparticles and specially the tendency to interact with APUH molecules caused by the formation of hydrogen bonds between the polar groups of APUH matrix and the polar groups on the surface of MOF nanoparticles. At higher MOF loadings, i.e. MMCM-5 (15 wt% UiO-66), m-MMCM-6 (20 wt% NH2-UiO-66), g-MMCM-6 (30 wt% GMA-UiO-66) and w-MMCM-4 (20 wt% GMAUiO-66), some particle aggregation is observed, which are circled with yellow dashed line. The aggregated MOF crystals create nonselective voids and provide an excess transport pathway in APUH matrix which can affect the gas diffusion through the membrane and dominate the permeability and separation performance of MMCMs.15,26 Fig. 2 displays the FESEM micrographs of the selective MMMs containing APUH and optimum MOF nanoparticles, which compares the effect of surface functionalization on the dispersion of MOF nanoparticles into polymer matrix. As can be observed from Fig. 2 and Fig S9, more fine dispersion with no aggregation even at higher MOF loading (25 wt%) is observed in APUH matrix when using GMA-UiO-66 nanoparticles as filler. This excellent dispersion of GMA-UiO-66 nanoparticles in APUH matrix can be attributed to multiple reasons as (1) uniform dispersion of MOF nanoparticles in the APU/HEMA solution 18


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because of the similar physico-chemical properties between this MOF and highly flexible APU chains which results in tightly wrapped MOF with APUH matrix after UV curing, (2) surface modification of NH2-UiO-66 nanoparticles with huge GMA group can increase the steric hindrance between the modified MOFs improving its dispersion and wettability with APUH matrix, (3) development of strong covalent bonds between GMA-UiO-66 and APUH, and eventually (4) partial penetration of APU/HEMA molecules into the pores of GMA-UiO-66 nanoparticles and enhancement of compatibility and interactions due to the formation of hydrogen bonds between polar groups of APU chain and inner nonfunctionalized amine groups.14,25,27,32

Fig. 2. FESEM images of the cross-section of MMCMs containing different MOFs. The presence of GMA-UiO-66 nanoparticles in the APUH matrix was further analyzed by performing energy dispersive X-ray spectroscopy (EDX) on typical sample, i.e. g-MMCM3, as presented in Fig. S11. The EDX scanning of Zr exhibits that GMA-UiO-66 nanoparticles are homogeneously dispersed in the APUH selective layer with no 19



penetration into the porous layer. Fig. 3 exhibits TEM micrographs of typical selective layers containing untreated UiO-66 and GMA-UiO-66 at the same loading, i.e. MMCM-5 and g-MMCM-3. As can be inferred, GMA-UiO-66 nanoparticles are dispersed individually in APUH matrix with firmly adhered interface without any defects. The high-resolution TEM images have shown individual rectangular GMA-UiO-66 shapes with particle sizes around 100-200 nm33. In the case of MMCM-5, aggregation of untreated UiO-66 particles which leads to larger clusters is visible in TEM images. This suggests that UiO-66 nanoparticles at this loading are not fully wetted by APUH matrix which can lead to formation of nonselective intra particle and interface voids.1,30 As can be observed, the size of UiO-66 nanoparticles in SEM images (Fig. S7) seems smaller than that in the TEM images (Fig. 3), which might be due to the differences in the nature of these techniques for imaging. However, at high MOF loading (MMCM-5 containing 15 wt% UiO-66), several MOF nanoparticles stick to each other and form clusters larger than 500 nm, which are circled with yellow dashed line in Fig. S7. Therefore, in this case several UiO-66 particles may overlap with each other and prevent the penetration of electron beam through the MMCMs, results to observed larger particles in TEM image.

20


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Fig. 3. TEM images of MMCM-5 and g-MMCM-3 at different magnifications. 3.3. Gel content Since the APUHs have been cross-linked by UV irradiation, their gel content was measured by Soxhlet extraction in acetone solution (Fig. S12). Accordingly, it can be found that the gel content of APUHs increases with increment the amount of HEMA loadings which might be due to the higher crosslinking density. For example, the gel content of w-APU is about 86% which increases up to 91% by adding 30% HEMA to the polymer solution. Additionally, it can be found that the gel content of MMMs containing UiO-66 and NH2UiO-66 decreased slowly with the increment of their concentrations, which might be due to the incomplete curing process caused by shield effect of these particles.30,34 The gel content of MMMs containing amine functionalized MOF is higher than that of MMMs containing untreated MOF, which could be due to the better dispersion of NH2-UiO-66 nanoparticles in the APUH matrix resulting in negligible shield effect. Contrary to UiO-66 and NH2-UiO-66 filled APUHs, the gel content of MMMs containing GMA-UiO-66 nanoparticles increases by increment of its concentration to 20 wt% and then 21



decreases with further increment of concentration. This observation might be due to the carbon double bonds of GMA-UiO-66 which can take part in the crosslinking reaction. The highest gel content (95%) is relevant to MMMs containing 20 wt% of GMA-UiO-66, because the higher carbon double bond content in GMA-UiO-66 phase. When the GMAUiO-66 content increases to 30 wt%, excessive free and also agglomerated particles create a shielding effect for UV light which decreases the curing speed leading to lower gel content.30,34 3.3.1. TGA analysis Fig. 4 shows the thermogravimetric analysis of neat APUH and typical MMMs. It is realized that the TGA thermogram of all membranes exhibits a two-step weight loss with similar decomposition trend. The first step weight-loss observed around 250-350°C may be attributed to the decomposition of urethane bonds in the hard segments, while the second sharp weight-loss around 350-500°C may be related to the decomposition of the soft polyol segments.27,34 According to Fig. 4a and b, APUH exhibits higher thermal degradation temperatures compared with w-APU suggesting that HEMA reactive diluent improves the thermal stability of the polymer. As HEMA reactive diluent increases the crosslinking density and promotes the micro-phase interference,34 such improvement in thermal stability is understandable. Indeed, by increasing the interaction between soft and hard segments, socalled phase interference, the mobility of polymer chains is reduced which results in the improvement of thermal stability of APUH compared with w-APU, as reported by other researchers.27 22


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As shown in Fig. 4, the thermal stability of MMMs is improved by introducing different UiO-66s into APUH. In these cases, the thermal degradation of urethane linkages in hard segments (first step weight-loss) shifts to higher temperature by incorporation of different MOF nanoparticles. This improvement in thermal stability would be intensified by using functionalized UiO-66 due to further loss in molecular mobility of polymers caused by the interfacial interaction between APUH matrix and modified UiO-66. The higher thermal degradation of urethane linkages is also observed for w-MMCM, which might be due to the improvement in interfacial interaction by participation of vinyl attached UiO-66 nanoparticles in radical polymerization in the course of UV irradiation.34 In this case the affinity of GMA-UiO-66 nanoparticles to disperse in hard segments increases which results in the decrease of MOF content in soft segment and causes lower thermal degradation of soft polyol for w-MMCM-3. While the thermal degradation of soft segments in other MMMs increases by incorporation MOF nanoparticles. The influence of GMA-UiO-66 concentrations on TGA curves of APUH/GMA-UiO-66 was also investigated and the results are depicted in Fig. S13. It is inferred that the thermal decomposition of g-MMCMs increases by the increment of GMA-UiO-66 concentration up to 25 wt% and then decreases with further increment of concentration to 30 wt%. For instance, the maximum degradation peak of the soft segments (Tdpeak) of neat APUH membrane is about 414°C, while the Tdpeak of g-MMCM-5 is about 425°C. Such behavior can be associated to (1) the increase in the crosslinking density due to the role of GMAUiO-66 as crosslinking agent, (2) excellent interfacial interaction between GMA-UiO-66 and polymer matrix and (3) the good dispersion of GMA-UiO-66 nanoparticles across the 23



membrane thickness, which was corroborated by FESEM images (Fig. S9). Comparatively lower Tdpeak of g-MMCM-6 can be attributed to agglomeration of GMA-UiO-66 at this composition which causes a lower interface area and creates a shielding effect for UV light leading to decrease of the crosslinking density.30,34

Fig. 4. TGA (a) and derivative TG (DTG) (b) curves for typical MMMs. 3.3.2. DSC analysis The DSC thermograms of typical MMMs are given in supporting information (Fig. S14), and Tg values obtained from these thermograms are listed in Table 3. Neat w-APU exhibits only one transition point in DSC thermogram showing one Tg at around -42°C, which is attributed to the Tg of PEG soft segment.35 Indeed, DSC cannot detect the hard segment of APU. By incorporation of GMA-UiO-66 into w-APU matrix, i.e. w-MMCM-3, Tg shifts to 24


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larger values; however, it still exhibits single transition peak. In fact, presence of GMAUiO-66 retards the molecular motion of w-APU through the interaction at the interface. As given in Table 3, APUH exhibits two different Tg values at low temperatures (Tg,S= 31.2 °C) which is correspondent with soft segment of APUH and at high temperatures (Tg,H=52.4 °C) attributable to hard phase in APUH matrix.5 Therefore, it can be realized that HEMA reactive diluent causes well separation between hard and soft segments leading to a two-phase morphology of APUH matrix. Compared with w-APU, it is deduced that addition of HEMA reactive diluent into APU shifts Tg,S to higher values which can be due to the increase in micro-phase interference35 and crosslinking density leading to lower chain mobility of soft segment. As can be realized from Table 3, incorporation of UiO-66 into APUH increases considerably the Tg value of both soft and hard segments. Micro-phase separation of APUH caused by incorporation of UiO-66 and its derivatives can only enhance Tg of hard segments. Therefore, the increment of Tg for both soft and hard segments can be attributed to establishment of interaction between UiO-66 with both hard and soft segments which restricts the molecular mobility of both phase and causes chain rigidification.29 Interestingly, the degree of increment in Tg of UiO-66 filled APUH is strongly dependent on the type of functionalization of UiO-66. As mentioned above, functionalized UiO-66 provides finer dispersion and can interact efficiently with APUH through the functional groups of functionalized UiO-66. As deduced from Table 3, the largest increment in Tg is associated with vinyl attached UiO-66, i.e. GMA-UiO-66, that exhibited finer dispersion in APUH and believed to create much efficient interaction with both hard and soft segments 25



via strong hydrogen and covalent bonding. Table 3. Glass transition temperature (Tg) of various MMMs prepared in this study. Subscripts “S” and “H” stand for soft and hard segments, respectively. Sample code w-APU APUH MMCM-5 m-MMCM-5 g-MMCM-3 w-MMCM-3

Tg,S (°C) -42.2

Tg,H (°C) NA

-31.2 -28.7 -24.3 -20.5 -33.4

52.4 57.2 61.7 66.3 NA

3.4. Gas permeation properties The permeability of pure N2 and CO2 gases through MMCMs synthesized in the present work was measured at 25°C and the feed pressure of 6 bar. All of the synthesized MMCMs were tested more than three times to assure reproducibility and reliability of the results. Fig. 5 presents the permeability of pure CO2 and N2 gases as well as the ideal CO2/N2 selectivity through neat APUH and MMCMs containing UiO-66 and NH2-UiO-66. It is found that the permeability of CO2 through APUH and all MMCMs is considerably higher than that of N2, which could be attributed to higher critical temperature as well as higher condensability, low kinetics diameter and more interaction of this polar gas (CO2) with the polar ethylen oxide groups in the repeating unit of soft segments via strong quadrupole-dipole interactions.5,15,25,28

26


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Fig. 5. Permeability and selectivity of the MMCMs as a function of (a) untreated UiO-66 concentration and (b) NH2-UiO-66 concentration and (c) CO2 adsorption isotherms of all UiO-66s at room temperature (298 K) and different pressures. Generally, in polyurethane membranes, the hard domain acts as an impermeable phase through which permeation of gases is restricted; however, the soft domain is permeable to gas molecules. Therefore, the micro-phase separation between these two phases is the key factor to control the gas separation performance of conventional polyurethane membranes and acrylated polyurethane membranes5. For MMCMs, UiO-66 and its derivatives might be localized in both soft and hard domains as corroborated above by FTIR-ATR, TGA and DSC analyses. Scheme 3 depicts the possible localization of UiO-66s in both hard and soft domains. Due to the inherent impermeable nature of hard domains, localization of UiO-66s in this phase cannot contribute considerably in overall permeability of MMCMs. However, it is speculated that localization of UiO-66s in soft domain would increase the gas permeability of MMCMs because of permeable nature of this phase and the porous structure of MOF nanoparticles.15

Scheme 3. The pathways for CO2 and N2 molecules transport in MMCMs based on APUH 27



and UiO-66s. As can be seen from Figs. 5a and b, the permeability of both gases increases steadily by increasing the content of UiO-66 and NH2-UiO-66 with a sharp increase at maximum loading where the particles start to agglomerate. In the case of CO2, the increment of permeability is much considerable compared with N2 and becomes more than twice at above 15 wt% loading. As the soft domain dominates the overall permeability of APUH, the increment of permeability can be associated to promotion of micro-phase separation by incorporation of UiO-66s (see Table 2) and localization of UiO-66s in soft domain which provides extra channels originated from the porous structure of UiO-66 nanoparticles. Furthermore, other reasons can be accounted for the increase of gas permeability with increasing the amount of UiO-66 and NH2-UiO-66 as the increase of free volume by disruption of the APUH chain packing and promotion of agglomeration at high concentration of UiO-66s. Interestingly, similar with permeability, ideal CO2/N2 selectivity increases by increasing the amount of UiO-66 and NH2-UiO-66 nanoparticle, as shown in Figs. 5a and b, which is not the case in MOF filled membranes so that the permeability and selectivity do not increase simultaneously.22,36 For instance, Nik et al. reported that by incorporation of NH2-UiO-66 and UiO-67 in polymeric membrane, the permeability and selectivity of CO2/CH4 mixtures exhibited inverse behaviors.36 The improvement in ideal CO2/N2 selectivity by increasing the amount of UiO-66 and NH2UiO-66 is due to the formation of a defect-free interface between these MOFs and APUH matrix due to the strong interaction between two phases. These interfacial interactions would be intensified using amine functionalized UiO-66 because of strong hydrogen bonds 28


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between NH2-UiO-66 nanoparticles and APUH as described above by FTIR-ATR analysis.6,26 Additionally, strong quadrupole-dipole interactions between polar CO2 molecule and amine groups on the surface of NH2-UiO-66 increase adsorption capacity, solubility and surface diffusion of CO2 in the membrane (see also Fig. 5c and Table S1).37 So, the maximum selectivity for APUH/UiO-66 is to be 38.78, while it is 44.30 for APUH/ NH2-UiO-66. As MOFs introduce extra channels in APUH matrix for gas permeation,15 it appears that CO2/N2 selectivity should decrease by increasing MOFs content because of the pore sizes of these MOFs (~6-7 Å) which are larger than the kinetics diameter of both gases (see Table S2). However, as the CO2 adsorption capacity of these MOFs is much larger than N2, it is concluded that these MOFs increase the surface diffusion of CO2 more than N2 due to their affinity to CO2 molecules. These observations are in agreement with the results reported by other researchers.26,37 As can be seen from Figs. 5a and b, the ideal CO2/N2 selectivity of MMCMs exhibits maximum value at a certain concentration of MOFs, i.e. 12 wt% UiO-66 and 15 wt% NH2UiO-66, and then decreases with further increment of concentration. Interestingly, this critical concentration is coincident with the promotion of agglomeration for each UiO-66, as confirmed by FESEM images (see Figs. S7 and S8). The agglomerated clusters of MOFs can form microvoids within the cluster particles which act as nonselective diffusion channels for the gas molecules resulting in lower selectivity of CO2 against N2. In this case the gas molecules prefer to diffuse through these nonselective microvoids rather than MOFs pores due to their bigger size. The dominant gas permeation mechanism through these 29



microvoids would be Knudsen diffusion, which permeates N2 molecule faster than CO2 molecule due to its lower molecular weight. Therefore, the increase in permeability of N2 would be more than CO2 resulting in lower CO2/N2 selectivity.14 Overall, it can be concluded that the agglomeration of MOF nanoparticles enhances considerably the permeability of MMCMs; however, it has a negative effect on the selectivity. As a balance between selectivity and permeability is basically required for practical applications, therefore, it is desirable to obtain APUH/MOF nanocomposite membrane at concentrations just below the critical value, i.e. below the agglomeration point. However, to benefit the porous structure of MOF for gas separation purposes in polymeric membrane, it would be desirable to increase the critical concentration of UiO-66 by appropriate surface modification. As mentioned above, amine functionalization of UiO66 increased the critical concentration of UiO-66 from 12 wt% to 15 wt%; so both permeability and selectivity increase for m-MMCMs compared to MMCMs. To further examine the role of surface functionality on improvement of gas separation performance of APUH, gas separation performance of APUH/GMA-UiO-66 is analyzed in the following section. As investigated above, the critical concentration of GMA-UiO-66 in APUH increases up to 25 wt%; therefore, it can be good candidate to examine the role of surface functionalization on the separation performances of APUH. Fig. 6 displays gas permeation properties of MMCMs containing GMA-UiO-66 nanoparticles. From Figs. 6a and b, it is found that neat w-APU membrane (with no HEMA reactive diluent) exhibits higher permeability respect to APUH, while its selectivity is lower. As the HEMA reactive diluent in APUH increases the gel content as well as the 30


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crosslinking density and enhances the content of impermeable hard segment and increases micro-phase interference (see Table 2 and Fig. S12), such behavior is understandable. These phenomena result in lower chain mobility, exclusively for soft domain of APUH matrix, which is reflected in higher Tg,S, as listed in Table 3, and lower free volumes. Therefore in this case, the diffusivity of both gases decreases with addition of HEMA reactive diluent.38 Furthermore, HEMA reactive diluent increases the fraction of insoluble hard segment which reduces the available region for gas dissolution (soft segments) resulting in decrease of the solubility coefficients of both gases (see Table S1). Consistent with MMCM and m-MMCM series, permeability of g-MMCMs and wMMCMs increases steadily by increasing the content of GMA-UiO-66 (see Figs. 6a and b); however, selectivity shows a maximum at critical concentrations because of promotion of agglomeration, as observed in FESEM micrographs. It is to be noted that the maximum selectivity for APUH is observed at higher GMA-UiO-66 loading, i.e. 25 wt%, compared with w-APU (with no reactive diluent) nanocomposite membrane (15 wt%). Therefore, it can be concluded that acrylated polyurethane diluted by reactive diluent is much desirable for preparation of MMCMs with MOFs.

Fig. 6. Permeability and selectivity of the MMCMs as a function of (a) GMA-UiO-66 with HEMA reactive diluent and (b) without HEMA reactive diluent, and (c) CO2 adsorption 31



isotherms of pure APUH and selected g-MMCMs at room temperature (298 K) and different pressures. Comparing m-MMCM-5 and g-MMCM-3 (compare Figs. 5 b and 6a), with the same MOF loading, it is realized that the CO2 permeability of g-MMCM-3 is almost similar with mMMCM-5, while the ideal CO2/N2 selectivity of g-MMCM-3 is higher than that of mMMCM-5. Meanwhile the maximum selectivity for g-MMCM, which is achievable at 25 wt% GMA-UiO-66 (g-MMCM-6), is 53 which is higher than 44 for m-MMCM-5. This indicates that GMA-UiO-66 can improve the selectivity while the permeability retains, compared with other UiO-66s. This behavior could be clarified by various characteristic properties as (1) fine dispersion of GMA-UiO-66 into APUH matrix up to 25 wt% with minor agglomeration and sedimentation; (2) no microvoids in the interface of APUH and GMA-UiO-66 due to strong interfacial interaction which is achieved by covalent bonding; (3) the increase of the gel content as well as crosslinking density due to vinyl functionality of GMA-UiO-66 nanoparticles taking part in polymerization; (4) increase of the adsoprtion capacity of CO2 and also adsorption selectivity of CO2 against N2 in presence of polar groups (hydroxyl, amine and ester) on the surface of GMA-UiO-66, and (5) partially blocking the pores of GMA-UiO-66 because of the penetration of bulky APU chains into its pores. As can be deduced from Figs. 5c and 6c, the adsorption capacity of CO2 in GMA-UiO-66 nanoparticles is greater than that in other UiO-66s, which could be due to the strong quadrupole–dipole interactions between this gas and polar groups like amine, hydroxyl, and ester on the surface of GMA-UiO-66 nanoparticles and also change the stoichiometry of CO2 in the chemical adsorption mechanism in the presence of hydroxyl groups.17,19 These 32


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phenomena increase the solubility coefficient, surface diffusion and eventually permeability of CO2 in the MMCMs containing GAM-UiO-66 nanoparticles, while their impact on N2 permeability is not so serious, resulting in the increase of the ideal CO2/N2 selectivity. In order to investigate the pore blockage of GMA-UiO-66 nanoparticles by APU chains, the CO2 adsorption capacity of neat APUH, g-MMCM-3 and g-MMCM-5 was measured and compared in Fig. 6c. It is deduced from Fig. 6c, that the adsorption capacity of MMCMs increases by increasing the amount of GMA-UiO-66 nanoparticles which indicates that the pores of modified MOF nanoparticles are not completely blocked by APUH polymer and the MMCMs are still porous. The similar results were also reported by Zhang et al.39 when grafting poly butyl methacrylate on the surface of functionalized NH2-UiO-66 nanoparticles. Similar with UiO-66 and NH2-UiO-66, MMCMs containing GMA-UiO-66 exhibits a relative decrease in selectivity at higher loading (see Fig. 6a) which is attributable to particle agglomeration. However, the agglomeration of GMA-UiO-66 is observed at higher concentrations, i.e. 30 wt%, compared with other UiO-66s investigated in this study. Higher loading of this MOF with no agglomeration enables one to obtain higher selectivity which is beneficial for CO2 separation. The similar scenario was also observed in the case of MMCMs containing GMA-UiO-66 nanoparticles as filler and APU polymer (without HEMA) as matrix (Fig. 6b). As mentioned above, distribution of GMA-UiO-66 nanoparticles in the hard domains is preferable because these nanoparticles act as crosslink agent due to their vinyl functionality. Basically crosslinking is gained in the hard domain where the carbon-carbon 33



double bond is available. So, it can be concluded that the large number of GMA-UiO-66 nanoparticles are distributed in the hard domains. Therefore, in this case the porous structure of MOF nanoparticles cannot contribute considerably in the gas permeability (see Scheme 3). Thus, the increase in permeability of two gases through MMCMs containing GMA-UiO-66 nanoparticles by increasing the MOF nanoparticles loading is lower than that NH2-UiO-66 and UiO-66 nanoparticles. Table S3 compares the role of UiO-66s to improve the permeability and selectivity of APUH matrix with other achievements reported in literature.10,14,15,26,33 Actually, the role of MOFs on the gas separation performance of PUs has not reported yet; however, influence of various MOFs on the gas separation performance of various polymer have been examined by researchers. As can be deduced, UiO-66, particularly GMA-UiO-66, exhibits very efficient improvement on the gas separation performance of APUH compared with other polymers, because both permeability and selectivity are improved simultaneously. 3.4.1. Gas diffusivity and solubility coefficients of MMCMs As the permeability of gas through polymeric membrane is a function of its diffusivity and solubility in the polymer;5 these parameters are taken into consideration for APUH based MMMs investigated in this study (see Table S1). As can be observed from Table S1, the solubility and diffusivity coefficients of CO2 in both APUH and w-APU are higher than that of N2, which could be attributed to higher critical temperature as well as higher condensability, low kinetic diameter and more interaction of this polar gas (CO2) with the polar groups in molecular structure of the polymers via strong quadrupole-dipole interactions.5 The solubility coefficient of CO2 in these membranes is much higher than the 34


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diffusivity coefficient and also the solubility selectivity (SCO2/SN2) is larger than diffusivity selectivity (DCO2/DN2). This indicates that the solubility dominates the gas transport in these membranes suggesting that APUH and w-APU membranes behave like rubber materials in gas permeation.5 Both diffusivity and solubility coefficients of CO2 and N2 increase upon incorporation of UiO-66 and its derivatives into APUH; however, the role of UiO-66s on the solubility and diffusivity of CO2 is much higher than that of N2. Basically, incorporation of UiO-66s increases the free volume of polymer matrix, creates interphase region, introduces extra channels within the porous UiO-66s and decreases gel content which all lead to the increment of diffusivity.5,6,37 Comparing the gas diffusivity of various MMCMs investigated in the present study, it is found that the functionalization of UiO-66 decreases the diffusivity of various MMMs respect to pristine UiO-66, so the order is as follows: MMCM> m-MMCM> w-MMCM> g-MMCM. This trend could be anticipated due to the development of stronger defect-free interface by modified UiO-66 which indeed retards the diffusion of gas molecules from the interphase region. Contrarily, the solubility coefficient is improved by surface functionalization of UiO-66, which is in agreement with the adsorption capacity of CO2 in these MOFs (see Figs; 5c and 6c). Therefore, it can be concluded that the adsorption capacity as well as solubility coefficient of CO2 in these MMCMs depend on the adsorption capacity of this gas in the MOF itself, as reported by other researchers as well.15,22,40 As reported in the previous works, CO2 adsorption capacity of NH2-UiO-66 and GMAUiO-66 is greater than that of UiO-66, while N2 adsorption capacity decreases by 35



functionalization, which could be due to the differences in the adsorption mechanism and the electrostatic interactions between nanoparticles and adsorbate.17,19 High adsorption capacity of CO2 might be attributed to its high quadrupole moment, more polarizable, small molecular size, and the strong interaction with the MOF nanoparticles. Moreover, CO2 with small kinetics diameter can be adsorbing on the sites that are in the small channels. Meanwhile, reversible chemical reaction between amine groups and CO2 molecules (scheme 4) could be the other reasons for such high CO2 adsorption capacity.17,19 According to reaction scheme (4-2), in the presence of hydroxyl groups on the surface of GMA-UiO-66 nanoparticles, one mole of amino group reacts with one mole of CO2 molecule, while without this group in NH2-UiO-66 nanoparticles; two moles of amino groups react with one mole of CO2 molecule as mentioned by reaction scheme (4-1). Thus, these phenomena increase adsorption selectivity of CO2 against N2 in modified UiO-66.

Scheme 4. Reaction of CO2 with amine groups in NH2-UiO-66 and GMA-UiO-66. 3.4.2. Long-term gas separation performance Long-term gas separation stability is an important factor for industrial applications, and it is the Achilles' heel of many membranes in practice.15 Therefore, long-term gas separation performance of neat APUH and g-MMCM-5 is also investigated to further investigate the 36


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structural stability of MMCMs introduced in this work. Fig. S15a depicts the gas permeability and ideal CO2/N2 selectivity of the selected membranes in time for 100 hours. It can be found that the permeability of CO2 in both membranes increases slightly with time, while the permeability of N2 is almost preserved. This observation might be due to condensable feature of CO2 against N2, which swells the APUH layer and acts as plasticizing agent resulting in greater free volume.26,33 Increment of CO2 permeability with upstream pressure, shown in Fig. S15b, also emphasizes the plasticizing role of CO2 in APUH and g-MMCM-5.18 It can be found that the increase in CO2 permeability with pressure (the slope of permeability vs. pressure) for pure APUH is higher than that of g-MMCM-5 (~2.5 times). This superior stability against pressure and also long-term operation for g-MMCM-5 could be attributed to the strong covalent bonds network between the GMA-UiO-66 nanoparticles with APUH matrix which enhance the structural stability and durability.15 3.4.3. Performance comparison of MMCMs with the Robeson’s curves The CO2 permeability and CO2/N2 selectivity data obtained for MMCMs based on APUH are compared with previously reported mixed-matrix membranes (MMMs) containing different MOFs in Robeson’s curve (see Fig. 7).4 As can be observed in Fig. 7, MMMs exhibits a wide range of permeability between 1-100 Barrer; however, the CO2/N2 selectivity is mostly concentrated within a narrower range between 30-50. It is obvious that by incorporation of UiO-66 into APUH, both permeability and selectivity increase simultaneously, and the gas separation data of APUH moves toward Robeson’s upper bound. APUH filled with GMA-UiO-66 at 25 wt% exhibits higher improvement in 37



terms of permeability and selectivity. However, based on the results obtained above, it is anticipated that further improvement in the gas separation performance of APUH/UiO-66 would be achievable when higher critical concentration of UiO-66 in APUH is attained. This situation can be obtained with functionalization of UiO-66 with more efficient functional groups. Therefore, further improvement in gas separation performance of APUH with UiO-66 would make APUH/UiO-66 commercially more attractive and promising candidate for membrane processes, because of many attractive features of APUH like thin film-forming ability, good water stability, environmental friendly (without any solvents), low energy cost (form at room temperature) and scalable.

Fig. 7. Comparison between the obtained permeation results of APUH/UiO-66s with Robeson’s curve and other reported MMMs. 4.

Conclusion

We presented a novel mixed-matrix composite membrane (MMCM) based on UV-curable acrylate polyurethane and functionalized UiO-66 nanoparticles for the first time. FESEM observation exhibited that UiO-66 could be dispersed uniformly in the APUH matrix up to 38


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12 wt% above which particle aggregation was appeared. This critical concentration could increase by surface functionalization of UiO-66. So, the critical concentration of NH2-UiO66 and GMA-UiO-66 increased to 15 wt% and 25 wt%, respectively. Moreover, functionalized UiO-66 enhanced further the micro-phase separation between hard and soft domains, improved interaction with both hard and soft segments and created stronger interfacial interaction compared with pristine UiO-66. Meanwhile, vinyl group attached on the surface of GMA-UiO-66 could take part in the crosslinking reaction via radical polymerization which improved interfacial interaction considerably. Interestingly, UiO-66 and its derivatives enhanced both permeability and selectivity simultaneously up to critical concentration. As the critical concentration of GMA-UiO-66 was higher than other UiO66s; therefore, APUH/GMA-UiO-66 exhibited higher improvement in gas separation performance in terms of permeability (130%) and selectivity (77%). In summary, it was concluded that UiO-66 is promising particle to improve the gas separation performance of APUH membrane, because it enabled to improve both permeability and selectivity simultaneously. However, appropriate surface functionalization of UiO-66 played vital role on the extent of improvement in gas separation performance by enhancing the critical concentrations and improving interfacial interactions. Although the performance of these MMCMs are still under the Robeson's curve, but their outstanding features such as structural stability, low cost, long life stability along with easy, fast and environmental friendly processing and strong adhesion to support layer make these MMCMs excellent candidates for CO2 separation.

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Supporting Information Procedure for synthesis and modifications of MOFs, Procedure for synthesis acrylate polyurethane (APU), Procedure for gas adsorption and permeation trough MMCMs, The images of apparatuses used for the adsorption capacity and permeation tests, Origin deconvoluted FTIR spectrum of all MMCMs, The whole cross-section SEM images of MMCMs, and The DSC thermograms of some MMCMs. Acknowledgment This work was financially supported by Niroo Research Institute. The authors gratefully appreciate their support. References

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Graphical abstract

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Mixed-Matrix Composite Membranes Based on UiO-66-Derived MOFs

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