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Mixed Matrix Membrane Based on Crosslinked Poly[(Ethylene Glycol) Methacrylate] and Metal-Organic Framework for Efficient Separation of Carbon Dioxide and Methane Liang Ma, Frantisek Svec, Tianwei Tan, and Yongqin Lv ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00459 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 7, 2018

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ACS Applied Nano Materials

Mixed Matrix Membrane Based on Crosslinked Poly[(Ethylene Glycol) Methacrylate] and Metal-Organic Framework for Efficient Separation of Carbon Dioxide and Methane Liang Ma,a Frantisek Svec,a Tianwei Tan,*a,b Yongqin Lv*a,b a.

Beijing Advanced Innovation Center for Soft Matter Science and Engineering,

Beijing University of Chemical Technology, Beijing 100029, China. b.

Beijing Key Laboratory of Bioprocesses, College of Life Science and Technology,

Beijing University of Chemical Technology, Beijing 100029, China. KEYWORDS. Mixed matrix membrane, crosslinked poly(ethylene glycol), metal-organic framework, CO2, methane, separation

ABSTRACT. The key in preparing mixed matrix membranes for the desired gas separation is to rationally select a suitable combination of inorganic fillers and polymers, and to develop fabrication techniques enabling formation of a continuous inorganic phase with dual transport pathway. Herein, we report the facile design of flexible

poly[poly(ethylene

glycol)

methacrylate-co-poly(ethylene

glycol)

dimethacrylate] membranes containing metal organic frameworks UiO-66 prepared 1 ACS Paragon Plus Environment

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from zirconium chloride and 2-aminoterephthalic and terephthalic acid varying in contents, shapes, and sizes. The surface chemistry effects of both polymer matrix and MOFs on permeability and selectivity were investigated. The bare polymer membrane exhibited a permeability for CO2 of around 117 barrer and a selectivity of up to 15. Addition of glycidyl methacrylate in the polymerization mixture led to membranes that were modified with hexamethylenediamine to provide for basicity. However, this modification did not improve performance of the membranes. In contrast, addition 35 wt.% UiO-66 octahedron enhanced both permeability and selectivity for CO2 to about 205 barrer and 19, respectively. By adjusting the size and shape of UiO-66, the best hybrid membrane containing 35 wt.% clusters of aggregated UiO-66 formed a close to continuous phase desirable for the dual transport mechanism, and exhibited a 247 % increase in CO2 permeability up to 365 barrer.

1. INTRODUCTION According to the Paris Agreement from 2015, which deals with greenhouse gases emissions mitigation, adaptation, and finance, most of the world countries agreed to manage their energy sources portfolio in ways to significantly decrease emissions. Carbon dioxide represents the largest contribution among the emitted greenhouse gases. Therefore, avoiding use of traditional fossil fuels such as coal and oil is highly desirable. Both natural gas and biogas produced from renewable raw materials contain methane as the active component are environmentally more friendly energy source 2 ACS Paragon Plus Environment

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since they produce a smaller carbon footprint compared to solid and liquid energy sources.1 However, these gases typically contain significant percentage of carbon dioxide on the top of minor percentages of other gases. CO2 decreases the energy content of the gas and reduces transport capacity of the pipelines.2 Upgrading these gases to the pipeline quality called sweetening is currently achieved via well-established technologies including water washing, pressure swing adsorption, selexol adsorption, and treatment with amines. Considerable efforts are currently focused on the design, development, and application of membrane processes for gas separation because membranes are relatively simple, less energy intensive, and easily scalable.3-5 The current unifying weakness of most of these membranes is the well-known ‘trade-off’ between permeability and selectivity. In other words, the membrane is typically not both highly selective and highly permeable.6 Membranes based on inorganic materials mostly afford a good performance but are difficult to produce thus being more expensive. Therefore, mixed matrix membranes that combine the advantages of polymeric matrix, i.e. low price and ease of production, with the selectivity of an inorganic filler are likely to meet the requirements of a viable separation process. A variety of fillers including silica, zeolites, graphene oxide, carbon nanotube, nanoparticles, frameworks, porous organic cages, as well as covalent and metal-organic frameworks (COFs and MOFs) have already been tested.7-30 MOFs are excellent fillers due to their crystalline characters with well-defined chemistry, large

3 ACS Paragon Plus Environment


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surface areas, and controlled porosity.31-33 MOF chemistry also facilitates interfacial interaction with polymeric matrix and enhances resistance to plasticization, thus further

improving

the

zirconium(IV)−carboxylate

membrane MOFs

performance.34,

(Zr-MOFs)

have

35

emerged

Particularly, in

membrane

technologies as the inorganic fillers of choice due to their exceptional chemical and thermal stability arising from the strong coordination bonds between the “hard-acid”−“hard-base” interactions between the Zr(IV) atoms and carboxylate oxygens.36-39 The typical Zr-MOFs include UiO-66 and UiO-66-NH2 that exhibit a good performance in CO2 separation.17, 23, 40-43 Numerous polymers have already been tested as mixed matrix membranes largely differing in their performances. Among other polymers, poly(ethylene glycols) (PEG) became popular since they dissolve substantial amounts of acid gases such as CO2 as a result of presence of polar oxygen atoms in the PEG chain. For example, PEG 300 dissolves 13.5 mg CO2/g, which is about 8 times more than water.44 The non-toxic PEGs are available at a moderate cost and do not evaporate. Thus, these polymers attracted significant interest as potential materials for the fabrication of membranes. Unfortunately, these polymers alone feature poor mechanical and thermal properties and do not allow formation of stable thin films. As a result, new strategies have to be designed to enable preparation of membranes with desired separation performance, including physical blending, polymer grafting, and crosslinking polymerization.


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Presence of PEG in a blended membrane enables both high permeability and selectivity while the other polymer enhances the robustness.45-48 For example, Kawakami et al. reported long time ago a continuous increase in the permeability and carbon dioxide permselectivity of cellulose nitrate/PEG blend membranes upon increasing the content of PEG.49 Hu et al. prepared well performing membranes from a blend of PEG and poly(phenylene oxide).50 The significant increase in permeability is typically attributed to enhanced solubility of the CO2 while the improvement in diffusivity is deemed to result from the plasticizing effect of PEG molecules in the blend.51 Davis and Sandall developed a model describing the facilitated transport of CO2 in amine-PEG membranes.52 Alternatively, Ahn et al. grafted poly(ethylene oxide methacrylate) on poly(vinyl chloride) via atom transfer radical polymerization (ATRP).53 The permeability for CO2 increased significantly with an increase in the amount of the grafted polymer. Another option is the preparation of polymer membranes from monomers containing PEG chains. Crosslinked poly(ethylene oxide methacrylate) shows high affinity for CO2 due to the presence of polar oxygen atoms in the PEG chains. For example, Fu et al. prepared supported ultra-thin nanocomposite film from crosslinked poly(ethylene oxide methacrylate) using a continuous assembly of polymer prepared via ATRP.54 The permeance of this polymer for CO2 was 1260 GPU and CO2/N2 selectivity more than 40. The separation performance of CO2 from natural gas was not shown in this study. So far, hybrid membranes enabling good separation performance formed from crosslinked poly(ethylene glycol) methacrylate containing inorganic filler were rarely 5 ACS Paragon Plus Environment


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reported.55 For example, Bae and Long prepared mixed matrix membranes from crosslinked poly(ethylene oxide methacrylate) and MOF prepared from magnesium chloride and 1,4-dioxido-2,5-benzenedicarboxylate. However, these membranes exhibited a decrease in permeability compared to the membrane prepared from the plain polymer. The authors explained this phenomenon by blockage of the pores in MOF with the flexible polymer chains.56 Similarly, admixing silica nanoparticles in the polymer prepared from poly(ethylene oxide) dimethacrylate reported by Urban’s group also led to a significant decrease in permeability, which was assigned to the favorable interaction between polymer chains and the hydroxyl groups at the surface of the silica.57 Later, this group reported the fabrication of mixed matrix membranes from polysulfone and UiO-66-NH2 MOF which exhibited enhanced permeation due to the formation of “dual transport pathways”.58 They demonstrated that this transport behavior was a new non-classical transport regime and an ideal mechanism to enhance the permeability of mixed matrix membranes. The inorganic filler exists as a continuous phase across the membrane. Inspired by this work, the main challenge in designing hybrid membranes remains to rationally select a suitable combination of inorganic filler and polymer,59,60 as well as to develop fabrication techniques enabling formation of a continuous MOF phase within the polymer matrix to provide dual transport pathway for the desired gas separation.58 In this study, we describes the preparation of new hybrid membranes via admixing MOFs UiO-66 and UiO-66-NH2 in the mixture of poly(ethylene glycol) monomethacrylate and poly(ethylene glycol) dimethacrylate followed by a simple 6 ACS Paragon Plus Environment

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thermally initiated free radical polymerization that is easy to scale up for the potential industrial applications. To the best of our knowledge, the hybrid membranes prepared from crosslinked PEGMA and UiO-66 have not been reported yet. So our work is the first study to describe the design of this new hybrid membrane and its application in CO2 separation from natural gas. Based on this, we also performed structural design and alternation of interfacial properties of the membranes to achieve better separation performances. We optimized the synthetic conditions and surface chemistry of both polymer matrix and MOF fillers varying in contents, sizes and shapes, which resulted in mixed matrix membranes containing up to 35 wt% MOF. As shown in Scheme 1, the hybrid membranes exhibited a 247 % increase in CO2 permeability by the formation of dual transport pathway.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Poly(ethylene glycol) methacrylate (PEGMA, average Mn=500), poly(ethylene glycol) dimethacrylate (PEGDMA, average Mn=750), glycidyl methacrylate (GMA), and azobisisobutyronitrile (AIBN) were all purchased from Sigma-Aldrich (Shanghai, China). Zirconium (IV) chloride anhydrous (ZrCl4, 98%), terephthalic acid (99%), and 2-aminoterephthalic acid (99%) were obtained from J&K Scientific Ltd. (Beijing, China). All other chemicals were supplied by Sinopharm Chemical Reagent Co., Ltd (Beijing, China). 2.2. Preparation of spherical UiO-66-NH2. This MOF was prepared following a slightly modified method published elsewhere.61 An autoclave was loaded with 0.5 g



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of zirconium chloride, 20 mL dimethylformamide (DMF), and 4 mL concentrated HCl, and the mixture sonicated until the solid was dissolved (~20 min). 2-Aminoterephthalic acid (0.54 g) and 40 mL DMF were then added and the mixture was sonicated for additional 20 min. The autoclave was sealed and the contents heated to 80 °C for a certain period of time. The heating time is included in the designation of the MOF. For example, UiO-66-NH2 prepared using reaction time of 3 h is called UiO-66-NH2-3h. The resulting solid was first washed with DMF (2 x 30 mL) and then with ethanol (2 x 30 mL). These crystals were activated by immersing them three times in ethanol and heating in pressurized reactor to 80 °C each time for 8 h. The final product was dried in vacuum overnight at 120 °C. The powder X-ray diffraction pattern of the activated UiO-66-NH2-3h showed an agreement with the simulated pattern. The surface area determined from nitrogen adsorption isotherms measured at 77 K and calculated using BET equation was 1136 m2/g, i.e. a value that is close to that published previously. The TGA plot was also in accordance with that reference.61 2.3. Preparation of spherical UiO-66. The method was identical with that used for the preparation of UiO-66-NH2 except for use of 0.49 g terephthalic acid instead of its amino derivative and reaction times were 3 and 6 h. The heating time is again included in the designation of the MOF. 2.4. Preparation of UiO-66-NH2 and UiO-66 clusters. The method was identical with that used for the preparation of both UiO-66-NH2 and UiO-66 except for the reaction time extended to 12 h.


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2.5. Preparation of octahedral UiO-66.Octahedral UiO-66 is prepared following a method described elsewhere.62 Zirconium chloride (0.75 g) was dissolved in a mixture of acetic acid (5.5 mL) and DMF (80 mL) in a 100 mL autoclave using sonication for 2 min. Then, terephthalic acid (0.53 g) was added and the mixture was sonicated until complete dissolution. After addition of water (0.24 mL), the autoclave was sealed and the mixture heated at 120 °C for 12 h. The resulting solid was first washed with DMF (2 x 30 mL) and then with ethanol (2 x 30 mL). These as-prepared octahedral UiO-66 crystals were activated by immersing them three times in ethanol in autoclave and heating to 80 °C for 8 h. The final product was dried in vacuum overnight at 120 °C. The SEM images revealed single nanoparticle morphology of octahedral UiO-66 with the size of about 200 nm. The powder X-ray diffraction patterns of the activated octahedral UiO-66 showed an agreement with the simulated patterns. The surface area determined from nitrogen adsorption isotherms measured at 77 K and calculated using BET equation was 1558 m2/g. Both surface area and the TGA plot were also in agreement with those reported earlier.62 2.6. Characterization of MOFs. Images of MOF particles were obtained using JEOL

JSM-7610F

field

emission

High-Technologies, Tokyo, Japan).

scanning

electron

microscope

(Hitachi

Particle size distributions of the crystals were

measured using dynamic light scattering (Mastersizer 2000, Malvern, UK). The particles were dispersed in methanol and sonicated for 10 min before the test. The surface area was determined using the V-Sorb2800P surface area and porosimetry analyzer (Gold APP Instruments Corporation, Beijing, China). The transmission 9 ACS Paragon Plus Environment


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electron microscopy images of metal-organic frameworks were observed using a JEOL 2100 transmission electron microscope (Hitachi, Ltd., Japan). X-ray powder diffraction patterns were collected using a D/max-UltimaIII (Rigaku Corporation, Japan). The thermal gravimetric analysis was carried out using a DTG-60A (Shimadzu, Japan) at a heating rate of 10 °C/min in a temperature range of 25-600 °C under air atmosphere. 2.7. Preparation of membranes. A typical polymerization mixture was prepared from 700 mg poly(ethylene glycol) methacrylate and 300 mg poly(ethylene glycol) dimethacrylate to produce membrane M-70-30. Alternatively, glycidyl methacrylate was added to the monomer mixture while percentage of PEGMA was decreased adequately to keep the percentage of crosslinker at 30%. The membrane was then designated M-X-30-(70-X). Next, 1.0 wt% azobisisobutyronitrile with respect to the total amount of PEGMA and PEGDMA was added to the mixture. The solution was then sonicated for 5 min, purged with nitrogen for 3 min, and sonicated for another 5 min. This mixture was then filled into a mold consisting of two glass plates clamped together and separated with two Teflon strips. The Teflon strips were 70 µm thick and defined the thickness of the membrane. Polymerization was carried out at 65 °C for 2 h. After the polymerization reaction was completed, the mold was immersed in water for 30 min to easily detach the membrane from the glass plates. Finally, the membranes were washed with methanol and dried at room temperature before further use.


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2.8. Functionalization of poly(PEGMA-co-PEGDMA-co-GMA) membranes. GMA containing membranes were modified using reaction of epoxy groups with hexamethylenediamine. The membrane was immersed in a 2 mol/L diamine solution in toluene, and kept at 60 °C for 5 h. After the reaction was completed, the membrane was washed with methanol and dried in air. 2.9. Preparation of hybrid membranes. Hybrid membranes were prepared the same way as poly(PEGMA-co-PEGDMA) membranes except for the addition of 10-35 wt% activated MOF crystals in the polymerization mixture. MOFs were dispersed in the monomer solution using vigorous agitation for 15 min to achieve their

uniform

distribution.

During

the

free

radical

polymerization

of

poly(PEGMA-co-PEGDMA), the dispersed MOF crystals in the starting monomers solution were embedded into the polymer network and formed hybrid membranes. 2.10. Characterization of membranes. JC2000D instrument (Powereach, Shanghai, China) was used to measure the static water contact angles of a 2 µL water droplet placed at the membrane surface. Each contact angle was measured at ambient temperature and repeated five times. The water uptake capacities and swelling ratios were evaluated by immersing the membranes in deionized water at room temperature for 24 h. After wiping off the excess water from the membrane using filter paper, the weight and length of membranes were measured. Then, the membranes were dried in a vacuum oven at 100 °C for 24 h, and measured their weight and length again.



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The equations for calculating water uptake (WU) and swelling ratio (SR) are as follows: The WU of the membrane is calculated by WU =

௠ೢ ି௠೏ ௠೏

× 100%, where mw and md

are the weights of wet membrane and dry membrane, respectively. The SR of the membranes can be calculated by SR =

௅ೢ ି௅೏ ௅೏

× 100%, where Lw and

Ld are the lengths of wet membrane and dry membrane, respectively. A binary gas mixture containing 10% of CO2 in CH4 was used as the feed gas for the gas transport measurements that were conducted at 35 °C under a pressure of 1 MPa. Permeability was determined from the rate of downstream pressure increase (dp/dt) using a custom built constant volume/variable pressure instrument (Suzhou Faith & Hope Membrane Technology Co., Ltd).63 The mixed gas steady-state permeation rate was calculated using the following equations: ‫۾‬۱‫۽‬૛ = ‫۾‬۱۶૝ =

‫ܡ‬۱‫۽‬૛ ‫ۺ܄‬ ‫ܘ܌‬૚ ૛ૠ૜ × ૚૙૚૙ ૠ૟ ૠ૟૙ ‫ܜ܌‬ ‫ ܂ۯ‬ቀ ૚૝. ૠቁ (‫ܠ‬۱‫۽‬૛ ‫ܘ‬૛ )

(૚ − ‫ܡ‬۱‫۽‬૛ )‫ۺ܄‬ ‫ܘ܌‬૚ ૛ૠ૜ × ૚૙૚૙ ૠ૟ ૠ૟૙ ‫ ܂ۯ‬ቀ૚૝. ૠቁ ((૚ − ‫ܠ‬۱‫۽‬૛ )‫ܘ‬૛ ) ‫ܜ܌‬

where P is the permeability of the membrane to a specific gas (barrer), V is the volume of the downstream chamber (cm3), L is the film thickness (cm), p1 the downstream permeate gas pressure (psia), p2 the upstream feed gas pressure (psia), A is the effective area of the film (cm2), T is the experimental temperature (K), and xCO2 is the molar fraction of CO2 in the feed gas (%). The separation factor α for CO2 to CH4 was calculated from equation α= PCO2/ PCH4 . 12 ACS Paragon Plus Environment

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3. RESULTS AND DISSCUSSION 3.1. Poly(PEGMA-co-PEGDMA) membranes. Copolymerization of PEGMA and PEGDMA in the mold produced a 70 µm thin transparent membrane with a good flexibility that could be curled by hand (Figure 1). Compared with the non-crosslinked membrane prepared from pristine PEGMA, its crosslinked counterpart exhibits significantly enhanced mechanical strength as indicated in Table S1. SEM images presented in Figure 2a,b demonstrate that the membrane is homogeneous with a smooth surface which roughness determined from AFM image (Figure 2c) has a surface roughness Ra of 1.32 nm. Hydrophilicity of this polymer was confirmed using contact angle measurement (Figure S1a,b in the supporting information). The water contact angle measured immediately after applying the droplet was 60o and rapidly decreased to 31o in 30 s while all the water was adsorbed in the membrane after about 1 min. Using mixed gas, the dry membrane exhibited a reasonable permeability of 117 barrer and a selectivity of 14.5 for CO2. This separation performance compares well with those published by Xiang et al.

19

and Su

et al.57. Fu et al. who achieved a CO2 permeability of 1260 GPU (201 barrer) using an ultra-thin (~100 nm) film prepared via atom transfer radical polymerization of PEG-based macro crosslinkers.54 However, this polymerization technique would be very difficult to apply on industrial scale. In contrast, we embraced the typical free-radical polymerization for the preparation of polymer membranes which is easily amenable to scale-up.



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3.2. Poly(PEGMA-co-PEGDMA-co-GMA) membranes. Concept of gas CO2 separation based on facilitated transport involves a reversible chemical reaction.5, 64 Aliphatic amine functionalities are known to be powerful immobilized carrier enabling facilitated transport.65-67 They react reversibly with CO2 and accelerate the permeation whereas methane can permeate only via solution-diffusion mechanism that is much less efficient. In order to test this approach, we prepared next series of membranes using polymerization mixture containing PEGMA, PEGDMA, and GMA. We kept the percentage of crosslinker PEGDMA constant at 30% while the percentage of GMA varied from 5 to 40%. This increase in the glycidyl methacrylate contents led to an increase in the quantity of reactive epoxide functionalities enabling following functionalization with diamine but also decreased flexibility of the membrane. Table 1 presents effect of glycidyl methacrylate content on separation performance. An increase in GMA contents reduced permeability of both gases while the effect on selectivity was small. This effect likely resulted from a decrease in content of poly(ethylene oxide) moieties in the polymer. Functionalization via reaction with hexamethylenediamine did not improve performance of the membranes either. Just in opposite, this reaction led to a significant decrease in permeability. For example, permeability of membrane M-65-30-5 for CO2 decreased from 90.6 to 55.9 barrer after its reaction with diamine. Explanation for this effect is presumably due to in an increase in crosslinking density since reaction of both amine functionalities of the diamine creates an additional crosslink that decreases the chain mobility. This conclusion can be confirmed from the slightly decreased swelling ratios of 14 ACS Paragon Plus Environment

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poly(PEGMA-co-PEGDMA-co-GMA) membranes after functionalization with hexamethylene-diamine (Table S2). Meanwhile, the water uptake capacity increased due to the introduction of hydrophilic functionalities. Thus, despite the presence of secondary amine groups, the crosslinking inhibits permeation of CO2, affects the free volume of the polymer, and decreases diffusion of the gas.

3.3. Poly(PEGMA-co-PEGDMA) mixed matrix membranes. 3.3.1. Membrane with UiO-66-NH2 and UiO-66-3h nanoparticles. The above results demonstrate that the polymer based membranes alone do not meet requirements of high performance no matter what chemistry they contain. Assuming the strong interaction of CO2 with aliphatic amine functionalities is the key factor for unsatisfactory performance, a decrease in the strength can be solution to this problem. Thus, we prepared mixed matrix membranes that combine the polymer with plurality of poly(ethylene glycol) moieties with metal organic frameworks (MOFs). UiO-66 and UiO-66-NH2 were selected as the inorganic fillers due to their excellent chemical and thermal stability, and good CO2/CH4 separation performances. 23, 41, 42, 58 We first used an aromatic amine group containing linker, i.e. 2-aminoterephthalic acid (or 2,4-anilinedicarboxylic acid) and a Zr salt for the preparation of UiO-66-NH2. Selection of the linker was inspired by the early work demonstrating excellent performance of membranes prepared from polyaniline coated with PEG.5, 65, 68 The synthetic approach for the preparation of MOF-polymer hybrid membranes were illustrated in Scheme 2. Figure 3a shows TGA plots for both individual components 15 ACS Paragon Plus Environment


of

the

mixed

matrix

membrane,

i.e.

Page 16 of 50

UiO-66-NH2-3h

and

poly(PEGMA-co-PEGDMA), as well as that of the mixed matrix membrane itself. While UiO-66-NH2-3h lost about 50% of its weight after heating to about 450 °C, poly(PEGMA-co-PEGDMA) began to decompose at 300 °C indicating its good thermal stability. Residual mass of the mixed matrix membrane heated to 450 °C is about 5% which corresponds well with the 10% loading of the MOF. Presence of the MOF in the polymerization mixture also changed morphology of the membrane as demonstrated in SEM image of Figure S2 and XRD pattern (Figure 4). However, added MOF did not change significantly hydrophilicity of this membrane (Figure S1c,d). The separation performance of the mixed matrix membrane did not improve as expected. On the opposite, the permeability for CO2 decreased to 76.8 barrer compared to that of the plain poly(PEGMA-co-PEGDMA) membrane, while selectivity remained almost unchanged at 16.6. Further extension of the reaction time to 12 h did not generate UiO-66-NH2 with dramatically superior separation performance. The permeability for UiO-66-NH2-12h incorporating polymer membrane is mere 88.5 barrer, and the selectivity is 15 (Table 2). We speculate that the reason for this result might be the favorable interaction between polymer chains and the MOF surface that restricts permeation of gas in the MOF as observed earlier.57 This explanation can be further confirmed from the better separation performance using the hybrid membrane containing the same amount of UiO-66-3h. Instead of using an amine containing linker, UiO-66-3h was prepared from terephthalic acid. The absence of amine groups features less interfacial interaction 16 ACS Paragon Plus Environment

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between the polymer chains and UiO-66 surface, and as a result reveals relatively larger permeability with a value of 97.8 barrer (Table 2). 3.3.2. Membrane with UiO-66 octahedrons. Since use of both UiO-66-NH2 and UiO-66-3h nanoparticles did not significantly improved performance of the membrane, we tested a different format of UiO-66, i.e. octahedrons. SEM and TEM images in Figure 5a, c show the shape of the UiO-66-octahedron crystals with an average size of 174 nm. The BET surface area of UiO-66 octahedron is 1558 m2/g calculated from N2 adsorption and desorption isotherm as shown in Figure S3a. Using the typical polymerization procedure, we prepared poly(PEGMA-co-PEGDMA) mixed matrix membranes containing 10 wt.% UiO-66-octahedron. Separation performance of these membranes is presented in Table 3. An increase in the UiO-66 octahedron content to 35 wt% enhanced permeability for CO2 and selectivity more significantly to about 205 barrer and 19, respectively. It is worth noting that we achieve this increase in permeability without sacrificing the selectivity. Our results compare favorably with reports published in the literature as shown in Table 4. The data shown in the Table 4 were mostly obtained using hybrid membranes containing MOFs embedded in rubbery polymers.19, 24, 56, 57, 69-75

3.3.3. Membrane with UiO-66 clusters. SEM images shown in Figure S4 confirmed that the MOF octahedrons were again homogeneously dispersed in the polymer at both percentages. Similar to the membranes containing UiO-66-NH2 and UiO-66 nanoparticles, the UiO-66 17 ACS Paragon Plus Environment


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octahedrons were separated by the polymer and did not contact each other. This disconnection was the likely reason why the dual transport mechanism could not be fully activated. In order to achieve the dual transport, which is thought to be an ideal mechanism for the mixed matrix membranes,58 and to enhance the permeability, we synthesized larger particles of UiO-66. Differing in the degree of aggregation, they were prepared by adjusting the reaction time to 3, 6, and 12 h, and designated as UiO-66-3h, UiO-66-6h, and UiO-66-12h shown in the SEM images of Figure S5. Their respective median particle sizes were 300, 700, and 1,300 nm as determined by the dynamic light scattering (Figure 6). N2 adsorption and desorption isotherm reveals a BET surface area of 1504 m2/g for UiO-66-12h as shown in Figure S3b. We then prepared hybrid matrix membranes containing 10 wt.% of UiO-66 prepared using different synthesis times. Performance of these membranes is presented in Table 2. The permeability increased along with the size of aggregated UiO-66. For example, performance for membrane containing 10% UiO-66-12h was characterized by a permeability for CO2 of 151.9 barrer and a selectivity of 17.8. Clearly, the aggregated state of UiO-66-12h shown in Figure 5b,d benefits the permeability. Values shown in Table 2 are higher than those determined for the polymer film with no MOF additive (Table 1). They also exceed those observed for hybrid membrane containing the other two types of MOFs incl. UiO-66-NH2-3h particles (PCO2=76.8 barrer, α=16.6) and UiO-66-octahedron (PCO2=88.5 barrer, α=14.9), which had a particle size of only about 200 nm. These results also indicate that the interaction between the UiO-66 and the polymer matrix is weaker compared 18 ACS Paragon Plus Environment

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to the MOF counterparts. Consequently, the transport of CO2 through the membrane is accelerated. Significant effect of UiO-66-12h has already been achieved using 10% addition. To appraise the effect of quantity of the MOF in the membranes, we prepared a series of membranes differing in the content of UiO-66-12h. The maximum content we could use was 35 wt.% since a polymerization mixture containing 40 wt.% had a play dough-like consistence and a good membrane could not be prepared as shown from the SEM images in Figure S8. However, even the membrane containing 35 wt.% MOF remains homogeneous and defect free (Figure 7). The contents of MOF were confirmed using TGA (Figure S6). While the XRD pattern of the polymer matrix is as expected amorphous featuring broad peaks at 13° and 18°, the characteristic peaks of mixed matrix membranes matched those of simulated UiO-66 and their intensity increased with the increase in the percentage of the MOF (Figure 4 and Figure S7). The significant peak shifts (2θ=13°, 20°) of hybrid membranes containing 10 wt% UiO-66 and 10 wt% UiO-66-NH2 are probably attributed to the preferential orientation of the crystal in the polymer matrix. To better understand how the MOF clusters interact with the polymer, the glass transition temperatures (Tg) of hybrid membranes containing different amounts of MOFs were examined using differential scanning calorimetry (DSC). As presented in Figure 3b, while the Tg of all hybrid membranes were similar to the bare polymer membrane, broadening of the thermograms were observed for the hybrid membranes containing 20, 30, and 35 wt.% UiO-66 clusters. This indicated favorable interfacial interaction between MOF and 19 ACS Paragon Plus Environment


Page 20 of 50

polymer which is advantageous to enhance the mechanical properties of the membrane.57, 64 Table 5 demonstrates that further increase in the percentage of UiO-66-12h enhanced permeability that grew to about 365 barrer for the membrane with the highest content of MOF. This represents a 247 % increase in CO2 permeability as presented in Table 4 and Figure 8. The interconnected UiO-66-12h clusters in that membrane form a close to continuous phase desirable for the dual transport mechanism. This parallel transport pathway favorably affects the gas permeation because the molecular diffusion through the rigid porous media like MOF is typically more intensive than that through the amorphous polymers as a result of the lower activation energy. While benefiting permeability, this increase had only a small effect on selectivity since with an increase in permeability for CO2, we also observed an increase in permeability for CH4. The highest selectivity of 17.8 was monitored for the membrane containing 10 wt.% of MOF.

4. CONCLUSION This work demonstrates that defect-free crosslinked membranes can be prepared using a simple free radical initiated polymerization of poly(ethylene glycol) moieties containing monomers, poly(ethylene glycol) methacrylate and poly(ethylene glycol) dimethacrylate. In contrast to the concept of facilitated transport,53, 67 we found that introduction of even rather basic aliphatic amine functionalities in the membrane as 20 ACS Paragon Plus Environment

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the fixed carrier did not contribute to an increase in performance of organic polymer-based membranes for the separation of carbon dioxide from methane. However, admixing zirconium-based metal-organic frameworks in the polymerization mixture followed

by

polymerization

produced

membranes

with

enhanced

performance. Application of organic linker containing aniline-like functionality in the terephthalic acid for the synthesis produced MOF which effect on the separation properties of the membrane was not significant. However, much larger improvement was observed for membranes including MOF prepared from neat terephthalic acid. Using extended reaction time, the MOF crystals formed clusters which, at high percentage present in the membrane, enabled dual transport separation mechanism characterized by a permeability for CO2 exceeding 300 barrer. We understand that the accuracy of our measurements may be limited by the relatively small area of the membrane given by our experimental setup. Nevertheless, our results clearly demonstrate the positive effect of the MOF. Thanks to the simplicity of the synthesis of MOFs, we are now preparing frameworks with different functionalities. Our continuing research is focused on testing these MOFs to achieve significant enhancements in selectivity of the separation in addition to the current membranes featuring rather high permeability.

ASSOCIATED CONTENT



Page 22 of 50

Supporting Information. Images of droplets used for measurement of water contact angles of poly(PEGMA-co-PEGDMA) film and its counterpart containing 10 wt% UiO-66-NH2-3h (Figure S1), SEM images of the cross-section of mixed matrix poly(PEGMA-co-PEGDMA) membrane containing 10 wt% UiO-66-NH2-3h with different magnifications, × 800 (a) and × 20,000 (b) (Figure S2), nitrogen adsorption/desorption isotherms for UiO-66 octahedron and UiO-66-12h cluster (Figure S3), SEM images of mixed matrix membranes containing 10 wt% UiO-66 octahedron and 35 wt% UiO-66 octahedron (Figure S4), SEM images of UiO-66 clusters prepared from different reaction time (Figure S5), the TGA plots of mixed matrix membranes varying in content of UiO-66-12h cluster (Figure S6), and XRD patterns (Figure S7), SEM images of surface and cross-section of mixed matrix membranes containing 40 wt.% UiO-66-12h cluster (Figure S8). Mechanical strength properties of membranes prepared from pristine PEGMA and crosslinked PEGMA-co-PEGDMA (Table S1), the water uptake and swelling ratio of poly(PEGMA-co-PEGDMA-co-GMA) membranes before and after functionalization with hexamethylene-diamine. (Table S2) are available in the Supporting Information.

AUTHOR INFORMATION Corresponding Author * Yongqin Lv, E-mail: [email protected]; Phone: +86 10 64454356 * Tianwei Tan, Email: [email protected]

Notes 22 ACS Paragon Plus Environment

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The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2015BAD15B07), National Natural Science Foundation of China (21576017, 21436002), 973 program (2014CB745100), and the Fundamental Research Funds for the Central Universities.

ABBREVIATIONS MOF,

metal-organic

framework;

COF,

covalent-organic

framework;

PEG,

poly(ethylene glycols); ATRP, atom transfer radical polymerization; PEGMA, poly(ethylene glycol) methacrylate; PEGDMA, poly(ethylene glycol) dimethacrylate; GMA,

glycidyl

methacrylate;

AIBN,

azobisisobutyronitrile;

DMF,

dimethylformamide; DSC, differential scanning calorimetry.

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(74) Zhou, T.; Luo, L.; Hu, S.; Wang, S.; Zhang, R.; Wu, H.; Jiang, Z.; Wang, B.; Yang, J. Janus Composite Nanoparticle-Incorporated Mixed Matrix Membranes for CO2 Separation. J. Membr. Sci. 2015, 489, 1-10. (75) Li, X.; Jiang, Z.; Wu, Y.; Zhang, H.; Cheng, Y.; Guo, R.; Wu, H. High-Performance Composite Membranes Incorporated with Arboxylic Acid Nanogels for CO2 Separation. J. Membr. Sci. 2015, 495, 72-80.



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Figures

Scheme 1. Schematic illustration of “dual transport pathway” formed in hybrid membranes containing MOFs.


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Scheme 2. The schematic illustration of the synthetic approach for the preparation of mixed matrix membranes based on crosslinked poly[(ethylene glycol) methacrylate] and metal-organic framework (MOF).



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Figure 1. Photograph of the bended poly(PEGMA-co-PEGDMA) membrane.


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Figure 2. SEM and AFM images of the poly(PEGMA-co-PEGDMA) film. (a) Surface of the film; (b) cross section of the film, (c) AFM of the surface.



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Figure 3. (a) TGA plots of UiO-66-NH2-3h (1), poly(PEGMA-co-PEGDMA) containing 10 wt% UiO-66-NH2-3h (2), and poly(PEGMA-co-PEGDMA) membrane (3), and (b) DSC thermograms of mixed matrix membranes containing 10, 20, 30, and 35 wt.% UiO-66 clusters. The thermograms are shifted by their respective Tg.


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Figure 4. XRD patterns of simulated UiO-66 (bottom), UiO-66-NH2-3h, UiO-66-3h, poly(PEGMA-co-PEGDMA) membrane, and mixed matrix membranes containing 10 wt% UiO-66-NH2-3h and 10 wt% UiO-66-3h.



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Figure. 5 SEM and TEM images of UiO-66 octahedrons (a,c) and cluster UiO-66-12h (b,d).


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Figure. 6 Particle size distribution of UiO-66 octahedron (a) and its clusters prepared using synthesis times of 3 (b), 6 (c), and 12 h (d).



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Figure. 7 SEM images of mixed matrix poly(PEGMA-co-PEGDMA) membrane containing (a) 10, (b) 20, (c) 30, and (d) 35 wt.% UiO-66-12h.


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Figure. 8 Mixed CO2 and CH4 permeation properties of membranes measured at 1 MPa upstream pressure and 35 °C (see Table 4 for the detailed information). The solid line represents the upper bound limit for polymeric membranes established in 2008.4



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Table 1. Mixed gas permeability and selectivity of poly(PEGMA-co-PEGDMA-co-GMA) membranes and their counterparts functionalized with hexamethylenediamine. P b, barrer

Membrane

Composition , %

N content wt .%

M-70-30-0

70:30:0

0

117.6

8.1

14.5

M-65-30-5

65:30:5

0

90.6

6.0

15.2

M-60-30-10

60:30:10

0

74.1

4.8

15.3

M-50-30-20

50:30:20

0

45.8

2.8

16.7

M-40-30-30

40:30:30

0

29.9

1.7

17.9

M-30-30-40

30:30:40

0

19.4

0.9

21.6

M-65-30-5-NH2

65:30:5

0.98

55.9

3.4

16.6

M-60-30-10-NH2

60:30:10

1.51

32.4

1.9

16.8

M-30-30-40-NH2

30:30:40

3.99

a

a

CO2

CH4

11.1

0.6

S c, CO2/CH4

18.1

Weight percentages of PEGMA, PEGDMA, and GMA; b Permeability; c Selectivity


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Table 2. Mixed gas permeability and selectivity of mixed matrix poly(PEGMA-co-PEGDMA) membranes containing 10 wt.% MOF. Permeability, barrer

Membrane

Selectivity

CO2

CH4

CO2/CH4

UiO-66-NH2-3h

76.8

4.6

16.6

UiO-66-NH2-12h

88.5

5.9

15.0

UiO-66-3h cluster

97.8

6.4

15.3

UiO-66-6h cluster

128.9

7.9

16.3

UiO-66-12h cluster

151.9

8.5

17.8

Table 3. Mixed gas permeability and selectivity of mixed matrix membranes containing UiO-66-12h cluster and UiO-66 octahedron. Membrane

Permeability (barrer)

Selectivity

CO2

CH4

CO2/CH4

10 wt% UiO-66 octahedrons

88.5

5.9

14.9

35 wt% UiO-66 octahedrons

205.4

10.6

19.4

10 wt% UiO-66-12h cluster

151.9

8.5

17.8

35 wt% UiO-66-12h cluster

364.7

29.2

12.5



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Table 4. Selected CO2 permeability % increase and selectivity values of hybrid membranes containing MOFs and rubbery polymers reported in literatures and this work. CO2/CH4

CO2/N2

selectivity

selectivity

(%)

(%)

29

71

-

19

37

269

-110

-

19

ZIF-7-NH2(70)/XLPEO

30

105

162

-

19

ZIF-7-NH2(70)/XLPEO

36

420

-24

-

19

Cu3(BTC)2/PDMS

10

22

9

6

70

Mg2(dobdc)/PDMS

20

-32

-

16

56

Mg2(dobdc)/XLPEO

10

-34

-

14

56

Silica/XLPEGDA

30

-61

4

0

57

IL@ZIF-8/ Pebax®1657

15

71

75

94

71

H_ZIF-8/PVC-g-POEM

30

790

-18

-

72

ZIF-7/ Pebax®1657

22

54

114

185

73

Janus/ Pebax®1657

10

62

36

-

74

CANs/ Pebax®1657

30

45

15

27

75

P@MOF2/PolyActive

40

110

-

27

24

CNT/P-PEG20000(40)

3

57

-76

-

69

CNT/P-PEGDME(40)

5

45

-

22

69

UiO-66-Octahedron/XLPEG

35

75

34

-

This work

UiO-66-Cluster/XLPEG

35

247

-14

-

This work

Inorganic

CO2

loading

permeability

(%)

(%)

ZIF-7/XLPEO

28

ZIF-7/XLPEO

Membrane a

a

Ref.

PEO – poly(ethylene oxide); XLPEO crosslinked poly(ethylene oxide); PDMS – poly(dimethyl

siloxane); XLPEGDA - crosslinked poly(ethylene glycol dimethacrylate); CNT – carbon nanotubes; XLPEG crosslinked poly(ethylene glycol)


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Table 5. Mixed gas permeability and selectivity of poly(PEGMA-co-PEGDMA) mixed matrix membranes containing different percentages of MOF UiO-66-12 h cluster. MOF content

Permeability, barrer

Selectivity

wt.%

CO2

CH4

CO2/CH4

0

117.6

8.1

14.5

10

151.9

8.5

17.8

20

203.6

14.3

14.2

30

255.6

19.4

13.2

35

364.7

29.2

12.5



Table of content 254x190mm (96 x 96 DPI)

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