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Polyethyleneimine-Grafted HKUST-type MOF/polyHIPE Porous Composites (PEI@PGD-H) as Highly Efficient CO2 Adsorbents Junjie Zhu, Linbo Wu, Zhiyang Bu, Suyun Jie, and Bo-Geng Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00213 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Industrial & Engineering Chemistry Research

Polyethyleneimine-Grafted HKUST-type MOF/polyHIPE Porous Composites (PEI@PGD-H) as Highly Efficient CO2 Adsorbents Junjie Zhu, Linbo Wu*, Zhiyang Bu, Suyun Jie*, Bo-Geng Li State Key Laboratory of Chemical Engineering at ZJU, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China [email protected] for Linbo Wu [email protected] for Suyun Jie

ABSTRACT Searching highly efficient and robust CO2 adsorbents is very important for CO2 capture, utilization and storage. In this study, new polyethyleneimine-grafted MOF-polymer composites, abbreviated as PEI@PGD-H, were synthesized via HIPE template polymerization of divinylbenzene and glycidyl methacrylate, followed by in-situ generation of HKUST-type MOF in the presence of hydrophobic-modified CuO nanoparticles via the reaction of CuO with 1,3,5-benzenetricarboxylic acid and then PEI functionalization via epoxy-amine reaction and amine-metal sites interaction. The coexistence of hierarchical interconnected porous skeleton of PGD, high specific surface area of MOF and chemical CO2 adsorption of PEI endows PEI@PGD-H with high CO2 adsorption capacity, rate and selectivity. PEI70@PGD-H shows a CO2/N2 separation factor as high as 76 and CO2 adsorption capacity of 4.3, 3.0 and 1.8 mmol CO2/g from pure CO2, simulated flue gas and air, respectively. After 20 CO2 adsorption-desorption cycles, the adsorbent still retained a high and stable CO2 capture capacity over 2.8 mmol CO2/g for simulated flue gas. The adsorbent also displays reasonably good thermal stability, excellent water endurance, low desorption energy and easy shaping. The desorption heat is as low as 48 kJ/mol CO2. These features make PEI@PGD-H highly efficient, robust and low-energy CO2 adsorbents for practical application in CO2 capture.

Keywords: metal-organic frameworks; HIPE porous polymers; polyethyleneimine; adsorbents; CO2 capture; gas adsorption

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INTRODUCTION Carbon capture, utilization and storage (CCUS) technology has received extensive attention in recent years for the purpose to control the concentration of carbon dioxide (CO2) in the air.1-3 The chemical absorption technology using aqueous organic amines, for instance, monoethanolamine (MEA), as an absorbent4-6 has been used to remove CO2 from industrial waste gas. Currently, it is the most important and effective industrial CO2 capture technology. However, high energy consumption, equipment corrosion and loss and degradation of amine are intractable problems to limit its large-scale application. Different from liquid absorbents, porous solid adsorbents such as metal−organic frameworks (MOFs)7,8 and porous polymers9,10 manifest much lower energy consumption, high specific surface area, multifunctional active groups, simple preparation process and low cost11. These features make them good candidates for CO2 capture. Amine functionalization of MOFs and porous polymers such as polyHIPE is usually used to enhance their interaction with CO2,12 and thus to improve adsorption capacity and selectivity of CO2. MOFs are three-dimensional porous crystalline materials made up of metal ions or clusters coordinated to organic linkers. They are characterized by large specific surface area (103 m2/g) and well-defined pore properties.13-16 The interaction between amines with the open metal sites of MOFs has been extensively used to enhance CO2 adsorption capacity of MOF-based adsorbents, such as N,N’-dimethylethylenediamine (mmen)/[Mg2(dobpdc)]17, ethylenediamine (ED)/ZIF-818, polyethyleneimine (PEI)/MIL-10119,20, tetraethylenepentamine (TEPA)/MIL-101-NH221, PEPA/Mg-MOF-7422 PEI/MIL-101-NH223 and PEI/HKUST24 composites. PolyHIPE is the abbreviation of porous polymers synthesized via high internal phase emulsion (HIPE) template method, which is characterized by interconnected hierarchical porous structure.25 Han et al26 reported an CO2 adsorption capacity as high as 4.85 mmol/g at 40 °C in high-humidity gas for PEI hydrogel impregnated poly(glycidyl methacrylate)HIPE. Wang et al27 also reported PEI-modified poly(styrene/divinylbenzene)HIPE with uniform and interconnected macropore and enhanced CO2 sorption. Among the polyamines, PEI possesses highly dense amine groups, especially easily accessible primary amine sites at chain end which can react with CO2 forming carbamates28, therefore it appears to be a very suitable candidate for amine functionalization. As fine powdery microcrystals, MOFs usually need to be shaped for practical use. In comparison, polyHIPE can be facilely shaped but usually has relatively low specific surface area (10-102 m2/g). High PEI loading in polyHIPE further leads to decrease in the specific surface area, therefore limiting the adsorption kinetics. To make full use of high specific surface area of MOF and interconnected hierarchical porous structure and facile shaping of polyHIPE, a number of MOF/polyHIPE composites have been synthesized by different routes, including conventional solvothermal synthesis of MOF crystals within the pre-formed polyHIPE support29 and incorporation of pre-formed MOF crystals into the HIPE followed by polymerization.30 However, the challenges still exist in both methods to improve the chemical and water stability of the resulting emulsion, to raise MOF content and to avoid clogging of MOF micropores. Wickenheisser et al31 utilized pre-polymerization of the N-isopropyl acrylamide HIPE emulsion before adding the MIL-MOF powder to minimize pore blocking effect. Simultaneous formation of MOF and polyHIPE was also reported, but the resulting composite 2


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had a low specific surface area (16.4 m2/g).32 Recently, Mazaj et al33 had prepared hybrid materials of MOF-5 or HKUST-1 and poly(dicyclopentadiene)HIPE from a metal salt-free technique, wherein the MOF was in situ formed from the ZnO and CuO nanoparticles by seed growth. The resulting composites contained MOF as high as 75 w% and exhibited excellent structural hydrostability and durable CO2 adsorption (2.45 mmol/g) under humid conditions. Although the great progress has been made, exploring CO2 adsorbents with superior and balanced performance (possessing high capacity/rate/selectivity, low energy, low cost, robustness/endurability and easy shaping at the same time) remains a challenging task for CCUS technology. Considering chemical CO2 adsorption of PEI, high specific surface area of MOF and hierarchical interconnected porous skeleton and easy shaping of polyHIPE, we anticipate that the new PEI-grafted MOF/polyHIPE composites could be such candidates deserving in-depth study. In this work, we report new PEI-grafted HKUST-type MOF/polyHIPE composites, PEI@PGD-H, as highly efficient CO2 adsorbents. The composite was synthesized via HIPE template polymerization of glycidyl methacrylate and divinylbenzene in the presence of hydrophobic-modified CuO nanoparticles followed by reaction of 1,3,5-benzenetricarboxylic acid with CuO to in-situ generation HKUST-type MOF in the PGD porous polymer. Then the composite (symbolized as PGD-H) was functionalized with PEI to prepare the PEI@PGD-H adsorbents via epoxy-amine reaction and amine-metal opening site interaction. The structure and morphology of the adsorbents were characterized, and the CO2 sorption behaviors under various conditions were investigated.

EXPERIMENTAL Materials Divinylbenzene (DVB, containing 20% ethyl styrene), glycidyl methacrylate (GMA, ≥97%), potassium persulfate (K2S2O8, 99.99%), copper dinitrate (Cu(NO3)2·3H2O, 99%), nanometer copper oxide (CuO, 99.5%, particle size of 100-200 nm), 1,3,5-benzenetricarboxylic acid (H3BTC, 98%), oleic acid (OA, analytical reagent) and branched polyethyleneimine (PEI, 99%, Mw = 600, viscous liquid, theoretical amino group content 32.9 w%, primary amino group content 12.8 w%) were purchased from Aladdin Chemical Co. Ltd. China. Toluene (Tol), sodium hydroxide (NaOH), anhydrous methanol, anhydrous ethanol, acetone, hexadecane (HD) and anhydrous calcium chloride (CaCl2) were all of analytical reagents and purchased from Sinopharm Chem. Reagent Co. Ltd. China. Synperonic PEL 121 (triblock copolymer of poly(propylene glycol) and poly(ethylene glycol), Mw = 4400, Sigma-Aldrich) was used as received. DVB was pre-treated to remove inhibitor before use34. The other chemicals were used as received. Carbon dioxide (CO2, ≥ 99.995%) and nitrogen (N2, ≥ 99.5%) were purchased from Jingong Specialty Gases Co. Ltd. China. Synthesis method The schematic diagram for the synthesis of PEI@PGD-H, namely, PEI-grafted HKUST-type MOF/polyHIPE composites, is shown in Scheme 1. The synthesis procedures are detailed as follows. 3



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PolyHIPE Containing CuO Nanoparticles (PGD-Cu) PGD-Cu was prepared via high internal phase emulsion (HIPE) template method 34 in the presence of CuO nanoparticles which was hydrophobic-modified with oleic acid (OA)33. The monomer GMA, crosslinker DVB, hydrophobic CuO nanoparticles, surfactant PEL 121, porogen toluene and hydrophobe hexadecane were added into a three-necked flask and stirred at 600 rpm for 30 min to obtain the organic phase. The initiator K2S2O8 (1 w% based on the sum of GMA and DVB) and co-stabilizer CaCl2 (2 w% based on aqueous phase) were dissolved in deionized water to obtain the aqueous phase. Then, a HIPE was formed by adding the aqueous phase dropwise to the organic phase within 10 min and stirring at 600 rpm for another 30 min. The HIPE was poured into a mould, then polymerized and crosslinked at 60 °C for 24 h. After Soxhlet extraction with deionized water and ethanol for 24 h respectively and vacuum drying at 60 °C for 24 h, a grey product (Fig. S1) symbolized “PGD-Cu” was obtained. For comparison, neat “PGD” polymer (pale yellow in color) was also prepared in the same way except the absence of CuO nanoparticles in the organic phase, some change in the recipe and slower stirring rate (400 rpm). The detailed recipes for syntheses of PGD and PGD-Cu are listed in Table 1.

Organic phase (15 w%) (GMA+DVB+n-CuO+121+C16+Tol) O O GMA

+

O

+

DVB

a. Mixing

n-CuO

b. Polymerization

Aqueous phase (85 w%) (H2O+CaCl2+K2S2O8) PGD-Cu O

O

HO

OH O

N

NH2

N

N H H 2N

N H

N

N

NH2 H N

Solvothermal reaction

OH

H3BTC

NH2

PEI Functionalization

PGD-H

PEI@PGD-H

Scheme 1. Schematic diagram for the preparation of PEI@PGD-H.

4


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Table 1. Recipes of two kinds of porous polyHIPEs, PGD and PGD-Cu. Sample

a.

Aq.a

GMAb

DVBb

HDb

Tolb

PEL 121b

CuOb

(w%)

(w%)

(w%)

(w%)

(w%)

(w%)

(w%)

PGDc

85

83

17

5

45

20

0

PGD-Cu

85

83

17

5

20

10

40

The mass fraction of aqueous phase.

b.

The mass fraction of GMA, DVB, HD, Tol, PEL 121 and modified CuO based on

c.

the sum of GMA and DVB. The recipe of PGD is the same of P4 in ref.34.

HKUST-type MOF/polyHIPE Composites (PGD-H) The PGD-Cu (1.0 g) was cut into small pieces and added to a solution of H3BTC (0.81 g) in 32 mL ethanol and 8 mL water. After reaction at 120°C for 48 h and cooling to room temperature, HKUST-type MOF was in-situ formed in the porous PGD-Cu polymer. The reaction mixture was filtered and the product was washed with acetone. Finally, the resulting pale blue composites of PGD and HKUST (symbolized as PGD-H) were dried under vacuum at 70 °C for 24 h. The content of HKUST in PGD-H was calculated to be 57.9 w% by the weight increment after the reaction. Neat HKUST (Cu3(BTC)2) was also prepared according to the procedure reported previously24,35 with some modifications. Under continuous stirring, a solution of H3BTC (2.1014 g, 10 mmol) in 40 mL ethanol was slowly added to 40 mL aqueous solution of Cu(NO3)2·3H2O (4.3488 g, 18 mmol). The homogenous mixture was heated at 120 °C for 18 h. The resulting product precipitated as blue microcrystalline powder. After cooling, centrifugating at 8000 rpm for 10 min and washing with deionized water and ethanol respectively, the powder was dried at 70 °C under vacuum for 24 h. The yield was 88%. PEI-grafted PGD-H (PEI@PGD-H) The PGD-H (1.0 g) was added into a PEI/methanol solution (0.6 or 0.7 g/15 mL). The reaction lasted for 12 h at 60°C, then the mixture was filtered. The product was washed with methanol and then dried under vacuum at 60°C for 12 h. The resulting deep blue product was symbolized as PEIx@PGD-H, where x was the mass percentage of feeding PEI based on PGD-H (x = 60, 70). For comparison, PEI60@PGD was also prepared in the same way except that PGD was used instead of PGD-H. Characterization FTIR spectra were recorded with a Nicolet 5700 infrared spectrometer using KBr disk samples. Elemental analysis was done with a Flash EA1112 elemental analyser (ThermoFinnigan Co.). Powder X-ray diffraction (PXRD) patterns were recorded with a PANalytical X’Pert X-ray diffraction system with Cu Kα radiation (1.54 Å), working at 40 kV and 40 mA. The samples were degassed at 100 °C for 24 h and then used for N2 adsorption isotherm measurement at 77 K with a Quantachrome Autosorb-1-C instrument. Scanning electron microscope (SEM, Utral 55, Carl zeiss, Germany) observation was conducted at an acceleration voltage of 1.5 kV, using samples whose fractured surfaces were coated with a thin gold layer prior to observe. Thermogravimetric curves were recorded on a Perkin-Elmer instrument Pyris 1 TGA analyser under N2 flow at 20 mL/min, heating 5



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from 50 to 850 °C at 10 °C min-1. CO2 Capture Adsorption/desorption experiments were carried out with the same method reported previously.34,36 Prior to CO2 adsorption, the adsorbent (about 1 gram) was heated to 100 °C and outgassed completely under vacuum. CO2 desorption heat (Qdes) was measured by differential scanning calorimetry (DSC, Q200, TA Instrument). The CO2-saturated samples (6-8 mg) were scanned from -20 oC to 220 oC at 10 oC/min under nitrogen flow.37 RESULTS AND DISCUSSION Synthesis and Structural Characterization In our previous study34, a series of porous crosslinked polymers with epoxy groups were synthesized from GMA and DVB via HIPE template method and then functionalized with sodium 3-amino-1,2,4-triazole to introduce CO2-responsive triazole groups. The resulting adsorbents exhibited reasonably good CO2 capture performance. In this study, PGMA polymers (named as “PGD” instead for simplicity in this study) were also synthesized and used as supports for preparing new CO2 adsorbents, PEI@PGD-H. PEI@PGD-H absorbents were synthesized by introducing HKUST-type MOF into the PGD porous polymers to produce a HKUST-PGD composite named as “PGD-H” and then grafting PEI onto the PGD-H, so as to enhance the specific surface area and improve the CO2 capture performance. Mazaj et al33 prepared hybrid materials of HKUST and poly(dicyclopentadiene)HIPE via in situ generating HKUST by seed growth from the metal-oxide nanoparticles other than metal salt. Inspired by this method, we synthesized the PEI@PGD-H adsorbents through a three-step process illustrated in Scheme 1. First, PGD-Cu, a polyHIPE containing CuO nanoparticles was synthesized via HIPE template polymerization of GMA and DVB in the presence of hydrophobic-modified CuO nanoparticles. Then, HKUST-type MOF crystals were in situ generated on the pore wall of the PGD-Gu via the reaction of CuO with H3BTC to form the so-called “PGD-H” composite of HKUST and PGD. Finally, the PGD-H composite was functionalized with PEI via the epoxy-amine reaction and amine grafting on metal opening sites of HKUST to yield two adsorbents named as PEIx@PGD-H, in which x (60, 70) means mass percentage of PEI based on PGD-H. For comparison, a PEI60@PGD adsorbent was also synthesized via HIPE template polymerization of GMA and DVB in the absence of CuO nanoparticles and then PEI grafting. For PEI60@PGD synthesis, a P4-type PGD polymer was synthesized at the same condition reported previously.34 For PEIx@PGD-H syntheses, the PGD-Cu polymer was synthesized with a modified recipe (see Table 1) in which the amount of porogen toluene was decreased from 45% to 20 w% to enhance mechanical property and the amount of surfactant PEL 121 was decreased from 20% to 10 w% considering that the CuO nanoparticles can also stabilize the emulsion as Pickering emulsions.38 The preparation conditions and results of the three adsorbents are listed in Table 2. The mass percentages of PEI, PEIexp,m and PEIexp,EA, were calculated from the mass increment after PEI grafting and from nitrogen content of the adsorbent (N) determined by elemental analysis (equation 1, 6


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where N(PEI)=30.74% is the theoretical nitrogen content of PEI), respectively. It can be seen that PEIexp,m and PEIexp,EA, accord well with each other (24.0-30.5% vs. 24.1-29.9 w%). From the PEIexp,EA values and the HKUST content in PGD-H (57.9 w%) calculated from the weight increment after HKUST formation, the mass percentages of PGD and HKUST were also determined. With increasing the PEI loading in PEI@PGD-H, both the nitrogen content and PEI content of PEI@PGD-H showed an upward trend. But all the experimental values of PEI content are lower than the theoretical values (37.5-41.2 w%), suggesting that only partial (64.3-75.5%) PEI was grafted onto the supports. Moreover, it can be seen that the PEI content and PEI grafting efficiency of PEI60@PGD-H is clearly higher than PEI60@PGD at the same PEI feeding, suggesting that the interaction between open Cu metal sites of HKUST and the amino groups of PEI also contributed to PEI grafting in PEI@PGD-H. PEIexp,EA 

N N (PEI)

(1)

Table 2. Preparation conditions and results of PEI@PGD and PEI@PGD-H. PEI/PGD-Ha

PEItheob

PEIexp,mc

Nd

PEIexp,EAe

PGDf

HKUSTf

(w%)

(w%)

(w%)

(w%)

(w%)

(w%)

(w%)

PEI60@PGD

60

37.5

24.0

7.4

24.1

75.9

0

PEI60@PGD-H

60

37.5

26.1

8.7

28.3

30.2

41.5

PEI70@PGD-H

70

41.2

30.5

9.2

29.9

29.5

40.6

Sample

a.

The mass percentage of PEI used for PEI grafting, based on PGD or PGD-H. b. Theoretical PEI content in the

adsorbent, calculated from PEI feeding. c. experimental mass percentage of PEI in the adsorbent, calculated by the mass increment after PEI grafting. d. Nitrogen content of the absorbent determined by elemental analysis. e. experimental mass percentage of PEI in the adsorbent, calculated by nitrogen content. f. experimental mass percentage of PGD and HKUST in the adsorbent.

The FTIR spectra shown in Fig. 1 provided clear evidences for successful synhthesis of PEI@PGD-H with expected chemcial structure. For PGD, the characteristic peaks at 908 and 848 cm−1 are attributed to epoxy group stretching vibration and that at 1728 cm−1 is ascribed to ester carbonyl group stretching vibration. In addition, the broad stretching vibration peak of hydroxy group appeared unexpectedly around 3400 cm−1 possibly because of partial hydrolysis of epoxy group under the experimental condition. The PGD-Cu showed a spectrum similar to the PGD. For HKUST, the C=O stretching vibration shifted to 1643 cm-1 due to the coordination of carboxyl with Cu2+,23 the characteristic vibration of C-C bonds in benzene ring appeared at 1373 cm-1 and the Cu-O stretching vibration at 729 cm-1. These characteristic vibrations also appear in PGD-H but not in PGD-Cu, indicating the successful in-situ generation of HKUST-type MOF in PGD-H. After PEI grafting, N-H bonds and secondary O-H bonds were formed due to the epoxy-amine reaction. As a result, for PEI@PGD-H, the two epoxy peaks disappeared, and at the same time, a new peak assigned to N-H deformation vibration appeared at 1561 cm−1 and the peak around 3400 cm−1 became broader and stronger. Besides, redshift appeared for the characteristic peaks at 1643 cm-1, 1373 cm-1 and 729 cm-1. Different from PEI@PGD-H, the characteristic peaks at 1643 cm-1, 1373 cm-1 and 729 cm-1 are absent in PEI60@PGD due to the absence of HKUST in it. 7



PGD

1728

908, 848

ester

PGD-Cu

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epoxy

PGD-H

PEI60@PGD 1561

PEI60@PGD-H

-NH-

PEI70@PGD-H HKUST

1643 1373 C-C

4000

3500

3000

1500

Wavenumbers (cm-1)

729

1000

Cu-O

500

Fig. 1 FTIR spectra of PGD, PGD-Cu, PGD-H, PEI@PGD, PEI@PGD-H and HKUST.

The PXRD patterns of PGD, PGD-Cu, PGD-H, PEI@PGD, PEI@PGD-H and HKUST are shown in Fig. 2. The as-synthesized HKUST showed narrow diffraction peaks in accordance with literature data23,34, confirming the successful synthesis of HKUST with high crystallinity. PGD-Cu show characteristic diffraction peaks of CuO crystals. The PXRD pattern of PGD-H is essentially identical to that of HKUST, indicating that HKUST-type MOF crystals have been successfully in-situ generated in PGD-H through the solvothermal reaction of CuO nanoparticles. After PEI grafting, the resulting PEI@PGD-H adsorbents show unchanged location but weakened intensity of the HKUST crystal diffraction peaks, indicating well maintained HKUST crystal structure in PEI@PGD-H. In comparison with PGD-H, the decrease of HKUST peak intensity in PEI@PGD-H is attributed to the decrease of HKUST content after PEI grafting. Other possible reasons include the interaction between amino groups in PEI and the opening metal sites in HKUST and possible destruction of the HKUST framework structure in alkali amine solution.23 Different from PGD-H and PEI@PGD-H, PGD and PEI@PGD hardly show crystallinity because of the absence of crystallizable component in them.

PEI70@PGD-H PEI60@PGD-H PGD-H PGD-Cu PEI60@PGD PGD HKUST 10

20

30

40

50

2θ (degree)

60

70

80

Fig. 2 Powder X-ray diffraction (PXRD) patterns of PGD, PGD-Cu, PGD-H, PEI@PGD, PEI@PGD-H and HKUST.

The nitrogen adsorption isotherms and pore size distribution curves of PGD, PGD-Cu, PGD-H, PEI@PGD, PEI@PGD-H and HKUST are shown in Fig. S2 and S3. The pore structure parameters determined from N2 adsorption isotherms are listed in Table 3. The PGD had a specific surface area 8


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of 22.9 m2/g, which is typical for a HIPE porous polymer. After PEI grafting, the specific surface area of PEI60@PGD decreased to 17.5 m2/g. In comparison with PGD, PGD-Cu had lower surface area (10.5 m2/g) despite the presence of CuO nanoparticles possibly because less porogen (20% vs. 45%) was used in PGD-Cu synthesis. However, the specific surface area of PGD-H increased significantly to 653 m2/g because of the presence of highly porous HKUST crystals. Actually, neat HKUST had specific surface area of 1632 m2/g and average pore size of 1.83 nm which is in microporous range. After PEI grafting, the specific surface areas of PEI60@PGD-H and PEI70@PGD-H decreased to 96.3 and 82.5 m2/g respectively. Meanwhile, the average pore size gradually increases from 2.73 nm to 4.93-5.29 nm and the pore volume decreases from 0.43 cm3/g to 0.12-0.10 cm3/g. The results suggest that PEI grafting remarkably reduced the surface area of HKUST because of the pore-filling effect of PEI on the macropores between MOF particles, making some micropores in MOF crystals no longer accessible for N2 adsorption. However, the PEI@PGD-H adsorbents still retained much higher specific surface area than PEI60@PGD, which was responsible for rapider CO2 adsorption kinetics that will be discussed in the next section. Table 3. Pore parameters of PGD, PGD-Cu, PGD-H, PEI@PGD, PEI@PGD-H and HKUST.

a.

Samples

Apa (m2/g)

Vpb (cm3/g)

Dpc (nm)

PGD

22.9

0.15

25.8

PGD-Cu

10.5

0.13

49.4

PGD-H

620

0.43

2.76

PEI60@PGD

17.5

0.09

21.2

PEI60@PGD-H

96.3

0.12

4.93

PEI70@PGD-H

82.5

0.10

5.29

HKUST

1632

0.75

1.83

specific surface area; b. total pore volume; c. average pore size.

The pore structure and morphology of the samples were observed by SEM. As shown in Figs. 3 and S4, the HKUST appears to be polyhedral prismatic crystals. As a typical polyHIPE mateiral, the PGD presents hierarchical interconnected porous structure, with many small pores on the wall of the macropores.39 For PEG60@PGD, the hierarchical porous structure remains but most of the small pores in PGD were occupied by PEI because of PEI grafting, which is responsible for the decrease of specific surface area. PGD-Cu also exhibits typical polyHIPE hierarchical porous structure, and most small pores were occupied with CuO nanoparticles. So, the specific surface area reduces and the average pore size increases as compared with PGD. The formation of polyhedral HKUST crystals, embedded in the macropore wall, is clearly observed in PGD-H though the crystallite size is significantly smaller than the HKUST crystals prepared separately. After PEI loading, PEI60@PGD-H basically retains its porous morphological structure, and the HKUST crystals show roughened surface, supporting the interaction of the Cu sites with PEI.23 For practical CO2 capture from flue gas emitted from fossil-fuel based thermal power plants, the adsorbents should have good enough thermal stability to endure high temperature at least 100-150 oC.40 TGA results (Fig. S5) indicate that all the PEI@PGD and PEI@PGD-H adsorbents are stable at 200 oC and therefore are able to endure the flue gas temperature. 9



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Fig. 3 SEM images of HKUST, PGD, PEI@PGD, PGD-Cu, PGD-H and PEI@PGD-H.

CO2 Adsorption Capacity, Rate and Selectivity over N2 CO2 adsorption performance of PGD-H, PEI@PGD and PEI@PGD-H at 25 °C and 1 atm is shown in Fig. 4A. The saturated CO2 adsorption capacity of PGD-H is 2.60 mmol CO2/g. It is slightly higher than the value (2.45 mmol CO2/g) reported for the hybrid materials of HKUST and poly(dicyclopentadiene).33 But it is lower than the value reported for HKUST at 25 °C and 1 atm (3.22 mmol/g) 23 because the PGD component has much lower specific surface area than the HKUST component. In comparison, the PEI@PGD-H adsorbents exhibit clearly higher adsorption capacity of 4.12-4.17 mmol CO2/g (Table 4) because of the contribution of the grafted PEI to CO2 adsorption. The CO2 adsorption capacity of PEI@PGD-H only shows slight increase with increasing PEI loading in the experimental range possibly because of the decrease in specific surface area. In comparison with PEI60@PGD, PEI60@PGD-H shows 28% higher CO2 adsorption capacity (4.12 vs. 3.23 mmol CO2/g) because of higher PEI loading and extra contribution of HKUST. Adsorption kinetics of CO2 adsorbents can be evaluated by the gradients of the adsorption curves, the adsorption halftime 41 or full time (the times required to reach half or saturated CO2 adsorption). From Fig. 4, it can be seen that the adsorption process can be roughly divided into the early rapid adsorption and late slow adsorption.41 All the adsorbents exhibited similar high adsorption rate at the early stage, but behaved very differently at the late stage. The slow adsorption process lasted about 22 min for PEI60@PGD, but only about 6 min for PGD-H and PEI@PGD-H. Obviously, such difference is related to the difference in the specific surface area of these adsorbents. The above results indicate that the presence of HKUST in PEI@PGD-H results in higher PEI loading and higher specific surface area and therefore leads to higher CO2 adsorption capacity and rate at slow adsorption stage.

10


Page 11 of 21

5 4 3 2

PGD-H PEI60@PGD PEI60@PGD-H PEI70@PGD-H

1 0 0

5

10

15

20

5

B

25 oC, 1 atm, 60 mL CO2/min

CO2 Uptake (mmol g-1)

A


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 oC, CO2+N2 10+60 mL N2/min

4 3 2 PGD-H PEI60@PGD PEI60@PGD-H PEI70@PGD-H

1 0 0

25

5

10

15

20

25

Time (min)

Time (min)

Fig. 4 CO2 adsorption of PGD-H, PEI@PGD and PEI@PGD-H at 25 °C and (A) 1 atm or (B) 0.15 atm (from CO2+N2 mixture containing CO2 15%, calibrated by the separation factor α). Table 4. CO2 adsorption properties of PGD-H, PEI@PGD and PEI@PGD-H. Adsorbent

Cs,1atma

αb

(mmol CO2/g)

Cs,expc

Cs,0.15atmd

Ee

Qdesf

(mmol gas/g)

(mmol CO2/g)

(J/g sorb)

(kJ/mol CO2)

25 °C

50 °C

25 °C

50 °C

25 °C

50 °C

25 °C

50 °C

PGD-H

2.60

2.19

9.5

9.6

1.85

1.54

1.67

1.39

86.0

36.9

PEI60@PGD

3.23

3.36

44.3

47.7

2.25

2.36

2.20

2.31

168.2

-

PEI60@PGD-H

4.12

4.22

67.9

69.3

2.95

3.03

2.91

2.99

162.8

46.7

PEI70@PGD-H

4.17

4.31

73.3

75.6

2.97

3.07

2.93

3.03

170.7

48.4

a. Experimental CO2 adsorption capacity measured at 1 atm. b. The CO2/N2 separation factor. c. Experimental gas sorption capacity measured in simulated flue gas (15 vol% CO2.). d. CO2 sorption capacity from simulated flue gas, calibrated by the separation factor α. e. The desorption enthalpy (J/g CO2-saturated adsorbent). f. The CO2 desorption heat.

Adsorption selectivity is also a key factor for a solid sorbent in practical use. To determine the selectivity2, the adsorption of pure N2 (60 mL/min) on PGD-H, PEI@PGD and PEI@PGD-H adsorbents was also conducted at 25 °C and 1 atm. The separation factor α, an indicator of adsorption selectivity of CO2 over N2, was calculated by equation (2), where Qi and Pi are the adsorption capacity and partial pressure of component i, respectively (here, Pi = 1 bar). The results are shown in Fig. S6 and listed in Table 4. PGD-H adsorbs N2 significantly (0.27 mmol g-1) because of its very high specific surface area, and therefore shows a low CO2/N2 separation factor α of 9.5. The value is even lower than a reported value of HKUST (1540). However, the PEI-grafted adsorbents adsorb much less N2 (0.06-0.07 mmol N2/g) as well as more CO2, therefore, the separation factor increases significantly to 44.3 for PEI@PGD and 67.9-73.3 for PEI@PGD-H. Therefore, the presence of PEI in the adsorbents not only enhances the CO2 adsorption capacity, but also contributes significantly to the selectivity. As a result, the α value is obviously improved with increasing PEI loading. 

QCO2 / PCO2 QN2 / PN2

(2) 11



CO2 adsorption from a mixture gas (CO2+N2, 70 mL/min, CO2 15%) used as simulated flue gas was further examined. As both CO2 and N2 contribute to the measured value (Cs,exp), the true CO2 sorption capacity was calibrated by the equation (3). The result is visualized in Fig. 4B. In comparison with pure CO2, both the adsorption rate and capacity of CO2 from the mixture gas decreased to some extent because the CO2 partial pressure in the mixed gas was reduced.42 PEI60@PGD, PEI60@PGD-H and PEI70@PGD-H show CO2 adsorption capacity of 2.20, 2.91 and 2.93 mmol CO2/g, respectively. Cs,0.15atm  Cs,exp 

 1

(3)

CO2 adsorption from air (CO2 concentration ca. 400 ppm) at 25 oC was also examined for the adsorbents. The results are shown in Fig. 5. At such an extra-low CO2 concentration, the CO2 adsorption capacity of PEI60@PGD, PEI60@PGD-H and PEI70@PGD-H reached 1.32, 1.67 and 1.78 mmol CO2/g in 30 h, respectively. Again, the PEI@PGD-H adsorbent showed higher CO2 adsorption rate and capacity than the PEI@PGD counterpart. The result indicates that the PEI@PGD-H materials are also effective adsorbents for CO2 separation from environment with ultra-low CO2 content. 2.0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 21

1.5

1.0

0.5

PEI60@PGD PEI60@PGD-H PEI70@PGD-H

0.0 0

10

20

30

40

50

Time (h)

Fig. 5 CO2 adsorption of the indicated adsorbents from air at 25 °C. In general, with increasing temperature, the equilibrium of CO2-amine reaction moves towards the reverse reaction direction so that the CO2 adsorption capacity is reduced. But for some PEI-incorporated adsorbents, such as PEI-impregnated silica (MCM-4143 and SBA-1544), it was observed that CO2 adsorption capacity at 50 oC or even 75 oC was abnormally higher than the adjacent temperature. Stretchable expansion and improved mobility of PEI segments at higher temperature may be responsible for the promotion of the CO2-amino reaction kinetics and therefore result in the improvement of the CO2 adsorption.44 For this reason, the CO2 adsorption at 50 oC was also examined. The adsorption curves are shown in Fig. 6, and the data are summarized in Table 4 too. At 50 °C and 1 atm, as expected, the CO2 adsorption capacity of PGD-H decreased by 15.8% because higher temperature depresses such an exothermic reaction. But the adsorption capacity of the three PEI-containing adsorbents all slightly increased by 4.0, 2.4 and 3.4%, respectively. The CO2 adsorption from CO2+N2 mixture gas at 50 oC also showed the same variation trend. As a result, one gram of PEI70@PGD-H adsorbent adsorbed 3.03 mmol of CO2 from the simulated flue gas containing 12


Page 13 of 21

15% CO2. Because cooling flue gas to room temperature is often energy intensive, the actual CO2 sorption temperature is often set above room temperature, for instance, the operation temperature for CO2 absorption using aqueous solution of organic amine is usually at 55 oC.4 Therefore, this result is clearly conducive to practical application because the adsorption can be performed at 50 oC rather than at room temperature (25 oC). Another advantage is that the CO2/N2 adsorption selectivity of the PEI-containing adsorbents is also improved slightly at 50 oC because the higher temperature results in not only higher CO2 absorption but also slight decrease in N2 adsorption at the same time. In fact, the separation factor α of PEI70@PGD-H increases from 73.3 at 25 oC to 75.6 at 50 oC. The CO2 adsorption performance of PEI70@PGD-H is compared with some other CO2 adsorbents reported in literature. As shown in Table 5, this adsorbent has quite good CO2 adsorption capacity and selectivity which are very desirable for practical CO2 capture from flue gas. 5

B 5

50 oC, 1 atm, 60 mL CO2/min


A


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 0 0

5

10

15

20

50 oC, 1 atm, CO2+N2 10+60 mL N2/min


1 0

25

0

5

10

15

20

25

Time (min)

Time (min)

Fig. 6 CO2 adsorption of PGD-H, PEI@PGD and PEI@PGD-H at 50 °C and (A) 1 atm or (B) 0.15 atm (from CO2+N2 mixture containing CO2 15%, calibrated by the separation factor α). Table 5. CO2 adsorption capacity and selectivity of various adsorbents. Tb

Adsorbenta

Csc (mmol/g)

αd

αe

Ref.

76

nr

this

(K)

1 bar

0.15 bar

PEI70@PGD-H

323

4.3

3.0

PEI (40%)/PAF-5

313

2.6

2.5

nr

2160

45

PEI (50%)−silica

348

3.1

3.0

nr

nr

46

0.70PEI@PDVB

298

3.2

2.4

nr

nr

47

PEI-PS polyHIPE

313

3.5

nr

26

nr

48

PEI-poly(GMA)HIPE

313

4.0

3.1

27

nr

26

PEI (70%)-poly(DVB)HIPE

348

5.5

4.6

nr

359

27

PEI@UiO-66

298

3.3

1.6

nr

nr

49

298

3.1

nr

nr

nr

PEI (2.5%)/HKUST

298

4.1

0.8

2

nr

24

en-[Mg2(dobpdc)]

298

4.5

3.5

nr

230

12

TEPA (50%)/NH2-MIL-101

work

21

13



a.

PAF: porous aromatic framework; PDVB: nanoporous poly(divinylbenzene). PS: poly(styrene-divinylbenzene),

modified with polyacrylic acid; b. Temperature. c. CO2 adsorption capacity at the indicated CO2 partial pressure. d. CO2/N2 selectivity: α = nCO2(1 atm)/nN2(1 atm). e. Calculated by ideal adsorption solution theory (IAST) model, α = [nCO2(0.15 bar)/nN2(0.85 bar)]*(0.85/0.15). nr: not reported.

CO2 Desorption Heat The CO2 desorption heat (Qdes) of PGD-H and PEI@PGD-H absorbents was determined from DSC. As shown in Fig. 7, the CO2-saturated samples all exhibited strong endotherm (86-171 Joule per gram CO2-saturated adsorbent) during first heating scan due to CO2 desorption. During the second heating scans, the adsorbents did not show observable endotherm, indicating that CO2 desorption had been completed in the first heating. The desorption temperature ranges from 50 oC to 200 oC. The desorption heat Qdes (kJ/mol CO2) of these adsorbents can be calculated from the desorption enthalpy (E, J/g CO2-saturated adsorbent), the CO2 adsorption capacity (Cs, mmol/g adsorbent) and the molar mass of CO2 (MCO2, 44 g/mol) by equation (4). The results are summarized in Table 4. The pristine PGD-H has a low CO2 desorption heat (36.9 kJ/mol CO2). After PEI loading, the value of PEI@PGD-H increases to 46.7-48.4 kJ/mol CO2, and the more PEI is grafted the higher CO2 desorption heat is. As HKUST has a relatively low CO2 desorption heat (28.1-35.0 kJ/mol CO250), it can be deduced that PEI contributes more than HKUST to the CO2 desorption heat of the adsorbents as CO2 is chemically adsorbed to PEI but physically to HKUST. Clearly, the PEI@PGD-H adsorbents will be very competitive as they have much lower desorption or regeneration energy consumption than some amine functionalized polymers (60-75 kJ/mol CO2)26,27 as well as lkanolamine aqueous solution (ca. 80 kJ/mol CO2)51,52.

Qdes  E

Heat Flow Exo Up (W/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 21

Cs Cs  MCO 2 1 1000

(4)

Heating at 10 oC/min

1 W/g CO2-PGD-H

2nd scan 1st scan

CO2-PEI60@PGD-H

CO2-PEI70@PGD-H

0

50

100

150

Temperature (oC)

200

Fig. 7 DSC thermograms of three CO2-saturated adsorbents.

Effect of Moisture on CO2 Adsorption The effect of moisture on CO2 adsorption performance should be considered when evaluating a 14


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


new absorbent, especially a hydrophilic one. Usually, there is 8-17% water vapor in real flue gas.53 As the PEI@PGD-H adsorbents have hydrophilic groups such as hydroxyl groups and amino groups, they would adsorb water vapor from the flue gas. At the same time, it is reported that HKUST crystals have certain hygroscopicity54 and exhibit high structural sensitivity to moisture55. It was found that PEI70@PGD-H adsorbed 5 w% water in 15 min (the same time for CO2-saturation) at 50 °C (the same temperature for CO2 adsorption) in an airtight moist space. Such treated sample was used as a wetted adsorbent (w-Sorb). Pure CO2 (d-CO2) or mixture gas (15 vol% CO2) were used to pass through dry and wetted adsorbents (d-Sorb and w-Sorb) separately. The results were shown in Fig. 8. Compared with the "d-Sorb", the CO2 adsorption rate was nearly unchanged when using the “w-Sorb”, but the adsorption capacity was slightly increased. The adsorption capacity at 1 atm and 0.15 atm reached 4.51 and 3.27 mmol CO2/g, increased by 4.6% and 6.5%, respectively. Therefore, the PEI@PGD-H adsorbent shows excellent moist endurance in CO2 adsorption. The promotion in CO2 adsorption is attributed to that the tertiary amino groups in PEI were able to adsorb CO2 to form bicarbonates56 in the presence of water. The result also suggests that the water stability of the HKUST in the adsorbent was improved possibly because the Cu sites were occupied by the amino groups.23 5


Page 15 of 21

4 3 2

d-Sorb, CO2 w-Sorb, CO2

1

d-Sorb, CO2+N2 w-Sorb, CO2+N2

0 0

5

10

Time (min)

15

20

Fig. 8 CO2 adsorption from pure CO2 and CO2+N2 mixture gas using dry or wetted PEI70@PGD-H.

Adsorption/Desorption Cycles Finally, CO2 desorption and reuse of PEI70@PGD-H was examined. The CO2 adsorption from simulated flue gas was conducted at 50 °C and the CO2 desorption was performed at 120 °C. N2 purging was used instead of CO2 purging in this study to promote CO2 desorption. From the six adsorption-desorption cycles of PEI70@PGD-H shown in Fig. 9A, it can be seen that CO2 desorption was completed rapidly in only 5 min, supporting the weak interaction between CO2 and PEI70@PGD-H at high temperature. The adsorbent retained 97.4% adsorption capacity at the 6th cycle. Then, the adsorption capacity remained essentially unchanged. Even after 20 cycles, the adsorbent still retained an effective CO2 adsorption capacity (defined as the difference between the highest and lowest values in each cycle shown in Fig. 9A) of 2.84 mmol CO2/g (Fig. 9B). In conclusion, the PEI@PGD-H adsorbent features superior durability as well as rapid and complete CO2 desorption. 15



4

adsorption at 50 oC, 1 atm, 10+60 mL (CO2+N2)/min desorption at 120 oC, 1 atm, 60 mL N2/min

3

2

1

0

0

20

40

60

80

100

120

B 4 Effective CO2 Uptake (mmol/g)

A

CO2 Uptake (mmol/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 21

3

2

1

0

0

Time (min)

5

10

15

20

Number of Cycle

Fig. 9. CO2 Adsorption/desorption cycles of PEI70@PGD-H.

CONCLUSIONS In this study, new PEI-grafted MOF-polyHIPE composites, abbreviated as PEI@PGD-H, were synthesized via HIPE template polymerization of GMA and DVB in the presence of hydrophobic-modified CuO nanoparticles followed by in-situ generation of HKUST-type MOF via the reaction of CuO with H3BTC and then PEI functionalization via epoxy-amine reaction and amine-metal sites interaction. The products were characterized and assessed as CO2 adsorbents. Benefited from the hierarchical interconnected porous skeleton of PGD, high specific surface area of MOF and chemical CO2 adsorption of PEI, these adsorbents manifest high CO2 adsorption rate, capacity and selectivity. At 50 oC, one gram PEI70@PGD-H can adsorb 4.3 and 3.0 mmol CO2 at 1 atm and 0.15 atm, respectively, with a high CO2/N2 separation factor of 76. It can even adsorb 1.8 mmol CO2 at room temperature from air where the CO2 concentration is as low as 400 ppm. The adsorbent also displays reasonably good thermal stability, low desorption energy, excellent water endurance and recyclability. It shows easy CO2 desorption with desorption heat as low as 48.4 kJ/mol CO2 and exhibits high and stable CO2 capture performance (>2.84 mmol CO2/g) in a 20-cycle adsorption-desorption assessment. In conclusion, the PEI@PGD-H materials appear to be highly efficient and robust CO2 adsorbents with potential applications in CO2 capture.

ASSOCIATED CONTENT Supporting Information The appearance, N2 adsorption isotherms, pore size distributions, SEM images and TGA curves of PGD, PGD-Cu, PGD-H, PEI@PGD, PEI@PGD-H and HKUST, and N2 adsorption of PGD-H, PEI@PGD and PEI@PGD-H at 25 °C and 50 °C were described in Supporting Information.

AUTHOR INFORMATION Corresponding Author 16


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*Tel +86 571 87952631; Fax +86 571 87951612; Email: [email protected].

ACKNOWLEDGEMENTS The authors thank the National High Technology Research and Development Program (2015BAC04B0103) and 151 Talents Project of Zhejiang Province for financial support.

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20


Page 21 of 21

TOC graphic

O

H3BTC O O

O HO

O GMA

PEI

N

OH

DVB

n-CuO Polymerization

O

NH2 N H

2 N NH H N HN N

H2N

OH

Solvothermal reaction

C ( mmol CO2/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N

NH2

Functionalization

4

PEI@PGD-H as CO2 adsorbents

at 0.15 bar

3 2 1 0 0

5

10

15

Number of Cycles

20

21


polyHIPE Porous

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