CO2 Capture by Porous Hyper-Cross-Linked Aromatic Polymers

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CO capture by porous hypercrosslinked aromatic polymers synthesized using tetrahedral precursors Pillaiyar Puthiaraj, and Wha-Seung Ahn Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b03963 • Publication Date (Web): 11 Nov 2015 Downloaded from http://pubs.acs.org on November 18, 2015

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Industrial & Engineering Chemistry Research

CO2 capture by porous hypercrosslinked aromatic polymers synthesized using tetrahedral precursors Pillaiyar Puthiaraj and Wha-Seung Ahn* Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, South Korea *E-mail: [email protected]

Abstract The low-cost synthesis of porous hypercrosslinked aromatic polymers (PHAPs) was achieved via the FeCl3 catalyzed Friedel-Crafts alkylation reaction between tetraphenylsilane or tetraphenylgermanium as a building block and formaldehyde dimethyl acetal as a cross-linker. The synthesized polymers were chemically and thermally stable and exhibited high surface areas of up to 1137 m2 g-1 (PHAP-1) and 1059 m2 g-1 (PHAP-2). The adsorption isotherms of the PHAPs revealed high CO2 adsorption capacity (104.3-114.4 mg g-1) with an isosteric heat of adsorption in the range, 26.5-27.3 kJ mol-1, and moderate CH4 adsorption capacity (12.6-13.8 mg g-1) at 273 K/1 bar. The PHAP networks also exhibited high CO2/N2 and CO2/CH4 relativities of 29.3-34.2 and 11.3-12.5, respectively at 273 K. Keywords: Hypercrosslinked polymer, aromatic polymer, Friedel-Crafts reaction, CO2 capture and separation, alkylation.

1. Introduction The rapid development of the world economy has been accompanied by concurrent increases in the anthropogenic emissions of CO2 due to the consumption fossil fuels, including coal, petroleum and natural gas, which are causing climate change and environmental disorders.11 ACS Paragon Plus Environment


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Consequently, a variety of strategies employing porous materials for CO2 capture and storage

(CCS) are attracting increasing attention to remedy the critical global warming problem. To this end, a range of different porous solid adsorbents including zeolites,4-5 silica,6 metal organic frameworks (MOFs),7 and porous organic polymers (POPs),3,8 have been proposed for CCS, which captures CO2 gas mostly via physisorption through the relatively weak van der Walls force. This process enables ready and reversible adsorption-desorption behavior that allows the efficient regeneration of the adsorbent at significantly reduced energy requirement than liquid amine absorption. Among the porous adsorbent materials, POPs exhibiting a high surface area, light-weight, adjustable pore size for specific applications, and good mechanical, thermal and chemical stability owing to the strong covalent linkage (B-O, C-C, C-H, C-N, etc.) between the ever-charging organic linkers,8-10 are being investigated vigorously for potential applications in CCS1,3,8 and catalysis.11-14 Recently, a range of POPs have been developed specifically for CCS applications based on their chemical nature and building blocks, including carbazole-based porous organic polymer (CPOP),15 benzimidazole-linked polymers (BILPs),16 covalent triazine frameworks (CTFs),17 polymers with intrinsic microporosity (PIMs),18 porous aromatic frameworks (PAFs),19 covalent organic frameworks (COFs),20 hypercrosslinked and conjugated microporous polymers (HCPs and CMPs),21-23 nanoporous organic polymers (NOPs),24 porous imine-linked networks (PINs),25 and porous polymer networks (PPNs).26 On the other hand, most POPs were synthesized via noble metal catalyzed polymerization processes, involving the high cost and multi-step synthesis of organic linkers, which has limited the scale-up preparation of these materials. Therefore, facile and cost effective preparation processes are needed for practical applications in the post-combustion capture of CO2.

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Recently, a versatile route of the Friedel-Crafts reaction catalyzed by inexpensive FeCl33,27 or AlCl328-30 for large scale synthesis of POPs was reported. Among them, HCPs, a subclass of POPs, have attracted increasing attention because they can be prepared by the low-cost FeCl3 catalyzed Friedel-Crafts alkylation reaction using formaldehyde dimethyl acetal (FDA) as an extensive cross-linker. The extensive cross-linked nature of materials enhances the stability in both organic and aqueous solvents, involves mild reaction conditions without the use of expensive metal catalysts and allows easy scale-up preparation, which can make promising materials for CCS. Tan et al. synthesized benzene-based HCPs with surface areas of 195-1391 m2 g-1, which exhibited 55-135 mg g-1 CO2 uptakes at 273 K/1 bar.31 Cooper et al. prepared the polystyrene32 and hydroxy-group-containing fused aromatic based HCPs33 with surface areas of 333-1015 m2 g-1 and CO2 uptakes of 81-174 mg g-1 at 273 K/1 bar. Zhu et al. developed PAF-32 from a tetraphenylmethane monomer with a surface area of 1679 m2 g-1 and CO2 adsorption of 73 mg g-1 at 273 K/1 bar.34 A triazine-based bifunctionalized task-specific porous polymer was reported to exhibit a surface area of 913 m2 g-1 and CO2 adsorption of 114 mg g-1 at 273 K/1 bar.35 Jiang et al. developed tetraphenylethylene36 and tetraphenylbiphenyldiamine-based37 HCPs with CO2 uptakes of 160 and 124 mg g-1, respectively, at 273 K/1 bar. Recently, phosphonium salt-incorporated HCPs showed surface area up to 1168 m2 g-1 with CO2 uptakes of 129 mg g-1 at 273 K/1 bar.38 On the other hand, these approaches made use of only limited aromatic precursors, and there is still scope for evaluating more diverse substrates for making more effective POPs for CCS. The controlled synthesis of silicon and germanium-containing nanoscale POPs are very difficult and still a challenge in this field, because their physicochemical properties depend strongly on the synthesis method. Zhu et al. reported the PAFs via an ionothermal reaction from 3 ACS Paragon Plus Environment


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a tetrakis(4-cyanophenyl)silicon and ZnCl2, which required high reaction temperature and long reaction time.39 Zhou et al. reported PPN-4(Si) and PPN-5(Ge) networks via the Yamamoto homocoupling of tetraphenylsilane and tetraphenylgermanium precursors with the flammable and expensive bis(1,5-cyclooctadiene)nickel(0) catalyst.26 Therefore, the synthesis of silicon and germanium-containing POPs with desirable textural properties remains problematic. With these considerations in mind, this study assessed an easy and low-cost strategy to achieve tetrahedral units with silicon and germanium-incorporated PHAPs via a FeCl3 induced Friedel-Crafts alkylation reaction (Scheme 1). These PHAPs possessed high surface areas with a combination of micro- and mesopores, good stability, and achieved promising performance in the adsorption of CO2 with excellent selectivity against N2 and CH4.

O

O

X

X FeCl3, DCE, 80 oC, 24 h

PHAP-1 (X = Si), PHAP-2 (X = Ge)

Scheme 1. Synthetic route for the porous hypercrosslinked aromatic polymers (PHAPs). 2. Experimental section 2.1.

Materials All gases used (CO2, CH4, N2 and Ar) were of ultrahigh-purity (99.999%) and purchased

from U-sung. Tetraphenylsilane (TPS) was purchased from Tokyo Chemical Industry Co., Ltd.


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Tetraphenylgermanium (TPG) was obtained from Alfa Aesar. Formaldehyde dimethyl acetal (FDA), anhydrous FeCl3 and 1,2-dichloroethane (DCE) were supplied by Sigma Aldrich. Unless specified otherwise, all other chemicals and solvents were purchased from commercial suppliers and used as received. 2.2.

Synthesis of PHAP-1 network TPS (1.00 g, 3 mmol), FDA (1.06 mL, 12 mmol) and DCE (20 mL) were added to an

oven-dried 100 mL three-necked round-bottom flask containing a magnetic stirring bar at room temperature. After 10 min, 1.95 g (12 mmol) of anhydrous FeCl3 was added with vigorous stirring. The resulting mixture was fitted with a condenser and heated to 318 K for 5 h, and then heated to 353 K for 19 h under a nitrogen atmosphere. After cooling to room temperature, the precipitated polymer was filtered and washed with dichloromethane, methanol (MeOH), distilled water, N,N’-dimethylformamide (DMF), and acetone, successively, until the filtrate was almost colorless. The product was purified further by Soxhlet extraction with DMF and MeOH for 24 h and dried in a vacuum at 433 K for 12 h to give the solvent free PHAP-1. 2.3.

Synthesis of PHAP-2 network The same procedure as above was followed using TPG (1.14 g, 3 mmol) instead of TPS

as the monomer. The polymerized PHAP-2 network was obtained as a black solid. 2.4.

Characterization Fourier transform infrared (FT-IR) spectra were performed using a VERTEX 80V FT-IR

spectrometer (Bruker, Germany). The powder X-ray diffraction (Powder XRD, Rigaku) patterns were obtained using CuKα (λ=1.54 Å) at a scan rate of 0.5o min-1. The C, H and N contents of the samples were measured using an elemental analyzer (EA, EA1112, Italy). The iron (Fe)



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contents were measured by inductively coupled plasma – optical emission spectroscopy (ICPOES, Optima 7300DV, USA). High resolution X-ray photoelectron spectroscopy (XPS, Thermo scientific, USA) was performed using a monochromatic Al Kα X-ray source and a hemispherical analyzer. The thermal stability of the samples was confirmed by thermogravimetric analysis (TGA, SCINCO thermal gravimeter S-1000, Japan) under an Argon (Ar) atmosphere over the temperature range, 300-1080 K, at a heating rate of 278 K min-1. The polymer morphologies were examined by field emission - scanning electron microscopy (FE-SEM, Hitachi S-4300). The N2 isotherms were measured using a BELsorp-Max (BEL, Japan) at 77 K. The samples were degassed at 433 K for 12 h under a high vacuum before measuring the isotherms. The surface areas were calculated using the Brunauer-Emmett-Teller (BET) method over the relative pressure range, 0.02 to 0.15, and the pore sizes of the samples were calculated using non-local density functional theory (NL-DFT) method assuming a slit pore geometry. 2.5.

CO2, CH4 and N2 adsorptions The CO2, N2 and CH4 adsorption isotherms under static conditions were obtained using a

BELsorp(II)-mini (BEL, Japan) at 273, 293, 298 and 303 K from 0 to 1 bar. During each measurement, the temperature was kept constant using a chiller circulator. The CO2/N2 selectivity was calculated using the ratio of the initial slope calculation from the gases adsorbed at low pressures. The isosteric heat of adsorptions was derived from the following ClausiusClapeyron equation.40 Qst = ܴ[∂݈݊ܲ/∂(1/ܶ)]ߠ

………………. (Eq. 1)

where P represents the pressure (in kPa), T is the temperature (in K), θ is the amount adsorbed, Qst is the isosteric heat of adsorptions and R is the universal gas constant.


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3. Results and discussion 3.1. Structural characterization of PHAP-1 and PHAP-2 The successful growths of PHAP-1 and PHAP-2 networks, their chemical composition and nature of the networks were examined using a range of analytical techniques. Figure 1 shows the FT-IR spectra of the PHAP networks and their corresponding monomers. The position and intensity of the C-C and C-H vibration bands changed completely after polymerization, and their changes and newly formed bands are as follows: (i) a new group of stretching bands appeared at 2918 and 2926 cm-1 in PHAP-1 and PHAP-2, respectively, which were assigned to aliphatic C-H vibrations originating from methylene linkers between the two aromatic rings; (ii) aromatic C=C stretching vibrations of the monomers were found at ca. 1582 cm-1, whereas the polymerized networks C=C stretching bands at ca. 1628 and 1660 cm-1 confirmed the presence of aromatic rings in the PHAPs networks; and (iii) the monomers C-C stretching bands (1427 cm-1) were red shifted completely in PHAP-1 (1437 cm-1) and PHAP-2 (1446 cm-1). EA, ICP-OES and XPS were carried out to determine the chemical composition of the networks. EA of PHAP-1 and PHAP-2 revealed the C, H and N contents quantitatively (Table S1 in the Supporting Information, SI), which were in close agreement with the calculated values of the polymer networks. Almost no Fe (ca. 0.002 wt.%) was detected by ICP-OES in both networks. In the XPS (Figure S1 in SI), the binding energies for silicon (2p) and germanium (3d) were detected at 102.8 and 30.0 eV, respectively, in the corresponding PHAPs, which confirmed the presence of silicon and germanium metal. Incidentally, no Fe binding energy peaks were observed by XPS (Figure S2 in SI), which clearly pointed out that the produced PHAPs were free of the Fe catalyst used for the synthesis. These FT-IR, EA, ICP-OES and XPS results confirmed the successful synthesis of the expected PHAP networks.



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Figure 1. FT-IR spectra of the PHAP networks and the monomers of TPS and TPG. Powder XRD was performed to determine the crystallinity, regularity and long-range structure of these PHAP-1 and PHAP-2 materials (Figure 2). The powder XRD pattern showed that the polymeric materials were basically amorphous in nature with only three broad peaks at approximately 11°, 23° and 43° 2θ, which suggests that the framework of the materials has intrinsic flexibility, disorder linkage, direct ring-ring interaction, and long-range structure due to the kinetics-controlled irreversible reaction processes.41-42 The morphology of the PHAPs was examined by FE-SEM (Figure 3), which shows that the both PHAP-1 and PHAP-2 networks had an irregular lamellar shape made of nanometer-sized subunits.

Figure 2. Powder XRD pattern of PHAP-1 and PHAP-2. 8 ACS Paragon Plus Environment

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Figure 3. FE-SEM image of (a) PHAP-1 and (b) PHAP-2. 3.2. Textural properties of PHAP-1 and PHAP-2 The N2 adsorption isotherms were measured at 77 K to determine the surface area and other textural properties of the PHAP materials (Figure 4). As shown in Figure 4a, PHAP-1 and PHAP-2 exhibited basically type I adsorption isotherms with sharp uptakes in the low pressure region, indicating the strong microporosity of the networks. On the other hand, the desorption isotherm displayed a significant hysteresis loop, which is consistent with the presence of some mesopores or elastic deformation after gas sorption. Table 1 lists the textural parameters, such as BET surface area, average pore size, total pore-, and micropore- volumes, which were derived from the N2 sorption isotherms. The specific surface areas of PHAP-1 and PHAP-2 networks calculated from the BET model were 1137 and 1059 m2 g-1, respectively. Figure 4b shows the pore size distribution of PHAP-1 and PHAP-2, which were calculated based on the nonlocaldensity functional theory (NL-DFT). The PHAP-1 network exhibited predominant micropore diameter of 1.51 nm with median mesopores peaks at 3.57 nm. Similarly, the PHAP-2 network showed a narrow pore size distribution centered at 1.42 and 1.83 nm with mesopores at 3.05 nm. The contribution of microporosity in PHAP-1 and PHAP-2 can be calculated from the ratio of the micropore volumes (V0.1) over the total pore volumes (Vtot).43 According to V0.1/Vtot, the 9 ACS Paragon Plus Environment


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microporosity of PCTP-1 and PCTP-2 were 53% and 56%, respectively, suggesting that the PHAP-1 and PHAP-2 networks contain almost equal amounts of micro- and meso-pores. Table 1. Textural parameters of the PHAP networks from the N2 isotherms at 77 K. Networks SABET (m2 g-1)a V0.1 (cm3 g-1)b Vtot (cm3 g-1)c V0.1/Vtot (%) Pore size (nm)d PHAP-1 1137 0.43 0.81 53 1.51, 3.57 PHAP-2 1059 0.41 0.73 56 1.42, 1.83, 3.05 a BET surface area at 77 K; bV0.1, pore volume at P/P0 = 0.1 at 77 K; cVtot, total pore volume at P/P0 = 0.995 at 77 K; dPore size distribution calculated by nonlocal-density functional theory (NL-DFT) model.

Figure 4. (a) N2 adsorption (solid symbol) - desorption (empty symbol) isotherm at 77 K and (b) pore size distribution of the PHAPs. Networks: PHAP-1 (black), PHAP-2 (red).

3.3. Thermal and chemical stability of PHAP-1 and PHAP-2 The stability of the sorbent is one of the main factors for their potential applications in CCS. The high stability of the sorbent can increase its effectiveness in recyclability, which reduces the energy consumption and cost.34 The thermal stability of the PHAPs was examined by TGA (Figure 5), which showed that the decomposition of the PHAPs started above 620 K under an argon atmosphere, suggesting the high thermal stability of the organic porous polymer networks. In addition, their chemical stability was confirmed by purification processes conducted 10 ACS Paragon Plus Environment

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for the PHAPs (Soxhlet extraction method with DMF and methanol) and tested in acetone, ethyl acetate, ethanol, chloroform, and water. These materials did not dissolve or decompose in these solvents.

Figure 5. TGA plot of PHAP-1 and PHAP-2. 3.4. CO2 and CH4 sorption properties of PHAP-1 and PHAP-2 The high surface area, large pore volume and good thermal and chemical stability against organic/aqueous solvents of the PHAP networks make them good candidates for CO2 and CH4 gas adsorption. Figure 6 shows the CO2, CH4 and N2 adsorption isotherms of the PHAP networks up to 1 bar at different temperatures and Table 2 represents their numerical data, which give the preferential binding of CO2 over CH4 and N2. As shown in Figure 6a and Table 2, the PHAP-1 exhibited a high CO2 adsorption capacity of 114.4 mg g-1 at 273 K, whereas the PHAP-2 was slightly lower at 104.3 mg g-1. As the temperature increased, the CO2 uptakes of the PHAPs decreased to 79.4, 73.1 and 64.9 mg g-1 at 293, 298 and 303 K respectively, for PHAP-1, and 74.4, 67.5 and 61.4 mg g-1, respectively, for PHAP-2. The CO2 uptakes capacity of PHAP-1 at 273 K was comparable or higher than that of some reported POPs, such as COF-1 (102 mg g-1),44 11 ACS Paragon Plus Environment


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COF-5 (58.9 mg g-1),44 COF-8 (62.9 mg g-1),44 COF-10 (53.2 mg g-1),44 COF-102 (68.6 mg g1 44

),

COF-103 (74.8 mg g-1),44 PPF-3 (91.9 mg g-1),45 PPF-4 (113.9 mg g-1),45 and PAF-18-OH

(110.0 mg g-1).46 Table S2 compares the CO2 uptakes for the resulting materials with some other previously reported POPs (in SI). The reversible CO2 adsorption-desorption isotherms (Figure S3 in SI) showed the effective regeneration of the materials without supplying external heat. These CO2 adsorption behaviors in aromatic POPs were reported to be a consequence of the large specific surface area with micropores and the chemical nature of the networks also played critical roles.3,36 In the same manner, CH4 and N2 adsorption over the PHAP networks were also explored up to 1 bar at 273 and 298 K (Figure 6b). At 1 bar, the CH4 uptakes were 13.8 mg g-1 at 273 K (8.1 mg g-1 at 298 K) for PHAP-1 and 12.6 mg g-1 at 273 K (7.0 mg g-1 at 298 K) for PHAP-2, which compared favorably with some previously reported POPs (Table S3 in SI). The CH4 adsorption of the PHAP networks increased almost linearly with pressure, indicating that the CH4 uptake is primarily influenced by the presence of micro- and mesopores and free porevolume in the PHAP networks as is usually the case for porous carbon-based materials.47-48 The N2 adsorption capacity by the PHAP networks remained almost flat and low, indicating that the PHAPs interact preferentially with CO2 than with N2. To determine the potential of gas separation by the PHAP networks, the CO2/N2 and CO2/CH4 selectivity were calculated (Figure S4 in SI), which were estimated by the ratio of the initial slope method using the single component adsorption isotherms at low pressure coverage (40 kJ mol-1),49 indicating that CO2 gas molecules interact moderately with the aromatic pore wall of the networks in physisorption. Similar Qst values were reported for other porous polymer adsorbents made of aromatic networks.3,22,34,50 In addition, the PHAP-2 has shown somewhat higher Qst values at low CO2 loading levels than PHAP-1 probably because of the corresponding basicities of the metal species in the PHAPs (silicon is acidic and germanium is amphoteric in nature); similar results were reported for the alkali or alkaline earth metal doped materials.51-52 Similarly, the Qst values for CH4 of PHAP-1 and PHAP-2 (Figure 7b) were 21.2 and 22.5 kJ mol-1, respectively, at low coverage (1.0 mg g-1), which were comparable to other reported POP materials, such as PAF-42 (25.6),30 PAF-43 (29.8),30 PAF-44 (22.9),30 ALP-7 (22.2),53 NPOF-4-NO2 (20.8).54 4. Conclusions Porous hypercrosslinked aromatic polymer networks were prepared by the FeCl3 catalyzed Friedel-Crafts alkylation using easily available economic starting materials of tetrahedral precursor units, and applied for CO2 and CH4 gas capture under 0 to 1 bar pressure conditions. The resulting adsorbent materials showed high surface areas, high thermal/chemical stability and good gas uptake (CO2 and CH4) abilities with low to moderate heats of adsorptions. The PHAP-1 network uptakes were 114.4 and 73.1 mg g-1 for CO2 and 13.8 and 8.1 mg g-1 for 14 ACS Paragon Plus Environment

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CH4 at 273 and 298 K/1 bar, respectively, whereas the selectivity of PHAP-2 for CO2-N2 (34.2) and CO2-CH4 (12.5) at 273 K, compared favorably against some other reported POPs. This costeffective synthesis of the PHAPs with high gas uptake performance is promising for the scale-up preparation for the removal of CO2 flue gas in industry. Supporting information: Detailed characterization of the prepared materials, CO2 adsorptiondesorption isotherms and selectivity plots, and comparison of their CO2 and CH4 adsorption capacities and corresponding heats of adsorption are provided. Acknowledgement This study was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant number: 2015042434). References 1. Wang, J.; Senkovska, I.; Oschatz, M.; Lohe, M. R.; Borchardt, L.; Heerwig, A.; Liu Q.; Kaskel. S. Highly porous nitrogen-doped polyimine-based carbons with adjustable microstructures for CO2 capture. J. Mater. Chem. A 2013, 1, 10951-10961. 2. Shi, Y.-Q.; Zhu, J.; Liu, X.-Q.; Geng, J.-C.; Sun, L.-B. Molecular template-directed synthesis of microporous polymer networks for highly selective CO2 capture. ACS Appl. Mater. Interfaces 2014, 6, 20340-20349. 3. Liu, G.; Wang, Y.; Shen, C.; Ju, Z.; Yuan, D. A facile synthesis of microporous organic polymers for efficient gas storage and separation. J. Mater. Chem. A 2015, 3, 3051-3058. 4. Díaz, E.; Muñoz, E.; Vega, A.; Ordonez, S. Enhancement of the CO2 retention capacity of y zeolites by Na and Cs treatments: Effect of adsorption temperature and water treatment. Ind. Eng. Chem. Res. 2008, 47, 412-418. 5. Grajciar, L.; Čejka, J.; Zukal, A.; Areán, C. O.; Palomino, G. T.; Nachtigall, P. Controlling the adsorption enthalpy of CO2 in zeolites by framework topology and composition. ChemSusChem 2012, 5, 2011-2022. 6. Liu, F.-Q.; Wang, L.; Huang, Z.-G.; Li, C.-Q.; Li, W.; Li, R.-X.; Li, W.-H.; Aminetethered adsorbents based on three-dimensional macroporous silica for CO2 capture from simulated flue gas and air. ACS Appl. Mater. Interfaces 2014, 6, 4371-4381.



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Graphical Abstract CO2 capture by porous hypercrosslinked aromatic polymers synthesized using tetrahedral precursors Pillaiyar Puthiaraj and Wha-Seung Ahn* Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, South Korea *E-mail: [email protected]

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CO2 Capture by Porous Hyper-Cross-Linked Aromatic Polymers

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