Article pubs.acs.org/IECR
CO2 Capture by Porous Hyper-Cross-Linked 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
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S Supporting Information *
ABSTRACT: Low-cost synthesis of porous hyper-cross-linked aromatic polymers (PHAPs) was achieved via the FeCl3catalyzed Friedel−Crafts alkylation reaction between tetraphenylsilane or tetraphenylgermanium as a building block and formaldehyde dimethylacetal 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 a 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 a moderate CH4 adsorption capacity (12.6−13.8 mg g−1) at 273 K and 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.
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 of fossil fuels, including coal, petroleum, and natural gas, which is causing climate change and environmental disorders.1−3 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,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 Waals force. This process enables ready and reversible adsorption−desorption behavior that allows efficient regeneration of the adsorbent at a 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 strong covalent linkages (B−O, C−C, C−H, C−N, etc.) between the ever-charging organic linkers8−10 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 POP,15 benzimidazole-linked polymers,16 covalent triazine frameworks,17 polymers with intrinsic microporosity,18 porous aromatic frameworks (PAFs),19 covalent organic frameworks (COFs),20 hyper-cross-linked and conjugated microporous polymers (HCPs and CMPs),21−23 nanoporous organic polymers,24 porous imine-linked networks,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 multistep synthesis of organic linkers, which has limited the scale-up preparation of these materials. Therefore, facile and cost-effective preparation © 2015 American Chemical Society
processes are needed for practical applications in the postcombustion capture of CO2. Recently, a versatile route of the Friedel−Crafts reaction catalyzed by inexpensive FeCl33,27 or AlCl328−30 for the largescale synthesis of POPs was reported. Among them, HCPs, a subclass of POPs, have attracted increasing attention because they can be prepared by a low-cost FeCl3-catalyzed Friedel− Crafts alkylation reaction using formaldehyde dimethylacetal (FDA) as an extensive cross-linker. The extensive cross-linked nature of the 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 uptake at 273 K and 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 uptake of 81− 174 mg g−1 at 273 K and 1 bar. Zhu et al. developed PAF-32 from a tetraphenylmethane monomer with a surface area of 1679 m2 g−1 and a CO2 adsorption of 73 mg g−1 at 273 K and 1 bar.34 A triazine-based bifunctionalized task-specific porous polymer was reported to exhibit a surface area of 913 m2 g−1 and a CO2 adsorption of 114 mg g−1 at 273 K and 1 bar.35 Jiang et al. developed tetraphenylethylene-36 and tetraphenylbiphenyldiamine-based37 HCPs with CO2 uptake of 160 and 124 mg g−1, respectively, at 273 K and 1 bar. Recently, phosphonium salt incorporated HCPs showed surface areas up to 1168 m2 g−1 with CO2 uptake of 129 mg g−1 at 273 K and 1 bar.38 On the other hand, these approaches made use of only limited aromatic Special Issue: International Conference on Carbon Dioxide Utilization 2015 Received: Revised: Accepted: Published: 7917
October 21, 2015 November 8, 2015 November 11, 2015 November 11, 2015 DOI: 10.1021/acs.iecr.5b03963 Ind. Eng. Chem. Res. 2016, 55, 7917−7923
Article
Industrial & Engineering Chemistry Research
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 the 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, 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 solvent-free PHAP-1. 2.3. Synthesis of the PHAP-2 Network. The same procedure as that 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 80 V FT-IR spectrometer (Bruker, Germany). Powder X-ray diffraction (XRD; Rigaku) patterns were obtained using Cu Kα (λ = 1.54 Å) at a scan rate of 0.5° min−1. The carbon, hydrogen, and nitrogen contents of the samples were measured by elemental analysis (EA; EA1112). The iron content was measured by inductively coupled plasma optical emission spectroscopy (ICP-OES, Optima 7300DV). 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 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 BELsorpMax (BEL, Japan) at 77 K. The samples were degassed at 433 K for 12 h under a high vacuum before the isotherms were measured. The surface areas were calculated using the
precursors, and there is still scope for evaluating more diverse substrates for making more effective POPs for CCS. The controlled synthesis of silicon- and germaniumcontaining nanoscale POPs is 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 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 (TPS) and tetraphenylgermanium (TPG) precursors with the flammable and expensive bis(1,5cyclooctadiene)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 porous hyper-cross-linked aromatic polymers (PHAPs) via a FeCl3-induced Friedel− Crafts alkylation reaction (Scheme 1). These PHAPs possessed Scheme 1. Synthetic Route for PHAPs
high surface areas with a combination of micropores and mesopores and good stability and achieved a promising performance in the adsorption of CO2 with excellent selectivity against N2 and CH4.
2. EXPERIMENTAL SECTION 2.1. Materials. All gases used (CO2, CH4, N2, and Ar) were of ultrahigh purity (99.999%) and were purchased from Usung. Tetraphenylsilane (TPS) was purchased from Tokyo Chemical Industry Co., Ltd. Tetraphenylgermanium (TPG) was obtained from Alfa Aesar. Formaldehyde dimethylacetal (FDA), anhydrous FeCl3, and 1,2-dichloroethane (DCE) were
Figure 1. FT-IR spectra of the PHAP networks and the monomers of TPS and TPG. 7918
DOI: 10.1021/acs.iecr.5b03963 Ind. Eng. Chem. Res. 2016, 55, 7917−7923
Article
Industrial & Engineering Chemistry Research Brunauer−Emmett−Teller (BET) method over the relative pressure range 0.02−0.15, and the pore sizes of the samples were calculated using the nonlocal density functional theory (NL-DFT) method assuming a slit pore geometry. 2.5. CO2, CH4, and N2 Adsorption. 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 heats of adsorption were derived from the following Clausius−Clapeyron equation.40 Q st = R[∂ ln P /∂(1/T )]θ
(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 adsorption, and R is the universal gas constant.
Figure 2. Powder XRD patterns of PHAP-1 and PHAP-2.
kinetics-controlled irreversible reaction processes.41,42 The morphology of the PHAPs was examined by FE-SEM (Figure 3), which showed that both the PHAP-1 and PHAP-2 networks had an irregular lamellar shape made of nanometer-sized subunits.
3. RESULTS AND DISCUSSION 3.1. Structural Characterization of PHAP-1 and PHAP2. The successful growth of PHAP-1 and PHAP-2 networks, their chemical composition, and the 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 CC stretching vibrations of the monomers were found at ca. 1582 cm−1, whereas the polymerized network C C stretching bands at ca. 1628 and 1660 cm−1 confirmed the presence of aromatic rings in the PHAPs networks; (iii) the monomer C−C stretching band (1427 cm−1) was 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 carbon, hydrogen, and nitrogen contents quantitatively (Table S1 in the Supporting Information, SI), which were in close agreement with the calculated values of the polymer networks. Almost no iron (ca. 0.002 wt %) was detected by ICP-OES in both networks. In XPS (Figure S1 in the 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 iron binding energy peaks were observed by XPS (Figure S2 in the SI), which clearly pointed out that the produced PHAPs were free of the iron catalyst used for the synthesis. These FT-IR, EA, ICP-OES, and XPS results confirmed the successful synthesis of the expected PHAP networks. 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 2θ = 11°, 23°, and 43°, which suggests that the framework of the materials has intrinsic flexibility, disorder linkage, direct ring− ring interaction, and long-range structure because of the
Figure 3. FE-SEM images 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 uptake 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 the BET surface area, average pore size, and total pore and micropore volumes, which were derived from the N2 sorption isotherms. The specific surface areas of the PHAP1 and PHAP-2 networks calculated from the BET model were 1137 and 1059 m2 g−1, respectively. Figure 4b shows the poresize distributions of PHAP-1 and PHAP-2, which were calculated based on NL-DFT. The PHAP-1 network exhibits a predominant micropore diameter of 1.51 nm with a median mesopore peak at 3.57 nm. Similarly, the PHAP-2 network shows a narrow pore-size distribution centered at 1.42 and 1.83 nm with a mesopore at 3.05 nm. The contribution of the microporosity in PHAP-1 and PHAP-2 can be calculated from the ratio of the micropore volume (V0.1) over the total pore volume (Vtot).43 According to V0.1/Vtot, the microporosities of PCTP-1 and PCTP-2 are 53% and 56%, respectively, suggesting that the PHAP-1 and PHAP-2 networks contain almost equal amounts of micropores and mesopores. 7919
DOI: 10.1021/acs.iecr.5b03963 Ind. Eng. Chem. Res. 2016, 55, 7917−7923
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Figure 4. (a) N2 adsorption (solid symbol)−desorption (empty symbol) isotherm at 77 K. (b) Pore-size distribution of the PHAPs. Networks: PHAP-1 (black); PHAP-2 (red).
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, PHAP-1 exhibits a high CO2 adsorption capacity of 114.4 mg g−1 at 273 K, whereas the PHAP-2 is slightly lower at 104.3 mg g−1. As the temperature is 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 uptake capacity of PHAP-1 at 273 K was comparable or higher than those of some reported POPs, such as COF-1 (102 mg g−1),44 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 g−1),44 COF-103 (74.8 mg g−1),44 PPF-3 (91.9 mg g−1),45 PPF4 (113.9 mg g−1),45 and PAF-18-OH (110.0 mg g−1).46 Table S2 compares the CO2 uptake for the resulting materials with those of some other previously reported POPs (in the SI). The reversible CO2 adsorption−desorption isotherms (Figure S3 in the 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 playing critical roles.3,36 In the same manner, CH4 and N2 adsorption over the PHAP networks was 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 those of some previously reported POPs (Table S3 in the 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 micropores and mesopores and free pore volume 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 rather than with N2. To determine the potential of gas separation by the PHAP networks, the CO2/N2 and CO2/ CH4 selectivities were calculated (Figure S4 in the SI), which were estimated by the ratio of the initial slope method using the
Table 1. Textural Parameters of the PHAP Networks from the N2 Isotherms at 77 K network
SABET (m2 g−1)a
V0.1 (cm3 g−1)b
Vtot (cm3 g−1)c
V0.1/Vtot (%)
PHAP-1 PHAP-2
1137 1059
0.43 0.41
0.81 0.73
53 56
pore size (nm)d 1.51, 3.57 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. Vtot, total pore volume at P/P0 = 0.995 at 77 K. dPore-size distribution calculated by the NL-DFT model. c
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
Figure 5. TGA plots of PHAP-1 and PHAP-2.
showed that decomposition of the PHAPs started above 620 K under an argon atmosphere, suggesting the high thermal stability of the PHAP networks. In addition, their chemical stability was confirmed by purification processes conducted for the PHAPs (soxhlet extraction method with DMF and MeOH) and tested in acetone, ethyl acetate, ethanol, chloroform, and distilled water. These materials did not dissolve or decompose in these solvents. 7920
DOI: 10.1021/acs.iecr.5b03963 Ind. Eng. Chem. Res. 2016, 55, 7917−7923
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Industrial & Engineering Chemistry Research
Figure 6. (a) CO2 uptake of the PHAPs. (b) CH4 and N2 uptake of the PHAPs at different temperatures. Networks: PHAP-1 (red); PHAP-2 (blue).
Table 2. Gas Uptake at 1 bar, Selectivities of CO2/N2 and CO2/CH4, and Isosteric Heats of Adsorption of CO2 and CH4 Gases for PHAP-1 and PHAP-2 CH4 uptake (mg g‑1)
CO2 uptake (mg g‑1)
CO2/N2 selectivitya
CO2/CH4 selectivitya
network
273 K
293 K
298 K
303 K
273 K
298 K
273 K
298 K
273 K
298 K
Qstb for CO2
Qstb for CH4
PHAP-1 PHAP-2
114.4 104.3
79.4 74.4
73.1 67.5
64.9 61.4
13.8 12.6
8.1 7.0
29.3 34.2
22.8 30.5
11.3 12.5
10.8 13.2
26.5−22.4 27.3-21.2
21.2−18.7 22.5−21.9
a Selectivities estimated from the ratio of the initial slopes of CO2, N2, and CH4 adsorption isotherms. bIsosteric heats of adsorption calculated using the Clausius−Clapeyron equation (gas coverage range: CO2, 1−40 mg g−1; CH4, 1−6 mg g−1).
Figure 7. (a) CO2 and (b) CH4 gas adsorption enthalpies of the PHAPs.
lower than those by the chemisorptive processes (>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, 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-earthmetal-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 and NPOF-4-NO2 (20.8).54
single-component adsorption isotherms at low-pressure coverage (
