Facile Carbonization of Microporous Organic Polymers into

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Facile Carbonization of Microporous Organic Polymers into Hierarchically Porous Carbons Targeted for Effective CO2 Uptake at Low Pressures Shuai Gu, Jianqiao He, Yunlong Zhu, Zhiqiang Wang, Dongyang Chen, Guipeng Yu, Chunyue Pan, Jianguo Guan, and Kai Tao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05170 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 28, 2016

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ACS Applied Materials & Interfaces

Facile Carbonization of Microporous Organic Polymers

into

Hierarchically

Porous

Carbons

Targeted for Effective CO2 Uptake at Low Pressures Shuai Gu,ab Jianqiao He,a Yunlong Zhu,a Zhiqiang Wang,a Dongyang Chen,a Guipeng Yu,*ab Chunyue Pan,a Jianguo Guan,*b and Kai Taoc a

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083,

China. E-mail: [email protected]. b

State Key Laboratory of Advanced Technology For Materials Synthesis and Processing,

Wuhan University of Technology, Wuhan 430070, China. c

Institute of Inorganic Materials, School of Materials Science & Chemical Engineering, Ningbo

University, Ningbo, Zhejiang 315211, China KEYWORDS: triptycene, 9,9’-spirobifluorene, microporous polymers, chemical activation, hierarchical pores, CO2/N2 selectivity

ABSTRACT: The advent of microporous organic polymers (MOPs) has delivered great potential in gas storage and separation (CCS). However, the presence of only micropores in these polymers often imposes diffusion limitations, which has resulted in the low utilization of MOPs in CCS. Herein, facile chemical activation of the single microporous organic polymers (MOPs) resulted in a series of hierarchically porous carbons with hierarchically meso-microporous

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structures and high CO2 uptake capacities at low pressures. The MOPs precursors (termed as MOP-7~10) with a simple narrow micropore structure obtained in this work possess moderate apparent BET surface areas ranging from 479 to 819 m2 g-1. By comparing different activating agents for the carbonization of these MOPs matrials, we found the optimized carbon matrials MOPs-C activated by KOH show unique hierarchically porous structures with a significant expansion of dominant pore size from micropores to mesopores, while their microporosity is also significantly improved, which was evidenced by a significant increase in the micropore volume (from 0.27 to 0.68 cm3 g-1). This maybe related to the collapse and the structural rearrangement of the polymer farmeworks resulted from the activation of the activating agent KOH at high temperature. The as-made hierarchically porous carbons MOPs-C show an obvious increase in the BET surface area (from 819 to 1824 m2 g-1). And the unique hierarchically porous structures of MOPs-C significantly contributed to the enhancement of the CO2 capture capacities, which are up to 214 mg g-1 (at 273 K and 1 bar) and 52 mg g-1 (at 273 K and 0.15 bar), superior to those of the most known MOPs and porous carbons. The high physicochemical stabilities and appropriate isosteric adsorption heats as well as high CO2/N2 ideal selectivities endow these hierarchically porous carbon materials great potential in gas sorption and separation.

INTRODUCTION Considering that the rapidly ascending CO2 concentrations in the atmosphere is wildly accepted as the predominant factor contributing to global warming, carbon capture and storage (CCS) technologies are potential solutions and highly desired.1-3 Recently, sorbents with large surface areas, high CO2 capture capabilities and excellent regeneration abilities are abstracting more and more attention for the separation of CO2 from N2 in flue or natural gas. For this purpose, various porous solids including zeolites,4 metal organic frameworks (MOFs),5 porous


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carbon6 and microporous organic polymers (MOPs)7-9 have been synthesized. They have also found to be of great potential for utilities in water treatment,10 energy storage,11 gas adsorption12 and drug delivery.13 Porous carbon possesses multifaceted advantages for CCS such as high physicochemical stabilities, good moisture stability, light weight, low cost and ease of regeneration. Unfortunately, commercially available porous carbon materials exhibit relatively low CO2 capture capacities, which damages their popularity in gas uptake.14 To enhance the CO2 sorption of porous carbon, various precursors (asphalt,15 organic polymers16 and biomass resources17), optimal carbonization processes18-19 and post-modification strategy6 have been thoroughly investigated. Major research efforts have been focused on developing high surface area and incorporating basic sites to engineer pore surface of carbon.20 However, recent studies have demonstrated that the pore size of sorbents has an important influence on the CO2 capture and storage especially in the case of low pressures which are most close to the real post-combustion situations. Gogotsi,21 Hu22 and our recent study23 pointed out that micropores (< 2 nm) especially ultramicropores (< 0.7 nm) should be responsible for high CO2 uptake below 1 bar considering the thermodynamic size of CO2. Wahby24 proposed that the mesopores (> 2 nm, < 50 nm), which can connect the micropores, could serve as a good passage for the facile pervasion and transport of the kinetic CO2 molecule. Hence, developing the hierarchically porous carbons (HPCs) integrating the advantages of micropores and mesopores would be a feasible strategy for CCS.2526

However, only several kinds of skeletons have been reported for the preparation of HPCs, such as Poly(benzoxazine-co-resol),27 MOFs,28-29 and pyridine,30 where only mesoporous solids are usually obtained, which undoubtedly limits the access to explore new structures and


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functions. As typical microporous materials, MOPs exhibit abundant micropores, which can be easily widened to mesopores via chemical activation17-18 for the preparation of HPCs. Additionally, most of the MOPs were obtained as powders or crystals and have no necessity for further energy-consuming crush17 and cockamamie sieve procedure17,31 that are always indispensable and crucial for HPCs derived from biomass resources before the activation process. Moreover, MOPs with tunable chemical composition allow the avoidance of undesired complex treatment with toxic and corrosive ammonia or liquid acid to introduce basic sites31,32 in the activation process. Hence, the development of HPCs derived from MOPs appears to be worthy of enormous efforts. Unfortunately, the carbonization of microporous organic polymer (such as PAF-133, CTF-134, and FCTF-134) in previous reports always leaded to a shrinking pore size and surface area, and failed to obtain carbons with a hierarchically porous feature. Herein, we report the facile synthesis of hierarchically porous carbons derived from microporous organic polymers via standard chemical activation. More specifically, the microporous organic polymers (MOPs, Scheme 1) with abundant micropores were synthesized through the simple Friedel-Crafts or Scholl polymerization reaction. Then the MOPs were chemically activated under optimized procedures, resulting in the significant expansion of the dominant pore size from micropores (< 2 nm) to mesopores (2 ~ 6 nm) and the concomitant formation of hierarchically porous carbons MOPs-C. Moreover, the BET surface areas and pore volumes of MOPs-C were obviously improved under the retention of the abundant micropores of their parent MOPs precursors. MOPs-C exhibit remarkably high CO2 uptake capacities at low pressures which are comparable with that of the best porous materials. This study provides a simple method for the preparation of hierarchically porous carbons and highlights the importance


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of hierarchically meso-microporous structure for effective CO2 capture under ambient conditions.

EXPERIMENTAL SECTION Materials Triptycene (TP), 9,9’-spirobifluorene (SBF), and 2,4,6-trichloro-1,3,5-triazine (TCT) were purchased from Alfa Aesar Chemical Inc. and used as received. Anhydrous aluminium chloride (AlCl3) was supplied from Aladdin Industrial Corporation (Shanghai, China). Anhydrous zinc chloride (ZnCl2) and Trichloromethane (CHCl3) were purchased from Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). CHCl3 was purified by distillation over P2O5 and stored with 4Å molecular sieves prior to use. All other solvents and reagents were purchased from Energy Inc. and used as received. Synthesis of MOP precursors The MOP precursor synthesized from the Friedel-Crafts condensation of 2,4,6-trichloro-1,3,5triazine (TCT) and triptycene (TP) is denoted as MOP-7, while that from condensation of TCT and 9,9’-spirobifluorene (SBF) is denoted as MOP-9. The MOP precursors from the Scholl polymerization of TP and SBF are labeled as MOP-8 and MOP-10, respectively. They were synthesized in similar procedures. For simplification, only the synthesis of MOP-7 is presented as an example. Under N2 atmosphere, a 50 mL round-bottomed flask was charged with anhydrous AlCl3 (1920 mg, 14.4 mmol), 2,4,6-trichloro-1,3,5-triazine (744 mg, 4 mmol), 15 mL of anhydrous CHCl3 and a magnetic stirrer. After stirred and heated to 60 °C for 0.5 h, the resultant solution was treated dropwise with triptycene (1017 mg, 4 mmol) dissolved in anhydrous CHCl3 (15 mL). The mixture was stirred at 60 °C for 24 h and left to cool to room


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temperature. The solid was obtained by filtration and subsequently rinsed with chloroform, acetone and methanol to remove unreacted monomers. The product was then immersed and stirred in 3 M HCl and then 3 M NaOH aqueous solution for 4 h, respectively, to remove catalyst residues. The crude product was soxhlet extracted with THF, methanol, acetone and chloroform and dried at 120 °C under vacuum for 24 h to give MOP-7 as tan powders in a yield of 93%. Typical procedure for preparation of HPCs from the MOP precursors All carbons materials were synthesized in an optimized procedure. In an agate mortar, polymers and KOH were evenly mixed with a weight ratio of KOH: polymer = 2:1. The mixture was added into a porcelain boat and then placed into a tubular furnace. The chemical activation was then carried out under N2 protection in a heating rate of 2 °C/min and kept at 700 °C for 2 h. After cooling to room temperature, the materials were thoroughly rinsed with 3 M HCl aqueous solution, followed by washing with deionized water until the water appears neutral. After being dried at 120 °C under vacuum, all HPC products were obtained as black powders. These carbons were denoted as MOP-7C to MOP-10C according to their corresponding MOP precursors, respectively (Scheme 1).

RESULTS AND DISCUSSION


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Scheme 1. The preparation routes of the MOPs and carbon materials. The synthetic routes of the MOP precursors (MOP-6~10) and their carbon materials are presented in Scheme 1. All precursors were obtained in high yields via Scholl35 and/or FriedelCrafts36,37 condensation chemistry using anhydrous AlCl3 as catalyst. They are insoluble in common organic solvents (such as methanol, acetone, chloroform, N-methylpyrrolidone and tetrahydrofuran) and even stable after refluxing in 3M HCl aqueous solution or 2M NaOH aqueous solution, implying their high chemical stability. Monoacidic bases were first considered as activating agents for the chemical activation of these precursors. Among them, CsOH and RbOH are relatively costly and unsafe. And the literature reported by Lillo-Rodenas38 et al. pointed out that the reaction between KOH and carbon possesses higher yield and needs lower temperature than that in the case of NaOH. This maybe indicate that KOH is the most efficient activating agent among them. Without prior crush or sieve procedure, such MOP precursors were pyrolyzed using KOH as the activating agent at 700 °C for 2 h. After simple rinse and desiccation, the hierarchically porous carbons MOPs-C were obtained as black powders. The known activation methods were compared in this study (ESI, S3 and S4). The material obtained


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by the direct pyrolyzation without activating agent is denoted as MOP-8C@Direct, while that material obtained by the pyrolyzation with activating agent ZnCl2 is denoted as MOP8C@ZnCl2. The chemical structure of these polymers was characterized by FT-IR (Figure S3, Supporting Information) and

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C CP-MAS NMR (Figure S4~S6, Supporting Information). Figure S3 show

FT-IR spectra of MOP-8 and MOP-10, in which the Scholl polymerization reactions were monitored by the diminution of the peaks at 746 cm-1 corresponding to the C-H deformation vibration of four adjacent ring hydrogens in the triptycene and spirobifluorene. In the FT-IR spectra of MOP-7 and MOP-9, the obvious depletion of intense C-Cl stretching vibration bands at 850 cm-1 is observed compared to that of the original 2,4,6-trichloro-1,3,5-triazine (TCT), suggesting the successful occurrence of the Friedel-Crafts polymerization reaction. And peaks at about 1700 cm-1 for the FT-IR spectra of MOP-7 and MOP-9 are characteristic C=N stretching modes for 1,3,5-triazine rings, which are in accordance with that of the monomer 2,4,6-trichloro1,3,5-triazine (TCT). The reduction in the intensity of the bands at around 746 cm-1 in cases of MOP-7 and MOP-9 indicates that hydrogens in the aromatic rings of triptycene and spirobifluorene have been displaced by 1,3,5-triazine rings or other aromatic rings. The structural information of these polymers was also provided by 13C CP-MAS NMR spectra (Figure S4-S6, Supporting Information). Signals at 124 and 126 ppm are assigned to unsubstituted phenyl carbons, while the intense resonances at 139, 140 and 143 ppm belong to substituted phenyl carbons bonding to other aromatic rings. As shown in the spectrum of MOP-10 (Figure S5, Supporting Information), the peak at 148 ppm is ascribed to the substituted phenyl carbons adjacent to the triazine rings and the signal of carbons of triazine rings was also collected at 176 ppm. In the spectrum of MOP-8 (Figure S4, Supporting Information), the clear signal at 53 ppm


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corresponds to the methylidyne bridge carbons of the triptycene.39 In addition, the resonance of quaternary carbon atoms of spirobifluorene-based units in the MOP-9 and MOP-10 (Figure S5 and S6, Supporting Information) was also be detected at 65 ppm.40

Figure 1. FT-IR spectra (a) and XPS spectra (b, c, and d) of MOP-8C.

FT-IR (Figure 1a) and X-ray photoelectron spectra (XPS) (Figure 1b~1d) analysis were conducted to estimate the chemical structure and elemental composition of the hierarchically porous carbons. Typically, the FT-IR spectrum for a HPC sample MOP-8C exhibit a strong adsorption peak at 1088 cm-1 for CO-O-C stretching vibrations, in contrast to that of its corresponding precursor (MOP-8). The peak at around 1400 cm-1 may be attributed to the -COOstretching vibrations, while the broader and overlapping peaks from 3100 to 3600 cm-1 were ascribed to the stretching vibrations of the -COOH and the adsorbed H2O. The XPS survey spectrum (Figure 1b) demonstrates the presence of carbon (77.6 at%), oxygen (21.3 at%), and chlorine (1.1 at%) species. The oxygen was resulted from the activating agent KOH, while the


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chlorine was introduced from the HCl aqueous solution at the washing process. In the highresolution C1s spectrum (Figure 1c), carbon atoms were found within four different environments in MOP-8C: C=C (284.8 eV), C-O (285.9 eV), C=O (288.1 eV), and O-C=O (289.2 eV). And the high resolution O1s spectrum (Figure 1d) further indicates the signals for C=O, C-O, C-OH configurations. The chemical structure of MOP-8C is quite different from that of MOP-8, indicating the drastic collapse of the polymer farmeworks and the structural rearrangement for the resultant hierarchically porous carbons occurred during the pyrolysis process. There are no fine signals found in the powder X-ray diffraction (PXRD) patterns of MOPs (Figure S7~S10, Supporting Information), indicating their amorphous nature. The thermal stability of both the resultant polymers (MOPs) and their carbon products (MOPs-C) were studied by Thermogravimetric analysis (TGA) under N2 atmosphere (Figure S11 and S12, Supporting Information). The 10% mass loss started at 523, 322, 379 and 370 ℃ for MOP-7, MOP-8, MOP-9 and MOP-10, respectively. Enhanced thermal stability were observed by the TGA curves of the carbon materials, which exhibit a 10% mass loss at 671, 800, 750 and 711 ℃ for MOP-7C, MOP-8C, MOP-9C and MOP-10C, respectively. Even heated to 800 ℃, all these carbon materials possess a mass residue above 85%, further implying their excellent thermal stability. These might be ascribed to the strong aromatic carbon–carbon covalent bonding network. Scanning electron microscopy images (Figure S13 and S14, Supporting Information) display that MOP-7 possesses a round cake micro-morphology, while for MOP-7C those small particles fused together during the activation process and exhibit a floppy and porous morphology. High-resolution transmission electron microscopy results (Figure S15~S22, Supporting Information) demonstrate that all the polymers and carbons have an alternately dark


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and bright microstructure, implying their porous structure. Elemental analysis (Table S3, Supporting Information) shows a nitrogen elimination during the carbonization process, which is coincident with the literatures.23,41 5.34% and 5.12% of nitrogen content were observed for MOP-7 and MOP-9, respectively, while those of the HPC samples significantly dropped to 0.58% and 0.69%, respectively.

Figure 2. (a) N2 adsorption (solid symbols)/desorption (open symbols) isotherms of the polymers at 77K and (b) NLDFT pore size distribution of the polymers. The porosity of these materials was investigated by nitrogen adsorption isotherms measured at 77 K (Figure 2a). All the isotherms of the polymers and carbons exhibit a rapid rise at low relative pressures (P/P0 < 0.01) and a slow rise at higher relative pressures (P/P0 = 0.01-0.9), which belongs to a typical characteristics of Type I isotherm,42 suggesting the existence of abundant micropores structure. The Brunauer-Emmett-Teller (BET) model calculation gives the surface areas of 819, 648, 479 and 635 m2 g-1 for MOP-7, MOP-8, MOP-9 and MOP-10, respectively (Table 1). The pore size distribution (PSD) curves (Figure 2b) obtained by utilizing the nonlocal density functional theory (NLDFT) and carbon split pore model indicated that the MOP precursors exhibit abundant micropores with the dominant pore width centered at 0.68 or 0.57 nm (Table 1), and narrow PSDs were also observed in these cases. The dominant


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micropores pore size of MOP-7, MOP-9 and MOP-10 were found to be located at 1.71, 0.99 and 1.59 nm, respectively, while that of MOP-8 was centered on around 1.00 and 1.59 nm.

Figure 3. (a) N2 adsorption (solid symbols)/desorption (open symbols) isotherms of the MOP8Cs at 77K and (b) NLDFT pore size distribution of the MOP-8Cs. Carbons MOP-8C@Direct and MOP-8C@ZnCl2 display a similar adsorption isotherms as their polymer precursor MOP-8 (Figure 3a) and BET surface areas of 531 and 734 m2 g-1, respectively, which are close to that of their precursor MOP-8 (648 m2 g-1). Similar to the literature,33 the shrinkage of the pore size was also observed in this work. The pore size distribution (PSD) curves (Figure 3b) demonstrate that the dominant pore width of MOP8C@Direct (< 0.68 nm) and MOP-8C@ZnCl2 (< 0.64 nm) are smaller than their precursor MOP-8 (Table S1, Supporting Information). This maybe attributed to the collapse of the relatively thermolabile parts in the polymer frameworks at high temperature.33 The higher BET surface area and pore volume of MOP-8C@ZnCl2 than that of MOP-8C@Direct could be explainable as that the molten ZnCl2 could aggravate the pyrolyzation of polymer and serve as “support bracket” to resist the collapse of polymer framework.


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Figure 4. (a) N2 adsorption (solid symbols)/desorption (open symbols) isotherms of the MOPs-C at 77K and (b) NLDFT pore size distribution of the MOPs-C. Compared with that of MOP-8C@Direct and MOP-8C@ZnCl2, the adsorption isotherms for the carbons MOPs-C obtained by the optimized chemical activation procedure using KOH as activating agent display a faster rise of the nitrogen adsorption at high relative pressures (P/P0 > 0.9) as well as a hysteresis phenomenon (Figure 4a ), which might be resulted from the emergence of mesopores or the interparticulate voids of the samples after carbonization.43 As well as that we expected, MOPs-C exhibits a significant increase in the BET surface areas, which are up to 1508, 1824, 1558 and 1348 m2 g-1 for MOP-7C, MOP-8C, MOP-9C and MOP-10C, respectively (Table 1). The pore size distribution curves (PSDs) (Figure 4b) were obtained by utilizing the nonlocal density functional theory (NLDFT) and carbon split pore model, whose reliability is demonstrated by a good fitting between the experimental data and that in theory (Figure S24~S27, Supporting Information). Moreover, it is interesting to observe that MOPs-C possess hierarchically porous structures. MOPs-C possess ample micropores with pore diameters range from 0.54 nm to 1.59 nm (Table 1). Especially, MOPs-C also exhibit mesoporous feature and their dominant mesopores diameters are centered at 2.52, 3.17, 3.79 and 2.99 nm (Table 1). And the macroporous structures of MOP-9C and MOP-10C were also revealed by the TEM


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images of low magnification (Figure S20 and S21, Supporting Information). The unique hierarchically porous structures may be originated from the break of covalent bonds and the collapse of frameworks in the polymer during the carbonization process, which were also accompanied with the drastic reduction of nitrogen (Table S2, Supporting Information).

Table 1. Pore parameters of the polymers and the carbons.

a

Sample

SBETa

SLANGb

VTotalc

VMicro

MOP-7 MOP-8 MOP-9 MOP-10 MOP-7C MOP-8C MOP-9C MOP-10C

819 648 479 635 1508 1824 1558 1348

886 744 566 853 1731 2109 2090 1615

0.43 0.34 0.26 0.37 0.70 0.91 0.93 0.84

0.27 0.22 0.16 0.23 0.56 0.68 0.48 0.39

Dominant Pore Sized nm 0.68, 1.71 0.68, 1.00, 1.59 0.57, 0.99 0.57, 1.59 0.94, 1.59, 2.52 0.92, 1.59, 3.17 0.55, 1.41, 3.79 0.54, 1.27, 2.99

Brunauer-Emmett-Teller surface area in m2 g-1. bLangmuir surface area in m2 g-1. cPore volume

determined from the N2 isotherm at P/P0=0.99 in cm3 g-1. dPore size derived from N2 isotherm with the NLDFT approach.

The enhanced BET surface areas of the hierarchically porous carbons could be attributed to the following two reasons: first, the chemical activation with KOH resulted in more micropores and mesopores in the carbon frameworks, which was indicated by the obviously enhanced pore volume, from 0.43, 0.34, 0.26 and 0.37 cm3 g-1 in the parent framework MOP-7, MOP-8, MOP-9 and MOP-10 to 0.70, 0.91, 0.93 and 0.84 cm3 g-1 in the carbons MOP-7C, MOP-8C, MOP-9C and MOP-10C, respectively; second, the chemical activation widened the pore size and produced hierarchically porous structures, especially the mesopores, which favor the pervasion of gas


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molecules24 and make that micropores (< 2 nm) and ultramicropores (< 0.7 nm) more accessible for the N2 probe molecules at 77 K. The KOH activation mechanism (KOH + C → K + H2 + K2CO3) of MOPs is similar to that of the carbonization of other precursors.38,44 In this work, the KOH as well as the later formed K metal etched the MOPs farmworks and resulted in the collapse of polymer frameworks at high temperature, while some micropores were coalesced to form larger pores (mesopores and macropores). Then the KOH and K metal permeated into the depth of the MOPs farmworks and contributed to the formation of extra micropores. And the finally formed K2CO3 in the carbon skeleton could serve as templates for the formation of additional pore canals when the washing process of HCl aqueous solution was carried out (S3.2, Supporting Information). The effect of the KOH/polymer weight ratio was also investigated in this work. MOP-8 was chemically activated with a KOH/polymer weight ratio of 1, 2, and 3, while their corresponding products were denoted as MOP-8C-1, MOP-8C-2 (MOP-8C), and MOP-8C-3, respectively (S3.2, Supporting Information). From MOP-8C-1 to MOP-8C-2, these materials show increasing BET surface areas (from 1062 to 1824 m2 g-1) and pore volumes (from 0.54 to 0.91 cm3 g-1), which maybe attributed to the increasing micropores and mesopores resulted from the increase of KOH weight in the activation process. However, activated with higher KOH/polymer weight ratio, MOP-8C-3 exhibits lower BET surface area (1372 m2 g-1) and pore volume (0.78 cm3 g-1) than that of MOP-8C-2 (Table S2, Supporting Information). This could be explained as that the excess KOH resulted in the collapse of the walls of that first formed pores and that many micropores were coalesced to form larger pores, which contribute little to the BET surface areas and pore volumes. This was implied by the decreasing ratio of micropore volume over the total pore volume (from 0.85 for MOP-8C-1 to 0.67 for MOP-8C-3) and further demonstrated by the


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pore size distribution curves (Fig. S2, Supporting Information). MOP-8C-1 exhibits dominant pore diameters at micropore part (at 0.69 and 1.41nm). And MOP-8C-2 diaplays hierarchically porous structures with dominant pore diameters at 0.92, 1.59, and 3.19 nm, while the values are even larger for hierarchically porous carbon MOP-8C-3 (2.01 and 5.46 nm).

Figure 5. CO2 adsorption isotherms of the polymers (a) and carbons (b). As summarized in Table 2, the gas uptake capacities of the polymers MOPs and carbons MOPs-C were investigated by CO2 adsorption isotherms (Figure 5) measured at 273 and 298 K under atmospheric pressure (1 bar). The MOPs exhibit moderate CO2 adsorption capacities ranging from 92 to 147 mg g-1 at 273 K and 1 bar, while those of the MOPs-C range from 147 to 214 mg g-1. With a relatively high micropore volume, MOP-7 possesses the highest CO2 uptake in the MOPs, while the capacity for porous carbon MOP-8C is superior to the other MOPs-C. Among the porous carbons, MOP-8C@Direct and MOP-8C@ZnCl2 still display poor capacities for the CO2 capture (81 and 137 mg g-1 at 273 K and 1 bar, respectively, Figure S1, Supporting Information) due to their low pore volume. Interestingly, the CO2 capture capacities were greatly enhanced after the carbonization processes using KOH as activating agent, such as from 99 mg g-1 for MOP-8 to 214 mg g-1 for MOP-8C. This may be attributed to the combination of the improved micropore volume (up to 0.68 cm3 g-1) and the hierarchically porous structure, which


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makes that ultramicropores more efficient for the kinetic CO2 molecules in the adsorption process. Notably, at 273 K and 1 bar, MOP-8C presents a high CO2 uptake up to 214 mg g-1, which surpasses most of the known porous organic polymers and the porous carbon materials, such as CPOPs (182 mg g-1),45-46 HCPs (181 mg g-1),47-48 CTFs (143 mg g-1),49 CMPs (122 mg g1 50

), nitrogen-doped graphene (114 mg g-1),51 HPNCs (207 mg g-1).52 Moreover, this value could

also be comparable with those of the porous carbon materials with high pore volume and nitrogen content, for example microporous carbon TPC-1 (216 mg g-1) 53 with a pore volume of 1.23 cm3 g-1 and a nitrogen content of 20.87 wt%, and mesoporous carbon OMC (132 mg g-1 at 278K)54 with a pore volume of 2.17 cm3 g-1. This might profit from their hierarchically porous structures, in which those micropores and mesopores result in a synergistic effect: Micropores, which possess strong framework-gas interreaction, are responsible to the fixation of the CO2 molecules. However, pure micropores in materials make against the diffusion and permeation of the CO2 molecules, restricting the access to active sites and more voids in the pores.25 Mesopores, some of which interconnect with micropores, are of weak framework-gas interreaction and efficaciously facilitating the transport of the CO2 molecules in the pores.24 To evaluate the realistic applications for postcombustion flue gas which usually contains ~ 15% CO2, the CO2 capture capabilities of MOPs-C at 0.15 bar (the partial pressure of CO2 in flue gas) were also studied (Table 2). A high CO2 capture of 52 mg g-1 at 273 K and 0.15 bar was observed for MOP-8C and it could be superior to those of the famous porous solids, such as CPOP16-19 (~ 50 mg g-1)45, PCTFs (~ 40 mg g-1)41, nitrogen-doped graphene (43 mg g-1)51, mesoporous carbon OMC (33 mg g-1 at 278 K),54 hierarchically porous polystyrene (HCPPolyHIPEs, 30 mg g-1).55 The parent microporous precursors (MOP-7~10) exhibit considerably lower CO2 uptake than their corresponding HPCs at some conditions, due to their low


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microporous volumes (Table 2). This could be reasonable considering that the micropores dominate the CO2 capture at low pressure because of the strong framework-gas interaction.33-34 With the presence of some mesopores (> 2 nm, < 50 nm) and macropores (> 50 nm), it is usually envisioned that CO2 molecules with a kinetic diameter of 0.33 nm could diffuse more easily to get into the pores. These results demonstrate that these hierarchically porous carbons are promising candidates for the effective CO2 capture at low pressure.

Table 2. CO2 and N2 sorption data of the polymers and carbons.

a

Sample

CO2 uptakea

CO2 uptakeb

N2 uptakea

Qstc

Qstd

SCO2/N2e

MOP-7 MOP-8 MOP-9 MOP-10 MOP-7C MOP-8C MOP-9C MOP-10C

147 99 107 92 194 214 192 147

49 35 32 30 50 52 47 32

4.0 3.9 3.1 3.9 13.6 13.8 14.1 15.6

33.8 32.7 30.7 27.9 34.7 38.9 38.8 34.8

21.5 21.3 24.4 22.2 15.3 18.0 20.9 16.2

80 33 55 31 21 20 13 6

Gas sorption in mg g-1 at 1 bar/ 273 K. bGas sorption in mg g-1 at 0.15 bar/ 273 K. cIsosteric

adsorption enthalpies (Qst) of CO2 in kJ mol-1 calculated by Clausius–Clapeyron equation at low uptake. dQst of CO2 in kJ mol-1 calculated by Clausius–Clapeyron equation at high uptake. eSCO2N2

is calculated by the IAST model from 85% N2 and 15% CO2, at 1 bar/ 273 K.


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Figure 6. Qst from CO2 adsorption at 273 and 298 K. To further investigate the CO2 storage properties of the MOPs and MOPs-C, isosteric heats of adsorption for CO2 (Figure 6) were calculated from the data collected at 273 K and 298 K (Figure 5) using the Clausius-Clapeyron equation (Section S2, Supporting Information). The isosteric heats obtained at low CO2 loadings were 33.8 kJ mol-1 for MOP-7, 32.7 kJ mol-1 for MOP-8, 30.7 kJ mol-1 for MOP-9 and 27.9 kJ mol-1 for MOP-10 (Table 2), while the values for MOP-7C, MOP-8C, MOP-9C and MOP-10C are 34.7, 38.9, 38.8 and 34.8 kJ mol-1, respectively. The enhanced isosteric heats of adsorption of MOPs-C at low CO2 loadings are due to the existence of abundant micropores, which provide a strong affinity toward guest gas molecules due to the overlap of the potential fields from both sides of the pore walls. At high CO2 loadings (1bar), isosteric heats of adsorption decrease significantly for all investigated samples, suggesting that the adsorption is governed by the CO2-CO2 interaction with a weaker affinity. Additionally, MOPs-C demonstrate a sharp decrease in isosteric heats of adsorption at high loadings relative to the MOP precursors. We ascribe this to the downward trend of the pore surface polarity which could be certified by the significantly decreased nitrogen content in elemental analysis.


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Figure 7. CO2 and N2 adsorption capacities for the polymers and carbons at 273 K and 1 bar. At 273 K and 1 bar, MOPs display a N2 uptake of 3.1-4.0 mg g-1, while those of MOPs-C range from 13.6 mg g-1 to 15.6 mg g-1 (Table 2, Figure 7). The CO2/N2 selectivities were calculated from the prevalent ideal adsorbed solution theory (IAST) model under an equilibrium partial pressure of 0.15 bar (CO2) and 0.85 bar (N2) in the bulk phase. As showed in Table 2, at 273 K and 1 bar, polymers MOP-7, MOP-8, MOP-9 and MOP-10 exhibit a CO2/N2 selectivities of 80, 33, 55 and 31 (Figure 8a), respectively. The high ideal IAST CO2/N2 selectivity of 80 is comparable to that of many porous polymers, such as MCTPs (68),36 NOPs (53-81),56 AzoCMPs (67-80)57 and DA-CMPs (60-63).57 The CO2/N2 selectivities dropped to 21, 20, 13 and 6 (Figure 8b) for MOP-7C, MOP-8C, MOP-9C and MOP-10C, respectively. This may be related to the broad pore size distribution of MOPs-C. Though the CO2/N2 selectivities are considerably lower than those of their precursors MOPs, the value of MOP-7C (21) is higher than those reported for many known porous carbons, such as NGA (12),51 PC (12.6),58 OMC (12.8)54 and CPC-700 (16).59


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Figure 8. IAST selectivities of CO2 over N2 for binary gas mixtures 15/85 molar composition in the polymers (a) and the carbons (b) at 273 K.

CONCLUSIONS In summary, we have developed a new strategy for the preparation of hierarchically porous carbons via a simple chemical activation of the single microporous organic polymers obtained in this work. The carbonization with the optimized activating agent KOH resulted in the structural rearrangement of the MOPs farmeworks and contributeded to a diplex improvement of the microporosity and mesoporosity as well as the concomitant formation of hierarchically porous structure. Compared to the MOPs precursors, MOPs-C present a improved porosity and a large BET surface areas (up to 1824 m2 g-1) after the optimized chemical activation. Moreover, the synergistic effect of the meso-microporous structure for the transfer and fixation of CO2 molecules significantly facilitated the improvement of CO2 capture capabilities at low pressures (up to 214 mg g-1 at 273 K and 1 bar, 52 mg g-1 at 273 K and 0.15 bar). The outstanding CO2/N2 selectivity (up to 21 at 273 K and 1 bar) of MOPs-C was revealed by the prevalent IAST calculation. The work offers a new juncture for the microporous organic polymers and


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hierarchically porous carbons in this field, and the as-obtained hierarchically porous carbons are of great potential in the CO2 capture and separation.

ASSOCIATED CONTENT Supporting Information Synthesis procedures, CO2 adsorption isotherms of the carbons derived from MOP-8, FTIR spectra, 13C CP-MAS NMR Spectra, powder X-ray diffraction, TGA curves, SEM images, TEM images, Elemental analysis data, etc.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: 86-731-88836961. (Guipeng Yu) *E-mail: [email protected]. Phone: 86-27-87218832. (Jianguo Guan) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS We acknowledge the financially support from the National Science Foundation of China (Nos. 21204103, 21376272, 51521001 and 51572272), State Key Laboratory of Fine Chemicals (KF1206), the Natural Science Foundation of Hubei Province (2015CFA003), the Top Talents Lead Cultivation Project of Hubei Province, and State Key Laboratory of Advanced Technolgy for Materials and Processing (2015-KF-8), the Fundamental Research Funds for the Central Universities of Central South University (2014zzts155) and Joint Funds of Hunan Provincial


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Natural Science Foundation and Zhuzhou Municipal Government of China (2015JJ5010). SG and JQH would like to thank the Mittal Students Innovative Projects of Central South University.

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Facile Carbonization of Microporous Organic Polymers into

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