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Microporous, Self-Segregated, Graphenal Polymer Nanosheets Prepared by Dehydrogenative Condensation of Aza-PAHs Building Blocks in the Solid State Fuyu Yuan,† Juan Li,§ Supawadee Namuangruk,*,‡ Nawee Kungwan,∥ Jia Guo,*,† and Changchun Wang† †

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, People’s Republic of China ‡ National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Pathumthani 12120, Thailand § Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, China ∥ Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand S Supporting Information *

ABSTRACT: A class of porous organic polymers (POPs), which are constructed by aryl−aryl linkages, has the wholly conjugated organic frameworks that can post-transform into twodimensional graphenal polymers by the intramolecular dehydrogenation. However, typical examples are difficultly defined on the molecular sizes, numbers, and distributions of graphene subunits within the networks, thereby giving rise to uncertainty in applications. Here we report a dehydrogenation fusion of polycyclic aromatic hydrocarbons (PAHs) into graphenal polymers under solvent-free and ionothermal conditions, by which 5,6,11,12,17,18hexaazatrinaphthylene (HATNA) is linked on itself to expand along the coplanar direction. During the reaction, the catalyst AlCl3 solids turn into the molten media to homogenize the reaction system, and alter the molecular configuration and reactivity of HATNA units, resulting in the formation of self-segregated nanosheets with the neighboring layers of the weakened π−π interaction. Besides, the obtained framework exhibits the intrinsic microporosity and exceptionally high surface area. We demonstrate that they can well perform on anhydrous proton conduction and catalytic cycloaddition of CO2 with epoxides. Therefore, this bottom-up strategy may constitute a step toward realizing innovative applications of POPs based on commercially available PAHs.

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INTRODUCTION

sacrifice of high surface areas and need harsh reaction conditions. Porous organic polymers (POPs) have recently achieved significant progress with the successive discovery of various reticular polymer frameworks including covalent organic frameworks (COFs),4−10 conjugated microporous polymers (CMPs),11−15 porous aromatic frameworks (PAFs),16−18 covalent triazine frameworks (CTFs),19−22 and so on. Irrespective of crystalline or amorphous arrangement in molecular structures, they are distinctly featured with high surface areas, adjustable pore sizes, controlled chemical structures and adaptable chemical functionalities. Accompanied with the significant progress in synthesis, PAHs, such as anthracene,8 triphenylene,4,6,7 pyrene,5,6,10,15 and hexabenzocoronene,9 have been covalently anchored on the different POPs to enhance π-electron immigration, whereas most of them are short of graphene-like structures. Thus, it has been explored that the wholly conjugated POPs with the aryl−aryl

Exploration of processable graphene in large scale has been challenging for years since it emerged as a rapidly rising star in the whole science community. Although the top-down strategy such as exfoliation of oxidative graphite has been dramatically developed in order to span applications, the inevitable issue encountered is to treat the numerous oxygen-containing domains on graphene. Thus, the bottom-up method to synthesis of defect-free graphene has been proposed to chemically create a class of two-dimensional (2D) graphenal materials. Polycyclic aromatic hydrocarbons (PAHs) are composed of laterally fused benzene rings, known as typical molecular graphenes that could serve as media to transfer electrons. Direct fusion of PAHs into nanographenes is synthetically challenged. Thus, an intramolecular cyclodehydrogenation is adopted to planarize the branched polyphenylenes into disk-type target molecules,1 but the yields are limited in the laboratory scale. High-temperature pyrolysis of PAHs is the alternative solution to yield the relatively pure graphitic carbon materials and facilitates the large-scale preparation.2,3 However, the kind of PAHs-based polymers with graphenal characteristics have been often built on the © 2017 American Chemical Society

Received: January 26, 2017 Revised: April 13, 2017 Published: April 13, 2017 3971

DOI: 10.1021/acs.chemmater.7b00353 Chem. Mater. 2017, 29, 3971−3979

Article

Chemistry of Materials

Figure 1. Ionothermal synthesis of two-dimensional Al-doped MGP starting from HATNA through the Scholl reaction.

the planar and discotic conformation with electron-deficient property. Hence HAT and its derivatives have been employed not only as ligands for metal complexes but also as building blocks in supramolecular systems for a variety of applications.29 Herein, we propose the synthesis of HAT-based graphenal polymers with the aim of mimicking the N-doped graphene for use in catalysis. Since pyrazine moieties of HAT impose the change of charge distribution of proximity carbon atoms just like N-doped graphene, it might generate the activation regions either to participate in catalytic reactions directly or to immobilize the metal nanoparticles with catalytic activity. However, the related performances of HAT-containing POPs have rarely been explored.30−32 We commenced the studies from the computational calculations to pursue the possibility of direct fusion of 5,6,11,12,17,18-hexaazatrinaphthylene (HATNA) in the presence of AlCl3 by the dehydrogenation reaction. Triggered by the theoretical results, the ionothermal system was adopted to

linked frameworks are subjected to high-temperature posttransformation into the so-called graphenal polymers that involve different sizes and numbers of graphene subunits (i.e., PAHs), while retaining the high surface areas and micropore characteristics.23−27 However, the pyrolysis method used for POPs generates the complicated existing terms of PAHs that would confuse the nature of the molecular configurations and functionalities of such materials. Recently, Tan and co-workers reported a dehydrogenative cross-linking polymerization of the simplest PAHs (i.e., naphthalene and pyrene) toward microporous aromatic networks.28 Nevertheless, with the larger discotic PAHs as monomers, the synthesis of PAH-related graphenal polymers have remained challenging to date because the stronger and more expanded π−π interaction among building blocks or oligomers impedes the construction of porous network structures. 1,4,5,8,9,12-Hexaazatriphenylene (HAT) is one of the smallest 2D polyheterocyclic aromatic systems, which have 3972



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polymerize HATNA in solid state using AlCl3 both as catalyst and medium, resulting in the 2D extended nanosheet through the aryl−aryl linkage. During the polymerization, AlCl3 chelated with the HATNA units not only alters the charge distribution of HATNA to favor the aryl−aryl coupling but also acts as shape regulator to segregate the neighboring layers for the significant 2D expansion. Also, the framework characteristic renders such materials the distinctive microporosity and high surface areas, largely promoting the abilities of adsorption and diffusion of guest molecules. Therefore, in light of their structural uniqueness and functionalities, they are termed microporous graphenal polymers (MGPs) and their potential utilizations have been rigorously evaluated for anhydrous proton conduction and CO2 uptake and transformation.

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Article

RESULTS AND DISCUSSION

Dehydrogenative coupling reactions of aromatic compounds such as the Scholl reaction and oxidative aromatic coupling have attracted particular interest, as they are easier and more economical without the need of prior installation of leaving functionalities in substrates.33 It has sparked the bottom-up route to synthesize nanographenes in excellent yields. Therefore, the typical Scholl reaction was conducted for polymerization of HATNA in CHCl3 under reflux in the presence of AlCl3 (Figure 1a), whereas there was nearly no product yielded even with prolonged reaction time. The reason underlying the low outcome was studied by the theoretical calculations, which were carried out by M06-2x/ccpVTZ method in the Gaussian 09 package (see calculation details in the Supporting Information). As shown in Figure 1b, the molecule HATNA contains six nitrogen atoms with sp2 hybridization, which could render the molecule an electrondeficient character but do not affect the charges of the peripheral carbon atoms on the three benzene (−0.21e and −0.22e). As HATNA has D3h symmetric structure, there are five possible distinct sites, i.e., A, B, C, D, and E, for the adsorption of AlCl3 to form complexes. At A site, Al atom is interacted with a terminal carbon atom of benzene ring to form a σ complex with the Al−C bond distance of 2.43 Å. The adsorption energy reaches 14.0 kcal mol−1. The charge on the interacted benzene is therefore reorganized, leading to uneven distribution of charge densities among the phenyl carbon atoms. The carbon atom closest to Al atom has the highest charge density of −0.40e, and the other adjacent carbon atoms give slightly depleted charges. Therefore, the benzene rings fusing around the HAT center are activated by AlCl3 and allow for the dehydrogenative coupling of HATNA. While migrating AlCl3 from B to E sites, the calculations revealed that the complexes were not stable at B, C, and D sites, but at E sites, Al atoms could settle down at the pockets of two pyrazine rings to form the stable complexes. This agrees with the charge density contour map of HATNA presented in Figure S1 (Supporting Information). Note that HATNA contains the three identical E sites that could accommodate the three AlCl3 molecules. The adsorption energies are up to 37.2, 70.6, and 97.8 kcal mol−1, for one, two, and three AlCl3 molecules, respectively. Also, the formation of E complex induces the reorganization of charge densities on the phenyl carbon atoms (−0.17e to −0.22e). After the three E sites are occupied, the reasonable position for AlCl3 is A site, and the formed σ complex could increase the charge density of the neighboring carbon to −0.46e and reduce to −0.15e and −0.16e for the other two adjacent carbon atoms. This large difference facilitates the aryl−aryl coupling of HATNA as aforementioned. Therefore, the calculation results reveal that the dehydrogenation process is more favorable on the 3Al-HATNA complexes instead of HATNA in the presence of AlCl3 as catalyst. On the other hand, we observed that upon coordination with the three AlCl3 molecules, the molecular configuration of HATNA was slightly bent and its aromaticity was compromised (Figure 1c). The side view of the complex shows that the up-to-down distance is 5.38 Å. Thus, the π−π interaction between the complexes would be largely attenuated so as to promote the accessibility of the fourth AlCl3 to the peripheral carbons of 3Al-HATNA complexes, allowing for the homopolymerization via the Scholl-reaction pathway. Therefore, the growing HATNA-based networks tend to evolve into a self-segregated

EXPERIMENTAL SECTION

Materials. Anhydrous aluminum(III) chloride (98.0%), 3,3′diaminobenzidine (97.0%), 1,2-phenylenediamine (98.0%), triquinoyl hydrate (95.0%), triphenylene (96.0%), tetrabutylammonium bromide (98.0%), and propylene oxide (99.0%) were purchased from TCI. All solvents including chloroform (99.5%), methanol (99.5%), ethyl acetate (98.5%), and hydrochloric acid (36.0−38.0%) were obtained from Sinopharm Chemical Reagent and used without further purification if not otherwise mentioned. All operations involving airand/or moisture-sensitive compounds were carried out in a glovebox or standard Schlenk line techniques under N2 atmosphere. General Procedures for Preparation of aza-MGP(Al). To a dried Pyrex tube (1 cm × 10 cm) was charged a given amount of HATNA and AlCl3. Because AlCl3 salts are easily hydrolyzed to generate H[AlCl3OH] and HCl in store, the produced traces of HCl or water could serve as Brønsted acids to accelerate the reaction. The mixture was applied to a vacuum pump for 5 min to remove air and moisture in vial. Afterward, a portable butane heater was used to seal off the tube, and the reaction was kept in a muffle furnace at a given temperature for 15 h. After reaction, the block solids turned black from the initial yellow color, and the product was buried in AlCl3 salts. Then the bulk solid was ground into powder, which was suspended in water to remove the excess amount of salts by ultrasonication treatment. The insoluble powders were collected by centrifugation, boiled in 1 M HCl solution, and exhaustively washed with water and ethanol, respectively. The black powder was dried overnight in an oven to give a 90% yield. Characterizations. 1H NMR spectra were recorded on a Bruker Advance III HD 400 MHz at 298 K. Solid-state 13C CP/MAS NMR measurements were performed on a Bruker 400 MHz NMR spectrometer at a MAS rate of 12 kHz and a CP contact time of 2 ms. Thermogravimetric analysis was conducted on a Pyris 1 thermogravimetric analyzer (PE, USA) at a heating rate of 20 °C min−1 from 100 to 800 °C under flowing air. N2 adsorption− desorption isotherms were collected by a TriStar II 3020 volumetric adsorption analyzer (Micromeritics, USA) at 77 K, while CO2 adsorption−desorption isotherms on a Quantachrome Autosorb-iQ gas adsorption and pore-size analyzer at 298 K. The samples were degassed at 200 °C for 24 h before measurement. Elemental analysis was carried out on a VARIO EL3 analyzer with the mode of CHN under 950 °C. Morphological characterizations were carried out by using transmission electron microscope (TEM; JEOL 2100F, Japan) and field-emission scanning electron microscope (FE SEM; Zeiss Ultra 55, Germany), respectively. The X-ray photoelectron spectroscopy (XPS) was performed using a PHI 5000C ESCA System X-ray photoelectron spectrometer. AFM image were obtained with a Bruker Multimode 8 scanning probe microscope, operating in tapping mode with phosphorus-doped Si tips (RTESPW, Bruker). Raman spectra were recorded on a LabRAM XploRA microscope (HORIBA Jobin Yvon) using a 532 nm laser. Powder X-ray diffraction (PXRD) patterns were collected on an X-ray diffraction spectrometer (Bruker D8 Advance, Germany) with Cu Kα radiation at λ = 0.154 nm operating at 40 kV and 40 mA. 3973


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Chemistry of Materials 2D polymer sandwiched by metal salts (Figure 1c). In contrast, the poor solubility of such complexes in solution might suppress the dehydrogenative reaction. Encouraged by the theoretical analyses, solid-state ionothermal condition was adopted to polymerize HATNA into a porous aromatic framework. AlCl3 solids with trace of water acted both as catalyst and reactive media to blend with HATNA, and the mixture was charged into a Pyrex tube. After being degassed and sealed off, the reaction proceeded in the molten AlCl3 salt at high temperature for 15 h. Overpressure in the sealed vial was detected for almost every experiment. The phenomena may imply the formation of hydrogen as a result of C−C coupling reactions. The crude product was boiled in a 1 M HCl solution and exhaustively washed with water and ethanol, respectively, resulting in the black and light powder with yield over 90%. Solid-state 13C CP/MAS NMR spectra confirmed the molecular structure of the target polymer. Compared with the starting material HATNA (Figure 2), there

Figure 2. Solid-state 13C CP/MAS NMR spectra of HATNA and azaMGP(Al). Figure 3. HR TEM (a) and AFM (b) images of the aza-MGP(Al) polymers with two-dimensional layered structure.

is one signal at 140.6 ppm for the six hydrogen-bearing C(1) atoms and one broad peak at the center of 128.3 ppm, which contains multiple signals assignable to the phenyl C(2) and the pyrazine C(3). The polymerization of HATNA was further confirmed by Raman spectrum (Figure S2 in the Supporting Information), wherein the two characteristic peaks similar to what has been reported for graphitic carbon materials are observed at 1346 and 1608 cm−1, which are assignable to the D band and G band, respectively. The D/G intensity ratio was calculated to be 0.89. The relatively strong G band reveals the formation of the overall graphitic structure, while a number of small domains of aromaticity responsible for the D band also exist in graphenal polymers.34 The HR TEM image from Figure 3a shows that the product has the sheet-like morphology with thin, large, and flexible features. Again, a large-area view from the FE SEM image reveals the uniform layered appearance of such materials (Figure S3 in the Supporting Information). The AFM image in Figure 3b gives a layer thickness of about 1.0 nm and shows the rough surface as a result of the adsorption of small-size 2D polymers. Since HATNA ligand is a rigid, planar, and aromatic discotic molecule, it is reasonable that the polymeric architecture could extend in a 2D direction, but as observed, they do not notably stack on one another. It validates that the starting complexes initiate a self-segregated pathway toward a 2D graphenal polymer well in line with the theoretical assumption.

Insight into the elemental compositions has been provided by X-ray photoelectron spectroscopy to acquire the surface nature of 2D polymers. Figure 4 exhibits that the raw data

Figure 4. XPS spectra of N 1s (a) and Al 2p (b) for aza-MGP(Al) polymers.

spectrum (dotted) in the energy ranges of N 1s and Al 2p are fitted with different components or chemical states. The N 1s signal with a binding energy of around 400 eV could be attributed to the formation of the two kinds of bonds, namely, N−C bond at 401.2 eV and N−Al bond at 399.4 eV, respectively (Figure 4a). For the Al 2p signal, deconvolution of this broad peak could afford three individual peaks at 76.8, 74.8, and 73.4 eV, corresponding to Al−N, Al−OH, and Al−Cl units, respectively (Figure 4b). Taken together, the results prove 3974


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Table 1. Porosity of a Series of aza-MGP Prepared under Different Conditions (Figure S5 in the Supporting Information) sample

ionic salt

molar ratio of salt and monomer

T (°C)

SBET (m2/g)a

Smicro(m2/g)b

Sext (m2/g)b

Vtot (cm3/g)d

aza-MGP(Al)-1 aza-MGP(Al)-2 aza-MGP(Al)-3 aza-MGP(Al)-4 aza-MGP(Al)-5 aza-MGP(Zn)-6 aza-MGP(Fe)-7 aza-MGP(Al)-8 TP-MGP

AlCl3 AlCl3 AlCl3 AlCl3 AlCl3 ZnCl2 FeCl3 AlBr3 AlCl3

20 30 40 20 20 30 30 30 30

400 400 400 450 500 400 400 400 400

595 (746) 1387 (1793) 896 (1132) 905 (1123) 1530 (1913) 214 (296) 150 (214) 1316 (1765) 105 (174)

479 940 698 729 1098 130 93 779 34

116 447 198 176 423 84 58 537 71

0.2808 0.6717 0.3664 0.4306 0.6779 0.1553 0.1438 0.8021 0.1041

a

Surface area is calculated from the N2 adsorption isotherm using the Brunauer−Emmett−Teller method, and the value in parentheses is the Langmuir surface area. bThe micropore (Smicro) and external (Sext) surface areas are obtained using the t-plot method based on the Halsey thickness equation. dTotal pore volume at P/P0 = 0.99.

conditions. Likewise, solution-phase reaction on TP was low yielding yet giving an insoluble oligomer without porosity. Figure 5a shows the N2 adsorption and desorption isotherms of

again that Al ions are coordinated with pyrazine of HATNA units to functionalize the polymer skeletons. Thermogravimetric analysis was carried out in air from 100 to 800 °C (Figure S4 in the Supporting Information). There are two weight-loss stages observed in the ranges of 100−300 and 350−750 °C, respectively. The thermal decomposition below 200 °C might be responsible for the bound water that was attached on the conjugated Al in micropores. The organic skeleton body was gradually degraded until 750 °C, and the decomposition temperature was found at 610 °C on the derivative curve. The residual weight derived from the survival Al2O3 was used to calculate the Al content in graphenal polymers, having reached 10.6 wt % close to the theoretical value (10.3%) from the 3Al-HATNA unit. The optimization of microporous structure could be accomplished by varying the reaction conditions including reaction temperatures, catalyst species, and molar ratios of salts and monomers. The measurement of N2 isotherm sorption was performed at 77 K, providing a series of pore parameters that have been compiled in Table 1. The Brunauer−Emmett−Teller (BET) model was applied to calculate the apparent surface area (SBET) and pore volume (Vtot), and de Boer statistical thickness (t-plot) analysis was used to evaluate the specific surface areas contributed by micropores (Smicro) and external pores (Sext). As the equivalents of AlCl3 salt were varied at 400 °C, the BET surface areas were increased to as high as 1387 m2 g−1 at 30 equiv of salt. Meanwhile, as the reaction temperature increased from 400 to 500 °C at 20 equiv of salt, the maximum surface area of 1530 m2 g−1 was obtained on aza-MGP(Al)-5, containing 72% of the micropore contribution. It is understandable that the increase of reaction temperatures could generate a huge amount of micropores due largely to the remarkable carbonization at elevated temperatures, but the effect of salts seems more complicated. When the other Lewis acids, i.e., FeCl3, ZnCl2, and AlBr3, were applied to undergo the ionothermal polymerization under identical conditions, only the aluminum salts gave the highest surface areas. Taken all together, we reason that the molten salts work as a dispersion phase in the solvent-free system, and among them, the aluminum salts may have the better polymer miscibility, thereby forming the extended graphenal structures with enhanced microporosity. Then the ionothermal coupling of triphenylene (TP) was carried out to study the effect of building blocks on the formation of porous aromatic networks in the solid-state Scholl reaction. With TP as monomer sources, only a very low conversion (ca. 10%) was observed under the same reaction

Figure 5. N2 sorption isotherms (a) and pore-size distributions (b) of aza-MGP(Al) and TP-MGP, respectively.

aza-MGP(Al) (sample 5 in Table 1) and TP-MGP prepared by the ionothermal route. Both give the type I gas sorption isotherms, which are indicative of a micropore character in accordance with the IUPAC classifications. Aside from the low uptake quantity on TP-MGP, there is an adsorption− desorption hysteresis found at low relative pressures. This implies a remarkable swelling of TP-MGP upon nitrogen adsorption in network, and the structural change is frequently observed in a soft network as increasing numbers of adsorbate molecules condense in the pores. The pore-size distribution was calculated by the nonlocal density functional theory model. As displayed in Figure 5b, the predominant pores within aza-MGP(Al) were found in the micropore region and narrowly distributed at 1.3 nm. In sharp contrast, TP-MGP displayed a broad pore-size distribution, implying that the structural change of TP-MGP gives rise to the pore deformation in measurement. The result indirectly turns out the presence of numerous defects in the TP-based network to impair intrinsic porosity. On the other hand, it has been reported that large pores could be formed due to the phase separation between organic polymers and salt melt under ionothermal conditions.35 As is evident from the pore analyses, the TP-MGP system might not be homogeneous in molten salts and, thereby, formed the foam-type monolith that made TP-MGP have the hierarchical pores (Figure S6 in the Supporting Information). On the contrary, the complexes of Al and HATNA not only alter the charge density of polycyclic aromatic components to promote reactivity but also are able to 3975


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as a proton source. From the N2 sorption isotherm, it was found that the surface area of H2SO4@aza-MGP(Al) was largely reduced as low as 656 m2 g−1, roughly 43% of the parent one (Figure S8 in the Supporting Information). The H2SO4@ aza-MGP(Al) sample was then compacted to the pellets for measurement, and the aza-CMP was used as control but hard shaped by similar treatment. According to the semicircles from the Nyquist plots in Figure 7a, the proton conductivity of

well disperse in molten salts to allow for the complete polymerization toward a microporous network. To our knowledge, only Jiang et al. has reported the dehydrate condensation between 1,2,4,5-benzenetetramine and triquinoyl hydrate which were applied to synthesize the azaCMP under ionothermal conditions.30 However, the side reaction is inevitable because ammonia can be eliminated to undergo the polymerization as the case of thermolysis of carbon nitride polymers shows.36 This leads to a significant diminution of nitrogen as well as the complicated compositions that are difficultly determined. In contrast, the dehydrogenative polymerization of HATNA may ensure the product purity to some extent. Elemental analysis was performed to calculate the molar ratios of C and N atoms for aza-CMP and aza-MGP(Al) samples (Figure 6). According to the theoretical values, the

Figure 7. Nyquist plots (a) and Arrhenius plots (b) of the proton conductivity for aza-MGP under anhydrous conditions with increase of measurement temperature. Least-squares fit in panel b is shown as a solid line.

H2SO4@aza-MGP(Al) shows an increasing tendency as the test temperature was elevated. The maximum conductivity was achieved as high as 2.49 × 10−4 S cm−1 at 120 °C under anhydrous conditions. The temperature-dependent proton conductivity values were employed to calculate the activation energy of the system. The fitted linear plot gives an activation energy of 0.187 eV for H2SO4@aza-MGP(Al) (Figure 7b). It is underlined that the value is comparable to those of the early reported MOFs and POPs among the best (Table S1 in the Supporting Information). The observation of the high proton conductivity and low energy value of H2SO4@aza-MGP(Al) suggests that it belongs to anhydrous fast-ion conductors. In light of the Lewis acid function of AlCl3 chelated on a skeleton, we investigated the catalytic performance of azaMGP(Al) as solid catalyst for CO2 transformation. To access the affinity of CO2 to the metalated MGP networks, the CO2 adsorption capability was evaluated using volumetric methods at 298 K. Commercially available active carbon (AC) and azaCMP were both utilized as references to establish an approximate activity ranking. As shown in Figure 8, the CO2 uptake reaches 76, 55, and 54 mg g−1 until the pressure approaches 1.0 bar for aza-MGP(Al), aza-CMP, and AC, respectively. Given that the three samples have similar surface areas, the comparatively high CO2 adsorption for aza-MGP(Al) is presumed owing to the numerous electrophilic sites, attributing to the chelated Al ions on the HATNA units. In comparison to the aza-CMP and AC, the multilayer 2D structure of aza-MGP(Al) also facilitates the rapid diffusion of CO2 because the interlayers are sandwiched by the chelating metals which attenuate the stacking interaction with each other. The results indicate the possibility for aza-MGP(Al) to efficiently catalyze the cycloaddition of CO2 with epoxides. The conversion of propylene oxide (PO) to propylene carbonate (PC) was carried out to examine the influence of reaction parameters on the yield of PC (Table 2). While tetrabutylammonium bromide (TBAB) was used as cocatalyst, aza-MGP(Al)-5 gave a moderate yield (49%) at 50 °C and under 1 MPa CO2 (entry 1). In general, the cycloaddition

Figure 6. Element analyses of the molar ratios of C and N atoms for the different aza-MGPs, HATNA, and aza-CMP. Theoretical values of C/N molar ratios are 2.5 and 4.0 for aza-CMP and aza-MGP(Al), respectively.

remarkable depletion of nitrogen content was found for azaCMP, but the C/N molar ratios of aza-MGP(Al) almost coincided with that of the monomer HATNA (C24N6H12), and the effect of polymerization conditions could be ignorable. Although the polymer subunits are defined, the PXRD pattern still reflects the amorphous structure of the aza-MGP(Al) that is similar to the reported aza-CMP (Figure S7 in the Supporting Information). The proton conductivity of aza-MGP(Al) was measured in an anhydrous medium in a temperature range of 25−120 °C. Prior to measurement, sulfuric acid (1 M aqueous solution) was adsorbed in the opening micropores of aza-MGP(Al) to either protonate the residual pyrazine or neutralize the conjugated Al(OH) ions so that the HSO4− anions were immobilized to act 3976


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efficient gas−liquid diffusion. The pure AlCl3 and 3Al-HATNA complexes were both used as control for the catalysis. As observed, they did not dissolve in the solvent-free mixtures, and the yields were only around 20% (entry 5 and entry 6). This elucidates that microporosity is a pivotal key for a processintensified conversion of CO2 with epoxides. Thus, the microporous aza-CMP and AC were mixed with equivalent AlCl3 for comparison with aza-MGP(Al). Their PC yields are just 47% and 35% (entry 7 and entry 8), respectively. This implies that the added AlCl3 salts might not cooperate with the two carriers during the catalysis process. Also, it is indirectly reflected that the excellent catalytic efficiency of aza-MGP(Al) is responsible for the synergistic action of 3Al-HATNA units with the opening micropores and unfolded sheet morphology.

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CONCLUSIONS To summarize, dehydrogenation polymerization of HATNA molecules toward microporous graphenal polymers has been conducted under ionothermal conditions. The presence of AlCl3 salt in the molten state helps form a homogeneous system due to the initial coordination with multidentate HATNA. Such metal complexes have the deformed molecular conformation and could be activated by AlCl3 salt to accelerate the polymerization on each other through the dehydrogenation process. Simultaneously, the steric hindrance of complexes inhibits the π−π stacking of HATNA units to allow for the 2D evolution of polymer frameworks. The resultant products are thus characteristic of N-doped, self-segregated, and graphenal nanosheets. The intrinsic microporosity gave the maximum surface area of as high as 1530 m2 g−1 and the narrow pore-size distribution (1.3−2 nm). Herein, the performances of anhydrous proton conduction and CO2 uptake and transformation were estimated by using the optimized aza-MGP(Al) in view of their structural advantages. Sulfuric acid was loaded within microporous networks to pump protons, giving a prominent high-temperature conductivity (2.49 × 10−4 S cm−1 at 120 °C) along with the low activation energy (0.187 eV). Also, it was found that the cycloaddition of CO2 with PO was efficiently catalyzed into PC (TOF, 2057 h−1). We are confident that the reported approach to polymerizing azaPAHs will open new perspectives for microporous polymers to mimic graphene toward scale-up applications in energy and environmental fields.

Figure 8. CO2 uptake of aza-MGP(Al), aza-CMP, and activated carbon (AC).

Table 2. CO2 Cycloaddition with PO Catalyzed by azaMGP(Al) and TBABa

entry b

1 2 3 4 5 6 7 8

catalyst

CO2 (MPa)

yield (%)

TON

aza-MGP(Al)-5 aza-MGP(Al)-5 aza-MGP(Al)-5 aza-MGP(Al)-1 3Al-HATNA AlCl3 aza-CMP + AlCl3 AC + AlCl3

1 1 3 3 3 3 3 3

49 72 93 75 22 16 47 35

350 514 637 536 157 114 335 250

All of the reactions were allowed to proceed at 100 °C for 1 h in the mixture of PO (5 mL), catalysts (48 mg), and TBAB (1740 mg) without added solvents. TON = (moles of product)/(moles of metal in the catalyst). bThe reaction was conducted at 50 °C.

a

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reaction is more sensitive to temperature compared to other similar catalytic systems. Thus, as the reaction was heated to 100 °C, the reaction efficiency was correspondingly enhanced to 72% yield (entry 2). While the CO2 pressure increased to 3 MPa, the highest yield was achieved (entry 3). Also, there has been no other product such as polycarbonate detected by 1H NMR analysis, implying the excellent selectivity toward PC. With such optimized reaction conditions, the reaction proceeded rapidly with 48% yield of PC in 10 min. This gives rise to a turnover frequency (TOF) of 2057 h−1. To our knowledge, this is among the highest TOFs hitherto obtained for a cycloaddition reaction of CO2 and PO with a heterogeneous catalyst. Moreover, we underline the likelihood of a further enhancement of TOF upon lowering the catalyst concentration because the activity of the catalyst is quite remarkable at low concentrations. Next, the comparison was made with the aza-MGP(Al)-1 that had the relatively low surface area (595 m2g−1). Evidently, its product yield was compromised under identical conditions (entry 4). We assume that the high surface area could significantly promote the CO2 uptake quantities and intensify the catalysis process by the

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00353. Detailed methods of calculations, synthesis, proton conductivity measurement and catalysis test, charge density contour maps, Raman spectrum, TGA curve, N2 sorption isotherms, pore-size distributions, PXRD pattern, FE SEM images, and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(S.N.) E-mail: [email protected]. *(J.G.) E-mail: [email protected]. ORCID

Jia Guo: 0000-0003-4869-9992 Changchun Wang: 0000-0003-3183-2160 3977


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Chemistry of Materials Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (Grant Nos. 21474015 and 51633001), STCSM (Grant No. 14ZR1402300), and the State Key Project of Research and Development (Grant No. 2016YFC1100300).

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Chemistry of Materials (36) Zheng, Y.; Lin, L.; Wang, B.; Wang, X. C. Graphitic Carbon Nitride Polymers toward Sustainable Photoredox Catalysis. Angew. Chem., Int. Ed. 2015, 54, 12868−12884.

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