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Heptazine-based porous polymer for selective CO2 sorption and visible light photocatalytic oxidation of benzyl alcohol
Microporous and Mesoporous Materials 282 (2019) 9–14
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Heptazine-based porous polymer for selective CO2 sorption and visible light photocatalytic oxidation of benzyl alcohol
T
Congying Xua, Li Qiana, Junyu Lina, Zhiyong Guoa,∗, Dan Yanb,∗∗, Hongbing Zhana,c a
College of Materials Science and Engineering, Fuzhou University, Fuzhou, 350116, PR China Testing Center, Fuzhou University, Fuzhou, 350116, PR China c Key Laboratory of Eco-materials Advanced Technology, Fuzhou University, Fujian Province, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous organic polymer s-Heptazine Gas separation Photocatalytic oxidation
Heptazine units with highly rich nitrogen sites were employed to fabricate a new porous organic polymer (POPHE) via direct coupling of 2,5,8-trichloro-s-heptazine and 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)tetraaniline with the mild base N, N′-diisopropylethylamine (DIPEA) in tetrahydrofuran (THF) solution. POP-HE displays good CO2 uptake capacity (50.6 cm3 g−1, 10 wt%, 1 bar/273 K) and highly selectively adsorptive separation of CO2 over CH4 under ambient conditions. Impressively, at 273 K, the predicted IAST selectivity of POP-HE is 47.0-13.4 for the natural gas mixture (CO2/CH4 = 5:95) at pressures varying from 0 to 1 atm. Meanwhile, the result of calculated band gap value based on Tauc plot motivated us to investigate its photocatalytic activity. POP-HE was applied as heterogeneous photocatalyst for visible light-induced selective oxidation of benzyl alcohol to benzaldehyde, and exhibited great conversion of 33.6% under irradiation of white LED light, which is much higher than that of g-C3N4. Moreover, POP-HE can be recycled up to three times without significant decrease in catalytic activity.
1. Introduction In recent years, polymeric carbon nitrides (PCNs) with unique physiochemical properties have attracted extensive interest in photocatalysis, including photocatalytic hydrogen production, selective oxidation of alcohols and nitrobenzene reduction [1–3]. Nevertheless, pure PCNs are usually restricted by their low surface area, limited light absorbance and rapid charge carrier recombination [4–6]. Several protocols such as texture modification, elemental doping, and copolymerization have been devoted to improve the photocatalytic performance of g-C3N4 [7–10]. However, the above strategy is usually hampered by the high condensation temperatures needed and the incomplete condensation of remaining isolated functionalities. On the contrary, using heptazine (C6N7) as building block to construct porous organic polymers (POPs) is a promising way to design new classes of PCNs with desirable photocatalytic and other properties [11]. POPs have exhibited great potential in heterogeneous catalysis, energy storage, gas storage and separation owing to their low skeleton density, excellent porosity, tuneable structure and high physicochemical stability [12–16]. However, unlike its parent polymeric carbon nitride (so-called g-C3N4) has been exploded with a wide variety of applications in the past two
∗
decades, the development and application of the porous polymeric materials that built directly from the heptazine core is still in the infancy stage. The main obstacle to construct the heptazine class of compounds lies in their synthesis protocols and their insolubility in the majority of organic solvents. Currently, the 2,5,8-trichloro-s-heptazine (cyameluric chloride, Cy) is the most widely available and best described precursor material which has three equivalent, very good leaving groups that can be exchanged by nucleophilic substitution reactions. In 2013, by utilizing cyameluric chloride as the starting building block, a heptazine based microporous polymer network was prepared by Thomas and Kailasam, which showed photocatalytic activity for the production of hydrogen from water under visible light illumination [17]. Since then, several porous polymers containing heptazine rings have been reported, of which showed favourable performance in organocatalysis, selective CO2 capture, luminescent sensing and ammonia detection [18–20]. To our best of knowledge, no example that using heptazine derived porous polymer as visible light photocatalyst in organic reaction has been reported. Herein, cyameluric chloride was used to reacted with 4,4′,4″,4‴(ethene-1,1,2,2-tetrayl) tetraaniline (ETTA), forming a heptazine-based porous organic polymer (POP-HE) via a simple nucleophilic
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Guo), [email protected] (D. Yan).
∗∗
https://doi.org/10.1016/j.micromeso.2019.03.011 Received 29 November 2018; Received in revised form 3 February 2019; Accepted 9 March 2019 Available online 12 March 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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2.3. Photocatalytic oxidation reactions
substitution reaction. POP-HE displayed permanent porosity and was found to be capable of highly selective capture CO2 from the natural gas mixtures (CO2/CH4 = 5:95), which may be attributed to the abundant nitrogen content in the polymer network. Furthermore, the band gap and the building units of POP-HE encourage us to study its photocatalytic performance under visible light.
The photooxidation of benzyl alcohol was performed in a 25 mL schlenk flask fitted with a condenser and a magnetic stirrer. In a typical reaction, 10 mg POP-HE was evacuated at 80 °C under vacuum to remove any absorbed species. The flask was then saturated with O2 and injected with 2 mL trifluorotoluene. The suspension was sonicated for 10 min to disperse the catalyst homogenously. Benzyl alcohol (0.2 mmol, 24 μL) and mesitylene (10 μL, internal GC standard) in 1 mL trifluorotoluene were then added to the reactor. An O2 balloon was used to ensure the atmospheric O2 pressure was maintained at 0.1 MPa. The suspension was stirred at room temperature for 30 min in the dark to achieve equilibrium. Then the reactor was irradiated with 4 white LED lamps (10 W) and heated at 100 °C for 1 day. After the reaction, the catalytic activity was monitored with an Agilent GC 7890B equipped with a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) with a flame ionization detector.
2. Experimental section 2.1. Materials and methods All chemical reagents and solvents were purchased from commercial suppliers (Sigma-Aldrich, Alfa Aesar and TCI) and used without further purification, among which ETTA (97%) was purchased from Bide Pharmatech Ltd. MIL125-NH2 and g-C3N4 were synthesized according to previous reports respectively [21,22]. The PXRD data was collected on a Rigaku Ultima III X-ray diffractometer using a Cu-Kα radiation at 35 kV and 30 mA. UV–Vis diffuse reflectance spectra were recorded on PE Lambda 950 UV–Vis spectrophotometer. The thermal stability was detected using a TGA unit (NETZSCH STA449-F5) under N2 atmosphere with a heating rate of 5 °C min−1 from 30 °C to 1000 °C. The Fourier Transform Infrared (FTIR) spectra were measured on a Thermo Fisher Scientific Nicolet 5700 FTIR spectrometer. Solid-state 13C NMR spectrum was measured on a Bruker AVANCE III 500. Transmission electron microscopy (TEM) micrographs were recorded on FEI Tecnai-G2-F20 Transmission electronic microscope. The X-ray photoelectron spectra (XPS) were recorded on a Thermo Scientific K-Alpha + spectrometer with a monochromatized Al Kα X-ray source (hν = 1486.6 eV). Elemental analyses were carried out using a Vario EL Cube device. Gas sorption isotherms were recorded volumetrically with a Micromeritics ASAP 2020 plus gas sorption analyser. N2 isotherms were measured at 77 K, while CO2 and CH4 sorption isotherms were measured at 273 K and 298 K. The dry samples were activated at 80 °C under dynamic vacuum prior to measurements. Highly pure gases (N2: 99.9999%; CO2: 99.999%; CH4: 99.99%) was used for the measurements. The specific surface area was calculated according to Brunauer–Emmett–Teller (BET) theory in the relative pressure range P/P0 = 0.05–0.30. The pore size distribution was calculated using the NLDFT method. The Mott-Schottky measurement was carried out in a quartz cell containing 60 mL Na2SO4 (0.2 M) aqueous solution with three-electrode system using Gamry Interface 1000E electrochemical workstation. An Ag/AgCl electrode and Pt wire served as the reference electrode and counter electrode, respectively. The working electrode was prepared by dropped 20 μL colloidal solution of POP-HE (5 mg mL−1 solution in N, N′-dimethylformamide) on the conductive side of the FTO glass and dried at room temperature.
2.4. Recycling test The catalyst was isolated at the end of the reaction and the liquid was removed. The catalyst was then washed thoroughly with ethanol and acetone, dried under vacuum and reused for the successive catalytic run. 2.5. Leaching experiments After the solid catalyst was separated from the hot solution right after reaction, the filtrate was transferred to another glass reactor. The reaction was continued with the filtrate in the absence of solid catalyst for an additional 60 h under the same reaction conditions. GC analysis was conducted every 12 h. 3. Results and discussion 3.1. Synthesis and characterization Nucleophilic substitution reactions between primary amines (aromatic and aliphatic) and 2,5,8-trichloro-s-heptazine have been successfully in building molecular materials and porous polymers [23]. However, the scope of nucleophiles is still been limited. Here, we successfully synthesized a heptazine-based polymer network (POP-HE) by treating 2,5,8-trichloro-s-heptazine with ETTA. Scheme 1 illustrated the formation of the networks POP-HE. The structure of POP-HE was further verified by 13C solid-state NMR characterization. As depicted in Fig. 1, the sharp signals at 162.2 ppm and 154.88 ppm can be assigned to the sp2 carbons of C]N bond in heptazine moieties [24]. In addition, a series of broad peaks were observed in the range of 110–150 ppm which corresponding to the aromatic carbon atoms, indicating the presence of benzene rings in the polymer [25]. Successful preparation of POP-HE was further confirmed by FTIR spectroscopy. As shown in Fig. S1, the peak at 1650 cm−1 is attributed to the stretching vibration of C]N bond in s-heptazine. The substitution of amine groups in ETTA
2.2. Synthesis of POP-HE ETTA (59 mg, 0.15 mmol), THF (10 mL), and N, N′-diisopropylethylamine (DIPEA, 0.1 mL) were mixed well under nitrogen atmosphere. 5 mL of 2,5,8-trichloro-s-heptazine (55 mg, 0.2 mmol) solution in THF was then added dropwise into the mixture with stirring after the mixture was cooled to 0 °C. Then the solution was warmed to room temperature for 2 h and later was refluxed for 24 h. After it was cooled down to room temperature, the precipitates were collected by filtration and washed with water and THF respectively. Finally, the product was extracted with THF in a Soxhlet apparatus for 2 days and dried at 80 °C under vacuum to give a yellow solid (84 mg, yield: 74%, calculated for C32H21N11·(THF) (H2O)6: C 58.4, H 4.0, N 21.0; Found: C 55.74, H 4.86, N 23.23.) POP-HE (1:1) and POP-HE (2:1) were also synthesized with molar ratios of 1:1 and 2:1 (Cy:ETTA) using the same method.
Scheme 1. Synthesis of POP-HE. 10
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Fig. 1.
13
C solid-state NMR spectroscopy of POP-HE.
was proved by the disappearance of the double peaks at 3360 cm−1 and the strong peak at 1619 cm−1, which attributed to NeH stretching vibration and bending vibration of primary amine, respectively. Meanwhile, the chlorine atoms in 2,5,8-trichloro-s-heptazine were substituted completely which could be confirmed by the absence of CeCl stretching vibration at 942 cm−1. Due to the irreversible kinetic control of the polymerization process, as shown in the figure of powder X-ray diffraction (PXRD, Fig. S2), the structure of POP-HE is non-ordered and amorphous. Thermogravimetric analysis (TGA, Fig. S3) in an N2 flow showed the high thermal stability of POP-HE up to 400 °C. The initial approximately 10% weight loss can be assigned to the loss of trapped THF molecules. TEM images in Fig. S4 showed that POP-HT powders were composed of particle agglomerates. Fig. S5 showed the XPS survey spectrum of POP-HE, and three elements (C, N, and O) were found in the spectrum. The high-resolution spectrum of C 1s (Fig. 2a) can be deconvoluted into two peaks with binding energy at 288.5 and 284.8 eV. The former intense peak is identified as sp2-bonded carbon in heptazine rings (NeC]N), while the later one was mainly resulted from the sp2 carbon in benzene rings. The N 1s spectra (Fig. 2b) can be fitted by three characteristic peaks with binding energy at 398.7, 400.2, 401.3 eV, which were assigned to CeN]C, N-(C)3 groups in heptazine rings and the bridged CeNeH groups, respectively [26,27]. The oxygen signal is presumably due to the absorbed O2 or H2O molecules on the polymer surface [28].
Fig. 3. N2 adsorption−desorption isotherms of POP-HE, POP-HE (1:1) and POP-HE (2:1) at 77 K.
adsorption above 0.8 (P/P0) indicates the presence of interparticle macroporous void, which is probably due to the loose packing of small particles [29]. In addition, a hysteresis was observed in the whole range of relative pressures, which could be ascribed to the swelling effect and softness of porous organic polymers [30]. The Brunauer−Emmett−Teller (BET) surface area of POP-HE was calculated to be 387.7 m2 g−1. Besides, the pore size distribution analysis derived from the nonlocal density functional theory (NLDFT) method shows that the dominant pore size in this polymer is 1.18 nm (Fig. S6). In addition, we found that the porosity of POP-HE varied greatly with the molar ratios of the starting monomers. The physical pore properties of POP-HE were summarized in Table S1. As the molar ratio of two reactant monomers (Cy:ETTA) changed from 1:1 to 2:1, the BET surface areas and pore volumes of POP-HE decreased distinctly. In contrast, the pore size distribution of the resulting polymers almost kept the same (Fig. S6). 3.3. Selective gas separation CO2 and CH4 adsorption isotherms of POP-HE measured at 298 K and 273 K were presented in Fig. 4. The absorption capacity of POP-HE for CO2 with values up to 50.6 cm3 g−1 (10 wt%) at 273 K and 35.5 cm3 g−1 (7.1 wt%) at 298 K while the adsorption volumes for CH4 only came up to 19.2 cm3 g−1 (1.4 wt%) at 273 K and 13.1 cm3 g−1 (0.9 wt%) at 298 K. The higher CO2 adsorption capacity is possibly arising from the fact that POP-HE has relatively high nitrogen content. Previous results shown that the nitrogen-containing structure such as Lewis base sites can increase the uptake capacity of acidic CO2 [31]. Probably because of low surface area, POP-HE (1:1) and POP-HE (2:1) displayed much lower CO2/CH4 adsorption capacity (Figs. S7 and S8)
3.2. Porosity measurements As shown in Fig. 3, POP-HE exhibits a typical type-II N2 gas adsorption isotherm, and the initial sharp uptake at relative low pressures (P/P0 < 0.01) implying the polymer's microporous nature. Simultaneously, the existence of mesoporosity can be found through the continuous increase from 0.2 to 0.8 (P/P0). The great increase in nitrogen
Fig. 2. High resolution XPS spectrums of POP-HE. (a) C 1s; (b) N 1s. 11
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dipole−quadrupole interactions [31]. Ideal adsorbed solution theory (IAST) calculations based on the dual site Langmuir-Freundlich (DSLF) model was further applied to evaluate its potential for gas separation [33]. In Fig. 5, the CO2/CH4 binary mixture with a molar ratio close to natural gas (CO2/CH4 = 5:95) was assumed to calculate the IAST selectivity. The predicted IAST selectivity for the natural gas mixture is 47.0-13.4 at 273 K and 16.7-7.8 at 298 K at pressures varying from 0 to 1 atm. Calculations from equimolar mixtures (CO2/CH4 = 50:50) were carried out as well, and it show the selectivity of this polymer is 29.58.7 at 273 K and 14.6-5.4 at 298 K. Interestingly, the calculated CO2/ CH4 selectivity of POP-HE can reach up to 47.0 at 273 K (CO2/ CH4 = 5:95), which is higher than some of reported porous polymers [34–39] (Table 1), indicating its potential application in natural gas purification.
3.4. Catalytic properties and possible photocatalytic mechanism As described in Fig. S12 (inset), the band gap calculated based on Tauc plot of POP-HE is about 2.2 eV, which provided a prerequisite that POP-HE might be a suitable catalyst for visible light-induced photocatalytic oxidation. The photocatalytic activity of POP-HE was then assessed by the selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation. As shown in Fig. 6, POP-HE exhibited decent activity for benzyl alcohol oxidation reaction with 33.6% conversion and 99% selectivity to benzaldehyde. In contrast, under the same reaction conditions, the pure g-C3N4 only converted about 3.1% benzyl alcohol to the corresponding aldehyde. Fig. 7 displayed the Mott–Schottky (M − S) plots of POP-HE. The flat-band potential determined from the slope of the linear part of the curve is about −1.76 V vs Ag/AgCl. Accordingly, the conduction band potential of POP-He is determined to be −1.56 V vs NHE, which is more negative than that of g-C3N4 (CB = −1.3 V vs NHE), indicating the electron redox ability of POP-HE to give superoxide radicals (∙O2−) is stronger than g-C3N4. Besides, since POP-HE displays higher BET surface area than g-C3N4 (∼10 m2g−1), it is reasonable to conclude that POP-HE can provide more photocatalytic reaction center than g-C3N4 [28]. The nanoporous structure is also good for improving the photoadsorption efficiency of the catalyst due to the easier mass transfer in the reaction medium [26]. As depicted in Fig. S13, POP-HE (1:1) and POP-HE (2:1) which possess low surface area, exhibited much lower catalytic performance than POP-HE in the same reaction. The controlled experiments were conducted in the absence of light or catalysts, and the result showed that merely a negligible thermocatalytic reaction occurred in the same system, which indicated that the reaction activity is mainly coming from the photocatalysis route. The catalytic stability of POP-HE was determined by the recycled experiments under the same conditions. As shown in Fig. S14, both the conversion and the selectivity were almost kept constant in three successive catalytic runs. Besides, no further
Fig. 4. CO2 and CH4 sorption isotherms of POP-HE at 298 K and 273 K.
Fig. 5. IAST selectivity of POP-HE for CO2/CH4 at 273 K and 298 K.
compared with POP-HE. The initial isosteric heats of adsorption (Qst) of CO2 and CH4 are 33.6 kJ mol−1 and 16.5 kJ mol−1, respectively, which was determined by Virial method (Fig. S11) [32]. The relative higher Qst value of CO2 for POP-HE might be relevant with its rich nitrogen content in the network, which reinforces the interactions between polarizable CO2 molecules and the functionalized pore surface by local
Table 1 Summaries of IAST adsorption selectivities for 5/95 (inside 50/50) CO2/CH4 mixture in porous polymers with different nitrogen content reported in literature at 1 bar. Polymers
POP-HE PCz–C5–Cz BTLP-5 Cz-POF-1 Cz-POF-3 TNP1 TNP2 PIN1 PIN2 CTF-DI-3 CTF-DI-4
BET /m2 g−1
387.7 768 705 2065 1927 1090 460 458 325 1877 1769
CO2 uptake/wt% (273 K,1 bar)
10 12.5 13.9 20.2 21 17.8 7 7.98 7.93 15.8 12.7
N%
23.23 – 6.54 6.5 5.97 15.13 15.93 14.8 13.3 – –
12
Qst of CO2 /kJ mol−1
33.6 21.07 29.1 25.3 27.8 37 38.5 31 30 38.7 40.1
IAST Selectivity
Ref
273 K
298 K
13.4 (8.7) 6.8 – 5.4 5.6 8 8 (4) (4) 6 (5) 8 (7)
7.8 (5.4) 5.5 5.7 – – 5 5 (5) (5) – –
this work [34] [35] [36] [36] [37] [37] [38] [38] [39] [39]
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conversion of benzyl alcohol was detected during the leaching test experiment (Fig. S15), which confirmed the heterogeneity of the catalyst. According to the empirical formula of Eg = EVB − ECB, the valence band potential of POP-HE is calculated to be 0.64 V vs NHE, which is inferior to the potential of benzyl alcohol (1.88 V vs NHE). Therefore, it is difficult to oxidize benzyl alcohol directly by the photogenerated holes on POP-HE [40]. On the other hand, the photoexcited electrons can reduce O2 to give ∙O2− easily because the conduction band of POPHE (−1.56 V vs NHE) is much more negative than the redox potential of O2/∙O2− (−0.33 V vs NHE) [41]. On the basis of above results, a proposed photocatalytic mechanism of POP-HE which catalysed benzyl alcohol to benzaldehyde was depicted in Fig. 8. The excited electrons transferred from valence band to conduction band upon irradiation of POP-HE with visible light, and subsequently reduced the molecular oxygen, generating ∙O2−, which further reacted with the cation radicals formed through the interaction between absorbed benzyl alcohol and positive holes, producing the corresponding aromatic aldehydes [42–44]. The exact photocatalytic mechanism of POP-HE and its further catalytic studies are ongoing in our lab. Fig. 6. Conversion and selectivity of benzyl alcohol oxidation reaction.
4. Conclusion In conclusion, a heptazine-based porous organic polymer POP-HE has been synthesized successfully via a common nucleophilic substitution reaction. POP-HE exhibits permanent porosity with IAST selectivities of 29.5-8.7 for the equimolar CO2/CH4 and 47.0-13.4 for the natural gas (CO2/CH4 = 5:95) at 273 K, suggesting that this polymer is a promising candidate for future natural gas upgrading. Moreover, POPHE showed high efficiency as a catalyst for visible-light-driven oxidation of benzyl alcohol to benzaldehyde with O2 under mild conditions, demonstrating superior activity than g-C3N4. Furthermore, POP-HE can be reusable for three times without obvious deactivation, demonstrating its potential application in heterogenous photocatalysis. We foresee a bright and promising prospect for this kind of microporous polymer in solving the increasing serious energy and environmental issues. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51703031).
Fig. 7. Mott-Sckottky plots of POP-HE.
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Fig. 8. Proposed mechanism for photocatalytic oxidation of benzyl alcohol to benzaldehyde over POP-HE (h+ = hole, e− = electron).
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Heptazine-based porous polymer for selective CO2 sorption and visible light photocatalytic oxidation of benzyl alcohol
T
Congying Xua, Li Qiana, Junyu Lina, Zhiyong Guoa,∗, Dan Yanb,∗∗, Hongbing Zhana,c a
College of Materials Science and Engineering, Fuzhou University, Fuzhou, 350116, PR China Testing Center, Fuzhou University, Fuzhou, 350116, PR China c Key Laboratory of Eco-materials Advanced Technology, Fuzhou University, Fujian Province, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous organic polymer s-Heptazine Gas separation Photocatalytic oxidation
Heptazine units with highly rich nitrogen sites were employed to fabricate a new porous organic polymer (POPHE) via direct coupling of 2,5,8-trichloro-s-heptazine and 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)tetraaniline with the mild base N, N′-diisopropylethylamine (DIPEA) in tetrahydrofuran (THF) solution. POP-HE displays good CO2 uptake capacity (50.6 cm3 g−1, 10 wt%, 1 bar/273 K) and highly selectively adsorptive separation of CO2 over CH4 under ambient conditions. Impressively, at 273 K, the predicted IAST selectivity of POP-HE is 47.0-13.4 for the natural gas mixture (CO2/CH4 = 5:95) at pressures varying from 0 to 1 atm. Meanwhile, the result of calculated band gap value based on Tauc plot motivated us to investigate its photocatalytic activity. POP-HE was applied as heterogeneous photocatalyst for visible light-induced selective oxidation of benzyl alcohol to benzaldehyde, and exhibited great conversion of 33.6% under irradiation of white LED light, which is much higher than that of g-C3N4. Moreover, POP-HE can be recycled up to three times without significant decrease in catalytic activity.
1. Introduction In recent years, polymeric carbon nitrides (PCNs) with unique physiochemical properties have attracted extensive interest in photocatalysis, including photocatalytic hydrogen production, selective oxidation of alcohols and nitrobenzene reduction [1–3]. Nevertheless, pure PCNs are usually restricted by their low surface area, limited light absorbance and rapid charge carrier recombination [4–6]. Several protocols such as texture modification, elemental doping, and copolymerization have been devoted to improve the photocatalytic performance of g-C3N4 [7–10]. However, the above strategy is usually hampered by the high condensation temperatures needed and the incomplete condensation of remaining isolated functionalities. On the contrary, using heptazine (C6N7) as building block to construct porous organic polymers (POPs) is a promising way to design new classes of PCNs with desirable photocatalytic and other properties [11]. POPs have exhibited great potential in heterogeneous catalysis, energy storage, gas storage and separation owing to their low skeleton density, excellent porosity, tuneable structure and high physicochemical stability [12–16]. However, unlike its parent polymeric carbon nitride (so-called g-C3N4) has been exploded with a wide variety of applications in the past two
∗
decades, the development and application of the porous polymeric materials that built directly from the heptazine core is still in the infancy stage. The main obstacle to construct the heptazine class of compounds lies in their synthesis protocols and their insolubility in the majority of organic solvents. Currently, the 2,5,8-trichloro-s-heptazine (cyameluric chloride, Cy) is the most widely available and best described precursor material which has three equivalent, very good leaving groups that can be exchanged by nucleophilic substitution reactions. In 2013, by utilizing cyameluric chloride as the starting building block, a heptazine based microporous polymer network was prepared by Thomas and Kailasam, which showed photocatalytic activity for the production of hydrogen from water under visible light illumination [17]. Since then, several porous polymers containing heptazine rings have been reported, of which showed favourable performance in organocatalysis, selective CO2 capture, luminescent sensing and ammonia detection [18–20]. To our best of knowledge, no example that using heptazine derived porous polymer as visible light photocatalyst in organic reaction has been reported. Herein, cyameluric chloride was used to reacted with 4,4′,4″,4‴(ethene-1,1,2,2-tetrayl) tetraaniline (ETTA), forming a heptazine-based porous organic polymer (POP-HE) via a simple nucleophilic
Corresponding author. Corresponding author. E-mail addresses: [email protected] (Z. Guo), [email protected] (D. Yan).
∗∗
https://doi.org/10.1016/j.micromeso.2019.03.011 Received 29 November 2018; Received in revised form 3 February 2019; Accepted 9 March 2019 Available online 12 March 2019 1387-1811/ © 2019 Elsevier Inc. All rights reserved.
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2.3. Photocatalytic oxidation reactions
substitution reaction. POP-HE displayed permanent porosity and was found to be capable of highly selective capture CO2 from the natural gas mixtures (CO2/CH4 = 5:95), which may be attributed to the abundant nitrogen content in the polymer network. Furthermore, the band gap and the building units of POP-HE encourage us to study its photocatalytic performance under visible light.
The photooxidation of benzyl alcohol was performed in a 25 mL schlenk flask fitted with a condenser and a magnetic stirrer. In a typical reaction, 10 mg POP-HE was evacuated at 80 °C under vacuum to remove any absorbed species. The flask was then saturated with O2 and injected with 2 mL trifluorotoluene. The suspension was sonicated for 10 min to disperse the catalyst homogenously. Benzyl alcohol (0.2 mmol, 24 μL) and mesitylene (10 μL, internal GC standard) in 1 mL trifluorotoluene were then added to the reactor. An O2 balloon was used to ensure the atmospheric O2 pressure was maintained at 0.1 MPa. The suspension was stirred at room temperature for 30 min in the dark to achieve equilibrium. Then the reactor was irradiated with 4 white LED lamps (10 W) and heated at 100 °C for 1 day. After the reaction, the catalytic activity was monitored with an Agilent GC 7890B equipped with a HP-5 capillary column (30 m × 0.32 mm × 0.25 μm) with a flame ionization detector.
2. Experimental section 2.1. Materials and methods All chemical reagents and solvents were purchased from commercial suppliers (Sigma-Aldrich, Alfa Aesar and TCI) and used without further purification, among which ETTA (97%) was purchased from Bide Pharmatech Ltd. MIL125-NH2 and g-C3N4 were synthesized according to previous reports respectively [21,22]. The PXRD data was collected on a Rigaku Ultima III X-ray diffractometer using a Cu-Kα radiation at 35 kV and 30 mA. UV–Vis diffuse reflectance spectra were recorded on PE Lambda 950 UV–Vis spectrophotometer. The thermal stability was detected using a TGA unit (NETZSCH STA449-F5) under N2 atmosphere with a heating rate of 5 °C min−1 from 30 °C to 1000 °C. The Fourier Transform Infrared (FTIR) spectra were measured on a Thermo Fisher Scientific Nicolet 5700 FTIR spectrometer. Solid-state 13C NMR spectrum was measured on a Bruker AVANCE III 500. Transmission electron microscopy (TEM) micrographs were recorded on FEI Tecnai-G2-F20 Transmission electronic microscope. The X-ray photoelectron spectra (XPS) were recorded on a Thermo Scientific K-Alpha + spectrometer with a monochromatized Al Kα X-ray source (hν = 1486.6 eV). Elemental analyses were carried out using a Vario EL Cube device. Gas sorption isotherms were recorded volumetrically with a Micromeritics ASAP 2020 plus gas sorption analyser. N2 isotherms were measured at 77 K, while CO2 and CH4 sorption isotherms were measured at 273 K and 298 K. The dry samples were activated at 80 °C under dynamic vacuum prior to measurements. Highly pure gases (N2: 99.9999%; CO2: 99.999%; CH4: 99.99%) was used for the measurements. The specific surface area was calculated according to Brunauer–Emmett–Teller (BET) theory in the relative pressure range P/P0 = 0.05–0.30. The pore size distribution was calculated using the NLDFT method. The Mott-Schottky measurement was carried out in a quartz cell containing 60 mL Na2SO4 (0.2 M) aqueous solution with three-electrode system using Gamry Interface 1000E electrochemical workstation. An Ag/AgCl electrode and Pt wire served as the reference electrode and counter electrode, respectively. The working electrode was prepared by dropped 20 μL colloidal solution of POP-HE (5 mg mL−1 solution in N, N′-dimethylformamide) on the conductive side of the FTO glass and dried at room temperature.
2.4. Recycling test The catalyst was isolated at the end of the reaction and the liquid was removed. The catalyst was then washed thoroughly with ethanol and acetone, dried under vacuum and reused for the successive catalytic run. 2.5. Leaching experiments After the solid catalyst was separated from the hot solution right after reaction, the filtrate was transferred to another glass reactor. The reaction was continued with the filtrate in the absence of solid catalyst for an additional 60 h under the same reaction conditions. GC analysis was conducted every 12 h. 3. Results and discussion 3.1. Synthesis and characterization Nucleophilic substitution reactions between primary amines (aromatic and aliphatic) and 2,5,8-trichloro-s-heptazine have been successfully in building molecular materials and porous polymers [23]. However, the scope of nucleophiles is still been limited. Here, we successfully synthesized a heptazine-based polymer network (POP-HE) by treating 2,5,8-trichloro-s-heptazine with ETTA. Scheme 1 illustrated the formation of the networks POP-HE. The structure of POP-HE was further verified by 13C solid-state NMR characterization. As depicted in Fig. 1, the sharp signals at 162.2 ppm and 154.88 ppm can be assigned to the sp2 carbons of C]N bond in heptazine moieties [24]. In addition, a series of broad peaks were observed in the range of 110–150 ppm which corresponding to the aromatic carbon atoms, indicating the presence of benzene rings in the polymer [25]. Successful preparation of POP-HE was further confirmed by FTIR spectroscopy. As shown in Fig. S1, the peak at 1650 cm−1 is attributed to the stretching vibration of C]N bond in s-heptazine. The substitution of amine groups in ETTA
2.2. Synthesis of POP-HE ETTA (59 mg, 0.15 mmol), THF (10 mL), and N, N′-diisopropylethylamine (DIPEA, 0.1 mL) were mixed well under nitrogen atmosphere. 5 mL of 2,5,8-trichloro-s-heptazine (55 mg, 0.2 mmol) solution in THF was then added dropwise into the mixture with stirring after the mixture was cooled to 0 °C. Then the solution was warmed to room temperature for 2 h and later was refluxed for 24 h. After it was cooled down to room temperature, the precipitates were collected by filtration and washed with water and THF respectively. Finally, the product was extracted with THF in a Soxhlet apparatus for 2 days and dried at 80 °C under vacuum to give a yellow solid (84 mg, yield: 74%, calculated for C32H21N11·(THF) (H2O)6: C 58.4, H 4.0, N 21.0; Found: C 55.74, H 4.86, N 23.23.) POP-HE (1:1) and POP-HE (2:1) were also synthesized with molar ratios of 1:1 and 2:1 (Cy:ETTA) using the same method.
Scheme 1. Synthesis of POP-HE. 10
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Fig. 1.
13
C solid-state NMR spectroscopy of POP-HE.
was proved by the disappearance of the double peaks at 3360 cm−1 and the strong peak at 1619 cm−1, which attributed to NeH stretching vibration and bending vibration of primary amine, respectively. Meanwhile, the chlorine atoms in 2,5,8-trichloro-s-heptazine were substituted completely which could be confirmed by the absence of CeCl stretching vibration at 942 cm−1. Due to the irreversible kinetic control of the polymerization process, as shown in the figure of powder X-ray diffraction (PXRD, Fig. S2), the structure of POP-HE is non-ordered and amorphous. Thermogravimetric analysis (TGA, Fig. S3) in an N2 flow showed the high thermal stability of POP-HE up to 400 °C. The initial approximately 10% weight loss can be assigned to the loss of trapped THF molecules. TEM images in Fig. S4 showed that POP-HT powders were composed of particle agglomerates. Fig. S5 showed the XPS survey spectrum of POP-HE, and three elements (C, N, and O) were found in the spectrum. The high-resolution spectrum of C 1s (Fig. 2a) can be deconvoluted into two peaks with binding energy at 288.5 and 284.8 eV. The former intense peak is identified as sp2-bonded carbon in heptazine rings (NeC]N), while the later one was mainly resulted from the sp2 carbon in benzene rings. The N 1s spectra (Fig. 2b) can be fitted by three characteristic peaks with binding energy at 398.7, 400.2, 401.3 eV, which were assigned to CeN]C, N-(C)3 groups in heptazine rings and the bridged CeNeH groups, respectively [26,27]. The oxygen signal is presumably due to the absorbed O2 or H2O molecules on the polymer surface [28].
Fig. 3. N2 adsorption−desorption isotherms of POP-HE, POP-HE (1:1) and POP-HE (2:1) at 77 K.
adsorption above 0.8 (P/P0) indicates the presence of interparticle macroporous void, which is probably due to the loose packing of small particles [29]. In addition, a hysteresis was observed in the whole range of relative pressures, which could be ascribed to the swelling effect and softness of porous organic polymers [30]. The Brunauer−Emmett−Teller (BET) surface area of POP-HE was calculated to be 387.7 m2 g−1. Besides, the pore size distribution analysis derived from the nonlocal density functional theory (NLDFT) method shows that the dominant pore size in this polymer is 1.18 nm (Fig. S6). In addition, we found that the porosity of POP-HE varied greatly with the molar ratios of the starting monomers. The physical pore properties of POP-HE were summarized in Table S1. As the molar ratio of two reactant monomers (Cy:ETTA) changed from 1:1 to 2:1, the BET surface areas and pore volumes of POP-HE decreased distinctly. In contrast, the pore size distribution of the resulting polymers almost kept the same (Fig. S6). 3.3. Selective gas separation CO2 and CH4 adsorption isotherms of POP-HE measured at 298 K and 273 K were presented in Fig. 4. The absorption capacity of POP-HE for CO2 with values up to 50.6 cm3 g−1 (10 wt%) at 273 K and 35.5 cm3 g−1 (7.1 wt%) at 298 K while the adsorption volumes for CH4 only came up to 19.2 cm3 g−1 (1.4 wt%) at 273 K and 13.1 cm3 g−1 (0.9 wt%) at 298 K. The higher CO2 adsorption capacity is possibly arising from the fact that POP-HE has relatively high nitrogen content. Previous results shown that the nitrogen-containing structure such as Lewis base sites can increase the uptake capacity of acidic CO2 [31]. Probably because of low surface area, POP-HE (1:1) and POP-HE (2:1) displayed much lower CO2/CH4 adsorption capacity (Figs. S7 and S8)
3.2. Porosity measurements As shown in Fig. 3, POP-HE exhibits a typical type-II N2 gas adsorption isotherm, and the initial sharp uptake at relative low pressures (P/P0 < 0.01) implying the polymer's microporous nature. Simultaneously, the existence of mesoporosity can be found through the continuous increase from 0.2 to 0.8 (P/P0). The great increase in nitrogen
Fig. 2. High resolution XPS spectrums of POP-HE. (a) C 1s; (b) N 1s. 11
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dipole−quadrupole interactions [31]. Ideal adsorbed solution theory (IAST) calculations based on the dual site Langmuir-Freundlich (DSLF) model was further applied to evaluate its potential for gas separation [33]. In Fig. 5, the CO2/CH4 binary mixture with a molar ratio close to natural gas (CO2/CH4 = 5:95) was assumed to calculate the IAST selectivity. The predicted IAST selectivity for the natural gas mixture is 47.0-13.4 at 273 K and 16.7-7.8 at 298 K at pressures varying from 0 to 1 atm. Calculations from equimolar mixtures (CO2/CH4 = 50:50) were carried out as well, and it show the selectivity of this polymer is 29.58.7 at 273 K and 14.6-5.4 at 298 K. Interestingly, the calculated CO2/ CH4 selectivity of POP-HE can reach up to 47.0 at 273 K (CO2/ CH4 = 5:95), which is higher than some of reported porous polymers [34–39] (Table 1), indicating its potential application in natural gas purification.
3.4. Catalytic properties and possible photocatalytic mechanism As described in Fig. S12 (inset), the band gap calculated based on Tauc plot of POP-HE is about 2.2 eV, which provided a prerequisite that POP-HE might be a suitable catalyst for visible light-induced photocatalytic oxidation. The photocatalytic activity of POP-HE was then assessed by the selective oxidation of benzyl alcohol to benzaldehyde under visible light irradiation. As shown in Fig. 6, POP-HE exhibited decent activity for benzyl alcohol oxidation reaction with 33.6% conversion and 99% selectivity to benzaldehyde. In contrast, under the same reaction conditions, the pure g-C3N4 only converted about 3.1% benzyl alcohol to the corresponding aldehyde. Fig. 7 displayed the Mott–Schottky (M − S) plots of POP-HE. The flat-band potential determined from the slope of the linear part of the curve is about −1.76 V vs Ag/AgCl. Accordingly, the conduction band potential of POP-He is determined to be −1.56 V vs NHE, which is more negative than that of g-C3N4 (CB = −1.3 V vs NHE), indicating the electron redox ability of POP-HE to give superoxide radicals (∙O2−) is stronger than g-C3N4. Besides, since POP-HE displays higher BET surface area than g-C3N4 (∼10 m2g−1), it is reasonable to conclude that POP-HE can provide more photocatalytic reaction center than g-C3N4 [28]. The nanoporous structure is also good for improving the photoadsorption efficiency of the catalyst due to the easier mass transfer in the reaction medium [26]. As depicted in Fig. S13, POP-HE (1:1) and POP-HE (2:1) which possess low surface area, exhibited much lower catalytic performance than POP-HE in the same reaction. The controlled experiments were conducted in the absence of light or catalysts, and the result showed that merely a negligible thermocatalytic reaction occurred in the same system, which indicated that the reaction activity is mainly coming from the photocatalysis route. The catalytic stability of POP-HE was determined by the recycled experiments under the same conditions. As shown in Fig. S14, both the conversion and the selectivity were almost kept constant in three successive catalytic runs. Besides, no further
Fig. 4. CO2 and CH4 sorption isotherms of POP-HE at 298 K and 273 K.
Fig. 5. IAST selectivity of POP-HE for CO2/CH4 at 273 K and 298 K.
compared with POP-HE. The initial isosteric heats of adsorption (Qst) of CO2 and CH4 are 33.6 kJ mol−1 and 16.5 kJ mol−1, respectively, which was determined by Virial method (Fig. S11) [32]. The relative higher Qst value of CO2 for POP-HE might be relevant with its rich nitrogen content in the network, which reinforces the interactions between polarizable CO2 molecules and the functionalized pore surface by local
Table 1 Summaries of IAST adsorption selectivities for 5/95 (inside 50/50) CO2/CH4 mixture in porous polymers with different nitrogen content reported in literature at 1 bar. Polymers
POP-HE PCz–C5–Cz BTLP-5 Cz-POF-1 Cz-POF-3 TNP1 TNP2 PIN1 PIN2 CTF-DI-3 CTF-DI-4
BET /m2 g−1
387.7 768 705 2065 1927 1090 460 458 325 1877 1769
CO2 uptake/wt% (273 K,1 bar)
10 12.5 13.9 20.2 21 17.8 7 7.98 7.93 15.8 12.7
N%
23.23 – 6.54 6.5 5.97 15.13 15.93 14.8 13.3 – –
12
Qst of CO2 /kJ mol−1
33.6 21.07 29.1 25.3 27.8 37 38.5 31 30 38.7 40.1
IAST Selectivity
Ref
273 K
298 K
13.4 (8.7) 6.8 – 5.4 5.6 8 8 (4) (4) 6 (5) 8 (7)
7.8 (5.4) 5.5 5.7 – – 5 5 (5) (5) – –
this work [34] [35] [36] [36] [37] [37] [38] [38] [39] [39]
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conversion of benzyl alcohol was detected during the leaching test experiment (Fig. S15), which confirmed the heterogeneity of the catalyst. According to the empirical formula of Eg = EVB − ECB, the valence band potential of POP-HE is calculated to be 0.64 V vs NHE, which is inferior to the potential of benzyl alcohol (1.88 V vs NHE). Therefore, it is difficult to oxidize benzyl alcohol directly by the photogenerated holes on POP-HE [40]. On the other hand, the photoexcited electrons can reduce O2 to give ∙O2− easily because the conduction band of POPHE (−1.56 V vs NHE) is much more negative than the redox potential of O2/∙O2− (−0.33 V vs NHE) [41]. On the basis of above results, a proposed photocatalytic mechanism of POP-HE which catalysed benzyl alcohol to benzaldehyde was depicted in Fig. 8. The excited electrons transferred from valence band to conduction band upon irradiation of POP-HE with visible light, and subsequently reduced the molecular oxygen, generating ∙O2−, which further reacted with the cation radicals formed through the interaction between absorbed benzyl alcohol and positive holes, producing the corresponding aromatic aldehydes [42–44]. The exact photocatalytic mechanism of POP-HE and its further catalytic studies are ongoing in our lab. Fig. 6. Conversion and selectivity of benzyl alcohol oxidation reaction.
4. Conclusion In conclusion, a heptazine-based porous organic polymer POP-HE has been synthesized successfully via a common nucleophilic substitution reaction. POP-HE exhibits permanent porosity with IAST selectivities of 29.5-8.7 for the equimolar CO2/CH4 and 47.0-13.4 for the natural gas (CO2/CH4 = 5:95) at 273 K, suggesting that this polymer is a promising candidate for future natural gas upgrading. Moreover, POPHE showed high efficiency as a catalyst for visible-light-driven oxidation of benzyl alcohol to benzaldehyde with O2 under mild conditions, demonstrating superior activity than g-C3N4. Furthermore, POP-HE can be reusable for three times without obvious deactivation, demonstrating its potential application in heterogenous photocatalysis. We foresee a bright and promising prospect for this kind of microporous polymer in solving the increasing serious energy and environmental issues. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51703031).
Fig. 7. Mott-Sckottky plots of POP-HE.
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Fig. 8. Proposed mechanism for photocatalytic oxidation of benzyl alcohol to benzaldehyde over POP-HE (h+ = hole, e− = electron).
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