Formation of Strong Basicity on Covalent Triazine Frameworks as

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Formation of strong basicity on covalent triazine frameworks as catalysts for the oxidation of methylene compounds Guozhi Zhu, Song Shi, Meng Liu, Li Zhao, Min Wang, Xi Zheng, Jin Gao, and Jie Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19001 • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Applied Materials & Interfaces

Formation of strong basicity on covalent triazine frameworks as catalysts for the oxidation of methylene compounds Guozhi Zhu,†,‡ Song Shi,*,† Meng Liu,†,‡ Li Zhao,†,‡ Min Wang,† Xi Zheng,† Jin Gao,† and Jie Xu*,† †

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ABSTRACT: Porous solid bases are increasingly attractive for applications in green chemistry and heterogeneous catalysis under relatively mild conditions. Here, covalent triazine frameworks (CTFs) were firstly applied as support for the porous solid strong bases through a redox process between the base precursor KNO3 and CTFs, leading to a relatively low calcination temperature (400 oC). As a result, porous organic frameworks possessing ordered microstructure as well as strong basic sites were successfully synthesized. The materials were characterized by XRD, FT-IR, HRTEM, CO2-TPD, etc. The obtained solid bases displayed remarkable catalytic activity in the aerobic oxidation of methylene compounds and the yield of fluorenones could reach 93.6% at 120 oC, which was nearly 3 times higher than that of control catalyst. The current research may present a new idea for the construction of porous organic polymers with strong basicity. KEYWORDS: Porous solid strong bases, covalent triazine frameworks, decomposition of KNO3, basic sites, aerobic oxidation of fluorenes ■ INTRODUCTION Porous solid strong bases have attracted increasing interest in economical catalysis and green chemistry due to their distinct advantages including much less corrosion, facile separation, and fewer waste production.1-3 They have shown good performance in the organic synthesis,4-7 bio-diesel production,8-10 utilization of CO2,11,12 etc. Nowadays, much effort have been made to fabricate new kinds of solid strong base catalysts with unique catalytic performance. Typically, the porous solid strong bases are made up of the strong basic sites and the porous support. And the catalytic performance is highly related to the properties of the support, such as the chemical structure, pore size, surface area, and alkali resistance. However, due to the corrosion nature and the preparation method (mostly in high temperature),13 the support must be stable towards the high temperature and strong basicity. A lot of research have been carried out to design and fabricate the suitable support for the solid strong bases.14-20 In recent years, Sun et al. have reported the formation of

strong basic sites on SBA-15 through a dualcoating strategy, which precoated a layer of carbon or metal oxide on SBA-15 surface before introduction of base sites.21-24 Although these strategies could obtained solid strong bases with ordered mesoporous frameworks, it is still a challenging task to form strong basic sites on the new kinds of porous supports with simple and efficient method. Porous organic polymers (POPs) are new kinds of developed porous materials in recent years. They gain a lot of advantages including high surface areas, adjustable pore size, variable functional groups and insoluble in most organic solvents, acid and basic solution.25,26 Recently, a lot of researchers have reported the triazine-based porous organic polymers or covalent organic frameworks were good candidates for the catalyst support,26-29 gas adsorption materials,29-31 and energy storage.32,33 However, due to the organic nature of porous organic polymers, most of the POPs could not be stable under high temperature required by the preparation of strong bases. Due to this characteristic, there was no report about the solid strong bases with POPs as support.

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Scheme 1. The Strong Basic Sites Generation on CTFs by a Modified Wetness Impregnation Method

Herein, as a kind of particularly important porous organic polymers, covalent triazine frameworks (CTFs)34 which were synthesized by the polymerization of aromatic dinitriles in ZnCl2 at 400 o C were applied as novel support to fabricate porous strong bases. The strong basic sites were formed on the framework of CTFs at a relatively low temperature (400 oC) without collapse of the organic frameworks during the preparation process (Scheme 1). Consequently, a new kind of material with characteristic of ordered organic frameworks and strong basic sites was successfully synthesized. And for all we know, there have been no report about introducing strong basic sites on porous organic polymers. The obtained porous strong bases showed good catalytic activity in the oxidation of methylene compounds. ■ EXPERIMENTAL SECTION Chemicals and Materials. Potassium nitrate (KNO3, Kermel), 1,4-Dicyanobenzene (Energy Chemical), Zinc chloride (ZnCl2, Kermel), γ-Al2O3 (Alfa Aesar) and activated carbon (AC Aladdin)1,4-dichlorobenzene (C6H4Cl2, Alfa Aesar), Fluorenes (Aladdin), Fluorenol (Alfa Aesar), Fluorenones (Ark Pharm), 3,6-Dimethoxyfluorene (Energy Chemical), 2-Phenylacetophenone (Adamas Reagent Co. Ltd.), 4-Methylbenzyl Phenyl Ketone (Energy Chemical), Deoxyanisoin (Alfa Aesar), Anthrone (Adamas Reagent Co. Ltd.), 9,10-Dihydroanthracene (Sigma-aldrich), Xanthene (Adamas Reagent Co. Ltd.), Thioxanthene (acros), Anthraquinone (Adamas Reagent Co. Ltd.), N,N-Dimethylformamide (DMF, Sinopharm Chemical Reagent Co. Ltd.) Materials Synthesis. Microporous covalent triazine-based frameworks were prepared according to previous report as follows.34 Typically, 1.0 g of 1,4-dicyanobenzene (8 mmol) and 1.1 g of ZnCl2 (8 mmol) were added into a pyrex ampoule (3 x 12 cm)

after they were grinded. The ampoules were sealed during evacuated, and transferred into the muffle maintaining at 400 oC for 40 h. Then the resulting black porous solid was washed completely with much deionized water to remove most of the ZnCl2. Finally, the black power was stirred in HCl (0.1 M) for 15 h to further purified. Base precursor KNO3 was introduced to CTFs by a modified impregnation method.35 Typically, a 100 mL flask was charged with 0.2 g of KNO3 and 15 mL of deionized water, then 0.8 g of support was put into the flask and stirred for 24 h at ambient temperature. Finally, the deionized water was removed by a rotary evaporator and solids dried at 80 oC in a vacuum oven all night. The obtained solids are denoted as nKCTF, where n is the mass percentage of KNO3 varied from 5% to 30%. Finally the obtained solid was calcined at 400 oC in a N2 flow for 60 min to decompose KNO3. The final samples were denoted as nKCTF-m, where m represents the calcination temperature varied from 300 oC to 600 oC. For comparison, samples of 20KMCM-400, 20KAl2O3-400, 20KAC-400 were prepared according to the same procedure above, using MCM-41, γ-Al2O3 and activated carbon as support, respectively. Characterization. Empyrean-100 powder diffraction system with Cu K ɑ radiation ( λ = 0.15406 nm) was employed to recorded X-ray diffraction (XRD) of the obtained materials between 5o and 80o (40 KV, mA). Fourier transform infrared (FT-IR) spectra were measured in KBr media on a Bruker Tensor 27. N2 adsorption-desorption isotherms were determined at 77 K on a Quantachrome instrument, and the obtained materials were degassed under 200 oC before determination. The morphology of the catalysts were studied by High-Revolution Transmission Electron Microscope (HRTEM) on JEM-2100F electron microscope. The basicity of the obtained samples and comparative materials were


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Fig. 1 XRD patterns (A) and IR spectra (B) of CTFs and 20KCTF-m after calcination, XRD patterns (C) and IR spectra (D) of CTFs and nKCTF-400. researched using temperature programmed desorption of CO2 (CO2-TPD) by a micromeritics autochem Ⅱ apparatus. The pathway for the conversion of KNO3 was investigated by temperature-programmed decomposition and mass spectrometer (TPDE-MS) on a micromeritics AutoChem 2910 apparatus. The amount of basic sites were measured according to a reported method.36 A 100 mL flask was charged with 100 mg of fresh sample after calcination at 400 o C, then 10 mL of standard HCl (0.2 M) was added into the flask. The obtained slurry was stirred for 24 h at ambient temperature, then centrifuged to obtain transparent liquid. Standard NaOH (0.05 M) was used to titrate the residual acid. Therefore, the amount of basic sites could be calculated according to the titration amount of standard NaOH. Reaction Procedures and Products Analysis. The obtained solid strong basic catalysts were applied in the aerobic oxidation of fluorenes to fluorenones. Oxidation reactions were conducted in a stainless autoclave (60 mL). Typically, 10 mL of N, N-Dimethylformamide was added into reactor as solvent, 166 mg of fluorenes (1 mmol) and 50 mg of catalyst were put into the stainless autoclave. The reactor was heated to 80 oC under stirring after being sealed and displacing air with pure O2 for three times

and maintained constant pressure (0.3 MPa) at 80 oC for 6 h. Upon O2 was consumed below 0.3 MPa, it should be fed to maintain 0.3 MPa. Products finally were analyzed by an Agilent 6890N GC/5973 MS and an Agilent 6890N GC outfitted with HP-INNOWAX column (30 m × 0.320 mm) capillary column. And internal standard was 1,4-dichlorobenzene. ■ RESULTS AND DISCUSSION Characterizations of Materials. The XRD patterns of the CTFs (Fig. 1A) display intense diffraction peak at 2θ of 7.2o attributed to (100) reflection, which suggests CTFs possess crystalline frameworks and are made up of hexagonal pores (Scheme 1). In addition, two weak peaks at 2θ of 12.7o, 14.6o originate from the (110) and (200) crystal plane, respectively. Furthermore, a broad peak at 26.0o corresponding to (001) aromatic sheets also exists.34 For the FT-IR of bare CTFs (Fig. 1B), the sharp C-N stretching vibrations signal (1521 cm-1) as well as ring stretching vibrations absorption band (1352 cm-1) are the characteristic of the aromatic triazine unit.34 Meanwhile, the peak of cyanogroup (2228 cm-1) is missing. The obtained CTFs possess a high surface area of 922 m2/g and the pore volume is 0.79 cm3/g (table S1, ESI†). All these characteristic



Fig. 2 HRTEM images (a) and STEM-HAADF image (b) of 20KCTF-400 and corresponding EDX mappings of 20KCTF-400 for the overlapped (c) C (d) N (e) O and (f) K. illustrate the successful preparation of CTFs. As for the KNO3-modified CTFs before calcination, intense characteristic diffraction lines of KNO3 at 2θ of 23.6o, 29.4o can be clearly observed13,21 (Fig. S1, ESI†). After the calcination (Fig. 1A), the peaks of KNO3 disappeared once the temperature was over 400 o C, meaning 400 oC was enough for the decomposition of KNO3. However, this temperature was lower than the thermally induced decomposition temperature of KNO3,24 which will be discussed in the following part. In addition, with the temperature over 600 oC, the characteristic diffraction lines of CTFs support disappeared, demonstrating the crystalline CTFs were destroyed. These results are also confirmed by the FT-IR (Fig. 1B). The intense band at 1384 cm-1 (asymmetric stretching vibrations of N–O of nitrate)13,37 disappeared over 400 oC. And the peaks at 1521 cm-1 and 1352 cm-1 which were the characteristic of the covalent triazine-based units, existed only when the calcination temperature was under 600 oC. Based on these experiments, the calcination was fixed at 400 oC, maintaining the crystalline structure of CTFs while decomposed KNO3. The samples with various KNO3 loadings confirmed the calcination temperature was appropriate (Fig. 1C, 1D). The obtained base samples were characterized by the HRTEM images and the corresponding HRTEM-EDX mapping (Fig. 2). It can be seen from Fig. 2(a) that the 20KCTF-400 sample exhibits a stacking or layer structures similar with CTFs support

(Fig. S2, ESI†), implying the main structure of 20KCTF-400 sample preserves well after calcination. In addition, HRTEM-EDX mapping images of CTFs show the C and N are dispersed uniformly on the organic frameworks. And EDX mappings of K and O images indicate that potassium species are uniformly dispersed on the support and no obvious agglomerates formed. Porosity parameters of the synthesized materials were acquired from the N2 sorption measurements (Fig. S3-S4, ESI†). All the isotherms of obtained samples display a steep rise at low relative pressure, indicating a microporous structure. The CTFs possess a high surface area of 922 m2/g and the pore volume is 0.79 cm3/g (table S1, ESI†). With an increasing amount of the basic sites, the surface area along with pore volume decreased stepwisely, nevertheless, the average pore diameter didn’t change evidently. These data illustrate though partly destroyed, the main porous framework structure of CTFs basically preserved. Temperature programmed desorption of CO2 (CO2-TPD) was applied to analyzed the distribution of basic sites (Fig. 3). CO2 desorption peak for CTFs support shows only a small peak at about 100 oC, indicating that there are weak basic sites on CTF support. For the sample of 20KCTF-400, two intense CO2 desorption peaks are observed at 120 oC and 500 o C. The 120 oC is supposed to the existence of weak basic sites, and the CO2 desorption peaks of 500 oC resulted from strong basic sites.38 According to above results, it is conclusive that KNO3 can 34


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Fig. 3 CO2-TPD of different samples with various supports. decompose to generate strong basic sites on CTFs calcination at 400 oC, which is much lower than the decomposition of KNO3. Three reference samples were prepared with the same method for comparison. Samples of 20KMCM-400, 20KAl2O3-400 almost do not exit any CO2 desorption on peaks above 200 oC, which demonstrates no strong base sites formed in MCM-41 and γ-Al2O3. Only 20KAC-400 sample shows a wide peak ranging from 400 oC to 600 oC, which means 20KAC-400 generated limited strong basic sites. The difference in the base strength could account for the different catalytic results in the following part. The base amounts of different samples were determined by the acid-base titration method showed in Table S1 (ESI†). As for the CTFs support, the amount of basic sites is 0.54 mmol/g, which possibly originates from the high number of basic nitrogen sites. And the amount of basic sites were enhanced to 1.29 mmol/g in the case of 10KCTF-400. As shown in Table S1, the amount of basic sites significantly increased with the increase of loaded KNO3 and attain 3.29 mmol/g for the sample of 30KCTF-400.

To investigate the formation of basic sites, gaseous products originating from the decomposition of KNO3 were monitored using temperature-programmed decomposition (TPDE) method. As for the CTF support, only one stage started from 600 oC during the heating process, and the predominant products is 17 and 28, which are typical signals of NH3 and N2 (Fig. 4A). These detected gases originate from the decomposition of CTFs. However, the decomposition of 20KCTF sample can be partitioned into two periods which center at 350 oC and 650 oC when KNO3 was introduced (Fig. 4B). During the first period of 350 oC, the intense signals of 18, 28, 30, 44 are attributed to H2O, CO, NO, CO2. These signals not only originate from the decomposition of KNO3 due to the existence of CO, CO2 and H2O. And 350 oC is far below the decomposition temperature of KNO3. In addition, it is obvious that KNO3 can completely decompose when temperature increasing to 400 oC, which is also supported by XRD and IR results above (Fig. 1). Based on the TPDE results and the previous reports.18,22 The CTFs support is believed participating in the decomposition of KNO3, resulting in a redox process. Therefore, the decomposition of KNO3 to basic sites on CTFs support could be described by eqn (1):

Catalytically Oxidative of fluorenes. The obtained solid strong basic catalysts were applied in the aerobic oxidation of fluorenes to fluorenones. Fluorenones are important building blocks to product various fine chemicals ranging from organic dyes, functional polymer, biomedical materials, and organic light emitting materials (OLEDs).39-41 Previously, stoichiometric base have been reported to be effective in the oxidation of fluorenes.42,43 However, little report was about the heterogeneous base with catalytic amount.

Fig. 4 MS-monitored conversion of (A) CTFs and (B) 20KCTF. ACS Paragon Plus Environment


Table 1. Substrate Scope of Methylene Compounds Catalyzed by 20KCTF-400 Catalyst a

Fig. 5 The catalytic conversion and yield on the different support materials. Reaction conditions: fluorene (1 mmol), catalyst (50 mg), DMF (10 mL), O2 (0.3 MPa), 80 oC, 6 h. As seen in Table S2 (ESI†), the blank experiment confirmed the reaction could not proceed without the catalyst, and the pure support including CTFs could also not take effect. The catalysts with MCM-41, Al2O3 as the support did not show any conversion (Fig. 5). And the catalyst with AC as the support showed the conversion at 22.5%. Meanwhile, the 20KCTF-400 catalyst showed the highest conversion of 57.6%. When conducting the reaction at 120 oC, 20KCTF-400 gave a conversion of 100% with all the fluorenes convert to fluorenones (Table S2, ESI†). The huge differences in the catalytic activity were highly related to the strong basic sites formed in the catalysts. As for the catalysts of 20KMCM-400 and 20KAl2O3-400, little strong basic sites were formed at the calcination of 400 oC (Fig. 3). As for the 20KAC-400, though strong basic site formed, the low catalytic activation may be ascribed to the limited amount of strong basicity. The relationship was also confirmed by the base samples with different amount of basic sites (Fig. 5). A clear trend could be observed that the conversion increased with the increase of base amount. The effect of solvent, O2 pressure and temperature were also investigated (Fig. S5, Fig. S6 and table S3, ESI†). Various methylene substrates were catalyzed by 20KCTF-400 and all of them could smoothly be oxidized to the desired ketone in excellent yields (Table 1). The reusability of the 20KCTF-400 catalyst for the aerobic oxidation of fluorenes was also evaluated (Fig. S7, ESI†). The 20KCTF-400 catalyst almost retained its activity for at least 3 recycling runs. This indicated the potential application of the solid bases in the catalytic oxidation of methylene compounds.

a

Reaction condition: substrate (0.2 mmol), catalyst (20 mg), DMF (5 mL), O2 (0.3 MPa), 120 oC, 6 h. b The results were presented as conversion/selectivity. c 80 oC; d catalyst (60 mg), 140 oC, 12 h.

■ CONCLUSIONS In summary, strong basic sites were successfully introduced on CTFs and the CTFs keep their ordered organic framework and also promote the decomposition of KNO3 at 400 oC. The decomposition temperature is unexpectable lower than that of thermally decomposition of KNO3 (over 600 oC). The redox process during the preparation process was responsible for the strong basic sites formation. The prepared solid strong bases performed excellent basic catalytic capacity of the selective aerobic oxidation of methylene compounds with catalytic amount. These catalysts have much potential to applications in heterogeneous base catalyzed reactions.

ASSOCIATED CONTENT Supporting Information. Characterization data and relative reaction data were supplied. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *Jie Xu: e-mail, [email protected]; *Song Shi: e-mail, [email protected]; Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT We are grateful to the National Natural Science Foundation of China (Grant no. 21603218, 21790331 and 21233008), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300). This work was also supported by DICP&QIBEBT UN201703, and the Key Research Program of Chinese Academy of Sciences (Grant no. ZDRW-CN-2016-1).

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Formation of Strong Basicity on Covalent Triazine Frameworks as

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