Heptazine-Based Porous Framework for Selective CO2 Sorption and

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Heptazine-Based Porous Framework for Selective CO2 Sorption and Organocatalytic Performances Qin-Qin Dang, Yu-Fen Zhan, Xiao-Min Wang, and Xian-Ming Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09441 • Publication Date (Web): 07 Dec 2015 Downloaded from http://pubs.acs.org on December 12, 2015

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ACS Applied Materials & Interfaces

Heptazine-Based Porous Framework for Selective CO2 Sorption and Organocatalytic Performances Qin-Qin Dang, Yu-Fen Zhan, Xiao-Min Wang, and Xian-Ming Zhang*

School of Chemistry& Material Science, Shanxi Normal University, Linfen, Shanxi 041004, China ABSTRACT: A new heptazine-based polymer network (Cy-pip) with highly rich nitrogen sites has been synthesized via catalyst-free direct coupling of cyameluric chloride (Cy) and piperazine (Pip). Cy-pip exhibits large CO2 uptake capacity (103.4 mg/g, 9.4 wt%, 1 bar/273 K), with high selectivity (117) towards CO2 over N2 selectivity. Furthermore, this framework with high Lewis basic nitrogen sites has also been exploited as heterogeneous catalyst for Knoevenagel reaction of aromatic and heterocyclic aldehydes with active methylene compounds. Moreover, the catalyst can recycle up to 4 times with only a minor loss of activity. KEYWORDS: heptazine, piperazine, CO2 adsorption, Knoevenagel condensation, catalysis. INTRODUCTION Porous organic frameworks (POFs) have attracted extensive attention in various applications, such as gas storage and separation1, heterogeneous catalysis2, luminescent sensor3 and photogenerated elecronics4 due to their intrinsic properties of low skeleton density, high surface area, high thermal and chemical stability. In recent years, significant progress have been reported in the formation of porous polymer networks.5 Several crystalline and amorphous porous polymers with tunable properties have been developed such as COFs (crystalline covalent

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organic frameworks)6, CTFs (triazine-based organic frameworks)7-8, and CMPs (amorphous microporous polymers)9-11, PIMs (polymers of intrinsic microporosity)12, PAFs (porous aromatic frameworks)13 and so on. Generally, intensive selection of structure motifs and variation the organic linkers has been recognized as one driving force to promote the major advance in POFs. An important goal for POFs is to enhance their performance by employing functional groups into the framework. In general, the building blocks play a crucial role in controlling the structures and properties of POFs. Heptazine consisting of three condensed triazine rings presents an interesting kind of building blocks with large p-conjugated system. During the past decades, heptazinebased polymeric carbon nitrides have attracted great attention because of their outstanding performance in many energy-related fields such as water splitting, catalysis, photocatalyst and so on14-16. Up to now, however, polymer network based on heptazine units have rarely been explored. Thomas and Kailasam prepared heptazine-based microporous network by coupling of aryl diamines with cyameluric chloride, which was used as photocatalysts of hydrogen evolution.17 Crystalline polymer-frameworks based on heptazine linkers have been produced using inothermal route by Antonietti.18 As far as we know, only these two POFs containing heptazine rings have been reported. Heptazine possesses three fused triazine rings with rigid planar structure, which can be used as a building unit to expand porosity of POFs. Therefore, it would be valuable to use heptazine as building unit to construct POFs with new properties such as CO2 capture and heterogenous catalysis. Carbon dioxide, emitted from the fossil fuels combustion, was believed to be the major cause of global warming. Selective CO2 capture and storage has attracted great interest. Many POFs have been presented as advanced materials for CO2 capture.10, 19-23 CO2 uptake capacity was not only influenced by the pore volume and surface area, but also on the polar functional groups. In this


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regard, the effective strategy to enhance CO2 uptake is to introduce special active sites or polar functional groups. For example, Zhou and coworkers demonstrated the incorporation of amine functional group into porous polymer networks displaying drastic increase of CO2 uptake and CO2/N2 selectivity24 at low pressure. Zhang and Wang reported the imine-linked porous polymer frameworks for high efficient capture of CO211, 25. The newly developed azo-bridged N2-phobic nanoporous covalent organic polymers can store a significant amount of CO2 and shows an extremely high CO2/N2 selectivity with increasing temperature.26-27 Several covalent triazine frameworks have been confirmed as excellent adsorbents for CO2 adsorption10, reported a series of

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carbazolic porous organic frameworks exhibit remarkable CO2 gas

adsorption capacity30. Heptazines with high nitrogen content is a suitable choice for CO2-specific gas capture. We anticipate that the coupling reaction of piperazine with mono-protic divalent nucleophiles could form extended polymer networks. With this consideration, herein we report the design and preparation of a heptazine-based polymer (Cy-pip) from a synthetically-accessible and a commercially available amine monomer, namely cyameluric chloride and piperazine. The rich nitrogen atoms in the network may enhance the interaction between adsorbent and CO2 molecules. Moreover, high nitrogen content of the materials prompts us to study catalytic performance towards Knoevenagel condensation reactions. As far as we know, the use of heptazine and piperazine functionalized porous polymers for CO2 capture and heterogeneous catalysis, has not been reported thus far. EXPERIMENTAL SECTION Materials and Methods. NH4SCN and piperazine were purchased from Aldrich chemical company. They were used directly without further purification. N,N-diisopropyl ethylamine and


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other reagents for catalysis were purchased form Aldrich chemical company. THF was distilled over Na/benzophenone. Characterization. The elemental analysis was measured by Elementar vario MACRO cube Elemental Analyzer. IR (infrared spectrum) was measured by Variann 640 FT-IR spectrometer with KBr pellets. The TGA (thermogravimetric analysis) was carried out by a METTLERTOLEDO thermal analyzer in N2 atmosphere. The heating rate is 10 oC/min. Solid-state NMR experiment was performed on a Varian Infinityplus-400 wide-bore (89 mm) NMR spectrometer using a 5 mm double-resonance HX CP/MAS NMR probe. The proton frequency is 399.7 MHz. 13

C chemical shifts were taken tetramethylsilane as the external reference and adamantane as a

secondary reference. We measured the N2 isotherm at 77K and 273 K, while CO2 sorption isotherms were measured at 273K and 298 K. The H2 sorption isotherms were measured at 77 K. The dry samples were activated at 150 oC under high vacuum prior to measurements. The specific surface areas were derived from N2 adsorption data based on BET model calculation. Pore size distributions were obtained from nitrogen adsorption isotherms using the NLDFT method. X-ray diffraction (XRD) were collected using a BrukerD8 Advance X-ray diffractometer equipped with CuK α1 irradiation. SEM micrographs were measured on JEOLJSM-6300 scanning electronic microscope. Synthesis of Heptazine-based Porous Frameworks. The synthesis was carried out from polymerization of cyameluric chloride (prepared according literature31, see Scheme S1, Supporting Information) and piperazine in homogeneous solution phase. Typically, cyameluric chloride (220mg, 0.8mmol) was dissolved in 30ml of THF and cooled to 0 °C before piperazine (103.4mg, 1.2mmol) and 1ml of N, N-diisopropyl ethylamine was added. Stirred the reaction 4 h at 0 °C, then raised to room temperature for 4 h and refluxed overnight. After cooling and


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filtration of the reaction mixture at room temperature, the precipitates were obtained and washed three times with water and THF. Finally, the resultant powder was extracted with THF in a Soxhlet apparatus at least 48 h and dried at 80°C under vacuum to afford light yellow powder. Yiled: 80%. The sample was named as Cy-pip. Elemental analysis of solvent-free Cy-pip samples: Calculated: C, 53.16; H, 0.04; N, 46.8. Found: C, 51.6; H, 0.1; N; 48.3. Heterogeneous catalytic Knoevenagel reaction. A 10 mL Schlenk tube was charged with malononitrile (1 eq), benzaldehyde (1 eq) and 3 mL dioxane/H2O (dioxane/H2O v:v=1:1) at room temperature, After stirring for ten minutes, Cy-pip (1mol%) was added. The mixture was reacted for a certain amount of time. The catalyst was recovered by filtration of the concentrated reaction mixture. The filtrate was subjected to GC analysis(Shimadzu GC-2014C) and quantified under the following conditions: GC was conducted using a HP-5 column (30 mm × 0.25 mm × 0.25 mm, Restek, USA). N2 was employed as a carrier gas with the flow rate of 1.0 mL min-1. The temperature of injector-port was programmed at 270 oC, while the column temperature was set to 280 oC and kept at that temperature for 5min from 40 oC with a heating rate of 20 oC min-1. Undecane was labelled as the internal standard. Recycling experiments. After the completion of the catalytic reaction, the Cy-pip was seperated from the reaction mixtures by fitration, washed thoroughly with dichloromethane. Then the recovered catalyst was desiccated under vacuum at 100 °C and reused in the next round of Knoevenagel reactions. RESULTS AND DISCUSSION Synthesis and Characterization. Cyameluric chloride has been chosen as a monomer to build up heptazine tectons because in the cyameluric chloride the C-Cl bond is highly reactive and


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could easily react with nucleophiles. For example, a series of symmetric and asymmetric aryl/alkyl-substituted heptazines have been prepared by reaction of cyameluric chlorides with secondary amines.32-33 Herein, we successfully synthesized a heptazine-based polymer network (Cy-pip) by treating cyameluric chloride with secondary diamine piperazine. The synthetic procedure of the framework is simple just involving refluxing THF of Cy, Pip with the mild base DIPEA. The reaction requires mild conditions without toxicity and other deleterious effects of metal residues. Scheme 1 shows the formation of the networks Cy-pip. The networks are similar with polymeric carbon nitride but introducing piperazine rings into the extended network, thus endowing the framework other properties such as gas storage and heterogeneous catalysis. It was insoluble in boiling water and other commonly used organic solvents, such as alkanes, chloroform, acetone, THF, methanol and DMF, which indicated good chemical stability. The chemical connectivity, component and thermal stability of Cy-pip were characterized by FTIR (Fourier transform infrared spectroscopy), solid state

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C/CP MAS NMR and TGA

(thermogravimetric analysis). In FT-IR spectra of Cy-pip, the typical bands of 1200-1600 cm-1 were assigned to the C-N stretching modes in the heptazine heterocycles (see Figure S1). The typical bands around 800 cm-1 were assigned to the breathing mode of the heptazine units. The absorption bands near 2930 cm-1 were assigned to C-H stretching in the piperazine. Meanwhile, the absence of stretching vibration at 1205 cm-1 indicated that no C-Cl bond existed in the Cy-pip, which confirmed that all chlorine atoms of cyameluric chloride had been substituted. Further the structure of Cy-pip was characterized by 13C CP/MAS NMR (Figure 1). The observed signals at 155.5 and 162.8 ppm were assigned to the sp2 carbon atoms within the heptazine ring and the carbon close to the bridging amino atoms.34 The signal at 44.9 ppm is assigned to the sp3 carbons in the piperazine units. The absence of C-Cl signals at 176 ppm17 also evidenced the complete


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substitution of all three chlorine atoms. TGA indicated that Cy-pip framework is stable even heating up to 400°C. The mass loss of 9% between 30-230 °C can be assigned to desorption of trapped THF molecules ( Figure S2). PXRD patterns confirmed Cy-pip networks are amorphous (Figure S3). This is in accordance with most porous polymers assembled under kinetic control due to irreversible condensation reaction2. Scanning electron microscopy (SEM) showed that the Cy-pip adopted a fused flake-like morphology (Figure S4). Adsorption Properties. In order to investigate the porous properties of guest free Cy-pip, N2 adsorption-desorption isotherms at 77K were measured. Before measurement, the solvent-free sample is heat treated at 150 oC in vacuum to remove any moisture. Nitrogen sorption isotherms (Figure 2) show that an initial sharp uptake at low relative pressures (P/P0

Heptazine-Based Porous Framework for Selective CO2 Sorption and

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