Synthesis of functional conjugated microporous polymers containing pyridine units with high BET surface area for reversible CO2 storage

Reactive and Functional Polymers 99 (2016) 95–99

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Synthesis of functional conjugated microporous polymers containing pyridine units with high BET surface area for reversible CO2 storage Jiake Zang, Zhaoqi Zhu, Hanxue Sun, Weidong Liang, An Li ⁎ College of Petrochemical Technology, Lanzhou University of Technology, Langongping Road 287, Lanzhou 730050, PR China

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Article history: Received 10 September 2015 Received in revised form 28 December 2015 Accepted 29 December 2015 Available online 30 December 2015 Keywords: Conjugated microporous polymers Pyridine Porosity CO2 uptake Functional

a b s t r a c t Novel and functional conjugated microporous polymers (PCMPs) with pyridine units in networks were synthesized from arylethynylenes and 2,6-bis(bromomethyl) pyridine by palladium-catalyzed Sonogashira–Hagihara crosscoupling chemistry. The resulting PCMPs exhibit good chemical and thermal stability with decomposition temperature above 400 °C. The choice of ethynyl monomer has an obvious influence on the specific surface areas of the resulting CMPs. A high BET surface area of 1136 m2 g−1 was achieved. Taking advantages of the good physicochemical stability and excellent porosity, the functional CMPs networks show superior adsorption performance for CO2 with uptake up to 109.0 mg g−1. Given the excellent porosity and gas uptake as well as good physiochemical stability, the as-synthesized PCMPs show great potentials in CO2 uptake. Our study may provide fundamental guidance in designing new CMPs-based materials for the application in CO2 capture and storage. © 2015 Published by Elsevier B.V.

1. Introduction The environmental issue from the emission of CO2 caused by burning fossil fuels has been garnered tremendous interest both in academia and industry [1]. The most popular process employed for CO2 capture is the adsorption using amine solutions such as monoethanolamine and triethanolamine [2], due to its high efficiency and simplicity. However, this process suffers from some drawbacks, such as toxicity, equipment corrosion and high regeneration costs [3]. To this end, porous solid materials including activated carbons, metal organic frameworks (MOFs) [4–7] and porous organic polymers (POPs) [8–11] have been investigated and proven to be viable alternatives for CO2 capture and storage. Conjugated microporous polymers (CMPs), first reported in 2007 by Copper et al. [12], are a subclass of POPs that consist of π-conjugated skeletons with nanoporous structures [4]. Different from other porous materials such as porous carbons, CMPs with π-conjugated skeletons and permanent nanoporous structures, has a high structural flexibility and have been attracted considerable attention for CO2 storage and capture [13–15]. CMPs typically were synthesized using metal-catalyzed cross coupling chemistry [16,17], which was favorable for the incorporation of functional groups into structures. And the functionalization of CMPs networks has been proven to be an effective method to modulate the interactions between the polymers and CO2, resulting in ⁎ Corresponding author. E-mail address: [email protected] (A. Li).

http://dx.doi.org/10.1016/j.reactfunctpolym.2015.12.016 1381-5148/© 2015 Published by Elsevier B.V.

high CO 2 uptakes. For example, Xie et al. reported cobalt/aluminum-coordinated CMPs that exhibit outstanding CO 2 capture at room temperature [18]. Cooper et al. developed a series of CMPs incorporated by a range of functional groups including amines, carboxylic acids, hydroxyl and methyl units for CO2 uptakes [19]. These functionalized CMPs are quite promising for CO2 capture due to their high tunabilities of surface functionalities and porosity, however, the design and synthesis of novel functionalized CMPs with high CO 2 uptake remain an ongoing challenge. Arab and others found that the introduction of nitrogen moieties into porous polymers or surface modification of porous polymers with amino would significantly enhance its CO2 uptake [20–22]. In view of these findings and taking advantage of high flexibility of functionalities designing of CMPs, here we report the synthesis of two novel CMP networks using Pd(0)/Cu(I)-catalyzed Sonogashira–Hagihara crosscoupling condensation of ethynyl monomer and 2,6bis(bromomethyl) pyridine. The nitrogen-rich pyridine units in CMPs networks could be advantageous for the improvement of affinity with CO2, as a result of enhancing in the CO2 uptake. 2. Experimental 2.1. Materials 2,6-bis(bromomethyl) pyridine, tetrakis(triphenylphosphine) palladium(0) and copper(I) iodide were obtained from J&K. 1,3,5-

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filtered and washed with chloroform, acetone, water and methanol for several times. Then the polymer was further purified by Soxhlet extraction with methanol for 72 h. The resulting product was dried at 60 °C for 24 h and named as PCMP-1. PCMP-2 were synthesized by the above mentioned procedure from 1,4-diethynylbenzene (283.9 mg, 2.25 mmol) and 2,6-bis(bromomethyl) pyridine (397.4 mg, 1.5 mmol) as monomers. 2.3. Characterization

Fig. 1. The synthesis of PCMP-1 and PCMP-2.

triethynylbenzene and 1,4-diethynylbenzene were obtained from TCI. All chemicals were used as received.

SEM images were obtained with a field emission gun scanning electron microscope (JSM-6701F, JEOL, Ltd.) after coating the sample with an Au film. FTIR spectra were recorded from in the range of 4000–400 cm−1 using the KBr pellet technique on a Nexus 670 spectrum instrument. N2 adsorption and desorption isotherms were recorded at 77 K on a pore and surface analyzer (ASAP 2020 apparatus). All samples were degassed at 120 °C for 12 h under vacuum before analysis. Thermogravimetric analysis (TGA) was performed on thermogravimeter analyzer (Perkin Elmer) from room temperature to 800 °C at a heating rate of 10 °C min− 1 under nitrogen atmosphere. 13 C cross-polarization magic angle Spinning (CP/MAS) NMR spectra were measured on Bruker AVANCE III 400 spectrometer operating at 100.6 MHz. Carbon dioxide isotherms were collected on the instrument (NOVA 1200e, Quantachrome) at 273 K and 298 K. All samples were degassed at 120 °C for 12 h under conditions of dynamic vacuum prior to analysis.

2.2. Synthesis 3. Results and discussion 1,3,5-triethynylbenzene (225.3 mg, 1.5 mmol), 2,6-bis(bromomethyl) pyridine (397.4 mg, 1.5 mmol), tetrakis (triphenylphosphine) palladium (0) (150 mg) and copper (I) iodide (50 mg) were placed in Et3N (7.5 mL) and toluene (7.5 mL) in 50 mL round-bottom flask, which was degassed with nitrogen gas for 0.5 h. Then the mixture was heated to 80 °C and stirred under nitrogen for 24 h. The resulting polymer was

We prepared two CMPs from 1,3,5-triethynylbenzene or 1,4diethynylbenzene and 2,6-bis(bromomethyl) pyridine via (A3 + B2) and (A2 + B2) Sonogashira–Hagihara cross-coupling reactions at a 1.5:1 M ratio of ethynyl to bromo functionalities with toluene as a solvent. The general synthetic route for the polymer networks was

Fig. 2. (a) FTIR spectra of the PCMP-1 and PCMP-2 samples. (b) 13C CP/MAS NMR spectra of PCMP-1. (c) 13C CP/MAS NMR spectra of PCMP-2.

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Fig. 3. SEM images of (a) PCMP-1 and (b) PCMP-2. Scale bar: 100 nm.

shown in Fig. 1. The molecular level structures of both PCMP-1 and PCMP-2 were assessed by FTIR and 13C CP/MAS solid-state NMR. As shown in Fig. 2a, the infrared spectra of both samples are basically similar. Three main adsorption regions were observed: a first absorption band in the 1400–1650 cm−1 regions are assigned to benzene ring and imine (−C_N–) stretch vibration [23], a second peak at approximate 2300 cm− 1 refers to –C`C– sites stretching, and a third peak close to 3000 cm− 1 regions corresponds to the –Ar–H stretching of CMPs. The network was further confirmed by solid state NMR. For PCMP-1 (Fig. 2b), a peak at ca. 157 ppm is ascribed to pyridyl (− HC_N–) [23]. The peaks at ca. 124 and 130 ppm can be ascribed to CAr–C and CAr–H sites. The sp.-hybridized CAr–C`C-units are observed at 90 ppm. The low-intensity peaks at 70–80 ppm can be assigned to the –C`CH end groups. The NMR spectrum of PCMP-1 is comparable with that of PCMP-2 (Fig. 2c), the main difference is that no obvious peaks at 70–80 ppm were visible, suggesting higher levels of polycondensation for PCMP-2 network. PCMP-1 was recovered as a yellow powder, and PCMP-2 was brown. And in both cases, PCMP-1 and PCMP-2 are totally insoluble in most solvents such as acetone, DMF and THF, and NaOH or HCl (Supporting Information, Fig. S1), suggesting high chemical stability. The thermal property of these two polymers was investigated by TGA measurement performed under a nitrogen atmosphere, as shown in Fig. S2. TGA curves showed good thermal stability of PCMP-1 and PCMP-2 with weight loss lower than 10 wt.%, even when the temperature reached up to 400 °C. The good chemical and thermal stability of the resulting PCMP-1 and PCMP-2 should be contributed to their rigid networks arising from the butadiynylene linkages, as seen in Fig. 1. From the SEM image, the PCMP-1 consists of agglomerated microgel particles, which formed a topographical micro/nanoporous structure (Fig. 3a). PCMP-2 is composed of uniform solid sub-micron spheres with an average size of ca. 50 nm (Fig. 3b). Furthermore, the SEM images of both PCMP-1 and PCMP-2 exhibit a porous structure due to the butadiynylene-linked network, as seen in Fig. 1. The porosity of the PCMPs samples was measured by collecting the nitrogen gas adsorption and desorption isotherms at 77 K from which the BET surface area was calculated. According to the IUPAC classification, the two networks gave rise to type-I N2 gas sorption isotherms with some H3 hysteresis loop character (Fig. 4a), indicating the presence of micropores and mesopores/small macropores, likely arising from interparticulate porosity, as seen in SEM images (Fig. 3). The surface areas of the PCMP-1 and PCMP-2 were calculated in the relative pressure (P/P0) range from 0.05 to 0.20, which results in apparent surface areas of 1136 m2 g−1 and 568 m2 g−1. Here it is worth emphasizing that PCMP-1 can achieve a high surface area of 1136 m2 g−1, which is higher than that of amine functionalized CMPs (600–800 m2 g−1) [19], tubular CMPs (b500 m2 g− 1) [24–26] and other functionalized poly(arylene ethynylene)s (PAEs) networks (b900 m2 g−1) [27]. The micropore surface areas derived using the t-plot method were

404 m2 g− 1 and 416 m2 g− 1 for PCMP-1 and PCMP-2. Fig. 4b is the pore size distribution (PSD) curves of PCMP-1 and PCMP-2, which exhibited similarly continuous curves. The pore volumes, estimated from the amount of gas adsorbed at P/P0 = 0.99, were 1.136 cm3 g−1 for PCMP-1 and 0.286 cm3 g−1 for PCMP-2. The CO2 gas sorption for PCMP-1 and PCMP-2 was measured at 273 K (Fig. 5a) and 298 K (Fig. 5b). As shown in Fig. 5, obvious hysteresis was observed between the absorption and desorption isotherms for both PCMP-1 and PCMP-2. The maximum CO2 uptake is 109.0 mg g−1 (2.48 mmol g−1) at 273 K for PCMP-1, nearly to that of 1,3,5-triazine based CMPs (2.62 mmol g−1) and largely than that of tube-liked CMPs (1.5–1.6 mmol g−1). Under the same temperature, PCMP-2 has a CO2 uptake of 61.6 mg g− 1, possibly resulting from its relatively lower special surface area and pore volume. From 273 K to 298 K, the CO2-

Fig. 4. (a) Nitrogen adsorption and desorption isotherms of PCMP-1 and PCMP-2 measured at 77.3 K. (b) Pore size distribution for PCMP-1 and PCMP-2, calculated according to desorption data.

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4. Conclusions Two novel and functional CMPs networks (PCMPs) with nitrogenrich pyridine units were designed and synthesized using Pd(0)/Cu(I)catalyzed Sonogashira–Hagihara crosscoupling polymerization of 1,3,5-triethynylbenzene or 1,4-diethynylbenzene and 2,6-bis(bromomethyl) pyridine. The resulting PCMPs exhibit good chemical and thermal stability due to their rigid networks arising from the butadiynylene linkages. The choice of ethynyl monomer has an obvious influence on the specific surface areas of the resulting CMPs. A high BET surface area of 1136 m2 g−1 was achieved for PCMP-1. Taking advantages of the good chemical and thermal stability as well as excellent porosity, the pyridine units based CMPs networks exhibit CO2 uptake up to 109.0 mg g−1. Our study may provide fundamental guidance in designing new CMPs-based materials for the application in CO2 capture and storage. Acknowledgments The authors are grateful to the National Natural Science Foundation of China (Grant No. 51263012, 51262019, 51462021 and 51403092), Gansu Provincial Science Fund for Distinguished Young Scholars (Grant No. 1308RJDA012), Support Program for Hongliu Young Teachers (Q201411), Hongliu Elitist Scholars of LUT (J201401) and Fundamental Research Funds for the Universities of Gansu Province. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.reactfunctpolym.2015.12.016.

Fig. 5. CO2 adsorption and desorption isotherms of PCMP-1 and PCMP-2 at (a) 273 K and (b) 298 K.

uptake capacity at 1 bar decreased to 61.6 mg g− 1 for PCMP-1 and 44.4 mg g−1 for PCMP-2, accordance with the exothermic process of CO2 sorption. To investigate the binding affinity of PCMP-1 and PCMP-2 towards CO2 molecular, we calculated the isosteric heats of adsorption (Qst) from the CO2 sorption dates measured at 273 K and 298 K. As shown in Fig. 6, PCMP-1 and PCMP-2 has a maximum Qst value of 35.0 kJ mol−1 and 32.6 kJ mol−1 at low adsorption values, which is consistent with the data reported for CO2 adsorption on CMPs (25–33 kJ mol−1) [28]. The higher Qst values of PCMP-1 results in its higher CO2 adsorption capacity, accordance with its higher BET surface area.

Fig. 6. Isosteric heats of CO2 adsorption of PCMP-1 and PCMP-2.

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