Carboxyl-Enriched Conjugated Microporous Polymers: Impact of Building Blocks on Porosity and Gas Adsorption

YAO Chan LI Guo-Yan XU Yan-Hong,2,*



Carboxyl-Enriched Conjugated Microporous Polymers: Impact of Building Blocks on Porosity and Gas Adsorption

YAO Chan1LI Guo-Yan1XU Yan-Hong1,2,*

(1;2)

Polar groups in the skeletons of conjugated microporous polymers (CMPs) play an important role in determining their porosity and gas sorption performance. Understanding the effect of the polar group on the properties of CMPs is essential for further advances in this field. To address this fundamental issue, we used benzene, the simplest aromatic system, as a monomer for the construction of two novel CMPs with multi-carboxylic acid groups in their skeletons (CMP-COOH@1 and CMP-COOH@2). We then explored the profound effect the amount of free carboxylic acid in each polymer had on their porosity, isosteric heat, gas adsorption, and gas selectivity. CMP-COOH@1 and CMP-COOH@2 showed Brunauer-Emmett-Teller (BET) surface areas of 835 and 765 m2∙g−1, respectively, displaying high potential for carbon dioxide storage applications. CMP-COOH@1 and CMP-COOH@2exhibited CO2capture capabilities of 2.17 and 2.63 mmol∙g−1(at 273 K and 1.05 × 105Pa), respectively, which were higher than those of their counterpart polymers, CMP-1 and CMP-2, which showed CO2capture capabilities of 1.66 and2.28mmol∙g−1, respectively. Our results revealed that increasing the number of carboxylic acid groups in polymers could improve their adsorption capacity and selectivity.

Conjugated microporous polymers; Carboxylic acid; Pore; Gas adsorption;selectivity

1 Introduction

Carbon dioxide is one of the main greenhouse gases that cause global issues, such as climate warming and increases in sea level and ocean acidity. Modern climate science predicts that the accumulation of greenhouse gases in the atmosphere will contribute to an increase in surface air temperature of 5.2 °C between the years 1861 and 2100. Carbon capture and sequestration (CCS), a process of CO2separation and concentration can contribute to solve. For this aim, the use of porous materials tailored for selective CO2absorption is energetically efficient and technically feasible. Among the numerous and diversified examples of novel porous materials, such as metal-organic frameworks1,2, zeolites3,4, and purely organic materials5,6are a class of porous organic materials that allow an elaborate design of molecular skeletons and a fine control of nanopores.

Conjugated microporous polymers (CMPs) are a unique class of porous organic materials that combine π-conjugated skeletons with permanent nanopores7–10, which is rarely observed in other porous polymers. CMPs have emerged as a powerful platform for synthesizing functional materials that exhibit excellent functional applications, such as heterogeneous catalysts11,12, guest encapsulation13–15, super-capacitive energy storage devices16,17, light-emitting materials18,19, and fluorescent sensors20,21and so on. Recently, CMPs have emerged as a designable material for the adsorption of gases, such as hydrogen, carbon dioxide, and methane22–24. Although great achievements in synthesizing CMPs have been realized, extremely high Brunauer-Emmet-Teller specific surface areas as high as 6461 m2·g−125, the other pore parameters, such as pore volume, pore size, and pore size distribution, are important in determining the gas sorption performance26,27. Moreover, previous work has shown the surface modification of porous polymers with polar group can significantly enhance their CO2binding energy, resulting in enhancement in CO2uptake and/or CO2selectivity28–30. Carboxylic-rich framework interaction is expected due to hydrogen bonding and/or dipole-quadrupole interactions between CO2and the functional groups of porous polymers31,32. Cooper.33,34reported increasing the heat of adsorption through the introduction of tailored binding functionalities could have more potential to increase the amount of gas adsorbed. Their results demonstrated that carboxylic groups functionalised polymer showed the higher isosteric heat of sorption for CO2. Torrisi35predicted that the incorporation of carboxylic groups would lead to the higher isosteric heat, challenging the current research emphasis in the literature regarding amine groups for CO2capture.

Herein, we report the synthesis and characterization two high carboxylic groups of porous polymers and investigate their performances in CO2storage application under high pressure and cryogenic conditions (Scheme 1, CMP-COOH@1 and CMP-COOH@2). The CMPs are highly efficient in the uptake of CO2by virtue of a synergistic structural effect, and that the carboxylic units improve the uptake, the high porosity provides a large interface, and the swellable skeleton boosts the capacity.

2 Experimental and computational section

2.1 Materials and Measurements

1,3,5-Triethynylbenzene (98%) was purchased from TCI, 2,5-dibromobenzoic-3-carboxylic acid (97%) and 2,5-dibromoterephthalicacid(97%) were purchased from Alfa. Tetrakis(4-ethynylphenyl)methane was synthesized according to the literature36. Tetrakis(triphenylphosphine)palladium(0), copper(I) iodide (CuI) and tetra(4-bromophenyl)methane (97%) were purchased from Aladdin.,-Dimethylformamide (DMF) (99.9%), triethylamine (99%), methanol (95%) and acetone (95%) were purchased from Aladdin.

Scheme 1 Schematic representation of synthesis of carboxylic polymers.

1H NMR spectra were recorded on Bruker Avance III models HD400 NMR spectrometers, where chemical shifts () were determined with a residual proton of the solventas standard.Fourier transform Infrared (FT-IR) spectra were recorded on a Perkin-Elmer spectrum one model FT-IR-frontier infrared spectrometer.The UV-visible analyzer was used for shimadzu UV-3600. Field-emission scanning electron microscopy (FE-SEM) images were performed on a JEOL model JSM-6700 operating at an accelerating voltage of 5.0 kV. The samples were prepared by drop-casting a tetrahydrofunan (THF) suspension onto mica substrate and then coated with gold.High-resolution transmission electron microscopy (HR-TEM) images were obtained on a JEOL model JEM-3200 microscopy.Powder X-ray diffraction (PXRD) data were recorded on a Rigaku model RINT Ultima III diffractometer by depositing powder on glass substrate, from 2= 1.5° up to 2= 60° with 0.02° increment. The elemental analysis was carried out on a EuroEA-3000. TGA analysis was carried out using a Q5000IR analyzer with an automated vertical overhead thermobalance. Before measurement, the samples were heated at a rate of 5 °C min−1under a nitrogen atmosphere. Nitrogen sorption isotherms were measured at 77 K with ASIQ (iQ-2) volumetric adsorption analyzer.Before measurement, the samples were degassed in vacuum at 150 °C for 12 h. The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas and pore volume. BET surface areas were calculated over the relative pressure (/0) range of 0.015–0.1. Nitrogen NLDFT pore size distributions were calculated from the nitrogen adsorption branch using a cylindrical pore size model. Carbon dioxide, methane and nitrogen sorption isothermswere measured at 298 or 273 K with a Bel Japan Inc. model BELSORP-max analyzer, respectively. In addition, carbon dioxide sorption isotherms were measured at 318 K and 5 × 106Pa with a iSorb HP2 analyzer. Before measurement, the samples were also degassed in vacuum at 120 °C for more than 10 h.

2.2 Synthetic procedures

2.2.1 Synthesis of CMP containing carboxylic groups

All of the polymer networks containing multi-carboxylic groups were synthesized by palladium(0)-catalyzed cross-coupling polycondensation. All the reactions were carried out at a fixed reaction temperature and reaction time (120 °C/48 h).

2.2.2 Synthesis of CMP-COOH@1 and CMP-COOH@2

2,5-Dibromoterephthalic acid (107 mg, 0.33 mmol) and 1,3,5-triethynylbenzene (50 mg, 0.33 mmol) (CMP-COOH@1)/tetrakis(4-ethynylphenyl)methane (104 mg, 0.25 mmol) (CMP-COOH@2) were put into a 50 mL round-bottom flask, the flask exchanged three cycles under vacuum/N2. Then added to 2 mL,-dimethylformamide (DMF) and 2 mL triethylamine (Et3N), the flask was degassed by threefreeze-pump-thaw cycles, purged with N2. When the solution had reached reaction temperature, a slurry of tetrakis(triphenylphosphine)palladium(0) (23.11 mg, 0.02 mmol) in the 1 mL DMF and copper(I) iodide (4.8 mg, 0.025 mmol) in the 1 mL Et3N (CMP-COOH@1)/(CMP-COOH@2) was added respectively, and the reaction was stirred at 120 °C under nitrogen for 48 h. The solid product was collected by filtration and washed well with hot reaction solvent for 4 times with THF, methanol, acetone, and water, respectively. Further purification of the polymer was carried out by Soxhlet extraction with methanol, and THF for 24 h, respectively, to give CMP-COOH@1(claybank solid, 98 mg, 94% yield), CMP-COOH@2(olivine solid, 142 mg, 90% yield). Elemental Analysis (%) Calcd. (Actual value for an infinite 2D polymer), (CMP-COOH@1) C 67.61, H 2.35. Found: C 64.84, H 2.05. (CMP-COOH@2) C 73.03, H 3.02. Found: C 70.02, H 2.19.

3 Results and discussion

Carboxylic-CMP was synthesized by the Sonogashira- Higihara reaction of 1,3,5-triethynylbenzene, tetrakis(4- ethynylphenyl)methane and 2,5-dibromoterephthalic acid in the presence of Pd(0) as catalyst. These two samples were unambiguously characterized by elemental analysis confirmed that the weight percentages of C and H contents are close to the calculated values expected for an infinite 2D polymer. The CMPs were further characterized by infrared spectroscopy (Fig.1). Band soft he primary bromo and borate groups of 2,5-dibromoterephthalic acid at about 598 and 1368 cm−1are absent, respectively. From 2900 to 3200 cm−1aromatic C―H stretching bands appear. A C＝C stretching mode at 1600 cm−1is also observed. All networks show the typical C≡C and COOH stretching mode at about 2200and 1700 cm−1, respectively, indicating the successful incorporation of the carboxylic and alkynyl groups into the polymer materials.

Fig.1 FT-IR spectra of 2,5-dibromoterephthalic acid (blue), CMP-COOH@1 (green) and SCMP-COOH@2 (red).

Fig.2 FE-SEM images of (a) CMP-COO H@1 and (b) CMP-COOH@2.

Field-emission scanning electron microscopy (FE-SEM) displayed that the CMPs adopt a spherical shape with sizes of 100–500 nm (Fig.2). High-resolution transmission electron microscopy (HR-TEM) revealed the homogeneous distribution of nanometer-scale pores in the textures (Fig.S1 (Supporting Information)). Powder X-ray diffraction (PXRD) revealed no diffraction, implying that all the polymers are amorphous (Fig.S2 (Supporting Information)). The TGA results show that the polymers have a good thermal stability, and the thermal degradation temperature is up to ca. 300 °C (Fig.S3 (Supporting Information)). The weight loss below 100 °C is generally attributed to the evaporation of adsorbed water and gas molecules trapped in the micropores.

The conjugated polymer networks were dispersed in THF to obtain UV/Vis spectra (Fig.S4 (Supporting Information)). The polymer CMP-COOH@1 shows mainly one wide absorption peak at about 396 nm. Compared to monomer 1,3,5-triethynylbenzene, with narrow absorption maxima at 305 nm, the polymer networks exhibit a large bathochromic shift of around 111 nm. CMP-COOH@2 show similar phenomenon, compared to tetrakis(4-ethynylphenyl)methane monomer, with absorption maxima at 325 and 345 nm, the polymer frameworks display a large bathochromic shift of around 68 and 48 nm, respectively. This indicates the effective enlargement of the-conjugated system through the polycondensation reaction.

The porosity of the polymer networks was probed by nitrogen sorption at 77 K. According to the IUPAC classification37, adsorption/desorption isotherms of two polymers showed mainly a type I isotherms. As seen in Fig.3(a), remarkably, the two polymer samples exhibit a steep uptake at a relative pressure of/0< 0.1, suggesting that these samples have micropores. There is a sharp rise in the isotherm for the CMP-COOH@1 at higher relative pressures (/0> 0.8), which indicates the presence of meso/macropores in the samples. These textural meso/macropores can be also found in the corresponding FE-SEM images (Fig.2(a)). However, the shape of the isotherm for the CMP-COOH@2 is significantly different from that of CMP-COOH@1, which displays a significant H2 type hysteresis loop in the desorption branch, characteristic of nanostructured materials with a mesoporous structure (Fig.3(a)). These meso/macropores can be ascribed mostly to interparticulate porosity that exists between the highly aggregated nanoparticles38.

The pore size distribution calculated from nonlinear density functional theory (NLDFT) shows that the two polymer networks have relatively broad pore size distribution (Fig.3(b)). CMP-COOH@1 and CMP-COOH@2 showed apparent peaks in the size range 0–2 nm, whereas small fluctuations can be observed at 2–12 nm regions. The pore size distribution curves agree with the shape of the N2isotherms (Fig.3(a)) and imply the presence of both micropores and mesopores in the two polymers. The contribution of microporosity to the networks can be calculated as the ratio of the micropore volume (micro), over the total pore volume (total). The microporosities of CMP-COOH@1 and CMP-COOH@2 are around 50.8% and 52.3%, respectively. This result indicates that the two carboxylic networks are predominantly microporous. In addition, the BET surface area of CMP-COOH@1 and CMP-COOH@2 were calculated to be 835 and 765 m2·g−1in the relative pressure range 0.015–0.1, respectively. The decreased surface area of CMP-COOH@2 compared to CMP-COOH@1 could be due to the CMPs constructed with longer connecting struts have lower BET surface areas39,40.

In view of the fact that the CMPs possess two key properties generally associated with high CO2uptake capacity, e.g., good porosity and abundant COOH sites, the CO2adsorption of the polymers were investigated up to 1.05 × 105Pa at both 298 K and 273 K (Fig.4(a, b)), respectively. Remarkably, CMP-COOH@1 and CMP-COOH@2 showed the CO2adsorption capacities of 1.61 and 1.92 mmol·g−1at 298 K and 1.05 × 105Pa, respectively (Fig.4(a)). When the temperature was elevated to 273 K, the polymers CMP-COOH@1 and CMP-COOH@2 displayed the higher CO2capture of 2.17 and 2.63 mmol·g−1(Fig.4(b)), respectively, which were comparable to that of other microporous hydrocarbon networks41. Despite CMP-COOH@2 with a lower surface area, but which adsorbed more CO2probably due to it has a higher pore volume. In addition, the isosteric heat of adsorption (st) of the polymers was calculated from the CO2uptake data at 273 K and 298 K by using Clausius-Clapeyron equation (Fig.4(c)). The two polymer networks show the isosteric heats of CO2adsorption around 35.5 and 30.9 kJ·mol−1. Because there is less carboxylic acid in the structural unit, the CO2stof CMP-COOH@2 is lower than that of CMP-COOH@1, which is consistent with that of the previous reported polymers33,34. Moreover, the high pressure CO2sorption properties of the two polymers were also investigated at 5 × 106Pa and 318 K. As seen in Fig.4(d), CMP-COOH@1 and CMP-COOH@2 show a nearly linear increase with the increasing pressure no obviously turning point. CMP-COOH@1 and CMP-COOH@2 show the higher CO2capture capacity of 498 and 434 mg·g−1at 318 K and 5 × 106Pa, respectively (Fig.4(d)). These results indicated that the CO2uptake in these networks at high pressures is not dependent solely on the surface area, pore volume or polar groups in the skeletons, but also the measuring pressure have a large effect on the uptake of gas.

Fig.3 (a) Nitrogen sorption curves (filled circles: adsorption, open circles: desorption, STP=standard temperature pressure) and (b) pore size distribution.

In order to investigate the amount of carboxylic group in the network whether affects CO2adsorption capacity of polymers. We synthesized another two carboxylic conjugated polymer with relatively low amount of carboxylic groups (scheme S1, CMP@1 and CMP@2 (Supporting Information)) based on 2,5-dibromobenzoic acid, 1,3,5-triethynylbenzene and tetrakis(4-ethynylphenyl)methane. They show the BET surface area of 979 and 876 m2·g−1(Fig.S5 (Supporting Information)), respectively, which is higher to that of counterpart CMP-COOH@1 and CMP-COOH@2. CMP@1 and CMP@2 showed the main pore size of 0.8–2.0 nm (Fig.S6 (Supporting Information)). The decreased surface area of CMP-COOH@1 compared to CMP@1 could be due to the volume of 2,5-dibromoterephthalic acid in CMP-COOH@1 is obviously larger than 2,5-dibromobenzoic acid in CMP@1, which made the bulky benzen–carboxylic moieties in CMP-COOH@1 occupy more cavity space. The similar phenomenon can be also observed in CMP-COOH@2 and CMP@2 system. As shown in Fig.4(b), at 273 K and 1.05 × 105Pa, polymers CMP@1 and CMP@2 show the CO2capture capacity of 1.66 and 2.28 mmol·g−1, respectively. The CO2uptake value of CMP-COOH@1 and CMP-COOH@2 is 1.31 and 1.15-times that of the counterpart CMP@1 and CMP@2, respectively, indicating that increasing amount of carboxylic groups in the CMP networks can improve CO2uptake. In addition, we calculated the isosteric heats of these polymers, they showed the following order (Fig.4(c)): CMP-COOH@1 > CMP-COOH@2 > CMP@1 > CMP@2. Because there is less carboxylic groups in the structural units of CMP@1 and CMP@2, the CO2stof CMP@1 and CMP@2 is lower than that of CMP-COOH@1 and CMP-COOH@2, respectively33,42. In addition, CMP-COOH@1 and CMP-COOH@2 show the higher CO2capture capacity than that of CMP@1 (447 mg·g−1) and CMP@2 (402 mg·g−1) at 318 K and 5 × 106Pa, respectively (Fig.4(d)). These results imply the amount of carboxylic groups effects BET surface area, pore volume and isosteric heats lead to different the uptake of gas.

As for carbon dioxide capture, high separation properties towards CH4and N2are also necessary and important in gas separation applications. In order to investigate the gas adsorption selectivity of the microporous polymer networks, CO2, N2, and CH4sorption properties were measured by volumetric methods at 273 K and 1.05 × 105Pa. It was found that the two porous polymer networks show significantly higher CO2uptake ability than N2and CH4in the whole measurement pressure range (Fig.S7 (Supporting Information)). CO2/CH4and CO2/N2selectivity was first evaluated by using the initial slope ratios estimated from Henry′s law constants for single-component adsorption isotherms. The CO2/CH4selectivity of CMP-COOH@1 and CMP-COOH@2 are calculated to be 6.9 and 6.2, respectively (Table S1 and Fig.S8 (Supporting Information)). In addition, two polymers exhibited the CO2/N2adsorption selectivity is 48.2 and 39.5, respectively (Table S1 and Fig.S9 (Supporting Information)). Meanwhile, the gas selective capture was also supported by the results from the ideal adsorbed solution theory (IAST), which has been widely used to predict gas mixture adsorption behavior in the porous materials43,44. Under simulated natural gas conditions (CO2/CH4, 50/50), the experimental CO2and CH4isotherms collected at 273 K for carboxylic CMP were fitted to the dual-site Langmuir model and the single-site Langmuir model, respectively (Fig.S10 (Supporting Information)). The calculated IAST data for carboxylic CMP are shown in Table S1. At 273 K and 1.05 × 105Pa, CMP-COOH@1 and CMP-COOH@2 exhibit an appreciably high selectivity of CO2over CH4 under natural gas conditions (5.5 and 5.2) (Fig.S10 (Supporting Information)), which is comparable to some reported MOPs, such as A6CMP (5.1)45, SCMP (4.4–5.2)30, and P-G1-T (5)46. Furthermore, the CO2/N2adsorption selectivities for CMP-COOH@1 and CMP-COOH@2 are calculated to be 45.4 and 37.8 at 273 K and 1.05 × 105Pa (Table S1 and Fig.S11 (Supporting Information)), respectively, which is comparable to some reported MOPs, such as ALP-1(35)38, PCN-TA (33)47, and PCN-DC (48)47. These excellent CO2selective capture performance of carboxylic CMPs evaluated by IAST are consistent with the results calculated from the initial slopes method. In addition, in light of the amount of carboxylic group effect for the uptake of gas, we reasoned that it might be effective for CO2/CH4and CO2/N2separations. At 273 K and 1.05 × 105Pa, CMP@1 and CMP@2 exhibit the selectivities of CO2/CH4(4.7 and 4.1) and CO2/N2(32.1 and 30.5) under natural gas conditions via the IAST method (Figs.S10 and S11 (Supporting Information)), respectively, which are lower that of counterpart CMP- COOH@1 and CMP-COOH@2. This result indicates that the amount of carboxylic groups effects selectivity of polymers. These data implys that increasing the amount of carboxylic unit of polymers can improve the adsorption capacity and selectivity of the materials, which suggested the possibility for the surface properties of microporous polymers to be controlled to interact with a specific gas by post-modification.

Fig.4 CO2 adsorption isotherms of CMP-COOH polymers collected at 298 K (a) and (b) 273 K, (c) Isosteric heats of adsorption of the CMP-COOH polymers, (d) CO2 adsorption isotherms of CMP-COOH polymers collected at 318 K at 5 × 106 Pa.

4 Conclusions

In summary, two carboxylic CMPs with relatively high surface area have been synthesized. The clean energy applications of the polymers have also been investigated and it was found that CMP-COOH@1 and CMP-COOH@2 can adsorb 2.17 and 2.63 mg·g−1of carbon dioxide at 1.05 × 105Pa and 273 K, respectively, which can be competitive with the reported results for porous organic polymers under the same conditions. The free carboxylicacid functionalized polymers show that increasing the amount of carboxylic group of polymers can improve the adsorption capacity and selectivity of the materials under the same conditions, which is a promising candidate for the separation and purification of CO2from various CO2/CH4mixtures such as natural gas and land-fill gas by adsorptive processes.

Supporting Information: available free of chargethe internet at http://www.whxb.pku.edu.cn.

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富羧酸基团的共轭微孔聚合物：结构单元对孔隙和气体吸附性能的影响

姚 婵1李国艳1许彦红1,2,*

(1吉林师范大学，环境友好材料制备和应用教育部重点实验室，长春 130103；2吉林师范大学，功能材料物理与化学教育部重点实验室，吉林 四平 136000)

共轭微孔聚合物(CMPs)骨架中的孔和极性基团对聚合物的气体吸附性能起着重要作用。阐明聚合物中极性基团的效果对该领域的进一步发展是必不可少的。为了解决这个根本问题，我们使用最简单的芳香系统-苯作为建筑单体，构筑了两个新颖的富羧酸基团的CMPs (CMP-COOH@1，CMP-COOH@2)，并探讨了CMPs中游离羧酸基团的量对其孔隙、吸附焓、气体吸附和选择性的深远影响。CMP-COOH@1和CMP-COOH@2显示的BET比表面积分别为835和765 m2∙g−1。这两种聚合物在二氧化碳存储方面显示了高潜力。在273 K和1.05 × 105Pa条件下，CMP-COOH@1和CMP-COOH@2的CO2吸附值分别为2.17和2.63 mmol∙g−1。我们的研究结果表明，在相同的条件下增加聚合物中羧基基团的含量可以提高材料对气体的吸附容量和选择性。

共轭微孔聚合物；羧酸；孔；气体吸附；选择性

O647

10.3866/PKU.WHXB201705112

April 6, 2017;

May 3, 2017;

May 11, 2017.

. Email: xuyh@jlnu.edu.cn; Tel: +86-431-81765151.

The project was supported by the National Natural Science Foundation of China (21501065), Science and Technology Program of Jilin Province, China (20160101319JC), Science and Technology Research Program of the Education Department of Jilin Province (2015229), and Science and Technology Program of Siping City (2015057).

国家自然科学基金(21501065)，吉林省科技发展计划(20160101319JC)，吉林省教育厅科学技术研究项目 (2015229)，四平市科技发展计划项目(2015057)资助项目