Macromolecular Research
Article
DOI 10.1007/s13233-017-5138-1
www.springer.com/13233 pISSN 1598-5032 eISSN 2092-7673
Influence of Nitrogen Moieties on CO2 Capture by Polyaminal-Based Porous Carbon Adeela Rehman Soo-Jin Park*
Department of Chemistry, Inha University, 100 Inharo, Incheon 22212, Korea Received March 23, 2017 / Revised June 4, 2017 / Accepted June 7, 2017
Abstract: The escalating level of CO2 in the atmosphere is the chief contributor to global warming and climate change. Existing technologies for post-combustion CO2 scavenging and air separation are inefficient and energy intensive. The cost-effective fabrication of adsorbents with efficient CO2 capture ability is the ultimate goal of the present work. Hence, a melamine-based porous organic polymer (MBPP) was synthesized by single-step condensation of isophthalaldehyde and 2,4,6-triamino-1,3,5-triazine using Schiff base chemistry. Pyrolysis of the as-prepared polymer at 800 oC produced nitrogen-rich porous carbon (NRC), which exhibited greater adsorption potential than the initial polymer. The fabricated materials were characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, thermal-gravimetric analysis, elemental analysis, textural analysis, and CO2 capture measurements. The moderately high surface area 445 m2·g-1 was exhibited by NRC with the CO2 capture of 128.37 mg·g-1 (2.91 mmol·g-1) at 273 K and 1 bar.
Keywords: porous polymers, pyrolysis, nitrogen-rich carbon, CO2 adsorption, CO2/N2 selectivity.
carbon energy sources. Additionally, many novel techniques have been developed globally for CO2 capture, sequestration, and utilization.3 Various techniques that have been introduced for CO2 capture in recent years include adsorption, physical and chemical absorption, membrane processing, and cryogenic distillation.4 Among the existing methods, post-combustion CO2 capture is one of the most widely applied technique.5 However, it is necessary to introduce some preventive measures to offset the dangers of global warming. Recently, porous polymers have gained scientific and technological interest owing to their valuable structural and functional attributes, along with the characteristics of both the porous materials and polymers. Not surprisingly, apart from the traditional applications, microporous organic polymers (MOPs) have grasped the attention of researchers owing to the wide range of non-traditional applications, including carbon dioxide capture, hydrogen storage, adsorption of volatile organic compounds, heterogeneous catalysis, and chemical sensing.6-8 In comparison with inorganic microporous materials for instance zeolites,9,10 as well as metal-organic frameworks (MOFs);11,12 characterized by a high specific surface area, microporous organic polymers lack metal elements and thus have a low skeleton density and exhibit excellent physicochemical stability. Moreover, a vast number of strategies have been developed for the synthesis of organic polymers that induce functionalities and flexibility in the molecular design of synthesized materials. Recently, a large number of microporous polymers with amorphous and ordered structures, have been reported. Most of them were synthesized from mono-
1. Introduction As the global energy crisis continues to escalate, the use of natural gas has become economically attractive. Conversely, the combustion of natural gas and fossil fuels to meet the world’s energy demands has greatly increased atmospheric greenhouse gas emission. Anthropogenic emissions lead to a continuous rise in the level of greenhouse gases, particularly carbon dioxide; the emission of carbon dioxide has surged by 30% over the past two centuries.1 The primary sources of CO2 are fossil fuel combustion and certain chemical industries.2 This alarming condition, if uncontrolled, can lead to environmental and economic disasters. According to recent data, the level of CO2 increased to 397 ppm in 2015. Furthermore, the energy demand has grown tremendously and is anticipated to increase by 53% by 2030. However, to meet the energy demands of society, fossil fuels are expected to persist as the primary source of energy, in contrast with other sources that are considered as environmentally-safe, such as fuels from biomass, solar, and nuclear energy. However, to improve the state of the environment, it is essential to curb greenhouse gas emissions. One of the method to reduce CO2 emissions is by using the lowAcknowledgments: This research was supported by The Leading Human Resource Training Program of the Regional Neo Industry through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT, and Future Planning (grant number: NRF2016H1D5A1909732). *Corresponding Author: Soo-Jin Park (
[email protected]) Macromol. Res.
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Macromolecular Research fabrication of nitrogen-based porous carbon by pyrolysis of nitrogen-containing polymers via direct synthesis has seldom been reported.27 Comparison of both methods reveals that the post-synthetic method is more facile with the generation of stable materials. Few examples reported in literature include the synthesis of porous nitrogen-enriched carbons from melamineformaldehyde and urea-formaldehyde resins; with the CO2 capture of 99 mg·g-1 at 25 oC, 1 atm.28 Recently, another nitrogen-containing porous carbon monolith designed by the carbonization of porous polymer with a CO2 capture of 138 mg·g-1 at STP. These studies reveal that both the porosity and the basic nitrogen moieties present in the carbon-based sorbents are responsible for adsorption. Herein, we report the fabrication of nitrogen-rich carbon via physical method, i.e., pyrolysis of a precursor polymer synthesized by condensation of isophthalaldehyde and melamine monomers. The polymer network contains basic nitrogen species in the backbone and exhibits microporosity. The carbon material produced by pyrolysis shows hierarchical micro-mesoporosity with a high percentage of nitrogen. The effectiveness of the designed material as a CO2 adsorbent is determined in terms of its thermal stability, adsorption ability, and CO2/N2 selectivity.
mers with multifunctional aromatic compounds having special geometrical conformations, such as monomers based on tetraphenyladamantane,13,14 tetraphenylmethane,15 tetraphenylsilane,16 triphenylamine,17 and triphenylbenzene.18 Despite the advantages, one of the main problems is purification of the synthesized polymers, which is time consuming and limits the large-scale production of microporous organic polymers. Thus, the development of a facile method of preparing MOPs using low-cost and commercially available raw materials is essential for actual industrial applications. Hypercross-linked microporous polyaminal networks are easily fabricated by the condensation of aromatic multialdehydes with trifunctional melamine.19,20 In this reaction, the imine bond (-C=N-) appears as an intermediate and reacts further with the active amino group to form a stable aminal linkage (-NH-CH-NH-). Additionally, the synthesis based on the melamine precursor is economically favorable, as melamine is a cheap material that is widely used in industry. According to the literature, adsorbents with amine groups, such as porous silica,21 metal-organic frameworks (MOFs),22 and porous polymers, exhibit excellent CO2 adsorption capacity and also exhibit selectivity for CO2 over different gases. This is ascribed to the strong interactions present between the pore walls and CO2 molecules. Studies on microporous polyaminals reveal that the presence of a large amount of secondary amines and N-hetero functionalities in the framework also plays a vital role in improving the CO2 capture capacity, thus making polyaminals one of the emerging candidates for mitigating global warming. Porous carbon materials with nitrogen functionalities have also been explored for CO2 capture.23,24 One-step direct synthesis and post-synthetic techniques are the methods of introducing nitrogen functionalities into the carbon framework. Impregnation, grafting, or ammoxidation are the various techniques used in the post-synthetic method. However, nitrogenenriched porous carbons fabricated by the impregnation and ammoxidation exhibit a decline in the CO2 adsorption capacity as compared to the original materials.25 One research group reported the successful synthesis of amine-rich carbon materials through grafting; however, the material exhibited very low CO2 uptake (>50 mg·g-1 at 25 oC, 1 atm).26 On the other hand,
2. Experimental 2.1. Materials Melamine (99%), isophthalaldehyde (99%), dimethylsulfoxide (DMSO>99.5%), anhydrous tetrahydrofuran, dichloromethane, and acetone were procured from Sigma-Aldrich and used without further purification. 2.2. Synthetic protocol for melamine-based porous polymer (MBPP) The polymeric framework was synthesized through Schiff base reaction (Scheme 1).29 In a pre-baked two-neck round-bottom flask, isophthalaldehyde (1.61 g, 12.0 mmol) and melamine (1.01 g, 8.0 mmol) were mixed in anhydrous DMSO (100 mL). The solution was mechanically stirred at 453 K under an inert gas.
Scheme 1. Synthetic route for MBPP and NRC. © The Polymer Society of Korea and Springer 2017
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Macromolecular Research After 72 h, the resultant mixture was allowed to cool up to room temperature. Consequently, the product was separated as an off-white precipitate by filtration and washed with acetone, tetrahydrofuran, and dichloromethane several times and subsequently dried at 353 K for 24 h under vacuum. The product was obtained as an off-white powder in 63% yield. The fabricated polymer was labeled as MBPP. 2.3. Synthesis of nitrogen-rich porous carbon (NRC) Pyrolysis of the as-prepared precursor was performed to produce the porous carbon material. The polymer was loaded into a ceramic boat and inserted into the furnace. Subsequently, pyrolysis was conducted under the optimum conditions of 800 oC for 1 h with a heating rate of 3 oC·min-1. The flow of dry argon was allowed to continue at a rate of 20 mL·min-1. The obtained char was cooled and employed for measurements, without any further modifications.
Figure 1. FTIR spectra of MBPP and NRC.
the polymeric network. The bands assigned to the unreacted precursors, including the NH2 stretching bands at 3470 and 3420 cm-1, NH2 deformation of the primary amine at 1650 cm-1, and the C-H and C-O stretching of the carbonyl group of isophthalaldehyde at 2720 and 1700 cm-1, respectively, were attenuated in the spectrum. This is an indication of successful fabrication of the polyaminal network. No intense band appeared at 1600 cm-1, revealing the absence of the C-N stretch in the FT-IR spectrum of MBPP. The spectrum of NRC indicates the successful transformation of the polymer into a carbon material. Overall, the FTIR spectra illustrate the formation of the melamine-based polymer (MBPP) with a high degree of polymerization, as well as the transformation of MBPP to NRC. X-ray photoelectron spectroscopy (XPS) is a technique commonly employed to explore the surface of analytes. Herein, the survey scan shows the presence of three kinds of atoms, namely, carbon, nitrogen, and oxygen. Comparison of the spectra of both the samples (Figure 2) shows a single peak in the core level scan of the N 1s region for MBPP, attributed to one type of nitrogen moiety in the framework. While for NRC, confirmed the presence of three peaks at 398.4, 400.8, and 402.7 eV. According to the literature, these peaks can be assigned to three distinct types of nitrogen functionalities, including alkyl nitrites, pyrrole, and pyridine nitrogen. Upon pyrolysis, nitrogen present in the polymer can generate pyrrole- and pyridinetype nitrogens that remain embedded in the layers of carbon. These distinct types of nitrogen possess Lewis basic character, and serve as active sites for scavenging carbon dioxide. Wide-angle X-ray diffraction (Figure 3) shows the amorphous nature of the synthesized polyaminal network, owing to the polymerization mechanism, which is considered as kinetically controlled. The absence of any sharp signals indicates the presence of random frameworks. Correspondingly, the XRD pattern of nitrogen-rich carbon (NRC) exhibits a shift in the peak towards higher angle. This change is quite obvious from the view point of carbonization. When MBPP is carbonized at high temperature, the condensation of three dimensional polymer takes place with the reduction in interplanar spacing (d
2.4. Characterization methods The synthesized material was studied by different characterization techniques. The FTIR spectrum was obtained in the range of 4000-400 cm-1 by Fourier transform-infrared vacuum VERTEX 80V spectrometer. The diffraction pattern of the samples was recorded on a D2 PHASER, BRUKER, X-ray diffractometer. Measurements were taken in the 2 range of 2o to 80o. The surface morphology of the samples was evaluated using field emission scanning electron microscopy (FESEM; Model SU8010, Hitachi Co., Ltd.). Elemental analysis was done by an EA1112 element analyzer. X-ray photoelectron spectroscopy (XPS, VG Scientific Co., ESCA LAB MK-II) was used for surface analysis. The thermal stability of the fabricated polymer was determined by thermal gravimetric analysis (TGA; TG209F3) using 2-5 mg of sample that was heated to 800 oC at a rate of 10 oC· min-1 under nitrogen atmosphere. The textural properties of the materials were explored using N2 adsorption-desorption isotherms obtained with a Model Belsorp Max instrument (BEL Japan, Inc.). The carbon dioxide capture capacity was determined at 273 and 298 K by a Model Belsorp Max instrument (BEL Japan, Inc.). To determine the CO2/N2 selectivity, the adsorption isotherms for both gases were obtained at 298 K using a Model Belsorp Max instrument (BEL Japan, Inc.). Before the adsorption measurements, the samples were degassed by heating for 120 oC under vacuum for 6 h.
3. Results and discussion 3.1. Morphological and structural analyses The successful fabrication of a three-dimensional polymer network along with complete transformation of the functional groups primarily existed in the monomers, was investigated by FTIR spectroscopy. In the FTIR spectrum of MBPP (Figure 1), the absorption peaks at 1550 cm-1 and the prominent semicircular stretching at 1476 cm-1, attributed to the triazine ring, clearly indicate the successful incorporation of melamine into Macromol. Res.
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Macromolecular Research
Figure 4. FESEM images of (a) MBPP and (b) NRC.
increases. Along with the peak shift, it can be seen that peak shown by NRC is intense suggesting higher density of micropores. Moreover, the graphitic nature of NRC can be confirmed by the diffraction of X-rays from (002) and (100) planes.30 The morphology of the materials was studied microscopically by field-emission scanning electron microscopy (FESEM) (Figure 4). The FESEM micrographs are in accordance with the XRD results. The studies show that the polymer sample is in the form of small particles with irregular shapes and no ordered arrangement. NRC also possesses a distorted structure with an irregular arrangement. Moreover, MBPP was insoluble in most of the organic solvents like chloroform, ethanol, N,N-dimethylformamide, dimethyl sulfoxide, and tetrahydrofuran. This is attributed to the high extent of polymerization and crosslinking. The percentage of each elemental component was determined by elemental analyses. The analyses indicated that MBPP contained 40.6 wt% carbon, 35.3 wt% nitrogen, 7.8 wt% sulfur, and 4.5 wt% hydrogen. The origin of the sulfur in the polymer sample is attributed to the DMSO used as the solvent for polymerization. Moreover, the high content of sulfur in MBPP is due to the porous nature of the polymer and its good storage ability. Furthermore, upon pyrolysis, the nitrogen and hydrogen content in NRC decreased to 26.8 and 0.6 wt%, respectively, with an increase in carbon content upto 61.10%. One of the consequences of pyrolysis is loss of the heteroatoms from the porous carbon.31 Similar to MBPP, NRC also contained a small percentage of sulfur. This is probably due to surface reactions between the polymer and DMSO at elevated temperature. This is advantageous for adsorption of CO2 because
Figure 2. XPS spectra of (a) N 1s of MBPP and (b) N 1s of NRC.
Figure 3. X-ray diffraction pattern of (a) MBPP and (b) NRC.
values). According to the Bragg’s equation: 2dsin = n
(1)
When the inter-layer spacing (d value) decreases, value of © The Polymer Society of Korea and Springer 2017
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Figure 5. TGA thermogram of MBPP under N2 atmosphere.
Figure 6. N2 adsorption-desorption isotherm of (a) MBPP and (b) NRC measured at 77 K (inset: pore size distribution curve).
sulfur doping can increase the interaction of gas molecules with the surface of the sorbent.32 Therefore, a higher nitrogen content, as well as the presence of the sulfur moiety, in NRC can enhance the adsorption capacity.
Table 1. Textural properties and elemental composition of the adsorbents prepared Samples
3.2. Thermogravimetric analysis
MBPP
545
0.81
0.34
1.35
35.34
40.69
NRC
445
0.51
0.28
1.08
26.87
61.10
a
Surface area calculated by BET method. bTotal pore volume determined at P/P0=0.98. cMicropore volume. dMaxima of the pore size distribution determined by the NLDFT method. eNitrogen content. fCarbon contents measured by the elemental analysis.
The thermal stability of the MBPP was determined by thermogravimetric analysis. The analysis was carried out under N2 atmosphere (Figure 5). Evaluation of the thermal stability of the synthesized material as a candidate for CO2 scavenging is critical for commercial applications. As shown in the thermogram, there was an initial weight loss at 155 oC, which is attributed to desorption of the entrapped solvent molecules from the sample pores. The figure shows a 5% weight loss around 180 oC. Upon further heating, the maximum percentage weight loss occurred at 400 oC, which is ascribed to decomposition of the polymer.33 The thermal studies indicates high stability of the fabricated material up to 400 oC, which is comparable to that of other reported polymers.34
observed for MBPP. This reflects the presence of interparticulate voids generated by the less compact packing of small particles. Contrary to this, previously reported microporous polymers exhibited very prominent hysteresis in the adsorption-desorption profiles. This is due to the softness of segments of the polymer and distortion of the pore structure during the measurements in liquid nitrogen.7,8 Nevertheless, a reversible adsorption-desorption isotherm was observed for the prepared material, suggesting that the network fabricated through aminal linkages is sufficiently rigid.36 Table 1 presents the data derived from the porosity measurement. The specific surface area of MBPP is 545 m2·g-1, with an average pore volume of 0.81 cm3·g-1 and micropore volume of 0.34 cm3·g-1. Furthermore, upon pyrolysis the capacity for nitrogen adsorption decreased markedly and some of the porosity parameters declined, including the specific surface area and pore volume, according to the trend previously reported.33 The nitrogen adsorption data for the nitrogen-rich porous carbon (NRC) showed a specific surface area of 445 m2·g-1, average pore volume of 0.51 cm3·g-1, and micropore volume 0.28 cm3·g-1. The micropore volume was determined by the Dubinin-Radushkevich (D-R) equation:37
3.3. Textural characterization Nitrogen sorption isotherms can provide information regarding the textural properties of the synthesized material, including the porosity and certain significant parameters, such as the surface area, pore size, and pore volume. The porous nature of the fabricated materials was explored after activating the samples at 120 oC for 6 h to eliminate any moisture and adsorbed gases before acquiring the isotherm. A steep rise corresponding to uptake of nitrogen was observed at very low relative pressure (P/P0
