CH4 Selectivity of Novel

Dec 17, 2018 - †School of Chemistry and Chemical Engineering, ‡The Key ... Results showed that CPDA@A-Cs exhibit a high Brunauer–Emmett–Teller...
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Cite This: Energy Fuels 2019, 33, 493−502

Enhanced CO2 Adsorption and CO2/N2/CH4 Selectivity of Novel Carbon Composites CPDA@A-Cs Wanwen Liang,† Zewei Liu,† Junjie Peng,‡ Xin Zhou,*,‡ Xun Wang,‡ and Zhong Li*,†,§ †

School of Chemistry and Chemical Engineering, ‡The Key Laboratory of Enhanced Heat Transfer and Energy Conversation Ministry of Education, and §State Key Lab of Subtropical Building Science of China, South China University of Technology, Guangzhou 510640, P. R. China

Energy Fuels 2019.33:493-502. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/17/19. For personal use only.

S Supporting Information *

ABSTRACT: New carbon composites (CPDA@A-Cs) were successfully prepared by carbonizing and activating the polydopamine (CPDA) and asphalt-based carbons (A-Cs) for CO2 capture and separation. The resulting CPDA@A-Cs were characterized, and the CO2, N2, and CH4 adsorption separation performances of CPDA@A-Cs were investigated systematically. Results showed that CPDA@A-Cs exhibit a high Brunauer−Emmett−Teller specific surface area of 2031 m2/g and a high total pore volume of 0.81 cm3/g, respectively. Boehm titration showed that the introduction of CPDA made the basic site concentration of CPDA@A-Cs increase in comparison with that of the parent A-C, and X-ray photoelectron spectroscopy analysis indicated that the N-containing groups mainly consisted of pyridinic N and pyridonic N. At the ambient pressure, the CO2 uptakes of CPDA@A-Cs amounted up to 6.89 mmol/g at 273 K and 4.05 mmol/g at 298 K, increasing by 34% compared with the parent A-C, and much higher than that of the most reported carbonaceous materials under the same adsorption conditions. Meanwhile, the CO2/N2 and CO2/CH4 adsorptive separation selectivities were significantly enhanced. For the CO2/N2 (0.15/0.85) mixture, its ideal adsorbed solution theory (IAST)-predicted selectivity at normal pressure and temperature was found to be 25.1, whereas for the CO2/CH4 (0.5/0.5) mixture, its IAST-predicted selectivity under the same conditions was calculated to be 5.1. Fixed-bed experiments showed that the CO2/N2 mixture and CO2/CH4 mixture can be well separated at room temperature. Density functional theory calculations revealed that surface pyridinic N and pyridonic N of the composites make a significant contribution to the enhanced CO2 capture capacity and CO2/N2 or CO2/CH4 selectivity.

1. INTRODUCTION Carbon dioxide (CO2), an anthropogenically emitted gas, is a primary greenhouse gas that leads to global warming.1 This warming trend has given rise to a series of ocean and ecosystem problems in the Arctic, such as above-average ocean temperature and loss of sea ice.2 Combustion of fossil fuels such as coal, oil, and natural gas is a crucial anthropogenic factor for CO2 emission. Therefore, development of affordable technologies for the reduction of greenhouse gas emissions, particularly CO2, is not only important for solving the environmental problems, but also significant for promoting sustainable economic development. Capture and sequestration technologies are indispensable for selectively capturing CO2 from emission sources such as the flue gas from power plants, which is composed of ∼85% N2 and 10−15% CO2 at a total pressure of ∼1 bar.3 In addition, natural gas and biogas, which are composed of 55−70% CH4, 30−45% CO2, and other impurities, are considered as an alternative and clean energy resource to replace traditional fossil fuels like petroleum and coal.4 Methane (CH4) is an efficient component of natural gas or biogas. However, the existence of unacceptable levels of CO2 (over 40%) will lower the calorific value of natural gas or biogas and corrode pipelines of facilities; therefore, separation of CO2 from CH4 can favor the production and application of natural gas. Currently, the most highly developed technology for CO2 capture is chemisorption by aqueous alkanolamine solutions.5,6 This technology suffers from pipeline corrosion issues and the © 2018 American Chemical Society

relatively high cost of regeneration. Therefore, the energyefficient alternative in relation to physisorption by solid porous materials has attracted significant attention recently. The solid porous materials with high working capacities and a low regeneration cost are promising in reducing the energetic and economic costs of CO2 capture. For this purpose, different kinds of adsorbents, such as metal organic frameworks (MOFs),7,8 porous polymers,9 activated carbon,10 zeolites,11 and covalent organic frameworks,12 have been investigated for CO2 capture. Recently, some novel carbonaceous materials with tunable porosity and excellent stability were well developed because of their excellent stability and CO2 adsorption performance. Researchers used a variety of carbon sources, such as porous aromatic frameworks, porous organic polymers (POPs), and MOFs,13 to prepare different kinds of porous carbons with excellent stability and decent adsorption and separation properties for CO2 capture. Xian et al.14 used polydopamine (PDA) to synthesize a new porous carbon adsorbent CPDA-3 with a high Brunauer−Emmett−Teller (BET) surface area of 2351 m2/g and a high CO2 adsorption capacity of 5.8 mmol/g under ambient conditions. Lee et al.15 prepared several chemically activated covalent triazine frameworks, a subclass of POPs, as CO2 adsorbents with the adsorption capacity Received: October 16, 2018 Revised: November 30, 2018 Published: December 17, 2018 493

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Figure 1. SEM images of (a) A-C and (b) 50CPDA@A-C.

collected to be 3.55 mmol/g at 298 K and 100 kPa. Zhu et al.9 prepared porous cross-linked polymers containing the relatively more pendant triazole groups PTz, and the CO2 uptake of these PTz materials reached 3.6 mmol/g at normal temperature and pressure. In addition, their CO2/N2 selectivity reached as high as 30. These previous works indicate that the carbonaceous materials with excellent thermal and chemical stability also possessed excellent CO2 capture property, which was perfectly comparable to that of MOFs. In addition, porous carbon materials are hydrophobic.16 However, the carbon sources or the carbon precursors mentioned above are relatively expensive. Therefore, it is necessary to search for cheap and abundant carbon sources, such as biomass and industrial byproducts, to develop new carbonaceous materials with decent CO2 capture property. Liu et al.17 used glucose to prepare an ultramicropore carbon adsorbent, and its CO2 uptake reached 3.82 mmol/g at normal temperature and pressure. Singh et al.18 used the mixture of Arundo donax and chitosan to synthesize a variety of porous biocarbons with nitrogen functionalities, and the CO2 adsorption capacity of these samples reached 2.1 mmol/g at 298 K and 100 kPa. Asphalt is a byproduct from crude oil refinery. It generally contains substances of naphthene aromatics and polar aromatics, and is readily available. Jalilov et al.19 used asphalt to synthesize porous carbon uGil-900 with a superhigh CO2 uptake of 35 mmol/g at 54 bar; however, its CO2 uptake at 1 bar was only about 2.5 mmol/g at 298 K. As CO2 is usually present in exhaust gas and some important commodities at low or trace concentrations, developing new carbon adsorbents with a high CO2 adsorption capacity at normal or low pressure is critical.20 The purpose of this work was to prepare new composites CPDA@A-Cs for efficient adsorption separation toward CO2/ N2 and CO2/CH4 mixtures. In this work, an asphalt-based carbon material (A-C) was prepared, followed by modifying its surface with the nitrogen-containing chemical polydopamine. The resultant CPDA@A-Cs were characterized. Equilibrium CO2, N2, and CH4 adsorption isotherms on CPDA@A-Cs were determined separately at 273, 288, and 298 K. The adsorption selectivities of CPDA@A-Cs for CO2/N2 and CO2/ CH4 were calculated separately on the basis of the ideal adsorbed solution theory (IAST) and difference of the isosteric heats (DIH). The dynamic separation behaviors of CPDA@ACs for CO2/N2 and CO2/CH4 mixtures were evaluated by means of breakthrough experiments. The mechanism of enhanced adsorption and selectivity toward CO2 of the samples was revealed by using molecule simulation. The

effects of the pore texture and surface chemistry properties on CO2 capture are discussed and reported herein.

2. EXPERIMENTAL SECTION 2.1. Preparation of CPDA@A-Cs. The detailed polydopamine (PDA) modification and preparation procedure can be found in our previous work21 (see Section S1, Supporting Information). In brief, the asphalt was carbonized at 673 K for 3 h under the protection of N2 atmosphere and then the polydopamine modification was performed at room temperature. The modified composites were activated under conditions of 973 K and 1 h. The resulting composites modified with polydopamine using different concentrations of dopamine hydrochloride are separately marked as 25CPDA@A-C, 50CPDA@A-C, and 75CPDA@A-C. The detailed preparation process of the parent unmodified asphaltbased carbon sample labeled as A-C can be found in our previous work.22 In brief, A-C was also activated under conditions of 973 K and 1 h, similar to CPDA@A-Cs. 2.2. Adsorbent Characterizations. The resultant carbonaceous sample textural properties were obtained by N2 adsorption/ desorption measurements (Micromeritics ASAP 2020 analyzer at 77 K). The specific surface area was computed according to the standard BET method. The total pore volume was obtained from the amount of N2 adsorbed at P/P0 = 0.990. The pore size distribution data were determined based on the density functional theory (DFT) method. Scanning electron microscopy (SEM) was carried out to observe the morphology of different samples on a Hitachi SU8200. X-ray photoelectron spectroscopy (XPS) measurements were performed to determine the contents and species of nitrogen on the different samples using Thermo-VG Scientific ESCALAB 250Xi. Boehm titration was carried out to examine the concentrations of basic sites on different samples on a Metrohm 702 Titrino titrator in the pH range from 2.0 to 11.0. Prior to the experiments, each sample was degassed at 423 K for at least 6 h. 2.3. Measurement of CO2, CH4, and N2 Adsorption, and Separation Performance. Equilibrium adsorption capacities of CO2, N2, and CH4 on the resultant carbonaceous samples at 273, 288, and 298 K were determined through the standard static volumetric methodology on a Micromeritics 3Flex Analyzer. Breakthrough experiments were performed on a home-built setup as shown in Figure S3. Compositions of the feed stream were designed as follows: CO2/N2 = 0.15/0.85 and CO2/CH4 = 0.50/0.50 at ambient temperature and pressure, which were similar to the composition of flue gas and raw natural gas, respectively. Detailed operation steps of the breakthrough experiment are shown in Section S10, Supporting Information. Prior to the experiments, each sample was degassed at 423 K for at least 6 h.

3. RESULTS AND DISCUSSION 3.1. Characterization of CPDA@A-Cs and A-C. The morphologies of A-C and 50CPDA@A-C were observed by means of SEM. As shown in Figure 1, both A-C and 50CPDA@A-C exhibited irregular morphologies and a rough 494

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In this work, the concentrations of basic sites in CPDA@ACs and the parent A-C were measured through the Boehm titration, and the results are listed in Table 1. It is clearly seen that the concentrations of basic sites on CPDA@A-Cs were higher than that on the parent A-C. Both the weight-basis and the surface area-basis basic sites’ concentrations followed the order of 75CPDA@A-C > 50CPDA@A-C > 25CPDA@A-C, which was consistent with the order of CPDA loading. 3.2. CO2, CH4, and N2 Adsorption Isotherms on CPDA@A-Cs and A-C. Figure 4 presents the CO2 adsorption isotherms separately based on the weight and surface area of CPDA@A-Cs and A-C at 298 K. It is noticed that the samples CPDA@A-Cs exhibited a CO2 adsorption capacity in the range from 3.61 to 4.05 mmol/g, which was significantly higher than the 3.05 mmol/g of the parent A-C material at 298 K and 100 kPa, implying that the CO2 adsorption capacity can be effectively improved by the introduction of CPDA in CPDA@ A-Cs. Among all CPDA@A-Cs, 50CPDA@A-C presented the highest CO2 uptakes of 4.05 and 6.89 mmol/g at 298 and 273 K, respectively, being much higher than most of the reported carbonaceous materials20,26−31 for CO2 capture as shown in Table S3. More importantly, CPDA@A-Cs in this work presented a decent CO2 adsorption capacity, which was comparable to those of many reported MOFs32−34 (Table S3). Figure 4a shows that the equilibrium CO2 adsorption capacities of CPDA@A-Cs and A-C were in the arrangement of 50CPDA@A-C > 25CPDA@A-C >75CPDA@A-C > A-C. This is contradictory to the arrangement of the specific surface area or total pore volume of the samples. However, when Figure 4a was converted into Figure 4b where the equilibrium CO2 adsorption capacities per surface area were plotted, it is clearly shown that the equilibrium CO2 adsorption capacities per surface area of CPDA@A-Cs and A-C followed the arrangement of 75CPDA@A-C > 50CPDA@A-C > 25CPDA@A-C > A-C, which is completely consistent with the arrangement of basic sites’ concentrations based on the specific surface area as shown in Table 1. It suggests that the basic sites played a dominant role in CO2 adsorption under certain circumstances since the CO2 (acid gas) can strongly interact with the basic sites on CPDA@A-Cs in this work.14,35 Also, it should be pointed out that the gas uptake at low pressure may also be partially attributed to the micropore volume and specific surface area.17,36 The CO2, CH4, and N2 uptakes of 50CPDA@A-C at different temperatures were collected from their adsorption isotherms as shown in Figure 5. From Figure 5a, it is visible that the equilibrium CO2 uptakes of 50CPDA@A-C were much higher than the corresponding equilibrium CH4 and N2 uptakes, suggesting that the interaction between 50CPDA@AC and CO2 is stronger than that between 50CPDA@A-C and CH4 or N2. From Figure 5a−c, it is noticeable that the equilibrium CO2, CH4, and N2 uptakes trended downward, with the adsorption temperature increasing, because of the physical adsorption effect between 50CPDA@A-C and these three types of gas molecules. To analyze the interaction strength between these three types of gas molecules and the surface of 50CPDA@A-C, the isosteric heat of adsorption (ΔHs) was determined based on the adsorption isotherm data at 273, 288, and 298 K by employing the Clausius−Clapeyron equation37 (see Section S5, Supporting Information). Figure 6 presents the ΔHs of CO2, CH4, and N2 adsorption on 50CPDA@A-C at different gas loadings. ΔHs showed a descending trend with an increase

surface with sharp edges and cavities, resulting from the KOH etching during the activation process at high temperature. However, 50CPDA@A-C showed smaller cavities compared with A-C. Figure S1 displays the N2 adsorption−desorption isotherms of the CPDA@A-Cs at 77 K (see Section S2, Supporting Information). For all CPDA@A-Cs, the isotherms exhibited type I curves, with N2 adsorption amounts increasing rapidly at a very low relative pressure, which is the characteristic of micropores’ existence. In addition, there were no hysteresis loops on isotherms in the whole P/P0 range, suggesting the lack of mesoporosity in the CPDA@A-Cs. The N2 adsorption amounts of the samples decreased with amounts of doped CPDA increasing. Figure 2 shows the pore size distribution

Figure 2. Pore size distributions of CPDA@A-Cs.

curves of different CPDA@A-Cs samples. Their pore sizes were distributed in the range from 0.6 to 2.0 nm with several distinctive peaks. The textual parameters of CPDA@A-Cs are listed in Table S1 (see Section S2, Supporting Information). The specific surface area (SBET) and total pore volume (Vtotal) of these CPDA@A-Cs decreased with the amounts of doped CPDA increasing, ranging from 2031 to 1539 m2/g and from 0.81 to 0.66 cm3/g, respectively. Table S1 also lists the surface elemental contents on different composites CPDA@A-Cs as determined by XPS analysis. It can be observed that the CPDA@A-Cs contained much more oxygen and nitrogen than the parent A-C.22 In addition, both the nitrogen and the oxygen contents increased with the increase in CPDA loading, suggesting that the nitrogen and oxygen elements had been effectively incorporated into these CPDA@A-Cs composites. High-resolution XPS was employed to further identify the species and amounts of N-containing groups on the surface of CPDA@A-Cs composites, and the results are shown in Figure 3. The N 1s spectra of CPDA@A-Cs composites can be deconvoluted into two peaks. The dominant peak at 397.7 (±0.1) eV is assigned to pyridinic nitrogen, and the peak at 400.0 (±0.1) eV is ascribed to pyridonic nitrogen or pyrrolic nitrogen or both of them.23,24 However, since the pyridonic nitrogen is more stable than pyrrolic nitrogen at high temperature, pyridonic nitrogen is more likely to be present in CPDA@A-Cs composites in this work.24,25 The quantitative analysis indicated that the amount of pyridinic nitrogen was higher than that of pyridonic nitrogen in CPDA@A-Cs as shown in Table S2 (see Section S3, Supporting Information). 495

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Figure 3. N 1s XPS spectra of CPDA@A-Cs.

Table 1. Concentrations of Basic Sites on CPDA@A-Cs and the Parent A-C samples

weight-basis conc. of basic sites (mmol/g)

surface area-basis conc. of basic sites (μmol/m2)

A-C 25CPDA@A-C 50CPDA@A-C 75CPDA@A-C

0.27 0.46 0.57 0.61

0.106 0.226 0.310 0.396

For predicting the adsorptive separation properties of 50CPDA@A-C and A-C toward flue gas (CO2/N2 = 0.15/ 0.85) and raw biomethane (CO2/CH4 = 0.5/0.5), the adsorption selectivities of 50CPDA@A-C and A-C for these two types of binary mixtures were computed through the ideal adsorbed solution theory (IAST) methodology38 (see Section S6, Supporting Information). IAST is widely employed to predict the adsorptive separation selectivity on porous carbon materials toward binary gas mixtures14,16 and other adsorbents such as MOFs.39,40 In addition, the parameters required by IAST calculation are obtained from fitting the experimental pure component isotherms; therefore, a precise model is needed to fit these isotherms. Here, the dual-site Langmuir− Freundlich (DSLF) model (see Section S6, Supporting Information) is applied to fit the CO2, CH4, and N2 adsorption isotherms on 50CPDA@A-C and A-C. The fitting parameters and correlation coefficients (R2) are listed in Table S4 (see Section S7, Supporting Information). The values of R2 were up

in the amounts of gases adsorbed, indicative of the surface energetic heterogeneity on 50CPDA@A-C. Meanwhile, the ΔHs of CO2 adsorption was remarkably higher than that of the other two gas molecules’ adsorption (CH4 and N2), which implies that the interaction between CO2 and 50CPDA@A-C was stronger than that between the other two gases and 50CPDA@A-C. The ΔHs of CO2 adsorption was in the range of 23.0−25.5 kJ/mol, significantly lower than that of chemisorption (60−90 kJ/mol), suggesting that the CO2 adsorption on 50CPDA@A-C was purely physical in nature. This moderate isosteric heat indicated that low energy consumption will be required for adsorbent regeneration and hence the CO2 capture process can be energy efficient and cost saving. 3.3. Adsorption Selectivity of CO2/N2 and CO2/CH4 Binary Mixtures on CPDA@A-Cs. The adsorption selectivity is an important criterion to estimate the separation efficiency of an adsorbent in a practically adsorptive separation process. 496

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Figure 4. CO2 adsorption isotherms of CPDA@A-Cs and A-C at 298 K based on (a) unit mass and (b) unit surface area.

Figure 5. (a) CO2/CH4/N2 adsorption isotherms of 50CPDA@A-C at 298 K and (b) CO2, (c) CH4, and (d) N2 adsorption isotherms of 50CPDA@A-C at 273, 288, and 298 K.

to 0.9990, implying that the CO2, N2, and CH4 adsorption isotherms on 50CPDA@A-C and A-C are fitted precisely by the DSLF model. Figure 7 presents the IAST-predicted selectivity of 50CPDA@A-C and A-C toward CO2/N2 and CO2/CH4

binary mixtures at 298 K. It can be seen that all selectivities gradually leveled off as the bulk pressure increased from 5 to 100 kPa. Both the CO2/N2 and CO2/CH4 selectivities on 50CPDA@A-C were much higher than that on the parent A-C material. Meanwhile, the CO2/N2 and CO2/CH4 selectivities 497

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idonic nitrogen) were calculated. The density functional theory (DFT) was applied to the calculation with the DMOL3 module of Materials Studio 2017 using GGA/PW91function, the DNP basis set, fine quality, and DFT-D3 correction. In the simulation, three types of binding sites (C-center/N1/N2) were considered, which is shown in Figure S2 as optimized adsorbate−adsorbent configurations of interaction sites on CPDA@A-Cs interacted with CO2/CH4/N2 molecules (see Section S9, Supporting Information). Figure 9 presents the calculated results of the binding energy (BE) between the adsorbates (CO2/CH4/N2) and different adsorption sites. It is obvious that the binding energy between CO2 molecules and pyridinic nitrogen/pyridonic nitrogen was lower than that between CO2 molecules and the C-center, implying that the presence of surface N-containing groups would enhance the interaction between CO2 molecules and the surfaces of samples. At the same time, it was noticed that the binding energy of N2/CH4 molecules with pyridinic nitrogen/ pyridonic nitrogen was higher than that with the C-center, indicating that the existence of surface N-containing groups would weaken the interaction of N2/CH4 molecules with the surfaces of the samples. To compare clearly the difference of binding energies between these adsorbates and the adsorptive sites, the absolute difference of binding energies (ΔBE) was defined as follows: ΔBE = |BEgas1| − |BEgas2|. Figure 9b shows the ΔBE between CO2 and CH4 or CO2 and N2 on three interaction sites. It can be seen that for the pair of adsorbates CO2/CH4 or CO2/N2, their ΔBE followed the order of ΔBEN1 > ΔBEN2 > ΔBEC‑center. A high value of ΔBE meant that the corresponding adsorption site possessed a high adsorption selectivity for CO2/CH4 or CO2/N2 mixtures. Therefore, the data above indicated that the nitrogen content, especially the pyridinic nitrogen (N1), is the dominant factor to enhance the adsorption selectivity of CPDA@A-Cs for CO2/N2 and CO2/ CH4. As a consequence of this, the introduction of more Ncontaining groups such as pyridinic nitrogen and pyridonic nitrogen would improve the CO2 adsorption capacity and CO2 adsorption separation selectivity. 3.5. Breakthrough Curves of CO2/N2 and CO2/CH4 Binary Mixtures on CPDA@A-Cs. In this work, the dynamic separation performances of 50CPDA@A-C and A-C toward

Figure 6. Comparison of the isosteric heats of CO2, CH4, and N2 adsorption on 50CPDA@A-C.

on 50CPDA@A-C were as high as 25.1 and 5.1, respectively, at 298 K and 100 kPa, indicating the potential of porous carbon 50CPDA@A-C in the selective adsorption of CO2. The difference of the isosteric heats (DIH) equation (see Section S8, Supporting Information) can predict the selectivity on various kinds of porous materials for CO2 over CH4/N2/H2 at a pressure ranging from 0 to 100 kPa with high accuracy.41,42 Here, the DIH equation is also employed to calculate the CO2/N2 and CO2/CH4 adsorption selectivities on 50CPDA@ A-C at 5−100 kPa, and the results are presented in Figure 8. The CO2/N2 and CO2/CH4 selectivities on 50CPDA@A-C calculated from the DIH equation were in the ranges of 45.0− 21.9 and 6.7−5.0, respectively, which were similar to the results calculated from IAST (correspondingly 45.4−25.1 and 7.6−5.1). 3.4. Binding Energies (BEs) of the Adsorbates (CO2/ CH4/N2) with Different Interaction Sites. To derive a better understanding of the mechanism of enhancing the CO2 capture performance (including the CO2 adsorption capacity and CO2-containing gas mixture adsorption selectivity), the binding energies (BEs) of the adsorbates (CO2/CH4/N2) with different interaction sites (C-center/pyridinic nitrogen/pyr-

Figure 7. IAST-predicted selectivities of 50CPDA@A-C and the parent A-C at 298 K for (a) CO2/N2 (0.15/0.85) and (b) CO2/CH4 (0.5/0.5) binary mixtures. 498

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Figure 8. DIH-predicted selectivities for (a) CO2/N2 (0.15/0.85) and (b) CO2/CH4 (0.5/0.5) binary mixtures on 50CPDA@A-C at 298 K.

Figure 9. (a) Binding energy of CO2/CH4/N2 with pyridinic nitrogen (N1)/pyridonic nitrogen (N2)/ C-center on the sample surfaces; (b) absolute difference of the binding energies (ΔBE) between CO2 and CH4 or CO2 and N2 on three interaction sites (N1, N2, and C-center).

Figure 10. Fixed-bed breakthrough curves of CO2/N2 (0.15/0.85) binary mixture through the fixed bed packed with (a) 50CPDA@A-C and (b) A-C at 298 K and 100 kPa.

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Figure 11. Fixed-bed breakthrough curves of CO2/CH4 (0.5/0.5) binary mixture through the fixed bed packed with (a) 50CPDA@A-C and (b) AC at 298 K and 100 kPa.

selectivities calculated from breakthrough curves were close to the corresponding selectivity values calculated from the IAST and DIH methodology, indicative of the accuracy of the IASTpredicted and DIH-predicted selectivities of 50CPDA@A-C toward CO2/N2 and CO2/CH4 binary mixtures.

CO2/N2 (0.15/0.85) and CO2/CH4 (0.5/0.5) mixtures were evaluated by breakthrough experiments (see Section S10, Supporting Information). Figures 10 and 11 show the breakthrough curves of CO2/N2 and CO2/CH4 binary mixtures through the fixed bed of 50CPDA@A-C and the parent A-C, respectively. CO2 and N2 or CO2 and CH4 concentrations in the outlet gas increased gradually with time. Generally, the breakthrough time of adsorbates through a fixed bed is an important parameter to estimate the dynamic adsorption and separation property of a sorbent, and the working adsorption capacity and practical adsorptive separation selectivity (S) of a sorbent can be calculated from the breakthrough time (see Section S10, Supporting Information). It is clearly visible from Figures 10 and 11 that a cleaner and sharper separation of the CO2/N2 and CO2/CH4 binary mixtures occurred on both samples 50CPDA@A-C and A-C. N2 or CH4 always eluted out first, and then CO2 eluted out. This is because the interaction between these samples and CO2 molecules is stronger than that of N2 or CH4. Noteworthy, the breakthrough time of CO2 through the fixed bed packed with the sample 50CPDA@A-C was longer than that through the fixed bed packed with the sample A-C, because 50CPDA@A-C possessed a higher CO2 working capacity and adsorption separation selectivity compared with A-C. Meanwhile, it was observed that there is a roll-up in the N 2 and CH 4 breakthrough curves because of the N2 and CH4 desorption stimulated by CO2 adsorption.43 The breakthrough curves of CO2/CH4 through the fixed bed showed a sharper roll-up compared with that of CO2/N2. This is because 50CPDA@AC possessed a higher CH4 adsorption capacity than N2, resulting in the desorption of more CH4 when they were replaced by incoming CO2. Here, we tried to calculate the adsorption selectivity (S) using eqs S8−S10 (see Section S10, Supporting Information) through the fixed-bed breakthrough curves obtained by experiments. The calculated results are listed in Table S5 (see Section S11, Supporting Information). The selectivities of 50CPDA@A-C for CO2/N2 and CO2/CH4 binary mixtures calculated from the breakthrough curve were 22.1 and 5.1, respectively, at 298 K and 100 kPa, implying that 50CPDA@AC possessed a high adsorption selectivity for CO2/N2 and CO2/CH4 mixture separation. In addition, the adsorption

4. CONCLUSIONS In this work, we modified the asphalt-based carbon (A-C) using polydopamine, and successfully synthesized a series of composites CPDA@A-Cs for CO2 capture and separation. These CPDA@A-Cs exhibited a microporous structure and a high surface area (up to 2031 m2/g) with abundant basic sites. XPS analysis indicated that the surface N-containing groups mainly consisted of pyridinic nitrogen and pyridonic nitrogen. The CO2 adsorption capacities of CPDA@A-Cs reached 4.05 and 6.89 mmol/g at 298 and 273 K, respectively, higher than that on the parent A-C and many reported porous materials. The isosteric heat of CO2 adsorption on CPDA@A-Cs was as low as 23.0 kJ/mol. This moderate isosteric heat meant that low energy would be required for CO2 desorption. More importantly, CPDA@A-Cs exhibited excellent adsorption selectivities for CO2/N2 and CO2/CH4 binary mixtures, reaching 25.1 for CO2/N2 (0.15/0.85) mixture and 5.1 for CO2/CH4 (0.5/0.5) mixture at 298 K and 100 kPa. DFT calculation showed that the presence of surface N-containing groups such as the pyridinic nitrogen/pyridonic nitrogen enhanced the interaction between CO2 molecules and the surfaces of samples, resulting in an enhanced CO2 capture performance of the sample. These excellent properties such as an enhanced CO2 adsorption capacity and selectivity relative to CH4 and N2 make CPDA@A-Cs promising and accessible adsorbents for the applications of CO2 capture and separation. However, the scaling up, costs, and life-cycle assessment still need systematic investigation before CPDA@A-Cs are put into practical application.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.8b03637. 500

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Preparation of CPDA@A-Cs, textual properties and surface elemental composition of the CPDA@A-Cs, N 1s XPS spectra of CPDA@A-Cs, CO2 adsorption capacities of various porous carbon materials, calculation of isosteric heat, ideal adsorbed solution theory (IAST) for calculating adsorption selectivity, fitting parameters of the DSLF model for CO2, N2 and CH4 isotherms, difference of the isosteric heats (DIH) equation for calculating adsorption selectivity, optimized adsorption configurations of binding sites, breakthrough experiment and selectivity calculated from breakthrough curves (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86 20 87113513 (X.Z.). *E-mail: [email protected]. Tel: +86 20 87110608. Fax: +86 20 87110608 (Z.L.). ORCID

Zewei Liu: 0000-0001-9770-4781 Xin Zhou: 0000-0002-4317-7354 Zhong Li: 0000-0001-6354-883X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Key Program of National Natural Science Foundation of China (No. 21436005), National Natural Science Foundation of China (No. 21808066), the Guangdong Province Science and Technology Project (No. 2016A020221006), the Research Foundation of State Key Lab of Subtropical Building Science of China (2018ZC08), and the Fundamental Research Funds for the Central Universities.

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DOI: 10.1021/acs.energyfuels.8b03637 Energy Fuels 2019, 33, 493−502

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DOI: 10.1021/acs.energyfuels.8b03637 Energy Fuels 2019, 33, 493−502