Highly Cost-Effective Nitrogen-Doped Porous Coconut Shell-Based

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Highly Cost-Effective Nitrogen-Doped Porous Coconut Shell-Based CO2 Sorbent Synthesized by Combining Ammoxidation with KOH Activation Mingli Yang,† Liping Guo,† Gengshen Hu,‡ Xin Hu,*,† Leqiong Xu,† Jie Chen,† Wei Dai,† and Maohong Fan*,§ †

College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, P. R. China Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, P. R. China § Department of Chemical and Petroleum Engineering, University of Wyoming, Laramie, Wyoming 82071, United States ‡

S Supporting Information *

ABSTRACT: The objective of this research is to develop a cost-effective carbonaceous CO2 sorbent. Highly nanoporous N-doped carbons were synthesized with coconut shell by combining ammoxidation with KOH activation. The resultant carbons have characteristics of highly developed porosities and large nitrogen loadings. The prepared carbons exhibit high CO2 adsorption capacities of 3.44−4.26 and 4.77−6.52 mmol/g at 25 and 0 °C under atmospheric pressure, respectively. Specifically, the sample NC-650-1 prepared under very mild conditions (650 °C and KOH/precursor ratio of 1) shows the CO2 uptake 4.26 mmol/g at 25 °C, which is among the best of the known nitrogen-doped porous carbons. The high CO2 capture capacity of the sorbent can be attributed to its high microporosity and nitrogen content. In addition, the CO2/N2 selectivity of the sorbent is as high as 29, higher than that of many reported CO2 sorbents. Finally, this N-doped carbon exhibits CO2 heats of adsorption as high as 42 kJ/mol. The multiple advantages of these cost-effective coconut shell-based carbons demonstrate that they are excellent candidates for CO2 capture.

1. INTRODUCTION CO2 capture by solid sorbents has been proposed as an effective technology of separating CO2 from flue gas.1 The key of this technology is to find sorbents with superior CO2 adsorption properties such as high CO2 uptake capacity, high CO2/N2 selectivity, fast kinetics, etc.2,3 To this end, intensive research interest has been focused on the development of new sorbents.4−8 Examples of the most widely used CO2 sorbents are carbons, 9−11 zeolites, 12 metal−organic frameworks (MOFs),13,14 and porous polymers.15 Among all types of sorbents, porous carbonaceous materials are especially promising due to their advantages such as inexpensive and easy preparation, high thermal and chemical stability, tunable pore structure, ease of regeneration, and inert to water vapor.9,16,17 Among various precursors used for synthesis of porous carbons, biomass resources are competitive because they are abundant, sustainable and cost-effective as carbon sources. As a renewable and inexpensive biomass feedstock, coconut shell has attracted lots of attention in the preparation of porous carbons.18−20 Previous studies showed that coconut shellbased porous carbons possessed less than 4 and 6 mmol/g CO2 adsorption capacities under 25 and 0 °C at atmospheric pressure, respectively.21−24 Thus, more efforts and new © XXXX American Chemical Society

synthesis strategies are needed to obtain coconut shell-based porous carbons with higher CO2 adsorption capacities. Recently, narrow micropores smaller than 1 nm in the porous carbons have been proposed to be effective for CO2 adsorption under ambient conditions (25 °C, 1 bar).9,11,25,26 However, many studies in this field have shown that the CO2 adsorption capacity of porous carbons can be improved by incorporation of nitrogen functional groups into their porous structures.27−31 Therefore, highly microporous nitrogen-doped carbonaceous sorbents could be promising for CO2 capture. This work was designed to develop an easy, low-cost method for synthesizing nitrogen-doped porous carbons, in which coconut shell was used as a carbon precursor first modified by a mixture of gases containing ammonia and oxygen (ammoxidation), and then activated by KOH under varying conditions. Ammoxidation is one of the most effective methods of nitrogen-enrichment to carbon materials, which includes a simultaneous oxidation and nitrogenation of the precursor.32 The nitrogen-doped carbons prepared by ammoxidation Received: March 14, 2015 Revised: April 29, 2015 Accepted: May 11, 2015

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DOI: 10.1021/acs.est.5b01311 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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data in the relative pressure range between 0.01 and 0.1. The total micropore volume (V0) was deduced from the N2 adsorption data by the t-plot method, and the total pore volume (Vt) was estimated from the adsorbed amount of liquid nitrogen at a relative pressure of 0.99. The micropore size distribution was calculated using the Harvath−Kawazoe (H−K) method. Adsorption isotherms of CO2 at 0 °C were also measured on the Beishide 3H-2000PS2 adsorption analyzer to assess narrow microporosity (size < 1 nm), where N 2 adsorption at −196 °C can be kinetically restricted. The pore volume corresponding to the narrow microporosity (Vn) was obtained by applying the Dubinin−Radushkevich (D−R) equation to the CO2 adsorption data at 0 °C. X-ray photoelectron (XPS) measurements were performed using an AXIS Nova spectrometer (Kratos Inc., NY, USA) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). XPS survey spectra were recorded with a pass energy of 160 eV, and high-resolution spectra with a pass energy of 40 eV. The CO2 adsorption isotherms were measured using the Beishide 3H2000PS2 sorption analyzer at 0 and 25 °C, respectively. Prior to each adsorption experiment, the sample was degassed for 12 h at 200 °C to remove the guest molecules from the pores. The uncertainty of CO2 adsorption results was less than 2% of the measured adsorption amount.

process have been used as effective sorbents to remove H2S, SO2, and NOx.33−35 Through the ammoxidation process, a significant amount of nitrogen groups was introduced into the carbon matrix. Further KOH activation can develop the micropore structures of the nitrogen-doped carbons. The resulting porous carbons thus featured a high fraction of narrow micropores ( N-Q for all nitrogen-doped samples. This may be beneficial to CO2 capture, as it has been documented that N-5 generally has a much greater contribution to CO2 capture than pyridinic nitrogen and quaternary nitrogen.29,36,41 3.3. Porous Textures. The porous textures of carbons in this work were characterized by nitrogen sorption at −196 °C and CO2 sorption at 0 °C, respectively. Figure 3 shows the nitrogen adsorption/desorption isotherms of the prepared nitrogen-doped carbons. All of the samples exhibit type I isotherms (according to the IUPAC classification) with significant adsorption below relative pressure P/P0 < 0.1. When KOH/NC = 1 and 2, the adsorption isotherm presents an abrupt knee at low relative pressure, which indicates that the microporosity of the samples is mainly composed of narrow micropores. When KOH/NC = 3 and 4, the carbons present a wider isotherm knee, indicative of wider micropores. The adsorption and desorption branches of the isotherms fit very well without any hysteresis loops in the medium P/P0 range. However, a small closed adsorption/desorption hysteresis loop is also observed for several samples with relative pressure P/P0 > 0.7, probably due to mesopores with a capillary condensation. For most carbons, in the range of P/P0 > 0.2, the isotherms show an almost flat sorption characteristic. These findings clearly indicate that micropores are dominant in these carbons. The porous textural characterization results of prepared carbons are given in Table 1. From the results, an increase in KOH/NC ratio and activation temperature are found to increase the BET surface area, total pore volume (Vt), and micropore volume (V0) of the carbons with values ranging from 879 to 2690 m2/g, from 0.38 to 1.33 cm3/g, and from 0.33 to 1.21 cm3/g, respectively (with a few minor exceptions). The volume of the narrow micropores (Vn) of the prepared carbons, which is proposed to be responsible for the CO2 uptake at ambient temperatures and pressures,9,11,17,26,42,43 was obtained from the CO2 adsorption data at 0 °C and shown in Table 1. It should be noted that the changing trend of Vn with respect to

Figure 3. N2 sorption isotherms of the samples prepared at different conditions (a) 600 °C, (b) 650 °C, and (C) 700 °C. Filled and empty symbols represent adsorption and desorption branches, respectively.

the synthesis conditions agrees with the other porous texture characteristics mentioned above such as BET surface area, Vt or V0 with the value ranging from 0.42 to 0.92 cm3/g for nitrogenD

DOI: 10.1021/acs.est.5b01311 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology doped carbons. These results indicate that these carbons have highly developed narrow microporosity. When compared to the values of Vn and V0 of the nitrogen-doped samples, these two values are rather similar except the carbons prepared at severe conditions such as NC-650-3, NC-650-4, and NC-700-4, suggesting a narrow and homogeneous microporosity. As shown in Figure S2, the pore size distribution (PSD) curves of the representative samples NC-650-1, NC-650-2, and NC-7001 show very narrow PSD centered at ca. 0.6 nm, whereas the sample NC-700-4 shows much broader PSD compared with above samples. Compared to NC-650-1, C-650-1 shows much lower porosity, suggesting that ammoxidation is an effective way to increase the porous textures of the carbons. As described in the above section, a significant amount of nitrogen groups introduced upon ammoxidation undergo decomposition on pyrolysis, whereas the others are transformed to more thermally stable species or are built into deeper layers of the NC structure. Upon KOH activation, these species react with the activating agent and undergo decomposition (as indicated by a decrease in the content of nitrogen), facilitating the activating agent penetration into the deeper layers of the NC, which leads to a greater development of the porous structure. This means that the doped nitrogen upon ammoxidation can additionally activate carbon samples, while this activation process belongs to the depth activation,44 which can increase the degree of microporosity. When comparing Vn values between NC-650-1 and C-650-1, the former shows much higher narrow microporosity than the latter indicating that ammoxidation is also an effective way to increase the narrow microporous textures of the carbons. 3.4. CO2 Adsorption Capacities and CO2/N2 Selectivity. CO2 adsorption capacities on prepared carbons were investigated at 25 and 0 °C under atmospheric pressure (1 bar). The CO2 adsorption isotherms for nitrogen-doped carbons measured under ambient conditions and two temperatures are presented in Figure 4, and the results are summarized in Table 1. All of the materials exhibited excellent CO2 capture capacities ranging from 3.44 to 4.26 mmol/g at 25 °C and 4.77 to 6.52 mmol/g at 0 °C under atmospheric pressure, respectively. The CO2 uptake of these N-doped porous carbons is among the best of the known N-decorated porous carbons30,31,45,46 and higher than many other microporous materials such as MOFs,13,14,47 COFs,48 and PAFs.49,50 For example, Ma et al. reported nitrogen-doped porous carbons synthesized with HNO3 as a catalyst and N source achieved a CO2 adsorption capacity of 4.30 mmol/g at 25 °C and 1 atm.45 Sevilla et al. synthesized N-doped activated carbons from algae and glucose with a CO2 adsorption capacity of 4.4 mmol/g at 25 °C and 1 atm.46 Xing et al. prepared N-doped activated carbons from bean dreg, which possess CO2 uptake capacity of 4.24 mmol/g at 25 °C and 1 atm.30 N-Doped zeolite-templated carbons developed by Xia et al. exhibited CO2 uptake capacity of ca. 4.4 mmol/g at ambient pressure and 25 °C.31 Some MOFs such as MOF-177,14 MOF-210,47 and IRMOF-114 only showed around 1 mmol/g at 25 °C and 1 bar. COFs48 developed by Yaghi et al. showed less than 3 mmol/g at 25 °C and 1 bar. Furthermore, a series of PAFs developed by Qiu’s group showed 1.1−1.8 mmol/g at 25 °C and atmospheric pressure.49,50 Besides the above typical solid sorbents, recently, liquid materials such as ionic liquids (ILs) have also been widely investigated.51−56 Some functionalized ionic liquids can possess a CO2 capacity of 1.65 mol per mol IL at 1 bar and around 25 °C.51 However,

Figure 4. CO2 adsorption isotherms at 25 °C (empty symbols) and 0 °C (filled symbols) for nitrogen-doped carbons prepared under different conditions (a) 600 °C, (b) 650 °C, and (C) 700 °C. E

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Environmental Science & Technology when the CO2 uptake value of ILs were transformed to gravimetric capacity, the maximum CO2 uptake is less than 4 mmol/g, which is inferior to our samples. Among all the samples, NC-650-1 possesses the maximum CO2 capture capacity of 4.26 mmol/g at 25 °C, while NC-700-4 prepared under the most severe conditions in this study shows the minimum CO2 capture capacity at 25 °C. The mild synthesis condition of NC-650-1 can greatly save the cost of sorbents preparation, which is preferred in the view of practical application. Furthermore, NC-700-1 and NC-650-2 show CO2 adsorbed capacities of 4.22 mmol/g, which is almost the same as that of NC-650-1 indicating that the optimum preparation parameters were probably not unique. When we further explore the relationship between the porous textures and CO2 sorption capacity for these carbons, it was found that the CO2 uptake at 25 °C and 1 bar was not directly correlated to each of the porous texture characteristics such as BET surface area, total pore volume, Vn, or V0. This result suggests that, in addition to the porous textures of the carbons, the Nfunctional groups on the carbon surface also play an important role in determining CO2 uptake for these nitrogen-doped carbons. NC-650-1, which has the third-lowest values in each parameter of porous textures (i.e., BET surface area, total pore volume, Vn, and V0) but the third-highest N content (4.56 wt %) among all of the samples, exhibited the highest CO2 uptakes of 4.26 mmol/g at 25 °C under ambient pressure. By contrast, although NC-700-4 possesses much greater developed porosity than that of NC-650-1, its lowest N content gives it the lowest CO2 capture capacity in this study. However, NC-600-1 has the highest N content among all the samples, but its lest developed porosity makes it second-lowest in CO2 adsorption capacity. From the above results, both porosity and N content clearly play a combined role in the high CO2 adsorption for these nitrogen-doped carbons. To further improve CO2 adsorption capacities at atmospheric pressure of coconut shell-based carbons, efforts have to be made to produce narrow microporosity with a large contribution of micropore volume coming from smaller micropores, together with high nitrogen content. For practical applications, in addition to a high CO2 adsorption capacity, carbon sorbents should be easy to regenerate and have high tolerance to recycling, and they also need to show high selectivity toward CO2. To show the regeneration and recyclability of the carbon sorbents studied, we performed five runs of CO2 adsorption/desorption measurements for NC-650-1 (see Figure S3) and obtained almost identical isotherm curves, indicating that this material shows excellent regenerability and cyclability. To determine CO2/N2 selectivity, the N2 adsorption capacity of carbons is also measured under the same experimental conditions. Figure 5 shows the CO2 and N2 isotherms of NC650-1 and C-650-1 at 25 °C. Selectivity of CO2 over N2 was calculated using Henry’s Law through the initial slopes of CO2 and N2 adsorption isotherms. Based on the initial slopes of N2 and CO 2 adsorption isotherms, the estimated CO 2 /N 2 selectivity of NC-650-1 is 29, comparable to or better than those obtained with reported nitrogen-doped carbons,2,10,41,43,57−59 in which CO2/N2 selectivity is also calculated using the same method described above. This selectivity is higher than that of C-650-1, which indicates that nitrogen doping is beneficial to the enhancement of CO2/N2 selectivity and thus flue gas separation.

Figure 5. CO2 (filled symbols) and N2 (empty symbols) adsorption isotherms of NC-650-1 and C-650-1 at 25 °C.

3.5. Isosteric Heat of Adsorption (Qst). The isosteric heats of adsorption (Qst) for representative N-decorated carbons were calculated from the CO2 sorption isotherms measured at 25 and 0 °C using the Clausius−Clapeyron equation. Figure 6 shows the curves of Qst for NC-650-1, NC-

Figure 6. Isosteric heat of CO2 adsorption on NC-650-1, NC-650-2, NC-700-1, and NC-700-4 calculated from the experimental adsorption isotherms at 0 and 25 °C.

650-2, UC-700-1, and UC-700-4. The initial Qst for the studied carbons varies in the range of 28−42 kJ mol−1 at low CO2 uptake, which is higher than previously reported values for typical carbonaceous adsorbents,11,31,46,60 but still in the value range for the physisorption process. The higher value of Qst means the enhanced interaction between CO2 molecule and adsorbent, which is often favorable for CO2 separation from flue gas, but a too high Qst (e.g., in the cases of chemisorption) would cause difficulty in sorbent regeneration. Our Qst values are well below the energy of covalent bonds and hence the desorption process is facile and reversible as discussed above. It was also found that Qst decreases with increased CO2 loading until a near plateau is achieved, which indicates that the binding energies of CO2 are heterogeneous in the pores, and that high F

DOI: 10.1021/acs.est.5b01311 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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initial Qst values may be due to the strong adsorbent−adsorbate interaction between the N-containing carbon framework and CO2 molecules indicating that the surface chemistry of the carbons has a beneficial effect on the initial gas adsorption. Moreover, the initial Qst for CO2 increases with the N content of the carbons, further emphasizing the importance of nitrogen functional groups for CO2 uptake. When the Qst between NC650-1 and C-650-1 (Figure S4) is further compared, it is found that nitrogen doping could improve the isosteric heat at lower coverage, which agrees with the above discussion. In summary, a carbonaceous CO2 sorbent with high microporosity and nitrogen-containing framework was successfully synthesized with coconut shell, which is first treated by ammoxidation and then KOH activation. These nitrogen-doped carbons exhibit high CO2 uptake capacities at atmospheric pressure with maximum value of 4.26 and 6.52 mmol/g at 25 and 0 °C, respectively. NC-650-1, prepared under very mild conditions, shows the highest CO2 uptake at 25 °C due to its characteristics of narrow micropores (