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Environmental and Carbon Dioxide Issues
Efficient CO2 adsorption on nitrogen-doped porous carbons derived from D-glucose Limin Yue, Linli Rao, Linlin Wang, Liying An, Chunyun Hou, Changdan Ma, Herbert DaCosta, and Xin Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01028 • Publication Date (Web): 20 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018
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Efficient CO2 adsorption on nitrogen-doped porous carbons derived from D-glucose Limin Yuea, Linli Raoa, Linlin Wangb, Liying Ana, Chunyun Houa, Changdan Maa, Herbert DaCostac, Xin Hu*,a, a
College of Chemistry and Life Sciences, Zhejiang Normal University, 688 Yingbin
Ave. Jinhua 321004, PR China b
College of Engineering, Zhejiang Normal University, 688 Yingbin Ave. Jinhua
321004, PR China c
Math, Science, and Engineering Division, Illinois Central College, 1 College Drive
East Peoria, IL 61635, USA *
Corresponding author’s e-mail:
[email protected]; phone: 86-151-0579-0257; fax:
86-579-8228-8269
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Abstract The synthesis of carbonaceous CO2 adsorbents doped with nitrogen were carried out via hydrothermal reaction of biomass D-glucose followed by urea treatment and K2CO3 activation. These carbons display high uptake of CO2 at 1 bar, 25°C and 0 °C, up to 3.92 and 6.23 mmol g-1, respectively. Additionally, the as-synthesized materials exhibit superior reusability, high CO2/N2 selectivity, fast CO2 adsorption kinetics, as well as excellent dynamic capture capacity at the experimental conditions used. The synthetic effect of nitrogen content and narrow microporosity decide the capture capacity for CO2 at 1 bar and 25°C for these N-enriched carbonaceous adsorbents. This study provides a viable method to prepare high-performance CO2 carbonaceous sorbents without using caustic KOH. In addition, this work gives further insights into the CO2 adsorption mechanism for nitrogen-doped porous carbon sorbents and hence inspires ways to synthesize novel carbonaceous materials for removing CO2 from combustion exhaust gases.
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Introduction In recent years, the frequency and intensity of extreme weather events such as storm, cold currents, heavy rains, heat waves and so on have greatly increased all round the world. Climate change has become a global issue, which is largely due to the emission of CO2, i.e. the main contributor of greenhouse gas. Thus mitigation of CO2 emission is an imminent task 1. Among various state-of-art CO2 capture techniques, adsorption via solid adsorbents was considered as an effectual technique of separating CO2 from flue gas due to its many advantages such as easy operation, high efficiency, low energy expenditure, and no corrosion problem, to name a few 2-6. The essential of this technique is finding solid materials possessing superior CO2 adsorption properties. In this respect, different solid adsorbents such as zeolites7, metal-organic frameworks (MOFs)8, carbons9, silicas10, and porous polymers
11, 12
have been comprehensively researched in CO2 capture. In the above mentioned solid materials, porous carbonaceous sorbents have manifested many advantages including facile preparation, low synthesis expense, large available surface area, controllable surface functionality and porosity, inertness to both bases and acids, and high resistance to moisture
13
. One attractive aspect of
porous carbonaceous sorbents is that they can be produced using various cheap carbon raw materials such as petroleum coke
14-16
, coal
17-19
, and various polymers
20-23
through a physical or chemical activation process. Among various carbon precursors, biomass materials stand out in terms of low cost, wide availability, and renewability, which should be the primary factors in choosing a precursor. Chemical activation by KOH is widely explored to synthesize porous carbons. In fact, most carbonaceous materials with high CO2 adsorption capacity (>4.5 mmol/g at 1 bar and 25°C) were synthesized by KOH activation24-29. KOH activation can 3
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effectively etch all kinds of carbon precursors to develop microstructure suitable for CO2 capture. However, activation with KOH has essentially two disadvantages that hamper its further application. One issue is KOH’s strong causticity, causing corrosion and equipment damage, particularly at high temperature. In addition, KOH has another environmental drawback, i.e. the as-synthesized products by KOH activation need to be washed to remove the residual inorganic materials, which could cause serious pollution problems. Thus, research on novel activation procedures using a mild activation reagent with environmentally friendly nature is crucial for further development of CO2 capture application systems. Of various substances used as activators, K2CO3 is a good alternative, which is not toxic and is a common food additive. Furthermore, K2CO3 can effectively etch carbon precursors to produce a microporous structure through redox reactions, in a similar manner of KOH activation but to a lesser degree
30
. Therefore, the study of the CO2 uptake performances of
porous carbons made by K2CO3 activation is appealing. Prior studies revealed the importance of the narrow micropores (with diameters < 1 nm) on the CO2 uptake abilities of porous carbonaceous sorbents under 1 bar and 25°C
13, 31-38
. Additionally,
integration of N elements into the carbon skeleton can enlarge the basicity and surface polarity, which could enhance the interplay between carbon and CO2 adsorbates
26,
39-44
. Considering the factors described above, in this paper, nitrogen-doped porous
carbonaceous sorbents were synthesized by K2CO3 activation using a typical biomass D-glucose as the raw material and urea as the nitriding agent. After the hydrothermal treatment of D-glucose, hydrochar is firstly achieved. Then the as-obtained hydrochar was subjected to urea treatment and K2CO3 activation under different conditions to synthesize N-enriched porous carbons. By varying the two synthesis parameters, i.e. 4
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K2CO3/precursors ratio and temperature of activation, the carbonaceous sorbents with a significant number of narrow micropores (under 1 nm) and a large quantity of nitrogenous functionalities were achieved. The effect of these two preparation parameters on the surface chemical properties and porosity and on the capacity of these as-synthesized nitrogen-enriched porous carbonaceous materials to uptake CO2 was investigated. This study provides a viable method to prepare high-performance CO2 carbonaceous sorbents without using caustic KOH, as well as gives more insights into the CO2 adsorption mechanism for nitrogen-doped porous carbon sorbents. Hence, it suggests means to exploit novel carbonaceous sorbent materials for removal of CO2 from combustion exhaust gases. 2. Experimental 2.1 Synthesis and Characterization of Carbonaceous Adsorbents All the chemicals in this study are obtained from Sinopharm Chemical Reagent Co., Ltd with the analytical purity. In a typical experiment, 15 g D-glucose was mixed to 150 mL water in a 200 mL Teflon-liner stainless steel autoclave. The reaction was carried out at 180ºC for 12 h. The resulting product was gathered, washed four times using water, followed by one wash using ethanol, then dried in an 80°C oven for 12 h. The obtained hydrochar was assigned as GC. The procedure for the modification with urea, described in our previous work 14, 42, was done at 350°C in air. The nitrided GC was named as NGC. Then, NGC were activated by K2CO3 under deferent conditions i.e. the activation temperatures of 600, 650, or 700ºC and K2CO3/NGC ratio of 2, 3 or 4. The activation process was carried out under the nitrogen atmosphere (400 mL/min). In the activation process, the activation time and the heating rate were set at 1 hour and 5ºC/min, respectively. The detailed K2CO3 activation process is represented in the supporting information. The yield of as-prepared carbonaceous 5
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adsorbents ranges from 30% to 65% at this stage under different synthesis conditions. The obtained nitrogen-enriched sorbents were represented as NGC-X-Y, of which the activation temperature is given by X and ratio of K2CO3 to NGC is represented by Y. For comparison, a reference sample was synthesized by directly reaction GC with K2CO3 at 650ºC and K2CO3/GC ratio of 4. Thus this sample was assigned as GC-650-4. The detailed sorbents’ characterization apparatus and methods can be found in the supporting information. 3. Results and Discussion 3.1 Chemical Composition and Surface Properties GC has a negligible nitrogen content (0.54 wt%). The nitrogen amount increased to 20.66 wt% for NGC after urea treatment. The following K2CO3 activation causes the decreased nitrogen content of the resulting samples, ranging from 12.27 wt% to 4.69 wt%. The higher activation temperature and K2CO3/NGC ratio lead to the lower nitrogen content of the resulting sorbents. This finding is consistent with results from previous work
26, 45
, on which some N-containing moieties were decomposed/reacted
during the K2CO3 activation process. Under more harsh conditions, the loss of nitrogen-containing functionalities became more
severe.
The
existence
of
N-containing groups was shown by FTIR spectra (Figure S1, Supporting information). The absorption bands at 1615 and 3429 cm-1 were ascribed to the N-H in-plane deformation and the stretching of N-H and/or –OH 46, respectively. Possible species of N-containing moieties on the surface of NGC and some representative N-doped porous samples were determined by XPS analysis. The XPS N1s spectrum of NGC can be deconvoluted into three subpeaks centered at 398.2, 399.2, and 400.2 eV, which can be appointed to pyridinic-N (N-6), amine, and pyrrole/pyridine-N (N-5) 47, respectively (Fig. 1a). Two peaks, i.e. N-5 and N-6, were found through the 6
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deconvolution of N1s XPS of NGC-600-2 (Fig. 1b). Under relatively mild pyrolysis condition, the unstable amine functionalities are converted to N-5 or N-6. When the pyrolysis temperature further increased to 650 and 700°C, part of N-5 and N-6 transformed to quaternary nitrogen (N-Q), as illustrated by the peak centered at 401.2 eV shown at the XPS spectra of NGC-650-4 (Fig. 1c) and NGC-700-4 (Fig. 1d). 3.2 Phase Structure and Surface Morphology The phase structure of the representative N-doped porous carbon (NGC-650-4) was examined by XRD. Two broad and weak diffraction peaks, almost negligible, at about 23° and 43° were found for this sample, which are usually referred to the (002) and (100) diffraction patterns of the amorphous carbon. (Figure S2, Supporting information)
48
. This finding suggests an amorphous nature in NGC-650-4. The
morphology of GC, NGC and NGC-650-4 were examined by SEM and TEM. GC is mostly composed of stacking spheroidal particles (Fig. 2a). After urea modification and further K2CO3 activation, the morphology of NGC (Fig. 2b) and NGC-650-4 (Fig. 2c) keep almost intact. TEM examinations can further confirm the spherical morphology of the activated sample (Fig. 2d and Fig. 2e). In addition, the randomly distributed wormhole-like micropores can be obviously seen in a magnified TEM image, as illustrated in Fig. 2f. It exhibits an amorphous microporous structure, which agrees well with the results of XRD analysis. 3.3 Texture Features of the Carbonaceous Sorbents The texture features of carbonaceous sorbent materials were measured by the physical adsorption–desorption of nitrogen at -196 ºC. Figure 3 illustrates all the nitrogen sorption isotherms of the N-enriched porous carbonaceous materials and Table 1 provides each of the porous features for all the samples. According to the IUPAC categorization, these samples displayed a type-I isotherm, demonstrating the 7
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microporous nature of these materials. The abrupt increase of the isotherm at low relative pressure (P/P00.2 indicates the multilayer adsorption on the external surface. There is an obvious adsorption ‘‘tail’’ at relative pressure approaching saturation for most of the isotherms, indicating the presence of macropores, which may be due to the stacking of the spherical particles. Additionally, it is found that at the activation temperature of 600ºC, the sorption isotherms have an acute knee at P/P0
