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A new route for conversion of corncob to porous carbon by hydrolysis and activation
Chemical Engineering Journal 225 (2013) 300–305
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
A new route for conversion of corncob to porous carbon by hydrolysis and activation Lili Ding a, Bo Zou a, Hequn Liu a, Yannan Li a, Zichen Wang a, Ying Su b, Yupeng Guo a,⇑, Xiaofeng Wang a,⇑ a b
College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, China The Department of Electronic Science, Changchun Institute of Engineering Technology, Huayuan Road 1, Changchun 130017, China
h i g h l i g h t s The new route could convert waste corncob to high performance carbon materials. This route was realized by the process of acid hydrolysis and alkali activation. 2
High surface area (up to 3611 m /g) was achieved. Porous carbon had specific capacitance of up to 236 F/g. This route could be readily extended to abundant lignocellulose biomass and industrial wastewater containing carbohydrates.
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Article history: Received 14 January 2013 Received in revised form 8 March 2013 Accepted 23 March 2013 Available online 30 March 2013 Keywords: Corncob Hydrolysis Conversion Activation Porous carbon
a b s t r a c t A new route for conversion of corncob to high performance porous carbon was proposed. This route involved hydrolysis of corncobs to dissolved sugars and degradation products of carbohydrates by sulfuric acid catalysis, transformation of the liquid hydrolyzate into solid hydrochar by dehydrating, polymerization and carbonization, fabrication of porous structures by activation. By using this route, high surface area (up to 3611 m2/g) and low ash content (0.51 wt%) were achieved. A cottony structure appeared on the surface of the porous carbon in a high KOH/hydrochar ratio. The specific capacitance for porous carbon was up to 236 F/g, which indicated the excellent electrochemical properties of these materials. Furthermore, the highly developed pore structures made the porous carbon possess potential applications in separation, catalysis, hydrogen storage and other fields. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Facing with serious environmental problems and gradual depletion of resources, the society requires replacement of petroleum based products. Biomass is the most abundant and the fastest growing renewable energy. Conversion of biomass into energy materials not only reduce environmental pollution but also produce enormous economic benefits. Corncob, one of the lignocellulose biomass, is the byproduct of corn production. In China, about 30 million tons of corncobs are produced annually. To realize the effective utilization of corncobs, some researchers utilized the main components (approximately 40% cellulose and 30% hemicellulose) of corncob to produce glucose, xylose, furfural, ethanol and so on by hydrolysis [1–4]. However, some unavoidable byproducts produced along with hydrolysis, such as polysaccharide, arabinose, ⇑ Corresponding authors. Tel./fax: +86 431 8515 5358. E-mail addresses: [email protected] (Y. Guo), [email protected] (X. Wang). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.090
galactose, mannose and some other degradation products of carbohydrates, these substances were hard to be isolated from the biomass hydrolyzates and generated harmful influences on the production of downstream materials. In addition, discharge of industrial wastewater containing carbohydrates will cause serious environmental pollution. In order to solve the separation problem of different hydrolysates and make full use of the biomass hydrolysates and the carbohydrates in industrial wastewater, we envisage new approaches that convert the carbohydrates and their degradation products in liquid biomass hydrolysates or industrial wastewater to solid carbonaceous material by dehydration, carbonization with sulfuric acid. Among all of the carbonaceous materials, porous carbons are especially promising owing to their wide applications. In most cases, the utility of the porous carbons come from relatively high surface areas and their unique porous structures [5–7]. The specific surface area of ordinary corncobs-based activated carbon is less than 2000 m2/g. To obtain larger surface area and better application properties, Cao et al. added soap to activator (KOH solution)
L. Ding et al. / Chemical Engineering Journal 225 (2013) 300–305
[8], Tseng et al. combined the KOH etching and CO2 gasification [9], the specific surface area of corncobs-based carbon could exceed 2000 m2/g. In addition to activating agent, the structure of precursor also generated tremendous effect on the porous carbon [10– 12]. In this paper, we propose a new route to convert corncob to useful porous carbon, which does not mix the biomass (or carbonized biomass) with activator directly, but transform the lignocellulosic corncob into hydrochars by hydrolysis first, and then fabricate porous structures by activation. The impurity in corncob has been removed by hydrolysis, which reduce the ash content of porous carbon. Furthermore, spherical hydrochar, the new porous carbon precursor, could largely enhance the specific surface area of the porous carbon compared to the lignocellulosic corncob that may give these porous carbons highly preferential application in many fields. The main aim of this work is to convert waste corncob into high performance energy materials. 2. Experimental 2.1. Materials and reagents Corncob was obtained from local farmer near Changchun, and grinded to particle size of 1–5 mm. The elemental analysis of corncob was 46.53 wt% C, 6.37 wt% H, 44.45 wt% O and 0.64 wt% N. The ash content was 2.01 wt%. The content of hemicellulose, cellulose and lignin in corncob were 40.5 wt%, 36.8 wt% and 20.7 wt%, respectively. Sulfuric acid and potassium hydroxide were analytical grade. 2.2. Acid hydrolysis of corncob The concept of sulfuric acid hydrolysis in a low temperature and atmospheric pressure come from the acid hydrolysis of rice husk reported in our previous work [13]. Here, sulfuric acid (68 wt%) was used as catalyst to hydrolyze the corncob. Fig. 1 shows the schematic diagram of the preparation process of porous carbon. The ratio of corncob to 68 wt% sulfuric acid was 1:10 (g/ml). Hydrolysis reaction was conducted at 55 °C for 10 min. Then the acid concentration of hydrolyate was diluted to 55 wt%. The liquid product was separated from solid by filtration. The hydrolysis solution containing dissolved sugars and some degradation products of carbohydrates was used for production of hydrochar. The hydrolysis solution was heated at 95 °C for 6 h. The black solid product formed in solution was separated from liquid product by filtering after the reaction. After washing several times with distilled water, the solid hydrochar was obtained. The liquid product containing sulfuric acid can be recycled. 2.3. Fabrication of porous structures Potassium hydroxide was used to fabricate porous structures in hydrochar by activation based on many Refs. [14–16] which used potassium hydroxide to create highly developed pore structures. Hydrochar was treated at 500 °C for 0.5 h under nitrogen atmosphere before activation. The yield was calculated by the mass ratio of the obtained porous carbon and the treated hydrochar.
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A two-step process of KOH activation was adopted for preparation of porous carbons. Firstly, the activation was carried out at 500 °C for 0.5 h, then in a temperature range of 700–900 °C for 0.5–2 h. The ratios of KOH/hydrochar were from 3:1 to 6:1. The porous carbons will be denoted according to the expression: C/ KOH:hydrochar/Temperature (°C)/Time (h). For example, the porous carbon obtained at 800 °C 2 h when the KOH:hydrochar ratio was 4:1, the porous carbon was denoted as C/4:1/800/2. All the porous carbons prepared from the above process were heated to 800 °C for 1 h under nitrogen atmosphere. 2.4. Characterizations The elemental analysis of corncob, hydrochars and porous carbon were performed on Perkin-Elmer 2400 C, H, N, O analyzer. The content of the hemicellulose, cellulose, lignin and ash in corncob were tested according to the China National Standards GB/T 2677.9-94, GB/T 2677.10-95, GB/T 747-2003 and GB/T 22427.12008. The morphology of the samples was characterized by a Hitachi H-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV, and a JSM-6700F scanning electron microscope (SEM). The Brunauer–Emmett–Teller (BET) surface area and the pore structure analysis were conducted on a Micromeritics ASAP 2010 Surface Analyzer. The pore size distributions were calculated by the Density Functional Theory (DFT) method. The X-ray Photoelectron Spectroscopy (XPS) measurements were performed on the ESCALAB250 using Al Ka excitation source (ht = 1486.6 eV). The quantitative analysis of O was based on the peak intensities of the O1s. The electrochemical measurements were conducted on a CHI 6600 electrochemical workstation in a three electrode configuration in 6 mol/L KOH aqueous solution.
3. Results and discussion The hydrochar contained 63.13 wt% C, 4.41 wt% H, 0.46 wt% N, 31.74 wt% O, and 0.21 wt% ash. The porous carbon, obtained at 800 °C 1 h at the KOH/hydrochar ratio of 4:1, contained 92.73 wt% C, 0.63 wt% H, 0.41 wt% N, 5.72 wt% O and 0.51 wt% ash. The yield of porous carbons was from 27% to 46% (Fig. 2), and the yield showed decreasing trend along with the increase of the activation time and temperature. The representative TEM (Fig. 3a and b) and SEM (Fig. 3c and d) images revealed that sample C/4:1/800/1 could maintain the original spherical structure of the hydrochar. The cottony structures appeared on the surface of the porous carbon (Fig. 3b) when the activation was carried out in a large amount of KOH. This suggested that the surface pyrolysis and interior etching processes occurred simultaneously during the preparation process. Fig. 3d confirmed that excessive dosage of KOH destroyed the spherical structure of the hydrochar. 3.1. Effects of activation conditions on textural properties Activation temperature, time and ratio of the KOH to hydrochar all affected the textural properties of porous carbons. Therefore, these factors were investigated.
Fig. 1. Schematic illustration of the preparation process of porous carbons.
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Yield (%)
40 30 20 10
1500 3 -1
700 C o 800 C o 900 C
a
Adsorbed volume (cm g )
50
4 5 KOH/Hydrochar Ratio
Adsorption isotherms Desorption isotherms
1200 900 600 o
700 C o 800 C o 900 C
300 0
0 3
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6
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700 C o 800 C o 900 C
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dV/dlogD (cm g )
50
0.5 h 1h 1.5 h 2h
0.06
0.04
0.02
0.00 0
0 3
4 5 KOH/Hydrochar Ratio
6
Fig. 2. Effects of KOH/hydrochar ratio and (a) activation temperature, (b) activation time on the yields of porous carbons.
10
20 30 40 Pore size (Å)
50
60
Fig. 4. (a) N2 adsorption and desorption isotherms, and (b) DFT pore size distribution curves of porous carbons obtained at different activation temperature when the activation time and KOH/hydrochar ratio were fixed at 1 h and 4:1.
Fig. 3. TEM micrographs of (a) C/4:1/800/1, (b) C/6:1/800/1, SEM micrographs of (c) C/4:1/800/1, and (d) C/6:1/800/1.
3.1.1. Effects of activation temperature Fig. 4 shows that the porous carbons obtained from different temperature mainly contained micropores and mesopores. As the activation temperature increased from 700 to 900 °C, the N2 adsorption/desorption isotherms (Fig. 4a) of the porous carbon
exhibited different type. In the case of porous carbon produced at 700 and 800 °C, the isotherms were similar to type I and II according to the IUPAC classification. When the temperature rose to 900 °C, the isotherm appeared obvious H4 hysteresis loop (occurred at the pressure from 0.4 to 0.9 P/P0) and displayed types I
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L. Ding et al. / Chemical Engineering Journal 225 (2013) 300–305 Table 1 Texture properties and the specific capacity of the porous carbons. Vmicd (cm3/g)
Smice (m2/g)
Sextf (m2/g)
Cg (F/g)
C/4:1/700/1 C/4:1/800/1 C/4:1/900/1 C/3:1/800/1 C/5:1/800/1 C/6:1/800/1 C/4:1/800/0.5 C/4:1/800/1.5 C/4:1/800/2
2322 2579 2825 3214 2567 3611 2705 2739 3220
1.57 1.84 2.17 2.08 1.78 2.68 1.95 1.39 2.35
2.70 2.84 3.08 2.59 2.77 2.96 2.88 2.77 2.92
0.30 0.31 0.29 0.47 0.29 0.35 0.32 0.34 0.36
1323 1366 1340 1918 1355 1756 1432 1526 1631
998 1213 1484 1296 1212 1855 1273 1267 1589
206 193 182 164 222 236 136 112 172
BET surface area. Total pore volume. Average pore diameter. t-method micropore volume. t-method micropore surface area. t-method external surface area. Specific capacity calculated by galvanostatic charge–discharge curves at 1.25 A/g.
3.1.2. Effects of activation time From the Table 1 we can see that the porous carbons obtained from 0.5 to 2 h did not appear big difference in micropore volume and average pore diameter. The changes of external surface area were small when the time increased from 0.5 to 1.5 h, and the total pore volume showed a downward trend and the micropore area presented a rising trend, which indicated that long activation time was conducive to formation of more micropores. By contrast, the external surface area and the total pore volume of the porous carbon obtained at 2 h displayed a noticeable increase, respectively. These results were consistent with the results of pore size distribution curves (Fig. 5), which reflected that the external surface area was mainly from the contribution of mesopores. We can deduced from the above results that the activation time only produced a slight effect on the textural properties of porous carbon in a short time (0.5–1.5 h), and the long time (2 h) had an important influence on the development of mesopores. There were three stages
-1
1500
a
Adsorption isotherms Desorption isotherms
3
and IV. Three adsorption isotherms presented steep increase at relative pressure below 0.1 P/P0, which was attributed to micropore adsorption. But the micropore adsorption only exhibited slight increase with the activation temperature increasing, which indicated they had similar micropore. The adsorption isotherms rapidly increased in the region of middle and high relative pressure and no platform appeared, it illustrated that porous carbons might contain mesopores. From Fig. 4b we can see the pore size distribution of mesopores became wide with increasing the temperature. The pore diameter of the micropores and mesopores were about 0.9–2 nm and 2– 5 nm, respectively. Table 1 lists the details of the texture properties of the porous carbons. Along with the increase of temperature, the BET surface area, total pore volume, average pore diameter and the external surface area of the porous carbons increased, and micropore volume and micropore area had no obvious change. The above results demonstrate that the high temperature is benefit to the development of mesopores. According to previous reports [17,18] about the KOH activation mechanism we know that KOH can be transformed into K2CO3 during KOH activation, K2CO3 could continue to take part in reaction with carbon. The melting point of K2CO3 (891 °C) is far higher than KOH (380 °C). Therefore, increasing the temperature could contribute to the reaction of K2CO3 and hydrochar, which leaded to further development of pores. In addition, Lozano-Castelló et al. [19] reported that the Gibbs free energy (DG) of the reaction between KOH and carbon became negative at about 570 °C, which indicated that the high temperature was benefit to the KOH activation.
Adsorbed volume (cm g )
f g
Davec (nm)
1200 900 600
0.5 h 1h 1.5 h 2h
300 0 0.0
0.2 0.4 0.6 0.8 Reletive pressure p/po
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Vtb (cm3/g)
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c d
SBETa (m2/g)
dV/dlogD (cm g )
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Sample
0.06 0.04 0.02 0.00 0
10
20 30 40 Pore size (Å)
50
60
Fig. 5. (a) N2 adsorption and desorption isotherms, and (b) DFT pore size distribution curves of porous carbons obtained at different activation time when the activation temperature and KOH/hydrochar ratio were fixed at 800 °C and 4:1.
in the pore development during activation, opening of inaccessible pores, creating of new pores and widening of the existing pores [20,21]. At the beginning of the activation, KOH reacted with hydrochar, the micropores were formed. With the etching time increasing, KOH continued to react with the carbon layer inside the pores, the existing micropores grew larger, large amounts of mesopores were generated. Although some micropores disappeared along with the formation of mesopores, meanwhile, new micropores were forming inside the existing mesopores and hydrochar, too. Therefore, micropores still increased with the increasing time. However, since the formation of mesopores was limited by the existing micropores, the changes of mesopores were small in a short time (0.5–1.5 h). In addition, the long time (2 h)
L. Ding et al. / Chemical Engineering Journal 225 (2013) 300–305
1600 1200 800
3:1 4:1 5:1 6:1
400 0
0.0
O1s
22000
0.2 0.4 0.6 0.8 Reletive pressure p/po
Counts per second (a.u.)
3
Adsorption isotherms Desorption isotherms
a
-1
Adsorbed volume (cm g )
304
20000 18000 16000 14000 12000 10000
1.0
8000 540
538
0.12
3:1 4:1 5:1 6:1
0.09
3
-1
dV/dlogD (cm g )
b
536 534 532 530 Binding energy (eV)
528
Fig. 7. O1s XPS spectra of the sample C/6:1/800/1.
0.06
account the impact of these factors, it is not surprising that the BET surface area of porous carbons decreased first, and then increased.
0.03
3.2. XPS analysis
0.00
0
10
20
30 40 Pore size (Å)
50
60
Fig. 6. (a) N2 adsorption and desorption isotherms, and (b) DFT pore size distribution curves of porous carbons obtained at different KOH/hydrochar ratios when the activation temperature and time were fixed at 800 °C and 1 h.
The relative concentration of surface oxygen in porous carbon (C/6:1/800/1) was 4.17 (at.%). Fig. 7 shows the O1s spectra of the sample C/6:1/800/1. The O1s spectra was deconvoluted into two peaks, the binding energy around 532.2 eV can be attributed to C@O carbonyl group and/or CAOAC ether groups, the binding energy around 534.6 eV can be assigned to chemisorbed oxygen
0.03
allowed a better contact between the melted KOH and carbon, therefore more micropores and mesopores produced. Current (A)
0.01 0.00 -0.01
3:1 4:1 5:1 6:1
-0.02 -0.03 -1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
Potential (V)
0.0
3:1 4:1 5:1 6:1
b
-0.2 Potential (V)
3.1.3. Effects of KOH/hydrochar ratio The KOH/hydrochar ratio is important for the development of porosity (Table 1 and Fig. 6). The micropore area, micropore volume, BET surface area and total pore volume of the porous carbon presented a downward trend with increasing the KOH/hydrochar ratio from 3 to 5. However, they rose abruptly when the KOH/ hydrochar ratio was changed from 5 to 6. The external surface area almost unchanged with the KOH/hydrochar ratio varying from 3 to 5, and then rose from 1212 to 1855 m2/g with further increasing the KOH/hydrochar ratio from 5 to 6. This is due to the formation of large amounts of mesopores. Tseng et al. proposed an interesting mechanism for KOH activation [22], it demonstrated that both physical surface pyrolysis and chemical KOH etching could be involved in the activation process when the KOH/char ratio was low, when the KOH/char ratio exceeded a certain amount, the KOH etching reaction was dominant. Our previous research [23] reported the BET surface area of a kind of hydrochar treated at 800 °C without KOH addition could reach 1034 m2/g, this result revealed that physical surface pyrolysis could contribute to the activation reported here. The competition between physical surface pyrolysis and chemical KOH etching is the most possible explanation for the discontinuity in the properties of the porous carbons when the KOH/hydrochar ratio changed from 3 to 6. Sample C/ 3:1/800/1 had so high specific surface area, which can be ascribed to the fully reaction of the physical surface pyrolysis and chemical KOH etching. When KOH/hydrochar ratio was between 4 and 6, the relatively high coverage of KOH onto hydrochar inhibited the physical surface pyrolysis and promoted KOH etching, excessive KOH entered into interior of hydrochar, new pores created. Taking into
a
0.02
-0.4 -0.6 -0.8 -1.0 -1.2
0
60
120
180
240
300
360
420
Time (s) Fig. 8. (a) CV curves and (b) galvanostatic charge–discharge curves of the porous carbons obtained at different KOH/hydrochar ratios when the activation temperature and time were fixed at 800 °C and 1 h.
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(COOH carboxylic groups) and/or water. The content of surface nitrogen in sample C/6:1/800/1 was below the detection limits. The results of XPS spectra demonstrated that the obtained porous carbons only had very small amounts of surface functional groups.
and Interdisciplinary (201003030).
3.3. Electrochemical performances of porous carbons
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In order to examine the electrochemical performances of the obtained porous carbons, the cyclic voltammetry (CV) and galvanostatic charge discharge measurements were carried out. Fig. 8 shows the CV curve (10 mV/s) and charge/discharge curve (1.25 A/g) of the porous carbon obtained at different KOH/hydrochar ratio when the activation temperature and time were fixed at 800 °C and 1 h. The CV curves show a quasi-rectangular shape, displaying typical characteristic of electric double layer capacitance. This result confirmed that the contribution of pseudocapacitance generated from the surface functional groups to the capacitance of the obtained porous carbon is not important. The CV curves enclosed areas increased with increasing the KOH/ hydrochar ratio, indicating the increase of capacitance. The charge/discharge curves exhibit similar symmetric triangular shape and an extremely small deviation from ideal voltage–time curves, which may be associated with the internal resistance during the changing of polarity. The specific capacitances obtained at the current density of 1.25 A/g are listed in Table 1. The highest specific capacitance of C/6:1/800/1 could achieve 236 F/g. It is interesting to note that the specific surface area of sample C/4:1/ 700/1, C/4:1/800/1 and C/5:1/800/1 (2322, 2579 and 2567 m2/g) are far smaller than C/6:1/800/1 (3611 m2/g), the specific capacitance of them also could reach 206, 193 and 222 F/g. Such excellent electrochemical performance illustrate they have more accessible surface area for the formation of electric double layer and are expected to be promising electrode materials. 4. Conclusions In summary, we have successfully realized the conversion of corncob into energy materials by a new fabrication route, which utilizes sulfuric acid to catalyze the hydrolysis of corncobs and KOH to fabricate the porous structures. This route is a suitable method for the production of porous carbon with low ash content (0.51 wt%), large specific surface area (2322–3611 m2/g) and highly developed pore structures. The textural properties of the porous carbons can be tuned by changing the activation conditions. The electrochemical performance of porous carbon has been explored, Sample C/6:1/800/1 exhibits the best capacitance that is up to 236 F/g. Such designed porous carbons are highly promising materials for electric double layer capacitor. In addition, they may be useful in other applications such as catalytic and adsorbent. Another advantage of this route is that both biomass and industrial wastewater containing carbohydrates can be used as the raw materials of hydrochar, which can be used for the production of functional carbonaceous materials. Acknowledgements This research work was supported by Scientific and Technological Planning Project of Jilin Province (20100326 and 20120311)
Research
Project
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Jilin
University
References
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
A new route for conversion of corncob to porous carbon by hydrolysis and activation Lili Ding a, Bo Zou a, Hequn Liu a, Yannan Li a, Zichen Wang a, Ying Su b, Yupeng Guo a,⇑, Xiaofeng Wang a,⇑ a b
College of Chemistry, Jilin University, Qianjin Street 2699, Changchun 130012, China The Department of Electronic Science, Changchun Institute of Engineering Technology, Huayuan Road 1, Changchun 130017, China
h i g h l i g h t s The new route could convert waste corncob to high performance carbon materials. This route was realized by the process of acid hydrolysis and alkali activation. 2
High surface area (up to 3611 m /g) was achieved. Porous carbon had specific capacitance of up to 236 F/g. This route could be readily extended to abundant lignocellulose biomass and industrial wastewater containing carbohydrates.
a r t i c l e
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Article history: Received 14 January 2013 Received in revised form 8 March 2013 Accepted 23 March 2013 Available online 30 March 2013 Keywords: Corncob Hydrolysis Conversion Activation Porous carbon
a b s t r a c t A new route for conversion of corncob to high performance porous carbon was proposed. This route involved hydrolysis of corncobs to dissolved sugars and degradation products of carbohydrates by sulfuric acid catalysis, transformation of the liquid hydrolyzate into solid hydrochar by dehydrating, polymerization and carbonization, fabrication of porous structures by activation. By using this route, high surface area (up to 3611 m2/g) and low ash content (0.51 wt%) were achieved. A cottony structure appeared on the surface of the porous carbon in a high KOH/hydrochar ratio. The specific capacitance for porous carbon was up to 236 F/g, which indicated the excellent electrochemical properties of these materials. Furthermore, the highly developed pore structures made the porous carbon possess potential applications in separation, catalysis, hydrogen storage and other fields. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Facing with serious environmental problems and gradual depletion of resources, the society requires replacement of petroleum based products. Biomass is the most abundant and the fastest growing renewable energy. Conversion of biomass into energy materials not only reduce environmental pollution but also produce enormous economic benefits. Corncob, one of the lignocellulose biomass, is the byproduct of corn production. In China, about 30 million tons of corncobs are produced annually. To realize the effective utilization of corncobs, some researchers utilized the main components (approximately 40% cellulose and 30% hemicellulose) of corncob to produce glucose, xylose, furfural, ethanol and so on by hydrolysis [1–4]. However, some unavoidable byproducts produced along with hydrolysis, such as polysaccharide, arabinose, ⇑ Corresponding authors. Tel./fax: +86 431 8515 5358. E-mail addresses: [email protected] (Y. Guo), [email protected] (X. Wang). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.090
galactose, mannose and some other degradation products of carbohydrates, these substances were hard to be isolated from the biomass hydrolyzates and generated harmful influences on the production of downstream materials. In addition, discharge of industrial wastewater containing carbohydrates will cause serious environmental pollution. In order to solve the separation problem of different hydrolysates and make full use of the biomass hydrolysates and the carbohydrates in industrial wastewater, we envisage new approaches that convert the carbohydrates and their degradation products in liquid biomass hydrolysates or industrial wastewater to solid carbonaceous material by dehydration, carbonization with sulfuric acid. Among all of the carbonaceous materials, porous carbons are especially promising owing to their wide applications. In most cases, the utility of the porous carbons come from relatively high surface areas and their unique porous structures [5–7]. The specific surface area of ordinary corncobs-based activated carbon is less than 2000 m2/g. To obtain larger surface area and better application properties, Cao et al. added soap to activator (KOH solution)
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[8], Tseng et al. combined the KOH etching and CO2 gasification [9], the specific surface area of corncobs-based carbon could exceed 2000 m2/g. In addition to activating agent, the structure of precursor also generated tremendous effect on the porous carbon [10– 12]. In this paper, we propose a new route to convert corncob to useful porous carbon, which does not mix the biomass (or carbonized biomass) with activator directly, but transform the lignocellulosic corncob into hydrochars by hydrolysis first, and then fabricate porous structures by activation. The impurity in corncob has been removed by hydrolysis, which reduce the ash content of porous carbon. Furthermore, spherical hydrochar, the new porous carbon precursor, could largely enhance the specific surface area of the porous carbon compared to the lignocellulosic corncob that may give these porous carbons highly preferential application in many fields. The main aim of this work is to convert waste corncob into high performance energy materials. 2. Experimental 2.1. Materials and reagents Corncob was obtained from local farmer near Changchun, and grinded to particle size of 1–5 mm. The elemental analysis of corncob was 46.53 wt% C, 6.37 wt% H, 44.45 wt% O and 0.64 wt% N. The ash content was 2.01 wt%. The content of hemicellulose, cellulose and lignin in corncob were 40.5 wt%, 36.8 wt% and 20.7 wt%, respectively. Sulfuric acid and potassium hydroxide were analytical grade. 2.2. Acid hydrolysis of corncob The concept of sulfuric acid hydrolysis in a low temperature and atmospheric pressure come from the acid hydrolysis of rice husk reported in our previous work [13]. Here, sulfuric acid (68 wt%) was used as catalyst to hydrolyze the corncob. Fig. 1 shows the schematic diagram of the preparation process of porous carbon. The ratio of corncob to 68 wt% sulfuric acid was 1:10 (g/ml). Hydrolysis reaction was conducted at 55 °C for 10 min. Then the acid concentration of hydrolyate was diluted to 55 wt%. The liquid product was separated from solid by filtration. The hydrolysis solution containing dissolved sugars and some degradation products of carbohydrates was used for production of hydrochar. The hydrolysis solution was heated at 95 °C for 6 h. The black solid product formed in solution was separated from liquid product by filtering after the reaction. After washing several times with distilled water, the solid hydrochar was obtained. The liquid product containing sulfuric acid can be recycled. 2.3. Fabrication of porous structures Potassium hydroxide was used to fabricate porous structures in hydrochar by activation based on many Refs. [14–16] which used potassium hydroxide to create highly developed pore structures. Hydrochar was treated at 500 °C for 0.5 h under nitrogen atmosphere before activation. The yield was calculated by the mass ratio of the obtained porous carbon and the treated hydrochar.
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A two-step process of KOH activation was adopted for preparation of porous carbons. Firstly, the activation was carried out at 500 °C for 0.5 h, then in a temperature range of 700–900 °C for 0.5–2 h. The ratios of KOH/hydrochar were from 3:1 to 6:1. The porous carbons will be denoted according to the expression: C/ KOH:hydrochar/Temperature (°C)/Time (h). For example, the porous carbon obtained at 800 °C 2 h when the KOH:hydrochar ratio was 4:1, the porous carbon was denoted as C/4:1/800/2. All the porous carbons prepared from the above process were heated to 800 °C for 1 h under nitrogen atmosphere. 2.4. Characterizations The elemental analysis of corncob, hydrochars and porous carbon were performed on Perkin-Elmer 2400 C, H, N, O analyzer. The content of the hemicellulose, cellulose, lignin and ash in corncob were tested according to the China National Standards GB/T 2677.9-94, GB/T 2677.10-95, GB/T 747-2003 and GB/T 22427.12008. The morphology of the samples was characterized by a Hitachi H-800 transmission electron microscope (TEM) at an accelerating voltage of 200 kV, and a JSM-6700F scanning electron microscope (SEM). The Brunauer–Emmett–Teller (BET) surface area and the pore structure analysis were conducted on a Micromeritics ASAP 2010 Surface Analyzer. The pore size distributions were calculated by the Density Functional Theory (DFT) method. The X-ray Photoelectron Spectroscopy (XPS) measurements were performed on the ESCALAB250 using Al Ka excitation source (ht = 1486.6 eV). The quantitative analysis of O was based on the peak intensities of the O1s. The electrochemical measurements were conducted on a CHI 6600 electrochemical workstation in a three electrode configuration in 6 mol/L KOH aqueous solution.
3. Results and discussion The hydrochar contained 63.13 wt% C, 4.41 wt% H, 0.46 wt% N, 31.74 wt% O, and 0.21 wt% ash. The porous carbon, obtained at 800 °C 1 h at the KOH/hydrochar ratio of 4:1, contained 92.73 wt% C, 0.63 wt% H, 0.41 wt% N, 5.72 wt% O and 0.51 wt% ash. The yield of porous carbons was from 27% to 46% (Fig. 2), and the yield showed decreasing trend along with the increase of the activation time and temperature. The representative TEM (Fig. 3a and b) and SEM (Fig. 3c and d) images revealed that sample C/4:1/800/1 could maintain the original spherical structure of the hydrochar. The cottony structures appeared on the surface of the porous carbon (Fig. 3b) when the activation was carried out in a large amount of KOH. This suggested that the surface pyrolysis and interior etching processes occurred simultaneously during the preparation process. Fig. 3d confirmed that excessive dosage of KOH destroyed the spherical structure of the hydrochar. 3.1. Effects of activation conditions on textural properties Activation temperature, time and ratio of the KOH to hydrochar all affected the textural properties of porous carbons. Therefore, these factors were investigated.
Fig. 1. Schematic illustration of the preparation process of porous carbons.
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Yield (%)
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Fig. 2. Effects of KOH/hydrochar ratio and (a) activation temperature, (b) activation time on the yields of porous carbons.
10
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Fig. 4. (a) N2 adsorption and desorption isotherms, and (b) DFT pore size distribution curves of porous carbons obtained at different activation temperature when the activation time and KOH/hydrochar ratio were fixed at 1 h and 4:1.
Fig. 3. TEM micrographs of (a) C/4:1/800/1, (b) C/6:1/800/1, SEM micrographs of (c) C/4:1/800/1, and (d) C/6:1/800/1.
3.1.1. Effects of activation temperature Fig. 4 shows that the porous carbons obtained from different temperature mainly contained micropores and mesopores. As the activation temperature increased from 700 to 900 °C, the N2 adsorption/desorption isotherms (Fig. 4a) of the porous carbon
exhibited different type. In the case of porous carbon produced at 700 and 800 °C, the isotherms were similar to type I and II according to the IUPAC classification. When the temperature rose to 900 °C, the isotherm appeared obvious H4 hysteresis loop (occurred at the pressure from 0.4 to 0.9 P/P0) and displayed types I
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L. Ding et al. / Chemical Engineering Journal 225 (2013) 300–305 Table 1 Texture properties and the specific capacity of the porous carbons. Vmicd (cm3/g)
Smice (m2/g)
Sextf (m2/g)
Cg (F/g)
C/4:1/700/1 C/4:1/800/1 C/4:1/900/1 C/3:1/800/1 C/5:1/800/1 C/6:1/800/1 C/4:1/800/0.5 C/4:1/800/1.5 C/4:1/800/2
2322 2579 2825 3214 2567 3611 2705 2739 3220
1.57 1.84 2.17 2.08 1.78 2.68 1.95 1.39 2.35
2.70 2.84 3.08 2.59 2.77 2.96 2.88 2.77 2.92
0.30 0.31 0.29 0.47 0.29 0.35 0.32 0.34 0.36
1323 1366 1340 1918 1355 1756 1432 1526 1631
998 1213 1484 1296 1212 1855 1273 1267 1589
206 193 182 164 222 236 136 112 172
BET surface area. Total pore volume. Average pore diameter. t-method micropore volume. t-method micropore surface area. t-method external surface area. Specific capacity calculated by galvanostatic charge–discharge curves at 1.25 A/g.
3.1.2. Effects of activation time From the Table 1 we can see that the porous carbons obtained from 0.5 to 2 h did not appear big difference in micropore volume and average pore diameter. The changes of external surface area were small when the time increased from 0.5 to 1.5 h, and the total pore volume showed a downward trend and the micropore area presented a rising trend, which indicated that long activation time was conducive to formation of more micropores. By contrast, the external surface area and the total pore volume of the porous carbon obtained at 2 h displayed a noticeable increase, respectively. These results were consistent with the results of pore size distribution curves (Fig. 5), which reflected that the external surface area was mainly from the contribution of mesopores. We can deduced from the above results that the activation time only produced a slight effect on the textural properties of porous carbon in a short time (0.5–1.5 h), and the long time (2 h) had an important influence on the development of mesopores. There were three stages
-1
1500
a
Adsorption isotherms Desorption isotherms
3
and IV. Three adsorption isotherms presented steep increase at relative pressure below 0.1 P/P0, which was attributed to micropore adsorption. But the micropore adsorption only exhibited slight increase with the activation temperature increasing, which indicated they had similar micropore. The adsorption isotherms rapidly increased in the region of middle and high relative pressure and no platform appeared, it illustrated that porous carbons might contain mesopores. From Fig. 4b we can see the pore size distribution of mesopores became wide with increasing the temperature. The pore diameter of the micropores and mesopores were about 0.9–2 nm and 2– 5 nm, respectively. Table 1 lists the details of the texture properties of the porous carbons. Along with the increase of temperature, the BET surface area, total pore volume, average pore diameter and the external surface area of the porous carbons increased, and micropore volume and micropore area had no obvious change. The above results demonstrate that the high temperature is benefit to the development of mesopores. According to previous reports [17,18] about the KOH activation mechanism we know that KOH can be transformed into K2CO3 during KOH activation, K2CO3 could continue to take part in reaction with carbon. The melting point of K2CO3 (891 °C) is far higher than KOH (380 °C). Therefore, increasing the temperature could contribute to the reaction of K2CO3 and hydrochar, which leaded to further development of pores. In addition, Lozano-Castelló et al. [19] reported that the Gibbs free energy (DG) of the reaction between KOH and carbon became negative at about 570 °C, which indicated that the high temperature was benefit to the KOH activation.
Adsorbed volume (cm g )
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Fig. 5. (a) N2 adsorption and desorption isotherms, and (b) DFT pore size distribution curves of porous carbons obtained at different activation time when the activation temperature and KOH/hydrochar ratio were fixed at 800 °C and 4:1.
in the pore development during activation, opening of inaccessible pores, creating of new pores and widening of the existing pores [20,21]. At the beginning of the activation, KOH reacted with hydrochar, the micropores were formed. With the etching time increasing, KOH continued to react with the carbon layer inside the pores, the existing micropores grew larger, large amounts of mesopores were generated. Although some micropores disappeared along with the formation of mesopores, meanwhile, new micropores were forming inside the existing mesopores and hydrochar, too. Therefore, micropores still increased with the increasing time. However, since the formation of mesopores was limited by the existing micropores, the changes of mesopores were small in a short time (0.5–1.5 h). In addition, the long time (2 h)
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1600 1200 800
3:1 4:1 5:1 6:1
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b
536 534 532 530 Binding energy (eV)
528
Fig. 7. O1s XPS spectra of the sample C/6:1/800/1.
0.06
account the impact of these factors, it is not surprising that the BET surface area of porous carbons decreased first, and then increased.
0.03
3.2. XPS analysis
0.00
0
10
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30 40 Pore size (Å)
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Fig. 6. (a) N2 adsorption and desorption isotherms, and (b) DFT pore size distribution curves of porous carbons obtained at different KOH/hydrochar ratios when the activation temperature and time were fixed at 800 °C and 1 h.
The relative concentration of surface oxygen in porous carbon (C/6:1/800/1) was 4.17 (at.%). Fig. 7 shows the O1s spectra of the sample C/6:1/800/1. The O1s spectra was deconvoluted into two peaks, the binding energy around 532.2 eV can be attributed to C@O carbonyl group and/or CAOAC ether groups, the binding energy around 534.6 eV can be assigned to chemisorbed oxygen
0.03
allowed a better contact between the melted KOH and carbon, therefore more micropores and mesopores produced. Current (A)
0.01 0.00 -0.01
3:1 4:1 5:1 6:1
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3.1.3. Effects of KOH/hydrochar ratio The KOH/hydrochar ratio is important for the development of porosity (Table 1 and Fig. 6). The micropore area, micropore volume, BET surface area and total pore volume of the porous carbon presented a downward trend with increasing the KOH/hydrochar ratio from 3 to 5. However, they rose abruptly when the KOH/ hydrochar ratio was changed from 5 to 6. The external surface area almost unchanged with the KOH/hydrochar ratio varying from 3 to 5, and then rose from 1212 to 1855 m2/g with further increasing the KOH/hydrochar ratio from 5 to 6. This is due to the formation of large amounts of mesopores. Tseng et al. proposed an interesting mechanism for KOH activation [22], it demonstrated that both physical surface pyrolysis and chemical KOH etching could be involved in the activation process when the KOH/char ratio was low, when the KOH/char ratio exceeded a certain amount, the KOH etching reaction was dominant. Our previous research [23] reported the BET surface area of a kind of hydrochar treated at 800 °C without KOH addition could reach 1034 m2/g, this result revealed that physical surface pyrolysis could contribute to the activation reported here. The competition between physical surface pyrolysis and chemical KOH etching is the most possible explanation for the discontinuity in the properties of the porous carbons when the KOH/hydrochar ratio changed from 3 to 6. Sample C/ 3:1/800/1 had so high specific surface area, which can be ascribed to the fully reaction of the physical surface pyrolysis and chemical KOH etching. When KOH/hydrochar ratio was between 4 and 6, the relatively high coverage of KOH onto hydrochar inhibited the physical surface pyrolysis and promoted KOH etching, excessive KOH entered into interior of hydrochar, new pores created. Taking into
a
0.02
-0.4 -0.6 -0.8 -1.0 -1.2
0
60
120
180
240
300
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Time (s) Fig. 8. (a) CV curves and (b) galvanostatic charge–discharge curves of the porous carbons obtained at different KOH/hydrochar ratios when the activation temperature and time were fixed at 800 °C and 1 h.
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(COOH carboxylic groups) and/or water. The content of surface nitrogen in sample C/6:1/800/1 was below the detection limits. The results of XPS spectra demonstrated that the obtained porous carbons only had very small amounts of surface functional groups.
and Interdisciplinary (201003030).
3.3. Electrochemical performances of porous carbons
[1] E. Bahcegul, E. Tatli, N.I. Haykir, S. Apaydin, U. Bakir, Selecting the right blood glucose monitor for the determination of glucose during the enzymatic hydrolysis of corncob pretreated with different methods, Bioresour. Technol. 102 (2011) 9646–9652. [2] X. Liu, N. Ai, H. Zhang, M. Lu, D. Ji, F. Yu, J. Ji, Quantification of glucose, xylose, arabinose, furfural, and HMF in corncob hydrolysate by HPLC–PDA–ELSD, Carbohyd. Res. 353 (2012) 111–114. [3] L. Bu, Y. Tang, Y. Gao, H. Jian, J. Jiang, Comparative characterization of milled wood lignin from furfural residues and corncob, Chem. Eng. J. 175 (2011) 176– 184. [4] K.K. Cheng, J.A. Zhang, E. Chavez, J.P. Li, Integrated production of xylitol and ethanol using corncob, Appl. Microbiol. Biot. 87 (2010) 411–417. [5] K. Xia, Q. Gao, J. Jiang, J. Hu, Hierarchical porous carbons with controlled micropores and mesopores for supercapacitor electrode materials, Carbon 46 (2008) 1718–1726. [6] R.Q. Sun, L.B. Sun, Y. Chun, Q.H. Xu, Catalytic performance of porous carbons obtained by chemical activation, Carbon 46 (2008) 1757–1764. [7] M. Sevilla, R. Mokaya, A.B. Fuertes, Ultrahigh surface area polypyrrole-based carbons with superior performance for hydrogen storage, Energy Environ. Sci. 4 (2011) 2930–2936. [8] Q. Cao, K.C. Xie, Y.K. Lv, W.R. Bao, Process effects on activated carbon with large specific surface area from corncob, Bioresour. Technol. 97 (2006) 110–115. [9] R.L. Tseng, S.K. Tseng, F.C. Wu, Preparation of high surface area carbons from corncob with KOH etching plus CO2 gasification for the adsorption of dyes and phenols from water, Colloids Surf. A: Physicochem. Eng. Aspects 279 (2006) 69–78. [10] T. Morishita, Y. Soneda, T. Tsumura, M. Inagaki, Preparation of porous carbons from thermoplastic precursors and their performance for electric double layer capacitors, Carbon 44 (2006) 2360–2367. [11] S.E. Chun, J.F. Whitacrea, The evolution of electrochemical functionality of carbons derived from glucose during pyrolysis and activation, Electrochim. Acta 60 (2012) 392–400. [12] O. Ioannidou, A. Zabaniotou, Agricultural residues as precursors for activated carbon production-A review, Renew. Sust. Energ. Rev. 11 (2007) 1966–2005. [13] L. Wang, Y. Guo, Y. Zhu, Y. Li, Y. Qu, C. Rong, X. Ma, Z. Wang, A new route for preparation of hydrochars from rice husk, Bioresour. Technol. 101 (2010) 9807–9810. [14] Y. Lv, F. Zhang, Y. Dou, Y. Zhai, J. Wang, H. Liu, Y. Xia, B. Tu, D. Zhao, A comprehensive study on KOH activation of ordered mesoporous carbons and their supercapacitor application, J. Mater. Chem. 22 (2012) 93. [15] S. Murali, J.R. Potts, S. Stoller, J. Park, M.D. Stoller, L.L. Zhang, Y. Zhu, R.S. Ruoff, Preparation of activated graphene and effect of activation parameters on electrochemical capacitance, Carbon 50 (2012) 3482–3485. [16] M. Choi, R. Ryoo, Mesoporous carbons with KOH activated framework and their hydrogen adsorption, J. Mater. Chem. 17 (2007) 4204–4209. [17] J. Alcañiz-Monge, M.J. Illán-Gómez, Insight into hydroxides-activated coals: chemical or physical activation?, J Colloid. Interface Sci. 318 (2008) 35–41. [18] E. Raymundo-Piñero, P. Azaïs, T. Cacciaguerra, D. Cazorla-Amorós, A. LinaresSolano, F. Béguin, KOH and NaOH activation mechanisms of multiwalled carbon nanotubes with different structural organisation, Carbon 43 (2005) 786–795. [19] D. Lozano-Castelló, J.M. Calo, D. Cazorla-Amorós, A. Linares-Solano, Carbon activation with KOH as explored by temperature programmed techniques, and the effects of hydrogen, Carbon 45 (2007) 2529–2530. [20] J. Wang, M. Chen, C. Wang, J. Wang, J. Zheng, Preparation of mesoporous carbons from amphiphilic carbonaceous material for high-performance electric double-layer capacitors, J. Power Sources 196 (2011) 552. [21] W.M.A.W. Daud, W.S.W. Ali, M.Z. Sulaiman, The effects of carbonization temperature on pore development in palm–shell-based activated carbon, Carbon 38 (2000) 1928. [22] R.L. Tseng, S.K. Tseng, F.C. Wu, C.C. Hu, C.C. Wang, Effects of micropore development on the physicochemical properties of KOH-activated carbons, J. Chin. Inst. Chem. Eng. 39 (2008) 37–47. [23] L. Wang, X. Wang, B. Zou, X. Ma, Y. Qu, C. Rong, Y. Li, Y. Su, Z. Wang, Preparation of carbon black from rice husk by hydrolysis, carbonization and pyrolysis, Bioresour. Technol. 102 (2011) 8220–8224.
In order to examine the electrochemical performances of the obtained porous carbons, the cyclic voltammetry (CV) and galvanostatic charge discharge measurements were carried out. Fig. 8 shows the CV curve (10 mV/s) and charge/discharge curve (1.25 A/g) of the porous carbon obtained at different KOH/hydrochar ratio when the activation temperature and time were fixed at 800 °C and 1 h. The CV curves show a quasi-rectangular shape, displaying typical characteristic of electric double layer capacitance. This result confirmed that the contribution of pseudocapacitance generated from the surface functional groups to the capacitance of the obtained porous carbon is not important. The CV curves enclosed areas increased with increasing the KOH/ hydrochar ratio, indicating the increase of capacitance. The charge/discharge curves exhibit similar symmetric triangular shape and an extremely small deviation from ideal voltage–time curves, which may be associated with the internal resistance during the changing of polarity. The specific capacitances obtained at the current density of 1.25 A/g are listed in Table 1. The highest specific capacitance of C/6:1/800/1 could achieve 236 F/g. It is interesting to note that the specific surface area of sample C/4:1/ 700/1, C/4:1/800/1 and C/5:1/800/1 (2322, 2579 and 2567 m2/g) are far smaller than C/6:1/800/1 (3611 m2/g), the specific capacitance of them also could reach 206, 193 and 222 F/g. Such excellent electrochemical performance illustrate they have more accessible surface area for the formation of electric double layer and are expected to be promising electrode materials. 4. Conclusions In summary, we have successfully realized the conversion of corncob into energy materials by a new fabrication route, which utilizes sulfuric acid to catalyze the hydrolysis of corncobs and KOH to fabricate the porous structures. This route is a suitable method for the production of porous carbon with low ash content (0.51 wt%), large specific surface area (2322–3611 m2/g) and highly developed pore structures. The textural properties of the porous carbons can be tuned by changing the activation conditions. The electrochemical performance of porous carbon has been explored, Sample C/6:1/800/1 exhibits the best capacitance that is up to 236 F/g. Such designed porous carbons are highly promising materials for electric double layer capacitor. In addition, they may be useful in other applications such as catalytic and adsorbent. Another advantage of this route is that both biomass and industrial wastewater containing carbohydrates can be used as the raw materials of hydrochar, which can be used for the production of functional carbonaceous materials. Acknowledgements This research work was supported by Scientific and Technological Planning Project of Jilin Province (20100326 and 20120311)
Research
Project
of
Jilin
University
References