第 20 卷 第 1 期
2014 年 2 月
燃 烧 科 学 与 技 术
Journal of Combustion Science and Technology
Vol.20 No.1
Feb. 2014
DOI 10.11715/rskxjs. R201308009
Preparation and Characterization of Activated Carbons
Based Alkali Lignin by KOH Chemical Activation
Xiao Gang，Wu Rongbing，Zhou Huilong，Ni Mingjiang，Gao Xiang，Cen Kefa
(State Key Laboratory of Clean Energy Utilization，Zhejiang University，Hangzhou 310027，China)
Abstract：Activated carbon(AC)with high specific surface area was prepared from alkali lignin by chemical activation
with KOH and its characteristics were investigated.The activation processes were conducted at various temperatures(T，
600—900 ℃)，KOH/lignin mass ratios(R，1∶1—4∶1)，activation times(t，1—3,h).Both scanning electron microscopy(SEM)and nitrogen adsorption isotherm as Brunauer-Emmett-Teller(BET)were used to identify the structural characteristics of the tested AC，and Fourier transform infrared spectroscopy(FT-IR)method was applied to analyze surface functional groups.Under experimental conditions，the optimum condition for preparing high specific surface AC of 2 684,m2/g
was R＝3∶1，T＝750,℃，t＝1,h.The results showed that R was the critical factor influencing the BET specific surface
area compared with T and t.Pore distribution of the prepared AC mainly concentrated on micropores and
mesopores.Chemical groups such as —OH and —C＝O were characterized by FT-IR.
Keywords：lignin；activated carbon；chemical activation；KOH
KOH 活化木质素制备高比表面积活性炭特性研究
肖 刚，吴荣兵，周慧龙，倪明江，高 翔，岑可法
(浙江大学能源清洁利用国家重点实验室，杭州 310027)
摘
要：以木质素为制备原料，采用 KOH 为活化剂制备高比表面积活性炭．通过探究不同活化温度、活化剂与木
质素质量比、活化时间对活性炭孔隙结构的影响，优化木质素活性炭的制备工艺．采用 BET、SEM、FTIR 等手段
对活性炭性能进行表征．实验结果表明，最佳制备工况为 KOH/木质素质量比为 3∶1，活化时间 1,h，活化温度为
750,℃．其中，KOH/木质素质量比对焦炭孔隙结构影响最大，活化温度次之，活化时间影响最小．制备的活性炭比
表面积最高可达 2,684,m2/g，孔径分布以中微孔居多，表面含有—OH、—C＝O 等基团．
关键词：木质素；活性炭；化学活化；KOH
中图分类号：TK6
文献标志码：A
Activated carbon(AC)is widely used due to its adsorptive capacity with a high specific surface area and
functional groups，such as adsorbent for removal of
pollutions[1-2]，catalyst support in catalysis[3-4] and gas
storage[5]. The world demand for AC was about 4.28
million metric tonnes in 2012 and it is expected to increase by more than 10% per year over the next five
years[6]. However，its wide applications are limited by
文章编号：1006-8740(2014)01-0014-07
high production cost. In recent years，many researchers
have prepared AC from many cheaper and renewable
materials. Agricultural wastes are promising sources of
AC because of their low cost and abundance. Presently，
quite a lot of literature has reported the use of nut
shells[7-8]，bamboo[9]，fruit shell[10]，lotus stalk[11]，etc.，
as raw materials owing to their high carbon and low ash
contents. Lignin is particularly suitable for preparation
收稿日期：2013-08-16.
基金项目：国家自然科学基金资助项目(51276167；50806013)．
作者简介：肖 刚（1979—
），男，博士，副教授．
通讯作者：肖 刚，[email protected]．
网络出版时间：2013-11-05. 网络出版地址：http://www.cnki.net/kcms/detail/12.1240.TK.20131105.1115.001.html．
2014 年 2 月
肖 刚等：KOH 活化木质素制备高比表面积活性炭特性研究
of AC because of its high phenolic content，which helps
to get higher carbon yield. Furthermore，lignin is the
second major component in biomass after cellulose and
produced in large quantities as a by-product from pulping industry. In the pulp and paper mills of China，at
least 5 million tons of waste lignin slurry are produced
every year[12].
The properties of AC are dependent on precursors
and preparation methods. Basically，there are two different processes for preparation of AC，namely physical
and chemical activations. Physical activation refers to
carbonization of carbonaceous precursor with activating
agent such as CO2 ，water steam，air，etc. Compared
with physical activation，chemical agent is impregnated
and inlaid into internal structure of precursor during
chemical activation. Through a series of crosslinking
and polymerization reaction，AC with developed porosity is obtained. Alkali chemical reagents，such as KOH，
NaOH，are often used as chemical activating agents due
to their suppression of production of tar，consequently
improving the yield of AC. In KOH activation process，
a considerable amount of K2CO3 is formed due to reaction between KOH and carbon，making pore structure
developed. Furthermore，potassium metal may be liberated at the reaction temperature intercalate and force
apart the separate lamellae of crystallite，making new
pores[13-14]. Many researchers have reported the use of
biomass to produce activated carbon by chemical activation with KOH，but there is little research on the
preparation of AC from lignin with KOH activation.
The objective of this study is to discuss the influence factors of preparation conditions including KOH/
lignin ratio(R)，activation time(t)and activation temperature(T). The obtained AC with developed porosity
can be used in air pollution control and waste water
treatment.
1
1.1
Materials and methods
Materials and preparation of AC
The alkali lignin powder(No.471003)of～50 µm ，
whose properties are listed in Table 1，is produced from
soft wood and supplied by Sigma-Aldrich (Shanghai，
China)Corp. The activating agent，massive KOH，is
purchased
from
Guoyao
Chemical
Reagent
Corporation，China. AC precursor was first impregnated
with KOH solution using a certain mass ratio：KOH/
·15·
alkali lignin(on a dry basis)，and then dried for 2,h in
an oven at 150,℃.The dried mixture was placed in a
corundum boat，and then carbonized in a nitrogen atmosphere at varying furnace，temperatures，as shown
in Fig.1.
Tab.1
Properties of lignin
Proximate analysis/%
Ultimate analysis/%
Mad
Aad
Vad
FC,ad
wC,ad
wH,ad
wO,ad
wN,ad
wS,ad
4.0
2.6
63.4
30.0
65.8
5.9
25.7
1.0
1.6
The reactor employed a corundum tube with a
length of 800,mm and an inner diameter of 50,mm，
which was covered by thermal insulation materials. Nitrogen(99.999% pure)was supplied from a cylinder to
drive away oxygen from the tube with a flow rate of
500,mL/min for 30,min after leak checking. Then the
reactor was heated from room temperature to the target
temperature with a heating rate of 10 ℃/min and kept
for a certain time with a flow rate of 100 mL/min of
nitrogen. The obtained carbonized materials were allowed to cool down naturally inside the reactor to room
temperature under nitrogen atmosphere. The resultant
AC was repeatedly washed with hot distilled water until
pH～7，and then dried at 120 ℃ to obtain the final
product.
Fig. 1
Schematic diagram of high temperature carboniza-tion
reactor
1.2 Characterization methods
To better understand the properties of AC，samples
were characterized by a variety of analytical techniques.
In order to observe the pyrolysis behavior of lignin，
TG-DTG was performed by thermogravimetric analyzer(TherMax 500)in relation to temperature and
time(heating rate：10 ℃/min，temperature range：20—
1,200 ℃，nitrogen atmosphere). Surface physical morphologies of samples were observed by scanning electron microscope(SEM，FEI-Sirion，Holland).
The specific surface area and pore structure of
samples were indicated by nitrogen adsorptiondesorption isotherms at 77 K with an automated specific
surface area and pore size analyzer(Autosorb-1-C). The
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samples were degassed at 250,℃ for 3,h prior to adsorption experiments. The Brunauer-Emmett-Teller(BET)
specific surface area was measured by applying BET
equation to adsorption-desorption data，and the total
pore volume and pore size distribution were measured
by Barrett-Joymer-Halenda (BJH)method，respectively.
Surface functional groups were qualitatively analyzed by Fourier transformed infrared spectroscopy(Nexus 670，USA). The KBr crystals were mixed
with samples，followed by pressing the mixture into a
pellet. The spectrum scope was within the range of
400—4000 cm 1.
2 Results and discussions
2.1 Analysis of thermal characteristic
Thermal analysis of raw material was performed to
examine its decomposition characteristics. TG-DTG
curves of alkali lignin carbonization were plotted in
Fig.2. From the DTG curve，three mass loss phases
were observed. The first stage，which occurred within
the temperature range from 20 ℃ to～200 ℃，mainly
involved the loss of moisture existing in the samples
with an approximate mass loss of 7%. The second stage
occurred between 200 ℃ and 600,℃，and the mass loss
was about 38% as a result of pyrolysis of lignin. A single
peak occurring at 330 ℃ was shown in DTG curve. At
this stage，lignin was partly transformed into tar and
combustible gas such as CH4，CO and CO2，remaining
ash and solid mixture of biochar[15]. In the third stage
between 600 ℃ and～800 ℃，the mass decreased by
further crack of C—C bond and C—H bond of charcoal.
Similar results had been reported in Ref.[16-17].
Fig. 2
TG and DTG curves of lignin
2.2 Effects of activation process parameters
2.2.1 Effect of activation temperature
The effects of activation temperature were determined within the range of 600 ℃ to 900 ℃. Remark-
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第 20 卷 第 1 期
able influences on the porous structure of AC were
shown in Fig.,3，which indicated the nitrogen adsorption-desorption isotherms at 77,K for AC prepared at
activation temperature of 600 ℃，700 ℃，750 ℃，800
℃，900 ℃，respectively. According to the IUPAC classification[18]，these isotherms belonged to type 1，which
were given by microporous solids having relatively
small external surfaces. The plateau of isotherm presented by AC-750 ℃-2-3(750 ℃ represented activation
temperature，2 represented activation time and 3 represented mass ratio of KOH/alkali lignin，the same below.)was higher than that of others，which presented
larger micropore volume. At relative pressures，higher
than 0.2，the slope of isotherms plateau of AC-750 ℃2-3 was higher than that of other AC prepared by same
process. Furthermore，a rather broad hysteresis loop
belonging to type H2[18] was observed within a wide
range of p/p0，ascribed to the development of mesoporosity. The similar results were shown in Fig.,4. The
pore size distribution of AC-750 ℃-2-3 was mainly
concentrated on mesopores ，whose proportion was
higher than that of others. Bigger mesopores were observed from AC-750 ℃-2-3，AC-800 ℃-2-3，AC-900
℃-2-3.
Fig. 3
Fig. 4
Nitrogen isotherms of AC at different activation
temperature
Pore size distribution of AC at different activation
temperature
Pore structure parameters were obtained from
Fig.,4 and summarized in Table 2. The specific surface
area of AC varied from 1,038 m2/g to 2,492 m2/g. AC
2014 年 2 月
肖 刚等：KOH 活化木质素制备高比表面积活性炭特性研究
prepared at 750 ℃for 2,h with a mass ratio of 3∶1
reached the optimal pore structure. When the value of
activation temperature was lower than 750 ℃，it led to
insufficient AC(fewer narrow pores，which led to a
smaller specific surface area). A higher value also led to
a wider pore structure，making a smaller specific surface area. The lignin is an aromatic macromolecule
compound whose heat-treatment may produce a mass of
volatiles such as CO，CO2 and H2O. The pores were
produced due to the overflow of the gases from the interior of raw materials[15]. When the activation temperature rose up to 750 ℃，metallic potassium was produced and transferred into the gas phase，leading to
more pores，consequently the specific surface area was
improved. However，the specific surface area was reduced with further increase of activation temperature，
which could be due to collapse and erosion of micropores. Some literature also reported similar results[19-20].
·17·
ratio of 3∶1. There are usually four stages in pore development during activation process：①opening of previously inaccessible pores；②creating of new pores by
selective activation；③expanding of the existing pores；
④merging of the existing pores due to pore wall breakage[20]. The reaction between the redundant KOH and
previou pores could explain the decrease of specific
surface area of AC prepared at 4∶1.
Fig. 5
Nitrogen isotherms of AC at different mass ratio
Tab. 2 Pore structure parameters at different activation
temperature
t/h T/℃ R
SBET/
(m2·g-1)
VT/
(cm3·g-1)
Average pore
size/nm
Yield/
%
2
600 3
1,038
0.571
2.201
28.3
2
2
2
2
700
750
800
900
2,065
2,492
2,077
1,543
1.138
1.602
1.165
0.893
2.208
2.571
2.244
2.314
26.2
25.4
24.7
23.2
3
3
3
3
2.2.2 Effect of mass ratio of KOH/alkali lignin
The mass ratio of KOH/alkali lignin is another key
factor affecting pore structure of AC. Experiment of
different ratios of KOH/alkali lignin(1—4)was carried
out to determine its effects on activation process. Nitrogen adsorption-desorption isotherms at 77,K for AC，
which were prepared at varied mass ratio，were shown
in Fig.,5. A great enhancement of N2 adsorption capacity at 77,K of AC-750 ℃-2-3 was observed. Hysteresis
loop given by the isotherms of AC-750 ℃-2-4 was the
evidence for the existence of mesopores. Pore size
distributions of AC prepared at various mass ratio of
KOH/alkali lignin were presented in Fig. 6. A sharp
increase of the mesoporosity proportion occurred at 3∶
1，while at the ratio of 4∶1，a few larger mesopores
appeared.
Table 3 clearly showed the pore structure parameters of AC prepared at different mass ratio. The results
indicated that the specific surface area of AC increased
with mass ratio of KOH/lignin until it reached the
maximum(2,492 m2/g)at 750 ℃ for 2,h with a mass
Fig. 6
Tab. 3
Pore size distribution of AC at different mass ratio
Pore structure parameters of AC at different mass
ratio
t/h T/℃ R
2
2
2
2
750
750
750
750
1
2
3
4
SBET/
(m2·g-1)
VT/
(cm3·g-1)
Average pore
size/nm
Yield/%
0 864.6
1 128.2
2 492.0
1 405.0
0.838 2
0.840 8
1.602 0
0.796 7
2.241
2.264
2.571
2.622
31.2
28.6
25.4
23.3
2.2.3 Effect of activation time
Compared with the effects of activation temperature and mass ratio of KOH/alkali lignin，activation
time has a relatively slight influence on the pore structure of AC in activation process. The results were obtained from Fig. 7，Fig. 8 and Table 4. The plateau of
nitrogen adsorption and desorption isotherms at 77 K
for AC prepared at 750 ℃ for 1 h with a mass ratio of
3∶1 was slightly higher than that of other materials.
The pore size distributions of these AC were mainly
concentrated on micropores and mesopores. The proportion of mesopores of AC-750 ℃-1-3，AC-750 ℃-2-3
·18·
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and AC-750 ℃-3-3 was larger than that of AC-750 ℃0.5-3. This could be explained by the fact that more
volatile compounds which were in inner part of particle
Fig. 7
Nitrogen isotherms of AC at different activation
times
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might evaporate during longer activation process. These
results agreed with previous findings which were concluded from activation of fruit shell by NaOH [10]. The
pore structure parameters of AC prepared by various
activation time were particularly presented in Table 4.
AC with highest specific surface area was obtained at
750 ℃ for 1 h with a mass ratio of 3∶1. A comparison
of the pore characteristics of AC between the present
study and other literature is listed in Table 5. It can be
observed from the data provided in table that the BET
surface area of AC prepared in the present study is much
higher than that reported previously. The properties of
AC are dependent on the precursors and preparation
methods. AC with a high specific surface area of 2,684
m2/g could be attributed to low ash content of lignin
compared with other precursors.
Tab. 4 Pore structure parameters of AC at different
activation times
t/h T/℃ R
Fig. 8
0.5
1
2
3
Pore size distribution of AC at different activation
times
750
750
750
750
3
3
3
3
SBET/
(m2·g-1)
VT/
(cm3·g-1)
Average pore
size/nm
Yield/%
1,856
2,684
2,492
2,048
1.018
1.643
1.602
1.162
2.195
2.405
2.571
2.637
28.3
26.7
25.4
24.6
Tab. 5 Comparison of the characteristics of porosity in AC between the present study and
other literature under optimum conditions
Raw material
Activation method
t/min
T/℃
SBET/(m2·g-1)
VT/(cm3·g-1)
Ref.
Lignin
Textile sewage sludges
Esparto grass
Durian shell
Fruit shell
Lotus stalk
Coconut shell
Chemical activation(KOH)
Chemical activation(KOH)
Physical activation(CO2)
Physical activation(H3PO4)
Chemical activation(NaOH)
Physical activation(H3PO4)
Physical activation(CO2)
60
60
60
20
120
60
120
750
500
800
500
800
500
900
2,684
69.23
1,602
1,404
1,873
1,114
1,703
1.643
0.210
0.290
—
1.312
1.170
1.032
Present study
[2]
[6]
[8]
[10]
[11]
[20]
2.3 Characteristics of AC
2.3.1 Surface physical morphology by SEM
SEM photographs of raw lignin(a)，AC-750 ℃
(b)，AC-750 ℃-1-3(c)were clearly shown in Fig. 9.
The AC-750 ℃ was obtained at the carbonization temperature of 750 ℃ for 1,h. Few pores were found on the
spherical surface of raw lignin，however，a large number of pores with different sizes and shapes were observed on the surface of AC-750 ℃-1-3. This phenomenon indicated that the activating agent used was effective in producing pores of raw precursor. In addition，
some pores could also be found on the surface of AC750,℃.
2.3.2 Surface functional groups by FTIR
The adsorption capabilities of AC not only rely on
the surface area but also on the surface chemical properties. The FTIR spectra of raw lignin，carbonized at 750
℃ without activation(AC)and AC-750 ℃-1-3，were
shown in Fig. 10. All the samples showed a strong and
wide absorption peak at 3,000—3,700 cm 1 with a
maximum at about 3,423,cm 1，ascribed to O—H of
alcoholic and phenolic functional groups stretching vibrations[16]. The peak at 2,848,cm 1 attributed to C—H
stretching of aromatic methoxyl groups of raw lignin
disappeared in the carbonization and activation processes as a result of removal of H element. The peak lo-
2014 年 2 月
肖 刚等：KOH 活化木质素制备高比表面积活性炭特性研究
·19·
can be obtained at 750,℃ for 1,h with a KOH/alkali
lignin ratio of 3∶1. At a suitable activation temperature
pores can be produced，and higher temperature may
lead to the erosion of pores. The results showed that the
optimal activation temperature is 750 ℃. Excessive
KOH/lignin ratio can lead to a lower BET specific surface area due to the reaction between redundant KOH
and previous pores，while insufficient KOH/lignin ratio
could not make ample pores. The optimal KOH/lignin
ratio is 3. The optimal activation time is 1,h in consideration of energy consumption. The KOH/lignin ratio is
the main factor influencing the BET specific surface
area of AC，followed by the activation temperature，
while the activation time is the lowest factor.
（a）raw lignin
（b）AC-750℃
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