Development of carbon adsorbents with high surface acidic and basic group contents from phosphoric acid activation of xylitol

Article abstract of DOI:10.1039/c5ra14579a

The present paper evaluated the feasibility of synthetizing activated carbons from xylitol with phosphoric acid activation at mild temperatures. Activation temperature (250-450 °C) and phosphoric acid to xylitol impregnation ratio (0.2-3 wt%) were varied during the synthesis of xylitol-based activated carbon, and the effects of these parameters on the textural and chemical properties of the final activated carbons were investigated by XRD, Raman, N₂ adsorption and desorption, SEM, XPS and Boehm's titration. The results of yield, XRD and Raman indicated that phosphoric acid activation enhanced the yields of activated carbons, and facilitated the formation of completely carbonized materials at low temperatures (around 250 °C) by comparing with charcoals derived from pyrolysis of xylitol. The porous structures of the activated carbons were developed after activation, and for each activation temperature, the carbons reached the maximum surface area at an impregnation ratio of 1.5. Due to the strong oxidizing radicals decomposed from phosphates, the produced carbons contain relatively high concentrations of acidic and basic surface groups. The total surface groups peaked at 6.08 mmol g^-1 for activated carbon obtained at an activation temperature of 350 °C and impregnation ratio of 1.5. The Ni(ii) adsorption capacity of the activated carbons was 4 to 7 folds that of the charcoals.

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Development of carbon adsorbents with high surface acidic and basic group
contents from phosphoric acid activation of xylitol
a,b
∗,a
a
a
a
a
a
Hai Liu , Jian Zhang , Li Jiang , Yan Kang , Cheng Cheng , Zizhang Guo , Chenglu Zhang
a
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of
Environmental Science and Engineering, Shandong University, Jinan 250100, China
b
Department of Chemical and Biomolecular Engineering, University of California, Berkeley,
California 94720, United States
Abstract
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The present paper evaluated the feasibility of synthetizing activated carbons from
xylitol with phosphoric acid activation at mild temperatures. Activation temperature
o
(250ꢀ450 C) and phosphoric acid to xylitol impregnation ratio (0.2ꢀ3 wt.%) were
varied during the synthesis of xylitolꢀbased activated carbon, and the effects of these
parameters on the textural and chemical properties of final activated carbons were
2
investigated by XRD, Raman, N adsorption and desorption, SEM, XPS and Boehm’s
titration. Results of yield, XRD and Raman indicated that phosphoric acid activation
enhanced the yields of activated carbons, and facilitated the formation of completely
o
carbonized materials at low temperatures (around 250 C) by comparing with
charcoals derived from pyrolysis of xylitol. The porous structures of the activated
carbons were developed after activation, and for each activation temperature, the
carbons reached the maximum surface area at impregnation ratio of 1.5. Due to the
*
Corresponding author: Tel.: + 86 531 88363015; Fax: +86 531 88364513; Eꢀmail address:
zhangjian00@sdu.edu.cn (J. Zhang); shandaliuhai@berkeley.edu (H. Liu)
1

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strong oxidizing radicals decomposed from phosphates, the produced carbons contain
relatively high concentrations of acidic and basic surface groups. The total surface
groups peaked at 6.08 mmol/g for activated carbon obtained at activation temperature
of 350 °C and impregnation ratio of 1.5. The Ni(II) adsorption capacity of the
activated carbons were 4 to 7 folds of the charcoals.
1. INTRODUCTION
Activated carbon has been well proven to be one of the most effective adsorbents
toward a wide variety of organic and inorganic pollutants from aqueous or gaseous
environment due to its highly developed porous structure, low acid/base reactivity,
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and wide spectrum of surface functional groups . In general, activated carbon
production consists of two processes: chemical activation and physical activation.
2 2
Due to the low reactivity between physical activating agents (H O or CO ) and carbon
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precursors, high activation temperatures (> 800 C) and prolonged times are required
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. Their high surface area comes at the cost of a high extent of char burnꢀoff,
resulting in the low product yield and scarce surface functional groups. The chemical
o
activation usually takes place at lower temperatures (about 400ꢀ700 C) in the
2 3 4
presence of an activating agent (ZnCl or H PO ). Therefore, the produced activated
carbons contain wellꢀdeveloped structure, high yield and favorable surface chemistry.
Given the environmental and economic effects, phosphoric acid activation has been
7, 8
well demonstrated to be a promising method
.
A various carbonaceous materials have been used as precursors to prepare
activated carbons by phosphoric acid activation, such as lignocellulosic materials
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synthetic polymers
, and coals
. Impregnation and activation are the main
processes involved in phosphoric acid activation. During impregnation stage, H
3
PO
4
permeates into the carbonaceous material and forms macromolecule composites. For
subsequent activation, H PO reacts with precursor, resulting in favorable
3
4
physicochemical properties. Thus, the degree of impregnation affects dramatically the
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activation effects
. Since these precursors are polymeric or extremely stable,
phosphoric acid is hard to react with them and penetrate into them at moderate
conditions. Zuo et al. has reported that the impregnation time of 10 h could result in
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an 80% increase in surface area . Therefore, in order to ensure good impregnation
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effects, the impregnation is required 6ꢀ24 h. It has been also reported the activation
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temperature should be controlled above 400 C to ensure the produced activated
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carbons with wellꢀdeveloped structure or good surface chemical properties
. As a
result, producing activated carbon with phosphoric acid activation is a time and
energyꢀconsuming process.
As the most important heterogeneous element, oxygen can form different
complexes with carbon, and result in acidꢀbase and electronꢀdonor/ꢀacceptor
properties of activated carbon. Surface functional groups of activated carbon give
significant contribution to its adsorption ability, especially for lowꢀsize and positively
21, 22
charged heavy metal ions
, primarily through providing adsorption sites of proton
exchange, electrostatic attraction, and surface complexation. Typical phosphoric acid
activated carbons exhibit a wide distribution of pore size, but relatively low metal ions
adsorption. Considering the huge influence of surface chemistry on the adsorption
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performance of carbon materials, various modification methods have been developed
to increase surface oxygen content through preꢀ and postꢀtreatment with gaseous or
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liquid chemicals
. However, these methods require an additional process and are
also time and energy consuming process.
Organophosphorus compounds, as a kind of ester of phosphoric acid and alcohol,
have been well demonstrated to be novel activating agents for developing activated
carbons from lignocellulosic materials with relatively high surface oxygenated groups
and heavy metal ions adsorption capacities in comparison with reference activated
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derived from H PO activation in our previous works
. These favorable results
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were mainly contributed to that radicals (R⋅ and RO⋅) decomposed from phosphates
further created the porosity and surface functional groups of final carbons. In general,
the different properties of phosphoric acidꢀactivated carbon mainly depend on the
precursor nature and synthesis conditions. Polyhydric alcohols contain multiple
hydroxyl groups, and they can condense with phosphoric acid via esterification to
form low molecular weight phosphates. These compounds can be easily decomposed
into phosphorus oxides and radicals. Based on these results, we deduce that these
reactions may promote the carbonization of polyhydric alcohols at low temperature,
and enhance the surface oxygen contents of the produced carbon materials, eventually
increasing their adsorption abilities toward heavy metal ions. In addition, polyhydric
o
alcohols can be melted at low temperatures (above 93 to 95 C) and dissolves easily in
29
phosphoric acid solution (eg. 85 wt.%) . Thus, via phosphoric acid activation, using
polyhydric alcohols as carbon precursors may avoid the problem of long impregnation
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time and high activation temperature for production of activated carbon from
conventional lignocellulosic materials. However, little information is available on
preparation of activated carbon from phosphoric acid activation of polyhydric
alcohols.
Accordingly, a common polyhydric alcohol, xylitol, was chosen as carbon
2 3 2
precursor. Xylitol (formula, CH OH(CHOH) CH OH) is categorized as a polyalcohol
or sugar alcohol. The main goals of the present work are to evaluate the feasibility of
preparation of activated carbon from xylitol with phosphoric acid activation, and to
provide a full understanding of the chemical properties and structural characteristics
of the carbons. For these purposes, the thermal behaviors of xylitol treated with
different impregnation ratios of phosphoric acid were studied. The effects of
impregnation ratio and activation temperature on the development of pore structure
and chemical characteristics were also investigated. The synthesized carbon materials
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were characterized by Xꢀray diffraction (XRD), N adsorption and desorption,
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scanning electron microscopy (SEM), Xꢀray photoelectron spectroscopy (XPS), and
analysis of Boehm’s titration.
2. EXPERIMENTAL (MATERIALS AND METHODS)
2.1. Synthesis of Carbon Adsorbents
In a typical synthesis, activated carbons were prepared by phosphoric acid
activation of xylitol. Five grams of xylitol was mixed fully with phosphoric acid
solution (85 wt.%) at a impregnation ratio of 0.2:1 ꢀ 3:1 (g H
3 4
PO / g xylitol). The
nomenclature for the impregnated samples (xylitolꢀR) was the xylitol followed by a
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number indicating the impregnation ratio. After mixing, xylitolꢀR samples were
o
immediately heated (heating rate of 10 C/min) from room temperature to the desired
temperature and maintained at this temperature for 1 hour under nitrogen atmosphere
(100 mL/min) in a tube furnace. After cooling to room temperature, the samples were
washed with deionized water until steady pH and absence of phosphate anions.
o
Finally, the samples were dried at 105 C for 9 h and grounded to obtain the particle
size of 100/160 mesh (Model Φ200). The activated carbons obtained at activation
o
temperature of X C with impregnation ratio of R were referred to as ACꢀXꢀR. In
order to evaluate the effect of phosphoric acid activation on the characteristics of final
carbon materials, the charcoals (CꢀX) were also prepared by pyrolysis of xylitol at the
same heating conditions.
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2.2. Characterization
The thermal behaviors of the untreated xylitol and xylitolꢀR samples during
carbonization/activation were evaluated with a thermal analyzer (TGAꢀ50 analyzer).
Samples were heated at a heating rate of 10 °C/min under nitrogen atmosphere (100
mL/min). The crystallinity of the produced ACꢀXꢀR samples was investigated by
using a Rigaku D/MAXꢀYA diffractometer with Niꢀfiltered Cu Ka radiation as Xꢀray
source. Raman spectra were collected with a Raman microscope (633 nm laser
excitation, Renishaw inVia Raman microscope, UK). The textural properties were
2
determined by N adsorption/desorption at 77 K with a surface area analyzer
(Quantachrome Corporation, USA). The structure and morphology were examined by
a scanning electron microscopy (SEM) JSMꢀ5610LV (10 kV, Jeol Company, Japan).
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The oxygenated acidic and basic surface groups were determined using the Boehm’s
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titration method . The surface elemental composition was studied by using an Xꢀray
photoelectron spectrometer (XPS, PerkinꢀElmer PHI 550 ESCA/SAM) with Mg Ka
irradiation source. All the spectra were calibrated by setting C 1s to 284.6 eV. The
adsorption abilities of the carbon adsorbents were evaluated by batch Ni(II)
adsorption experiment.
3. RESULTS AND DISCUSSION
3.1. Thermal Analysis
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a
450
xylitol
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xylitol-0.2
xylitol-0.5
xylitol-1.0
xylitol-1.5
xylitol-2.0
xylitol-3.0
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Time (min)
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xylitol
xylitol-0.2
xylitol-0.5
xylitol-1.0
xylitol-1.5
xylitol-2.0
xylitol-3.0
250
200
150
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10
100
50
-12
0
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60
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Time (min)
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Fig. 1. TGA (a) and DTG (b) curves for pyrolysis of xylitol and xylitol-R samples.
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The thermogravimetric analysis (TGA) and derivative thermogravimetric (DTG)
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curves of the original and H PO ꢀinpregnated xylitol samples are shown in Fig. 1. It
can be seen from Fig. 1a that the weight loss for original xylitol began at temperature
o
slightly higher than its boiling point of 216 C, its weight loss rate slowed down as
o
temperature above 350 C (Fig. 1b), and a slight weight was observed when
o
temperature was kept at 450 C, indicating that the carbonization process was
completed. However, the xylitolꢀR samples showed a major weight loss at
o
temperature below 200 C, which reflected that phosphoric acid promoted
o
carbonization at low temperatures. From room temperature to about 100 C, an initial
weight loss was mainly due to the loss of moisture present in the xylitolꢀR samples.
Some reactions were expected to be responsible for the dramatic weight loss for
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pyrolysis of xylitolꢀR samples at temperature blow 250 C: (1) esterification between
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phosphoric acid and xylitol, which took place at temperatures above about 90 C; (2)
etherification between hydroxyl groups of xylitol catalyzed by phosphoric acid; and
(3) condensation between phosphoric acid molecules (occurred at temperature above
o
213 C). Meanwhile, the produced phosphates with low polymerization degree were
instable at high temperature, and subsequently volatilized and decomposed into
31, 32
radicals and P
2
O
5
, causing a further weight loss.
The thermal behaviors of xylitolꢀR samples were obviously different to other
31, 33
phosphoric acidꢀimpregnated lignocellulose/coal samples
. During activation,
xylitol formed easily phosphates with low polymerization degree, which tended to
volatilize and decompose and led to less phosphoric compounds remaining in the
samples. The loss of phosphorous species and production of radicals by
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decomposition of phosphates resulted in: (1) carbonization of xylitol at low
temperature (see Fig. 2) (2) low yield (see Fig. 3); (4) suppressed formation of porous
structure (see Fig. 4); (3) low surface phosphorus content (see Fig. 6); and (4) large
surface oxygen content (see Fig. 6) for final carbon materials.
3.2. Formation of Carbon Materials
a
charcoal
b
150 C
o
R = 0.2
R = 0.5
R = 1.0
R = 1.5
R = 2.0
O
T = 250
T = 300
T = 350
T = 450
C
O
C
O
C
O
C
R = 3.0
0
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0
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o
o
c
200 C
d
250 C
R = 0.2
R = 0.5
R = 0.2
R = 0.5
R = 1.0
R = 1.0
R = 1.5
R = 2.0
R = 1.5
R = 2.0
R = 3.0
R = 3.0
0
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80
90
0
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20
30
40
50
0
60
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80 90
0
2
theta ( )
2 theta ( )
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Fig. 2. XRD patterns of charcoals (a) derived from pyrolysis of xylitol and AC-X-R samples
o
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produced at activation temperatures of 150 (b), 200 (c), and 250 C (d).
o
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As shown in Fig. 2, a broad peak at around 2θ = 23 can be found from XRD
patterns of some samples, which was attributed to (002) planes of the graphitic
34
crystallites , and revealing that these xylitol or xylitolꢀR samples have been
carbonized or activated into carbon materials. All samples exhibited an intense and
o
sharp peak at around 2θ = 10 , which was attributed to the water molecular or
oxygenꢀcontaining groups between the layers of the graphite. It can be seen from Fig.
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2a that for the CꢀX samples obtained at carbonization temperature below 350 C, they
exhibited a XRD pattern without (002) diffraction peak, reflecting that the final
materials were carbonized incompletely. After phosphoric acid activation, absent,
inconspicuous and broad (002) diffraction peaks were recognized for the ACꢀ150ꢀR,
ACꢀ200ꢀR and ACꢀ250ꢀR samples (Fig. 2b, c and d), respectively. These results
meant that phosphoric acid activation promoted the carbonization of xylitol at low
temperatures, and the onset of complete carbonization of xylitolꢀR samples took place
o
between 200 and 250 C. The ACꢀ250ꢀR samples also exhibited typical Raman
spectra of nonꢀgraphitic carbon materials (Shown in Supporting Information (SI)
Figure S1). This can be also confirmed by the yield analysis that the yields of
ACꢀ200ꢀR samples were slightly lower than that of ACꢀ250ꢀR samples (Fig. 3).
Obviously, high activation temperature will promote the weight loss of xylitolꢀR
samples (see Fig. 1). However, xylitol could not be incompletely carbonized at low
activation temperature, hence some dissolved organic matters were removed from the
solid products by DI water washing. Accordingly, it also can be deduced that
ACꢀ200ꢀR samples were incomplete carbonized products. It is well known that xylitol
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has a very low melting point (93∼95 C) and can be dissolved in phosphoric acid
solution easily. Thus, xylitol can be mixed fully with phosphoric acid, which
promoted the reactions between xylitol and phosphoric acid. In addition, the formed
phosphates decomposed into strong oxidizing radicals, and further enhanced the
carbon structure forming at low temperature. Accordingly, the precursor exhibited
high weight loss and formed carbonized structures at low temperatures.
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00;
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300;
350;
450
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00 C
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o
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50 C
o
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50 C
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Impregnation ratio
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Fig. 3. Yields of the produced carbon materials (Yield is defined as the % ratio of weight of
carbon produced to the weight of xylitol utilized for activation/pyrolysis).
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The yields of the carbon materials are represented in Fig. 3. The yields of ACꢀXꢀR
samples were obviously higher than that of CꢀX samples, especially at high activation
or carbonization temperatures, meaning the good flameꢀretardant effect of phosphoric
acid. This was due to that phosphoric acid promoted formation of carbon materials at
low temperatures, hence increasing the yields of ACꢀXꢀR samples. For each
activation temperature, with increasing impregnation ratio up to 3, the yield of the
samples increased to a maximum at impregnation ratio of 1.5 and decreased slightly at
higher impregnation ratios, because suitable dose of phosphoric acid had a positive
effect on the yield, while the excessive dose resulted in the highly aggressive
chemical reactions between the xylitol and phosphoric acid. For each impregnation
ratio, the yield decreased dramatically as the reaction temperatures increased due to
the enhanced decomposition of precursor at higher temperatures. It should be noted
that ACꢀ250ꢀR, ACꢀ300ꢀR and ACꢀ350ꢀR samples had relatively high yields (30ꢀ45%)
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due to the low activation temperatures. However, the samples produced at activation
o
temperature of 450 C had low yields (10ꢀ20%). The yields of xylitolꢀbased activated
carbons were much lower than the activated carbon derived from lignocellulose
o
18
materials (30ꢀ50%) at activation temperature of 450 C, such jackfruit peel waste ,
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Kraft lignin , date stones , and rubber wood sawdust . As proposed above, the
oxidizing radicals derived from decomposition of phosphate esters accelerated the
weight loss of xylitolꢀR samples, and the volatilization of organophosphorus esters
also caused the relatively low yields of final carbon materials. Taking into account the
yield and completely carbonized products, the activation temperature was
o
1
1
1
recommended in the range of 250 ꢀ 450 C for phosphoric acid xylitolꢀbased carbon
material production.
3.3. Physical Properties of Activated Carbons
o
o
o
o
2
50 C;
300 C;
350 C;
450 C
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00
00
00
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2
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20
10
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Impregnation ratio
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4
Fig. 4. BET surface area calculated by the standard BET method of the carbon materials.
1
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1
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The effects of impregnation ratio and activation temperature on the BET surface
1
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area (SBET) of the samples are shown in Fig. 4. At each activation temperature, the
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SBET of carbons reached a maximum at an impregnation ratio of approximately 1.5,
which was coincident with the results of yield. The same tendency was observed
previously for activated carbons produced from phosphoric acid activation of cotton
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stalks , hydrochars , and cellulose . It can be seen that phosphoric acid activation
developed the porous structure of final carbon materials (ACꢀXꢀR) as compared to the
2
CꢀX samples (less than 10 m /g), particularly for the ACꢀXꢀR samples with R beyond
a value of 0.5. Such results can be also observed from the SEM images for the CꢀX
and AC samples (shown in SI Figure S2). With impregnation ratio below 0.5, the
carbonization/activation was incomplete/ inadequate for original xylitol and
2
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1
2
2
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xylitolꢀXꢀ0.2 samples, demonstrated by their low surface area (less than 30 m /g).
Some phosphoric acid or phosphorous species in xylitolꢀR was lost via decomposition
and volatilization of phosphates. Thus, detectable activation in the terms of porous
structure was not obtained for xylotilꢀXꢀ0.2 samples. Such phenomenon can be also
observed from the SBET of ACꢀXꢀ0.5 samples that the SBET values of ACꢀ250ꢀ0.5 and
ACꢀ300ꢀ0.5 samples were much larger than ACꢀ350ꢀ0.5 and ACꢀ450ꢀ0.5 due to the
loss of phosphoric acid and the consequent damage of pore structure at high activation
temperatures. An increase in impregnation ratio enhanced degree of polymerization of
phosphates, and restrained their thermal decomposition, resulting in the higher yield
and surface area. However, the excessive phosphoric acid addition aggravated such
32
activation effect and destroyed some pores . The surface area of carbons increased
with increasing activation temperature was mainly due to the enhanced interactions of
phosphoric and xylitol. This trend was generally consistent with previous reports for
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40
41, 42
1
phosphoric acid activation of Kraft lignin , chestnut wood , and rice straw
.
o
o
o
o
2
50 C;
300 C;
350 C;
450 C
280
a 2
40
200
160
120
80
40
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
P/P0
2
o
o
o
o
2
50 C;
300 C;
350 C;
450 C
0.30
b
0.25
0.20
0.15
0.10
0.05
0.00
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Pore width (nm)
3
4
2
Fig. 5. N adsorption/desorption isotherms (a) and DFT pore size distributions (b) for
5
6
carbons prepared with impregnation ratio of 1.5 at different activation temperatures.
7
8
9
0
1
2
Nitrogen adsorption and desorption isotherms and DFT pore size distributions of
the carbons produced at different activation temperatures using an impregnation ratio
of 1.5 are summarized in Fig. 5. The isotherms for ACꢀ250ꢀ1.5 and ACꢀ300ꢀ1.5 were
Type I, practically without hysteresis loop, which was characteristic of highly
microporous carbons. The isotherms for ACꢀ350ꢀ1.5 and ACꢀ450ꢀ1.5 were the
1
1
1
0
mixture of Type I and IV, with a hysteresis at P/P above 0.4 (Fig. 5a), indicating a
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microꢀmesoporous structure. These results also can be confirmed from Fig. 5b that the
pore widths of a large part of pores for ACꢀ350ꢀ1.5 and ACꢀ450ꢀ1.5 were in the range
of 2ꢀ6 nm, as will be demonstrated by the evaluated parameters (see Table 1). The
structural characterization results listed in Table 1 showed that with the increase of
temperature from 250 to 450 °C, the micropores and mesopores of the carbons were
developed, but the contribution of micropore to the total porosity (Vmic/Vmes
)
decreased, suggesting the enlargement of existing micropores as well as the
continuous pore creation.
9
0
1
1
1
Table 1-Textural parameters of activated carbons prepared with impregnation ratio of 1.5 at
different activation temperatures.
S
BET
2
S
mic
2
S
ext
2
V
tot
V
mic
3
V
ext
3
Carbon
%
%
%
%
3
(
m /g) (m /g)
(m /g)
(cm /g) (m /g)
(m /g)
AC-250-1.5 377
AC-300-1.5 432
AC-350-1.5 509
AC-450-1.5 663
372
380
403
433
98.7
5
1.3
0.171
0.165 96.5 0.006
0.169 83.7 0.033
0.173 69.8 0.075
0.178 47.3 0.198
3.5
88.0 52
79.2 106
65.3 230
12.0 0.202
20.8 0.248
34.7 0.376
16.3
30.2
52.7
The micropore surface area (Smic), external surface area (Sext) and micropore volume (Vmic) were
evaluated by the t-plot method. Total pore volume calculated for P/P = 0.95. Mesopore volume
0
(Vmes) was calculated by Vtot-Vmic.
1
1
2
3
The simultaneous increases of micropore and mesopore volumes were related to
1
1
1
1
1
1
4
5
6
7
8
9
the geometry of polymeric species (polyphosphoric acids) and the production of
volatile substances. By condensation and dehydration, phosphoric acid (H PO )
3
4
converted to polycondensed forms: pyrophosphoric acid (H
4
P
2
O
7
) and polyphosphoric
) (>
o
acid (Hn+2
P
n
O
3n+1) at temperature above 213 C, and metaphosphoric acid ((HPO
3
)
n
o
31
300 C) . After thermal treatment, the acids occupied different volumes in the
carbonized materials were extracted by washing, and produced different porosities.
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7
The higher activation temperature promoted the condensation and dehydration of
phosphoric acid, hence causing the formation of larger pores. Thus, evident pore
evolution occurred as activation temperature was above 300 °C. Meanwhile,
phosphoric acid formed crossꢀlinked structure with polyhydric alcohol (xylitol), and
the enhanced elimination of highly volatile matters at higher activation temperatures
further promoted the porosity of final carbons.
3.4. Chemical Characteristics
O
O
O
O
2
50 C;
300 C;
350 C;
450 C
3.6
AC-250-1.5
AC-300-1.5
3
.4
3.2
C-300
C-350
3.0
2.8
AC-350-1.5
2.6
2.4
AC-450-1.5
C-450
2.2
2
.0
0
200 400 600 800 1000
0
200 400 600 800 1000
0.5
1.0
1.5
2.0
2.5
3.0
Binding energy (eV)
Impregnation ratio
O
O
O
O
C
O
O
O
O
C
2
50 C;
300 C;
350 C;
450
250 C;
300 C;
350 C;
450
3
.0
6.2
.0
.8
.6
.4
5.2
.0
.8
.6
.4
4.2
6
2.8
5
2.6
5
2.4
5
2.2
5
2.0
4
1.8
4
1.6
4
1.4
0.5
1.0
1.5
2.0
2.5
3.0
0.5
1.0
1.5
2.0
2.5
3.0
Impregnation ratio
Impregnation ratio
8
9
0
1
Fig. 6. XPS survey spectra of C-X and AC-X-1.5 samples (a). The Boehm’s titration results
of the carbons: acidity (b), basicity (c), and total groups (d).
1
1
1
1
1
1
1
2
3
4
5
6
The surface compositions of the carbons were also determined by XPS shown in
Fig. 6. The spectra indicated the presence of two distinct peaks for carbon and oxygen
elements, and the contribution of other elements was insignificant, meaning the
functionalities were derived from the complexes of carbon and/or oxygen. Obviously,
in comparison with CꢀX samples, the much higher intensity of O 1s peaks for the
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0
1
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0
1
2
activated carbons derived from same heating conditions indicated that they contained
more oxygen containing groups. The XPS can only reflect the oxygen functionalities
on the outer surface of the samples. The activated carbons exhibited porous structure
and the CꢀX samples were almost nonporous. It meant the activated carbons exhibited
extra more total oxygen groups than CꢀX samples. For the phosphoric acid activated
7, 43, 44
carbons, high phosphorus content (1ꢀ8 wt.%) were detected on their surfaces
.
However, the activated carbon studied in present paper was absent of phosphorus
species, which can be attributed the formation, and subsequent volatilization and
decomposition of phosphate esters.
1
1
1
1
1
1
1
1
1
1
2
2
2
In order to evaluate quantitatively the surface functionalities of the carbon
influenced by the preparation parameters, both acidic and basic groups of the
activated carbon were determined by Boehm’ titration. It is known that the acidity of
activated carbon mainly derive from the carboxylic acids, lactones and phenolic
hydroxyls, and basic groups originate from the complex aromatic system (delocalized
π electrons) of graphene surface. During activation, many strong oxidizing radicals
were produced, and eventually caused the high oxygen content of the final carbon
materials. For these samples, increasing activation temperature reduced the amount of
acidic groups and promoted the production of basic groups (see Fig. 6a and b). The
high temperature led to the decomposition of acidic groups into CO and CO , since
2
39
they are unstable at high temperature . Meanwhile, the high temperature promoted
1
8
the formation of delocalized π electrons, resulting in the high basicity . When
impregnation ratio increased from 0.5 to 1.0, both acidic and basic groups increased
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dramatically and reached a maximum at impregnation ratio of 1.5, then with
impregnation ratio greater than 1.5, the amounts of these groups exhibited a slight
decrease and kept high levels. As well demonstrated above that the phosphates
decomposed and volatilized easily at low temperatures, during activation, less
phosphoric compounds were left in the xylitolꢀ0.2 and xylitolꢀ0.5 samples, thus the
formed oxygen groups were susceptible to the high temperature and decomposed into
carbon gases. The larger impregnation ratio promoted the formation of phosphates
with high degree of polymerization that tended to be stable at high temperature,
whereas the large amount of phosphoric acid could hinder the thermal degradation of
the oxygen groups. Accordingly, the carbon samples exhibited a slightly decrease in
acidic groups with increasing impregnation ratio.
1
1
1
3.5. Evaluation of Ni(II) Adsorption Ability
Charcoal-250
a 70 AC-250-1.5
Charcoal-300
AC-300-1.5
Charcoal-350
AC-350-1.5
Charcoal-450
AC-450-1.5
60
50
40
AC-PA (reference material)
30
Q
20
10
0
0
10
20
30
40
50
60
Ce (mg/g)
1
1
3
4
Fig. 7. Adsorption isotherms of Ni(II) for the carbons, fitted with Langmuir isotherm model
(dosage = 0.2 g/L, initial Ni(II) concentrations = 20-60 mg/L, initial pH = 6.0 ± 0.02,
1
1
5
6
o
temperature = 25 ± 2 C, and ionic strength = 10 mM NaCl).
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Table 2- Ni(II) adsorption capacity of activated carbons developed from various precursor with
phosphoric acid activation.
Precursor
Impregnation
ratio
Activation
Adsorption
mg/L
SBET
Qm
References
2
o
m /g mg/g
Temperature C
Time
C
0
pH
Xylitol
1.5
1.5
1.5
1.5
1.5
2
250
300
350
350
450
450
450
500
450
550
1 h
20ꢀ60
20ꢀ60
20ꢀ60
20ꢀ60
20ꢀ60
ꢀꢀꢀ
6.0
6.0
6.0
6.0
6.0
ꢀꢀꢀ
377
432
509
663
52.9
62.9
69.4
64.5
This work
This work
This work
This work
This work
Xylitol
1 h
Xylitol
1 h
Xylitol
1 h
Phragmites australis
Lotus stalk
1 h
1220 35.2
1220 31.0
1638 7.6
1194 25.8
4
4
4
4
4
5
6
7
8
9
1 h
Prosopis Ruscifolia wood
Arundo donax L.
Date stone
2
0.5 h
0.5 h
2 h
ꢀꢀꢀ
ꢀꢀꢀ
2
5ꢀ100
10ꢀ100
10ꢀ60
5.8
ꢀꢀꢀ
1.75
1.5
826
472
24.4
54.0
Wool waste
40 min
ꢀꢀꢀ
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
As discussed above, phosphoric acid activation of xylitol could produce activated
carbons with high surface acidity and basicity. This is very interesting considering that
the produced activated carbons have application potentials in heavy metal ions
removal from aqueous solution. Thus, in the present study, the adsorption properties
of the xylitolꢀbased activated carbons were evaluated by studying uptake of nickel
ions from aqueous solutions. Adsorption isotherms were performed by a batch method,
where 10 mg of carbon sample was added into a 150ꢀmL conical flask with 50 mL
solution of Ni(II) with an initial solution pH of 6.0. To estimate the maximum
adsorption capacities of the produced activated carbons, the adsorption data were
1
1
1
1
1
1
1
1
fitted with the Langmuir model (Q
Ni(II) equilibrium adsorption capacity (mg/g) and concentration in solution (mg/L),
is the monolayer adsorption capacity of the adsorbent (mg/g), K is the Langmuir
isotherm constant (L/mg).
The fitting Langmuir data for Ni(II) adsorption onto the activated carbons and a
e m L e L e e e
= Q K C /(1+K C )), where Q and C are the
Q
m
L
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0
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3
4
5
6
7
8
9
0
1
2
reference activated carbon (ACꢀPA) prepared from phosphoric acid activation of
2
3
Phragmites australis (Properties: SBET = 1220 m /g, Vtot = 1.098 cm /g, Vmic
=
3
0.235cm /g, acidic groups = 1.644 mmol/g, basic groups = 1.125 mmol/g) are shown
2
in Fig. 7. The relatively high values of R (> 0.98, SI Table S1) and good
representation of the data (Fig. 7) indicated that the Langmuir isotherm model was
m
favorable to describe the adsorption of Ni(II) on the carbon adsorbents. The Q of the
carbons followed an order of ACꢀ350ꢀ1.5 > ACꢀ450ꢀ1.5 > ACꢀ300ꢀ1.5 > ACꢀ250ꢀ1.5 >
ACꢀPA ≫ Cꢀ250 > Cꢀ300 > Cꢀ350 > Cꢀ450. In comparison with charcoal samples, the
4ꢀ7 times higher Ni(II) adsorption capacities of activated carbons was clear evidence
that phosphoric acid activation improved dramatically their Ni(II) adsorption
capacities. Compared with the reference material (ACꢀPA), the 170ꢀ230% larger Ni(II)
adsorption capacities indicated that ACꢀXꢀR could be promising adsorbents in Ni(II)
ions pollution cleanup. For the sake of further comparison, Table 2 also shows the
Ni(II) adsorption capacities of other activated carbons by phosphoric acid activation.
The ACꢀXꢀ1.5 samples in this study showed relatively good adsorption capacities
1
1
1
1
1
1
1
1
1
1
2
2
2
compared to the adsorbents. The difference in Q
m
of the adsorbents seems to depend
26, 50, 51
primarily on the surface functions, as proposed by previous papers
. Since the
ionic diameter of Ni(II) is very low (0.138 nm), the surfaces of adsorbents are easily
accessible for Ni(II) ions, and the narrow micropore could not withdraw Ni(II)
effectively. Strong chemical interactions existed between the functional groups of
activated carbon and Ni(II) cations, such as electrostatic attraction between the
deprotonated acidic oxygenized groups (carboxylic, lactonic and phenolic groups) and
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+
2+
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
Ni ions, ion exchange between acidic groups and Ni ions, and complexation
2
+
between oxygen complexes and Ni ions. Meanwhile, the basic groups (the
delocalized π electron systems of graphene layer) can form electron donorꢀacceptor
2+
complexes with Ni ions via the Lewisꢀacidꢀbase interaction.
4. CONCLUSION
The results of this study demonstrated that phosphoric acid activation of xylitol
could produce promising carbon adsorbents with high contents of surface groups and
large Ni(II) adsorption capacities. The complete carbonization for phosphoric
o
o
acidꢀtreated xylitol samples took place between 200 C and 250 C. For each
activation temperature, the carbon samples reached the maximum surface area, and
content of surface groups at impregnation ratio of 1.5. Compared with the charcoal
obtained from pyrolysis of xylitol, the activated carbon showed much larger yields,
surface area, surface groups content, and Ni(II) adsorption capacity.
ASSOCIATED CONTENT
1
1
1
1
1
1
1
1
1
1
2
2
2
Supporting Information Available
Table S1 summarizes the Langmuir model fitting parameters of the nickel adsorption
onto the carbons. Figure S1 presents Raman spectra and intensity ratios of D to G
bands of the activated carbons. Figure S2 presents SEM micrographs of CꢀX and AC
samples.
ACKNOWLEDGEMEMNTS
This work was supported by the Independent Innovation Foundation of Shandong
University (2012JC029), Natural Science Foundation for Distinguished Young
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0
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9
0
1
2
Scholars of Shandong Province (JQ201216) and National Water Special Project
(2012ZX07203ꢀ004). The authors gratefully acknowledge the fund from Shanghai
Tongji Gao Tingyao Environmental Science and Technology Development
Foundation.
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FIGURE CAPTIONS:
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Fig. 1. TGA (a) and DTG (b) curves for pyrolysis of xylitol and xylitolꢀR samples.
Fig. 2. XRD patterns of charcoals (a) derived from pyrolysis of xylitol and ACꢀXꢀR
o
samples produced at activation temperatures of 150 (b), 200 (c), and 250 C (d).
Fig. 3. Yields of the produced carbon materials (Yield is defined as the % ratio of
weight of carbon produced to the weight of xylitol utilized for activation/pyrolysis).
Fig. 4. BET surface area calculated by the standard BET method of the carbon
materials.
2
Fig. 5. N adsorption/desorption isotherms (a) and DFT pore size distributions (b) for
carbons prepared with impregnation ratio of 1.5 at different activation temperatures.
Fig. 6. XPS survey spectra of CꢀX and ACꢀXꢀ1.5 samples (a). The Boehm’s titration
results of the carbons: acidity (b), basicity (c), and total groups (d).
Fig. 7. Adsorption isotherms of Ni(II) for the carbons, fitted with Langmuir isotherm
model.
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7