CONVERSION OF ALMOND SHELL TO ACTIVATED CARBONS:
METHODICAL STUDY OF THE CHEMICAL ACTIVATION BASED ON A
EXPERIMENTAL DESIGN AND RELATIONSHIP WITH THEIR
CHARACTERISTICS
Mª Teresa Izquierdoa, *, Alicia Martínez de Yusob, Begoña Rubioa, Mª Rosa Pinob.
a
Instituto de Carboquímica, CSIC. c/Miguel Luesma, 4. 50018 Zaragoza. Spain
b
University of San Jorge. Autovía A23 Zaragoza-Huesca km 510. 50830 Villanueva de
Gállego, Zaragoza. Spain.
*corresponding author
e-mail mizq@icb.csic.es
voice 34 976 733977
fax
34 976 733318
ABSTRACT
The objective of this work is to carry out a methodical study of the preparation conditions
of almond shell based chemically activated carbons and its influence on the characteristics
of the activated carbons. An experimental design was used to optimize the preparation
conditions of activated carbons from almond shell via chemical activation with phosphoric
acid. Temperatures from 400º to 800ºC, impregnation ratios in the range 0.5-1.5 and
carbonization times varying from 30 to 120 min were defined as continuous parameters to
be introduced in a simplex mixture experimental design giving a total of 12 experiments to
carry out.
The response surface methodology was applied in order to study the influence of all the
production parameters on the selected responses. The optimization of all the
characteristics of the activated carbons under the same experimental conditions is not
possible because the influence of activation temperature, impregnation ratio and activation
time is different. However using the response surface methodology it is possible to
determine the ranges of each experimental preparation condition to obtain optimal
characteristics of the activated carbon.
KEYWORDS: Agricultural waste, chemical activation, activated carbon, porosity,
functional groups, response surface
1
1. INTRODUCTION
Activated carbons can be produced from virtually any carbonaceous precursor. Coal and
lignite [1-4], wood [5-9], agricultural wastes [10-15] and industrial wastes [16-23] are raw
materials widely used to produce activated carbons by different routes: physical or
chemical activation [24, 25].
Activated carbons have recognized adsorption capacity and versatility for different
applications [26, 27] not only as adsorbents but also as catalysts and catalysts supports.
The adsorption capacity of an activated carbon will depend on surface area, pore volume
and microporosity [25]. Another important factor for the removal of polar compounds is
surface chemistry [28]. These characteristics can be modified by controlling the
production process giving best features of activated carbons depending on their use: gas or
liquid phase emissions purification.
Last years a great amount of papers regarding the preparation of activated carbons using
low cost precursors as waste materials can be found in the literature (a review can be
found in [29]). Recent studies of activated carbons production cost [30] reveal that one
important factor to minimize the cost of the activated carbon is the raw material cost and
other relevant factor for the economy of the overall process is the quality (adsorption
capacity) of the activated carbon because selling price by weight is very sensitive to the
quality. Research is on the way to use waste materials because their low cost. Moreover,
the demand of high adsorption capacity of the activated carbons for the different
applications involves the development of a methodical methodology for the preparation of
activated carbons and its relationship with the physicochemical characteristics of them.
2
Chemical activation has recognized advantages for the production of activated carbons
with high developed porosity [31]; in addition, this procedure leads to high yields and the
use of low activation temperatures. Main parameters of the preparation method affecting
the characteristics of activated carbons are impregnation ratio, activation temperature and
activation time [32]. Other authors have also studied the effect of the activating
atmosphere [33].
The impregnation ratio (defined as the ratio of the weights of chemical agent and the
precursor) is a variable that highly affects not only the pore size distribution of the
resulting activated carbon but also the surface area. A general trend is that at increasing
impregnation ratio the surface area of the activated carbon increases; however it seems
that high impregnation ratios lead to a reduction in pore volume and surface area [34],
indicating a collapse of micropores due to weakness of pore walls after intensive
dehydration.
The activation temperature in chemical activation ranges from 400ºC to 800ºC for
different lignocellulosic precursors [29]. As a general trend, for a given impregnation ratio
an increase in the activation temperature leads to an increase in surface area due to the
volatilization process. This process highly affects the product yield, an important factor
from an economical point of view [30].
The activation time, i.e. the time that the sample is maintained at the fixed activation
temperature, should be enough to allow the evolution of volatiles from the precursor in
order to enable the pore development. However, too long activation times enables pore
enlargement causing decrease in surface area. Moreover, the control of the activation time
minimizes the energy required for activated carbon production improving the economy of
the process [30].
3
According to the compilation of information carried out in this work, no study has been
done on optimization of the production of activated carbons from almond shells by
experimental design, considering simultaneously as variables impregnation ratio,
activation temperature and activation time. In this context, the objective of this work is to
carry out a methodical study of the preparation conditions of almond shell based
chemically activated carbons and its influence on the characteristics of the activated
carbons. An experimental design was used for the preparation conditions of activated
carbons from almond shell via chemical activation with phosphoric acid and the response
surface methodology was applied to optimize the experimental conditions.
2. EXPERIMENTAL
2.1. Preparation of activated carbons
Raw material for the preparation of activated carbons was almond shell (from Vera del
Moncayo, Zaragoza, Spain) crushed and sieved at 0.2-1 mm. The ash content of the
almond shell is 2.5 %. More details of the characterization of raw material are given
elsewhere [35].
Chemical activation with (orto)-phosphoric acid was carried out by impregnation method
followed by one step carbonization-activation in N2 atmosphere. An experimental design
was used to obtain the conditions to prepare the activated carbons. A commercial software
was used to obtain the experimental conditions to prepare the activated carbons. A mixture
design simplex based on a centroid design with interior points and two center runs was
used. The three mixture variables introduced were impregnation ratio (defined as: weight
of phosphoric acid / weight of precursor), temperature and time of activation, defined all
4
of them as a continuous function in the range of 0.5-1.5, 400ºC-800ºC and 30-120 min,
respectively, giving a total of 12 experiments. As non-design variable either BET surface
area (as representative of development of porosity) or CO or CO2 evolved groups from
TPD experiments (as representative of surface chemistry) were used as response of the
system.
The procedure to prepare the activated carbons was as follows: the almond shell was
mixed with a determined amount of phosphoric acid of 89 wt % concentration to reach the
impregnation ratio desired. The suspension was shaken at room temperature during 1 h.
The impregnated samples were further thermally treated at different temperatures ranging
from 400º to 800ºC during times of carbonization ranging from 30 to 120 min. The sample
was abruptly brought to the desired temperature. An inert flow of N2 was passed through
the sample during carbonization step, which was kept during both heating and cooling.
Solid pyrolysis residues were water washed in Soxhlet until 6<pH<7. The resulting
activated carbons were dried at 100ºC until constant weight and stored under Ar.
Table 1 reports the label of the samples with their preparation conditions.
2.2. Characterization of activated carbons
Samples were characterized by ultimate analysis, temperature programmed desorption
coupled to mass spectrometry (TPD-MS), N2 adsorption and thermogravimetry (TGA).
Ultimate analysis of the activated carbons was carried out in a Thermo Flash 1112
microanalysis apparatus. Oxygen content was obtained by difference. Table 2 gives results
for elemental analysis composition obtained for the activated carbons as well as yield to
activated carbon (calculated as 100 x mass of AC/mass of precursor).
5
The TPD runs were carried out with a custom built set-up, consisting of a tubular quartz
reactor placed inside an electrical furnace. TPD experiments were carried out by heating
the samples up to 1100ºC in Ar flow at a heating rate of 10ºC /min, recording the amounts
of CO and CO2 evolved at each temperature with a quadrupole mass spectrometer from
Pfeiffer. The calibrations for CO and CO2 were carried out by standards diluted in Ar. In a
typical run 0.5 g of carbon was placed in a vertical quartz tube reactor under a stream of
30 ml/min of Ar.
Nitrogen adsorption-desorption isotherms at -196ºC were obtained in a Micromeritics
ASAP 2020 automatic adsorption apparatus. Prior each analysis the samples were
outgassed at 150ºC and up to a vacuum of 10-6 mm Hg. The volume of adsorbed nitrogen
was measured from a relative pressure of 10-7 up to 0.995. Surface area was determined
by BET method [36] applied to the adsorption branch of the isotherm. Pore volume
corresponding to micropores was calculated using the t-plot method. Volume of
mesopores was obtained from BJH method [37]. The total pore volume was taken from the
measure of adsorbed nitrogen at a relative pressure of 0.995.
Thermogravimetric (TG) curves were obtained in a TA Instruments thermobalance.
Approximately 20 mg of sample was placed in a Pt crucible and was heated at 10ºC/min
up to 1000ºC under a flow of Ar.
3. RESULTS AND DISCUSSION
The conditions used to prepare each activated carbons according to the experimental
design change not only surface area but also other important characteristics of the
activated carbons as porosity and surface chemistry. Results concerning the evaluation of
6
these properties may help to discuss the influence of the preparation method on the final
performance of an activated carbon when it is further applied to gas or water emissions
control.
BET surface area was chosen as a common non-design variable for the experimental
design because is easy to compare with that reported in the literature. Moreover, BET
surface area gives a rapid way to follow the development of the textural characteristics of
the activated carbons with the different experimental conditions. The amount of CO and
CO2 evolved during TPD experiments was chosen as a response for surface chemistry
characteristics of the activated carbons.
Before the application of the response surface methodology, a preliminary study of the
characteristics of the activated carbons was carried out in order to help fixing axis in the
response surface graphs and thus to choose most influencing variable on response surface
among those studied.
3.1. Characterization of samples.
Lignocellulosic materials consist mainly of hemicellulose, cellulose and lignin [38].
Lignin and hemicellulose are the components that decompose first. Lignin decomposes at
low temperature and at low rate up to about 900ºC. Hemicellulose is a light fraction that
decomposes at low temperature and cellulose decomposes at higher temperature. In the
case of almond shells [39], DTG curves gives three zones of decomposition; the first peak
observed corresponds to hemicellulose (centred around 330ºC), the second peak
corresponds to cellulose (centred around 380ºC) whereas the decomposition of lignin takes
place over a broad range of temperatures starting around 220ºC, overlapping with the
7
temperatures of decomposition of hemicellulose and cellulose. However, the addition of
chemicals to some of the components of almond shell or to the almond shell itself lead to a
variation in the temperatures of the decomposition and in the amount of residue obtained
[39, 40].
Figure 1 a and b shows DTG curves for activated carbons obtained at low activation
temperature and at high activation temperature, respectively. The inspection of these
curves can help to discuss the results obtained from surface chemistry and porosity
analysis.
At the temperatures used to prepare the activated carbons, the process corresponding to the
decomposition of the hemicellulose is not present in any of the activated carbons prepared.
Despite of the cellulose decomposes around 300ºC and this temperature is lower than
those used to prepare the activated carbons, DTG curve of sample AT400R15t30 exhibits
a small peak at 300ºC and a wide broad band that is not present in any of the activated
carbons prepared. The peak can be attributed to cellulose decomposition because
activation time has been not enough to complete degradation at low temperature. The
broad band centred at about 450ºC can indicate that the degradation process of the lignin
during activation has been incomplete due to the low activation time used. Increasing the
activation time for the same activation temperature it can be observed a shift of this broad
band to higher temperatures, indicating that the degradation of the lignin is being
completed. At increasing activation temperature, this broad band disappears and a band
around 800ºC is becoming more intense (Figure 1 a). This fact can indicate that the
crosslinking process to produce polyaromatic units becomes important. This band around
800ºC is shifted to higher temperatures when the highest activation temperatures are used
(Figure 1 b). At activation temperatures higher than 600ºC the crosslinking process has
8
started during activation of the samples and during TGA experiments this process is
completed at temperatures around 900ºC.
These results can be compared with those obtained by TPD experiments (detailed below),
where the most important evolution of CO is obtained with samples prepared at the higher
activation temperatures.
The conditions used to prepare the activated carbons change the amount and type of
surface oxygen functional groups present on them. Figure 2 and 3 show the profiles of CO
and CO2 evolution during a TPD experiment for low and high temperature activated
carbons, respectively. The total amount of CO and CO2 evolved up to 1100ºC in a TPD
run for each activated carbon is reported in Table 3.
The amounts of CO and CO2 desorbed and the temperatures at which these gases released
have been found to be characteristics of various oxygen complexes. In general it is agreed
that each type of oxygen surface group decomposes to a defined product. It has been
proposed that CO2 derives from complexes like carboxylic acids, anhydrides and lactones
and CO derives from complexes like phenolic and quinonic groups. The complexes
yielding CO2 have been shown to decompose typically over a range of temperatures 150º600ºC and complexes yielding CO at temperatures in the range of 600º-1000ºC [28].
The variation of the operation conditions used to prepare the activated carbons lead to a
change in TPD spectra of the activated carbons not only in the amount evolved but also
the shape of CO2 and CO peaks. The profiles of CO2 of samples obtained at lower
temperatures of carbonization show quite symmetric peaks centered at about 500º and
800ºC. First peak can be attributed to lactones with contribution of carboxyls and second
peak to anhydrides. Sample series AT400- exhibit only a small shoulder not a well defined
peak at 800ºC, indicating that more stable functional groups are only formed at increasing
9
severity of activation conditions. The profiles of CO of samples obtained at lower
temperatures of carbonization show two peaks. First peak, near 600ºC, can be attributed to
phenolic and ether groups. This peak is more asymmetric and is shifted to about 625ºC for
samples obtained at increasing activation temperatures, indicating the destruction of less
thermally stable phenolic and ether groups. Second CO peak, near 800ºC, can be attributed
to anhydrides, carbonyls and quinones decomposition. Similar temperatures of evolution
of second CO peak are obtained compared with second CO2 peak. However, the shape and
the intensity changes because of the contribution of other surface complexes rather than
anhydrides. The contribution of carbonyls and quinone-like groups seem to be higher for
sample series AT467- and sample AT533-R083-t60, indicating the presence of more
stable oxygen functional groups at increasing the severity of the activation conditions.
The profiles obtained for CO2 evolution in TPD experiments for the samples obtained at
higher temperatures of carbonization (600 to 800ºC) show a low intensity broad band up
to approximately a temperature of desorption of 600ºC, indicating the disappearance of
less stable functional groups, as carboxyls and lactones, at higher activation temperatures.
The peak around 850ºC is attributed to anhydrides, increasing its intensity compared to
that exhibit by carbons obtained at lower temperature of carbonization. The profiles
obtained for the evolution of CO during TPD experiments show a intense peak at about
850ºC and a small shoulder at around 1000ºC. The peak at 850ºC exhibit asymmetry at
low temperatures, indicating small contribution of phenols and ethers. Moreover, the
intensity of the peak is twice the CO2 peak, attributed to anhydrides, indicating the
presence of thermally stable groups as quinones and carbonyls.
The introduction of surface groups that yield CO increase as a general trend at increasing
carbonization temperature as can be seen in Table 3. In the case of CO2 evolution, there is
10
not a relationship between carbonization temperature and the total amount of CO2 evolved.
The impregnation ratio and the time of activation are important parameters influencing the
introduction of CO2-evolving groups.
Table 4 summarizes the characterization of porosity of samples. The percentage of
microporosity can be obtained as the ratio between total micropore volume, calculated by
applying Dubinin-Radushkevich equation [43] to the adsorption branch of the N2 isotherm
and volume adsorbed at relative pressure of 0.995 [44]. As can be seen, at intermediate
impregnation ratios, lower carbonization temperatures are needed to obtain higher surface
areas. On the other hand, the use of impregnation ratios in the extreme of the range studied
need higher carbonization temperatures to obtain high surface areas.
Yield to activated carbons reported in Table 2 have not a direct relationship with the
development of porosity. It can be obtained activated carbons high surface areas and high
yields an important factor from an economical point of view.
Summarizing, BET surface area reach highest values in the range of impregnation ratios
1.0-1.17, independently of the activation temperature used; the amount of oxygen surface
groups yielding CO highly depend on the temperature whereas the amount of oxygen
surface groups yielding CO2 does not exhibit a clear relationship with a fixed preparation
condition variable.
3.2. Response analysis and interpretation
11
In order to study and describe the effect of the preparation experimental conditions on the
differenr non-design variables studied, the response surface methodology was chosen
using a second-order polynomial model. In a first approach, the complete model was used:
Y=A0+A1T+A2R+A3t+A12TR+A13Tt+A23Rt+A11T2+A22R2+A33t2
[1]
being Y the response (either SBET or CO-CO2 TPD evolving groups), T, R and t, activation
temperature, impregnation ratio and activation temperature, respectively and A0 is the
constant, A1, A2, A3 are the linear coefficients, A12, A13, A23 are the interactive coefficients
and A11, A22, A33 are the quadratic coefficients.
Based on the analysis of variance (ANOVA) of the complete quadratic model, terms of
equation [1] with no significant effect on the response can be established. The observation
of p-value (significance level) is used to assess the significance of the coefficients on the
response. The limits taken were 0.05<p-value<0.15. If p-value <0.05 it can be considered
that the effect on the response is not due to random variations and it may be concluded that
it is a significant effect. On the contrary, p-values greater than 0.15 indicate that the
coefficient is not significant, and thus the contribution of the corresponding variable(s)
either.
In present case based on the complete model, quadratic effect of the activation time as
well as the interactive effect between temperature and impregnation ratio exhibit p-values
greater than 0.15 for the responses SBET and amount of CO2 evolved in TPD. These
parameters were removed from the model, resulting:
Y1=B0+B1T+B2R+B3t+ B13Tt+B23Rt+B11T2+B22R2
[2]
being Y1 responses either SBET or the amount of CO2 evolved in TPD and Bij the
coefficients for each variable or combination of variables.
12
In the case of the amount of CO evolved in TPD used as response, p-values of coefficients
of interactions between temperature and time as well as the quadratic term of impregnation
ratio are higher than 0.15. These parameters were removed from the complete model,
resulting:
Y2=C0+C1T+C2R+C3t+C12TR+ C23Rt+C11T2+ C33t2
[3]
being Y2 the response amount of CO evolved in TPD and Cij the coefficients for each
variable or combination of variables.
In order to check the quality of the model predicted values are plotted versus measured
values for SBET and CO and CO2 evolved groups in Figure 4. Typical error for each sample
is also depicted.
Using those limited quadratic models, isoresponse surface are represented graphically
relating each response and two factors, keeping the third factor constant. The optimization
of all responses (SBET and CO and CO2 evolved groups) under the same experimental
conditions is impossible because the influence of activation temperature, impregnation
ratio and activation time is different. However, it is possible to determine the ranges of
each experimental preparation condition to obtain the desired activated carbon.
Main characteristic desired for an activated carbon is large surface area, and it has been
shown in the previous section that the main factor influencing surface area is impregnation
ratio, existing a maximum in SBET between impregnation ratio 1.00 and 1.17. So, in Figure
5 it can be found the response surface of surface area as a function of activation
temperature and time, keeping constant impregnation ratio at different values in the range
studied (0.95-1.20). As can be deduced from this Figure 5 the optimal range for BET
surface area indicated by the model is activation temperature in the range 400º-550ºC
(common for the range of impregnation ratio studied) and activation time in the range 85
13
min (lower value for impregnation ratio 0.95) and 120 min (higher value for the range of
impregnation ratio studied). Despite there are not methodological studies in the literature
concerning the production of activated carbons from chemical activation of almond shells,
some data of activated carbons production with this precursor and using phosphoric acid
can be found in order to compare the results obtained in this study [15, 41, 45].
Following the goal to obtain an ideal activated carbon, the volume of micro and mesopore
can be an important parameter depending on the type of pollutant to be removed as well as
the media from they have to be removed. Thus, the response surface methodology for
Vmicro and Vmeso using the same model than that used for response SBET was applied. The
idea is to test if the range of preparation conditions to obtain an optimal Vmicro and Vmeso
are in the range of those obtained to obtain optimal SBET. Figure 6 depicts the response
surface for volume of micro and mesopore when impregnation ratio fixed at 1.10. Optimal
Vmeso is obtained in similar range of activation temperature and time than that obtained for
SBET. However, optimal Vmicro are obtained at higher activation temperatures in the same
range of activation time. Thus, in order to obtain an activated carbon with high surface
area and well developed structure (high mesopore and micropore volumes) it is needed to
work at the temperature intersection among intervals of the three responses studied, i.e.
550ºC and activation times of 100 min. However, depending on further application of the
activated carbon (in particular, molecular size of the pollutant to be removed) activation
temperature can be shifted to lower values giving higher surface areas as well as mesopore
volumes.
Surface chemistry in an important characteristic of activated carbons for the removal of
polar compounds [28]. Thus, the amount of CO and CO2 evolving groups from TPD
experiments has been used as a response not only for the amount of surface groups present
14
on the activated carbon but also the type of surface groups present depending on the
activation conditions.
Figure 7 depicts estimated response surface for the amount of CO2 evolving groups at
impregnation ratios 1.00 and 1.20. The optimal range for this response indicated by the
model is activation temperature in the range 400º-550ºC (common for the range of
impregnation ratio studied) and activation time in the range 95-120 min. Both ranges can
be combined with the previous responses studied.
Figure 8 depicts estimated response surface for the amount of CO evolving groups at
impregnation ratios 1.00 and 1.20. The optimal range for this response indicated by the
model is activation temperature in the range 400º-665ºC (common for the range of
impregnation ratio studied) and activation time in the range 108-120 min. Both ranges can
be combined with the previous responses studied.
Summarizing, the analysis all the responses allows to obtain a compromise among the
preparation conditions of the activated carbons in the range studied in this paper.
CONCLUSION
Experimental design methodology has been used to study the influence of activation
temperature, impregnation ratio and activation time on the surface area, porosity and
surface chemistry of the activated carbons prepared. Response surface methodology has
allowed the definition of a range of optimal experimental conditions for the preparation of
activated carbon. This range is wide if only surface area and mesopore volume has to be
optimal (being the intersection of the intervals for both responses temperature 400-550ºC,
time 90-120 min). When a well developed porous activated carbon is needed for a
15
determined application, the range of temperature shifts to higher temperatures and the
range of activation time becomes narrower (temperature 600-800ºC, time 112-120 min) to
reach the optimal micropore volume. The amount of less stable acidic surface groups that
evolves CO2 during TPD experiments is optimal in similar range of preparation condition
than that used to optimize surface area response. However, the amount of more stable
acidic surface groups that evolves CO during TPD experiments is optimal in a wide range
of temperatures but a narrow interval of activation times (temperature 400-665ºC, time
108-112). Thus, to prepare a high surface activated carbon with a well developed porosity
and a high amount of oxygen surface groups, the intersection of the above defined
intervals gives a temperature of 550ºC (there is not intersection between temperature for
optimal response of Vmicro and the rest of the responses), impregnation ratio 1.10 and
activation time of 112 min.
Acknowledgements
The financial support from Spanish Ministry of Environment (contracts 439/2006/3-11.2
and B030/2007/2-11.2) is duly recognized.
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20
FIGURE CAPTIONS
Figure 1. (a) DTG curves for decomposition of low temperature activated carbons. 1:
AT400R05t120; 2: AT400R1t75; 3: AT400R15t30; 4: AT467R067t90; 5: AT467R117t45;
6: AT533R083t60. (b) DTG curves for decomposition of high temperature activated
carbons. 7: AT600R05t75; 8: AT600R1t30; 9: AT667R067t45; 10: AT800R05t30; 11:
AT800R15t120.
Figure 2. : Profiles of CO2 and CO evolved in TPD experiments for low temperature
activated carbons. 1: AT400R05t120; 2: AT400R1t75; 3: AT400R15t30; 4:
AT467R067t90; 5: AT467R117t45; 6: AT533R083t60;
Figure 3. Profiles of CO2 and CO evolved in TPD experiments for high temperature
activated carbons. 7: AT600R05t75; 8: AT600R1t30; 9: AT667R067t45; 10:
AT800R05t30; 11: AT800R15t120.
Figure 4. Predicted values (using the corresponding model) versus measured values. a)
SBET; b) amount of surface groups yielding CO during TPD experiments; c) amount of
surface groups yielding CO2 during TPD experiments.
Figure 5. Response surface for BET surface area at fixed impregnation ratios: a) 0.95 b)
1.00 c) 1.10 d) 1.20
Figure 6. Response surface for a) mesopore volume and b) micropore volume at fixed
impregnation ratio 1.10.
Figure 7. Response surface for CO2 evolving groups (in TPD experiments) at fixed
impregnation ratios: a) 1.00 b) 1.20
Figure 8. Response surface for CO evolving groups (in TPD experiments) at fixed
impregnation ratios: a) 1.00 b) 1.20
21
Table 1. Preparation conditions of the activated carbons and labeling of samples
Sample
T
R*
(ºC)
t
(min)
AT400R05t120
400
0.50
120
AT400R1t75
400
1.00
75
AT400R15t30
400
1.50
30
AT467R067t90
467
0.67
90
AT467R117t45
467
1.17
45
AT533R083t60
533
0.83
60
AT533R083t60(2)
533
0.83
60
AT600R05t75
600
0.50
75
AT600R1t30
600
1.00
30
AT667R067t45
667
0.67
45
AT800R05t30
800
0.50
30
AT800R15t120
800
1.50
120
*impregnation ratio: amount phosphoric acid (g)/ amount almond shell (g)
22
Table 2. Ultimate analysis of activated carbons (% in dry basis)
Sample
C
H
N
S
O*
Yield**
AT400R05t120
69.10
3.57
0.40
n
26.93
72.5
AT400R1t75
75.20
2.42
0.31
n
22.07
68.4
AT400R15t30
72.40
3.13
0.49
n
23.98
69.2
AT467R067t90
69.90
2.81
0.34
n
26.95
71.7
AT467R117t45
70.00
2.72
0.30
n
26.98
70.6
AT533R083t60
68.70
2.12
0.38
n
28.80
72.9
AT533R083t60(2)
68.11
2.57
0.50
n
28.82
73.6
AT600R05t75
67.57
2.12
0.45
n
29.86
74.1
AT600R1t30
68.47
2.05
0.42
n
29.06
73.2
AT667R067t45
66.97
2.02
0.55
n
30.46
74.8
AT800R05t30
60.30
1.33
0.37
n
38.00
71.3
AT800R15t120
71.44
1.93
0.37
n
26.26
67.3
*by difference
**mass of activated carbon/mass of precursor x 100
n: negligible
23
Table 3. Total amount of CO and CO2 evolved up to 1100ºC in TPD experiments.
Sample
CO
CO2
mmol/g
mmol/g
AT400R05t120
1.98
0.57
AT400R1t75
2.17
1.32
AT400R15t30
1.80
0.75
AT467R067t90
2.59
1.04
AT467R117t45
2.18
1.18
AT533R083t60
2.23
0.39
AT533R083t60(2)
2.18
0.36
AT600R05t75
2.83
0.52
AT600R1t30
3.04
0.45
AT667R067t45
2.86
0.48
AT800R05t30
4.13
1.92
AT800R15t120
3.72
1.29
24
Table 4. Results from N2 physisorption
Sample
SBET
Vmeso1
Vmicro2
Vp
Microporosity3
(m2/g)
(cm3/g)
(cm3/g)
(p/p0=0.995)
%
(cm3/g)
AT400R05t120
626±29
0.124
0.202
0.376
89.1
AT400R1t75
1128±50
0.385
0.259
0.670
77.8
AT400R15t30
789±34
0.268
0.176
0.482
71.0
AT467R067t90
785±33
0.259
0.209
0.497
78.9
AT467R117t45
1117±44
0.494
0.181
0.724
70.9
AT533R083t60
891±36
0.341
0.194
0.571
74.6
AT533R083t60(2)
903±36
0.378
0.182
0.593
72.0
AT600R05t75
502±23
0.106
0.176
0.301
86.7
AT600R1t30
926±37
0.375
0.182
0.624
69.7
AT667R067t45
624±28
0.176
0.183
0.440
71.4
AT800R05t30
723±34
0.147
0.253
0.425
88.9
AT800R15t120
990±38
0.511
0.145
0.694
64.6
1 BJH method
2 t-plot method
3 according to Lillo-Rodenas et al.. 2005
25
Figure 1
1,5
a
d(M/M0)/dt (min-1)
1,25
1
3
0,75
1
0,5
2
4
5
6
0,25
0
200
400
600
T (ºC)
800
1000
3
b
2,5
d(M/M0)/dt (min-1)
9
10
2
7
1,5
11
8
1
0,5
0
200
400
600
T (ºC)
800
1000
26
Figure 2
2
CO2
%
1,5
2
1
5
4
3
1
0,5
6
0
0
200
400
600
Tª (ºC)
800
1000
1200
4,5
CO
4
3,5
3
5
2,5
%
2
2
1,5
1
1
0,5
3
4
6
0
0
200
400
600
Tª (ºC)
800
1000
1200
27
Figure 3
5
CO2
4
10
%
3
2
11
1
7
9
8
0
0
200
400
600
Tª (ºC)
800
1000
1200
10
CO
9
8
10
7
11
9
%
6
5
4
8
3
2
7
1
0
0
200
400
600
Tª (ºC)
800
1000
1200
28
Figure 4
4,4
1200
a
1100
1000
3,6
predicted (CO)
predicted (S BET)
b
4,0
900
800
700
3,2
2,8
2,4
600
2,0
500
400
400
1,6
500
600
700
800
900 1000 1100 1200
1,6
2,0
2,4
2,8
3,2
3,6
4,0
4,4
m easured (CO)
m easured (SBET)
2,2
c
predicted (CO 2)
1,8
1,4
1,0
0,6
0,2
0,2
0,6
1,0
1,4
1,8
2,2
m easured (CO2)
29
Figure 5
b
a
c
d
30
Figure 6
b
a
31
Figure 7
a
b
32
Figure 8
a
b
33