Materials Sciences and Applications, 2011, 2, 331-339
doi:10.4236/msa.2011.25043 Published Online May 2011 (http://www.SciRP.org/journal/msa)
331
Novel Activated Carbon Monoliths for Methane
Adsorption Obtained from Coffee Husks
Liliana Giraldo1, Juan Carlos Moreno-Piraján2*
1Departamento de Química, Facultad de Ciencias, Universidad Nacional de Colombia, Bogotá. Colombia; 2Grupo de Investigación
en Sólidos Porosos y Calorimetría, Universidad de los Andes, Bogotá, Colombia.
Received March 6th, 2011; revised March 16th, 2011; accepted March 24th, 2011.
ABSTRACT
Methane adsorption by different forms of activated carbon obtained from coffee husks, including monolith honeycomb
and disc types, was studied by activation with zin c salts and potassium hydroxide at 298.15 K and 303.15 K and pres-
sures up to 30.00 a tm in a volumetric adsorptio n apparatus. We observed increased methane adsorption capacity on a
mass basis in the different activated carbon monoliths with increasing surface area, total pore volume and micropore
volume, with the honeycomb type displaying the highest methane absorption capacity. The maximum volumetric meth-
ane uptake by the synthesised carbon monoliths was observed to be 130 V/V at 298.15 K and 30.00 atm for honeycomb
monoliths synthesised with zinc chloride (ZnCl2) and Polyvinyl alcohol (PVA) as the binder. Adsorption calorimetry
results were used to describe the interaction between guest molecules and the adsorbent at low surface coverage and
the energetic heterogeneous surface nature of the adsorbent.
Keywords: Coffee Husk, Activated Carbon, Isotherm Adsorption, Carbon Monolith, Methane Adsorption, Isosteric Hea t
of Adsorption, SEM
1. Introduction
The global crisis over the coming shortage of fossil fuels
is increasing, while the environmental effects of contin-
ued fossil fuel use are already being felt in the form of
the greenhouse effect and global climate change. Be-
cause of this environmental crisis, researchers have pro-
posed several interesting solutions to avoid the massive
use of fossil fuels, which will hopefully result in a
cleaner planet. One of these proposals is to develop ma-
terials that enable storage and subsequent release of less
harmful fuels, such as methane. In fact, published re-
search on this topic has been increasing over the past few
years, and several ingenious developments have emerged.
Various solids with very high porosity have been devel-
oped, and, through materials engineering, highly specific
structural designs have been created in an attempt to op-
timise substrates for methane storage. Activated carbon
is widely used in adsorption separation processes, be-
cause of its attractive adsorption properties [1-5]. As
such, large amounts of data have been published con-
cerning the adsorption isotherms of pure and multicom-
ponent gases on activated carbon [6-10]. Natural gas
fu-els, which satisfy both environmental and economical
concerns, are one of the most desired fuels in the energy
field, especially in comparison with liquid fossil fuels
such as gasoline and diesel. Natural gas is abundant and
the cleanest burning alternative transportation fuel avail-
able.
After testing at the laboratory level, pilot natural gas
vehicles (NGVs) have been evaluated, with some amaz-
ing results. In fact, investigators reported that NGVs
have been certified to meet the most demanding envi-
ronmental emission standards. Although methane is
more convenient than liquid fuel, due to its low energy
density at standard temperature and pressure (STP) con-
ditions, the storage of large amounts of methane in a
given volume is very difficult. Thus, methane storage
has become an important challenge for researchers in
this field [11]. In addition, the development of natural
gas storage and transportation technologies will be very
important for the future. Although several authors have
proposed that well- known conventional methods, such
as liquefied natural gas (LNG) and compressed natural
gas (CNG), may be the solution for methane storage
purposes, these methods are associated with both safety
and cost effectiveness problems [12-14]. For example,

Novel Activated Carbon Monoliths for Methane Adsorption Obtained from Coffee Husks 332
even though storing high density methane in LNG vehi-
cles is possible at cryogenic temperatures, the special-
ised container design and refuelling procedure required
are undesirable for vehicular fuel applications. Similarly,
CNG, with its low energy density compared to gasoline,
requires multi-stage compression that increases its cost
and necessitates the use of a relatively large fuel tank
[15]. Compared to CNG and LNG, adsorbed natural gas
(ANG) is one of the best alternatives for natural gas
storage and for the inevitable technologies for storing
natural gas at 3.5 MPa at room temperature [15-17].
Indeed, because of its low capital and low operating
costs versus CNG, the ANG process is being considered
for on-board technology in NGVs [18]. One of the big-
gest obstacles to the implementation of ANG technology
is the development and evaluation of adsorbents [16].
While the majority of carbon adsorbents are granular,
powder, or fibres, an adsorbent with a large volume of
pores and a high surface area is required for maximal
adsorption on a mass basis [19]. Activated carbon ex-
hibits the largest adsorption capacity and highest ANG
energy: highly microporous carbons are the most prom-
ising adsorbents for natural gas storage.
Evaluation of pure-gas adsorption isotherms is an easy
and straightforward method for experimentally measure-
ing adsorption levels with high accuracy. Natural gas is
considered to be one of the most promising alternative
fuels, because of its abundance, low cost and minimal
emissions. However, natural gas requires a special stor-
age system [1], because of its low volumetric energy
density [12,13,20-24], and the adsorbed natural gas
(ANG) storage represents a feasible solution to this tech-
nological challenge [22-30]. ANG methanol is a method
where both the adsorption and compression processes are
carried out simultaneously, in order to store natural gas
under convenient temperatures and pressures compared
to conventional methods [10,23,28-39]. Activated carbon
(AC) has been extensively used in the ANG process as
an adsorbent, because of its highly microporous surface
[22-24,31,37]. Activated carbon has been prepared from
various precursors, including different biomasses [37].
Unfortunately, highly porous activated carbons display
relatively lower natural gas adsorption capacities on a
volume basis, because of their lower packing density
[7,9,13,30]. Thus, monolithic forms of activated carbon
have been employed in ANG storage systems, to achieve
improved storage capacity [16,38-40]. These monolithic
forms of activated carbon display increased packing den-
sity through a reduction of excess void volume [40], and
offer increased compactness and easy handling. Carbon
monoliths are normally made by compressing either ac-
tivated carbon [22], or a mixture of activated carbon and
a binder [3,10,40]. In the present study, we investigated
the methane storage capacities of two forms of activated
carbon, a powder and a monolith (honeycomb and disc),
obtained by chemical activation of waste coffee husks:
an abundant waste product. Evaluations were conducted
using published procedures reported in the literature
[22-24,37,39], to enable comparison of our results with
previously published results. Carbon monoliths were pre-
pared using binders and an impregnating agent [5,41,42].
After careful screening, the activated carbon monoliths
were selected for methane storage analyses. The physical
properties of the carbon monolith samples were characte-
rized by N2 (77 K) adsorption isotherms [43,44]. Heats of
adsorption for pure-component adsorption may be ob-
tained experimentally from adsorption calorimetry [45-
47]. The methane adsorption capacities of the carbon
monoliths were assessed using a volumetric adsorption
method at two constant temperatures (298.150 and
303.150 K) and a pressure up to 35.00 atm. In addition to
methane adsorption behaviour on the carbon monoliths,
the interaction between the adsorbent and adsorbate at
low surface loading, and the heat evaluation during ad-
sorption were examined using Clausius-Clapeyron equa-
tions [19,48-51].
2. Experimental
2.1. Preparation of the Carbon Monoliths
Husks from Colombian coffee beans were impregnated
with an aqueous solution of phosphoric acid following a
variant of the incipient wetness method, as described
elsewhere [51]. Briefly, aqueous solution (1.4 ml/g cof-
fee bean husks) was added drop wise (while stirring the
solid to facilitate homogeneous absorption of the liquid)
to produce swelling, resulting in incipient wetness.
H3PO4 (J. T. Baker (Phillipsburg, USA) was used at a
concentration of 150% wt% [51,52] in the aqueous solu-
tion, defined as follows: g H3PO4 per g coffee bean husks)
× 100. After impregnation, samples were dried for 4 h at
383 K in air. Thermal treatments were carried out in a
vertical tubular reactor made of quartz, using 10 g of
impregnated and dried material in all cases. All treat-
ments were performed at a constant heating rate of 283
K/min with an argon (99.999% pure) flow of 50 STP
cm3/min, which was maintained during both heating and
cooling. An activation temperature of 723 K and a soak-
ing time of 1 h was used. After cooling the solid pyroly-
sis residue to room temperature, samples were washed
with milli-Q distilled water until the conductivity of the
washing liquids was reduced to </5 μS/cm (as measured
by a pH/conductivity meter, Mettler Toledo, model
MPC227). Resulting activated carbon samples (ACs)
were dried at 383 K for 12 h in a vacuum furnace. This

Novel Activated Carbon Monoliths for Methane Adsorption Obtained from Coffee Husks 333
sample is marked in this work as AC-RC.
The activated carbon monolith samples (MHZ-RC:
Monolith honeycomb activated with zinc chloride; MHK-
RC: Monolith honeycomb activated with potassium hy-
droxide, MDZ-RC: Monolith type disc activated with
zinc chloride; and MDK-RC: Monolith type disc acti-
vated with potassium hydroxide) were synthesised under
the different conditions shown in Table 1. The monoliths
were prepared by pressing at 0.02 atm at a temperature of
423.15 K. The carbon monolith samples were made from
activated carbon samples obtaining from coffee husks
using ZnCl2 and KOH as chemical activators. In final
form, all four monoliths were 5 mm thick, with a diame-
ter of 12 mm.
2.2. Adsorption Measurements
The surface structural characteristics of the carbon
monoliths were determined from N2 (77 K) adsorption
isotherm results (AUTOSORB 3B Quantachrome, Miami,
FL, US). The apparent surface area (SBET) of each carbon
monolith was calculated by analysing N2 adsorption data
according to Brunauer-Emmett-Teller (BET) theory (re-
lative pressure range: 0.01 - 0.05) [40]. Micropore vol-
umes (Vo) were obtained from the Dubinin-Raduskevich
equation, and total pore volumes (VT) were evaluated
from nitrogen adsorption data obtained at a relative
pressure of 0.99 using the Gurvistsch rule. The mesopore
volume (Vmes) was determined using the Barrett-Joiner-
Halenda (BJH) equation. Once the BET surface area
(SBET) and total pore volume (VT) were determined, the
average pore width was calculated using the following
expression: (4VT/SBET) [40]. Resulting packing densities
are also listed in Table 1. Dry materials were outgassed
at 473 K overnight: however, to avoid water evaporation,
wetted materials were not subjected to this treatment. A
thermostatic device was used to maintain the temperature
within a tolerance of 2 ± 0.1 K. Porosity development
and void volume were analysed using a field emission
JEOL JSM 6510 scanning electron microscope (SEM)
(Japan). Scanning electron microscope images of the two
types of adsorbents (activated carbon honeycomb and
disc) are shown in Figures 1(a) and 1(b), respectively.
The channels of honeycombs are regular with side leng-
ths between 453 and 532 μm, while the channel walls
have a thickness between 367 and 387 μm; for monoliths
in the form disc a diameter of 10 mm × 5 mm was ob-
tained. High-pressure methane adsorption studies were
conducted in a volumetric adsorption apparatus at two
temperatures (298.150 and 303.150 K) and at pressures
up to 35.00 atm. The experimental procedure and the
volumetric apparatus have been previously described
[4,5].
(a)
(b)
Figure 1. (a) Scanning electron microscope image of an ac-
tivated carbon honeycomb monolith; (b) Scanning electron
microscope image of an activated carbon disc.
The experimental apparatus used for methane adsorp-
tion at high pressure is shown in Figure 2. Initially, a
high vacuum was created in the experimental device,
including the cell-sample container (parts 6, 7 and 9 in
Figure 2). This vacuum was achieved using a system of
pumps (primary and secondary pumps, part 6) overnight.
A residual pressure of 1.40 × 10−7 Pa was observed in the
system after the activated carbon monoliths were backed
out and evacuated for the adsorption experiment. After-
wards, the CH4 valve was opened and methane was in-
jected in small quantities into the introduction compart-

Novel Activated Carbon Monoliths for Methane Adsorption Obtained from Coffee Husks 334
Table 1. Textural properties of activated carbon type honeycomb monoliths.
N
2 a 77 K
Sample SBET m2/g Vo cm3/g Vmeso
cm3/g Vt cm3/g V0/Vt Average pore width (APW)(Å)Cell packing density (g/cm3) Storage capacity
(V/V)
MHZ-RC 1326 0.84 0.02 0.86 0.98 22.6 0.598 130
MHK-RC 1187 0.80 0.01 0.83 0.96 31.3 0.561 125
MDZ-RC 1059 0.76 0.02 0.79 0.96 30.4 0.576 118
MDK-RC 1000 0.72 0.03 0.77 0.93 29.6 0.551 104
AC-CR 890 0.59 0.01 0.64 0.92 19.8 0.349 98
Figure 2. Experimental set-up of the equipment used to obtain high pressure isotherms, 1. Gauge, 2. pressure transducer, 3.
reservoir gas, 4. cold trap, 5. gas, 6. vacuum pump, 7. cell container, 8. valves, 9. micrometric valve.
ment (parts 1, 2, 3 and 4 in Figure 2). The pressure value
was monitored using pressure gauges (gauge 1: range
from 0 to 121.31 Pa and gauge 2: range from 1 to 35.45
kPa, part 3 in Figure 2). Following this step, the valve
separating the introduction compartment from the cell-
sample container was opened and the pressure value was

Novel Activated Carbon Monoliths for Methane Adsorption Obtained from Coffee Husks 335
read once equilibrium was established.
3. Results and Discussion
3.1. Evaluation of the Methane Storage Capacity
of Activated Carbon Honeycomb Monoliths
Characterizations data obtained from N2 adsorption/de-
sorption isotherms are presented in Table 2, and the cor-
responding adsorption isotherms are shown in Figure 3.
Isotherms were defined as type I, according to the Inter-
national Union of Pure and Applied Chemistry (IUPAC)
classification of adsorption isotherms: where type I iso-
therms display rapidly increasing low pressure which
plateaus to a constant value of relatively high pressure.
Obtained isotherms are consistent with the microporous
solids in all cases.
The Brunauer-Emmett-Teller (BET) surface area of
the carbon monoliths increased from 890 m2/g (for acti-
vated carbon obtained from coffee husks, AC-RC) to
1326 m2/g (honeycomb monoliths treated with zinc chlo-
ride, MHZ-RC) under the activation conditions used
(impregnation chemical agent and chemical activation).
The micropore volume of the carbon monoliths also in-
creased from 0.59 cm3/g (AC-RC) to 0.89 cm3/g (MHZ-
RC). Chemical activation to obtain the honeycomb type
monoliths, using ZnCl2 (MHZ-RC) or KOH (MHK-RC)
and PVA as a binder, resulted in increased surface area,
micropore volume and total volume (see Table 2). Disc-
type monoliths, obtained by chemical activation using
KOH with PVA as the binder, developed a smaller sur-
face area and reduced micropore and total volumes. The
effect of ZnCl2 treatment was observed in MHZ-RC and
MDZ-RC monoliths. ZnCl2 treatment enabled generation
of a monolith with a surface area and pore volume of the
same order of magnitude as the original activated carbon,
AC-RC. Similarly, monoliths MDK-RC and MDK-CR
developed surface areas and pore volumes of the same
order of magnitude as AC-CR.
These results are likely due to chemical activation with
ZnCl2, which has been reported to produce a larger
number of pores during the activation process, as well as
a greater surface area and micropore volume [5]. Activa-
tion with KOH also promoted development of the proc-
ess surface area and porosity, but to a lesser degree. This
is shown in Table 2, which illustrates the difference be-
tween the honeycomb and disc forms. Specifically, the
geometric effect of the channels present in the honey-
comb monoliths appear to allow for greater diffusion of
gases (and therefore adsorption) than the disc-shaped
monoliths.
The main goal of the present work was to obtain
monoliths from coffee husks with good microporosity,
which is a desirable characteristic for methane storage of
methane. With these aims in mind, the CHZ-RC and
MHZ-RC monoliths obtained using ZnCl2 and KOH,
respectively, were the most effective monoliths tested.
Methane adsorption studies were performed at a pressure
of 35.45 kPa and at two different temperatures (298.150
K and 303.150 K), and methane adsorption results are
shown in Figures 4 and 5. Figure 5 shows the methane
adsorption capacity by mass of MHZ-RC at 298.15 K
and 303.15 K and 35.45 kPa. It can be seen that the
amount of methane adsorption by MHZ-RC was signify-
Table 2. Characteristics of sy nthesised monoliths fr om c o ffe e husks.
PARAMETER ACTIVATION I
AC-RC
ACTIVATION II
MHZ-RC
ACTIVATION I
MHZ-RC
ACTIVATION II
MDZ-RC
ACTIVATION III
MDK-RC
Binder -
Polyvinyl alcohol
(PVA)-5 wt%
Polyvinyl alcohol
(PVA)-5 wt%
Polyvinyl alcohol
(PVA)-5 wt%
Polyvinyl alcohol
(PVA)-5 wt%
Precursor Coffee husk Coffee husk Coffee husk Coffee husk Coffee husk
Activation gas Ar (50 l/min) N2 (100 ml/min) N2 (100 ml/min) N2 (100 ml/min) N2 (100 ml/min)
Temperature of
activation 723 K 1073 K 1073 K 1073 K 1073 K
Temperature ramp 283 K/min 283 K/min 283 K/min 283 K/min 283 K/min
Residence time 1 hour 2 hours 2 hours 2 hours 2 hours
Temperature of
impregnation 358 K 358 K 358 K
Chemical agent for
impregnation
ZnCl2 (ratio 2 ml of ZnCl2
32% solution to 1 g of
precursor binder mixture)
KOH (ratio: 4 g of KOH
to 1 g precursor binder
mixture)
ZnCl2 (ratio 2 ml of
ZnCl2 32% solution to
1 g of precursor)
KOH (ratio: 4 g of KOH
to 1 g precursor binder
mixture)
Impregnation
method Impregnated in plate with magnetic stirring
Impregnated in plate
with magnetic stirring
Impregnated in plate
with magnetic stirring
Impregnated in plate
with magnetic stirring
Dry Furnace 110˚C (more
than 12 hours)
Furnace 110˚C (more
than 12 hours)
Furnace 110˚C (more
than 12 hours)
Furnace 110˚C (more
than 12 hours)
Furnace 110˚C (more
than 12 hours)

Novel Activated Carbon Monoliths for Methane Adsorption Obtained from Coffee Husks 336
Figure 3. Adsorption isotherms of activated carbon honey-
comb monolith.
Figure 4. Methane adsorption capacity (by mass) of acti-
vated carbon monoliths at 298.15 K and 30.00 atm.
Figure 5. Methane adsorption capacity (by mass) of ACM1
at 298.15 K and 303.15 K and 30.00 atm.
cantly greater than the AC-RC starting material. In addi-
tion, because of their larger surface area, pore volume
and total volume, monoliths obtained by chemical active-
tion and with PVA as a binder were able to adsorb 9.34
mmol/g of methane. In contrast, the original activated
carbon (AC-RC) obtained by physical activation only
adsorbed 5.06 mmol/g.
The adsorption capacity for methane was increased
from 98 V/V (AC-RC) to 130 V/V (MHZ-RC). These
results are novel for this type of material, and the high
adsorption capacity for methane observed for the MHZ-
RC sample was found to be associated with its increased
surface area and pore volume, which was developed via
the synthesis route described. Our results show that
provides greater storage capacity of methane using coffee
husks. Other results using activated carbons synthesized
from lignocellulosic residues such as waste olive oil have
storage capacities of 59 V/V. Other authors have reported
storage capacities between 64-94 V/V using coconut
shells. Capacities up to 95 V/V with acti- vated carbon
from coconut shell activated with phospho- ric acid have
also been reported. Other values for acti- vated carbon
derived from waste tires have reached storage capacity of
methane between 45 - 50 V/V.
3.2. Isosteric Heats of Adsorption
The variations in the isosteric heat of adsorption for the
different adsorbate loadings were calculated using the
Clapeyron equation. The isosteric heat of adsorption cor-
responds to the energy released during the adsorption
process, and depends on the temperature and surface area.
Figure 6 shows the isosteric heat of adsorption for
methane for each of the different forms of activated car-
bon. Isosteric heats calculated from the experimental data
varied with the different amounts of methane adsorbed.
The isosteric heat of adsorption is a valuable thermo-
dynamic property, which provides useful information
about the interactions between the adsorbent and the ad-
sorbate, as well as the energetic heterogeneity of the ad-
sorbent surface [4,7,10,13,24]. The isosteric heat of ad-
sorption was determined based on the Clausius-Clapey-
ron equation [5,52-56]:
2
ln
st
N
qP
T RT








 (1)
where qst is the isosteric heat of adsorption, R is the gas
constant, T is the temperature, P is the pressure and N is
the adsorbed amount. In the present work, several iso-
therms at different temperatures were employed for de-
termination of the dependence of the isosteric heat on the
concentration of adsorbate. Figure 6 shows isosteric
heats obtained in the present study for methane adsorp-
tion as a function of the amount of methane adsorbed. As

Novel Activated Carbon Monoliths for Methane Adsorption Obtained from Coffee Husks 337
Figure 6. Isosteric heats of adsorption for methane for each
of the different forms of activated carbon.
can be seen, the heat of adsorption decreases as the
amount loading increases, consistent with the energetic
heterogeneity of the carbon honeycomb monolith sur-
faces obtained under different synthesis conditions. Dur-
ing methane adsorption onto the different carbon mono-
liths, heats of adsorption varied with surface loading as
follows: 40.00 - 8.00 kJ/mol for MHZ-RC; 26.8 - 5.2
kJ/mol for MHK-RC; 18.5-3.3 KJ/mol for MDZ-RC;
13.5 - 2.00 kJ/mol for MDK-RC; and [8,10,30] kJ/mol
for AC-RC, 11.5 - 0.8 kJ/mol.
Overall, a mainly linear relationship was observed be-
tween methane adsorption capacities (by mass), adsorp-
tion affinities and isosteric heat adsorptions for the acti-
vated carbon honeycomb monoliths. For the MHZ-RC
and MHK-RC monoliths, observed qst values were lower
than those reported in the literature [5]. This dif- ference
may be due to textural characteristics of the monoliths
synthesised in this study, because surface area, pore
volume and pore width affect diffusion of methane to the
interior pores of these samples, resulting in lower qst val-
ues.
4. Conclusions
Activated carbon monoliths were synthesised from cof-
fee husks and different materials, using PVA as a binder.
The best methane storage capacity was observed for the
MHZ-RC sample, which had the greatest surface area and
total pore volume. Under the experimental conditions
used in this work, methane storage capacity was a func-
tion of the total surface area and pore volume.
Measurement of isosteric heats demonstrated that the
synthesised monoliths had heterogeneous surfaces from a
thermodynamic point of view. Additionally, the obtained
isosteric heats were consistent with the textural charac-
teristics of the monoliths.
5. Acknowledgments
The authors thank the Departments of Chemistry of Uni-
versidad Nacional de Colombia, Universidad de Los An-
des (Colombia) and the Master Agreement established
between these institutions. Special gratitude is expressed
to the Fondo Especial de Investigaciones de la Facultad
de Ciencias de la Universidad de Los Andes (Colombia)
for partial financing.
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