Influence of activated carbon modifications on their catalytic activity in methanol and ethanol conversion reactions

Article abstract of DOI:10.2478/s11532-011-0078-7

Activated carbons containing different surface functionalities have been investigated as catalysts in conversion reactions of ethanol and methanol. These carbon materials were prepared from Polish brown coal by chemical activation with potassium hydroxide and modified by the oxidation or reaction with ammonia or chlorine. The main process upon ethanol decomposition was its dehydrogenation, while in the process of methanol decomposition only a few samples were catalytically active, and the only product was dimethyl ether (a product of dehydration). Versita Sp. z o.o.

Full text of DOI:10.2478/s11532-011-0078-7

Cent. Eur. J. Chem. • 9(5) • 2011 • 925-931
DOI: 10.2478/s11532-011-0078-7
Central European Journal of Chemistry
Influence of activated carbon modifications
on their catalytic activity in methanol
and ethanol conversion reactions
Research Article
Jadwiga Jasińska, Beata Krzyżyńska, Mieczysław Kozłowski*
Faculty of Chemistry, Adam Mickiewicz University, 60-780 Poznań, Poland
Received 1 February 2011; Accepted 15 June 2011
Abstract:
Activated carbons containing different surface functionalities have been investigated as catalysts in conversion reactions
of ethanol and methanol. These carbon materials were prepared from Polish brown coal by chemical activation with potassium
hydroxide and modified by the oxidation or reaction with ammonia or chlorine. The main process upon ethanol decomposition was its
dehydrogenation, while in the process of methanol decomposition only a few samples were catalytically active, and the only product
was dimethyl ether (a product of dehydration).
Keywords: Dehydration • Dehydrogenation • Alcohols • Activated carbon • Catalytic activity
© Versita Sp. z o.o.
1. Introduction
Szymański et al. [23] carried out the process of ethanol
decomposition over different carbon catalysts obtained
from poly(furfuryl alcohol). Apart from the fundamental
products of ethanol decomposition (ethylene, diethyl
ether and acetic aldehyde) they also obtained 1,1’-
diethoxyethane. The same authors proved that those
samples oxidised with concentrated nitric acid showed
greater activity and selectivity towards dehydration, than
those with the addition of Ni2+ ions which resulted in the
appearance of new centres active in dehydrogenation.
According to authors, the dehydrogenation occurred with
simultaneous participation of Lewis acid and base sites.
Catalytic activity increases with increasing temperature
of the process, but for temperatures higher than
593 K, a drop in the catalytic activity caused by thermal
decomposition of surface oxygen groups is observed.
Grunewald and Drago [24] who checked the behaviour
of carbon molecular sieves in the reaction of ethanol
decomposition noted that when this process is conducted
exclusively in the neutral gas atmosphere the catalytic
activity decreases, and the main reaction product is
ethylene. The catalyst activity can be recovered when
a mixture of the carrier gas and air is used; then the
dominant products are ethyl acetate and acetic aldehyde.
The dehydrogenation of ethanol was established to take
One of the catalytic processes taking place on acid/base
active centres is the decomposition of alcohols. From
among reactions of this type described in literature,
much interest has been devoted to dehydration and the
dehydrogenation ofaliphatic alcohols(mainlyisopropanol
[1,2]). Taking into regard the selectivity of these reactions
towards acid or base catalytic centres, these processes
are classified as the so-called models and often used for
characterisation of the chemical character of the surface
[3-5]. These processes are of interest regarding their
cognitive aspect as well as their prospective applications
[6,7] due to the significant industrial importance
of the products obtained in ethanol and methanol
decompositions (such as ethylene, acetaldehyde, diethyl
ether, dimethyl ether and formaldehyde) [6,7].
A large number of catalytic systems has been studied
in the reactions of methanol and ethanol decomposition,
starting from the catalysts based on metal oxides
[8-10] (transient metals included [7]), through zeolites
[11-13] to metals or metal oxides supported on activated
carbon [14-22]. There are only a few reports on the
decomposition of ethanol and methanol on activated
carbon which is used as catalyst in its own right [22-27].
* E-mail: mkozlow@amu.edu.pl
925

Influence of activated carbon modifications on their catalytic
activity in methanol and ethanol conversion reactions
place on Lewis acid sites; their activity decreased with
time at the neutral gas flow through the catalyst bed.
The other major product observed (ethyl acetate) was
the result of a secondary condensation reaction.
2. Experimental procedure
The precursor used to obtain the 13 samples of different
surface carbon catalysts was Polish brown coal from
the Konin colliery. The initial procedure to obtain the
activated carbon catalysts included grinding of the
precursor to the size ≤0.2 mm, its demineralisation by
the concentrated HCl and HF and chemical activation
by potassium hydroxide at 1123 K. The activated carbon
was then treated with various chemical reagents such
as nitric acid, sulphuric acid, peroxyacetic acid (PAA),
air, gas ammonia and chlorine. The aim of these
modifications was to introduce new surface functional
groups or change (as to the type and abundance) of
the present active centres. Details on the conditions
of precursor activation and modification as well as full
characterisation of the samples obtained (elemental
analysis, FTIR results, porous structure) have been
given in our previous paper [28]. The samples obtained
were labelled to include the following information: K /
modifying agent / time of modification / modification
temperature.
The catalytic tests in the decomposition of ethanol
and methanol were performed in a glass fixed-bed
reactor. Small volumes (0.2 µL) of ethyl or methyl alcohol
were introduced in pulses into the reactor containing
a layer of a carbon catalyst (0.02 g). The reaction
temperature varied from 423 to 723 K. The carrier gas
was helium at a flow rate of 30 cm3 min-1. Products of
ethanol and methanol decomposition were analysed on
a gas chromatograph connected on-line with the reactor
and equipped with flame-ionisation detector (FID) and
a 2 m long column packed with 30% Emulphor O on
Chromosorb W.
Ethanol decomposition has also been studied by the
research group of Carrasco–Marín [25]. They studied
the effect of active carbon oxidation with (NH4)2S2O8 on
their activity and selectivity in ethanol decomposition
reaction. As follows from their results, the activated
but unoxidised carbon is able to catalyse exclusively
the dehydrogenation reaction whose main product
is acetic aldehyde. Dehydrogenation takes place on
both acid and base centres. This reaction can also be
catalysed by ether groups on the surface of the carbon
catalysts. When oxidised activated carbons are used,
alcohol decomposition can result in its dehydration and
dehydrogenation. The dehydration process increases
with an increased total acidity of the carbon surfaces
and takes place mainly at the active centres of the
Brönsted type. The main product of the reaction is ether
which easily converts to ethylene.
Not many authors have been interested in the
decomposition of methanol on carbon materials.
Zawadzki et al. revealed that the pure, unoxidised carbon
itself is unreactive in the decomposition of methanol
[22]. Catalytic activity appeared only after carbon
oxidation by gas oxygen. The results provided evidence
for the participation of methoxides and formaldehyde
intermediates in the decomposition of methanol. The
reaction of methanol decomposition catalysed by
activated carbons oxidised by different oxidising agents
was studied by Moreno–Castilla et al. [26,27]. They
reported that activated carbons oxidised with (NH4)2S2O8
have the strongest acid groups and are the most active
in the dehydration reaction of methanol to dimethyl
ether. An increase in the catalyst heating temperature
results in a rapid decrease in its activity in methanol
dehydration. The reason is a gradual decomposition of
3. Results and discussion
carboxyl groups present on the catalyst surface which 3.1. Catalyst characterization
already becomes unstable at 453 K.
The most important data characterising the catalysts
As follows from the survey of literature, reactions of tested (ash content, presence of heteroatoms, specific
ethanol and methanol decompositions can be a source surface area, pore volume and area contributions) are
of information on the chemical nature of the carbon shown in Table 1. The ash content in carbon decreases
surface. Moreover, to the best of our knowledge, these as a result of its treatment with concentrated acids: nitric
reactions have not been performed over carbon catalysts acid (samples K/HNO3/8 h/353 K, K/HNO3/16 h/353 K),
obtained by chemical activation by KOH and modified sulphuric acid (samples K/H2SO4/8 h/523 K, K/H2SO4/16
by concentrated H2SO4, peroxyacetic acid, ammonia or h/523 K), and even after contact with peroxyacetic acid
chlorine. The aim of the study was to establish the effect (sample K/PAA/6 h/313 K). According to literature [29],
of the initial activated carbon modification (liquid or gas treatment with acids also results in increased acidity
phase oxidation and ammonia or chlorine treatment) and hydrophilous character of the carbon surface.
on its catalytic properties in ethanol and methanol Modifications of the initial activated carbon performed
decompositions.
by reagents in the gas phase do not remove mineral
926

J. Jasińska, B.Krzyżyńska, M.Kozłowski
to 2800 m2
components but to the contrary, for the majority of
samples, lead to an increased ash content.
g-1 (Table 1). This carbon had a distinct
microporous structure as follows from the calculated
Further analysis of data shown in Table 1 proves ratio of micropore area to the total surface area and the
that oxidation of activated carbon by oxidising agents contribution of the volume of micropores in the total pore
both in gas and liquid phase brings an increase in the volume. Activated carbon modifications with different
oxygen content relative to that in the initial sample. chemical agents did not cause essential changes in
According to the literature, carbon oxidation in the gas the carbon porous structure; however its exposure to
phase increases mainly the concentration of hydroxyl concentrated nitric acid led to considerable destruction
and carbonyl groups, while the wet oxidation leads of the carbon surface, which is surely related to a very
mainly to an increased content of carboxyl groups strong oxidising effect of this acid.
[30]. The unmodified activated carbon obtained from
3.2. Catalytic activity
of carbons
in
brown coal by the chemical activation with potassium
hydroxide shows a slight content of heteroatoms
(O, H, N, Cl, S) [28]. The number of oxygen groups
significantly increases with an increasing oxidation
temperature and an extended duration of the process
(Table 1). The most effective oxidising agent proved
to be concentrated nitric acid as the treatment with
this acid resulted in the incorporation of the greatest
number of oxygen groups into the carbon structure. As
follows from literature data, the carbon oxidation with
concentrated nitric acid enhances the acidity of the
active centres while decreasing the number of basic
centres [31]. On the other hand, oxidation with nitric
acid leads to the formation of weakly acidic functional
groups [32]. According to Shim et al. [33], these weakly
acidic groups are carboxyl, lactone and phenol ones.
Moreover, the oxidation process with nitric acid besides
oxygen incorporation leads to a nitrogen attachment to
the carbon structure (Table 1). Nitrogen can be attached
in the form of nitric groups [34]. Oxidation of the initial
activated carbon with concentrated sulphuric acid,
peroxyacetic acid or air led to a much smaller increase
in the oxygen content (Table 1).
The treatment with ammonia (Table 1) results in the
deep removal of oxygen from the structure of activated
carbon and the introduction of considerable amounts
of nitrogen (e.g. in the form of pyridine functionalities
[35]). The full elimination of oxygen is explained by the
fact that upon heating in high temperatures, the oxygen
functional groups present on the surface of the catalysts
undergo thermal decomposition. The modification of the
activated carbon with gas chlorine was effective, and
a greater amount of chlorine was introduced into the
carbon structure at the lower modification temperature
(Table 1). It is assumed that chlorine is incorporated
into the carbon structure mainly through addition or by
substitution of hydrogen or oxygen atoms [36].
decomposition of ethanol
According to the literature, ethanol decomposition leads
not only to ethylene, diethyl ether and acetic aldehyde,
but also produces ethyl acetate and 1,1-dietoxyethane
[23-25]. The two latter compounds are produced in
the secondary reactions between acetic aldehyde and
ethanol. The reaction of dehydrogenation takes place in
a simultaneous presence of Lewis base and acid centres,
while that of dehydration involves only acid centres [23].
Results of the catalytic tests performed in this study
proved that all activated carbon samples that were
studied acted as catalysts of ethanol decomposition,
but to a different degree. The main products of
decomposition are ethylene and acetic aldehyde. Diethyl
ether is formed in trace amounts (and that is why its
presence is not shown in the plots), whereas formation
of 1,1-dioxyethane and ethyl acetate is not detected.
Catalytic activity of the activated carbon samples was
evaluated on the basis of the temperature dependencies
of alcohol conversion (Figs. 1-7). As follows from the
results, for the initial sample and for the majority of
the modified activated carbons , the decomposition of
ethanol proceeds mainly by dehydrogenation (which is
in contrast to the decomposition of isopropanol [28]).
The exceptions are the carbon samples oxidised with
concentrated HNO3 (Fig. 2), whose use as a catalyst
leadsmainlytotheformationofethylene(bydehydration).
The latter observation confirms the literature reports
which state that oxidation with nitric acid increases the
activity of carbon catalysts and their selectivity towards
dehydration of lower alcohols [23]. These samples also
yielded the highest amount of ethylene (about 35-50%).
The reaction of ethanol dehydration over activated
carbons oxidised with concentrated nitric acid (Fig. 2)
demonstrated a clear maximum of activity at 600 K,
followed by a decrease in activity probably caused by
the decomposition of certain acidic active centres, i.e.
carboxyl groups located on the catalyst’s surface. This
observation is in agreement with the results published
by Szymański et al. [37], who reported that carboxylic
groupsdecomposetoCO2 at373-673K.Atimeextension
Porous structure of the carbon catalysts was
characterised on the basis of nitrogen adsorption
isotherms measured at 77 K. The use of KOH in the
chemical activation process permitted obtaining the
initial activated carbon of specific surface area close
927

Influence of activated carbon modifications on their catalytic
activity in methanol and ethanol conversion reactions
Table 1. Characteristics of activated carbons investigated.
Ash
(dry
basis)
Atomic
ratio
O/C
Atomic
ratio
N/C
Atomic
ratio
Cl/C
Sample
SBET
Smicro/SBET
Vmicro/Vtot
(m2 g-1
)
Activated carbon (K)
K/HNO3/8 h/353 K
K/HNO3/16 h/353 K
K/H2SO4/8 h/523 K
K/H2SO4/16 h/523 K
K/PAA/6 h/313 K
K/PAA/6 h/333 K
K/AIR/4 h/673 K
3.3
1.1
1.0
1.5
0.3
1.5
3.3
2.4
5.1
1.9
5.9
3.5
2.7
0.014
0.367
0.461
0.042
0.108
0.069
0.072
0.056
0.065
0.047
0.005
0.000
0.000
0.001
0.012
0.013
0.002
0.003
0.004
0.003
0.006
0.003
0.004
0.002
0.009
0.007
-
2802
415
0.954
0.978
-
0.894
0.905
-
-
-
3
-
2369
1952
2746
2457
2826
2639
2374
2498
2986
2967
0.954
0.968
0.961
0.959
0.958
0.960
0.966
0.968
0.947
0.916
0.879
0.900
0.901
0.901
0.897
0.899
0.913
0.923
0.878
0.816
-
-
-
-
K/AIR/8 h/673 K
-
K/Cl2/4 h/723 K
0.032
K/Cl2/4 h/923 K
0.028
K/NH3/2 h/1073 K
K/NH3/2 h/1223 K
-
-
Figure 1. Catalytic activity of unmodified activated carbon in the
Figure 2. Conversion of ethanol for samples modified with nitric
decomposition of ethanol.
acid.
from 8 to 16 hours for carbon oxidation with nitric acid and related to small differences in the content of oxygen
increased the content of oxygen (Table 1), but decreased functional groups. Slightly greater differences in the
the catalytic activity both towards ethylene and acetic catalytic activity were noted for the samples oxidised
aldehyde (Fig. 2). This decrease was a consequence of with sulphuric acid or air.
a drastic decrease in the surface area caused by rapid
When the reaction was performed over chlorinated
oxidation. The other oxidised samples (Figs. 3-5) were carbon catalysts (Fig. 6) the domination of the process
considerably less active and showed higher selectivity of dehydrogenation over dehydration was greater than
towards dehydrogenation, which could be explained by for oxidised carbon samples (Figs. 3-5). Pérez-Cadenas
a lower content of acidic oxygen groups. The influence et al. [38] claimed that chlorination leads to increased
of modification temperature on the catalytic activity of acidity of the Lewis acid centres on the activated carbon
the sample treated with peroxyacetic acid was small but, on the other hand, to reduced acidity of the Brönsted
928

J. Jasińska, B.Krzyżyńska, M.Kozłowski
Figure 5. Conversion of ethanol for samples modified by contact
Figure 3. Conversion of ethanol for samples modified with
with air.
peroxyacetic acid.
Figure 6. Conversion of ethanol for samples modified by contact
Figure 4. Conversion of ethanol for samples modified with sulfuric
with chlorine.
acid.
centres in the surface oxygen complexes, which well from our results, methanol decomposition takes place by
explains the relations noted. A clear domination of its dehydration, and the only product is dimethyl ether.
dehydrogenation over dehydration can be also noted for No other possible products of methanol decomposition
the samples of activated carbon modified with ammonia (such as formaldehyde) were detected. Only a few
(Fig. 7). Moreover, these samples showed the lowest activatedcarbonsamplesprovedtobecatalyticallyactive
catalytic activity from among all samples studied, (K/H2SO4/16 h/523 K, K/HNO3/16 h/353 K, K/AIR/4
which is most probably a consequence of almost total h/673 K, K/AIR/8 h/673 K), while the others were almost
absence of the oxygen groups in the structure of these totally inert. Thus, the results indicated that only the
carbons (Table 1). It is worth noting that for the majority oxidised carbon samples were active as catalysts in
of the samples studied (exceptions are those modified methanol decomposition, which is in agreement with the
with nitric acid), its catalytic activity increases with results reported by Zawadzki et al. [22]. From among the
increasing temperature of the process, similarly as for abovementionedcatalyticallyactivesamples,thehighest
the decomposition of isopropanol [28].
activity towards dimethyl ether formation was observed
for the sample oxidised with concentrated nitric acid for
16 h (Fig. 8). The activated carbon samples oxidised
with air or sulphuric acid were characterised by a much
lower degree of methanol conversion to dimethyl ether.
The catalytic activity of the oxidised carbon samples
was also found to increase with increasing temperature.
A comparison of the catalytic data (Figs. 1-8) with
3.3. Catalytic activity
of carbons
in
decomposition of methanol
Decomposition of methanol over the activated carbon
catalysts was much less effective than that of ethanol
or earlier studied isopropanol [28], which suggests that
this reaction needs stronger catalytic centres. As follows
929

Influence of activated carbon modifications on their catalytic
activity in methanol and ethanol conversion reactions
4. Conclusions
Results of the above described catalytic tests have
proved that all activated carbon samples, modified with
different chemical agents, catalyse the decomposition
of alcohols, however, to
a different degree. No
correlation between the specific surface area of the
activated carbons and their catalytic activity in alcohol
decomposition confirmed our earlier observations. The
majority of tested catalysts showed activity towards
acetic aldehyde, except for the activated carbon oxidised
with concentrated nitric acid which showed the highest
content of oxygen in the carbon structure. This sample
proved the most effective in the ethanol decomposition
towards ethylene (dehydration process). On the other
hand, chlorinated activated carbon was highly effective
in the ethanol decomposition towards acetic aldehyde.
Only a few of the activated carbon samples were
found catalytically active in the methanol decomposition;
these were the activated carbon samples oxidised
with nitric acid, sulphuric acid and air. The conversion
of methanol to dimethyl ether was only observed
over these catalysts (dehydration process), while no
even trace amounts of formaldehyde – a product of
dehydrogenation – were detected.
Figure 7. Conversion of ethanol for samples modified by contact
with ammonia.
As follows from the results obtained for the
decomposition of ethanol, methanol and isopropanol
[28] over the activated carbon catalysts, in the
same conditions of the reactions the processes of
decomposition of different alcohols take place with
different yields. The highest degree of conversion was
obtained for isopropanol, then ethanol and next for
methanol.
Figure 8. Conversion of methanol.
textural properties of individual samples (Table 1) leads
to a conclusion that a specific surface area does not
directly determine the catalytic activity of the activated
carbons.
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