Nitrogen-Doped, Metal-Free Activated Carbon Catalysts for Aerobic Oxidation of Alcohols

Article abstract of DOI:10.1021/acscatal.5b00375

Various nitrogen-doped carbon materials were prepared via treatments of an activated carbon (AC) with ammonia and hydrogen peroxide, and their catalytic performance was tested for aerobic oxidation of several alcohols in ethanol. The amount and nature of doped nitrogen-species were examined by X-ray photoelectron spectroscopy to discuss the genesis of active species by nitrogen doping. The nitrogen-doped AC catalysts are active for the oxidation of such alcohols as benzyl alcohol, cinnamyl alcohol, and 5-(hydroxymethyl)-2-furaldehyde, and in some cases, they are even more selective to the oxidation of the hydroxyl group compared with conventional Pt/C and Ru/C catalysts, for which coupling products with the ethanol solvent are formed at low conversion levels. Graphite-type doped nitrogen species are significant for the formation of active sites on the surface of AC. The present results demonstrate the potential of nitrogen-doped AC materials as metal-free, carbon-based catalysts useable for organic synthetic reactions. (Chemical Equation Presented).

Full text of DOI:10.1021/acscatal.5b00375

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Article
Nitrogen-doped Metal-free Activated Carbon
Catalysts for Aerobic Oxidation of Alcohols
Hiroyuki Watanabe, Sayaka Asano, Shin-ichiro Fujita, Hiroshi Yoshida, and Masahiko Arai
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b00375 • Publication Date (Web): 23 Mar 2015
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Nitrogen-doped Metal-free Activated Carbon Catalysts for Aerobic
Oxidation of Alcohols
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Hiroyuki Watanabe, Sayaka Asano, Shinꢀichiro Fujita, Hiroshi Yoshida, Masahiko Arai*
Division of Chemical Process Engineering, Faculty of Engineering, Hokkaido University,
Sapporo 060ꢀ8628, Japan
* Eꢀmail marai@eng.hokudai.ac.jp
ABSTRACT: Various nitrogenꢀdoped carbon materials were prepared via treatments of an
activated carbon (AC) with ammonia and hydrogen peroxide and their catalytic
performance was tested for aerobic oxidation of several alcohols in ethanol. The amount
and nature of doped nitrogenꢀspecies were examined by Xꢀray photoelectron spectroscopy
to discuss the genesis of active species by nitrogen doping. The nitrogenꢀdoped AC
catalysts are active for the oxidation of such alcohols as benzyl alcohol, cinnamyl alcohol,
and 5ꢀ(hydroxymethyl)ꢀ2ꢀfuraldehyde and, in some cases, they are even more selective to
the oxidation of hydroxyl group compared to conventional Pt/C and Ru/C catalysts, for
which coupling products with the ethanol solvent are formed at low conversion levels.
Graphiteꢀtype doped nitrogen species are significant for the formation of active sites on the
surface of AC. The present results demonstrate the potential of nitrogenꢀdoped AC
materials as metalꢀfree carbonꢀbased catalysts useable for organic synthetic reactions.
KEYWORDS: activated carbon, nitrogen doping, metal-free carbon catalyst, oxidation,
alcohol
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1. INTRODUCTION
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Nitrogenꢀdoped carbon and carbon nitride are interesting functional materials that may be
useful as electrochemicalꢀ and chemicalꢀcatalysts and supports for transition metal
catalysts.1ꢀ4 Those carbonꢀbased materials can be prepared in different methods, for
example, chemical vapor deposition, wellꢀdesigned organic synthesis, and doping of
nitrogen to parent bulk carbon materials. In addition to their effectiveness in
electrochemical applications, those carbonꢀbased materials will serve as metalꢀfree catalysts
in synthetic organic reactions. For example, Thomas et al. indicate that their wellꢀdesigned
synthetic carbon nitride materials can catalyze FriedelꢀCraft and cyclization reactions.1, 5, 6
Jin et al.7 and Xu et al.8 show porous carbon nitride materials, prepared using porous silica
materials as templates, to be active for Knonevenagel and transesterification reactions,
which are usually catalyzed by base catalysts. Bitter et al. 9 and Wang et al.10 used chemical
vapor deposition using nitrogenꢀcontaining compounds (nitriles, pyridine, amines) to
prepare nitrogenꢀdoped carbon nanotubes. These were observed to be active catalysts for
Knoevenagel condensation. Bitter et al. suggest the importance of pyridine type of doped
nitrogen species for the genesis of catalytic activity. 9 A theoretical study indicate that
carbon atoms at edge sites of graphite sheets may interact with oxygen molecules with the
aid of adjacent doped nitrogen atoms and the nitrogenꢀdoped carbon can then work similar
to Pt metal,11 resulting in a good performance as a Ptꢀfree electrode in proton exchange
membrane fuel cells.12, 13
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The present authors doped nitrogen to an activated carbon (AC) by treating in a
mixture of ammonia and air at temperatures of 400 – 800 ºC. The nitrogenꢀdoped AC
materials are active catalysts for liquidꢀphase reactions including Knoevenagel
condensation,14 transesterification,14 chemoselective reduction of nitrobenzene, styrene, and
3ꢀnitrostyrene with hydrazine,15, 16 and aerobic oxidation of xanthene,17 which are catalyzed
by bases, metals, and metal oxides.18 Fujita et al. suggest that, for the oxidation of xanthene,
pyridineꢀtype N species in a large graphitic sheet is important for the activity of
nitrogenꢀdoped AC catalysts on the basis of Xꢀray photoelectron spectroscopy (XPS)
measurements and reaction runs with melamine, pyridine, and 2ꢀpyridone. 17 Recently, Liao
et al. have reported that the liquid phase aerobic oxidation of cumene to cumene
hydroperoxide can effectively be catalyzed by metalꢀfree carbon nanotube materials
synthesized by chemical vapor deposition using mixtures of xylene and aniline 19 and heat
treatment of commercial carbon nanotubes with nitric acid. 20 They suggest the significance
of interactions between hydroperoxide cumene and the surface of nitrogenꢀdoped carbon
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nanotubes for their high catalytic activity.
Those results indicate the importance of
nitrogen and oxygen species doped on the surface of carbon materials for the catalytic
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performance observed 14ꢀ20 but their chemical structure and role in the catalysis are still not
clear in depth. Their catalytic activity depends on the conditions of nitrogen doping to AC,
in our cases, such as ammonia concentration and treatment temperature, which change the
amount and nature of nitrogen species doped on its surface. 14ꢀ17 Long et al. investigated the
catalysis of nitrogenꢀdoped graphene nanoꢀsheets for aerobic oxidation of benzyl alcohol
and a few other benzylic alcohols. 21 They reported that the graphiteꢀtype nitrogen species
contributed to the formation of active sites and discussed reaction mechanisms. Recently,
nitrogenꢀcontaining grapheneꢀbased catalysts were applied for oxidation reactions using
hydrogen peroxide and tertꢀbutyl hydroperoxide as oxidants.22ꢀ24
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Thus, one can say that the nitrogenꢀdoped metalꢀfree carbon materials would be
promising catalysts for organic synthetic reactions, which could replace conventional base,
metal, and metal oxide catalysts. After considering the previous results obtained so far, it is
still desirable to test their catalytic activity for other synthetic reactions and to make an
effort at clarifying the active sites in more detail. Using a porous bulk material of AC, the
present work has been undertaken to investigate the potential of nitrogenꢀdoped AC
materials for aerobic oxidation of several alcohols, one of industrially important reactions,
and the nature of doped nitrogen species involved in active sites through surface
characterization by XPS. The catalyst recyclability has also been examined because it is
one of merits of using heterogeneous catalysts in organic synthetic reactions. Furthermore,
conventional carbonꢀsupported Pt and Ru catalysts have also been used for the oxidation
reactions and the features of the metalꢀfree nitrogenꢀdoped AC catalysts have been
compared to those of the supported noble metal catalysts.
2. EXPERIMENTAL SECTION
2.1. Preparation of Nitrogenꢀdoped Carbon
A commercial AC powder (GL Science Co.) was used as a starting carbon source and
treated in the following procedures. 14, 15 About 500 mg of AC sample was set in a quartz
tube reactor and an NH3 stream of pure NH3 or a mixture of NH3 (90 %) in air was passed
over the sample at 100 cm3 minꢀ1 for 30 min. The reactor was heated at 10 ºC minꢀ1 up to a
certain temperature lower by 100 ºC than the desired temperature and then at 5 ºC minꢀ1 up
to the desired temperature, at which the sample was treated in the NH3 stream for 1 h. The
AC sample was cooled in the same NH3 stream to 300 ºC and, after switching the NH3
stream to pure N2, the sample was further cooled to room temperature. The AC powder was
also treated by 30% hydrogen peroxide: 300 mg AC powder was dispersed in 15 cm3 of
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30% hydrogen peroxide and the mixture was treated in a Teflon tubular reactor in a
stainless steel autoclave at 100 or 130 ºC for 5 h while stirring by a magnetic stirrer. After
the treatment, the AC powder was cooled to room temperature, separated by filtration,
washed with anhydrate ethanol (Wako), and dried under ambient conditions and then at 100
ºC overnight. In some cases, AC powder was treated by the hydrogen peroxide followed by
the NH3 treatment.
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For comparison, commercially available carbonꢀsupported noble metal catalysts, 5
wt.ꢀ% Pt/C and 5 wt.ꢀ% Ru/C (Wako) were also used. The degree of metal dispersion
measured by CO chemisorption (BELꢀMETALꢀ1) was 0.25 for the former and 0.56 for the
latter. The CO pulse chemisorption was measured after pretreatment with H2 at 100 ºC for
30 min and then with He at 110 ºC for 30 min. A stoichiometry of metal/CO = 1/1 was
assumed.
2.2 Characterization of Carbonꢀbased Catalysts
The textural properties of AC materials prepared were measured by N2 adsorption/
desorption on Quantachrome NOVA 100. 14,15 The specific surface area was determined by
the BrunauerꢀEmmetꢀTeller equation. The surface of nitrogenꢀdoped AC samples was
examined by XPS (JOEL JPSꢀ9200) using monochromatic Al Kα radiation.14,15 A pelletized
AC sample on an iridium foil was subjected to the XPS measurement without any
pretreatment and the chargeꢀup shift corrections of binding energies were made by
reference to C1s of 284.0 eV.
2.3. Aerobic Oxidation of Alcohols
The catalytic performance of those nitrogenꢀdoped AC samples was tested for liquid phase
aerobic oxidation of alcohols according to similar procedures as described previously.16 The
reaction runs were conducted in a Teflon tube in a stainless steel reactor (100 cm3), which
was loaded with 1 mmol substrate, 100 mg ACꢀbased catalyst, and 5 cm3 ethanol (solvent)
and closed. Air in the reactor, its pressure being 0.1 MPa at room temperature, was used as
an oxidant. The other reaction conditions used (temperature and time) will be indicated later.
The reaction mixture was analyzed by a gas chromatograph (GL Science GCꢀ390B) with a
high resolution capillary column (30 m, 0.25 mm OD, 0.25 ꢀm ID) packed with TCꢀWAX
and a flame ionization detector. The substrates used were benzyl alcohol, cinnamyl alcohol,
3ꢀphenylꢀ1ꢀpropanol, 1ꢀheptanol, cyclohexylmethanol, and 5ꢀ(hydroxymethyl)ꢀ2ꢀ
furaldehyde, the former four compounds being purchased from Wako and the latter two
from Aldrich. These substrates were used as received.
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3. RESULTS AND DISCUSSION
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Various nitrogenꢀdoped AC catalyst samples were prepared under different conditions given
in Table 1, which includes the results of their BET surface area measurements. The
treatment with 90% NH3 and 10% air burned off AC to some extent. The weight loss was
found to be 27% by the treatment at 800 ºC and smaller at lower temperatures. Note that the
surface area is not a significant factor determining the catalytic performance.14,17 These
samples were applied for aerobic oxidation of benzyl alcohol, cinnamyl alcohol, and others.
In addition, the results with the nitrogenꢀdoped AC catalysts were compared to those with
conventional carbonꢀsupported noble metal catalysts of 5 wt.ꢀ% Ru/C and 5 wt.ꢀ% Pt/C, for
which the degree of metal dispersion was 0.56 and 0.25, respectively, measured by CO
chemisorption.
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Table 1
3.1. Oxidation of Benzyl Alcohol
The results of benzyl alcohol oxidation (Scheme 1) with various nitrogenꢀdoped AC
catalysts are also given in Table 1. Under the conditions used, benzaldehyde was observed
to form without any other byproducts and no reaction occurred in the absence of catalyst
(blank test). The parent and H2O2 – treated AC samples were less active (#1 – #3). A sample
treated with NH3 at 800 ºC was a little active (#4). When AC was treated by H2O2 followed
by NH3, the oxidation activity was enhanced significantly (#6 – #8) and a maximum
conversion of 23% was obtained with the AC catalyst prepared through the H2O2 treatment
at 130 ºC and the subsequent one with NH3 at 800 ºC (#8). The other AC catalysts prepared
with 90% NH3 were less active (#9, #10) and those prepared through the treatments with
H2O2 and 90% NH3 were moderately active (#11ꢀ #13). Thus, the catalyst preparation
conditions were crucial for determining the oxidation activity of ACꢀbased catalysts. The
large difference in the catalytic performance observed cannot be explained by that in the
BET surface area (Table 1).
Scheme 1
The surface of those AC samples was examined by XPS and three different
nitrogen species were detected to exist, which were pyridineꢀ, pyrroleꢀ, and graphiteꢀtype
nitrogen species (Figure 1). 21, 25ꢀ28 The XPS results measured with the samples #4 ꢀ #13 are
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presented in Supporting Information (Figure S1). Figure 2 shows the surface concentration
of these three different nitrogen species doped on the surface of various ACꢀbased catalysts
(#4 ꢀ #13 of Table 1). No XPS analysis was made for the AC samples #1 ꢀ #3 that were not
treated by NH3. The conversion of benzyl alcohol measured is plotted against the amounts
of those different nitrogen species in Figure 3. The amount of a certain nitrogen species in a
carbon sample was estimated by the product of the surface concentration (mole fraction) of
the nitrogen species measured by XPS and the BET surface area of the sample (m2 gꢀ1). The
conversion cannot be correlated with the amount of either pyridineꢀtype or pyrroleꢀtype
nitrogen species but there is a good relationship with the amount of graphiteꢀtype nitrogen
species, which should contribute to the genesis of active sites on the surface of AC for the
oxidation of benzyl alcohol. This is in accordance with the result of Long et al. using
nitrogenꢀdoped graphene nanoꢀsheet catalysts for the same oxidation reaction. 21 There were
oxygen species on the surface of those carbon materials but the conversion of benzyl
alcohol was not related to the amount of oxygen species measured by XPS (Figure S2 in SI).
XRD was used to examine their structural features but no significant difference was
observed among those carbon samples (Figure S3 in SI). It was previously observed for
baseꢀcatalyzed Knoevenagel and transesterification reactions that the catalytic activity was
correlated with the ratio of surface nitrogen species against surface oxygen ones for the
nitrogenꢀdoped AC catalysts.14 This is different from the correlation of Figure 3(c), due to
the difference in the nature of the reactions studied.
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Figure 1, Figure 2, Figure 3
There is a scatter at small amounts of graphiteꢀtype nitrogen species (< 0.15 %) for
the relationship of Figure 3(c). This suggests the presence of other active sites on the
surface AC samples. Hayashi and coworkers examined various aerobic oxidation reactions
over untreated AC catalysts.29, 30 They reported that surface oxygenꢀcontaining groups,
which were evolved as CO at high temperatures, were active sites for oxidative
cyclization.30 In addition, the present authors recently indicated that pyridineꢀtype nitrogen
species were active for the aerobic oxidation of xanthene.17 Those other surface species
should catalyze the aerobic oxidation of alcohols but less active compared to the
graphiteꢀtype nitrogen species. This would cause a scatter in the range of small amount of
graphiteꢀnitrogen species as observed in Figure 3(c).
3.2. Oxidation of Cinnamyl Alcohol
A few selected ACꢀbased catalysts (#1, #4, #7, and #8 in Table 1) were applied for the
oxidation of a different substrate of cinnamyl alcohol at 80 ºC (Scheme 2). Cinnamaldehyde
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was observed to form in 100% selectivity under the conditions used. These AC catalysts
were active and the conversion was also correlated with the amount of graphiteꢀtype
nitrogen species, as seen in Figure 4. This suggests again that these nitrogen species are
involved in the formation of active sites.
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Scheme 2, Figure 4
3.3. Oxidation of Other Alcohols
The catalytic performance of a selected nitrogenꢀdoped AC catalyst #8, most active among
those listed in Table 1, was examined in the aerobic oxidation of other alcohols. Table 2
summarizes the results obtained with several alcohols including benzyl and cinnamyl
alcohols. As already described, the nitrogenꢀdoped AC catalyst can oxidize benzyl alcohol
and cinnamyl alcohol to corresponding aldehydes with 100% selectivity (entries 1, 2). For
3ꢀphenylꢀ1ꢀpropanol, cyclohexylmethanol, and 1ꢀheptanol, the nitrogenꢀdoped AC was
inactive (entries 3, 4, 5). For the oxidation of 5ꢀ(hydroxymethyl)ꢀ2ꢀfuraldehyde, a
conversion of 24% was achieved with the nitrogenꢀdoped AC catalyst in 15 h with a
selectivity of 93% (entry 6). The byproduct was the acetal from the substrate and the
solvent
of
ethanol.
For
benzyl
alcohol,
cinnamyl
alcohol,
and
5ꢀ(hydroxymethyl)ꢀ2ꢀfuraldehyde (entries 1, 2, 6), the carbon bonding to hydroxyl group
may be activated by its adjacent conjugated system and so they can be easily oxidized. In
contrast, the other alcohols (entries 3, 4, 5) do not have such a conjugated system so that
they cannot be activated and oxidized.
Table 2
3.4. Catalyst Recyclability
The catalyst reusability was examined using a nitrogenꢀdoped AC catalyst #8 and a model
substrate of benzyl alcohol. The sample was separated by filtration after the first oxidation
run and used for the second one under the same conditions. Figure 5 gives the results of
repeated oxidation runs, indicating a decrease in the conversion obtained, in particular after
the first run. The surface of fresh and recycled (three times) catalyst samples was examined
by XPS. Figure 6 displays the XPS N 1s spectra and Table 3 gives the surface concentration
of nitrogen species as measured by XPS. The results show a significant decrease in the
amount of graphiteꢀtype species on the recycling, which should be involved in the active
sites as aboveꢀmentioned (Figures 3 and 4). These nitrogen species was found to change to
oxideꢀtype ones, as indicated from the XPS results (Figure 6). The amounts of oxygen
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species measured by XPS were 5.3% and 7.0% for the fresh and recycled samples. As
mentioned later, the presence of gaseous oxygen was needed to improve the rate of benzyl
alcohol oxidation. It was assumed, therefore, that the surface oxygen species were
insignificant for the catalyst deactivation observed (Figure 5). The catalyst deactivation due
to the disappearance of graphiteꢀtype nitrogen species should occur during the oxidation
reaction. Attempts were made to regenerate the nitrogenꢀdoped AC catalyst #8 after the first
reaction run, which was treated in ethanol at 1 MPa H2 and 120 ºC for 8 h or by hydrazine
(8 cm3) at 120 ºC for 2 h and then used for the second run. The conversion values observed
were 13% and 7.5% after the former treatment and the latter one, respectively, which were
50% and 30% of the initial performance of the fresh catalyst. These two treatments failed to
activate the catalyst that lost its activity after the first reaction run.
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Figure 5, Figure 6, Table 3
The occurrence and extent of catalyst deactivation of nitrogenꢀdoped AC catalysts
depends on the type of reaction and the type of active sites involved. The catalysts lose their
activity for oxidation reactions of xanthene 17 and alcohols (this work) and Knoevenagel
condensation 14 in which water is produced and could have an effect in the deactivation. In
14
contrast, no catalyst deactivation occurs for transesterification reaction,
which is a
baseꢀcatalyzed reaction similar to Knoevenagel condensation. The catalyst recyclability
was not mentioned by Long et al. for their nitrogenꢀdoped graphene nanoꢀsheet catalysts. 21
The group of Peng et al. prepared nitrogenꢀdoped carbon nanotubes by chemical vapor
deposition using aniline and xylene as nitrogen and carbon sources.31, 32 They showed that
these carbon materials were metalꢀfree active catalysts for liquidꢀphase oxidation of an
aromatic alkene of cyclohexene using molecular oxygen and, in addition, these catalysts
were recyclable for five runs, in contrast to the present case (Figure 5). Nitrogen species
should exist in the structure of graphene sheets for Peng’s carbon nanotubes while they are
likely to be doped to such structure irregular sites as edge ones for our ACꢀderived
materials. This difference in the structural features might explain the absence and presence
of catalyst deactivation in the oxidation reactions.
3.5. Oxidation Processes over Nitrogenꢀdoped AC Catalysts
For nitrogenꢀdoped graphene nanoꢀsheet catalysts, Long et al. pointed out the significance
of graphiteꢀtype nitrogen species for their catalysis in the oxidation of alcohols. 21 These
authors propose a Langmuir – Hinshelwood process, in which a sp2 N – O2 adduct is
formed on the graphiteꢀtype nitrogen active sites and this is a key step for the oxidation. A
model calculation of Ikeda et al. indicates that an oxygen molecule may be activated on a
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carbon atom bonded to a doped nitrogen atom. 11 The present results also indicate that the
oxidation of alcohols proceeds over the surface of nitrogenꢀdoped AC catalysts and the
graphiteꢀtype nitrogen species contribute to the formation of active sites. Additional
reaction runs were conducted to consider reaction mechanisms. A reaction with benzyl
alcohol was conducted with the nitrogenꢀdoped AC catalyst #8 in the presence of a radical
scavenger of pꢀbenzoquinone (0.1 mmol) in the reaction mixture under the standard
conditions (benzyl alcohol 1.0 mmol, for others see caption of Table 1). The conversion
was observed to be only 3%, which was even smaller than 23% obtained in the absence of
pꢀbenzoquinone. This suggests that radical species may be included in the oxidation
processes. Another additional run was conducted in an atmosphere of pure N2 instead of air.
The oxidation was found to occur but with an even smaller conversion of 9% as compared
to 23%, suggesting that a source of oxidant is the gaseous O2 in the air but not the doped O
species. After considering those additional results and the aboveꢀmentioned previous
works11, 21 possible reaction pathways may be envisaged as follows (Scheme 3). An oxygen
molecule dissolved in the liquid phase is adsorbed on a carbon site adjacent to the
graphiteꢀtype nitrogen and/or on the carbon atom and the nitrogen atom. Then, some
oxygen radical is formed and reacts with an alcohol molecule, which might be activated by
interactions between graphiteꢀtype nitrogen and C1 atom of benzyl alcohol, producing
benzaldehyde and regenerating the active sites. In some cases, undesired process should
occur, in which the graphiteꢀtype nitrogen is changed to the oxidizedꢀtype nitrogen during
the oxidation reaction, causing the unavoidable catalyst deactivation. The formation of a
byproduct of water might play a role in the catalyst deactivation, similar to the case of
Knoevenagel condensation. 14
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The group of Peng et al. report that the nitrogenꢀdoped carbon nanotube materials
are active for the liquidꢀphase oxidation of cyclohexene, in which the nitrogen doping
enhances the rate of oxidation and the selectivity to 2ꢀcyclohexeneꢀ1ꢀone. 31, 32 It is noted
that these catalytic features are explained by their capability of promoting the radical chain
propagation via stabilizing peroxyl and cycloxyl radicals and, as a result,
2ꢀcyclohexeneꢀ1ꢀol is further converted to 2ꢀcyclohexeneꢀ1ꢀone as well. As
aboveꢀmentioned, the wellꢀorganized carbon nanostructure (graphene wall) of their
catalysts could be different in the catalytic performance with our commercial ACꢀbased
nitrogenꢀdoped carbon catalysts.
Scheme 3
In a previous work, 17 the aerobic oxidation of xanthene was investigated with
nitrogenꢀdoped AC catalysts, which were prepared in similar manners but without the
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pretreatment with hydrogen peroxide and so they contained a trace amount of graphiteꢀtype
nitrogen species on their surface. It was assumed that the presence of pyridineꢀtype nitrogen
species gave Lewis base nature to the neighboring carbon atoms and these carbon sites
were more effective for the activation of methylene group of the substrate. As a result, the
catalytic activity tended to increase with an increase in the amount of surface pyridineꢀtype
nitrogen species. Although the graphiteꢀtype species were little doped, the activation of
oxygen should occur on those ACꢀbased catalysts but less effectively as compared with the
present ACꢀbased catalysts containing the graphiteꢀtype nitrogen species. This is a reason
for that a higher reaction temperature of 100 ºC is required for the oxidation of xanthene 17
and the activation oxygen should be less significant than that of xanthene under the reaction
conditions used.
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3.6. Comparison with Conventional Noble Metal Catalysts
The catalytic performance of a selected nitrogenꢀdoped AC catalyst #8, most active among
those listed in Table 1, was examined in the aerobic oxidation of other alcohols and
compared with that of conventional carbonꢀsupported noble metal catalysts, 5 wt.ꢀ% Ru/C
and 5 wt.ꢀ% Pt/C. Table 4 summarizes the results obtained with several alcohols including
benzyl and cinnamyl alcohols. Benzyl alcohol was oxidized to benzaldehyde in 23%
conversion in 5 h with 100 mg nitrogenꢀdoped AC catalyst (entry 1) while a similar
conversion was achieved with a smaller amount (10 mg) of Ru/C in a shorter time (3 h)
(entry 5). It is interesting to note that benzaldehyde was formed in 100% selectivity with
the former but in a lower selectivity of 60% for the latter at similar conversion levels, with
which an acetal was produced in a large quantity from the oxidized product (benzaldehyde)
and the solvent (ethanol). The Pt/C was less active for the oxidation of benzyl alcohol
(entries 2, 4). The nitrogenꢀdoped AC catalyst was inactive for the oxidation of
cyclohexylmethanol (entry 6); the supported noble metal catalysts were active but the
desired product of cyclohexyl aldehyde was not detected (entries 7, 8). For the oxidation of
an aliphatic alcohol of 1ꢀheptanol, the nitrogenꢀdoped AC was inactive (entry 9). A small
conversion of 9% was obtained with 100% selectivity with Ru/C under the conditions used
(entry 11). For cinnamyl alcohol, the metaꢀfree AC catalyst (100 mg) gave a conversion of
21% and a selectivity of 100% in 15 h (entry 12). A similar conversion of 17% was
obtained in a shorter time of 7 h with a smaller amount of Ru/C (entry 14). Similar to the
case of benzyl alcohol, the selectivity to the desired product (cinnamaldehyde) was even
low (38%), which gave an acetal from cinnamaldehyde and ethanol. Again Pt/C was less
active (entry 13). For the oxidation of 5ꢀ(hydroxymethyl)ꢀ2ꢀfuraldehyde, a conversion of
24% was achieved with the nitrogenꢀdoped AC catalyst (100 mg) in 15 h with a selectivity
of 93% (entry 16). Both Pt/C and Ru/C were active but their selectivity values to the
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desired oxidation product were low (entries 17ꢀ19), which gave an acetal from the substrate
and the solvent, in contrast to the cases of benzyl and cinnamyl alcohols.
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The comparison was further made among the nitrogenꢀdoped AC and
carbonꢀsupported noble metal catalysts in the oxidation of benzyl alcohol. Table 4 shows
that the performance of Pt/C (entry 3) was not good as compared to Ru/C (entry 2) under
the same reaction conditions, as aboveꢀmentioned. The ACꢀbased and Ru/C catalysts gave
very similar conversion levels under the conditions used and so these two can be compared
(entries 1, 2). For the Ru/C catalyst, TOF is 29 hꢀ1 and the selectivity to the desired
oxidation product of benzaldehyde is 60%, in which the acetal is formed from
benzaldehyde and the solvent (ethanol) in a large amount. For the nitrogenꢀdoped AC
catalyst, the graphiteꢀtype nitrogen species can be assumed to contribute to the formation of
active sites and so TOF was determined using the number of these species determined by
XPS (see footnote b of Table 4). The metalꢀfree AC catalyst produces benzaldehyde in
100% selectivity but has a TOF of 1.6 hꢀ1, which is even smaller by more than one order of
magnitude compared to 29 hꢀ1 for the Ru/C catalyst. In addition, apparent activation energy,
Ea, was measured at temperatures of 100 – 130 ºC for the nitrogenꢀdoped AC and 90 – 110
ºC for Ru/C using the data collected at conversion < 15%. Arrheniusꢀtype plots of reaction
rate against 1/temperature are presented in Figure 7. The Ea value was determined to be 22
kJ molꢀ1 and 32 kJ molꢀ1 for the metalꢀfree AC and Ru/C catalysts, respectively. The Ea
value is smaller by 10 kJ molꢀ1for the former than the latter; however, TOF is even smaller.
These results suggest that the reaction mechanisms are different between the two catalysts
or the rateꢀdetermining step is the same but the required activation energy is different
depending on the nature of active sites. A detailed kinetics analysis was made by Long et al.
for the oxidation of benzyl alcohol over their nitrogenꢀdoped graphene nanoꢀsheet catalysts.
21 Their oxidation reactions were conducted in the presence of water in the reaction mixture
at the start of reaction. An activation energy of 56 kJ molꢀ1 was reported for the oxidation of
benzyl alcohol over the nitrogenꢀdoped graphene nanoꢀsheet catalysts, which was close to
51 kJ molꢀ1 for a Ru/Al2O3 catalyzed oxidation using PhCF3 solvent reported in the
literature. 33 The apparent activation energy determined for the nitrogenꢀdoped AC catalyst
in the present work is different from those previous values 21, 29 and from that of a reference
of Ru/C catalyst (Table 5). Those results indicate again that the catalytic actions of
nitrogenꢀdoped carbon materials are different depending on the carbon structure,
wellꢀstructured graphene and nanotubes or lessꢀstructured carbons like AC used in the
present work, which determines the location and environment of nitrogen dopants. More
detailed reaction kinetics analysis is in progress to shed light on the catalysis of the
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metalꢀfree nitrogenꢀdoped bulk AC catalyst of our interest.
Table 5, Figure 7
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4. CONCLUSIONS
The nitrogenꢀdoped metalꢀfree AC catalyst is active for the aerobic oxidation of alcohols
such as benzyl alcohol, cinnamyl alcohol, and 5ꢀ(hydroxymethyl)ꢀ2ꢀfuraldehyde to the
desired aldehydes but inactive for the oxidation of 3ꢀphenylꢀ1ꢀpropanol, 1ꢀheptanol and
cyclohexylmethanol under the conditions used. In other words, the alcohols in which the
carbon bonding to hydroxyl group may be activated by its adjacent conjugated system can
be oxidized by the nitrogenꢀdoped AC catalysts. On nitrogenꢀdoping, different types of
nitrogen species are formed on the surface of AC, which are pyridineꢀ, pyrroleꢀ, and
graphiteꢀtype. The graphiteꢀtype nitrogen species are involved in the genesis of catalytically
active sites, which should adsorb and activate an oxygen molecule. The nitrogenꢀdoped
metalꢀfree AC catalyst has a smaller TOF value by more than one order of magnitude
compared to those for carbonꢀsupported Ru and Pt catalysts. But, the apparent activation
energy is smaller for the former than the latter, suggesting different reaction mechanisms.
The graphiteꢀtype nitrogen species are likely to change to the less active oxidized ones,
causing the catalyst deactivation during the oxidation reaction, which could not be
avoidable owing to the nature of reaction.
ASSOCIATED CONTENT
Supporting Information
The following file is available free of charge on the ACS Publications website at DOI:
○○○○○○○○○○.
Catalyst characterization results (XPS, XRD) and relation between conversion in benzyl
alcohol oxidation and amount of surface oxygen species (PDF)
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Table 1. List of nitrogenꢀdoped metalꢀfree AC catalysts prepared and their catalytic
5
6
performance for aerobic oxidation of benzyl alcohol
7
8
9
Conversion a
(%)
Treatment
Catalyst
number
Surface area
(m2 gꢀ1)
First
Second
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(H2O2 temp.)
(NH3 conc., temp.)
1
2
1031
1188
3.0
7.0
4.0
8.1
15
－
－
100 ºC
130 ºC
－
－
b
3
－
－
4
100%, 800 ºC
100%, 600 ºC
100%, 700 ºC
100%, 800 ºC
100%, 800 ºC
90%, 400 ºC
90%, 600 ºC
90%, 800 ºC
90%, 600 ºC
90%, 800 ºC
1205
1050
707
5
100 ºC
100 ºC
100 ºC
130 ºC
－
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12
7
1276
1198
971
19
8
23
9
2.8
3.2
10
10
11
12
13
1075
1172
803
－
－
100 ºC
100 ºC
7.2
10
1063
First: AC was treated in H2O2 at temperatures given for 5 h.
Second: AC was then treated in a stream of NH3 and/or air at temperatures given for 1 h.
a. Reaction conditions: substrate 1.1 mmol, catalyst 100 mg, solvent (ethanol) 5 cm3,
temperature 120ºC, time 5 h.
b. Not measured.
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Scheme 1. Oxidation of benzyl alcohol to benzaldehyde
(b) Pyrroleꢀ
type
NH
(a) Pyridineꢀ
type
N
N+
(c) Graphite type
Figure 1. A typical N1s XPS spectrum showing the presence of pyridineꢀtype (a),
pyrroleꢀtype (b), and graphiteꢀtype (c) nitrogen species doped on the surface of a
nitrogenꢀdoped AC catalyst #8
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0
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Concentration of nitrogen species (%)
Composition of nitrogen species (%)
Figure 2. Composition of nitrogen species, pyridineꢀtype (a), pyrroleꢀtype (b), and
graphiteꢀtype (c), on the surface of nitrogenꢀdoped AC catalyst samples. No XPS analysis
was made for the samples #1 ꢀ #3 given in Table 1, which were parent AC and AC samples
treated with H2O2 but not with NH3.
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(a)
(c)
(b)
0
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10 15 20 25 30 35
Nb (m2 g−1
0
1
2
3
4
5
6
Na (m2 g−1
)
)
Nc (m2 g−1
)
Figure 3. Plots of total conversion in oxidation of benzyl alcohol to benzaldehyde against
the amount of pyridineꢀtype (Na), pyrroleꢀtype (Nb), and graphiteꢀtype (Nc) nitrogen species
on the surface of nitrogenꢀdoped AC catalysts listed in Table 1.
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Scheme 2. Oxidation of cinnamyl alcohol to cinnamaldehyde
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Figure 4. Plot of total conversion in oxidation of cinnamyl alcohol to cinnamaldehyde at 80
ºC for 15 h against the amount of graphiteꢀtype (Nc) nitrogen species on the surface of a
few selected nitrogenꢀdoped AC catalysts (#1, #4, #7, #8) listed in Table 1
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Table 2. Results of aerobic oxidation of several alcohols with the nitrogenꢀdoped metalꢀfree
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AC #8 catalyst
T a
t a
X a
S a
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Entry
1
Substrate
Product
(ºC)
(h)
(%)
(%)
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Reaction conditions: solvent ethanol 5 cm3, substrate 1.1 mmol, catalyst 100 mg.
a
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Run number
Figure 5. Results of recycling test of a nitrogenꢀdoped AC catalyst #8 in the aerobic
oxidation of benzyl alcohol. Reaction conditions: Initial weight of catalyst 100 mg, benzyl
alcohol 1.1 mmol, ethanol 5 cm3, 120 ºC, 5 h.
Recycled
a
b
d
c
Fresh
b
a
c
396 397 398 399 400 401 402 403
Binding energy (eV)
Figure 6. N1s XPS spectra of fresh and recycled (three times) nitrogenꢀdoped AC based
catalyst #8. (a) pyridineꢀtype, (b) pyrroleꢀtype, (c) graphiteꢀtype, (d) oxideꢀtype nitrogen
species.
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Table 3. Concentration of surface nitrogen species in a nitrogenꢀdoped AC catalyst #8
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9
Surface concentration (atom %)
Catalyst
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58
59
60
Total N
Pyridineꢀtype N
Pyrroleꢀtype N
Graphiteꢀtype N
Total O
5.3
Fresh
3.2
0.87
1.9
0.41
Recycled
1.8
0.57
1.2
0.06
7.0
Reaction conditions: Initial weight of catalyst 100 mg, benzyl alcohol 1.1 mmol, ethanol 5
cm3, 120 ºC, 5 h.
Scheme 3. Possible reaction pathways for aerobic oxidation of benzyl alcohol over
nitrogenꢀdoped AC catalyst: (a) adsorption of oxygen molecule, (b) formation of oxygen
radical, (c) oxidation of alcohol producing aldehyde and regenerating active sites, (d)
change of active sites (graphiteꢀtype nitrogen) into inactive sites (oxideꢀtype nitrogen)
causing catalyst deactivation
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Table 4. Results of aerobic oxidation of alcohols with the nitrogenꢀdoped metalꢀfree AC #8
5
6
7
and conventional carbonꢀsupported noble metal catalysts
Entry
w a
T a
t a
X a
S a
Catalyst
8
9
(mg)
(ºC)
(h)
(%)
(%)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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41
42
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45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(a)
1
2
3
4
5
NꢀCarbon
Pt/C
100
10
5
0.5
3
23
7
100
100
87
120
100
20
9
Ru/C
1
9
100
60
10
3
22
(b)
6
7
NꢀCarbon
Pt/C
100
10
5
5
5
0
20
21
－
0
120
8
Ru/C
10
0
(c)
9
NꢀCarbon
Pt/C
100
10
15
5
0
< 1
9
－
－
80
10
11
Ru/C
10
5
100
(d)
12
13
14
15
NꢀCarbon
Pt/C
100
10
15
15
7
21
7
100
100
38
80
Ru/C
10
17
43
10
15
21
(e)
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8
9
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NꢀCarbon
Pt/C
100
10
15
5
24
29
28
44
93
0
80
Ru/C
10
2
14
12
10
11
12
13
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60
10
5
Reaction conditions: solvent ethanol 5 cm3, substrate 1.1 mmol.
a
w: weight of catalyst used, T: reaction temperature, t: reaction time, X: total conversion,
S: selectivity to the desired oxidation product
Table 5. Comparison of a nitrogenꢀdoped AC catalyst sample #8 with conventional
supported noble metal catalysts for aerobic oxidation of benzyl alcohol at 120 ºC
Weight Time Conversion Selectivity a
TOF b
(hꢀ1)
1.6
Ea
(kJ molꢀ1)
22
c
Entry
Catalyst
(mg)
100
10
(h)
5
(%)
23
(%)
100
60
1
2
3
Nꢀdoped AC #8
5% Ru/C
3
22
29
32
5% Pt/C
10
3
8.6
87
49
－
Reaction conditions: see caption for Figure 1.
a. Selectivity to benzaldehyde
b. TOF to the consumption of benzyl alcohol. The number of exposed active sites was
determined as follows: the number of exposed Ru and Pt atoms was measured by CO pulse
chemisorption assuming a stoichiometry of CO/metal = 1. The degree of metal dispersion
determined was 56% and 25% for the Ru/C and Pt/C samples, respectively. The number of
graphiteꢀtype nitrogen species was estimated from the surface composition (0.41%)
measured by XPS and a BET surface area of 1198 m2 gꢀ1 assuming a density of a carbon
atom per 0.02625 nm2 for graphitic surface.
c. Apparent activation energy determined at 100 ºC – 130 ºC and at 90 ºC – 110 ºC for the
Nꢀdoped AC and 5% Ru/C catalysts, respectively, using the data of conversion < 15%
(Figure 7)
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0.4
0.3
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(a)
(b)
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6
5
0.2
0.0024
0.0026
1/T (K−1
0.0028
0.0026
0.0027
1/T (K−1
0.0028
)
)
Figure 7. Arrhenius type plots of the rate of reaction against 1/reaction temperature for the
oxidation of benzyl alcohol over the nitrogenꢀdoped metalꢀfree AC catalyst #8 (a) and a
carbonꢀsupported Ru one (b)
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Graphical Abstract
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Nitrogen-doped Metal-free Activated Carbon Catalysts for Aerobic
Oxidation of Alcohols
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