Highly efficient carbon catalyzed aerobic selective oxidation of benzylic and allylic alcohols under transition-metal and heteroatom free conditions

Article abstract of DOI:10.1039/c4ra11148f

The aerobic oxidation of benzylic and allylic alcohols has been realized with carbon material as the catalyst in the absence of transition-metal and heteroatoms. Under the optimized reaction conditions, aldehyde and acid can be selectively synthesized with up to 99% yields. The carbon catalyst can be easily recovered from the reaction mixture and can be reused for 5 runs without deactivation.

Full text of DOI:10.1039/c4ra11148f

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Highly efficient carbon catalyzed aerobic selective
oxidation of benzylic and allylic alcohols under
transition-metal and heteroatom free conditions†
Cite this: RSC Adv., 2014, 4, 59754
a
a
a
Huimin Yang,ab Xinjiang Cui, Youquan Deng and Feng Shi
Received 25th September 2014
Accepted 3rd November 2014
*
DOI: 10.1039/c4ra11148f
www.rsc.org/advances
The aerobic oxidation of benzylic and allylic alcohols has been realized higher chemical stability, larger surface area, and lower cost.27,28
with carbon material as the catalyst in the absence of transition-metal Up to now, most reports of metal-free carbon materials are
and heteroatoms. Under the optimized reaction conditions, aldehyde carbon-catalysts including graphene oxide,29–31 nitrogen-doped
and acid can be selectively synthesized with up to 99% yields. The graphene32,33 and carbon nanotubes.19,34 Recently, the applying
carbon catalyst can be easily recovered from the reaction mixture and of graphene33,35–37 in the oxidation of alkane and alcohol has
can be reused for 5 runs without deactivation.
been made good progress.22 Wang and coworkers37 reported
that the multilayer graphene nano-sheets doped by N-
heteroatoms can efficiently catalyze the aerobic selective
oxidation of primary alcohols. The authors suggest that the
oxygen molecules may be activated by the N-graphene to
possibly form a sp2 N–O2 adduct transition state, which can
oxidize alcohols to aldehydes. There is rare report about mes-
oporous and metal-free carbon material (not ordered grapheme
and carbon nanotubes) without heteroatoms modication in
the selective oxidation of alcohols. Herein, we rst demonstrate
new chemical function of transition metal-free carbon materials
which can catalyze the controllable synthesis of aldehydes or
acids from alcohols by altering the amount of base. As far as we
know, it was also the rst time using transition metal-free
carbon materials to achieve chemoselective oxidation of
alcohol to organic acid. This method offers a new alternative
methodology for the development of sustainable catalysts.
In this work, the catalyst was prepared as following steps.
First, RF gels (prepared by sol–gel polymerization of resorcinol
and formaldehyde) were prepared through a hydrothermal
method as the previous report,38 which is a sol–gel polymeri-
zation of resorcinol and formaldehyde with Na2CO3 as a cata-
lyst. Next, RF gels were mixed together with base and were
The aerobic oxidation of alcohols with high selectivity is a
challenge with huge relevance from a fundamental point of
view, and is also of great importance for industry due to the
marketing value of the resulting compounds.1–4 This reaction
has been performed by using heterogeneous catalysts5–10 and
homogeneous metal complexes,11–16 as catalysts with O2 or
peroxide as the oxidant. Usually, the most attractive way for
alcohol oxidization is to use O2 as a terminal oxidant, where
only H2O is generated as a side-product. However, a persistent
challenge of this transformation is the over-oxidation of alde-
hyde.8,17–20 It seems that due to the unavoidable formation of
H2O2 as a byproduct in metal-containing catalyst systems,
which can further react with aldehyde to produce acid and lead
to selectivity loss. In order to address this problem, and also for
the sake of sustainability and the rational use of resources, it
would be highly desirable to develop nonmetallic catalysts for
the oxidation of alcohols to the corresponding aldehydes and
ketone.21,22
Catalysis by metal-free carbon has currently become a hot
topic of much current interests in chemistry, since they have a
number of advantages over the inorganic metal oxide materials
(TiO2, Fe2O3, SiO2, and CeO2 oxide supports8,23–26), such as
ꢀ
treated at 800 C under nitrogen ow. Finally, the carbonized
sample was washed by deionized water, and the carbon material
was obtained. Several carbon materials were prepared with this
method by varying the types and amount of bases. The RF gels
itself was carbonized at 800 ꢀC and was denoted as RF-HT-C-
0 (RF: RF gels prepared by sol–gel polymerization of resor-
cinol and formaldehyde; HT: hydrothermal treatment; C:
carbonization via thermal treatment). The carbon materials
being treated by a mass ratio of 1 : 1 of RF and varied bases, i.e.
KOH, NaOH, K2CO3 and Na2CO3, were denoted as RF-HT-C-1,
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Centre for Green
Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy
of Sciences, No. 18, Tianshui Middle Road, Lanzhou, 730000, China. E-mail: fshi@
licp.cas.cn; Fax: +86-931-82770882
bUniversity of the Chinese Academy of Sciences, No. 19A, Yuquanlu, Beijing, 100049,
China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra11148f
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Table 1 Properties of the typical carbon catalysts
RF-HT-C-2, RF-HT-C-3 and RF-HT-C-4, respectively. RF-HT-C-5
and RF-HT-C-6 were obtained by varying the mass ratio of RF
and KOH to 1 : 0.25 and 1 : 1.25.
Pore volume/
Catalysts
SBET/m2 gꢁ1
cm3 gꢁ1
Pore size/nm
The structures of the carbon materials were characterized by
SEM, BET, XPS and XRD. The morphology and structure of
these as-made carbon catalyst samples were studied by SEM
analysis. As shown in Fig. 1, the samples activated by different
bases exhibit different porous structure aer carbonization.
BET characterization further indicates that the porosity of
carbon material is well related to their surface areas as showed
RF-HT-C-0
RF-HT-C-1
RF-HT-C-2
RF-HT-C-3
RF-HT-C-4
RF-HT-C-5
220.5
1220.9
408.1
841.8
407.7
582.0
0.044
0.15
0.29
0.12
0.08
0.10
4.85
3.44
4.36
3.78
2.66
3.61
in Table 1. The surface area of RF-HT-C-0 was only 220.5 m2 gꢁ1
,
which was un-activated by any base. The surface areas of other
catalysts such as RF-HT-C-1, RF-HT-C-2, RF-HT-C-3, RF-HT-C-4,
and RF-HT-C-5 were 1220.9, 408.1, 841.8, 407.7 and 582.0 m2
gꢁ1, respectively. Moreover, no regular structures were observed
in RF-HT-C-0, RF-HT-C-3 and RF-HT-C-4. The bonding cong-
urations of C and O atoms in these carbon materials were
further studied by XPS. As shown in Table S1 and Fig. S1,† C1s
peaks in the XPS spectra of the RF-HT-C-1 to RF-HT-C-5 samples
can be tted into one peak centered at 284.7 ꢂ 0.1 eV, indicating
that there are similar surface carbon species in these carbon
materials samples. XRD diffraction patterns of the carbon
materials are similar to the amorphous carbon materials,
Fig. S2.† So the treatment of base only altered the porosity of the
carbon materials without changing the crystal structure of
carbon in the nal samples.
oxygen as an oxidant. As summarized in Table 2, the catalytic
performance of the carbon materials was studied under various
conditions. First, different kinds of catalysts were explored and
it was clear that the selectivities of all catalysts were similar with
Table 2 Catalytic performance of the different conditions for benzyl
alcohol aerobic oxidationa
Entry
Catalyst
Base/mmol
Yieldb/%
Selectivityb/%
In a preliminary experiment, benzyl alcohol (1 mmol) was
treated with carbon materials (100 mg) with KOH as cocatalyst
at 100 ꢀC in toluene for 12 h in a sealed vessel using molecular
1
2
3
4
5
6
7
8
RF-HT-C-0/100
RF-HT-C-1/100
RF-HT-C-2/100
RF-HT-C-3/100
RF-HT-C-4/100
RF-HT-C-5/100
RF-HT-C-6/100
RF-HT-C-1/20
RF-HT-C-1/50
RF-HT-C-1/200
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
—
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
RF-HT-C-1/100
AC/100
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
—
38
95
85
76
72
45
96
67
80
96
81
14
54
10
36
60
92
60
21
0
96
95
93
91
95
93
97
90
93
97
92
54
85
96
90
98
96
62
24
0
9
10
11c
12d
13e
14
15
16
17
18f
19f
20f
21g
22h
23i
24j
25k
26l
KOH/0.2
KOH/0.05
KOH/0.1
KOH/0.5
KOH/1
KOH/2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
KOH/0.2
80
96
74
90
93
25
93
97
90
93
91
94
a
1 mmol benzyl alcohol, 100 mg catalyst, 0.2 mmol KOH, 2 mL toluene,
b
1 atm oxygen, 100 ꢀC, 12 h. Determined by GRF-HT-C-FID using
biphenyl as external standard. The main byproduct was the
c
d
corresponding carboxylic acid. Xylene was used as solvent. Water
e
f
was used as solvent. Dioxane was used as solvent. The byproduct
was benzoic acid. The isolated yield of benzoic acid was 92% when
g
h
0.2 equiv., of KOH was used. Reacted at 80 ꢀC. Reacted at 120 ꢀC.
i
l
j
k
ꢀ
Reacted for 6 h. Reacted for 24 h. Reacted at 80 C for 24 h.
Commercial sample, Vulcan XRF-HT-C-72 SBET ¼ 235 m2 gꢁ1
.
Fig. 1 SEM images of the carbon materials: (a) RF-HT-C-0, (b) RF-HT-
C-1, (c) RF-HT-C-2, (d) RF-HT-C-3, (e) RF-HT-C-4 and (f) RF-HT-C-5.
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Table 3 Oxidation of various alcohols to aldehydesa
different conversions. Low activity of catalyst RF-HT-C-0 was
observed for the selective oxidation of benzyl alcohol to the
corresponding benzaldehyde. The yield of benzaldehyde was
only 38% (entry 1). The yields were 95%, 85%, 76%, 72% and
45% if RF-HT-C-1, RF-HT-C-2, RF-HT-C-3, RF-HT-C-4 and RF-
HT-C-5 were used as catalysts, respectively (entries 2–7).
Therefore, the treatment of the carbon material with bases can
improve the catalytic performance signicantly. From results
given in Table 2, RF-HT-C-1 and RF-HT-C-6 had similar catalytic
activity, but RF-HT-C-1 was the more suitable catalyst than RF-
HT-C-6 because it needs fewer bases during the treatment. Next,
the inuence of the catalyst loading, solvent, temperature, and
reaction time were also studied. Clearly, low yield would be
obtained if the catalyst loading less than 100 mg, therefore, the
suitable quantity of catalyst should be 100 mg (entries 8–10, 15).
Among the solvents tested, good yield was observed in xylene
(81%) and the selectivity remained high (92%) (entry 11). If
water was employed as solvent, 26% conversion was obtained
but the selectivity was only 54% because benzoic acid as the
byproduct was formed (entry 12). So the presence of large
amount of water might deactivate the catalyst. The conversion
of benzyl alcohol was 64% and the selectivity of benzaldehyde
was 85% if using dioxane as solvent (entry 13). Moreover, the
reaction temperature and reaction time optimization indicated
that the reaction could be conducted completely at 100 ꢀC for 12
h. Finally, the amount of cocatalyst was investigated and it had a
remarkable effect on the reaction. The yield was only 10% if no
base was added, but it was increased remarkably when 10%
mmol or 20% mmol KOH was added (entries 1, 14 and 17). If the
base amount reached to 50% mmol or higher, i.e. 100% mmol,
the yield and selectivity of benzaldehyde decreased, and a large
amount of benzoic acid was produced (entries 18, 19). Note-
worthy when the quantity of KOH reached 200% mmol, no
benzaldehyde was generated, because the benzyl alcohol was
totally transformed to benzoic acid, and 92% isolated yield of
benzoic acid was achieved (entry 20). The above results showed
that aldehydes and acids can be synthesized from the corre-
sponding alcohols with high selectivity by adjusting the amount
of base. Under similar operating conditions, the catalytic
performance of RF-HT-C-1 was found to be much better than
the commercial carbon material such as activated carbon (entry
26). The lower yield using commercial AC as catalyst might be
caused by the adsorption of base.
Entry
Substrate
Time/h
12
Con./%
Yieldb/%
1
2
99
92
90
80
12
3
4
5
6
7
12
12
12
12
18
97
98
88
92
92
85
89
76
81
84
8
18
18
18
82
85
93
73
76
70
9
10
11
12
13
14
15c
18
18
24
24
24
80
88
85
75
72
68
71
71
60
49
16c
17d
24
89
78
Aer the realization of benzyl alcohol oxidation to benzal-
dehyde catalyzed by carbon material, we applied RF-HT-C-1 in
the selective oxidation of a series of benzylic alcohols having
substitutions ranging from primary and secondary alcohols
under the optimized reaction conditions for aldehyde synthesis
(Table 3). For all these derivatives, good conversions and
selectivities could be easily obtained. In order to compare the
electronic effect of different substituents, benzyl alcohol deriv-
atives contain electron-donating groups and electron-
withdrawing groups were tested. The reaction results indi-
cated that CH3 or OCH3 group had a benecial effect on the
reaction, but F, Cl and NO2 were negative (entries 1–14). Benzyl
alcohols with p-CH3 and p-OCH3 groups had conversions of
around 99% and 98% at 100 ꢀC aer 12 h, respectively. On the
24
24
93
90
83
77
18d
a
1 mmol alcohol, 100 mg RF-HT-C-1, 0.2 mmol KOH, 2 mL toluene, 1
b
atm O2, 100 ꢀC, 12 h. Determined by GRF-HT-C-FID using biphenyl
as external standard. c Reacted at 120 ꢀC. d 1H NMR yield.
other hand, benzyl alcohols with p-F, p-Cl, p-Br and p-NO2
groups gave conversions of 92%, 93%, 85% and 75%, respec-
tively. Consequently, cinnamyl alcohol and 1-phenylethyl
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Table 4 Oxidation of various alcohols to carboxylic acidsa
alcohol could also be oxidized to the desired products with 49%
and 78% yields, respectively (entries 15–16). This catalyst was
also active for the oxidation of heteroaromatic benzylic alco-
hols. For example, the yields to the desired products were 83%
and 77% when using furan-2-ylmethanol and pyridin-3-
ylmethanol as starting materials (entries 17–18). However, no
oxidation reaction occurs if using aliphatic alcohols, such as 1-
hexanol and 2-hexanol as starting materials.
Entry
Substrate
Yieldb/%
92%
1
2
99%
Following, we further investigated the selective oxidation of
alcohols to carboxylic acids. To the best of our knowledge, to
date, the metal-free carbon material has not been reported as an
efficient catalyst in the synthesis of carboxylic acid via alcohol
oxidation. The range of suitable substrates for this catalytic
system was investigated, and the results were listed in Table 4.
RF-HT-C-1 can effectively catalyze the oxidation of aromatic
alcohols to the corresponding carboxylic acids. Benzyl alcohol
3
4
83%
85%
5
96%
ꢀ
was quantitatively converted to benzoic acid at 100 C for 12 h
6
7
8
85%
98%
80%
(entry 1). Moreover, benzyl alcohols with electron-donating
groups such as p-CH3 and p-OCH3 can be easily converted to
the corresponding acids with 85–99% isolated yields (entries 2–
5). For benzylic alcohols with F, Cl and Br substituents, 80–98%
yields of the corresponding carboxylic acids can be obtained
(entries 6–10). For the oxidation of p-isopropyl benzyl alcohol,
91% yield of the corresponding carboxylic acid was realized
(entry 11). Finally, 90% yield of p-triuoromethyl benzoic acid
was also achieved via the oxidation of p-triuoromethyl benzyl
alcohol (entry 12). In addition, the position of the substituent
also affected the reactivity of benzyl alcohol derivatives. If taking
methyl benzyl alcohol as an example, the reactivity decreased in
the order of p-CH3, m-CH3 and o-CH3 (entries 2–4), and the same
reactivity order could be found in chlorobenzyl alcohol (entries
7–9). These results indicated that the substituent at the ortho-
position might hinder its reactivity. In addition, furan-2-
ylmethanol and pyridin-3-ylmethanol can be oxidized into the
corresponding acids with 85% and 95% yields (entries 13–14).
The above reaction results show that RF-HT-C-1 is effective for
the oxidation of alcohols to corresponding carboxylic acids.
In order to check the reusability of the carbon material
catalyst, ve reaction runs were carried out using benzyl alcohol
oxidation as model reaction. Aer each reaction the catalyst was
separated by ltration, washed with water and ethanol for three
times, and dried at 80 ꢀC for 6 h. The recovered catalyst was
used in the recycling study under the same reaction conditions.
It can be seen that the yield of benzaldehyde at the 5th run was
almost the same as the fresh catalyst (Table 5), indicating that
RF-HT-C-1 is stable and recyclable.
9
92%
92%
10
11
91%
90%
12
13
85%
14
95%c
a
1 mmol alcohol, 100 mg RF-HT-C-1, 2 mmol KOH, 2 mL toluene, 1 atm
b
O2, 100 ꢀC, 12 h. Isolated yield, extracted by ethyl acetate aer
neutralizing the mixture with diluted hydrochloric acid. NMR yield.
c
In summary, carbon material has been demonstrated to be
an active catalyst in the selective oxidation of various alcohols to
aldehydes or acids by molecular oxygen under metal- and
heteroatom-free conditions. These reactions were found to
proceed under relatively mild conditions and the controlled
synthesis of the desired product (aldehyde or acid) was realized.
Moreover, catalyst recovery was found to be both convenient
and effective using simple ltration techniques. To the best of
our knowledge, these results constitute the rst example to
facilitate synthetically useful transformations via alcohol
oxidation.
Table 5 Reusability test of RF-HT-C-1 for benzaldehyde synthesisa
Entry
Run
Yieldb/%
1
2
3
4
5
First
95
91
93
90
91
Second
Third
Fourth
Fih
a
1 mmol benzyl alcohol, 100 mg RF-HT-C-1, 0.2 mmol KOH, 2 mL
b
toluene, 1 atm O2 100 ꢀC, 12 h. GC yield.
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