Base-free oxidation of alcohols to esters at room temperature and atmospheric conditions using nanoscale Co-based catalysts

Article abstract of DOI:10.1021/cs502101c

The direct oxidation of alcohols to esters with molecular oxygen is an attractive and crucial process for the synthesis of fine chemicals. To date, the heterogeneous catalyst systems that have been identified are based on noble metals or have required the addition of base additives. Here, we show that Co nanoparticles embedded in nitrogen-doped graphite catalyze the aerobic oxidation of alcohols to esters at room temperature under base-free and atmospheric conditions. Our Co@C-N catalytic system features a broad substrate scope for aromatic and aliphatic alcohols as well as diols, giving their corresponding esters in good to excellent yields. This apparently environmentally benign process provides a new strategy with which to achieve selective oxidation of alcohols.

Full text of DOI:10.1021/cs502101c

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Article
Base-free oxidation of alcohols to esters at room temperature
and atmospheric conditions using nanoscale Co-based catalysts
Wei Zhong, Hongli Liu, Cuihua Bai, Shijun Liao, and Yingwei Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs502101c • Publication Date (Web): 12 Feb 2015
Downloaded from http://pubs.acs.org on February 18, 2015
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Base-free oxidation of alcohols to esters at room temperature and
atmospheric conditions using nanoscale Co-based catalysts
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Wei Zhong, Hongli Liu, Cuihua Bai, Shijun Liao, and Yingwei Li*
Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering,
South China University of Technology, Guangzhou 510640, P.R. China.
ABSTRACT: The direct oxidation of alcohols to esters with molecular oxygen is an attractive and crucial process for
the synthesis of fine chemicals. To date, the heterogeneous catalyst systems that have been identified are based on noble
metals or have required the addition of base additives. Here, we show that Co nanoparticles embedded in nitrogenꢀ
doped graphite catalyze the aerobic oxidation of alcohols to esters at room temperature under baseꢀfree and atmospherꢀ
ic conditions. Our Co@CꢀN catalytic system features a broad substrate scope for aromatic and aliphatic alcohols as well
as diols, giving their corresponding esters in good to excellent yields. This apparently environmentally benign process
provides a new strategy with which to achieve selective oxidation of alcohols.
KEYWORDS: alcohols, cobalt, heterogeneous catalysis, metalꢀorganic frameworks, oxidation
such as gold5 and palladium.6 Moreover, satisfactory reꢀ
sults were obtained in only limited cases, in which a
INTRODUCTION
The selective oxidation of alcohols is widely recognized
as one of the most important transformations in organic
synthesis because of the importance and versatility of the
corresponding carbonyl products for the fine and bulk
chemical industry. Among these carbonyl compounds,
esters represent an abundant class of chemicals, widely
utilized in fine chemicals, natural products, pharmaceuꢀ
ticals, agrochemicals, and food additives.1 Traditionally,
esters are prepared by the reaction of carboxylic acids or
activated acid derivatives (acyl chlorides and anhydrides)
with alcohols, a multistep process often with the forꢀ
mation of large amounts of undesired byproducts.2 In
recent years, a great deal of research effort has been deꢀ
voted to the development of environmentally benign and
costꢀeffective procedures for the synthesis of esters as
alternatives for traditional protocols; e.g., the catalytic
esterification of aldehydes with alcohols.3 Of the wellꢀ
established methodologies, the oxidative direct esterifiꢀ
cation of alcohols with molecular oxygen may be most
preferred and promising,4 which could represent a big
step forward toward green, economic, and sustainable
processes because of the availability and low cost of alꢀ
cohols compared to their oxidation products such as alꢀ
dehydes and acids.
large excess of base additives was required, and this was
usually achieved at relatively high temperatures and/or
high pressures. Therefore, the development of reusable
catalysts (ideally based on nonꢀnoble metals) for the oxiꢀ
dative direct esterification of alcohols under mild condiꢀ
tions (preferably at room temperature and atmospheric
conditions) is an attractive and challenging subject in
both green chemistry and organic synthesis.
Here, we report the first example of a highly efficient
and reusable cobaltꢀbased catalyst for catalytic direct
oxidation of alcohols to esters at room temperature and
atmospheric conditions without the assistance of any
base additives. In comparison with the already disclosed
synthesis methodologies, this novel catalytic system
would hold multiple advantages including: costꢀeffective
(using nonꢀprecious metals as catalyst and air as oxiꢀ
dant), environmentally benign (at room temperature and
atmospheric conditions without the addition of base adꢀ
ditives), and simplicity (easy operation under ambient
conditions and facile magnetic separation of the cataꢀ
lyst). Moreover, the proposed catalytic system features a
broad substrate scope for aromatic and aliphatic alcohols
as well as diols, providing good to excellent yields to the
target products with high selectivities. These make the
catalyst system great potential for industrial application
in production of esters from alcohols direct aerobic oxiꢀ
dation.
Many catalytic systems, including both homogeneous
and heterogeneous catalysts, have been developed for
the oxidation of alcohols to esters. From an economic
and environmental viewpoint, the use of heterogeneous
catalysts would be beneficial with respect to catalyst sepꢀ
aration and recycling. Nevertheless, most of the heteroꢀ
geneous catalysts for the oxidative esterification of alcoꢀ
hols with molecular oxygen were based on noble metals
The Co@CꢀN (carbonꢀnitrogen embedded cobalt nanoꢀ
paticles) materials were prepared by simple thermolysis
of a Coꢀcontaining metalꢀorganic framework (MOF),
ZIFꢀ67. MOFs are an emerging class of ordered porous
materials built with metal ions and organic ligands. Owꢀ
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ing to their high surface area, porosity, and chemical
proved active surface. Conversion of ZIFꢀ67 into Co@Cꢀ
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tunability, MOFs have shown great potential for applicaꢀ
tions in a wide range of fields such as catalysis, luminesꢀ
cence, gas storage, and separation.7 On the other hand,
taking advantage of their ordered structures and relativeꢀ
ly low thermal stability, MOFs could be utilized for the
preparation of new metal oxides or carbon nanomateriꢀ
als by thermal decomposition. In MOFs, the highly orꢀ
dered metal ions are isolated by organic ligands regularꢀ
ly, which will play an important role in preventing metal
aggregation during thermolysis. These MOFꢀderived
materials have shown excellent performances in a variety
of applications such as heterogeneous catalysis,8 electroꢀ
chemistry,9 and gas adsorption.10 To the best of our
knowledge, so far the reports on the use of such MOFꢀ
derived materials as catalyst for liquidꢀphase organic
synthesis are rare.
N materials was performed by direct thermal treatments
under a flow of argon. As observed from the TGAꢀDSC
curves (Figure S3), ZIFꢀ67 began to decompose when the
temperature was increased to ca. 500 °C under argon.
Therefore, four different target temperatures (i.e., 600
°C, 700 °C, 800 °C, and 900 °C) were set with a heating
rate of 1 °C/min from room temperature. The ZIFꢀ67
crystals were annealed at the final temperatures for 8 h
to produce the Co@CꢀN composites. The prepared mateꢀ
rial is denoted as Co@CꢀN(x), where x indicates the MOF
pyrolysis temperature. SEM images (Figure S2) of the
Co@CꢀN(x) exhibited relatively smaller particles and
more and more rough surface as compared to the parent
ZIFꢀ67, indicating the gradual decomposition and carꢀ
bonization of the frameworks. The Co contents in the
Co@CꢀN(x) materials were about 30ꢀ40 wt% (Table S1).
The EDS elementꢀmapping revealed the uniform distriꢀ
butions of Co, C, and N elements in the Co@CꢀN materiꢀ
als (Figure S4). Nitrogen adsorption isotherms at 77 K
(Figure S5) showed typically microporous structures of
the Co@CꢀN(x) materials with specific surface areas of
around 300ꢀ400 m2 g−1 (Table S1).
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ZIFꢀ67 (Co(MeIM)2, MeIM = 2ꢀmethylimidazole) was
selected as the MOF precursor due to its good thermal
stability, and high nitrogen and carbon contents while
zero oxygen content. The strong coordination interaction
between Co and N atoms would allow a stepwise collapse
of the MOF structure during the slow heating procedures
to prevent a serious aggregation of Co. The MeIM linkers
will be carbonized gradually and the resulting carbonꢀ
nitrogen composite will play an important role in isolatꢀ
ing the Co species (Figure 1). Since nitrogen doping can
be an effective way to enhance the performance of carꢀ
bon materials,11 the nitrogen content remained in the
final material could be maximized due to the absence of
oxygen atom in the selected MOF.
Figure 2. Powder XRD patterns of (a) Co@CꢀN(600), (b)
Co@CꢀN(700), (c) Co@CꢀN(800), and (d) Co@CꢀN(900).
Figure 1. Schematic illustration of formation of a nitrogenꢀ
dopedꢀgraphite embedded Co catalyst from oneꢀstep pyrolyꢀ
sis of ZIFꢀ67.
The XRD patterns of the Co@CꢀN composites (Figure
2) showed five diffraction peaks at around 44.2°, 51.6°,
76.0°, 92.4°, and 97.8°, respectively, characteristic of
metallic Co (JCPDS No. 15ꢀ0806). The improved intensiꢀ
ties of the Co diffraction peaks for the materials prepared
at higher pyrolysis temperatures indicated the producꢀ
tion of Co phase with a higher crystallization degree. The
XRD peak at ca. 25.8° observed clearly at relatively high
temperatures corresponds to the interlayer d spacing of
3.42 Å, which can be assigned to the turbostratic orderꢀ
ing of the carbon and nitrogen atoms in the graphite layꢀ
ers.15
RESULTS AND DISCUSSION
ZIFꢀ67 was prepared from cobalt nitrate and 2ꢀmethyl
imidazole by a roomꢀtemperature precipitation methꢀ
od.12 The powder Xꢀray diffraction (XRD) patterns of asꢀ
synthesized ZIFꢀ67 (Figure S1) matched well with the
simulated and also the published XRD patterns, conꢀ
firming the formation of pure ZIFꢀ67 crystals.13 The MOF
material showed clearly the typical rhombic dodecaheꢀ
dron shape with particle sizes of around 300 nm, as can
be seen from the scanning electron microscopy (SEM)
images (Figure S2a). The formation of the MOF crystals
with a smaller particle size as compared to those preꢀ
pared by using the general synthesis method (normally
with a crystal size of subꢀmillimetre),14 may be beneficial
for the preparation of small Co@CꢀN particles and
thereby for enhanced catalytic activity because of imꢀ
The catalytic activities of the prepared materials were
tested using the oxidative cross esterification of pꢀ
nitrobenzyl alcohol and methanol as model reaction.
Reactions were performed at room temperature (25 °C)
and atmospheric pressure using air as oxidant under
baseꢀfree condition. The results are summarized in Table
1. The parent ZIFꢀ67 gave no conversion of pꢀnitrobenzyl
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mixture of commercial Co and carbon powder, the Co and C
contents are the same as those of Co@CꢀN(800).
alcohol, suggesting that cobalt ions coordinated with 2ꢀ
methylimidazole were not active for alcohols oxidation
under the investigated conditions (Table 1, entry 1). To
our delight, ZIFꢀ67 pyrolyzed materials were highly acꢀ
tive in this transformation, and Co@CꢀN(800) exhibited
the highest efficiency with respect to both conversion
and selectivity (Figure S6), affording the desired product
methyl pꢀnitrobenzoate in >99% yield (Table 1, entries 2ꢀ
5). Such a high reactivity and selectivity in the oxidative
esterification of alcohols at room temperature and atꢀ
mospheric conditions in the absence of base additives by
using nonꢀnoble metals as catalyst has not been reported
to date. Upon further increasing the thermolysis temperꢀ
ature to 900 °C, the reactivity of the resulting material
decreased significantly (Table 1, entry 5). pꢀ
Nitrobenzaldehyde was detected as the main byꢀproduct,
especially at lower conversions (Figure S6), which is conꢀ
sistent with a twoꢀstep mechanism involving the first
oxidation of alcohol to aldehyde and further reaction
with alcohol to produce ester.5a And the latter reaction
might be the rateꢀdetermining step in the direct oxidaꢀ
tive esterification of alcohols. It is noteworthy that the
reaction could also proceed at lower methanol loadings
(even stoichiometrically), albeit less efficiently (Table 1,
entries 6ꢀ7). As expected, the addition of base remarkaꢀ
bly accelerated the reaction and shortened the time
needed to complete the conversion (Table 1, entry 8).
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Although Co3O4 nanoparticles from direct thermolysis
of MOFs have been described as a highꢀperformance
material for supercapacitors,16 lithium ion battery,17 cataꢀ
lytic oxidation of CO,18 gas sensing,19 and gas adsorpꢀ
tion.20 Thus far, report on the use of such MOFꢀderived
Co materials for liquidꢀphase catalytic reactions is rare.
In this regard, the recent work by Beller and coꢀworkers
is noteworthy, which reports a novel Co3O4−N@C mateꢀ
rial by pyrolysis of nitrogenꢀligated cobalt(II) acetate
supported on commercial carbon for the direct oxidative
esterification of alcohols.21 Good to excellent yields were
achieved using pure oxygen as oxidant at 60ꢀ120 °C in
the presence of catalytic amounts of K2CO3 as base over
the Co3O4−N@C catalyst. For example, the reaction of pꢀ
nitrobenzyl alcohol and methanol gave 88% yield within
24 h at 60 °C.21 Under identical conditions, the present
Co@CꢀN(800) catalyst furnished a quantitative yield of
methyl pꢀnitrobenzoate in 8 h. Note that in the present
study although the time to complete conversion of pꢀ
nitrobenzyl alcohol was a little long (i.e. 96 h) at 25 °C
under air in the absence of basic promoters, it could be
significantly shortened at slightly higher temperatures,
e.g., within 20 h at 60 °C over the Co@CꢀN(800) cataꢀ
lyst (Table 1, entry 9).
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The recyclability of the catalyst was investigated beꢀ
cause it is crucial that a highly active catalyst may be reꢀ
used. After the reaction with Co@CꢀN(800), a magnet
was placed close to the reactor wall. Very quickly, all the
powders were adsorbed on the wall by the magnet (Figꢀ
ure S7), implying the good magnetic property of the Coꢀ
based material. Thus, the catalyst could be easily sepaꢀ
rated from the reaction mixture just by pouring the soluꢀ
tion. The remained solids were washed with methanol
and then reused as catalyst under identical reaction conꢀ
ditions. The oxidative esterfication result over the reused
catalyst showed that the activity was significantly deꢀ
creased. However, to our delight, the reactivity could be
fully restored when the reused catalyst was treated in H2
at 400 °C for 1 h, and no apparent loss in efficiency was
observed for up to five runs (Figure S8). PXRD results
(Figure S9) indicated that the Co diffraction peaks of the
reused catalyst without H2 treatment were much weaker
than those of the fresh one, suggesting that the metallic
Co was probably partially oxidized during the course of
the reaction by oxygen in air. After reduction, it was obꢀ
served that the strengths of the Co diffraction peaks inꢀ
creased to the same levels as the fresh one. This result
could account for the recovery of the catalytic activity of
Co@CꢀN(800) upon reduction treatment.
Table 1. Oxidative esterification of p-nitrobenzyl
alcohol with methanol.a
Entry Catalyst
Conversion (%)
Yield (%)b
1
ZIF-67
−
−
2
Co@C-N(600)
Co@C-N(700)
Co@C-N(800)
Co@C-N(900)
Co@C-N(800)
Co@C-N(800)
Co@C-N(800)
Co@C-N(800)
Co@C-N(800)-H+
Co@C-N(800)-air
Co + C
76
89
>99
55
98
72
>99
>99
−
74
88
>99
52
97
61
>99
>99
−
3
4
5
6c
7d
8e
9f
10
11
12g
5
4
A leaching experiment was performed to further check
the stability of the Co nanoparticles and to verify the hetꢀ
erogeneous nature of the reaction. The reaction with the
solution after filtration at approximately 35% conversion
essentially stopped, strongly suggesting that the reaction
was predominantly heterogeneous (Figure S10). Moreoꢀ
ver, the Co@CꢀN(800) showed negligible cobalt leaching
during the reaction. We considered that the Co nanoparꢀ
ticles were well embedded in the CꢀN composite (see
−
−
a
Reaction conditions: pꢀnitrobenzyl alcohol (0.5 mmol),
CH3OH (1 mL), nꢀhexane (4 mL), catalyst (Co 15 mol%), p =
b
1 atm, 25 °C, under air, 96 h. GC yield based on pꢀ
c
d
nitrobenzyl alcohol. CH3OH (2.5 mmol). CH3OH (0.5
e
f
g
mmol). K2CO3 (0.1 mmol), 48 h. 60 °C, 20 h. Physical
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layers. Picture (e) shows an enlarged image of a Co particle
in (c).
TEM images as shown below), and thus the cobalt leachꢀ
ing could be efficiently prohibited. These results demonꢀ
strated that the highly active Co@CꢀN(800) catalyst was
stable and reusable under the mild reaction conditions.
To elucidate reasons for the remarkable catalytic activꢀ
6
To identify the active sites for aerobic oxidative esteriꢀ
fication of alcohols, we examined the catalytic efficiency
of Co@CꢀN(800) after removing the metallic Co or CꢀN
composite under identical conditions. First, the asꢀ
synthesized Co@CꢀN(800) was immersed in aqua regia
to completely remove Co (Table S1 and Figure S11), and
the yielded material is denoted as Co@CꢀN(800)ꢀH+.
The Co@CꢀN(800)ꢀH+ was not active in the oxidation of
alcohols, demonstrating the requirement of a metal to
catalyze the reaction (Table 1, entry 10). On the other
hand, the Co@CꢀN(800) material was subjected to heatꢀ
ing at 400 °C for 0.5 h in air and then for 1 h in hydroꢀ
gen. The CꢀN composite was mostly burned (Table S1
and Figure S11) after this heating treatment, and the reꢀ
sulting material is denoted as Co@CꢀN(800)ꢀair. The
XRD of Co@CꢀN(800)ꢀair exhibited a mixture of faceꢀ
centred cubic (fcc) and hexagonaly closed packed (hcp)
cobalt that was often observed for Co after treatments at
high temperatures.22 Co@CꢀN(800)ꢀair showed little
activity and produced methyl pꢀnitrobenzoate in only 4%
yield (Table 1, entry 11). Interestingly, a physical mixture
of commercial metallic cobalt (20ꢀ30 nm) and activated
carbon also showed no reactivity (Table 1, entry 12). Theꢀ
se control experiments suggest the important synergic
interactions between CꢀN composite and Co in determinꢀ
ing the activity of the Co@CꢀN(800) in the oxidative esꢀ
terification.
ity of the Co@CꢀN(800) material, the effects of pyrolysis
temperature on the structure of the catalysts were studꢀ
ied in detail by highꢀresolution transmission electron
microscopy (HRTEM) and xꢀray photoelectron spectrosꢀ
copy (XPS). No significant aggregation of Co nanopartiꢀ
cles was observed for Co@CꢀN pyrolyzed even at a temꢀ
perature as high as 800 °C (Figure 3 and Figure S12),
which could be attributed to the separation effect of carꢀ
bons formed from carbonization of the 2ꢀ
methylimidazole linker in ZIFꢀ67. The size of Co crystalꢀ
line was in good agreement with that estimated from
XRD for each sample. Each Co nanoparticle in the
Co@CꢀN(800) was surrounded by graphitized carbon
tightly. This phenomenon is consistent with the literaꢀ
ture works that report on the use of transition metals for
the catalytic graphitization of carbon.23 The graphiteꢀ
enclosed Co nanoparticles in Co@CꢀN(800) was highly
crystallized (Figure 3e), which exhibited two fringe spacꢀ
ings of 2.05 and 1.79 Å, corresponding to the (111) and
(200) plane in fcc cobalt respectively.24 Besides the simiꢀ
lar structures as Co@CꢀN(800), we observed the forꢀ
mation of a large number of carbon nanotubes when the
pyrolysis temperature was increased to 900 °C (Figure
S12d), as was also evident in the SEM images (Figure
S2e). The Co nanoparticles staying at the tip of tubes
suggests that the growth of carbon nanotubes was cataꢀ
lyzed by the generated Co nanoparticles. As seen from
the TEM images, the growth of tubes resulted in serious
aggregation of Co nanoparticles, which may be the cruꢀ
cial reason for the low catalytic activity of the Co@Cꢀ
N(900) in the oxidative esterification reaction.
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To gain further insight into the catalyst structure and
especially the location of nitrogen, XPS characterization
was performed on the samples. In the cobalt region, only
peaks characteristic for metallic Co were observed (Figꢀ
ure S13) with the typical binding energies of 793.6 and
778.8 eV, assigned to Co 2p1/2 and Co 2p3/2 electrons of
Co metal, respectively. In the parent ZIFꢀ67, one N1s
peak occurred at 398.7 eV (Figure 4), which is related to
pyridinic nitrogen coordinating to a cobalt ion.25 A new
N1s peak at 401.0ꢀ402.0 eV was observed in the Co@CꢀN
materials, which may be assigned to nitrogen in a graꢀ
phitic structure.26 Moreover, the ratio of graphitic nitroꢀ
gen to pyridinic nitrogen in Co@CꢀN was increased when
enhancing the pyrolysis temperature (e.g., 0.4 for 700
°C, 1.2 for 800 °C, and 1.4 for 900 °C), implying a higher
graphitization degree at higher temperatures. This result
is in good agreement with the TEM and XRD observaꢀ
tions.
Figure 3. HRTEM images of (a) Co@CꢀN(600), (b) Co@Cꢀ
N(700), (c) Co@CꢀN(800), and (d) Co@CꢀN(900). The red
lines in (c) indicate a Co nanoparticle wrapped by graphite
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tion reaction and afforded the heterocyclic carboxylic
3
acid esters in up to 93% yield (Table 2, entries 17ꢀ19).
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6
7
Table 2. Oxidative esterification of aromatic al-
cohols with methanol.a
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Entry
1
Alcohol
Product
Yield (%)b
>99
2
3
4
5
>99
>99
>99
94
410
407
404
401
398
395
392
Electron binding energy (eV)
Figure 4. N1s spectra for (a) ZIFꢀ67, (b) Co@CꢀN(700), (c)
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Co@CꢀN(800), and (d) Co@CꢀN(900).
6c
7
86
The scope of the presented methodology was then exꢀ
tended to the oxidative esterification of a series of strucꢀ
turally diverse aromatic alcohols under similar reaction
conditions using the Co@CꢀN(800) catalyst. As can be
seen from Table 2, various benzylic alcohols were selecꢀ
tively converted into the corresponding methyl esters in
excellent yields at room temperature and atmospheric
conditions without the addition of any base additives.
The desired esters were obtained in >99% yields when
employing the benzylic alcohols substituted with the
strongly electronꢀdonating groups such as pꢀOMe (Table
2, entries 2 and 3). The different substituted positions on
the phenyl ring had a marked influence on the reactivity
of the reaction. As an example, the methyl group at meta
and ortho positions showed a lower activity to the deꢀ
sired ester than that in the para position (Table 2, entries
4ꢀ6). Apart from electronꢀdonating groups, benzylic alꢀ
cohols substituted with various electronꢀwithdrawing
groups, such as –CO2CH3, ꢀNO2, ꢀF, ꢀCl, ꢀBr, and ꢀI, all
reacted with methanol smoothly, giving the desired esꢀ
ters in >90% yields (Table 2, entries 7ꢀ14). Notably, even
more sensitive allylic alcohols such as cinnamic alcohol
underwent the oxidative esterification in a quantitative
yield (Table 2, entry 15). Furthermore, diols could be
completely converted into the corresponding diester in
>99% yield (Table 2, entry 16).
>99
8
>99
98
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12
>99
>99
>99
13
14
94
90
It is worth noting that heterocycles such as thiopheneꢀ
2ꢀmethanol, furanꢀ2ꢀmethanol, and pyridineꢀ2ꢀmethanol
could also be used as substrate in the oxidative esterificaꢀ
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a Reaction conditions: aromatic alcohol (0.5 mmol), CH3OH
(1 mL), nꢀhexane (4 mL), Co@CꢀN(800) (Co 15 mol%), p = 1
atm, under air, 25 °C, 96 h. b GC yield based on benzylic
7c
8
c
alcohol. Aromatic aldehydes were the main byꢀproducts.
Same as ‘a’ at 108 h. d Same as ‘a’ with 25 mol% Co at 120 h.
After successful application of the cross esterification
to methanol with various benzylic alcohols, we tried to
extend this novel catalytic system to the oxidative esteriꢀ
fication of longꢀchain aliphatic alcohols. Various linear
aliphatic alcohols including ethyl, nꢀpropyl, nꢀbutyl, nꢀ
pentyl, nꢀhexyl, nꢀheptyl, and even nꢀoctyl alcohol unꢀ
derwent the esterification smoothly to furnish the correꢀ
sponding esters in good to excellent yields at room temꢀ
perature under atmospheric and baseꢀfree conditions
(Table 3, entries 1ꢀ7). A gradual decrease in product yield
was observed when the carbon chain length of the aliꢀ
phatic alcohol increased. However, a high yield of the
desired product could still be achieved simply by proꢀ
longing the reaction time, e.g., for the oxidative esterifiꢀ
cation of nꢀoctyl alcohol (Table 3, entry 7). The present
catalytic system is also applicable to the oxidative esteriꢀ
fication of branched alcohols. Not surprisingly, such
cross esterifications are difficult to achieve a high yield
and have been rarely investigated in the literature due to
the possible oxidation of one alcohol in the presence of
another. The desired ester was obtained in 81% yield
when isobutanol was employed (Table 3, entry 8). The
yield to the cross esterification product decreased reꢀ
markably as the steric hindrance of alcohol increased,
e.g., for tertiary butanol, the desired product was obꢀ
tained in 62% yield under identical conditions (Table 3,
entry 9).
9
a
Reaction conditions: pꢀnitrobenzyl alcohol (0.5 mmol),
aliphatic alcohol (2 mL), nꢀhexane (3 mL), Co@CꢀN(800)
(Co 40 mol%), p = 1 atm, under air, 25 °C, 96 h. Aldehydes,
and esters from aliphatic alcohols (e.g., ethyl acetate) were
observed. b GC yield based on benzylic alcohol. Same as ‘a’
c
at 120 h.
The application of the newly developed oxidative esterꢀ
ification protocol was further extended to the lactonizaꢀ
tion of diols. Lactones are ubiquitous as structural eleꢀ
ments in various bioactive natural products and synthetꢀ
ic organic compounds. The oxidative lactonization of
diols using molecular oxygen as oxidant is an attractive
methodology for the synthesis of lactones. Nevertheless,
so far the developed catalytic protocols were usually
based on homogeneous catalysts or noble metals such as
Au, Ru, Pd, or Ir.27 Here, 1,2ꢀbenzenedimethanol was the
first diol used in the oxidative lactonization reaction over
the Co@CꢀN(800) catalyst. The desired lactone was obꢀ
tained in 97% yield at room temperature and atmospherꢀ
ic conditions without the assistance of any base additives
(Table 4, entry 1). The catalyst system was also applicaꢀ
ble to the oxidative lactonization of aliphatic α,ωꢀdiols
ranging from 1,4ꢀ to 1,8ꢀdiols, affording the correspondꢀ
ing lactones in excellent yields under mild reaction conꢀ
ditions (Table 4, entries 2ꢀ6). It is noteworthy that there
are only few reports on catalytic synthesis of lactones
with largeꢀmembered rings from lactonization of diols up
to date.28 The reaction results of different alcohols (Taꢀ
bles 2ꢀ4) demonstrated the general applicability of the
novel catalyst system in the selective oxidative esterificaꢀ
tion.
Table 3. Oxidative esterification of p-nitrobenzyl
alcohol with various aliphatic alcohols.a
Entry Aliphatic alcohol
Product
Yield (%)b
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Table 4. Lactonization of various diols.a
ASSOCIATED CONTENT
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Supporting Information
Experimental details, characterization data and spectra of
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
7
8
9
Yield (%)b
97
AUTHOR INFORMATION
Corresponding Author
Entry
1
Diol
Product
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liyw@scut.edu.cn
Notes
The authors declare no competing financial interests.
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ACKNOWLEDGMENTS
We thank the National Natural Science Foundation of China
(21322606 and 21436005), Doctoral Fund of Ministry of
Education of China (20120172110012), and Guangdong
Natural
Science
Foundation
(S2011020002397,
2013B090500027, and 10351064101000000) for financial
support.
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a
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Co@CꢀN(800) (Co 40 mol%), p = 1 atm, under air, 25 °C, 96
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We performed 10ꢀmmol scale reactions for selected subꢀ
strates and the results are shown in Table S2. It can be
seen that all these substrates underwent the oxidative
esterification smoothly, furnishing the corresponding
esters in 95ꢀ97% isolated yields under scaleꢀup condiꢀ
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for esters synthesis from oxidation of alcohols is scalable
under the investigated conditions.
CONCLUSIONS
In summary, we have developed a general, simple, costꢀ
effective, and environmentally friendly protocol for diꢀ
rect aerobic oxidative esterification of alcohols, using a
novel nitrogenꢀdopedꢀgraphite enclosed cobalt material
as catalyst. The catalyst is prepared by oneꢀpot thermolyꢀ
sis of a Coꢀbased MOF with high nitrogen and carbon
contents. The catalytic system features a broad substrate
scope for aromatic and aliphatic alcohols as well as diols,
giving their corresponding esters in good to excellent
yields at room temperature and atmospheric conditions
without the assistance of any base additives. The initial
success in achieving selective oxidative esterification of
alcohols using MOFꢀpyrolyzed materials might provide a
new avenue for developing new types of composite mateꢀ
rials for highly efficient catalytic organic transforꢀ
mations.
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Base-free oxidation of alcohols to esters at room tempera-
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catalysts
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