Efficient and Highly Selective Solvent-Free Oxidation of Primary Alcohols to Aldehydes Using Bucky Nanodiamond

Article abstract of DOI:10.1002/cssc.201700968

Selective oxidation of alcohols to aldehydes is widely applicable to the synthesis of various green chemicals. The poor chemoselectivity for complicated primary aldehydes over state-of-the-art metal-free or metal-based catalysts represents a major obstacle for industrial application. Bucky nanodiamond is a potential green catalyst that exhibits excellent chemoselectivity and cycling stability for the selective oxidation of primary alcohols in diverse structures (22 examples, including aromatic, substituted aromatic, unsaturated, heterocyclic, and linear chain alcohols) to their corresponding aldehydes. The results are comparable to reported transition-metal catalysts including conventional Pt/C and Ru/C catalysts for certain substrates under solvent-free conditions. The possible activation process of the oxidant and substrates by the surface oxygen groups and defect species are revealed with model catalysts, ex situ electrochemical measurements, and ex situ attenuated total reflectance. The zigzag edges of sp² carbon planes are shown to play a key role in these reactions.

Full text of DOI:10.1002/cssc.201700968

Accepted Article
Title: Efficient and Highly Selective Solvent-Free Oxidation of Primary
Alcohols to Aldehydes Using Bucky Nanodiamond
Authors: Yangming Lin, Kuang-Hsu (Tim) Wu, Linhui Yu, Saskia
Heumann, and Dangsheng Su
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ChemSusChem
10.1002/cssc.201700968
FULL PAPER
Efficient and Highly Selective Solvent-Free Oxidation of Primary
Alcohols to Aldehydes Using Bucky Nanodiamond
Yangming Lin,[a,b,c] Kuang-Hsu (Tim) Wu,[a] Linhui Yu, Saskia Heumann, and Dang Sheng Su*
[d]
[c]
[a,e]
Dedication ((optional))
Abstract: Selective oxidation of alcohols to aldehydes is widely
applicable to the synthesis of various green chemicals. The poor
chemo-selectivity for complicated primary aldehydes over state-of-
the-art metal-free or metal-based catalysts represents a major
obstacle for industrial application. Here we report on bucky
nanodiamond as a potential green catalyst which exhibits excellent
chemo-selectivity and cycle stability in the selective oxidation of
primary alcohols in diverse structures (22 examples, including
aromatic, substituted aromatic, unsaturated, heterocycle and linear
chain alcohols) to their corresponding aldehydes; the results are
even comparable to the reported transition metal catalysts and
conventional Pt/C and Ru/C catalysts for certain substrates under
solvent-free conditions. The possible activation process of surface
oxygen groups and defect species to oxidant and substrate are
revealed with model catalysts, ex-situ electrochemical measurement
desired aldehydes further hinders their industrial application.9-15
Over the past few years, nanocarbon catalysis are recognized
as promising candidates to meet the requirement of sustainable
and green chemistry.16-19 Some achievements have shown the
positive effects of nanocarbon materials for the oxidation
reaction of simple aromatic alcohols (e.g.benzyl alcohol,
substituted benzyl alcohol) by using various organic reagents as
solvents and in the presence of particular oxidants, such as nitric
acid, t-butylhydroperoxide (TBHP),
H
2
O
2
and O
2
,20-26 but
there are few studies on the feasibility of carbon materials for the
complicated alcohols (e.g., unsaturated, heterocycle and linear
chain alcohols). Moreover, most of recent research objectives
still focuses on the catalytic performance of newly developed
catalysts for simple aromatic alcohols rather than on
understanding the activation process of reactants or oxidants by
the used catalysts. In this case, only a few studies could provide
2
and ex-situ attenuated total reflectance. The zigzag edges of sp
carbon planes have been shown to play a key role in these reactions. convincing evidence for the activation of the oxidant (e.g., TBHP)
and substrates over carbon materials (e.g,. surface active sites).
The roles of surface different oxygen groups and defects in the
reactions have rarely been reported, and the interaction at
interface between catalyst and oxidant or substrate remains
Introduction
elusive and controversial. It is therefore highly important to
Selective oxidation of alcohol to aldehyde is a key reaction in
clarify the active sites and the corresponding activation process
organic synthesis and will likely play a significant role in the
into more detail as an effort to elucidate their underlying catalytic
development of value-added chemicals.1-4 Metal-based,
activity for other complicated substrates.
especially noble-metal materials, have been considered to be
the most important catalysts for heterogeneous alcohol
oxidation,5-8 but the toxicity, unsustainability and the complex
preparation process of catalysts or the low chemo-selectivity for
2
3
Bucky nanodiamond (BND), a class of sp /sp hybrid material,
3
2
consists of a sp carbon core covered by a few sp graphite-like
layers that can be prepared by graphitizing purified
nanodiamond (ND) at different temperatures under an inert
atmosphere or in vacuum (Scheme 1, T<1500 °C). Such hybrid
structure has both remarkable properties of surface graphitic
nanomaterials and intrinsic nature of a diamond core.27 In the
previous studies, BND has been shown to be a promising
catalyst in gas-phase reactions due to its unique structure and
electronic properties.28-29 Here, we demonstrate the excellent
catalytic activity of BND for selective oxidation of complicated
alcohols (22 examples) to their corresponding aldehydes. To the
best of our knowledge, this is the first time that the alcohols in a
range of diverse structures can be catalyzed only by
nanocarbons with decent activity, even under the solvent-free
condition. Through comparative experiments with model
catalysts, ex-situ electrochemical measurement and ex-situ
attenuated total reflectance, we are able to show the possible
activation process of the oxidant (TBHP) and substrate (benzyl
alcohol as probe) by the surface oxygen groups and the defects,
with a proposition of possible catalytic mechanism.
[
a]
Dr. Y.M. Lin, Dr. K.-H. (Tim) Wu, Prof. Dr. D.S. Su
Institute of Metal Research, Chinese Academy of Sciences
72 Wenhua Road, Shenyang, 110016, P.R.China
E-mail: dssu@imr.ac.cn
[b]
[c]
[d]
[e]
Dr. Y.M. Lin
School of Chemistry and Materials Science, University of Science
and Technology of China, Hefei, 230001, P. R. China
Dr. Y.M. Lin, Dr. S. Buller
Max Planck Institute for Chemical Energy Conversion,
Stiftstrasse 34–36, Mülheim an der Ruhr, 45470, Germany
Dr. L. Yu
Research Institute of Photocatalysis, Fuzhou University,
Fuzhou 350002, P. R. China
Prof. Dr. D.S. Su
Department of Inorganic Chemistry, Fritz Haber Institute of the Max
Planck Society, Faradayweg 4-6, Berlin, 14195, Germany
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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ChemSusChem
10.1002/cssc.201700968
FULL PAPER
0
.1% Fe/
2 [
0
h]
i]
386
386
TFT
TFT
13.9
12.5
>99
>99
74.5
71.8
1300BND
0
.1%Cr/
2 [
1
1300BND
[
a] Reaction condition: 10 mg catalyst, 2.0 mmol substrate, 2 mmol 65 wt%
tert-butyl hydroperoxide (TBHP), 5 mL trifluorotoluene (TFT), 70 °C, 4 h. [b] 24
h. [c] Ref. 26, 1.1 mmol substrate, 100 mg catalyst, 120 °C, 3 h, O as terminal
oxidant. [d] Ref. 26, 1.1 mmol substrate, 20 mg catalyst, 120 °C, 3 h, O as
2
Scheme 1. The phase transformation of ND to BND and onion-like carbon
2
(
OLC). The surface of BND and OLC should be rather irregular sp2 carbon
terminal oxidant. [e] Ref. 32, 47 mmol substrate, 47 mmol TBHP,
metal/substrate=6500, 80 °C, 24 h. [f] Ref. 33, 52 mmol substrate, 78 mmol
TBHP, 500 mg catalyst, 94 °C, 2 h. [g] Ref. 34, 52 mmol substrate, 78 mmol
fragments, although multiple perfect graphitic shells were displayed for
simplicity.
TBHP, 500 mg catalyst, 94 °C, 0.5 h. [h] Fe(NO
73K for 1h. [i] Cr(NO ·9H O as precursor, annealed at 573K for 1h.
Desired product: benzaldehyde.
3 3
) as precursor, annealed at
5
)
3 3
2
Results and Discussion
Table 1 is a summary of the selective oxidation of benzyl
alcohol to benzaldehyde over various carbon materials under
different conditions. Without catalysts, only 2.5% benzyl alcohol
could be converted. The corresponding selectivity towards
benzaldehyde is 90.3% (Table1, entry 1) in the presence of
TBHP. Among the three BND samples treated at different
temperatures (entries 3-5), 1300BND (treated at 1300 ° C)
offered the best catalytic performance with a benzyl alcohol
conversion of 13.6% under the optimized reaction condition
The HRTEM images of ND, BND and onion-like carbon (OLC)
are shown in Figure S1. The surface of purified ND is covered
by amorphous and disordered carbon. The identified interlayer
spacing of 0.340 nm in the outer shell and the lattice spacing of
0
.206 nm in the core can be assigned to the (002) plane of
graphite and (111) plane of diamond, respectively.30-31
Depending on the calcination temperature, the number of
graphite-like shells of BND increases from two layers of 900BND
to several layers of 1300BND. When the temperature is further
increased to above 1500 °C (Scheme 1), BND could completely
(
entry 5). As references samples, other carbon materials, such
as ND (entry 2), 1500OLC (treated at 1500 ° C, entry 6),
600OLC (treated at 1600 °C, entry 7), graphene (entry 8) and
1
2
transform into OLC that has multiple layers of curved sp carbon
graphite oxide (entry 9) showed a lower conversion of benzyl
alcohol (4.9%~11.4%) and selectivity for benzaldehyde under
the same reaction conditions. The decomposition efficiency of
TBHP over 1300BND is apparently higher than that of the listed
carbon materials, suggesting that the surface structure of
1300BND possesses a good activation capability for TBHP. It
should be noted that the conversion of benzyl alcohol can be
further improved to 62.3% along with a decent selectivity by
prolonging the reaction time (entry 10).
concentric shells. The particle sizes of all ND, BND and OLC
samples are about 5-8 nm.
Table 1. Selective oxidation of benzyl alcohol by various samples under
[
a]
different conditions.
S
(m
g )
----
BET
2
Decomposition
efficiency (%)
of TBHP
Con.
%)
Sel.
(%)
Entry
Catalyst
Solvent
(
-
1
1
2
----
ND
TFT
TFT
2.5
4.5
90.3
98.2
1.8
313
14.9
The effect of solvent to catalytic performance was also
investigated. The results in Table S1, entries 3-9 indicate that
there is no direct relationship between the catalytic activity and
the polarity value of the solvents. Moreover, 1300BND also
displays an appreciable conversion of benzyl alcohol (13.4%,
Table 1, entry 11) and selectivity of benzaldehyde (>99%) under
solvent-free condition, which is comparable to commercial 5 wt%
Pt/C and 5 wt% Ru/C (Table 1, entries 12, 14) catalysts, also in
the absence of solvent, but being much better than the reported
3
4
5
6
7
900BND
1100BND
1300BND
1500OLC
1600OLC
342
350
386
463
476
TFT
TFT
TFT
TFT
TFT
7.0
8.1
13.6
11.4
9.3
>99
>99
>99
98.3
95.9
28.5
38.7
69.5
56.8
43.2
8
Graphene
492
TFT
7.8
96.8
34.6
Graphite
oxide
1300BND
1300BND
9
201
TFT
4.9
97.3
24.9
0[b]
1
386
386
TFT
-----
62.3
13.4
97.1
>99
>99
68.8
11
5
wt% Pt/C
Alfa)
wt% Pt/C
Wako)
wt%Ru/C
Alfa)
wt%Ru/C
Wako)
5
wt% Pt/C, 5 wt% Ru/C and other noble-metal supported
1
2
----
----
-----
16.3
9
85.2
87
>99
----
(
catalysts (Table 1, entries 13, 15-19) under similar reaction
conditions.22,32-34 In the previous studies, the trace amount of
metal impurities (e.g., Fe<50 ppm, Cr< 10 ppm) has been found
in 1300BND by an inductively coupled plasma optical emission
spectrometer. Here, in order to exclude the roles of metal
impurities, 0.1% Fe-, Cr-containing 1300BND samples were
prepared by an annealing approach. The results indicate that
metal-containing 1300BND catalysts cannot improve the
catalytic performance in the selective oxidation of benzyl alcohol
(Table 1, entries 13, 20-21).
5
5
5
1
3[c]
ethanol
(
14
----
-----
13.7
81.3
>99
(
1
5[d]
6[e]
7[e]
----
----
----
ethanol
----
22
60
----
----
----
(
1
Au-Pt/AC
Pd-Pt/AC
~21
~17
~50
~45
1
1
----
[
f]
8
Au/TiO
Au/U
2
3.5
7
----
----
63
100
79
85
----
----
[
g]
19
3
O
8
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ChemSusChem
10.1002/cssc.201700968
FULL PAPER
Table 2. Selective oxidation of various substituted alcohols over 1300BND
catalyst.
18[c]
[
a]
20
10.8
87.1
Con.
%)
Sel.
(%)
Entry
Substrate
Product
t (h)
12
(
1 [
9
d]
1
2
14.9
21.1
77.2
73.2
1
43.3
30.2
98.2
>99
2
0
0
2
0
1
4.2[
g]
62.1
79.0
[g]
3
2[b]
6
6
2
21.7
2
[h]
[h]
37.4
60.2
[
a] Reaction condition: 10 mg catalyst, 2.0 mmol substrate, 2.0 mmol TBHP,
solvent-free, 70 °C. [b] 5 mL TFT as solvent. [c] 5 mL TFT as solvent, 1.0
mmol substrate, 2.0 mmol TBHP. [d] 1.0 mmol substrate, 2.0 mmol
TBHP,solvent-free. [e] Ref 35, 1.5 mmol substrate, 20 mg Mn
organic polymers as catalyst, 10 mL CH CN, 3 mmol TBHP, 80 °C, 8 h. [f] Ref
36, n(substrate): (cat) =50, n(TBHP):(substrate)=2, [Fe(bipy) ](OTf) as
catalyst, 25 °C, 24 h. [g] The reaction conditions are the same as stated in
footnote a, except the catalyst used is 5 wt% Pt/C. [h] The reaction conditions
are the same as stated in footnote a, except the catalyst is 5 wt% Ru/C.
3[b]
20.7
23.7
21.6
18.3
93.1
92.6
89.5
93.5
3 4
O /porous
3
4
12
10
3
2
5[b]
6[b]
7
Since the preliminary results for the selective oxidation of
benzyl alcohol seems positive over 1300BND catalyst, the
substrate scope of the oxidation reaction was further examined.
In Table 2, the results show that 1300BND catalyst was effective
for the alcohols with mono-, di-, multi-substituted, heterocyclic,
linear-chain and unsaturated structures. There were reasonable
conversion and selectivity outputs under the optimized reaction
conditions for all of the primary alcohols studied, especially
under solvent-free conditions. For example, the selective
oxidation of para-substituted alcohols, including 4-methylbenzyl
alcohol, 4-nitrobenzyl alcohol, 4-methoxybenzyl alcohol, 4-
8
8
17.1
72.4
81.8
80.1
40
[
e]
[e]
~
~
70
~75
100[
f]
~66[f]
8[b]
10
13.5
88.8
9[b]
8
8
22.7
17.5
97.2
94.7
fluorobenzyl
biphenylmethanol resulted in
alcohol,
4-chlorobenzyl
alcohol
substrate conversion of
and
4-
a
1
0[b]
17.5%~43.3%, accompanying by excellent selectivities (>89.5%)
towards the desired products (Table 2, entries 1-5, 10).
Moreover, reasonable catalytic activity for the oxidation of
unsaturated alcohols, such as cinnamic alcohol and 4-
cyanobenzyl alcohol, could be achieved (Table 2, entries 7-8). It
is worth to note that the selectivity for cinnamyl aldehyde (81.8%)
is higher than that of the reported transition-metal catalysts.3
Moreover, the conversion of cinnamic alcohol can be further
improved to 72.4% along with a decent selectivity by prolonging
the reaction time (entry 7). Recently, the production of
aldehydes relies on the use of carbocatalysts and metal-based
catalysts that possess selective oxidation of alcohol groups in
the presence of some easily oxidizable functionalities. Here,
1300BND catalyst is also competent in the oxidaiton of di- and
tri- substituted alcohols (Table 2, entries 12-17), those include
methyl (e.g., 3, 4-dimethylbenzyl alcohol)-, methoxy (e.g., 3, 4-
dimethoxybenzyl alcohol, 2, 3, 4-trimethoxybenzyl alcohol)-,
halogenated (e.g., 3-fluoro-4-methoxybenzyl alcohol, 4-fluoro-2-
methylbenzyl alcohol)-, nitro-groups (e.g., 2-chloro-5-nitrobenzyl
alcohol), even in heterocycle-substituted alcohols (Table 2,
entries 6, 18~19, e.g., 3-pyridinecarboxaldehyde, 1-
benzothiophen-2-ylmethanol, cyclohexylmethanol) under solvent
or solvent-free condition. The conversion of the alcohols and the
selectivity for the corresponding aldehydes reached 10.9%~18.3%
and 77.2~99%, respectively. In addition, the oxidation of linear-
chain alcohols (e.g., 5-hexen-1-ol, 1-hexanol) could achieve a
1
1[c]
8
19.1
15.8
98.5
>99
5-36
2[c]
1
12
1
3[d]
1
2
2
12. 2
10.9
90.9
91.3
4[c]
1
1
5[c]
1
2
0
2
17.2
12.8
88.1
84.3
1
6[c]
1
1
7[d]
20
14.9
89.2
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ChemSusChem
10.1002/cssc.201700968
FULL PAPER
comparable selectivity to 5 wt% Pt/C and 5 wt% Ru/C (Table 2,
entries 20-21) under solvent-free condition. It is clear that
Previous studies in metal-free catalysis have shown that the
surface functional groups (e.g., carbonyl groups) and defects in
nanocarbons play important roles in promoting catalytic
reactions and thus are considered to be possible active sites for
the particular reactions.37-40 In order to study the origin of the
catalytic performance, the surface properties of the catalysts
were characterized by XPS, FTIR and Raman spectroscopy.
Figure 2a is the XPS O 1s spectra of ND, BND andOLC
materials, deconvoluted into three peaks corresponding to
unsaturated carbonyl group (C=O, ~531.8 eV), carbon-oxygen
ester-like single group or anhydride (C-O, ~532.9 eV) and
phenolic group (C-OH, ~534.0 eV). The total concentration of
oxygen decreases from 9.8 at% of ND to 0.6 at% of 1500OLC.
There is only little phenolic group found in 1500OLC. The FTIR
spectra confirm the results from XPS and reveal the same
1300BND as an efficient carbocatalyst could deliver an excellent
performance in the selective oxidation of various alcohols to
their corresponding aldehydes.
Figure 1. Catalytic stability of 1300BND for the selective oxidation of a) benzyl
alcohol under solvent condition and b) cinnamic alcohol under solvent-free
condition, 8 h.
-1
surface groups over ND, such as C-O (1124~1405 cm ), C=C
-1
-1
-1
(
~1635 cm ), C=O (~1780 cm ) and C-H (~2985 cm ), as
shown in Figure 2b. The intensity weakening of various carbon-
related signal peaks over BND should be attributed to its the
decrease of the content of the surface groups. No obvious peaks
could be observed for 1500OLC. On the aspect of structure-
activity relationship, it may be found that 1300BND, which
exhibited the best catalytic acitivity, has a low oxygen content
The catalytic stability of catalyst was studied as well. The
conversions of representative benzyl alcohol and cinnamic
alcohol over 1300BND are 12.4% and 16.9 % along with the
corresponding aldehydes selectivity of 96.5% and 80.0%,
respectively, even after six successive runs (Figure 1). These
indicated that 1300BND exhibits good stability upon recycling.
The carbon balances of each run were verified by taking into
account the unreacted alcohols, the produced aldehydes, the
unreacted oxidant and the main product from the decomposition
of oxidant. The carbon balance was calculated to be ~ 95 % of
benzyl and cinnamic alcohol reactions and the small loss may
be ascribed to the production of open-ring aromatic compounds
and carbon oxides deriving from over-oxidation of the aldehydes.
(
only 1.3 at%). On the contrary, ND, which carries the highest
concentration of surface oxygen, showed a poor conversion of
substrate (Table 1, entry 2). The correlations within different
kinds of oxygen species (C=O, C-O and C-OH) and catalytic
performance were studied in detailed in Figure S2, and the
results suggest that the content of surface oxygen species was
not proportional to the mass-normalized activity. In other words,
there may be no direct relationship between surface oxygen
groups and catalytic performance. To verify this hypothesis,
1600OLC, which has a similar content of oxygen species (0.54
at%, see Figure S3a) to 1500OLC (0.61 at%), was used as a
reference, and the corresponding catalytic activity reflected that
the 1600OLC exhibited a lower conversion of substrate than that
of 1500OLC (Table 1, entries 6-7). All the results suggest that
the surface oxygen groups may be inconsequential to the
selective oxidation of alcohols.
Raman spectroscopy is a powerful tool to study surface defect
of the samples. The intensity of the D-band was allowed as
probes for the formation of
a disordered graphitic lattice
4
1
structure (graphene layer edges). As shown in Figure 2c, the
D-bands (~1325 cm-1) of BND and OLC significantly enhance
with increasing the synthesis temperature from 900 °C to
1
500 °C, which are attributed to the phase transformation of
sp /sp hybridized BND to sp2 hybridized OLC.42 Besides,
2
3
comparing with 900 BND and 1100 BND, the appearance of 2D-
-1
band (~2675 cm ) over 1300BND and 1500OLC suggests that
their surface carbon structures should consist of stacked
graphene layers. It can be found that 1300BND, with a better
catalytic acitivity, shows a stronger D-band than that of 900 BND
and 1100 BND. In this case, it seems that the concentration of
graphitic edges can affect the catalytic performance. The value
of the ID/IG ratio was further used as a quantitative measure of
Figure 2. (a) O 1s XPS spectra, (b) FTIR spectra and (c) Raman spectra
of ND, BND and OLC samples. The O1s XPS spectra are divided by
fitting the peak maximum within ± 0.1 eV and applying a full width at half-
maximum (FWHM) of 1.4~1.6 eV. The value of the mixed Gaussian-
Lorentzian is maintained at 40%.
the structural defects. Compared with 1500OLC (I
D G
/I =1.12,
calculated by the intensity ratio), 1300BND (I /I =1.31) also
D
G
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ChemSusChem
10.1002/cssc.201700968
FULL PAPER
exhibited better catalytic activity. It is worth noting that when
directly used to simulate the structures of carbon catalysts and
to identify specific active sites, such as zigzag and armchair
configurations. Here we use several small molecules with
defined structures as model catalysts to further investigate
possible active defect sites and the corresponding activation
capacities for oxidant. As mentioned, the roles of oxygen groups
in the reaction have been excluded based on XPS and IR
techniques. As shown in Table 3, model catalysts with C=O, C–
OH and C–O groups (entries 2-3 and 10) do not exhibit a
prominent catalytic activity as compared to the blank experiment.
This result well supported the non-participation of the surface
oxygen groups for selective oxidation of benzyl alcohol. We also
applied a few model catalysts with specific armchair and zigzag
configurations to identify the types of active defect sites. Two
model catalysts with an armchair structure did not exhibit an
improved catalytic activity (Table 3, entries 4-5); the selectivity
towards benzaldehyde was even lower than that of the blank
experiment. In contrast, the conversion of benzyl alcohol
D G
1600OLC, having a smaller I /I value (1.05, Figure S3b), was
introduced into this reaction, the corresponding conversion of
benzyl alcohol (9.3%, Table 1, entry 7) is lower than that of
1
1
500OLC (11.4%, Table 1, entry 6). The comparative results of
300BND, 1500OLC and 1600OLC provided an evidence for the
important roles of structural defects in the selective oxidation of
benzyl alcohol.
Table 3. Catalytic performance of benzyl alcohol over various model molecule
catalysts.[a]
Decomp
osition
Ent
ry
Mimicked
structure
Con.
(%)
Sel.
(%)
Yield
(%)
Model catalyst
Blank
efficiency
%) of
(
TBHP
1[b]
----
2.5
2.9
90.3
84.7
2.3
2.5
1.8
2
phenol
2.6
4.0
(
(
3.4%~8.1%) and the selectivity towards benzaldehyde
89.8%~94.5%) were remarkably improved with increasing of the
3
quinones
3.9
84.5
3.3
zigzag unit of the model catalysts (Table 3, entries 6-9). All
these facts indicate that zigzag configuration should have an
important role in facilitating the selective oxidation of benzyl
alcohol. In addition, there was no conspicuous synergistic effect
between the zigzag and oxygen species (C–O, C=O) since the
observed yields were only 3.3 % and 4.5 % on xanthenes and
pentacenequinone catalysts (entries 10-11), respectively. The
decomposition efficiency of TBHP over different model catalysts
was evaluated in detail. As displayed in Table 3, the model
4
5
armchair
armchair
3.5
3.8
83.6
86.9
2.9
3.3
2.4
2.9
6
7
8
9
zigzag
zigzag
zigzag
zigzag
3.4
4.8
6.3
8.6
4.1
91.4
89.8
92.0
94.5
81.1
3.1
4.3
5.9
8.1
3.3
6.9
17.7
28.2
41.4
3.7
catalysts
with
zigzag
configuration
exhibited
better
decomposition efficiencies of TBHP (6.9~41.4%) than other
catalysts, which are directly proportional to their catalytic
performances (entries 6-9). The same relationship can be found
in used nanocarbon catalysts (Table 1, entries 2-9). These
observations provided evidence to the fact that the zigzag
configuration has an excellent activation capacity for TBHP and
thus might be active site in the selective oxidation of alcohols.
zigzag
ether
1
0
1
zigzag
quinones
1
5.1
89.2
4.5
18.8
Redox potential is an important descriptor in electrochemistry
that reflects the extra potential energy required to initiate or
activate reactant species, i.e., the oxidant or substrate. Catalytic
systems with a smaller onset potential is more likely to proceed
under conditions with less energy input. Cyclic voltammetry (CV)
was firstly performed to measure the ability to activate the
substrate (BA) and oxidant (TBHP) by the catalyst (Cat,
anthracene was used as a representative catalyst) under an ex-
situ reaction condition.Figure 3a shows the voltammograms of
the system components and the oxidation onset potentials are
determined. It is clear that the oxidation onset potentials for the
BA and TBHP are above 1.3 V (vs. Ag/Ag+). When TBHP and
BA were together introduced to the system, the slight
enhancement in the current and reduction in onset potential are
indications of their interaction during the electrochemical
oxidation. Such interaction adequately explains for the 2.5% BA
conversion and the TBHP decomposition efficiency of 1.8% in a
blank experiment. Interestingly, as the catalyst was added,
[
a] Reaction conditions: 1 mmol model catalyst, 2 mmol substrate, 2
mmol TBHP, 5 mL TFT, T= 70 °C, 4 h.
[
b] Conducted in the absence of catalyst. Desired product: benzaldehyde.
Defects are abundant in carbon materials and have been
considered as an essential constituent of carbon network
structure with abundant chemical information. Although possible
roles of defects in metal-free catalysis have been studied by
using Raman measurements and theoretical calculations over
the past few years,1
9, 40, 43-44
there is still a lack of an effective
method to identity the type of defects and to give an accurate
description of the amounts of each type, not even to mention the
total number of defects per unit area. As such, revealing the
structure-activity relationship of defects in a reaction process is
still a great challenge. As compared to nanocarbon materials,
aromatic organic molecules (e.g., anthracene, phenanthrene)
have a simliar carbon network structure and a tunable electronic
conjugated π system (by extending the carbon unit) that can be
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ChemSusChem
10.1002/cssc.201700968
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species and armchair configuration are higher/lower than that of
anthracene (Figure 3c), respectively. This can be attributed to (1)
zigzag edge may be more reactive than the armchair edge due
to its unique electronic structure and unpaired electrons;4
5-46
(2)
the incorporation of oxygen to edge structure may deteriorate
the π-conjugated system of graphitic lattice and thus weaken the
activation ability for the oxidant. The electrochemical results
further excluded the roles of oxygen species and armchair edge
structure in the catalytic reaction. Moreover, there is a directly
proportional relationship between the yield and the oxidation
current density (Figure 3d), suggesting a good correlation
between electrochemical and thermal reactions. It is therefore
reasonable to expect that the zigzag edges are the active
structure in the selective oxidation of benzyl alcohol, thanks to
the excellent decomposition efficiency for oxidant and the
activation ability for substrate thereby lower the onset oxidation
potential and enhance the higher oxidation current relative to
other species.
Figure 3. Ex-situ cyclic voltammograms (100 mV/s) of benzyl alcohol
oxidation in 0.045 M TBAPF /TFT electrolyte under (a) systems with
6
different components, (b) systems using model catalysts with different
zigzag structure and (c) systems using model catalysts with varying
oxygen species and armchair structures. (d) The dependence of
apparent current density (at 1.8 V) on the catalytic activity over different
model catalysts with zigzag structure for the selective oxidation of benzyl
alcohol; Cat used in (a) is anthracene and BA denotes benzyl alcohol.
The test conditions: 0.02 mmol model catalysts (Cat), 2 mmol benzyl
6
alcohol (BA), 2 mmol TBHP, 5 mL TBAPF /TFT electrolyte, 25 °C.
the onset potential decreased from c.a. 1.45 V (BA) to 1.25 V
Cat+BA) for the BA system, while that for TBHP system
(
decreased from c.a. 1.35 V (TBHP) to 1.3 V (Cat+TBHP). At the
same time, the corresponding current densities for both systems
showed significant improvement with the catalyst as compared
to that when without. The higher oxidation current and the lower
oxidation onset potential of Cat+BA system to BA imply that
catalyst may activate the hydrogen atom of –CH2OH group of
BA. Similarly, the obvious differences of Cat+TBHP system to
TBHP on current and oxidation potential suggest that edge
structure could facilitate the decomposition of TBHP into tBuOO•,
tBuO• and •OH radicals under reaction conditions. It is worth
noting that the current enhancement in Cat+BA/BA is more
prominent to that in Cat+TBHP/TBHP. This suggests that BA is
more readily activated on the catalyst and thus being more
vulnerable to oxidation. The enhancements may be ascribed to
the edge structures of the catalyst and the interaction with the
reactants. These observations demonstrate that anthracene as a
model catalyst can effectively activate substrate and oxidant in
the selective oxidation of benzyl alcohol. This is consistent with
the experimental results of the improved decomposition
efficiency of TBHP and the catalytic activity over anthracene. It
should be noted that other model catalysts with elongated
zigzag edges exhibited further enhancement in terms of the
reduced onset potential and increased Faradaic current density
Figure 4. Ex-situ attenuated total reflectance (ATR) spectra of
anthracene for the activation process of benzyl alcohol. The spectra
were normalized by subtracting the signals of TFT. The test conditions:
0
.02 mmol model catalyst (Cat, red line), 2 mmol benzyl alcohol (BA,
black line), 5 mL TFT , 70 °C, 4 h.
As mentioned above, zigzag configuration may activate the
hydrogen atom of –CH OH group of BA. Here, we performed ex-
2
(
Figure 3b). For instance, the onset potential for the system with
situ ATR experiments to monitor the chemical state at the
working surface. As shown in Figure 4a, compared with pure BA,
the introduction of Cat does not change the bands position of -
pentacene is close to 0.8 V, which is far less than that of
anthracene (1.1 V).
Besides,
the
corresponding
onset
oxidation
-1 47
2 2
CH stretching vibration of -CH OH group (2800~2900 cm ).
potentials/currents for model catalysts with different oxygen
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On the contrary, the blue-shift (from 3312 cm-1 of BA to 3317 cm-
of Cat+BA) of -OH band indicates that Cat can weaken the
elucidated by the model catalysts, the ex-situ electrochemical
and the ex-situ attenuated total reflectance studies. The best
oxidation onset potential of model catalysts with a zigzag
structure is as low as 0.8 V (i.e., a rather low extra potential for
activation reaction). It was revealed that the surface oxygen
groups do not carry a key role in the reaction. This work is
anticipated to form a basis for the insight into the roles of defects
and oxygen species in carbon materials in liquid-phase catalytic
oxidation reactions.
1
interaction between O and H atoms, that is, -OH may be
activated. The similar blue-shift (from 3047 cm-1 of pure Cat to
3
051 cm-1 of Cat+BA, see the inset of Figure 4a) of -CH stretch
of benzene ring (Ar-H) shows that there is a weak interaction
between -OH and Cat. As displayed in Figure 4b, the peaks
located at about 1008 cm-1 and 1450, 1533, 1620 cm are
assigned to the stretching vibration band of C-O and the C=C
vibration bands of benzene ring, respectively. Compared with
pure BA and pure Cat, both the blue-shifts of C-O peak (shifts
-1
-
1
-1
from 1008 cm to 1011cm of Cat+BA, see the inset of Figure
4
Experimental Section
-1
b) and C=C peak (at 1450 cm ) of Cat+BA suggest that the
weak interaction may be Ar•••H-O-CH -R, that is, Cat can
activate the hydrogen atom of -OH group rather than -CH group.
2
Experimental Materials
2
The •OH radicals derived from decomposition of TBHP over
zigzag edge defect may capture the activated hydrogen atom on
Benzyl alcohol (99%+), anthraquinone (99%), phenanthraquinone (99%),
trifluorotoluene (TFT, 99%), tert-butyl hydroperoxide (TBHP, ~65 wt% in
water), phenanthrene (99%), anthrone (97%), anthracene (99%),
dimethyl carbonate (DMC, 99%) and 5% Pt/C were purchased from Alfa.
Picene (purified by sublimation, 99.5%), xanthene (98%), naphthacene
2 2
the -OH of the -CH OH, along with the release of H O and the
formation of a -CH O group (Figure 5). On the other hand, it has
2
been reported that the tBuOO•, tBuO• radicals from the
(
97%) and fluorene (97%) were supplied by TCI. Pentacene (98%), 6,13-
pentacenequinone (99%), tetrabutylammonium hexafluorophosphate and
% Ru/C were supplied by Acros. 9-phenanthrenol was obtained from
decomposition of TBHP could extract an hydrogen atom derived
from the C-H bond of the -CH
leading to the formation of aldehydes (-CHO group), tert butyl
2
O group,48-49 and subsequently
5
Sigma-Aldrich. Graphite was provided by ACHESON Co. Ltd. Acetonitrile,
tetrahydrofuran (THF), dimethyl formamide (DMF), dioxane, hexane and
dichloromethane (DCM) were all from Sinopharm Chemical Reagent Co.,
Ltd. All reagents were used without further purification. Purified ultra-
dispersed nanodiamond (ND) was bought from Beijing Grish Hitech Co.
t
t
t
alcohol ( BuOH) and BuOOH. The produced BuOOH can be
reused in the reaction. The detailed process can be found in
Figure 5.
(China), produced by detonation and purified by acid washing; the
average particle size is ~5 nm. The noncombustible contaminations were
tested by an inductively coupled plasma optical emission spectrometer
(ICP-OES) and included Fe < 50 ppm, Cr < 10 ppm, Al < 50 ppm, Cu <
10 ppm, Mg < 10 ppm, Ti < 10 ppm, and Ca <50 ppm. Graphene (99%)
was supplied by Tanmei Ltd, Co. (China).
Samples preparation
900BND, 1100BND and 1300BND with graphite-like shell structures were
produced by annealing purified ND at 900, 1100 and 1300 °C for 4 h in
argon atmosphere, respectively. Similarly, 1500OLC and 1600OLC can
be obtained by annealing ND at 1500 and 1600 °C for 30 min in an Ar
atmosphere, respectively.
Characterization
Figure 5. The possible mechanism diagram of catalytic reaction for
selective oxidation of alcohols on BND (graphite planes with zigzag
configuration is used as an ideal catalyst surface).
High-resolution transmission electron microscopy (HRTEM) was
conducted on a FEI Tecnai G2 F20 microscope. The X-ray photoelectron
spectroscopy (XPS) was performed on an ESCALAB 250 XPS system
with
2
a monochromatized Al K-alpha X-ray source. N adsorption-
Conclusions
desorption was carried out on the Micromeritics ASAP2020 setup and
analyzed applying Brunauer-Emmett-Teller (BET) theory. Raman spectra
In summary, an effective catalytic reaction pathway for the
2
of samples on SiO /Si were collected with a LabRam HR800
spectrometer and a He/Ne laser at 532 nm (50×objective) was selected
as the excitation source. Moreover, the laser power and exposure time
were maintained low to avoid local damage of samples by the heating. All
selective
oxidation
of
alcohols
with
various
functionalities and structures has been investigated with bucky
nanodiamond (BND) as catalyst. The excellent selectivity for
desired products (>73.2%) can be achieved at a mild reaction
condition, even in a solvent-free condition. It was found that the
edge defects with a zigzag configuration play an important role
in the catalytic process, based on the excellent decomposition
capability for TBHP and the effective activation for substrate, as
electrochemical
measurements
were
operated
by
Epsilon
Electrochemical workstation (PAR2273) at 25 °C. The measurements
were performed with a three-electrode system using polished glassy
carbon electrodes as both the counter electrode and working electrode,
and a non-aqueous Ag/Ag+ reference electrode stabilized in a 0.045 M
6
tetrabutylammonium hexafluorophosphate (TBAPF )/trifluorotoluene
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ChemSusChem
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(
TFT). The whole electrochemical measurement system was almost
similar to the alcohol oxidation reaction conditions except reaction
temperature. TBAPF was used as a conductive additive. Therefore, the
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Entry for the Table of Contents (Please choose one layout)
FULL PAPER
Yangming Lin,[a,b,c] Kuang-Hsu (Tim)
Wu, [a] Linhui Yu, [d] Saskia Heumann,[c]
and Dang Sheng Su [a, e] *
Page No. – Page No.
Efficient and Highly Selective
Solvent-Free Oxidation of Primary
Alcohols to Aldehydes Using Bucky
Nanodiamond
An effective catalytic reaction pathway for the selective oxidation of primary
alcohols with various functionalities and structures has been developed with bucky
nanodiamond as catalyst. The zigzag edges of sp carbon planes have been shown
2
to play a crucial role in these reactions.
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