Aerobic alcohol oxidation catalyzed by CuO-rectorite/TEMPO in water

Article abstract of DOI:10.1007/s11164-018-3618-3

An environmentally benign CuO-rectorite was prepared by calcining the co-precipitation product of Cu²⁺ with an acid-activated rectorite at pH 6. It could be well dispersed in water and used as a co-catalyst with TEMPO to selectively aerobically oxidize alcohols to aldehydes. The conversion of most primary benzylic alcohols was very high under optimized conditions. Interestingly, the oxidation protocol also allowed highly selective transformation of benzylic alcohols containing a phenolic hydroxyl group into the corresponding aldehydes. Additionally, very little copper was leached from the catalyst during the catalytic reaction. CuO-rectorite could be reused by centrifugation and washed with acetonitrile and water. At the same time, its catalytic activity remained high.

Full text of DOI:10.1007/s11164-018-3618-3

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-018-3618-3
Aerobic alcohol oxidation catalyzed by CuO‑rectorite/
TEMPO in water
Wanghui Liu · Jianhong Yang1 · Jun Cai2
1
Received: 2 June 2018 / Accepted: 28 September 2018
©
Springer Nature B.V. 2018
Abstract
An environmentally benign CuO-rectorite was prepared by calcining the co-precip-
2
+
itation product of Cu with an acid-activated rectorite at pH 6. It could be well
dispersed in water and used as a co-catalyst with TEMPO to selectively aerobically
oxidize alcohols to aldehydes. The conversion of most primary benzylic alcohols
was very high under optimized conditions. Interestingly, the oxidation protocol also
allowed highly selective transformation of benzylic alcohols containing a phenolic
hydroxyl group into the corresponding aldehydes. Additionally, very little copper
was leached from the catalyst during the catalytic reaction. CuO-rectorite could be
reused by centrifugation and washed with acetonitrile and water. At the same time,
its catalytic activity remained high.
Keywords CuO-rectorite · TEMPO · Aldehyde · Selective transformation
Introduction
Carbonyl compounds are important raw materials and intermediates of ꢀne chemi-
cal and pharmaceutical synthesis, which are mainly used in the perfume, dye and
7
pharmaceutical industries [1–3]. Their annual output in the world is over 10 tons.
These compounds are mainly derived from alcohol oxidation which is tradition-
ally performed by using stoichiometric Cr(VI) [4], Mn(VII) [5] or V(V) [6] com-
pounds as oxidants. Although these conventional processes have high conversion
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4
-018-3618-3) contains supplementary material, which is available to authorized users.
*
Jianhong Yang
yangshenyang3@163.com
1
2
School of Environmental and Safety Engineering, Changzhou University, Changzhou 213164,
China
Key Laboratory of Fermentation Engineering (Ministry of Education), Hubei University
of Technology, Wuhan 430068, China
Vol.:(012
1
3456789)
3

W. Liu et al.
eꢁciency, there are considerable drawbacks such as the toxicity of the oxidants and
some by-products generated due to over-oxidation. It is desirable to replace them
by environmentally friendly oxidants. In 1984, Semmelhack and co-workers devel-
oped a process for the oxidization of alcohols to aldehydes with CuCl/TEMPO
(
2,2,6,6-tetramethyl-1-piperidine-N-oxyl) as a co-catalyst and O as an oxidant in
2
DMF [7]. So far, the copper/TEMPO catalytic system has been widely studied and
proven to be an economical and eꢂective method for aerobic oxidation of various
alcohols to aldehydes [8–16]. However, most research was involved in homogene-
ous catalytic processes, and soluble metal complexes in solvents were generally used
as the catalysts. An inevitable problem is that it is diꢁcult to separate the catalysts
from the reaction mixture for reuse. In addition, the reactions are often performed in
organic solvents. These will cause economic and environmental concerns in indus-
trial applications.
In recent years, some heterogeneous catalysts, such as Cu–Mn oxides [17],
Cu(II)-based metal–organic framework [18, 19], Cu@C [20], mesoporous silica-
supported mononuclear copper(II) complex [21], MCM-41-immobilized bipyridine
copper(I) complex [22] and silica-functionalized Cu(II) [23], had been prepared and
used as co-catalysts with TEMPO for aerobic alcohol oxidation. A nitroxyl radical-
modiꢀed polymer, which integrated the functions of copper compound and TEMPO,
had also been synthesized by Li and his coworkers [24]. These catalysts have high
catalytic eꢁciency for the aerobic oxidation of alcohols in organic solvents. After
the reactions are completed, they can be easily separated from the mixtures and
reused. However, the synthesis processes of these catalysts are very complex, lead-
ing to their high costs. Thus, the development of a cheap and eꢁcient heterogeneous
catalyst for the aerobic oxidation of alcohols still attracts considerable attention.
In 2001, potassium/proton-containing octahedral molecular sieves (K-OMS-2 and
H-K-OMS-2) were used for alcohol oxidation [25]. Clay minerals have structural
features similar to K-OMS-2 and H-K-OMS-2, and they are widely found in nature
and are low in price. They are nanoparticles with interlayer structures. Their inter-
+
+
2+
+
lamellar spaces are occupied by cations such as Na , K and Ca . These interla-
3+
2+
mellar cations can be easily replaced by other cations such as H , Fe and Cu
through very simple procedures. Some clay minerals and their cation-modiꢀed prod-
ucts have successfully been applied in ꢀne chemistry. Acid-treated clay minerals, as
solid acids, showed high selectivity for the acylation of thiophene with p-ꢃuoroben-
3
+
zoyl chloride [26]. Some heavy metal ion-modiꢀed products, such as Fe /montmo-
rillonite-K10, have been used as heterogeneous oxidation catalysts in organic reac-
tions [27, 28]. But these heterogeneous metal ion-modiꢀed catalysts prepared by ion
exchange will be also subject to metal ion leaching during the catalytic reactions,
which will aꢂect their reuse and cause environmental pollution. Furthermore, there
2
+
is no report about the catalytic aerobic alcohol oxidation using Cu -clay mineral/
TEMPO.
CuO is a stable, cheap and readily available metal oxide. It has been used as a
co-catalyst with TEMPO to aerobically oxidize benzyl alcohol to benzaldehyde in
dichloromethane [29]. However, in case of a large amount of CuO and TEMPO, its
catalytic eꢁciency was not high even at 120 °C. In addition, the use of the dichlo-
romethane also brought about some environmental concerns. In this paper, we
1
3

Aerobic alcohol oxidation catalyzed by CuO-rectorite/TEMPO…
prepared an environmentally benign CuO-rectorite (CuO-REC). It could be used as
a co-catalyst with TEMPO to selectively aerobically oxidize alcohols to aldehydes
in water at 50 °C. In this system, the heterogeneous catalyst can be easily separated
and recovered by centrifugation.
Experimental
Materials and method
Calcium-rectorite (Ca-REC) was obtained from Hubei Zhongxiang Mingliu Rec-
torite Inc., Co. (China). Other chemicals and solvents were purchased from Sigma-
Aldrich, Macklin and Aladdin without further puriꢀcation. X-ray diꢂraction (XRD)
patterns were recorded on a Rigaku D/max-2500PC diꢂractometer (Japan) at 40 kV
and 100 mA for a Cu-target tube (λ=1.5406 Å). Scanning electron microscopy
(
SEM) images were obtained using a Zeiss SUPRA55 scanning electron micro-
scope equipped with QUANTAX energy-Dispersive x-ray spectrometer (Ger-
many). Speciꢀc surface area (S ) and total pore volume (V ) were determined at
BET
total
−
195.85 °C using a Micromeritics ASAP 2020 instrument (USA). Before analysis,
the samples were dried for 12 h in a 120-°C oven. X-ray photoelectron spectroscopy
XPS) analysis was performed on the ESCALAB 250XI spectrometer (Thermo,
(
1
13
USA) equipped with an Al Kα radiation source at 12.5 kV and 16 mA. H and
NMR spectra were determined on an AVANCE III 500 MHz NMR spectrometer
Bruker, Germany) in CDCl as solvent with TMS as an internal standard.
C
(
3
Preparation of acid treated rectorite
−
1
Ca-REC (10 g) and hydrochloric acid (150 mL, 1.2 mol L ) were added into a
round ꢃask (250 mL). The suspension was stirred at 90 °C for 8 h and ꢀltered after
cooling. The REC was repeatedly washed with deionized water until obtaining free
−
Cl and dried at 90 °C overnight. The acid-treated REC was named H-REC.
Preparation of CuO‑rectorite
−
1
H-REC (1 g) was dispersed into 150 mL of Cu(NO ) solution (0.01 mol L ). After
3
2
it was stirred for 1 h at 30 °C, pH was adjusted to 6 and maintained by slowly adding
−1
0
.1 mol L of NaOH aqueous solution. The suspension was further stirred for 23 h
and centrifuged. The precipitate was washed with deionized water three times and
then dried at 90 °C overnight. Finally, it was calcined in a pre-heated muꢄe furnace
at 500 °C for 5 h. The product was recorded as CuO-REC. Its copper content was
determined by ꢃame atomic absorption spectrophotometry (FAAS) using an AA300
ꢃame atomic absorption spectrophotometer (Jena, Germany) (See ESM †).
1
3

W. Liu et al.
Catalytic reaction
In a typical experiment, 2.0 mL of solvent, 31 mg of CuO-REC, 0.1 mmol of
TEMPO, 0.1 mmol of K CO and 2.0 mmol of alcohol were mixed in a ꢃask with
2
3
a magnetic stirrer. The mixture was stirred at 50 °C for 24 h in oxygen atmosphere
and then extracted with ethyl acetate three times (10 mL×3). The combined organic
layer was concentrated and applied to a silica gel (200–300 mesh) column to sepa-
rate the product. In addition, during the reaction, the conversion was monitored via
high-performance liquid chromatography (HPLC) with an InertSustain C18 column
(
See ESM Fig. S1–S21†).
Results and discussion
Catalyst characterization
XRD patterns of Ca-REC, H-REC and CuO-REC are shown in Fig. 1. The Ca-
REC exhibited an intense reꢃection at 2θ=3.521° (Fig. 1a) and its basal spacing
(
d
) was 2.507 nm based on the Bragg’s equation. It is necessary for Ca-REC to be
001
activated with acid. When Ca-REC was activated in HCl aqueous solution, it fully
swelled and formed a slurry-like mass. After acid activation, it was also found that
the 001 reꢃection of H-REC shifted to 2θ=3.820° (Fig. 1b), its intensity was sig-
niꢀcantly lower than that of Ca-REC (Fig. 1a) and its basal spacing was 2.311 nm.
+
This conꢀrmed that H had been intercalated into the interlayer of Ca-REC [30–35].
For CuO-REC prepared from H-REC, its 001 reꢃection shifted to 2θ=4.300° and
was very weak (Fig. 1c). Its basal spacing was 2.048 nm, indicating that partial
Fig. 1 XRD patterns of Ca-REC (a), H-REC (b) and CuO-REC (c)
1
3

Aerobic alcohol oxidation catalyzed by CuO-rectorite/TEMPO…
2+
Cu had inserted into the interlayer of REC [32–34]. In addition, compared with
the Ca-REC and H-REC, several new weak reꢃections at 2θ=35.440°, 38.740° and
4
8.830° appeared in the XRD pattern of the CuO-REC, which were the character-
istic reꢃections of CuO [36]. This proved the presence of CuO which resulted from
calcination of Cu(OH) deposited on the REC. Based on FAAS results, the copper
2
content of CuO-REC was 77.8 mg/g. Furthermore, the states of the Cu species in the
CuO-REC were also further characterized by XPS. The results are shown in Fig. 2,
indicating the peaks of CuO such as Cu 2p peak at 935.24 eV, Cu 2p peak at
3
/2
1/2
9
55.04 eV and two satellite peaks at 942.43 eV and 962.68 eV. These indicate the
existence of CuO in the material [37]. The Cu 2p peak was deconvoluted into two
3
/2
peaks at 935.24 eV and 936.11 eV, suggesting that two chemical environments of
2
+
2+
Cu existed. The former resulted from CuO. The latter was due to Cu ions con-
nected to the –OH groups on the surface of CuO particles and from the clay min-
eral layers [38]. Based on the intensities of two deconvoluted peaks, the content of
2+
Cu cations connected with –OH groups was about 36 mg/g. In addition, the lack
of peaks at 932.0–932.7 eV indicated the absence of Cu O [39, 40].
2
The SEM images of Ca-REC, H-REC, CuO and CuO-REC samples are shown
in Fig. 3. The SEM image of Ca-REC shows a typical layered structure of platy
aggregates of compact parallel layers (Fig. 3a). After Ca-REC was modiꢀed in HCl
aqueous solution, then treated by Cu(NO ) at pH 6 and calcined at 500 °C, H-REC
3
2
and CuO-REC still maintained the layered structure (Fig. 3b, e). Furthermore, pure
CuO particles were linked together and looked like a string of beads (Fig. 3c, d). In
the SEM image of CuO-REC, it can be observed that some CuO particles were suc-
cessfully deposited on the surface of REC, but unlike pure CuO particles connected
together, they were separate (Fig. 3f). The EDS analysis in the diꢂerent positions
of CuO-REC was also conducted and the results are shown in Fig. 4. It was found
that the average content of Cu element in the surface of CuO-REC was 11.06 wt%.
The values in three diꢂerent positions (Fig. 4a, position 1, 2 and 3) were 15.58 wt%,
1
3.30 wt% and 11.85 wt%, respectively, indicating that copper mainly existed on
the surface of REC and its distribution was uneven. The elemental mapping analy-
sis by EDS of CuO-REC also showed copper was distributed over the whole REC
Fig. 2 Cu 2p XPS spectra of CuO-REC
1
3

W. Liu et al.
Fig. 3 SEM images of Ca-REC (a), H-REC (b), CuO (c, d) and CuO-REC (e, f)
Fig. 4 Quantitative analysis (a) and element mapping (b) of Cu by SEM–EDS of CuO-REC
surface (Fig. 4b), suggesting some smaller CuO particles could existed on REC sur-
2
+
faces or some Cu ions connected to the –OH groups also existed as mentioned
previously. In addition, the speciꢀc surface areas of H-REC, CuO-REC and CuO
were also determined (Table 1). Their speciꢀc surface areas were 27.9, 27.3 and
2
2
3.1 m /g, respectively. The speciꢀc surface area of CuO-REC was consistent with
that of H-REC, indicating Cu was well dispersed on the surface of REC. In compari-
son with CuO particles, CuO-REC also had bigger speciꢀc surface area, suggesting
that it could show better catalytic activity for aerobic alcohol oxidation.
1
3

Aerobic alcohol oxidation catalyzed by CuO-rectorite/TEMPO…
Table 1 Speciꢀc surface area
SBET (m2 g−1)
3
−1 a
Sample
Vtotal (cm g
)
and total pore volume of the
catalysts
H-REC
CuO-REC
CuO
27.9
27.3
23.1
0.029
0.038
0.031
a
Single-point adsorption total pore volume of pores (less than
0.3122 nm diameter at P/Po=0.95)
4
Catalytic performance
The catalytic oxidation of alcohols to aldehydes mediated by CuO-REC was evalu-
ated. Benzyl alcohol (2 mmol) was chosen as a model substrate and some reaction
conditions such as solvent, temperature, the amount of K2CO3 and CuO-REC were
optimized (Table 2). All experiments were carried out in 2 ml of solvent under a
dioxygen atmosphere. As shown in Table 2, the aerobic oxidation of benzyl alcohol
did not occur in the absence of any CuO-REC or TEMPO catalyst (Table 2, entries
1
and 2). But CuO-REC/TEMPO can catalytically oxidize benzyl alcohol to ben-
zaldehyde using O2 as an oxidant, which conꢀrmed that CuO-REC in combination
with TEMPO as a co-catalyst was very necessary. As to the solvent, diꢂerent from
the reports in some literatures [10, 14, 16, 18, 19], CH3CN wasn’t a good solvent for
this reaction. Compared with CH3CN, H2O as solvent could cause a higher conver-
sion of benzyl alcohol (Table 2, entries 3 and 4). When CH3CN/H2O was used as
the solvent, the catalytic eꢁciency of CuO-REC/TEMPO was also higher than that
with CH3CN as the solvent (Table 2, entry 5). This was because CuO-REC with the
layered structure is strongly hydrated in the presence of water, which makes it more
easily disperse to form stable suspension in water than in CH3CN (Fig. 5a, c and e).
Therefore, water as a solvent was conducive to enhancing the catalytic activity of
CuO-REC.
The base is also essential for copper/TEMPO catalyzed aerobic oxidation of alco-
hol [8, 13, 19, 21, 22, 41]. The experiments have been conducted in the presence
of K CO (Table 2, entries 6–11). When water was used as the solvent, addition
2
3
of K CO markedly promoted the reaction eꢁciency. But its optimum amount was
2
3
0
.1 mmol (concentration, ~0.05 M; alcohol/base, 20:1, mol/mol), which was much
lower than that reported in the literature (concentration, 0.2–0.5 M; alcohol/base,
:1, mol/mol) [19, 22, 42]. In addition, when the reaction was carried out without
K CO , the aggregation of CuO-REC was very serious in water (Fig. 5g). However,
1
2
3
addition of K CO solved the problem (Fig. 5b, h). The addition of K CO caused
2
3
2
3
CuO-REC to carry more negative charges on its surfaces, which made it more sta-
ble in water and reduce the amount of adsorption of benzyl alcohol [43, 44]. When
the reaction was carried out in CH CN, addition of K CO decreased the conver-
3
2
3
sion (Table 2, entries 4 and 10). This was due to its insolubility in CH CN (Fig. 5f).
3
What is more, K CO also inhibited the conversion of benzyl alcohol in CH CN/
2
3
3
H O (Table 2, entries 5 and 11), which was related to the poor dispersion of CuO-
2
REC (Fig. 5d). These further indicated that water was a good solvent for the CuO-
REC/TEMPO co-catalyst.
1
3

W. Liu et al.
Table 2 The optimization of reaction condition of CuO-REC-catalyzed alcohol oxidation
Entry CuO-
REC
K CO
Solvent
T (°C) Time (h) Conversion
Selectivity
(%)
TONc
2
3
b
(mmol)
(%)
(
mg)a
1d
2
3
4
5
31
–
31
31
31
–
–
–
–
–
H O
50
50
50
50
50
24
24
24
24
24
<1
<1
52.8
18.8
40.2
–
–
>99
>99
>99
–
–
27.8
9.9
21.2
2
H O
2
H O
2
CH CN
3
H O/
2
e
CH CN
3
6
7
8
9
31
31
31
31
31
31
0.05
0.1
0.2
0.4
0.2
0.2
H O
50
50
50
50
50
50
24
24
24
24
24
24
86.1
90.6
84.8
67.1
6.8
>97
>99
>99
>99
>99
>99
45.3
47.7
44.6
35.3
3.6
2
H O
2
H O
2
H O
2
1
1
0
CH CN
3
1
H O/
34.6
18.2
2
e
CH CN
3
12
13
14
15
16
17
18
19
31
31
21
41
31
31
31
–
0.2
0.2
0.2
0.2
0.1
0.1
0.1
0.2
H O
40
60
50
50
50
50
50
50
24
24
24
24
4
8
12
24
77.5
90.2
72.6
80.6
22.0
37.0
53.0
25.4
>99
>99
>99
>99
>99
>99
>99
>99
40.8
47.5
38.2
42.4
11.6
19.4
27.9
13.4
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
2
f
H O
2
Reaction conditions: benzyl alcohol (2 mmol), TEMPO (0.1 mmol) in 2 mL of solvent, O as the oxidant
2
a
3
1 mg CuO-REC (37.6 μmol Cu atoms)
b
c
d
e
f
Conversion was determined by HPLC analysis (See ESM Fig. S1–S5†)
Turnover number (number of moles of aldehyde formed per mole of Cu
Without TEMPO
v/v, 1:1
CuO (3.2 mg) as the catalyst
As to the reaction temperature, 50 °C was suitable (Table 2, entries 7, 12, 13). For
the CuO-REC, the increase of its amount from 21 to 31 mg improved the conver-
sion of benzyl alcohol (Table 2, entries 8, 14). However, more CuO-REC couldn’t
lead to higher conversion (Table 2, entry 15). It was similar to copper on boehmite
catalyst, which could be because, as distance between substrates and oxo species
might be longer [45]. And then the conversion increased with time (Table 2, entries
7
, 16–18). In addition, compared with CuO-REC/TEMPO, the catalytic eꢁciency
was lower only by using CuO/TEMPO as the co-catalyst (Table 2, entry 19). This
result was consistent with previous report [29]. This could be related to the contact
eꢁciency of copper atoms with TEMPO and substrate. From the optimization stud-
ies, it was found that the optimized reaction conditions were as follows: 31 mg of
1
3

Aerobic alcohol oxidation catalyzed by CuO-rectorite/TEMPO…
Fig. 5 The photographs of CuO-REC/TEMPO catalytic oxidation of benzyl alcohol in diꢂerent solvents.
a Before reaction in H O; b before reaction in H O with K CO ; c before reaction in CH CN/H O (v/v,
2
2
2
3
3
2
1
:1); d before reaction in CH CN/H O (v/v, 1:1) with K CO ; e before reaction in CH CN; f before reac-
3 2 2 3 3
tion in CH CN with K CO ; g after reaction in H O; h after reaction in H O with K CO
3
3
2
3
2
2
2
CuO-REC, 0.1 mmol of K CO , 2 mL of water as solvent, O as the oxidant, 50 °C
2
3
2
and 24 h. Additionally, 0.1 mmol of TEMPO was selected throughout the research
process.
The substrate range of aerobic alcohol oxidation catalyzed by CuO-REC/TEMPO
was investigated under the optimized reaction conditions and the results are shown
in Table 3. It can be seen that most tested primary benzylic alcohols containing
electron-donating or electron-withdrawing groups, such as –Me, –OMe, –NO , –Cl
2
and –Br on the benzene ring, could be selectively converted to the corresponding
aldehydes with high conversion (Table 3, entries1–16) and there were no acids
and other by-products by HPLC analysis. All products have been isolated and
1
13
their structure has also been conꢀrmed by H NMR and C NMR analysis (See
ESM Fig. S22–S53†). Furthermore, there was no signiꢀcant diꢂerence in the con-
version between the substrates with electron-withdrawing groups and those with
electron-donating groups. This suggested that the electronic eꢂect was not a main
inꢃuencing factor for the reaction. But a noteworthy phenomenon was that some
ortho-substituted benzyl alcohols such as 2-methylbenzyl alcohol and 2-nitrobenzyl
alcohol showed lower conversion in the aerobic oxidation compared to para-substi-
tuted analogs (Table 3, entries 2 and 3, 4 and 6). This could be due to the eꢂects
of steric hindrance. At the same time, it was also interesting that benzylic alcohols
with a phenolic hydroxyl group could be smoothly converted to the correspond-
ing aldehydes and over-oxidized by-products weren’t observed (Table 3, entries
1
4–16). This indicated that CuO-REC/TEMPO catalyzed aerobic alcohol oxida-
tion had a good selectivity. However, the conversion of 4-hydroxybenzyl alcohol
and 4-hydroxy-3-methoxybenzyl alcohol was much lower than that of 3-hydroxy-
4
-methoxybenzyl alcohol. This was because the aggregation of CuO-REC occurred
during their catalytic oxidation and the aqueous solution of 4-hydroxybenzyl alcohol
was very viscous as well. Although the TON value of CuO-REC is higher than the
general homogeneous catalysis [8–10, 14] and heterogeneous catalysis [19, 22, 23],
it is more important for the heterogeneously catalyzed catalyst to consider recycling
and environmentally friendly.
1
3

W. Liu et al.
Table 3 CuO-REC catalyzed aerobic oxidation of primary alcohols to aldehydes
Entry Alcohol
Production
Conversion (%)a
Selectivity (%) Yield (%)b TONc
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
PhCH OH
PhCHO
90.6
74.5
91.8
55.6
80.5
98.4
99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
95
85.3
70.2
88.2
49.8
74.6
93.2
95.6
96.2
80.2
94.1
80.1
73.2
77.8
59.8
51.7
47.6
39.2
48.3
29.3
42.4
51.8
52.1
52.1
44.8
52.1
46.4
41.5
44.5
36.8
31.7
2
2-MeC H CH OH
2-MeC H CHO
6
4
2
6
4
4-MeC H CH OH
4-MeC H CHO
6
4
2
6 4
2-O NC H CH OH
2-O NC H CHO
2 6 4
2
6
4
2
3-O NC H CH OH
3-O NC H CHO
2
6
4
2
2 6 4
4-O NC H CH OH
4-O NC H CHO
2 6 4
2-MeOC H CHO
4-MeOC H CHO
2
6
4
2
2-MeOC H CH OH
6
4
2
6 4
4-MeOC H CH OH
99
6
4
2
6
4
3,5-(MeO) C H CH OH 3,5-(MeO) C H CHO 85.2
2
6
3
2
2
6
3
0
1
2
3
4
5
3-BrC H CH OH
3-BrC H CHO
99
6
4
2
6
4
4-BrC H CH OH
4-BrC H CHO
88.1
78.8
84.5
70.0
60.2
6
4
2
6
4
3-ClC H CH OH
3-ClC H CHO
>99
>99
>99
>99
6
4
2
6
4
4-ClC H CH OH
4-ClC H CHO
6
4
2
6 4
4-HOC H CH OH
4-HOC H CHO
6 4
6
4
2
3-MeO-4-OHC H-
3-MeO-4-HOC H-
6
6
CH OH
CHO
4
2
4
1
6
4-MeO-3-HOC H-
4-MeO-3-HOC H-
4
94.5
>99
84.2
49.7
6
6
CH OH
CHO
4
2
Reaction conditions: alcohol (2 mmol), CuO-REC (31 mg, Cu content:37.6 μmol), K CO (0.1 mmol),
2
3
TEMPO (0.1 mmol), water (2 mL), O as the oxidant at 50 °C for 24 h
2
a
Conversion was determined by HPLC analysis (See ESM Fig. S6–S21†)
Isolated yield
b
c
Turnover number (number of moles of aldehyde formed per mole of Cu
Heterogeneous nature and recycling of catalyst
Under the optimized conditions, benzyl alcohol was aerobically oxidized to verify
whether the catalyst was CuO-REC or the leached copper in water. A ꢀltration
test was performed according to the method in the literature [46, 47]. After 4 h of
catalytic oxidation, the conversion of benzyl alcohol to benzaldehyde was 22.0%
(
Table 2, entry 16). Then CuO-REC was ꢀltered out. After a further reaction for 8 h,
it was found that the conversion was 22.1% and almost remained unchanged. In addi-
tion, after CuO-REC-catalyzed aerobic oxidation of benzyl alcohol was performed
for 12 h, its conversion was 53% (Table 2, entry 18). Meanwhile, copper leaching
from the heterogeneous catalyst was also determined by FAAS analysis. Only 0.13%
of copper was leached into water within 24 h. These results indicated that the copper
leached was not suꢁcient to catalyze the oxidation of benzyl alcohol to benzalde-
hyde and the aerobic alcohol oxidation was mainly catalyzed by CuO-REC.
For a heterogeneous catalytic system, it is very important to separate and reuse
the catalyst. The recyclability of CuO-REC for the oxidation of benzyl alcohol under
the optimized conditions was investigated as well (Table 4). After the reaction was
over, CuO-REC was collected by centrifugation, washed with acetonitrile and water,
and used for the next run. It was found that the reused CuO-REC had almost the
same activity for ꢀve consecutive cycles. This was mainly because the amount of
1
3

Aerobic alcohol oxidation catalyzed by CuO-rectorite/TEMPO…
Table 4 The recyclability of
Number of reuses
Conversion (%)a
1
2
3
4
5
CuO-REC
90.6
87.8
87.2
88.0
87.4
Reaction conditions: benzyl alcohol (2 mmol), CuO-REC (31 mg,
Cu content: 37.6 μmol), K CO (0.1 mmol), TEMPO (0.1 mmol),
2
3
water (2 ml), O as the oxidant at 50 °C for 24 h
2
a
Conversion was determined by HPLC analysis
copper leached was very small after each reaction. Furthermore, after CuO-REC
was reused for two times, the copper leached out in water couldn’t be detected pre-
cisely by FAAS analysis, indicating that CuO-REC was very stable.
Proposed catalytic mechanism
Although the mechanism for the Cu(II)/TEMPO oxidation of the alcohols is not
especially clear, based on Samanta’s report [48], the reduction of copper(II) to
copper(I) takes place in the presence of the alcohol and TEMPO under an inert
atmosphere, and the catalytic eꢁciency of the reaction is very low. However, the
catalytic eꢁciency increases rapidly when oxygen is introduced, indicating that oxy-
gen plays a large role in the cycle of the reaction. This is consistent with the cooper-
atively with redox-active organic co-catalyst reported by Stahl [49]. So, a reasonable
mechanism for the oxidation of benzylic alcohols is show in Scheme 1. First, K CO
2
3
is responsible for the deprotonation of alcohols, which can promote the formation of
Scheme 1 Proposed catalytic cycle
1
3

W. Liu et al.
Cu(II)-alkoxide. Meanwhile, TEMPO also coordinates to the copper(II) ion. Inter-
molecular abstraction of hydrogen with TEMPO leads to the formation of aldehyde
and TEMPOH, and it suggest that an electron transfer to copper(II)-REC produces
Cu(I)-REC. Finally, Cu(II)-REC and TEMPO are regenerated by aerobic oxidation
of Cu(I)-REC and TEMPOH, producing water as a byproduct, resulting in the com-
pletion of the catalytic cycle.
Conclusions
CuO-REC was prepared and used as a co-catalyst with TEMPO to catalyze aerobic
oxidation of alcohols to aldehydes in water. The new catalytic system is environ-
mentally benign, safe, highly eꢁcient, high-yielding and operationally simple. Thus,
it will be hopefully used for aerobic alcohol oxidation in industrial production.
Acknowledgements This work was supported by the National Natural Science Foundation of China
(
no. 31270616) and the Key Laboratory of Fermentation Engineering (Ministry of Education) Open
Foundation.
Compliance with ethical standards
Confict oꢀ interest The authors declare that they have no conꢃict of interest.
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