12
T. Chattopadhyay et al. / Catalysis Communications 12 (2010) 9–13
Table 6
Table 8
Reuse of Cu-ONT complex 1 with H2O2 and TBHP for 2-octanol oxidation.
Reuse of Cu-ONT complex 1 with TBHP for TMP oxidation.
a
H2O2
TBHPb
TBHPa
Reused number
Conversion (%)c
1
17
2
19
3
18
4
16
1
29
2
28
3
26
4
26
Reused number
bConversion (%)
bSelectivity (%)
1
52
93
2
50
91
3
49
93
4
51
92
a
Complex 1 (0.01 mmol), 2-Octanol (1 mmol), 30% H2O2 (15 mmol), and CH3CN
(5 ml), were stirred at 60 °C for 5 h in air.
a
Complex 1 (0.01 mmol), substrate (1 mmol), 70% TBHP (2 mmol), 5 ml CH3CN,
stirring 5 h at 65 °C.
b
Complex 1 (0.01 mmol), alcohols (1 mmol), 70% TBHP (2 mmol), and CH3CN
b
(5 ml), were stirred at 60 °C for 5 h in air.
Determined by GC analysis on the basis of substrates charged with biphenyl as
c
Determined by GC analysis on the basis of substrates charged with biphenyl as
internal standard.
internal standard.
3.2.3. Phenol oxidation
For phenol oxidation, we used 2, 3, 6-trimethyl phenol (TMP) as a
test substrate. It would be oxidize to trimethyl-1, 4-benzoquinone
(TMQ) which is a key intermediate for production of vitamin E. We
observed best conversion of TMP (52%) when we used two folds
excess of 70% TBHP compared to substrates (see Table 3). But we did
not observe any oxidation with H2O2.
All the results suggested that the Cu-ONT complex 1 reacted with
H2O2 and formed the brown color Cu-ONT complex 3 which was
peroxo complex. Although the Cu-ONT complex 3 was stable in dry
state, it slowly decomposed in solution with bubbling and regener-
ated Cu-ONT complex 1. The bubbling may be molecular oxygen,
displaced from Cu-ONT complex 3. Possibly, the Cu-ONT complex 2
was a reactive intermediate. But at this moment, it is difficult to
analyze the exact composition. Heat treatment accelerated the cycle
of color changes from 24 h to 6 h.
3.2.4. Alkane and alkyl benzene oxidation
We got 30% and 70% conversion of cyclohexane when we used 15
folds excess of 30% H2O2 and two folds excess of 70% TBHP
respectively (see Table 4). We got 25-32% conversion of the alkyl
benzenes with two folds excess of 70% TBHP. But we observed almost
no oxidation result with H2O2 (see Table 4).
3.2. Catalytic oxidation
We investigated catalytic activity of Cu-ONT complex 1 for
oxidation of a wide range of organic substrates such as alcohols, α,
β-unsaturated alcohols and aldehydes, phenol, alkane, alkyl benzenes,
alkenes and terpene. The details are described below.
3.2.5. Alkenes and terpene epoxidation
Here, we took cyclohexene, styrene and α-pinene as test
substrates. We got 15-21% conversion of the substrates to
corresponding epoxide with 15 folds excess of 30% H2O2 while 26-
41% conversion with two folds excess of 70% TBHP (see Table 5).
According to the results, oxidation by TBHP provided higher
conversion than H2O2. This difference was due to decomposition of
H2O2 [31] via formation of Cu-ONT complex 3 (see Scheme 1). This
side reaction competed with the substrates oxidation on Cu-ONT
complex 1 and reduced the oxidation product.
3.2.1. Alcohol oxidation
Cu-ONT complex 1 can act as a selective oxidation catalyst for
various primary and secondary alcohols to corresponding aldehydes
and ketones (see Table 1). When we used 15 folds excess of 30% H2O2
and two folds excess of 70% TBHP compared to substrates, we got best
conversion. The conversions of primary and secondary alcohols were
ranging from 20-50% with TBHP while 3-25% with H2O2 with good
selectivity.
3.2.6. Reuse property of the Cu-ONT complex 1
Cu-ONT complex 1 was easily reused in the oxidation of 2-octanol,
cinnamyl alcohol, TMP, cyclohexane and styrene (see Tables 6-10).
For all cases, conversion and selectivity of the catalytic reactions were
not significantly different through first to fourth use of the Cu-ONT
complex 1.
3.2.2. α , β-unsaturated alcohols and aldehydes oxidation
We took some α, β-unsaturated alcohols and aldehydes as
substrates for oxidation and got corresponding aldehydes and
carboxylic acids without byproducts such as epoxide. Selective
oxidation of cinnamyl alcohol to aldehyde needed two folds excess
of 10% H2O2 or TBHP compared to substrates. Higher concentration of
H2O2 or TBHP produced many byproducts. In contrast, oxidation of
cinnamaldehyde required three folds excess of 30% H2O2 or TBHP. We
found same tendency on stepwise oxidation of trans-hex-2-ene-1-ol
to corresponding carboxylic acid. Table 2 clearly shows that the Cu-
ONT complex 1 can control the oxidation reactions depending on the
concentration of oxidants.
4. Conclusions
In conclusion, Cu-ONT complex 1 can function as a heterogeneous
oxidation catalyst with H2O2 and TBHP. The large surface area of
nanotube made Cu-ONT complex 1 a good catalyst. Due to its
operational simplicity, generality and efficacy, this material is
applicable to the oxidation of a variety of organic compounds. It is
Table 9
Reuse of Cu-ONT complex 1 with H2O2 and TBHP for cyclohexane oxidation.
Table 7
TBHPb
a
H2O2
Reuse of Cu-ONT complex 1 with H2O2 and TBHP for cinnamyl alcohol oxidation.
Reused number
Conversion (%)c
Selectivity (%)c
Cyclohexanone
Cyclohexanol
1
30
2
28
3
31
4
31
1
70
2
70
3
68
4
71
a
H2O2
TBHPb
Reused number
Conversion (%)c
Selectivity (%)c
1
17
87
2
16
86
3
17
87
4
15
85
1
33
91
2
32
92
3
32
90
4
33
91
53
47
51
49
52
48
53
47
71
29
70
30
72
28
71
29
a
a
Cu-ONT complex 1 (0.01 mmol), substrate (1 mmol), 10% H2O2 (2 mmol), 5 ml
Complex 1 (0.01 mmol), substrate (1 mmol), 30% H2O2 (15 mmol), 5 ml CH3CN,
stirring 5 h at 60 °C .
CH3CN, stirring 5 h at 60 °C.
b
b
Cu-ONT complex 1 (0.01 mmol), substrate (1 mmol), 10% TBHP (2 mmol), 5 ml
Complex 1 (0.01 mmol), substrate (1 mmol), 70% TBHP (2 mmol), 5 ml CH3CN,
CH3CN, stirring 5 h at 60 °C.
stirring 5 h at 60 °C.
c
c
Determined by GC analysis on the basis of substrates charged with biphenyl as
Determined by GC analysis on the basis of substrates charged with biphenyl as
internal standard.
internal standard.