Commercial Gold(I) and Gold(III) Compounds Supported on Carbon Materials as Greener Catalysts for the Oxidation of Alkanes and Alcohols

Article abstract of DOI:10.1002/cctc.201701886

Strategies are presented for single-pot efficient oxidative functionalization of cyclohexane and alcohols, under mild conditions, catalysed by Au(I) or Au(III) compounds supported on different carbon materials with three different surface treatments. The obtained materials were tested for the oxidation of cyclohexane under mild conditions (room temperature and atmospheric pressure), using an environmentally friendly oxidant (tert-butyl hydroperoxide, TBHP, 70 % aqueous solution). All materials were very selective to the production of cyclohexanol and cyclohexanone with no trace of by-products detected. The catalysts were also tested in the selective oxidation of methyl benzyl alcohol, cyclohexanol, and 2-octanol with TBHP, under microwave irradiation, to the corresponding aldehydes or ketones. In general, better results are obtained for the heterogenised complexes and that the most efficient support is CNT-ox-Na. This is the first report on the oxidation of C?H bonds using the mononuclear gold complexes used; only the oxidation of unsaturated units had been reported previously. The sp³-C?H activation is much more difficult than the oxidation of unsaturated molecules with molecular oxidants.

Full text of DOI:10.1002/cctc.201701886

Accepted Article
Title: Commercial gold(I) and gold(III) compounds supported on carbon
materials as greener catalysts for the oxidation of alkanes and
alcohols
Authors: Sonia A. C. Carabineiro, Luísa M.D.R.S. Martins, Armando
J.L. Pombeiro, and José L Figueiredo
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ChemCatChem
10.1002/cctc.201701886
FULL PAPER
Commercial gold(I) and gold(III) compounds supported on carbon
materials as greener catalysts for the oxidation of alkanes and
alcohols
Sónia A.C. Carabineiro,* Luísa M.D.R.S. Martins,* Armando J.L. Pombeiro, José L. Figueiredo[a]
[
a]
[b]
[b]
Abstract: The present paper reports strategies for single-pot Introduction
efficient oxidative functionalization of cyclohexane and alcohols,
under mild conditions, catalysed by Au(I or III) compounds, namely,
dichloro(2-pyridinecarboxylato)gold(III) (1), tetrabutylammonium
tetrachloroaurato(III) (2), chlorotrimethylphosphinegold(I) (3),
Although heterogeneous catalysis by (supported) gold
nanoparticles is still predominant, an explosive growth of
homogeneous catalysis by gold took place a few years ago and
keeps increasing.[ Homogeneous gold catalysts (in the form of
1]
chlorotriphenylphosphinegold(I)
(4)
and
1,3-bis(2,6-
soluble complexes) can have high activity, enantioselectivity[1a,
[
2]
diisopropylphenyl)imidazol-2-ylidenegold(I) chloride (5) supported on
different carbon materials: activated carbon (AC), multi-walled
carbon nanotubes (CNT) and carbon xerogel (CX), with three
different surface treatments: original, as purchased/prepared,
oxidized with nitric acid (-ox), and oxidized with nitric acid and
subsequently treated with NaOH (-ox-Na). The obtained materials
were tested for the oxidation of cyclohexane under mild conditions
3
]
and well characterized structures, but heterogeneous gold
systems show longer lifetimes, easier separation of the catalyst
from the products, possible catalyst recycling and adaptation to
continuous flow processes (for the problems with continous flow
processes in homogeneous gold catalysis, see [4]). Thus, the
possibility to combine the remarkable properties of
homogeneous catalysts with the well-known advantages of
heterogeneous systems by anchoring soluble gold complexes
(
room temperature and atmospheric pressure), using an
environmentally friendly oxidant (tert-butyl hydroperoxide, TBHP,
0% aqueous solution). All materials were very selective to the
7
on solid supports, is
research.[5]
a very promising interface area of
production of cyclohexanol and cyclohexanone with no trace of by-
products detected. The same catalysts were also tested in the
selective oxidation of methyl benzyl alcohol, cyclohexanol and 2-
octanol with TBHP, under microwave irradiation, to the
corresponding aldehydes or ketones. The obtained results showed
that, in general, better results are obtained for the heterogenised
complexes and that the most efficient support is CNT-ox-Na for both
reactions. The best result for cyclohexane oxidation was obtained for
Heterogenisation was already accomplished for complexes of
several metals, however, examples of heterogenisation of Au
complexes are still not much abundant.[5] They dealt mostly with
3 2 5 7 2
the work of Gates et al. using the [Au(CH ) (C H O
)] complex,[6]
Corma et al.[ and Parida et al. heterogenised Au complexes
7]
[8]
that contained Au(III) Schiff bases. Corma et al. also used
[
9]
compounds with ONN-tridentate and CNN-tridentate pincer-
[
10]
5/CNT-ox-Na, which showed recyclability up to 5 cycles without
type ligands,
chiral (NHC)-dioxolane-amino Au(I) and Au(III)
complexes[ and a mononuclear asymmetrical N-heterocyclic
carbene-Au complex.[12] A N-heterocyclic carbene-Au(I) complex
was also reported by Fujita and co-workers.[13] Ganai et al.[14]
used a Au(III) complex with a 1,2,3-triazole linkage, which can
be used as bridge to the support. A triazole-Au(I) complex was
also reported by Cai and co-workers.[15] Mallissery et al.[16] used
phosphinine Au(I) complexes. Dubarle-Offner et al.[17]
synthesised π-complexes of phosphino ligands, [Cp*Ru(η6-
11]
decrease of activity. The highest activity for methyl benzyl alcohol
oxidation was found for the 3/CNT-ox-Na material, which could be
recycled to 7 cycles without loss of activity. This is the first report so
far on the oxidation of C,H bonds using the mononuclear gold
complexes above (as only the oxidation of unsaturated units had
3
been reported until now). The sp -C,H activation is much more
difficult than the oxidation of unsaturated molecules with molecular
oxidants.
arene-PAr
attached to
electron-donating methyl group (−PAr
withdrawing trifluoromethyl group (−PAr
2
)][OTf], in which the diarylphosphine unit was
metalated π-arene scaffold, with either an
= −P(p-tol) ) or electron-
= −P(p-C CF ).
a
2
2
2
6
H
4
3 2
)
Those metallo ligands were converted to heterodinuclear gold
6
[
[
a]
b]
Dr. S.A.C. Carabineiro,* Prof. J.L. Figueiredo
complexes upon treatment with [AuCl(tht)]. [Cp*Ru(η -p-
Laboratório de Catálise e Materiais, Laboratório Associado LSRE-
LCM, Faculdade de Engenharia, Universidade do Porto, Rua Dr.
Roberto Frias 4200-465 Porto, Portugal
6
CH
3
C
6
H
4
-P(p-tol)
2
-Au-Cl)][OTf]
and
[Cp*Ru(η -C
6
H
5
-P(p-
[
17]
C
6
H
4
CF
3
)
2
)-Au-Cl][OTf] materials were obtained. Cao and Yu
a (benzotriazole)(triphenylphosphine)Au(I) complex
Anchoring of a gold
chiral sulfinamide monophosphine complex was also reported by
used
prepared using a pre-anchored ligand.
*
Email: sonia.carabineiro@fe.up.pt
[
18]
Prof. L.M.D.R.S. Martins,* Prof. A.J.L. Pombeiro
Centro de Química Estrutural, Instituto Superior Técnico,
Universidade de Lisboa, Av. Rovisco Pais 1049-001 Lisboa,
Portugal
Chen et al.,[
19]
while the immobilisation of phosphine Au(I)
complexes was reported by other authors.[ Vriamont et al.
reported the heterogenisation of the gold complex
20]
*
Email: luisamargaridamartins@tecnico.ulisboa.pt
Supporting information for this article is given via a link at the end of
the document.
[
Tf
2
NAuPPh
3
22]
CH
2
NH
2
][21]
and
new
pyrene-tagged
gold
moiety was
[
complexes.
A cationic Au(I) complex with a BF
4
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ChemCatChem
10.1002/cctc.201701886
FULL PAPER
reported by Somorjai’s and Toste’s groups,[23] while other
materials (activated carbon, carbon xerogel and carbon
nanotubes). Four of these compounds have the advantage of
being commercially available, which avoids the need to
troublesome often low yield syntheses. The hybrid catalysts
were tested in the oxidation of cyclohexane and alcohols under
mild conditions. Those are reactions of industrial importance for
the synthesis of fine chemicals and intermediates.[31] So far,
such mononuclear gold complexes have only been reported for
authors
reported
heterogenisation
of
Au(I)-carbene
[
24]
compounds. Immobilisation of a Au(III)-bipy complex was also
reported by Yang et al.[25] and of strongly luminescent cationic
cyclometalated Au(III) complexes with various dimensions by
other authors.[26]
[12a, 14, 20c,
Several supports were used for these works, like silica,
2
3]
[6b]
[6a]
[6f]
[16]
[7a]
γ-Al
2
O
3
,
MgO,
La
2
O
3
,
TiO
porous organic polymers,
magnetic nanoparticles (Fe
metal-organic frameworks,[26] etc. Yet, to the best of our
2
,
CeO
2
,
2 3
Y O ,
zeolites,[
12a]
polystyrene,
[18-19, 24]
[15]
the oxidation of unsaturated units, but not for C,H bonds. It is
[32]
[
17]
),[13,
3
metalated arene scaffold,
3
O
4
worth to point out that the sp -C,H activation is a more difficult
2
0a, 20b, 25]
challenge than the “easier” oxidation of unsaturated molecules
knowledge, comparatively, not much work has been done on
heterogenization of Au complexes on carbon materials (although
carbon materials have advantages as supports since their
with molecular oxidants.
texture and surface chemistry can be easily tuned as needed[27]). Results and Discussion
The first report on the heterogenization of gold complexes was a
publication of ours, dealing with Au (C-scorpionate) complexes
immobilized on activated carbon, carbon xerogel and carbon
nanotubes,[28] that showed superior performance to Au
nanoparticles on similar supports[29] for the oxidation of
cyclohexane to cyclohexanol and cyclohexanone. Also the yield
achieved with supported gold complexes was much better than
that obtained with the unsupported (soluble) complexes, and the
best reported result was also larger than the obtained in the
currently used industrial process (that uses a homogeneous
cobalt species as catalyst and dioxygen as oxidant, at a
Characterisation of carbon supports
The carbon supports were used in their original forms: as
purchased (AC and CNT) or as prepared (CX), oxidised (-ox)
and oxidised with nitric acid and subsequently treated with
sodium hydroxide (-ox-Na). The characterization details of the
nine carbon samples are displayed in Table 1 and Figure 1.
As expected, AC has the largest surface area and has
[27a, 27b, 27d, 27e, 33]
abundance of micropores,
while CX is
Carbon nanotubes
[29, 33]
mesoporous and has a large pore size.
have a cylindrical structure and the pores result from the free
considerably high temperature of 150 ºC, with selectivities of ca.
space in the bundles, so they show lower surface areas than the
other carbon materials.[29, 33]
5%).[
28]
Since then, the only other reports on the
8
heterogenisation of gold compounds on carbon materials were
from Vriamont et al., which dealt with the heterogenisation of a
[
Tf
Au(I) complex
also reported gold complexes of general formula [(NHC)AuX]
where X = Cl, Br) immobilized on reduced graphene oxide by π
stacking interactions.[22b] Thus, to the best of our knowledge,
2
NAuPPh
3
CH
2
22a]
NH
2
] complex[21] and a new pyrene-tagged
on carbon nanotubes. Ventura-Espinosa et al.
Table 1. Description and characterisation of (powder) carbon samples:
[
surface area (SBET), total pore volume (V
p
), average mesopore width (L),
micropore volume (Vmicro), external area (Sext), obtained by adsorption of N
2
at -196 ºC, and amounts of CO and CO
(
2
desorbed, as determined by TPD
(
-
adapted from [ ).
34]
V
(cm /
g)
p
V
(cm /
g)
micro
3
CO
(µmol/
g)
CO
2
(µmol
/g)
S
BET
2
3
L
(nm)
S
ext
2
those four publications are the only reports so far on the subject.
Sample
(
m /g)
(m /g)
AC
974
914
610
257
400
0.67
0.62
0.35
2.89
1.89
1.45
0.91
0.80
0.75
-
0.348
0.324
0.251
~0
260
643
179
AC-ox
AC-ox-Na
CNT
-
247
80
4930
5012
142
2596
2883
89
-
-
257
400
350
604
512
496
CNT-ox
-
~0
1475
1079
492
729
CNT-ox-
Na
3
50
-
~0
838
CX
604
570
560
13.7
18.8
17.6
~0
135
Scheme 1. Dichloro(2-pyridinecarboxylate)gold(III) (1) – firstly used as a
catalyst by Hashmi et al.,[30] tetrabutylammonium tetrachloroaurate(III) (2),
chlorotrimethylphosphinegold(I) (3), chlorotriphenylphosphinegold(I) (4) and
CX-ox
CX-ox-Na
0.038
0.036
4609
3720
3774
3793
1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidenegold(I) chloride (5) complexes.
In order to contribute to the development of this area, herein, we
report on the heterogenization of five different Au(I) and Au(III)
complexes (Scheme 1) on three different types of carbon
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ChemCatChem
10.1002/cctc.201701886
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chemistry of the supports, but the obtained materials were able
to anchor all the Au complexes, although with different
Anhydrides Phenols
Carbonyls/
Quinones
Carboxylic
acids
Anhydrides Lactones
AC-ox-Na
efficiencies (Table S1). It can be seen that complex 1
AC-ox
heterogenises equally on all supports. The other compounds, in
general, heterogenise better on –ox-Na or –ox samples, than on
pristine supports. It is well known that functionalization of the
AC-ox-Na
AC
AC-ox
AC
supports increases the number of surface groups which act as
28, 34-36]
“
anchors” for the complexes.[
Anhydrides Phenols
Carbonyls/
Quinones
Carboxylic Anhydrides Lactones
acids
Cyclohexane oxidation
CX-ox
The catalytic systems were based on the chlorogold complexes
1-5, bearing phosphinic or N,O-donor ligands, supported on the
different carbon supports with different surface chemistries,
TBHP (70% aqueous solution) as the oxidizing agent, and
CX-ox-Na
CX-ox
CX-ox-Na
CX
CX
acetonitrile as the solvent, in a slightly acidic medium (HNO
room temperature and 6 h reaction time.
3
) at
Anhydrides Phenols
Carbonyls/
Quinones
Carboxylic Anhydrides Lactones
acids
It was found that complexes 1-5 supported on carbon materials
exhibit a remarkable catalytic activity as catalysts for the single-
pot peroxidative oxidation of cyclohexane to a cyclohexanol and
cyclohexanone mixture (final products), under mild conditions
(Table 2). These compounds are important reagents for the
production of adipic acid and caprolactam used for the industry
of manufacture of nylon.[28-29, 37]
CNT-ox-Na
CX-ox
CNT-ox
CX-ox-Na
CNT
CX
Figure 1. TPD profiles for the AC (top), CX (middle), and CNT (bottom)
materials. Desorption of CO (left) and CO (right), with identification of types of
All heterogenised systems show activities higher than the
homogenous counterparts, independently of the type of support
and surface chemistry. The positive effects of immobilisation of
complexes on solid supports include confinement effect, site-
isolation, prevention of dimerization, cooperative effect of
support, among others.[ The largest increases are about 10-
fold (comparing the homogenous catalyst with that supported on
CNT-ox-Na), reaching almost 14-fold for the 2/CNT-ox-Na,
compared with unsupported 2 (entries 20 and 11, respectively,
of Table 2).
2
groups desorbing at different temperature ranges. The different colour bars
are only indicative of the temperature ranges expected for desorption of
different groups, and do not provide any information on their amounts
(
adapted from [34]).
38]
Not surprisingly, the -ox treatment greatly increases the CO and
CO amounts (Table 1 and Figure 1). The CO desorption profiles
Figure 1, left) show that the desorption process starts around
2
(
350 °C for the –ox materials, being slightly higher for the -ox-Na
samples. That can be explained by destruction of carboxylic
anhydrides, which are known to desorb as CO (and CO ) in that
temperature range,[27a-d, 33a] which also contribute to the increase
of the carboxylate groups.[35] The peaks of the -ox-Na samples
sharper, more intense and have their maxima at a temperature
higher than the -ox materials, suggesting that phenol groups
It was also shown that the complexes supported on CNT-ox-Na
yield the best results (Table 2, entries 10, 20, 30, 40 and 50).
The presence of more stable phenolate and carboxylate
moieties, originated by the –ox-Na treatment, might explain the
good results, since these groups can act as coordination sites
for the complexes.[28, 34-36]
The best results were always obtained with the CNT materials
(Table 2, entries 8-10, 18-20, 28-30, 38-40, 48-50). This might
be due to a textural effect, the absence of porosity of these
materials allowing easier accessibility of the reactants to the
active centers, or to an electronic effect, since the more graphitic
structure of these materials might lead to enhanced interactions
with the reactants or intermediate radicals.[28]
2
(
that desorb as CO in this temperature range[27a-d, 33a]) are
converted into phenolates, that are more stable.[35]
The CO desorption profiles (Figure 1, right) of the -ox and -ox-
Na supports show a substantial increase in the carboxylic acids
that are known to decompose around 200-350 ºC[27a-d, 33a] in
2
(
compared with the pristine materials. The peaks of -ox-Na
supports are generally narrower with centers slightly shifted to
higher temperatures, which suggests possible conversion of the
carboxylic acids to (sodium) carboxylates, which increases
stability.[35]
Depending on the complexes, CX or AC are the less adequate
supports (entries 2, 12, 22, 32, 42 of Table 2). However
functionalization is beneficial, as –ox materials (entries 3, 6, 9,
1
3, 16, 19, 23, 26, 29, 33, 36, 39, 43, 46, 49 of Table 2) are
always more active than the pristine samples (entries 2, 12, 22,
2, 42 of Table 2). And the –ox-Na treatment is even better
entries 10, 20, 30, 40 and 50 of Table 2), as already explained
Heterogenisation efficiency
The gold complexes were heterogenised on the different carbon
materials (~2% Au was the target value, but different loadings
were obtained as will be explained ahead) on the different
carbon materials, their original forms (AC, CX and CNT),
oxidised with refluxing HNO 7 M (-ox) and oxidised with HNO
3 3
followed by refluxing with NaOH 20 mM (-ox-Na). As seen above,
the different treatments affect the porous structure and surface
3
(
above. That is valid not just for CNT, but also for CX and AC.
The oxidation treatments provide sites to anchor of the
complexes. However, it was found that the carbon supports (with
no anchored complexes) showed negligible catalytic activity
(
results not shown in Table 2 for simplicity).
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10.1002/cctc.201701886
FULL PAPER
Table 2. Selected optimized resultsa for the oxidation of cyclohexane with TBHP.
It was found that the best results were obtained for the 5/CNT-
ox-Na material with overall yields up to ca. 26% (Table 2, entry
50), five times more than those of the industrial process [39] and
overall turnover number (TON, mol of product/mol of catalyst) of
653 after 6 h reaction. This suggests that the imidazole moiety is
able to efficiently assist the catalytic oxidation reaction. The
reaction was also monitored with time. Figure S1 shows those
results. It can be seen that the yield increases sharply up to 4-5
h, with 6 h being the optimum. Leaving the reaction more time is
not needed and detrimental.
Au
compound
Entry
carbon support
TONb
Total yieldc /%
1
2
3
4
5
6
7
8
9
-
58
2.3
3.1
4.8
7.3
4.2
5.2
9.8
9.7
15.1
24.3
0.5
3.7
4.4
4.9
3.0
7.8
8.5
6.3
7.2
CX
78
CX-ox
CX-ox-Na
AC
AC-Ox
AC-Ox-Na
CNT
CNT-Ox
CNT-Ox-Na
-
120
195
105
130
245
243
388
608
13
1
Unsupported 5 was also the best among the homogenous
catalysts. The second result was achieved for complex 1,
showing also the good effect of the pyridine group. The
presence of nitrogen in the ligands is beneficial, since it can
assist proton-transfer steps that are believed to be involved in
key catalytic processes.[
10
11
12
13
14
15
16
17
18
19
CX
93
CX-Ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
110
123
75
195
213
158
180
40]
2
This reaction is carried out through formation of the cyclohexyl
hydroperoxide (primary product), according to Scheme 2. The
formation of CyOOH is proved by using the method proposed by
Shul'pin.[
CNT-ox
41]
20
CNT-ox-Na
240
9.6
21
22
23
24
25
26
27
28
-
35
58
1.4
2.3
4.2
6.1
5.2
8.6
9.7
6.2
CX
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
105
153
130
215
243
155
3
Scheme 2. Oxidation of cyclohexane to cyclohexanol and cyclohexanone.
A high selectivity towards the formation of cyclohexanol and
cyclohexanone is exhibited by our systems, since no traces of
by-products were detected by GC-MS analysis of the final
reaction mixtures. The well-known recognized promoting effect
of an acidic medium on the peroxidative oxidation of alkanes
catalyzed by homogeneous complexes of several metals,[
2
9
CNT-ox
238
9.5
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
CNT-ox-Na
-
CX
333
43
13.3
1.7
4.5
113
153
203
180
223
303
173
355
405
63
118
150
153
78
125
180
303
330
653
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
6.1
8.1
7.2
8.9
12.1
6.9
37a-d, 42]
pyrazole–rhenium complexes supported on modified silica
gel[37e] or by other metallic catalysts[43] is also observed here.
4
The favourable effect of the acid presumably can be associated
with: (i) the activation of the metal centre by further unsaturation
upon protonation of a ligand, (ii) the enhancement of oxidative
properties of metal complexes, and (iii) the stabilization of the
peroxide towards decomposition and promotion of peroxo (or
hydroperoxo)-complexes formation. This is unlike what was
found for the C-scorpionate complexes where the presence of
acid (either organic or mineral) has an inhibiting effect on the
CNT-ox
CNT-ox-Na
-
14.2
16.2
2.5
4.7
6.0
CX
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
6.1
3.1
5.0
7.2
12.1
13.2
26.1
5
catalytic activity.[28] In that work, H
TBHP), but the results were not so good as those reported here
2 2
O was used (instead of
CNT-ox
CNT-ox-Na
(
the highest activity was ca. 10% with a TON of 300 for the
2
[
AuCl
2
( -TpmOH)]Cl [TpmOH= HOCH
2
C(pz)
3
, pz = pyrazol-1-yl]
a
Reaction conditions: cyclohexane (5.0 mmol), TBHP (10.0 mmol), 2 μmol
0.04 mol% vs. substrate) of 1 - 5, HNO (1.40 mmol), acetonitrile (3.0 mL), 6
h, 20 ºC; 2 mol of 1-5 supported on different carbon materials (CM): carbon
xerogel (CX), activated carbon (AC) or multi-walled carbon nanotubes (CNT),
used in their original forms, oxidised with nitric acid (-ox) or oxidised with nitric
acid and subsequently treated with sodium hydroxide (-ox-Na). Percentage of
yield, TON determined by GC analysis (upon treatment with PPh
Turnover number (moles of product per mol of Au catalyst). Molar yield (%)
based on substrate, i.e. moles of products (cyclohexanol and cyclohexanone)
per 100 mol of cyclohexane.
complex heterogenised on CNT-ox-Na[28]).
(
3
The activities (yield) of supported gold complexes were higher
than the values found for gold nanoparticles (NP) on carbon
supports.[29] These systems also required the presence of acid.
The values were also higher than those reported for Au
b
3
).
nanoparticles supported on several oxides (yields varying from
c
31]
1
.3 to 13.5%)[ . A comparison is not straightforward since H
O
2 2
was also used in these systems, instead of TBHP. The
advantage of the Au/oxide systems is that they did not require
the presence of acid.[31]
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ChemCatChem
10.1002/cctc.201701886
FULL PAPER
The results are presented in terms of TON, however TOF
Table 3. Selected optimized resultsa for the MW-assisted solvent-free
(
turnover frequency, moles of product per mol of Au catalyst per
oxidation of selected secondary alcohols to the respective ketones
time) is also an important measurement.[44] TOF is the reaction
rate, normalised by the amount of active centres, and can be
calculated from the rate constants after adequate kinetic
analysis.
b
c
Entry
Au compound
CM
TON
Total yield /%
methyl benzyl alcohol
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
19
2
2
22
2
2
25
2
2
28
2
3
-
604
619
714
48.3
49.5
57.1
82.5
37.8
35.3
52.7
42.9
66.1
89.0
26.1
31.1
30.0
31.5
40.7
61.8
71.5
45.6
57.8
68.1
55.8
69.4
71.3
73.6
64.5
67.6
76.9
66.8
78.8
96.1
45.5
53.1
62.4
79.4
60.4
71.2
78.6
80.8
87.5
94.8
60.7
65.6
65.5
71.1
61.8
74.0
77.7
68.2
78.4
92.7
Catalyst recyclability was tested for up to five consecutive cycles
for the most active catalyst: 5/CNT-ox-Na. After completion of
each cycle, the products were analyzed as normally and the
hybrid catalyst was recovered by filtration from the reaction
mixture, washed several times with acetonitrile and dried
overnight. The subsequent cycle was initiated upon addition of
new standard portions of all other reagents. The filtrate was
tested in a new reaction (by addition of fresh reagents), and no
oxidation products were detected, thus excluding the hypothesis
of catalyst leaching. The recyclability of the system 5/CNT-ox-Na
is shown in Figure 2: it maintains almost the original level of
activity after several reaction cycles (in a second, third, fourth
and fifth run, the observed activities were 99.1, 98.3, 97.5 and
CX
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
CNT-ox
CNT-ox-Na
-
1031
473
441
659
536
826
1113
326
389
375
394
509
773
894
570
723
851
698
868
891
920
806
845
961
835
985
1201
569
664
780
993
755
890
983
1010
1094
1185
759
820
819
889
773
925
971
853
980
1159
1
2
3
4
5
0
1
2
3
4
5
6
7
8
CX
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
CNT-ox
CNT-ox-Na
-
0
1
9
5.8% of the initial one, respectively) to cyclohexanol and
CX
3
4
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
CNT-ox
CNT-ox-Na
-
cyclohexanone.
A small amount of Au (near 0.97 % of the initial amount loaded)
was found by ICP in the supernatant liquid after 5 cycles. This
explains the small decrease in the activity, together with small
losses of the solid material, when the catalyst is separated by
filtration and re-used again.
6
7
9
0
31
3
3
3
2
3
4
CX
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
CNT-ox
CNT-ox-Na
-
700
600
500
400
300
200
100
0
35
3
3
3
3
6
7
8
9
40
4
4
4
4
1
2
3
4
CX
CX-ox
CX-ox-Na
AC
AC-ox
AC-ox-Na
CNT
45
4
4
4
4
5
6
7
8
9
0
cycle 1 cycle 2 cycle 3 cycle 4 cycle 5
CNT-ox
CNT-ox-Na
Figure 2. Effect of the catalyst recycling on the overall yield of products for
cyclohexane oxidation catalysed by 5/CNT-ox-Na. Reaction conditions: C6H12
(
5.00 mmol); NCMe (3.0 mL); TBHP (10.00 mmol); HNO3 = (1.40 mmol), 20
cyclohexanol
3
ºC; 6 h.
5
5
5
1
2
3
CNT-ox-Na
CNT-ox-Na
CNT-ox-Na
1104
871
88.3
69.7
52.9
Oxidation of secondary alcohols
2-octanol
3
The oxidation of secondary alcohols was carried out using
microwave (MW) irradiation and TBHP as the oxidant. The
oxidation of methyl benzyl alcohol to acetophenone (Scheme 3)
was tested for all the catalysts (Table 3). This ketone is an
important industrial organic synthesis precursor to other organic
3-octanol
3
661
a
Reaction conditions: 2.50 mmol of substrate, 5.00 mmol of TBHP (2 eq.,
0% in H O), 2 μmol (0.08 mol% vs. substrate) of 1 - 5, 80 ºC, 1 h of MW
7
2
b
irradiation (25 W power).
per mol of catalyst precursor. c Moles of ketone product per 100 moles of
alcohol.
Turnover number = number of moles of product
compounds that range from pharmaceuticals to plastic
additives.[31, 36, 45]
It was found that, in general, the supported complexes show
higher catalytic activity than the homogenous counterparts
(
Table 3), possibly on account of the stabilizing effect of the
support which, e.g., can hamper the formation of less active di-
or polynuclear species. However that effect is not as intense as
the one found for cyclohexane oxidation (Table 2), as the
increase is only around 2-fold (while from cyclohexane it varied
Scheme 3. Oxidation of methyl benzyl alcohol to acetophenone using MW
irradiation.
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ChemCatChem
10.1002/cctc.201701886
FULL PAPER
from 10- to 14-fold). The only exception is complex 1, in which
heterogenisation on AC and AC-ox (entries 5 and 6 of Table 3)
gave worse results than unsupported 1 (entry 1 of Table 3). That
can be due to the fact that 1 is a small compound that can
possibly penetrate inside the micropores of activated carbon
preventing access to the reagents. Unsupported 1 (and similar
complexes) have already been investigated in the oxidation of
alcohols with TBHP by Hashmi et al.[46] The methyl benzyl
alcohol oxidation in the absence of a chorogold catalyst was
evaluated and it was found that it occurred in a very low extent.
In fact, only 12.4% of acetophenone was obtained after 1 h of
MW irradiation (25 W power) under the above conditions (but
without catalyst).
mixture, washed and dried overnight. The subsequent cycle was
initiated upon addition of new standard portions of all other
reagents. The filtrate was tested in a new reaction (by addition of
fresh reagents), and no oxidation products were detected, thus
excluding the hypothesis of catalyst leaching. The recyclability of
the system 3/CNT-ox-Na is shown in Figure 3: it maintains almost
the original level of activity until seven reaction cycles, starting to
loose activity on the 8 , which is decreased on the 10 . Just like
with the cyclohexane oxidation experiments, the decrease in the
activity on the 10th cycle can be due to some small leaching,
together with small losses of the solid material, when the catalyst
is separated by filtration and re-used again.
th
th
In general, the complexes supported on CNT-ox-Na yield the
best results (Table 3, entries 10, 30, 40 and 50), similarly to
what was found for cyclohexane oxidation (Table 2). As
explained above, that can be due to the presence of more stable
phenolate and carboxylate groups[28, 34-36] and to a textural or
electronic effect of CNT.[28] The exception is complex 2, since
the best result was obtained for AC-ox-Na (entry 17 of Table 3),
which was slightly higher than the obtained for CNT-ox-Na (entry
1
00
90
80
70
60
50
40
20 of Table 3).
In general, functionalization is beneficial, as complexes
supported on –ox materials (entries 3, 9, 16, 19, 23, 26, 29, 33,
30
20
3
6, 39, 43, 46, 49 of Table 2) are more active than the
complexes supported on pristine samples (entries 2, 12, 22, 32,
2 of Table 2). The exception is 1/AC-ox (entry 6) and 2/CX-ox
1
0
0
4
cycle 1 cycle 2 cycle 3 cycle 4 cycle 5 cycle 6 cycle 7 cycle 8 cycle 9 cycle 10
(
(
entry 13), that are less active than 1/AC (entry 5) and 2/CX
entry 12). However the –ox-Na treatment always provides
Figure 3. Effect of the catalyst recycling on the overall yield of products for the
MW-assisted oxidation of methyl benzyl alcohol by 3/CNT-ox-Na. Reaction
conditions: Reaction conditions: 2.50 mmol of alcohol, 5.00 mmol of TBHP, 80
ºC, 1 h of MW irradiation (25 W power).
better catalysts (entries 10, 20, 30, 40 and 50), than the –ox or
pristine counterparts. As already explained that treatment
provides more efficient anchorage sites for catalysts. It was
found that the carbon supports (with no anchored complexes)
showed negligible catalytic activity (results not shown in Table 3
for simplicity).
Conclusions
The best results were obtained for the 3/CNT-ox-Na material with
overall yields up to ca. 96.1% (entry 30). This reaction also
monitored with time, as shown in Figure S2. The yield increases
very fast up to 1 h, and then is kept constant up to 2 h (the
maximum studied).
However unsupported 5 was the best homogenous catalyst (just
like it had been for cyclohexane oxidation), and 3 was only the
second best (among the homogenous catalysts). The selectivity
was always very high in all cases, with no trace of byproducts
detected.
The heterogenised chlorogold complexes were tested as
catalysts for cyclohexane and alcohol oxidation, with TBHP as
the oxidizing agent. The highest catalytic activities were
observed in the presence of CNT-ox-Na with complexes 5 and 3,
respectively. The advantages of the present systems are the
recyclability (up to 5 cycles without decrease of activity for
5
3
/CNT-ox-Na in cyclohexane oxidation and up to 7 cycles for
/CNT-ox-Na in methyl benzyl oxidation) using TBHP. The ease
of separation of the catalysts from the reaction mixtures
minimises additional work-up procedures, reducing costs and
decreasing the production of toxic waste.
The best catalytic system (3/CNT-ox-Na) was also tested for the
oxidation of other alcohols: cyclohexanol, 2-octanol and 3-octanol
(
Table 2). The highest yield was obtained for cyclohexanol
oxidation (TON = 1104, yield = 88.3%), which was smaller than
the obtained for methyl benzyl alcohol (TON = 1201, yield =
Experimental Section
9
6.1%). This value is higher than those reported for Au
nanoparticles supported on several oxides, obtained at 100ºC
TON=1095).[31]
General reagents
Cyclohexane (Merck), methyl benzyl alcohol (Aldrich), cyclohexanol (Alfa
Aesar), 2-octanol (Aldrich), 3-octanol (Aldrich), acetonitrile (Riedel-de-
Haën), nitric acid (65%, Riedel-de-Haën), diethyl ether (LabScan),
cycloheptanone (Riedel-de-Haën), tert-butyl hydroperoxide (TBHP, 70%,
Aldrich), were used as received from the suppliers.
(
Catalyst recyclability was tested for up to ten consecutive cycles
for the most active catalyst: 3/CNT-ox-Na. After completion of
each cycle, the products were analyzed as normally and the
hybrid catalyst was recovered by filtration from the reaction
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ChemCatChem
10.1002/cctc.201701886
FULL PAPER
Gold complexes
The temperature of injection was 240 ºC. For alkane oxidation
experiments, the initial temperature was maintained at 100 ºC for 1 min,
then raised 10 ºC/min to 180 ºC, while for the alcohol oxidation tests the
initial temperature was maintained at 120 ºC for 1 min, then raised 10
ºC/min to 200 ºC and held at this temperature for 1 min.
Different Au(III) and Au(I) complexes were used (Scheme 1). Concerning
the Au (III) compounds, dichloro(2-pyridinecarboxylato)gold(III) (1) was
purchased from Aldrich, while tetrabutylammonium tetrachloroaurate(III)
(
2) was synthesized by adding dropwise, under stirring, 1 mL of the
aqueous HAuCl solution (0.01 mmol) to an equimolar quantity (0.01
mmol) of [nBu N][BF ] dissolved in 4 mL of water. The [nBu N][AuCl ] (4)
4
GC-MS analyses were performed using a Perkin Elmer Clarus 600 C
instrument (using He as the carrier gas). The ionization voltage was 70
eV. GC experiments were conducted in the temperature-programming
mode, using a SGE BPX5 column (30 m×0.25 mm×0.25 µm). Reaction
products were identified by comparing their retention times with known
reference compounds, and by comparison of their mass spectra to
fragmentation patterns obtained from the NIST spectral library stored in
the computer software of the mass spectrometer.
4
4
4
4
yellow precipitate was formed within a few minutes. The Au(I) complexes
chlorotrimethylphosphinegold(I) (3), chlorotriphenylphosphinegold(I) (4)
and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidenegold(I) chloride (5)
were purchased from Strem Chemicals.
Carbon supports
The carbon supports used in this work were the following: activated
carbon Norit® RO 0.8 (AC) from Sigma-Aldrich, multi-walled carbon
nanotubes NC3100 (CNT) from NanocylTM, and carbon xerogel (CX)
Typical procedures for the catalytic oxidation of alkanes and products
analysis
prepared by
a
protocol described elsewhere.[33a, 47] These carbon
In typical conditions the reaction mixtures were: catalyst (0.1-10 mol,
0.002-0.2 mol% vs. cyclohexane) in acetonitrile (3.00 mL), then 10.00
materials were used in their original forms, as purchased (AC and CNT)
or as prepared (CX); oxidised with nitric acid (-ox); and oxidised with
nitric acid with a subsequent treatment with NaOH (-ox-Na). The -ox
supports were obtained by treating the pristine ones with 75 mL of a 5 M
nitric acid solution per gram of carbon material for 3 h in reflux. The solid
was then filtrated and washed with deionized water until a neutral value
of pH was obtained.[28, 34, 36, 48] The -ox-Na materials were achieved by
subsequent treatment of the -ox ones with 75 mL of a 20 mM NaOH
aqueous solution (per gram of carbon material) for 1 h in reflux, followed
by filtration and washing until neutral pH.[28, 34-36, 48]
3
mmol of oxidant, HNO (0.10 mL) and 5.00 mmol of cyclohexane (0.54
mL) were added (by this order) and the reaction solution was stirred at
the desired temperature (usually room temperature) and time at normal
pressure. For the products analysis, 90 L of cycloheptanone (internal
standard for cyclohexane oxidation) and 5.0 mL of diethyl ether (to
extract the substrate and the organic products from the reaction mixture)
were added. The obtained mixture was stirred during 10 min and then a
sample (1nL) was taken from the organic phase and analyzed by GC
using the internal standard method. Blank experiments confirmed that no
cyclohexanol or cyclohexanone were formed without the presence of
catalyst. The amount of alkyl hydroperoxide was estimated from the
variations in the alcohol and ketone yields, determined by GC, analyses,
Supports characterisation
The carbon materials were analysed by N
2
adsorption at -196 ºC using a
Quantachrome Nova 4200e apparatus. The specific surface area (SBET
)
3
upon addition of PPh to the final reaction solution, according to a
was calculated by the Brunauer-Emmett-Teller (BET) equation, the
external surface area (Sext) and the micropore volume (Vmic) were
determined by the t-method using an appropriate standard isotherm, the
method reported by Shul'pin.[41]
Typical procedures for the catalytic oxidation of alcohols and product
analysis
total pore volume (V
mesopore size (dmeso) was calculated using area and volume data
assuming cylindrical pores). The surface chemistry was characterised by
p 0
) was determined at P/P = 0.99, and the average
Oxidation reactions of the alcohols were carried out in sealed cylindrical
Pyrex tubes under focused microwave irradiation as follows: the alcohol
substrate (2.50 mmol), catalyst (0.1-15 mol, 0.004-0.6 mol% vs.
(
temperature programmed desorption (TPD) using an Altamira AMI-300
apparatus, coupled with a Ametek Dycor DyMaxion quadrupole mass
spectrometer.
t
substrate) and a 70 % aqueous solution of Bu OOH (5.00 mmol) were
introduced in the tube, which was placed in the microwave reactor. The
system was stirred and irradiated (25 W) for 0.25-1 h at 80 ºC. After the
reaction, the mixture was cooled to room temperature. 150 L of
benzaldehyde (internal standard) and 2.50 mL of NCMe (to extract the
substrate and the organic products from the reaction mixture) were
added. The obtained mixture was stirred during 10 min and then a
sample (1 L) was taken from the organic phase and analysed by GC
using the internal standard method.
Heterogenisation procedure
The complexes were anchored onto the nine different carbon materials
(0.15 g). The necessary amount to obtain ~3% Au per mass of carbon
was weighted and dissolved in 25 mL of methanol. Samples were left
stirring at room temperature for overnight, filtered, washed with methanol
and dried overnight at 40 °C, under in a vacuum oven. The gold loading
was confirmed by inductively coupled plasma (ICP) by the Analytical
Services of Instituto Superior Técnico.
Acknowledgements
Catalytic tests
The catalytic tests for alcohol oxidation were performed in a microwave
CEM discover reactor (25 W), using a 10 mL reaction tube with 13 mm of
internal diameter.
We are grateful for the financial support received from Fundação
para a Ciência e a Tecnologia (FCT), Portugal, and its projects
UID/QUI/00100/2013,
PTDC/QEQ-QIN/3967/2014. This work is a result of project
AIProcMat@N2020 Advanced Industrial Processes and
Materials for a Sustainable Northern Region of Portugal 2020”,
PTDC/QEQ-ERQ/1648/2014
and
A FISONS Instruments GC 8000 gas chromatograph (GC) equipped with
a DB-624 (J&W) capillary column (FID detector) and Jasco-Borwin v.1.50
software was used for the GC measurements. Helium was the carrier gas.
“
−
This article is protected by copyright. All rights reserved.

ChemCatChem
10.1002/cctc.201701886
FULL PAPER
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Entry for the Table of Contents
Layout 1:
FULL PAPER
[
a]
Single-pot efficient oxidative
Sónia A.C. Carabineiro,* Luísa
M.D.R.S. Martins,*[ Armando J.L.
Pombeiro, [c] José L. Figueiredo[a]
b,c]
functionalization of cyclohexane and
alcohols, under mild conditions, was
catalysed by Au(I) or Au(III)
complexes supported on activated
carbon, carbon xerogel and multi-
walled carbon nanotubes.
Page No. – Page No.
Commercial gold(I) and
gold(III)compounds supported on
carbon materials as greener
catalysts for the oxidation of
alkanes and alcohols
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