Highly selective cycloalkane oxidation in water with ruthenium nanoparticles

Article abstract of DOI:10.1002/cctc.201500805

Ruthenium(0) nanospecies, with small sizes of approximately 1.75 nm, proved to be active, selective, and retrievable nanocatalysts for the oxidation of various cycloalkanes in neat water, using tert-butylhydroperoxide as an oxidant and at room temperature. Relevant conversions and selectivities (up to 97 %) were achieved towards the major formation of the ketone product, which constitutes a high-value-added intermediate for polymer or fine chemistry. The lifetime of the catalyst has been checked over several runs, with no significant loss of activity and selectivity. Kinetic and mechanistic investigations proved that radical species are involved in the oxidation process. A literature comparison showed the relevance and the usefulness of the present ruthenium nanocatalytic system in a benign reaction context. Active, selective, and retrievable! A sustainable oxidation process of cycloalkanes to the ketones with an easy-to-handle and reusable catalyst, in neat water, and under ambient conditions is described. The active catalyst is a ruthenium(0) nanospecies. t-BHP=tert-butylhydroperoxide.

Full text of DOI:10.1002/cctc.201500805

DOI: 10.1002/cctc.201500805
Full Papers
Highly Selective Cycloalkane Oxidation in Water with
Ruthenium Nanoparticles
Audrey Denicourt-Nowicki,*[a, b] Anastasia Lebedeva,[a, b] ClØment Bellini,[a, b] and
Alain Roucoux*[a, b]
Ruthenium(0) nanospecies, with small sizes of approximately
1.75 nm, proved to be active, selective, and retrievable nanoca-
talysts for the oxidation of various cycloalkanes in neat water,
using tert-butylhydroperoxide as an oxidant and at room tem-
perature. Relevant conversions and selectivities (up to 97%)
were achieved towards the major formation of the ketone
product, which constitutes a high-value-added intermediate
for polymer or fine chemistry. The lifetime of the catalyst has
been checked over several runs, with no significant loss of ac-
tivity and selectivity. Kinetic and mechanistic investigations
proved that radical species are involved in the oxidation pro-
cess. A literature comparison showed the relevance and the
usefulness of the present ruthenium nanocatalytic system in
a benign reaction context.
Introduction
The selective oxidation of saturated hydrocarbons constitutes
an outstanding value-creating synthetic strategy, ranging from
the production of fine chemicals to the replacement of current
petrochemical feedstocks by less expensive and more readily
available alkanes.[1–3] The ensued oxygenated molecules could
be used as building blocks in various branches of the chemical
industry, ranging from polymer synthesis to medicinal chemis-
try.[4] In particular, cyclohexane oxidation remains a large-
scaled industrial process, producing approximately 106 ton per
year of cyclohexanone and cyclohexanol, also known as K-A
oil,[5] which are mostly used in the manufacture of nylon-6 and
nylon-6,6.[6] However, this process is among the least efficient
industrial chemical processes, owing to the difficulty in control-
ling the selectivity toward the target products.[7] In the present
cobalt-catalyzed industrial process, a very low conversion of
less than 5% is preferentially required to achieve a 80% selec-
tivity to K-A oil and to avoid the deep oxidation into over-oxi-
dized by-products.[8,9] Therefore, in recent years, many research
efforts have been devoted towards the search of alternative
and more environment-friendly methodologies to achieve
a high conversion in cyclohexane oxidation, while maintaining
selectivity and reducing energy consumption.[10–13]
potentially advanced catalysts owing to their outstanding in-
trinsic properties.[14–16] Besides their potential recovery poten-
tialities, they could provide relevant catalytic activities owing
to a high number of surface-exposed metal atoms and en-
hanced selectivities owing to a good shape control during the
synthesis.[17–19] As a consequence, metallic nanospecies have
found great applications in various catalytic reactions, such as
hydrogenation, carbonÀcarbon coupling or oxidation reac-
tions.[20,21] Over the last decade, fewer nanocatalysts,[22–25] such
as Fe and/or Co nanostructured catalysts, supported gold
nanoparticles or Au–Pd alloys, have been reported for the se-
lective cyclohexane oxidation, but still suffer from low conver-
sions and/or recyclability owing to metal leaching.
Herein, we report the use of ruthenium(0) nanospecies as
catalysts in the liquid-phase oxidation of various cycloalkanes
into the high-value-added ketone/alcohol products under mild
conditions, in neat water as a suitable industrial green solvent
(Scheme 1).[26] The tert-butylhydroperoxide (t-BHP) has been
In that context, nanoheterogeneous catalysis could consti-
tute a pertinent approach in the quest towards more sustaina-
ble processes for these oxidation transformations. In fact,
nanometer-sized particles have been intensively pursued as
Scheme 1. Ru-catalyzed oxidation of various cycloalkanes in neat water.
chosen as an oxidant because it has emerged as a suitable oxi-
dant for cyclohexane oxidation, possessing higher solubility
than H2O2 or molecular oxygen.[27] After optimization of the re-
action conditions, kinetic and mechanistic investigations have
been performed.
[a] Dr. A. Denicourt-Nowicki, A. Lebedeva, C. Bellini, Prof. A. Roucoux
Ecole Nationale SupØrieure de Chimie de Rennes
CNRS, UMR 6226
11 AllØe de Beaulieu
CS 50837, 35 708 Rennes Cedex 7 (France)
E-mail: audrey.denicourt@ensc-rennes.fr
alain.roucoux@ensc-rennes.fr
[b] Dr. A. Denicourt-Nowicki, A. Lebedeva, C. Bellini, Prof. A. Roucoux
UniversitØ EuropØenne de Bretagne (France)
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Results and Discussion
Table 1. Influence of the oxidant ratio and addition on the cyclohexane
oxidation.
Aqueous suspensions of ruthenium colloids were obtained
from ruthenium(III) trichloride hydrate in neat water. The TEM
analyses showed the presence of well-dispersed metallic parti-
cles with a mean diameter of 1.75 nm and 77% of the nanoob-
jects between 1.50 nm and 1.75 nm (Figure 1a,b). The rutheni-
um colloids have been further identified by high-resolution
(HR) TEM experiments (Figure 1c). The analyses of the fast
Fourier transformation of the HRTEM picture mainly revealed
the interplanar distances of 0.234, 0.214, and 0.206 nm corre-
sponding to the (100), (002), and (101) planes lattice of
ruthenium, respectively. The reduction of the Ru3+ species into
Ru0 colloids was checked by UV/Vis spectroscopic analyses,
with the displacement of the original peak (398 nm) into blue
shifts, as recently reported by Chakraborty et al.[28] This reduc-
tion could be assigned to a redox process with water, as re-
ported with rhodium and ruthenium species in high oxidation
states.[29]
The Ru0 colloidal system was evaluated in the cyclohexane
oxidation at room temperature (Scheme 2), in the presence of
tBHP as oxidant, in a pure biphasic media (water/substrate).
The conversion and the selectivity were determined by GC
analyses. In this study, we focused on the target products, cy-
clohexanone 2 and cyclohexanol 3, considering the selectivity
towards One/Ol only (cyclohexanone and cyclohexanol). How-
ever, other products could potentially be formed, in particular
the (tert-butylperoxy)cyclohexane 4, as previously reported in
the literature and from our previous works.[30–32] As a reference
experiment, no oxidation products were demonstrated in the
absence of ruthenium species under typical reaction condi-
tions, proving an expected catalyzed oxidation reaction.
In a first set of experiments, the oxidant ratio was optimized,
as well as the addition technique, to achieve relevant conver-
sions and to limit the formation of coproducts (Table 1).
Entry
Oxidant
ratio
Total reaction
time [h]
Conv.[c]
[%]
Select.[d] [%]
2/3
ratio
2
3
4
1[a]
2[a]
3[a]
4[b]
5[b]
6[b]
7[b]
1.0
1.5
3.0
4
4
4
1.5
3
35
46
71
68
73
85
97
21
30
40
33
43
56
90
5
7
6
8
5
6
–
9
4.2
4.3
6.7
4.1
8.6
9.3
1
9
25
27
25
23
7
1”3.0
2”1.5
3”1.0
6”0.5
4.5
9
[a] Reaction conditions: cyclohexane (2.23 mmol, 1 equiv.), t-BHP (n”
2.23 mmol, n equiv.), substrate/metal=125, 3 mL H2O, 208C, 4 h. [b] Reac-
tion conditions: cyclohexane (2.23 mmol, 1 equiv.), t-BHP (6.9 mmol,
3 equiv.), substrate/metal=125, 3 mL H2O, 208C, 1.5 h per t-BHP addition
plus 1.5 h after last t-BHP addition. [c] Conversion=% 2+% 3+% 4.
[d] Determined by GC analyses.
First, whatever the conditions were, the desired products,
cyclohexanone 2 and cyclohexanol 3, were formed, and (tert-
butylperoxy)cyclohexane 4 was also identified as a coproduct.
Moreover, no reaction occurred without any colloidal rutheni-
um species, thus clearly showing their catalytic role during the
oxidation reaction. In a same manner, in the absence of t-BHP,
no oxidation products were observed. The increase in the t-
BHP amount, from 1.0 to 3.0 equivalents (Table 1, entries 1 to
3), leads to an improvement of the cyclohexane conversion
(up to 71%), but also to a higher amount of coproduct 4. Sec-
ondly, to limit its formation, the addition of small quantities of
oxidant at regular intervals (1.5 h) was investigated, based on
a global amount of 3 equivalents of t-BHP (entries 4 to 7). This
study shows that the addition of 6”0.5 equivalents of t-BHP in
9 h leads to a quasi-complete conversion (97%), checked by
a good mass balance control, with a high selectivity (90%) to-
wards cyclohexanone (entry 7). It is noteworthy that an in-
crease of the temperature from 208C (entry 2) to 758C leads to
a lower conversion (17%) and selectivity (13%) towards the
ketone, probably owing to a more rapid decomposition of the
oxidant at higher temperatures.
The durability of the Ru0 colloidal system was investigated
through successive cyclohexane oxidation runs. The results are
gathered in Figure 2. Throughout four consecutive runs, con-
Scheme 2. The model cyclohexane oxidation.
Figure 1. a) Transmission electron micrograph (scale bar=20 nm), b) size distribution histogram of ruthenium colloids, and c) high-resolution transmission
electron micrograph (scale bar=2 nm).
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Table 2. Investigation on various cycloalkanes using optimized reaction
conditions.[a]
Entry
Cycloalkane
Conv.[b]
[%]
Selectivity[c] [%]
Alcohol
Ketone
Alkane[d]
1
2
3
4
5
6
7
cyclopentane[e]
cyclohexane
cycloheptane
cyclooctane[f]
cyclooctane[g]
cyclooctane[h]
cyclodecane[i]
45
97
72
47
46
34
35
37
90
72
47
46
34
35
8
0
–
–
–
–
–
–
7
–
–
–
–
–
Figure 2. Conversion (columns) and cyclohexanone selectivity (line) as a func-
tion of the recycle runs with Ru colloids. Reaction conditions: substrate
(2.23 mmol), t-BHP (6”0.5 equiv., 1.5 h per addition), substrate/metal=125,
3 mL H2O, 208C, 9 h.
[a] Reaction conditions: cycloalkane (2.23 mmol, 1 equiv.), t-BHP (6”
0.5 equiv.), substrate/metal=125, 3 mL H2O, 208C, 1.5 h per t-BHP addi-
tion, total reaction time=9 h. [b] Conversion=% ketone+% alcohol+
% (tert-butylperoxy)alkane. [c] Determined by GC Analyses. [d] (tert-
Butylperoxy)alkane. [e] After 24 h of reaction time. [f] 57% conversion in
24 h [g] In the presence of HEA16Cl (ammonium surfactant). [h] In the
presence of a randomly methylated b-cyclodextrin. [i] 35% conversion in
24 h.
versions remained high (>90%) whereas the selectivity to-
wards cyclohexanone slightly decreased and reached an pla-
teau at approximately 75%. TEM imaging after the fourth run
revealed that the particle size remains unchanged (1.75 nm),
with 57% of the nanoobjects between 1.25 nm and 1.75 nm
(Figure 3).
time, into the cyclopentanone, an important fine chemical in-
termediate and mainly used for the production of jasmon.
Though the conversion is medium, the Ru0 colloidal suspen-
sion developed in this work seems pertinent, considering the
very recent work reported for this substrate.[36]
The reaction kinetic profile (Figure 4) in optimized conditions
(6”0.5 equiv. t-BHP, 9 h) showed that the cyclohexanol amount
remains limited all over the reaction. The alcohol is a potential
intermediate in the reaction, because the cyclohexanol oxida-
tion in similar conditions leads to the formation of the corre-
sponding ketone in 4 h. Moreover, the reaction profile clearly
showed an initiation period of 3 h, in which the conversion re-
mains inferior to 10%.
The presence of such an induction time has already been re-
ported in the literature for liquid-phase cyclohexane oxida-
tion.[22,37] This phenomenon could be attributed to an insuffi-
cient amount of oxidant at an early stage or to the fact that
cyclohexanone (the reaction product) could catalyze the initia-
tion of the autoxidation process.[38] To check these hypotheses,
Figure 3. Transmission electron micrograph (scale bar=20 nm) and the size
distribution histogram of Ru0 colloidal system after the fourth catalytic run.
The scope of the oxidation reaction using the Ru0 colloidal
suspension was extended to other cyclic alkanes possessing 5
to 10 carbon atoms, at room temperature and in neat water
(Table 2). Encouraging results in terms of activity and selectivity
were obtained for these various substrates. All the substrates
studied afforded the corresponding ketones as major product
with negligible or no formation of the respective alcohol. How-
ever, increasing the size of the ring leads to lower conversions,
as also reported in the literature with some catalysts.[23,33] No
significant increase in conversion was observed after longer re-
action times (Table 2, entries 4 and 7). This result could not be
attributed to the higher lipophilicity of the substrate, because
adding a mass-transfer agent, such as an ammonium surfac-
tant[34] or a randomly methylated b-cyclodextrin[35] (entries 5,
6), did not increase the conversion. Moreover, for cyclopentane
(entry 1), a conversion of 45% was achieved in 24 h of reaction
Figure 4. Cyclohexane conversion and product selectivity as a function of
time (1=cyclohexane, 2=cyclohexanone; 3=cyclohexanol; 4=(tert-butyl-
peroxy)cyclohexane. Reaction conditions: substrate (2.23 mmol, 1 equiv.), t-
BHP (6”0.5 equiv., 1.5 h per addition), substrate/metal=125, 3 mL H2O,
208C, 9 h.
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Scheme 3. Proposed radical mechanism for cyclohexane oxidation with t-
Figure 5. Influence of the oxidant ratio (method A: 0.5+5”0.5 equiv.,
method B: 1+4”0.5 equiv.).
BHP.
C
ical species, the tert-butoxyl (tBuO ) and tert-butylperoxyl
[27,41]
C
(tBuOO ) radicals, as already reported in the literature.
two complementary experiments were performed. In a first ex-
periment, the addition of a higher amount of oxidant at the
beginning of the reaction (1 equiv. instead of 0.5, method B vs.
method A) leads to an increase in the conversion at the earlier
stage with a quite linear conversion (Figure 5).
C
tBuO abstracts hydrogen from cyclohexane, affording cyclo-
hexyl radicals, which could react with molecular oxygen from
air to form the cyclohexylperoxy radicals. The cyclohexyl radical
C
could also react with tert-butylperoxyl (tBuOO ) radical to
In a second set of experiments, cyclohexanone (0.1 and
0.5 equiv.) was added to the early stage of the reaction, thus
increasing the reaction rate with a linear conversion and no in-
duction time (Figure 6). We demonstrated that the starting
amount of t-BHP in the reaction mixture was the predominant
parameter to control the kinetics of the catalyzed oxidation re-
action.
afford 4, as already reported in the literature.[27] With regards
to recent literature,[42–44] we could presume that the spontane-
ous decomposition of the unstable product formed by a-H ab-
straction of the cyclohexylhydroperoxide by radical species
C
C
(tBuO and CyOO ) is the major ketone source and could also
explained the formation of alcohol. We could suggest a catalyt-
ic mechanism for the cyclohexylhydroperoxide decomposition
to ketone, because high cyclohexanone/cyclohexanol ratio
were observed.[38] Moreover, in the absence of O2, a very low
yield (4%) of cyclohexanone was observed, proving the role of
O2 in the proposed mechanism. Finally, during the catalytic
process, the cyclohexanol 3 could be further oxidized into the
corresponding ketone
conditions.
2 under the investigated reaction
Finally, the catalytic performances of the ruthenium(0) col-
loids developed in the present paper were compared with
those of heterogeneous catalysts (Table 3), already reported in
the literature, considering the cyclohexane oxidation with clas-
sical oxidants (t-BHP, H2O2, O2). Compared with these various
catalytic systems, the present ruthenium(0) nanocatalyst dem-
onstrated promising performances in terms of conversion and
selectivity, using an inexpensive organic oxidant (t-BHP), at am-
bient temperature, and in water as a benign and environment-
friendly reaction media. Based on these results, our system
proved to be an advanced model and thus, this easily pre-
pared, active, stable, and reusable catalyst could be promising
to develop more sustainable hydrocarbon compounds oxida-
tions.
Figure 6. Influence of the addition of cyclohexanone (0.1 and 0.5 equiv.).
In most cases, the reaction pathway for cyclohexane oxida-
tion has been described, in the literature, through a radical
mechanism.[39,40] To check the general type of mechanism in-
volved in the catalytic oxidation of cyclohexane with rutheni-
um(0) colloids, the reaction was performed in the presence of
radical traps such as the butylated hydroxytoluene or hydro-
quinone, which yielded no cyclohexanone product (<1%).
This result strongly indicates that the oxidation reaction pre-
ponderantly goes through a radical mechanism. Accordingly,
a potential radical mechanism of the Ru0 colloids-catalyzed oxi-
dation of cyclohexane is proposed in Scheme 3. In the initial
period of the reaction, the ruthenium active nanospecies cata-
lyze the homolytic decomposition of t-BHP, generating the rad-
Conclusions
An easy-to-handle, stable, and highly efficient ruthenium(0)
nanocatalyst has been developed for the oxidation of saturat-
ed cyclic hydrocarbons, possessing 5 to 10 carbon atoms, in
a pure biphasic (water/substrate) medium. High conversions
and nearly quantitative selectivities toward the formation of
the high-value-added ketone product were achieved according
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Table 3. Catalytic activities of different catalysts in the cyclohexane oxidation.
Entry
Catalyst
Oxidant
Solvent
T [8C]
Conv. [%]
Ketone [%]
Alcohol [%]
Ref.
1
2
3
4
5
6
7
8
Ru nanoparticles
Pt/Al2O3
Fe–MIL-101
H–K–OMS-2
Ru colloids
CuPd/TiO2
t-BHP
t-BHP
t-BHP
t-BHP
t-BHP
H2O2
H2O2
O2
H2O
CH3COOH
–
CH3CN
cyclohexane
CH3CN
CH3CN
–
20
70
70
80
20
50
50
140
150
125
97
96
38
59.9
1.75
88
70
11
90
4
24
56.3
64
61
85
37
50.2
9.9
–
17
25
34.3
28
39
–
58
27.7
19.1
this work
[45]
[46]
[27]
[32]
[47]
[23]
[48]
[22]
CuCr2O4
AuPd/MgO
AuPd/MIL-101
Fe nanowires
9
10[a]
O2
O2
CH3CN
acetone
50.8
36.7
[49]
[a] Adipic acid was also observed with 60.5% selectivity.
Recycling experiments
to the substrate. Furthermore, the ruthenium(0) colloidal sus-
pension could be easily recovered and reused, maintaining
a high activity and selectivity over the recycling experiments
under the investigated reaction conditions, which constitutes
sine qua non conditions for industrial applications. Mechanistic
investigations proved that radical intermediates are likely to be
involved in the oxidation reaction. This sustainable and mild
route to cyclohexanone production may be a potential alterna-
tive to the existing conventional processes.
For the durability experiments, after reaction, the Ru colloids were
extracted from the reaction mixture. The catalytic system was
reused for a next run, by adding the desired amount of cyclo-
alkane and aqueous t-BHP solution.
TEM experiments
TEM analyses were performed on a JEOL TEM 100CXII electron mi-
croscope at an accelerating voltage of 100 kV. The samples were
prepared by the addition of a drop of the ruthenium colloidal solu-
tion on a copper grid coated with a porous carbon film. The size
distributions were determined through a manual analysis of en-
larged micrographs with ImageJ software using Microsoft Excel to
generate histograms of the statistical distribution and a mean
diameter.
Experimental Section
Materials
Ruthenium(III) trichloride hydrate, RuCl3·3H2O, was obtained from
Strem Chemicals. All cyclic alkanes and the aqueous t-BHP solution
(70%) were purchased from Aldrich or Acros and used without fur-
ther purification. Water was distilled twice before use through
usual method. All the oxidation reactions were performed in a Rad-
leys Discovery Technologies carousel.
Acknowledgements
The authors thank Patricia Beaunier from UniversitØ Pierre et
Marie Curie for performing TEM analyses.
Preparation of the colloidal ruthenium suspension
Keywords: alkanes · nanoparticles · oxidation · ruthenium ·
The RuCl3·3H2O (0.1 mmol) was dissolved in 5 mL H2O, affording
a homogeneous colloidal suspension with 0.02 molLÀ1 concentra-
tion. The so-obtained ruthenium species were directly used in the
oxidation reaction.
water
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Received: July 21, 2015
Published online on November 18, 2015
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