Selective Decomposition of Cyclohexyl Hydroperoxide using Homogeneous and Heterogeneous CrVI Catalysts: Optimizing the Reaction by Evaluating the Reaction Mechanism

Article abstract of DOI:10.1002/cctc.201701909

In this study, known homogeneous and heterogeneous chromium(VI) catalyst systems were investigated with respect to the favored formation of cyclohexanone during the decomposition of cyclohexyl hydroperoxide (CHHP). The focus was on mechanistic studies using different spectroscopic methods as well as DFT calculations to further optimize the reaction conditions. As in previous decomposition studies, a mechanism via the formation of a metal alkylperoxido intermediate is probable. In situ spectroscopic studies revealed that in case of both the soluble and insoluble catalyst, the selective decomposition happens via a non-radical, non-redox mechanism at the Cr^VI stage through the formation of a cyclohexylperoxychromium(VI) complex. The proposed mechanism is supported by thorough DFT calculations.

Full text of DOI:10.1002/cctc.201701909

Accepted Article
Title: Selective decomposition of cyclohexyl hydroperoxide using
homogeneous and heterogeneous Cr(VI) catalysts: Optimizing
the reaction by evaluating the reaction mechanism
Authors: Jessica Nadine Hamann, Marko Hermsen, Anna-Corina
Schmidt, Saskia Krieg, Jasmin Schießl, Dominic Riedel,
Joaquim Henrique Teles, Ansgar Schäfer, Peter Comba, A.
Stephen K. Hashmi, and Thomas Schaub
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ChemCatChem
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Selective decomposition of cyclohexyl hydroperoxide using
homogeneous and heterogeneous Cr(VI) catalysts: Optimizing the
reaction by evaluating the reaction mechanism
[a]
[a,b]
[a]
[c]
Jessica Nadine Hamann, Marko Hermsen,
Anna-Corina Schmidt, Saskia Krieg, Jasmin
[d]
[e]
[e]
[b]
[c]
Schießl, Dominic Riedel, Joaquim Henrique Teles, Ansgar Schäfer, Peter Comba, A. Stephen
K. Hashmi,[a,d] Thomas Schaub*[a,e]
Abstract: In this study, known homogeneous and heterogeneous
chromium(VI) catalyst systems were investigated with respect to the
favored formation of cyclohexanone during the decomposition of
cyclohexyl hydroperoxide (CHHP). The focus was on mechanistic
studies using different spectroscopic methods as well as DFT
calculations to further optimize the reaction conditions. As in previous
decomposition studies, a mechanism via the formation of a metal
alkylperoxido intermediate is probable. In situ spectroscopic studies
revealed that in case of both the soluble and insoluble catalyst, the
textile industry in the manufacture of yarn or as thin plastic film for
packaging purposes.[2] On an industrial scale, the polymer is
produced via ring-opening polymerization of ε-caprolactam at
high temperatures.[1,2,4] The required starting material for the
synthesis of ε-caprolactam is cyclohexanone that reacts with
hydroxylamine in a condensation reaction to the corresponding
cyclohexanone oxime and through an acid-catalyzed Beckmann
rearrangement providing the desired ε­caprolactam (Scheme 1).[
4]
selective decomposition happens via
a non-radical, non-redox
mechanism at the Cr(VI) stage through the formation of
a
cyclohexylperoxychromium(VI) complex. The proposed mechanism is
supported by thorough DFT calculations.
Introduction
Nylon 6 is one of the most widely used polymers and is produced
on a large industrial scale (~ 4 million t/a).[1-4] This important
polymer has a wide range of applications, for example in the
Scheme 1. Industrial process for the production of Nylon 6 starting from
cyclohexanone.
The well-established industrial process to obtain cyclohexanone
is the autoxidation of cyclohexane, either non-catalyzed or
catalyzed by different transition metal compounds like soluble
cobalt catalysts.[5-15] The liquid-phase oxidation of cyclohexane is
conducted at temperatures up to 130 °C as well as high
pressures are required (125-165 °C, 8-15 bar). These conditions
are applied to shorten the retention time, in order to avoid
oxidative side reactions. For the same reason, the cyclohexane
conversion is limited to 10-12 % per cycle.[3] One common
intermediate in the oxidation procedure is cyclohexyl
hydroperoxide which further decomposes to a mixture of the
corresponding ketone and alcohol.[16-18] In detail, in a radical
[
a]
Dr. J. N. Hamann, M. Hermsen, Dr. A.-C. Schmidt, Prof. Dr. A. S. K.
Hashmi, Dr. T. Schaub
Catalysis Research Laboratory (CaRLa)
Im Neuenheimer Feld 584
69120 Heidelberg (Germany)
E-mail: thomas.schaub@basf.com
M. Hermsen, Dr. A. Schäfer
BASF SE, Quantum Chemistry & Molecular Simulation Catalysis
Carl-Bosch-Straße 38
[
b]
c]
67056 Ludwigshafen (Germany)
[
S. Krieg, Prof. Dr. P. Comba
Institute of Inorganic Chemistry & Interdisciplinary Center for
Scientific Computing
Heidelberg University
Im Neuenheimer Feld 275
chain reaction cyclohexane is oxidized to cyclohexyl
hydroperoxide by air at elevated temperature.[
8,19]
In the next step,
69120 Heidelberg (Germany)
[
d]
e]
J. Schießl, Prof. Dr. A. S. K. Hashmi
Institute of Organic Chemistry
Heidelberg University
intermediately formed cyclohexyl hydroperoxide decomposes to
an ~ 1:1 mixture of cyclohexanone (ketone K, CyO) and
cyclohexanol (A, CyOH), which is known as KA oil and is
produced on a scale of more than 7 million tons per year (Scheme
2).[8]
Im Neuenheimer Feld 270
69120 Heidelberg (Germany)
[
Dr. D. Riedel, Dr. J. H. Teles, Dr. T. Schaub
BASF SE, Synthesis and Homogeneous Catalysis
Carl-Bosch-Straße 38
67056 Ludwigshafen (Germany)
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201701909
FULL PAPER
supported transition metal catalysts.[35,44-49] For example,
following the activity of soluble cobalt catalysts, poly(4-
vinylpyridine) (PVP) supported Co(II) catalysts have been
prepared and investigated for the oxidation of cyclohexane.[
44-49]
3
In the same way, CrO immobilized on PVP was prepared for the
selective decomposition of cyclohexyl hydroperoxide and seemed
to be a promising candidate for this kind of reaction.[ In 2003,
Buijs et al. showed that homogeneous and heterogeneous Cr(VI)
catalysts exhibit a comparable reactivity with respect to the
selective decomposition of cyclohexyl hydroperoxide towards
cyclohexanone.[ The early work focused only on achieving high
conversions as well as high yields of cyclohexanone. However,
so far there is no clear picture about the mechanism using Cr(VI)
catalysts and furthermore, nothing is known about the oxidation
state of chromium at the end of the catalysis. To close this gap,
we focused on investigating the mechanism of decomposition of
cyclohexyl hydroperoxide. We investigated homogeneous and
35]
Scheme 2. Usual oxidation of cyclohexane to obtain KA oil and cyclohexyl
hydroperoxide (CHHP).
41]
Cyclohexanol and cyclohexanone can be separated by distillation
and, depending on the requirements, the alcohol can be either
used for the production of adipic acid or dehydrogenated to
cyclohexanone.[8] With the target of reducing the number of
complexity in the production of cyclohexanone, it would be
desirable to increase the selectivity towards the formation of the
ketone. Oxidation of cyclohexane without any metal catalyst
forms cyclohexyl hydroperoxide as the major product.[8] Therefore, heterogeneous catalysts using different spectroscopic methods
a few patents and studies with regard on the development of
catalysts for the selective decomposition of cyclohexyl
hydroperoxide to cyclohexanone have been published.[
During the last years, especially Cr, V, Co, Fe and Mn catalysts
have been investigated for the selective dehydroperoxidation and
seem to be the most promising.[27,28,30-33,35-37] In our previous work,
we presented different vanadium(V) catalysts with dipicolinic acid
and compared the experimental results with results from DFT
calculations. Especially, the structure of the reactive intermediate
is of interest. Based on the results, the dehydroperoxidation
reaction can be understood and optimized for a possible
application in industry. A possible mechanism is proposed based
on the combination of experimental and theoretical investigations.
20-34]
(
dipic)-based ligands or Schiff base ligands for the selective
[
21]
decomposition of different alkyl hydroperoxides. Nevertheless,
in the case of cyclohexyl hydroperoxide, a higher selectivity
towards the ketone could not be achieved. However, the obtained
results point out that transition metal oxo species (M=O) seem to
be necessary in the design of an efficient catalyst. The terminal
oxo group supports a non-radical mechanism via a hydrogen
transfer from the secondary hydroperoxide to form the ketone in
one step with a high overall selectivity.[21] Another mandatory
aspect is the stability of the catalyst, even at temperatures higher
than 120 °C. As attractive systems, Cr(VI) compounds have been
identified as promising leads, which fulfill these requirements and
Results and Discussion
Catalyst screening
In order to find the most active chromium catalyst for the
dehydroperoxidation step and to investigate a possible influence
of the ligand framework, an extensive catalyst screening was
performed under different reaction conditions. Therefore, several
known Cr(III) and Cr(VI) compounds were used in the
decomposition of neat cyclohexyl hydroperoxide (Scheme 3).
enable the selective decomposition of cyclohexyl hydroperoxide
to cyclohexanone.[
38-42]
3
Especially chromium trioxide (CrO ) is
known to perform the desired reaction with very good
selectivity.[39,40] The use of neat CrO
as catalyst to decompose
3
cyclohexyl hydroperoxide was first mentioned in a German patent
in 1973.[
Scheme 3. Selective decomposition of CHHP to cyclohexanone and water
39]
using chromium catalysts.
3
In the last years, soluble CrO has been investigated in
comprehensive studies as a potential catalyst, but to the best of
our knowledge, the Cr(VI) catalyst is not applied industrially in the
production of cyclohexanone. The main drawbacks are the high
environmental risks associated with the use of Cr(VI), which is the
most toxic form of chromium.[41] An important aspect of designing
a catalyst is to cover all possibilities of increasing the efficiency
and recyclability. Therefore, another approach to use the high
activity and in particular to benefit from high selectivity is the use
of heterogeneous chromium(VI) catalysts.[35,41,43] Besides
With focus on an industrial application, short reaction times at high
temperatures are of particular interest and for this reason
temperatures up to 130 °C were included in the screening.
Table 1. Different chromium(III) and chromium(VI) compounds used as
catalysts in the decomposition of CHHP.
Catalyst
Molar
ratio
Solv.a
Condit.
[t, T]
Conv.
[%]b
Select.
[%, K:A]
b,c
chromium species supported on zeolites and mesoporous
Cr:CHHP
chromium aluminophosphates (CrAlPO-5),[
30,41]
the oxidation of
cyclohexane as well as the selective decomposition of cyclohexyl
hydroperoxide have been investigated extensively with polymer-
a) Cr(acac)
3
1:184
Cy-d12
12 h,
rt
42
90:10
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ChemCatChem
10.1002/cctc.201701909
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on (i) the used solvent and (ii) depending on the solubility of the
used catalyst at room temperature. In some cases an influence
on the selectivity was observed by increasing the temperature.
For example, using Cr(tBu-acac) as catalyst at 100 °C, a
3
conversion of 85 % (after 20 min) was observed, but the
selectivity decreased drastically to 81:9 % (see SI). Using
b) Cr(salen*)
1:181
CDCl
CDCl
CDCl
CDCl
3
3
3
3
2 h,
88
53
91
100
80
13
55
94
44
64
45
100
88
94
99
99
98
98
69:31
29:71
55:45
66:34
96:4
40 °C
c) Cr
2
O
3
1:183
1:200
1:287
1:230
1:424
1:298
1:1511
1:766
1:415
1:388
1:400
1:441
1:477
1:512
1:380
1:560
1:398
72 h,
60 °C
d) CrCl
e) CrCl
3
24 h,
60 °C
3
CrO *py at elevated temperatures, a positive evolution in the
conversion could be observed along with small losses in
selectivity. Starting at 80 °C, a conversion of 88 % was observed.
Increasing the temperature by 10 °C, the conversion could be
increased to 94 %, whereas the same selectivity was obtained.
Finally, the best combination of good selectivity and nearly full
conversion was detected by running the reaction at 100 °C.
Overall, reactions at higher temperatures are favored relating to
the industrial process, as the stream obtained directly from the
liquid-phase oxidation also has temperatures in the range of 100
to 130°C. Therefore, it is beneficial from a process point of view if
a selective dehydroperoxidation could be applied directly on this
stream without changing the temperature (avoids additional heat
exchangers).
3 2
*6H O
2 h,
90 °C
f) Cr(tBu-acac)
3
CDCl
CDCl
CDCl
3
3
3
16 h,
rt
g) Cr(acrylate)
3
24 d,
rt
30:70
91:9
3
h) Cr(CF -acac)
3
8 d,
rt
i) Cr(Ph,Me-
acac)
Cy-d12
12 h,
60 °C
88:12
62:38
56:44
58:42
98:2
3
j) K
Cr O
2 2 7
CDCl
CDCl
CDCl
CDCl
3
3
3
3
6 h,
As shown in Table 1, the best results regarding high
conversion and high selectivity towards the ketone as well as
short reaction times at elevated temperatures were obtained
60 °C
3
k) CrO *bipy
30 min,
20 °C
1
using a soluble CrO
excellent results observed with pure CrO
temperature (Table 1, entry m), the use of a chromium-pyridine
complex as catalyst was necessary because neat CrO is
insoluble in the required non-polar solvents and addition of CrO
3
-pyridine complex as catalyst. Despite the
3
in CDCl at room
3
3
l) CrO *phen
30 min,
20 °C
1
3
m) CrO
3
10 min,
rt
3
to cyclohexyl hydroperoxide at room temperature leads to a highly
exothermic reaction. At higher temperatures, the precipitation of
an insoluble black solid was observed, due to the rapid formation
of unknown inactive chromium species. The results lead to the
assumption that (i) a Cr(VI) center is necessary but (ii) a fancy
ligand framework is not required for an efficient chromium catalyst.
Moreover, using bidentate N-donor ligands like 2,2'-bipyridine
n) CrO
o) CrO
p) CrO
q) CrO
3
*py
*py
*py
*py
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
2 h,
96:4
80 °C
3
3
3
2 h,
96:4
90 °C
1 h,
00 °C
95:5
1
(
bipy) or 1,10-phenanthroline (phen), the conversion and
selectivity decreased drastically. The dehydroperoxidation with
45 min,
20 °C
95:5
1
Cr(VI) works equally at room and elevated temperatures. Figure
1
1
shows as an example the H NMR spectra measured before
3
r) CrO *py
30 min,
30 °C
90:10
84:16
3
and after the catalytic run using CrO *py at 100 °C. After one hour,
the substrate is completely decomposed and only small amounts
of cyclohexanol are detected.
1
3
s) CrO *py
30 min,
50 °C
1
In contrast to our previous studies with vanadium(V)
complexes, the yield of cyclohexanone is not significantly
influenced by higher reaction temperatures. The slightly lower
selectivity observed when the reaction is run at 130 °C is probably
due to the competing uncatalyzed thermal decomposition of
CHHP via a radical mechanism. This non-catalyzed unselective
decomposition lowers the yield of the desired cyclohexanone. In
a separate experiment, the half-life for thermal decomposition of
CHHP at 130 °C was determined as 29 min.
[
[
[
a] 0.5 mL deuterated solvent.
b] Results based on NMR data with naphthalene as internal standard.
c] K = cyclohexanone, A = cyclohexanol.
*
A modified salen ligand was used.
The dehydroperoxidation reactions of CHHP were performed in
different deuterated solvents. In a general procedure, a solution
containing the catalyst, substrate and naphthalene as internal
standard was prepared in a NMR tube. Depending on the reaction
conditions, the tube was heated or stored at room temperature. In
case of strong exothermic reactions, the NMR tube was stored in
an ice bath. The reaction mixture was analyzed by NMR
spectroscopy. The reaction temperatures were chosen depending
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At 5 °C a first UV/vis spectrum was measured, which is shown
in Figure 2 (black line). As expected for Cr(VI), no intensive
absorption band is observed. After addition of stoichiometric
amounts of CHHP, a UV/vis spectrum was recorded every
After 1 h@100 °C
CyO
10 seconds. During the first two minutes of reaction, the rapid
formation of an absorption band at 480 nm was observed.
According to the literature, this band is assigned to a peroxy to
chromium charge transfer transition and indicates the formation
of the expected cyclohexylperoxychromium(VI) complex.[
After the rapid formation of the peroxychromium(VI) complex, the
formed intermediate decomposed at a slightly slower rate. This
behavior was observable by a shift of the absorption band to lower
wavelength (420 nm) within 10 min (Figure 2, inset black line). An
additional measurement after 48 hours revealed that the complex
had been completely decomposed (inset red line) to another
chromium species with different intensity of absorption. Boitsov et
al. also observed the same behavior in case of the formation of a
tert-butylperoxychromium complex.[51]
*
CyOH
Before
50,51]
CHHP
*
1
Figure 1. H NMR spectra before and after the catalytic run using 1 eq. CrO
as catalyst at 100 °C for 1 h, measured in cyclohexane-d12. Signals marked with
3
*py
an asterisk belong to the internal standard (naphthalene).
Mechanistic studies
3
Using the optimized catalyst (CrO -pyridine complex),
different spectroscopies were used to gain insight into the
chromium-catalyzed dehydroperoxidation. Initially, in situ UV/vis
measurements were performed to identify the oxidation state of
the metal center during catalysis. A solution of the catalyst in
cyclohexane was prepared and the UV/vis spectrum was
measured at ambient temperatures. The absorption spectrum of
the pure catalyst shows no intensive absorption bands in the
range between 300 and 1000 nm, which is in agreement with the
oxidation state +6 of chromium (see SI). After the addition of
2
min
1
0 min
8 h
4
500 eq. of substrate at room temperature, the fast catalytic
reaction was followed for 60 min. The intensity of the weak
absorption at ~ 450 nm decreases due to the coordination of the
substrate to the metal center, but during the following catalytic
decomposition of the cyclohexyl hydroperoxide, no further
absorption bands were observed. Especially, in the interesting
area around 600 nm no new absorption bands appear. This
implies that the oxidation state of chromium does not change
during the catalytic decomposition reaction. At the end of the
reaction, the oxidation state is still +6, as in case of a reduction to
Figure 2. UV/vis spectra detected during the rapid stoichiometric reaction of a
.93 mM solution of CHHP in α,α,α-trifluorotoluene and 1 eq. CrO *py at 5 ° C.
Inset: Spectrum of formed cyclohexylperoxychromium(VI) complex after 2 min
(blue line); decomposition and shift to lower wavelength after 10 min (black line)
and full decomposition after 48 h (red line); l = 1 cm.
0
3
3
Cr(III) two spin allowed d-d-transitions of the d configuration in
the range of 420 - 450 nm and 580 - 620 nm are expected.
Based on this first indication concerning the oxidation state of
the chromium catalyst, the reactive intermediate was
characterized by low-temperature measurements. In accordance
with our proposed mechanism for the vanadium(V) catalysts,[
the formation of an alkylperoxychromium(VI) complex was
expected. Following the procedure by Boitsov et al. to detect a
The cyclohexylperoxychromium(VI) complex formed when a
stoichiometric amount of CHHP is added decomposes slowly to
the ketone and CrO (OH) over time. In the presence of water, the
2 2
formation of chromic acid is favored and stabilized in the aqueous
environment, especially at low temperatures. As Boitsov et al.
showed in their study, an acidic environment influences the
21]
tert-butylperoxychromium(VI)
complex,
in
situ
UV/vis
[
51]
measurements were performed at low temperatures. Therefore, stability of the investigated tert-butylperoxychromium(VI) complex
drastically.[51] This implies that the formation and stability of the
cyclohexylperoxychromium(VI) complex is influenced by the
strongly acidic reaction mixture. The following experiment
supported this assumption: 10 equivalents of cyclohexyl
hydroperoxide were added to the solution containing the
a solution of CrO *py in anhydrous α,α,α-trifluorotoluene was
3
prepared and cooled to 5 °C. The non-coordinating solvent
α,α,α-trifluorotoluene (BTF) was chosen to further slowdown the
rate of reaction.[
51]
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ChemCatChem
10.1002/cctc.201701909
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decomposed cyclohexylperoxychromium(VI) complex at 5 °C, but
the formation of a new peroxy complex could not be observed.
The decomposition of the substrate to cyclohexanone was not
observed with the inhibited catalyst at low temperatures. On the
other hand, adding 100 eq. cyclohexyl hydroperoxide to the
inhibited cyclohexyl-peroxychromium(VI) complex and running a
catalytic reaction at 120 °C, the selective formation of
cyclohexanone was observed. That implies, that the regeneration
of the cyclohexylperoxychromium(VI) complex is suppressed at
low temperatures but takes place at higher temperatures. By
using an excess of CHHP like under the conditions of the catalytic
CHHP decomposition, the equilibrium seems to be shifted to the
active form of the catalyst.
cyclohexylperoxychromium(VI) complex and is catalytically
inactive. Addition of an excess of cyclohexyl hydroperoxide did
not form the cyclohexylperoxychromium(VI) complex and
decomposition of the cyclohexyl hydroperoxide was not observed.
The
formation
of
the
reactive
intermediate
(cyclohexylperoxychromium(VI) complex) was also detected by
NMR spectroscopy. Due to the observed rapid formation and
decomposition of the reactive intermediate, measurements were
8
performed at -20 °C in deuterated toluene (toluene-d ). To gain
insights into the coordination of cyclohexyl hydroperoxide to the
chromium center, in the first step a 1H NMR spectrum of pure
CHHP was measured at -20 °C (Figure 3 lower part). The focus
was put on the proton signal at 3.72 ppm (red label), which was
assigned to the CH-group next to the OOH group. Immediately
after addition of stoichiometric amounts of the catalyst, a
Another experiment was also performed by adding
2.5 equivalents of substrate to 1 equivalent of Cr(VI). The same
absorption band as described in Figure 2 (blue line) was observed, downfield shift to 4.09 ppm of this signal was observed (Figure 3
but its formation and subsequent decomposition happened twice
as fast. In further experiments, the stability of the reactive
cyclohexylperoxychromium(VI) complex was investigated by the
addition of water. To the reactive intermediate one equivalent of
water was added that leads immediately to the formation of
undefined chromium(VI) species. NMR investigations of the
reaction mixture showed, that cyclohexanone was not formed
during the water-induced rapid conversion at low temperatures.
This implies that the catalyst is inhibited by the presence of water
at low temperatures. To further investigate the influence of formed
water during the catalytic decomposition of CHHP, a catalytic
reaction was performed in the presence of a 25-fold excess of
water (7500 eq according to Cr(VI)). The obtained results showed
that larger quantities of water are inhibiting the catalytic activity of
Cr(VI) indicated by the observed lower conversion (99 % vs.
upper part).
3
Addition of CrO *py @ -20°C
OOCH
OH
OOH
Pure CHHP @ -20°C
46 %) and selectivity (95 % vs. 71 % ketone, for further details
OOCH
see SI). This aspect is in line with the reported removal of
formed water from the reaction to avoid catalyst inhibition.[41]
The addition of two equivalents cyclohexanol to the
stoichiometrically
formed
cyclohexylperoxychromium(VI)
(
480 nm) complex led to a shift of the charge transfer transition to
1
lower wavelength (455 nm) but no decrease in intensity. The blue
shift may be due to ligand exchange (CHHP to cyclohexanol).[
This formed cyclohexyl chromate complex is more stable than the
Figure 3. Low temperature H NMR spectra of pure CHHP (lower part) and after
the addition of stoichiometric amounts of CrO *py (upper part), measured in
toluene-d . Marked are significant changes and shifts due to the formation of a
cyclohexylperoxychromium(VI) complex.
51]
3
8
Table 2. Comparison of experimental and calculated 1H NMR shifts,
which
are
observable
during
the
formation
of
the
This is induced by coordination of the peroxide group to the
metal center, leading to a reduction of the electron density and
the proton becomes more deshielded. The broadening of the
signal, due to the metal environment, supports this aspect.
Furthermore, the H signal of the OOH group at 5.83 ppm
disappeared, and a new broad signal at 11.64 ppm appeared
cyclohexylperoxychromium complex.
NMR shifts / ppm
Pure CHHP
OOH
Experimental
Calculated[51]
1
5.83
3.72
6.62
3.84
(
blue label). In order to verify the observed changes in the NMR
OOCH
spectra, DFT calculations of the expected shifts were carried
[
52]
out. A comparison of the relevant experimental and calculated
Coordinated
species
1H NMR shifts is provided in Table 2.
Although there is a systematic shift between measured and
calculated shifts, there is an excellent linear correlation between
measured and calculated shift, with a slope of 1 and a correlation
2
CrO (OH)(OOCy)
OH
11.64
4.09
11.82
4.19
2
coefficient r = 0.992. The observed downfield shift of relevant
OOCH
signals
is
induced
by
the
formation
of
the
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ChemCatChem
10.1002/cctc.201701909
FULL PAPER
cyclohexylperoxychromium(VI) intermediate. In agreement with
the calculated shifts, the OOCH-signal shifts by 0.37 ppm. Based
on the calculated results, the new broad signal at 11.64 ppm is
assigned to the OH group on Cr(VI). Finally, the disappearance of
the hydroperoxide group signal further supports that the substrate
is coordinated to the hexavalent chromium center. In the 13C
spectra, the calculation predicts only small shifts in the range of
cyclohexyl hydroperoxide was investigated with a focus on the
mechanism as well as leaching properties.
In accordance to Lončarević et al., insoluble poly(4-vinylpyridine)
supported chromium(VI) catalysts were prepared by adding CrO
3
to a suspension of PVP in water.[35] To investigate the influence
of the metal loading, catalysts with chromium(VI) contents in the
range of 0.9 until 7.3 wt. % were used. Using the same chromium
to substrate molar ratio (1:470) in the following catalytic studies at
0.1-0.7 ppm upon coordination. Still these are in good agreement
with the experimentally obtained 13C NMR data (for details see SI). 60 °C, the influence of the chromium loading was determined. As
Besides UV/vis and NMR measurements, the dehydro-
peroxidation reaction was also investigated using in situ IR
spectroscopy. At different temperatures, the reaction of
shown in Figure 4, the specific activity of the chromium catalyst
decreases with an increasing metal content on PVP.
3
cyclohexyl hydroperoxide and CrO was monitored in order to
detect possible vibrations of the cyclohexylperoxychromium(VI)
intermediate. Therefore, the reaction was performed in both a
stoichiometric as well as in a catalytic mode. Relevant but weak
chromium(VI)-oxygen vibrations should be observable in the
-1
-1
range of 700 cm to 900 cm , but despite comprehensive studies,
no characteristic vibrations, which belong to the reactive
[
50]
intermediate, could be detected. However, during the catalytic
run, an increasing formation of cyclohexanone could be observed
due to the decomposition of CHHP (see SI). After the addition of
1
3
eq. CrO to 250 eq. cyclohexyl hydroperoxide at -74 °C and
warming to room temperature, an intensive vibration appears in
-1
the range of 1700 to 1750 cm , which increases in intensity with
reaction time. Due to the characteristic range of carbonyl
stretches, this new intensive band is assigned to the C=O stretch
of emerging cyclohexanone (see SI).
Figure 4. Comparison of the observed CHHP conversion after 80 min at 60 °C
at the same Cr:CHHP ratio of 1:470 in cyclohexane-d12 using catalysts with
different Cr(VI) loadings on PVP.
With respect to the obtained mechanistic insights, it is
possible to propose that the dehydroperoxidation of cyclohexyl
hydroperoxide happens via (i) the formation of an
alkylperoxychromium(VI) intermediate and (ii) Cr(VI) is not
reduced during the catalysis.
For example, using CrPVP with a chromium content of 7.3 wt. %,
the conversion of CHHP was only 45 % after 80 min at 60 °C (see
SI). A conversion of 98 % was achieved only after 5.5 h, leading
to a ketone:alcohol:dicyclohexyl peroxide ratio of 55:32:13 (Table
3). Using the lowest loaded CrPVP catalyst, a nearly full
Heterogeneous Cr(VI) catalysis
conversion was obtained after 2.5 h with a selectivity of 74 %
ketone. With a higher chromium content, the rate of reaction
decreases and the formation of dicyclohexyl peroxide could be
detected by NMR spectroscopy. Due to a lower catalytic activity,
uncatalyzed radical reactions are favored to form cyclohexyloxy
radicals, which induce the formation of this coupled side-product
(Figure 5).
In the context of a potential industrial application, a dissolved
Cr(VI) catalyst would require an elaborated recycle concept and
efficient disposal and treatment to remove or reduce the toxic
Cr(VI) species in the work-up, implicated its toxicity. In order to
use the desirable properties of chromium(VI) catalysts, a possible
approach is the use of heterogeneous chromium(VI) catalysts.[
Especially the use of Cr(VI) immobilized on the well-known poly(4-
vinylpyridine) (PVP) presents the simplest approach to
35]
heterogenize the CrO
3
*py complex used above.[53-56] Poly(4-
vinylpyridine) supported Cr(VI) complexes have already been
studied as catalysts for the decomposition of cyclohexyl
hydroperoxide. In 2010, Lončarević et al. presented a study using
poly(4-vinylpyridine)-supported cobalt(II) and chromium(VI)
catalysts in the oxidation of cyclohexane and decomposition of
cyclohexyl hydroperoxide under mild conditions.[35] In the case of
the dehydroperoxidation, CrPVP showed a higher selectivity
towards cyclohexanone, whereas the activity of CoPVP was
higher. Possible effects due to leaching of metal ions were only
investigated for the cobalt catalysts. Regarding the mechanism
using PVP supported catalysts, no clear picture was given. To
address this point, the CrPVP catalyzed decomposition of
Figure 5. Structure of the side-product dicyclohexyl peroxide, whose formation
increases from 6 % to 14 % (after 80 min )with a Cr(VI) loading of 7.3 wt. %.
Table 3. Catalytic decomposition reaction of CHHP using CrPVP with different
Cr loadings.
Catalyst
CrPVP
Molar
ratio
Cr:CHHP
Solv.a
Condit.
[t, T]
Conv
Select.
[%,
.
[%]b
b
K:A:CyOOCy]
,c
0.9 wt.%
1:469
Cy-d12
2.5 h,
98
74:20:6
60 °C
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CHHP was performed at different temperatures and the reaction
mixtures analyzed by NMR spectroscopy using naphthalene as
internal standard. These results are listed in Table 4. Reaction
temperatures between 80 °C and 150 °C were chosen which are
in the range of temperatures used in the dehydroperoxidation in
industrial plants.[8] With increasing temperature, the rate of
reaction also increases. At 150 °C the reaction time required for
full conversion could be reduced to 30 min.
2
3
7
.6 wt.%
.5 wt.%
.3 wt.%
1:471
1:470
1:469
Cy-d12
Cy-d12
Cy-d12
3 h,
98
98
98
75:19:6
65:26:9
55:32:13
60 °C
3.5 h,
60 °C
5.5 h
0 °C
6
[
[
[
a] 0.5 mL deuterated solvent.
b] Results based on NMR data with naphthalene as internal standard.
c] K = cyclohexanone, A = cyclohexanol, CyOOCy = dicyclohexyl peroxide.
Table 4. Catalytic decomposition reaction of CHHP using CrPVP (2.6 wt% Cr-
loading) at different temperatures.
An explanation of these results is the formation of less-
Solv.a
Catalyst
Molar
ratio
Cr:CHHP
Condit.
[t, T]
Conv.
[%]b
Select.
[%, K:A]
b,c
reactive dichromate species due to an overloading with chromium.
In order to clarify this point, a dichromate-loaded quaternized PVP
was prepared. The quaternized PVP was used to rule out
coordination of the chromium in the dichromate to the N-donor
ligand. The quaternized PVP precursor was prepared by
t) CrPVP
u) CrPVP
v) CrPVP
w) CrPVP
x) CrPVP
y) CrPVP
1:545
1:412
1:647
1:589
1:611
1:450
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
2 h,
99
99
99
98
98
98
98:2
80 °C
well-known procedure.[57] The
2 h,
98:2
methylating PVP using
a
90 °C
successful quaternization was followed by IR spectroscopy to
detect the disappearance of the vibration of the pyridine ring at
1 h,
100 °C
98:2
-1
~
1600 cm-1 and the appearance of a new one at ~ 1640 cm ,
which belongs to the quaternized pyridine ring.[ The methylated
PVP was then used to prepare a PVP-supported dichromate
catalyst (CrMePVP, Scheme 4) containing 3.2 wt. % Cr.[59]
58]
45 min,
92:8
120 °C
30 min,
30 °C
91:9
1
30 min,
50 °C
82:18
1
[
[
[
a] 0.5 mL deuterated solvent.
b] Results based on NMR data with naphthalene as internal standard.
c] K = cyclohexanone, A = cyclohexanol.
Scheme 4. Reaction overview to obtain the dichromate catalyst supported on
However, at temperatures higher than 130 °C, the selectivity
decreases due to the competing thermal decomposition of CHHP.
The best results were obtained using moderate reaction times at
100 °C. The same behavior was observed running the reaction
MePVP.
When running a similar catalytic decomposition reaction using
CrMePVP as catalyst, comparable results were obtained to the
with the homogeneous CrO
3
*py catalyst. A comparison of the
3
experiment with the highest loading of CrO : 55 % conversion of
evolution of selectivity depending on the reaction temperature is
CHHP and an increased formation of cyclohexyl peroxide (see SI). given in Table 5.
In the literature, different dichromate PVP species are known as
oxidizing agents, but only for the oxidation of alcohols. However,
in the case of cyclohexanol, long reaction times (more than 20 h)
and high temperatures are required for its oxidation yielding 72 %
cyclohexanone.[59] Therefore, it can be assumed that the
decreasing activity at a higher chromium content is connected
with the formation of dichromate species that are less effective for
the catalytic decomposition of cyclohexyl hydroperoxide.
To gain insights into the CrPVP-catalyzed dehydro-
peroxidation, different studies including kinetic studies were
performed. Measurements with the 0.9 wt. % and 2.6 wt. %
catalysts at 100 °C revealed a slightly higher conversion (after
1
9
h) by using the higher loaded CrPVP (99 % with 2.6 wt.% vs.
7 % with 0.9 wt.%). The selectivity is the same for both systems
(
98:2) Therefore, in the following experiments, the catalyst used
had a chromium loading of 2.6 wt. %. The decomposition of
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ChemCatChem
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Table 5. Comparison of the evolution of selectivity depending on the
Table 6. Reusability of CrPVP in the catalytic decomposition reaction of
temperature using homogeneous and heterogeneous Cr(VI) catalysts.
CHHP over 10 cycles at 100 °C.
3
CrO *py
Conv.
Select.
[%, K:A]
CrPVP
Conv.
[%]
Select.
[%, K:A]
Run Catalyst
Molar
ratio
Solv.a
Condit.
[t, T]
Conv.
[%]b
Select.
[%, K:A]
b
b,c
b
b,c
b,c
[%]
Cr:CHH
P
8
9
1
1
1
1
0 °C
88
96:4
96:4
80 °C
99
99
99
98
98
98
98:2
98:2
98:2
92:8
91:9
82:18
0 °C
94
99
99
98
98
90 °C
1.
CrPVP
CrPVP
CrPVP
CrPVP
CrPVP
CrPVP
CrPVP
CrPVP
CrPVP
CrPVP
1:507
1:531
1:498
1:487
1:501
1:505
1:517
1:499
1:506
1:503
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
Cy-d12
1 h
00 °C
99
98
98
95
95
93
92
90
87
85
98:2
97:3
95:5
93:7
91:9
91:9
90:9
89:11
87:13
85:15
1
00 °C
20 °C
30 °C
50 °C
95:5
100 °C
120 °C
130 °C
150 °C
2.
1 h
95:5
100 °C
1 h
90:10
84:16
3.
1
00 °C
4.
1 h
00 °C
1
In previous studies on heterogeneous chromium(VI) catalysts,
leaching of active chromium ions was the reason for the
observable selective catalysis.[41] Due to the high toxicity of Cr(VI),
special interest was put on a possible leaching behavior of CrPVP.
In a first attempt to rule out leaching of chromium ions, the
colorless product solution was investigated using ICP-MS. The
chromium concentration was below the detection limit of the
method used (<0.1 mg kg ). Further evidence to exclude
leaching of active chromium was obtained by running a split test.
Therefore, a dehydroperoxidation reaction was performed at
5.
1 h
100 °C
6
.
.
1 h
100 °C
7
1 h
00 °C
1
-1
8.
1 h
100 °C
1 h
5
5 °C to slow down the decomposition of CHHP. After 25 min, the
9.
hot clear supernatant reaction mixture was divided into two parts.
Both samples, a) with catalyst and b) without catalyst were heated
to 55 °C for further 40 min. The following NMR spectroscopic
investigation revealed that in case of sample a) the substrate was
further decomposed to the ketone (for details see SI). In contrast,
in sample b) the decomposition of CHHP stagnated during further
heating to 55 °C. Moreover, no changes in the compared NMR
spectra are observable (see SI). The same procedure was
performed to exclude possible leaching behavior at elevated
temperatures by running a split test at 100 °C. To slow down the
fast reaction, a Cr:CHHP ratio of 1:1100 was applied. After 2 min
of reaction, the hot supernatant solution was separated and
investigated as described above. In case of sample a) the
decomposition further happened. In case of sample b) the
selective decomposition mainly stagnated, but due to the
uncatalyzed self-decomposition of CHHP at elevated
temperatures, traces of dicyclohexyl peroxide were detected. The
obtained results underline that active chromium ions were not
leaching during the catalysis, even at elevated temperatures, and
1
00 °C
10.
1 h
00 °C
1
[
[
[
a] 0.5 mL deuterated solvent.
b] Results based on NMR data with naphthalene as internal standard.
c] K = cyclohexanone, A = cyclohexanol.
By reusing the catalyst ten times, the selectivity towards
cyclohexanone decreased by 13 %, whereas the conversion
declined by 14 %. Following the recycle tests using NMR
spectroscopy, the formation of small traces of dicyclohexyl
peroxide started after five cycles (see SI).
3
Kinetic studies using CrO *py and CrPVP catalysts
The results with homogeneous and heterogeneous
chromium(VI) catalysts show that both types of catalysts
as expected, the decomposition happens by
heterogeneous pathway.
a
purely
decompose
cyclohexyl
hydroperoxide
selectively
to
cyclohexanone. The mechanistic studies concerning the
homogeneous pathway strongly suggest dehydroperoxidation
occurs via the formation of a cyclohexylperoxychromium(VI)
species.
To compare both catalytic systems and to gain insights into
the reaction order, kinetic studies using 1H NMR spectroscopy
were performed. The reaction was followed by 1H NMR in
cyclohexane-d12 at temperatures between 50 °C and 66 °C to
obtain the rate constants k at different temperatures. Due to the
slower reaction rate, the catalytic decomposition of cyclohexyl
For an industrial application, the recyclability of the CrPVP
catalyst is also a matter of interest and a reusability test was
conducted in ten cycles with the CrPVP at 100 °C (each run 1 h).
After running a catalytic decomposition of CHHP, the supernatant
clear solution was removed and the catalyst was washed with
cyclohexane (3 x). Fresh substrate was added and a further
catalytic dehydroperoxidation was performed. As shown in Table
6
, the reusability could be confirmed with slight loss in activity (see
Table 6).
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ChemCatChem
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hydroperoxide could conveniently be monitored during the first
8
0 minutes of reaction. The 1H NMR spectra using CrPVP as
catalyst at 50 °C are shown in Figure 6.
1
80 min
60 min
40 min
*
*
*
2
0 min
min
*
*
Figure 7. Plot of ln[A]
t
/[A]
0
vs. time for the decreasing concentration of CHHP
during the catalytic conversion using CrPVP as catalyst. The rate constants k
are obtained from the slope.
5
After 80 min of reaction, the maximum conversion was
determined to 75 % at 339 K. Due to the incomplete
decomposition of CHHP, the relative small amounts of formed
water are negligible and have no influence on the reactions rates
regarding possible inhibition effects. As shown above, a 25-fold
excess of water is necessary to see a significant inhibition effect
under the conditions of the catalysis. According to transition state
theory, the activation parameters ΔH and ΔS were determined
from the temperature dependence of the obtained rate constants
k. A representative Eyring plot, ln (k/T) vs. 1/T for the CrPVP
system, is given in Figure 8.
1
Figure 6. H NMR spectra recorded following the catalytic dehydroperoxidation
of CHHP using CrPVP at 50 °C for 80 min, measured in cyclohexane-d12
.
Signals marked with an asterisk belong to the internal standard (naphthalene).
As shown in Figure 6, with increasing conversion of CHHP,
the concentration of cyclohexanone increases. The reaction rates
for the decomposition of cyclohexyl hydroperoxide at different
temperatures were obtained using the differential equation
describing first-order kinetics.[60] Both catalytic systems
‡
‡
(
homogeneous and heterogeneous) follow first-order kinetics as
revealed by the linear plots in Figure 7 and the determined rate
constants for the decomposition of CHHP are given (Table 7).
Table 7. Rate constants for the decomposition of cyclohexyl
hydroperoxide into cyclohexanone following first-order-kinetics.
T / K
k / 10-5 s-1
CrPVP
T / K
k / 10-5 s-1
3
CrO *py
323
331
339
15.6 ± 1.53
24.2 ± 1.49
29.6 ± 2.05
323
331
339
16.1 ± 1.01
27.3 ± 1.65
47.2 ± 1.88
0
The corresponding linear plots of ln {[CHHP]/[CHHP] versus
time, at 323, 331 and 339 K for the CrPVP system are shown in
Figure 7. The values of the rate constants k were obtained from
the slope of the fitted linear equation. A comparable plot for the
Figure 8. Eyring plot for the decomposition of CHHP to cyclohexanone using
CrPVP as catalyst.
3
soluble CrO *py system is given in the SI. In order to confirm that
‡
the decomposition follows a first-order kinetics depending on the
catalyst concentration, further studies with different chromium
concentrations were performed at 323 K. The results based on
the linear plots confirm a clear correlation between the reaction
rates k and the chromium concentration. This aspect leads to the
conclusion that the dehydroperoxidation follows first-order
kinetics depending on (i) substrate and (ii) chromium
concentration (see SI).
Using the Eyring plot, the value of ΔH can be obtained from
‡
the slope whereas the value of ΔS is obtained from the intercept.
For the decomposition reaction using CrPVP, the activation
parameters were estimated to ΔH = 59.2 ± 2 kJ mol and
ΔS = -137.3 ± 5 J K mol . At 323 K, the Gibbs free energy of
activation was calculated to be ΔG = 102.8 ± 3 kJ mol .
Interestingly, for the homogeneous CrO *py system (see SI for
details), the Gibbs free energy of activation was nearly identical
ΔG = 102.7 ± 8 kJ mol ) although the activation enthalpy and
entropy are different (ΔH = 34.8 ± 7 kJ mol and ΔS = -213.7 ±
‡
-1
‡
-1
-1
‡
-1
3
‡
-1
(
‡
-1
‡
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ChemCatChem
10.1002/cctc.201701909
FULL PAPER
-1
-1
2
2 J K mol ). The strongly negative entropy of activation
additionally shows that the transition state has a higher degree of
organization than the reactants, which is also in accordance with
the proposed mechanism.
∆
G
O
H
kJ/mol
O
O
Cr
HO
O
H
O
The values for the activation parameters indicate that the
homogeneous as well as the heterogeneous Cr(VI) systems
follow the same mechanism for the catalytic decomposition of
cyclohexyl hydroperoxide to cyclohexanone, and both reactions
can be described by a kinetic which is first-order in both CHHP
and Cr. To gain further insight into the mechanism as well as the
activation parameters, comprehensive DFT studies were
performed.
TS4
O
H
O
O
H
Cr
O
O
O
O
RO
Cr
HO
O
O
O
TS6
H
H
H
TS3
OOH
2
H O
R =
H
83.7
O
O
O
H
83.
O7
Cy
85.3 (TS7)
Cr
HO
O
O
O
H
OOCy 87.8 (TS8)
H
O
TS2
Computational investigations
Cr
HO
OH
O
O
1
Cr
O
HO
O
DFT calculations were used to further elucidate the mechanism of
the chromium-catalyzed dehydroperoxidation. Based on the
obtained computational results, the following mechanism can be
proposed for the dehydroperoxidation reaction using
chromium(VI) catalysts.
O
5
O
O
Cr
HO
OH
+
O
1
The key step is a hydrogen transfer from the peroxide
α-carbon to the oxo ligand on the chromium(VI) center with
simultaneous cleavage of the oxygen-oxygen bond. This
mechanism was suggested earlier by Buijs et al. who studied this
particular step in the reaction at the B3LYP/6-31G** level.[41] Our
investigation shows that hydrogen transfer and O-O bond
cleavage is indeed the rate-determining step, considering
different variants of chromic acid esters and alkyl peroxide
species that may be present in the reaction mixture as well as
complexes and adducts that can be formed in the presence of
pyridine (as a model for poly(4-vinylpyridine), PVP). As given in
Figure 9, starting from chromic acid 1, addition of cyclohexyl
hydroperoxide can proceed through a proton relay mechanism
with cyclohexanol (TS2) with a barrier of 54.5 kJ mol-1 to form the
alkyl peroxide chromium(VI) complex 5, which is exergonic by
Figure 9. Mechanism for the dehydroperoxidation of cyclohexyl hydroperoxide
by chromic acid and derivatives, energies given are free enthalpies in kJ mol-1
The rate-determining step is the release of ketone from the alkyl peroxide
complex 5. Addition of cyclohexyl hydroperoxide to the free acid is greatly
accelerated by proton relay agents such as cyclohexanol or water.
.
The barriers for the cyclohexyl ester or dialkyl peroxide are
very similar (85.3 kJ mol-1 and 87.8 kJ mol-1, respectively). In the
presence of pyridine, a chromium(VI) complex with distorted
trigonal planar coordination geometry can be formed, resembling
the Collins reagent’s structure.[61] The activation barriers of the
dehydroperoxidation for the pentacoordinated complexes with
pyridine depend stronger on the second ligand (due to interaction
with the additional pyridine ligand) but are lower in the case of the
-1
dialkyl peroxide (74.1 kJ mol , see the SI).
-1
2
3.1 kJ mol . Addition via water (TS3) is also feasible with an
We also looked at dichromic acid and the chromate and
dichromate anions as potential catalysts, which could be
generated upon deprotonation through pyridine. The mechanistic
picture is quite similar but the anions exhibit lower barriers for
-1
activation energy of 75.1 kJ mol and even direct addition without
-1
proton relay agents is possible (TS4, 86.2 kJ mol ). Note, that the
direct addition has a similar energetic barrier as the following
deperoxidation step (TS6/TS7/TS8) so that even without proton
relay agents, the overall barrier of the reaction would not change.
Under the catalytic conditions however, and regarding the
generation of water from the reaction itself, we suggest that proton
relays are taking place. Since intermediates 1 and 5 are in
equilibrium, a large excess of water prevents formation of the
alkylperoxo intermediate 5 and is therefore disadvantageous.
Similarly, an addition can occur to monosubstituted chromic
acid derivatives or directly to chromium trioxide (see SI), albeit
being higher in energy. Intermediate 5 can then go through the
-1
-1
dehydroperoxidation (79.0 kJ mol for chromate, 71.5 kJ mol for
dichromate, see SI), driven by the higher basicity of the oxo group
in the anions. Overall, the discussed mechanism should be
feasible for both monomeric chromic acid as well as its dimer and
the respective deprotonated anions. However, as shown in our
experiments with dichromates we observed a drop in yield and
selectivity, which we attribute to side reactions only possible with
the dinuclear catalyst while the regular chromic acid/chromate
gives clean conversion to the ketone.
We also investigated a mechanism proposed by Boitsov et al.,
who suggested a mechanism on the basis of NMR experiments,
the ketone is formed from the corresponding alcohol via an
alkoxide/alkyl peroxide complex (see Figure 10).[51]
-1
six-membered transition state TS6 with a barrier of 83.7 kJ mol ,
in which the hydrogen of the peroxide carbon is transferred to an
accepting oxo group on the chromium(VI) center, while the O-O
bond is cleaved to release cyclohexanone.
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ChemCatChem
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FULL PAPER
mechanistic studies including DFT calculations revealed that the
chromium(VI)-catalyzed
hydroperoxide happens
cyclohexylperoxychromium intermediate. These observations are
in agreement with our previous work using different vanadium(V)
complexes as catalysts.[21] The initial hypothesis of the
decomposition
via the
of
cyclohexyl
of a
∆
G
O
kJ/mol
formation
Cr
O
O
O
O
O
Cr
O
O
O
O
O
H
H
H
TS10
H
[21]
importance of a terminal M=O group
for a non-radical and
TS11
therefore selective dehydroperoxidation was confirmed using
CrO as catalyst in this work. Previous studies using chromium(VI)
(
258.4 kJ/mol)
3
as catalyst presented different mechanisms regarding the
selective decomposition of cyclohexyl hydroperoxide.[
41,51]
A
comparison of the soluble and insoluble catalysts with respect to
their kinetic behavior showed that both systems follow the same
mechanism.
The DFT calculations performed on the chromium-pyridine
complex are supporting that the selective decomposition of
cyclohexyl hydroperoxide proceeds via the expected non-radical,
OH
O
O
Cr
+
+
O
O
O
Cr
O
non-redox
mechanism
through
formation
of
the
O
O
O
cyclohexylperoxychromium complex as the reactive intermediate.
Furthermore, the experimentally obtained activation parameters
are in good agreement with the calculated values, indicating that
the proposed mechanism is feasible.
9
Figure 10. “Cooperative” alcohol/hydroperoxide mechanism suggested by
[
51]
[62]
Boitsov et al.
intermediate resulting in the same net reaction. The process is however
energetically far less feasible than the mechanism shown in Figure
In this mechanism, the corresponding alcohol is merely an
Due to the increasing demand for polyamides, the development
of efficient as well as cheap catalysts for the key step, the
formation of cyclohexanone, is of current interest in industry. As
presented, the use of insoluble chromium(VI) catalysts provides
an alternative way to reduce the steps in the oxidation of
cyclohexane to cyclohexanone. Furthermore, environmental risks
due to leaching of toxic Cr(VI) ions could be excluded. Finally, the
presented heterogeneous polymer-supported chromium(VI) is a
cheap as well as reusable catalyst and seems to be a promising
system for an industrial application.
9
-
1
-1
(
168.9 kJ mol vs. 83.7 kJ mol ).
Formation of chromium trioxide, cyclohexanol and cyclohexanone
from the cyclohexanol cyclohexyl hydroperoxide diester of
chromic acid (9) is exergonic by 22.3 kJ mol , but the barrier for
this reaction is 168.9 kJ mol- via TS10 (the proton relay through
water, TS11, is even higher in energy with 258.4 kJ mol ), making
it energetically not feasible. Several effects likely play into this: the
lower basicity of the peroxide oxygen to accept the incoming
proton from the alkoxide, the weaker acidity of the proton on the
alkoxide relative to the hydrogen on the peroxide and the
-1
1
-1
relatively strong Cr-O bond, which needs to be broken, while the Experimental Section
previous mechanism only cleaves the weak O-O bond.
General Methods. Unless otherwise specified, reactions were carried out
under air in sealed NMR tubes or round bottom flasks. NMR spectra were
recorded using Bruker 200 at CaRLa or Bruker AVANCE III 300, Bruker
AVANCE III 400, Bruker AVANCE III 500 and Bruker AVANCE III 600
spectrometers at the Institute of Organic Chemistry / Heidelberg University.
Chemical shifts are given in ppm referenced to the solvent (1H, 13C). IR
vibrational spectra were recorded from 3500 to 400 cm-1 (Varian 2000,
Scimitar Series, FTS2000) as KBr pellets at room temperature. In situ (low
temperature and room temperature measurements) IR spectra were
recorded from 2000 to 650 cm-1 using a Mettler Toledo React IR 10. UV/vis
measurements (low temperature and room temperature) were performed
using an Agilent 8453 spectrophotometer with a Unisoku USP-203-A
Cryostat. The measurements were performed in solution in quartz glass
cuvettes, l = 1 cm. The chromium contents were determined in the central
analytical laboratory of BASF SE in Ludwigshafen using ICP-MS.
Conclusions
Well-known homogeneous and heterogeneous chromium(VI)
catalysts were investigated regarding their behavior in the
selective decomposition of cyclohexyl hydroperoxide towards
cyclohexanone. This reaction plays an important role in the
industrial cyclohexane oxidation and an understanding of the
mechanism provides the opportunity to use very efficient
chromium(VI) catalysts for this reaction. With this study, the
mechanism
dehydroperoxidation reaction was clarified. The chosen catalysts
CrO *py and CrPVP were used in dehydroperoxidation reactions
behind
the
chromium(VI)
catalyzed
3
under the same conditions and it was found that both systems
have a similar behavior regarding their catalytic activities and
selectivities towards cyclohexanone. Especially at higher
temperatures, relevant for possible industrial applications, the
chromium(VI) catalysts show high catalytic activity with almost no
decrease in the yield of the desired product. The comprehensive
Materials. Unless otherwise specified, reagents and solvents were
purchased from commercial suppliers (Aldrich, abcr, TCI) and used without
further purification. All chemicals were used as received.
Computational details. All structures were optimized at the BP86/def2-
SV(P) level,[63-65] with final energies computed at the PBE0-D3(BJ)/def2-
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QZVPP level of theory.[66] For all electronic structure calculations, the
TURBOMOLE program[67] was used. Zero-point vibrational energies and
thermal corrections for the gas phase were obtained at the level of
geometry optimization and 298.15 K / 1 bar with TURBOMOLE’s freeh
program. Free enthalpies of solvation at infinite dilution in cyclohexane
were computed with COSMO-RS theory[68] using the COSMOtherm
program (Version C3.0, Release 1501, revision 1744).[69]
chloroform or cyclohexane, which are used as purchased and handled
under air. The reactions were carried out in sealed NMR tubes at different
temperatures. The ratios of ketone to alcohol and the conversion of CHHP
were measured by integration of the corresponding characteristic signals
in 1H NMR spectra and naphthalene was used as internal standard. In a
general procedure, a NMR tube was charged with the catalyst (1 eq.), the
substrate cyclohexyl hydroperoxide (150-1500 eq.), naphthalene and
deuterated solvent (0.5 mL) at room temperature. The samples were
stored at room temperature or heated up and the reactions were followed
by NMR measurements.
Preparation of the catalysts. Preparation of CrO
6.6 µmol) was dissolved in pyridine-d (5.00 mL). The light-yellow
solution was stored at 5 °C.
3 3
*py. CrO (4.66 mg,
4
5
In situ UV/vis measurements. Catalytic reaction. For following the
3
decomposition reaction, a solution of CrO *py in cyclohexane (0.93 mM)
Preparation of CrPVP. The CrPVP catalysts were prepared according to
(1.00 g, 10.0 mmol)
was added in small portions to a suspension of PVP (10.0 g) in H
100 mL). The light-yellow suspension was stirred overnight at room
temperature. After filtration, the yellow powder was washed several times
with H O until the filtrate became colorless. The catalyst was dried in
the procedure reported by Lončarevic et al.[35] CrO
was prepared. After measuring an UV/vis spectrum of the pure catalyst,
cyclohexyl hydroperoxide (500 eq.) was added and the rapid
decomposition was followed for 60 min recording an absorption spectrum
every 10 seconds. Stoichiometric reaction. For following the formation of
3
2
O
(
the reactive intermediate, a solution of CrO
3
*py in α,α,α-trifluorotoluene
2
(
0.93 mM) was prepared. After measuring an UV/vis spectrum of the pure
vacuum. The content of chromium on PVP was 2.6 wt. % determined by
ICP-MS. The CrPVP catalyst with a higher or lower chromium loading were
prepared in a similar manner.
Preparation of MePVP. Poly(4-vinylpyridine) (10.0 g) was suspended in
ethylene carbonate (35.0 mL). The mixture was stirred at 35 °C for 1 h.
Methyl methanesulfonate (12.0 mL, 0.142 mmol) was added. The mixture
was heated to 90 °C for 24 h. Additional methyl methanesulfonate (4.0 mL,
catalyst at 5 °C, cyclohexyl hydroperoxide (1 eq.) was added and the rapid
formation of the intermediate was followed for 2 min recording an
absorption spectrum every 10 seconds at 5 °C.
Low temperature NMR measurements. For following the formation of the
reactive intermediate, a solution of cyclohexyl hydroperoxide (0.8 mM) in
toluene-d
substrate at -20 °C, CrO
1
0
4
.05 mmol) was added and the mixture was heated to 90 °C for further
8 h. After filtration, the solid was washed with cold H
O and diethyl ether
8
was prepared. After measuring a H NMR spectrum of the pure
2
3
*py (1 eq.) was added and the rapid formation of
several times. The beige solid was dried in vacuum.
Preparation of CrMePVP. The CrMePVP catalyst was prepared according
to a procedure described by Tamami et al.[59]
Cr (1.75 g, 5.95 mmol)
was added in small portions to a suspension of MePVP (2.00 g) in H
the intermediate was followed for 3 h recording a NMR spectrum 10 min
at -20 °C.
K
2
2 7
O
2
O
In situ IR measurements. For following the formation of the reactive
(50 mL). The yellow suspension was stirred overnight at room temperature.
intermediate, cyclohexyl hydroperoxide (250 eq.) was charged in a flask
2
After filtration, the dark yellow powder was washed several times with H O
3
and cooled to -74 °C. CrO (1 eq.) was added and the reaction was
until the filtrate became colorless. The catalyst was dried in vacuum. The
content of chromium on MePVP was 3.2 wt. % determined by ICP-MS.
followed recording an IR spectrum every 15 seconds. During the reaction,
the mixture was slowly warmed to room temperature and the formation of
cyclohexanone was followed for 3.5 h.
Preparation of cyclohexyl hydroperoxide. Pure cyclohexyl
hydroperoxide was isolated from the crude KA oil mixture (2 L, containing
Kinetic measurements. For running temperature depending NMR
measurements, the catalytic decomposition of cyclohexyl hydroperoxide
was followed at 50 °C, 58 °C and 66 °C. A NMR tube was charged with
cyclohexyl hydroperoxide (400 eq.) and naphthalene followed by the
addition of 1 eq. catalyst. The catalytic reaction was monitored for 80 min.
The ratios of ketone to alcohol and the conversion of CHHP were
>
95 % cyclohexane, < 5 % cyclohexanol, cyclohexanone and cyclohexyl
hydroperoxide) obtained from the plant at BASF SE, Ludwigshafen. The
KA oil mixture was cooled to 5 °C and sodium hydroxide (40 % solution,
500 mL) was slowly added. Caution! The product of this step can be
explosive! The sodium salt was filtered and washed with acetone (3 x 400
mL) and cyclohexane (400 mL). The slightly yellow solid was dried in
vacuum. The solid was then dissolved in water (800 mL) and cyclohexane
measured by integration of the corresponding characteristic signals in the
1H NMR spectra using naphthalene as internal standard.
(400 mL). During cooling with ice, phosphoric acid (20 % solution) was
added until pH 7 was reached. The formed solid was washed with
cyclohexane (2 x 100 mL). The organic phase was dried over sodium
sulfate and evaporated under reduced pressure. The obtained yellowish
oil was purified by column chromatography (hexane: ethyl acetate, 10:1).
Acknowledgements
Cyclohexyl hydroperoxide could be obtained as a clear, colorless oil. Yield:
CaRLa (Catalysis Research Laboratory) is co-financed by the
Heidelberg University and BASF SE.
1
(
8.42 g, 72.5 mmol). R
f
= 0.23. H NMR (200 MHz, CDCl
3
): δ = 7.67 (s, 1H,
) ppm. 13C NMR (50 MHz,
2
), 25.8 (1C, CH ), 23.8 (2C, CH )
OH), 3.96 (m, 1H, CH), 1.96 – 1.28 (m, 10H, CH
2
CDCl
ppm.
3
): δ = 83.4 (1C, CH), 30.2 (2C, CH
2
2
Conflict of interest
The NMR data of the isolated cyclohexyl hydroperoxide are in perfect
agreement with the synthesized cyclohexyl hydroperoxide using
a
The authors declare no conflict of interest.
procedure according to Kundu et al.[70] In the catalytic decompositions,
both cyclohexyl hydroperoxides show an identical behavior.
Keywords: Cyclohexane oxidation • cyclohexyl
hydroperoxide • chromium • homogeneous catalysis •
kinetic
Dehydroperoxidation of cyclohexyl hydroperoxide. Each catalytic
reaction was carried out several times, and herein, the results are
presented as average values. The reactions were performed in deuterated
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
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A look behind the scenes: Known
homogeneous and heterogeneous
Cr(VI) systems were used to
investigate the mechanism behind the
transition metal-catalyzed selective
decomposition of cyclohexyl
Jessica Nadine Hamann, Marko
Hermsen, Anna-Corina Schmidt, Saskia
Krieg, Jasmin Schießl, Dominic Riedel,
Joaquim Henrique Teles, Ansgar
Schäfer, Peter Comba, A. Stephen K.
Hashmi, Thomas Schaub*
hydroperoxide. Based on
comprehensive experimental as well
as DFT studies, a non-radical, non-
redox mechanism via a
cyclohexylperoxychromium(VI)
complex is proposed.
Page No. – Page No.
Selective decomposition of
cyclohexyl hydroperoxide using
homogeneous and heterogeneous
Cr(VI) catalysts: Optimizing the
reaction by evaluating the reaction
mechanism
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