A multifunction Pd/Sc(OTf)3/ionic liquid catalyst system for the tandem one-pot conversion of phenol to ε-caprolactam

Article abstract of DOI:10.1021/cs400009w

A multifunction Pd/Sc(OTf)₃/ionic liquid catalyst for the tandem one-pot conversion of phenol to ε-caprolactam is reported. Pd and Sc(OTf)₃ cooperate to catalyze the hydrogenation of phenol to cyclohexanone with excellent conversion (>99.9%) and selectivity (>99.9%), whereas Sc(OTf)₃ and an ionic liquid, [bmim][PF₆], cooperate to catalyze the tandem transformation of the resulting cyclohexanone to cyclohexanone oxime and the Beckmann rearrangement affording ε-caprolactam.

Full text of DOI:10.1021/cs400009w

Letter
pubs.acs.org/acscatalysis
A Multifunction Pd/Sc(OTf)3/Ionic Liquid Catalyst System for the
Tandem One-Pot Conversion of Phenol to ε‑Caprolactam
Ju Yeon Shin, Da Jung Jung, and Sang-gi Lee*
Department of Chemistry and Nano Science (BK21), Ewha Womans University, Seoul 120-750, Korea
S
* Supporting Information
ABSTRACT: A multifunction Pd/Sc(OTf)3/ionic liquid catalyst for the tandem
one-pot conversion of phenol to ε-caprolactam is reported. Pd and Sc(OTf)3
cooperate to catalyze the hydrogenation of phenol to cyclohexanone with excellent
conversion (>99.9%) and selectivity (>99.9%), whereas Sc(OTf)3 and an ionic
liquid, [bmim][PF6], cooperate to catalyze the tandem transformation of the
resulting cyclohexanone to cyclohexanone oxime and the Beckmann rearrange-
ment affording ε-caprolactam.
KEYWORDS: multifunction catalyst, tandem catalysis, hydrogenation, phenol, caprolactam
Scheme 1. Multifunction Pd/Sc(OTf)3/Ionic Liquid Catalyst
for Tandem One-Pot Selective Conversion of Phenol to ε-
Caprolactam
ecause of the problems associated with energy shortages
Band environmental pollution that confront our societies,
the design and implementation of tandem catalytic reactions
have become increasingly important.1 In biological systems,
tens of enzymes selectively cooperate to catalyze a tandem
transform of a simple molecule to complex molecules.2
Mimicking of such multienzymatic tandem catalysis could
make synthetic chemistry more sustainable. In this paper, we
report the selective cooperation of a multifunction Pd/
Sc(OTf)3/ionic liquid (IL) catalytic system, which made
possible the tandem one-pot conversion of phenol to ε-
caprolactam, a key precursor of nylon-6. Pd and Sc(OTf)3
cooperate to catalyze the hydrogenation of phenol to
cyclohexanone under extremely mild conditions (1 atm of H2
pressure at room temperature) with excellent conversion
(>99.9%) and selectivity (>99.9%) within 2 h. Sc(OTf)3 and
an ionic liquid, 1-butyl-3-methylimidazolium hexafluorophos-
phate, [bmim][PF6], also cooperate to catalyze the tandem
transformation of the resulting cyclohexanone to ε-caprolactam
in three-step overall yield of 67% (Scheme 1). It is noteworthy
that this represents the first example of tandem one-pot
conversion of phenol to ε-caprolactam, providing useful
grounds for the future design of more practical catalysts.
ε-Caprolactam is produced worldwide on one of the largest
scales among commercial chemical products. Its preparation is
typically carried out in a three-step procedure:3 (a) synthesis of
cyclohexanone either by oxidation of cyclohexane4 or by
hydrogenation of phenol in the gas phase5 or liquid phase,6 (b)
formation of cyclohexanone oxime,7 and (c) Beckmann
rearrangement of cyclohexanone oxime to ε-caprolactam.8
Because of the ever-increasing industrial demand for ε-
caprolactam, extensive efforts have been devoted to maximizing
the efficiency of each reaction step. For example, the
development of an efficient catalyst for selective hydrogenation
of phenol to cyclohexanone has received a great deal of
attention in recent years.9 Phenol can be hydrogenated to
cyclohexanone in a “one-step” or a “two-step” process. The
two-step process involves hydrogenation of phenol to cyclo-
hexanol, followed by dehydrogenation to cyclohexanone.
Although the direct one-step selective hydrogenation of phenol
to cyclohexanone is more efficient than the two-step process, it
is difficult to achieve high selectivity at high levels of conversion
because cyclohexanone is easily hydrogenated to cyclohexanol
under the reaction conditions.
Recently, by taking advantage of cooperative catalytic activity
of dual-function Lewis acid/Pd catalyst,10 Liu and co-workers
have achieved both excellent conversion (>99.9%) and
selectivity (99.9%) in the hydrogenation of phenol to
cyclohexanone by using a Lewis acid/Pd−C catalyst, in which
Received: January 3, 2013
Revised: February 8, 2013
Published: February 18, 2013
© 2013 American Chemical Society
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dx.doi.org/10.1021/cs400009w | ACS Catal. 2013, 3, 525−528

ACS Catalysis
Letter
a hydrogen molecule and phenol can be selectively activated by
the Pd−C and the Lewis acid, respectively.9d More recently,
Wang and co-workers developed a catalyst made of Pd
nanoparticles supported on a mesoporous graphite carbon
nitride that achieves the hydrogenation of phenols to the
corresponding cyclohexanones with high selectivity.9e Corma
and co-workers also achieved the selective conversion of phenol
to cyclohexanone using Pd on Al2O3 with a high surface area.9b
Although these catalytic systems could potentially solve the
problems associated with overreduction of phenol to cyclo-
hexanol, they are effective only for the first step of the process
and may not be applied to formation of cyclohexanone oxime
and subsequent Beckmann rearrangement.
Many Lewis acids, such as AlCl3, are sensitive to water, which
is formed alongside with cyclohexanone oxime in the reaction
of cyclohexanone with hydroxylamine or its salts. Moreover, the
AlCl3-catalyzed Beckmann rearrangement of cyclohexanone
oxime is notorious for its extremely low yields of ε-
caprolactam.11 A one-step process for the conversion of
cyclohexanone to ε-caprolactam has been developed by
Thomas and Raja using a bifunctional catalyst, MnIIIMgIIAlPO4,
which catalyzed air oxidation of ammonia to hydroxylamine,
reacting with cyclohexanone to form cyclohexanone oxime and
the Beckmann rearrangement of the resulting oxime to ε-
caprolactam.12 However, this catalyst could not be applied to
the selective hydrogenation of phenol. Therefore, the develop-
ment of a catalytic system for the tandem one-pot synthesis of
ε-caprolactam from phenol has remained a challenge.
Table 1. Selective Hydrogenation of Phenol to
Cyclohexanone using Pd/Ln(OTf)3 Catalyst
a
c
selectivity
b
c
entry
Ln(OTf)3
time (h)
conv. (%)
CO
C−OH
d
1
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Ce(OTf)3
Pr(OTf)3
Nd(OTf)3
Sm(OTf)3
Eu(OTf)3
Gd(OTf)3
Tb(OTf)3
Dy(OTf)3
Ho(OTf)3
Er(OTf)3
Tm(OTf)3
Yb(OTf)3
Lu(OTf)3
Y(OTf)3
2.0
3.5
2.0
3.5
3.5
3.0
3.5
2.5
2.5
3.0
2.5
2.5
3.0
3.0
2.0
2.5
2.5
2.5
3.0
5.0
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
97.7
<0.1
2.3
e
2
f
3
>99.9
94.2
<0.1
5.8
g
4
5
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
La(OTf)3
Zn(OTf)2
For the tandem one-pot conversion of phenol to ε-
caprolactam, the catalyst should catalyze not only the selective
hydrogenation of phenol to cyclohexanone, but also the oxime
formation and Beckmann rearrangement. In addition, the
catalysts must be stable to water, which is generated during
oxime formation. On the basis of these considerations, we
chose lanthanide triflates as a Lewis acid partner for the Pd-
catalyzed selective hydrogenation of phenol to cyclohexanone
because they are well-known Lewis acids with water
tolerance.13 Moreover, although they were not effective in the
synthesis of ε-caprolactam, it has been reported that some
lanthanide triflates, such as Yb(OTf)3, Y(OTf)3, and Nd-
(OTf)3, could catalyze the Beckmann rearrangement of oximes
to lactams.14 In the course of our ongoing study on lanthanide
triflate-catalyzed reactions in imidazolium-based ILs, we found
that the catalytic activity of lanthanide triflates is often
significantly increased in the ILs having noncorrdinating anions,
such as SbF6, PF6, and NTf2.15 Considering the positive effects
of such ILs on the lanthanide triflate-catalyzed reactions, oxime
formation and Beckmann rearrangement,16 we set about to
investigate imidazolium-based ILs bearing noncoordinating
anions as one of the components of the catalytic system.
We began our investigation to find a catalytic system for the
selective hydrogenation of phenol to cyclohexanone using Pd−
C and lanthanide triflates. These results are summarized in
Table 1. As shown in Table 1, all lanthanide triflates can, in the
presnece of a Pd catalyst, perform the selective hydrogenation
of phenol to cyclohexanone within a few hours in dichloro-
methane under 1 atm of H2 at room temperature. More
specifically, the catalyst composed of Sc(OTf)3/Pd−C showed
extremely high catalytic activity and hydrogenated phenol to
afford cyclohexanone with >99.9% conversion and 99.9%
selectivity in only 2 h (entry 1, Table 1). In addition, the Pd−
C/Sc(OTf)3 catalyst can be reused four times without any loss
of selectivity (entry 1, Table 1). The reaction rate and
a
Unless otherwise noted, the reaction was carried out with 0.5 mmol
of phenol in the presence of 10 mol % (based on Pd contents) of Pd/
C (10 wt %) and 10 mol % of lanthanide triflate in dichloromethane
under 1 atm of H2 at room temperature (see the Supporting
Information for experimental details). Time for complete conversion
of phenol by GC. Determined by GC using n-decane as an internall
b
c
standard (see the Supporting Information for experimental details).
d
e
The catalyst was reused four times. Reaction was carried out in the
f
presence of 1.0 mol % of Sc(OTf)3. Reaction was carried out in
dichloroethane. Pd on alumina was used instead of Pd/C.
g
selectivity (97.7%) were decreased as the amount of Sc(OTf)3
decreased to 1.0 mol % (entry 2, Table 1). This result suggests
that activation of phenol with Sc(OTf)3 is crucial to obtain high
selectivity.
The reactivity and selectivity were retained in high-boiling
1,2-dichloroethane solvent, which is a necessity in Beckmann
rearrangement (entry 3, Table 1); however, it was found that
the support material could influence the reaction efficiency,9b,e
and thus, the use of palladium on alumina decreased reactivity
and selectivity, requiring 3.5 h to afford 94.2% of cyclohexanone
along with 5.8% of overreduced cyclohexanol (entry 4, Table
1). Under the same reaction conditions, Zn(OTf)2 also
catalyzed the selective hydrogenation of phenol to cyclo-
hexanone but required a prolonged reaction time (entry 20,
Table 1). The Pd−C/Sc(OTf)3 catalyst can hydrogenate
methyl-substituted phenols to the corresponding methylated
cyclohexanones with full conversion and high selectivities at
room temperature within hours (Scheme 2).
With the optimal dual-function catalyst, Pd−C/Sc(OTf)3, in
hand for the selective hydrogenation of phenol to cyclo-
hexanone, we then attempted to find a multifunction catalytic
system for the one-pot conversion of cyclohexanone to ε-
caprolactam using Sc(OTf)3 as a catalyst (Table 2).
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Letter
Scheme 2. Selective Hydrogenation of Methylated Phenols
to the Corresponding Cyclohexanones using Pd−C/
Sc(OTf)3 Catalyst
Table 3. Tandem One-Pot Conversion of Phenol to ε-
a
Caprolactam Using Pd−Sc(OTf)3/Ionic Liquid Catalyst.
b
entry
[bmim][X]
yield (%)
1
2
3
4
5
[bmim][SbF6]/200 mol %
[bmim][BF4]/200 mol %
[bmim][NTf2]/200 mol %
[bmim][PF6]/200 mol %
[bmim][PF6]/300 mol %
34
<5
22
63
67
a
Reaction conditions: the reaction was carried out with 0.5 mmol of
phenol in the presence of 10 mol % (based on Pd contents) of Pd/C
(10 wt %), 10 mol % of Sc(OTf)3, and [bmim][X] in dichloroethane
(6.0 mL) under 1 atm of H2 pressure at room temperature. After
complete conversion of phenol to cyclohexanone, determined by GC,
hydroxylamine (1.65 mmol, 1.1 equiv) was added and stirred for 21 h
at 100 °C (see the Supporting Information for experimental details).
Table 2. One-Pot Conversion of Cyclohexanone to ε-
Caprolactam Using Sc(OTf)3/[bmim][SbF6] Catalyst
a
b
Determined by GC using n-decane as an internal standard.
at room temperature, only 75% of the phenol was converted to
cycohexanone during 6 h. Fortunately, the catalytic activity of
Pd/C and Sc(OTf)3 was restored when IL was used as an
additive. With addition of 200 mol % of [bmim][SbF6], phenol
was completely converted to cyclohexanone with >99.9%
selectivity within 3 h. Unfortunately, the tandem reaction of the
resulting cyclohexanone with hydroxylamine hydrochloride
afforded the desired ε-caprolactam in only 34% yield (entry
1, Table 3). Changing the counteranion of the ionic liquid to
[BF4] (entry 2, Table 3) or [NTf2] anion (entry 3, Table 3) did
not improve the yield of ε-caprolactam. To our delight, when
[bmim][PF6] was used as an additive, the tandem one-pot
conversion of phenol to ε-caprolactam could be achieved with
overall 63% yield (entry 4, Table 3). In the presence of 300 mol
% of [bmim][PF6], the yield was further increased to afford ε-
caprolactam in 67% overall yield (entry 5, Table 3); however,
the catalytic activity of the recovered catalyst was dramatically
decreased, and the origin of the decreased catalytic activity is
not clear at the present time. Nevertheless, it is clear that the
Lewis acidic Sc(OTf)3 could accelerate the hydrogenation of
phenol to cyclohexanone, but suppress its reduction to
cyclohexanol.9d,10 In addition, IL may play a dual function:15a
increasing Lewis acidity of Sc(OTf)3 via anion exchange and
lowering the activation energy for Beckmann rearrangement via
stabilization of the charged intermediate formed during
Beckmann rearrangement to ultimately afford ε-caprolactam.
In summary, a new multifunction catalyst, consisting of Pd−
C/Sc(OTf)3/[bmim][PF6], has been developed for the tandem
one-pot synthesis of ε-caprolactam from phenol in 67% overall
yield. Similar to the multienzymatic catalytic reactions, Pd
catalyzed the hydrogenation of phenol to cyclohexanone in the
presence of Sc(OTf)3 with >99.9% selectivity in >99.9%
conversion. Furthermore, Sc(OTf)3 catalyzed the oxime
formation and Beckmann rearrangement in the presence of
[bmim][PF6] to afford ε-caprolactam. The cooperative action
of multicomponent catalytic systems will therefore be a valuable
concept in the design of future catalytsts demonstrating greater
practicality.
catalyst
b
entry
Pd/C
Sc(OTf)3
IL
oxime
%
lactam %
c
1
10 mol %
10 mol %
93
2
3
4
IL
IL
IL
5
96
7
89
3
10 mol %
10 mol %
88
a
Unless otherwise noted, a solution of cyclohexanone (1.5 mmol) and
hydroxylamine hydrochloride (1.65 mmol) in a mixture of 1,2-
dichloroethane (5.0 mL) and [bmim][SbF6] (1.0 mL) was stirred for
18 h at 100 °C in the presence of catalyst (see the Supporting
Information for experimental details). Determined by GC using n-
decane as an internal standard. Reaction was carried out in 1,2-
b
c
dichloroethane (6.0 mL) only.
Through the systematic variation of catalyst compositions,
we found that both Sc(OTf)3 and an ionic liquid are critical for
the one-pot conversion of cyclohexanone to ε-caprolactam. In
the reaction of cyclohexanone with hydroxylamine hydro-
chloride in dichloroethane at 100 °C for 18 h in the presence of
10 mol % of Sc(OTf)3, only cyclohexanone oxime was formed
in 93% yield. No sign for the formation of ε-caprolactam was
observed (entry 1, Table 2). When the same reaction was
carried out in a mixture of dichloroethane and [bmim][SbF6]
(5/1, v/v), the desired ε-caprolactam was formed in 89% yield
along with 5% of cyclohexanone oxime (entry 2, Table 2). In
contrast, only tiny amounts (∼3%) of ε-caprolactam were
formed in the absence of Sc(OTf)3, and cyclohexanone oxime
was formed as a major product in 96% conversion (entry 3,
Table 2). The reaction efficiency was not diminished in the
presence of Pd catalyst (entry 4, Table 2).
Finally, the tandem one-pot conversion of phenol to ε-
caprolactam was investigated using the multicomponent
catalytic system composed of Pd−C/Sc(OTf)3/IL (Table 3).
When phenol was hydrogenated in a mixture of dichloro-
ethane/[bmim][SbF6] (5/1, v/v, 0.3 M) in the presence of 10
mol % of Pd/C and 10 mol % of Sc(OTf)3 under 1 atm of H2
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Letter
Z.; Zhang, S.; Li, D.; Li, Z.; Deng, Y. Green Chem. 2006, 8, 296.
(f) Zhu, M.; Cha, C.; Deng, W.-P.; Shi, X.-X. Tetrahedron Lett. 2006,
47, 4861. (g) Marthala, V. R. R.; Jiang, Y.; Huang, J.; Wang, W.;
ASSOCIATED CONTENT
* Supporting Information
Details of the experimental procedures and GC chromatograms
for the conversion of phenol (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
■
S
Glaser, R.; Hunger, M. J. Am. Chem. Soc. 2006, 128, 14812. (h) Furuya,
̈
Y.; Ishihara, K.; Yamamoto, H. J. Am. Chem. Soc. 2005, 127, 11240.
(i) Li, W.-C.; Lu, A.-H.; Palkovits, R.; Schmidt, W.; Spliethoff, B.;
Schuth, F. J. Am. Chem. Soc. 2005, 127, 12595. (j) Boero, M.; Ikeshoji,
̈
T.; Liew, C. C.; Terakura, K.; Parrinello, M. J. Am. Chem. Soc. 2004,
AUTHOR INFORMATION
Corresponding Author
*E-mail: sanggi@ewha.ac.kr.
Notes
■
126, 6280.
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Chem. 2012, 36, 1085. (b) Perez, Y.; Fajardo, M.; Corma, A. Catal.
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Commun. 2011, 12, 1071. (c) Matos, J.; Corma, A. Appl. Catal., A
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Korean Research Foundation
(KRF-20120005673). We thank to Prof. J. Bouffard for helpful
comments.
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