Au-Pd alloy nanoparticles supported on layered double hydroxide for heterogeneously catalyzed aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones to phenols

Article abstract of DOI:10.1039/c6sc00874g

Phenol, an important industrial chemical, is widely produced using the well-developed cumene process. However, demand for the development of a novel alternative method for synthesizing phenol from benzene has been increasing. Herein, we report a novel system for the synthesis of phenols through aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones, including ketone-alcohol (KA) oil, catalyzed by Mg-Al-layered double hydroxide (LDH)-supported Au-Pd alloy nanoparticles (Au-Pd/LDH). Alloying of Au and Pd and basicity of LDH are key factors in achieving the present transformation. Although monometallic Au/LDH, Pd/LDH, and their physical mixture showed almost no catalytic activity, Au-Pd/LDH exhibited markedly high catalytic activity for the dehydrogenative phenol production. Mechanistic studies showed that β-H elimination from Pd-enolate species is accelerated by Au species, likely via electronic ligand effects. Moreover, the effect of supports was critical; despite the high catalytic performance of Au-Pd/LDH, Au-Pd bimetallic nanoparticles supported on Al₂O₃, TiO₂, MgO, and CeO₂ were ineffective. Thus, the basicity of LDH plays a deterministic role in the present dehydrogenation possibly through its assistance in the deprotonation steps. The synthetic scope of the Au-Pd/LDH-catalyzed system was very broad; various substituted cyclohexanols and cyclohexanones were efficiently converted into the corresponding phenols, and N-substituted anilines were synthesized from cyclohexanones and amines. In addition, the observed catalysis was truly heterogeneous, and Au-Pd/LDH could be reused without substantial loss of its high performance. The present transformation is scalable, utilizes O₂ in air as the terminal oxidant, and generates water as the only by-product, highlighting the potential practical utility and environmentally benign nature of the present transformation. Dehydrogenative aromatization of cyclohexanols proceeds through (1) oxidation of cyclohexanols to cyclohexanones; (2) dehydrogenation of cyclohexanones to cyclohexenones; and (3) disproportionation of cyclohexenones to afford the desired phenols. In the present Au-Pd/LDH-catalyzed transformation, the oxidation of the Pd-H species is included in the rate-determining step.

Full text of DOI:10.1039/c6sc00874g

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Au–Pd alloy nanoparticles supported on layered
double hydroxide for heterogeneously catalyzed
aerobic oxidative dehydrogenation of
Cite this: DOI: 10.1039/c6sc00874g
cyclohexanols and cyclohexanones to phenols†
*
Xiongjie Jin, Kento Taniguchi, Kazuya Yamaguchi and Noritaka Mizuno
Phenol, an important industrial chemical, is widely produced using the well-developed cumene process.
However, demand for the development of a novel alternative method for synthesizing phenol from
benzene has been increasing. Herein, we report a novel system for the synthesis of phenols through
aerobic oxidative dehydrogenation of cyclohexanols and cyclohexanones, including ketone–alcohol (KA)
oil, catalyzed by Mg–Al-layered double hydroxide (LDH)-supported Au–Pd alloy nanoparticles (Au–Pd/
LDH). Alloying of Au and Pd and basicity of LDH are key factors in achieving the present transformation.
Although monometallic Au/LDH, Pd/LDH, and their physical mixture showed almost no catalytic activity,
Au–Pd/LDH exhibited markedly high catalytic activity for the dehydrogenative phenol production.
Mechanistic studies showed that b-H elimination from Pd-enolate species is accelerated by Au species,
likely via electronic ligand effects. Moreover, the effect of supports was critical; despite the high catalytic
performance of Au–Pd/LDH, Au–Pd bimetallic nanoparticles supported on Al2O3, TiO2, MgO, and CeO2
were ineffective. Thus, the basicity of LDH plays a deterministic role in the present dehydrogenation
possibly through its assistance in the deprotonation steps. The synthetic scope of the Au–Pd/LDH-
catalyzed system was very broad; various substituted cyclohexanols and cyclohexanones were efficiently
converted into the corresponding phenols, and N-substituted anilines were synthesized from
cyclohexanones and amines. In addition, the observed catalysis was truly heterogeneous, and Au–Pd/
LDH could be reused without substantial loss of its high performance. The present transformation is
scalable, utilizes O2 in air as the terminal oxidant, and generates water as the only by-product,
highlighting the potential practical utility and environmentally benign nature of the present
transformation. Dehydrogenative aromatization of cyclohexanols proceeds through (1) oxidation of
cyclohexanols to cyclohexanones; (2) dehydrogenation of cyclohexanones to cyclohexenones; and (3)
disproportionation of cyclohexenones to afford the desired phenols. In the present Au–Pd/LDH-
catalyzed transformation, the oxidation of the Pd–H species is included in the rate-determining step.
Received 25th February 2016
Accepted 6th May 2016
DOI: 10.1039/c6sc00874g
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competitive and widely employed commercial process for
phenol, the development of new processes that avoid these
Introduction
problems is highly desirable. In this context, processes for the
one-step direct oxygenation of benzene to phenol using various
oxidants such as H2O2, N2O, and O2 have attracted considerable
attention as promising alternatives for producing phenols.3
However, little success has been achieved in the eld of direct
selective hydroxylation of benzene to phenol, largely because of
the inertness of C–H bonds in benzene and the more facile
oxidation of phenol than benzene by oxidizing agents, leading
to overoxidation.3 Dehydrogenative aromatization of a mixture
of cyclohexanone and cyclohexanol—specically, an unrened
mixture of cyclohexanone and cyclohexanol, known as ketone–
alcohol (KA) oil—can provide an alternative approach to access
phenol from benzene. KA oil is readily available through
a well-developed two-step commercial process involving the
Phenol and its derivatives are very important industrial chem-
icals and key structural motifs in various pharmaceuticals,
agrochemicals, plastics, and resins.1 Annually, millions of tons
of phenol are produced mainly by the well-established three-
step cumene process starting from benzene (Scheme 1).2
However, this traditional cumene process suffers from several
drawbacks, including the formation of acetone co-product and
the involvement of an explosive cumene hydroperoxide inter-
mediate.2 Although the current cumene process is the most
Department of Applied Chemistry, School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: tmizuno@mail.ecc.u-tokyo.
ac.jp; Fax: +81-3-5841-7220
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6sc00874g
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Scheme 1 Synthetic procedures for phenol.
hydrogenation of benzene to cyclohexane and aerobic autoxi- multi-component noble-metal nanoparticle catalysts have oen
dation of cyclohexane (Scheme 1). shown catalytic activities superior to those of their mono-
Recently, several homogeneous Pd-based catalytic systems metallic counterparts because of the synergistic effects between
for the dehydrogenation of cyclohexanones to phenols using O2 different components.8 In recent years, Au-based multi-metallic
as the terminal oxidant under mild reaction conditions have nanoparticles have been among the most extensively studied
been developed.4 These systems are highly valuable for the catalysts.9 For example, Au–Pd bimetallic nanoparticle catalysts
synthesis of phenols with various substitution patterns given (1) have shown signicantly higher catalytic activity than Au or Pd
the limitations of traditional electrophilic substitution reac- monometallic nanoparticles catalysts for oxygen-related trans-
tions for the regioselective introduction of substituents onto formations such as the aerobic oxidations of CO to CO2,10
a phenol ring as a consequence of the strong electron-directing alcohols to carbonyl compounds,11 and H2 to H2O2.12 The high
effects of hydroxy groups and (2) the easy accessibility of catalytic activity of multi-metallic nanoparticles is attributable
cyclohexanones with various substituents on cyclohexyl rings to the ligand and ensemble effects, which can affect the local
through a wide range of chemical transformations. Acceptorless electronic and geometric structures of nanoparticles, thereby
dehydrogenation of cyclohexanones using a homogeneous Ir- leading to enhancement of the catalytic performance.8 In the
based catalyst has also been developed.5 Despite the high effi- case of supported multi-metallic nanoparticles, in addition to
ciency of these homogeneous systems, they suffer several their structures (e.g., core–shell, hetero, or alloyed structures),
drawbacks, including the requirement of rather sophisticated the choice of appropriate supports is also key to achieving high
ligands and/or high reaction temperatures and difficulties catalytic performance. The support can not only stabilize the
associated with the separation and reuse of the catalysts. Quite highly dispersed metal nanoparticles, but also substantially
recently, a supported Pd-nanoparticle-catalyzed dehydrogena- promote the reaction through metal-support cooperation.13
tion of cyclohexanones has also been developed either using O2 Therefore, we envisage that Pd nanoparticles alloyed with other
(5 atm) as a hydrogen acceptor6a or under acceptorless con- metals (e.g., Au) supported on suitable support materials could
ditions.6b However, in these cases, the need for large amounts of promote the aerobic oxidative dehydrogenation of cyclo-
bases as additives and/or leaching of Pd species is problematic hexanols and cyclohexanones to phenols.
in terms of catalyst stability and reusability. Therefore, the
Herein, we report for the rst time that Au–Pd bimetallic
development of novel catalytic systems using easily separable alloy nanoparticles supported on Mg–Al-layered double
and reusable heterogeneous catalysts under ligand- and addi- hydroxide14 (Au–Pd/LDH) can efficiently promote the aerobic
tive-free conditions is highly desirable from practical, oxidative dehydrogenation of various structurally diverse
economical, and environmental viewpoints. Despite remark- cyclohexanols to phenols via a consecutive triple-dehydrogena-
able progress in the dehydrogenation of cyclohexanones to tion process. Interestingly, whereas the bimetallic Au–Pd/LDH
phenols in recent years, as far as we know, the literature exhibits high catalytic performance, monometallic Au/LDH and
contains no reports of efficient catalytic systems that directly Pd/LDH and their physical mixture exhibit almost no catalytic
convert cyclohexanols to phenols with useful yields under activity toward the dehydrogenation. The catalysis for the
relatively mild conditions. Therefore, the development of novel present dehydrogenative aromatization was truly heteroge-
catalytic systems that facilitate dehydrogenation of cyclo- neous, and the Au–Pd/LDH catalyst could be reused at least 10
hexanols to phenols would not only enable the synthesis of times with retention of its high catalytic performance.
phenol directly using KA oil as the feedstock without tedious
isolation of cyclohexanone, but would also complement the
aforementioned synthesis of substituted phenols via the dehy-
drogenation of cyclohexanones.
Results and discussion
Preparation and characterization of LDH-supported Au–Pd
alloy nanoparticles
Over the past several decades, (supported) metal nano-
particles have attracted much attention as efficient catalysts for Initially, we prepared several Au–Pd/LDH catalysts with
a wide variety of aerobic oxidation reactions.7 In particular, different Au/Pd ratios using the deposition–precipitation
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Fig. 1 (a) TEM image of Au9Pd1/LDH. (b) The size distribution of Au9Pd1/LDH. (c) HAADF-STEM image of Au9Pd1/LDH. EDS intensity line profiles
are shown in red (Au) and white (Pd) lines. (d) EDS elemental map of Au. (e) EDS elemental map of Pd. (f) Overlap of EDS elemental map of Au and
Pd.
Table 1 Oxidative dehydrogenation of 4-methylcyclohexanol (1a) to
method. Specically, Au and Pd species were co-precipitated
onto the LDH surface as hydroxide species and these metal
species were subsequently reduced using NaBH4 (for details on
the preparation procedures for the Au–Pd/LDH catalysts, see the
Experimental section). As revealed by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analysis, the
Au/Pd molar ratios of the Au–Pd/LDH catalysts slightly deviated
from those of the initial metal precursors used to prepare the
catalysts. Hereaer, Au–Pd/LDH catalysts with an initial solu-
tion Au/Pd molar ratio of x/y are designated as AuxPdy/LDH. The
metal contents of AuxPdy/LDH are summarized in Table S1.†
X-ray diffraction (XRD) patterns of the catalysts showed that
the layered structure of LDH was well maintained aer immo-
bilization of the Au–Pd bimetallic nanoparticles onto the
surface (Fig. S1†). Transmission electron microscopy (TEM)
analysis revealed that the average Au–Pd bimetallic nano-
particle size of Au9Pd1/LDH was 3.2 nm (Fig. 1a and b; for the
average metal nanoparticle sizes of other catalysts, see Table
S1†). In addition, high-angle annular dark-eld scanning TEM
(HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS)
analyses of Au9Pd1/LDH indicated that Au–Pd alloy nano-
particles were formed on the surface of LDH.15 This result was
also supported by the UV-vis spectrum of Au9Pd1/LDH; the
intensity of the absorption band at approximately 520 nm,
which is derived from plasmon resonance absorption of Au
nanoparticles, was markedly decreased compared to that of
monometallic Au/LDH (Fig. S2†).16 Similarly, UV-vis spectra of
other supported bimetallic nanoparticle catalysts AuxPdy/LDH
also indicated the formation of Au–Pd alloy nanoparticles
(Fig. S2†).
4-methylphenol (2a) with various catalystsa
Yield (%)
Entry
Catalyst
Conv. (%)
2a
3a
Au/Pdb
1
2
3
4
5
6
7
8
Au/LDH
55
94
87
52
59
56
<1
<1
<1
31
17
60
32
73
2
nd
91
79
2
55
2
4
3.6/0
Au9Pd1/LDH
Au4Pd1/LDH
Au1Pd1/LDH
Au1Pd4/LDH
Pd/LDH
Au9Pd1/Al2O3
Au9Pd1/TiO2
Au9Pd1/CeO2
Au9Pd1/MgO
Au4Pd1/Al2O3
Au4Pd1/Al2O3 + K2CO3
Au4Pd1/Al2O3 + LDH
Au/LDH + Pd/LDH
Au9Pd1/TiO2 + LDH
Au9Pd1/LDH
None
3.1/0.5
2.6/1.0
1.5/2.1
0.7/2.9
0/3.6
3.2/0.4
3.1/0.5
3.2/0.4
2.9/0.7
2.8/0.8
2.8/0.8
2.8/0.8
3.1/0.5
3.1/0.5
3.1/0.5
—
29
43
45
nd
nd
nd
21
11
35
22
53
2
nd
nd
nd
nd
nd
nd
nd
nd
nd
5
9
10
11
12c
13d
14e
15d
16f
17
nd
nd
nd
3
21
3
nd
a
Reaction conditions: 1a (0.5 mmol), catalyst (total amount of metals:
3.6 mol%), DMA (2 mL), 130 ꢀC, air (1 atm), 2.5 h. Conversion and
yields were determined by GC analysis. DMA
¼
N,N-
b
c
dimethylacetamide. Amount of metals (mol%). K2CO3 (0.5 mmol).
d
e
LDH (100 mg). A physical mixture of Au/LDH (3.1 mol%) and Pd/
LDH (0.5 mol%). f Ar (1 atm).
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Fig. 2 Effect of removal of the Au9Pd1/LDH catalyst on the present
oxidative dehydrogenation. The reaction conditions were the same as
those described in Table 1. GC yields are shown here. Closed squares
and circles indicate the yields of 2a and 3a, respectively, without
removal of the catalyst; open squares and circles indicate the yields of
2a and 3a, respectively, after removal of the catalyst.
Fig. 3 Au9Pd1/LDH reuse experiments. The reaction conditions were
the same as those described in Table 1. GC yields are shown here.
Generally, the retrieved catalyst was washed with water and ethanol.
However, the retrieved catalysts after 5th, 8th, and 9th reuse experi-
ments were washed with a 0.1 M NaOH solution in water/EtOH (1 : 1).
ratio was the same as that of Au9Pd1/LDH) only gave a trace
amount of 2a and a moderate yield of 3a (Table 1, entry 14),
Optimization of reaction conditions
Next, various supported Au–Pd alloy nanoparticles were which indicates that formation of the bimetallic alloy is
applied to the dehydrogenative aromatization of 4-methyl- indispensable for the effective transformation of 3a to 2a. In
cyclohexanol (1a) to 4-methylphenol (2a) (Table 1). The reaction addition, the reaction with a physical mixture of Au4Pd1/Al2O3
was carried out in N,N-dimethylacetamide (DMA) at 130 ꢀC or Au9Pd1/TiO2 and LDH did not give 2a at all, thus suggesting
under open-air conditions. Although monometallic Au/LDH that highly dispersed Au–Pd bimetallic alloy nanoparticles on
and Pd/LDH promoted the oxidation of 1a to 4-methyl- the surface of LDH are the key to achieving highly efficient
cyclohexanone (3a), the desired 2a through the dehydrogen- dehydrogenative aromatization of 3a to 2a (Table 1, entries
ative aromatization of 3a was not obtained in either case (Table 13 and 15). The reaction under an Ar atmosphere gave only
1, entries 1 and 6). Surprisingly, when the reaction was carried a trace amount of 3a and did not give 2a, which suggests that
out with Au9Pd1/LDH as the catalyst, 91% yield of 2a was ob- O2 in air functioned as the terminal oxidant in the present
tained (Table 1, entry 2). The effect of the Au/Pd molar ratio on system (Table 1, entry 16). In the absence of a catalyst, the
the present transformation was signicant. The catalytic reaction did not proceed (Table 1, entry 17). Among the various
activity decreased with decreasing Au/Pd molar ratio (Table 1, solvents examined, i.e., DMA, N,N-dimethylformamide (DMF),
entries 1–6), and Au1Pd1/LDH and Au1Pd4/LDH hardly gave 2a N-methylpyrrolidone (NMP), dimethylsulfoxide (DMSO), mon-
(Table 1, entries 4 and 5). The effect of the support on the ochlorobenzene, and mesitylene, DMA best promoted the
present reaction was also critical. Au–Pd bimetallic nano- present dehydrogenation (Table S2†).17
particles supported on other materials such as Al2O3, TiO2, and
The conditions for the reaction starting from 3a were also
CeO2 did not exhibit catalytic activity for the dehydrogenation; optimized. As shown in Table S3,† Au/LDH and Pd/LDH
in these cases, neither 2a nor 3a was formed (Table 1, entries exhibited almost no catalytic activity toward the dehydroge-
7–9). Although the dehydrogenation of 1a gave 3a when using nation of 3a (Table S3,† entries 1 and 6). For the reaction of 3a,
MgO as the support for Au–Pd alloy nanoparticles, 2a was not Au9Pd1/LDH also exhibited the best performance among
formed at all (Table 1, entry 10). Notably, the aerobic oxidation various LDH-supported Au–Pd alloy nanoparticle catalysts
of 1a to 3a by Au4Pd1/Al2O3 was signicantly promoted by (Table S3,† entries 2–5). Au–Pd alloy nanoparticles supported
addition of bases such as K2CO3 (stoichiometric amount, 0.5 on Al2O3, TiO2, CeO2, and MgO gave only trace amounts of 2a
mmol) and LDH (100 mg) to the reaction mixture (Table 1, (Table S3,† entries 7–10). The dehydrogenation of 3a using
entries 11–13). Thus, the basicity of the LDH support likely Au4Pd1/Al2O3 as the catalyst was substantially promoted by
plays a key role in promoting the oxidation of 1a to 3a through the addition of K2CO3 or LDH (Table S3,† entries 11–13).
its assistance of the abstraction of the proton from the hydroxy These results strongly support the hypothesis that the basicity
group of 1a.14 The dehydrogenative aromatization of 1a with of LDH plays the key role in the dehydrogenative aromatiza-
a physical mixture of Au/LDH and Pd/LDH (where the Au/Pd tion of 3a, likely via its assistance in the abstraction of a-H
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from 3a.14 The physical mixture of Au/LDH and Pd/LDH phenols. The phenol products were readily isolated; their yields
did not exhibit catalytic activity for the dehydrogenative are summarized in Table 2. The dehydrogenation of simple
aromatization (Table S3,† entry 14), and the physical mixture cyclohexanol afforded phenol in 89% yield (Table 2, entry 1).
of Au9Pd1/TiO2 and LDH afforded a lower yield of 2a than The dehydrogenation of cyclohexanols with various substitu-
Au9Pd1/LDH (Table S3,† entry 15), indicating the highly ents at 2-, 3-, and 4-positions smoothly proceeded (Table 2,
dispersed Au–Pd alloy nanoparticles on the surface of LDH are entries 2–12). 2-Substituted cyclohexanols were not as reactive
indispensable for the dehydrogenative aromatization of 3a. as 3- and 4-substituted ones, suggesting that steric effects on the
The transformation of 3a under an Ar atmosphere did not give dehydrogenation are substantial (Table 2, entries 2, 3, and 8).
2a, which suggests that O2 in air is the terminal oxidant (Table Various disubstituted cyclohexanols were also suitable
S3,† entry 16). These results related to the transformation of substrates for the present reaction (Table 2, entries 8 and 9).
3a agree well with those obtained starting from 1a.
Heterogeneous catalysis and reusability of Au9Pd1/LDH
Table 2 Substrate scope for the dehydrogenation of cyclohexanols
with Au9Pd1/LDHa
To verify whether the observed catalysis was truly heteroge-
neous, the Au9Pd1/LDH catalyst was removed by hot ltration
when the conversion of 1a reached approximately 40%, and the
reaction was repeated using the ltrate under the same reaction
conditions. As shown in Fig. 2, the reaction was completely
stopped by the removal of the catalyst. Furthermore, the ICP-
AES analysis results conrmed that Au and Pd species were
hardly present in the ltrate (below the instrumental detection
limits: Au < 0.01%, Pd < 0.38%). On the basis of these experi-
mental results, we ruled out the possibility that the catalysis
originated from metal species leached from the catalyst; thus,
the observed catalysis was truly heterogeneous.18
Aer the dehydrogenation of 1a to 2a was completed,
Au9Pd1/LDH was easily retrieved from the reaction mixture by
simple ltration, with greater than 95% recovery, for each reuse
experiment. The retrieved catalyst could be reused at least 10
times without substantial loss of its high catalytic perfor-
mance. Even at the 10th reuse experiment, 85% yield of 2a was
still obtained (Fig. 3). The catalytic performance could be
restored by simply washing the used catalyst with a 0.1 M
NaOH solution in water/ethanol (1 : 1). XRD and TEM analyses
of the fresh as-prepared Au9Pd1/LDH catalyst and the catalyst
aer the 10th reuse experiment show that the LDH structure
and the average size of the Au–Pd bimetallic alloy nanoparticles
were almost unchanged (fresh: 3.2 nm, aer the 10th reuse
experiment: 3.3 nm) (Fig. S3 and S4†). The difference UV-vis
spectrum between fresh Au9Pd1/LDH and Au9Pd1/LDH aer the
10th reuse experiment revealed a new absorption band
centered at approximately 250 nm, which is attributable to
aromatic compounds (possibly 2a) (Fig. S5†). These results
indicate that the gradual decrease of the catalytic activity is
likely caused by the absorption of acidic phenol onto the
surface of the catalyst. Thus, the catalytic ability (basicity) of
the LDH support was readily recovered by washing with the
NaOH solution, thereby restoring the catalytic activity of
Au9Pd1/LDH.
a
Substrate scope
Reaction conditions: 1 (0.5 mmol), Au9Pd1/LDH (total amount of
metals: 3.6 mol%), DMA (2 mL), 130 ꢀC, air (1 atm). Conversion and
yields were determined by GC analysis. The values in the parentheses
With the optimized reaction conditions in hand, we next
investigated the scope of the present Au9Pd1/LDH-catalyzed are the isolated yields. b Au9Pd1/LDH (total amount of metals: 7.2
mol%). c For 10 mmol-scale synthesis, reaction conditions: Au9Pd1/
dehydrogenative aromatization of cyclohexanols. Under the
optimized reaction conditions, various structurally diverse
LDH (total amount of metals: 1 mol%), DMA (10 mL), 130 ꢀC, air (1
atm). The isolated yield is shown. d Mesitylene/DMA (1 : 1, 2 mL).
cyclohexanols were efficiently converted into the corresponding
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Table 3 Substrate scope for the dehydrogenation of cyclohexanones
2-Cyclohexen-1-ol also reacted well to give phenol in excellent
yield (Table 2, entry 10). Amide groups on the 4-positions
remained intact under the reaction conditions (Table 2, entries
11 and 12). The present dehydrogenation could also be scaled
up; for the 10 mmol-scale dehydrogenation of 1a, the amount of
the catalyst could be reduced to 1 mol%, and 2a was obtained in
90% isolated yield (Table 2, entry 5).
with Au9Pd1/LDHa
Cyclohexanones could also be utilized as starting materials
for the synthesis of phenols, and simple cyclohexanone and
its derivatives with various substituents at 2-, 3- and 4-posi-
tions were all suitable substrates (Table 3). Disubstituted
phenols could also be synthesized starting from the corre-
sponding disubstituted cyclohexanones (Table 3, entry 8). In
the case of a glycol-protected cyclohexanone, the dehydroge-
nation proceeded without deprotection of the carbonyl group
(Table 3, entry 9). In addition, cyclohexanones substituted
with an ester, amide, or alkoxy group were dehydrogenated
efficiently without hydrolytic decomposition of these func-
tional groups (Table 3, entries 10–12). Enones, such as
2-cyclohexen-1-one and carvone, could also be utilized as
substrates for the present dehydrogenation (Table 3, entries
13 and 14).
Recently, Pd-catalyzed dehydrogenative aromatization has
emerged as an attractive method for the synthesis of
N-substituted anilines from cyclohexanones and amines.19
Encouraged by the unique activity of Au9Pd1/LDH for the
dehydrogenative aromatization of cyclohexanols and cyclohex-
anones, we also synthesized various N-substituted anilines
using the dehydrogenative aromatization strategy. Under reac-
tion conditions similar to those used for the aforementioned
dehydrogenation, cyclohexanones with various substituents on
the cyclohexyl rings were efficiently reacted with various alkyl
and benzylamines to give the corresponding N-substituted
anilines (Table 4).
To further demonstrate the potential practical applicability
of the present Au9Pd1/LDH-catalyzed dehydrogenative aroma-
tization, we attempted to synthesize phenol and anilines
directly from KA oil. When a 1 : 1 mixture of cyclohexanol and
cyclohexanone was used as the starting material, 96% yield of
phenol was obtained (Scheme 2a). In addition, KA oil was
reacted with n-octylamine to give N-octylaniline in high yield
(Scheme 2b).
Mechanistic studies
a
Reaction conditions: 3 (0.5 mmol), Au9Pd1/LDH (total amount of
The reaction prole for the Au9Pd1/LDH-catalyzed oxidative
dehydrogenation of cyclohexanol (1b) showed that cyclohexa-
none (3b) was initially produced, followed by the dehydroge-
nation of 3b to 2-cyclohexen-1-one (4b); the further
dehydrogenation of 4b gave phenol (2b) as the nal product
(Fig. 4a). The dehydrogenation starting from 3b also gave 2b in
high yield, which suggests the reaction proceeded through 3b
metals: 3.6 mol%), DMA (2 mL), 130 ꢀC, air (1 atm). Conversion and
yields were determined by GC analysis. The values in the parentheses
are the isolated yields. b Au4Pd1/LDH (total amount of metals: 3.6
mol%).
as the intermediate (Fig. 4b). The reaction prole for the throughout the reaction, which indicates that the dispropor-
dehydrogenation starting from 4b revealed that 2b was tionation of 4b is much faster than the dehydrogenation of 3b
produced through the disproportionation of 4b (Fig. 4c). to 4b (Fig. 4b).
Furthermore, the time course of the dehydrogenation starting
To verify whether the dehydrogenation proceeded through
from 3b showed that only a trace amount of 4b was detected radical intermediates, the Au9Pd1/LDH-catalyzed dehydrogenation
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Table
4 Substrate scope of the N-arylation of amines with
cyclohexanonesa
a
Reaction conditions: 3 (1.0 mmol), 5 (0.5 mmol), Au9Pd1/LDH (total
amount of metals: 3.6 mol%), DMA (2 mL), 130 ꢀC, air (1 atm).
Conversion and yields were determined by GC analysis. Au4Pd1/LDH
(total amount of metals: 3.6 mol%). 3 (2.5 mmol).
b
c
Scheme 2 Syntheses of phenol and N-octylaniline starting from KA
oil. Reaction conditions: (a) 1b (0.5 mmol), 3b (0.5 mmol), DMA (2 mL),
130 ꢀC, air (1 atm), 4 h. (b) 1b (0.5 mmol), 3b (0.5 mmol), n-octylamine
(0.5 mmol), DMA (1 mL), mesitylene (1 mL), 130 ꢀC, air (1 atm), 3 h.
Yields were determined by GC analysis. The parenthetical value is the
isolated yield.
of 1b was carried out in the presence of a stoichiometric amount
of a radical scavenger, 2,2,6,6-tetramethylpiperidine 1-oxyl
(TEMPO). In this case, the reaction was not suppressed, but was
substantially promoted by the addition of TEMPO and the
formation of 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEM-
POH) was observed (Fig. 5a). Similarly, the dehydrogenations
starting from 3b and 4b were both accelerated in the presence of
TEMPO, with the concomitant formation of TEMPOH (Fig. 5b
and c). Furthermore, control experiments showed that TEMPO
only or LDH together with TEMPO (in the absence of catalysts)
could not promote the dehydrogenation of 3b (Fig. 5b). All these
results indicate that the involvement of radical intermediates is
unlikely in the present transformation. TEMPO has been re-
ported to act as a one-electron oxidant to abstract a hydrogen
atom from metal hydride species, where TEMPO itself is
reduced to TEMPOH.20,21 Therefore, the aforementioned
Fig. 4 Reaction profiles of dehydrogenations starting from (a) 1b, (b)
3b, and (c) 4b. Reaction conditions: substrates (0.5 mmol), Au9Pd1/
LDH (total amount of metals: 3.6 mol%), DMA (2 mL), 130 ꢀC, air (1 atm).
GC yields are shown here.
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experimental results strongly support the involvement of metal
hydride species such as Au–H20 and Pd–H21 in the consecutive
dehydrogenations; the reactions were promoted by the oxida-
tion of these metal hydride species by TEMPO. In particular, for
the initial stage of the Au9Pd1/LDH-catalyzed disproportion-
ation of 4b, the formation of 3b was substantially suppressed by
the addition of TEMPO compared to the reaction in the absence
of TEMPO (Fig. 5c, Table S5,† entries 3 and 4). These results
suggest that the ability for the oxidation of the metal hydride
species (Pd–H) increases in the order O2 < 4b < TEMPO.
As shown in Table 1, for the oxidative dehydrogenation of 1a,
either Au/LDH or Pd/LDH could promote the dehydrogenation
of 1a to 3a (Table 1, entries 1 and 6); however, their substrate
conversions were much lower than that achieved when Au9Pd1/
LDH was used as the catalyst (Table 1, entry 2). These results
indicate that both Pd and Au species in Au9Pd1/LDH intrinsi-
cally possess catalytic activity toward the dehydrogenation of 1a
to 3a and that the dehydrogenation ability was improved by
alloying Au and Pd; this observation is in good agreement with
results previously reported by other research groups.11
Although Au/LDH did not show catalytic activity for the
dehydrogenative aromatization of 3b, Pd/LDH could promote
the transformation into 2b, albeit in low yield (Fig. S6†). In
addition, with respect to the disproportionation of 4b, Pd/LDH
exhibited high catalytic activity, whereas Au/LDH afforded only
a small amount of 2b (Fig. S7†). These results suggest that Pd
species in the Au–Pd alloy nanoparticles likely play the main
role in the dehydrogenation of 3b to 4b and the successive
disproportionation of 4b. In addition, the disproportionation of
4b using Pd/LDH as the catalyst was promoted by the addition
of TEMPO, which suggests the involvement of the Pd–H species
in the disproportionation (Table S5,† entries 5 and 6).
On the basis of the aforementioned experimental results, we
herein propose a plausible reaction mechanism for the dehy-
drogenation of cyclohexanols to phenols. For the oxidation of
cyclohexanols to cyclohexanones (Scheme 3, cycle 1), depro-
tonative coordination of cyclohexanols to the metal species
(either Au or Pd) promoted by LDH generates metal alkoxy
species (Scheme 3, cycle 1, step 1), followed by b-H elimination
to give cyclohexanones with the concomitant formation of
metal hydride species (either Au–H or Pd–H) (Scheme 3, cycle 1,
step 2). The metal hydride species can then be oxidized by O2
(Scheme 3, cycle 1, step 3). For the dehydrogenation of cyclo-
hexanones to cyclohexenones (Scheme 3, cycle 2), depro-
tonative coordination of cyclohexanones to Pd with the
assistance of basic LDH affords Pd-enolate species (Scheme 3,
cycle 2, step 4) followed by b-H elimination to give cyclo-
hexenones (Scheme 3, cycle 2, step 5). The oxidation of Pd–H
can then regenerate the catalyst (Scheme 3, cycle 2, step 6).22
Finally, the disproportionation of cyclohexenones proceeds
through a catalytic cycle similar to that of the dehydrogenation
of cyclohexanones to cyclohexenones (Scheme 3, cycle 3).
Specically, LDH-assisted a-C–H cleavage generates Pd-enolate
species (Scheme 3, cycle 3, step 7) followed by b-H elimination
to give the nal phenol products with the concomitant
formation of the Pd–H species (Scheme 3, cycle 3, step 8). In the
Fig. 5 Effects of TEMPO on dehydrogenations starting from (a) 1b, (b)
3b, and (c) 4b. Reaction conditions: substrates (0.5 mmol), Au9Pd1/
LDH (total amount of metals: 3.6 mol%), DMA (2 mL), 130 ꢀC, air (1 atm).
GC yields are shown here.
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Scheme 3 A plausible reaction mechanism. M ¼ Au or Pd.
cycle 3, the hydrogen acceptor is cyclohexenones rather than O2 reported to result in a net electron transfer from Pd to Au; thus,
(Scheme 3, cycle 3, step 9). the Au species become more electron-rich and the Pd species
Because of the mechanism of alcohol oxidation by supported become more electron-poor.9–12 Therefore, the b-H elimination
Au–Pd alloy nanoparticles has been extensively investigated by step in the dehydrogenation of 3b to 4b likely become more
other research groups,11 we herein mainly focused on the favorable with increasing Au/Pd molar ratio of the catalysts.
elucidation of the detailed roles of Au–Pd alloy nanoparticles in Consequently, the catalytic activity of the AuxPdy/LDH catalysts
the dehydrogenation of cyclohexanones to phenols. As previ- possibly increased with increasing Au/Pd molar ratio, and thus
ously mentioned, Pd/LDH exhibited signicant catalytic activity Au9Pd1/LDH exhibits much higher catalytic activity than other
for the oxidation of 1a to 3a (Table 1, entry 6) but did not exhibit catalysts toward the present dehydrogenation (Tables 1 and S3†).
catalytic activity for the dehydrogenative aromatization of 3a to
Kinetic studies on the Au9Pd1/LDH-catalyzed dehydrogena-
2a (Table S2,† entry 6). Furthermore, the disproportionation of tion of 3b showed the saturation kinetics of the reaction rate
4b was efficiently promoted by Pd/LDH (Fig. S7†). These results based on the concentration of 3b (22.9–478.9 mM, Fig. 6a),
suggest that the Au atoms in Au–Pd alloy nanoparticles play an which indicates that a pre-equilibrium exists between Pd
important assisting role on the dehydrogenation of cyclohexa- species and Pd-enolate species for cleavage of an a-C–H bond.
nones to cyclohexenones by Pd (Scheme 3, cycle 2).
The dehydrogenation of cyclohexanone-2,2,6,6-d4 (97% deute-
In addition, the dehydrogenation of 3b with Pd/LDH was not rium labeling at the 2- and 6-positions) gave phenol with 65%
accelerated by TEMPO even though TEMPO is known to promote deuterium labeling at the 2- and 6-positions (Scheme S1†); this
the oxidation of Pd–H species (Table S4,† entries 5 and 6);21 this deuterium loss also supports the existence of the pre-equilib-
result suggests that the low activity of Pd/LDH for the dehydro- rium. In addition, the kinetic studies revealed a rst-order
genation of 3b is not likely due to the inertness of Pd–H species dependence of the reaction rate on the partial pressure of O2
toward oxidation by O2 (Scheme 3, cycle 2, step 6) but is instead (0.1–1.0 atm, Fig. 6b), which suggests that the oxidation of the
due to slow b-H elimination (Scheme 3, cycle 2, step 5). The Au Pd–H species is included in the rate-determining step for the
species in Au–Pd alloy nanoparticles possibly promote b-H dehydrogenation of 3b. The aforementioned signicant
elimination by Pd, likely via electronic ligand effects.9–12 The promotion effects of TEMPO also suggest that the oxidation of
greater electronegativity of Au compared to that of Pd has been the Pd–H species is included in the rate-determining step.
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by-product, which highlights the environmentally benign
nature of the present transformation. The substrate scope was
very broad with respect to both cyclohexanols and cyclohexa-
nones, and N-substituted anilines could also be synthesized
starting from cyclohexanones and amines. Furthermore, the
present catalytic system could be scaled up and applied to the
direct conversion of KA oil to phenol. Thus, we demonstrated
the potential practical applicability of the present catalyst
system for the synthesis of phenol from benzene in industrial
processes.
Experimental section
Instruments and reagents
Gas chromatography (GC) analyses were performed on a Shi-
madzu GC-2014 equipped with a ame ionization detector (FID)
and an InertCap-5 or a TC-WAX capillary column. GC mass
spectrometry (GC-MS) spectra were recorded on a Shimadzu
GCMS-QP2010 equipped with an InertCap 5 capillary column at
an ionization voltage of 70 eV. Liquid-state NMR spectra were
recorded on a JEOL JNM-ECA-500 spectrometer. 1H and 13C
NMR spectra were measured at 495.1 and 124.5 MHz, respec-
tively, using tetramethylsilane (TMS) as an internal reference
(d ¼ 0 ppm). ICP-AES analyses were performed on a Shimadzu
ICPS-8100. TEM observations were performed on JEOL JEM-
2010HC. HAADF-STEM and EDS images were obtained using
JEOL JEM-ARM 200F operating at 200 kV. XRD patterns were
collected on a Rigaku SmartLab diffractometer (CuKa, l ¼
˚
1.5405 A, 45 kV, 200 mA). UV-Vis diffuse reectance spectra were
recorded on a Jasco V-570DS. Mg–Al LDH (Mg6Al2(OH)16CO3-
$4H2O, BET surface area: 51 m2 gꢁ1, Tomita Pharmaceutical
Co., Ltd.), CeO2 (BET surface area: 111 m2 gꢁ1, Cat. no. 544841-
25G, Aldrich), Al2O3 (BET surface area: 160 m2 gꢁ1, Cat. no.
KHS-24, Sumitomo Chemical), MgO (BET surface area: 28 m2
gꢁ1, Cat. No. P0082, Ube Industries Ltd.), and TiO2 (BET surface
area: 316 m2 gꢁ1, Cat. No. ST-01, Ishihara Sangyo Kaisya) were
acquired from commercial sources. Solvents and substrates
were obtained from Kanto Chemical, TCI, Wako, or Aldrich
(reagent grade) and were puried before use (if required).23
Fig. 6 Dependence of reaction rates of the dehydrogenation of 3b on
(a) the concentration of 3b and (b) the partial pressure of O2, line fit: R0
¼ 17.9P (O2), R2 ¼ 0.98. Reaction conditions: (a) 3b (22.9–478.9 mM),
Au9Pd1/LDH (total amount of metals: 3.6 mol%), DMA (2 mL), 130 ꢀC,
air (1 atm); (b) 3b (250 mM), Au9Pd1/LDH (total amount of metals: 3.6
mol%), DMA (2 mL), 130 ꢀC, O2 (0.1–1 atm). GC yields are shown here.
Conclusion
Preparation of catalysts
We successfully developed a novel heterogeneously-catalyzed
oxidative dehydrogenation reaction for the conversion of Au–Pd/LDH with different Au/Pd ratios, such as Au9Pd1/LDH,
cyclohexanols and cyclohexanones into phenols using Au9Pd1/ was typically prepared as follows. First, Mg–Al LDH (2.0 g) was
LDH. Monometallic Au or Pd supported on LDH exhibited added to 60 mL of an aqueous solution of HAuCl4$4H2O (7.50
almost no activity toward the dehydrogenation, whereas mM), PdCl2 (0.83 mM), and KCl (2 equiv. with respect to PdCl2,
marked enhancement of the catalytic activity was observed for 1.67 mM). The resulting slurry was then stirred vigorously at
alloyed Au and Pd. Mechanistic studies showed that b-H elim- room temperature for 12 h. The solid was then ltered off,
ination from Pd-enolate species is accelerated by Au species, washed with water (3 L), and dried in vacuo to afford the sup-
likely via electronic ligand effects; this acceleration is the key to ported hydroxide precursor. The hydroxide precursor was
achieve the high activity of Au9Pd1/LDH. The basicity of LDH redispersed in 50 mL water and reduced with NaBH4 (70 mg).
also plays a deterministic role in the present dehydrogenation The resulting slurry was then stirred vigorously at room
through its assistance in the deprotonation steps. Au9Pd1/LDH temperature for 2 h. The solid was then ltered off, washed with
functioned as a truly heterogeneous catalyst and could be water (2 L), and dried in vacuo overnight, giving Au9Pd1/LDH as
reused at least 10 times with retention of its high catalytic a dark-gray powder (Au content: 0.148 mmol gꢁ1, Pd content:
performance. The present dehydrogenation utilizes O2 in air 0.024 mmol gꢁ1). Au4Pd1/LDH (Au content: 0.134 mmol gꢁ1
as the terminal oxidant and generates water as the only Pd content: 0.051 mmol gꢁ1), Au1Pd1/LDH (Au content:
,
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0.091 mmol gꢁ1, Pd content: 0.122 mmol gꢁ1), and Au1Pd4/LDH
(Au content: 0.044 mmol gꢁ1, Pd content: 0.192 mmol gꢁ1) were
prepared via the same method used to prepare Au9Pd1/LDH.
Preparation methods for other catalysts are described in the
ESI.†
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Catalytic dehydrogenation
In a typical dehydrogenation procedure, Au–Pd/LDH (105 mg,
total amount of metals: 3.6 mol%), cyclohexanone (0.5 mmol),
DMA (2.0 mL), and a Teon-coated magnetic stir bar were
successively placed in a Pyrex glass reactor (volume: ca. 20 mL)
ꢀ
and the reaction mixture was vigorously stirred at 50 C under
1 atm of air. Aer the reaction was completed, an internal
standard (diphenyl) was added to the reaction mixture and the
conversion of cyclohexanone and the yield of phenol were
determined by GC analysis. In cases where phenol products
were isolated, an internal standard was not added. Aer the
reaction, the catalyst was ltered off (>95% recovery). EtOAc
(15 mL) and n-hexane (5 mL) were then added to the ltrate,
which was washed with brine (10 mL) three times. The organic
phase was collected and evaporated to remove solvents. The
crude product was subjected to column chromatography on
silica gel (for phenols, n-hexane/acetone was typically used as an
eluent; for N-octylaniline, n-hexane/ethylacetate was used),
giving the pure phenols. The products were identied by GC-MS
and NMR (1H and 13C) analyses. The recovered catalyst was
washed with water and ethanol (or 0.1 M NaOH in 1 : 1 water/
ethanol) before being used in the reuse experiment.
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Acknowledgements
This work was nancially supported by Grant-in-Aid for Scien-
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with Customized Scaffolding” (15H05797) and Research Activity
start-up (15H06143) from MEXT, Japan.
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surface, and we cannot exclude the possibility at this time.
We reason from the ineffectiveness of Pd/LDH (see entry 6,
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