New approach for highly selective hydrogenation of phenol to cyclohexanone: Combination of rhodium nanoparticles and cyclodextrins

Article abstract of DOI:10.1016/j.catcom.2015.10.005

The effect of cyclodextrins on the activity and selectivity of a catalytic system based on rhodium nanoparticles stabilized by polyacrylic acid (PAA) in the hydrogenation of phenol in aqueous solution and ionic liquid was reported. It was found that the r

Full text of DOI:10.1016/j.catcom.2015.10.005

Catalysis Communications 73 (2016) 63–68
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Catalysis Communications
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Short Communication
New approach for highly selective hydrogenation of phenol to
cyclohexanone: Combination of rhodium nanoparticles
and cyclodextrins
Sergey Kuklin a, Anton Maximov a,b, Anna Zolotukhina a, Eduard Karakhanov a,
⁎
a
Department of Chemistry, Moscow State University, Moscow 119991, Russian Federation
b
Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow 119991, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2015
Received in revised form 11 September 2015
Accepted 2 October 2015
Available online 9 October 2015
The effect of cyclodextrins on the activity and selectivity of a catalytic system based on rhodium nanoparticles
stabilized by polyacrylic acid (PAA) in the hydrogenation of phenol in aqueous solution and ionic liquid was
reported. It was found that the reaction medium and the nature of the cyclodextrin (CD) essentially affect the
rate of reaction and the distribution of reaction products. The use of the system based on rhodium nanoparticles
stabilized by cyclodextrins makes it possible to rapidly and efficiently prepare cyclohexanone from phenol with
yields up to 100% under relatively mild conditions (1 h; T = 80°°C, p(H2) = 10–40 bars).
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Phenol
Selective hydrogenation
Cyclohexanone
Cyclodextrin
Ionic liquid
Rhodium nanoparticles
1. Introduction
C in terms of activity. Analogous results were obtained for the Pd/Al2O3
catalyst doped with AlCl3 in supercritical carbon dioxide [6].
The hydrogenation of phenols and their derivatives is one of the
most important processes in industrial organic chemistry and the re-
newable feedstock processing. The reduction of phenol to cyclohexa-
none or cyclohexanol is the primary stage of the production of adipic
acid and caprolactam [1]. Thus, in the hydrogenation of phenol, it is rea-
sonable to obtain cyclohexanone, which can be further directly convert-
ed into cyclohexanone oxime and caprolactam [1–2].
As a rule, the hydrogenation of phenol is carried out in the presence
of different platinum group metals (palladium, rhodium, and rutheni-
um) supported onto silicon oxide, aluminum oxide, or activated carbon
[3–4]. The reaction occurs by a consecutive-parallel mechanism:
cyclohexanol is formed both as a result of the direct hydrogenation of
phenol and from cyclohexanone, an intermediate product of the reac-
tion [5–7]. In this context, the selective production of cyclohexanone
(N90%) at a high phenol conversion (N80%) is a nontrivial problem.
Earlier, it was found that the high yields of cyclohexanone can be
reached in the presence of a bimetallic palladium–gold catalyst support-
ed on the mesoporous hollow spheres of silica (Pd-Au/MHSS) [8]: the
selectivity of this catalyst for the ketone was 95–97% at a 93–97% con-
version in 45 min at 50°°C and 10 atm of H2. This catalyst was much su-
perior to both corresponding monometallic catalysts and traditional Pd/
Palladium supported onto mesoporous aluminum oxide made it
possible to obtain cyclohexanone in high yields (98% selectivity at a
96% conversion) even after half an hour at a temperature of 100°°C
and a hydrogen pressure of 5 bars [9]. A special feature of this catalytic
system was the use of water as a solvent: taken in a large excess relative
to the substrate, water readily displaced less polar cyclohexanone from
the hydrophilic surface of mesoporous aluminum oxide. Aqueous solu-
tion was also used for the hydrogenation of phenol and its derivatives in
the presence of a palladium catalyst supported onto poly(s-triazine)
(Pd@mpg-C3N4) [10].
The dispersions of metal nanoparticles in a reaction medium stabi-
lized by surfactants and polymers are an alternative to supported sys-
tems in the hydrogenation of aromatic compounds and phenols [11–
13]. Thus, it was found that phenol was hydrogenated to cyclohexanol
on catalysis by Rh nanoparticles stabilized with tetraalkylammonium
[14] and tetraalkylphosphonium [15] ionic liquids. In the latter case,
the reaction occurred much more slowly, with hydrogenolysis. With
the use of Rh nanoparticles stabilized by polymers [16–17], cyclohexanol
was the main reaction product. Simultaneously, high selectivity for
cyclohexanone was observed at the slow hydrogenation of phenol in
the presence of Pd nanoparticles stabilized by polyvinylpyrollidone
in water [18] and on Pd nanoparticles stabilized by a polymer based
on ionic liquid modified with sulfogroups in the presence of
heteropolyphosphoric acid [19].
⁎
Corresponding author.
E-mail address: kar@petrol.chem.msu.ru (E. Karakhanov).
http://dx.doi.org/10.1016/j.catcom.2015.10.005
1566-7367/© 2015 Elsevier B.V. All rights reserved.

64
S. Kuklin et al. / Catalysis Communications 73 (2016) 63–68
The selectivity of hydrogenation can be affected with the use of ad-
synthesized in a tetraalkylammonium ionic liquid by the reduction of
an aqueous solution of rhodium trichloride with sodium borohydride
in the presence of polymer at the ratio Rh: COOH = 1: 4 (Scheme 1)
with the subsequent addition of β-CD.
ditional ligands capable of forming inclusion complexes of the host–
guest type [20–23]. Cyclodextrins and calixarenes belong to ligands of
this type [20,21]. Noël et al. [21,24] studied the stabilization of rhodium
nanoparticles by a polymer consisting of carboxylic acid and β-
cyclodextrin (β-CD) fragments. In this case, the rhodium nanoparticles
exhibited high stability and catalytic activity in the hydrogenation of
olefins and aromatic compounds (xylenes and styrene) in aqueous solu-
tions. Depending on the Rh/β-CD ratio, styrene could be hydrogenated
to ethylbenzene or ethylcyclohexane.
Cyclodextrins can serve as effective phase-transfer agents not only in
water but also in ionic liquids [20]. Here, we report the results of the se-
lective hydrogenation of phenol to cyclohexanone in water and a
tetraalkylammonium ionic liquid in the presence of rhodium nanoparti-
cles and cyclodextrins (free or immobilized on a polymer).
According to the TEM data, the particle size distribution in the sys-
tem obtained in the absence of β-CD was monomodal and unsymmetri-
cal, with particle size of 1.5 nm [26; also see Fig. S1 in the
Supplementary material]. Upon the addition of free cyclodextrin to the
system, the average size considerably increased to reach 4.2 nm
(Fig. 1, top); the distribution became more symmetrical. We assume
that this phenomenon is related to the formation of inclusion complexes
between cyclodextrins and the hydrophobic fragments of the
tetraalkylammonium constituents of ionic liquids. In this case, a cyclo-
dextrin molecule can form an inclusion complex simultaneously with
two tetraalkylammonium ions (Fig. 1, bottom) [30]. Consequently, this
latter leaves the near-surface layer to cause an increase in the average
particle size (the local ligand/metal ratio decreases). For polymer 1 the
average size was 1.5 nm in ionic liquid and 2.7 nm in water that
corresponded to the Noël et al. data [21].
The XPS data for the sample containing cyclodextrin (Table 1;
Figs. S2–S5 in the Supplementary Material) confirm the above hypoth-
esis on the structure of the Rh–PAA–ionic liquid–CD system. Thus, the
3d signal of rhodium was very weak, and its atomic surface concentra-
tion was only 0.16%. Rhodium predominantly occurred in an oxidized
form, which corresponds to its stabilization by polyacrylic acid [31]
and, as a result, to the formation of Rh–O bonds, as additionally con-
firmed by the deconvolution of the 1s spectrum of oxygen [32]; Fig. S3
in the Supplementary Material]. Cyclodextrins can also take part in the
stabilization of rhodium nanoparticles [33] due to adsorption and
charge transfer from donor OH groups. The low nitrogen content of
the catalyst surface and the almost complete absence of metal–
nitrogen bonds are also indicative of the predominant occurrence of
ionic liquid in an interlayer between PAA and cyclodextrin molecules.
Cyclohexanone was the only product of the reaction performed in
the ionic liquid in the presence of β-CD (Fig. 2b). We found that a con-
version of about 80% at 100% selectivity for cyclohexanone could be
reached even in 1 h; in this case, a further increase in the reaction
time did not lead to the formation of cyclohexanol. As the pressure of
hydrogen was decreased to 10 bars, the degree of conversion insignifi-
cantly decreased to 70–80% in 3 h. Note that the addition of cyclodextrin
led to an increase in the overall rate of the reaction, as compared with an
analogous system without cyclodextrin [26]. Thus, the TOF values were
401 h−1 and 104 h−1 for the reactions with and without cyclodextrin
correspondingly.
2. Experimental
Commercial reagents from Fluka, Aldrich, Reachem, and Acros Or-
ganics were used in this study. The n-hexyltriethylammonium bromide
ionic liquid (N6222Br) was synthesized in accordance with a published
procedure [25]. The synthesis of poly(mono-(β-cyclodextrin-2-yl)-ma-
leate-alt-maleate-alt-methyl vinyl ether) (polymer 1) was carried out
using a published procedure [24]. The structures of the ionic liquid
and polymer 1 were confirmed by 1H and 13C NMR spectroscopy.
The catalyst in the ionic liquid was prepared in accordance with a
previously reported procedure [26]. Then, corresponding cyclodextrin
(200 mg) was added to the system. The catalyst in water was prepared
analogously. On the stabilization of rhodium nanoparticles with poly-
mer 1, this polymer (200 mg) was added in place of PAA without the
use of free cyclodextrin in this case. For details, please see Supplementa-
ry Material.
The studies by X-ray photoelectron spectroscopy (XPS) and trans-
mission electron microscopy (TEM) were performed on a Kratos Axis
Ultra DLD instrument equipped with an OPX-150 analyzer of photoelec-
trons with retarded potential and on a LEO912 AB OMEGA transmission
electron microscope with a cathode potential of 100 kV, respectively.
The catalytic experiments on the hydrogenation of phenols were car-
ried out according to a procedure described elsewhere [26]. The reaction
products were analyzed by gas–liquid chromatography on a Chrompack
CP9001 instrument with an SE-30 column (30 m × 0.2 mm) and a flame-
ionization detector.
The catalyst activity (turnover frequency, TOFs) was defined as the
number of moles of the substrate converted per mole of surface rhodi-
um per unit of time from the formula:
Cyclohexanone can also be selectively prepared in the presence of
various β-CD derivatives (Table 2). We found that the use of cyclodex-
trins containing polar modifying groups (SO−3 and maltosyl — Entries
7–8 and 10–11 respectively, Table 2) increased the rate of hydrogena-
tion, as compared with unmodified β-CD, whereas, on the contrary, hy-
drophobic cyclodextrins (methyl-β-cyclodextrin and heptakis(2,6-di-
O-methyl)-β-cyclodextrin — Entries 3–6 and 9 respectively, Table 2)
considerably inhibited the process. The activity for system with 6-O-
Maltosyl-β-CD was 1827 h−1 at 40 bars and 988 h−1 at 10 bars. The
conversion was 98% for 0.5 h at pressure of 40 bars. It is likely that this
tendency is related to an increase in the stability constants of the inclu-
sion complexes of phenol with hydrophobic cyclodextrins and, as a re-
sult, the hindered adsorption of phenol on the surface of metal
nanoparticles.
conv: ꢀ csubstr:
1
TOFsðsubstrÞ ¼
ꢀ D
;
vRHꢀt
M
where DM is the ratio of surface Rh atoms to the total number of Rh
atoms in nanoparticles. DM was calculated as d/0.901, where d is a nano-
particle diameter [27]. Also TOFs(H2) for the hydrogen consumed has
been calculated. In this case the TOF value obtained was multiplied on
3 for the cyclohexanol as product and for 2 for the cyclohexanone, tak-
ing into account the corresponding selectivities. All the reactions, both
in water and ionic liquid, in presence and in the absence of CD, were
conducted under the kinetic control, where the reaction rate has not al-
ready depended on the stirring rate.
It is likely that the high selectivity for cyclohexanone reached in ionic
liquids in the presence of cyclodextrins can be explained by the struc-
ture of a near-surface layer of ligands adjacent to rhodium nanoparticles
(Scheme 2). Cyclohexenol, which is an intermediate product of phenol
hydrogenation, forms an inclusion complex with cyclodextrin; as a con-
sequence, it is desorbed from the metal surface. The probability of the
repeated adsorption of cyclohexenol is very small because of a high sta-
bility constant of this complex with cyclodextrin [34]. The isomerization
3. Results and discussion
The system based on rhodium nanoparticles stabilized by polyacrylic
acid (PAA) and modified with cyclodextrins was used as a catalyst for
the hydrogenation of phenol. An analogous catalytic system without cy-
clodextrins was successfully used previously in an aqueous-alcoholic
solution and ionic liquids [26,28–29]. The rhodium nanoparticles were

S. Kuklin et al. / Catalysis Communications 73 (2016) 63–68
65
Scheme 1. Synthesis of rhodium nanoparticles.
of cyclohexenol to cyclohexanone may occur directly within the cavity
of cyclodextrin.
cyclohexenol, which is not involved in the formation of inclusion com-
plexes, is not desorbed from the metal surface, and it undergoes further
reduction to cyclohexanol.
Note that cyclohexanone is slowly hydrogenated to cyclohexanol in
a tetraalkylammonium ionic liquid in the absence of cyclodextrin [26].
In the presence of cyclodextrins ketone is not hydrogenated as was
shown in special experiments. Thus, it is believed that both cyclohexa-
none and its enol form can be adsorbed on the surface of rhodium nano-
particles under these conditions to undergo further transformations;
nevertheless, the probability of this process is much lower than that of
electron-rich phenol.
The hydrogenation of phenol in water in the presence of cyclodex-
trin, leads to the preferred formation of cyclohexanol with selectivity
to 100% (Fig. 2a). Because cyclohexanone did not undergo reduction
under analogous reaction conditions (80°°C; 40 bars of H2; 3 h), we con-
cluded that cyclohexanol in water is formed by the direct hydrogena-
tion of phenol.
The predominant formation of cyclohexanol in an aqueous solution
can be explained by the fact that the immobilization of cyclodextrin in
the surface layer is hindered due to competition with water molecules.
Water can be easily adsorbed on the surface of rhodium nanoparticles;
it can also form relatively stable hydrogen bonds with the polymeric
molecules of PAA and cyclodextrins in the test system. As a result,
Cyclodextrins can be immobilized in the near-surface layer of metal
nanoparticles with the use of specially synthesized copolymers contain-
ing different cyclodextrin fragments. Noël et al. [24] found that rhodium
nanoparticles stabilized by such copolymers are effective catalysts for
the hydrogenation of alkenes and aromatic compounds. We studied
the effect of such a polymer (polymer 1) on the rate and selectivity of
phenol hydrogenation in both an ionic liquid and water. We established
that cyclohexanone was the main product of reaction performed in the
ionic liquid as in the presence of free cyclodextrin. Note that the activity
of this system was much higher than that of the system containing free
cyclodextrin: a 98% conversion of phenol was achieved already after half
an hour (TOFs 652 h−1). The nearly 100% selectivity for cyclohexanone
did not decrease as the reaction time was further increased (Fig. 3b); the
subsequent conversion of cyclohexanone did not occur in this system. It
is likely that a ketone should undergo isomerization into an enol for the
further hydrogenation, and this isomerization is impossible under the
reaction conditions. We found that this system makes it possible to se-
lectively hydrogenate phenol at a lower pressure of hydrogen (10
bars): in this case, the yield of cyclohexanone was 100% in 3 h
(Table 2, Entry 15).
Cyclohexanone was also the main product of the reaction performed
in water in the presence of a catalyst based on polymer-bound cyclo-
dextrin (Fig. 3a); however, the catalyst activity was noticeably lower,
and the conversion after 3 h was as low as 57%. It should be noted,
that the amount of the polymer-bounded cyclodextrin is strongly affect-
ed the selectivity of the phenol hydrogenation. Thus, when Rh/CD ratio
was 2/1, the cyclohexanol was the major reaction product, when the cy-
clohexanone part was only 12% (under the 40 bars of hydrogen and
after 3 h, since reaction began). At Rh/CD ratio of 1/5 the cyclohexanol
was still the predominant product, and only at Rh/CD = 1/20 the selec-
tivity of 100% for cyclohexanone was achieved. Therefore, the further
studies in presence of polymer-bounded cyclodextrin were conducted,
using this ratio.
Table 1
XPS data for the catalyst with cyclodextrin.
Spectrum Total
Binding
Bond type
Fraction,
at.%
concentration, energy, eV
at.%
C 1s
65.93
285.0
286.5
С–С/C–H (PAA, N6222Br, CD) 14.88
CH2N+ (N6222Br)
C–O–C’/C–OH (CD)
O–C′–O (CD)
41.6
287.9
289.3
531.1
532.7
8.74
0.71
COOH (PAA)
O 1s
N 1s
31.24
1.47
Rhsurf → O, C(_O)OH (PAA) 0.44
C(_O)OH (PAA)
C–OH (CD)
28.17
533.6
399.7
402.3
1071.4
68.3
O–C–O (CD)
Rh ← N, C–N
2.63
0.17
1.3
0.17
1.03
–
NR+4 (N6222Br)
Na+ (NaBH4, Na2B4O7)
Br− (N6222Br)
Na 1s
0.17
1.03
–
Br 3d5/2
Rh 3d5/2
Rh 3p3/2
309.0
497.3
[Rh2(H2O)(COOH)]
[Rh2(H2O)(COOH)]
Fig. 1. Micrograph of rhodium nanoparticles stabilized by PAA in the ionic liquid N6222Br in
the presence of cyclodextrin.
0.16
0.16

66
S. Kuklin et al. / Catalysis Communications 73 (2016) 63–68
Fig. 2. Hydrogenation of phenol in the presence of β-CD in (a) water and (b) N6222Br. Reaction conditions: T = 80°°C; p(H2) = 40 bars; substrate/Rh = 100/1.
Increasing the selectivity on cyclohexanone at the increased
at phenol conversions of 82% for 1 h and 98% within 30 min in presence
of free and bound β-cyclodextrins correspondingly (Rh loading is
1 mol%). At the same time analogous results were earlier reached only
for 8–16 h at less substrate to catalysts ratio (5 mol%), in particular, for
Pd nanoparticles, stabilized by poly(vinylpyrrolidone) [18]. Herein, the
calculated TOF values appeared to be as 19.3 h−1 for surface Pd
atoms. Such low TOF values, as well as high selectivity on cyclohex-
anone (95–99%) were achieved at very low hydrogen pressure
(1 bar). For our catalysts the high selectivity on cyclohexanone
(100%) is maintained even under the increased hydrogen pressures
(40 bars), and catalyst turnover frequencies reached here up to
1827 h−1 for 6-O-Maltosyl-β-CD.
amounts of polymer-bounded cyclodextrin is likely a result of the com-
petitive adsorption of water molecules on the surface of rhodium,
whereas the major portion of phenol molecules predominantly oc-
curred as inclusion complexes with cyclodextrins; this is consistent
with an induction period (Fig. 3a). Consequently, the rate of reaction de-
creased. At the same time, cyclodextrins immobilized on the polymer
were retained in a near-surface layer, and they easily formed inclusion
complexes with the intermediate product cyclohexenol to facilitate its
desorption from the surface of rhodium nanoparticles. Thus, although
a long reaction time was necessary for the achievement of a 100% con-
version under the given conditions, the selectivity for cyclohexanone
remained at a level of 100% (Table 2 Entry 17).
It should be noted, that our Rh catalysts, synthesized in situ in pres-
ence of the ionic liquid and β-cyclodextrin (especially, polymer-bound
cyclodextrin), are much more effective, than the previously described
in literature [18–19]. Thus, we obtain 100% selectivity on cyclohexanone
4. Conclusions
We found that the presence of cyclodextrin significantly affects the
activity and selectivity of a catalyst based on rhodium nanoparticles in
Table 2
Effect of cyclodextrins on the rate and selectivity of phenol hydrogenation.
Entry
Catalyst
Cyclodextrin
T, °C
t, h
p, bar
Sa, %
C, %
TOFs, h−1 (H2)
TOFs, h−1 (substr.)
_O
–OH
Ionic liquid N6222Br
1
2
3
4
5
6
Rh-PAA
Rh-PAA
Rh-PAA
Rh-PAA
Rh-PAA
Rh-PAA
β-CD
80
80
80
80
80
80
2
3
0.5
2
0.5
2
40
15
15
40
40
40
100
100
100
100
100
100
–
–
–
–
–
–
86
78
17
97
39
401
242
317
452
755
466
201
121
159
226
378
233
Methyl-β-CD
Methyl-β-CD
Methyl-β-CD
Methyl-β-CD
Sulfonated β-CD,
sodium salt
100
7
8
Rh-PAA
Rh-PAA
Sulfonated β-CD,
sodium salt
Heptakis-
80
80
0.5
2
40
40
100
100
–
–
51
31
950
145
475
73
(2,6-di-O-methyl)-β-CD
6-O-Maltosyl-β-CD
6-O-Maltosyl-β-CD
Polymer 1
Polymer 1
Polymer 1
9
Rh-PAA
Rh-PAA
Rh
Rh
Rh
80
80
60
80
80
80
0.5
0.5
3
0.25
0.5
3
40
10
40
40
40
10
100
100
100
100
100
100
–
–
–
–
–
–
98
53
88
70
98
1827
988
98
932
652
111
914
494
49
466
326
56
10
11
12
13
14
Rh
Polymer 1
100
Water
15
16
17
18
Rh-PAA
Rh
Rh
Pd-PVP [18]
Pd-PVP [18]
β-CD
80
80
80
80
90
0.5
7
0.5
16
2
40
40
40
1
–
100
–
–
1.5
1.0
100
99
12
96.5
29.5
3330
84
144
7.9
1110
41
72
Polymer 1
Polymer 1
No
100
100
98.5
99.0
3.9
9.6
19
No
1
19.3
Reaction conditions: substrate/Rh = 100/1; C is conversion, and S is selectivity.
a
_O indicates cyclohexanone and –OH indicates cyclohexanol.

S. Kuklin et al. / Catalysis Communications 73 (2016) 63–68
67
Scheme 2. Hydrogenation of phenol (a) in N6222Br and (b) in N6222Br in the presence of cyclodextrin.
Fig. 3. Phenol hydrogenation in the presence of polymer 1 (a) in water and (b) in the ionic liquid N6222Br. Reaction conditions: T = 80°°C; p(H2) = 40 bars; substrate/Rh = 100/1; CD/
Rh = 20/1.
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Acknowledgments
This study was supported by the Russian Foundation for Basic
Research (grant No. 12-08-01187).
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Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2015.10.005.
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