Hydrogenation of phenols in ionic liquids on rhodium nanoparticles

Article abstract of DOI:10.1134/S0965544113030043

A new catalyst system based on rhodium nanoparticles stabilized by polyacrylic acid have been suggested for the hydrogenation of phenols in ionic liquids. It has been shown that high near-quantitative yields of reaction products are achieved in ionic liquids containing a tetraalkylammonium cation. By the TEM and XPS techniques it has been revealed that the use of ionic liquids substantially decreases the particle size and reduces the aggregation of nanoparticles through the inclusion of the ionic liquid cations into the surface layer along with polyacrylic acid.

Full text of DOI:10.1134/S0965544113030043

ISSN 0965ꢀ5441, Petroleum Chemistry, 2013, Vol. 53, No. 3, pp. 157–163. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © A.L. Maksimov, S.N. Kuklin, Yu.S. Kardasheva, E.A. Karakhanov, 2013, published in Neftekhimiya, 2013, Vol. 53, No. 3, pp. 177–184.
Hydrogenation of Phenols in Ionic Liquids
on Rhodium Nanoparticles
A. L. Maksimov, S. N. Kuklin, Yu. S. Kardasheva, and E. A. Karakhanov
Faculty of Chemistry, Moscow State University, Moscow, 119991 Russia
eꢀmail:kar@petrol.chem.msu.ru
Received November 30, 2012
Abstract—A new catalyst system based on rhodium nanoparticles stabilized by polyacrylic acid have been
suggested for the hydrogenation of phenols in ionic liquids. It has been shown that high nearꢀquantitative
yields of reaction products are achieved in ionic liquids containing a tetraalkylammonium cation. By the
TEM and XPS techniques it has been revealed that the use of ionic liquids substantially decreases the particle
size and reduces the aggregation of nanoparticles through the inclusion of the ionic liquid cations into the surꢀ
face layer along with polyacrylic acid.
Keywords: hydrogenation, phenols, rhodium nanoparticles, ionic liquids
DOI: 10.1134/S0965544113030043
Hydrogenation of phenols plays an important role the hydrogenation of phenols and other aromatic comꢀ
both in the petrochemical industry and in the processꢀ pounds in the aqueous medium.
ing of various types of renewable raw materials. This
reaction is the first stage of the commercialꢀscale proꢀ
EXPERIMENTAL
duction of caprolactam, adipic acid, and polyamide
To carry out the hydrogenation of phenols, two types
resins. Hydrogenation products of phenol derivatives
of ionic liquids containing imidazolium and tetraalkyꢀ
are used as feedstock for the manufacture of synthetic
lammonium cations were chosen as solvents (Fig. 1).
lubricants, oil and fuel additives, special solvents, surꢀ
Tetrabutylammonium bromide (C4H9)4NBr (N4444Br
)
factants, and synthetic fragrances. Upon processing of
biological feedstock, the hydrogenation of various types
of substituted phenols plays an important role to proꢀ
duce hydrocarbons or alcohols. In the latter case, works
aimed at processing phenols which are models of lignin
fragments deserve consideration [1–2]. Systems conꢀ
taining metal nanoparticles, either supported or immoꢀ
bilized in alternative media, exhibit a high catalytic
activity in the hydrogenation of phenols [3–10].
Aldrich, 99%, methyltri(
CH3(C8H17)3NCl (N8881Cl
lammonium bromide (C16H33(CH3)3NBr (N16111Br
n
ꢀoctyl)ammonium chloride
) Aldrich, cetyltrimethyꢀ
),
(
)
)
Merck, 97%, were used without previous purification.
The synthesis of 1ꢀbutylꢀ3ꢀmethylimidazolium chloꢀ
ride ([Bmim]+[Cl]–) was performed according to [16];
1ꢀbutylꢀ3ꢀmethylimidazolium
tetrafluoroborate
ꢀhexyltriethyꢀ
(
[Bmim]+[BF4]–), according to [17];,
n
lammonium bromide (N6222Br), according to [18]. The
structure of the ionic liquids was confirmed by 1H and
13C NMR spectroscopy. The obtained data were consisꢀ
tent with the literature.
Triethyl(propylꢀ3ꢀsulfonyl) ammonium hydrogen
sulfate ([Et3N+(CH2)3SO3H]HSO4⎯) was prepared as
follows. 1,3ꢀpropanesultone (12.2 g), triethylamine
(15.3 mL), and toluene were placed in a 100 mL flask.
When the reaction is carried out in water, various
stabilizing ligands may be used; however, it is necessary
to use a number of special stabilizers to create stable
hydrogenation nanocatalysts in the case of ionic liquids
(ILs) [11–12]. Thus, the catalytic activity of rhodium
nanoparticles in nonfunctionalized imidazolium ionic
liquids without further stabilization by ligands
decreased sharply because of aggregation into larger
particles [13]. Successful hydrogenation of phenol and
other arenes over rhodium nanoparticles using special
polymers as stabilizers was described in [11]. In [12],
preliminary modification of ionic liquids was proposed
for the dissolution in them of conventional stabilizers of
nanoparticles, such as polyvinylpyrrolidone.
The mixture was refluxed for 12 h at 90°С and constant
stirring. After the completion of the reaction, toluene
was removed on a rotary evaporator to the complete
drying of the resulting white amorphous precipitate.
22.3 g of zwitterion was obtained, the yield was 100%.
The obtained zwitterion (5 g) and H2SO4 conc.
(1.2 mL) were placed into a flask. The reaction mixture
was refluxed twentyꢀfour hours at 70
stance was obtained, the yield was 99%.
1H NMR (DMSOꢀD6), δ, ppm: 1.2 (t, 9H,
°С. 7 g of a subꢀ
In this study, we have examined a catalytic system,
based on rhodium nanoparticles, stabilized with polyꢀ
acrylic acid, in the hydrogenation of phenols Such sysꢀ
.
tems have been shown [14–15] to have a high activity in CH2CH3); 1.9 (m, 2H, CH2CH2CH2); 2.5 (t, 4H,
157

158
MAKSIMOV et al.
Cl
BF4
+
+
ꢀ
ꢀ
N
N
N
N
[Bmim]+[Cl]–
[Bmim]+[BF4]–
1ꢀbutylꢀ3ꢀmethylimidazolium
tetrafluoroborate
1ꢀbutylꢀ3ꢀmethylimidazolium
chloride
N
ꢁ
Br
ꢁ
N
ꢀ
N
ꢁ
Br
HSO4
ꢀ
ꢀ
SO3H
[Et3N+(CH2)3SO3H]HSO4–
N4444Br
N6222Br
triethyl(propylꢀ3ꢀsulfonyl)
ammonium hydrosulfate
tetrabutylammonium bromide
hexyltriethylꢀ
ammonium bromide
C6H13
C6H13
N
N
Br
ꢁ
ꢁ
ꢀ
Cl
ꢀ
C6H13
C14H29
N16111Br
N8881Cl
nꢀoctyl)ammonium chloride
methyltri(
cetyltrimethylammonium bromide
Fig. 1. Structures of the ionic liquids used in the study.
CH2CH2CH2); 3.1 (m, 6H, NCH2CH3); 3.2 (m, 2H, perature of the reaction mixture was maintained using a
NCH2CH2); 3.3 (quartet, 6H, NCH2CH3). thermostat with an accuracy of no less than 0.5 . The
catalyst was prepared in situ: 5 mg of polyacrylic acid
°
С
13C NMR (DMSOꢀD6),
, ppm: 7.5 (NCH2CH3);
δ
was dissolved in 150 L of water with stirring, an ionic
µ
18.4 (NCH2CH2) 47.7 (NCH2CH2CH2); 52.2
(NCH2CH3) 55.5 (NCH2CH2)
liquid (~0.5 mL) and 3 mg of rhodium chloride were
added. Red coloring of the solution was observed upon
the dissolution of rhodium chloride. Then 2 mg of
NaBH4 was dissolved in 100 L of water and added to
the solution of rhodium chloride, the solution turned
black.
During the catalytic experiments the following order
of operations was used: a catalyst in the form of rhodꢀ
ium nanoparticles immobilized in an ionic liquid and
.
The study of the obtained samples of nanoparticles
by Xꢀray photoelectron spectroscopy (XPS) was carried
out using a LAS_3000 electronic device equipped with
a photoelectron analyzer with an OPXꢀ150 retarding
potential. The photoelectron excitation was carried out
µ
using Xꢀray emission of an aluminum anode (Al
K =
α
1486.6 eV) with a tube voltage of 12 kV and an emission
current of 20 mA. Photoelectron peaks were calibrated
by carbon C 1s line with binding energy 285 eV. The
studies by the method of transmission electron microsꢀ
copy (TEM) were performed using a LEO912 AB
Omega transmission electron microscope.
stabilized by polyacrylic acid, and a substrate (1.06
×
10–3 mol of substrate per 1.06 10–5 mol of the catalyst)
×
were placed into a glass cartridge of an autoclave. The
autoclave was sealed, filled with hydrogen to a pressure
of from 5 to 40 bar and then thermostated for 0.5–
3 hours at 80°С. After the experiment the reactor was
cooled and an excess pressure was relieved. The reacꢀ
tion products were analyzed by GLC using a
Procedure for Catalytic Experiments
Catalytic experiments on hydrogenation of phenols ChromPack CP9001 gas chromatograph with a flame
were carried out at an elevated pressure and vigorous ionization detector and a column 30 m 0.2 mm coated
×
stirring of the reaction mixture in a stainless steel autoꢀ with SEꢀ30 stationary phase. Chromatograms were
clave under a hydrogen atmosphere. A constant temꢀ recorded and analyzed on a computer using a Maestro
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HYDROGENATION OF PHENOLS IN IONIC LIQUIDS
159
50 nm
50 nm
(b)
(а)
Fig. 2. Micrographs of rhodium nanoparticles stabilized by PAA in (a) water and (b) the ionic liquid N
Br.
4444
1.4 program. The conversion was monitored using cles stabilized by polyacrylic acid (PAA) was used as a
nonane as an internal standard.
catalyst system for the hydrogenation of phenol [14–
15]. The synthesis of nanoparticles was carried out in an
aqueous medium by the reduction of a rhodium trichloꢀ
ride solution with sodium borohydride in the presence
RESULTS AND DISCUSSION
A wellꢀestablished earlier for the use in an aqueous–
alcoholic medium system based on rhodium nanopartiꢀ of the polymer at a Rh : COOH ratio = 1 : 4 (scheme 1).
–O
–O
+
Na
O
O
–O
+
–O
N+a
NR4
O
NR4 +
O
–O
–O
N+a O
+
NR4
O
–O
–O
+
+
NR4
O
–O
+
NR4
NaBH4
N+a
NR4
+
–Na
O
RhCl3 · 4H2O
–O
N+a
O
O–
O
O–
–O N+a
+
NR4
O
–O
NR4
O
+
+ N+a
Na
+
ONR4
O
O
O
O–
O–
Scheme 1. Synthesis of rhodium nanoparticles.
The resulting dispersion was added to the correꢀ N4444Br (Fig. 4) evidence in favor of the suggestion that
sponding ionic liquids for subsequent reaction. Rhodꢀ on the surface of the particles a layer of polyacrylic acid
ium nanoparticles synthesized were characterized using and its associates with cations of ionic liquids is formed
transmission electron microscopy (TEM) (Fig. 2). The in the ionic liquid, which prevents the aggregation of the
average size of the particles in water is 2.2 nm, while in nanoparticles. Additionally, the surface layer contains a
the ionic liquid it was 1.3 nm (Fig. 3). It is substantial small amount of carboxylate complexes of rhodium (II)
that in water the association of metal nanoparticles is [Rh2(CH3COO)4]ClO4 [19]. A signal corresponding to
observed to occur in a significantly greater extent. This 3d5/2 binding energy for both the metal, and the comꢀ
suggests that the ionic liquids act as not only solvent, but plex of rhodium is present in the spectrum. The C : O :
also as a stabilizer of metal nanoparticles. Xꢀray photoꢀ N ratio indicates that polyacrylic acid on the surface is
electron spectroscopy data for particles isolated from in the form of associates with cations of the ionic liquid.
PETROLEUM CHEMISTRY Vol. 53
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160
MAKSIMOV et al.
Number of particles
(b)
Number of particles
(a)
180
160
140
120
100
80
60
40
20
1000
800
600
400
200
0
0.5
2
4
6
8
10 12 14
, nm
1.0 1.5 2.0 2.5 3.0 3.5
d, nm
d
Fig. 3. Size distribution of rhodium nanoparticles stabilized by PAA in (a) water and (b) the ionic liquid N
Br.
4444
Phenol was chosen as a model substrate for the acidic ionic liquids. In the case of the first ionic liquid,
hydrogenation reaction. It has been shown that the use cyclohexanone was formed with 100% selectivity. When
of ionic liquids based on imidazolium cation does not using an acidic ionic liquid, cyclohexanol (selectivity
make it possible to achieve acceptable yields in the reacꢀ 54%), and cyclohexane (selectivity 46%) were formed
tion of phenol hydrogenation. The conversion of the as the main products.
substrate for 3 h is only a few percent.
The separation of the catalyst after the reaction in
the ionic liquid showed that the particles are in a finely
dispersed state (Fig. 5). According to the TEM data, the
size distribution of the nanoparticles is bimodal (averꢀ
age size, 0.9 nm and 2.1 nm). In water, insignificant
aggregation of small nanoparticles is detected (Fig. 6).
Apparently, the ionic liquid exerts an additional stabiꢀ
lizing effect on the nanoparticles in this case.
The study of the kinetics of the process shows that
both cyclohexanol and cyclohexanone are formed
already in the first period of the reaction; cyclohexꢀ
anone is then slowly hydrogenated to the corresponding
alcohol (Table 2).
Special experiments have shown that even for three
hours of the reaction with cyclohexanone as a substrate
the rate of its hydrogenation was low, the yield of alcoꢀ
hol was less than 1%. Apparently, most of the cyclohexꢀ
anol is formed via a parallel route.
A similar pattern is observed when using cetyltrimeꢀ
thylammonium bromide (N16111Br) (Table 3). In this
case, the conversion reached 60% for half an hour, and
the selectivity for cyclohexanone is 100%.
It has been shown that the used catalyst system is
active in the hydrogenation of substituted phenols and
phenol derivatives. Upon the hydrogenation of paraꢀ
and orthoꢀethylphenol the conversion achieved 100%
for 3 h. The hydrogenation of cresols proceeds at much
lower rates. Upon the hydrogenation of anisole and
phenetole 36% selectivity for cyclohexanone is
observed, for diphenyl ether, it is 10% (Table 4). These
data indicate the occurrence of hydrogenolysis.
Note that according to published data, acceptable
conversions of the substrate by catalysis with nanopartiꢀ
cles of platinum metals in imidazolium ionic liquids can
be attained only over a very long reaction time (more
than 20 h). Apparently, this is caused by the interaction
of NCN carbenes formed in the reaction conditions
with the metal surface, which results in the blocking of
the surface of metal nanoparticles. When replacing the
imidazolium ionic liquids by tetraalkylammonium ILs,
the rate of the reaction increases by two or more orders
of magnitude and quantitative conversion is achieved.
Cyclohexanol and cyclohexanone are formed as the
main reaction products. The alcohol/ketone ratio difꢀ
fered significantly depending on the ionic liquid nature
(Table 1). Unusual results were obtained using
methyl(trioctyl)ammonium chloride (N8881Cl) and
at %
C1s, Rh 3d
C1s
С—82
O—10.4
Rh—0.7
N—6.9
Rh 3d
5/2
3/2
When using (N16111Br) as an ionic liquid, the selecꢀ
tivity for methoxycyclohexane, the product of anisole
hydrogenation, was substantially higher than when
using N6222Br and was more than 80%.
320 315 310 305 300 295 290 285 280 275
Binding energy, eV
We also investigated the effect of pressure on the
reaction of hydrogenation (Table 5). Upon carrying out
Fig. 4. XPS data for a sample of the catalyst.
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HYDROGENATION OF PHENOLS IN IONIC LIQUIDS
Table 1. Phenol hydrogenation in various ionic liquids
161
Run no.
Ionic liquid
6222Br
N8881Cl
N16111Br
Product
Selectivity, %
Conversion, %
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
cyclohexane
36
64
100
51
49
32
68
46
54
–
N
1
2
3
100
100
100
N4444Br
4
5
6
7
100
91
[Et3N+(CH2)3SO3H]HSO4–
[Bmim]+[Cl]–
cyclohexanol
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
1–3
1–3
–
–
[Bmim]+[BF4]–
–
Reaction conditions:
T
= 80
°
C,
p
(H ) = 40 bar, 3 h, substrate/Rh ratio = 100.
2
50 nm
(b)
50 nm
(а)
Fig. 5. Micrographs of rhodium nanoparticles stabilized by PAA in (a) water and (b) the ionic liquid N
Br after the reaction.
4444
Number of particles
80
Number of particles
600
(а)
(b)
500
400
300
200
100
60
40
20
0
0
1
2
3
4
5
6
7
d
8
, nm
1
2
3
4
5
6
d, nm
Fig. 6. Size distribution of rhodium nanoparticles stabilized by PAA in (a) water and (b) the ionic liquid N
tion.
Br after the reacꢀ
4444
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162
MAKSIMOV et al.
Table 2. Kinetics of phenol hydrogenation in N6222Br
Run no.
1
Time, h
0.5
Products
Selectivity, %
Conversion, %
46
TOF*, h–1
213
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
68
32
70
30
40
60
36
64
2
3
4
2
90
96
104
100
88
2.5
3
100
Reaction conditions:
T = 80°C, p(H ) = 40 bar, substrate/Rh ratio = 100.
2
n
consumed H2
* TOF = ꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀꢀ.
ncatalist × t
Table 3. Phenol hydrogenation in N16111Br
Run no.
1
Time, h
0.5
Products
Selectivity, %
Conversion, %
59
TOF, h–1
236
cyclohexanone
cyclohexanone
cyclohexanol
100
51
2
3
100
83
49
Reaction conditions:
T = 80°C, p(H ) = 40 bar, substrate/Rh ratio = 100.
2
Table 4. Hydrogenation of various substrates in N6222Br
Run no.
1
Substrate
Products
cyclohexanone
Selectivity, %
Conversion, %
100
36
64
phenol
anisole
cyclohexanol
methylcyclohexyl ether
cyclohexanone
64
2
100
36
3
4
5
6
7
benzene
cyclohexane
100
100
100
100
100
36
100
38
9
o
p
o
p
ꢀcresol
2ꢀmethylcyclohexanol
4ꢀmethylcyclohexanol
2ꢀethylcyclohexanol
4ꢀethylcyclohexanol
cyclohexanone
ꢀcresol
ꢀethylphenol
ꢀethylphenol
84
99
8
phenetole
27
ethoxycyclohexane
cyclohexanone
64
10
9
diphenyl ether
dicyclohexyl ether
phenylcyclohexyl ether
21
50
69
Reaction conditions:
T = 80°C, p(H ) = 40 bar, 3 h, substrate/Rh ratio = 100.
2
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HYDROGENATION OF PHENOLS IN IONIC LIQUIDS
163
Table 5. Effect of pressure on the process of phenol hydrogenation in N6222Br
Run no.
1
Pressure, bar
5
Products
Selectivity, %
Conversion, %
57
TOF, h–1
41
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
84
16
66
34
36
64
2
10
40
98
76
88
3
100
Reaction conditions:
T = 80°C, 3 h, substrate/Rh = 100.
Table 6. Effect of the IL/water ratio on phenol hydrogenation in N6222Br
Run no.
1
IL/water ratio
1/1
Products
Selectivity, %
Conversion, %
100
TOF, h–1
76
cyclohexanone
cyclohexanol
cyclohexanone
cyclohexanol
cyclohexanone
72
28
36
2
2/1
4/1
100
40
88
27
64
3
100
Reaction conditions:
T = 80°C, p(H ) = 40 bar, 3 h, substrate/Rh = 100.
2
4. D. V. Sokol’skii and N. I. Popova, Zh. Obshch. Khim.
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(2010).
the reaction for three hours the conversion was close to
100%, right up to 10 bar. Significant is the fact that the
selectivity for alcohol increases (from 16% to 64%) as
the pressure increases.
It has been shown that when running the reaction in
the presence of water an increase in its quantity results
in an increased conversion of the substrate (Table 6).
Apparently, at high concentrations of the ionic liquid in
water a partial blocking of the surface by the formation
of a double adsorption layer of organic cations is
observed.
7. A. A. Krichko, N. I. Artemova, A. V. Artemov, and
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based catalysts known in the literature in the activity in
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11. C. Zhao, H. Wang, N. Yan, et al., J. Catal. 250, 33
(2007).
12. X. Yang, N. Yan, Z. Fei, et al., Inorg. Chem. 47, 7444
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ACKNOWLEDGMENTS
Chem. Eur. J. 9, 3263 (2003).
This work was supported by the Russian Foundation
for Basic Research, project nos. 12ꢀ03ꢀ00325 and
11ꢀ03ꢀ00817.
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