Selective phenol hydrogenation to cyclohexanone over a dual supported Pd-Lewis acid catalyst

Article abstract of DOI:10.1126/science.1179713

Cyclohexanone is an industrially important intermediate in the synthesis of materials such as nylon, but preparing it efficiently through direct hydrogenation of phenol is hindered by over-reduction to cyclohexanol. Here we report that a previously unappr

Full text of DOI:10.1126/science.1179713

Selective Phenol Hydrogenation to Cyclohexanone Over a Dual
Supported Pd −Lewis Acid Catalyst
Huizhen Liu et al.
Science 326, 1250 (2009);
DOI: 10.1126/science.1179713
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REPORTS
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Materials and Methods
Figs. S1 to S3
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We report here that Pd/C, Pd/Al2O3, and
Pd/NaY zeolite (18) (NaY zeolite is hereafter
denoted NaY) catalysts and solid Lewis acids
show excellent synergy in the hydrogenation of
phenol to cyclohexanone, together substantially
enhancing both activity and selectivity. The re-
action can be carried out effectively at temper-
atures as low as 30°C, and >99.9% conversion of
phenol is observed with >99.9% selectivity to
cyclohexanone. Separation of the product from
the catalyst–Lewis acid system is simple, and the
catalyst system can be reused directly. This route
has great potential for industrial application.
Our experiments (18) showed that dichloro-
methane is the best reaction solvent among sev-
eral tested (table S2). Table 1 presents the results
of phenol hydrogenation under different con-
ditions over Pd/C, Pd/Al2O3, and Pd/NaY cata-
lysts with and without AlCl3. The conversion of
phenol was very low, and considerable byproduct
Selective Phenol Hydrogenation to
Cyclohexanone Over a Dual Supported
Pd–Lewis Acid Catalyst
Huizhen Liu, Tao Jiang,* Buxing Han,* Shuguang Liang, Yinxi Zhou
Cyclohexanone is an industrially important intermediate in the synthesis of materials such as nylon,
but preparing it efficiently through direct hydrogenation of phenol is hindered by over-reduction to
cyclohexanol. Here we report that a previously unappreciated combination of two common commercial
catalysts―nanoparticulate palladium (supported on carbon, alumina, or NaY zeolite) and a Lewis acid
such as AlCl3―synergistically promotes this reaction. Conversion exceeding 99.9% was achieved with
>99.9% selectivity within 7 hours at 1.0-megapascal hydrogen pressure and 50°C. The reaction was
accelerated at higher temperature or in a compressed CO2 solvent medium. Preliminary kinetic and
spectroscopic studies suggest that the Lewis acid sequentially enhances the hydrogenation of phenol to
cyclohexanone and then inhibits further hydrogenation of the ketone.
yclohexanone is a key raw material in the the reaction can be conducted in either the gas was produced when only the Pd/C catalyst was
synthesis of many useful chemical inter- phase or liquid phase. The gas-phase phenol hy- used (Table 1, entry 1), and the reaction did not
mediates, such as caprolactam for nylon 6 drogenation is usually performed in the temper- occur at all when only Lewis acid (AlCl3) was
C
and adipic acid for nylon 66 (1, 2). The industrial ature range of 150° to 300°C over supported Pd used (Table 1, entry 2). When Pd/C and AlCl3
production of cyclohexanone typically involves catalysts (5–9), and different supports have been were used at the same time, the reaction pro-
either the oxidation of cyclohexane (3, 4) or the used, including alumina, that may act as Lewis ceeded with a selectivity of >99.9% up to com-
hydrogenation of phenol. The former route re- acids (9). The gas-phase process can be carried plete conversion at 1.0 MPa of H2 and a
quires high temperature and generates bypro- out easily in continuous reactors for higher temperature at or below 50°C (Table 1, entries 3
ducts such as cyclohexanol and organic acids that throughput. Liquid-phase phenol hydrogenation to 5). At higher temperature, the reaction reached
complicate purification, and the yield of cyclo- offers cost and energy savings, because the reac- completion in 1 hour and the selectivity remained
hexanone is usually low. The phenol hydrogen- tion can be performed at relatively low tempera- >99% (Table 1, entries 9 and 10). With other
ation route is undertaken through either two-step tures (10–15). Many researchers have contributed conditions fixed, an increase in hydrogen pres-
or one-step processes. In the two-step procedure, to this area, and multiple catalysts have been sure (Table 1, entries 6 to 12) shortened the time
phenol is first hydrogenated to cyclohexanol, screened, such as Rh/C (10), Rh/C nanofiber to completion but slightly reduced the selectivity
which in turn is dehydrogenated to cyclohexa- (11), Pd/hydrophilic C (12), Ru/poly(N-vinyl-2- to cyclohexanone. Other Lewis acids were also
none at high temperature. The one-step selective pyrrolidone) (PVP) (13), Pd/Mg and Pd/Fe (14), effective in promoting the reaction [Table 1, en-
hydrogenation of phenol to cyclohexanone is mesoporous Ce-doped Pd (15), and Pd/C (16). tries 13 to 18; see also fig. S7 and supporting
advantageous from an efficiency standpoint, and However, the attainment of high selectivity (>95%) online material (SOM) text]. The activity and
at elevated conversion (>80%) with a satisfactory selectivity of the reaction using Pd/Al2O3 and
rate is a great challenge (8, 17), because the Pd/NaY (18) were also enhanced effectively by
cyclohexanone product can be further hydro- AlCl3 (Table 1, entries 19 to 26). The prospects
Beijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
genated to cyclohexanol under the reaction con- for recycling Pd/C-ZnCl2 were tested (18), and
*To whom correspondence should be addressed. E-mail:
Jiangt@iccas.ac.cn (T.J.); Hanbx@iccas.ac.cn (B.H.)
ditions (7, 15).
the results indicated that the catalyst system could
1250
27 NOVEMBER 2009 VOL 326 SCIENCE www.sciencemag.org

REPORTS
be reused after simple separation without a de- a standard pseudo–first-order rate dependence on minimization, ease of product separation, and
crease in conversion and selectivity (Table 1, phenol concentration (fig. S9 and SOM text). pressure tunability (19–21). The Pd/C–Lewis
entries 16*, 16†, and 16‡). Preliminary kinetics We also explored the use of CO2 as a reaction acid systems were very efficient catalysts for the
measurements indicated that the reaction exhibits solvent for its characteristic advantages of waste reaction in compressed CO2 at suitable pressures
(Table 2, entries 3 to 12). The reaction in CO2
Table 1. Hydrogenation of phenol under different conditions. Conversions and selectivities were
determined by gas chromatography (GC). Reactants and products were identified by GC mass spectrometry
(MS) as well as by comparing retention times to respective standards in GC traces. At >99.9% conversion,
the minimum time required for reaction completion is given. Cyclohexanone and cyclohexanol were the
only reaction products observed. Reaction conditions were as follows: phenol, 1.0 mmol; Pd [5 weight
percent (wt %) in Pd/C or Pd/Al2O3; 2.5 wt % in Pd/NaY], 5 mol % relative to phenol; Lewis acid, 10 mol %
relative to phenol; solvent, 1 ml of dichloromethane. p, pressure; T, temperature. Dashes indicate that no
catalyst or Lewis acid was added. C=O indicates cyclohexanone, and OH denotes cyclohexanol.
also follows standard pseudo–first-order kinetics
with respect to phenol (fig. S9). The pressure of
CO2 had a significant effect on the apparent reac-
tion rate. At optimized pressures, both the con-
version and selectivity were >99.9% (Table 2,
entries 3 to 12), and the apparent reaction rate was
faster than that in dichloromethane (Table 1, entry
3, versus Table 2, entries 3 to 6; Table 1, entry 13,
versus Table 2, entries 7 to 9; Table 1, entry 15,
versus Table 2, entries 10 to 12), which may
result mainly from the larger diffusion coefficient
of the reactants in CO2 and the higher miscibility
of gaseous H2 with CO2 than with liquid solvents
(22). In addition, CO2 is a weak Lewis acid,
which may also enhance the reaction. To further
explore the effect of CO2 pressure on the reaction
rate, we observed the phase behavior of the re-
action system directly by using a high-pressure
view reactor (21). Avapor phase (a mixture of H2
and CO2) and a solid phase (a mixture of catalyst,
Lewis acid, and phenol) were evident at total pres-
sures of 5.0 MPa (Table 2, entry 1) and 6.5 MPa
(Table 2, entry 2). The phenol, Pd, and Lewis
acid could not come in contact efficiently, and
therefore the reaction proceeded slowly. At higher
pressures, a liquid phase formed in addition to
the vapor phase, due to the liquefaction of CO2.
The catalyst and Lewis acid were dispersed and
the reactants dissolved under stirring, allowing
the reaction to proceed effectively. Above 7 MPa,
however, the apparent reaction rate decreased
with increasing pressure (Table 2, entries 3 to 6),
which may have resulted from dilution as the
volume of the liquid phase increased. Separation
of CO2, product, and catalyst proved quite facile,
because the product could be extracted in situ by
CO2, whereas the solid catalyst and Lewis acid
remained in the reactor (18). At 30°C and a total
H2/CO2 pressure of 8.0 MPa, the Pd/C-AlCl3
system could be recycled three times with no loss
in conversion or selectivity (Table 2, entries 5 and
13 to 15).
Selectivity (%)
Conversion
(%)
Entry Catalysts Lewis acids PH (MPa) T (°C) Time (hours)
2
C=O
OH
1
2
3
4
5
6
7
8
Pd/C
–
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/C
Pd/Al2O3
Pd/Al2O3
Pd/Al2O3
Pd/Al2O3
Pd/NaY
Pd/NaY
Pd/NaY
Pd/NaY
–
1
1
1
1
1
1
2
3
0.5
1
2
3
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
30
30
30
40
50
80
80
80
100
100
100
100
30
12
12
12
11
7
3
2.5
2
12.6
0
93.8
0
6.2
0
<0.1
<0.1
<0.1
0.7
3.2
4.9
0.8
1.0
2.7
3.8
<0.1
0.7
<0.1
1.3
1.7
1.3
1.4
<0.1
1.1
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
InCl3
InCl3
ZnCl2
ZnCl2
ZnCl2
ZnCl2
ZnCl2
SnCl2
SnCl2
–
>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
15.5
>99.9
>99.9
>99.9
99.3
96.8
95.1
99.2
99.0
97.3
96.2
>99.9
99.3
>99.9
98.7
98.3
98.7
98.6
>99.9
98.9
89.5
>99.9
99.3
99.0
82.3
>99.9
>99.9
99.3
9
1
10
11
12
13
14
15
16
16*
16†
16‡
17
18
19
20
21
22
23
24
25
26
0.5
0.3
0.2
16
4
20
6
6
6
6
22
8
12
8
100
30
100
100
100
100
30
100
30
30
50
80
30
30
10.5
<0.1
0.7
AlCl3
AlCl3
AlCl3
–
AlCl3
AlCl3
AlCl3
>99.9
>99.9
>99.9
10.6
>99.9
>99.9
>99.9
3
2
12
9
4
1.0
17.7
<0.1
<0.1
0.7
50
80
3
*Second run to test the reusability of Pd/C-ZnCl2 under the conditions of entry 16.
†Third run to test the reusability of Pd/C-ZnCl2
under the conditions of entry 16. ‡ Fourth run to test the reusability of Pd/C-ZnCl2 under the conditions of entry 16.
Prior research on the hydrogenation of phenol
has indicated that this reaction proceeds mainly
in a sequential manner (Fig. 1A) (15, 23, 24). The
benzene ring of phenol is first partially hydro-
genated to an enol in step 1, which is unstable
and isomerizes rapidly to form cyclohexanone
(23); cyclohexanone can then be further hydro-
genated to form cyclohexanol. Cyclohexanone
and cyclohexanol were the only reaction products
observed in the present study, and under all
reaction conditions, selectivity approached 100%
at sufficiently low conversion (fig. S9). We con-
clude from the results in Tables 1 and 2 that under
the likely operation of this two-step mechanism,
the supported Pd catalyst–Lewis acid systems are
very active for the first step but are not active for
the second step under the experimental condi-
tions. This is further discussed below.
OH
OH
O
A
OH
Step 2
H2
Step 1
H2
fast
Lewis acid
H
HO
B
Lewis acid
O
OH
OH
H2
Lewis acid
Pd, H2
H
H
Pd
Fig 1. (A) General reaction pathway for hydrogenation of phenol. (B) Possible mechanism of dual
activation in phenol hydrogenation and stabilization of cyclohexanone by Lewis acid.
www.sciencemag.org SCIENCE VOL 326 27 NOVEMBER 2009
1251

REPORTS
It is known that Pd nanoparticles themselves anone. This synergy also enhances the selectivity,
are not active for the hydrogenation of benzene at because the reaction in the first step becomes activity and selectivity of the dual catalyst sys-
low temperature (25), and they have very low relatively faster than that in the second step. tems can be explained reasonably, as shown in
activity for phenol hydrogenation (Table 1, en- Inhibition of cyclohexanone hydrogenation Fig. 1B. The Pd, Lewis acid, and reactants contact
On the basis of the above discussion, the high
tries 1, 19, 23). It has also been reported that (Fig. 1A, step 2) is another crucial factor under- each other when stirred. Lewis acid coordination
Lewis acids can activate aromatic rings (26–28). lying the high selectivity that we observed. We makes the benzene ring of phenol more active,
Electrophilic aromatic substitution reactions conducted the hydrogenation of cyclohexanone while Pd activates H2, and cyclohexanone is
have been extensively studied using experimental under some typical conditions using Pd/C and formed quickly. At the same time, acid-base inter-
and theoretical techniques. Koltunov et al. (26) Pd/C-AlCl3 as the catalysts (Table 3). The con- action between the Lewis acid and cyclohexanone
studied the reaction of 5-amino-1-naphthol with version of cyclohexanone catalyzed by Pd/C- inhibits further hydrogenation to cyclohexanol.
benzene in the presence of AlCl3. Tarakeshwar AlCl3 is much lower than that catalyzed by Pd/C.
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and C6H6-AlCl3. Their results clearly indicated interaction between cyclohexanone and AlCl3 in
that one of the carbons in benzene tends to be- dichloromethane by Fourier transform infrared
come highly nucleophilic, thereby facilitating at- spectroscopy (fig. S7 and SOM text). The ab-
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electrophile. Furthermore, it is well known that shifts from 1714 cm−1 in the absence of AlCl3 to
Pd can activate H2 (29). Based on the experimen- 1624 cm−1 in the presence of AlCl3. This shift is
tal data presented here and the literature results, we consistent with coordination of the Lewis basic
can conclude that in the hydrogenation of phenol, C=O group to the Lewis acid (30). We believe
these two types of activation work cooperatively, that this Lewis acid–base interaction inhibits
resulting in high activity for producing cyclohex- further hydrogenation of cyclohexanone.
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Time
Conversion
(%)
2
2
Entry
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(MPa)
(hours)
Cyclohexanone
Cyclohexanol
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1
2
3
4
5
6
7
8
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
AlCl3
InCl2
InCl2
InCl2
ZnCl2
ZnCl2
ZnCl2
AlCl3
AlCl3
AlCl3
5.0
6.5
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13.0
1.7
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5.5
8.0
9.5
10.0
7.0
7.3
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4.0
4.0
4.0
49.7
63.9
98.9
98.2
1.1
1.8
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>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
>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
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*Second run to test the reusability of Pd/C-AlCl3 under the conditions of entry 5.
†Third run to test the reusability of Pd/C-AlCl3
under the conditions of entry 5. ‡Fourth run to test the reusability of Pd/C-AlCl3 under the conditions of entry 5.
29. M. Benkhaled et al., Appl. Catal. Gen. 346, 36 (2008).
30. D. Cook, Can. J. Chem. 41, 522 (1963).
31. We thank the National Natural Science Foundation of
China (grants 20773144, 20733010, and 20633080),
the National Key Basic Research Project of China (grant
2006CB202504), and the Chinese Academy of Sciences
(grant KJCX2.YW.H16) for financial support.
Table 3. Hydrogenation of cyclohexanone under different conditions. Conversions and yields were
determined by GC. Reaction conditions were as follows: cyclohexanone, 1.0 mmol; pH , 1.0 MPa; Pd
2
(5 wt % in Pd/C), 5 mol % relative to cyclohexanone; AlCl3 (if used), 10 mol % relative to cyclohexanone;
solvent, 1 ml of dichloromethane.
Entry
Catalysts
T (°C)
Time (hours)
Conversion (%)
Yield of cyclohexanol (%)
1
2
3
4
5
6
7
8
Pd/C
Pd/C-AlCl3
Pd/C
Pd/C-AlCl3
Pd/C
Pd/C-AlCl3
Pd/C
Pd/C-AlCl3
30
30
50
50
100
100
100
100
12
12
7
0.7
<0.1
12.9
0.4
37.3
1.8
0.7
<0.1
12.9
0.4
37.3
1.8
Supporting Online Material
www.sciencemag.org/cgi/content/full/326/5957/1250/DC1
Materials and Methods
SOM Text
Figs. S1 to S10
Tables S1 and S2
References
7
0.5
0.5
4
53.7
2.8
53.7
2.8
27 July 2009; accepted 30 September 2009
10.1126/science.1179713
4
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27 NOVEMBER 2009 VOL 326 SCIENCE www.sciencemag.org