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Y. Pérez et al. / Catalysis Communications 12 (2011) 1071–1074
O
plastic glass. Then, propylene oxide was added and the resulting
solution stirred for 10 min, and allowed to form the gel by stopping
stirring. The aged gel (24 h) was washed in a bath of ethanol for 24 h.
The gel was lyophilizated for 8 h to get alumina aerogel, which was
used without calcining.
Incorporation of palladium on different supports was performed
by the impregnation method with PdCl2(PhCN)2 as the source of Pd,
following the experimental procedure detailed in Ref. [26].
OH
OH
II
Cyclohexanone
+ H2
+ 2H2
I
IV
+ H2
2.2. Catalytic experiments
OH
Phenol
III
Cyclohexenol
A reinforced glass batch reactor (8 mL) was used to carry out the
liquid phase hydrogenation. Phenol (100 mg, 1.06 mmol) dissolved in
5 mL of water was placed into the reactor followed by catalyst
addition (see the amount of catalyst in Table 1). The reactor was
purged two times with hydrogen and then pressurized with 5 bars of
hydrogen. The reaction mixture was heated up to 100 °C and stirred at
500 rpm. The products were periodically extracted (100 μL samples),
analyzed and identified by GC and GC–MS.
Cyclohexanol
Fig. 1. Reaction showing steps in the hydrogenation of phenol.
small Pd metal nanoparticles. In the first part of the work, and in order
to elucidate the influence of the support on hydrogenation of phenol
in aqueous phase, supports of different nature: hydroxyapatite (HA),
carbon (C), alumina (γ-Al2O3, acidic properties) and alumina with
high surface area (Al2O3-CWE), have been used. Then, catalytic
properties have been related not only with the nature of the support
but with the size of the Pd nanocrystals obtained on the different
carriers.
3. Results and discussion
Pd (1.5 wt.%) was supported on hydroxyapatite (HA) by impreg-
nation with an acetone solution of PdCl2(PhCN)2 [26]. The same
method of impregnation was applied to prepare Pd/C (Pd content:
1.5 wt.%), Pd/γ-Al2O3 (Pd content: and 0.2 wt.%) and Pd/Al2O3-CWE
(Pd content: 1.5 wt.%). The catalyst Pd/Al2O3-CWE presents higher
surface area (406 m2 g−1) than Pd/γ-Al2O3 and Pd/HA (237 m2 g−1
and 98 m2 g−1, respectively). The average pore diameters are 43 , 41
and 57 for Pd/Al2O3-CWE, Pd/γ-Al2O3 and Pd/HA, respectively. The
crystal size of the supported Pd was determined by TEM, and
representative pictures with the corresponding size distribution are
given in Figs. 2 and 3 for Pd/γ-Al2O3 and Pd/Al2O3-CWE and in
supplementary data Figs. A3 and A4 for Pd/HA and Pd/C. As can be
seen there, both catalysts Pd/γ-Al2O3 and Pd/Al2O3-CWE show
narrow crystallite size distribution with average palladium crystal
sizes of 4.6 and 5 nm, respectively.
The catalysts were tested for the hydrogenation of phenol in
aqueous media under mild conditions, as described in Experimental.
Moreover, the catalysts prepared here were compared with the
commercially available 5% Pd/C-com, 10% Pd/C-com and 10% Pd/
Al2O3-com, for the hydrogenation of phenol at 5 bars of hydrogen and
100 °C, and the results obtained are showed in Fig. 4 and Table 1.
Results from Fig. 4 clearly show that the catalysts synthesized here
give better activity and selectivity to cyclohexanone than the
commercially prepared Pd on carbon and alumina. Moreover, Pd
supported over alumina and hydroxyapatite, was more active and
more selective to cyclohexanone than the catalysts supported on
2. Experimental
2.1. Materials
Phenol, cyclohexanone, cyclohexanol, propylene oxide, AlCl3, and
PdCl2(PhCN)2 were purchased from Sigma-Aldrich and used as
received. Acetone, etanol (Scharlau), and water (Milli-Q, Millipore)
were used as solvents, whereas hydrogen 5.0 (Abelló Linde S.A.,
99.999%) was used as reducing agent. The carbon support was
purchased from Aldrich with 1600 m2 g−1 surface area while γ-Al2O3
(238 m2 g−1) was obtained with bohemite from Condea at 500 °C for
10 h. Hydroxyapatite with 98 m2 g−1 surface area was prepared
following the procedure reported in Ref. [26]. For comparative
purposes the commercial catalysts: 5% Pd/C-com (palladium on
activated charcoal from Fluka), 10% Pd/C-com (palladium on activated
charcoal from Fluka) and 10% Pd/Al2O3-com (palladium on activated
alumina from Aldrich) were also used.
The synthesis of high surface area Al2O3-CWE using propylene oxide
in ethahol/water (50/50) has been reported previously [27]. In a typical
synthesis, AlCl3 was dissolved in 20 mL of ethahol/water (50/50) in a
Table 1
Comparative catalytic performance of different palladium catalyst.
Conversiona(%)
Selectivity (%)
Catalyst
Pdb (wt.%)
mcatalyst (g)
10 min
30 min
50 min
90 min
120 min
C=O cyclohexanona
C–OH cyclohexanol
Pd/Al2O3-CWE
Pd/γ-Al2O3
Pd/γ-Al2O3
Pd/Al2O3-com
Pd/HA
Pd/C
Pd/C-com
Pd/C-com
1.5
0.2
1.5
10
1.5
1
0.188
1.41
70
26
35
8
21
17
5
96
30
89
31
87
46
13
21
100
37
100
60
100
73
28
100
40
100
58
98
100
97
78
97
98
74
86
3
–
0.188
0.028
0.188
0.282
0.056
0.028
100
100
100
100
80
100
100
100
100
100
100
3
2
2
26
14
5
10
9
35
61
Reaction conditions: phenol (100 mg and 1.06 mmol) dissolved in water (5 mL) in the presence the solid catalyst, stirred in a glass reactor (8.0 mL), pressurized with H2 (5 bars) and
heated to 100 °C.
a
The conversion was determined by GC analysis and the products identified by GC–MS with the external standard method.
wt.% of palladium measured by inductively coupled plasma (ICP) chemical analysis.
b