2
J. Chen et al. / Applied Catalysis A: General 470 (2014) 1–7
method [6]. Two aqueous solutions were ready at first. AlCl3·9H2O
(50 mmol) and MgCl2·6H2O (150 mmol) were dissolved in 100 mL
of deionized water (solution a). Na2CO3 (60 mmol) and NaOH
(210 mmol) were dissolved in 60 mL of deionized water (solution
b). Solution a was slowly added into solution b and the emulsion
was heated at 65 ◦C for 18 h with vigorous stirring. Thereafter, the
white slurry was cooled to room temperature. The solids were fil-
tered and washed with about 1.6 L deionized water, finally dried at
120 ◦C overnight.
Pd/C was purchased from Aladdin Reagent Corporation. The rest
supported-palladium catalysts were synthesized by the impreg-
nation method. The support was immersed into a fresh aqueous
solution of PdCl24−, and the mixture was stirred for 4 h. Thereafter
the mixture was allowed to stand for 6 h and then dried at 60 ◦C
under stirring. The powdery sample was dried at 120 ◦C over night
and calcined at 300 ◦C for 0.5 h. For a hydrotalcite supported cata-
lyst, the calcination was not carried out in order to keep the special
layer-to-layer structure. XRD patterns confirmed the hydrotalcite
structure on Pd/HT.
In a typical reaction, 2 wt% Pd/HT catalyst (0.05 g, 2 mol% Pd
relative to nitrobenzene) was added to a pressure tube (25 mL)
equipped with a stir bar. The tube was sealed with a rubber sep-
tum, and vacuumed for several times. Substrates nitrobenzene
(0.5 mmol, 50 L), benzyl alcohol (1.5 mmol, 150 L), and solvent
toluene (2 mL) were added into the tube though the rubber sep-
tum using syringes, and then the septum was replaced by a Teflon
screwcap under a nitrogen flow. The tube was sealed and the mix-
ture was allowed to stir in a preheated oil bath at 130 ◦C for 24 h.
After the reaction was completed, a GC internal standard dodecane
(10 L) was added to the mixture. The mixture was diluted with
ethyl acetate and filtered through a pad of silica gel. The pad of the
silica gel was washed again with small amount of ethyl acetate and
the final volume of the clear solution was marked up to 25 mL. The
liquid products were then analyzed using a gas chromatography
equipment (Agilent 7820A).
For the cycling performance test when the reaction mixture
was cooled down, the solid catalyst was separated by centrifuge,
washed with ethanol several times and dried at 120 ◦C. Then the
recycle catalyst was ready for the next reaction. After the reac-
tion the solid catalyst was filtered, and the filtrate was analyzed
for palladium by ICP. And the palladium loading in the 2 wt% Pd/HT
catalyst was also determined by ICP.
2.2. Catalyst characterization
The specific surface area of varied supports were determined
by micropore analyzer ASAP 2020. The palladium loading in the
catalysts before and after reaction were determined by inductively
coupled plasma OES spectrometer (ICP) using a Jobin Yvon Ultima
2. Transmission electron microscope (TEM) measurements were
performed on a JEM 2010 electron microscope operated at an accel-
eration voltage of 200 kV. Samples for TEM measurements were
suspended in ethanol and dispersed ultrasonically. Drops of the
suspensions were applied on a copper grid coated with carbon.
For the analysis of the nitrobenzene and benzyl alcohol deriva-
tives, an aliquot (2 mL) was removed from the reaction mixture in
the pressure tube, and it was concentrated under reduced pressure.
NMR internal standard paraldehyde was added to it. The product
was then determined by 1H NMR spectroscopy.
4. Results and discussion
4.1. Optimization of catalytic systems and recycling of catalysts
3. Catalytic test
The optimization study was first performed on a series of palla-
dium catalysts loaded on various supports (Table 1). The reaction of
nitrobenzene (1a) with 3 equiv. of benzyl alcohol (2a) was chosen
as the model system, in which the 1:3 ratio of 1a to 2a was calcu-
lated on the basis of the minimum theoretical amount of hydrogen
needed for the nitrobenzene reduction. Among the palladium
All reactions were performed under nitrogen atmosphere using
Schlenk techniques. The reagents used in the experiments were
purchased from Sigma-Aldrich Co., Acros Organics, Avocado and
Alfa Aesar and used as received.
Table 1
Direct synthesis of imine from nitrobenzene and benzyl alcohol on various catalystsa.
Entry
Catalyst b
Conv. of 2a (%)c
Conv. of 1a (%)c
Yield (%)c
3a
4a
5a
1
2
3
4
5
2 wt% Pd/␥-Al2O3
2 wt% Pd/TiO2
10 wt% Pd/C
2 wt% Pd/HAP
2 wt% Pd/MgO
2 wt% Pd/HT
2 wt% Pd/HT
1 wt% Pd/HT
1 wt% Pd/HT
1 wt% Pd/CoAl-HT
HT
83
91
51
44
43
>99
>99
47
97
23
23
81
–
20
20
25
22
15
>99
97
32
94
19
6
8
3
18
5
2
15
2
4
9
7
7
5
13
5
0
0
2
0
0
0
0
1
1
2
0
2
0
2
6
93
88
23
79
12
0
7d
8e
9
10
11
12f
13g
0
6
7
1 wt% Pd/HT
2 wt% Pd/HT
75
35
63
19
a
Reaction conditions: nitrobenzene (0.5 mmol), benzyl alcohol (1.5 mmol), toluene (2 mL), and catalyst (metal: 2 mol%), 130 ◦C, 24 h.
b
c
d
e
f
Pd/C was purchased from Aladdin Reagent Corporation. The other catalysts were prepared by wet impregnation methods (see the supporting information).
Yield was based on nitrobenzene conversion, determined by GC using n-dodecane as the internal standard.
12 h.
The catalyst was calcined at 300 ◦C for 0.5 h.
The palladium precursor was Pd(CH3CN)2Cl2.
g
Nitrobenzene (0.3 mmol), benzaldehyde (0.9 mmol), toluene (2 mL), and catalyst (Pd 2 mol%) with the substitution of about 1 mmol H2 for N2 at an atmospheric pressure.