148
Y. Ding et al. / Catalysis Communications 28 (2012) 147–151
Table 1
reach 98% within 1 h, but obtaining the medium selectivity of 62% to
benzyl alcohol, due to side-reaction to form 1-(ethoxymethyl)benzene
and 2-ethoxy-1,2-diphenylethanone detected by GC–MS (Table 1,
entry 1). To avoid the side reaction, the reaction was performed in cy-
clohexane. Notably, a 99% conversion of benzaldehyde with 83% selec-
tivity within 1 h was achieved (Table 1, entry 2). Nevertheless, there
still has room to increase the selectivity. When the hydrogenation of
benzaldehyde was carried out in acetic acid, mediocre conversion and
selectivity were achieved (Table 1, entry 3).
Considering that water is an environmentally benign solvent, and
the solubility of benzaldehyde in water is low but that of benzyl alco-
hol is relatively high, we anticipate that Pt/CMK-3 catalyst can work
well in water for the hydrogenation of benzaldehyde. However, due
to high hydrophobicity of CMK-3 OMCs, the Pt/CMK-3 catalyst cannot
be soaked at all in neat water, so that the catalyst fine powders were
floated on water surface and cannot contact the substrates dispersed
in water. Therefore, the mixed solvents containing water and ethanol
with different ratios were used in the benzaldehyde hydrogenation.
To our delight, the selectivity was increased to 91% in a mixed sol-
vent containing water and ethanol with an equal volume, although
the conversion of benzaldehyde was not complete, obtaining 88%
conversion within 1 h (Table 1, entry 4). If the volume ratio of water
was further increased in the mixed solvent, nearly full conversion
with >99% selectivity was obtained within 1 h (Table 1, entry 5), which
is consistent with our previous observations for benzaldehyde hydroge-
nation on Pt/FDU-14 catalyst [17]. Hence, we chose the mixed solvent
containing 18 mL water and 2 mL ethanol as an optimal solvent in the fol-
lowing researches.
Optimization of reaction conditions for benzaldehyde hydrogenation with Pt/CMK-3
catalyst
a
.
Entry
Solvent
Catalyst
(mg)
Time
(h)
Conv.
(%)
Sele.
(%)
b
b
1
2
3
4
5
6
7
8
Ethanol
Cyclohexane
Acetic acid
10 mL water+10 mL ethanol
18 mL water+2 mL ethanol
18 mL water+2 mL ethanol
18 mL water+2 mL ethanol
18 mL water+2 mL ethanol
100
100
100
100
100
100
50
1
1
1
1
98
99
73
88
99
99
58
99
62
83
72
91
>99
>99
>99
>99
1
0.5
0.5
1
50
a
Reaction conditions: Pt/CMK-3 catalyst; 21 mmol benzaldehyde; 4.0 MPa H2;
1200 rpm; r.t.
b
Determined by GC.
100
80
60
40
20
Pt/CMK-3
Pt/CMK-8
Besides the solvent, the catalyst dosage and reaction time were
also optimized. When the reaction time was reduced to 0.5 h, to our
surprise, the benzaldehyde conversion still reached 99% (Table 1,
entry 6). To further explore the catalytic property, the catalyst
amount was decreased to half amount. Within 0.5 h, the conversion
of benzaldehyde was 58% with 50 mg catalyst (Table 1, entry 7).
When prolonged to 1 h, a 99% conversion was furnished with 50 mg cat-
alyst (Table 1, entry 8). Therefore, 50 mg Pt catalyst, 1 h, 18 mL water
and 2 mL ethanol were determined as the standard parameters in the
following studies.
0
0
10
20
30
40
50
Reaction time (min)
Fig. 1. Kinetic profiles of benzaldehyde hydrogenation (a) with Pt/CMK-3 catalyst and
(b) with Pt/CMK-8 catalyst. Reaction conditions are identical to Table 3 except for re-
action time.
3.2. Hydrogenation of benzaldehyde on Pt/CMK-3 and Pt/CMK-8
catalysts
a designated temperature after 4.0 MPa hydrogen was introduced into
the autoclave. The reaction was stopped after an allotted period and
the products were analyzed by GC–FID (GC-2014, Shimadzu Co.)
equipped with a capillary column (DM-WAX, 30 m×0.32 mm×
0.25 μm).
The Pt/CMK-8 catalyst was more active for the chiral hydrogena-
tion of ethyl pyruvate and ethyl 2-oxo-4-phenylbutyrate due to the
large pore volume compared with Pt/CMK-3 catalyst [12]. Conse-
quently, we firstly compared the kinetic profiles of benzaldehyde hy-
drogenation with Pt/CMK-3 and Pt/CMK-8 catalysts, respectively.
As displayed in Fig. 1, the conversion of benzaldehyde with Pt/CMK-3
catalyst was about 7–13% higher than that obtained with Pt/CMK-8 cat-
alyst within the same time. This can be interpreted mainly in terms of the
chemical and physical properties of CMK-3 and CMK-8 OMCs.
3. Results and discussion
3.1. Optimization of reaction conditions for benzaldehyde hydrogenation
In order to optimize the reaction conditions, benzaldehyde was
chosen as a model substrate and Pt/CMK-3 catalyst was chosen as a
model catalyst.
The solvents were firstly screened for the hydrogenation of benzal-
dehyde (Table 1). In ethanol, the conversion of benzaldehyde could
As summarized in Table 2, the Pt/CMK-3 and Pt/CMK-8 catalysts
had similar structural properties, including the specific surface area,
pore diameters and Pt particle sizes, except that Pt/CMK-8 catalyst
had relatively larger pore volume. The similar Raman spectra of CMK-3
and CMK-8 OMCs also indicate that both CMK-8 and CMK-3 OMCs had
Table 2
Relevant parameters of Pt/CMK-3 and Pt/CMK-8 catalysts.
a
a
Sample
SBET (m2 g−1
)
DP (nm)a
VP (cm3 g−1
)
Pt size (nm)a
Pt disp. (%)a
Surface comp. (%)b
C
O
Pt
Pt/CMK-3
Pt/CMK-8
1290
1318
3.5
3.7
1.18
1.43
2.6
2.3
43.7
48.8
94.1
91.7
5.56
7.91
0.34
0.35
a
Quoted from Refs. [11,12].
Calculated by XPS analysis.
b
Ding, Yue
