Chemoselective hydrogenation of aromatic aldehydes over SiO2 modified Co/γ-Al2O3

Article abstract of DOI:10.1016/j.apcata.2014.02.011

With benzaldehyde as a model compound, the hydrogenation of aromatic aldehydes to corresponding benzyl alcohols was investigated in a fixed-bed reactor. Co/γ-Al₂O₃ doped with a amount of SiO ₂ displayed excellent catalytic performance for this reaction, Co/γ-Al₂O₃ is modified by SiO₂ in two ways, the strong interaction between metal oxide and support is obviously reduced, and also a large number of acid sites are diminished on the catalyst. The first aspect apparently facilitated the reduction of metal oxide, and improved the hydrogenation activity of the catalyst; while the second aspect greatly inhibited the hydrogenolysis of the CO bond, and thus increased the selectivity toward benzyl alcohol.

Full text of DOI:10.1016/j.apcata.2014.02.011

Applied Catalysis A: General 476 (2014) 34–38
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Chemoselective hydrogenation of aromatic aldehydes over SiO2
modified Co/␥-Al O
2
3
a
b,c,∗
Xiangjin Kong , Ligong Chen
a
Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell Technology, School of Chemistry and Chemical Engineering, Liaocheng
University, Liaocheng 252059, China
School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China
b
c
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
With benzaldehyde as a model compound, the hydrogenation of aromatic aldehydes to corresponding
benzyl alcohols was investigated in a fixed-bed reactor. Co/␥-Al2O3 doped with a amount of SiO2 displayed
excellent catalytic performance for this reaction, Co/␥-Al2O3 is modified by SiO2 in two ways, the strong
interaction between metal oxide and support is obviously reduced, and also a large number of acid
sites are diminished on the catalyst. The first aspect apparently facilitated the reduction of metal oxide,
and improved the hydrogenation activity of the catalyst; while the second aspect greatly inhibited the
Received 3 December 2013
Received in revised form 21 January 2014
Accepted 8 February 2014
Available online 18 February 2014
Keywords:
hydrogenolysis of the
C O bond, and thus increased the selectivity toward benzyl alcohol.
Chemoselective hydrogenation
Aromatic aldehyde
Benzyl alcohol
©
2014 Elsevier B.V. All rights reserved.
Hydrogenolysis
Co/␥-Al2O3–SiO2
1
. Introduction
Benzyl alcohols are a key class of intermediates in organic chem-
developed and exhibited excellent performance for the hydrogena-
tion of carbonyl compounds [15,19–21]. These results encouraged
us to study the chemoselective hydrogenation of aromatic alde-
hydes over transition metals catalysts.
In this work, benzaldehyde is used as a model molecule to eval-
uate catalyst for reduction of aromatic aldehydes to corresponding
benzyl alcohols. Several Co based catalysts were examined, and
istry [1–3]. For example, benzyl alcohol is a commercially important
chemical, employed as solvent for inks, paints, lacquers and as pre-
cursor for the manufacture of a range of esters in the cosmetics
and flavoring industries [4]. However, the traditional production
methods for these alcohols are always present a serious impact
on the environment [5,6]. As an alternative route, chemoselec-
tive hydrogenation of aromatic aldehydes to obtain benzyl alcohol
has drawn people’s more attention. Several noble metals (Pd, Pt,
Rh and Ru) have been reported for this reaction [7–11]. Unfortu-
nately, the hydrogenation selectivity of C O is still a challenge,
since catalytic hydrogenolysis of benzyl alcohols to correspond-
ing methyl arenes easily occur [11,12]. Nowadays, transition metals
in which SiO2 modified Co/␥-Al O3 exhibited excellent perfor-
2
mance. The effects of SiO2 doped in Co/␥-Al O3 were studied by
2
X-ray diffraction (XRD), H -temperature programmed reduction
2
(H -TPR), NH -temperature programmed desorption (NH -TPD)
2
3
3
and nitrogen adsorption/desorption measurement. Additionally,
the selected catalyst is applied in the reduction of a collection of
representative aromatic aldehydes.
(
Cu, Co and Ni) are frequently employed for hydrogenation reaction
instead of the expensive noble catalyst [13–15]. Whereas, there are
only a few reports concerned about the chemoselective hydrogena-
tion of aromatic aldehydes over transition metals catalysts, and
which often suffered from poor benzyl alcohols yield [16–18]. In
our previous studies, a series of Ni and Cu based catalysts have been
2
. Experimental
2.1. Materials and catalysts
Aromatic aldehydes were purchased from Tianjin Guangfu Fine-
chemical institute. Pseudo-boehmite was supplied by Jiangyan
Chemical Auxiliary Factory (Jiangyan, China). All the commercially
available solvents and reagents were used without further purifi-
cation.
∗ Corresponding author. Tel.: +86 22 27406314; fax: +86 22 27406314.
E-mail address: lgchen@tju.edu.cn (L. Chen).
http://dx.doi.org/10.1016/j.apcata.2014.02.011
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

X. Kong, L. Chen / Applied Catalysis A: General 476 (2014) 34–38
35
Table 1
The reaction results of benzaldehyde over Co based catalysts.
Catalyst
Conversion (%)
Selectivity (%)
Methylcyclohexane
Toluene
Benzyl alcohol
Other
Co20/␥-Al2O3
85.5
90.6
98.0
95.8
90.5
0.0
0.0
0.0
0.0
0.9
57.6
25.6
4.6
8.5
10.6
41.9
73.8
95.0
91.1
88.0
0.5
0.6
0.4
0.4
0.5
Co/␥-Al2O3–48%SiO2
Co/␥-Al2O3–64%SiO2
Co/␥-Al2O3–72%SiO2
Co/SiO2
◦
In the present studies, the cobalt weight percent on the cat-
130 C. A solution of aromatic aldehydes (concentration in 1,4-
dioxane = 20.0 wt%) was dosed into the reactor at a flow rate of
0.2 mL/min by a pump. The components of the reaction mixture was
confirmed by GC–MS (Polaris Q, Thermo Finngan, America, HP-1
capillary column, 30 m × 0.25 mm, 0.2 ␮m film thickness) equipped
with an ion trap MS detector. The composition of the reaction
mixture was monitored by GC with an OV-1701 capillary column
(30 m × 0.25 mm, 0.2 ␮m film thickness).
alyst is 20%. Co/␥-Al O3 was prepared by kneading a mixture of
pseudo-boehmite together with the corresponding precipitates
2
of metal carbonates or hydroxides [15]. Co/␥-Al O –mSiO (m
2
3
2
means the weight percent of SiO2 in the catalyst) was prepared
by kneading a mixture of SiO and pseudo-boehmite together with
2
the precipitates of cobalt carbonates or hydroxides. For example,
Co20/␥-Al O –64%SiO (namely, the weight percent of SiO2 in the
2
3
2
support is 80%) was prepared as follows: 20.2 g metal carbonates
were kneaded with a mixture of 32.0 g SiO2 and 12.1 g pseudo-
boehmite, followed by molding to bars (with a diameter of 3 mm)
3. Results and discussion
◦
by an extruder. After drying in air at 110 C for 6 h, the catalysts were
◦
3.1. Catalyst modification
calcined at 500 C for 4 h. The Co based catalysts were reduced at
00 C for 4 h in a stream of hydrogen under 1.0 MPa before use.
◦
4
Co based catalysts were frequently employed in the hydro-
2.2. Catalysts characterization
genation reaction, and ␥-Al2O3 was a widely used support for
metallic catalyst [20–23]. Therefore, Co/␥-Al O3 was prepared and
2
Powder X-ray diffraction (XRD) was performed on a Rigaka
employed for the reduction of benzaldehyde firstly; the obtained
results are listed in Table 1. Besides the desired product benzyl alco-
hol, a considerable amount of toluene was also detected. Barbelli
et al. [24] and Wu et al. [25] had reported that strong acidic sites on
D/max 2500 X-ray diffractometer, using Cu K␣ radiation and
◦
◦
scanning 2ꢀ from 15 to 85 , operated at 40 kV and 100 mA.
H -temperature programmed reduction (H -TPR) was measured
2
2
using a micromeritics 2910 apparatus with a temperature ramp
catalysts are helpful for the hydrogenolysis of C O bond. Consid-
◦
◦
◦
ering of this, we inferred that the toluene was possibly generated
from the hydrogenolysis of benzyl alcohol. According to the above
results, the reaction pathway was deduced and shown in Scheme 1.
As well known, SiO2 has lower acidity than ␥-Al2O3. Thus, it was
added to Co/␥-Al2O3 to decrease its acidity, and the results over
these modified catalysts are shown in Table 1.
from 130 C to 850 C at 10 C/min, and a gas flow of 5% H in
2
argon (20 mL/min). A thermal conductivity detector (TCD) was
used to determine the amount of hydrogen consumed during
temperature ramping. NH -temperature programmed desorption
3
(
NH -TPD) was carried out on a TP-5000 instrument with a thermal
3
conductivity detector (TCD). The BET (Brunauer–Emmett–Teller)
specific surface areas, pore-size distributions, total pore vol-
umes and sorption isotherms of the materials were measured
via nitrogen adsorption/desorption using a NOVA 2000e analyzer
As described in Table 1, with the addition of 64 wt% SiO2 to the
catalyst, the selectivity of benzyl alcohol increased from 41.9% to
95.0%, while the conversion of benzaldehyde increased from 85.5%
to 98.0%. Table 1 also displayed that when the percent of SiO2 in
the catalyst is more than 64 wt%, both the selectivity of benzyl alco-
hol and conversion of benzaldehyde were slightly decreased. These
results demonstrated that the catalytic properties of the catalyst in
hydrogenation and hydrogenolysis were evidently influenced by
the introduction of SiO , and Co /␥-Al O doped with 64 wt% SiO
(
Quantachrome, US).
2
.3. Catalytic test
Activity measurement was performed in a fixed bed reactor,
which was loaded with about 18.0 g catalysts (cylinder cata-
lyst particle with diameter of 3 mm and height about 2–3 mm),
under an atmosphere of 3 MPa H2 and a reaction temperature of
2
20
2
3
2
displayed an excellent catalytic performance for the present reac-
tion. To better understand the effects of doped SiO , Co/␥-Al O ,
2
2
3
Scheme 1. The reaction pathway for chemoselective hydrogenation of benzaldehyde.

3
6
X. Kong, L. Chen / Applied Catalysis A: General 476 (2014) 34–38
o
#
CoO
Co
3
19.9 C
+
+
#
o
+
+
+
326.4 C
(
c)
(c)
#
+
+
#
(b)
o
(b)
3
64.1 C
#
#
(a)
(a)
2
0
40
60
80
200
400
600
800
2
Theta (deg)
o
Temperature( C)
Fig. 1. XRD patterns for reduced (a) Co/␥-Al2O3, (b) Co/␥-Al2O3–SiO2 and (c)
Co/SiO2.
Fig. 2. H2-TPR curves for oxides precursor of (a) Co/␥-Al2O3, (b) Co/␥-Al2O3–SiO2
and (c) Co/SiO2.
Co/␥-Al O –64%SiO (denoted as Co/␥-Al O –SiO ) and Co/SiO
2
2
3
2
2
3
2
are investigated by some physical methods.
3
.2. Catalysts characterization
.2.1. XRD
3
The XRD patterns of reduced Co/␥-Al O , Co/␥-Al O –SiO and
Co/SiO2 are depicted in Fig. 1. No peaks of SiO2 or any other Si
compound were present, possibly due to the amorphous state of
2
3
2
3
2
(a)
the added SiO . However, both peak of metallic Co and CoO were
2
observed on the XRD patterns of these catalysts. With the sequence
of Co/␥-Al O , Co/␥-Al O –SiO and Co/SiO , the peak density of
2
3
2
3
2
2
(
b)
c)
Co becomes more and more strongly and the peak density of CoO
greatly decreased. These results indicated that the reduction degree
of these Co based catalyst are obviously improved through the addi-
(
tion of SiO . As reported, the activity of cobalt catalysts depends
2
solely on the number of active sites located on the surface of crys-
talline metal formed by reduction, and the number of active sites
is mainly determined by the reduction degree of the Co precur-
sors [26,27]. In other words, the higher reduction degree would
result in a higher metallic cobalt surface area and excellent cat-
alytic activity. Based on the above analysis and the reaction results
in Table 1, it is easily to understand that why Co/␥-Al O –SiO
2
00
300
400
500
600
700
800
Temperature(ºC)
Fig. 3. NH3-TPD curves for oxides precursors (a) Co/␥-Al2O3, (b) Co/␥-Al2O3–SiO2
and (c) Co/SiO2.
2
3
2
demonstrated a better catalytic performance than Co/␥-Al O . But,
2
3
◦ ◦
altered from 364.1 C for the original Co/␥-Al O catalyst to 326.4 C
2 3
it is hard to explain why Co/SiO2 has lower catalytic ability than
Co/␥-Al O –SiO .
◦
2
3
2
and 319.9 C for Co/␥-Al O –SiO and Co/␥-Al O –SiO catalyst.
2
3
2
2
3
2
As reported by some previous studies [31–33], the interaction of
the supported-cobalt precursors with alumina support is stronger
than SiO2 support. So, based on the obtained results from H2-TPR,
we inferred that the addition of SiO2 to the catalyst decreased
the strength of interaction between cobalt oxides and the support,
and thus enhanced the reduction degree of the Co based catalysts.
Clearly, this conclusion is in accordance with the results from XRD
characterizations.
3
.2.2. H -TPR
2
In order to study the reducibility of the active metallic
phase, temperature-programmed reduction (TPR) analysis was
performed. In TPR spectra of various catalyst’s oxides precursors,
as shown in Fig. 2, two major peaks can be observed, which are
located at 200–500 C and 500–800 C, respectively. It appears to
be difficult to perfectly assign the broad TPR peaks to the reduction
of the well-defined cobalt species. However, the following gener-
ally accepted assignment can be applied for the above-mentioned
peaks. The TPR peak between 200 and 500 C is related to the
two-step reduction of Co to Co and synchronous reduction
from Co to Co, respectively. While the peak between 500 and
8
◦
◦
3.2.3. NH -TPD
3
◦
In the present investigation, NH -TPD is used to compare the
3
3+
2+
acidity distinction of Co/␥-Al O , Co/␥-Al O –SiO and Co/SiO2.
2
3
2
3
2
2
+
Fig. 3 shows that Co/␥-Al O3 catalyst exhibited a typical double-
2
◦
00 C is related to the reduction of these cobalt oxide species
peak. The two desorption peaks at the lower temperature
3+
2+
◦
◦
(
Co and Co ) strongly interacting with the support [28–30]. With
(200–300 C) and the higher temperature (350–600 C) correspond
to the weak and strong acid sites on the catalyst surface, respec-
tively. As for Co/␥-Al O –SiO and Co/SiO , the higher temperature
addition of SiO2 to the catalyst, both of these peaks moved grad-
ually toward low temperature side. And the centered value for
the peak temperature for the broad peak between 200 and 500 C
2
3
2
2
◦
peak could not be observed obviously.

X. Kong, L. Chen / Applied Catalysis A: General 476 (2014) 34–38
37
Table 2
displayed a lower selectivity of benzyl alcohol; this result seems
contradictive with the above conclusion. Thus, further studies of
these catalysts are required to clear some misunderstandings.
Temperature programmed desorption of NH3 for various catalysts.
Sample
Weak acid
mmol NH3/g)
Strong acid
(mmol NH3/g)
Total acid
(mmol NH3/g)
(
Co/␥-Al2O3
Co/␥-
Al2O3–SiO2
Co/SiO2
0.28
0.21
0.09
0.00
0.37
0.21
3.2.4. N2 sorption
The nitrogen sorption isotherms of Co/␥-Al O , Co/␥-
2
3
Al O –SiO and Co/SiO at 77 K are depicted in Fig. 4. All isotherms
2
3
2
2
0.09
0.00
0.09
displayed an evident hysteresis loop, being indicative sign of the
mesoporosity of these catalysts. The uptake of N2 at high relative
pressure only slightly decreased, with the addition of 64 wt% SiO2
The acidic capacity of the samples is listed in Table 2, it can be
to the catalyst. But for Co/SiO , the uptake of N2 was decreased to
2
found that the total acidity decreased from 0.37 mmol NH /g in
only about 80% of that of Co/␥-Al O , demonstrated that the pore
3
2
3
curve (a) to 0.21 mmol NH /g in curve (b), and further reduced to
structure of the catalyst has been greatly changed when using SiO2
as the catalyst support. The texture characteristics calculated from
N2 adsorption/desorption isotherms of these Co based catalysts
are summarized in Table 3.
3
0
.09 mmol NH /g in curve (c). Combination with the above results,
3
it is clearly that SiO has a lower acidity than ␥-Al O . And, the addi-
2
2
3
tion of SiO2 to Co/␥-Al O3 dramatically decreased the total acidity,
2
especially the strong acidity of this catalyst. As shown in Table 1,
The BET specific surface area and total pores volume of Co/␥-
Al O –SiO are only slightly lower than that of Co/␥-Al O . But,
with the addition of 64 wt% SiO2 to Co20/␥-Al O , the selectivity of
2
3
2
3
2
2
3
benzyl alcohol increased from 41.9% to 92.3% while the selectivity
of toluene decreased from 57.6% to 7.1%. Thus, it can be concluded
that the lower acidity is responsible for inhibiting the hydrogenol-
ysis of benzyl alcohol, namely, the addition of SiO2 can increase
the selectivity of benzyl alcohol, and these are in consistent with
the results reported by Saadi et al. [34]. From Table 2, it is inter-
esting that Co/SiO2 with lower total acidity than Co/SiO -␥–Al O
when it comes to Co/SiO , the BET specific surface area and total
2
pores volume both obviously decreased due to the formed micro
pores on this catalyst. The activities and selectivity of the catalyst
in reaction are dependent on the surface area and structure of the
applied supports, respectively [15]. For the current results, the pore
size of the Co/␥-Al O –SiO decreased very slightly, indicating that
2
3
2
the addition of SiO did not change the pore structure of the alumina
2
2
3
2
Table 3
N2 sorption characteristics of Co based catalysts.
2
3
3
Sample
SBET (m /g)
Vtotal (cm /g)
Vmicrop (cm /g)
Daverage (nm)
Co/␥-Al2O3
Co/␥-Al2O3–SiO2
Co/SiO2
234.6
227.8
189.2
0.529
0.512
0.431
0.000
0.012
0.084
9.1
8.7
9.1
Table 4
The reaction results of aromatic aldehydes over Co/␥-Al2O3–SiO2.
Entry
Substrate
Product
Conversion (%)
Selectivity (%)
95.0
1
98.0
98.3
2
3
4
95.2
95.1
95.7
97.9
98.2
5
99.6
95.5

3
8
X. Kong, L. Chen / Applied Catalysis A: General 476 (2014) 34–38
of SiO2 apparently reduced the strong interaction between metal
adsorption
desorption
oxide and support, and also diminished a large number of weak
and strong acid sites on the catalyst. The first aspect apparently
facilitated the reduction of metal oxide, and improved the hydro-
genation activity of the catalyst; while the second aspect greatly
inhibited the hydrogenolysis of the C O bond, and thus increased
the selectivity of benzyl alcohol.
1
00
00
(c)
1
(b)
Acknowledgement
This work is supported by program for scientific research inno-
vation team in colleges and universities of Shandong Province.
1
00
(a)
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2
2
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alyst for the reduction of benzaldehyde to benzyl alcohol in a
continuous process. After an intensive study on the prepared cata-
lysts via several physical measurements, it was found that addition
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9–85.
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7