Effect of polymers and alkaline earth metals on the catalytic performance of Ni-B amorphous alloy in benzophenone hydrogenation

Article abstract of DOI:10.1016/j.catcom.2012.02.031

Soluble polymers are found to be effective in inhibiting the agglomeration of the active Ni particles in Ni-B amorphous alloy catalysts and then increase the catalysts activity in benzophenone hydrogenation. However, the acidic nature of polyethylene glycol 1000 will cause the dehydration of benzhydrol and decrease its yield. Here, we found that the addition of alkaline earth metals can tune the acid-base properties of the catalysts and overcome this shortcoming. Ba and polyethylene glycol 1000 doped Ni-B was found to exhibit excellent activity and selectivity to benzhydrol due to its larger surface areas, more surface active Ni species, and better acidic properties.

Full text of DOI:10.1016/j.catcom.2012.02.031

Catalysis Communications 23 (2012) 34–38
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Effect of polymers and alkaline earth metals on the catalytic performance of Ni–B
amorphous alloy in benzophenone hydrogenation
Guoyi Bai ⁎, Zhen Zhao, Libo Niu, Huixian Dong, Mande Qiu, Fei Li, Qingzhi Chen, Guofeng Chen
Key Laboratory of Medicinal Chemistry and Molecular Diagnosis of Ministry of Education, College of Chemistry and Environmental Science, Hebei University, Baoding,
Hebei, 071002, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 January 2012
Received in revised form 29 February 2012
Accepted 29 February 2012
Available online 8 March 2012
Soluble polymers are found to be effective in inhibiting the agglomeration of the active Ni particles in Ni–B
amorphous alloy catalysts and then increase the catalysts activity in benzophenone hydrogenation. However,
the acidic nature of polyethylene glycol 1000 will cause the dehydration of benzhydrol and decrease its yield.
Here, we found that the addition of alkaline earth metals can tune the acid–base properties of the catalysts
and overcome this shortcoming. Ba and polyethylene glycol 1000 doped Ni–B was found to exhibit excellent
activity and selectivity to benzhydrol due to its larger surface areas, more surface active Ni species, and better
acidic properties.
Keywords:
Polymer
Alkaline earth metal
Synergistic effect
Hydrogenation
Benzophenone
© 2012 Elsevier B.V. All rights reserved.
1
. Introduction
Since 1980s, amorphous alloys have attracted much attention be-
In this paper, a series of polymers were first used as modifiers in
Ni–B amorphous alloy and tested in benzophenone hydrogenation
to benzhydrol, an important pharmaceutical intermediate [13]. It
was found that a polyethylene glycol 1000 (PEG(1000)) modified
Ni–B catalyst exhibited higher activity than the undoped Ni–B, but
the selectivity to benzhydrol was lower due to the dehydration of
benzhydrol. We then found that alkaline earth metals can increase
the selectivity to benzhydrol effectively. The synergistic effect of poly-
mers and alkaline earth metals on the catalytic performance of Ni–B
amorphous alloy was studied and clarified based on necessary
characterizations.
cause of their interesting intrinsic properties, e.g., short-range order,
long-range disorder, and high dispersion, as well as their potential appli-
cations in catalysts and hydrogen storage materials [1]. In particular, Ni–
B amorphous alloys have exhibited excellent catalytic activity and selec-
tivity equivalent or superior to those of conventional catalysts in various
hydrogenations. Since agglomeration of the tiny particles in amorphous
alloys usually decreased their activities, many types of modifiers, includ-
ing metal, organic, and inorganic modifiers, have been used to overcome
this shortcoming [2–6]. For example, Li and coworkers have found Ce-
doped Ni–B amorphous alloy catalyst exhibiting higher activities than
the undoped one for liquid-phase hydrogenation of furfural [6].
2
. Experimental
Polymers have been widely used in the field of catalysis due to
their unique properties of macromolecular structure [7–9]. Though
polymers are widely applied as carriers in the catalysts, only a few re-
sults have been reported on polymers used as modifiers. Particularly,
the role of polymers as modifiers in amorphous alloy has not been
reported. On the other hand, the alkaline earth metals have also
been applied as effective modifiers to tune the acid–base properties
of the catalysts due to their basic nature [10–12]. Yet, the synergistic
effect of polymers and alkaline earth metals has never been reported.
2
.1. Preparation of catalysts
The polymer-modified Ni–B amorphous alloy was prepared by
adding 50 mL of a 1.0 M aqueous KBH
wise to 10 mL of a 0.5 M aqueous NiCl
4
containing 0.2 M NaOH drop-
·6H O containing 1.0 g poly-
2
2
mer with vigorous stirring while cooling in an ice bath to furnish a
black precipitate. The precipitate was washed by deoxygenated dis-
tilled water until the washings were pH=7. The catalyst was further
washed with absolute ethanol three times to replace residual water
and then stored under absolute ethanol. The obtained catalysts were
denoted as Ni–B-p (p is the type of the polymer). When
3 2 2 2 2
X(NO ) ·nH O was also added in the NiCl ·6H O and polymer solu-
tions during the preparation, the catalysts so obtained were denoted
⁎
Corresponding author. Tel.: +86 312 5079359; fax: +86 312 5937102.
E-mail address: baiguoyi@hotmail.com (G. Bai).
as Ni–X-B-p (X=Mg, Ca, Sr, Ba, molar ratio X:Ni=1:8, n=0, 4 or 6).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.02.031

G. Bai et al. / Catalysis Communications 23 (2012) 34–38
35
Table 1
Effect of polymers on Ni–B amorphous alloys in benzophenone hydrogenation.
3. Results and discussion
Catalyst
Conversion of Selectivity to Selectivity to
benzophenone benzhydrol diphenylmethane benzhydrol
Yield of
3.1. Catalyst selection
b
(
%)
(%)
(%)
(%)
Table 1 shows the results of benzophenone hydrogenation over
the Ni–B amorphous alloys. The undoped Ni–B exhibited a relatively
low conversion (57.0%) and moderate selectivity to benzhydrol
Ni–B
Ni–B-PVP
Ni–B-PAA
Ni–B-PVA
Ni–B-PAM
57.0
79.8
63.7
24.6
8.7
81.3
73.6
72.3
92.9
92.0
84.8
72.3
14.1
21.4
22.6
3.3
4.8
9.1
46.3
58.7
46.1
22.9
8.0
a
a
(
81.3%), together with small amount of diphenylmethane as the
a
a
major byproduct. Fortunately, most polymers improved the conver-
sion of benzophenone; only polyvinyl alcohol (PVA) and polyacryl-
amide (PAM) modified amorphous alloys revealed terribly low
activity. Considering that both PVA and PAM partly dissolved in pre-
cursor solutions, the in-situ prepared Ni–B particles should mostly
adsorb on the surface of undissolved polymers and then lower the ac-
tivity of these modified catalysts. In contrast, soluble polymers im-
proved the dispersion of the active Ni species in the modified Ni–B
amorphous alloys and then increased their activities. Interestingly,
the conversion of benzophenone first increased and then decreased
with the increase of polyethylene glycol (PEG) molecular weight for
the PEG-modified Ni–B amorphous alloys. Ni–B-PEG(1000) showed
the best conversion, but lower selectivity to benzhydrol (72.3%) com-
pared with the undoped Ni–B. We ascribed this result to the acidic
nature of PEG(1000), which will induce dehydration of the obtained
benzhydrol to diphenylmethane (Scheme 1). Thus, some alkaline
earth metals were added to tune the acid–base properties of Ni–B-
PEG(1000) and the results are listed in Table 2.
Ni–B-PEG(400) 74.8
63.4
69.5
Ni–B-PEG
1000)
Ni–B-PEG
2000)
Ni–B-PEG
10000)
96.1
73.6
60.2
21.3
(
89.0
89.5
7.7
7.0
65.5
53.9
(
(
Reaction conditions: 6.0 g benzophenone, 0.5 g catalyst, 200 mL methanol, temperature at
03 K, initial P(H )=2.5 MPa, and reaction time 2 h.
PVP: polyvinylpyrrolidone; PAA: polyacrylic acid; PVA: polyvinyl alcohol; PAM:
polyacrylamide.
4
2
a
b
Yield of benzhydrol=(conversion of benzophenone)×(selectivity to benzhydrol)/100.
2
.2. Catalysts characterization
Bulk compositions were identified by inductively coupled plasma
analysis (ICP) on a VISTA-MPX spectrometer. BET surface area was
measured using a Micromeritics Tristar II 3020 surface area and
pore analyzer. X-ray diffraction (XRD) patterns were acquired on a
Bruker D8-ADVANCE X-ray diffractometer. Scanning electron micros-
copy (SEM) was carried out on a JEOL JSM-7500 electron microscope.
The Fourier transform infrared spectra (FT-IR) were recorded with a Bru-
Just as we have expected, the selectivity to benzhydrol all in-
creased and reached at least 93.8% by the addition of alkaline earth
metals. The conversion of benzophenone also increased with the in-
crease of the basic properties of the alkaline earth metals and Ni–
Ba-B-PEG(1000) showed the best conversion (96.8%), even a little
higher than Ni–B-PEG(1000). Furthermore, the yield of benzhydrol
over Ni–Ba-B-PEG(1000) (90.8%) was obviously higher than that of
Ni–Ba-B (56.0%) (Table 2). Thus, Ni–Ba-B-PEG(1000) is proven to
have the highest yield for the hydrogenation of benzophenone to
benzhydrol due to the synergistic effect of PEG(1000) and Ba.
ker VECTOR 22 Fourier transform spectrophotometer. H
temperature programmed desorption of H (H -TPD) and temperature
programmed desorption of NH (NH -TPD) were performed on a TP-
000 instrument from Xianquan Ltd. X-ray photoelectron spectroscopy
XPS) measurements were recorded with a PHI 1600 spectroscope.
2
-chemisorption,
2
2
3
3
5
(
3.2. Catalyst characterizations
2
.3. Catalyst activity test
2
The results of compositions, BET surface areas and H -chemisorption
Benzophenone hydrogenation was conducted as follows: benzo-
of Ni–B, Ni–B-PEG(1000) and Ni–Ba-B-PEG(1000) are summarized in
Table 3. From the ICP analysis, it is found that Ni–B-PEG(1000) contains
similar B with Ni–B and the B contents increase remarkably with the ad-
dition of Ba. With the addition of PEG(1000), the BET surface area and
pore volume of Ni–B only increased slightly, whereas both of them in-
creased noticeably with further addition of Ba. Thus, Ni–Ba-B-
PEG(1000) has the largest surface area among the catalysts studied,
probably owing to the synergistic effect of PEG(1000) and Ba. With re-
phenone (6.0 g), catalyst (0.5 g), methanol (200 mL) were mixed in
a 500 mL stainless steel autoclave equipped with a mechanical stirrer
and electrical heating system. The air in the autoclave was first
replaced with H
Then the autoclave was pressurized with H
2
three times in succession at room temperature.
to 2.5 MPa, and heated
2
to 403 K at a rate of 4 K/min. On reaching 403 K, hydrogenation was
started by stirring the reaction mixture vigorously and proceeded
for 2 h. Reaction mixtures were analyzed by a gas chromatography
using a 30 m SE-54 capillary column and the product structures were con-
firmed by a gas chromatograph-mass spectroscopy (GC-MS) on Agilent
2
spect to H -chemisorption, the two PEG-modified catalysts show simi-
lar values and are both higher than the undoped Ni–B, which means
the PEG-modified catalysts have larger number of active Ni species, in
accordance with their activities in benzophenone hydrogenation.
5
975 C.
Scheme 1. Possible mechanism of benzophenone hydrogenation.

36
G. Bai et al. / Catalysis Communications 23 (2012) 34–38
Table 2
Effect of alkaline earth metals on Ni–B-PEG(1000) and Ni–B amorphous alloys in ben-
zophenone hydrogenation.
Catalyst
Conversion of
benzophenone benzhydrol
Selectivity to Selectivity to
Yield of
a
diphenylmethane benzhydrol
(
%)
(%)
(%)
(%)
Ni–B
Ni–Ba-B
Ni–B-PEG
57.0
62.8
96.1
81.3
89.2
72.3
14.1
4.4
21.3
46.3
56.0
69.5
(
1000)
Ni–Mg-B-PEG 82.3
1000)
Ni–Ca-B-PEG
1000)
Ni–Sr-B-PEG
1000)
Ni–Ba-B-PEG
1000)
94.9
94.4
94.5
93.8
3.1
3.9
3.6
3.0
78.1
86.6
87.7
90.8
(
91.7
92.8
96.8
(
(
(
Reaction conditions: 6.0 g benzophenone, 0.5 g catalyst, 200 mL methanol,
temperature at 403 K, initial P(H )=2.5 MPa, and reaction time 2 h.
Yield of benzhydrol=(conversion of benzophenone)×(selectivity to benzhydrol)/100.
2
a
Table 3
Physicochemical properties of the Ni–B amorphous alloys.
Catalyst
Composition
(
Surface area Pore volume
2
H -chemisorption
cm /g
atomic ratio)a m /g
2
cm /g
3
3
Ni–B
Ni–B-PEG
Ni
Ni
1
1
B
B
0.43
0.42
20.7
20.9
0.059
0.071
0.28
0.43
(
1000)
Ni–Ba-B-PEG Ni
1000)
1
Ba0.012
B
0.58
37.5
0.089
0.42
(
a
Based on ICP results.
Fig. 1 shows the XRD patterns of Ni–B, Ni–B-PEG(1000) and Ni–
Ba-B-PEG(1000). The three catalysts were all present in the amor-
phous structure, as only one broad diffraction peak around 2θ=45°
appeared [14,15], indicating that the addition of PEG(1000) and Ba
had not changed the amorphous structure of these Ni–B catalysts.
Fig. 2 shows SEM images of the amorphous alloy catalysts. Each
sample displayed approximately spherical morphology, consistent
with other reported Ni-based amorphous alloy catalysts [16]. It is ob-
vious that the particle sizes of the PEG-modified samples became
smaller than the undoped Ni–B. Particularly, the particle sizes of Ni–
Ba-B-PEG(1000) decreased to 30 to 70 nm and were homogeneously
o
Fig. 2. SEM images of the amorphous alloy catalysts. (a) Ni–B, (b) Ni–B-PEG(1000),
c) Ni–Ba-B-PEG(1000).
45
(
c
dispersed on its surface, explaining the highest surface area and pore
volume of this catalyst. On the other hand, no evidence for the exis-
tence of PEG(1000) on the surface of the two PEG-modified samples
can be found from the SEM results. So we proposed that most of
PEG(1000) may be removed during washing in the preparation pro-
cess and it indeed prevented the agglomeration of the amorphous
alloy particles mainly in the reduction process of preparation to attain
more active Ni species. The results that no characteristic peaks of
PEG(1000) was detectable in the FT-IR spectroscopy and XRD curves
of the PEG-modified catalysts proved this assumption.
b
a
The electronic states of surface Ni in the three catalysts are deter-
mined by XPS (Fig. 3). The characteristic peaks of Ni 2p centered at
2
0
30
40
50
60
70
80
2
theta / deg.
8
53.4 eV and 856.9 eV in all samples can be assigned to metallic and
oxidized Ni, respectively. Moreover, the peak areas of metallic Ni 2p
in Ni–B-PEG(1000) and Ni–Ba-B-PEG(1000) become much larger
Fig. 1. XRD patterns of the amorphous alloy catalysts. (a) Ni–B, (b) Ni–B-PEG(1000),
c) Ni–Ba-B-PEG(1000).
(

G. Bai et al. / Catalysis Communications 23 (2012) 34–38
37
a
853.4
b
c
856.9
a
c
b
a
8
48
850
852
854
856
858
860
862
864
400
450
500
550
600
650
700
750
800
Binding energy (eV)
Temperature/K
Fig. 3. XPS spectra of Ni 2p in the amorphous alloy catalysts. (a) Ni–B, (b) Ni–B-
PEG(1000), (c) Ni–Ba-B-PEG(1000).
b
than that in Ni–B, indicating that more active Ni species were formed
on the catalysts surface due to PEG(1000) modification. These conclu-
sions were consistent with the H
2
-chemisorption results.
Fig. 4 shows the H -TPD profiles of the three samples. Ni–B ex-
2
f
e
hibits two obvious peaks at about 540 K and 650 K, indicating the
presence of two adsorbing sites on its surface [5,15]. In contrast,
both Ni–B-PEG(1000) and Ni–Ba-B-PEG(1000) have a sharp peak
d
c
(
6
at around 550 K), together with a very smooth peak (at around
00–800 K), thus it can be concluded that the active Ni species of
the catalysts have a trend to become uniform in the presence of
PEG(1000) [2,17].
3
NH -TPD curves of the Ni–B amorphous alloy catalysts are presented
in Fig. 5. For Ni–B, the peaks centered at 580 K are assigned to the weakly
acidic sites and the peaks centered approximately at 660 K are assigned
to the moderately-strong acidic sites; whereas, Ni–B-PEG(1000) and Ni–
3
50 400 450 500 550 600 650 700 750 800 850
Temperature/K
3
Ba-B-PEG(1000) only showed one strong NH desorption peak centered
3
Fig. 5. NH -TPD spectra of the amorphous alloy catalysts (a: undoped and PEG(1000)
at around 650 K and 670 K, respectively (Fig. 5a). It is apparent that Ni–
B-PEG(1000) has more acidic amounts than Ni–B, calculated from their
peak areas, which can be ascribed to the acidity of PEG(1000), explaining
the lower selectivity to benzhydrol over Ni–B-PEG(1000). Furthermore,
Ni–Ba-B-PEG(1000) showed lower acidic amounts than Ni–B-
PEG(1000) due to the tuning of Ba, and in turn, exhibited the best
doped ones; b: alkaline earth metal and PEG(1000) doped ones). (a) Ni–B, (b) Ni–B-
PEG(1000), (c) Ni–Ba-B-PEG(1000), (d) Ni-Sr-B-PEG(1000), (e) Ni-Ca-B-PEG(1000),
(f) Ni–Mg-B-PEG(1000).
selectivity to benzhydrol among them. On the other hand, Ni–Ba-B-
PEG(1000) showed the lowest desorption peak and the smallest acidic
amounts among the four alkaline earth metals modified Ni–B-
PEG(1000) catalysts, which was in good agreement with the basic se-
quence of the alkaline earth metals (Fig. 5b). Thus, it can be concluded
that the basic properties of the alkaline earth metals modified Ni–B-
PEG(1000) amorphous alloy catalysts increased with the increase of
the basic properties of the alkaline earth metals.
c
4
. Conclusion
Soluble polymers are proven to inhibit the agglomeration of the
b
active Ni particles in Ni–B amorphous alloys during the preparation
process, thus improving the dispersion of the active Ni species and
then increasing the catalysts activity. On the other hand, the acidic
nature of PEG(1000) will cause the dehydration of the obtained benz-
hydrol to diphenylmethane and lower its selectivity. Fortunately, the
addition of alkaline earth metals can effectively adjust the acid–base
properties of Ni–B-PEG(1000) and increase the selectivity to benzhy-
drol. It was found that Ni–Ba-B-PEG(1000) showed excellent activity
and selectivity for the hydrogenation of benzophenone to benzhydrol
due to its larger surface areas, more homogeneously dispersed sur-
face active Ni species, and better acidic properties, demonstrating
the synergistic effect of PEG(1000) and Ba.
a
3
00
400
500
600
700
800
900 1000 1100
Temperature/K
Fig. 4. H
(
2
-TPD spectra of the amorphous alloy catalysts. (a) Ni–B, (b) Ni–B-PEG(1000),
c) Ni–Ba-B-PEG(1000).

38
G. Bai et al. / Catalysis Communications 23 (2012) 34–38
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