A NiPdB-PEG(800) amorphous alloy catalyst for the chemoselective hydrogenation of electron-deficient aromatic substrates

Article abstract of DOI:10.1002/cctc.201300865

A new Pd and polyethylene glycol 800 [PEG(800)]-modified NiB [NiPdB-PEG(800)] amorphous alloy catalyst was prepared, which demonstrated excellent activities, similar to those of noble metal catalysts, in the chemoselective hydrogenation of a series of electron-deficient aromatic substrates in water. The addition of small amounts of Pd to NiB markedly improved its activity. The Pd not only benefits the dispersion of active species but also contributes to the activity of the catalyst. The accompanying agglomeration can be inhibited with the further addition of PEG(800), which results in the largest surface area, the smallest particle size, and the greatest number of active species, resulting in optimum H₂- chemisorption and accounting for its highest activity. The key factors determining the main reaction products depend not only on the structures of the substrates but also on the character of the solvents. Water is found to be the most effective solvent for most of the substrates. Disperse and be active! A Pd and polyethylene glycol 800 [PEG(800)]-modified NiB [NiPdB-PEG(800)] amorphous alloy catalyst demonstrates excellent activities, similar to those of noble metal catalysts, in the chemoselective hydrogenation of a series of electron-deficient aromatic substrates in water. The addition of Pd benefits the dispersion of active species and contributes to the activity of the catalyst. Copyright

Full text of DOI:10.1002/cctc.201300865

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DOI: 10.1002/cctc.201300865
A NiPdB-PEG(800) Amorphous Alloy Catalyst for the
Chemoselective Hydrogenation of Electron-Deficient
Aromatic Substrates
[
a]
Guoyi Bai,* Zhen Zhao, Huixian Dong, Libo Niu, Yalong Wang, and Qingzhi Chen
A new Pd and polyethylene glycol 800 [PEG(800)]-modified NiB
NiPdB-PEG(800)] amorphous alloy catalyst was prepared,
addition of PEG(800), which results in the largest surface area,
the smallest particle size, and the greatest number of active
[
which demonstrated excellent activities, similar to those of
noble metal catalysts, in the chemoselective hydrogenation of
a series of electron-deficient aromatic substrates in water. The
addition of small amounts of Pd to NiB markedly improved its
activity. The Pd not only benefits the dispersion of active spe-
cies but also contributes to the activity of the catalyst. The ac-
companying agglomeration can be inhibited with the further
species, resulting in optimum H -chemisorption and account-
2
ing for its highest activity. The key factors determining the
main reaction products depend not only on the structures of
the substrates but also on the character of the solvents. Water
is found to be the most effective solvent for most of the
substrates.
Introduction
Cyclohexane and its derivatives are important intermediates in
industry; in particular, cyclohexanol and cyclohexanone are key
Amorphous alloys have attracted much attention owing to
their interesting intrinsic properties such as short-range order,
long-range disorder, and high dispersion, and so they have
[
1]
intermediates for the manufacture of nylon 66. The prepara-
tion of cyclohexane derivatives through the catalytic hydroge-
nation of aromatic precursors has attracted much attention in
[8]
been widely used in catalytic hydrogenation. Generally, amor-
phous alloys, especially NiB amorphous alloys, are prepared
through the reduction of transition-metal salt solutions with
potassium borohydride or sodium hypophosphite, which dem-
onstrate excellent catalytic performance in the hydrogenation
[
2]
recent years, both in the laboratory and in industry, owing to
the green nature of this process, the ready availability of suit-
able feedstocks, and the directness of the approach. Noble
metals such as Pt, Pd, and Rh have been studied extensively
[
9]
of alkenes and carbonyl compounds. The addition of metals,
[
3]
[10]
[11]
for these transformations. For example, Hubert et al. have ob-
tained good results under mild experimental conditions in the
catalytic hydrogenation of benzene derivatives over Rh-based
complexing species, and polymers as well as the applica-
[12]
tion of ultrasound during the catalyst preparation process
can play a key role in improving the activity of amorphous
alloy catalysts. For example, Chen and Sasirekha have found
that appropriate proportions of Fe in a NiB amorphous alloy
catalyst could suppress the growth of crystalline NiFeB and the
Ni possessed more d-band electrons and higher electron densi-
ty than the undoped NiB amorphous alloy, which leads to
higher activity in the hydrogenation of p-chloronitroben-
[
4]
nanocatalysts in water.
Taniya et al. have investigated
Sn-modified SiO -coated Pt catalysts for the chemoselective hy-
2
drogenation of crotonaldehyde and found that the role of the
Sn promoter was to activate the aldehyde group of croton-
[
5]
aldehyde. Snelders et al. have achieved chemoselectivities ex-
ceeding 90% in the hydrogenation of phenylacetone to cyclo-
hexylacetone over Rh-based catalysts poisoned by phosphine
[13]
zene. However, there are few reports on the hydrogenation
of aromatic substrates over amorphous alloy catalysts and
such reports are mainly limited to electron-rich aromatic sub-
[
6]
ligands. Unfortunately, the application of such catalysts in in-
dustry has been limited owing to the high price of noble
metals. Meanwhile, reports of related studies using non-noble
metal catalysts are scarce and such catalysts can achieve good
[14]
strates, such as phenol, owing to the relatively lower catalyt-
ic activity of NiB amorphous alloys compared with noble metal
catalysts. To our knowledge, no detailed studies on the hydro-
genation of electron-deficient aromatic substrates over amor-
phous alloy catalysts have been reported.
[
7]
results only under forcing reaction conditions.
[
a] Prof. G. Bai, Z. Zhao, H. Dong, L. Niu, Y. Wang, Q. Chen
Key Laboratory of Chemical Biology of Hebei Province
College of Chemistry and Environmental Science
Hebei University
[15]
In the continuation of our previous work,
we report
herein our recent research into the hydrogenation of aromatic
carboxylic acids and their derivatives over amorphous alloy
catalysts and the selective hydrogenation of ethyl benzoate to
ethyl cyclohexanecarboxylate was chosen as a model system.
Considering the low activity of the pure NiB amorphous alloy
catalyst, different modifications were surveyed to improve its
Baoding, Hebei 071002 (P.R.China)
Fax: (+86)312-5937102
E-mail: baiguoyi@hotmail.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300865.
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activity. A Pd and polyethylene glycol 800 [PEG(800)]-modified
NiB [NiPdB-PEG(800)] amorphous alloy catalyst demonstrated
excellent catalytic performance, similar to that of noble metal
catalysts, in this reaction. The effects of the amount of Pd
added and the solvent were studied. Finally, to expand the
scope of this protocol, we also investigated the hydrogenation
of other aromatic carboxylic acids and their derivatives over
NiPdB-PEG(800) under optimal conditions and efficient and se-
lective conversions were observed. The result of these studies
is the development of a new catalytic system, which demon-
strates efficiency and chemoselectivity similar to those of
noble metal-based catalysts but requires significantly reduced
levels of Pd for use in water.
A conversion of ethyl benzoate of 70.9% and a selectivity
toward ethyl cyclohexanecarboxylate of 95.1% are good re-
sults for a NiB amorphous alloy doped with such a small
amount (0.2 mmol) of Pd, although it was still lower than that
in standard noble metal catalysts. Considering the benefit of
[15]
polymer modification in our previous studies,
a series of
polymers were then used as modifiers for the NiPdB amor-
phous alloy catalyst to improve its activity, and the results are
listed in Table 2. As expected, the addition of PEG improved
Table 2. Hydrogenation of ethyl benzoate over the polymer-modified
[
a]
amorphous alloy catalysts.
[
b]
Catalyst
NiPdB
Conversion [%]
Selectivity [%]
Yield [%]
70.9
40.4
48.0
7.5
70.7
80.7
96.1
91.9
86.0
84.9
37.4
1.0
95.1
98.3
98.9
99.6
97.2
95.1
95.3
95.9
94.9
95.8
95.9
100.0
98.6
90.2
67.4
39.7
47.5
7.5
68.7
76.7
91.6
88.1
81.6
81.3
35.9
1.0
NiPdB-PVP
NiPdB-PVA
NiPdB-PAA
Results and Discussion
Hydrogenation of ethyl benzoate
NiPdB-PEG(400)
NiPdB-PEG(600)
NiPdB-PEG(800)
NiPdB-PEG(1000)
NiPdB-PEG(2000)
NiPdB-PEG(10000)
NiB-PEG(800)
c-NiPdB-PEG(800)
Pd/C
Raney Ni
Owing to the reluctance of electron-deficient aromatic sub-
strates to undergo hydrogenation, the undoped NiB amor-
phous alloy demonstrated low activity in the hydrogenation of
ethyl benzoate to ethyl cyclohexanecarboxylate. Considering
the benefit of metal modification on amorphous alloys,
a series of metal-doped NiB amorphous alloys were prepared
and tested in this reaction, and the results are listed in Table 1.
[c]
[
d]
78.5
29.4
77.4
26.5
[
a] Reaction conditions: Ethyl benzoate (5.0 g, 33 mmol), ethanol (70 mL),
catalyst amount (0.2 g), P(H )=5 MPa, 400 rpm, 423 K, 5 h; [b] Molar ratio
2
of Ni to Pd is 20:1; [c] Catalyst was crystallized at 973 K; [d] 0.5 g, 5 wt%
Pd.
Table 1. Hydrogenation of ethyl benzoate over the metal-doped amor-
phous alloy catalysts.
[
a]
Catalyst
NiB
Conversion [%]
Selectivity [%]
Yield [%]
15.2
40.5
31.6
49.7
51.3
70.9
36.2
38.7
98.2
97.7
97.1
90.3
93.9
95.1
88.5
93.5
14.9
39.6
30.7
44.9
48.2
67.4
31.9
36.2
[
b]
the activity of the NiPdB amorphous alloy catalyst, whereas
polyvinyl alcohol (PVA)-, polyvinylpyrrolidone (PVP)-, and espe-
cially polyacrylic acid (PAA)-modified amorphous alloys re-
vealed minimal activity. We ascribed this result largely to the
solubility of the polymers. Considering that PVA, PVP, and PAA
partly dissolved in the precursor mixtures to give slightly vis-
cous solutions, we propose that the in situ prepared NiPdB
particles would mainly adsorb onto the surface of the undis-
solved polymers, leading to a decrease in the activity of these
modified amorphous alloy catalysts. In contrast, PEGs, especial-
ly low molecular mass PEGs, are soluble polymers and can pre-
vent the agglomeration of the particles during the reduction
process to attain more active Ni species. PEGs are also consid-
ered as reducing agents and stabilizers in the preparation of
NiCoB
[
b]
NiFeB
[
b]
NiLaB
[
b]
NiZrB
NiPdB
[
b]
[
c]
PdB
NiB+PdB
[
d]
[a] Reaction conditions: Ethyl benzoate (5.0 g, 33 mmol), ethanol (70 mL),
catalyst amount (0.2 g), P(H )=5 MPa, 400 rpm, 423 K, 5 h; [b] Molar ratio
of Ni to other metals is 20:1; [c] The amount of PdB is 0.023 g; [d] The
amount of NiB and PdB are 0.177 and 0.023 g, respectively.
2
As expected, all metal-doped NiB amorphous alloys demon-
strated better catalytic performance than the undoped amor-
[
16]
[17]
phous alloy mainly owing to their dispersive effect. The addi-
tion of small amounts of Pd to the NiB amorphous alloy (molar
ratio of Ni to Pd is 20:1) markedly increased the conversion of
ethyl benzoate from 15.2 to 70.9%. Furthermore, NiPdB dem-
onstrated much better catalytic performance than the physical
mixture of NiB and PdB, with the same proportion of Ni and
Pd as in NiPdB. The physical mixture of NiB and PdB demon-
strated activity similar to that of PdB of the same weight
nanoparticles, as Harraz et al. have reported previously. Fur-
thermore, the conversion of ethyl benzoate first increased and
then decreased with the increase in molecular mass of PEG
used for the PEG-modified NiPdB amorphous alloys. Of all the
PEG-modified NiPdB amorphous alloys, NiPdB-PEG(800) dem-
onstrated the best conversion (96.1%), together with a high
selectivity (95.3%) toward ethyl cyclohexanecarboxylate, which
is ascribed to the appropriate molecular mass and good solu-
bility of PEG(800). The activity of NiPdB-PEG(800) is higher than
that of Pd/C and commercial Raney Ni under similar reaction
conditions (Table 2), but lower than that of the Ru-based cata-
(
Table 1). Thus, we suggest that the small amount of Pd not
only leads to fine dispersion of Ni species but also contributes
to the high activity of NiPdB.
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[
18]
lyst. Given that the conversion of ethyl benzoate over NiB-
PEG(800) was only 37.4%, Pd is believed to have a greater
effect on the high activity of NiPdB-PEG(800) than PEG(800).
Further studies showed that the activities of NiPdB-PEG(800)
increased with the increase in the amount of Pd in NiPdB-
PEG(800). The conversion of ethyl benzoate increased from
52.1 to 98.3% with the decrease in the molar ratio of Ni to Pd
from 40:1 to 10:1. Thus, the addition of a small amount of Pd
and PEG(800) can remarkably improve the activity of the NiB
amorphous alloy owing to the synergistic effect between Pd
and PEG(800), and NiPdB-PEG(800) was finally chosen as the
catalyst of choice for the chemoselective hydrogenation of
ethyl benzoate to produce ethyl cyclohexanecarboxylate.
The reaction conditions were also preliminarily optimized
(see the Supporting Information). It was proven that there was
no mass or heat transfer limitation while the stirring rate was
[
19]
400 rpm or more.
The conversion of ethyl benzoate in-
creased with the increase in hydrogen pressure and reaction
temperature. Eventually, 5 MPa hydrogen pressure, 423 K, and
400 rpm is considered an appropriate reaction condition for
this hydrogenation process.
Catalyst characterization
The compositions, Brunauer–Emmett–Teller (BET) surface areas,
H -chemisorptions, average particle sizes, and turnover fre-
2
quencies (TOFs) of NiB, NiPdB, and NiPdB-PEG(800) were mea-
sured, and selected results are summarized in Table 3. From
the inductively coupled plasma (ICP) analysis, it was found that
NiPdB contained the same ratio of B in the bulk as NiB and the
contents of both Pd and B increased slightly with the further
addition of PEG(800). As the noble metal Pd has a greater ca-
pacity for adsorbing and releasing hydrogen than the transi-
[
20]
tion-metal Ni, more Pd helps to improve the catalyst’s activi-
ty in hydrogenation. Although the BET surface area and H2-
chemisorption of NiB increased to some extent with the addi-
tion of Pd, both increased markedly with the further addition
of PEG(800). The BET surface area increased from 14 to
Figure 1. XRD patterns of the amorphous alloy catalysts. A: a) NiB, b) NiPdB,
c) NiPdB-PEG(800). B: c) NiPdB-PEG(800), d) NiPdB-PEG(800) treated at 573 K,
e) NiPdB-PEG(800) treated at 973 K. C: SAED image of the NiPdB-PEG(800).
2
À1
2
0
7 m g
and H -chemisorption increased from 0.33 to
counts for its highest activity in the hydrogenation of ethyl
benzoate and is in agreement with the TOF values of these
amorphous alloy catalysts.
2
3
À1
.61 cm g owing to the dispersive effect and reductive char-
acter of PEG(800). In contrast, the average particle size de-
creased from 9.5 to 9.3 and 8.7 nm with the addition of Pd
and PEG(800), respectively. Of the catalysts studied, NiPdB-
PEG(800) has the largest surface area, the greatest number of
active species, and the smallest particle size, probably owing
to the synergistic effect between Pd and PEG(800), which ac-
The X-ray diffraction (XRD) patterns of NiB, NiPdB, and
NiPdB-PEG(800) are shown in Figure 1. A broad peak at 2q=
458 was observed for each freshly prepared catalyst, which is
[15]
indicative of a typical NiB amorphous structure and can also
be confirmed by a successive diffraction halo in the selected
area electron diffraction (SAED)
[22]
image (Figure 1C). This finding
indicates that the addition of Pd
and PEG(800) had not changed
the amorphous structure of
these NiB catalysts. In contrast,
increasing the treatment tem-
perature of NiPdB-PEG(800) to
Table 3. Selected physicochemical properties of the amorphous alloy catalysts.
[
b]
[c]
Catalyst
Composition
Surface area
H
[cm g
2
-chemisorption
Particle size
[nm]
TOF
[
a]
2
À1
3
À1
2
À1
(
atomic ratio)
[m g
]
]
[10 h ]
NiB
NiPdB
NiPdB-PEG(800)
Ni1.00
B
0.38
11
14
27
0.24
0.33
0.61
9.5
9.3
8.7
2.34
5.69
6.26
Ni1.00Pd0.04
B
B
0.38
0.41
Ni1.00Pd0.05
5
73 K in a N atmosphere led to
2
[
a] Based on ICP results; [b] Estimated from TEM results; [c] TOF values were calculated on the basis of the
the appearance of weak charac-
teristic peaks corresponding to
number of moles of ethyl benzoate converted per mole of active metal per hour, according to Ref. [21].
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[
23]
crystalline Ni and Ni B.
With the increase to 973 K, these
surface area and H -chemisorption of this catalyst. In contrast,
3
2
peaks became sharp, together with the disappearance of the
characteristic peak of the amorphous alloy (Figure 1B), which
demonstrates the transformation of nanoparticles from an
amorphous state to a crystalline state. Furthermore, the
shoulder peak at 2qꢀ408 in the Pd-doped catalysts can be as-
no evidence for the existence of PEG(800) on the surface of
NiPdB-PEG(800) could be found from the TEM investigation.
Thus, we propose that most of the PEG(800) polymer has been
removed during washing in the preparation process and its
main role was to prevent agglomeration of the amorphous
alloy particles in the reduction process, which resulted in more
active Pd and Ni species; this observation is also reported in
[
24]
cribed to the characteristic peak of PdB, which can disperse
and activate the amorphous Ni, giving the catalyst higher sur-
[15]
face area and H -chemisorption value. This peak disappeared
our previous work and is in accordance with the ICP and
2
after the high-temperature treatment, probably because the
peaks corresponding to crystallized Pd were obscured by the
H -chemisorption results.
2
The temperature-programmed desorption of hydrogen
[
25]
characteristic peaks of Ni B. A comparative experiment es-
(H -TPD) profiles of the three samples are shown in Figure 3.
2
3
tablished that NiPdB-PEG(800) that had crystallized at 973 K
demonstrated no activity in the hydrogenation of ethyl ben-
zoate (Table 2), which proved that the amorphous structure of
NiPdB-PEG(800) was crucial to its high activity.
NiB shows three peaks at approximately 447, 539, and 605 K,
which indicate the presence of three adsorbing sites. In con-
trast, both NiPdB and NiPdB-PEG(800) show one strong peak
at approximately 600 K, together with a small peak at approxi-
The surface morphology of the amorphous alloy catalysts
was recorded by using transmission electron microscopy (TEM;
Figure 2). It appears that each sample demonstrated approxi-
mately spherical morphology, consistent with other reported
2
Figure 3. H -TPD spectra of the amorphous alloy catalysts: a) NiB, b) NiPdB,
and c) NiPdB-PEG(800). TCD=thermal conductivity detector.
Figure 2. TEM morphologies of the amorphous alloy catalysts: a) NiB,
b) NiPdB, and c) NiPdB-PEG(800).
[
11]
Ni-based amorphous alloy catalysts. The particle distribution
decreased from 6.1–14.8 to 6.0–12.8 nm upon addition of Pd,
although some agglomeration still took place in NiPdB. The ag-
glomeration may result in some coverage of active centers and
consequently diminish the extent of H -chemisorption. Howev-
2
er, the effect of Pd addition appears to be advantageous on
the catalyst’s structure, which results in a larger surface area
and higher H -chemisorption as compared with those of NiB.
2
Moreover, this agglomeration phenomenon could be inhibited
with the further addition of PEG(800). The particles in NiPdB-
PEG(800) were well dispersed and the particle distribution de-
creased further to 7.1–10.7 nm, which resulted in the highest
Figure 4. XPS spectra of Ni2p and Pd3d of the amorphous alloy catalysts:
a) NiB, b) NiPdB, and c) NiPdB-PEG(800).
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mately 450 K, and thus it can be
concluded that active Ni species
of the catalysts tend to become
uniform in the presence of
[
a]
Table 4. Hydrogenation of ethyl benzoate and benzoic acid in different solvents over NiPdB-PEG(800).
Substrate
Solvent
Conversion [%]
Selectivity [%]
Byproduct
Main product
[
26]
Pd.
Furthermore, the hydro-
gen desorption peaks of NiPdB-
PEG(800) are much larger than
those of the other two catalysts,
especially the peaks at approxi-
mately 600 K, which indicate
that the addition of PEG(800)
could favor the dispersion of
nanoparticles in the catalyst, re-
sulting in the formation of more
active centers, which is in agree-
water
100.0
75.6
95.3
22.4
1.7
ethanol
dioxane
n-hexane
water
96.1
25.5
89.2
97.6
100.0
49.2
83.5
100.0
97.1
–
–
–
ment with the H -chemisorption
2
results. In combination with the
experimental results, it reveals
that the high-temperature hy-
drogen-absorbing sites are more
beneficial to the hydrogenation
of ethyl benzoate.
100.0
47.4
The X-ray photoelectron spec-
troscopy (XPS) spectra of Ni2p
and Pd3d of the amorphous
alloy catalysts are shown in
ethanol
dioxane
n-hexane
46.8
Figure 4.
The
characteristic
100.0
73.4
–
peaks of Ni2p centered at ap-
proximately 852 and 856 eV in
all samples can be assigned to
metallic and oxidized Ni, respec-
tively. The characteristic peaks
of Pd3d centered at approxi-
mately 335 eV confirm the pres-
ence of metallic Pd in the cata-
26.6
[
a] Reaction conditions: Substrate (33 mmol), solvent (70 mL), catalyst (0.2 g), P(H
2
)=5 MPa, 400 rpm, 423 K,
5 h.
[
27]
lysts, which are active in hydrogenation. Thus, metallic Ni is
compared with dioxane, probably owing to the difference in
polarity of these two aprotic solvents.
[
15]
considered as active species of the catalysts and the added
Pd not only benefits the dispersion of active Ni species but
also contributes to the activity of the catalysts.
Furthermore, the hydrogenation of benzoic acid yielded dif-
ferent results in water and ethanol (Table 4). The hydrogena-
tion of benzoic acid proceeded effectively in water with 97.6%
conversion and 100.0% selectivity toward cyclohexanecarbox-
ylic acid, which we ascribe to the solvating and orientating
effect of water on the carboxylic group, as also described by
Effects of solvent and substrate
To test the effect of the solvent, the hydrogenation of ethyl
benzoate and benzoic acid was studied in various solvents
over NiPdB-PEG(800) under the same reaction conditions
[28]
Anderson et al. The conversion of benzoic acid was 100.0%
in ethanol; however, ethyl benzoate and ethyl cyclohexanecar-
boxylate were detected as the two main products owing to
the concomitant esterification of benzoic acid with the subse-
quent hydrogenation of ethyl benzoate thus formed, but cy-
clohexanecarboxylic acid was not detectable in the reaction
mixture. Thus, it would appear that the catalyst promotes
esterification before the hydrogenation in ethanol. In the case
of dioxane, the hydrogenation of benzoic acid showed moder-
ate conversion (49.2%) but 100.0% selectivity toward cyclohex-
anecarboxylic acid, whereas in the case of n-hexane, the hydro-
genation proceeded faster with 83.5% conversion but 73.4%
selectivity. Thus, water is regarded as the best solvent in the
hydrogenation of benzoic acid owing to the high selectivity
(Table 4). The hydrogenation of ethyl benzoate proceeded
faster with 100.0% conversion in water than in ethanol; how-
ever, the selectivity toward ethyl cyclohexanecarboxylate de-
creased and cyclohexanecarboxylic acid was detected as the
major byproduct owing to the hydrolysis of the resulting ethyl
cyclohexanecarboxylate or the successive hydrolysis and hy-
drogenation of ethyl benzoate in water. In contrast, in the case
of dioxane, the extent of hydrogenation was limited but the
selectivity was excellent and only the ring hydrogenated prod-
uct was detected. However, in the case of n-hexane, the hydro-
genation proceeded faster with 89.2% conversion but lower
selectivity toward ethyl cyclohexanecarboxylate (97.1%) as
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[
28]
demonstrated by the catalyst in this solvent and the overall
green nature of the system.
sults for Pd on carbon nanofibers (CNFs). In contrast, the hy-
drogenation of acetophenone and benzophenone showed dif-
ferent reactivities. 1-Cyclohexylethanol was detected as the
main product in the hydrogenation of acetophenone, which
yielded cyclohexyl methyl ketone as the main byproduct; this
finding indicates that NiPdB-PEG(800) was active for the hydro-
genation of both the aromatic ring and the carbonyl group in
acetophenone. In contrast, cyclohexylmethylbenzene was ob-
tained as the main product in the hydrogenation of benzophe-
none arising from the successive hydrogenation of diphenyl-
methane, the main byproduct, which in turn was obtained
from the direct hydrogenolysis of the carbonyl group of
To determine whether the high activity of NiPdB-PEG(800)
was specific to ethyl benzoate and benzoic acid, a number of
other electron-deficient aromatic substrates were studied
(Table 5). NiPdB-PEG(800) demonstrated excellent activities
with all substrates with 100.0% conversion in most cases
except benzoic acid (97.6% conversion) and phenyl acetic acid
(95.4% conversion). However, the selectivities varied markedly,
depending on the structures of the substrates. In the case of
methyl benzoate, cyclohexanecarboxylic acid was the main
product instead of methyl cyclohexanecarboxylate, as com-
pared to the case of ethyl benzoate. This finding is consistent
with the higher susceptibility of methyl esters to aqueous hy-
[30]
benzophenone.
[
29]
drolysis compared to ethyl esters. For various aromatic car-
boxylic acids, increasing the length of the carbon chain did
not decrease the excellent conversion and selectivity toward
the ring hydrogenated products. However, in the case of cin-
namic acid, the side-chain olefinic double bond was also hy-
drogenated in the main product obtained. The best result was
obtained in the hydrogenation of benzamide with both
Conclusions
The presence of small amounts of Pd in a NiB amorphous alloy
can introduce larger surface area, smaller particle size, and
higher H -chemisorption, although some agglomeration still
2
occurred, and this markedly increased the conversion of ethyl
benzoate on hydrogenation. The accompanying agglomeration
could be inhibited with the further addition of PEG(800), which
decreases the coverage of Ni centers and results in the highest
100.0% conversion and selectivity toward cyclohexane carbox-
amide over NiPdB-PEG(800), which eclipsed the reported re-
H -chemisorption, the largest
2
surface area, and the smallest
[
a]
Table 5. Hydrogenation of different aromatic substrates over NiPdB-PEG(800) in water.
particle size. NiPdB-PEG(800)
demonstrated the highest activi-
ty in the chemoselective hydro-
genation of ethyl benzoate
comparable with noble metal
catalysts; this finding indicates
a synergistic effect between Pd
and PEG(800), as confirmed by
Substrate
Conversion [%]
Selectivity [%]
Main product
Byproduct
1
1
00.0
00.0
75.6
78.5
22.4
21.5
XPS and H -TPD analyses. Fur-
2
thermore, ethyl benzoate and
benzoic acid showed different
hydrogenation performance in
different solvents. Finally, NiPdB-
PEG(800) demonstrated excel-
lent activities and selectivities in
the hydrogenation of various
electron-deficient aromatic sub-
strates in water and is thus po-
tentially applicable to large-
scale productions. The signifi-
cant reduction in the necessary
amount of Pd is advantageous
in this respect.
9
9
1
7.6
100.0
100.0
96.0
–
–
5.4
00.0
4.0
1
1
1
00.0
00.0
00.0
100.0
63.6
50.9
–
36.4
34.5
Experimental Section
Catalyst preparation
Unless otherwise stated, all chemi-
cals were purchased from Baoding
Huaxin Reagent and Apparatus
Co. Ltd. and were used as received
without further purification. Metal-
[
2
a] Reaction conditions: Substrate (33 mmol), water (70 mL), catalyst (0.2 g), P(H )=5 MPa, 400 rpm, 423 K, 5 h.
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and polymer-modified NiB amorphous alloys were prepared
through chemical reduction. A typical method is as follows:
NiCl ·6H O (0.94 g, 4.0 mmol) and the requisite amount of the solu-
mounted on the sample holder, in which the background pressure
was lower than 2.7 mPa. All the binding energy values were correct-
ed by the C1s peak of contaminant carbon at 284.6 eV.
2
2
ble salts of Co, Fe, La, Zr, and Pd (X) in a molar ratio of Ni/X=20:1
were dissolved in distilled water (7.5 mL), and then the solution
was treated with ultrasound for 5 min to ensure good mixing. Sub-
sequently, KBH4 (aq) (20 mmol, 17 mL, 1.3m) containing NaOH
Catalyst activity test
The hydrogenation of ethyl benzoate was performed as follows:
ethyl benzoate (5.0 g, 33 mmol), ethanol (70 mL), and the catalyst
(
0.22m) was added dropwise to the above solution with vigorous
stirring while cooling in an ice bath until gas evolution ceased. The
black precipitate thus obtained was washed with deoxygenated
distilled water until the washings were pH 7. The catalyst was fur-
ther washed 3 times with absolute ethanol to replace residual
water and then stored in absolute ethanol. The catalysts so ob-
tained are labeled NiXB. Upon the addition of the polymer (1.0 g)
to the NiCl ·6H O and metal salt solutions during the preparation,
(0.2 g) were successively added into a 100 mL stainless steel auto-
clave equipped with a mechanical stirrer and an electrical heating
system. The autoclave was first flushed 3 times with H , followed
by evacuation, at RT and then pressurized with H to 5 MPa and
heated to 423 K. Hydrogenation was then performed by stirring
the reaction mixture vigorously for 5 h. Hydrogenations of other
aromatic carboxylic acids and their derivatives were performed
under the same reaction conditions in water.
2
2
2
2
the catalysts so obtained are labeled NiXB-p, in which p represent-
ed the nature of the polymer.
Reaction mixtures were analyzed by using GC on a 30M SE-54 ca-
pillary column, and product structures were confirmed by using
GC–MS on an Agilent 5975C spectrometer.
Pure NiB and PdB were also obtained through chemical reduction
[21]
as reported previously. The physical mixture of NiB and PdB was
obtained by physically mixing the previously prepared NiB and
PdB. Otherwise, the Pd/C catalyst was purchased from Tianjin
Kermel Chemical Reagent Co. Ltd. Raney Ni was prepared through
alkali leaching of the NiÀAl alloy by using the conventional
Acknowledgements
[31]
method.
We thank Prof. Laurence M. Harwood for his kind assistance in
preparing this article. We gratefully acknowledge financial sup-
port from the National Natural Science Foundation of China
Catalyst characterization
(
21376060) and the Natural Science Foundation of Hebei
Bulk compositions of the amorphous alloy catalysts were deter-
mined from ICP analysis on a Varian Vista MPX spectrometer. All
samples were first treated with concentrated HNO /H SO solution
Province (B2011201017).
3
2
4
(
1:1 volume ratio) at RT, then diluted to the requisite concentration,
Keywords: amorphous
compounds · hydrogenation · palladium · polymers
materials
·
electron-deficient
and finally analyzed to obtain the elemental composition.
The BET surface areas of the catalysts were measured by using N2
physisorption at 77 K with a Micromeritics TriStar II 3020 apparatus.
Before measurements, the samples were dried under vacuum at
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Received: October 10, 2013
1
Published online on December 27, 2013
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