Polymers as novel modifiers for supported metal catalyst in hydrogenation of benzaldehydes

Article abstract of DOI:10.1016/j.jcat.2010.10.008

Polyethylene glycol (PEG) was impregnated on a palladium metal supported on silica gel and used in a catalyst for hydrogenation of benzaldehydes. In the vapor-phase flow reaction, the PEG modification improved catalytic activity and selectivity for a partially hydrogenated product, benzyl alcohol. In isoprene hydrogenation, selectivity for partially hydrogenated products, monoenes, was also enhanced. X-ray photoelectron spectroscopy (XPS) analysis of the modified catalysts revealed that the modification with PEG makes the palladium surface negatively charged, possibly leading to an increase in the selectivity for the partially hydrogenated product caused by enhancement of its desorption from the surface. In the liquid-phase hydrogenation of benzaldehyde, the PEG modification also increased the selectivity for benzyl alcohol.

Full text of DOI:10.1016/j.jcat.2010.10.008

Journal of Catalysis 276 (2010) 423–428
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Polymers as novel modifiers for supported metal catalyst in hydrogenation
of benzaldehydes
⇑
Masaki Okamoto , Tomoyuki Hirao, Tatsuya Yamaai
Department of Applied Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 July 2010
Revised 12 October 2010
Accepted 12 October 2010
Available online 9 November 2010
Polyethylene glycol (PEG) was impregnated on a palladium metal supported on silica gel and used in a
catalyst for hydrogenation of benzaldehydes. In the vapor-phase flow reaction, the PEG modification
improved catalytic activity and selectivity for a partially hydrogenated product, benzyl alcohol. In
isoprene hydrogenation, selectivity for partially hydrogenated products, monoenes, was also enhanced.
X-ray photoelectron spectroscopy (XPS) analysis of the modified catalysts revealed that the modification
with PEG makes the palladium surface negatively charged, possibly leading to an increase in the selectiv-
ity for the partially hydrogenated product caused by enhancement of its desorption from the surface. In
the liquid-phase hydrogenation of benzaldehyde, the PEG modification also increased the selectivity for
benzyl alcohol.
Keywords:
Polyethylene glycol
Palladium metal catalyst
Partial hydrogenation
Modifier
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
the catalytic activity. Thus, PEG is a prospective modifier to pro-
duce a negatively charged metal catalyst for various metal-cata-
The development of high-performance catalysts is imperative
for energy conservation and effective utilization of resources. The
use of modifiers for heterogeneous catalysts is an efficient method
for improving product selectivity and catalytic activity [1]. Many
types of modifiers used with supported metal catalysts for hydro-
genation reactions have been reported [2–4], including metal [2,3],
organic [2–4], and inorganic [2,3] modifiers. These modifiers exert
primarily an electronic effect on the metal surface, poison-specific
active sites for the formation of by-products, and alter the mor-
phology of the metal catalysts. The variety of modifiers allows var-
ious selective hydrogenations, and thus, the development of new
modifiers is important for widening the range of possible selective
hydrogenations.
In this study, we examined polymers such as polyethylene gly-
col (PEG) as novel modifiers for metal catalysts. Because a polymer
has low vapor pressure at higher temperatures, it has an advantage
as a modifier for vapor-phase reactions. Furthermore, oxygen in an
ether bond can be coordinated to metal atoms on the metal surface
[5,6], possibly because the metal surface is negatively charged by
electrons donated from oxygen in PEG. Generally, since oxygen is
weakly coordinated to the metal atoms, ordinary molecules con-
sisting of oxygen, such as ethers, cannot remain on the surface
for a long duration. However, polymers containing oxygen must re-
main on the metal surface or in its vicinity to continuously affect
lyzed reactions. The use of PEG with metal catalysts has been
previously reported [7–9]. In these reports, the metal catalyst
was highly dispersed in PEG and showed high hydrogenation activ-
ity because of formation and stabilization of Pd nanoparticles in
PEG. However, the role of PEGs as a modifier on metal catalysts
has not been examined.
In this study, we report on the use of polymers as effective mod-
ifiers for a conventional supported palladium metal catalyst in the
partial hydrogenation of benzaldehyde. Benzaldehyde is reduced
to afford benzyl alcohol, which is subsequently converted to tolu-
ene and benzene. To obtain benzyl alcohol selectively, many metal
catalysts such as Pd [10–12], Ni [13,14], Cu [15], and Pt [16–18]
have been reported. Sulfur poisoning of palladium [10] and inter-
calation of palladium nanoparticles into bentonite [11] increased
the selectivity for partial hydrogenation. This work demonstrates
that modification with PEG improved benzyl alcohol selectivity
as well as benzaldehyde conversion, and that in isoprene hydroge-
nation, selectivity for monoenes also increased. These results indi-
cate that PEG modification could potentially be applied to various
hydrogenations.
2. Experimental
2.1. Catalyst preparation
⇑
A silica-supported palladium metal catalyst, Pd/SiO2, was pre-
pared in the following manner. Silica gel Q-15 (surface area
Corresponding author. Fax: +81 3 5734 2625.
E-mail address: mokamoto@apc.titech.ac.jp (M. Okamoto).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.10.008

424
M. Okamoto et al. / Journal of Catalysis 276 (2010) 423–428
194 m2 gÀ1, pore volume 0.95 mL gÀ1, from Fuji Silysia Chemical
Ltd.) was dried at 200 °C for 24 h in an oven. Tetraamminepalla-
dium(II) chloride (Pd(NH3)4Cl2ÁH2O, 98%, Aldrich) was impregnated
on the dried silica using the incipient wetness method. The amount
of palladium was 0.1% (w/w) of silica gel. After drying at 40 °C in
vacuum (20 Torr), the catalyst was calcined at 400 °C for 12 h. Be-
fore the hydrogenation, palladium was reduced at 150 °C for 1 h in
a stream of hydrogen. PEG-modified Pd/SiO2, PEG-Pd/SiO2 (Pd 21%
(w/w)), was prepared by impregnation of PEG (0.42 g, average
3. Results and discussion
3.1. Vapor-phase hydrogenation of benzaldehyde
3.1.1. Vapor-phase hydrogenation of benzaldehyde over Pd/SiO2
modified with PEG
Table 1 shows the results of vapor-phase hydrogenation of
benzaldehyde using the polymer-modified Pd/SiO2. The use of an
unmodified catalyst resulted in the formation of toluene as a by-
product, together with benzyl alcohol. Modification with 21% (w/
w) PEG increased benzyl alcohol selectivity to 100% and increased
benzaldehyde conversion; PPG also improved the selectivity. At al-
most 100% benzaldehyde conversion, the addition of the polymer
enhanced the selectivity remarkably, as shown in Table 2. The
selectivity increased with an increase in the amount of polymer;
however, an increase in polymer amount also decreased the con-
version, possibly because a larger amount of palladium present
on the surface was covered with the polymer. A catalyst prepared
by impregnation with an aqueous mixture of PEG and tetraam-
minepalladium(II) chloride solution gave almost the same conver-
sion and selectivity. This indicates that differences in catalyst
preparation procedures do not affect the catalytic property of pal-
ladium. PEG (21% (w/w))-modified Pd/SiO2 showed constant con-
version and selectivity for 5 h on stream.
molecular weight: 1000, specific gravity: 1.13 g mLÀ1
, Kanto
Chemical Co., Inc.) using its aqueous solution (4.5 mL) on calcined
Pd(NH3)4Cl2-impregnated silica (2 g). After drying at 40 °C in vac-
uum (20 Torr), the catalyst was reduced at 150 °C for 1 h in a
stream of hydrogen. Long-chain PEGs (PEG(8200), average
molecular weight: 8200, Kanto Chemical Co., Inc.; PEG(500,000),
average molecular weight: 500,000, Kanto Chemical Co., Inc.) and
polypropylene glycol (PPG, average molecular weight: 400,
Wako Pure Chemical Industries) were impregnated using their
aqueous solutions, whereas ethanol and toluene were used as
solvents for impregnation of polydimethylsiloxane (DMPS,
SH200, Dow Corning Toray Co.) and polyvinyl butyral (PVB, aver-
age molecular weight: 300, Wako Pure Chemical Industries),
respectively.
Thermogravimetric analysis of the used catalyst revealed that
almost all PEG still remained on the catalyst. We have reported
that PEG was used as a liquid medium in a supported liquid-phase
2.2. Reaction procedure
Vapor-phase hydrogenation was performed in an atmospheric
fixed-bed flow reactor. The catalyst, 0.05 g (excluding polymer
weight), was placed in a quartz reactor (id: 10 mm). After reduc-
tion of the catalyst at 150 °C for 1 h in a stream of hydrogen
(25 mL minÀ1), benzaldehyde vapor (0.43 kPa) and hydrogen
(33.3 kPa) were fed into the reactorÀ, 1with nitrogen as a balance
gas. The contact time, W/F [L/(Lliquid h )], was 3.5 h. Products were
collected for 1 h and analyzed by gas chromatography and gas
chromatography–mass spectrometry.
Liquid-phase hydrogenation was performed in a 50-mL auto-
clave. After reduction of the catalyst (0.2% (w/w) Pd/SiO2) at
150 °C for 1 h in the quartz tube reactor in a stream of hydrogen
(25 mL minÀ1), 0.05 g (excluding polymer weight) of the catalyst
was transferred into the autoclave in a nitrogen atmosphere. Benz-
aldehydes (2 mL) and dodecane (25 mL) as a solvent were put in
the autoclave and hydrogen was charged at 3 MPa. The hydrogena-
tion was performed at 180 or 190 °C. After the reaction, the auto-
clave was cooled, and butanol as an internal standard was added
to the reaction mixture.
Table 1
Vapor-phase hydrogenation of benzaldehyde using polymer-modified Pd/SiO2.
Catalyst
Polymer amount
[% (w/w)]
Benzaldehyde
conversion (%)
Selectivity (%)
Benzyl
alcohol
Toluene
a
Pd/SiO2
PEG-Pd/
SiO2
0
21
59
71
92
100
8
0
a
PEG-Pd/
SiO2
66
21
57
59
100
100
0
0
a
PPG-Pd/
a
SiO2
b
Pd/SiO2
PEG-Pd/
0
21
25
45
90
98
0
2
b
SiO2
a
0.1% (w/w) Pd/SiO2 0.05 g, W/F = 3.5 h, reaction temperature 100 °C, benzalde-
hyde 0.43 kPa, H2 33.3 kPa.
b
1% (w/w) Pd/SiO2 3.5 mg, W/F = 0.037 h, reaction temperature 100 °C, benzal-
dehyde 0.22 kPa, H2 14.4 kPa.
2.3. Catalyst characterization
Nitrogen adsorption was measured with a BELSORP-mini (BEL
Japan). Prior to the measurement, the catalyst was dried at
120 °C for 3 h under evacuation. Pore size distributions were calcu-
lated using desorption isotherms. X-ray diffraction (XRD) patterns
Table 2
Benzaldehyde conversion and product selectivity in vapor-phase hydrogenation of
benzaldehyde at almost 100% benzaldehyde conversion.
Polymer
Polymer amount Benzaldehyde
Selectivity (%)
of the catalysts were recorded using Cu Ka with a MiniFlex (Rigaku
[% (w/w)]
conversion (%)
Co.). XRD samples were not ground to avoid changes in the catalyst
due to heat generated by grinding. The amount of CO adsorbed on
palladium was measured by consumption of CO in several pulses
(0.63 mL) of 20% CO (in helium) at room temperature, after 0.2 g
of the catalyst was reduced at 150 °C for 1 h in a hydrogen stream.
Transmission electron microscopy (TEM) images were taken using
a JEM-2010F (JEOL) operated at an accelerating voltage of 200 kV.
X-ray photoelectron spectroscopy (XPS) measurements were per-
Benzyl
alcohol
Toluene
None
PEG
PEG
0
100
99
98
91
100
99
49
89
95
93
92
89
88
51
11
5
7
8
2.1
21
66
21
21
PEG
PEGa
PEG(8200)
PEG(500,000) 21
11
12
12
formed using an ESCA-3200 (Shimadzu Co.). Mg K
a (1253.6 eV,
a
Prepared by impregnation with a mixture of PEG and Pd(NH3)4Cl2 0.1% (w/w)
8 kV, 30 mA) was used as incident radiation. The Au 4f 7/2 binding
energy of 83.8 eV was used as an internal standard to correct for
electrostatic charging.
Pd/SiO2 0.15 g, W/F = 4.9 h, reaction temperature 120 °C, benzaldehyde 0.86 kPa, H2
67.1 kPa.

M. Okamoto et al. / Journal of Catalysis 276 (2010) 423–428
425
catalyst for vapor-phase reactions at higher reaction temperature
than 120 °C (hydrogenation temperature in Table 2) [19–21]. In
this catalyst system, PEG was not lost through vaporization during
the reaction. These results strongly suggest that the PEG-modified
catalyst is stable. When the PEG-modified Pd/SiO2 was reused after
the first reaction, the catalyst still showed the same conversion
after three times; however, the selectivity decreased slightly from
95% to 91%, which is still higher than that of the reaction catalyzed
by Pd/SiO2. This indicates that the PEG-modified catalyst is
reusable.
3.1.2. Pd/SiO2 modified with various polymers
The effects of various polymers on the reaction were examined.
Modification of PEG(8200) or PVB also improved the selectivity;
however, it was still low, as shown in Tables 2 and 3. Further
lengthening of the PEG chain (PEG(500,000)) decreased the
conversion remarkably. The particles of Pd/SiO2 modified with
PEG(500,000) were aggregated because their external surface was
adhesive. This indicates that the external surface of the catalyst
was covered with PEG and that PEG hindered access of the reactant
to the palladium metal surface. When 1,2-diethoxyethane (DEE),
which is one unit of PEG, was fed to the reactor with benzaldehyde
and hydrogen, selectivity was also observed to improve. Improve-
ment in the selectivity by modification with PEG, PPG, or PVB, or
through co-feed of DEE indicates that coordination of oxygen in
an ether bond to palladium metal atoms affects the selectivity.
Polydimethylsiloxane (PDMS) having oxygen in a siloxane bond
decreased the conversion slightly and did not affect the selectivity.
The external surface of the catalyst modified with PDMS was also
adhesive. Impregnation of PDMS only on the external surface of
the catalyst possibly causes no effect of PDMS modification. Thio-
phene was adsorbed by feeding gaseous thiophene (2.5% of palla-
dium amount) to the reactor before the hydrogenation. Sulfur in
thiophene did not influence the selectivity.
Fig. 1. Pore size distribution of PEG-modified Pd/SiO2. PEG amount (a) 0%, (b) 21%
and (c) 66% (w/w).
Table 3
Vapor-phase hydrogenation of benzaldehyde using Pd/SiO2 catalyst with various
modifiers.
Modifier
Modifier
Benzaldehyde Selectivity [%]
Amount [%
(w/w)]
Conversion
(%)
Benzyl
alcohol
Toluene Benzene
None
PEG
PVB
–
24
29
35
25
27
32
26
91
96
94
92
94
91
8
3
5
7
5
8
1
1
1
1
1
1
3
3
PDMS
DEE
6 kPaa
Thiophene 2.5%b
3.2. Vapor-phase hydrogenations of isoprene and crotonaldehyde
0.1% (w/w) Pd/SiO2 0.06 g, W/F = 1.3 h, reaction temperature 120 °C, benzaldehyde
2.0 kPa, H2 32 kPa.
Isoprene hydrogenation was also performed using polymer-
modified Pd/SiO2. Modification with PEG increased the selectivity
for pentenes to 91%. Although change in the isoprene conversion
with the PEG modification was small and close to the experimental
errors, the conversion increased from 30% to 35% (Table 4).
Improvement in the selectivity for partial hydrogenation (probably
also in the catalytic activity) was consistent with the results for
benzaldehyde hydrogenation. However, in crotonaldehyde hydro-
genation at 90 °C, 2.9 kPa of crotonaldehyde and 16 kPa of hydro-
gen, the modification with PEG decreased the conversion from
54% to 19%, and only butyraldehyde was formed, regardless of
the PEG used. The modification is likely to have no effect on the
selectivity for crotonalcohol formed via a different path.
a
1,2-Diethoxyethane was fed with benzaldehyde and hydrogen.
Thiophene was adsorbed before the hydrogenation. The amount of adsorbed
b
thiophene was 2.5% of the surface palladium atoms.
Table 4
Vapor-phase hydrogenation of isoprene using PEG-modified Pd/SiO2.
Catalyst
Isoprene
Selectivity (%)
conversion (%)
2-Methyl- 2-Methyl- 3-Methyl- Isopentane
1-butene 2-butene 1-butene
Pd/SiO2
30
12
19
57
65
4
7
27
9
PEG-Pd/SiO2 35
0.1% (w/w) Pd/SiO2 0.016 g, PEG 0.21% (w/w). W/F = 0.030 h, reaction temperature
70 °C, time on stream 5 h, isoprene 13 kPa, H2 49 kPa.
3.3. Characterization of the PEG-modified catalyst
3.3.1. Pore size distribution
The effect of PEG amount on pore size distribution and pore vol-
ume was examined using nitrogen adsorption–desorption. Pore
size distributions of the PEG-modified catalysts and pore volumes
are shown in Fig. 1 and Table 5, respectively. Pore volume de-
creased with increasing PEG amount, and a decrease in pore vol-
ume was close to the volume of PEG loaded on silica, indicating
that the pore openings are not plugged with PEG. Modification
with 21% (w/w) PEG decreased pore size. When a higher amount
(66% (w/w)) of PEG was used, pore size was the same as that using
21% (w/w) PEG and peak intensity of pore size decreased. This indi-
cates that using 21% (w/w) PEG, pore walls are uniformly coated
with PEG; however, use of 66% (w/w) PEG leads to partially filling
Table 5
Pore volume of PEG-modified Pd/SiO2.
PEG amount [% (w/w)]
PEG volume (mL gÀ1
)
Pore volumea (mL gÀ1
)
0
21
66
0
0.19
0.58
1.15
1.01
0.65
a
Excluding polymer weight.
pores with PEG. This causes lower catalytic activity of 66% (w/w)
PEG-modified catalysts (Tables 1 and 2). When 21% (w/w) PEG

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M. Okamoto et al. / Journal of Catalysis 276 (2010) 423–428
was used for preparation of the catalyst, which was the most effec-
tive among the prepared catalysts, the thickness of the PEG layer
was calculated to be ca. 1 nm.
(TEM results) show that 56% of the surface atoms were covered
with PEG. In the measurement of CO adsorption at room tempera-
ture, PEG was solid. However, under reaction conditions, PEG be-
came liquid. It is possible that the reactants dissolved in the
liquid PEG and were exposed to the palladium surface through
PEG overlayers. Supposing that there are no reactants passing
through the PEG layers, the turnover frequency of the surface
3.3.2. Palladium particle size
To clarify the effect of PEG modification, palladium particle size
was examined. Transmission electron microscope (TEM) analysis
revealed that palladium particles are spherical and that palladium
particle sizes of the unmodified and modified Pd/SiO2 were the
same, i.e. 5 nm (Fig. 2a and b). It is known that in PEG medium me-
tal nanoparticles are formed and stabilized [7–9]. However, on the
silica support, PEG did not affect the particle size. This indicates
that enhancement in the conversion and the selectivity by PEG
modification is not due to a change in the size of the palladium par-
ticles. Thus, PEG modification affects properties of surface palla-
dium atoms.
atoms increased remarkably (from 2.9 Â 103 hÀ1 to 7.5 Â 103 hÀ1
,
as shown in Table 1) when modified with PEG.
3.3.4. X-ray photoelectron spectroscopy
Coordination of oxygen in the polymer to the palladium metal
atoms was expected to improve the catalytic activity. The elec-
tronic state of the palladium surface was examined by X-ray pho-
toelectron spectroscopy (XPS). Fig. 3 shows the Pd 3d spectra of the
unmodified and modified catalysts. Since the amount of palladium
in the catalyst used for the reaction was very low (0.1% (w/w), it
was below the detection limit of XPS. Therefore, the catalyst of
1% (w/w) of palladium was used for XPS analysis. When 1% (w/
w) palladium catalysts were used for the hydrogenation of benzal-
dehyde, the modification with PEG enhanced the product yield and
the selectivity for benzyl alcohol (Table 1). In the unmodified cat-
alyst, peaks at 335.0 and 340.4 eV assigned to palladium metal
were observed (Fig. 3). Modification with PEG decreased the bind-
ing energy of Pd 3d peaks. This indicates a decrease in electron
density of Pd. In the spectrum of the PEG-modified catalyst, one
Pd 3d5/2 peak was observed and its peak intensity was low, indicat-
ing that all Pd particles were partially covered with PEG. Therefore,
the hypothesis that some Pd particles were coated and some were
not is incorrect.
3.3.3. Adsorption of carbon monoxide
To estimate the amount of palladium atoms on the surface, the
amount of carbon monoxide (CO) adsorbed at room temperature
was measured. Assuming that one molecule of CO is adsorbed on
one palladium metal atom, the amount of surface palladium atoms
in the unmodified catalyst was 1.6 Â 10À6 mol gcatÀ1. On the other
hand, the cÀa1talyst modified with 21%(w/w) PEG had 7.1 Â
10À7 mol gcat of surface palladium atoms. Thus, CO adsorption
results and the fact that there was no change in particle size
3.3.5. Schematic of PEG-modified catalyst
TEM observation and nitrogen adsorption showed that palla-
dium particles are spheres with ca. 5 nm diameters and that the
pore wall is uniformly coated with 1-nm layers of PEG, respec-
tively. Fig. 4 shows our proposed schematic of PEG-modified cata-
lyst. As mentioned above, CO adsorption results show that 56% of
the surface atoms were covered with PEG. We speculated that
the bottom of the palladium particles is buried in PEG layers and
the top is partially coated with thin PEG layers. Benzaldehyde pos-
sibly accesses the palladium surface through openings of the PEG
layers on the palladium particles.
Generally, the effects of modifiers on catalytic properties of me-
tal catalysts are changes in size and morphology of metal particles,
Fig. 3. X-ray photoelectron spectra of the Pd 3d region of Pd/SiO2 and PEG-modified
Pd/SiO2 after reduction at 150 °C for 1 h in a stream of hydrogen. Pd and PEG
amounts were 1% and 21% (w/w), respectively.
Fig. 2. TEM images of: (a) Pd/SiO2 and (b) PEG-modified Pd/SiO2. Pd amount was
0.1% (w/w).

M. Okamoto et al. / Journal of Catalysis 276 (2010) 423–428
427
adsorption of thiophene on the catalyst did not improve the
selectivity.
Pd
5 nm
Piperidine was reported as an electron-donating modifier for
palladium catalysts in hydrogenations of 1-butene and 1,3-butadi-
ene, and the modification resulted in a remarkable decrease in a
turnover number for 1-butene hydrogenation and a slight change
in that for 1,3-butadiene [24], indicating that electron-rich palla-
dium catalysts show high selectivity for monoenes in diene hydro-
genation. This is consistent with the result that PEG modification
increased selectivity for the monoenes in isoprene hydrogenation
(Table 4).
1 nm
PEG
SiO2
In isoprene hydrogenation, 2-methyl-2-butene is formed by
1,4-addition of hydrogen to isoprene and also by simultaneous
isomerization of 2-methyl-1-butene and 3-methyl-1-butene. Adu-
riz et al. reported that 2-methyl-2-butene selectivity increased
with increasing conversion of isoprene due to the isomerization
of 2-methyl-1-butene and 3-methyl-1-butene [25]. Modification
with PEG slightly decreased the product distribution of 2-
methyl-2-butene among pentenes (Table 4) even at a higher iso-
prene conversion. It is plausible that repulsion between negatively
charged palladium and methyl-1-butenes enhances their desorp-
tion from the catalyst before the isomerization.
Fig. 4. Proposed schematic of PEG-modified Pd/SiO2.
steric effects on reaction intermediates, and changes in electronic
state of metal surface. When PEG was used as the modifier, the
modification did not affect particle size and morphology of palla-
dium particles. PEG weakly interacts with palladium atoms on
the surface because coordination strength between oxygen and
the metal is much weaker than sulfur and nitrogen. Thus, at the
reaction temperature, it is possible that PEG can move on the sur-
face, leading to no steric effect.
3.4. Liquid-phase hydrogenation of benzaldehydes
To prepare PEG-modified Pd/SiO2, PEG was impregnated on the
reduced Pd/SiO2. It is possible that palladium atoms on the surface
are partially oxidized during PEG impregnation, and Chen and
Chen reported that adsorbed oxygen on Pd/SiO2 enhanced the for-
mation of ethylbenzene in acetophenone hydrogenation [22]. We
tried to partially oxidize Pd/SiO2 in a 4% O2(4%)–He mixture stream
at 100 °C for 30 min. Pre-oxidation did not improve the conversion
nor the selectivity in vapor-phase hydrogenation of benzaldehyde
because the oxidation temperature was lower than that in the re-
port [22] and reduction of the catalyst was carried out prior to the
hydrogenation. This indicates that partial oxidation of palladium
metal during PEG impregnation does not cause the improvement.
XPS analysis of the catalysts revealed that modification with
PEG makes the palladium surface negatively charged, and we con-
cluded that this is the most important effect of PEG modification
on catalytic properties. It was reported that the Langmuir–Hinshel-
wood mechanism is applicable to benzaldehyde hydrogenation
and that low surface coverage of benzyl alcohol brings about high
selectivity for benzyl alcohol [17]. We speculated that repulsion
between the oxygen of benzyl alcohol and the negatively charged
palladium surface promotes benzyl alcohol desorption from the
surface, leading to the inhibition of further hydrogenation. Thio-
phene can also be coordinated to the metal surface; however, thi-
ophene is an electron acceptor [23]. As shown in Table 3, pre-
To examine the effect of PEG modification on hydrogenations of
various benzaldehydes, the reactions of p-anisaldehyde and p-tol-
ualdehyde were carried out in a liquid phase, because their vapor
pressures were too low to feed them by vapor to the reactor at a
reaction temperature. Dodecane was used as a solvent because
the solubility of PEG in it is very low. The results are summarized
in Table 6. To compare the selectivities for partial hydrogenation at
the same conversion, the catalyst amount and reaction time were
adjusted. Improvement in the selectivity for the partial hydrogena-
tion was observed, indicating that PEG modification can be applied
to hydrogenations of various benzaldehydes. However, the cata-
lytic activity decreased, which is contrary to the results in the va-
por-phase flow reaction.
It is plausible that in the vapor-phase reaction, PEG holds benz-
aldehyde to increase its residence time in the catalyst bed in the
flow reactor, causing an increase in the conversion. On the other
hand, in the liquid-phase reaction, it is likely that charging palla-
dium negatively by PEG modification also causes improvement in
the selectivities. Decreasing the benzaldehyde conversion is prob-
ably caused by decreasing the number of surface palladium atoms
by PEG modification and the fact that an increase in the residence
times of the reactants in the flow system is not observed in the
batch system.
Table 6
Liquid-phase hydrogenation of benzaldehydes using PEG-modified Pd/SiO2.
Polymer
Catalyst amount (mg)
Reaction time (h)
Benzaldehydes conversion (%)
Selectivity (%)
Benzaldehyde (R: H)a
None
PEG
50
80
2
2
84
84
98
100
2
0
p-tolualdehyde (R:CH3)b
None
PEG
50
90
5
5
18
23
75
95
25
5
p-anisaldehyde (R:CH3O)b
None
PEG
50
300
1.5
6
35
32
98
100
2
0
a
Palladium 0.2% (w/w), reaction temperature 180 °C, benzaldehydes 2 mL, H2 3 MPa, dodecane 25 mL.
Palladium 0.2% (w/w), reaction temperature 190 °C, benzaldehydes 2 mL, H2 3 MPa, dodecane 25 mL.
b

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M. Okamoto et al. / Journal of Catalysis 276 (2010) 423–428
Table 7
Liquid-phase hydrogenation of acetophenone using PEG-modified Pd/SiO2 and PEG-modified Pt/SiO2.
Polymer
Catalyst amount (mg)
Reaction time (h)
Acetophenone conversion (%)
Selectivity (%)
Pd/SiO2
None
PEG
50
80
3
3
64
72
97
99
3
1
0
0
0
0
Pt/SiO2
None
PEG
50
50
1
15
14
13
59
90
0
0
38
10
3
0
Palladium 0.2% (w/w), reaction temperature 180 °C, acetophenone 2 mL, H2 3 MPa, dodecane 25 mL.
When the catalyst was recycled four times in the liquid-phase
hydrogenation of benzaldehyde, neither the conversion nor the
selectivity decreased, indicating that the catalyst is reusable in
the liquid phase.
polymers, particularly PEG, has the potential to be applied to var-
ious hydrogenations in a liquid phase as well as in a vapor phase.
It is notable that PEG is suitable for the modifier because PEG is
inexpensive, non-toxic, readily available, and stable.
3.5. Liquid-phase hydrogenation of acetophenone catalyzed
by PEG-modified Pt/SiO2
Acknowledgments
The TEM observations were performed at the Center for Ad-
vanced Materials Analysis in Tokyo Institute of Technology. The
authors thank Prof. Michikazu Hara and Dr. Kiyotaka Nakajima
for XPS measurement.
Modification of a supported platinum catalyst with PEG also af-
fected the product selectivity. When the palladium catalyst was
used for liquid-phase hydrogenation of acetophenone, products
were 1-phenylethanol and ethylbenzene. PEG modification slightly
increased the selectivity for 1-phenylethanol (Table 7). On the
other hand, when the platinum catalyst was used, a benzene ring
of acetophenone was also hydrogenated; i.e. products were
1-phenylethanol, 1-phenylethanone and 1-cyclohexylethanol.
PEG modification of the platinum catalyst inhibited the hydrogena-
tion of the benzene ring. Repulsion between the benzene ring of
acetophenone and the negatively charged palladium surface possi-
bly inhibits the hydrogenation of the benzene ring.
Appendix A. Supplementary material
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.jcat.2010.10.008.
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