Phenanthroline-stabilized palladium nanoparticles in polyethylene glycol - An active and recyclable catalyst system for the selective hydrogenation of olefins using molecular hydrogen

Article abstract of DOI:10.1016/j.molcata.2004.07.021

1,10-Phenanthroline-stabilized palladium nanoparticles dispersed in a polyethylene glycol (PEG) matrix was synthesized, which is stable and active catalyst for the selective hydrogenation of olefins (e.g., cyclohexene, 1-octene, styrene, 1,5-cyclooctadiene) using molecular hydrogen under mild reaction conditions. The phenanthroline-Pd-PEG had comparable hydrogenation activity with that of the polyvinylpyrrolidone-Pd-PEG and 1-hexadecylaniline-Pd-PEG under identical reaction conditions. Hydrogenation of 1,5-cyclooctadiene afforded the mono hydrogenated product selectivity at 30°-40°C and 8 hr reaction time. The selectivity of the di-hydrogenated product increased with increasing reaction time.

Full text of DOI:10.1016/j.molcata.2004.07.021

Journal of Molecular Catalysis A: Chemical 222 (2004) 153–158
Phenanthroline-stabilized palladium nanoparticles in polyethylene
glycol—an active and recyclable catalyst system for the selective
hydrogenation of olefins using molecular hydrogen
Unnikrishnan R. Pillai1, Endalkachew Sahle-Demessie∗
National Risk Management Research Laboratory (NRMRL), Clean Processes Branch, MS 443,
United States-Environmental Protection Agency (US-EPA), Cincinnati, OH 45268, USA
Received 27 May 2004; received in revised form 14 July 2004; accepted 28 July 2004
Available online 11 September 2004
Abstract
1,10-Phenanthroline-stabilized palladium nanoparticles dispersed in a polyethylene glycol (PEG) matrix is synthesized which is found to
be a stable and active catalyst for the selective hydrogenation of olefins using molecular hydrogen under mild reaction conditions. A variety
of olefins are hydrogenated to their corresponding alkanes with good to excellent yield at room temperature. The catalyst is easily separable
and recyclable.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Palladium nanoparticles; Phenanthroline; Polyethylene glycol; Olefin hydrogenation
1. Introduction
stabilizing ligands for transition metal nanoparticles and used
as catalysts for the hydrogenation of arenas [6–11]. Disper-
sion or supporting these stabilized nanoparticles on to some
matrix is very important to ensure its maximum effectiveness.
Different types of ligand-stabilized metal nanoparticles have
been synthesized and used as catalysts after being anchored
on to some solid supports [12–18]. Reduction and capping
of metal nanoparticles by multifunctional molecules such as
hexadecyl aniline which in turn may be dispersed in an or-
ganic solvent for the effective hydrogenation of arenes is also
described [19]. Dendrimer-encapsulated metal nanoparticles
are also synthesized in which metal ions are sorbed into the
interior of dendrimers and then chemically reduced to yield
nano-sized metal particles [20].
A method of enhancing the catalytic activity of these
nanoparticle catalysts is to mobilize them using some liq-
uid polymeric matrices thereby achieving the advantages
of both homogeneous and heterogeneous catalysis. Re-
cently, phenanthroline ligand-stabilized palladium nanopar-
ticles dispersed in ionic liquid has been reported to be very
active for the hydrogenation of olefins [21]. However, ionic
The development of novel nano-structured materials has
attracted great attention recently due to their unique charac-
teristics that differ from individual atoms and bulk materials
[1]. Nanoparticles have different physical (heat capacity, va-
por pressure, melting point), chemical, optical and electronic
properties [1–3]. Their increased number of edges, corners
and faces give them a high surface-to-volume ratio and there-
fore are highly useful as catalysts with a higher activity and
selectivity during catalytic reactions [4,5].
Transition metal nanoparticles have wide ranging applica-
tions in catalysis. However, due to their large surface area and
surface energy, they tend to agglomerate during the reactions
and therefore need to be stabilized for effective utilization.
Polyoxoanions and tetrabutyl ammonium have been used as
∗
Corresponding author. Tel.: +1 513 569 7739; fax: +1 513 569 7677.
E-mail address: Sahle-Demessie.Endalkachew@epa.gov
(E. Sahle-Demessie).
1
Present address: New Technology Research, Philip Morris USA, Rich-
mond, VA 23234, USA.
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.07.021

154
U.R. Pillai, E. Sahle-Demessie / Journal of Molecular Catalysis A: Chemical 222 (2004) 153–158
liquid is not only very expensive but also not yet accepted
as an environmentally friendly medium and there are reports
of it being toxic in nature in spite of several studies in this
medium [22].
8453) and also by transmission electron microscopy (Philips
320 TEM).
2.3. Hydrogenation
The search for an alternative environmentally friendly re-
action medium in place of the volatile and toxic organic sol-
vents is an important goal of green chemistry research with
significant environmental consequence. Therefore, it is desir-
able to develop nano-structured catalytic systems supported
or dispersed on a suitable mobile phase that are very active,
selective, stable, non hazardous and recyclable. Polyethylene
glycol (PEG) has been emerging as a very convenient support
for the synthesis of a variety of catalysts, ligands and organic
compounds [23]. It is relatively inexpensive, environmen-
tally benign and can readily be functionalized. It is soluble in
many organic solvents but insoluble in some others such as
diethyl ether. These properties render it very attractive as a
mobile support wherein a reaction catalyzed by the PEG sup-
ported catalyst can be carried out under homogeneous condi-
tion while the PEG-catalyst system can be easily recovered
after extracting the reaction mixture with diethyl ether.
Herein, we report such a catalyst system where
phenanthroline-stabilized palladium nanoparticles are dis-
persed in a PEG matrix for the selective hydrogenation of
olefins using molecular hydrogen. There have been numer-
ous studies on the synthesis of metal nanoparticles in PEG
(polyol method) where PEG functions as a reducing agent
[24–27]. However, in this study, PEG is used not only as a
reducing agent but also as a dispersing medium for the ligand-
stabilized metal nanoparticles. The catalyst is easy to prepare,
easy to recover and reusable. Hydrogenation of olefins con-
tinues to be of great interest among catalytic processes due
to the increasing industrial demand for low-aromatic diesel
fuels and in the synthesis of intermediates for several phar-
maceuticals and chemicals [28–30].
Hydrogenation of olefins was conducted in liquid phase
in a 25 mL round-bottomed flask equipped with a reflux con-
denser and a magnetic stirrer. In a typical reaction procedure,
10 mmol of the substrate was added to the above catalyst
mixture and was purged with oxygen. The mixture was then
vigorously stirred in the presence of hydrogen at atmospheric
pressure for the desired time period at the desired tempera-
ture. Afterthereaction, themixturewasextractedwithdiethyl
ether and analyzed by a Hewlett-Packard 6890 Gas Chro-
matograph using a HP-55% phenyl methyl siloxane capillary
column (30 m × 320 ␮m × 0.25 ␮m) and a quadropole mass
filter equipped HP 5973 mass selective detector. All the prod-
ucts are known compounds. Identification of the compounds
was carried out by comparing the retention time of the stan-
dard and also by GC-MS. Quantification of the products was
obtained by a peak area ratio method.
In the catalyst recycling experiments, the mixture after
the reaction was cooled to room temperature and the organic
products were extracted into diethyl ether and removed by
simple decantation. The ether-immiscible PEG layer contain-
ing the metal nanoparticle catalyst was recovered followed
by warming up in a rotavapor to remove any organics. The
catalyst thus recovered was used for the next reaction cycle.
3. Results and discussion
Phenanthroline-stabilizedpalladiumnanoparticlesinPEG
are synthesized using a procedure similar to that reported by
Han and co-workers for preparing the same system in ionic
liquid [21]. The conversion of cyclohexene hydrogenation
using the above catalyst system under different conditions is
shown in Table 1. It is apparent that phenanthroline-stabilized
palladium nanoparticles are highly active for the hydrogena-
tion of cyclohexene using molecular hydrogen under mild
conditions. Cyclohexene conversion of up to 84% is obtained
at 35 ◦C (entry 3) by using palladium acetate and phenanthro-
line in PEG. Studies on the effect of temperature show that in-
creasing temperature increases the conversion (entries 4–6),
however, tends to precipitate out palladium from the colloidal
state over 70 ◦C. The effects of varying the amounts of pal-
ladium, phenanthroline and PEG are also given. Of the three
palladium precursors studied, using the palladium acetate has
resulted in the most effective catalyst for the hydrogenation
of cyclohexene. The molecular weight of PEG does not seem
to be very influential in determining the catalyst activity (not
shown in Table 1). However, the higher the molecular weight
of PEG, the greater is the boiling point (lower vapor pres-
sure), which reduces the evaporation loss and also facilitates
its separation after the reaction for recycling. Other metal-
lic systems are also tested for the reaction (entries 14–17);
2. Experimental
2.1. Catalyst preparation
Phenanthroline-stabilizedpalladiumnanoparticlesinPEG
was synthesized by the following procedure. Approximately
5 mg palladium acetate was mixed with 1.5 g of 1,10-
phenanthroline in 4 g PEG in a 25 mL round-bottomed flask.
The mixture is stirred under an atmosphere of hydrogen
at room temperature for about 15 min when the palladium
nanoparticles are formed indicated by a dark color of the
mixture. Polyvinylpyrrolidone(PVP)and1-hexadecylaniline
(HAD) stabilized palladium nanoparticles in PEG are also
prepared in a similar fashion.
2.2. Catalyst characterization
The synthesized catalyst was characterized by UV–vis
spectroscopy using a HP UV-vis spectrophotometer (Model

U.R. Pillai, E. Sahle-Demessie / Journal of Molecular Catalysis A: Chemical 222 (2004) 153–158
155
Table 1
Phenanthroline-stabilized metal nanoparticles in PEG for the hydrogenation of cyclohexene using molecular hydrogena
Entry
Metal Salt
Pd acetate (mg)
Phenan-throline (mg)
Solvent (g)
Temperature (◦C)
Conversion (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
a
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd acetate
Pd chloride
Pd acetyl acetonate
Cu acetate
Ni acetate
Co acetate
Zn acetate
2
5
1
4
4
4
4
4
4
1
2
3
6
4
4
4
4
4
4
4
35
35
35
35
50
70
35
35
35
35
35
35
35
35
35
35
35
10
74
84
53
45
59
20
31
50
72
36
54
10
46
30
35
10
1.5
2.5
5
5
5
1.5
1.5
1.5
1.5
0
1.5
1.5
3
10
10
10
10
5
5
5
5
5
5
5
10
10
10
10
3
3
3
Reaction conditions: 10 mmol cyclohexene, stir, H2 balloon, 4 h.
however, under the reaction conditions used, catalysts pre-
pared with palladium are found to be the most active for the
reaction.
as cinnamyl alcohol (entries 22 and 23) and citral (entries
24 and 25) form the C C hydrogenated product selectively
without affecting the C O group. Only one C C bond is
selectively hydrogenated at short reaction times. However,
the selectivity of the di-hydrogenated product increases with
reaction time (20 h, entry 25). The selectivity of the catalyst
system is ascribed to the nanoparticle size and the nature of
the metal. The calculated turn over numbers (TON, the num-
ber of moles of product formed per mol of palladium) for this
catalyst system are relatively higher than some of the other
systems.
The catalyst system is also examined for its recy-
clability and the results are shown in Table 3. The
phenanthroline–stabilizedpalladiumnanoparticles–PEGcat-
alyst system is easily separable from the reaction mixture
by extraction of the products with diethyl ether and can be
reused several times without any loss of activity or selectiv-
ity. The activity of the non-stabilized Pd particles in PEG
is much lower than the phenanthroline-stabilized Pd in PEG.
The phenanthroline ligand stabilizes the nanoparticles of pal-
ladium by preventing their agglomeration during the reaction.
This is verified by comparing the activity and recyclabil-
ity of phenanthroline-stabilized palladium and non-stabilized
The application of the catalyst system is further verified
by extending the reaction to a series of aliphatic, alicyclic and
aromatic olefins as illustrated in Table 2. The catalyst system
is found to be active and selective for the hydrogenation of
a variety of olefins with good to excellent conversion under
mild reaction conditions. For example, acyclic, aliphatic, and
aromatic olefins are easily converted to their corresponding
alkanes within 8 h reaction period at 30–50 ◦C. Hydrogena-
tion of 1,5-cyclooctadiene affords the mono hydrogenated
product selectively (entries 15 and 16) at 30–40 ◦C and
8 h reaction time. The selectivity of the di-hydrogenated
product increases with increase in the reaction time (entry
16). Similarly, hydrogenation of unsaturated ketones, such
Fig. 1. Comparison of conversion and reusability of phenanthroline–
Pd–PEG, polyvinyl pyrrolidone–Pd–PEG and hexadecylaniline–Pd–PEG
system for cyclohexene hydrogenation using hydrogen at 35 ◦C (reac-
tion conditions: 10 mmol substrate, 5 mg Pd acetate, 1.5–3.0 mg 1,10-
phenanthroline/PVP/HDA, 4 g PEG (400), stir, 4 h, H2 balloon).
Fig. 2. UV–vis absorption spectra of phenanthroline-stabilized palladium
nanoparticles in polyethylene glycol.

156
U.R. Pillai, E. Sahle-Demessie / Journal of Molecular Catalysis A: Chemical 222 (2004) 153–158
Table 2
Phenanthroline-stabilized palladium nanoparticles in PEG for the hydrogenation of olefins using molecular hydrogena
Entry
1
Substrate (g)
Cyclopentene
Product
Temp (◦C)
Time (h)
Conversion (%)
Selectivity (%)
TON
Cyclopentane
30
8
20
8
20
4
8
4
8
8
20
8
20
8
20
8
20
8
20
4
8
8
20
8
20
8
20
4
8
8
20
8
20
8
20
4
8
20
4
4
8
20
4
8
8
20
8
20
8
50
100
44
100
61
100
74
100
5
18
22
54
13
38
89
100
64
100
69
100
38
78
65
100
24
50
46
100
32
54
83
100
61
100
100
43
100
100
100
100
100
100
100
100
100
100
100
100
62b
87b
95b
94b
100
100
100
100
100
100
100
100
100
100
100
100
62c
70c
89c
38c
100
100
100
100
100
100
100
100
100
97d
94d
100
100
82e
15e
100
100
100
100
100
242
448
197
448
274
448
332
448
22
81
99
242
58
170
399
448
287
448
309
448
170
350
291
448
108
224
206
448
143
242
372
448
274
448
448
193
404
448
448
161
448
318
448
54
2
3
1-Hexene
n-Hexane
30
50
30
40
50
40
50
40
50
40
50
40
50
30
40
30
1-Hexene
n-Hexane
4
Cyclohexene
Cyclohexane
Methyl cyclohexane
Methyl cyclohexane
Phenyl cyclohexane
Phenyl cyclohexane
n-Octane
5
1-Methyl cyclohexene
1-Methyl cyclohexene
1-Phenyl cyclohexene
1-Phenyl cyclohexene
1-Octene
6
7
8
9
10
11
12
13
14
15
16
17
1-Octene
n-Octane
2-Octene
n-Octane
2-Octene
n-Octane
Cyclooctene
Cyclooctane
Cyclooctane
Cyclooctene
Cyclooctene
Ethyl benzene
Cyclooctene
1,5-Cyclooctadiene
1,5-Cyclooctadiene
Styrene
18
19
Styrene
Stilbene
Ethyl benzene
Dibenzyl
50
40
90
20
21
22
Stilbene
Norbornylene
Cinnamyl alcohol
Dibenzyl
Norborane
3-Phenyl propanol
50
40
40
100
100
36
100
71
100
12
23
85
100
92
94
94
23
24
25
Cinnamyl alcohol
Citral
3-Phenyl propanol
Citronellal
50
40
50
103
381
448
413
422
422
229
448
Citral
Citronellal
26
27
28
29
3-Methyl-2-butenal
Mesityl oxide
Cyclohexenone
3-Methyl butenal
4-Methyl-2-pentanone
Cyclohexanone
40
40
40
40
8
8
8
20
3-Methyl cyclohexene-1-ol
3-Methyl cyclohexanol
51
100
aReaction conditions: 10 mmol substrate, 5 mg Pd acetate, 1.5–3.0 mg 1,10-phenanthroline, 4 g PEG (400), stir, H2 balloon. bremaining biphenyl, cremaining
cyclooctane, dremaining cinnamaldehyde, eremaining hydrocitronellal, TON: turn over number, calculated as the number of moles of product formed per mol
of palladium.
palladium in PEG. Pd–PEG without the ligand showed a
much lower initial activity (Table 3). However, the recycla-
bility was not affected due to the absence of the ligand even
though the activity was much lower than the ligand-stabilized
palladium in PEG. Control experiments in the absence of pal-
ladium did not show any activity. These results suggest that
phenanthroline can stabilize the palladium nanoparticles in
PEG which can act as a mobile supporting phase thereby
achieving high stability and high activity for the catalyst
system.

U.R. Pillai, E. Sahle-Demessie / Journal of Molecular Catalysis A: Chemical 222 (2004) 153–158
157
Fig. 3. TEM picture of (a) phenanthroline-stabilized palladium nanoparticles in PEG and (b) palladium particles in PEG with no phenanthroline (200 kV,
Philips 320 CM).
The phenanthroline–Pd–PEG catalyst system has also
been compared with other active ligand-stabilized palla-
dium nanoparticles reported earlier for hydrogenation viz.,
polyvinyl pyrrolidone and 1-hexadecylaniline-stabilized pal-
ladium nanoparticles dispersed in PEG as shown in Fig. 1
[6,8]. The results reveal that the phenanthroline–Pd–PEG
has comparable hydrogenation activity with that of the
other two systems (polyvinylpyrrolidone–Pd–PEG and 1-
hexadecylaniline–Pd–PEG) under identical reaction condi-
tions. An increase in the activity is observed for the second
run when compared to the first run which may be ascribed to
the palladium precatalyst not being fully converted to nan-
oclusters in the first run. Similar observation has been made
earlier [1].
continua extending throughout the visible-near-ultraviolet re-
gion. This suggests the formation of palladium colloid hav-
ing particle size less than 10 nm. A TEM image (Philips
CM 20) of the freshly prepared sample after reduction for
15 min shows the formation of well-defined, discrete palla-
dium nanoparticles of 2–6 nm size (Fig. 3a). On the other
hand, the particle sizes were in 500–1000 nm range in the
case of palladium particles dispersed in PEG in the absence
of phenanthroline (Fig. 3b).
The reduction of palladium acetate in polyethylene glycol
in the presence of phenanthroline ligand may proceed in two
steps: (i) ligand exchange around Pd2+ and (ii) reduction of
Pd2+ to Pd metal. PEG can also act as a reducing agent similar
to ethylene glycol in the polyol method.
The phenanthroline-stabilized palladium nanoparticles
in PEG has been characterized by both UV–vis spec-
troscopy and by TEM analysis. The UV–vis spectrum of
phenanthroline-stabilized palladium shows absorption max-
imum at around 265 nm, which is characteristic of nanopar-
ticles (Fig. 2). The peak at 275 nm also appeared after the
reduction, which may be due to the formation of some inter-
mediate species formed during the reduction of Pd2+. The
UV–vis spectrum of gray colloid showed broad absorption
4. Conclusions
In summary, this study demonstrates that phenanthroline-
stabilized palladium nanoclusters in PEG is a stable, active
and recyclable catalyst for the hydrogenation of olefins us-
ing molecular hydrogen under mild reaction conditions. The
phenanthroline–Pd–PEG catalyst system has comparable hy-
drogenation activity like other reported ligand-stabilized
metal nanoparticles such as polyvinyl pyrrolidone and 1-
hexadecylaniline-stabilized metal nanoparticles.
Table 3
Recyclability of phenanthroline-stabilized Pd nanoparticles in PEG for the
hydrogenation of cyclohexenea
Cycle number
Conversion in
phenanthroline–Pd–PEG (%)
Conversion in
Pd–PEGb (%)
1
2
3
4
5
6
a
84
100
100
100
100
100
54
49
43
46
42
43
Acknowledgements
URP is a postgraduate research participant at the National
Risk Management Research Laboratory administered by the
Oak Ridge Institute for Science and Education through an in-
teragency agreement between the US Department of Energy
and the US Environmental Protection Agency.
Reaction conditions same as that given for Table 2.
Pd acetate in PEG (no phenanthroline).
b

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[16] M.-K. Kim, Y.-M. Jeon, W.S. Jeon, H.-J. Kim, S.G. Hong, C.G.
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