Green Synthesis of Rhodium Nanoparticles that are Catalytically Active in Benzene Hydrogenation and 1-Hexene Hydroformylation

Article abstract of DOI:10.1002/cctc.201701644

Rhodium nanoparticles (Rh NPs) were prepared according to a green method based on the reduction of (acetylacetonato)dicarbonylrhodium(I), Rh(acac)(CO)₂, in water at 80 °C. The nanoparticles, which were obtained without the addition of a reducing agent, were stabilized by polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA) polymers and characterized by TEM (transmission electron microscopy), XPS (X-ray photoelectron spectroscopy), and XRD (X-ray powder diffraction) methods. The excellent catalytic activity of these Rh NPs was evidenced in the hydrogenation of benzene to cyclohexane. In the presence of PPh₃, Rh NPs formed a highly active system in the hydroformylation of 1-hexene. In this system, they acted as a source of soluble rhodium species. Rh NPs were also synthesized in water using rhodium(II) acetate, Rh₂(OAc)₄, and rhodium(III) chloride, RhCl₃, as rhodium sources, and their catalytic activity was compared with that of the rhodium precursors.

Full text of DOI:10.1002/cctc.201701644

Accepted Article
Title: Green synthesis of rhodium nanoparticles, catalytically active in
benzene hydrogenation and 1-hexene hydroformylation
Authors: Waleed Alsalahi, Wlodzimierz Tylus, and Anna Trzeciak
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Green synthesis of rhodium nanoparticles, catalytically active in benzene hydrogenation
and 1-hexene hydroformylation
W. Alsalahia, W. Tylusb, A.M. Trzeciaka
a University of Wrocław, Faculty of Chemistry, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
b Department of Advanced Material Technologies, Faculty of Chemistry, Wrocław University of Science
and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wroclaw, Poland
Abstract
Rhodium nanoparticles (Rh NPs) were prepared according to a novel green methodology based on
the reduction of (acetylacetonato)dicarbonylrhodium(I), Rh(acac)(CO)2, in water at 80 °C. The
nanoparticles, obtained without the addition of a reducing agent, were stabilized by
polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA) polymers and characterized by TEM
(transmission electron microscopy), XPS (X-ray photoelectron spectroscopy), and XRD (X-ray
powder diffraction) methods. The excellent catalytic activity of these Rh NPs was evidenced in
the hydrogenation of benzene to cyclohexane. In the presence of PPh3, Rh NPs formed a highly
active system in the hydroformylation of 1-hexene. In this system, they acted as a source of soluble
rhodium species.
Rh NPs were also synthesized in water using rhodium(II) acetate, Rh2(OAc)4, and rhodium(III)
chloride, RhCl3, as rhodium sources, and their catalytic activity was compared with that of the
rhodium precursors.
Key words: rhodium; nanoparticles; hydrogenation, hydroformylation;
1. Introduction
Nanocatalysis is a rapidly growing field which involves the use of transition metal nanoparticles
(NPs) as catalysts for a diversity of organic and inorganic reactions. Transition metal nanoparticles
have been synthesized, according to the literature, using five general synthetic methods: (I)
chemical reduction of transition metal salts, (II) ligand reduction and displacement from
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organometallics, (III) thermal, photochemical, or sonochemical decomposition, (IV)
electrochemical reduction, and (V) metal vapor synthesis. The most widely used method of NP
preparation is the reduction of transition metal salts in the presence of stabilizing agents such as
polymers or surfactants preventing NP agglomeration and crystal growth.[1–8] Concerning
polymers, polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA) are commercially available,
relatively cheap, nonpoisonous, water-soluble, and very effective stabilizing agents for NPs.[9–10]
While PVP was used very often for the stabilization of NPs, the application of PVA was only
scarcely reported.[10–12]
A wide range of reducing agents have been used in NP preparation, such as hydrogen or carbon
monoxide, hydrides or salts (i.e. sodium borohydride or sodium citrate), or even oxidizable
solvents, such as alcohols or glycols.[4, 13–16]
Rhodium is one of the rarest and the most expensive transition metals. Nevertheless, this noble
metal exhibits extraordinary and often unique catalytic properties in comparison with other noble
metals, in particular in hydrogenation, hydroformylation, and oxidation reactions. Rh NPs are
located at the boundary between homogeneous and heterogeneous catalysts.[1, 16–17, 24] Their
catalytic activity and selectivity depend on the shape, size, and composition of the stabilizing
agents and ligands.[4]
Hirai et al. reported the use of water-and-alcohol mixtures as reducing agents in the preparation
of Rh, Pt, Pd, Os, or Ir NPs.[18–24] Interestingly, Hirai observed that Rh NPs were not obtained in
anhydrous methanol or in the absence of PVA. In contrast, RhCl3 was reduced to Rh NPs after the
addition of water to methanol. The presence of water during synthesis was essential for both metal
reduction and stabilizing polymer dissolution.[18] Rh NPs have also been prepared using ethylene
glycol[25] and vanadocene (Cp2V) as reducing agents.[26–27] All these NPs have been stabilized by
extensively used organic polymers, PVP or PVA.
Interestingly, the size of Rh NPs depends on the amount of water used in the synthesis, the
number of smaller NPs increases when the water content increases.[28–29]
The hydrogenation of benzene to cyclohexane is one of the most important and intensively
investigated reactions, in both laboratory and industrial synthetic chemistry. Cyclohexane is an
important and essential chemical in the manufacturing of caprolactam,-caprolactone,
polyamides, polyesters, nylon-6, and nylon-66. Millions of tons of cyclohexane are produced
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annually by means of benzene hydrogenation,[30–32] and several catalytic systems based on Rh NPs
have been reported for this reaction.[33–39]
There are a few papers dealing with the application of Rh NPs in the hydroformylation of
olefins. In 1980, Smith used Rh NPs stabilized with a styrene/butadiene copolymer in the
hydroformylation of 1-hexene at 100 °C and 42 bar of syngas and obtained 95% conversion to
aldehydes with an n/iso ratio of 2.[40] Liu et al. reported the hydroformylation of propylene
catalyzed by a Rh/PVP colloid in a biphasic aqueous system.[41] Bruss et al. obtained an n/iso ratio
of 25 using 5 nm Rh NPs with the addition of Xantphos. They also observed Rh-carbonyl
catalytically active species formed during the reaction.[42] Rh NPs stabilized by poly(ethylene
imine)/amide, employed in the hydroformylation of 1-hexene, gave 96% selectivity toward
aldehydes and a 1.4 n/iso ratio. The formation of catalytically active complexes of the type
[HRh(CO)nLm] was proposed in this reaction.[43] Rh NPs stabilized with diphosphite ligands
catalyzed the hydroformylation of styrene forming soluble active species.[44] Han et al. reported
the use of Rh NPs stabilized with (R)-BINAP ligands in the asymmetric hydroformylation of
olefins, with high regioselectivity toward iso-aldehyde.[45] Behr et al. described Rh NPs stabilized
in a thermomorphic solvent as a catalyst for 1-dodecene hydroformylation with an n/iso ratio of
2.6, the n/iso ratio reaching 24 after the addition of a biphephos ligand.[46] Rh NPs stabilized in an
ionic liquid catalyzed the hydroformylation of 1-octene in a biphasic system, with 91% selectivity
toward aldehydes, with a 1.4 n/iso ratio.[47] Rh NPs immobilized in ZIF-8 exhibited high catalytic
activity for the hydroformylation of alkenes and were easily separated and recycled five times
without any remarkable loss in activity.[48] High regioselectivity toward iso-aldehyde was reported
for the hydroformylation of vinyl acetate with Rh NPs stabilized in TiO2 nanotubes.[49] The
formation of soluble active species during hydroformylation was reported for Rh NPs stabilized
by tetraoctylammonium bromide.[50]
We have previously reported excellent chemoselectivity to aldehydes at a very high activity of
the Rh catalyst in hydroformylation performed under water and solventless conditions.[51–53]
In this paper, we present a new, simple and green, methodology of Rh NP synthesis by the
reduction of rhodium species, Rh(acac)(CO)2, Rh2(OAc)4, and RhCl3, in a water medium, in the
absence or in the presence of PVP or PVA polymers as stabilizing agents. The catalytic activity of
the synthesized Rh NPs was successfully confirmed for the hydrogenation of benzene and the
hydroformylation of 1-hexene in water.
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2. Results and discussion
2.1. Synthesis of RhNPs
The unexpected formation of Rh NPs in reaction with water can be described by equations (1)–
(4). Thus, water reacts with the carbonyl group according to the water gas shift scheme to produce
carbon dioxide and hydrogen. Next, hydrogen acts as a reducing agent donating its electrons to
Rh(I), which allows rhodium ions to be reduced to Rh NPs.
To the best of our knowledge, the efficient reduction of Rh(I) to Rh(0) by water has not been
reported before. However, the important role of water in the reduction process was underlined by
Hirai et al.[26] They found that anhydrous alcohols were not efficient in the reduction of rhodium,
whereas the presence of water enabled the successful synthesis of Rh NPs. This is in full agreement
with our observations.
2.2. IR studies
The IR spectrum of the product obtained after heating Rh(acac)(CO)2 in a water medium without
a polymer showed only a base line without any peaks characteristic for CO groups or for
coordinated acac ligands (Fig. S1.). Consequently, the removal of those ligands from rhodium was
evidenced. Similarly, the IR spectrum of the rhodium product obtained in the presence of PVP
(Fig.S2-9) or PVA (Fig.S10-11) showed only peaks originating from the polymer.
2.3. TEM studies
The morphology and nanoparticle size distribution were investigated by TEM analyses. Figures
1a–e show typical TEM images of freshly prepared Rh NPs synthesized by the reduction of
Rh(acac)(CO)2 in a water medium with different stabilizing agents or without them. They
displayed uniform NP size distribution without any aggregation. The mean size of Rh NPs
supported on polymers PVP-K30, PVP-K15, and PVA was 2–2.5 nm, while slightly bigger Rh
NPs, 2.5–3 nm in diameter, were obtained in the absence of any stabilizing agent as shown in the
histogram plot.
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Similar TEM micrographs were also obtained for the product prepared from Rh2(OAc)4 as the
rhodium precursor (Fig. 1 f). However quite different results were observed by using RhCl3·3H2O.
In this case the TEM micrograph showed the significant aggregation of Rh NPs (Fig. 1 g).
a)
5 nm
b)
5 nm
c)
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10 nm
d)
20 nm
e)
10 nm
f)
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10 nm
g)
20 nm
Fig. 1. Morphology and NP size distribution histograms of Rh NPs synthesized by the
reduction of Rh(acac)(CO)2 in a water medium at 80 °C: a. PVP-K30; b. PVP-K30 at
room temperature; c. PVP-K15; d. PVA; e. without any stabilizing agent; f. Rh2(OAc)4 as
a precursor with PVP-K30; g. RhCl3·3H2O as a precursor with PVP-K30; More TEM
images at low and high magnification and EDS spectra are in supplementary material
(Fig. S18-24)
3.4. XRD studies
XRD measurements were performed for different time of XRD analysis with grinding and without
grinding the sample (Fig. S12-17). Fig. 2. Shows the X-ray diffractograms of Rh NPs prepared by
the reduction of Rh(acac)(CO)2 in water showed a barely recognizable small peak at 2θ = 41.1°
corresponding to the Rh(111) plane. No peaks were observed at 47.8°, 69.9°, and 84.5°
corresponding to the (200), (220), and (311) planes, due to the small size of the nanoparticles (2
nm).[56–57]
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300
200
100
0
b
a
30
40
50
60
70
80
90
100
2 theta [degrees]
Fig. 2 X-ray diffraction pattern of rhodium nanoparticles synthesized by reducing
Rh(acac)(CO)2 as a precursor in a water medium: (a) without any stabilizing agent; (b) with
PVP-K30.
3.5. XPS analysis
The Rh NPs have been analyzed by XPS to investigate the oxidation state of rhodium. The XPS
spectrum showed two prominent bands at low BE values, 307 and 311.74 eV, typical for Rh(0),
Rh(3d5/2) and Rh(3d3/2) , respectively. Two additional bands at higher energy, 308.74 and 313.5
eV, are consistent with Rh2O3 and assigned to Rh(III) 3d5/2 and Rh(III) 3d3/2, respectively.[58–59]
Figure 3 presents the XPS spectrum of Rh/PVP as received (a) and after sputtering with argon ions
(b). The spectrum shows an increase in the intensity of two bands of Rh(0) and a decrease in the
intensity of Rh(III) bands after the surface was cleaned with argon.
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Fig.3 XPS spectra for Rh/PVP-K30; a) as received and b) after Ar+ cleaning (120s, 1.5 keV, 2 /cm2)
The sample of Rh NPs prepared without a stabilizing agent contained more of Rh(0), 71.8%.
After sputtering with argon ions, it demonstrated only bands assigned to Rh(0) (Fig. 4 ).
Table 1 collects XPS data for the analyzed samples obtained from different rhodium precursors.
As expected, when the same samples were analyzed after 2.3 months, the amount of oxidized Rh
species increased, whereas the Rh(0) content decreased. Also, two additional bands were observed,
at 309.75 and 314.49 eV, which can be attributed to RhO2 and correspond to Rh(IV) 3d5/2 and
Rh(IV) 3d3/2, respectively, (Table S1 and Fig. S25-27).[58–59] The rhodium oxides (Rh2O3 and
RhO2) could be formed during sample preparation and/or on the surface of rhodium nanoparticles
upon exposure to air.[60–61]
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Fig.4 XPS spectra for Rh NPs without stabilizing agent (polymer); a) as received and b) after Ar+
cleaning (120s, 1.5 keV, 2 /cm2)
Table 1. XPS data of rhodium samples prepared by reduction of different rhodium precursors in water.
Rh0
Rh2O3
Rh+2
Precursor
Stabilizing agent
A%
17.8
71.8
0
B%
79.3
100
A%
82.2
28.2
0
B%
20.7
0
A%
0
B%
0
PVP
Without
PVP
Rh(acac)(CO)2
0
0
87.3
54.9
43.5
0
100
100
100
12.7
45.1
56.5
Rh2(OAc)4
RhCl3
Without
PVP
0
0
0
0
0
0
A - as received; B – after sputtering with argon ion
In Table 2 there are XPS data for different rhodium species collected in this work and found in
the literature.
Table 2. Summary of XPS (Rh 3d5/2) data for different rhodium species.
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Binding energy
3d5/2 (eV)
307
Sample
Ref.
This work, [58, 59, 61]
307.25
[62]
[63]
Rh0
Rh metal
307.3
Rh(acac)(CO)2
307.8
307.9
309.61
309.75
309.02
310
[64]
Rh(CO)2/γ-Al2O3
Rh+
Rh2+
Rh3+
Rh4+
+
Rh(CO)2 / Al2O3–ZrO2
[65]
Rh(CO)P+/ Al2O3–ZrO2
Rh2(OAc)4
This work,
[66-68]
RhCl3
310.3
308.7
308.5
309.75
This work, [58, 59]
[63]
Rh2O3
RhO2
This work, [58, 59]
3.6. Hydrogenation of benzene
The catalytic activity of as-prepared Rh NPs was tested in the hydrogenation of benzene in water
under relatively mild conditions: 80 °C and 20 bar of H2. In all experiments, cyclohexane was
formed as the only product.
Scheme 1. Hydrogenation of benzene
The obtained results illustrating the activity of different Rh NPs are summarized in Table 3. It
was found that 19% of cyclohexane was formed when the reaction was carried out with Rh/PVP-
K30 at 80 °C using 1.5 mL of benzene and 1.5 mL of water under 15 bar of H2 (Table 3, entry 1).
The yield of cyclohexane increased to 64% when the H2 pressure increased to 20 bar (Table 3,
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entry 2). An excellent conversion of benzene to cyclohexane (91%) was obtained when smaller
amounts of benzene and water (1 mL of each) were used with the same catalyst (Table 3, entry 3),
However, the activity of catalyst decreased by carrying out the same experiment after 2 months
under the same condition, only 61 % of cyclohexane where obtained (Table 3. Entry 4) due to
oxidation of catalyst. Increasing the concentration of catalyst, resulted in excellent conversion 95%
(Table 3. Entry 5). Lower activity was noted when using Rh/PVP-K30 synthesized at room
temperature as a catalyst. In that case, 64% of cyclohexane was formed (Table 3, entry 6). An
excellent result, 100% yield of cyclohexane, was also obtained when Rh NPs immobilized on PVP-
K15 was applied (Table 3, entry 7). However, when PVA was used as a support for Rh NPs, only
47% of cyclohexene was formed (Table 3, entry 8).
Good results were also obtained with Rh NPs used without any stabilizing agent. The yield of
cyclohexane was 55 and 78% under solventless and under on-water conditions, respectively (Table
3, entries 9 and 10).
The next two catalysts, prepared from Rh2(OAc)4 and RhCl3·3H2O and stabilized by PVP-K30,
gave similarly good result, 60 and 88% of cyclohexane, respectively (Table 3, entries 11 and 12).
In that case, it was interesting to compare whether pre-made NPs exhibit similar activity to those
formed in situ from the same precursors. Interestingly, the application of RhCl3·3H2O without any
pre-treatment resulted in a higher yield of cyclohexane, namely 100% (Table 3, entry 14). It also
catalyzed hydrogenation under solventless conditions; however, the obtained conversions were
lower (Table 3, entry 13). The in situ reduction of RhCl3·3H2O to Rh NPs was confirmed by the
TEM analysis of the black suspension formed in the water phase during the hydrogenation of
benzene. The TEM images demonstrated the presence of small Rh NPs with a symmetric size
distribution, the mean size of NPs being located at 2.5–3 nm (Fig. 6). Bigger agglomerates were
also observed. The organic phase did not contain any nanoparticles.
Table 3. Hydrogenation of benzene catalyzed by Rh NPs and Rh compounds.
Entry Precursor for Rh NPs synthesis
Polymer
Cyclohexane% TOFh, h-1
1
19a
65b
95
Rh(acac)(CO)2
2
PVP-K30
325
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3
4
91
61c
455
305
238
320
216
93
5
95d
6
64e
7
PVP-K15
65 (1 run)
56 (2 run)
35 (3 run)
38 (4 run)
47f
7a
7b
7c
8
58
63
PVA
470
275
390
300
440
225
500
9
-
55g
10
11
12
13
14
-
78
Rh2(OAc)4
PVP-K30
60
PVP-K30
88
RhCl3.3H2O
-
-
45g
100
Reaction conditions: benzene (1 mL), water (1 mL), [benzene]/[Rh] = 2000, t = 4 h, T = 80 °C, P = 20
bar; a P = 15 bar; a and b benzene (1.5 mL), water (1.5 mL); c catalyst was stored in air for 2 months before
f
using; d [benzene]/[Rh]= 1000, e catalyst have been synthesized at r.t., [benzene]/[Rh]= 4000; g
reaction carried out under solventless conditions and h TOF= (mole of cyclohexane)/(mole of catalyst x
reaction time)
a
b
20 nm
50 nm
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c
Fig 6. Morphology and nanoparticle size distributions of Rh NPs formed in situ during
hydrogenation reaction of benzene in water medium catalyzed by RhCl3. H2O. a) non-aggregated
nanoparticles, b) aggregated nanoparticles and c) histogram of Rh NPs size distribution.
Next, Rh NPs were tested in the hydrogenation of different arene derivatives in a water medium
and under solventless conditions at 80 °C and under 20 bar hydrogen pressure. Rh NPs stabilized
with PVP-K15 exhibited good activity in the hydrogenation of toluene, forming 70% of
methylcyclohexane. In contrast, Rh/PVP-K30 achieved 30% yield (Table 4, entries 1 and 2,
respectively). Unfortunately, Rh/PVP-K15 was inactive on hydrogenation of iodobenzene in
water. Hydrogenation of nitrobenzene catalyzed by Rh/PVP-K30 in water and under solventless
conditions resulted in its complete conversion to aniline (93.6% and 95.5%) and to
cyclohexylamine (6.4 and 4.6%) (Table 4, entries 3 and 4).
Table 4. Hydrogenation of different substrates catalyzed by Rh NPs.
Entry Substrate
1
Catalyst
Solvent
Yield% TOFa, h-1
Products
70
Rh/PVP-K15
350
Water
2
3
Rh/PVP-K30
Rh/PVP-K30
30
150
Water
6.4/93.6
468b
4
Solventless 4.5/95.5
477.5b
Reaction conditions: substrate (1 mL), water (1 mL), [substrate]/[Rh] = 2000, t = 4 h, T = 80 °C, P = 20
bar, a TOF= (mole of product)/(mole of catalyst x reaction time) and b TOF for aniline product.
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3.7. Hydroformylation of 1-hexene
The hydroformylation of 1-hexene under on-water conditions was investigated by applying Rh
NPs with a 13-fold excess of hydrophobic phosphine, PPh3. In all experiments, aldehydes were
the main products with a small amount of 2-hexene, the isomerization product (ca. 2–7%).
Hydrogenation did not occur, and, consequently, hexane was not found. It was unexpected since
Rh NPs are very active catalysts for the hydrogenation of alkenes and aromates. Clearly, CO
efficiently inhibited hydrogenation.
Scheme 2. Hydroformylation of 1-hexene
First, we studied the reaction using Rh/PVP-K30 alone; however, no conversion of 1-hexene
was observed (Table 5, entry 1). However, already a 6-fold excess of PPh3 was sufficient to achieve
79% conversion to corresponding aldehydes with an n/iso ratio of 3.2 (Table 5, entry 2). An
increase in the amount of PPh3 to a 13-fold excess resulted in a high conversion of 1-hexene (96%)
with 90% selectivity to aldehydes (75% of 1-heptanal and 15% of 2-methyl-hexanal) (Table 5,
entry 3). Thus, an excess of PPh3 caused a remarkable increase in the conversion of 1- hexene and
an increase in the n/iso ratio from 3.2 to 5. This is in agreement with the literature data: that a high
yield of linear aldehydes increases with higher phosphine excess.[69] The same excellent results
were obtained when other Rh NPs synthesized from Rh(acac)(CO)2 and stabilized by PVP-K15
and PVA were applied (Table 5, entries 5 and 6). Rhodium nanoparticles used without a stabilizing
agent gave 83% and 94% conversion on water and under solventless conditions, respectively
(Table 5, entries 7 and 8). Very good results were obtained when applying Rh/PVP-K30
synthesized from Rh2(OAc)4, with the n/iso ratio of 5.1 at 88% conversion (Table 5, entry 9). In
contrast, the catalyst obtained by the reduction of the RhCl3·3H2O complex, Rh/PVP-K30, was
not active in the hydroformylation of 1-hexene on water, probably due to the aggregation of
nanoparticles (Table 5, entry 10).
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Table 5. Hydroformylation of 1-hexene catalyzed by rhodium nanoparticles.
Precursor for
Rh NPs
[PPh3]/[Rh]
2-
hexene
%
Conv.
%
Aldehydes
%
TOFe,
n/iso
h-1
Entry
Polymer
synthesis
1
2
0
6
0
0
79
96
96
88
96
83
94
84
0
79
90
89
83
92
74
91
79
3.2 197.5
PVP-K30
3
13
13
13
13
13
13
19.6
13
6
7
5
4
9
3
5
5
225
320
220
608
4a
5
5.4
5
Rh(acac)(CO)2
PVP-K15
6b
7
PVA
3.4
-
3.1 207.5
8c
9d
10
-
3.9
5.1
235
315
0
Rh2(OAc)4
PVP-K30
RhCl3.3H2O
Reaction conditions: 1-hexene (1.5 mL), water (1.5 mL), [1-hexene]/[Rh] = 1000, T = 80 °C, P = 10 bar
(H2/CO=1), t = 4 h, a and b t = 3h, c catalyst prepared at room temperature; b and d [sub]/[Rh] = 1900 and
1500, respectively; c reaction carried out under solventless conditions and e TOF= (mole of
aldehydes)/(mole of catalyst x reaction time).
3.8. Studies of catalysts composition after catalytic reaction
The Rh/PVP catalysts separated after catalytic reactions were analyzed by XPS and TEM methods
to determine their structure. The catalyst Rh/PVP-K30 examined after benzene hydrogenation
contained 87% of Rh(0) and 13% of Rh(III) (Fig. 7a). The Rh/PVP-K30 catalyst isolated from the
hydroformylation reaction mixture contained 92.5% of Rh(0) and 7.5% of Rh(III) (Fig. 7b).
The Rh/PVP-K15 catalyst recovered after four subsequent runs of benzene hydrogenation
was also analyzed. According to TEM micrographs the sample is composed of organic compounds
and small Rh(0) nanocrystallites (Figure 8). The Rh(0) nanoparticles are approximately 2 nm large.
They are arranged in branched chain-like species with an overall length about 20-30 nm.
These experiments confirmed good stability of the studied catalysts under catalytic reaction
conditions.
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Fig.7 XPS spectra for a) Rh/PVP-K30 after benzene hydrogenation b) Rh/PVP-K30 after 1-hexene
hydroformylation
Fig 8. TEM micrographs of the sample of Rh/PVP-K15 after benzene hydrogenation.
4. Conclusions
Rhodium nanoparticles were prepared from Rh(acac)(CO)2 suspended in water, in the presence of
a water-soluble polymer (such as polyvinylpyrrolidone, PVP, or polyvinyl alcohol, PVA) as the
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stabilizing agent at room temperature and at 80 °C. This new methodology is especially attractive,
operationally simple and green, as it is highly effective without any addition of a reducing agent
or an organic solvent. Rhodium nanoparticles were obtained with water as the only reducing agent.
The same method was also applied to get Rh NPs from other rhodium substrates, Rh2(OAc)4
and RhCl3. In the samples isolated after heating in water, Rh(0) was not found by XPS; however,
TEM confirmed the presence of Rh NPs. The sputtering of the surface with argon resulted in the
appearance of XPS bands characteristic of Rh(0).
The diameter of Rh NPs obtained by water reduction was ca. 2 nm, and they were uniformly
distributed in the polymer. Only in the case of RhCl3 was the agglomeration of nanoparticles
observed.
Rh/PVP catalysts display excellent catalytic performance in the hydrogenation reaction of
benzene to cyclohexane under biphasic conditions (benzene/water). Similarly, high activity was
also noted for Rh NPs prepared without a stabilizing agent.
The same catalysts showed very high catalytic activity and high regioselectivity to heptanal
(linear aldehyde) in the hydroformylation of 1-hexene under on-water conditions. In those
reactions, Rh NPs acted as a source of soluble, catalytically active species.
5. Experimental
5.1. Materials
The rhodium complexes Rh(acac)(CO)2 and Rh2(OAc)4 was prepared according to the
55]
literature.[54,
RhCl3. 3H2O was purchased from Riedel-de Haën, polyvinylpyrrolidones
(K15(Mw ~ 10,000) and K30(Mw ~ 40,000)) were purchased from Fluka, polyvinyl alcohol was
purchased from Sigma-Aldrich; triphenylphosphine (PPh3) was purchased from Avocado; 1-
hexene was purchased from Merck; benzene was purchased from Sigma-Aldrich; hydrogen (H2,
99.999%) and carbon monoxide (CO, 99.97%) were procured from Air Products. All chemicals
were used without any additional purification. Distilled water was used as the reaction medium.
5.2. Synthesis Rh NPs in water
A solution of polyvinylpyrrolidone (K-30, Mw ~ 40 000; or K-15, Mw ~ 10 000) or polyvinyl
alcohol (0.48 g) dissolved in water (10 mL) was added to a suspension of Rh(acac)(CO)2 (2.4 ×
10-4 mol) in water (10 mL). The mixture was stirred for 2 h at 80 °C and for 24 h at room
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10.1002/cctc.201701644
ChemCatChem
temperature and filtered. The resulting solution was evaporated to dryness using a rotary vacuum
evaporator yielding a film of polymer-stabilized Rh NPs (Table 6, entries 1–4). Table 6 shows the
colors of the products and the content of rhodium determined by ICP. The same procedure was
used to obtain rhodium nanoparticles from other rhodium species and without a stabilizing agent
(Table 6, entry 5).
Attempts to obtain Rh NPs by the same method, namely heating a rhodium compound in a
water medium, were also performed using Rh2(OAc)4 and RhCl3·3H2O as the rhodium sources
(Table 6, entries 6 and 7).
The product obtained from Rh(acac)(CO)2 in water at room temperature was yellow, whereas
all the other products, prepared at 80 °C, were black. However, the use of Rh2(OAc)4 or
RhCl3·3H2O as rhodium precursors afforded dark blue or dark brown products, respectively. The
content of rhodium in the obtained materials was determined by means of ICP. The rhodium
content was the lowest, 1.9 wt.% Rh, when the reaction was carried out at room temperature, while
it increased to 3.7% when the synthesis was performed at 80 °C, using PVP-K30 as the stabilizing
agent (Table 6, entries 1 and 2). The amount of rhodium in the final material also varied with the
change of the stabilizing polymer and the rhodium precursor (Table 6).
Table 6. Synthesis of rhodium nanoparticles in a water medium.
Particle
Entry
Precursor
Polymer T, oC
t, h
Color
Rh%a
size[nm]b
1
2
3
4
5
6
7
Rt.
PVP-K30
80
24
2
Yellow
1.9
3.7
4.4
3.2
2-2.5
2-2.5
Rh(acac)(CO)2 PVP-K15
80
80
80
80
80
2
2-2.5
Black
PVA
-
2
2-2.5
2
2.5-3
Rh2(OAc)4
PVP-K30
20
2
blue
7.7
3.9
2-2.5
RhCl3.3H2O
brown
aggregation
a Rhodium content determined by ICP; b Average size of rhodium particles determined by counting the
size of approximately 200–300 rhodium nanoparticles from several TEM images.
5.3. Sample preparation for ICP measurements
A 0.02 g sample of Rh/PVP was placed in a 25 mL volumetric flask, and the flask was filled to 25
ml with distilled water and then stirred until the complete dissolution of solids.
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10.1002/cctc.201701644
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5.4. Hydrogenation of benzene
Hydrogenation reactions were carried out in a 50 mL stainless steel autoclave equipped with a
manometer, a thermostat, a magnetic stirrer, and a gas inlet/outlet system. The catalyst (5.5 × 10-6
mol) was introduced in the autoclave under nitrogen atmosphere. 1 mL of benzene (0.011 mol)
and 1 mL of water were placed in the autoclave. The autoclave was closed, filled with hydrogen
(5 bar) three times, and then pressurized with hydrogen to 20 bar and heated to 80 °C for 4 h. After
the reaction was finished, the autoclave was cooled down to room temperature and depressurized.
The organic products were separated by a vacuum transfer procedure and analyzed by means GC
and GC-MS.
5.5. Hydroformylation of 1-hexene
Hydroformylation experiments were carried out in a 50 mL stainless steel autoclave equipped with
a manometer, a thermostat, a magnetic stirrer, and a gas inlet/outlet system. The catalyst (1.2 × 10-
5 mol of Rh) and PPh3 (1.56 × 10-4 mol) were introduced in the reactor under nitrogen atmosphere,
and 1.5 mL (0.012 mol) of 1-hexene and 1.5 mL of water were introduced. The autoclave was
closed, flushed with hydrogen (5 bar) three times, and then pressurized with the synthesis gas
(H2: CO = 1: 1) to 10 bar and heated to 80 °C for 4 h. Afterwards, the autoclave was cooled down
to the ambient temperature and depressurized. The organic products were separated by a vacuum
transfer procedure and analyzed by means GC and GC-MS.
Acknowledgements
XPS studies were financed by a statutory activity subsidy from the Polish Ministry of Science and Higher
Education for the Faculty of Chemistry of Wrocław University of Technology.
Authors thank to prof. L. Kępiński and to dr W. Gil for performance of TEM measurements and to dr A.
Gniewek for technical assistance.
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