Ligand effect in the Rh-NP catalysed partial hydrogenation of substituted arenes

Article abstract of DOI:10.1039/c3cy00388d

The Rh nanoparticles Rh1-Rh4 stabilised by the mono- and bidentate phosphine and phosphite ligands I-IV were synthesised, characterised and applied as catalysts in the partial hydrogenation of substituted arenes. In the case of disubstituted arenes, selectivities for the corresponding cyclohexene derivatives of up to 39% were achieved at ca. 40% conversion. The effect of parameters such as temperature and pressure was also examined. In the hydrogenation of styrene, very high selectivities for ethylbenzene were achieved with TOF values up to ca. 23500 h^-1. All these results show that the catalytic performance of small Rh-NPs can be modulated by the appropriate choice of stabilising agents.

Full text of DOI:10.1039/c3cy00388d

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Ligand effect in the Rh-NP catalysed partial
hydrogenation of substituted arenes†
Cite this: Catal. Sci. Technol., 2013,
3, 2828
´
Jessica Llop Castelbou, Aitor Gual, Elisabet Mercade, Carmen Claver* and
Cyril Godard*
The Rh nanoparticles Rh1–Rh4 stabilised by the mono- and bidentate phosphine and phosphite ligands
I–IV were synthesised, characterised and applied as catalysts in the partial hydrogenation of substituted
arenes. In the case of disubstituted arenes, selectivities for the corresponding cyclohexene derivatives of
up to 39% were achieved at ca. 40% conversion. The effect of parameters such as temperature and
pressure was also examined. In the hydrogenation of styrene, very high selectivities for ethylbenzene
were achieved with TOF values up to ca. 23 500 hÀ1. All these results show that the catalytic performance
of small Rh-NPs can be modulated by the appropriate choice of stabilising agents.
Received 6th June 2013,
Accepted 31st July 2013
DOI: 10.1039/c3cy00388d
www.rsc.org/catalysis
stabilisers for Ru-NPs and their application in the hydrogenation
of aromatics, with interesting ligand effects observed.7
Introduction
Over the last decades, transition metal-nanoparticles (M-NPs)
have received a great deal of attention as catalysts since they
potentially combine the advantages of heterogeneous and
homogeneous catalysts, exhibiting high activities while retain-
ing tunability and selectivity through their well defined com-
position with a narrow size distribution.1–3 In particular,
Ru-NPs have been extensively studied and relevant advances
in both their characterisation and catalytic applications have
been reported.4 The stabilisation of M-NPs can be realised in
the presence of polymers, surfactants or ligands, which allows
the control of their size, shape and dispersion as well as their
surface state. The choice of an appropriate stabiliser for the
M-NPs is thus of critical importance in tailoring their catalytic
performance. For instance, polymer stabilised Rh and Ru-nano-
particles were shown to be highly active in the hydrogenation of
arenes, and produce exclusively cyclohexanes.5 Interestingly,
Dyson and co-workers recently showed that the addition of
phosphines to Rh-NPs of ca. 3 nm in diameter previously synthe-
sised in the presence of PVP improves the selectivity of the
NP hydrogenation reactions.6 They postulated that the selective
coordination of these ligands could modify the steric hindrance
at specific sites of the NPs and hence generate selectivity.
Van Leeuwen, Chaudret and co-workers also recently reported
the use of carbene and large bite angle diphosphine ligands as
In the hydrogenation of arenes, the formation of partially
hydrogenated products, namely cyclohexenes, is of high inter-
est for the straightforward formation of cyclohexanol via hydra-
tion (Scheme 1). In this field, several strategies to improve the
chemoselectivity for cyclohexenes vs. cyclohexanes have been
developed and usually involve the competitive displacement of
the cyclohexene product from the metal surface prior to its
complete hydrogenation.8–11 For this purpose, modification of
heterogeneous catalysts by organic and/or inorganic additives
was shown to be efficient. To date, the employment of M-NPs in
the partial hydrogenation of arenes has been limited to the use
of aqueous reaction media12 or stabilisers like ionic liquids
(ILs)13–15 where the solubility of the substrate is higher than
that of the cyclohexene product. However, high selectivities for
cyclohexenes are usually only achieved at low conversions.
´
Masdeu-Bulto and co-workers reported the use of Rh and
Ru-NPs stabilised by PPh3 and its fluorinated derivative
P[3,5-(CF3)2C6H3]3 in the partial hydrogenation of methylanisoles
Department of Physical and Inorganic Chemistry, Universitat Rovira I Virgili,
C/Marcel.li Domingo s/n, Campus Sescelades, 43007, Tarragona, Spain.
E-mail: cyril.godard@urv.cat; Fax: +34 977 559563
Scheme 1 Industrial application of the products formed during the hydrogena-
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cy00388d
tion of arenes.
c
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to cyclohexene derivatives with a selectivity of up to 15% at 21%
conversion (yield up to ca. 3%) in a sCO2 reaction medium.16
In recent years, our research group has been interested in
the use of phosphorus donor ligands as NP stabilisers in
selective catalysis. We reported the hydrogenation of arenes
using Rh and Ru-NPs stabilised by diphosphites bearing a
carbohydrate backbone.17 These catalytic systems are active in
the hydrogenation of methylanisoles in pentane; however, no
conversion was obtained when coordinating solvents such as
THF and acetonitrile were used.
Here, we report the synthesis and characterisation of a series
of P-donor stabilised Rh-NPs and their application in the
partial hydrogenation of xylenes and methylanisoles with high
selectivity for cyclohexene derivatives, even at relatively high
conversion, and in the absence of additives. The objective of this
work is the study of the effect of these stabilisers (mono- and
bidentate phosphines and phosphites), which is analysed via
comparison with the catalytic results obtained using NPs stabi-
lised by a mixture of THF and MeOH. The selectivity of these NPs
Fig. 1 TEM micrographs and the corresponding size histograms of nano-
particles Rh1–Rh4.
towards the hydrogenation of vinylic vs. aromatic double bonds cubic (fcc) lattice structure. No reflections arising from
was also investigated using styrene as the substrate.
rhodium oxides were observed, and coherence lengths with
respect to TEM analysis were obtained. XPS measurements
were performed on freshly prepared catalyst samples and
revealed distinct ratios of Rhd+/Rh0 at the surface of Rh1, Rh2
and Rh4, while in Rh3 all of the surface atoms were found to be
in the zero-valent state. The theoretical peaks for Rh 3d 5/2 and
3d 3/2 are 307.2 eV and 311.9 eV respectively. These peaks were
observed slightly displaced, which may be due to a strong inter-
action between the Rh nanoparticles and the phosphorus atoms of
the stabilising agents. These results are in agreement with XPS data
reported for soluble Rh NPs.18 Elemental analysis of these NPs
confirmed the presence of the P-ligands in the NP samples.
Next, the nanoparticles Rh1–Rh4 were tested in the hydro-
genation of methylanisoles 1a–c at 80 1C for 16 h (Table 1,
entries 1–12). In all cases, the cis isomers of the corresponding
cyclohexane derivatives were the major products. In most cases,
under these conditions, the Rh1 system bearing the triphenyl-
phosphine I was found to be the most active catalyst with the
highest TON (entries 1, 5 and 9), while the Rh2 system,
stabilised by the triphenylphosphite ligand II, was totally
inactive (entries 2, 6 and 10). The latter catalyst was also
inactive when xylenes were tested as substrates, which indi-
cated that the ligand II efficiently blocks the NP surface sites
which are responsible for the hydrogenation of the aromatic
ring. This result can be explained by the high ligand coverage
observed for the Rh2 NPs (see elemental analysis in ESI†), when
compared to the Rh1 NPs bearing the ligand I. When the
systems bearing bidentate stabilisers Rh3 and Rh4 were used,
lower conversions were observed relative to those obtained
with Rh1. However, Rh4 produced the partially hydrogenated
products with high selectivity (ca. 40%) at ca. 40% conversion,
which corresponds to ca. 15% yield. Interestingly, no partially
hydrogenated products were detected when the system bearing
the diphosphine ligand III was used as catalyst.
Results and discussion
Soluble Rh-NPs stabilised by the phosphorus donors I–IV
(P : Rh = 0.4) (Scheme 2) were synthesised by decomposition
of [Rh(Z3-C3H5)3] in THF at 40 1C under 6 bar of H2. The Rh-NPs
were isolated as black powders after precipitation with pentane.
These Rh-NPs were characterised by transmission electron
microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS) and elemental analysis.
TEM micrographs of Rh1–Rh4 revealed in all cases the
formation of small Rh-NPs of similar size (ca. 1.6 nm diameter,
Fig. 1). The Rh4 NPs were observed to present some degree of
agglomeration, which could be due to the ability of the ligand
IV to act as a bridge between nanoparticles. The mean diameter
and size distribution of Rh1 stabilised by PPh3 are in agreement
with those reported by Masdeu-Bulto and co-workers.16 Diffuse
´
peaks in the XRD pattern were observed as expected for a homo-
geneous distribution of very small particles with a face-centred
These results clearly demonstrate that the properties of the
P-donor stabilising agents strongly affect the activity and
Scheme 2 P-donor ligands used in this study to stabilise the Rh-NPs Rh1–Rh4.
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Table 1 Rh-NP (Rh1–Rh4) catalysed hydrogenation of o-, m-, p-xylene (1a–c)
and o-, m-, p-methylanisole (1d–f)a
TEM analysis revealed the presence of small nanoparticles of
ca. 1.2 nm diameter, slightly smaller than Rh1–Rh4. Surpris-
ingly, when the nanoparticles Rh5 were used in the hydro-
genation of p-xylene, no catalytic activity was detected. These
results indicated that either the substitution of P-donor stabi-
lisers at the surface of these NPs by THF–MeOH or their smaller
size was inhibiting the hydrogenation reaction.
To distinguish between these two possibilities, larger NPs
were synthesised using the seeded growth method.20 The TEM
micrographs of these NPs revealed the formation of nano-
particles of 1.9 and 2.5 nm diameter. Again, no catalytic activity
was observed when these new systems were applied in the
hydrogenation of p-xylene under the same conditions. It was
therefore concluded that the activity and selectivity of Rh1–Rh4
are mainly governed by the presence of the P-donor stabilising
agents I–IV and that substitution of these by the mixture of THF
and MeOH molecules has an inhibiting effect on their hydro-
genation performance.
Next, the effects of temperature and H2 pressure were
studied using the Rh4 system in the hydrogenation of p-xylene
during 16 h in heptane. The results are shown in Fig. 2. Under
40 bar of H2 pressure, when the temperature was increased
from 60 1C to 120 1C, a decrease in conversion from 51% to 15%
was observed. Conversely, the selectivity towards the partially
hydrogenated product increased from 6% to 32%.
Subst. NPs %Conv.b cis–transb %Sel.3/[%yield]b TONc
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1b
1b
1b
1b
1c
Rh1 499
98/2
—
—
—
36 [13]
—
—
—
39 [15]
—
—
147
—
53
124
147
—
Rh2
Rh3
Rh4
—
—
40
35
499
499
58/42
Rh1 499
Rh2
Rh3
Rh4
—
—
—
19
38
63/37
68/32
66/34
25
135
147
—
Rh1 499
10 1c
11 1c
12 1c
13 1d
14 1e
15 1f
Rh2
Rh3
Rh4
—
32
49
71/29
71/29
75/25
75/25
67/33
—
42
21 [10]
8 [8]
15 [8]
22 [8]
174
351
181
128
Rh4 499
Rh4
Rh4
51
36
a
General conditions: 1.24 mmol of substrate, 3.5 mg of Rh-NPs, 10 mL
b
of heptane, T = 80 1C, P = 40 bar H2, t = 16 h. Determined by GC.
c
TON was defined as the number of mol of substrate (1a–f) converted
per mol of surface Rh. The substrate/Rh ratio was calculated based on
elemental analysis data of each NP.
These results indicated that the desorption of both the
substrate and the cyclohexene derivative product from the NP
selectivity of Rh-NP catalysts in the hydrogenation of methyla- surface is facilitated by increased temperature, thus lowering
nisoles. The selectivity for the partially hydrogenated products the conversion and increasing the selectivity towards the
in the hydrogenation of methylanisoles observed for the Rh4
system was found to follow the trend ortho E meta 4 para,
which was attributed to the greater steric hindrance induced by
the substituents in ortho and meta positions.
To investigate the effect of the presence of a coordinating
oxygen atom in the substrates 1a–1c, the corresponding xylene
substrates 1d–1f were examined using Rh4 as catalyst (Table 1,
entries 13–15). Although similar selectivity to cyclohexene
derivatives was observed for both para-substituted substrates
1c and 1f (Table 1, entry 12 vs. 15), higher yields of partially
hydrogenated products were obtained in the case of ortho- and
meta-methylanisoles compared to their xylene analogues
(Table 1, entry 4 vs. 13 and entry 8 vs. 14). Higher conversions
were obtained with xylenes as substrates, which could explain
these differences in selectivity. However, when benzene was
used as a substrate using Rh4 as catalyst, cyclohexane was the
only reaction product. No significant changes in the size of the
NPs were observed by TEM microscopy after catalysis.
To further probe the effect of the stabilising agent, new
Rh-NPs were synthesised in the absence of P-donor stabilisers.
These NPs were formed following the procedure reported by
Chaudret and co-workers to obtain analogous ruthenium nano-
particles.19 The proportion of solvent (THF–MeOH) was chosen
to obtain nanoparticles with similar size to those stabilised with
the P-donors I–IV. The same rhodium precursor Rh(Z3-C3H5)3 was
Fig. 2 Effect of pressure and temperature on the partial hydrogenation of
decomposed in the presence of THF and a small amount of
MeOH (ratio 97.5 :2.5), under 4 bar of H2 at ambient temperature.
p-xylene using Rh4 as catalyst (general conditions: 1.24 mmol of substrate,
3.5 mg of Rh-NPs, 10 mL of heptane, t = 16 h; [a] P = 40 bar of H2; [b] T = 80 1C).
c
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Table 3 Rh-NP (Rh1–Rh4) catalysed hydrogenation of styrene at RTa
partially hydrogenated product, as previously reported for
heterogeneous catalysts.21 At 80 1C, when the pressure was
increased from 3 to 40 bar of H2, conversion increased from 7%
to 36%. Interestingly, the selectivity for the partially hydro-
genated product was also improved and reached a maximum of
27% at 20 bar of H2, but decreased again when 40 bar of H2
pressure was used. It is accepted that hydrogenation of aromatics
occurs in a consecutive manner and hydrogen pressure affects
the different steps of aromatic ring hydrogenation in different
ways. Odenbrand and Lundin reported that the hydrogenation
of cyclohexenes to cyclohexanes is less dependent on H2
pressure than the hydrogenation of aromatics to cyclohexenes.22
With an increase in hydrogen pressure, the rate of hydrogena-
tion of the aromatic ring to cyclohexene apparently increases
faster than the rate of hydrogenation of cyclohexene to cyclo-
hexane, resulting in the higher selectivity for cyclohexene
formation. However, when the pressure is further increased,
both rates are higher, which results in a higher yield of total
hydrogenation.
t (h)
0.25
1
16
0.25
1
16
0.25
1
16
0.25
1
16
0.25
1
16
NPs
%Conv.b
%4b
%6b
TONc
TOFc (hÀ1
)
1
2
3
4
5
6
7
8
Rh1
Rh1
Rh1
Rh2
Rh2
Rh2
Rh3
Rh3
Rh3
Rh4
Rh4
Rh4
Rh5
Rh5
Rh5
27
100
100
5
24
84
23
54
100
46
100
100
65
0
8
15
0
0
0
0
0
2
0
0
11
0
0
21
100
92
85
1460
5409
5409
699
5842
—
—
2795
3354
—
4414
2591
—
23 696
—
—
9705
—
—
100
100
100
100
100
98
100
100
89
3354
11 738
1103
2591
4797
5924
12 878
12 878
2426
3733
3733
9
10
11
12
13
14
15
100
100
79
100
100
a
General conditions: 3.5 mg of Rh-NPs, 54 mmol of styrene (5 mL),
To compare the selectivity of our catalysts in the hydrogena-
tion of vinylic and aromatic CQC double bonds, the Rh NPs
Rh1–Rh5 were tested in the hydrogenation of styrene. The
results are shown in Tables 2 and 3.
b
c
5 mL of heptane, T = RT, P = 40 bar H2. Determined by GC. TON was
defined as the number of moles of substrate (3) converted per mol of
surface Rh. TOF was calculated at styrene conversions lower than 65%
and it is defined as the mol of substrate (3) converted per mol of surface
Rh per hour. The substrate/Rh ratio was calculated based on elemental
analysis data of each NP.
When the catalytic reactions were performed at 80 1C
(Table 2), full conversions were obtained with all the systems
after 1 h. In most cases, the selectivity for the totally hydrogenated
product 4 increased once the vinylic group was reduced. Inter-
estingly, under these conditions, the aromatic ring of styrene
was hydrogenated even using the Rh5 system, which was totally
inactive in the hydrogenation of p-xylene at 80 1C (entries 9 and
10). This result suggests that increased steric hindrance in the
case of the xylene substrate was inhibiting the reaction. In
contrast, total selectivity for ethylbenzene 6 was achieved using
Rh2 as catalyst (entries 3 and 4), even after 16 h of reaction
time, suggesting that the monophosphite ligand II efficiently
blocks the coordination sites of the Rh-NPs required for the
hydrogenation of the aromatic ring of styrene. Under these
conditions, yields of up to 18% of partially hydrogenated
products were detected when Rh1 or Rh4 was employed. It is
noteworthy that these values were obtained at complete
conversion of the substrate. The higher activities obtained with
styrene compared to those obtained with disubstituted sub-
strates (Table 1) can be explained by the lower steric hindrance
associated with the presence of only one substituent on the
aromatic ring.
To obtain selectivity data at lower conversions, the reaction
was performed at ambient temperature using a much larger
ratio of substrate to Rh (54 mmol of styrene, 1 : 1 volume ratio
with heptane) (Table 3). In all cases, ethylbenzene 6 was the
major reaction product. No partially hydrogenated products
were detected when the reaction was performed at this temp-
erature, in agreement with the results presented in Fig. 2a. After
15 minutes of reaction time, conversions between 5 and 65%
were obtained with total selectivity for ethylbenzene 6, corre-
sponding to TOFs up to ca. 23 500 hÀ1 using Rh4 (Table 3,
entry 10). Rh2, stabilised by P(OPh)3 II, was once again the least
active catalyst: after 16 h at this temperature, the totally
hydrogenated product 4 was detected in all cases except for
this catalyst (Table 3, entry 6). These results clearly show that
the nature of the stabilising ligand affects the catalytic perfor-
mance of these highly active nanocatalysts.
Table 2 Rh-NP (Rh1–Rh5) catalysed hydrogenation of styrene at 80 1Ca
t (h)
NPs
%Conv.b
%4b
%5b
%6b
TONc
1
2
3
4
5
6
7
8
9
1
16
1
16
1
16
1
16
1
Rh1
Rh1
Rh2
Rh2
Rh3
Rh3
Rh4
Rh4
Rh5
Rh5
100
100
100
100
100
100
100
100
100
100
27
65
2
0
18
0
0
0
0
9
16
8
3
73
17
98
97
71
0
24
8
78
5
124
124
320
320
110
110
295
295
86
3
29
100
67
76
14
91
10
16
86
Conclusions
a
General conditions: 3.5 mg of Rh-NPs, 1.24 mmol of styrene, 10 mL of
b
c
heptane, T = 80 1C, P = 40 bar H2. Determined by GC. TON was
defined as the number of mol of substrate (3) converted per mol of
surface Rh.
In summary, the hydrogenation of substituted arenes such as
styrene, xylenes and methylanisoles was investigated using
c
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soluble Rh-NPs as catalysts, and results showed relevant differ- previously prepared Rh-NPs (0.14 mmol of Rh). The reaction
ences in activity and selectivity depending on the properties of mixture was placed in a Fischer–Porter reactor, which was then
the NP-stabilising P-ligand used. For instance, large differences pressurised under 4 bar of H2 and stirred at room temperature
in activity and selectivity were observed for Rh1 and Rh2, for 16 h. A small amount (approx. 5 drops) of the solution was
stabilised by triphenylphosphine I and triphenylphosphite II, deposited under an argon atmosphere on a carbon-covered
respectively. The presence of the P-ligand was shown to be copper grid for TEM analysis. The rest of the solution was
critical: the THF–MeOH-stabilised Rh5 was inactive in the concentrated under reduced pressure. Precipitation and wash-
hydrogenation of xylenes, although hydrogenation of both the ing with pentane (3 Â 15 mL) was then carried out, to obtain a
aromatic ring and the vinylic group of styrene was observed. In black precipitate.
the case of disubstituted arene substrates, selectivities for the
corresponding cyclohexene derivatives of up to 39% were
General procedure for the hydrogenation reactions
achieved at ca. 40% conversion. The effect of parameters such
as temperature and pressure was also examined. In the hydro-
genation of styrene, very high selectivities for ethylbenzene
were achieved with TOF values up to ca. 23 500 hÀ1. All these
results show that the catalytic performance of small Rh-NPs can
be modulated by the appropriate choice of stabilising agents.
In a typical experiment, a 5 entries autoclave was charged in the
glove-box with Rh nanoparticles (3.5 mg) and the substrate
(1.24 mmol, approx. substrate to Rh ratio = 100) in 10 mL of
heptane. Molecular hydrogen was then introduced until the
desired pressure was reached. The reaction was stirred for 16 h
at the appropriate temperature. The autoclave was then depres-
surised. The solution was filtered over Celite and analysed by
gas chromatography. The conversion and the cis-selectivity
of the product were determined using a Fisons instrument
(GC 9000 series) equipped with a HP-5MS column.
Experimental section
General procedure for the synthesis of the Rh-NPs
In a typical procedure, the [Rh(Z3-C3H5)3] (64 mg, 0.28 mmol)
was placed into a Fischer–Porter reactor at À110 1C (acetone/N2
bath) in 64 mL of dry and deoxygenated (by freeze–pump–thaw
Acknowledgements
´
´
cycles) THF in the presence of the appropriate ligand (0.2 equiv. We are grateful to the Spanish Ministerio de Educacion (Ramon
for bidentate ligands and 0.4 equiv. for monodentate ligands). y Cajal to C.G., CTQ2010-15835) for financial support.
The Fischer–Porter reactor was then pressurised under 6 bar of
H2 and stirred for 30 minutes at room temperature. The
solution was then heated to 40 1C and stirred at this tempera-
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