A mild route to solid-supported rhodium nanoparticle catalysts and their application to the selective hydrogenation reaction of substituted arenes

Article abstract of DOI:10.1039/c5cy00599j

A clean route is described for the preparation of 1.3% (w/w) supported rhodium nanoparticle (3.0 ± 0.7 nm) catalysts onto commercial ion-exchange resins. Their application to the liquid-phase hydrogenation reaction of C=C bonds shows the most active species are obtained under catalytic conditions at room temperature and 1 bar H₂. The heterogeneous catalyst shows excellent activity, selectivity and reusability in the hydrogenation reaction of alkenes and substituted arenes under very undemanding conditions. The results are discussed in terms of support effect on the catalytic efficiency.

Full text of DOI:10.1039/c5cy00599j

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PAPER
A mild route to solid-supported rhodium
nanoparticle catalysts and their application to the
selective hydrogenation reaction of substituted
arenes†
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
Carmen Moreno-Marrodan,a Francesca Liguori,a Elisabet Mercadé,b Cyril Godard,b
Carmen Claverb and Pierluigi Barbaro*a
DOI: 10.1039/x0xx00000x
www.rsc.org/catalysis
A clean route is described for the preparation of 1.3 % (w/w) supported rhodium nanoparticle (3.0
± 0.7
nm) catalysts onto commercial ion-exchange resins. Their application to the liquid-phase
hydrogenation reaction of C=C bonds shows the most active species are obtained under catalytic
conditions at room temperature and 1 bar H2. The heterogeneous catalyst shows excellent activity,
selectivity and reusability in the hydrogenation reaction of alkenes and substituted arenes under very
undemanding conditions. The results are discussed in terms of support effect on the catalytic
efficiency.
Introduction
conditions or sophisticated equipments, ii) lack of control over
MNP size and distribution, iii) lack of reproducibility, iv) lack
of stability under reaction conditions, v) applicability only to
specific reaction-support combinations.7,8
Compared to other insoluble support materials, ion-
exchange resins offer a number of advantages for the
production of MNP-based catalysts:9,10
Rhodium nanoparticles (RhNP), either colloidal dispersions or
solid-supported, are pivotal in catalysis because of the unique
performance provided in several oxidation, carbonylation,
coupling and hydrogenation reactions, compared to other
transition
metals.1
Particularly,
the
(stereo)selective
hydrogenation reaction of substituted, monocyclic arenes to
cyclohexane derivatives is a reaction of much practical interest
in the polymer, fuel and fine-chemicals sector, that is usually
achieved under mild and more drastic conditions using
homogeneous and heterogeneous Rh catalysts, respectively.2
However, the preference of the chemical industry for solid
catalysts due to environmental and economic constraints,3,4
combined with the cost and rarity of rhodium,5,6 pushes
chemists to devise innovative protocols for heterogeneous
RhNP catalysts which comply with sustainability criteria. The
task is challenging since solid-supported metal nanoparticles
(MNP) are usually affected by multiple drawbacks, which
include: i) synthetic procedures requiring toxic reagents, harsh
- low cost,
- commercial availability in various chemical and physical
modifications,
- satisfactory chemical, mechanical and thermal resistance,
- ease of handling,
- straightforward, non-covalent metal anchoring,
- MNP stabilization by the dual effect of charged functional
groups (electrostatic stabilization),11 and porosity (steric
stabilization),12
- facile integration in existing reactor equipments,
- versatility in terms of accessible reactions.
In particular, low-cross linked resins (0.5-4%) develop a
microporous gel structure when swollen in an appropriate
solvent, allowing for the accommodation of narrow size
distributions of MNP.13 Despite of these favourable features,
ion-exchange resin-supported RhNP catalysts are almost
unexplored, most applications focussing on palladium, platinum
and gold particles.14,15
Based on the green strategy we have developed for the
immobilization of PdNP onto ion-exchange resins,16,17 herein
we report on the synthesis and characterisation of resin-
a Consiglio Nazionale delle Ricerche , Istituto di Chimica dei Composti
Organo Metallici, Via Madonna del Piano 10, 50019 Sesto Fiorentino,
Firenze, Italy. E-mail: pierluigi.barbaro@iccom.cnr.it.
b Department of Physical and Inorganic Chemistry, Universitat Rovira I
Virgili,C/Marcel.li Domingo s/n, Campus Sescelades, 43007, Tarragona,
Spain.
†
Electronic Supplementary Information (ESI) available: catalytic
experiment results table. See DOI: 10.1039/ b000000x/
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supported RhNP, and their use in the heterogeneous, liquid- analyses were performed on a Shimadzu GC-2010 gas
phase catalytic hydrogenation reaction of substituted arenes.
chromatograph equipped with a flame ionization detector and
either a Varian VF-WAXms (30 m, 0.25 mm ID, 0.25 µm FT)
or a Supelco SPB-1 (30 m, 0.25 mm ID, 0.25 µm FT) capillary
column. GC-Ms analyses were performed on a Shimadzu
QP2010S spectrometer equipped with identical capillary
columns. The metal content in the resin-supported catalysts was
determined by Atomic absorption spectrometry (AAS) using a
AANALYST200 spectrometer. Each sample (50-100 mg) was
treated in a microwave-heated digestion bomb (Milestone,
MLS-200, 20 min.@ 220 °C) with concentrated HNO3 (1.5
mL), 98% H2SO4 (2 mL), and 0.5 mL of H2O2 30%. The
content of metal leached in the solutions recovered after
catalysis was determined by Inductively Coupled Plasma
Atomic Emission Spectroscopy (ICP-OES) with a Varian
720ES instrument at a sensitivity of 500 ppb. The detection
limit for Rh was 0.01 ppm. The solutions were analysed
directly after 1:5 dilution in 0.1 M hydrochloric acid.
Experimental
General information
Unless otherwise stated, all manipulations were performed
under nitrogen atmosphere by using standard Schlenk
techniques. DOWEX® 50WX2-100 (H+ form, 2% cross-linked,
gel-type, 50-100 mesh [150-300 µm] bead size, 4.8 meq/g
exchange capacity), DOWEX® 50WX2-400® strong cation-
exchange resins (H+ form, 2% cross-linked, gel-type, 200-400
mesh [38-75 µm] bead size, 4.8 meq/g exchange capacity) were
obtained from Aldrich. The commercial resins were treated as
follows prior of use to remove incidental impurities. 20 g of
resin were washed with refluxing deionised water for 2 h and
with refluxing methanol for 1 h using a Soxhlet apparatus in air.
After cooling to room temperature, the resin was washed with
dichloromethane (3 x 100 mL), methanol (3 x 100 mL) and
diethyl ether (3 x 100 mL). The resin was dried under a stream
of nitrogen and stored under nitrogen. The lithiated resin was
prepared by adding 5 g of the purified protonated resin to a 1 M
solution of lithium hydroxide (150 mL) in air atmosphere. The
mixture was stirred at 150 rpm at room temperature for 24 h
using an orbital stirrer. The resin obtained was placed in a glass
filter and washed repeatedly with deionised water (5 x 100 mL)
until neutral pH of the washings. Then, it was washed with
methanol (3 x 100 mL) and diethyl ether (3 x 100 mL) and
dried in a stream of nitrogen. The product obtained as white
beads was stored under nitrogen. [Rh(NBD)2]BF4 was obtained
Preparation of resin-supported rhodium(I) species
In a typical procedure, 1 g of dry cation-exchange resin, either
lithiated or protonated form, was swollen in 30 mL of degassed
methanol followed by the addition of a degassed solution of
bis(norbornadiene) rhodium(I) tetrafluoroborate (54.4 mg,
0.145 mmol, ratio Rh/sulfonic groups = 1/33) in methanol (26
mL). The mixture was stirred under nitrogen atmosphere and
room temperature for 2 h using an orbital stirrer. The resin
obtained was transferred into a glass filter via a Teflon tube
under nitrogen and washed sequentially with methanol (3 x 75
mL) and diethyl ether (3 x 75 mL), before being dried in a
stream of nitrogen overnight. The rhodium containing resin
obtained as yellow beads was stored under nitrogen in the dark.
AAS analysis showed the resin to contain 1.3 % (w/w) of Rh,
corresponding to a 87% metal uptake.
from Alfa Aesar and used as received (NBD
=
bicyclo[2.2.1]hepta-2,5-diene). All other reagents were
commercial products and were used as received without further
purifications.
ESEM (Environmental Scanning Electron Microscopy)
Preparation of resin-supported rhodium(0) species
measurements were performed on
a FEI Quanta 200
Method a) Reduction with NaBH4: solid NaBH4 (82.5 mg, 2.18
mmol) was slowly added to 0.5 g of 1.3% (w/w) rhodium(I)-
resin in 28 mL of methanol. The resin became immediately
black. The suspension was stirred at 200 rpm at room
temperature for 2h, using an orbital stirrer. The resin obtained
was transferred into a glass filter via a Teflon tube under
nitrogen and washed with methanol (3 x 75 mL) and diethyl
ether (3 x 75 mL), and then dried in a stream of nitrogen
overnight. The product, obtained as black beads, was stored
under nitrogen in the dark. Method b) Reduction with a flow of
H2: 0.5 g of 1.3% (w/w) rhodium(I)-resin were suspended in 28
mL of methanol under nitrogen. Hydrogen gas was bubbled at 1
bar at room temperature for 2 h under orbital stirring at 170
rpm. The resin became slowly black (ca. 10 min.). After that
time, the resin was transferred into a glass filter under nitrogen
via a Teflon tube, it was washed with methanol (3 x 75 mL),
diethyl ether (3 x 75 mL) and then dried in a stream of nitrogen
overnight. The product, obtained as black beads, was stored
under nitrogen in the dark. Method c) Reduction with a flow of
H2 in the presence of an excess of substrate (catalytic conditions):
microscope operating at 25 KeV accelerating voltage in the
low-vacuum mode (1 torr) and equipped with an EDAX Energy
Dispersive X-ray Spectrometer (EDS). X-ray maps were
acquired on the same instrument using a 512x400 matrix, 25
KeV accelerating voltage and 350 µm horizontal full width.
TEM (Transmission Electron Microscopy) measurements were
carried out using a CM12 PHILIPS instrument. The sample
preparation was carried out by dispersing the grinded resin into
1 mL of ethanol and treating the solution in an ultrasonic bath
for 30 min. A drop of solution was deposited onto a carbon
coated Cu TEM grid and the solvent left to evaporate.
Statistical nanoparticle size distribution analysis was typically
carried out on 300-400 particles. XRD (X-ray Diffraction)
spectra were recorded with a PANanalytical XPERT PRO
powder diffractometer, employing CuKα radiation (λ =
1.54187 Å), a parabolic MPD-mirror and a solid state detector
(PIXcel). Reactions under batch conditions were performed
using a stainless steel autoclave constructed at ICCOM-CNR
(Firenze, Italy) and equipped with a magnetic stirrer, a Teflon
® inset and a pressure controller for higher pressures. GC
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a
N BH4
Rh
re uc on
d
ti
Rh NBD +BF4
-
[
(
)2]
SO3-H+
+
Rh DH
@
me a a on
t ll ti
H2
SO3-H+
SO3-H+
SO3-H+
SO3 Rh NBD
)2]
-
+
[
(
X
re uc on
d
ti
LiOH
Rh NBD +BF4
-
H2
[
(
)2]
Rh
me a a on
t ll ti
re uc on
d
ti
SO3-Li+
SO3-Li+
SO3-Li+
SO3 Rh NBD
)2]
SO3-Li+
-
+
[
(
+
Rh DLi
@
Scheme 1 Sketch of ion-exchange resins-supported RhNP synthesis with labelling scheme adopted.
0.05 g of 1.3% (w/w) rhodium(I)-resin were swollen in 5 mL of reactions carried out at higher temperatures and/or pressures,
methanol under nitrogen. A substrate solution in methanol was the supported Rh species (50 mg, ca. 1.3% Rh w/w, ca. 0.006
then added and the mixture heated and pressurized with H2 gas mmol of rhodium) was placed in a metal free autoclave that
at the desired temperature and pressure. After completion of the was immediately closed and degassed with
3 cycles
catalytic reaction, the resin was transferred into a glass filter vacuum/nitrogen. A solution of the substrate (1.45 mmol) in
under nitrogen via a Teflon tube, washed with methanol (3 x 75 methanol (9.1 mL) was then transferred under nitrogen into the
mL), diethyl ether (3 x 75 mL) and then dried in a stream of reactor using a cannula and nitrogen was replaced by hydrogen
nitrogen overnight. The product, obtained as black beads, was with 3 cycles pressurization/depressurization. After setting up
stored under nitrogen in the dark.
the desired temperature and pressure, the solution was
Irrespective of the reduction method, AAS analysis showed the magnetically stirred at 170 rpm.
Rh content in the resin was unchanged 1.3 % (w/w).
Results and discussion
Hydrogenation reactions and catalyst recycle
Catalyst synthesis and characterization
In a typical experiment, the resin-supported Rh species, either
Rhodium(0) or Rhodium(I) (50 mg, ca. 1.3% Rh w/w, ca. 0.006
mmol of rhodium), was swollen in 5 mL of degassed methanol.
After 5 minutes a solution of the substrate (1.45 mmol) in
methanol (4.1 mL) was added under nitrogen. Hydrogen gas
was then bubbled at 1 bar H2 and 10 mL/min at room
temperature, under stirring at 170 rpm with an orbital stirrer.
The H2 inlet was taken as the start time of the reaction. After
the desired time, the methanol solution was completely
removed under a stream of hydrogen using a gas-tight syringe.
A sample of this solution (0.5 µl) was used for GC and GC-MS
analysis, while the remaining aliquot was used for the catalyst
leaching test and ICP-OES measurement. A fresh solution of
the substrate (1.45 mmol) in methanol (9.1 mL) was then
transferred under hydrogen into the flask containing the
recovered supported species. The mixture was stirred at 170
rpm and room temperature under hydrogen flow and, after the
desired time, the mixture was treated as described above. The
same recycling procedure was used in the subsequent
hydrogenation cycles. After use in catalysis, the solid species
was washed with methanol (3 x 10 mL) and diethyl ether (3 x
10 mL), dried in a stream of nitrogen overnight and stored
under nitrogen for later characterization. The reaction products
were identified by GC retention times and GC-MS analysis. For
We have recently reported a friendly method for the growth of
PdNP within the pores of gel-type ion-exchange resins.16 In the
present work, we have extended the methodology to the
synthesis of RhNP. The approach involves metallation of the
resin by ion-exchange using an appropriate metal precursor, i.e.
a soluble cationic specie whose metal centre can be smoothly
reduced,18 followed by reduction under 1 bar H2 and room
temperature. A schematic representation of the two-step
procedure is reported in Scheme 1. The resins used were the
commercial, strong cation-exchange sulfonic gels DOWEX®
(2% cross-linkage), either 50-100 mesh (150-300 µm) or 200-
400 mesh (38-75 µm) bead size. The polymers were employed
in their protonated form as manufactured, or converted into the
corresponding lithium salt.19 [Rh(NBD)2]BF4 was selected as
metal precursor due to its solubility in methanol, required for
optimal resin swelling, and to the known propensity to H2 metal
reduction via norbornene elimination.20 Indeed, first step
metallation in methanol using a 33 : 1 = meq exchange capacity
: mmol Rh ratio afforded a yellow material, whose treatment
with H2 under very mild conditions (rt, 1 bar H2, ca. 10 min.)
gave rhodium(0)-containing black beads featuring a typical
1.3% bulk Rh loading (w/w, ICP-OES), which corresponds to a
metal uptake of ca. 87%. Importantly, the H2 reduction was
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Table 1 Resin-supported RhNP catalysts prepared
Resin a
Reduction
Rh c
(%)
Label
Ionic form
Size (µm) b
150-300
38-75
method
NaBH4
NaBH4
Rh@D100H+
Rh@D400H+
Rh@D100Li+
Rh@D400Li+
H+
H+
Li+
Li+
Li+
1.3(1)
1.3(1)
150-300
38-75
H2 (rt, 1 bar H2) 1.3(1)
H2 (rt, 1 bar H2) 1.3(1)
Rh@D400Li+ in
38-75
H2, in situ d
1.3(1)
a
b
c
DOWEX 50WX2, 100 or 400 mesh. Bead diameter. Rh loading (w/w)
from ICP-OES. d Under the conditions of catalytic reaction. 0.16 M substrate
solution in CH3OH, substrate / Rh molar ratio = 230.
successful only in the case of the lithiated resins, since use of
NaBH4 was required to achieve metal reduction on the
protonated polymers. This finding can be attributed to the
different reduction potentials of rhodium in acidic medium.21
Further, in order to directly use the metallated resins in
catalysis without a pre-reduction step (vide infra), and to
ascertain the effect of the preparation method on the catalyst
activity, as previously explored for PdNP,16 the synthesis of
RhNP was also accomplished in-situ by carrying out the
reduction of rhodium(I)-supported species under catalytic
hydrogenation conditions, i.e. in the presence of H2 and an
excess of substrate in methanol. Catalyst characterization was
performed at the end of the catalytic reaction, in that case. A
summary of the catalysts prepared is reported in Table 1.
Fig. 1 ESEM images (left, backscattered electrons) and EDS X-ray maps of oxygen
(center, O Kα1) and rhodium (right, Rh Lα1) of: (a) rhodium(I)@D100Li+ bead section
before reduction (b) Rh@D100Li+ after Rh reduction, (c) Rh@D100H+ after Rh reduction
(25 KeV, 650 magnifications).
40
30
20
10
0
1
2
3
4
5
6 7
Diameter (nm)
Previously reported methods for solid-supported RhNP
synthesis involve impregnation-reduction methods using RhCl3
or Rh(NH3)6Cl3 and sodium borohydride (montmorillonite,22
silica,23 CNT24), Rh(CH3CO2)2, Rh(NO3)3 or RhCl3 followed
by calcination and high-temperature H2 reduction (873 K
Fig.
2 Representative TEM image (left) and RhNP size distribution (right) of
Rh@D400H+.
metal distributions in the case of Rh@DLi+ (H2 reduction) and
Rh@DH+ (NaBH4 reduction), respectively This is clearly
visible from the backscattered electron images and from the
oxygen and Rh maps, reported for comparison in Fig. 1, before
and after rhodium reduction.39 A similar finding was previously
observed for other resin-supported MNP species (M = Pd, Ru,
Au)40 and was ascribed to difference in the rate of metal cation
diffusion within the solid matrix, as a consequence of the
concentration gradient generated by its reduction, and the rate
of diffusion of the reducing agent.41 Egg-shell distributions
may be observed in the instance that the diffusion of the cation
is faster than that of the reductant.
alumina,25 973
K
ceria,26 1273
K K
zirconia27, 1123
MgAl2O4,28 673
K
silica29). Other methods include
heterogenization of preformed RhNP (silica,30 zirconia,31
alumina,32 titania33), vapour deposition (silica34), flame spray
pyrolysis (alumina35) and laser ablation (silica36) techniques.37
It is worth mentioning that RhNP onto magnesium oxide have
been recently obtained by deposition-reduction of [Rh(COD)(µ-
OCH3)]2 by H2 under slightly heavier conditions than those
herein reported (50 °C, 3.4 bar).38
In the present study, all Rh-containing resins were
characterized in the solid state by a combination of microscopic
and scattering techniques.
Irrespective of synthetic method or the resin type, XRD and
TEM analyses showed the presence of agglomerates up to 30
nm diameter consisting of embedded single RhNP having a
mean size of 3.0 ± 0.7 nm. Fig. 2 reports a typical TEM image
and the corresponding RhNP size distribution in Rh@DH+
species. The RhNP size calculated from the characteristic
reflexion of the [111] crystallographic plane (2θ = 41º) in the
XRD pattern was 3.1 nm (Fig. S1), in complete agreement with
the TEM values of single particles. RhNP agglomeration was
ESEM experiments showed that the resin beads are not
damaged by metallation, reduction or use in catalysis, as no
signs of breakage or cracking were detected anyhow. EDS
maps recorded on sections of rhodium(I) contained beads
showed a homogeneous metal distribution within the solid
support, in both the protonated and lithiated resins, indicating
that the solvent diffuses thoroughly into and out the bead during
the metallation step.19 On the contrary, maps of pre-reduced
supported rhodium(0) species revealed egg-shell and uniform
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a
t
C
conditions in methanol solutions, albeit with remarkable
differences depending on various factors affecting their
performance. To ascertain the catalysts versatility, a number of
probe substrates bearing different functionalities were
scrutinized.
H2
a
1
1
a
t
C
H2
a
2
2
2b
NHCOCH3
C
ATALYST ACTIVITY, SELECTIVITY AND REUSABILITY
In order to get a first evaluation of the efficiency of the
catalysts family, the hydrogenation of simple substrates subject
NHCOCH3
CO2CH3
a
t
C
CH
3
H2
CO2CH3
to minor selectivity issues, i.e. cyclohexene
cyclooctadiene ( ), methyl-2-acetamidoacrylate (
phenyl-3-buten-2-one ( ) and styrene ( ) was carried out using
(
1
), 1,5-
3
a
3
O
O
OH
2
3), trans-4-
4
5
a
t
C
Rh@D100H+ species under 1 bar H2 and room temperature. A
sketch of the substrates investigated and the reaction products
detected is reported in Scheme 2. Irrespective of the substrate,
the catalyst showed to be quite active under the above reaction
conditions affording nearly quantitative conversions within
reasonable timeframes. Representative data are reported in
Table 2 in terms of conversion, selectivity and turnover
frequency (TOF) based on bulk rhodium content.
H2
a
4
4
4b
a
t
C
H2
a
5
5
5b
Scheme 2 Sketch of probe substrates examined and products detected with labelling
scheme
The hydrogenation of
1 to cyclohexane 1a was achieved
with a productivity of 838.9 h-1, that likens to that previously
reported for the parent resin-supported Rh@MonoBor system
under comparable flow conditions (rt, 1.8 bar H2, 1020 h-1).44
The analogous Pd@D100 catalyst was less active under the
same reaction conditions (TOF = 414 h-1).45 The solvent-free
hydrogenation of cyclohexene by solid-supported RhNP was
recently described with hydroxyapatite (rt, 1 bar H2, TOF = 10
h-1)46, silica-coated magnetite nanoparticles (25 °C, 1 bar H2,
TOF = 2638 h-1)47 and 5% Rh@MgO (25 °C, 5 bar H2, TOF =
15667 h-1 calculated on exposed Rh).38
Table 2 Catalytic hydrogenation reaction using Rh@D100H+ under1 bar H2
pressure and room temperature
a
Conversion
(%)
TOF b
(h-1)
Selectivity c
(%)
Entry Substrate
Product
1a
1
2
3
4
5
1
2
3
4
5
98.1
91.0
99.8
93.7
94.9
838.9
259.3
56.9 e
82.1
98.7 d
63.8
2a
3a
100.0
92.4
4a
The hydrogenation of
2 gave 63.8% of the partial
hydrogenation product cyclooctene 2a at 91.0% conversion,
which is less efficient than the corresponding Pd catalyst
(96.7% sel. at 98.5% conv., TOF = 418 h-1).45
270.4
5a
100.0
a Reaction conditions: methanol, substrate : Rh = 570 : 1 molar ratio, 50 mg
catalyst 1.3 % (w/w) Rh. Data from GC analysis. b Turnover frequency, as:
Chemoselectivity of C=C versus C=O hydrogenation was
(mol substrate converted) / (mol Rh × h) calculated at the conversion
c
tested with
4 resulting in 92% selectivity to 4-phenyl-butan-2-
indicated and on bulk Rh content.
For example, 2a selectivity =
2a/(2a+2b). d Benzene ca. 1%. e Substrate / Rh ratio = 114.
one (4a) at 94% conversion, with formation of 4b and traces of
ketals by-products. Best results for the partial hydrogenation of
4
were previously obtained using the resin-supported
already described for other solid-supported systems, that was
suggested to take place during the formation of metallic
rhodium.22b,38 No major changes were observed in TEM and
XRD data for Rh@D after use in catalysis, which indicates the
sinter-stability of the resin-supported particles. Chemisorption
studies were hampered by the limited accessibility of gas
reactants to gel-type resin-embedded metal NP.42,43
Pd@MonoBor catalyst under continuous flow (rt, 1.5 bar H2,
96.0% sel. at 99.5% conv., TOF = 166 h-1),44 and Pd@SiO2 in
batch (70 °C, 1 bar H2, 99.0% sel. at 100% conv.).48
The hydrogenation of
productivity (TOF
5
gave ethylbenzene 5a with good
=
270.4 h-1), with no traces of
ethylcyclohexane 5b detected under these conditions.
The supported catalyst could be quantitatively recovered by
decantation and reused by addition of identical amounts of
substrate solution under hydrogen. Recycling experiments
showed negligible activity decay upon recycle over five
consecutive runs under the same reaction conditions.
Representative results are reported in graphical format in Fig. 3
Catalytic hydrogenation reactions
The resin-supported RhNP were tested as catalysts in
hydrogenation reactions under batch conditions, either as pre-
reduced species or prepared in-situ. In the latter case, the
rhodium(I)-metallated resins were directly added to the
substrate solution under nitrogen, before the mixture was
exposed to the desired H2 pressure and temperature, which was
taken as the start time of the catalytic reaction. All supported
catalysts showed to be effective under very undemanding
for
2 and 5. Rh leaching in solution was negligible in each run
(ICP-OES), while the absence of catalytic activity of the
recovered reaction solutions indicated the catalyst to be truly
heterogeneous, ruling out the contribution of homogeneous
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100
100
80
60
40
20
0
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 1
Rh@D400H+
Cycle 2
80
60
40
Rh@D400Li+ in
Rh@D400Li+
Cycle 3
Cycle 4
Cycle 5
20
0
0
30 60 90 120 150 180 210 240 270 300 330
Time (min.)
5
2
0
20
40
60
80 100
0
20 40 60 80 100
Time (min.)
Fig. 5 Hydrogenation reaction of 3 using Rh@D400 catalysts. Reaction conditions:
substrate 0.16 M in CH3OH, 50 mg catalyst 1.3 % (w/w) Rh, substrate / catalyst molar
ratio 290, rt, H2 1 bar. Data from GC analysis.
Time (min.)
Fig. 3 Reuse of Rh@D100H+ catalyst in the hydrogenation reaction of 2 (left) and 5
(right). Reaction conditions: substrate 1.03 M in CH3OH, 50 mg catalyst 1.3 % (w/w) Rh,
substrate / catalyst molar ratio 570, rt, 1 bar H2. Data from GC analysis.
identical reaction conditions (rt, 1 bar H2, substrate:metal =
molar ratio 290:1). Regardless the substrate, two major outcomes
were apparent. First, the pre-reduced catalysts obtained by
NaBH4reduction were more active than those obtained by H2
100
Rh@D400H+
pre-treatment. Data for the hydrogenation reaction of
3 using
80
Rh@D100H+
Rh@D400 catalysts are reported in graphical format in Fig. 5,
as representative example. Thus, Rh@D400H+ gave a 94.5%
conversion after 1 h, whereas Rh@D400Li+ resulted in 5%
conversion after the same reaction time. An explanation for this
result is not straightforward. A proton-acceleration effect by
resin acid sites has been invoked for some metal-reaction
combinations (Pd-hydroxylation,15b,54 Ru-hydrogenation55), but
not for others (Pd-hydrogenation16). On the other hand, lithiated
resin supports usually show a higher efficiency compared to
protonated ones, that was attributed to the better swelling due
the higher solvation of lithium.19,56 In the case in our hands, the
different metal distributions within the beads (egg-shell or
homogeneous) generated by the different preparation method
may have consequences on their performance, although
difficultly rationalizable. Small differences in the size of
supported RhNP may also play a role, even if the lack of direct
connection between size of RhNP and their catalytic activity
was demonstrated.1c
60
40
20
0
0
20
40
60
80
100
120
Time (min.)
Fig. 4 Hydrogenation reaction of 3 using catalysts with different bead size. Reaction
conditions: substrate 0.16 M in CH3OH, 50 mg catalyst 1.3 % (w/w) Rh, substrate /
catalyst molar ratio 290, rt, H2 1 bar. Data from GC analysis.
species to the catalytic conversion.49 No significant changes in
catalyst selectivity was observed upon reuse. The above
findings are in line with those previously reported for the parent
resin-supported PdNP hydrogenation catalysts,16 and indicate
the strong metal anchoring on the support under the conditions
of catalysis.50
Second, most importantly, the activity of the catalysts
prepared in-situ under catalytic conditions was invariably
higher than that of the corresponding pre-reduced species. In
E
FFECT OF THE PARTICLE SIZE
Since it is known that gel-type ion-exchange resins are affected
by internal mass-transfer limitations,51 we compared the
activity of the catalysts supported onto resins of different bead
size under the same experimental conditions.52,53 Indeed, use of
38-75 µm beads, instead of 150-300 µm, invariably resulted in
higher productivity, thus confirming that the kinetic of the
hydrogenation reaction is ruled by the diffusion of the reagents
inside the support. The finding is shown in Fig. 4 in which the
the case of Rh@D400 catalysts, the hydrogenation reaction of
3
was completed within 150 min. using the in-situ prepared
Rh@D400Li+in catalysts, whereas the conversion was as low as
12% using the pre-reduced specie Rh@D400Li+ (Fig. 5).57 An
analogous behaviour was previously observed for the parent
PdNP supported catalysts, which was attributed to an “excess of
substrate stabilizing effect” responsible both for the limited
growth of Pd nanoparticles under in-situ conditions, and for the
stabilization of metastable Pd species, whose reactivity is
higher than that of conventional Pd0.16,45 Inspection of Fig. 5
shows a short induction period (ca. 30 min. in the case in our
hand), that can be safely ascribed to the time required to reduce
the supported rhodium(I) to Rh0, followed by the generation of
the specie featured by the highest catalytic activity.58 Table 3
hydrogenation data of
3
using Rh@D100H+ and Rh@D400H+
catalysts are reported. Despite of this, resins of larger size were
preferably used in recycling experiments, due to their easier
separation.
E
FFECT OF THE PREPARATION METHOD
The performance of supported catalysts of the same bead-size,
but prepared with different methods, was compared under
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Table 3 Hydrogenation reaction of 3 by ion-exchange resin-supported MNP
100
80
60
40
20
0
catalysts a
5
Conversion
(%)
Time b
(min)
TOF c
(h-1)
5a
5b
Entry Catalyst
Ref.
Rh@D400H+
1
Rh@D400H+
42.8
5.8
20
20
20
20
20
20
369
50
this work
this work
this work
this work
45
2
Rh@D400Li+in
Rh@D400Li+
Rh@D100H+
Pd@D100Li+in
Pd@D100Li+
3
1.4
12
4
6.0
41
100
80
60
40
20
0
Rh@D400Li+in
5d
100
97.0
750
727
5
6d
45
5a
5b
a
Reaction conditions: methanol, 1 bar H2 pressure, room temperature
substrate : metal = 290 : 1 molar ratio, 50 mg catalyst, 1.3% (w/w) Rh,
b
c
1.25% (w/w) Pd. Data from GC analysis. Reaction time. Turnover
frequency, as: (mol substrate converted) / (mol metal × h) calculated at the
reaction time indicated and on bulk metal content. d Substrate : metal = 250
: 1 molar ratio.
0
30 60 90 120 150 180 210 240 270 300 330 360 390
Time (min.)
summarizes the efficiency values reported in the literature for
Fig. 6 Catalytic hydrogenation reaction of 5 by Rh@D400H+ (top) and Rh@D400Li+ in
(bottom). Reaction conditions: substrate 0.16 M in CH3OH, 50 mg catalyst 1.3 % (w/w)
Rh, substrate / catalyst molar ratio 230, rt, H2 1 bar. Data from GC analysis.
the hydrogenation reaction of
3 by resin-supported Rh and Pd
catalysts under the same experimental conditions. TOF values
are calculated at the same reaction time for comparative
purposes.
100
C
ATALYST SCOPE: HYDROGENATION OF ARENES
Rh@D400Li+in
80
Having established an efficient benchmark catalyst for the
hydrogenation of olefins, we then turned our attention to the
hydrogenation reaction of more challenging substrates, i.e.
substituted arenes. We started our investigation from the
Rh@D400H+
60
40
20
0
hydrogenation reaction of styrene (5) under 1 bar H2 and room
temperature using both Rh@D400H+ and Rh@D400Li+in
species. The time evolution of the reaction products detected in
the two instances is shown in graphical format in Fig. 6. In
agreement with findings discussed in the previous section, the
0
30 60 90 120 150 180 210 240 270 300 330
Time (min.)
partial hydrogenation of
5 to give ethylbenzene (5a) was 3.5-
Fig. 7 Catalytic hydrogenation reaction of 6 by Rh@D400Li+ in and Rh@D400H+.
Reaction conditions: substrate 0.16 M in CH3OH, 50 mg catalyst 1.3 % (w/w) Rh,
substrate / catalyst molar ratio 230, rt, H2 1 bar. Data from GC analysis.
fold faster for Rh@D400H+ (100% conversion after 40 minutes),
compared to Rh@D400Li+in (2 hours, including the usual
induction period). In both cases, the hydrogenation of the
aromatic ring commenced after almost complete substrate
consumption, what allowed to obtain 5a in 100% selectivity
using the protonated support and in 97% in the case of the
lithiated resin. Surprisingly, the subsequent conversion of
ethylbenzene to ethylcyclohexane (5b) followed the opposite
trend, that is, it was much faster for Rh@D400Li+in (90% yield
after 380 min), rather than for Rh@D400H+ (10% yield). No
other partial hydrogenation products were detected under these
hydrogenation activity, including the synthetic procedure,
consequently size, shape and distribution of embedded
MNP,1c,59 the metal-support interactions,60 the mechanism of
hydrogen transfer.61 The latter, when involving a ionic
hydrogen transfer mechanisms, may result in an activity
enhancement for the protonated form of the catalyst. In our
case, the diverse preparation methods required for supported
MNP prevents us from a full understanding of our observations.
Various models have been proposed to rationalise the role of
acidic support in the catalytic activity of arene
hydrogenations.62 In the case of the protonated resin, we may
speculate that the aromatic molecules are adsorbed on the acid
sites in the form of incipient arenium ions,63 that would reduce
the strength of substrate chemisorption onto the rhodium
surface, resulting in a low hydrogenation activity. An analogous
“deactivation” effect of aromatic substrates by Brønsted acidic
conditions. Direct conversion of
5 to 5b with no intermediate
detection of 5a was possible using Rh@D400Li+in at higher
temperatures.
The different behaviour exhibited by Rh@DLi+ and Rh@DH+
catalysts in olefins versus aromatic bonds hydrogenation is very
difficult to justify. Extreme care should be taken when
comparing the role of the support in MNP-based heterogeneous
catalysts since a variety of additional factors may affect the
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Table 4 Selected data for the catalytic hydrogenation reactions of arenes by Rh@D400Li+in.a
T
(°C)
P H2
(bar)
Time
(min)
Conversion
(%)
Selectivity b
(%)
cis / trans
ratio
TOFc
(h-1)
Substrate
Product
5
6
5b
6a
rt
rt
1
1
380
345
99.8
95.0
89.9
-
-
36
38
100.0
O
O
O
rt
10
15
10
10
240
240
240
240
98.0
63.7
71.7
97.2
100.0
-
-
-
-
56
58
7
8
9
7b
8a
9a
60
rt
100.0
73.8
O
O
O
d
-
O
O
60
100.0
58
57
O
O
rt
10
240
99.3
7.2
-
d
10
11
10a
11a
rt
rt
10
10
240
240
7.5
17.3
-
-
100.0
100.0
76:24
58
d
rt
10
15
240
240
63.2
100.0
100.0
94:6
92:8
-
12
13
12a
13a
60
100.0
58
58
O
O
O
O
40
rt
15
10
240
240
100.0
31.7
70.9
85.2
97:3
100
d
O
O
O
O
14
15
14a
15a
-
O
O
d
60
40
4320
76.2
37.1
91:9
-
NH2
NH2
a
b
Reaction conditions: 0.16 M methanol solution, substrate : metal = 230 : 1 molar ratio, 50 mg catalyst, 1.3% (w/w) Rh. Data from GC analysis.
Selectivity at the given conversion, as: (mol product indicated) / (mol substrate converted). c Turnover frequency, as: (mol substrate converted) / (mol Rh
× h) calculated at the conversion indicated and bulk Rh content. d Not calculated.
OCH3
OCH3
OH
OCH3
supports was previously reported for the hydrogenation of
styrene by RhNP onto either H+ or Na+-exchanged mesoporous
aluminosilicates.64 A similar behaviour was described for the
hydrodechlorination reaction of 1,2,4-trichlorobenzene by
PdNP onto alkali-modified hydrotalcite.65
H3CO
OCH3
a
t
C
+
+
+
H2 CH3OH
,
7
c
7
a
7
7
b
7d
Assuming that Rh@D400Li+in is the most efficient of the
prepared catalyst in the hydrogenation of arenes, we then
extended our study to the hydrogenation reaction of other
substituted monocyclic arenes. Selected data obtained under
mild reaction conditions are summarized in Table 4, wherein
selectivity to the indicated products and TOF values calculated
at ≥ 95% conversions are reported for comparative purposes.
Rh leaching in solution was below the ICP-OES detection limit
in any case.
Scheme 3 Products of catalytic hydrogenation reaction of 7 by Rh@D400Li+in.
to methylcyclohexane (6b) was fully achieved with comparable
efficiency (TOF 38 h-1) under 1 bar H2 and room temperature,
whereas the Rh@D400H+ catalyst was much less effective.
Toluene hydrogenation by solid supported-Rh(0) catalysts was
previously described with various materials, including carbon
nanotubes,66 alumina67 and silica.68 The catalytic activity of
Rh@D400Li+in well compares with the reported systems, while
adding the advantages of easy recover and reuse. Best TOF
The reaction profile for the hydrogenation of toluene (
6) was
similar to that above described for (Fig. 7). Selective conversion
5
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values were reported for 5% RhNP onto MgO (solvent-free, 100 have also seen contradictory results, e.g. in the hydrogenation
°C, 10 bar H2)38 and for tetrahedral RhNPs onto charcoal (in of
methanol, rt, 1 bar H2), the latter after a troublesome shape-
controlled synthesis, however.69
7 vs. 8.
In order to ascertain the stereoselectivity of the
Rh@D400Li+in catalyst, the hydrogenation of disubstituted
The hydrogenation of anisole (7) was investigated under 10 arenes was finally tested. Whatever the substrate, the
bar H2 and room temperature showing good activity, but preferential formation of the kinetically favoured cis isomer
modest selectivity to the desired hydrogenation product was observed over the thermodynamically favoured trans one
cyclohexyl methyl ether (7b, Scheme 3), since variable (Table 4), as usually reported for comparable heterogeneous
amounts of the hydrogenolysis products 7c, 7d and the acetal systems, wherein the cis/trans ratio is affected by the nature
7a, that is formed by the acid-catalysed addition of methanol to and the position of the substituents and by the reaction
the partial hydrogenation product 1-methoxy-cyclohexene (not temperature.77 Thus, the hydrogenation of para- (11) and ortho-
detected), were observed, as already reported for other xylene (12) gave the dimethyl-cyclohexane derivatives 11a and
rhodium(0)-based catalysts.70 Residual acid catalytic activity of 12a in a cis/trans ratio of 76:24 and 92:8 at full substrate
Rh@D400Li+in could be attributed to spurious Brønsted acidity conversion, respectively, tough slightly harder reaction
or to the intrinsic Lewis acidity of the rhodium metal sites.71 conditions were required to achieve comparable reaction rate in
Selectivity could be improved by the adoption of slight the latter case (60° C and 15 bar vs. rt and 10 bar). Similar
overhydrogenation conditions (60 °C, 15 bar) to give 7b in 72% results were previously reported for RhNP onto boehmite
yield.
nanofibers78 or Rh@MWCNT,24 and justified in terms of
The hydrogenation of substrates bearing electron- limited Rh-sites accessibility of C=C bonds due to the larger
withdrawing substituents (EWG) was also examined using steric hindrance of 12. Compared to the hydrogenation reaction
methyl benzoate (
The reaction of
8
8
), acetophenone (
9) and benzophenone (10). of the parent monosubstituted arene, i.e. toluene (6), both 11
under 10 bar H2 and room temperature and 12 showed to be hydrogenated with much less efficiency
resulted in incomplete substrate conversion (74%) and traces under the same reaction conditions (rt and 1 bar, TOF = 20 h-1
(1.5%) of the partial hydrogenation product methyl-1,2,3,6- and 3 h-1, respectively), as commonly found with increasing the
tetrahydrobenzoate. Full conversion with 100% selectivity to number of substituents.2b
methyl-cyclohexanecarboxylate (8a) required 60 °C and 10 bar
H2. Similarly, the hydrogenation of under 10 bar H2 and room line with the selectivity results above described for
temperature resulted in incomplete aromatic ring the desired aromatic hydrogenation product 1-methoxy-2-
The hydrogenation reaction of 2-methylanisole (13) was in
9
7
. Hence,
hydrogenation, with concurrent preferential formation of methylcyclohexane (13a) was obtained in 71% yield at 100%
deoxygenation and C=O bond reduction by-products, to give 2- substrate conversion under 40° C and 1 bar H2, with a cis/trans
cyclohexylethanone (9a) in 7.2% selectivity at 99% substrate ratio of 97:3. By-products due to acetalization and
conversion. Ring deactivation effect and scarce resistance of hydrogenolysis were observed. A similar performance was
ketonic group toward reduction was confirmed by the previously reported for rhodium colloids impregnated onto
hydrogenation
of
10
which
provided silica which afforded 13a in a 98:2 cis/trans ratio.79 The
cyclohexyl(phenyl)methanone (10a) in 1.3% yield under the hydrogenation of methyl-2-methylbenzoate (14) was possible
above conditions. Similar performances were previously under room temperature and 10 bar, although with slow kinetic,
reported in the hydrogenation of
supported and colloidal RhNP catalysts under comparable, or
8
,
9
and 10 using other solid- to give selectively methyl-2-methylcyclohexanecarboxylate
14a) as single cis isomer, together with minor amounts of
(
more drastic, reaction conditions.2b Particularly, in a recent dihydrogenated products. The hydrogenation of methyl-2-
study the low chemoselectivty observed in acetophenone aminobenzoate (15) was very slow, as 76.2% conversion was
hydrogenation was justified by the fact that interaction of the achieved after 72h, resulting in the formation of 15a with a
substrate with rhodium occurs via the aromatic ring, so that cis/trans ratio of 91:9 and considerable amount of
proximity of the carbonyl group results in the reduction of the dihydrogenated products. Deactivation of 15 was previously
two groups at comparable rates.72
attributed to strong substrate adsorption on the metal surface
The rate of aromatic hydrogenation at Rh surface in through the amino group.80 An 80% yield with a cis/trans ratio
monosubstituted arenes was previously related to both steric of 6 after 210 h reaction time was reported using Ru colloids.
and electronic factors. While reduction is usually comparatively To the best of our knowledge no supported rhodium catalysts
slower for longer alkyl substituents,73 effect of EWG and have been reported for the hydrogenation reaction of 14 and 15
electron-donating groups (EDG) is less certain.74 It has been
.
proposed that the hydrogenation is faster for EDG substituents Conclusions
because of the better adsorption of electron-rich substrates on
On the route to a sustainable nanocatalysis, we have developed
a friendly and reproducible method for the growth of RhNP
within the pores of commercially available insoluble polymers.
The synthetic procedure involves the atomic-level distribution
of an appropriate Rh precursor within the solid matrix by
the metal surface.75 On the other hand, low rates observed for
EDG groups have been attributed to the higher energy barrier
toward hydrogenation due to π-complexation of the aromatic
ring.76 Substantially both hypotheses could explain our data
since, despite pointing out to a EWG-deceleration effect, we
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impregnation,81 followed by reduction using clean reducing
agents (H2), low energy inputs (rt, 1 bar) and without
stabilizers or long conditioning steps, thus accomplishing with
most Principles of Greener Nanomaterial Production.82 The as-
prepared supported RhNP have shown to efficiently catalyse
the hydrogenation reaction of C=C bonds under very
undemanding conditions, as well as to be easily recovered with
neither activity loss nor metal leaching in solution upon reuse.
Following the same paradigm pursued for the corresponding
palladium systems, we have explored the effect of both the
preparation method and the ionic form of the support on the
catalyst performance. It has been shown that, while the
hydrogenation of alkenes is best achieved using the supported
catalyst in the protonated form, the hydrogenation of aromatics
is much more favourably accomplished using the lithiated
catalyst prepared in-situ under catalytic conditions. The latter
system allows for the heterogeneous-phase hydrogenation of
mono- and di-substituted arenes to take place in one-pot at
temperatures below 60 °C and H2 pressures below 15 bar, thus
representing a sustainable option for this reaction.
9
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