A new approach for the preparation of well-defined Rh and Pt nanoparticles stabilized by phosphine-functionalized silica for selective hydrogenation reactions

Article abstract of DOI:10.1039/c6cc10338c

In this work, a new methodology for the synthesis of well-defined metallic nanoparticles supported on silica is described. This methodology is based on the surface control provided by SOMC. The nanoparticles are formed via the organometallic approach and are catalytically active in the hydrogenation of p-xylene, 3-hexyne, 4-phenyl-2 butanone, benzaldehyde, and furfural.

Full text of DOI:10.1039/c6cc10338c

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
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New approach for the preparation of well defined Rh and Pt
nanoparticles stabilized by phosphine-functionalized silica for
selective hydrogenation reactions
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
b
b
b
a,c
b
DOI: 10.1039/x0xx00000x
J. Llop Castelbou, K. C. Szeto, W. Barakat, N. Merle, C. Godard,* M. Taoufik* and C.
Claver*
a,c
www.rsc.org/
In this work, a new methodology for the synthesis of well defined phosphorus based ligands have been extensively used in
metallic nanoparticles supported on silica is described. This homogeneous catalysis and recently, their use as efficient
methodology is based on the surface control provided by SOMC. stabilizers of soluble Pd, Ru, Ir and Rh nanoparticles was
6
The nanoparticles are formed via the organometallic approach reported. These catalysts have controllable size and shape and
and are catalytically active in the hydrogenation of p-xylene, 3- show interesting activities and selectivities in various catalytic
hexyne, 4-phenyl-2 butanone, benzaldehyde, and furfural.
reactions. It was therefore thought that the treatment of the
support by efficient M-NPs stabilizing groups through SOMC
could provide well defined and monodispersed materials.
Several examples of supported metallic nanoparticles on silica
Nowadays, the synthesis of well defined metallic nanoparticles
(M-NPs) has become a major challenge since they are utilized
7
in a variety of fields with applications in biomedicine, optical
1
electronics and catalysis. M-NPs are constituted by several
are reported in the literature. However, in this work, a new
methodology combining the surface control provided by SOMC
and the advantages of the organometallic approach is
described, producing supported nanoparticles with controlled
size, shape and environment. The catalytic performance of the
resulting materials was examined in the hydrogenation of p-
metallic atoms, possess a nanometric scale (1-100 nm of
diameter) and can be stabilized by a wide range of compounds
such as polymers, surfactants, ionic liquids, silane or electron
2
donor ligands. The main disadvantage when dealing with
soluble nanoparticles in catalysis is the loss of activity caused
by agglomeration and the impossibility of their recycling and
reuse. The use of supported catalysis is therefore required to
ensure the sustainability of the processes. Several approaches
were reported for the immobilization of metal nanoparticles
xylene, 3-hexyne, 4-phenyl-2-butanone, benzaldehyde
, and
furfural. This latter substrate is an abundant feedstock derived
8
from biomass.
Initially, the silica supported rhodium nanoparticles were
synthesized under the conditions previously reported for non-
3
6
f,6g
such as impregnation, co-precipitation or deposition.
supported systems.
as wt%, in analogy with the classical heterogeneous
supported catalysts. The rhodium precursor [Rh(ɳ -C
reduced under 4 bar of hydrogen at 40 °C in the presence of
triphenylphosphine (PPh ) and silica previously dehydroxylated
The amount of rhodium was calculated
Nevertheless, low control on the size and distribution of the
supported species is often observed following these
methodologies. For example, classical impregnation on
conventional supports usually provides various dispersions of
the metal and the size of the obtained species can be limited
5
3
3 5 3
H ) ] was
3
at 700 °C (SiO2-700) (Figure 1). The solution turns black after a
few hours and after washings with pentane and drying under
4
by the pore size of the supports.
Surface Organometallic Chemistry (SOMC) is an established
methodology that affords well defined heterogeneous
catalysts through the grafting of organometallic species on
surface hydroxyls of classical supports such as alumina, silica,
2
vacuum, the material Rh-NPs@SiO was obtained.
5
mesoporous silica, silica-alumina, etc... On the other hand,
a
Departament de Química Física
Marcel·li Domingo s/n, Campus Sescelades, 43007, Tarragona, Spain.
i Inorgànica, Universitat Rovira I Virgili, C/
b
C2P2, team COMS (CNRS-UMR 5265) Université Lyon 1, ESCPE Lyon 43 Boulevard
du 11 Novembre 1918, 69626 Villeurbanne Cedex, France.
Centre Tecnologic de la Química, C/Marceli Domingo s/n, Campus Sescelades,
Figure 1 Scheme of the synthesis of silica supported rhodium PPh
NPs@SiO
3
stabilized NPs M-
c
2
.
4
3007 Tarragona, Spain
Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available:
See
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According to TEM analysis, the nanoparticles Rh-NPs@SiO
Journal Name
2
were formed with spherical shape, a mean size of 2.55 ± 0.42
nm and moderate distribution (Figure2, left).
9
Based on the size obtained by TEM and the VHH model, it was
calculated that 45 % of the rhodium atoms are at the surface
of these nanoparticles. A corresponding experiment in the
absence of silica provided nanoparticles with smaller size and
narrower distribution (1.52 ± 0.21, Figure S5). Note that Rh-
NPs@SiO contains a small fraction of formed nanoparticles
2
with no interaction with the support (Figure 2, right). The same
methodology was applied to obtain platinum nanoparticles
2
Figure 2 TEM micrographs and size distribution of nanoparticles Rh-NPs@SiO . (20 nm scale bar)
(
Figure 1). The organometallic precursor Pt(dba)
3
was reduced
and
in THF under 3 bar of H , at 25 °C in the presence of PPh
2
3
The new material Rh-NPs@P-SiO
2
was characterized by TEM
silica previously dehydroxylated at 700 °C (SiO2-700). The
(Figure 3) and a homogeneous distribution of the
solution darkened after a few hours. The resulting material, Pt-
nanoparticles on the support was observed. It is noteworthy
that for this sample, all the NPs detected were situated onto
2
NPs@SiO , was then washed three times with pentane. After
evacuation of the volatiles and drying under vacuum, a brown
powder was obtained. TEM microscopy confirmed the
2
the P-SiO support, in contrast with the results obtained for
unfunctionalized silica (Figure 2). In addition, using this new
strategy, NPs with spherical shape, a mean diameter of 1.17±
2
formation of the platinum nanoparticles Pt-NPs@SiO that
exhibited spherical shape and a narrow size distribution, with a
mean diameter of 1.60± 0.40 nm (figure S7). However, as
previously observed for rhodium, some metal particles were
observed outside the support.
0
.18 nm and narrow size distribution were obtained.
Interestingly, the supported NPs were slightly smaller than the
6f,6g
non-supported analogous previously reported (1.5 nm),
indicating the potential stabilizing role of the phosphine
The synthesis of the nanoparticles Rh-NPs@SiO
2
and Pt-
2
modified silica surface P-SiO . This result is in agreement with
NPs@SiO stabilized by PPh in silica700 was thus completed.
3
2
previous reports describing that supported NPs are usually
12
smaller than the corresponding unsupported systems.
31
The material was also analyzed by P CP MAS solid state NMR,
However, the presence of supported and unsupported NPs in
both cases was expected to constitute an issue in catalysis.
In view of these results, it was thought that the appropriate
functionalisation of the support could overcome this issue.
revealing a main signal at 21 ppm, a small resonance at 26
ppm and a very broad signal at 42 ppm (Figure S8). The
chemical shift of the main peak was attributed to Rh
coordinated phosphorus atoms surrounded by phenyl moieties
Recently,
a method was used for the anchoring of
functionalized groups on silica surface, leading to new hybrid
10
materials used as supports. Since phosphines efficiently
stabilized such Rh NPs, the use of a silica support containing
grafted triphenylphosphine units at the surface was explored.
The use of such material as catalyst was recently published by
13
based on reported data. However, a contribution of the
oxidized form of the stabilizing ligand could be present in this
broad band as previously observed for ruthenium
14
nanoparticles. The broad signal at 42 ppm could correspond
11
our groups. This functionalized support, defined in this work
as P-SiO , was therefore used for the grafting of the rhodium
nanoparticles.
to partially reduced ligand interacting with the nanoparticles
2
(Figure S8). Hydrogenation of the stabilizing ligands and
3
Initially,
[Rh(ɳ -C
3
H
5
)
3
]
and
Pt(dba)
3
stabilization by their oxidized form were previously reported
15,16
organometallic precursors were reduced under the same
previously described conditions in the presence of the
for rhodium and ruthenium nanoparticles.
Significantly, the
peak corresponding to the P-SiO at ca. -6 ppm was not
2
phosphine modified silica P-SiO
2
.
TEM measurements
observed, revealing the complete interaction of the phosphine
anchored to the support with the rhodium nanoparticles. The
elemental analysis of this material revealed a content of 1.27
confirmed the formation of the nanoparticles. However, a
significant increase in size was observed for both systems, ca.
4
nm for Rh and 1.9 nm for Pt (see S3). Moreover, some
wt% in phosphorus and 4.36 wt% in rhodium. Based on the
15
VHH model, the Rh-NPs@P-SiO system contains 76 % of the
2
degree of agglomeration was detected. It was concluded that
the high dispersion of phosphine at the surface could not
ensure an efficient stabilization of the nanoparticles alone.
To enhance the nanoparticles stabilization, triphenylphosphine
ligand was added in addition of the phosphine contained on
Rh atoms at their surface. This value is considerably higher
than that of the non-modified silica Rh-NPs@SiO (45%).
2
The same procedure was employed for the synthesis of the
corresponding platinum nanoparticles with 5 wt% in metal.
2
the support. The phosphine modified silica support P-SiO ,
The organometallic precursor Pt(dba)
under 3 bar of H at 25 °C in the presence of PPh
material P-SiO . The solution turned dark after a few hours.
The resulting material Pt-NPs@P-SiO was then washed with
3
was reduced in THF
3
3 5 3
excess triphenylphosphine and [Rh(ɳ -C H ) ] were placed in
2
3
and the
THF and treated overnight under 4 bar of hydrogen at 40 °C.
The solution turned black after a few hours. Washings with
pentane were performed and after evacuation of the volatiles
2
2
pentane and dried under vacuum. A brown powder was
obtained. TEM microscopy confirmed the formation of the
a black powder corresponding to the material Rh-NPs@P-SiO
2
was obtained.
platinum nanoparticles Pt-NPs@P-SiO
2
homogeneously
2
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distributed on the support. These NPs exhibited spherical system was studied: after the third run, the catalyst was
shape, a diameter of 1.47±0.25 nm and a narrow size filtered and more substrate was added to the remaining
distribution (Figure S9).
solution, and the same catalytic conditions were applied (entry
). The metal leaching in the solution was also measured by
4
ICP OES, confirming no relevant Rh leaching during catalysis. In
this experiment, no conversion was observed, indicating the
absence of leached nanoparticles in solution.
Table 1 Catalytic hydrogenation using the supported Rh and Pt-NPs.
Figure 3 TEM micrograph and size distribution of nanoparticles Rh-NPs@P-SiO
material. (50 nm scale bar)
2
, scheme of the
All the NPs detected were onto the support. The material Pt-
NPs@P-SiO was also characterized by solid state NMR
2
spectroscopy (Figure S10). The elemental analysis of this
material revealed a content of 0.61 % in phosphorus and 3.46
%
a
% full
hyd.
(Cis/
entry
NPs
Subs.
% a
% ket.
b
%
in platinum. The number of surface atoms for the Pt-
Conv.
Trans)
NPs@P-SiO is of 68 %, which smaller than the Rh system due
2
1
2
3
4
5
6
7
8
9
Rh-NPs@P-SiO
Rh-NPs@P-SiO
Rh-NPs@P-SiO
-
2
2
2
1
1
100
100
95
-
-
-
-
-
-
-
-
100
100
100
-
58/42
to the slightly larger diameter of these nanoparticles.
These results indicated that the phosphine anchored on the
silica support plays an important role in the stabilization of
both Rh- and Pt-NPs. The synthesis of the nanoparticles
described in this work can therefore be considered an optimal
methodology to obtain controlled systems of supported
nanoparticles of small size and well distributed.
61/39
1
62/38
1
0
-
Rh-NPs@P-SiO
Rh-NPs@P-SiO
Rh-NPs@P-SiO
Rh-NPs@P-SiO
2
2
2
2
4
100
97
100
42
3
-
7
5
-
53
97
100
-
-
To evaluate their catalytic performance, the supported
2 2
nanoparticles Rh-NPs@P-SiO and Pt-NPs@P-SiO were tested
11
15
1
99
-
in the hydrogenation of p-xylene, 3-hexyne, 4-phenyl-2
butanone, benzaldehyde, and furfural. Although a negative size
effect was previously reported in the hydrogenation of
16
-
-
-
Pt-NPs@P-SiO
Pt-NPs@P-SiO
2
32
-
100
13
-
100/0
1
7
xylenes, these NPs were selected based on previous results
from our group which showed that such small NPs were
1
0
2
7
100
-
87
100
-
-
efficient catalysts for the hydrogenation of substituted arenes 11 Pt-NPs@P-SiO2 15
100
-
6
g
such as xylenes and methylanisoles. The reactions were General conditions: 2 mol % NPs, 2 ml of heptane, T = 80 °C,P= 40 bar H
2
, t= 4 h;
carried out at 80 °C under 40 bar of H
2
during 4h using 2 mol %
a
Determined by GC. % a: amount of product from the hydrogenation of the arene group; %
catalyst. The preliminary results using p-xylene
1
as substrate ket.: % of product from the hydrogenation of the ketone group; % full hyd. : amount of fully
b
are summarized in table 1. When the rhodium nanoparticles hydrogenated product; Cis/trans selectivity
Rh-NPs@P-SiO
2
were used, full conversion was obtained
When 3-hexyne
conversion was observed and the only product detected was
hexane . Next, the substrate 4-phenyl-2 butanone 7, which
4 was tested as substrate (entry 5), full
under these conditions (entry 1). The products were detected
by GC-MS in a 58/42 cis/trans ratio. In contrast, the platinum
catalyst only provided 32% conversion but with total selectivity
to the cis product, in agreement with reported results (entry
6
contains two reducible groups, namely a ketone and an
aromatic ring, was tested using both catalysts. The results
obtained are summarized in Table 1 (entries 6 and 10). When
the rhodium nanoparticles Rh-NPs@P-SiO2 were used, almost
full conversion was obtained (97%) providing 53 % of 4-phenyl-
1
8
1
0). The Rh-NPs@P-SiO
2
catalyst was therefore more active
than the Pt-NPs@P-SiO counterpart and in both cases, the cis
2
isomer was the preferred product. However, the platinum
system only formed this product. No partially hydrogenated
products were obtained using these catalysts, in contrast with
previous work on colloidal phosphine stabilized rhodium
2
butanol
9, 42 % of the completely reduced product 10 and
only 5 % of the cyclohexylketone derivative
8. It was concluded
from this results that there was no significant preference for
the hydrogenation of the ketone or that of the aromatic ring
using this Rh catalyst. Using the platinum catalyst, full
conversion was observed with 87 % selectivity for 4-phenyl-2-
6
f,6g
nanoparticles.
In view of these results using Rh-NPs@P-SiO
recyclability test was performed. The catalyst was reused
entries 2 and 3), observing a slight decrease in the conversion
2
as catalyst, a
(
butanol
9
and 13 % to the completely hydrogenated product
were detected
after the third run (entry 3), no significant differences in terms
of selectivity was detected. Additionally, the stability of the
1
0. No traces of the cyclohexyl derivative 8
using this catalyst. In the case of the platinum catalyst, there is
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clearly a preference for the hydrogenation of the ketone group 1 G. Schmid, in Nanoparticles: from theory to application, Ed
Wiley-VCH, Germany, 2004, vol.1, ch.2, pp 4-44.
over that of the aromatic ring. Benzaldehyde 11, that contains
2
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an aromatic and an aldehyde was used as substrate with the
Rh system (entry 7). In this case, excellent conversion (99 %)
was observed with preferential hydrogenation of the aldehyde
group, producing benzyl alcohol as main product (97 %).
2
3
a) C. J. Jia, F. Schuth, Phys. Chem. Chem. Phys. 2011, 13
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,
457. b) M.E. Grass, y. Zhang, D.R. Butcher, J.Y. Park, Y. Li H.
In view of the selectivity obtained with these systems as
catalysts, the more challenging substrate furfural 15 was
Bluhm, K.M. Bratile, T. Zhang, G.A. Somorjai, Angew. Chem.
Int. Ed. 2008, 47, 8893.
1
4
tested. The hydrogenation of furfural can provide furfuryl
alcohol that is an important precursor in the synthesis of a
4
5
C. Hubert, E. Guyonnet Bilé, A. Denicourt-Nowicki, A.
Roucoux, Appl. Catal. A: General, 2011, 394, 215.
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1
9
wide variety of chemicals. The selective formation of this
product by hydrogenation of furfural is still a challenge.
Several authors have explored the modification of supported
catalysts by tuning the size, shape, supports and surface
modifiers to promote this selectivity. For example, the
modification of silica supported Pt nanoparticles with Sn
improves the selectivity from 36 to 98 % to furfuryl alcohol 16
6
a) D. Conzález- Gálvez, P. Nolis, K. Philippot, B. Chaudret,
P.W.N.M. van Leeuwen, ACS Catal. 2012,
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2, 317. b) P. Lara, K.
5, 28. c) B.
6
2
0
2
by poisoning the sites responsible for the side reactions.
Gual, R. Axet; K. Philippot, B. Chaudret, A. Denicourt-
Nowicki, A. Roucoux, S. Castillon, C. Claver, Chem. Commun.
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Blondeau, B. Chaudret, S. Castillón, C. Claver, C. Godard,
In this work, we investigated whether the phosphine units
present in our systems could have an analogous effect. Both
systems showed remarkable selectivities to the desired
product (Table 1, entries 8 and 11), showing a beneficial effect
of the phosphorus present in our systems. Both catalytic
systems provided total selectivity to furfuryl alcohol 16
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3
,
2
7
,
although Rh-NPs@P-SiO
than the Pt-NPs@P-SiO
2
provided lower conversion (16 %)
system that fully converted the
5, 3512. c) S.L.
2
1
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In conclusion, a new methodology for the preparation of silica
supported rhodium and platinum nanoparticles has been
successfully developed using
a
phosphine functionalized
8
9
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synthesized in a one-pot manner using the organometallic
approach. The characterization of these systems showed a
homogeneous distribution of the metallic nanoparticles on the
support, exhibiting a small size of ca. 1.2 and 1.5 nm for Rh and
Pt respectively. These new supported systems were active in
the hydrogenation of aromatics and ketones. Remarkably,
promising results were obtained for the selective
hydrogenation of furfural to furfuryl alcohol, with high
activities in the case of the Pt catalyst. The phosphine
stabilized moiety could play an important role in the selectivity
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The authors are grateful the Spanish Ministerio de Economía y
Competitividad and the Fondo Europeo de Desarrollo Regional
FEDER (CTQ2016-75016-R, AEI/FEDER, UE) and the Generalitat
de Catalunya (2014SGR670) for financial support.
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1
,
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Gustafson, Y. Pei, Z. Qi, D.D. Johnson, S. Zhang, F. Tao, W.
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Notes and references
4
| J. Name., 2012, 00, 1-3
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