One-Pot Cooperation of Single-Atom Rh and Ru Solid Catalysts for a Selective Tandem Olefin Isomerization-Hydrosilylation Process

Article abstract of DOI:10.1002/anie.201915255

Realizing the full potential of oxide-supported single-atom metal catalysts (SACs) is key to successfully bridge the gap between the fields of homogeneous and heterogeneous catalysis. Here we show that the one-pot combination of Ru₁/CeO₂ and Rh₁/CeO₂ SACs enables a highly selective olefin isomerization-hydrosilylation tandem process, hitherto restricted to molecular catalysts in solution. Individually, monoatomic Ru and Rh sites show a remarkable reaction specificity for olefin double-bond migration and anti-Markovnikov α-olefin hydrosilylation, respectively. First-principles DFT calculations ascribe such selectivity to differences in the binding strength of the olefin substrate to the monoatomic metal centers. The single-pot cooperation of the two SACs allows the production of terminal organosilane compounds with high regio-selectivity (>95 %) even from industrially-relevant complex mixtures of terminal and internal olefins, alongside a straightforward catalyst recycling and reuse. These results demonstrate the significance of oxide-supported single-atom metal catalysts in tandem catalytic reactions, which are central for the intensification of chemical processes.

Full text of DOI:10.1002/anie.201915255

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition
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Accepted Article
Title: One-pot cooperation of single-atom Rh and Ru solid catalysts for
a selective tandem olefin isomerization-hydrosilylation process
Authors: Bidyut B Sarma, Jonglack Kim, Jonas Amsler, Giovanni
Agostini, Claudia Weidenthaler, Norbert Pfaender, Raul
Arenal, Patricia Concepción, Philipp Plessow, Felix Studt, and
Gonzalo Prieto
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201915255
Angew. Chem. 10.1002/ange.201915255
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10.1002/anie.201915255
Angewandte Chemie International Edition
RESEARCH ARTICLE
One-pot cooperation of single-atom Rh and Ru solid catalysts for
a selective tandem olefin isomerization-hydrosilylation process
Bidyut B. Sarma[a], Jonglack Kim,[a] Jonas Amsler[b], Giovanni Agostini[c], Claudia Weidenthaler,[a]
Norbert Pfänder[d], Raul Arenal[e,f,g],Patricia Concepción[h], Philipp Plessow[b], Felix Studt[b], and
Gonzalo Prieto*[a,h]
supported metal for catalysis while displaying a higher site structural
homogeneity –which is expected to translate into superior catalytic
selectivity– compared to supported catalysts based on metal (oxide)
clusters or nanoparticles. Atomically dispersed supported metals
often exist in a cationic state, as their full reduction to a zero-valent
state implies that bonds to the oxide support are cleaved, which is
typically followed by high adatom surface mobility and agglomeration
even at relatively mild temperatures.[2] Their cationic nature,
monoatomicity and defined coordination environment make oxide-
supported SACs excellent candidates to bridge the gap between the
disciplines of heterogeneous and homogeneous catalysis,[3]
particularly in a number of areas which have been traditionally
dominated by molecular complex catalysts applied in solution.
Hence, SACs have been recently explored for reactions classically
catalyzed by cationic metal salts or complexes, including olefin
Abstract: Realizing the full potential of oxide-supported single-atom
metal catalysts (SACs) is key to successfully bridge the gap between
the fields of homogeneous and heterogeneous catalysis. Here we
show that the one-pot combination of Ru1/CeO2 and Rh1/CeO2 SACs
enables
a highly selective olefin isomerization-hydrosilylation
tandem process, hitherto restricted to molecular catalysts in solution.
Individually, monoatomic Ru and Rh sites show a remarkable
reaction specificity for olefin double-bond migration and anti-
Markovnikov α-olefin hydrosilylation, respectively. First-principles
DFT calculations ascribe such selectivity to differences in the binding
strength of the olefin substrate to the monoatomic metal centers.
The single-pot cooperation of the two SACs allows the production of
terminal organosilane compounds with high regio-selectivity (>95%)
even from industrially-relevant complex mixtures of terminal and
internal olefins, alongside a straightforward catalyst recycling and
reuse. These results demonstrate the significance of oxide-
supported single-atom metal catalysts in tandem catalytic reactions,
which are central for the intensification of chemical processes.
hydroformylation,[5]
hydrochlorination,[7] or C-C coupling.[8]
olefin
hydrosilylation,[6]
alkyne
A relevant area where SACs can have a profound impact is
tandem catalysis, i.e. the integration of two catalysts in a single pot
to achieve sequential transformations in a direct manner.[9] Tandem
catalytic processes hold the promise for a multifold contribution to
chemical process intensification by (i) circumventing the need for
energy and cost-intensive isolations of intermediate products; (ii)
improving safety and selectivity by minimizing the residence time of
highly reactive or unstable intermediate products in the reaction
medium; and (iii) overcoming thermodynamic bounds to reaction
yields, e.g. driving reversible reactions to completion via the in situ
processing of a reaction product in a subsequent irreversible
catalytic step. The concept of tandem catalysis originated in the field
of homogeneous catalysis with soluble molecular complexes.[11]
Initial approaches to combine two different catalysts in the same
reaction medium, while preventing undesired (often self-
deactivating) mutual interactions, relied on the compartmentalization
of the molecular catalysts in soft dendrimer or micelle
nanocapsules.[12] Oxide-supported SACs hold the potential to
inaugurate an advanced generation of tandem catalysts as they
reconcile key features of most organometallic catalysts, i.e. well-
Introduction
Isolated metal atoms stabilized on the surface of oxide carriers
attract great attention as active sites in heterogeneous catalysis.[1]
Often referred to as single-atom catalysts (SACs), these materials
hold the potential to achieve a quantitative surface exposure of the
[a]
Dr. Bidyut B. Sarma, Jonglack Kim, Dr. Claudia Weidenthaler, Dr.
Gonzalo Prieto
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
E-mail: Prieto@mpi-muelheim.mpg.de
[b]
Jonas Amsler$, Dr. Philipp Plessow$, Dr. Felix Studt$,#
$Institute of Catalysis Research and Technology (IKFT)
#Institute for Chemical Technology and Polymer Chemistry (ITCP)
Karlsruhe Institute of Technology (KIT)
Hermann-von-Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen,
(Germany)
[c]
[d]
[e]
Dr. Giovanni Agostini
ALBA Synchrotron Light Source
Carrer de la Llum 2-26,Cerdanyola del Vallès, Barcelona (Spain)
defined
monoatomic
sites,
with
an
intrinsic
catalyst
Norbert Pfänder
Max-Planck-Institut für Chemische Energiekonversion
Stiftstraße 34-36, 45470 Mülheim an der Ruhr (Germany)
compartmentalization in non-contacting solid matrices and a fully
inorganic composition, which endows them with superior mechanical
and thermal stability and facilitates catalyst reuse.
Dr. Raul Arenal
Herein we show that two solid catalysts based on Rh and Ru
isolated metal atoms, respectively, stabilized on the surface of CeO2
create a synergetic effect when combined in a single pot, enabling a
highly selective tandem olefin isomerization-hydrosilylation process.
Terminal organosilane compounds, which are of utmost
technological significance in areas such as functional coatings,
polymer cross-linking and the manufacture of a wide variety of
composite materials,[13] can be produced with similarly high regio-
selectivities from both terminal and internal olefin substrates, as well
as mixtures thereof.
Laboratorio de Microscopias Avanzadas (LMA), Instituto de
Nanociencia de Aragon (INA), Universidad de Zaragoza, Mariano
Esquillor s/n, 50018 Zaragoza, Spain.
[f]
Instituto de Ciencias de Materiales de Aragon, CSIC-Universidad de
Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain.
[g]
[h]
ARAID Foundation, 50018 Zaragoza, Spain.
Dr. Patricia Concepción, Dr. Gonzalo Prieto
ITQ Instituto de Tecnología Química, Universitat Politécnica de
València-Consejo Superior de Investigaciones Científicas (UPV-
CSIC), Av. Los Naranjos s/n, 46022 Valencia (Spain).
Supporting information for this article is given via a link at the end of the
document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201915255
Angewandte Chemie International Edition
RESEARCH ARTICLE
Figure 1: Synthesis of M/CeO2 catalysts by oxidative metal redispersion. Representative TEM micrographs for a) unannealed CeO2, b) CeO2 after
annealing in air at 1073 K, c) 0.2Rh/CeO2 catalyst, and d) 0.5Rh/CeO2 catalyst. e) Evolution of the BET specific surface area with the surface-specific metal
content for the series of M/CeO2 catalysts. As a reference, the specific surface area for the unannealed CeO2 support is also included in the plot.
Catalyst synthesis and characterization.
is decreased, likely via the binding of metal species to high-energy
surface sites, and thus CeO2 sintering is hampered. Such
stabilization is effective up to δ=1.0-2.0 Mat nm-2, beyond which
further metal loading does not add to surface stabilization and metal
agglomeration sets in, presumably due to the saturation of those
binding sites on CeO2 which stabilize dispersed metal species.
X-Ray photoelectron spectroscopy (XPS) and X-ray
Absorption Near-Edge spectroscopy (XANES) proved that all metals
exist in a cationic state in catalysts synthesized with metal contents
at which no crystalline metal (oxide) species develop, i.e. ≤2.0 Mat
nm-2 (Figures S7-S10). Core electron binding energies could be
ascribed to Pt(II), Rh(III) and Ru(IV) formal oxidation states,
respectively. Only in the case of Pt/CeO2 materials were additional
contributions from Pt(IV) oxide and metallic Pt(0) species detected at
metal contents >2.0 Pt nm-2, evidencing that Pt species aggregate
into PtO2 at these metal loadings. This oxide is known to be unstable
at the applied annealing temperature,[17] and it thus decomposed
partially into Pt0 crystals via the emission of lattice oxygen.
Cerium oxide was applied as a support for the synthesis of
SACs owing to its reported ability to stabilize transition metal cations
at high temperatures.[14] Initially, platinum was selected as active
metal, given the established dominance of Pt molecular complexes
as (pre)catalysts in conventional hydrosilylation processes.[15] In
addition, two series of materials were synthesized incorporating Rh
and Ru, respectively. High metal dispersions on the CeO2 surface
were induced via oxidative re-dispersion at 1073 K in stagnant air.[16]
The nominal metal content, expressed hereafter as a surface-
specific loading (δ) i.e. metal atoms per unit CeO2 surface area, was
systematically adjusted within 0.2-10.0 Mat nm-2 (M=Pt, Rh or Ru).
As illustrated in Figure 1, annealing of the metal-free CeO2
support at 1073 K resulted in significant crystal sintering. Ceria
nanocrystallites (5-20 nm) grew into larger (50-300 nm) and highly
faceted crystals, resulting in a 15-fold decrease in specific surface
area (90 to 6 m2 g-1). However, the deposition of transition metals
prior to annealing, already from very low metal contents, inhibits
sintering and stabilizes smaller CeO2 crystals after annealing.
Increasing the metal content from 0.2 Mat nm-2 to about 1.0-2.0 Mat
nm-2 led to a progressive increment in SBET up to 40-60 m2 g-1
(Figure 1e). This trend, which was observed regardless of the
identity of the metal deposited on CeO2, leveled off on further
increasing metal content, attaining a plateau surface area. Analysis
of the metal-loaded materials by Raman spectroscopy provided
evidences for the formation of M-O-Ce linkages upon annealing
(Figures S1-S3). Powder X-ray diffraction showed no diffraction
peaks other than those for CeO2 (Fm-3m) at metal contents <2.0 Mat
nm-2 for Pt/CeO2 and Ru/CeO2 catalysts, and <5.0 Mat nm-2 for
Rh/CeO2 catalysts, respectively, indicating that at lower loadings
metal species are highly dispersed as structures lacking long-range
order (Figures S4-S6). At higher metal contents, weak diffraction
signals for Rh2O3 and RuO2 emerged for the series of Rh/CeO2 and
Ru/CeO2, respectively, and increased in relative intensity with δ. In
the case of Pt/CeO2 catalysts, sharp diffractions for metallic Pt0 were
detected for δ>2.0 Pt nm-2, indicating also metal agglomeration and
crystallization. It is hence inferred from these results that, regardless
of the metal identity, metal species interact strongly with the CeO2
support upon high-temperature annealing. The oxide surface energy
Extended X-ray Absorption Fine Structure (EXAFS)
spectroscopy was applied to gain insight into the atomicity and
coordination environment of metal species. Figure 2 shows the
EXAFS spectra for M/CeO2 catalysts synthesized with various
surface metal contents. As a reference, data for the corresponding
bulk oxide and metal are also depicted. The corresponding spectra
in k-space are given in Figures S11-13. Regardless of the nature of
the supported metal, no discernible second-shell M-O-M scattering
contributions (r>2 Å) could be inferred for catalysts with δ≤1.0 Mat
nm-2, as an evidence for the existence of isolated metal atoms as the
only metal species. Contributions from second-shell M-O-M
coordination became apparent at δ≥2.0 Mat nm-2, and increased in
relative amplitude upon further increasing metal loading. In line with
XRD results, this shows that a fraction of metal species aggregate
into oxide (and metallic in the case of Pt) clusters beyond a metal
content, which is somewhat metal-dependent but in all cases >1.0
Mat nm-2. As EXAFS is sensitive also to species lacking long-range
atomic order, it reveals metal clustering already at contents at which
no metal (oxide) crystallites were detectable by XRD, e.g. δ=2.0 Rhat
nm-2 for Rh/CeO2 catalysts. The first-shell M-O average coordination
number (CN) was minimum for δ=1.0 Mat nm-2 regardless of the
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10.1002/anie.201915255
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RESEARCH ARTICLE
Table 1: Catalytic results for the hydrosilylation of 1-octene with different catalysts.
Et3SiH
+
+
SiEt3
1
3
2
Product
selectivity (%)
Entry
T
Time
(h)
Xa
Catalyst
Silane
(K)
(%)
1
2
3
1b
2
1.0Rh/CeO2
1.0Rh/CeO2
1.0Pt/CeO2
1.0Pt/CeO2
1.0Ru/CeO2
1.0Ru/CeO2
5.0Rh/CeO2
10Rh/CeO2
Et3SiH
-
393
393
393
393
393
393
393
393
393
393
393
2
2
2
2
2
2
5
5
5
5
5
98
n.d.f
99
96
-
4
-
-
-
3
Et3SiH
-
40
-
58
-
2
-
4
n.d.
99
5
Et3SiH
-
1
99
-
-
6
<1
-
-
7
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
99
94
93
81
97
70
6
-
8
53
6
1
-
c
9
1.0Rh/CeO2
99
19
2
d
10
11
Rh2O3
79
1
3
Rh/Ce
40
27
Reaction Conditions: 1-octene (5 mmol ), triethylsilane (5 mmol), catalyst (2 μmol, metal basis), P=10 bar ( N2, 99.999% purity). a Olefin conversion.b The 1H-NMR
spectra of the crude product is given in Figure S16 (Supplementary Information) c Catalyst activated by reduction at 623 K in flow of 20% H2/N2. d As received
from Sigma-Aldrich (99.8% purity). e As received from Sigma-Aldrich, 5 wt%Rh. fn.d.: not detected.
Figure 2: Atomicity of metal species in M/CeO2 catalysts. |FT| of the k3-weighted χ(k) EXAFS function in R-space for a) Pt/CeO2, b) Rh/CeO2 and c) Ru/CeO2
catalysts, as a function of the surface metal content (Mat nm-2). Radial distances are not phase-corrected. See Figs. S11-S13 for the corresponding spectra in k-
space. The spectra for bulk-type metal oxides and metallic foils have also been included for reference. Amplitude scale bars are identical for catalysts and
reference materials in each series. d-g) Representative Cs-HAADF-STEM micrographs for 1.0Pt/CeO2. Panel g) shows a close-up view of a nanoscale region in
panel f, where isolated Pt atoms have been red-circled as a guide to the eye. The corresponding 3D map of Z-contrast for the same region is given in panel h.
Arrows point to the atomic-size high-Z-contrast objects ascribed to isolated Pt atoms.
metal nature, suggesting that isolated metal centers are maximum at
this content (Tables S1-S3 and accompanying discussion in SI).
On the basis of our EXAFS results, a metal content of 1.0 Mat
nm-2 was deemed to maximize the abundance of atomically
dispersed species, while avoiding the coexistence of metal (oxide)
aggregates. Therefore, selected catalysts with δ=1.0 Mat nm-2 were
further investigated using Scanning-Transmission Electron
Microscopy (STEM) to get insights into the atomicity and spatial
distribution of the metal species. As shown in Figures 2f-h, isolated
Pt atoms could be identified on CeO2 owing to their comparatively
high Z-contrast. On the contrary, Rh (like Ru) contribute a notably
lower Z-contrast, which precluded a direct visualization of metal
atoms (Figure S14). Nevertheless, local EDX analysis proved the
presence of Rh species in nanoscale areas where no other
crystalline lattices aside that of the cubic CeO2 structure could be
discerned. In agreement with EXAFS analysis, these results indicate
that metal species did not form aggregates. Conversely, analysis of
catalysts with δ>2.0 Mat nm-2, showed plainly the presence of metal
SACs relies on an interplay between bulk (sub-surface) and surface
saturation phenomena. Catalysts exposing exclusively isolated metal
atoms on their surface can be synthesized at an intermediate metal
content of ca. 1.0 Mat nm-2, i.e. high enough to achieve solid-solution
saturation, albeit low enough to prevent "saturation" of metal-binding
centers on the CeO2 surface, beyond which metal clustering occurs.
Olefin hydrosilylation catalysis.
In order to evaluate the catalytic performance of atomically
dispersed metals, catalysts with 1.0 Mat nm-2 were tested in the
hydrosilylation of 1-octene in the presence of triethylsilane (Et3SiH)
as silylating reagent. The results are compiled in Table 1 (entries 1-
6). 1.0Pt/CeO2 showed to be active, and reached near quantitative
olefin conversion after 2 hours at 393 K. Nevertheless, the selectivity
to internal olefins (58%) exceeded that to the terminal 1,1,1-triethyl-
1-octylsilane (40%), indicating that olefin isomerization and
hydrosilylation pathways occur at comparable rates on this catalyst.
In marked contrast, with 1.0Rh/CeO2 the reaction proceeded to
(oxide) nanoparticles on the CeO2 surface (Figure S15). Collectively, almost full olefin conversion (96%), remarkably with 96% selectivity
our characterization results indicate that the synthesis of M/CeO2 to the anti-Markovnikov terminal octylsilane product. In the absence
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10.1002/anie.201915255
Angewandte Chemie International Edition
RESEARCH ARTICLE
Table 1: Catalytic results for the hydrosilylation of 1-octene with different catalysts.
Et3SiH
+
+
SiEt3
1
3
2
Product
selectivity (%)
Entry
T
Time
(h)
Xa
Catalyst
Silane
(K)
(%)
1
2
3
1b
2
1.0Rh/CeO2
1.0Rh/CeO2
1.0Pt/CeO2
1.0Pt/CeO2
1.0Ru/CeO2
1.0Ru/CeO2
5.0Rh/CeO2
10Rh/CeO2
Et3SiH
-
393
393
393
393
393
393
393
393
393
393
393
2
2
2
2
2
2
5
5
5
5
5
98
n.d.f
99
96
-
4
-
-
-
3
Et3SiH
-
40
-
58
-
2
-
4
n.d.
99
5
Et3SiH
-
1
99
-
-
6
<1
-
-
7
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
99
94
93
81
97
70
6
-
8
53
6
1
-
c
9
1.0Rh/CeO2
99
19
2
d
10
11
Rh2O3
79
1
3
Rh/Ce
40
27
Reaction Conditions: 1-octene (5 mmol ), triethylsilane (5 mmol), catalyst (2 μmol, metal basis), P=10 bar ( N2, 99.999% purity). a Olefin conversion.b The 1H-NMR
spectra of the crude product is given in Figure S16 (Supplementary Information) c Catalyst activated by reduction at 623 K in flow of 20% H2/N2. d As received
from Sigma-Aldrich (99.8% purity). e As received from Sigma-Aldrich, 5 wt%Rh. fn.d.: not detected.
of Et3SiH, under otherwise identical reaction conditions, 1.0Rh/CeO2
afforded only 16% olefin conversion to isomer products, whereas
1.0Pt/CeO2 led to negligible 1-octene conversion. 1.0Ru/CeO2
displayed barely any reactivity in the absence of Et3SiH. However,
under hydrosilylation conditions, it proved remarkably selective
towards olefin isomerization, affording 99% selectivity to internal
olefins. On the one hand, these results show that active sites for
olefin isomerization develop in the presence of the hydrosilane
reactant. On the other hand, they reveal vast differences in
performance for the different metals atomically dispersed on CeO2,
period, the Rh-specific initial hydrosilylation reaction rate showed a
volcano dependence with the metal content. A maximum activity was
registered for δ=1.0 Rhat nm-2, a coverage which is hence inferred to
maximize the density of the most active sites. Further increasing the
Rh content led to a progressive increase in the induction period
alongside a decline in reaction rate. This finding suggests that
polynuclear Rh oxide clusters, which exist on the catalyst surface at
δ>2.0 Rhat nm-2, are notably less effective sources of active sites. In
all cases, similarly high selectivities (>90%) to the terminal
octylsilane were obtained, indicating that differences in catalytic
performance arise from differences in the number of effective active
sites rather than in their intrinsic behavior.
as
a result of differences in the relative reaction rates for
hydrosilylation and isomerization pathways.
Encouraged by the excellent olefin hydrosilylation performance
exhibited by 1.0Rh/CeO2, additional studies followed to ascertain the
nature of the optimal rhodium active sites. First, the performance of
Rh/CeO2 catalysts was studied as a function of δ. No activity was
detected with 0.2Rh/CeO2, presumably due to the fact that a majority
of the Rh atoms are coordinatively saturated, e.g. in sub-surface
positions, and thus not accessible to reactants as suggested by CO-
FTIR spectroscopy (Figure S17). Catalysts with higher Rh contents
(0.5-10 Rhat nm-2) proved to be active. In all cases, a reaction
induction period was observed, whose duration was a function of δ
(Figure 3a). Activating the material in H2 prior to catalysis eliminated
this induction period (Figure S18), whereas experiments with
deuterated Et3Si-D led to longer induction times under otherwise
identical reaction settings. Slurry-phase EXAFS spectroscopy
applied on 1.0Rh/CeO2 prior to and after catalysis induction
discarded Rh dimerization or oligomerization as processes which
precede activity (Figure S19). Similarly to what has been proposed
for molecular catalysts,[18] the reaction induction period can thus be
ascribed to a slow (partial) reduction of the metal centers, i.e. the
cleavage of Rh-O bonds, necessary for the oxidative addition of the
hydrosilane reagent. The duration of this induction period correlated
with the average Rh-O coordination number derived from EXAFS
analysis (Figure S20), which suggests that Rh centers in higher
coordination positions, e.g. most stable (confined) surface sites on
ceria at the lowest metal contents or RhOx clusters at the highest
metal contents, are more difficult to activate for catalysis via Rh-O
bond cleavage. As depicted in Figure 3b, following the induction
To elucidate the optimal Rh speciation for catalysis, various
catalysts bearing Rh species with different oxidation state and
nuclearity were tested (Table 1, entries 7-11). Tests with 10Rh/CeO2
and Rh2O3, both bearing polynuclear Rh(III) oxide species, resulted
in not only lower activity and/or selectivity to the terminal silane
compared to 1.0Rh/CeO2, but also noticeable leaching of rhodium
into the liquid reaction medium. Moreover tests with catalysts
bearing metallic Rh0 nanoparticles, either supported on CeO2
(1.0Rh/CeO2 pre-reduced in H2 at 623 K) or on a rather inert carbon
carrier (commercial 5wt%Rh/C), led to lower reaction rates and,
most notably, greater selectivities to undesired internal olefin
isomerization products by a factor of >4. These findings suggest that,
even though reduction treatments might eliminate catalysis induction,
the presence of metallic Rh0 species in the activated catalyst
undesirably enhances the rate of olefin isomerization over that of
hydrosilylation. This is in keeping with previous studies which have
associated an unbalanced olefin isomerization activity to active
centers on metallic nanoparticles[19] or nascent polynuclear metallic
clusters in solution originating from the decomposition of molecular
catalysts.[20] Taken together, these results furnish evidence that
atomically dispersed Rhδ+ species, whose contribution on the
surface of CeO2 is maximized at a metal content of ~1.0 Rhat nm-2,
are optimal sites for hydrosilylation. Moreover, atomically dispersed
Rh/CeO2 displays also a remarkable substrate scope and tolerance
to various functional groups in the α-olefin substrates (Table S4).
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10.1002/anie.201915255
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RESEARCH ARTICLE
Figure 3: Metal speciation-dependent catalytic performance. a) Time-resolved evolution of the olefin conversion in the hydrosilylation of 1-octene with
Et3SiH employing Rh/CeO2 catalysts synthesized with different surface metal content. b) Dependence of the initial metal-specific olefin hydrosilylation rate
(extrapolated to conversion onset) of 1-octene with Et3SiH with the surface-specific Rh content for Rh/CeO2 catalysts. c) Dependence of the initial metal-specific
olefin isomerization rate for 1-octene under olefin hydrosilylation reaction conditions with the surface-specific Ru content for Ru/CeO2 catalysts. Reaction
conditions: 1-octene (5 mmol), triethylsilane (5 mmol), catalyst (2 μmol, Rh or Ru metal basis), P=10 bar (N2, 99.999% purity), T=393 K.
Tandem olefin isomerization-hydrosilylation catalysis.
catalysis, is lower than for the Rh/CeO2 system. The reaction rate
decreased for Ru contents >1.0 Ruat nm-2, in parallel to the
development of aggregated Ru(IV) oxide species on the catalyst
surface. It is hence inferred that, also in this case, atomically
dispersed Ru on the CeO2 surface is the most efficient active site.
Density Functional Theory (DFT) calculations at the PBE-D3
level of theory (with U=5eV to describe the Ce 4f electrons) were
performed to get molecular level insight into the reaction specificity
displayed by single-atom Rh/CeO2 and Ru/CeO2 catalysts for olefin
hydrosilylation and isomerization pathways, respectively. We use a
CeO2(211) surface, that connects two (111) facets of ceria, to model
the stoichiometric type II edge (Figure S23). This step edge has
been previously described as an adsorption site for atomically
dispersed Pt on CeO2 surfaces.[26] Isomerization pathways of
propene as the model olefin were compared for Ru1/CeO2(211) and
Rh1/CeO2(211) sites (see Figure S24 for the optimized active site
structures). The results are given in Figures 4a-e. In line with
experimental results, which show the presence of Et3SiH in the
reaction medium to be essential for reactivity, the effective energy
barriers for isomerization, at a temperature of 393 K, decreased by
44 kJ mol-1 for Ru1 and by 31 kJ mol-1 for Rh1 when oxidative
insertion of the hydrosilane on the monatomic sites preceded olefin
binding. After activation of the metal centers by silane addition, the
lowest effective reaction energy barrier of 38 kJ mol-1 was found for
Ru1/CeO2(211), i.e. about 34 kJ mol-1 lower than that computed for
the Rh analog. Since the intrinsic energy barriers for olefin insertion
into the metal hydride bond (2-›TS) are very similar for Ru and Rh
centers (13 vs. 10 kJ mol-1), the effective barrier height is mainly
determined by the binding strength of the olefin substrate to Ru and
Rh. This is in agreement with scaling relations previously proposed
by Wodrich et al.[27] who found that Ru complexes tend to bind to
olefin substrates stronger than Rh counterparts. These
computational findings provide an explanation for the notably higher
olefin isomerization activity observed experimentally for 1.0Ru/CeO2.
Having identified the foundations for the role of Ru in olefin
isomerization, the second half of the tandem reaction was
investigated next. Figure 4f shows the free energies computed for
elementary steps across the entire tandem process, i.e. olefin
isomerization on Ru1/CeO2, and subsequent hydrosilylation
catalyzed by Rh1/CeO2. The calculated hydrosilylation pathway was
consistent with the so-called Chalk-Harrod mechanism involving
RhH species as the resting state of the catalytic center.[24] This
active species was found to be energetically feasible, and in line with
various experimental observations, e.g. an induction period for the
In view of the different activities exhibited by SACs based on
different metals for olefin isomerization and hydrosilylation pathways,
it stood to reason to study the potential of these catalysts for a
tandem olefin isomerization-hydrosilylation process. Whereas the
production of technologically relevant terminal silanes via
hydrosilylation of α-olefins is typically uncomplicated and highly
selective, particularly with optimized molecular catalysts, a tandem
catalytic conversion is highly desired to achieve the selective
conversion of more challenging, unconventional olefin feedstocks,
e.g. those derived from paraffin dehydrogenation,[21] the Fischer-
Tropsch synthesis[22] or cross-metathesis upgrading processes from
low-value ethylene oligomerization products,[23] which typically
consist of complex mixtures of various olefin regio-isomers.
Essential, in this case, is to integrate olefin double-bond migration
and hydrosilylation activities in a single reaction medium. Moreover,
whilst these reactions often compete on the same active sites under
18, 24]
hydrosilylation conditions,[15c,
in this case they should ideally
reside on independent and non-interacting active sites, so that their
relative reaction rates can be independently adjusted in order to
optimize the selectivity and time-yield of the tandem process to the
desired terminal silane products. The combination of different
organometallic complexes in a single pot has been reported to be an
effective approach to accomplish a tandem (dehydrogenative)
isomerization-hydrosilylation process.[25] However, next to those
issues intrinsically associated to homogeneous catalysis, i.e. catalyst
recovery and recycling, the long-term stability of the molecular
catalysts as well as the minimization of hydrogenation and
isomerization side-products remain genuine challenges.
Studies with the herein developed solid SACs on 2-propen-1-
ol as substrate proved that olefin hydrosilylation proceeds >30 times
faster than isomerization on isolated Rh sites (Figure S21 and Table
S5). On the contrary, Ru centers in 1.0Ru/CeO2 are exceptionally
selective for olefin isomerization under hydrosilylation conditions. For
Ru/CeO2 catalysts the metal-specific reaction rate showed an
evolution with the metal surface content which is qualitatively similar
to that observed for olefin hydrosilylation on Rh/CeO2 catalysts
(Figure 3c). In this case, however, no induction period was
observed, pointing to a kinetically more facile development of the
active Ru species after contact with the Et3SiH reactant (Figure
S22). Moreover, full reactivity was observed already from δ=0.2 Ruat
nm-2, and retained up to δ=1.0 Ruat nm-2, i.e. the compositional range
for which Ru is atomically dispersed, suggesting that the fraction of
metal atoms in sub-surface positions, and thus inaccessible for
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10.1002/anie.201915255
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RESEARCH ARTICLE
Figure 4: DFT calculations of the reaction mechanisms. a-d) Computed reaction pathways and e) the corresponding free energy diagrams with the most
stable state set to zero in each case. Comparison of olefin isomerization on Ru1/CeO2 and Rh1/CeO2 single-atom sites stabilized at the step-edge of the
corrugated CeO2 (211) surface, prior to (a,b) and after (c,d) activation by oxidative addition of HSiMe3. f) Computed free energy diagram of the tandem olefin
isomerization/hydrosilylation process. Olefin isomerization is catalyzed by Ru1/CeO2 while subsequent hydrosilylation with HSiMe3 is catalyzed by Rh1/CeO2
single-atom sites. Reactants considered: propene as model olefin, HSiMe3 as silylating agent. T=393 K, P=10 bar (see computational details in the SI).
reaction, which is eliminated upon pre-treatment with H2 (vide supra). combination of 1.0Ru/CeO2 and 1.0Rh/CeO2 catalysts increased the
Experiments with 1-octene as olefin reactant and isotopically
labelled Et3Si-D as silylating agent showed the incorporation of
deuterium in various carbon positions in the terminal silane product,
as well as its scrambling within four carbon atoms in the octene
isomer byproducts (Figure S25), suggesting the hydrosilane as the
hydride source and the reversibility of olefin addition to the Rh sites.
Alternative reaction mechanisms previously proposed in
literature,[28] and which assume olefin coordination to precede
oxidative addition of the hydrosilane reagent, were also explored, but
were found to be subjected to comparatively higher overall free
energy barriers (Figure S26). On the basis of the computational
results, the olefin hydrosilylation rate is proposed to be limited by
olefin insertion, with an effective barrier of 84 kJ mol-1, while the
reductive elimination of the organosilane product is predicted with a
lower barrier of 62 kJ mol-1.
reaction yield to 70% (for an equimolar Ru/Rh ratio) and 80% (for
Ru/Rh=2), notably while preserving a high selectivity in excess of
92% to the terminal organosilane. Additional experiments showed
that this slight drop in selectivity was not related to the tandem
approach but simply due to the much higher conversion degree
achieved (Figure S27). These results highlight the synergistic effect
achieved by the integration of the complementary olefin
isomerization and hydrosilylation reactivities of each catalyst,
respectively, in a single pot. The performance level achieved via the
tandem combination of Ru- and Rh-based SACs was beyond reach
for the Pt-based counterpart, which showed comparable rates for
olefin isomerization and hydrosilylation routes (Table 1 and Table
S5), either alone or in combination with 1.0Ru/CeO2, or
a
conventional Pt-based molecular Karstedt's catalyst (Table 2,
entries 6-8). These results are in line with the lower performance
expected when olefin double-bond chain-walking and terminal
hydrosilylation routes compete (at similar rates) on the same active
sites –as previously observed for molecular Pt catalysts–[15c, 18] which
makes olefin on-site residence time less effective and it thus lowers
hydrosilylation turnover frequencies.
The performance of the two combined SACs in the tandem
process was assessed via the conversion of internal olefin
substrates and the results are summarized in Table 2. Under the
standard reaction conditions applied herein (393 K), the application
of 1.0Rh/CeO2 on 2-octene resulted in a 35% yield to silanes after
18 hours, with a 97% selectivity to the terminal octylsilane product.
This result reveals that the minor olefin double-bond migration
activity of the Rh-based single-atom catalyst contributes to the
conversion of 2-octene. The Ru-based counterpart, on the contrary,
showed barely any activity towards hydrosilylation, in line with the
performance observed with α-olefins. Remarkably, the one-pot
The performance of the tandem reaction on 2-octene
prompted us to assess the conversion of a further internal n-olefin
such as 3-octene, which is both more challenging as a substrate
when terminal regioselectivity is sought after, and more
representative of target industrial olefin feedstocks derived e.g. from
metathetic olefin redistribution processes on oligomerization educts,
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10.1002/anie.201915255
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Table 2. Catalytic results for the tandem olefin isomerization/hydrosilylation of internal olefins and olefin isomer mixtures.
Et3SiH
R
R
SiEt3
n
n
T:B
Silaneb
(-)
T
Time
(h)
Entry
Catalyst
Olefin
Silane
Ya
(%)
33k
<1
70
(K)
1
2
3
4
5
6
1.0Rh/CeO2
1.0Ru/CeO2
2-octene
2-octene
2-octene
2-octene
2-hexene
2-octene
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 393
18
18
18
18
18
18
96:4k
-
1.0Rh/CeO2 + 1.0Ru/CeO2
c1.0Rh/CeO2 + 1.0Ru/CeO2
1.0Rh/CeO2 + 1.0Ru/CeO2
1.0Pt/CeO2
93:7
92:8
97:3
96:4
80
73
39
7
1.0Pt/CeO2 + 1.0Ru/CeO2
Pt Karstedt catalyste
2-octene
2-octene
3-octene
3-octene
3-octene
3-octene
3-octene
3-octene
4-octene
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 393
Et3SiH 413
Et3SiH 413
18
18
18
18
18
18
18
18
18
18
49
10
5
93:7
98:2
96:4
-
8
9
1.0Rh/CeO2
10
11
12
13
14
15
16
1.0Ru/CeO2
<1
20
29
32
50
37
50
1.0Rh/CeO2 + 1.0Ru/CeO2
c1.0Rh/CeO2 + 1.0Ru/CeO2
d1.0Rh/CeO2 + 1.0Ru/CeO2
1.0Rh/CeO2 + 1.0Ru/CeO2
1.0Rh/CeO2 + 1.0Ru/CeO2
1.0Rh/CeO2 + 1.0Ru/CeO2
91:9
90:10
89:11
88:12
82:18
98:2
trans-Propenylbenzene Et3SiH 393
8-bromooctene
Et3SiH 393
isomers mixf
17
18
19
1.0Rh/CeO2 + 1.0Ru/CeO2
18
18
18
70
83
43
91:9
95:5
96:4
Octene
g
1.0Rh/CeO2 + 1.0Ru/CeO2
Et3SiH 393
isomers mixh
Neodene® 8/9/10
Et3SiH 393
isomers mixj
i
1.0Rh/CeO2 + 1.0Ru/CeO2
Reaction Conditions: olefin (5 mmol), triethylsilane (5 mmol), catalyst (4 μmol (total metal basis) unless otherwise stated), P=10 bar (N2, 99.999% purity). For tests
combining two catalysts, equimolar amounts of the two metals were applied, unless otherwise indicated with footnotes. For additional catalytic results, i.e. full
screening of Ru/Rh molar ratio for the tandem isomerization/hydrosilylation of 3-octene, see Figure S28 in the Supporting Information. aYield to silanes (remaining
products are olefin isomers). bTerminal-to-branched molar ratio within organosilane products.c Ru/Rh molar ratio of 2.0. d Ru/Rh molar ratio of 4.0. eCommercially
f
available platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane (Sigma-Aldrich). Isomers mixture generated from the corresponding α-olefin (8-bromooct-1-en) by
reaction with 1.0Ru/CeO2 as catalyst. gOlefin (5 mmol, excluding n-octane), triethylsilane (5 mmol), catalyst (Ru/Rh molar ratio of 2). hOctene isomers/n-octane
mixture (20% Octane, 42% 1-Octene, 38% 2-Octene (cis+trans)) representative of the crude olefin product obtained by transfer dehydrogenation of n-octane
i
employing a state-of-the-art Ir-based pincer catalyst (see main text). Reaction conditions: industrial olefin mixture (~25 mmol), triethylsilane (25 mmol), catalyst
(30 μmol of metal, Ru/Rh molar ratio of 2.0), P=10 bar (N2, 99.999% purity). jIndustrial internal/terminal olefin mixture, containing mainly C8-C10 linear olefins,
produced as part of the Shell Higher Olefins Process (SHOP) and associated olefin redistributon unitary operations (see main text). kThe standard error for yield
and L:B selectivity was ±2% and ±1%, respectively, as determined from 3 independent tests for selected reaction conditions.
which contain significant shares of 3-ene and further internal olefins.
In this case, the use of either of the Rh- or Ru-based SACs alone
resulted in barely any reactivity (yield to organosilanes <5% after 18
h, Table 2 entries 9 and 10), as a result of their poor individual
significantly higher intrinsic (metal-specific) activity of 1.0Rh/CeO2
for hydrosylilation of α-olefins compared to that of 1.0Ru/CeO2 for
double-bond isomerization (Figure 3b,c). On the other hand, the >6-
fold increase in product yield compared to single-catalyst tests
activity for olefin double-bond migration and hydrosilylation pathways, (Figure S28), clearly illustrates how the tandem system opens the
respectively. Remarkably, the tandem combination of the two
catalysts achieved a significant activity on this substrate. As shown
in Figure S28, a full screening of the tandem metal composition
enlightened a clear volcano dependence of the organosilane yield
with the Ru/Rh molar relative abundance. A maximum yield of 32%
after 18 h was achieved with an optimal Ru/Rh ratio of 4.0, while
≥90% selectivity to the terminal silane product was retained within
the entire compositional range studied. On the one hand, the optimal
catalyst blending, notably enriched in Ru, is a consequence of the
door to a process which is beyond reach for either of the two
catalysts individually. Excellent results were also obtained via the
cooperation of the two single-atom catalysts on sterically more
hindered substrates such as trans-propenylbenzene (50% yield with
98% selectivity to terminal silane, entry 16). Reaction yields could be
increased by a factor of 3, and the conversion extended to further
internal olefins such as 4-octene, by increasing the reaction
temperature to 413 K, in this case with comparatively lower product
regioselectivities, yet in excess of 80% (entries 14 and 15). This
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10.1002/anie.201915255
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RESEARCH ARTICLE
catalytic system proved also efficient to selectively convert complex
mixtures of terminal and internal olefins into terminal organosilanes.
A mixture of 8-bromooctene isomers, generated by isomerization of
the corresponding α-olefin (8-Bromo-1-octene) with 1.0Ru/CeO2,
was converted with 91% selectivity to the terminal 1,1,1-triethyl-8-
bromooctylsilane. Similarly high regioselectivities were also achieved
from industrially relevant olefin mixtures, representative for output
conducted at the Laboratorio de Microscopias Avanzadas, Instituto de
Nanociencia de Aragon, Universidad de Zaragoza, Spain. R.A. gratefully
acknowledges the support from the Spanish Ministry of Economy and
Competitiveness (MINECO) through project grant MAT2016-79776-P
(AEI/FEDER, UE) and from the European Union H2020 programs
“ESTEEM3” (823717). The authors acknowledge support by the state of
Baden-Württemberg through bwHPC (bwUnicluster and JUSTUS, RV
bw17D01), by the GRK 2450 and by the Helmholtz Association. This
research received funding from the Max Planck Society, and the Fonds
der Chemische Industrie of Germany. Funding from the Spanish Ministry
of Science, Innovation and Universities (Severo Ochoa program SEV-
2016-0683 and grant RTI2018-096399-A-I00) is also acknowledged.
B.B.S. acknowledges the Alexander von Humboldt Foundation for a
postdoctoral scholarship.
streams
from
mild-temperature
paraffin
dehydrogenation
processes,[4] and olefin oligomerization/metathesis operations, as in
the commercial Shell Higher Olefin Process®.[10, 23] In all cases, the
cooperation of Ru/CeO2 and Rh/CeO2 single-atom catalysts in
tandem led to a highly selective (>95%) production of terminal
organosilanes (Table 2, entries 18 and 19). The tandem process
relies on the in situ processing of terminal olefins –generated by the
isomerization catalyst– further on the hydrosilylation active catalyst,
and its efficiency cannot be paired by a two-step process where a
first, independent isomerization step favors olefin mixtures enriched
in the thermodynamically most stable internal isomers.
Keywords: Single-atom-catalysis • tandem catalysis • olefin valorization
• structure-performance relationships • DFT
References
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(Figure S29). EXAFS analysis of the solid catalysts after the
sequence of reaction batches showed no evidences of second-shell
M-O-M coordination scattering (Figure S29), whereas no metal
clusters or nanoparticles could be visualized by Cs-HAADF-STEM
(Figure S30). These observations provide strong evidence for the
perseverance of the atomically isolated metal centers and their
stability against clustering.
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Acknowledgements
X-ray absorption experiments were performed at the ALBA Synchrotron
Light Source (Spain), experiments 2018082961 and 2019023278. L.
Simonelli and C. Marini (CLAESS-ALBA beamline) are thanked for
beamline setup. E. Andrés, M.E. Martínez, M. García, and I. López (ITQ),
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Raman spectroscopy. J.M. Salas (ITQ) is gratefully acknowledged for his
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10.1002/anie.201915255
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
RESEARCH ARTICLE
Singles in tandem: The one-pot integration of
Ru1/CeO2 and Rh1/CeO2 single-atom solid
catalysts enables a highly selective olefin
isomerization-hydrosilylation tandem process
for the conversion of terminal and internal
olefins, as well as mixtures thereof, into
industrially relevant terminal organosilane
compounds.
This article is protected by copyright. All rights reserved.