Metal Nanoparticles Catalyzed Selective Carbon-Carbon Bond Activation in the Liquid Phase

Article abstract of DOI:10.1021/jacs.6b03977

Understanding the C-C bond activation mechanism is essential for developing the selective production of hydrocarbons in the petroleum industry and for selective polymer decomposition. In this work, ring-opening reactions of cyclopropane derivatives under hydrogen catalyzed by metal nanoparticles (NPs) in the liquid phase were studied. 40-atom rhodium (Rh) NPs, encapsulated by dendrimer molecules and supported in mesoporous silica, catalyzed the ring opening of cyclopropylbenzene at room temperature under hydrogen in benzene, and the turnover frequency (TOF) was higher than other metals or the Rh homogeneous catalyst counterparts. Comparison of reactants with various substitution groups showed that electron donation on the three-membered ring boosted the TOF of ring opening. The linear products formed with 100% selectivity for ring opening of all reactants catalyzed by the Rh NP. Surface Rh(0) acted as the active site in the NP. The capping agent played an important role in the ring-opening reaction kinetics. Larger particle size tended to show higher TOF and smaller reaction activation energy for Rh NPs encapsulated in either dendrimer or poly(vinylpyrrolidone). The generation/size of dendrimer and surface group also affected the reaction rate and activation energy.

Full text of DOI:10.1021/jacs.6b03977

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Article
Metal Nanoparticles Catalyzed Selective Carbon-
Carbon Bond Activation in the Liquid Phase
Rong Rocky Ye, Bing Yuan, Jie Zhao, Walter T. Ralston, Chung-
Yeh Wu, Ebru Unel Barin, F. Dean Toste, and Gabor A. Somorjai
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03977 • Publication Date (Web): 20 Jun 2016
Downloaded from http://pubs.acs.org on June 23, 2016
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Rong (Rocky) Ye, Bing Yuan, Jie Zhao, Walter T. Ralston, Chung-Yeh Wu, Ebru Unel Barin,
F. Dean Toste,* and Gabor A. Somorjai*
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Department of Chemistry, University of California, Berkeley, California 94720, United States, and Chemical Science
Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California, 94720, United States
Catalysis; C-C bond activation; ring opening; nanocatalysis
ABSTRACT: Understanding the C-C bond activation mechanism is essential for developing the selective production of
hydrocarbons in the petroleum industry and for selective polymer decomposition. In this work, ring opening reactions of
cyclopropane derivatives under hydrogen catalyzed by metal nanoparticles (NPs) in the liquid phase were studied. 40-
atom rhodium (Rh) nanoparticles (NPs), encapsulated by dendrimer molecules and supported in mesoporous silica, cata-
lyzed the ring opening of cyclopropylbenzene at room temperature under hydrogen in benzene, and the turnover fre-
quency (TOF) was higher than other metals or the Rh homogeneous catalyst
counterparts. Comparison of reactants with various substitution groups showed
that electron donation on the 3-membered ring boosted the TOF of ring open-
ing. The linear products formed with 100% selectivity for ring opening of all re-
actants catalyzed by the Rh NP. Surface Rh(0) acted as the active site in the NP.
The capping agent played an important role in the ring opening reaction kinet-
ics. Larger particle size tended to show higher TOF and smaller reaction activa-
tion energy, for Rh NPs encapsulated in either dendrimer or poly(vinylpyrrolidone) (PVP). The generation/size of den-
drimer and surface group also affected the reaction rate and activation energy.
1. INTRODUCTION
crystal or powder catalysts in the gas phase was often ac-
companied by hydrogenolysis, forming smaller molecules.
It was posited that the reaction mechanism was metal-
dependent, proceeding through different intermediates
including a mono-adsorbed radical,9 1,3-diadsorbed spe-
cies on Ni catalysts10 or metallocycle intermediate on Pt.11
Carbon-carbon (C-C) bond activation in the gas phase
plays an important role in the chemical industry. Crack-
ing and reforming reactions in crude oil refineries prac-
tice C-C bond activation on a large scale daily1. Polyeth-
ylene and polypropylene are currently the mostly widely
used plastics, but their decomposition requires thermal
C-C bond activation2. Due to thermodynamic and kinetic
considerations, selective C-C bond activation is challeng-
ing3-6. Currently, these reactions are operated at high
temperatures in industry (above 600 K). Undifferentiated
thermal activation of C-C bond leads to multiple product
formation simultaneously, requiring further product sep-
aration.
In order to improve the product selectivity, efforts have
been made to study C-C bond activation reactions using
metal complexes of Rh12-16, Pt17, Pd18, Cu19, and other met-
als20,21 in solution under relatively mild conditions. For
example, Bart and Chirik14 recently reported examples of
ring opening of cyclopropane derivatives catalyzed by
organometallic Rh catalysts in the liquid phase, and pro-
posed the rhodium-metallocycle as the intermediate.
To overcome the low reactivity of C-C bonds, a practi-
cal strategy is to exploit the ring strain of (three- or four-
membered) cyclic compounds as a driving force, thereby
allowing the study of the factors controlling catalyst activ-
ity and selectivity. Early work7,8 showed that catalytic cy-
clopropane hydrogenation (ring opening) by metal single
In this work, the advantages of both types of catalysis
on the ring opening reaction are combined: the high ac-
tivity of heterogeneous catalysts and the high selectivity
of homogeneous catalysts. We investigated the effect of
different variables on the metal nanoparticles catalyzed
C-C bond activation in the liquid phase. Rh NPs catalyzed
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the ring opening of cyclopropylbenzene (1) at room tem- prop-1-en-1-ylbenzene at 80 °C without formation of
1
2
3
4
perature under an atmospheric pressure of H2. The turno-
ver frequency (TOF) of the reaction was correlated with
reactant and metal NP properties and the factors govern-
ing the catalytic activity and selectivity were analyzed.
propylbenzene, but they were inactive at room tempera-
ture. Rh40/G4OH/SBA-15 also catalyzed the reaction in
other solvents (toluene, dichloromethane and methanol)
with similar TOF to that in benzene.
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6
7
8
Table 1. Catalytic ring opening of cyclopropylbenzene
(1) with various catalysts under 1 atm of H2 in C6D6
A
B
9
En-
try
Catalyst
TOF (h-1)a
at 20 oC
TOF (h-1)a
at 80 oC
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1
Rh40/G4OH/SBA-15
Pd40/G4OH/SBA-15
Au40/G4OH/SBA-15
(PPh3)3RhCl
2.24
1.70
0
11.4
22.1
0.20
0.38b
0.10
0
10 nm
10 nm
2
3
4
5
6
C
Rh(0)
Rh(III)
0.10
0
[Rh(CO)2Cl]2
Rh40/G4OH/SBA-15
+ PhICl2
a Numbers of active sites for TOF calculation are estimated
0
from simple hard-sphere counting models. b This is the sum
TOF for the formation of linear and branch products.
3d3/2
3d5/2
2. RESULTS AND DISCUSSION
2.1. Catalyst screening
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312
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308
306
304
Binding Energy (eV)
The catalytic ring opening of 1 in the presence of H2 was
used as a model reaction to study the C-C bond activation
reaction under mild conditions. The TOF of several po-
tential catalysts for ring opening reactions under atmos-
pheric pressure of hydrogen in C6D6 was explored at 20 °C
and 80 °C (Table 1). TOF of metal NPs was calculated by
the number of molecules reacted in unit time (an hour)
divided by the number of undercoordinated metal atoms,
which was estimated from simple hard-sphere counting
models22. 40-atom nanoparticles of Rh, Pd, and Au encap-
sulated by fourth-generation polyamidoamine (PAMAM)
dendrimer with hydroxyl terminal groups (G4OH) were
synthesized and loaded into mesoporous silica SBA-1523-26
(Entry 1-3 in Table 1). Among these three catalysts,
Rh40/G4OH/SBA-15 offered the highest turnover frequen-
cy at room temperature. Pd NPs were less active catalysts
than Rh NPs at room temperature, but were more active
at elevated temperature. The Au NPs showed very low
activity as catalysts for this reaction. Rh(I) complexes
such as the Wilkinson’s catalyst (PPh3)3RhCl, and
[Rh(CO)2Cl]2 had lower TOF compared to the Rh NPs.
(PPh3)3RhCl at 80 °C was the only case in Table 1 where
the branch product cumene formed along with the linear
product propylbenzene (branch:linear ratio about 1:2).
Rh(III) compounds RhCl3 or Rh(acac)3 failed to catalyze
the reaction. Oxidizing Rh in Rh40/G4OH/SBA-15 by add-
ing an organic oxidizer PhICl2 completely inhibited the
activity of the catalyst (Entry 6 in Table 1). Pure mesopo-
rous SBA-15 could not catalyze the reaction, which could
be considered as the blank reaction. On the other hand,
solid acids, such as H+-Al-SBA-15 (acidified mesoporous
silica SBA-15) and H+-MFI (acidified mesoporous MFI zeo-
lite), catalyzed the rearrangement of 1 to form only (E)-
D
Rh(0)
3d3/2
3d5/2
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312
310
308
306
304
Binding Energy (eV)
Figure 1. Characterization of Rh40/G4OH/SBA-15. TEM imag-
es of the sample before (A) and after (B) three cycles of reac-
tions. The scale bars are 10 nm. (C) and (D) are the corre-
sponding XPS spectra. XPS spectra are not normalized.
2.2. Characterization of Rh40/G4OH/SBA-15
TEM images of Rh40/G4OH/SBA-15 before and after
three cycles of reactions with 1 are shown in Figure 1A and
1B, respectively. The size of Rh NPs was 1.6 ± 0.2 nm by
averaging over 100 particles. The NPs were stable after
three cycles of reactions. No leaching was observed, as
confirmed by the inductively coupled plasma optical
emission spectrometry (ICP-OES) of the filtrate of the
reaction mixture. Hydrogenolysis was not observable dur-
ing the reaction, which was confirmed by the absence of
small gas molecules in the headspace (the gas above the
product solution) GC analysis. The mass balance (total
amount of product and unreacted reactant after the reac-
tion matched the amount of reactant before the reaction)
provides additional support for the lack of hydrogenolysis.
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The rate of ring opening of 1 in the second and third cycle
of the recycled catalyst was 97% and 96% of that in the
first cycle, respectively. The as-synthesized
tion is even smaller. The observation that ring opening
1
2
3
4
5
6
7
8
was accelerated by electron donating and decelerated by
electron withdrawing groups on the phenyl ring of the
reactant is consistent with a hypothesis that a cationic
intermediate is involved in the rate-limiting step of the
reaction. This hypothesis held for molecules without phe-
nyl rings. Rings attached to electron donating groups
opened quickly at a low temperature, while those at-
tached to electron withdrawing groups required higher
temperatures to exhibit an observable reaction rate of C-C
bond scission.
Rh40/G4OH/SBA-15 sample contained surface rhodium
oxide species, according to the XPS spectrum (Figure 1C).
The oxidation state of Rh after the reaction was purely 0,
as shown by the 307.3 eV binding energy24 in the XPS
spectrum (Figure 1D). To explore the effects of these rho-
dium oxides on the kinetics of the ring opening reaction,
a set of control experiments with different pretreatment
conditions were performed. Keeping all other variables
the same, one catalyst was mixed with H2 at 100 °C for 24
h, one was mixed with Ar at 100 °C for 24 h, and the other
without pretreatment. When these three catalysts were
used in the reaction at 20 °C, they exhibited the same re-
action rate/TOF on the ring opening of 1. This implies
that the reduction of the surface rhodium oxides to me-
tallic rhodium was a fast process under reaction condi-
tions.
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The TOF values in Scheme 1 were measured at various
temperatures, which was a compromise of the low reac-
tion rate at a low temperature and reactant/solvent boil-
ing at a high temperature. Reactants 6 and 7, rings at-
tached to a hydroxyl group, underwent the ring opening
reaction with the hydroxyl group transformed to a car-
bonyl group. The formation of a C=O bond, which was
not further hydrogenated under the reaction condition,
provided an additional driving force for the ring opening
reaction. Thus, the TOF of 6 was high, and the typically
inert 4-membered ring in 7 was activated. In another ex-
ample of 4-membered ring C-C bond activation, bi-
phenylene (8) was transformed to biphenyl at room tem-
perature, a reaction for which homogeneous catalysts
typically required elevated temperatures17.
Scheme 1. Reactants for ring opening reactions cata-
lyzed by Rh40/G4OH/SBA-15 and the turnover fre-
quency under specified reaction condition.
The products from reactants 1~5 in Scheme 1 were en-
tirely the linear/proximal ones. In contrast, Bart and Chi-
rik14 reported that the branch/distal products were fa-
vored by homogeneous Rh catalysts, unless a reactant
with the ability to chelate with the Rh catalyst was used.
The unique selectivity toward linear products by NPs in
this work is likely attributed to a different reaction mech-
anism between the reported homogeneous catalysts and
NP catalysts employed in this work.
2.4. Kinetics study of the ring opening reaction of 1
The size of dendrimer encapsulated NPs was tuned by
modifying the Rh:G4OH ratio during the NP synthesis.
Rh15G4OH, Rh40G4OH, and Rh50G4OH were synthesized
to explore the size effects of Rh NPs on the catalytic per-
formance. However, synthesis of even larger sizes of
G4OH-encapsulated NPs was limited to 4.5 nm in diame-
ter by the size of G4OH. Figure 2A displays the tempera-
ture dependence of the ring opening TOF of 1 catalyzed
by Rh NPs between 293 K and 333 K in C6D6 under 1 atm
of H2. TOF increased as temperature increased for the
same catalyst as expected. Further control experiments
(data not reflected in the figure) with faster stirring rates
gave the same TOF, suggesting that the reactions were
not limited by diffusion under these reaction conditions.
The apparent activation energy of ring opening catalyzed
by each catalyst was determined from the slopes of the
linear fit plots, and plotted in Figure 2C. Keeping every-
thing else constant, increasing the size of Rh NPs from 0.8
nm to 2.1 nm increased the TOF from 2.4 h-1 to 13.4 h-1 (at
333 K), and decreased the reaction activation energy from
29.8 kJ/mol to 17.7 kJ/mol. The trend that larger particle
2.3. Applications on other cyclic compounds
Reactants that underwent the ring opening reaction
catalyzed by Rh40/G4OH/SBA-15 are listed in Scheme 1
along with the TOF values under the specified conditions.
Under the same reaction condition, the more electron-
rich 1a reacted faster than 1, but reaction of electron-
deficient 1b was notably slower. It should be noted that a
small amount of 1b was reduced to 1a during the reaction
process, and the TOF reported for 1b does not discount
this pathway. Thus, the real TOF of 1b ring opening reac-
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size offered higher TOF has been previously observed in
encapsulated in polyvinylpyrrolidone (PVP). The Arrhe-
the catalytic hydrogenation of allyl alcohol27 and alkynol28,
and dehydrogenation of ammonia borane29. The same
size-dependence of TOF and activation energy held for
the hydrogenation of styrene, an example of C-C π-bond
hydrogenation (Figure S1).
nius plots of ring opening of 1 catalyzed by Rh/PVP NPs
were shown in Figure 2B. The size effects of PVP capped
NPs were the same as G4OH capped NPs. Here, the large
Rh/PVP NPs (6.0 nm and 9.1 nm) had only a small per-
centage of undercoordinated surface atoms. With compa-
rable amount of Rh loading, reactions catalyzed by large
NPs had lower yield, due to scarcity of surface Rh atom,
leading to a relatively large measurement error of TOF by
NMR.
1
2
3
4
5
6
7
8
Rh15/G4OH 0.8 nm
Rh40/G4OH 1.6 nm
3
9
A
Rh50/G4OH 2.1 nm
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As shown in Figure 2C, comparing 2 nm Rh NPs capped
by G4OH and PVP, the former had greater TOF. There
were several possible reasons for this observation: 1) steric
effect, the capping agent might block a portion of under-
coordinated Rh atoms due to steric hindrance ; 2) elec-
tronic effect, the interaction of the capping agent might
tune the electronic structure of the Rh atoms, and thus
affected the energetics of the reaction process.
2
1
0
3.1
3.2
3.3
3.4
1000/(T/K)
Rh40/G3OH
1.9 nm Rh/PVP
3.0 nm Rh/PVP
6.0 nm Rh/PVP
9.1 nm Rh/PVP
A
Rh40/G4OH
Rh40/G6OH
Rh40/G4NH2
Rh40/G4SA
2
1
3
2
1
0
B
0
-1
3.2
3.3
3.4
3.0
3.1
3.2
3.3
3.4
1000/(T/K)
1000/(T/K)
40
30
20
10
0
B
G4OH-capped Rh
PVP-capped Rh
30
25
20
15
10
5
C
0
Rh40/G4SA
Rh40/G6OH
Rh40/G3OH
0
2
4
6
8
10
Rh40/G4OH
Rh40/G4NH2
Size/nm
Figure 3. (A) Arrhenius plots of ring opening of 1 in C6D6
under H2, catalyzed by Rh NPs with different generation
number of dendrimer (G3OH, G4OH, and G6OH), and G4
dendrimer with surface groups of primary amine (G4NH2) or
succinamic acid (G4SA). (B) Calculated ring opening activa-
tion energy catalyzed by the corresponding Rh NPs. Inset
shows the structure of dendrimer surface groups used.
Figure 2. Arrhenius plots of ring opening of 1 in C6D6 under
H2, catalyzed by Rh NPs of various sizes encapsulated in (A)
G4OH and (B) PVP. (C) Plots of particle size and calculated
ring opening activation energy catalyzed by Rh NPs. The
cartoons of dendrimer encapsulated NPs and PVP-capped
NPs were shown in the bottom left and top right, respective-
ly.
In order to elucidate the effects of the capping agent,
Rh NPs encapsulated in different dendrimers were syn-
thesized. Third generation G3OH and sixth generation
G6OH were used in order to vary the size of the den-
On the basis of these observations, we hypothesized
that the increased abundance of Rh-Rh bond on the sur-
face of larger Rh NPs led to their higher catalytic activity.
We tested the hypothesis with Rh NPs of various sizes
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drimer without changing its chemical properties. The properties. For Rh NPs encapsulated in G4OH and PVP,
1
2
3
4
5
6
7
8
impact of the chemical properties of the dendrimer sup-
port was studied by replacing the terminal hydroxyl group
in G4OH with primary amine and succinamic acid (SA)
groups by using G4NH2 and G4SA, respectively. Rh NPs
were synthesized with these dendrimers, keeping the
same amount of Rh and the Rh:dendrimer ratio as 40:1.
TEM images of these Rh NPs indicated that the sizes were
constant within error. Arrhenius plots of ring opening of 1
catalyzed by these NPs are given in Figure 3A, and the
corresponding activation energy in Figure 3B. Modifying
the dendrimer generation from G3OH to G6OH, the rate
of ring opening tended to decrease and the corresponding
activation energy generally increased, likely due to the
steric effect. G4NH2 and G4OH had similar size but dis-
tinct Lewis and Brønsted basicity, so the lower activity of
Rh40/G4NH2 might result from an electronic effect. Both
G4OH and G4SA have 64 surface groups, but -SA is much
bulkier than -OH as shown in Figure 3B. Rh40/G4SA
showed smaller TOF than Rh40/G4OH, mainly caused by
the steric effects.
larger particle size tends to show higher TOF and smaller
reaction activation energy. For dendrimer encapsulated
Rh NPs, both the dendrimer surface group and the den-
drimer generation played a role in the reaction rate and
activation energy of ring opening. The study sheds light
on the fundamental factors controlling C-C bond activa-
tion activity and selectivity.
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Supporting Information. Details of the experimental pro-
cedure, and supplementary figures. This material is available
free of charge via the Internet at http://pubs.acs.org.
fdtoste@berkeley.edu; somorjai@berkeley.edu
The authors declare no competing financial interest.
3. EXPERIMENTAL SECTION
3.1. Chemicals.
We acknowledge support from the Director, Office of Sci-
ence, Office of Basic Energy Sciences, Division of Chemical
Sciences, Geological and Biosciences of the US DOE under
contract DEAC02-05CH11231. We thank Dr. Kyungsu Na for
the synthesis of mesoporous zeolite, and Dr. Joyce Rodrigues
De Araujo for XPS fitting. We thank Profs. A. Paul Alivisatos
and Peidong Yang for the use of TEM. We thank the Molecu-
lar Fondry of the Lawrance Berkeley National Laboratory
(Proposal 3806) for using their facilities.
Information about chemicals, including synthesis of
supported nanoparticles, organic reactants, and charac-
terization methods, can be found in the Supporting In-
formation.
3.2. Representative procedure for catalytic reactions.
To a dry 10 mL Schlenk flask, equipped with a stir bar,
was
added
cyclopropylbenzene
(8.9 mg,
(1) Gary, J. H.; Handwerk, G. E.; Kaiser, M. J. Petroleum refining:
technology and economics; CRC press, 2007.
(2) Beyler, C. L.; Hirschler, M. M. SFPE handbook of fire protection
engineering 2002, 2, 110.
0.075 mmol), Rh40/G4OH/SBA-15 (50 mg, 0.003 mmol,
4 mol%), mesitylene (2.3 µL, internal standard, Aldrich)
and benzene-d6 (1 mL). The reaction mixture was de-
gassed, placed under 1 atm of H2 and heated with stirring
at the desired temperature for 2 hours. The mixture was
then cooled to room temperature and the solid catalyst
filtered using a polytetrafluoroethylene syringe filter. The
filtrate was transferred to a NMR tube for analysis. The
conversions were controlled to be low (typically between
5-20%) to allow assessment of the initial rates.
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134, 17823.
4. CONCLUSIONS
The ring opening reactions of cyclopropane derivatives
catalyzed by metal nanoparticles under hydrogen were
investigated. Rh40/G4OH/SBA-15 showed a TOF of 2.24 h-1,
which was higher than the homogeneous counterparts,
toward the ring opening of cyclopropylbenzene at room
temperature. This catalyst was stable and could be recy-
cled at least three times. The reaction was accelerated by
electron-donating groups on the three-membered ring,
while electron-accepting groups decreased the TOF. In
contrast to ring opening reactions catalyzed by homoge-
neous rhodium catalysts, the products of the nanoparticle
catalyzed reactions were 100% linear. The apparent acti-
vation energy of the reaction was affected by the catalyst
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