Selective Catalytic Chemistry at Rhodium(II) Nodes in Bimetallic Metal–Organic Frameworks

Article abstract of DOI:10.1002/anie.201908761

We report the first study of a gas-phase reaction catalyzed by highly dispersed sites at the metal nodes of a crystalline metal–organic framework (MOF). Specifically, CuRhBTC (BTC^3?=benzenetricarboxylate) exhibited hydrogenation activity, while other isostructural monometallic and bimetallic MOFs did not. Our multi-technique characterization identifies the oxidation state of Rh in CuRhBTC as +2, which is a Rh oxidation state that has not previously been observed for crystalline MOF metal nodes. These Rh²⁺ sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and the MOF structure stabilizes the Rh²⁺ oxidation state under reaction conditions. Density functional theory calculations suggest a mechanism in which hydrogen dissociation and propylene adsorption occur at the Rh²⁺ sites. The ability to tailor the geometry and ensemble size of the metal nodes in MOFs allows for unprecedented control of the active sites and could lead to significant advances in rational catalyst design.

Full text of DOI:10.1002/anie.201908761

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Communications
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International Edition: DOI: 10.1002/anie.201908761
German Edition: DOI: 10.1002/ange.201908761
Heterogeneous Catalysis
Hot Paper
Selective Catalytic Chemistry at Rhodium(II) Nodes in Bimetallic
Metal–Organic Frameworks
Deependra M. Shakya, Otega A. Ejegbavwo, Thayalan Rajeshkumar, Sanjaya D. Senanayake,
Amy J. Brandt, Sharfa Farzandh, Narayan Acharya, Amani M. Ebrahim, Anatoly I. Frenkel,
Ning Rui, Gregory L. Tate, John R. Monnier, Konstantinos D. Vogiatzis,* Natalia B. Shustova,*
and Donna A. Chen*
Abstract: We report the first study of a gas-phase reaction
catalyzed by highly dispersed sites at the metal nodes of
a crystalline metal–organic framework (MOF). Specifically,
CuRhBTC (BTC3À = benzenetricarboxylate) exhibited hydro-
genation activity, while other isostructural monometallic and
bimetallic MOFs did not. Our multi-technique characteriza-
tion identifies the oxidation state of Rh in CuRhBTC as + 2,
which is a Rh oxidation state that has not previously been
observed for crystalline MOF metal nodes. These Rh2+ sites are
active for the catalytic hydrogenation of propylene to propane
at room temperature, and the MOF structure stabilizes the Rh2+
oxidation state under reaction conditions. Density functional
theory calculations suggest a mechanism in which hydrogen
dissociation and propylene adsorption occur at the Rh2+ sites.
The ability to tailor the geometry and ensemble size of the
metal nodes in MOFs allows for unprecedented control of the
active sites and could lead to significant advances in rational
catalyst design.
geometries connected by organic linkers. Furthermore, the
high porosity (> 1000 m2 gÀ1 surface area)[1] of MOFs provides
a sufficient number of highly dispersed active sites for
catalytic reactions.
MOFs are reported to be excellent catalysts for a number
of reactions in the liquid phase, including hydrogenation,
oxidation, and epoxidation.[2] However, gas-phase heteroge-
neous catalysis is preferred over reactions in the liquid phase
due to the ease of separating the catalyst from the products in
the former case. Given their extraordinarily high surface
areas, MOFs have been used as supports for metal particles in
gas phase reactions.[3] Similarly, catalytically active metal
complexes or clusters have been coordinated to the MOF
nodes or linkers by postsynthetic modification.[4] In compar-
ison to these catalysts that employ the MOF as a support, the
use of metal nodes intrinsic to the MOF structure as the
catalytically active sites has several advantages. These
include: a homogeneous distribution of the metal nodes
within the framework; and a well-defined structure of the
metal nodes, which can be fully characterized by crystallog-
raphy and can be investigated theoretically to predict the
possible mechanisms of catalytic transformations.
T
he use of crystalline metal–organic frameworks (MOFs) as
heterogeneous catalysts provides the unique opportunity to
tailor active site geometries and ensemble sizes on the atomic
scale. MOFs are crystalline hybrid inorganic/organic materi-
als with structures consisting of metal nodes of specific
In this work, we present the first study of gas-phase
catalytic activity occurring at the metal nodes of a crystalline
MOF and investigate possible reaction mechanisms using
density functional theory (DFT) calculations. Specifically,
(CuxRh1Àx)3(BTC)2 (abbreviated CuRhBTC, where BTC3À
=
[*] D. M. Shakya, O. A. Ejegbavwo, Dr. A. J. Brandt, S. Farzandh,
N. Acharya, Prof. N. B. Shustova, Prof. D. A. Chen
Department of Chemistry and Biochemistry, University of South
Carolina, Columbia, SC 29208 (USA)
benzenetricarboxylate) is found to catalyze propylene hydro-
genation to propane with relatively high activity and excellent
long-term stability, whereas the other bimetallic and mono-
metallic MOFs investigated exhibit no activity. CuRhBTC is
prepared via solid-state metathesis using Cu3(BTC)2
(HKUST-1) as a precursor. To the best of our knowledge,
this is the first report of a + 2 oxidation state for isolated Rh
E-mail: shustova@sc.edu
dachen@sc.edu
Dr. T. Rajeshkumar, Prof. K. D. Vogiatzis
Department of Chemistry, University of Tennessee
Knoxville, TN 37996 (USA)
ions in a MOF; these Rh2+ ions are not stabilized by the
E-mail: kvogiatz@utk.edu
[5]
Dr. S. D. Senanayake, Prof. A. I. Frenkel, Dr. N. Rui
Chemistry Division, Brookhaven National Laboratory
Upton, NY 11973 (USA)
À
presence of Rh Rh bonds at Rh dimer nodes, and are
produced through the reduction of postsynthetically incorpo-
rated Rh3+. This unusual Rh2+ oxidation state provides the
active sites for the reaction, given that activity increases with
Rh concentration. DFT studies were used to model the active
Rh2+ sites within the CuRhBTC structure and to identify an
energetically favorable reaction mechanism in which H2
dissociation and propylene adsorption occur at Rh2+ sites.
Propylene hydrogenation experiments were carried out
on the monometallic CuBTC and NiBTC MOFs, as well as
a variety of Cu-containing bimetallic MOFs ((CuxM1Àx)3-
A. M. Ebrahim, Prof. A. I. Frenkel
Department of Materials Science and Chemical Engineering, Stony
Brook University, Stony Brook, NY 11794 (USA)
G. L. Tate, Prof. J. R. Monnier
Department of Chemical Engineering, University of South Carolina
Columbia, SC 29208 (USA)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under https://doi.org/10.
1002/anie.201908761.
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Figure 1. a) Average propylene hydrogenation activity for the following
bimetallic CuMBTC and monometallic MBTC MOFs at 208C: CuBTC,
CuRh(33%)BTC, CuRh(18%)BTC, CuRu(6%)BTC, CuCo(50%)BTC, and
CuIr(5%)BTC; and at 1008C: CuNi(6%)BTC and NiBTC; and b) activity
for propylene hydrogenation as a function of time on stream over
CuRhBTC with the following Rh concentrations: 18% Rh at 208C and
508C; 33% Rh at 208C; and 0% Rh (pure CuBTC) at 208C.
(BTC)2) where M = Rh, Co, Ni, Ir and Ru, as shown in
Figure 1a. Only the CuRhBTC exhibited significant activity
for propane production at 208C ( ꢀ 1600 mmolgcata-
catalystÀ1 minÀ1 for CuRh(33%)BTC). Specifically, the pure
CuBTC MOF showed no activity under the same reaction
conditions, indicating that the Cu2+ ions are not active for
propylene hydrogenation. Pure NiBTC also had no appreci-
able hydrogenation activity. Note that monometallic RhBTC
was not studied due to its loss of crystallinity upon exposure to
air, and the instability of RhBTC is attributed to the fact that
the paddle-wheel configuration of the metal nodes is highly
strained for Rh ions.[6] For all MOF catalysts, powder X-ray
diffraction (PXRD) data were collected immediately after
reaction, and catalysts exhibited the same high crystallinity
observed before reaction.
The activity of the CuRh(33%)BTC catalyst is also stable
over a 24 hour period on stream (Figure 1b). Moreover,
CuRhBTC with a lower bulk concentration of Rh (18%) has
45% of the activity of the CuRh(33%)BTC, which indicates
that activity scales with Rh concentration and suggests that
the Rh ions are the active sites for reaction. At 508C, the
reaction rate for CuRh(18%)BTC is 3.2 times higher than at
208C and is stable over the entire period on stream. The
apparent activation energy for propylene hydrogenation on
CuRhBTC was determined to be 6.3 kcalmolÀ1 based on data
collected at 20–508C (Figure S1 in the Supporting Informa-
tion).
Figure 2. X-ray photoelectron spectroscopy data for the Rh(3d) region
for the CuRh(33%)BTC MOF: after activation by heating at 1608C in
vacuo for 24 hours; after exposure to flowing H2 at room temperature
for 2 hours; after heating in flowing H2 at 508C for 2 hours; and after
propylene hydrogenation at 208C and 508C in the reactor (MOF
catalyst from Figure 1). Spectra for a Rh2(OAc)4/SiO2 catalyst prepared
by incipient wetness impregnation (ꢀ7), crystalline Rh2(OAc)4 (ꢀ0.5)
and RhCl3 (ꢀ0.5) are shown for comparison of binding energies.
consistent with Rh3+ oxidation states reported in the liter-
ature,[7] whereas Rh1+ is expected at 308.0–308.4 eV.[7,8] There
is no evidence for metallic Rh at 307.0–307.4 eV in the
spectrum of CuRhBTC.[7,9] In addition, the Rh2+ ions in
CuRhBTC are stable under reducing conditions since expo-
sure of CuRhBTC to flowing H2 at room temperature and
508C results in almost no change in binding energy. The
nearly identical Rh(3d) peak shapes for the as-prepared MOF
and the post-reaction catalyst used for the hydrogenation
experiment in Figure 1 are also consistent with a stable Rh2+
oxidation state during reaction.
The X-ray absorption near edge structure (XANES) data
provide further evidence for the assignment of the Rh2+
oxidation state in CuRhBTC. As shown in Figure 3, the Rh
X-ray photoelectron spectroscopy (XPS) experiments
address the oxidation state of the Rh ions in CuRhBTC
(Figure 2). For the as-prepared sample, the Rh(3d5/2) peak has
a binding energy of 309.2 eV. The spectrum of dirhodium (II)
tetraacetate (Rh2(OAc)4) provides a standard for the rela-
tively uncommon Rh2+ oxidation state, which appears at
308.9 eV for Rh2(OAc)4 and is similar to the value observed
for CuRhBTC. Furthermore, the Rh(3d) spectra for highly
dispersed Rh2+ ions prepared from incipient wetness impreg-
nation of Rh2(OAc)4 on silica shows identical binding
energies and peak widths to that of CuRhBTC; therefore
the relative broadness of the Rh(3d5/2) peak for CuRhBTC
cannot be attributed to mixed oxidation states since Rh2-
(OAc)4/SiO2 is equally broad and contains only Rh2+. In
contrast, Rh3+ in RhCl3 appears at 310.1 eV, which is
Figure 3. Rh K-edge XANES data for CuRhBTC and Rh2(OAc)4.
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K-edge is nearly identical for CuRhBTC and the Rh2(OAc)4,
pre-reaction MOF suggests that there is no change in
which has an oxidation state of + 2. In contrast, the Rh K-
edges for RhCl3 and [RhI(CO)2Cl]2 have distinctly different
XANES features compared to those in the CuRhBTC
spectrum (Figure S2a). The Cu K-edge for CuRhBTC is
identical to that for CuBTC; a comparison with the spectrum
of CuII oxide indicates that the Cu oxidation state in
CuRhBTC is consistent with Cu2+, whereas the CuI oxide
and metallic Cu0 standards exhibit different spectra (Fig-
ure S2b). EXAFS data were also collected and analyzed for
the Cu and Rh K-edges to provide complementary informa-
tion to the XANES experiments (see Supporting Informa-
tion).
oxidation state of the Cu ions after propylene hydrogenation
(Figure S3). Moreover, exposure of CuRhBTC to H2 at room
temperature does not cause significant reduction of Cu2+.
Consequently, there is no direct evidence that reduction of
Cu2+ plays a role in propylene hydrogenation, particularly
since CuBTC, which also contains both Cu1+ and Cu2+, is
inactive for propylene hydrogenation.
In order to understand the intrinsic activity of the Rh2+
site, propylene hydrogenation was also studied on dirhodium-
(II) tetraacetate (Rh2(OAc)4); this species served as a molec-
ular model for the pure Rh paddle-wheel node. At 208C,
crystalline Rh2(OAc)4 exhibits stable hydrogenation activity
(11000 mmolgcatÀ1 minÀ1), which confirms that Rh2+ ions
provide the active sites (Figure S4a). The apparent activation
energy for the Rh2(OAc)4 catalyst determined from activity
measurements at 0–408C is 5.4 kcalmolÀ1 (Figure S4b), which
is a value similar to that of CuRhBTC and suggests a common
mechanism. Hydrogenation activity for Rh2+ is consistent
with reports in the literature for Rh2(OAc)4,[11] as well as for
Rh2+ ions in amorphous coordination polymers containing
dinuclear rhodium paddle-wheel units.[6,12] However, the Rh2+
active site in Rh2(OAc)4 is unstable under reaction conditions,
in contrast to the high stability of Rh2+ in CuRhBTC.
Specifically, Rh2(OAc)4 is reduced to metallic Rh after
exposure to H2 at 508C (Figure S5), and the unstable
propylene hydrogenation activity for Rh2(OAc)4 at 508C
(Figure S4a) is attributed to reduction of Rh2+ to metallic Rh,
as confirmed by post-reaction XPS and XRD studies.
Hydrogenation activity of the CuRhBTC cannot be
attributed to the presence of residual Rh3+ ions retained in
the MOF pores after exposure to RhCl3 during ion exchange
because RhCl3 itself has no activity for propylene hydro-
genation at 208C (Figure S6). Furthermore, Rh3+ becomes
reduced at 508C under hydrogenation conditions, as demon-
strated by the activity data as well as XPS data for RhCl3
exposed to H2 at 508C. Further discussion of the Rh3+ activity
is presented in the Supporting Information.
Ambient pressure (AP)-XPS studies establish that the Rh
oxidation state in CuRhBTC does not change during expo-
sure to the reactant gases or pure H2. Figure 4 shows the
Figure 4. X-ray photoelectron spectroscopy data for the Rh(3d) region
for CuRhBTC: after pumping down in the UHV chamber; after heating
at 1508C in UHV for 1.5 hours; during exposure to 32 mTorr H2/
8 mTorr propylene at room temperature; in UHV after exposure to H2/
propylene; and during exposure to 20 mTorr H2 at room temperature.
In order to better understand propylene hydrogenation on
CuRhBTC, DFT studies were carried out to investigate the
reaction mechanism. All calculations were performed with
the Gaussian09 program package[13] using the M06-L[14]
functional and the def2-TZVPP[15] basis set (see Supporting
Information). The full reaction profile for propylene hydro-
genation catalyzed by the bimetallic CuRh node of
CuRhBTC is shown in Figure 5. Table S1 provides relative
energy differences for the intermediates and transition states
with respect to the CuRh node and the reactants, as well as
key bond distances that demonstrate the evolution of the
reaction.
The first step in the reaction mechanism is the coordina-
tion of propylene at the open site of the Rh2+ cation
(intermediate 1) via p-backbonding; propylene adsorbs
more strongly than H2 at Rh2+ sites by ꢀ 13 kcalmolÀ1 (see
Supporting Information). In intermediate 1, Rh can be
considered as a hexacoordinated metal after accounting for
a weak bond with the Cu2+ cation (see Supporting Informa-
tion), and therefore the mechanism for H2 dissociation is not
obvious. Several possible mechanisms were considered,
Rh(3d) region for the CuRhBTC and this same sample: after
activation by heating at 1508C for 1.5 hours in ultrahigh
vacuum (UHV); during exposure to 32 mTorr H2/8 mTorr
propylene at room temperature; and in UHV following the
H2/propylene exposure. The Rh(3d) spectrum collected
during exposure to 20 mTorr H2 is also shown for another
CuRhBTC sample that was initially activated at 1508C in
UHV. In all cases, the Rh(3d) spectra exhibit no appreciable
change in peak shape or binding energy, implying that the Rh
oxidation state remains Rh2+ under all conditions, and this
behavior is also consistent with the Rh(3d) spectrum of the
post-reaction CuRhBTC (Figure 2).
Both the XPS and AP-XPS studies demonstrate that the
CuRhBTC contains Cu2+ as the main species although Cu1+ is
also present (Figure S3: Cu(2p), Cu AES and C(1s) regions).
Previous studies by our group have established that reduction
of Cu2+ to Cu1+ in pure CuBTC is induced by the combination
of X-ray irradiation and heating to remove coordinating
solvent molecules.[10] The fact that the post-reaction
CuRhBTC exhibits a Cu(2p3/2) peak identical to that of the
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well as the first report of Rh2+ MOF nodes prepared via the
reduction of postsynthetically incorporated Rh3+. Propylene
hydrogenation activity at room temperature is observed for
CuRhBTC, whereas CuBTC itself and CuMBTC (M = Co, Ir,
Ni, Ru ions) have no appreciable activity, thus illustrating the
unique hydrogenation activity of Rh2+ in the CuBTC frame-
work. The Rh2+ ions in the bimetallic MOF structure are
stable under reaction conditions for up to 24 hours and are
essential for the reaction pathways, as demonstrated by DFT
calculations. The main role of the Cu ions in CuRhBTC is
believed to be the stabilization of the MOF framework, which
prevents incorporated Rh2+ from becoming reduced to
metallic Rh under reaction conditions. This work provides
fundamental understanding for the rational design of MOF
catalysts, which exhibit the exceptionally high selectivity
typically associated with homogeneous catalysts, but are more
stable under reaction conditions than their molecular analogs.
Figure 5. Reaction profile for propylene hydrogenation catalyzed at the
bimetallic CuRh node of CuRhBTC.
including H2 dissociation on pentacoordinated Rh2+ and
direct H-transfer to propylene for the formation of a propyl
radical (see Supporting Information). However, the lowest
energy pathway for H2 dissociation involves cleavage of the
Acknowledgements
This research was supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences,
under Award DE-SC0019360. In addition, we thank the
University of South Carolinaꢀs Advanced Support for Inno-
vative Research Excellence program for funding preliminary
studies. We also acknowledge USCꢀs XPS user facility, as well
as Dr. Stavros Karakalos for his help at the facility. The work
carried out at Brookhaven National Laboratory was sup-
ported by the U.S. Department of Energy under contract no.
DE-SC0012704. S.D.S. is partially supported by a U.S. Depart-
ment of Energy Early Career Award. A.I.F and A.M.E were
supported by the U.S. Army Research Laboratory and the
U.S. Army Research Office under grant number W911NF-15-
2-0107. This research used beamline 7-BM (QAS) of the
National Synchrotron Light Source II, a U.S. DOE Office of
Science User Facility operated for the DOE Office of Science
by Brookhaven National Laboratory under Contract No. DE-
SC0012704. Beamline operations were supported in part by
the Synchrotron Catalysis Consortium (U.S. DOE, Office of
Basic Energy Sciences, Grant No. DE-SC0012335).
À
Rh O bond to create a second vacant site in the coordination
sphere of Rh2+ for the binding of H2. Although this step has
a relatively high activation energy, the decoordinated oxygen
atom facilitates H2 dissociation in the next step via the
formation of OH.[16] Furthermore, the reaction barrier for H2
dissociation is only 0.5 kcalmolÀ1 (TS2) and leads to the
thermodynamically more stable intermediate 3. After the H2
bond is cleaved, a Rh monohydride and a hydroxyl group are
formed. Next, the hydrogen atom of the Rh-hydride is
transferred to propylene (TS3) to form the Rh-propyl
intermediate 4. Finally, a second hydrogen atom from the
hydroxyl group is transferred to the propyl radical (TS4) for
the formation of the final product. The desorption of propane
À
and reformation of the Rh O bond close the catalytic cycle.
À
Note that the activation energy barrier for Rh O bond
dissociation ( ꢀ 15 kcalmolÀ1) is higher than the experimen-
tally measured activation energy of 6.3 kcalmolÀ1. A possible
explanation for this discrepancy is that CuRhBTC serves as
a pre-catalyst, where species 2 provides the active sites, and
À
the Rh O bond is not reformed in the catalytic cycle. In that
case, addition of hydrogen to adsorbed propyl would be the
rate-limiting step, and the calculated barrier of 5.8 kcalmolÀ1
agrees well with the experimental value. This hypothesis will
be further discussed in a subsequent publication. Investiga-
tions were also conducted on Rh2(OAc)4 as a model system
for addressing activity at monometallic Rh2+ sites, and
a similar mechanism for reaction on Rh2(OAc)4 is reported
in Figure S7.
Conflict of interest
The authors declare no conflict of interest.
Keywords: DFT calculations · gas-phase reactions ·
heterogeneous catalysis · metal–organic frameworks · rhodium
Spin density plots for the CuRh node of CuRhBTC
(Figures S8 and S9) demonstrate the lack of significant
overlap between the half-filled 4dz orbital on Rh2+ and the
half-filled 3dx2-y2 on Cu2+. These results suggest minimal
electronic interaction between the two metal centers, but
a more detailed investigation of the bimetallic interactions
and their role in the reaction is the topic of an ongoing study.
In summary, this study represents the first report of gas-
phase catalysis at the metal nodes of a crystalline MOF, as
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Manuscript received: July 14, 2019
Revised manuscript received: August 15, 2019
Version of record online: && &&, &&&&
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Heterogeneous Catalysis
Selective catalytic hydrogenation of pro-
pylene to propane at room temperature
occurs in the gas phase at the metal
nodes of a crystalline CuRh bimetallic
metal–organic framework. The RhII ions
are identified as the catalytically active
sites.
D. M. Shakya, O. A. Ejegbavwo,
T. Rajeshkumar, S. D. Senanayake,
A. J. Brandt, S. Farzandh, N. Acharya,
A. M. Ebrahim, A. I. Frenkel, N. Rui,
G. L. Tate, J. R. Monnier, K. D. Vogiatzis,*
N. B. Shustova,*
D. A. Chen*
&&&& — &&&&
Selective Catalytic Chemistry at
Rhodium(II) Nodes in Bimetallic Metal–
Organic Frameworks
6
www.angewandte.org
ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 1 – 6
These are not the final page numbers!