Single-site metal-organic framework catalysts for the oxidative coupling of arenes: Via C-H/C-H activation

Article abstract of DOI:10.1039/c8sc05510f

C-H activation reactions are generally associated with relatively low turnover numbers (TONs) and high catalyst concentrations due to a combination of low catalyst stability and activity, highlighting the need for recyclable heterogeneous catalysts with stable single-atom active sites. In this work, several palladium loaded metal-organic frameworks (MOFs) were tested as single-site catalysts for the oxidative coupling of arenes (e.g. o-xylene) via C-H/C-H activation. Isolation of the palladium active sites on the MOF supports reduced Pd(0) aggregate formation and thus catalyst deactivation, resulting in higher turnover numbers (TONs) compared to the homogeneous benchmark reaction. Notably, a threefold higher TON could be achieved for palladium loaded MOF-808 due to increased catalyst stability and the heterogeneous catalyst could efficiently be reused, resulting in a cumulative TON of 1218 after three runs. Additionally, the palladium single-atom active sites on MOF-808 were successfully identified by Fourier transform infrared (FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopy.

Full text of DOI:10.1039/c8sc05510f

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DOI: 10.1039/C8SC05510F
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ARTICLE
Single-Site Metal-Organic Framework Catalysts for the Oxidative
Coupling of Arenes via C-H/C-H activation
Niels Van Velthoven,a Steve Waitschat,b Sachin M. Chavan,cd Pei Liu,e Simon Smolders,a Jannick
Received 00th January 20xx,
Accepted 00th January 20xx
Vercammen,a Bart Bueken,a Sara Bals,e Karl Petter Lillerud,cd Norbert Stock bd and Dirk E. De Vos *a
DOI: 10.1039/x0xx00000x
C-H activation reactions are generally associated with relatively low turnover numbers (TONs) and high catalyst
concentrations due to a combination of low catalyst stability and activity, highlighting the need for recyclable heterogeneous
catalysts with stable single-atom active sites. In this work, several palladium loaded metal-organic frameworks (MOFs) were
tested as single-site catalysts for the oxidative coupling of arenes (e.g. o-xylene) via C-H/C-H activation. Isolation of the
palladium active sites on the MOF supports reduced Pd(0) aggregate formation and thus catalyst deactivation, resulting in
higher turnover numbers (TONs) compared to the homogeneous benchmark reaction. Notably, a threefold higher TON could
be achieved for palladium loaded MOF-808 due to increased catalyst stability and the heterogeneous catalyst could
efficiently be reused, resulting in a cumulative TON of 1218 after three runs. Additionally, the palladium single-atom active
sites on MOF-808 were successfully identified by Fourier transform infrared (FTIR) and extended X-ray absorption fine
structure (EXAFS) spectroscopy.
www.rsc.org/
compared to conventional coupling reactions, efficient recovery
and recycling of the precious homogeneous palladium catalyst
is a key aspect in the eventual implementation of this new
synthetic strategy.14 Moreover, since the valence state of
Introduction
palladium changes between Pd(II) and Pd(0) in the catalytic
cycle, formation of Pd(0) aggregates is commonly recognized as
an important deactivation pathway (Scheme 1), highlighting the
need for solid catalysts with stable single-atom active sites.15–17
Nevertheless, only very few heterogeneous catalysts for the
oxidative coupling of arenes via C-H/C-H activation have been
reported so far.18–22
The synthesis of biaryls has attracted much attention over the
past decades since these motifs are abundantly present in
pharmaceuticals, natural products, agrochemicals, specialty
monomers and other fine chemicals.1–4 Typically, the synthesis
of the biaryl groups in these compounds involves conventional
coupling reactions, such as the Suzuki reaction, which requires
pre-functionalized substrates and produces stoichiometric
amounts of salt waste.1 In recent years, formation of biaryls via
palladium catalyzed cross-dehydrogenative coupling (CDC)
reactions has been proposed as a more cost-efficient and
environmentally benign alternative, since simple arenes can be
used as substrate and water is the only byproduct if O2 is used
as the oxidant.5–7
Metal-organic frameworks (MOFs), which are coordination
polymers made up of inorganic secondary building units (SBUs;
metal ions or clusters) and organic linkers, are an interesting
group of porous, crystalline materials that can be transformed
into heterogeneous single-site catalysts.23–26 Besides any
catalytic activity inherent to MOFs, common strategies to imbue
MOFs with well-defined and isolated active sites include
anchoring active transition metals on the organic linkers27–31 or
grafting them on open coordination sites of the SBUs31–40. Due
to their excellent stability, combined with high surface areas
and tunable porosity, Zr-based MOFs have already been proven
The primary focus of the scientific community regarding C-H
activation of aromatic C(sp2)-H bonds has been so far on
increasing the catalysts’ activity and regioselectivity, besides
expanding the substrate scope.8–13 However, given the
relatively low turnover numbers (TONs) and high catalyst
concentrations generally associated with CDC reactions
a.Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F p. o.
box 2461, 3001 Leuven (Belgium).
b.Institute of Inorganic Chemistry, Christian-Albrechts University Kiel, Max-Eyth-
Straße 2, 24118 Kiel (Germany).
c. Department of Chemistry, University of Oslo, p. o. box 1033 Blindern, 0315 Oslo
(Norway).
d.ProfMOF AS, Kirkegårdsveien 45, 3616 Kongsberg (Norway).
e. Electron Microscopy for Materials Science, University of Antwerp,
Groenenborgerlaan 171, 2020 Antwerp (Belgium)
† Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, optimization of the reaction conditions, additional characterization
data, substrate scope. See DOI: 10.1039/x0xx00000x
Scheme 1. Schematic representation of catalyst deactivation
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Chemical Science
-OH)4(btc)2(CH3COO)6], btc3- = 1,3,5-benzenetricarVbieowxAyrltaictleeO)4n8li,n4e9
to be interesting heterogeneous scaffolds to anchor metals for
several applications ranging from sensing to catalysis.41–45
DOI: 10.1039/C8SC05510F
Herein, we present the first heterogeneous MOF-based
catalysts for the oxidative coupling of arenes via C-H/C-H
activation, which exhibit superior TONs compared to their
homogeneous analogues due to isolation of the Pd(II) centers
on the MOF supports.
Results and discussion
Scheme 2. The oxidative coupling of o-xylene under the standard reaction conditions: o-
xylene (16.58 mmol), Pd(OAc)2 (8.29 µmol), MOF support (8.29 µmol), 1-propanesulfonic
acid (82.85 µmol), acetic acid co-solvent (2.07 mmol), 90 °C, 16 bar O2, 17 h.
Screening of MOF Supports
The oxidative homocoupling of o-xylene via C-H/C-H activation
(Scheme 2) is considered to be a scientifically and industrially
relevant CDC model reaction, since the 3,3',4,4'-
tetramethylbiphenyl product is an important intermediate in a
more cost-efficient route to prepare the high-performance
polyimide resin Upilex.7,14,46 Industrially, the reaction is stopped
at low conversion to avoid oligomerization of the products and
the starting feedstock is recycled. This justifies evaluating the
reaction using a TON value instead of a single-pass yield.7 As a
first step, several Zr-MOFs were evaluated for their potential as
heterogeneous supports in the oxidative coupling of o-xylene
(Table 1). Generally, an equimolar amount of MOF (based on its
structural formula) with respect to Pd(OAc)2 was added, i.e. one
Zr6-cluster of UiO-66 or MOF-808 per Pd atom. This ensures that
a 4 – 6 fold excess of anchoring sites per Pd center are present
in the frameworks (Table S1). Additionally, 1-propanesulfonic
acid was employed as strongly acidic additive, since recent
findings by the group of Stahl reveal that strong acids or their
corresponding salts dramatically increase the activity of Pd(II)
for this reaction.12 Inspired by the excellent results obtained
with 2,2’-bipyridine-grafted mesoporous silica for the related
Pd(II)-catalyzed oxidative Heck coupling,17 a MOF containing
2,2'-bipyridine-5,5'-dicarboxylate (bpydc2-) linkers (UiO-67-bipy;
Table 1. Screening of the Different MOF Supports under the Standard Reaction
Conditions (cfr. Scheme 2).
chemo-
selectivity selectivity
(%)a
88
regio-
MOF
support
yield
(%)c
TOF
entry
TONd
(h-1)e
(%)b
71
1
2
3
4
5
6
7
8
/
4.9
0.3
7.4
9.2
8.4
7.1
5.1
6.5
97
6
8.1
1.1
UiO-67-bipy
74
92
92
93
93
90
89
66
73
74
74
73
75
75
UiO-66-
COOH
149
183
168
142
103
130
10.6
11.2
11.1
11.0
8.3
MOF-808
UMCM-
309a
Zr-abtc
UiO-66
f
ZrO2
10.5
aChemoselectivity is defined as the percentage of biaryls relative to all formed
[Zr6(µ3-O)4(µ3-OH)4(bpydc)6])27 was tested. However, the products (oxidation products, biaryls and triaryls). bRegioselectivity is defined as
the percentage of 3,3',4,4'-tetramethylbiphenyl relative to all three biaryls. cYield
addition of this MOF resulted in a dramatic decrease in activity
versus the homogeneous reaction (Table 1; entry 1 and 2). A
similar deactivation effect was found for the analogous
dissolved heterocyclic nitrogen-containing ligand, 4,4’-
was determined by GC-FID with hexadecane as internal standard. dTON is defined
as TON = 2 x mole (biaryl) / mole (Pd). eTOF was determined after 4 h. f10 mg of
zirconium oxide was added.
are 6-fold coordinated by btc3- linkers, resulting in up to 6 open
sites per cluster, after removing the acetate modulators using a
simple acid treatment. A closely related MOF with 6-fold
coordinated Zr-clusters is UMCM-309a ([Zr6(µ3-O)4(µ3-
dimethyl-2,2’-bipyridine (Table S2). Consequently, other MOFs
featuring anchoring sites which correspond more closely to the
carboxylates in the active Pd(OAc)2 complex were investigated.
For instance, a UiO-66 analogue with pendent carboxylic acid
groups as Pd(II) anchoring sites (UiO-66-COOH; [Zr6(µ3-O)4(µ3-
OH)4(btb)2(HCOO)6],
btb3-
=
1,3,5-(4-
OH)4(bdc-COOH)6],
bdc2--COOH
=
1,2,4-
carboxylphenyl)benzene),50 which features a stable two-
dimensional layered network instead of a three-dimensional
framework. Recently, a new 8-connected Zr-MOF with 3,3′,5,5′-
azobenzene-tetracarboxylate (abtc4-) linkers ([Zr6(µ3-O)4(µ3-
OH)4(abtc)2(OH)4(H2O)4]) was reported,51 which contains 4 open
sites per cluster and could be synthesized following a newly
developed water-based green synthesis procedure (Figure S2).
In line with the increase in activity for UiO-66-COOH, the
addition of MOFs with open sites on the Zr-clusters resulted in
significantly higher TONs compared to the homogeneous
reference case (Table 1; entries 4-6). In addition, UiO-66
([Zr6(µ3-O)4(µ3-OH)4(bdc)6], bdc2- = 1,4-benzenedicarboxylate),52
benzenetricarboxylate)47 was synthesized and tested. In
contrast to UiO-67-bipy, a significantly higher TON than in the
homogeneous case was obtained after the same reaction time
(Table 1; entry 3), highlighting the positive effect of active site
isolation by anchoring the active Pd(II) centers on the pendent
carboxylic acid groups. Inspired by previous research in which
platinum-group metal complexes were grafted onto the
inorganic SBUs of a MOF support,35,38,40 several Zr-MOFs with
available coordination sites on the clusters (hydrogen-bonded
OH/OH2 pairs)39 were synthesized (MOF-808, UMCM-309a and
Zr-abtc). The hexanuclear Zr-clusters of MOF-808 ([Zr6(µ3-O)4(µ3
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a well-known non-functionalized Zr-MOF, was tested as a dimensional pore structure, like MOF-808. Furthermore, an
reference, since it does not contain significant amounts of acrylic acid grafted polyolefin fiber (Smopex-102V),iewonAreticloefOntlhinee
DOI: 10.1039/C8SC05510F
pendent carboxylic acid groups or open sites on the Zr-clusters very few already reported heterogeneous supports for the
to which Pd(II) could coordinate. The TON observed in the oxidative coupling of arenes,22 was tested using a similar ratio
presence of UiO-66 was similar to that observed for of 1 Pd(II) center per 6 pendent carboxylic acid groups. The
homogeneous Pd(OAc)2 (Table 1; entry 7). This confirms that cumulative TON of this Pd-loaded polymer with pendent
the increase in TON observed for other MOFs like MOF-808 is carboxylic acid groups is substantially lower than the TON of its
due to active site isolation of the Pd on the anchoring sites of MOF analogue (UiO-66-COOH), which might be due to inferior
the MOFs. Besides MOFs with anchoring sites, a moderate accessibility of the carboxylic acid groups. In addition, the
increase in TON could also be achieved by the addition of high- stability of the MOFs was evaluated by comparing the powder
surface area zirconium oxide. Finally, higher chemoselectivities X-ray diffraction patterns before and after the reaction (Figures
were obtained in the presence of MOFs with open sites on the S5-10). No significant decrease in crystallinity was observed for
Zr-clusters or pendent carboxylic acid groups since less triaryl MOF-808, UMCM-309a, Zr-abtc, UiO-66-COOH and UiO-66 after
side products were formed. The formation of these bulky four runs at 90 °C. In contrast, UiO-67-bipy largely lost its long-
triaryls may be suppressed in the presence of MOFs due to pore range order after exposure to the reaction solvent and
confinement.
conditions.
Heterogeneity of the Single-Site MOF Catalysts
Substrate Scope
The heterogeneity of the single-site MOF catalysts was studied In view of its superior activity, heterogeneity and facile
by recycling tests and metals analysis of the reaction solution. synthesis procedure, MOF-808 was selected as model support
After the reaction, the sample was centrifuged and the MOF to expand the substrate scope (Figure S11). High TONs were
crystals were separated from the reaction solution. Fresh obtained for the homocoupling of toluene and tert-
reactant (e.g. o-xylene), co-solvent (e.g. acetic acid), internal butylbenzene, while the activity was significantly lower for the
standard and strongly acidic additive (e.g. 1-propanesulfonic coupling of p-xylene and 1,2-difluorobenzene due to steric
acid) were added to the recycled MOF and the reaction was hindrance and electron-withdrawing effects, respectively.
resumed for a second run. Materials with open sites on the Zr-
clusters (MOF-808, UMCM-309a and Zr-abtc) retained their
Optimizing the Reaction Conditions
activity better than UiO-66-COOH with pendent carboxylic acid
groups, resulting in higher cumulative TONs after 4 runs (Figure
1). In line with these results, a lower cumulative TON was
noticed for UiO-66, which does not contain anchoring sites. In
addition, the palladium and zirconium contents of the reaction
solution were measured after the first run by inductively
coupled plasma optical emission spectrometry (ICP-OES) (Table
S3). Generally, palladium leaching could be minimized to
approximately 5 % for MOFs with open sites on the Zr-cluster
(MOF-808 and Zr-abtc) and no significant zirconium leaching
was detected. However, more leaching was observed for
UMCM-309a, presumably due to its two-dimensional layered
structure. Higher leaching values were also obtained for UiO-66-
COOH and UiO-66, indicating that palladium is best retained on
MOFs with open sites on the Zr-clusters and with a three-
Significantly higher TONs for the oxidative coupling of o-xylene
could be obtained for the heterogeneous system with MOF-808
by increasing the reaction temperature (Figure 2). Notably, at
110 °C, the TON is 2.9 times higher for the MOF-808 system than
for the analogous homogeneous system, which is a clear benefit
of active site isolation enabled by the MOF. At even higher
temperatures, the MOF support started to decompose (Figure
S12), resulting in lower TONs, which were more comparable to
those of the homogeneous system. After further optimizing the
palladium loading of the MOF and the amount of 1-
propanesulfonic acid and co-solvent (Table S2), a TON of 436 ±
17 could be reached after the first run (Figure 3) with only 2 %
Figure 1. Recycling experiments of several heterogeneous supports for the Pd-catalyzed oxidative coupling of o-xylene performed under the standard reaction conditions.
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Active site isolation
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DOI: 10.1039/C8SC05510F
The binding mode of Pd(II) to the zirconium clusters of MOF-808
was studied by Fourier transform infrared spectroscopy (FTIR).
Upon removal of a capping acetate from the cluster, a
hydrogen-bonded OH/OH2 pair occupies the place of the
missing carboxylate. This OH/OH2 pair displays a characteristic
IR band at 2744 cm-1, while the sharp IR band at 3672 cm-1
results from a combination of non-hydrogen-bonded OH groups
and µ3-OH groups inside the cluster (Figure S15).53,54 In
agreement with previous research,33,35,36,38,55 the characteristic
IR band at 2744 cm-1 disappears after chemisorption of the
transition metal complex and the sharp IR band at 3672 cm-1
broadens, indicating that the palladium species interact with
these open sites on the Zr-clusters. Moreover, the local
environment of the MOF-supported Pd(II) was studied by X-ray
absorption spectroscopy (XAS). In line with the FTIR data, the fit
between the experimental and simulated extended X-ray
absorption fine structure (EXAFS) data is excellent for the
structure in which Pd(II) is anchored on the open site of the Zr-
Figure 2. Effect of temperature on the TON of the oxidative coupling of o-xylene with
and without MOF-808 performed under the standard reaction conditions.
Figure 4. The magnitude (blue) and real component (orange) of the Fourier transform of
the experimental k2-weighted Pd K-edge EXAFS spectra of preloaded MOF-808 (1 Pd per
Zr-cluster in o-xylene) (hollow circles) and the corresponding fits of the structure model
(solid lines) in R-space. The vertical dashed lines indicate the fitting range. The Zr, O, C,
H and Pd atoms are represented in the structure model by turquoise, red, gray, white
and dark blue spheres, respectively.
Figure 3. Oxidative coupling of o-xylene performed under optimized reaction conditions
and with reactivation of the Pd loaded MOF support under vacuum (1 mbar) at room
temperature for 24 h in between consecutive runs. Conditions: o-xylene (16.58 mmol),
Pd(OAc)2 (8.29 µmol), MOF-808 (41.43 µmol), 1-propanesulfonic acid (331.40 µmol),
acetic acid (2.07 mmol), 110 °C, 20 bar O2, 17 h. The TONs are the average of three
experiments.
Scheme 3. The proposed catalytic cycle (X = acetate or 1-propanesulfonate).
Pd leaching (Table S4). Since oxygen is used as terminal oxidant,
a stoichiometric amount of water is produced and partially
absorbed by the hydrophilic MOF material. Product inhibition
experiments revealed that the formation of water did not lead
to a significant decrease in activity at short reaction times
(Figure S13). However, after many catalytic cycles (TONs > 400),
a large amount of water is formed and product inhibition due to
water formation is observed (Table S2). Nevertheless, the
activity of the solid MOF-based catalyst could be restored by
reactivating the MOF materials after each run under vacuum (1
mbar) at room temperature for 24 h (Figure 3). A cumulative
TON of 1218 ± 36 with an overall regioselectivity of 73 % for
3,3',4,4'-tetramethylbiphenyl was obtained after three runs,
which is well beyond the state-of-the-art.12,22 Moreover, only 5
% of Pd was leached after three runs (Table S4).
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cluster after exchanging with the proton of the -OH2 group and
liberating acetic acid (Figure 4). Furthermore, the absence of a
clear peak around 2.6 Å confirms that the trimeric Pd(OAc)2
complexes are converted into monomeric, MOF-supported
Pd(II) species (Figure S16).17,22 Hence, the Pd(II) species formed
after the reoxidation step and possibly also the Pd(II) complex
formed after the first C-H activation step, can be anchored on
the MOF support (Scheme 3). High-angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM)
images of MOF-808 and UiO-66 after reaction in combination
with energy-dispersive X-ray spectroscopy (EDX) showed that
In this work, we have shown for the first time that palladium
loaded Zr-MOFs can be efficient single-site solid hybrid catalysts
View Article Online
DOI: 10.1039/C8SC05510F
for the oxidative coupling of arenes via C-H/C-H activation.
MOFs with various anchoring sites were screened first and the
heterogeneity of these solid catalysts was studied by recycling
tests and metals analysis of the reaction solution, which
indicated that the heterogeneous catalysts could efficiently be
reused. After optimization of the reaction conditions, a
threefold higher TON could be achieved for the MOF-808
system compared to the analogous homogeneous system and
palladium anchoring reduces Pd nanoparticle aggregation and Pd leaching was minimized to 2 %. The activity of the solid MOF-
thus deactivation after the reductive elimination step (Figures
S19-22).
based catalyst could be retained by reactivating the MOF
materials by drying in between consecutive runs, resulting in a
cumulative TON of 1218 after three runs. Finally, analysis of the
reaction kinetics revealed that the superior TONs result from
isolation of the active sites on the MOF material, which prolongs
catalyst lifetime, and the isolated palladium single-atom active
sites on MOF-808 were successfully identified by FTIR and
EXAFS spectroscopy. These results show that the TON of
palladium in C-H activation reactions can be significantly
increased by developing stable heterogeneous single-site
catalysts.
Reaction Mechanism
Catalyst deactivation was studied by analyzing the reaction
kinetics (Figure 5). Although the homogeneous system was
found to be the most active at short reaction times, only
moderate yields were obtained at longer reaction times due to
catalyst deactivation. On the contrary, almost no deactivation
was observed when MOF-808 was added, with an essentially
constant rate up to 24 h reaction time. This implies that catalyst
lifetime can be considerably prolonged by isolating the active
sites on the MOF material. Moreover, a maximum TON was
achieved for the heterogeneous system if more than 8 bar O2
was applied, indicating that deactivation is negligible under
these conditions. In contrast, a clear dependence of the TON on
the oxygen pressure could be seen in the homogeneous system
for a wide range of oxygen pressures (Figure S23). To gather
further insights into the catalytic mechanism, the kinetic isotope
effect (KIE) was evaluated by comparing the conversion of o-
xylene with o-xylene-d10 at short reaction times. The measured
kH/kD values were 1.4 and 1.3 for the homogeneous and
heterogeneous case, respectively, indicating a similar rate-
limiting step for both systems. These modest KIE values are in
accordance to the literature for the oxidative coupling of o-
xylene under neat conditions.14
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The research leading to these results has received funding from
the NMBP-01-2016 Program of the European Union's Horizon
2020 Framework Program H2020/2014-2020/ under grant
agreement n° [720996]. N. V. V., S. S., J. V., B. B. and D. E. D. V.
thank the FWO for funding (SB, Aspirant and postdoctoral
grants). The electron microscopy work was supported by FWO
funding G038116. D. E. D. V. is grateful for KU Leuven support in
the frame of the CASAS Metusalem project and a C3 type
project. The XAS experiments were performed on beamline
BM26A at the European Synchrotron Radiation Facility (ESRF),
Grenoble, France. We are grateful to D. Banerjee at the ESRF for
providing assistance in using beamline BM26A. Johnson
Matthey and S. Bennett are gratefully acknowledged for
providing Smopex-102.
Notes and references
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Figure 5. Kinetic profiles of the oxidative coupling of o-xylene performed under the
standard reaction conditions with MOF-808 (blue), UiO-66-COOH (green) and the
homogeneous reaction without MOF support (orange). Lines were added as a guide to
the eye.
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Chemical Science
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Chemical Science
A. Lago, V. Guillerm, C. Hemsley, J. F. Eubank, M.
ARTICLE
View Article Online
DOI: 10.1039/C8SC05510F
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Graphical abstract
Heterogeneous palladium single-site MOF catalysts for the
oxidative coupling of arenes via C-H/C-H: superior TONs due to
active site isolation.
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