Probing the Evolution of Palladium Species in Pd@MOF Catalysts during the Heck Coupling Reaction: An Operando X-ray Absorption Spectroscopy Study

Article abstract of DOI:10.1021/jacs.8b03505

The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (¹H NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl^- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.

Full text of DOI:10.1021/jacs.8b03505

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Article
Probing the evolution of palladium species in Pd@MOF catalysts during the
Heck coupling reaction: An operando X-ray absorption spectroscopy study
Ning Yuan, Vlad Pascanu, Zhehao Huang, Alejandro Valiente, Niclas Heidenreich, Sebastian Leubner,
A. Ken Inge, Jakob Gaar, Norbert Stock, Ingmar Persson, Belen Martin-Matute, and Xiaodong Zou
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03505 • Publication Date (Web): 11 Jun 2018
Downloaded from http://pubs.acs.org on June 11, 2018
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Probing the evolution of palladium species in Pd@MOF cata-
lysts during the Heck coupling reaction: An operando X-ray
absorption spectroscopy study
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$
$
†,
†,¤
Ning Yuan,†, Vlad Pascanu,†,¤, Zhehao Huang, Alejandro Valiente, Niclas Heidenreich,⊥ Sebasꢀ
‡,
‡
§,
,
†,
¤
†,¤
‡
tian Leubner,⊥ A. Ken Inge, Jakob Gaar, Norbert Stock,⊥ Ingmar Persson, * Belén MartínꢀMatute, *
§
,
‡
†,
Xiaodong Zou *
† Berzelii Center EXSELENT on Porous Materials, Stockholm University, SEꢀ106 91 Stockholm, Sweden
‡ Department of Materials and Environmental Chemistry, Stockholm University, SEꢀ106 91 Stockholm, Sweden
§ Department of Molecular Sciences, Swedish University of Agricultural Sciences, P.O. Box 7015, SEꢀ750 07 Uppsala, Sweden
¤ Department of Organic Chemistry, Stockholm University, SEꢀ106 91 Stockholm, Sweden
⊥ Institut für Anorganische Chemie, ChristianꢀAlbrechtsꢀUniversität zu Kiel, DEꢀ24118 Kiel, Germany
ABSTRACT: The mechanism of the Heck C–C coupling reaction catalyzed by Pd@MOFs has been investigated using operando
Xꢀray absorption spectroscopy (XAS) and powder Xꢀray diffraction (PXRD) combined with transmission electron microscopy
(TEM) analysis and nuclear magnetic resonance (1H NMR) kinetic studies. A customꢀmade reaction cell was used allowing operꢀ
ando PXRD and XAS data collection using highꢀenergy synchrotron radiation. By analyzing the XAS data in combination with ex
situ studies, the evolution of the palladium species is followed from the asꢀsynthesized to its deactivated form. An adaptive reaction
mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction,
which then gradually transform into Pd nanoclusters with 13ꢀ20 Pd atoms on average in later catalytic turnovers. Consumption of
available reagent and substrate leads to coordination of Cl– ions to their surfaces, which causes the poisoning of the active sites. By
understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.
regardless of the aggregation state of the sample (solid, liquid
or gas). The concentration of the investigated element can be
as low as a few mM. These properties make XAS an ideal
INTRODUCTION
Sustainable catalysis by transition metals1 is essential for
further advances in the largeꢀscale production of specialty
chemicals for pharmaceuticals, advanced materials or agroꢀ
chemicals. In this context, C–C coupling reactions remain
indispensable for creating the backbone of organic molecules.
To minimize, recycle or replace the heavy metals involved in
these processes, for economic and environmental reasons, a
thorough understanding of reaction mechanisms is required.
The Heck coupling is one such process of utmost importance,
forging new C–C bonds between aryl halides and olefins, in
the presence of a metal catalyst, via a twoꢀelectron redox
cycle.2
method to obtain detailed information about catalytically
active centers supported in solid matrices such as MOFs.
Moreover, operando XAS methods provide great opportuniꢀ
ties to monitor in real time the structural changes of the cataꢀ
lytic center while avoiding interference from the solid supꢀ
port.7
Constant improvements in the time resolution of XAS methꢀ
ods led to various reactors being designed and adapted to
various reaction conditions. In situ/operando XAS measureꢀ
ments became possible in both gasꢀsolid8 and liquidꢀsolid9
reaction systems. In the latter case, Pd nanoparticles supported
on active carbon, Al2O3 or polymers were studied by operando
XAS for oxidation10 and C–C coupling11 reactions, revealing
mechanistic aspects that were inaccessible with other techꢀ
niques. Surprisingly, catalytic reactions starting from atomic
Pd preꢀcatalysts are less explored, although they could shed
light in the debate over the heterogeneity of Pdꢀcatalyzed
processes.12
Detailed investigations into the catalyst activation, nature of
active species, factors that govern reactivity and possible
deactivation pathways are more difficult to perform in the case
of emerging heterogeneous systems such as metalꢀorganic
framework (MOF)ꢀsupported catalysts,3 for which solutionꢀ
specific characterization methods are not suitable. Heterogeꢀ
neous mechanisms are notoriously difficult to monitor4,5 and
adapted operando methods are highly demanding to achieve a
deeper understanding.6
Meanwhile, MOFs evolved to become the most prolific
supports in heterogeneous catalysis today, owing to their recꢀ
ordꢀbreaking porosity and their unprecedented degree of tunaꢀ
Xꢀray absorption spectroscopy (XAS) is a powerful elementꢀ
specific technique for investigating the oxidation state and
coordination environment of particular atoms in a material,
,
bility,13 14 which allow more complex catalysts to be embedded
in a controlled manner.15 Our groups have previously develꢀ
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oped MOFꢀsupported Pd catalysts that exhibited excellent target temperature in less than 1 min. The actual temperature
reactivity and recyclability for C–C crossꢀcoupling reactions,16
and for the functionalization of aromatic C–H bonds,17 which
are essential tools for creating new organic molecules. Howꢀ
ever, preliminary ex situ investigations revealed that despite
the apparent recyclability, the catalyst compositions showed
obvious differences before and after the reaction.18 In addition,
the same MOFꢀsupported catalysts suffered from an unexꢀ
plainable rapid deactivation in other reactions (e.g. the Heck
coupling). These peculiar observations highlighted the necesꢀ
sity to identify better techniques for studying the behavior of
MOFꢀsupported catalysts.
of the reaction mixture was constantly monitored using a
PTFEꢀcoated thermocouple and kept close to the target temꢀ
perature through a combination of resistive heating and direct
cooling of the heating mantle with compressed air. To initiate
very fast reactions or alter chemical parameters during the
reaction, the injection of reagents was triggered remotely via
two tubes embedded into the reactor’s cap. The tubes were
connected to a neMESYS syringe pump fitted with 5 mL glass
syringes. A magnetic stirrer was placed under the base of the
aluminum casing to ensure homogeneous distribution of partiꢀ
cles in the pathway of the Xꢀray beam.
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A custom reactor was earlier developed by us19 for the simulꢀ
taneous acquisition of operando XAS and powder Xꢀray difꢀ
fraction (PXRD) data. This setꢀup enables the correlation of
information about the active catalytic species with the changes
in the structural stability of the crystalline solid support. The
reactor was designed for operation at synchrotron beamlines.
It includes an inꢀbuilt miniaturized stirring plate and parameꢀ
ters like temperature, pressure as well as the addition of reaꢀ
gents can be controlled remotely. Duplicate experiments under
identical conditions were followed by ex situ NMR spectrosꢀ
copy, transmission electron microscopy (TEM), scanning
transmission electron microscopy (STEM), and energy disperꢀ
sive spectroscopy (EDS). Combining the information extractꢀ
ed from these methods we have unambiguously probed the
entire “lifetime” evolution of the Pd species in Pd(II)@MOF
preꢀcatalysts during the Heck coupling reaction. This “lifeꢀ
time” includes activation of the preꢀcatalyst, catalysis driven
by different types of active species and deactivation of the
catalysts. This information further enabled us to manipulate
and prolong the activity of our catalyst.
The method described herein is widely applicable to the
study of complex catalytic systems promoted by the majority
of supported transition metal complexes, where traditional
spectroscopic techniques are not sufficient.
Figure 1. Customꢀmade reactor for combined operando XAS and
PXRD measurements.
Operando XAS and PXRD experiments
Operando XAS and PXRD data were collected on the beamꢀ
line BM01B at the ESRF, Grenoble, France. Detailed descripꢀ
tions of data collection and analysis are given in Section S2.
PXRD data were collected on a Dexela 2D detector using Xꢀ
rays with an energy of 24.55 keV (λ = 0.505 Å).21 XAS measꢀ
urements were performed at the Pd Kꢀedge (24.35 keV) in
transmission mode with an energy range from 24.00 to 25.25
keV. During the catalytic reactions XAS and PXRD data were
collected in bundles of 5 scans each, alternating between the
two modes of the beamline. The acquisition of five XAS specꢀ
tra required ca. 17 min (3.4 min/scan). The time necessary to
switch from XAS to PXRD, collect five diffraction patterns
and switch back to XAS was ca. 2.4 min. This sequence was
repeated for the entire duration of the experiment, as described
in Figure 2.
EXPERIMENTAL SECTION
Materials
Pd(II)@MILꢀ101ꢀNH2, Pd(0)@MILꢀ101ꢀNH2, Pd(II)@MILꢀ
88BꢀNH2 and Pd(0)@MILꢀ88BꢀNH2 (Pd@MOF, all with ca.
7–8 wt% Pd loading) were synthesized according to proceꢀ
dures previously reported.16a,20 Details of synthesis and characꢀ
terization are given in Section S1 of the Supporting Inforꢀ
mation (SI). All reagents and solvents were used as obtained
from commercial suppliers without further purification.
Reactor for operando experiments
Reactions were carried out in a customꢀbuilt reactor (Figure 1)
developed at the ChristianꢀAlbrechts University (Kiel, Germaꢀ
ny) in cooperation with the beamline staff at the P08 beamline,
PETRA III, DESY (Hamburg, Germany).19 The reactor allows
the analysis of chemical reactions and crystallographic transꢀ
formations under solvothermal conditions using synchrotronꢀ
based characterization techniques. It consists of an aluminum
casing that holds Duran© glass vials with a maximum volume
of 6 mL. The inner diameter of the vials used was 10 mm and
the thickness of the glass wall was 1.0 mm. The whole reactor
was aligned on the beamline in transmission geometry. The
aluminum casing was surrounded by a heating mantle made of
copper wires. Typically, the reaction mixture was brought to
For PXRD data, minimal changes were observed between
the five consecutive frames. Therefore, intensities from the
five frames were summed up in order to improve the intensity
statistics. For XAS data, all scans were treated individually as
the sample composition changed with time. The data treatꢀ
ment, including preꢀedge subtraction, spline removal, normaliꢀ
zation and Fourier transformation, was performed with the
EXAFSPAK package.22 The experimental k3ꢀweighted
EXAFS oscillations were analyzed by nonꢀlinear leastꢀsquares
fits of the data to the EXAFS equation, refining the model
parameters, number of backscattering atoms (N), mean interaꢀ
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tomic distances (R), DebyeꢀWaller factor coefficients (σ2), and
Catalytic reactions for operando experiments
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threshold energy (Eo). The spectrum of a palladium metal foil
was recorded simultaneously in transmission mode as referꢀ
ence with the first inflection point of the absorption edge deꢀ
fined as 24.350 keV.23 The theoretical phases and amplitudes
used in the refinements were calculated using the FEFF7 proꢀ
gram.24 The standard deviations reported for the refined paꢀ
rameters were obtained from k3 weighted leastꢀsquares refineꢀ
ments of the EXAFS function χ(k), and without including
systematic errors. These statistical error values allow reasonaꢀ
ble comparisons, e.g. of the significance of comparing relative
shifts in the distances. However, the variations in the refined
parameters, including the shift in the Eo value (with k = 0),
using different models and data ranges, indicate that the accuꢀ
racy of the distances given for an individual complex is beꢀ
tween ±0.005 and ±0.02 Å for wellꢀdefined interactions. The
“standard deviations” have been increased accordingly to
include estimated additional systematic errors.
To achieve XAS data of sufficient quality and a rapid compleꢀ
tion of the organic reaction, the total concentration of reagents
was doubled and the catalyst loading was further increased to
ca. 18 mol%. pꢀIodobenzonitrile (0.4 mmol), tertꢀbutyl acryꢀ
late (1.5 equiv.), NaOAc (2 equiv.), and Pd@MOF catalyst
(0.075 mmol) were mixed in H2O (1 mL) and DME (3 mL) in
6 mL vials. Full conversion was typically achieved within 1 h.
At higher overall concentrations, an increased viscosity of the
reaction medium impeded stirring. Control experiments conꢀ
firmed that besides an increased reaction rate, no further difꢀ
ferences were observed compared to those under standard
conditions. Therefore, the conclusions derived from operando
studies can be considered relevant for general catalytic experꢀ
iments.
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The following sequence of operations was used as illustrated
in Figure 2: 1. (t < 0 min): pꢀIodobenzonitrile (0.4 mmol, 91
mg), sodium acetate (2 equiv., 66 mg) and Pd@MOF catalyst
(0.075 mmol, ca. 100 mg for 8 wt% Pd loading in MOF) toꢀ
gether with H2O (1 mL) and DME (2 mL) were added to the
reaction cell. XAS and PXRD data acquisition was already
initiated while the mixture was allowed to stir to homogenize
for 10−20 min. 2. (t = 0 min): tertꢀbutyl acrylate (1.5 equiv.)
was injected with the remaining solvent (DME, 1 mL) and 1
mL headspace gas was simultaneously extracted to avoid
overpressure. 3. (t = 0−10 min): the temperature was increased
to 60 °C in less than 1 min and held constant for 10 min. 4. (t
= 10−30 min): the temperature was further increased by 10 °C
every 10 min up to 90 °C. 5. (t = 30−90 min): the temperature
was maintained constant at 90 °C for 60 min to achieve comꢀ
pletion of the organic reaction. 6. (t > 90 min): the mixture
was cooled down gradually to room temperature (RT) during
which the XAS and PXRD data acquisition was continued.
NMR and TEM experiments
The experiments at the beamline were repeated in the laboraꢀ
tory under identical conditions in order to acquire NMR and
TEM data and correlate it with that obtained by XAS. Small
aliquots (<50 µL) were taken from the hot reaction mixture at
regular intervals, extracted with cold CDCl3 and analyzed by
1
1H NMR spectroscopy to measure conversion. The H NMR
spectra were recorded at 400 MHz on a Bruker Advance specꢀ
trometer. Alternatively, a droplet of the aliquot was transferred
to a Cu grid without further treatment. The grid was immediꢀ
ately placed into the vacuum chamber of the microscope for
TEM and EDS analysis. TEM observations and EDS mapping
were performed on a JEOL JEMꢀ2100F microscope equipped
with a Schottky field emission gun, and operated at 200 kV.
Images were recorded with a Gatan Ultrascan 1000 CCD
camera.
Results and discussion
Catalytic reactions under standard conditions
Aryl iodide (0.1 mmol), olefin (1.3 equiv.), sodium acetate (2
equiv.), and Pd@MOF (0.003 mmol) were stirred in a mixture
of deionized water (0.5 mL) and dimethoxyethane (DME, 1.5
mL) in a vial sealed with a septumꢀfitted cap. The reactions
were performed in air. Minimum reactivity was observed at 60
°C and optimal reactivity was observed at 90 °C. Under these
conditions, the catalyst was completely deactivated after one
run and could not be recycled. Details about the optimization
of reaction parameters and the substrate scope are given in
Sections S3 and S4. pꢀIodobenzonitrile and tertꢀbutyl acrylate
were chosen as model substrates for the operando experiments
(Scheme 1).
1. Catalytic activity (NMR Spectroscopy)
Kinetic profiles of the Heck reaction using Pd(II)@MILꢀ101ꢀ
NH2 as preꢀcatalyst are presented in Figure 3a, where both
conversion and temperature are plotted as a function of time.
Intriguingly, after an initial short period of rapid conversion at
60 °C, a plateau was reached at ca. 70–80 °C (20–30 min after
olefin addition) with a clearly diminished activity (Figure 3a,
solid blue line). Upon further heating, the rate increased again
and the reaction reached completion in ca. 60–70 min. This
unusual behavior is indicative of different mechanisms being
dominant under different reaction conditions. At the attempt to
isolate a single operating mode, we performed additional
experiments maintaining the temperature constant at 60 °C. To
our surprise, the reaction stopped after only one turnover (Figꢀ
ure 3a, dashed blue line).
Scheme 1. Model Heck coupling for operando experiments.
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Figure 2. Timeline of the operando XAS and PXRD experiments.
addition. Furthermore, these Pd species became mobile
and could agglomerate into clusters and further into nanoꢀ
particles. Therefore, the plateau observed during the stepꢀ
wise heating process (ca. 20–30 min, Figure 3a, solid blue
line) is likely due to the decreased amount of mononuclear
Pd complexes coordinated to the MOF linker while the
mobile Pd species are unreactive at low temperature (beꢀ
low 90 °C). We anticipated that in order to catalyze the
reaction at 60 °C, Pd might need to coordinate to a more
electron rich ligand (such as ArꢀNH2). This was confirmed
by adding aniline in stoichiometric amounts to Pd into the
reaction mixture. Aniline represents a small homogeneous
model for the MOF linker that can simulate the same
coordination mode. Under these conditions, oxidative
addition is faster than aggregation, and full conversion
was achieved at 60 °C (Figure 3a, dotted blue line). The
shape of the reaction profile suggests a completely homoꢀ
geneous reaction. This is also confirmed by TEM and
EDS studies at various stages of the catalytic reaction; no
Pd nanoparticles were observed before the completion of
the catalytic reaction (Figures S1ꢀ2, Section S5).
To establish the generality of this behavior, we also
tested Pd(II)@MILꢀ88BꢀNH2, and found that it operates
through a related mechanism, with subtle but important
differences (Figure 3b, Figure S3, Section S6). When the
catalyst was tested at 60 °C, the first turnover occurred at
a high rate, as in the case of Pd(II)@MILꢀ101ꢀNH2. Afꢀ
terwards the reaction continued, although at a slower rate,
instead of being completely suppressed as for
Pd(II)@MILꢀ101ꢀNH2 (dashed blue line). When using the
stepwise temperature ramp to 90 °C, a similar behavior as
for Pd(II)@MILꢀ101ꢀNH2 was observed (solid green line),
but the reactivity plateau was less pronounced. We have
previously shown that Pd(II) in MILꢀ88BꢀNH2 is coordiꢀ
nated by two amino ligands in a chelating fashion, not
only one as in the case of MILꢀ101ꢀNH2.20 In this way,
there is a higher probability that Pd remains partly coordiꢀ
nated to amino donors in the MOF, even after the first
turnover and does not agglomerate as fast as in MILꢀ101ꢀ
NH2. Thus, although the activity was diminished, this
effect was not as drastic as for Pd(II)@MILꢀ101ꢀNH2.
Furthermore, a sustained activity due to coordination to
the second MOF linker is also in agreement with literature
reports showing that bipyridineꢀtype and other chelating
Figure 3. Reaction profile for the Heck coupling catalyzed by
Pd(II)@MILꢀ101ꢀNH2 (a) and Pd(II)@MILꢀ88BꢀNH2 (b).
We hypothesized that during one of the catalytic steps,
after oxidative addition in the first catalytic turnover, Pd
lost its coordination to the MOF linker, due to the poorer
coordinating ability of anilines in the MOF compared to
ancillary ligands such as phosphines.25 This also agrees
with our result from XAS showing that the Pd–N mean
distance decreased from 2.114(8) Å (Pd coordinated to
Ar–NH2 in the MOF) to 1.993(9) Å (Pd coordinated to R–
CN and/or C ligands) at 47 min (vide infra, Table 1).26
Without the nitrogen donor, Pd species did not have the
necessary electron density to perform a second oxidative
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ligands could catalyze the Heck coupling reaction more
being reꢀoxidized by the aryl iodide substrate in the initial
oxidative addition step of the catalytic cycle (vide infra,
Figure 8).
efficiently.27 It is worth mentioning that the kinetic proꢀ
files for Pd(II)@MILꢀ101ꢀNH2 and Pd(II)@MILꢀ88ꢀNH2
at the early stage (up to 15 min) of the reaction have alꢀ
most perfect overlap (Figure S4, Section S6). This indiꢀ
cates that the reaction mechanism was the same for both
materials at this early stage, and the first catalytic turnover
occurred inside the pores.
The XANES features of asꢀsynthesized Pd(II)@MILꢀ
101ꢀNH2 and their evolution as a function of time under
Heck conditions is presented in Figure 4. The overview of
all XANES spectra recorded during one operando experꢀ
iment (as described in Figure 2) is shown in Figure 4a.
The XANES spectra have the same edge features apꢀ
peared as one smooth peak at the beginning of the measꢀ
urement. Then the smooth peak gradually split into two
smaller peaks. This overview indicates an initial stable
state and thereafter a transformation stage of Pd species
during the measurement.
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Importantly, when the reaction mixture was heated to 90
°C, the reactivity was restored for both catalysts (Figures
3aꢀ3b). This indicates that asꢀsynthesized Pd centers may
have changed their coordination environment to form
mobile mononuclear Pd complexes and Pd clusters or
nanoparticles under the reaction conditions, which altered
the reactivity of the catalysts at 90 °C. We anticipate that
such drastic changes in the Pd coordination environment
should be reflected in the XAS spectra. Indeed, the operꢀ
ando XAS monitoring of the Heck reaction revealed preꢀ
viously unseen details, which support a dynamic, adaptive
behavior of Pd centers upon exposure to physical and
chemical stimuli in the reaction mixture. The XAS data is
presented in the following section.
To follow the changes in detail, representative XANES
spectra were selected and compared with asꢀsynthesized
Pd(II)@MILꢀ101ꢀNH2 and a metallic Pd foil reference
(Figures 4b and 4c). Figure 4b focuses on the initial stages
of the reaction. It is noted that the first XANES spectrum
recorded under operando measurement condition shows
significant changes compared with the XANES spectrum
of the asꢀsynthesized Pd(II)@MILꢀ101ꢀNH2 (Figure 4b,
green line). This reveals the changes of the coordination
environment of Pd atoms when the asꢀsynthesized catalyst
was added into the reaction mixture. From the start of the
operando data acquisition until the reaction temperature
reached 80 °C, the XANES spectra overlap and show
almost identical edge positions at 24354 eV and similar
profiles after the edge. This indicates that the observed Pd
species have a similar coordination environment from the
start of the operando measurement at RT to 80 °C. During
this period, edge positions are located close to the asꢀ
synthesized Pd(II)@MILꢀ101ꢀNH2 and about 4 eV above
the Pd metal foil reference, which has the oxidation state
zero and Kꢀedge position at 24350 eV. Considering that a
oneꢀstep change in oxidation number commonly introducꢀ
es a Kꢀedge shift to a higher energy by 2.0 to 2.5 eV, these
results confirm that Pd mainly maintained an oxidation
state of +II during this reaction period.30 These observaꢀ
tions are in agreement with the NMR data (vide supra).
We can conclude that the freshly reduced Pd(0) was shortꢀ
lived and reacted immediately upon heating to 60 °C. It is
thus reasonable that at sufficiently high concentration of
aryl iodide, Pd is reꢀoxidized much faster than the time
resolution of the present XAS experiments (ca. 3.4
min/scan). A cationic resting state as a Pd(II) complex
during the catalytic cycle is likely to be the one observed
by XAS, thus not showing major differences from the preꢀ
catalyst. Figure 4c includes selected XANES spectra of
Pd(II)@MILꢀ101ꢀNH2 covering the entire experiment.
The spectrum of the recycled catalyst and that of the Pd
foil are also included. When the spectra were observed
2. Nature of the active species and catalyst
deactivation (X-ray absorption spectrosco-
py)
Xꢀray absorption spectroscopy (XAS) revealed that clear
changes at the Pd centers occurred as soon as the catalyst
was added to the reaction mixture. Both XANES and
Fourier transformed (FT) EXAFS of asꢀsynthesized
Pd(II)@MILꢀ101ꢀNH2, with the formula [Pd(Arꢀ
NH2)(CH3CN)Cl2] are markedly different from those
recorded in the first operando XAS scan (Figures 4 and 5,
Figures S5aꢀb, Section S7). Even though the Heck reacꢀ
tion had not yet started, anionic Cl– ligands were already
replaced by neutral Nꢀligands, presumably pꢀ
iodobenzonitrile, which form more stable complexes by
several orders of magnitude with Pd(II) in aqueous media
(details can be found in Section S2).28
We found that the Pd(II) in the resulting cationic comꢀ
plex was rapidly reduced upon olefin addition, in a Wackꢀ
erꢀtype process.29 This was proved by carrying out the
oxidation of the olefin using Pd(II)@MILꢀ101ꢀNH2 (30
mol% Pd) under the same XAS reaction conditions. Deꢀ
spite the low stability of the resulting αꢀformylacetate
derivative in basic media and high temperature, we were
able to observe characteristic aldehyde signals (9.9ꢀ9.6
1
ppm) in the H NMR spectrum (Figure S6, Section S8).
This process required both olefin and water (Figure S7,
Section S8). In the absence of H2O, Pd remained in oxidaꢀ
tion state +II, even upon heating to 60 °C, and was homoꢀ
geneously distributed in the MOF crystals as indicated
from the TEM images (Figure S7a, Section S8). The in
situ generated atomic Pd(0) triggers the Heck coupling,
compared
to
those
recorded
at
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Figure 4. Normalized Pd Kꢀedge XANES spectra of Pd(II)@MILꢀ101ꢀNH2 catalyst as a function of measurement time. (a) All XANES
spectra displayed in threeꢀdimensions. (b) Selected XANES spectra showing the asꢀsynthesized Pd(II)@MILꢀ101ꢀNH2, the heating steps
and Pd reference. (c) Selected XANES spectra at RT, 90 °C, cooling back to RT, recycled and Pd reference.
analysis are described in Figure S8 and Section S9. The fairly
good fit results confirm that the catalysis system consisted of
mononuclear Pd(II) complexes and Pd(0) nanoclusters. The
fractions of different Pd components during the operando
experiment are displayed in Figure 6. At the start of the operꢀ
ando measurement, the catalyst consisted of 100% mononuꢀ
clear Pd(II) complexes and 0% nanoclusters. The complexes
started transforming into nanoclusters from 31 min when the
temperature reached 90 °C. Generally, the rate of transforꢀ
mation was high at the beginning, appearing as steep slopes
and then decreased. The transformation reached completion in
the recycled catalyst where only Pd(0) nanoclusters were
present.
Figure 5. Fourier transformed k3ꢀweighted EXAFS data as a
function of time showing the coordination environment of Pd
in Pd(II)@MILꢀ101ꢀNH2. The spectra are not phase corrected.
* The peak at ca. 1.9 Å comprises the signal of Pd–Cl single
scattering and the side peak of Pd–Pd single scattering.
system was heated to 90 °C, changes in the XANES shifted
towards a lower energy. These changes became even more
pronounced for the recycled catalyst. Its edge position further
Figure 6. Fractions of mononuclear Pd(II) complexes and
shifted to 24352 eV indicating the presence of Pd in both
Pd(0) nanoclusters in the catalyst derived from the linear comꢀ
metallic Pd(0) and oxidized Pd(+II) forms. These observations
bination fit of the XANES spectra.
suggest a gradual increase in the degree of transformation of
The specific coordination environment of Pd atoms in the
MOF was further investigated by analyzing the corresponding
FT EXAFS spectra from the same operando experiment (Figꢀ
ure 5). The FT spectrum of the asꢀsynthesized Pd(II)@MILꢀ
101ꢀNH2 is shown in Figure 5 (ꢀ22 min) which contains two
major peaks corresponding to the Pd–N and Pd–Cl bonds.
mononuclear Pd(II) complexes into Pd(0) nanoclusters, and
part of the Pd atoms in recycled catalyst remained in an oxiꢀ
dized form.
The composition of Pd species during this transformation
and their corresponding ratios were studied by linear combinaꢀ
tion fit (LCF) of the XANES spectra. Detailed LCF fit and
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Refinement
Journal of the American Chemical Society
suggests formula
of
the
data
a
shells of PdꢀꢀꢀPd, one shell of Pd−N/C and one shell of Pd−Cl
distances were introduced to optimize the EXAFS fitting. FT
spectra of Na2PdCl4 and Pd metal foil were used as references
to confirm the Pd−Cl and Pd−Pd bonds in our system. The
model for fitting PdꢀꢀꢀPd was the crystalline structure of meꢀ
tallic palladium, with faceꢀcentered cubic packing. The experꢀ
imental EXAFS spectra and their best fits are shown in Figꢀ
ures S5, Section S7. The final fitting parameters are given in
Table 1. The average size (i.e. average number of Pd atoms) of
the Pd clusters was estimated from the coordination numbers
(CN) of Pd–Pd in the first shell refined from the EXAFS daꢀ
ta.31 It shall be noticed that the CN values determined from
EXAFS refinements are average values without taking into
account of the fraction of each Pd species. The true CN values
for Pd–Pd equal to the CN values from EXAFS refinement
divided by the corresponding fractions of the Pd nanoclusters.
It shall be stressed that the Pd nanoclusters transformed from
mononuclear Pd complexes are on average smaller than 1 nm
in size and each cluster consists of ca. 13 Pd atoms. They are
1
[Pd(ArNH2)(CH3CN)Cl2]. During the first part of the operanꢀ
do measurement, the temperature of the reaction system was
increased from RT to 90 °C (up to 31 min) and the FT spectra
are very similar with only one major peak corresponding to
Pd–N/C bonds with an average coordination number of 4. The
Pd–Cl bonds disappeared immediately when the asꢀ
synthesized catalyst was added into the reaction mixture. This
indicates that the initial Pd complex containing Pd(II)ꢀCl
bonds did not participate in the catalytic process. EXAFS
spectra recorded at 24 min (at 80 °C) and 34–38 min (at 90
°C) are excluded because of the untreatable data quality (this
phenomenon is discussed in Section S2). Interestingly, after
the temperature was stabilized at 90 °C, the EXAFS data qualꢀ
ity was quickly recovered. At this stage (at 31 min), a peak
corresponding to Pd–Pd bonds appeared and later became
clearly visible with increasing intensity. Importantly, as the
Pd–Pd peak became more prominent, a Pd–Cl peak also
emerged, while the Pd–N/C peak decreased in intensity. The
formation of Pd–Cl bonds indicates the presence of Pd(II) in
the system, which is caused by oxidation of Pd atoms on the
surface of nanoclusters, as later confirmed by EXAFS refineꢀ
ment. It should be noted that the shoulder side peak of Pd−Pd
bond in the Fourier transform is at about the same position of
the peak corresponding to Pd(II)−Cl bond. A thorough
EXAFS data analysis proving the existence of Pd−Cl bonds
can be found in Figures S9ꢀS12 and Section S10. In addition
to the confirmation from EXAFS data, inductivelyꢀcoupled
plasma atomicꢀemission spectroscopy (ICPꢀOES) was used to
determine the element composition of the recycled catalyst.
The results showed that significant amount of chloride (1.05
wt%) remained in the recycled catalyst. The molar ratio of
Pd/Cl derived from ICPꢀOES results is ca. 1.2. This is in good
agreement with the result from the EXAFS refinement (Table
1) which implies that each Pd atom was bound to one Clꢀ ion
on average. These observations from FT spectra are consistent
with the information obtained from XANES spectra suggestꢀ
ing the formation of mononuclear Pd(II) complexes from the
asꢀsynthesized catalyst followed by the continuous transforꢀ
mation of mononuclear Pd(II) complexes into Pd(0) nanoclusꢀ
ters when the reaction system was heated to 90 °C. The existꢀ
ence of Pd−Cl also explains the oxidation state of Pd in the
recycled catalyst determined by its Kꢀedge position.
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considerably smaller than the nanoparticles in commercial
8f,10a,32b
catalysts, such as Pd@C,11a,32 Pd@Al2O3
and
Pd@polymers.11c Meanwhile, the mean Pd−Pd bond distances
in the Pd nanoclusters were also determined from EXAFS
refinements (Table 1), and they are between 2.65 – 2.67 Å.
Interestingly, these bond distances are significantly shorter
than the distances found in Pd foil (2.74 Å) and Pd nanopartiꢀ
cles with size of ca. 2 nm (≥ 2.71 Å).10,11 It is known that Pd
atoms on the surface of Pd nanoparticles form shorter Pd−Pd
bonds compared with Pd atoms at interior due to lower coorꢀ
dination number of the surface atoms.33 The average bond
distances of Pd−Pd and their corresponding particle sizes had
also been investigated by an accurate allꢀelectron density
functional approach.34 These previous studies strongly support
the Pd nanoclusters model we proposed here including the
bond distance and average nanocluster size. With an average
cluster size of ca. 13 atoms, most of the Pd atoms are on the
surface of the nanoclusters. Therefore, such small Pd
nanoclusters should still be highly catalytically active, judged
by size alone35 and by the number of Pd atoms with coordinaꢀ
tion sites available. However, the rise of the Pd–Cl peak in the
Fourier transforms offers a first hint into the real motive beꢀ
hind the catalyst deactivation.
Local structures of the Pd atoms during the operando measꢀ
urement were determined by fitting of the EXAFS spectra
corresponding to the FTs in Figure 5. A maximum of three
Table 1. Refined distances (d/Å), and mean number of distances (N) in selected scans using the Pd(II)@MILꢀ101ꢀNH2 catalyst.
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d(Pd-N/C)
(Å)a
d(Pd-Cl)
d(Pd···Pd)
d(Pd···Pd)
d(Pd···Pd)
CN
Catalyst
CNb
CNb
CNb,c
CNb
(Å)a
b
1st shell (Å)a
2nd shell (Å)a
3rd shell (Å)a
1
2
3
4
5
6
7
8
As synth.
First scan
31 min
44 min
47 min
50 min
57 min
63 min
73 min
2.114(8)
2.083(6)
2.056(4)
2.037(6)
1.993(9)
2.026(8)
1.980(6)
1.956(5)
1.979(7)
1.959(7)
1.978(7)
2.0
4.0
3.8
3.4
2.9
2.8
1.3
1.0
0.8
0.9
0.8
2.298(1)
ꢀ
2.0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2.710(7)d
2.674(5)
2.666(7)
2.655(4)
2.654(4)
2.653(4)
2.662(3)
2.662(3)
2.662(2)
2.689(1)
0.4(8.0)d
0.8(4.0)
1.8(4.5)
2.7(6.0)
2.8(5.6)
3.1(5.6)
3.2(4.9)
3.7(4.9)
4.2(5.3)
6.5
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
0.4
0.7
1.0
1.0
1.0
1.0
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3.68(2)
3.82(2)
3.76(2)
3.74(4)
3.76(1)
3.78(2)
3.76(2)
3.815(6)
3.904(8)
1.0
1.0
1.0
1.0
1.0
1.0
2.0
3.0
6.0
2.37(1)
2.35(1)
2.38(2)
2.38(1)
2.384(7)
2.415(3)
4.68(2)
4.73(2)
4.73(1)
4.75(1)
4.85(1)
4.747(3)
4.767(2)
2.0
2.0
2.0
2.0
3.0
6.0
24.0
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85 min
115 min
Recycled
Pd foil
2.741(1)
12.0
a The standard deviations in parentheses were obtained from k3ꢀweighted least square refinements of the EXAFS function χ(k) and do not
include systematic errors of the measurement. b The estimated error of the coordination is ca. 25% of the given value when refined. Underꢀ
scored parameters have been optimized and fixed in the refinements. c The CN values in parentheses are the true coordination numbers of
Pd−Pd 1st shell when the fractions of Pd nanoclusters were taken into account. These values were used to determine the average size of Pd
nanoclusters. d The bond distance and CN value of Pd−Pd 1st shell at 31 min contain relatively big errors because of the weak EXAFS signal
when Pd nanoclusters just started to form.
Figure 7. Proposed Pd species at different stages during the Heck coupling reaction using Pd(II)@MILꢀ101ꢀNH2 as a catalyst. The red
trace shows the temperature and the blue trace shows conversion to the desired product. The value of CN in the figure refers to the coordiꢀ
nation number of Pd–Pd in the first shell.
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Figure 8. Proposed mechanism for the Heck coupling reaction catalyzed by Pd(II)@MILꢀ101ꢀNH2.
distance of a 2 nm Pd nanoparticle. It is reasonable to propose
The average coordination number of Pd−N/C maintained at
4.0 when there was no transformation of Pd(II) species. As the
mononuclear Pd(II) started to reconstruct into Pd(0) nanoclusꢀ
ters, the average CN of Pd−N/C decreased to ca. 3.4 at 44 min
and to ca. 0.8 at 115 min indicating a continuous transforꢀ
mation process. The first Pd−Cl bonds were observed after 57
min with an average CN of ca. 0.4. The reaction conversion
had at this time already reached ca. 95% (Figure 3a). The
average CN then further increased and ended up at ca. 1.0. It
shall be noted that the bond distances of the reformed Pd−Cl
bonds were 2.35ꢀ2.42 Å, which is significantly longer than
those in Pd(II) complexes20. This indicates that the Pd atoms
coordinated to Clꢀ were surface atoms of the Pd nanoclusters,
where each Pd atom bound to several Pd atoms and the Pd−Cl
bonds became longer in comparison with Pd(II) complexes.
This also indicates that when Clꢀ ions are coordinated to the
relatively bulky and less mobile Pd nanoclusters, they could
not easily be replaced by the amino groups of the MOF linkꢀ
ers. In addition, no EXAFS signals corresponding to
Pd−Cl−Cl and Pd−Cl−Pd−Cl multiple scattering paths were
observed, which excludes the presence of PdCl2. Furthermore,
no EXAFS signal corresponding to Pd−O single scattering was
observed in the recycled catalyst, which suggests that surface
Pd of the nanoclusters were coordinated to Clꢀ ions. Otherwise,
they would have been oxidized in air to form Pd−O bonds.
that at this stage of the process, Pd nanoclusters became the
absolute dominating species with an average size of ca. 20
atoms,34 and the surfaces of these Pd nanoclusters were almost
covered by Clꢀ ions leading to the deactivation of the catalyst.
It shall be noted that the coordination numbers and the estiꢀ
mated sizes of Pd clusters obtained from the EXAFS fitting
are an average of various Pd species. In reality, it is likely that
both larger Pd nanoparticles with higher coordination numbers
and smaller Pd clusters with lower coordination numbers
coexist in the sample.
Based on the operando XANES and EXAFS analyses, the
evolution of Pd species was followed and the catalytically
active species under Heck conditions were identified. The
kinetic profile (vide supra) of the identified Pd species are
illustrated in Figure 7. The active species were mononuclear
Pd(II) complexes coordinating to the linkers of the MOF at the
beginning of the reaction below 90 °C. The mononuclear
Pd(II) complexes then dissociated from the linkers after one
run and their activity was lost when the temperature was lower
than 90 °C. As the temperature was raised and maintained at
90 °C, these mobile mononuclear Pd(II) complexes gradually
converted to Pd nanoclusters of ca. 13 Pd atoms on average. A
mixture of the mobile mononuclear Pd(II) complexes and Pd
nanoclusters coꢀexisted and became the active species to cataꢀ
lyze the reaction. The activity of the mobile mononuclear Pd
complexes at 90 °C was confirmed by adding a second load of
reagents at 70 min, when the system consisted of ca. 40%
mononuclear Pd complexes and 60% Pd nanoclusters covered
and poisoned by coordinated Cl– ions (Figure 7). A full conꢀ
version of the new reagents was achieved in another 70 min at
90 °C. This experimental result proved that the mobile Pd
mononuclear complexes are catalytically active after the reacꢀ
tion system was heated to 90 °C during the operando measꢀ
The recycled Pd(II)@MILꢀ101ꢀNH2 was measured by ex situ
EXAFS (Figure S5f, Section S7). No EXAFS signal from
Pd−N/C distances was found from the refinement and average
coordination numbers of Pd−Pd and Pd−Cl bonds were deterꢀ
mined to be ca. 6.5 and ca. 1.0, respectively. The Pd−Pd bond
distance is longer (ca. 2.69 Å) than Pd−Pd bond distances
(2.65 – 2.67 Å) during operando measurement (44 – 115 min)
indicating larger clusters, but still shorter than the Pd−Pd
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Page 10 of 14
urement. At the beginning of this stage, mononuclear Pd comꢀ (25% conversion at 70 °C) was probably caused by transforꢀ
1
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7
8
plexes appeared also as the major Pd species compared to the
Pd nanocluster. The mononuclear Pd complexes have even
better activity than the corresponding Pd nanoclusters considꢀ
ering the size and the number of coordination sites available.
This continuous conversion of the Pd species also indicated
that mononuclear Pd complexes had a relatively high activity
because it could compete with the aggregation process under
the reducing conditions. After Cl– ions were bound to the
surface of the Pd nanoclusters, the remaining mononuclear
Pd(II) complexes catalyzed the reaction to its completion.
When the substrates were completely consumed, mononuclear
Pd complexes could not react and they selfꢀaggregated into Pd
nanoclusters, which were quickly poisoned by remaining Clꢀ
ions in the system. A similar evolution of Pd species was also
observed from Pd(II)@MILꢀ88BꢀNH2 catalyzed reaction. The
corresponding XANES and EXAFS spectra can be found in
Figure S13, Section S11.
mation of Pd complexes into Pd nanoclusters during the rapid
cooling and interruption of the catalytic process. Considering
the significantly smaller average size of Pd species derived
from EXAFS (Table 1), the presence of such large Pd nanoꢀ
particles is partially due to the promoted aggregation of the Pd
nanoclusters transformed from resting Pd complexes especialꢀ
ly under the condition of solvent evaporation. The large partiꢀ
cle size also implies that much smaller Pd clusters should also
be present in the MOF crystals. Indeed, the small Pd clusters
and mononuclear Pd complexes were captured by EDS mapꢀ
ping which showed clear Pd signals within the entire MOF
particles (Figures 9b2 ꢀ 9b4). The EDS spectra also confirmed
that Pd species remained inside the MOF crystals at all stages
of the reaction, although the Pd/Cr ratio gradually decreased
with time (Figures S14ꢀS15, Section S12).
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A mechanism derived from the aboveꢀdescribed investigaꢀ
tions is proposed and shown in Figure 8. The freshly produced
Pd(0) complex (I) represents the active species that initiate the
catalytic cycle by oxidative addition in the C–I bond, leading
to the Pd(L2)ArI intermediate (II). The absence of any Pd–I
distances in the EXAFS data is not surprising. The oxidative
addition is rapidly followed by a ligand exchange reforming
the favored cationic species stabilized by a neutral Nꢀligand,
following a similar preference as observed in the early stages
of the experiment. This leads to an intermediate complex (III),
to which the olefin coordinates to form a πꢀcomplex (IV).
Intermediates III and IV contain solely Pd−C/N bonds, in
agreement with the XAS data. Based on this analysis, it is
reasonable to conclude that the resting state of Pd species
observed by XAS in the early stages of the measurement corꢀ
responds to intermediate IV, in which Pd has an oxidation
state of +II. This is also in agreement with numerous reports
that pointed out the migratory insertion step to be turnoverꢀ
limiting. In the following step, a βꢀhydride elimination yields
the final organic product and a palladium hydride species (VI)
that upon reductive deprotonation returns to Pd(0), completing
this cycle.
Figure 9. TEM images (a) and STEM image and EDS mapping
(b) of Pd(II)@MILꢀ101ꢀNH2 at various stages during the Heck
reaction: at 0% (a1ꢀb1), 25% (a2ꢀb2), 54% (a3ꢀb3) and > 99%
(a4ꢀb4) conversion. While Pd nanoparticles are observed in all
samples except the initial one, smaller Pd species (monomers,
nanoclusters) are found to be homogenously distributed in the
MILꢀ101ꢀNH2 crystals throughout the reaction. The arrows in
Figure 8b3 indicate a Pd nanoparticle on the surface of the MOF.
The lack of recyclability of the MOF under the Heck couꢀ
pling conditions indicates that not only the possible large Pd
nanoparticles observed outside the MOF crystals by TEM, but
also the small Pd species within the MOF crystals were alꢀ
ready inert. This information is extremely important as it
proves that it was neither the Pd leaching nor the Pd aggregaꢀ
tion itself that led to the deactivation of the catalyst. Instead,
the deactivation was caused by the poisoning of Clꢀ ions. Furꢀ
thermore, the operando PXRD data proves that both MILꢀ101ꢀ
NH2 and MILꢀ88BꢀNH2 were completely stable and no strucꢀ
tural changes occurred under these reaction conditions (Figure
S16, Section S13). Based on all information acquired, a clear
picture emerges. The deactivation of the catalyst is not of a
physical nature. Different from the structural degradation
factors commonly suggested in the literature such as Pd leachꢀ
ing and agglomeration or MOF decomposition, the predomiꢀ
nant deactivation pathway here is of a chemical nature. The
irreversible capping of small Pd clusters with chloride ligands
makes the reactive sites inaccessible to the reaction partners,
long before Pd leaching or aggregation becomes a major issue.
3. Framework stability and catalyst distribution
inside the MOFs (TEM, EDS, PXRD)
Duplicate experiments were recreated as described in Figure 2
and monitored by TEM and STEM/EDS. The distribution of
Pd in MILꢀ101ꢀNH2 crystals was investigated at various stages
during the Heck coupling reaction (Figure 9). To minimize
possible Pd aggregation introduced during the TEM sample
preparation, a droplet of the hot reaction mixture was directly
deposited onto the TEM grid and transferred into the vacuum
chamber of the microscope. At the initial stage, no Pd nanoꢀ
particles were visible in the TEM images, both inside and
outside MILꢀ101ꢀNH2 crystals (Figure 9a1). In addition, EDS
mapping showed the presence of homogenously distributed Pd
throughout the framework (Figure 9b1). This confirms that the
preꢀcatalyst consisted of Pd(II) complexes, embedded inside
the MILꢀ101ꢀNH2. After the Heck reaction was initiated, TEM
images recorded at various stages of the reaction (25%, 54%
and > 99% conversion) showed formation of Pd nanoparticles
of 2–15 nm in size on the surfaces of the crystals (Figures 9a2 ꢀ
9a4). The presence of the Pd nanoparticles shown in Figure 9a2
4. Recyclability
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sions can be derived for Pd(II)@MILꢀ88BꢀNH2 although the
With the above information in hand, we revisited the issue of
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recyclability. Initial studies showed a dramatic loss of activity
in the second cycle, after the catalyst was recovered, washed
and dried. However, when fresh reagents were added during
the reaction phase of high activity, before a complete inhibiꢀ
tion by Cl– ions was achieved, a high TOF could be mainꢀ
tained for at least 3 cycles. Despite the disturbance introduced
by fresh reagents (temperature, solubility, reagent ratios) the
reaction was faster than what would have been necessary to
convert three batches of starting materials by fresh catalysts
(Section S14). This experiment was purely illustrative and
does not represent a practical experimental procedure. Howevꢀ
er, it suggests that operation under continuous flow would be a
suitable way to improve the total turnover of the catalyst. Cl–
ions could be washed away at the beginning of the experiment
and a constant feed of fresh reagents would postpone the deacꢀ
tivation process.
chelating coordination mode of the MOF and the more conꢀ
fined pore space attenuated the deactivation process.
This study fits into a larger effort to bring a better underꢀ
standing to the complex behavior of palladium under reaction
conditions specific for C–C and C–heteroatom bond forꢀ
mation.36 In recent years, it has become evident that the actual
landscape of active species is more diverse and dynamic than
previously believed. Starting from mononuclear Pd catalysts,
formation of Pd clusters is often unavoidable. However, these
particulate species not only represent an alternative resting
state for Pd but can also get involved directly in the second
stage of catalysis. The presence of rapidly interꢀconvertible
species that possess different levels of reactivity makes it
challenging to quantify precisely their contribution to the total
observed catalytic effect.37 Moreover, commonly employed
heterogeneity tests may not always provide reliable results.38
Without a better understanding of these phenomena, a rational
design of superior catalysts would not be achievable.
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Even though the recyclability of the material did not work as
expected, we observed remarkable differences when comparꢀ
ing the activity of the Pd(II)@MOF with its homogeneous
counterparts. When the reaction was performed using the same
palladium precursors (i.e. Na2PdCl4 and PdCl2(MeCN)2) as
catalysts without being impregnated into the MOF support, the
yields dropped to 42ꢀ43% for both Pd(II) salts. This clearly
shows the beneficial effect of the MOF support in this catalyꢀ
sis.
Towards this goal, the method presented herein can reveal
fine details of the reaction mechanism by monitoring more
precisely the evolution of different Pd species and the effects
of Pd phase transitions on its catalytic activity. Moreover, we
are continuously working to expand the applicability of the
method. Further development of the in situ reactor will make
the acquisition of fluorescence XAS data in operando mode
feasible. This would be especially beneficial to measure highly
diluted samples or catalysts with relatively low absorption
edge energies. Furthermore, the new generation of synchrotron
radiation sources will provide great opportunities to improve
the time resolution and investigate reactions with significantly
shorter halfꢀlifetimes.
5. Discussion
The correlation of data from multiple characterization techꢀ
niques portrays Pd@MILꢀ101ꢀNH2 and Pd@MILꢀ88BꢀNH2 as
heterotopic multifaceted catalysts with more than one mode of
action. In their asꢀsynthesized dry form, both Pd(II)@MOF
preꢀcatalysts contain Pd centers in a +II oxidation state and
square planar coordination geometry, bearing two anionic Cl–
ligands. When the catalyst was mixed with the other reagents
except for olefin, the chloride ligands were immediately reꢀ
leased and replaced by neutral nitrogen ligands, forming a
cationic complex favored by the polar aqueous environment.
Upon olefin addition in the presence of water, Pd(II) was
rapidly reduced to Pd(0) even at room temperature. The freshꢀ
ly reduced Pd underwent a fast oxidative addition, initiating
the catalytic cycle. The Heck coupling product was already
detectable immediately after reaching 60 °C, proving that
mononuclear Pd(II) complexes attaching to the linkers of the
MOFs were competent active species to catalyze this transꢀ
formation at relatively low temperatures.
Conclusions
The mechanism of a Pd@MOFꢀcatalyzed Heck C–C coupling
reaction was investigated in detail using a newly developed
reaction cell suitable for operando studies of heterogeneous
catalysts. For the first time we probed the entire lifetime of the
catalyst and revealed fine details of the reaction mechanism
that would otherwise remain hidden. Different active species
operating at different reaction stages and under different reacꢀ
tion conditions have been identified, together with their activaꢀ
tion and deactivation pathways. The irreversible deactivation
of the catalyst was provoked neither by leaching of the active
species, nor by decomposition of the crystalline framework.
Instead a chemical deactivation mechanism was identified, in
which Clꢀ ions cover the surface of transformed Pd clusters
and nanoparticles, blocking the access of starting materials to
the active sites. This information can be used to prolong the
lifetime of the catalyst and to design improved catalysts and
processes in the future. Importantly, the method described is
widely applicable to study diverse chemical reactions cataꢀ
lyzed by transition metals, including both homogeneous and
heterogeneous systems, and has the potential to provide unꢀ
precedented insight into the fine details of the mechanism.
Other complex mechanisms of MOFꢀcatalyzed processes are
currently under investigation in our laboratories.
The first catalytic turnovers have taken place inside the
MOF pores, and proceeded with comparable rate for both
Pd@MILꢀ101ꢀNH2 and Pd@MILꢀ88BꢀNH2 catalysts. During
the first cycle, the coordination to the aminoꢀterephthalate
linker was lost in the case of MILꢀ101ꢀNH2, leading to the
formation of an even more electronꢀdeficient Pd species,
which could travel through the pores. A second oxidative
addition could not be initiated by this form of electrophilic Pd,
which instead gradually agglomerated into small nanoclusters.
The resulting mixture of the mobile mononuclear Pd complexꢀ
es and the Pd nanoclusters was inert at 60 °C, and became
active in the catalytic cycle at 90 °C. However, upon depletion
of the starting materials and further upon cooling the reaction
mixture, the surface of the Pd clusters was covered by Cl– ions
and their catalytic properties were finally lost. Similar concluꢀ
ASSOCIATED CONTENT
Supporting Information Catalysts syntheses; optimization of
reaction conditions, substrate scope and recycling experiments;
1
additional studies of kinetic profiles by H NMR; details on the
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Journal of the American Chemical Society
Page 12 of 14
XAS and PXRD data collection; XAS data treatment and refineꢀ
ment results; additional TEM and EDS results. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
Soc. Rev. 2017, 46, 3134–3184; j) Leus, K.; Liu, Y. Y; Voort P. V. D.
Cat. Rev. Sci. Eng. 2014, 56, 1–56.
(4) For recent reviews, discussing the complexity of heterogeneous
catalytic mechanisms, see: a) Eremin, D. B.; Ananikov, V. P. Coord.
Chem. Rev. 2017, 346, 2–19; b) Schauermann, S.; Freund, H.ꢀJ. Acc.
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Shalkhutdinov, S.; Freund, H.ꢀJ. Acc. Chem. Res. 2013, 46, 1673–
1681; d) Yang, X.ꢀF.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T.
Acc. Chem. Res. 2013, 46, 1740–1748.
(5) Metzger, E. D.; Comito, R. J.; Hendon, C. H.; Dinca, M. J. Am.
Chem. Soc. 2017, 139, 757–762.
(6) Morris, R. J.; Russell Jr., J. N.; Karwacki C. J. J. Phys. Chem.
6
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*xzou@mmk.su.se
*belen.martin.matute@su.se
*ingmar.persson@slu.se
Author Contributions
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$
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
Lett. 2015, 6, 4923–4926.
ACKNOWLEDGMENT
(7) a) Bordiga, S.; Groppo, E.; Agostini, G.; van Bokhoven, J. A.;
Lamberti, C. Chem. Rev. 2013, 113, 1736–1850; b) Singh, J.;
Lamberti, C.; van Bokhoven, J. A. Chem. Soc. Rev. 2010, 39, 4754–
4766; c) Bordiga, S.; Bonino, F.; Lillerud, K. P.; Lamberti, C. Chem.
Soc. Rev 2010, 39, 4885–4927.
This work has been supported by the MATsynCELL project
through RöntgenꢀÅngström Cluster, supported by the Swedish
Research Council (VR) and the German Federal Ministry of
Education and Research (BMBF). We are also thankful to the
Berzelii Center EXSELENT, the Swedish Research Council and
the Project Management Organization at DESY (Deutsches Elecꢀ
tronenꢀSynchrotron). The allocation of beamtime at BM01B
(SNBL), European Synchrotron Radiation Facility (ESRF) is
gratefully acknowledged. A.K.I. is supported by the Knut and
Alice Wallenberg Foundation (KAW) through the MAX IV postꢀ
doctoral scholarship. B.M.ꢀM. was supported by VINNOVA
through a VINNMER grant. We thank KAW for the project grant
3DEMꢀNATUR and a grant for purchasing the TEMs. We thank
Prof. Lynne McCukser for assistance during the beamtime appliꢀ
cations, and the staff of beamline BM01B (SNBL), particularly
Dr. Michela Brunelli and Dr. Hermann Emerich at the ESRF for
the assistance during data collection. We also thank Dr A. Bermeꢀ
jo Gómez for helpful discussions.
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