Understanding Ligand-Directed Heterogeneous Catalysis: When the Dynamically Changing Nature of the Ligand Layer Controls the Hydrogenation Selectivity

Article abstract of DOI:10.1002/anie.202103960

We present a mechanistic study on the formation and dynamic changes of a ligand-based heterogeneous Pd catalyst for chemoselective hydrogenation of α,β-unsaturated aldehyde acrolein. Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active and close to 100 % selective toward propenol formation by promoting acrolein adsorption in a desired configuration via the C=O end. Employing a combination of real-space microscopic and in-operando spectroscopic surface-sensitive techniques, we show that an ordered active ligand layer is formed under operational conditions, consisting of stable N-butylimine species. In a competing process, unstable amine species evolve on the surface, which desorb in the course of the reaction. Obtained atomistic-level insights into the formation and dynamic evolution of the active ligand layer under operational conditions provide important input required for controlling chemoselectivity by purposeful surface functionalization.

Full text of DOI:10.1002/anie.202103960

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Accepted Article
Title: Understanding Ligand-Directed Heterogeneous Catalysis: When
the Dynamically Changing Nature of the Ligand Layer Controls
the Hydrogenation Selectivity
Authors: Swetlana Schauermann, Carsten Schröder, Marvin C.
Schmidt, Philipp A. Haugg, Ann-Katrin Baumann, and Jan
Smyczek
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10.1002/anie.202103960
Angewandte Chemie International Edition
Understanding Ligand-Directed Heterogeneous Catalysis: When the Dynamically
Changing Nature of the Ligand Layer Controls the Hydrogenation Selectivity**
Carsten Schröder, Marvin C. Schmidt, Philipp A. Haugg, Ann-Katrin Baumann, Jan Smyczek
and Swetlana Schauermann*
Institute of Physical Chemistry, Christian-Albrechts-University Kiel, Max-Eyth-Str. 2, 24118
Kiel, Germany
Abstract
We present a mechanistic study on formation and dynamic changes of a ligand-based
heterogeneous Pd catalyst for chemoselective hydrogenation of -unsaturated aldehyde
acrolein. Deposition of allyl cyanide as a precursor of a ligand layer renders Pd highly active
and close to100 % selective toward propenol formation by promoting acrolein adsorption in a
desired configuration via the C=O end. Employing a combination of real space microscopic
and in operando spectroscopic surface sensitive techniques, we show that an ordered active
ligand layer is formed under operational conditions, consisting of stable N-butylimin species.
In a competing process, unstable amine species evolve on the surface, which desorb in the
course of the reaction. Obtained atomistic-level insights into the formation and dynamic
evolution of the active ligand layer under operational conditions provide important input
required for controlling chemoselectivity by purposeful surface functionalization.
* schauermann@pctc.uni-kiel.de
**
This work has been supported by the German Science Foundation (DFG, Grant SCHA
1477/6-1).
1
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One of the major challenges in heterogeneous catalysis is a precise control over the selectivity
of a catalytic process, which depends on subtle differences in the energy barriers of competing
reaction pathways. A promising strategy to tackle this problem is to introduce a specific
selective interaction between a reactant and an active site, which governs the chemical
transformations in the desired direction. This idea has been successfully realized in
enantioselective heterogeneous catalysis via functionalization of metal surfaces with ligand-
like adsorbates  chiral modifiers  with many of these systems being nowadays well-
understood at the mechanistic level.[1] Recently, a new type of ligand-directed heterogeneous
catalysis – chemoselective catalysis  gained more attention as new highly chemoselective
powdered catalysts were developed for partial hydrogenation of multi-unsaturated oxygenates
or semi-hydrogenation of alkynes.[2] These developments demonstrate a great potential of this
emerging technology to controllably tune the surface structure and the lateral reactant-ligand
interactions in order to provide the tailor-made environment for the desired reaction steps.
Despite the impressive progress in this field demonstrated on powdered materials, the
mechanistic details of the underlying ligand-directed surface processes remain only poorly
understood for the most systems. Recently, we reported a mechanistic study on selective
hydrogenation of an -unsaturated aldehyde acrolein over Pd(111), in which the C=O bond
was hydrogenated with 100 % selectivity to produce the target product propenol.[3] This
chemistry occurs, however, not on pristine Pd(111), but on the surface densely packed with
oxopropyl ligands, which are spontaneously formed on the early stages of partial acrolein
hydrogenation. Although this mechanistic study provided some starting ideas about the
microscopic origin of the ligand-related effects, an in-depth atomistic understanding of the
active ligand layer is largely missing. Specifically, the elementary steps leading to formation
of an active ligand layer, it’s chemical composition and the possible dynamic changes under
the reaction conditions needs to be understood, in order to approach rational design of this new
promising class of materials with tailor-made catalytic properties.
Here, we report the proof-of-principal mechanistic study on purposeful formation of a
catalytically highly efficient ligand layer based on model Pd(111) catalyst functionalized with
allyl cyanide (AC) for selective hydrogenation of acrolein. By combination of in operando
infrared reflection absorption spectroscopy (IRAS), scanning tunnelling microscopy (STM)
and molecular beam techniques, formation and dynamic changes of the ligand layer under
reaction conditions were investigated at the atomistic level. While AC forms a self-assembled
2D layer under non-reactive conditions, in which the flat lying molecules largely preserve their
chemical nature, strong changes occur under the reactive conditions both in terms of the
chemical nature and the spacial distribution of the ligand species. Specifically, AC transforms
to N-butylimin species resulting from partial hydrogenation of the original ligand, which are
adsorbed in a more upright configuration and form a more densely packed layer, adopting the
hexagonal symmetry of the underlying metal. On this active layer, acrolein nearly
instantaneously forms the desired propenoxy reaction intermediate followed by evolution of
the target product propenol. The dynamic changes of the ligand layer induced by the reaction
conditions were identified as a crucial process rendering the surface highly active and selective
2
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Figure 1. (a) Competing reaction pathways for acrolein hydrogenation (b) Formation rates of propenol on
pristine and AC-precovered Pd(111) at 250 K; (c) formation rates of propenol and propanal on Pd(111)
precovered with 0.8 L AC at 250 K.
in partial hydrogenation of acrolein at the C=O bond. Obtained insights provide a deep
atomistic level understanding of the most important working principles of ligand-directed
heterogeneous catalysis.
Selective partial hydrogenation of acrolein, leading to formation of propenol as a target and
propanal as a non-desired product (Fig. 1a), was investigated on Pd(111) surface functionalized
with AC. The catalytic activity was probed by molecular beams combined with quadrupole
mass spectrometry (QMS) under isothermal conditions in an apparatus described elsewhere.[4]
Figure 1b shows the time evolution of the propenol formation rate in the gas phase for pristine
and AC-precovered Pd(111) at different AC coverages. In each experiment, the surface was
first exposed continuously to a H2 beam, at a time indicated as zero the second molecular beam
was opened to dose acrolein (for details see Supporting Information, SI; in the following, the
simultaneous exposure to H2 and acrolein at the reaction temperature will be denoted as
“reaction conditions” for the reactivity measurements). On pristine Pd(111), a prolonged
induction period is observed before the onset of propenol evolution. Previously, we have shown
that during the induction period an oxopropyl ligand layer is formed as a result of a single H
insertion into the C=C bond of adsorbed acrolein, which turns the surface close to 100 %
selective toward C=O bond hydrogenation.[3, 5] On Pd(111) functionalized with AC prior the
reaction, the induction period becomes either strongly reduced at intermediate AC coverages
or even completely vanishes at coverages close to a monolayer. For all functionalized surfaces
propenol is formed with the estimated selectivity in the range 95 - 100 %, which will be denoted
in the following as “close to 100%”, and fairly high conversion (up to 32 %) as exemplarily
3
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Figure 2. (a) Formation rate of propenol on Pd(111) precovered with 1.0 L AC at 250 K measured in a pulsed
experiment; (b) IR spectra of 1.0 L AC prior the beginning of the reaction (spectrum 0) and IR spectra obtained
on the same surface after the beginning of the reaction. The spectra 1 – 5 were obtained during the time slots
indicated in (a) as 1 – 5; full details can be found in Chapter II in SI.
shown in Figure 1c. These observations demonstrate that surface functionalization with AC
renders it highly selective and can be used for rational design of ligand-containing catalysts
without relying on spontaneous formation of the oxopropyl ligand layer.
To get more insights into the formation of the active ligand layer and the reaction
intermediate, a combination of in operando IRAS and kinetic studies via molecular beams was
performed in a broad reaction parameter range. Figure 2a shows the propenol formation rate
over AC-functionalized Pd(111) at 250 K upon pulsing of acrolein onto the surface
continuously exposed to H2. During the indicated periods of time (1-5), IR spectra were
obtained on this surface, shown in Figure 2b as spectra 1-5. The spectrum 0 corresponds to the
same surface prior to exposure to acrolein. On this non-reactive surface, two most important
peaks are observed at 1746 and 1582 cm-1, which are not present in non-perturbed AC and are
related to a partly hydrogenated derivatives of AC. The band at 1746 cm-1 was previously
described as a (C=N···Pd) stretching vibration for structurally similar acetonitrile and methyl
isocyanide.[6] The band at 1582 cm-1 is typical of (NH2) deformation vibration in simple
amines.[7] While the exact nature and formation of these species will be discussed the detail
later, it is important to point out that AC does not retain its original chemical structure in
presence of H2 and transforms to two new species with spectroscopic signatures characteristic
of imines and amines. Upon dosing of acrolein (spectra 1-4), a new band at 1166 cm-1 appears
nearly instantaneously on AC-functionalized Pd(111), which presence strongly correlates with
the evolution of propenol in the gas phase, and vanishes when the acrolein beam is interrupted
(spectrum 5). Following the assignment made in our previous study on pristine Pd(111), we
attribute this band to a C–O vibration in a direct reaction intermediate (RI) propenoxy-species
CH2=CH–CH–O···Pd , in which the C–O entity is attached to Pd through the O atom (for
4
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Figure 3. (a) STM image of allyl cyanide on pristine Pd(111) at 230 K (15.9×19.1 nm2); (b) close-up image
(5.8×4.6 nm2) from (a) indicating three main rows in the 2D self-organized AC layer; (c) close-up image
showing the unit cell of the 2D AC layer; (d) STM image of allyl cyanide on H-precovered Pd(111) at 250
K (10.6×10.8 nm2). Full experimental details can be found in Chapter II in SI.
details see SI, Chapter IV). Importantly, on the AC-covered surface, this band shifts by 46 cm-
1 pointing to a strong lateral interaction between the RI and the ligand layer.
Another striking observation is that the evolution of two prominent AC-related surface species
is not directly correlated with the reactant supply or evolution of the target product propenol:
the imine species containing a C=N···Pd bond (1746-1755 cm-1) remain constantly present on
the surface irrespectively of the reactant supply, while the species containing a -NH2 group
slowly disappear in the course of the reaction. Additionally, a band at 1330 cm-1 evolves, which
results from partial decomposition of acrolein to ethylidyne-like species.[8] The evolution of
these species is responsible for reactivity decrease after the initial period of high propenol
formation rate (more details on reactivity decrease in Chapter VIII in SI).
These datasets provide a clear indication that formation of the direct RI propenoxy (CH2=CH–
CH–O···Pd) species and the target product propenol is strongly affected by the ligand formed
under the reaction conditions on the AC-functionalized surface.
To get more atomistic level insights into the chemical and geometric structure of the active
ligand layer forming and dynamically changing under the reaction conditions, a combined
microscopic and spectroscopic study was performed on AC adsorbed on pristine and H-
containing Pd(111) in a broad temperature range. Figure 3 shows the STM images obtained
under non-reactive (Fig. 3 a-c) vs. reactive conditions (Fig. 3d). Note that the term “reaction
conditions” for STM measurements is related to AC adsorbed on Pd(111) in presence of H2 at
the specific reaction temperature. In both cases, AC adsorbates form ordered layers on these
surfaces, exhibiting, however, strongly different patterns. To the best of our knowledge, this is
the first experimental proof of the formation of an active ligand layer obtained by real space
microscopy (Fig. 3d), which exhibits a high degree of ordering and by this provides a specific
5
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Figure 4. (a) IR spectra of AC adsorbed on Pd(111) at indicated temperatures. The spectrum 1 is obtained on
pristine and the spectra 2 – 4 on H-precovered Pd(111). Spectrum 5 is recorded during continuous H2 exposure
under the reaction conditions (full details can be found in Chapter II in SI); (b) Reaction scheme showing
chemical transformations of AC according to two competing reaction pathways as deduced from (a).
confinement, allowing for a desired adsorption configuration of the reactant. In our previous
studies on acrolein hydrogenation, formation of an ordered oxopropyl ligand layer was
hypothesized[3a], but was not yet proven experimentally. Complementarily, the chemical nature
of the adsorbed species was investigated by IRAS on both types of surfaces (Fig. 4).
Under non-reactive conditions, AC forms a 2D self-assembled layer (Fig 3a-c), which consists
of individual entities, each showing a brighter (head) and darker (tail) protrusions. Generally,
each adsorbate in this layer exhibits a 2-point interaction, which is realized via interaction of
each head with two tails and each tail with two heads. Figure 3b shows this structure in more
detail: there are two parallel rows R1 and R2 of adsorbates, which are inclined with respect to
the axis R3 by 69±1°. The distance between the heads in the row R3 d1 = 8.3±0.2 Å, which
corresponds to (3.0±0.1)×dPd-Pd with dPd-Pd being the spacing between two Pd atoms (2.75 Å).
The distance d2 between two heads along the rows R1 and R2 amounts to (4.8±0.1)×dPd-Pd. The
main axis R4 of the molecules lying in the row R1 is inclined with respect to the axis R3 by
=61±1°, while the main axis R5 of the molecules constituting the row R2 is tilted by the angle
=37±1° with respect to R3. Figure 3c shows the unit cell of the AC 2D self-organized layer
with the lattice parameters d2 = (4.8±0.1)×dPd-Pd; d3 = (7.6±0.1)×dPd-Pd and = 106±1°.
The IR spectra related to non-reactive conditions are shown in Figure 4a (spectrum 1, 100 K)
and in Figure SI_2 in SI (100 – 250 K). At 100 K, only two major vibrational bands are
observed: at 1648 ((C=C)) and 1419 cm-1 ((CH2)), while the normally intense band related
to the CN-group (2250 cm-1) is not seen.[9] The latter observation originates most likely from
the nearly parallel orientation of the CN-group with respect to the metal plane making it
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invisible for IRAS due the metal surface selection rule (MSSR).[10] At elevated temperatures,
the intensity of both bands (1648 and 1419 cm-1) strongly decrease most probably due to the
changes in the adsorption configuration making the entire molecule oriented nearly flat with
respect to the metal plane (for additional discussion see SI, Chapter V), which is consistent
with the elongated form of the adsorbates imaged by STM. Combined microscopic and
spectroscopic information result in the atomistic model of this ligand layer under non-reactive
conditions shown in Figure 5a. The adsorbates are separated by 5×dPd-Pd along the row R3,
while the distance between R1 and R2 corresponds to the separation by 3×dPd-Pd. The angles
between the molecular axes with respect to R3 ( and  amount to ≈37° and 61°, respectively.
The mutual intermolecular interactions between the adsorbates are most likely based on H-
bonding between the N-atom of each molecule and the H atoms of the terminal CH2 group of
two neighboring adsorbates (visualized by red dotted lines). It should be noted that while the
exact position of the CN groups cannot be derived unambiguously from the STM data, it is
likely that the electron rich CN group is related to the brighter part of the molecule, i.e. the
head-position.
Upon transition to the reactive conditions, the 2D self-assembled ligand layer undergoes
dramatic changes both in terms of the geometric structure and the chemical nature of the
adsorbates. Here, an ordered layer of the hexagonal symmetry is formed (Fig. 3d) with the
adsorbates exhibiting a nearly round footprint in STM. The spacing between the adsorbates
along the axes shown in Figure 3d is b1 = (2.7±0.3)×dPd-Pd and b2 = (3.1±0.1)×dPd-Pd (red vectors
b1 and b2 show the unit cell of this layer;  = 53±2°). This transition is also accompanied by
substantial changes in the chemical composition of the originally deposited AC ligand. Figure
4a shows the IR spectra obtained on H-precovered Pd(111) surface at successively increasing
temperatures (spectra 2 and 3), which finally reaches the state of the catalyst under operational
conditions (spectra 4 and 5). At 100 K, AC adsorbed on H-covered surface still retains two
peaks typical of molecularly adsorbed AC: 1648 ((C=C)) and 1419 ((CH2)) cm-1.[9]
Importantly, a new peak at 1755 cm-1 appears, which is present neither in multilayer nor in the
gas phase AC.[9] Previously, this vibration was observed for structurally similar molecules
acetonitrile and methyl isocyanide upon adsorption on Pd(111)[6b] and was attributed to imine
species, in which the band at 1755 cm-1 is related to the stretching vibration of the (C=N) bond
in the (-C=N···Pd) fragment. This observation suggests partial reduction of the C≡N triple to
the C=N double bond accompanied by formation of the species strongly bound to Pd.
Generally, two possible configurations of these species are possible: bound either through the
N or the C atom, which are shown in the scheme of the Figure 4b and indicated as N- and C-
butenylimine (S2 and S3, respectively).
At 160 K, the further hydrogenation step occurs giving rise to (i) disappearance of the peaks at
1648 ((C=C)) and 1419 ((CH2)) cm-1 and (ii) appearance of a new peak at 1454 cm-1
corresponding to the CH3 deformation vibration ((CH3)).[11] The combination of these
observations suggest that the C=C bond becomes hydrogenated in this temperature range and
a new terminal -CH3 group appears.
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Figure 5. Model of adsorbed AC-derived species under (a) non-reactive and (b) operational conditions in
presence of H2 and acrolein. For additional discussion on the AC structure in (a), see SI, Chapter V.
At 250 K (spectra 4 and 5), further hydrogenation steps occur mainly resulting in (i) significant
growth of the peak at 1755 cm-1 (C=N···Pd) and (ii) appearance of two new peaks at 1580
and 1280 cm-1, which are characteristic of -NH2 and HNC deformation vibrations,
respectively.[12] Simultaneous appearance of these bands means that both imine and amine
species are formed and co-exist in this temperature range. It also suggests that at least two
reaction pathways run in parallel to transfer the originally deposited AC to co-existing imine
and amine surface species.
Figure 4b displays a model for the proposed AC reaction pathways, which is consistent with
all spectroscopic observations. For all species, the characteristic bands employed for
assignment are given next to the proposed structure. At 100 K, AC deposited on pristine
Pd(111) exhibits an adsorption geometry with the CN group oriented nearly parallel to the
surface, while the C=C group is inclined and hence visible in IRAS (species S1). When H is
co-adsorbed at 100 K, a fraction of these species is capable of forming the imine group
(C=N···Pd, 1755 cm-1). The formed butenylimine species can be bound to the surface either
via the N-end (N-butenylimin, S2 species) or via the C-atom (C-butenylimin, S3 species) and
undergo further partial hydrogenation steps. In the first pathway, the N-butenylspecies (S2) is
hydrogenated at the C=C bond producing the N-butylimin (S4) species in the temperature range
160 – 250 K. In the second pathway, the C-butenylimin (S3) species are also hydrogenated at
the C=C bond forming C-butylimin species (S5). However, hydrogenation does not stop at this
stage and in the fraction of S5 species, the imine group is hydrogenated to amine forming C-
butylamin species (S6). The species S5 and S6 co-exist on the surface and evolve in a very
similar way: they appear in the same temperature range and both disappear in a correlated way
upon prolonged H2 exposure in the course of the reaction as can be deduced from the
comparison of the spectra 4 and 5 in Figure 4b. Note that the characteristic vibrations used to
identify the species S5 and S6 ((HNC) at 1280 and (NH2) at 1580 cm-1) cannot originate
from the same surface species. On the other hand, the nearly identical evolution of these species
upon temperature variation and prolonged exposure to H2 suggest that the species S5 and S6
are strongly correlated, most likely as a precursor (S5) and the product (S6). Thus, the
combination of all spectroscopic data suggest that the originally adsorbed AC species S1
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becomes partly hydrogenated under the reaction conditions along two competing reaction
pathways: the first one leads to formation of the N-butylimin (S4) species, while the second
results in the formation of the mixture of C-butylimin (S5) and C-butylamin (S6) species.
It is important to emphasize that one type of species - N-butylimin (S4) - remains present on
the catalytic surface under the reaction conditions and serves as an active ligand layer. The
other two species  C-butylimin (S5) and C-butylamin (S6) – quickly disappear after prolonged
H2 exposure (see Figure 4b, spectra 4 and 5; Figure 2, spectra 0-5), most likely as a result of
successive hydrogenation of the C=N bond of C-butylimin (S5) to C-butylamin (S6) followed
by H insertion into the C···Pd bond to produce molecular butylamine that is able to desorb (for
additional discussion on the stability of S5 and S6 species see SI, Chapter X; the stability of
the S4 species is additionally discussed in SI, Chapter III). This observation suggests that
species building an active ligand layer under the reaction conditions are the stable N-butylimin
(S4) species, while the mixture of the unstable species C-butylimin (S5) and C-butylamin (S6)
are the spectators disappearing in the course of the reaction without effecting the catalytic
efficiency.
Figure 5b shows the model of the geometric orientation of the active N-butylimin (S4) species
based on the STM data: all ligand molecules are separated by approx. 3×dPd-Pd, meaning that
each third Pd atom along the main axes is covered by the ligand. The clearly visible band at
1755 cm-1 characteristic of the imine species suggests a rather upright adsorption geometry,
which is also reflected in this model.
Summarizing, we provide for the first time a comprehensive atomistic level description of a
ligand-based heterogeneous catalyst for chemoselective hydrogenation of -unsaturated
aldehyde acrolein. Deposition of allyl cyanide as a precursor of an active ligand layer renders
the catalyst highly active and close to 100% selective toward propenol formation and eliminates
the induction period observed on pristine surfaces. Formation and dynamic evolution of the
active ligand layer under the reaction conditions were investigated by combination of in
operando spectroscopic and microscopic surface sensitive techniques. On pristine Pd and under
non-reactive conditions, AC forms a self-assembled 2D layer with the species oriented close
to parallel to the surface and exhibiting a two-point interaction between the –CN and the CHx
groups of neighboring molecules. Upon transferring to the reaction conditions, both the
chemical nature of the adsorbates and the geometric configuration of the ligand layer drastically
change. In two competing reaction pathway, both stable N-butylimin active ligand species and
unstable spectator species are formed. These latter species comprising C-butylamin and C-
butylimin desorb in the course of the reaction after prolonged H2 exposure. The stable N-
butylimin species forms an ordered ligand layer acting under the reaction conditions, in which
every third Pd atom is covered by N-butylimin adsorbate. With this we provide for the first
time a direct microscopic proof that ordered layers of active ligand species are formed, which
govern the interaction of acrolein with the catalyst by lateral interactions and by this render the
surface highly chemoselective.
Obtained atomistic-level insights highlight the exceptional importance of controlling the
chemical and geometrical structure of dynamically changing active ligand layer under
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operational conditions. Related effects are expected to play a key role for chemoselective
hydrogenation of all types of multi-unsaturated hydrocarbons over ligand-functionalized
catalysts including practically relevant powdered materials.
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TOC
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