Porous-Organic-Polymer-Triggered Advancement of Sustainable Magnetic Efficient Catalyst for Chemoselective Hydrogenation of Cinnamaldehyde

Article abstract of DOI:10.1002/cctc.202000072

In this study, we adopted a cost-effective, facile and metal & template-free strategy for the successful synthesis of hydroxyl enriched POP (denoted as TPT). An integrated catalyst, Pd?Fe₃O₄@TPT, has been developed for the liquid phase selective hydrogenation cinnamaldehyde (CAL). Pd?Fe₃O₄@TPT exhibited excellent catalytic performance, providing 100 % selectivity towards hydrocinnamaldehyde (HCAL) under mild reaction conditions (with relatively low hydrogen pressure and very short reaction time), whereas Fe₂O₃@TPT appeared inert. Compared with the conventional catalytic systems, our newly designed catalyst was superior in many aspects, owing to the rigid nature of TPT-POP, which prevents aggregation and leaching of the metal nanoparticles.

Full text of DOI:10.1002/cctc.202000072

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Porous-Organic-Polymer-Triggered Advancement of Sustainable
Magnetic Efficient Catalyst for Chemoselective Hydrogenation of
Cinnamaldehyde
Authors: John Mondal, Ratul Paul, Chitra Sarkar, Yong Yan, Quang
Thang Trinh, Bolla Srinivasa Rao, Chih-Wen Pao, Jyh-Fu
Lee, and Wen Liu
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To be cited as: ChemCatChem 10.1002/cctc.202000072
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0
1/2020

ChemCatChem
10.1002/cctc.202000072
Porous-Organic-Polymer-Triggered Advancement of Sustainable
Magnetic Efficient Catalyst for Chemoselective Hydrogenation of
Cinnamaldehyde
[
a]
[a]
[b],[c]
[c]
[a]
Ratul Paul, Chitra Sarkar, Yong Yan,
Quang Thang Trinh, Bolla Srinivasa Rao, Chih-
[
d]
[d]
[b],[c]
[a]
Wen Pao, Jyh-Fu Lee, Wen Liu,
and John Mondal*
[a] R. Paul, C. Sarkar, Dr. B. S. Rao and Prof. Dr. J. Mondal
Catalysis & Fine Chemicals Division, CSIR-Indian Institute of Chemical Technology, Uppal
Road, Hyderabad-500007, India.
Email: johncuchem@gmail.com; johnmondal@iict.res.in
[b] Dr. Y. Yan, Prof. W. Liu
School of Chemical and Biomedical Engineering, Nanyang Technological University, 62
Nanyang Drive, Singapore 637459.
[c] Dr. Q. T. Trinh
Cambridge Centre for Advanced Research and Education in Singapore (CARES), Campus for
Research Excellence and Technological Enterprise (CREATE), 1 Create Way, 138602,
Singapore.
[d] Dr. C. W. Pao, Dr. J. F. Lee
National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu 30076, Taiwan.
Abstract: Intriguing characteristics including high specific surface area, adjustable chemical
functionality, flexibility of molecular design, high hydrothermal and mechanical stability
of Porous-Organic-Polymers (POPs) present new opportunities for them to become a versatile
platform in heterogeneous catalysis. Cinnamaldehyde (CAL), a representative α,β-unsaturated
aldehyde, is an essential molecule, because its partially hydrogenated products, namely cinnamyl
alcohol (COL) and hydrocinnamaldehyde (HCAL) are key intermediates for the production of
fine chemicals including perfumes, flavorings and pharmaceuticals. In this study, we adopted a
cost-effective, facile and metal & template-free strategy for the successful synthesis of hydroxyl
enriched POP (denoted as TPT). The TPT was prepared by the simple one-pot condensation of
triphenyl amine and terephthaldehyde in the presence of p-toluene sulphonic acid, which served
as an organic catalyst. Accordingly, an integrated catalyst, Pd-Fe
for the liquid phase selective hydrogenation cinnamaldehyde (CAL). Pd-Fe
excellent catalytic performance, providing 100% selectivity towards hydrocinnamaldehyde
HCAL) under mild reaction conditions (with relatively low hydrogen pressure and very short
reaction time), whereas Fe @TPT appeared inert. Compared with the conventional catalytic
3 4
O @TPT, has been developed
3 4
O @TPT exhibited
(
2 3
O
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ChemCatChem
10.1002/cctc.202000072
systems, our newly designed catalyst was superior in many aspects, owing to the rigid nature of
TPT-POP, which prevents aggregation and leaching of the metal nanoparticles. We identify Pd
nanoparticles (-NPs) to have two decisive roles, namely (I) transformation of Fe
Fe through metal-support interaction and (II) triggering the catalytic activity. A decrease in
the Pd-Pd bond distance (~2.7 Å to ~2.49 Å) along with the shift of Fe K-edge to a lower energy
was observed by the synchrotron EXAFS analysis of Pd-Fe @TPT, revealing that Pd
facilitated the reduction of Fe to Fe . The significant enhancement in catalytic performance
2 3
O to
3 4
O
3 4
O
2
O
3
3 4
O
for the selective hydrogenation of C=C over C=O bond could be ascribed to the flat adsorption
of C=C over the Pd surface and C=O bond shortening, as suggested by in-situ ATR-IR and
DRIFTS spectroscopy studies. Density Functional Theory (DFT) calculations reveal two
possible ways by which TPT-POP could contribute to the highly selective hydrogenation of C=C
bond on the Pd-Fe
3
O @TPT catalyst. Firstly, TPT acts as a structural template for generating
4
highly concentrated Pd step sites and other low coordinated sites, which possess high activity
towards C=C bond hydrogenation. Secondly, the charge transfer from TPT to the Pd clusters
increases the negative charge density of the Pd sites, consequently enhancing their C=C
hydrogenation activity. The present findings provide new inspirations for designing easily
realizable, low-cost, and high performance catalysts for sustainable chemistry via effective
surface/interface engineering.
Keywords:
Porous-Organic-Polymer
(POP),
Cinnamaldehyde
hydrogenation,
Terephthaldehyde, Magnetic Nanocatalyst, Density Functional Theory (DFT) calculations.
Introduction
Chemoselective hydrogenation of C=C and, or C=O in α,β-unsaturated aldehydes is an
indispensable approach in fine chemical manufacturing. Cinnamaldehyde (CAL), a
representative α,β-unsaturated aldehyde, is an industrially important molecule, because its partial
hydrogenation products, viz. cinnamyl alcohol (COL) and hydrocinnamaldehyde (HCAL) are
key intermediates for the synthesis of high-value products, including perfumes, flavorings and
[
1-3]
pharmaceuticals.
For example, HCAL is demonstrated to be an important intermediate in the
[
4]
formulation of a HIV drug. Cinnamaldehyde (α,β-unsaturated aldehyde), a typical model
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ChemCatChem
10.1002/cctc.202000072
component representing the coniferyl aldehydes and sinapaldehydes derived from lignin-, could
be produced by electron abstraction of the phenoxy radicals followed by disruption of the C -C
α
β
[
5]
bond through retro-aldol cleavage of lignin. Therefore, lignin-derived chemicals can serve as
platform molecules towards the production of a wide range of value-added chemicals, whose
implementation will motivate the transformation towards a sustainable chemical industry.
With the advancing knowledge and developments in the field of engineering heterogeneously
supported metal catalysts for diverse chemical applications, various research groups reported that
[6]
[7]
[8]
[9]
[10]
[11]
Pt, Au, Ru, Ir, Co, and Cu based supported metal catalyst are active for the selective
hydrogenation of CAL to COL. Liu and coworkers discovered that Pd metal supported on multi-
walled carbon nanotube is an active catalyst for the selective hydrogenation of cinnamaldehyde
to saturated aldehyde, showing 91.3% selectivity under 40 bar H
2
and 148 bar CO
pressure at 60 °C. Pd nanoparticles deposited on N-doped mesoporous carbon is also an
effective catalyst for producing partially hydrogenated HCAL with 93% selectivity under low H
2
partial
[12]
2
[13]
pressure at room temperature. Remarkably, Wei et al. reported that Co based inter-metallic
compounds, derived from layered double hydroxide, exhibited 96% selectivity towards cinnamyl
[14]
alcohol. By evaluating the sustainability and greenness of selective hydrogenation reaction,
Kubo et al. reported boronate micro-particle supported nano palladium and nano gold catalyst for
the chemoselective reduction of C=C and C=O bonds of cinnamaldehyde, respectively, using
[
15]
environmentally benign solvents. Recently, Hu et al. established the strategy of using metal
organic frameworks (MOFs) and metal nanoparticles shelled structures e.g.
MOFs@NPs@MOFs for the partial hydrogenation of CAL with 97.3% selectivity towards
-1 [16]
cinnamyl alcohol, with an elevated turn over frequency to 1516.1 h .
A distinct class of
porous materials have been fabricated by combining porous Pt-Ni nano-wires and MOFs to
[
17]
intensify the catalytic activity for producing cinnamyl alcohol with 83.3% selectivity. Indeed,
Yang et al. recently employed water as a hydrogen exchange bridge over carbon nanotube
supported bimetallic Pt-Fe catalyst for the successive synthesis of partially hydrogenated
[
18]
cinnamyl alcohol from cinnamaldehye. Although all the accomplished work on the partial
reduction of cinnamaldehyde report good selectivity along with high conversion, there remain
challenges associated with catalyst recyclability, leaching of metal nanoparticles and the high
cost of noble metals.
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To address the above challenges, some researchers have started exploring the material space
[
19]
of micro porous polymeric network with functional building blocks, such as covalent organic
[
20]
[21]
frameworks(COFs),
polymers with intrinsic micro porosity (PIMs),
conjugated micro
[
22]
[23]
porous polymers (CMPs), hyper-cross-linked polymers (HCPs), porous organic polymers
[
24]
[25]
(POPs),
covalent triazene frameworks (CTFs)
and covalent molecular organic cage
[26]
[27]
(MOC),
owing to their high thermal and chemical stability,
excellent surface
[
28]
hydrophobicity,
low skeletal density, and large specific surface area. Amongst the various
synthetic approaches available for preparing hyper-cross-linked porous organic network, acid
catalyzed condensation polymerization is the most suitable for constructing a ameliorate
polymeric backbone with -OH functional groups, which are readily available for further
modification.
With these considerations, we report herein a truly cost-effective facile, metal-free, and
template-free synthetic strategy to develop -OH group functionalized Porous-Organic-Polymer
(TPT) via the one-pot condensation of triphenyl amine and terephthaldehyde. This synthetic
strategy avoids the use or generation of substances that are toxic to humans and,or the
environment. Our research aligns with the globally emerging interest in developing
environmentally benign, sustainable, and low-metal consumption chemical synthesis
[29]
processes. Also, the one pot synthesis method used involves the incorporation of all the raw
materials into the final product, thereby facilitating atom economy. Specifically, we demonstrate
this simple method for the synthesis of diphenyl-methanol bearing, magnetic monohybrid
3 4
catalyst, viz. Pd-Fe O @TPT, which could be implemented effectively for the catalytic
chemoselective hydrogenation of cinnamaldehyde (CAL) to hydrocinnamaldehyde (HCAL),
with preference to attack the conjugated C=C group, whilst leaving the C=O bond untouched.
Through polymerization, the diphenylmethanol based TPT-POP framework may provide higher
proportion of active surface sites for enhanced catalytic activity and strengthened catalyst
durability. Here, functionalized and magnetically retrievable catalysts mitigate the needs for
complex catalyst recovery steps (e.g. filtration and repeated washing) at the end of the reactions,
thus providing cost-effective, energy efficient and sustainable pathways for the catalytic
[
30]
conversion of chemical feedstocks. The evolution of catalytic efficiency and structure-activity
relationship have been established based on comprehensive characterization results, including
13
2
wide-angle powder X-ray diffraction (XRD), N physisorption, C cross-polarization (CP) solid-
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ChemCatChem
10.1002/cctc.202000072
state magic angle spinning (MAS), nuclear magnetic resonance (NMR), Fourier transform
infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron
microscopy (HR-TEM), field emission scanning electron microscopy (FE-SEM), energy
dispersive X-ray (EDX) mapping, extended X-ray absorption fine structure (EXAFS), attenuated
total reflectance-infrared spectroscopy (ATR-IR), and diffusive reflectance infrared Fourier
transform spectroscopy (DRIFT). The remarkable electronic metal-support interaction (EMSI),
which modulates the local atomic structure and electronic environment of the Pd-Fe
brings unique synergies that promote the catalytic activity, selectivity and stability of the Pd-
Fe @TPT catalyst.
3 4
O interface,
3 4
O
EXPERIMENTAL SECTION
Materials
Triphenylamine, terepthaldehyde, palladium acetate (Pd(OAc)
2
), ferric chloride (FeCl ),
3
polyvinyl pyrrolidone (PVP), p-toluene sulphonic acid were purchased from Sigma-Aldrich and
used as received unless noted otherwise. 1,2-dichlorobenzene, ethylene glycol and methanol
solvents were all dried before using in reactions.
Synthesis of Porous-Organic-Polymer (TPT):
Porous organic polymer (TPT-POP) was synthesized through typical one-step condensation
polymerisation in a sealed tube under high vacuum condition, p-toluene sulphonic acid and
dichlorobenzene were added as catalyst and solvent, respectively, with a slight modification from
[
31]
the previous report.
In a typical synthesis, triphenylamine (1.0 mmol, 245 mg),
terephthalaldehyde (1.5mmol, 201mg) and p-toluene sulphonic (290 mg) acid were first charged
into a 25 ml sealed tube, followed by the addition of 1,2-dichlorobenzene (5 ml). To ensure the
complete dispersion of all the reactants in the solvent, the reaction mixture was sonicated in an
ultrasound bath for 30 mins. The well-dispersed solution was then degassed by three freeze-
2
pump-thaw cycles at 77 K using liquid N . Next, the tube was flame sealed and kept in an oven
at 150 °C for 12 hours. After cooling to room temperature, the bluish-green solid was collected
and washed thoroughly with methanol, acetone, dichloromethane, and THF, respectively.
Finally, the solid product was dried overnight in a vacuum oven at 100 °C .
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2 3 3 4
Synthesis of Fe O @TPT and Pd-Fe O @TPT:
At first, the as-synthesized 150 mg TPT-POP was dispersed in ethylene glycol (30 ml), followed
by the addition of a solution of polyvinyl pyrrolidone (PVP, 25 mg) in ethylene glycol (5 ml) and
water (5 ml). The reaction mixture was kept stirred for at least two hours. Then, a solution of
FeCl (0.567 mmol, 92 mg) in ethylene glycol (10 ml) was prepared and added drop-wise into
3
the previous mixture under constant stirring. After two hours, the resulting mixture was taken
into an autoclave and heated at 150 °C for 8 hours. After cooling to room temperature, the
brownish-black solid product (Fe
and washed with methanol. Finally, the solid product was dried overnight in vacuum at 80 °C.
For the preparation of the black colored Pd-Fe @TPT, we added PdCl (0.189 mmol, 35mg)
in the solution of polyvinyl pyrrolidone (25 mg) in ethylene glycol (5 ml) and water (5 ml), then
followed the same procedure for the synthesis of Fe @TPT.
2
O @TPT) was separated from the mixture by centrifugation
3
O
3 4
2
3 4
O
Catalytic activity for Cinnamaldehyde Hydrogenation:
The catalytic hydrogenation of cinnamaldehyde was performed in a 100 ml Parr autoclave
reactor, which is protected by stainless steel shell, fitted with an built-in pressure gauge set up
(measuring 0-12 MPa) and a temperature rating of 573K. In a typical experiment,
cinnamaldehyde (0.25 ml, 2mmol), along with the catalyst (0.03 g), were dissolved or dispersed
in 30 ml isopropanol. The reaction mixture was charged into the reactor. The reactor was then
sealed and evacuated using vacuum pump for 15 min at room temperature to remove the
2
dissolved oxygen. The required H pressure of 10 bar was achieved by purging the reactor with
pressurized H for at least 10 times. The reaction was conducted for a specified period with
2
continuous stirring at 1000 rpm at the constant temperature of 60 °C. After the completion of the
reaction, the reactor was kept aside to cool down. The progress of the reaction was monitored by
sampling at regular time intervals and analyzing the product mixtures using an Agilent 6980 gas
chromatograph (GC) equipped with a flame ionization detector and an SE-54 capillary column
(30 m × 0.32mm × 1.0 μm). During GC analysis, toluene was used as an internal standard with a
standard deviation of less than 2%.
RESULTS AND DISCUSSIONS
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Catalyst Synthesis and Characterization
The TPT-POP material has been synthesized through organic acid catalyzed, metal & template-
free, one-pot condensation of triphenyl amine and terepthaldehyde under sealed-tube conditions
to produce a bluish-green solid (Scheme 1). The rate of polymerization towards a 3D cross-
linking framework is accelerated in a closed glassware system. The highly active triphenyl amine
monomer, each bearing three electron rich phenyl units, contributed to the electrophilic aromatic
substitution with two aldehyde groups of terephthaldehyde, where each triphenyl amine
molecules could be linked together by their para position with two terephthaldehyde units. A
huge amount of -OH functional groups in the polymeric backbone, could be found in the
resulting TPT-POP. These -OH groups are available for further functionalization. The as-
prepared diphenylmethanol based TPT-POP is very stable and insoluble in common organic
solvents, attributing to the successful formation of the highly rigid and cross-linked network. To
the best of our knowledge, we have utilized condensation-polymerization inspired chemistry to
demonstrate the feasibility of our novel approach with rigid aromatic building blocks and
external linker based TPT-POP. Sequential synthesis procedure involving solvothermal reduction
was employed, where Pd and Fe
framework (designated as Fe @TPT& Pd-Fe
intense black was observed during the formation of the Fe
3
O
4
NPs were homogeneously incorporated in the OH-rich POP
@TPT). Color change from brownish black to
@TPT and the Pd-Fe @TPT
, which is
O
2 3
3 4
O
O
2 3
O
3 4
2 3 3 4
nano-hybrid, probably owing to the in situ phase transformation from α-Fe O to Fe O
mediated by the Pd nanoparticles.
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Scheme 1:SchematicIllustration for the synthesis of Porous-Organic-Polymer and the sequential
synthetic approach for Pd-Fe based integrated catalyst.
3 4
O
In order to examine the thermal stability of the as-synthesized TPT-POP, TGA experiments were
carried out. The prominent decrease of wt% below 100 °C belongs to the loss of water or gas
molecules trapped in the organic framework. Distinct weight loss with increasing temperature
after 100 °C indicates the release of free -OH groups from the porous skeleton (Figure S2).
Furthermore, weight loss continued beyond 400 °C, probably owing to temperature assisted C-C
bond cleavage along with burning of organic framework present in the porous organic network.
The parent TPT contains 77.28 wt% C, 3.85 wt% H and 2.59 wt% N, as established by elemental
(
C, H, N) analysis. After the incorporation of iron oxide, the carbon content of Fe
Pd-Fe @TPT decreases drastically to 59.61 wt% and 55.04 wt%, respectively, along with the
hydrogen content (4.10 and 3.46 wt%, respectively) and the nitrogen content (4.12 and 3.75
wt%, respectively) (Table S1). Pd and Fe contents in the Pd-Fe @TPT are 0.173 & 0.285
2
O
3
@TPT and
3 4
O
3 4
O
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10.1002/cctc.202000072
mmol/g, respectively, as measured by Inductively Coupled Plasma Mass Spectrometry (ICP-
13
MS). C CP solid-state MAS NMR spectrum analysis was performed to obtain the molecular
connectivity and the chemical environment of the carbon nuclei of the polymeric skeleton of the
three as-synthesized porous materials, namely TPT, Fe
2 3 3 4
O @TPT and Pd-Fe O @TPT (Figure
1
a). We distinguished a common resonance peak for all three samples near 55 ppm, attributed to
the tertiary aliphatic carbon-containing hydroxyl group (-OH). The unsubstituted aromatic
2
Sp carbon peak of the as-synthesized TPT polymer appears at 127 ppm, while two peaks at 138
and 144 ppm are ascribed to the C-substituted and N-substituted aromatic carbon. The absence of
the aldehyde (-CHO) carbon resonance peak around 190 ppm clearly indicates the complete
[
30]
incorporation of aldehyde groups into the polymer material. In the parent TPT-POP, the sharp
peak at 144ppm corresponds to the N-substituted carbon atom of the benzene ring, which has
been shifted to the higher values of 146 ppm and 149 ppm for Fe
respectively. This alteration in the δ-value could be associated with the intrinsic synergistic
effect developed by the in situ incorporation of Fe and bimetallic Pd-Fe on the TPT-POP
2 3
O @TPT, Pd-Fe
3 4
O @TPT,
O
2 3
3 4
O
surface, which regulates the electronic environment around the N-atom as a result of the charge
transfer from N to Fe and Pd. Therefore, the electron density around the N-atom contracts, and
eventually withdraws electrons from the adjacent C-atom, making the latter more de-shielded,
hence shifting the peak to a higher δ-value. However, we discriminate some broad peaks near 50
ppm for Fe
on the surface of Fe
surrounding electronic environment.
2 3
O @TPT, probably attributed to the presence of large numbers of hydroxyl groups
2 3
O , as hydrogen bonding interaction between them can influence the
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ChemCatChem
10.1002/cctc.202000072
13
Figure 1: (a) C CP solid state MAS NMR spectra, (b) FT-IR spectra of TPT, Fe
Pd-Fe @TPT.
2 3
O @TPT and
3 4
O
The polymeric framework construction with covalent bonding in all the TPT-POP based
-1
materials have been elucidated by FT-IR spectroscopy (Figure 1b). A peak around 3420 cm ,
characteristic of hydroxyl functional groups of the porous framework, is observed for the parent
[
30]
TPT as well as the Fe
2 3
O @TPT and Pd-Fe
3
O
4
@TPT nanocomposites. The corresponding
2
3
-1
C
sp -H and Csp -H bond (coupled to the –OH group) frequencies are detected around 3020 cm
-1
and 2928 cm , respectively. C-N stretching peak of triphenyl amine and –COOH of
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ChemCatChem
10.1002/cctc.202000072
-1
-1
-1
terepthaldehyde can be seen around 1600-1674 cm and 1500 cm & 1263-1318 cm ,
respectively, both clearly indicating the fabrication of the porous organic network, in good
[
32,33]
agreement with previous works.
In Fe
2
O
3
@TPT, the formation of α–Fe
2
O
3
is confirmed by
@TPT
single absorption peak is noticed at 546 cm after deposition of iron oxide
-
1
-1
the characteristic peaks of the Fe-O bond at 547 cm and 470 cm , whereas for Pd-Fe
3 4
O
-1
nano-hybrid, Fe
O
3 4
[34]
by solvothermal method.
2
N adsorption-desorption isotherm analysis at 77 K was employed to
evaluate the permanent porosity and the specific surface area present on the TPT-POP based
materials. The accordingly analyzed pore size distribution curves are presented in Figure 2a &
2c, which shows that the as-synthesized TPT-POP, Fe
2 3 3 4
O @TPT and Pd-Fe O @TPT all exhibit
behaviors resembling a combination of type I and IV isotherms (according to IUPAC
[35]
classification), suggesting the dominance of mesopores. The large hysteresis loop at the high
pressure region, probably due to the swelling of polymeric units upon gas adsorption and the
subsequent elastic deformation of pores and inter-particle voids, made the whole gas adsorption
process irreversible in nature. Brunauer-Emmett-Teller (BET) surface areas of TPT-POP,
2
2
2
Fe
2 3 3 4
O @TPT and Pd-Fe O @TPT are measured to be 532.8 m /g, 462.3 m /g & 410.7 m /g,
3
-1
3
-1
respectively, with the corresponding pore volumes of 1.593 cm g , 1.213 cm g and 1.106
3
-1
cm g , respectively. We have observed a diminishment in the BET surface area owing to the
accommodation of the NPs inside the porous channel and a mass increment of the parent POP,
2
thereby inhibiting N uptake. Our observation is fully consistent with the previous report by
[36]
Wang et.al. The pore size distribution curve, as derived by the NLDFT (Non-Local-Density-
Functional-Theory) fitting of the adsorption branches model (Figure 2c), clearly demonstrates
the mesoporous nature of the three as-synthesized materials, with a distinct sharp peak
predominately appearing in the 2-3 nm range. We also find a contraction of pore-width from
2
.58, 2.4 & 2.07 nm, respectively, which could be attributed to the clogging of the pore-channels
by the high density metal NPs.
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Figure 2: (a) Nitrogen adsorption/desorption isotherms, (b)Wide-angle powder X-ray diffraction
patterns,(c) pore-size distributions as determined by NLDFT method of TPT, Fe @TPT and
Pd-Fe @TPT, respectively.
2 3
O
3 4
O
The wide-angle powder XRD of the as-synthesized TPT based POP material was displayed in
°
°
Figure S1. The two distinct broad peaks at 25 and 44 correspond to the π-stacking of the
aromatic building blocks in small domains and the formation of the amorphous structure, which
is composed of randomly polymerized TPA units. These peaks could be also nicely indexed by
[
37]
the diffraction planes of the carbon (002) & (101), respectively. The PXRD patterns of
Fe @TPTand Pd-Fe @TPT are shown in Figure 2b, which clearly depicts the
rhombohedral structure of α-Fe with the characteristic Bragg’s diffraction peaks at 2=
4.1, 30.1, 35.5, 40.8, 49.3, 53.9, 57.5, 62.3& 64.0, corresponding to the (012), (104),
O
2 3
3 4
O
2 3
O
2
[
38]
(110), (113), (024), (116), (214), (300) diffraction planes, respectively.
For the Pd-
Fe @TPT nano-hybrid, distinct peaks of Pd nanoparticles at 40.18, 46.76 and 68.16 are
3
O
4
indexed as Pd(111), Pd(200) and Pd(220), respectively. The inverse spinel structure of magnetite
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ChemCatChem
10.1002/cctc.202000072
[
39]
is confirmed by the assigned (220), (311), (400), (422), (511), (440) reflections. Interestingly,
the synthesis of Fe @TPTand Pd-Fe @TPTnano-catalyst were performed in the same
reaction condition, despite having ended up with two different phase of iron oxides. We note that,
the Pd NPs played the crucial role for the formation of magnetite (Fe ), which are generated
O
2 3
O
3 4
3 4
O
through in situ reduction, facilitated by PVP from the metal precursor. Pal et al. described the
redox properties of Pd NPs and indicated that the relatively small Pd NPs can act as effective
[
40]
reducing agents. Recently, the promoted reduction of hematite to magnetite by Pd NPs was
[41]
reported by Esproet al. The surface morphology is explored through FE-SEM analysis, which
clearly indicates the formation of highly rough surface of TPT-POP (Figure S3). Random
aggregation of particles, formed during the high temperature synthesis, lead to the observation of
2 3 3 4
chunks on the TPT-POP surface. FE-SEM images of Fe O @TPT and Pd-Fe O @TPT(Figure
S5 & S6) reveal the pearl millet like morphology, making the surface rough in texture. By
evaluating the lower magnification FE-SEM images, we can clearly observe parched earth-type
morphology, indicating the formation of iron oxide-wrapped porous organic polymer. The
average size of the conjugated spherical Fe
in the range of 21-65 nm and 7-25nm, respectively. Energy dispersive X-ray (EDX) analysis
Figure S7) of the selected areas of the FE-SEM micrographs clearly signifies the presence of N,
2 3 3 4
O @TPT and Pd-Fe O @TPT particles appears to be
(
O, Fe and Pd in the respective spectra. In order to further investigate the surface morphology and
the electronic properties, TEM analysis was performed (Figure 3). Close inspection of the TEM
image of the parent TPT-POP (Figure S4) reveals cloud-like surface morphology, resulting in a
hyper-branched porous nature. At 50 nm scale, spider web-type morphology was noticed,
probably owing to the hyper-cross-linking between two monomers, along with the irregular
dispersion of metal nanoparticles over the porous network. The two different phases of iron
oxide (Fe
network, as shown in Figure 3a &3e. In figure 3f, dark regions represent the presence of Pd
nanoparticles, whereas Fe particles are intertwined with the relatively brighter polymeric
network of Pd-Fe @TPT (). HR-TEM images of Fe @TPT and Pd-Fe @TPT
nanoclustures show lattice fringes of the Fe (104) & Fe (012) and Fe (311) & Pd (111),
2 3 3 4
O , Fe O ) and Pd nanoparticles are distributed randomly over the fibrous polymer
3 4
O
O
3 4
2
O
3
3 4
O
O
2 3
O
2 3
3 4
O
respectively crystal planes , consistent with the PXRD data. The selected area electron
diffraction (SAED) patterns reveal d-spacings that correspond to lattice fringes of rhombohedral
α-Fe
O
2 3
and spinel Fe
O
3 4
& Pd metal, as shown in Figure 3d & 3h, respectively; this observation
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ChemCatChem
10.1002/cctc.202000072
is also in good agreement with the JCPDS nos. (33-0664), (19-0629), and (46-1043). STEM
micrographs (Figure 3i & j) of Fe @TPT & Pd-Fe @TPT depict comparably large
O
2 3
O
3 4
stationary metal phases distributed randomly amongst the somewhat extensive agglomeration of
nanoparticles.
Figure 3: TEM images of Fe
with clear crystalline lattice fringes of Fe
area electron diffraction) patterns were also provided for Fe
h); STEM images of Fe @TPT (i) and Pd-Fe @TPT (j) with the corresponding elemental
mapping of O, N, Fe and Pd.
2
O
3
@TPT (a & b) and Pd-Fe
@TPT (c) & Pd-Fe
@TPT (d) and Pd-Fe
3
O
4
@TPT (e & f). HR-TEM images
@TPT (g). SAED (selected
@TPT
O
2 3
3 4
O
2
O
3
3 4
O
(
O
2 3
3 4
O
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ChemCatChem
10.1002/cctc.202000072
Figure 4: High-resolution TEM images at different magnifications of Pd-Fe
3
O @TPT.
4
Additionally, elemental mapping of both integrated catalysts reveals that each of the O, N, Fe,
and Pd elements is locally concentrated at specific regions. The representative high resolution
3 4
TEM image (Figure 4a) depicts considerably rough spherical particles Fe O of 9.97-12.42 nm in
diameter, which can be spotted on the exterior surface of the TPT-POP after Pd-mediated
reduction. Moreover, the TEM image of the near-surface region (Figure 4b) demonstrates two
phases of Fe
3
O with different electron densities, owing to the high surface energy compared to
4
that of TPT-POP with low roughness.
X-ray photoelectron spectroscopy (XPS) was analyzed to shed light on the elemental
composition and oxidation state of each element on the surface of Fe
2
O @TPT and the Pd-
3
Fe @TPT nano-hybrid. The characteristic peaks of N, O, Fe and Pd in the as-synthesized
3
O
4
porous material was indicated by the XPS survey spectra (Figure S9). The highest binding
energy peak at ~399.6 eV (Figure S8b & S8d) in the N 1s spectrum corresponds to the N-atom
connected to the benzene ring of triphenyl amine monomer (C=C-N) for both nano-
[42]
composites.
In the O 1s spectrum, binding energy peaks appear at ~530eV and ~531.66
eV (Figure S8a, Figure S8c), which can be attributed to the lattice oxygen (Fe-O) and the
[
43-45]
hydroxyl group on the polymers’ surface.
2 3 2 3
The presence of α‑Fe O phase in Fe O @TPT
was confirmed by the binding energy peaks at ~711.83 eV and ~725.51 eV (Figure 5a), which
correspond to Fe 2p3/2 and Fe 2p1/2, respectively, along with a shake-up satellite peak at ~719.74
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10.1002/cctc.202000072
[
.
46]
eV
A slight positive shift in binding energy relative to bare Fe
O
2 3
is noticed; this is in good
[
43]
+3
agreement with the literature, attributed to the interaction of Fe with the hydroxyl groups the
N atoms in the polymeric framework. For Pd-Fe @TPT, the characteristic binding energy
3
O
4
[
43]
peaks at ~711.92 eV and ~725.67 eV (Figure 5b) reveal the presence of magnetite. Binding
energy peaks indexed at ~336.19 eV and ~341.18 eV (Figure 5c) in the XPS spectra corresponds
0
to the Pd 3d5/2 and Pd 3d3/2 orbitals, respectively, of metallic Pd in Pd-Fe
3 4
O @TPT. However,
comparing the XPS data of Pd-Fe
3 4
O @TPT with bare Pd-NP, the negative shift of around ~0.3
0
eV signifies the electron enrichment of Pd due to the interaction between N-atom and the Pd-
[
47]
3 4
NPs in Pd-Fe O @TPT.
Figure 5: XPS spectra of Fe-2p for Fe
Pd-Fe @TPT (c).
2 3 3 4
O @TPT(a)& Pd-Fe O @TPT (b); Pd-3d spectrum for
3 4
O
3 4
To unfold the reason behind the superior catalytic activity of Pd-Fe O @TPT, detailed
information, including local structure, oxidation state of each element, and Fe, Pd density were
extracted from X-ray absorption near edge structure analysis (XANES). The Pd K-edge
3 4
XANES/EXAFS spectra of the Pd-Fe O @TPT sample look similar to those of Pd foil,
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ChemCatChem
10.1002/cctc.202000072
[47]
indicating that most Pd atoms are present in metallic state and aggregated to form Pd clusters.
However, the Pd-Pd coordination number of the Pd-Fe @TPT sample is 10.2, significantly
O
3 4
smaller than 12 for Pd foil with a bulk fcc structure (as reflected by the reduced EXAFS intensity
both in k-space and in R-space). The Pd-Pd coordination number of 10.2 corresponds to an
average Pd cluster size of roughly 3 nm. The Pd-Pd and Fe-Pd coordination in the first
[48]
coordination sphere is represented by the peak at around ~2.49 Å (Figure 6b). Comparing with
the typical Pd-Pd bond distance of around ~2.7 Å, the observed value of 2.49 Å signifies the
[
50]
formation of Pd-Fe bonds in the Pd-Fe
3 4
O @TPT sample. As the radius of Fe atom (1.27 Å) is
small compare to the radius of Pd atom (1.38 Å), the overall lattice constant is reduced and is
[
48,49]
responsible to the observed interatomic distance.
theoretical value of 2.6 Å may be due to the interaction between Pd NPs, Fe
POP material. The Fe K-edge spectra of the Fe @TPT sample are very similar to those of
standard alpha-Fe . However, the Fe-O bond length of 1.92 Å of the Fe @TPT sample is
slightly longer than that the 1.89Å of the Fe standard, probably resulting from interactions
between Fe and TPT-POP. Such interactions caused the Fe lattice to expand to some
extent. Both XANES and EXAFS spectra of the Fe K-edge of the Pd-Fe @TPT sample are
similar to those of standard Fe . At a closer examination, either the white-line peak or the pre-
edge peak of the Pd-Fe @TPT sample shifted to a lower energy relative to the standard Fe
but still higher than those of the standard FeO). It seems that the overall spectral features were
contributed by both Fe and FeO phases (with the former being in majority). The Fourier
transform of the EXAFS (FT-EXAFS) spectrum of Pd-Fe @TPT exhibits a strong peak
between 1.0-2.0 Å in R-space, which also supports the above speculation. Another strong peak
between 2.5-3.5 Å represents the Fe-Fe shell, which is confirmed by the reference Fe
The deviation in Fe-Pd bond from the
3 4
O
and the TPT-
O
2 3
O
2 3
2 3
O
2 3
O
O
2 3
2 3
O
3 4
O
3 4
O
O
3 4
3 4
O
(
3 4
O
3 4
O
3 4
O
[51]
spectrum. On the whole, this XAS observations indicate that the Fe species in Pd-Fe
is similar in nature to Fe . Here, Pd NPs play a vital role in the preparation of Pd-Fe
as it promotes the reduction of Fe to Fe
3
O
4
@TPT
O
3 4
3
O
4
@TPT,
O
2 3
3 4
O .
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10.1002/cctc.202000072
Figure 6: The normalized XANES spectra (a) and k
spectra (b) at the PdK-edge of Pd-Fe
c) and k -weighted Fourier transform EXAFS spectra (d) of the samples (Pd-Fe
Fe @TPT) and the relative standards (α-Fe , Fe and FeO) at the Fe K-edge.
2
-weighted Fourier transform EXAFS
3
O
4
@TPT and bulk Pd foil. The normalized XANES spectra
@TPT and
(
3
3 4
O
O
2 3
O
2 3
3 4
O
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10.1002/cctc.202000072
3 4 2 3
Selective Hydrogenation of Cinnamaldehyde over Pd-Fe O @TPT and Fe O @TPT:
Selective hydrogenation of α, β-unsaturated aldehyde is crucial for the production of useful
intermediates in the pharmaceutical, cosmetics and food industries. Here, cinnamaldehyde was
selected as a model compound, which consists of distinct types of unsaturated functional groups,
such as C=C and C=O, along with the extended conjugation with the benzene ring. Selective
hydrogenation of cinnnamaldehyde gives rise to different types of unsaturated aldehyde and
alcohol, i.e. double bond reduction leads to the formation of hydrocinnamaldehyde (HCAL)
whereas C=O reduction produces cinnamylalcohol (COL). Initial reaction study was carried out
for the selective hydrogenation of cinnamaldehyde (2 mmol, 0.25 ml) in 30 ml isopropanol over
3 4 2
Pd-Fe O @TPT (30 mg) and 10 bar H at 60 °C for 1h with constant stirring. The comparison
study was performed under identical conditions to optimize the reaction temperature, pressure
and time. At the end of each reaction, the products obtained were analyzed by GC-FID. The
3 4
progress of the catalytic hydrogenation of cinnamaldehyde over Pd-Fe O @TPT as a function of
time at 60 °C and 80 °C are shown on Figure 7a and Figure 7b, respectively. After the initial 0.5
h of reaction at 60 °C and 10 bar pressure, 94% conversion of cinnamaldehyde, accompanied by
100% selectivity towards hydrocinnamaldehyde, was observed. Complete conversion of
cinnamaldehyde along with 100% hydrocinnamaldehyde selectivity was obtained with 1 h
advancement of the reaction, although further increment in reaction time facilitates the
production of phenyl propanol as the byproduct from hydrocinnamaldehyde. Time on stream
profile at 80 °C reveals no further conversion of cinnamaldehyde after the initial 0.5 h. The same
trend of increasing conversion and selectivity with time is also noticed at 80 °C. The full
conversion of cinnamaldehyde was attained after 1h, followed by 82% and 18% selectivities
towards hydrocinnamaldehyde and phenyl propanol, respectively. The prolongation of reaction
time to 3 h decreased the selectivity of hydrocinnamaldehyde to 73% while increased the
byproduct selectivity to 27%.
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10.1002/cctc.202000072
Figure 7: Evolution of reactant and product distributions against time for selective
hydrogenation of cinnamaldehyde at two different temperature of 60 °C (a) and 80 °C (b); Effect
of H
2
pressure (c) and Influence of reaction temperature (d) over the Pd-Fe
3 4
O @TPT catalyst.
Reaction conditions: Cinnamaldehyde (0.25ml, 2 mmol), Pd-Fe
3 4
O @TPT (30 mg), 60 °C, 1 h in
30 ml of isopropanol; the other product refers to phenyl propanol.
2
A series of reactions were also carried out at different H pressures to evaluate the optimum
pressure for the reaction at 60 °C with 1 h reaction time (Figure 7c). Interestingly, 12%
conversion of cinnamaldehyde and 100% selectivity towards hydrocinnamaldehyde was
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ChemCatChem
10.1002/cctc.202000072
achieved at 1 bar H
2
pressure. At 5 bar, the conversion of CAL and selectivity of HCAL
increased to 95% and 100%, respectively. Finally, the full conversion of cinnamaldehyde with
00% hydrocinnamaldehyde selectivity was secured at 10 bar H pressure. Although the full
conversion of cinnamaldehyde occurred at 15 bar H pressure, the selectivity of
1
2
2
hydrocinnamaldehyde degraded to 95% with a byproduct selectivity of 5%. The progress of the
cinnamaldehyde hydrogenation as a function of temperature was provided in Figure 7d for Pd-
3 4
Fe O @TPT. The initial 65% conversion of cinnamaldehyde, accompanied by 100%
hydocinnamaldehyde selectivity was observed at 40 °C, after 1 h of reaction. At 60 °C and 10
bar pressure, complete conversion of cinnamaldehyde to hydrocinnamaldehyde was detected in 1
h. When the temperature was raised to 80°C, hydrocinnamaldehyde selectivity lowered to 82%,
owing to the further conversion to other byproducts (18%) at higher temperatures. The trend of
increasing selectivity towards other byproduct remains unaltered even at 100 °C, giving rise to
24% selectivity towards phenyl propanol. We have also conducted the catalytic reaction at room
temperature 25 ºC (Table 1, entry 9). In this case, we have achieved 40% conversion of
cinnamaldehyde with 100% selectivity of hydrocinnamaldehyde under optimized reaction
conditions.
To shed light on the key factors contributing to the enhanced catalytic activity of the Pd-
Fe
3
O @TPT catalyst, we investigated the interaction of cinnamaldehyde with the surface of the
4
catalyst by ATR-IR spectroscopy. Horiuti-Polayni mechanism suggests that the hydrogenation of
2
α, β- unsaturated aldehyde involves three different adsorbed states, correspondingly di-σC = Oη ,
2
2
4
[52-55]
di-σC = Cη and di-πη (η ).
. Also, the metal d-band center plays a crucial role in
determining the product selectivity during the hydrogenation of α, β- unsaturated carbonyl
[56]
compounds. From Huckel calculations, it has already been established that the interaction
between the metal surface and the conjugated C=C bond increases relative to that with the C=O
bond with decreasing energy level of the metal d-band center, which is in the order: Pd < Pt < Ir
[
57,58]
≈
Os.
DFT calculations
through C=C bond with Pd (111) instead of C=O bond. These factors suggest that the C=C bond
Selectivity of the interaction between metals and C=C bond has been established by
[
59]
[60]
and HREELS study , which showed that maleic anhydride interacts
2
of CAL has more affinity towards hydrogenation by Pd metal-activated H than the C=O bond.
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ChemCatChem
10.1002/cctc.202000072
Figure 8: (a) In situ ATR spectra of CAL hydrogenation on Fe
b) DRIFTS spectra of CAL adsorption on Pd-Fe @TPT sample after drop of CAL at 60 C
and subsequently flushing with He for 60 min at 250 C, (c) DRIFTS spectra of CO adsorption
on Fe @TPT and Pd-Fe @TPT, (d) Plausible mechanistic pathway for the selective
hydrogenation of CAL to HCAL. Reaction conditions: Cinnamaldehyde (0.25 ml, 2 mmol), Pd-
Fe @TPT (30 mg), solvent (30 mL), 10 bar H pressure, 60°C; Time (1 h).
2 3 3 4
O @TPT and Pd-Fe O @TPT,
o
(
3 4
O
o
O
2 3
3 4
O
O
3 4
2
From the ATR analysis, characteristic peaks of liquid phase CAL on Pd-Fe
3 4
O @TPT are shown
-
in Figure 8a. Flowing hydrogen over Pd-Fe
3 4
O @TPT led to the appearance of a peak at 1722 cm
1
[61]
,
which could be attributed to the C=O vibration of hydrocinnamaldehyde. Further reaction in
a sealed H
2
atmosphere resulted in an increase in the intensities of this peak, accompanied by a
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ChemCatChem
10.1002/cctc.202000072
gradual disappearance of all the characteristic peaks of CAL. In contrast, the IR spectra of CAL
hydrogenation over Fe @TPT remained unchanged, manifesting the essential role of Pd in
activating H . The observations by in situ ATR reflected a clear reaction pathway, i.e. the
preferential hydrogenation of the C=C bond over the C=O bond over Pd-Fe @TPT. Initially,
CAL molecule is adsorbed on the Pd metal surface through the C=C bond. Simultaneously, H
O
2 3
2
3 4
O
2
molecules dissociatively adsorb onto the electronically promoted Pd NPs. Then, activated
hydrogen species attaches the adsorbed C=C bond of the, followed by desorption of the HCAL
product. Accordingly, Figure 8d presents the mechanistic pathway of selective hydrogenation of
CAL to HCAL over Pd-Fe
3 4
O @TPT. On the other hand, the presence of mesoporous channel on
the as-synthesized catalyst facilitated the rapid removal of hydrocinnamaldehyde from the active
[13]
sites, thus preventing the further hydrogenation of HCAL to PPL. The CAL adsorption
DRIFTS spectra in Figure 8b shows an obvious blue shift of peaks assigned to the vibrations of
-1
-1
C=O bonds, from 1669 cm to 1675cm over the Pd-Fe
3 4
O @TPT, which suggests that the
[62-64]
adsorption CAL on the surface of Pd-Fe @TPT resulting a shortening of the C=O bond.
3
O
4
-1
Therefore, the hydrogenation of C=O was hindered. In contrast, the peak at 1624 cm , assigned
to the C=C bonds, remained almost unchanged compared to the liquid phase standard. Figure 8c
2 3
reveals three DRFIT CO adsorption peaks over the Fe O @TPT sample, which could be
[
65]
attributed to the CO adsorption on Fe of different valences. Nonetheless, a sharp peak at 2042
-1
3 4
cm is observed over the Pd-Fe O @TPT catalyst; this peak is ascribed to the ontop CO
[
66]
adsorbed on Pd.
We have also screened the use of various solvents with hydrogen-bond-acceptance ability
HBA), whilst fixing the temperature at 60 °C. The progress of the catalytic reaction with CAL
(
conversion, along with the various products selectivity, are presented in Table S2. Isopropanol
was found to be the best solvent among all the solvents screened. However, the selectivity of
HCAL decreased and whilst that of PPL increased when MeOH was the solvent. However, high
selectivity toward HCAL was obtained in water, despite the low conversion of CAL after 1 h.
We subsequently conducted the chemoselective hydrogenation of cinnamaldehyde using
different catalysts, as listed in Table 1. We could observe that, in the absence of any catalytic
active metal sites, bare TPT appears to be completely inactive (Entry 1, Table 1). When the
reactions were conducted with monometallic Pd or Fe
3 4
O -NPs, poor catalytic activities have also
been encountered (Entry 2 & 3, Table 1). When the Pd-Fe
3 4
O
NPs were mechanically mixed
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ChemCatChem
10.1002/cctc.202000072
with TPT-POP, it displayed an almost similar CAL conversion to that of unsupported Pd NPs,
but despite a slight improvement (38%) (Entry 5, Table 1). The promoted performance could be
3 4
attributed to the presence of the Pd-Fe O interface, which modulates the electronic property of
metal sites, effectively enhancing the catalytic activity. However, no further improvement in the
conversion of CAL was noticed for the Pd@TPT catalyst, which preferred the over
hydrogenation product of 1-phenyl propanol (PPL) (Entry 7, Table 1). When the Fe
deposited on the TPT-POP support (Fe @TPT), both the conversion and selectivity for the
catalyst were low (Entry 6, Table 1). Xiao et al. also reported the inertness of bare
Fe & Fe @C towards the hydrogenation of cinnamaldehyde, in strong agreement with our
findings of Fe
compared to that on Pd-Fe
electronically enriched Pd NPs interact with the Fe oxide species, which promotes the splitting of
molecule to form non-polar H atoms over the Pd surface, thereby accelerating the catalytic
conversion. In a control experiment, we synthesized a Ni-Fe @TPT catalyst, which achieved
low cinnamaldehyde conversion (~5%) under optimized reaction conditions (Table 1, entry 8);
this observation further highlight the importance of the Pd-Fe interface. Based on our
systematic investigation, we can confidently conclude that the combination of TPT-POP support
with the Pd-Fe interface plays a decisive role in promoting the catalytic activity.
3 4
O was
3 4
O
O
3 4
3 4
O
[69]
3
O
4
@TPT. Moreover, the dissociative activation of H
2
on Fe
3 4
O was sluggish
3
O
4
, resulting in the poor catalytic activity of Fe O
3 4
@TPT. In contrast,
H
2
3 4
O
3 4
O
3 4
O
Table 1: Catalytic activity of various catalysts for chemoselective Hydrogenation of
Cinnamaldehdye (CAL)
Con of CAL Sel of HCAL Selectivity of Selectivity of
Entry
Catalyst Used
(%)
(%)
PPL (%)
COL (%)
1
2
3
TPT
0
0
0
0
Pd-NPs
26
15
20
10
6
2
0
3
3 4
Fe O -NPs
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ChemCatChem
10.1002/cctc.202000072
4
Pd-Fe
Pd-Fe
3
O
4
@TPT
100
38
100
36
0
2
0
0
5
3 4
O
@TPT
(
Physical Mixture)
Fe @TPT
Pd@TPT
6
7
8
O
3 4
10
72
5
10
65
0
7
0
0
0
0
0
0
Ni-Fe
3
O
4
@TPT
100
100
9*
Pd-Fe
3
O
4
@TPT
40
Reaction conditions: Cinnamaldehyde (0.25 ml, 2 mmol), Catalyst (30 mg), isopropanol solvent
(
(
30 mL), 10 bar H
25 ºC).
2
pressure, 60 °C; Time (1 h). *Reaction was carried out at room temperature
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ChemCatChem
10.1002/cctc.202000072
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10.1002/cctc.202000072
Figure 9:Evolution of reactant and product distribution against time of different supports Pd-
Fe @SiO (a), Pd-Fe @C (b), bare Pd-Fe -NPs(c), respectively. Reaction Conditions:
O
3 4
2
O
3 4
3 4
O
Cinnamaldehyde (0.25ml, 2 mmol), 60°C, 30 ml of isopropanol.
To demonstrate the importance of the TPT-POP support, a comparative study was performed
3 4 2 3 4
using other synthetic catalysts including Pd-Fe O @SiO and Pd-Fe O @C. All catalysts were
prepared following similar synthesis procedures. The evolutions of reactant and product
distributions over time for the three catalysts tested are plotted Figure 9. It can be clearly
observed that the CAL to HCAL conversion become sluggish in comparison to the Pd-
Fe
3 4
O @TPT-POP catalyst. The high catalytic activity of Pd-Fe
3 4 3 4 2
O @TPT and Pd-Fe O @SiO
compared to the Pd-Fe
3 4
O @C could be attributed to the presence of surface -OH groups, which
binds strongly to the hydrophilic COL, thereby hindering its desorption and promoting the
selectivity towards HCAL. The presence of porous channels leads to the faster diffusion of CAL
and enables the easy access of the reactants to the active Pd sites to furnish the desired products.
We also note that Pd-Fe
et.al have developed a magnetically responsive core-shell Pd/Fe
the hydrogenation of cinnamaldehyde. Their Pd/Fe @C catalyst achieved 70% selectivity to
hydrocinnamaldehyde under 14 bar pressure at 80 °C after 10 h reaction time. In comparison,
Pd-Fe @TPT-POP attained hydrocinnamaldehyde with 100% selectivity at 60 ºC after only 1
3
O
4
@TPT appears to be far more active than Pd-Fe
3 4 2
O @SiO . Xiao
3 4
O @C composite catalyst for
3 4
O
[
71]
O
3 4
h reaction time, demonstrating the superiority of our catalyst design strategy. Based on the
results discussed above, we can rationalize that, the - interactions between the aromatic
organic substrate and support matrix, as well as the large surface area favoring the hydrogenation
of the intermediate to the desired product with fast and easy diffusion, facilitates the efficient
[70]
chemical conversion.
To verify the function of TPT-POP in promoting the selectivity during the Pd-catalyzed
cinnamaldehyde hydrogenation, we performed Density Functional Theory (DFT) calculations to
computationally examine the interaction between a monomer of the POP, triphenylamine (TPA),
with the Pd cluster. All first-principle calculations were performed using periodic boundary
conditions and plane-wave pseudopotential implemented in the Vienna ab-initio simulation
[
72,73]
package (VASP), developed at the FakultätfürPhysik of the Universität Wien.
Details of the
calculations are described in the Supporting Information. We chose the optB88-vdW functional
[
74,77]
developed by Klimešet al.
for the DFT calculations since it was reported to be the best level
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ChemCatChem
10.1002/cctc.202000072
of theory describing correctly the interaction between aromatic compounds with the surface of
[78,81]
transition metals.
The model of Pd332 cluster was used to represent the Pd NP with a size of
which was consistent with the average Pd cluster size of ca. 3 nm, estimated based on
a Pd-Pd coordination number of 10.2, obtained by fitting the Pd K-edge XAS spectra of the Pd-
Fe @TPT sample, as shown in Figure 6.
[
82,80]
3
nm,
O
3 4
According to DFT calculations, three different binding configurations of TPA monomer with the
Pd332 cluster are shown in Figure 10. The adsorption of TPA molecule on the flat terrace sites via
the coordination of only one aromatic ring (Fig. 10a) has an strong adsorption energy of -189
kJ/mol; this value is in good agreement with earlier experimental and theoretical results reported
in the literature for the adsorption on benzene on Pd(111) surface and demonstrates the very
[
79,83]
strong interaction between aromatic compound with transition metal surfaces.
When the
adsorption occurs by the coordination of two aromatic rings of TPA at the step edge (Fig. 9b),
the binding energies become significantly stronger by ~75 kJ/mol. The computed adsorption
energy attains a maximum at 301 kJ/mol when two aromatic rings of TPA interact with Pd
cluster at the corner site (Fig. 9c). The strong interaction of Pd cluster with the TPA compound at
the step-edge and corner binding modes could be a factor leading to the high density of step sites
on the fabricated Pd NPs. Due to the tilted structure of the TPA monomer (three aromatic rings
are not planar and form corrugated structure around N atom) and the very strong binding to the
Pd facets, the as-synthesized Pd NPs could be nucleated and grown along those corrugated sites
between the tilted phenyl rings and finally form NP structures with high density of step sites.
Consequently, the presence of the step sites could be an important factor contributing to the high
selectivity of the Pd-Fe
3
O @TPTto the cinnamaldehyde hydrogenation.
4
Indeed, the enhancement in the selectivity towards C=C hydrogenation in cinamaldehyde and in
α, β-unsaturated aldehydes on steps sites and lower coordinated sites has been reported in
[
84-86]
literature.
While the hydrogenation of C=O bond is structure-insensitive, the hydrogenation
of C=C bond was reported to be structure-dependent and promoted significantly at the step
[
87,84]
[88]
sites.
Serrano-Ruiz et al.
achieved a hydrocinnamaldehyde selectivity of 82% for
cinamaldehyde hydrogenation on 3 nm Pt catalyst, which possesses high ratio of corner & edges
atoms to terrace atoms. In contrast, 10 nm Pt catalysts with much lower density of step and edge
[
89]
sites gives a HCAL selectivity of only 49%. Zhang et al. also reported great enhancement in
the hydrogenation of cinamaldehyde towards hydrocinnamaldehyde on Pd cluster nanowire
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ChemCatChem
10.1002/cctc.202000072
catalyst with the enrichment of high index facets and step sites on the surface, whereas the
activity and selectivity of cinamaldehyde hydrogenation on commercial Pd/C and Pd/γ-Al
2
O
3
catalysts were more than one order of magnitude lower due to the lack of those active sites.
Furthermore, in the hydrogenation of 3-methylcrotonaldehyde, the Pd(533) stepped surface was
[
90]
shown to be twice more active than the terrace Pd(111) plane in the study of Birchem et al.
Also in that study, when the step sites on Pd(533) stepped surface were blocked by a sub-
monolayer of pre-adsorbed Sn, the selectivity towards the C=C bond hydrogenation was
drastically reduced. The aforementioned studies emphasize the higher activity and selectivity of
the step sites towards C=C bond hydrogenation to saturated aldehydes. Since the TPT polymer
frame structure could act as the structural template for growing Pd NP with high density of step
and corner sites, it could be responsible for the high HCAL selectivity during cinamaldehyde
hydrogenation on our fabricated Pd-Fe
3
O @TPT catalyst.
4
In addition to the structural factors discussed above, DFT calculations also revealed charge
transfer between the TPA and the Pd332 cluster. From the Bader charge calculations, it is shown
that there is a net transfer of 0.416 electron from the TPA monomer to the Pd332 cluster in their
most stable interaction configuration, i.e. at corner site (Fig.10c). The charge transfer of electron
from TPA to the Pd cluster induces the negative charge on the surface of the Pd cluster, which is
also consistent with the observed negative shift of ~0.3 eV in the Pd 3d5/2 XPS spectra of Pd-
3 4
Fe O @TPT (Fig.5c). As reported by recent studies, the C=C bond hydrogenation is very
sensitive to the charge transfer. Accordingly, the charge transfer from the support to the metal
cluster, imposing negative charges on the surface metal atoms, was found to dramatically
[
91]
increase the C=C bond hydrogenation activity. This finding provides additional evidence that
the modification of electronic properties induced by the TPT-POP promotes the selectivity
towards hydrocinnamaldehyde formation during the cinamaldehyde hydrogenation on Pd-
Fe
3
O @TPT.
4
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