Understanding the roles of variable Pd(II)/Pd(0) ratio supported on conjugated poly-azobenzene network: From characteristic alteration in properties to their cooperation towards visible-light-induced selective hydrogenation

Article abstract of DOI:10.1016/j.jcat.2020.03.014

Selective hydrogenation of organic functionalities at environmentally benign conditions using visible light is of great industrial and economic significance. Herein we report visible-light-induced rapid, almost quantitative and selective hydrogenation of olefins to respective mono-reduced products using cooperative performance of Pd(0) nanoparticles (NPs) and Pd(II) ions evenly distributed on a newly synthesized conjugated mesoporous poly-azobenzene network. Role of variable Pd(0)/Pd(II) ratio on the properties of polymeric networks and their overall catalytic abilities is critically investigated. This is the first proposed example of cooperative hydrogenation by simultaneous activation of H₂ and unsaturated substrates using Mott-Schottky heterojunction between Pd NPs and the semiconducting polymer, with the help of Pd(II)-site-mediated η-coordination. A control over selective mono-reduction of diene with identical double bonds was also obtained. The catalytic activity retained for other non-olefinic functionalities as well.

Full text of DOI:10.1016/j.jcat.2020.03.014

Journal of Catalysis 385 (2020) 120–128
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Understanding the roles of variable Pd(II)/Pd(0) ratio supported on
conjugated poly-azobenzene network: From characteristic alteration in
properties to their cooperation towards visible-light-induced selective
hydrogenation
b
a
Ipsita Nath a,b, Jeet Chakraborty a,b, , Gaoke Zhang , Cheng Chen , Somboon Chaemchuen a,c, Jihae Park d,
⇑
Serge Zhuiykov d, Taejun Han d, Francis Verpoort a,c,d,
⇑
a Laboratory of Organometallics, Catalysis and Ordered Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of
Technology, Wuhan 430070, China
b School of Resources and Environmental Engineering, Wuhan University of Technology, Wuhan 430070, China
c National Research Tomsk Polytechnic University, Lenin Avenue 30, Tomsk 634050, Russia
d Ghent University Global Campus, 119 Songdomunhwa-Ro, Yeonsu-Gu, Songdo, Incheon 406-840, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Selective hydrogenation of organic functionalities at environmentally benign conditions using visible
light is of great industrial and economic significance. Herein we report visible-light-induced rapid, almost
quantitative and selective hydrogenation of olefins to respective mono-reduced products using coopera-
tive performance of Pd(0) nanoparticles (NPs) and Pd(II) ions evenly distributed on a newly synthesized
conjugated mesoporous poly-azobenzene network. Role of variable Pd(0)/Pd(II) ratio on the properties of
polymeric networks and their overall catalytic abilities is critically investigated. This is the first proposed
example of cooperative hydrogenation by simultaneous activation of H2 and unsaturated substrates using
Mott-Schottky heterojunction between Pd NPs and the semiconducting polymer, with the help of Pd(II)-
Received 26 December 2019
Revised 19 February 2020
Accepted 15 March 2020
Available online 1 April 2020
Keywords:
Azobenzene
Porous organic polymer
Selective hydrogenation
Visible light
site-mediated
g-coordination. A control over selective mono-reduction of diene with identical double
bonds was also obtained. The catalytic activity retained for other non-olefinic functionalities as well.
Ó 2020 Elsevier Inc. All rights reserved.
Cooperative mechanism
Mott-Schottky interface
g
-coordination
1. Introduction
a significant source of environmental pollution [8], photochemical
reactions are practiced with a final goal of harvesting sunlight as an
Hydrogenation is a commonly employed reaction in synthetic
chemistry lab and of great industrial relevance [1,2]. Particularly,
selective hydrogenation of olefins in the presence of multiple redu-
cible functionalities is of extreme interest [3–5]. An in situ discrim-
ination between two or more olefin units present in the same
substrate is even more challenging, and such examples are, to date,
extremely rare [6,7]. Since most of the conventional hydrogenation
reactions require harsh conditions including high pressure and
temperature, photocatalytic protocols are gaining certain atten-
tion. As high energy consumption by chemical industries remain
alternative sustainable source of energy and converting it directly
to photo-energy [9]. Also, photochemistry often allows the attain-
ment of products which are difficult to access by conventional
methods [10].
Infinitely conjugated porous organic polymers (POPs) have
recently found surging applications in versatile photocatalytic
reactions [11–13]. The suitably oriented yet tailorable valance
band (VB) and conduction band (CB) potentials of these materials
open up routes for unique reaction mechanisms. Pd NPs have been
loaded onto POP surface lately to generate Mott-Schottky hetero-
junction at the noble metal-semiconductor interface [14,51].
Utilizing the photo-amplified electron transfer from the CB of POPs
to the metal NP fermi level, classic CAC cross-coupling reaction has
been performed. By drawing a chemical analogy to the surface
plasmon resonance (SPR) exhibited by coinage metal NPs [15–
22], we envisioned that the Schottky effect of POP-Pd NPs can, in
principle, be used to activate the unsaturated substrate molecules
⇑
Corresponding authors at: Laboratory of Organometallics, Catalysis and Ordered
Materials, State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Center for Chemical and Material Engineering, Wuhan University of
Technology, Wuhan 430070, China.
E-mail addresses: jeet.chakraborty@hotmail.com (J. Chakraborty), francis.
verpoort@ghent.ac.kr (F. Verpoort).
https://doi.org/10.1016/j.jcat.2020.03.014
0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

I. Nath et al. / Journal of Catalysis 385 (2020) 120–128
121
too, to ease the following olefin hydrogenation process, while the
VB of the polymer can activate the H2 molecules. Furthermore,
the presence of Pd(II) was anticipated to benefit the reactions in
a cooperative manner by stimulating the substrates through
water could serve as gentle reducing agent gradually converting
strongly polymer-coordinated Pd(II) to Pd(0), and by simply
adjusting the boiling time, different Pd(0)/Pd(II) ratio can be
obtained.
in situ
With this idea, we herein report the synthesis of a novel
azobenzene-based POP, B3-Azo2, and series of its post-
synthetically Pd-incorporated analogs. Owing to the near UV-
active absorption band of azobenzene, its successful incorporation
g
-coordination.
Accordingly, fixing the boiling time to 45 min lead to the for-
mation of Pd-B3-Azo2 [Pd(II):Pd(0) = 85:15]. XPS analysis, per-
formed to validate our postulate, confirmed the existence of
both Pd(0) to Pd(II) species (Fig. 1b). The Pd3d5/2 peaks for Pd
(0) and Pd(II) appeared at 335.5 and 337.1 eV respectively, while
that for Pd3d3/2 emerged at 340.9 and 342.5 eV (Fig. 1b). A 15:85
distribution ratio of Pd(0) and Pd(II) was calculated from peak
integrations. Moreover, a noticeable 1.3 eV negative shift of the
Pd(II)3d5/2 binding energy peak compared to free Pd(OAc)2
(338.4 eV) indicated strong coordination of Pd(II) with the poly-
mer, making it more electron-rich [25]. The N1s spectrum of
the materials before and after Pd-incorporation exhibiting ca.
30% decrease in the N-C peak area further endorsed this Pd-
polymer coordination. The existence of acetate counter anion
for Pd(II) cations originating from Pd(OAc)2 can also be observed
from the O-C-O peak at 288.7 eV in C1s spectral region, while
peaks for conjugated C = C and C-N appeared at 284.8 and
286.4 eV respectively.
With these results in hand, consequent adjustments of boiling
time for B3-Azo2 À Pd(OAc)2 mixture in water to 1.5 h and 4 h
resulted in Pd-B3-Azo2 [Pd(II):Pd(0) = 80:20] and Pd-B3-Azo2 [Pd(
II):Pd(0) = 50:50] respectively, both possessing Pd(0) and Pd(II)
mix-distributions (Fig. 1b). Pd(0)/Pd(II) ratios of 20:80 and 50:50
were found for the second and third materials respectively from
similar XPS peak area calculations. Boiling the mixture for 8 h
led to the synthesis of Pd-B3-Azo2 [Pd(II):Pd(0) = 25:75] with
75:25 Pd(0)/Pd(II) ratio. On the other hand, Pd-B3-Azo2 [Pd(II):Pd
(0) = 0:100] featuring completely reduced Pd-species was obtained
after 12 h boiling of B3-Azo2 À Pd(OAc)2 mixture, whereas Pd-B3-
Azo2 [Pd(II):Pd(0) = 100:0] containing only Pd(II) cations was syn-
thesized by stirring the polymer À Pd(OAc)2 mixture at room tem-
perature without boiling. The Pd-oxidation states of each of these
materials were confirmed from the peak positions of respective
XPS analysis (Table S1, Fig. 1b, Figs. S2–S7). Similar to Pd-B3-Azo2
[Pd(II):Pd(0) = 85:15], strong coordination of Pd with polymeric
backbone can be confirmed from Pd3d peak positions of Pd-B3-
Azo2 [Pd(II):Pd(0) = 80:20], -[Pd(II):Pd(0) = 50:50], -[Pd(II):Pd(0) =
25:75], and -[Pd(II):Pd(0) = 100:0] relative to free Pd(OAc)2
(Table S1). Moreover, a comparative diagram of N1s peaks for the
pristine polymer and its Pd-incorporated analogs demonstrates
that with a gradual decrease in Pd(II) content of the materials,
the N-C peak area gradually intensified, suggesting increased elec-
tron density on N centers (Fig. 1c). This observation is in accor-
dance with our expectation, as more Pd(II) are converted to Pd
(0), the azo-N atoms shifts back to their original ‘‘non-
coordinated” form reinstating the pristine N1s spectral pattern.
Similarly, the existence of O-C-O units for every Pd(II)-containing
composites can also be seen in the comparative C1s spectral region
of the XPS (Fig. 1d). These observations all together confirmed that
by simply adjusting reaction time, we were able to selectively
reduce polymer-coordinated Pd(II) to Pd(0).
a
into conjugated POP skeleton was anticipated to increase overall p-
conjugation affording a higher visible light absorption cross-
section of the material. Moreover, we have shown recently that
the electronic state potentials of POPs can be manipulated for bet-
ter photo-catalysis by incorporating azobenzene units in the skele-
ton [23,37]. Keeping the total amount of metal same, different ratio
of Pd(0) and Pd(II) were immobilized on B3-Azo2 surface to assess
the impact of metal oxidation state variations on physical, photo-
electrochemical and overall catalytic properties of the materials.
Interestingly, despite having similar physical properties, compos-
ites bearing different Pd(0)/Pd(II) ratio exhibited significantly
altered photo-absorption and HOMO-LUMO potentials. This, quite
evidently, resulted in considerably discrete photocatalytic activity
and apparent quantum efficiency (AQE) of these materials. Among
these composites, 50:50 Pd(0)/Pd(II) loading on B3-Azo2 showed
the optimum activity for exceptionally rapid, selective and cooper-
ative visible-light-mediated hydrogenation of unsaturated organic
functionalities. Conventional thermal methods were tested with
our catalyst as well, for providing
overview.
a thorough comparative
2. Result and discussion
2.1. Synthesis of B3-Azo2
The polymeric network B3-Azo2 (Fig. 1a) was synthesized by
Sonogashira-Hagihara cross-coupling of 1,3,5-triethynyl benzene
and (E)-1,2-bis(4-iodophenyl)diazene (Scheme S1) following a
classic high dilution technique. We have previously confirmed that
adaptation of such synthetic protocol could lead to a less interpen-
etrated, low-density network by minimizing the chances of kinet-
ically controlled prompt coupling of macromolecular polymeric
intermediates [23]. Accordingly, the desired polymer was obtained
as dark brown powder insoluble in common solvents. Successful
generation of B3-Azo2 skeleton was confirmed by solid-state 13C
CP-MAS NMR spectra demonstrating typical chemical shifts at
153.79 and 93.01 ppm, corresponding to the carbon bound to the
azo-nitrogens and the ethynyl carbons, respectively (Fig. S1). Exis-
tence of other aromatic carbon atoms can also be seen from the fig-
ure. The disappearance of residual peaks at 80–82 ppm region
supports the absence of unreacted ethynyl ends, i.e. a high degree
of cross-coupling.
2.2. Synthesis of Pd-incorporated B3-Azo2
Pd was loaded in B3-Azo2 following an aptly modified earlier-
known water-acetone dual solvent technique using Pd(OAc)2 as
the metal source [24]. The dual solvent approach was found bene-
ficial for better diffusion of metal ions throughout the organic
skeleton of polymers. Given the well-established transition metal
ion complexation ability of azo-benzenes, we envisioned that sim-
ple stirring of B3-Azo2 À Pd(OAc)2 mixture in said solvent for cer-
As an additional proof to our claim of generating B3-Azo2 net-
works bearing different Pd(0)/Pd(II) ratio, XRD analysis of all mate-
rials were performed. Despite the amorphous nature of the
polymeric skeleton, presence of Pd(0) can be clearly confirmed
from the appearance of two characteristic peaks at 2h = 40° and
46° originating from the x-ray diffraction by 111 and 200 planes
of Pd(0) nano-crystals [26,27]. No such peaks were observed for
pure B3-Azo2 and Pd-B3-Azo2 [Pd(II):Pd(0) = 100:0] verifying the
non-existence of reduced Pd-species in those materials, while
those peak intensities steadily increase with increasing Pd(0) con-
tent for other four materials (Figs. 2a and S57).
tain time would result in Pd(II)-loaded B3-Azo2 network.
A
follow-up heating to ~95 °C at N2-atmosphere with constant gas
flow could force the acetone present in the solvent mixture to leave
while bringing the water to boil. It was proposed that the boiling

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I. Nath et al. / Journal of Catalysis 385 (2020) 120–128
Fig. 1. (a) Chemical structure of B3-Azo2; (b) Pd3d XPS patterns of Pd-B3-Azo2 [Pd(II):Pd(0)] composites with gradual increase in Pd(0) content (top to bottom); (c) N1s XPS
patterns of B3-Azo2 (top) and Pd-B3-Azo2 [Pd(II):Pd(0)] composites with gradual increase in Pd(0) content (top to bottom); and (d) C1s XPS patterns of B3-Azo2 (top) and Pd-
B3-Azo2 [Pd(II):Pd(0)] composites with gradual increase in Pd(0) content (top to bottom).
Fig. 2. (a) XRD patterns of B3-Azo2 (top) and Pd-B3-Azo2 [Pd(II):Pd(0)] composites with gradual increase in Pd(0) content (top to bottom); and (b) HRTEM image of Pd NP
present in Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50].
Particular attention was given to keep the total Pd-content sim-
ilar for all six Pd-loaded materials by using the same amount of Pd
(OAc)2 and B3-Azo2 in each case, for obtaining a comparative
assessment on the role of Pd(0)/Pd(II) ratio on the physical and
photochemical properties of materials. As a result of this cautious
effort, ICP analysis confirmed ca. 2.1 ( 0.2) wt% Pd-loading in all
materials (Table S4). On the other hand, FTIR analysis of all six net-
works showed similar spectral patterns inferring the retention of
the structural integrity of the polymeric backbone before and after
Pd-loading (Figs. S8–S13). Characteristic peaks for C„C stretching
and aromatic CAH stretchings can be observed at 2300–2400 cmÀ1
and 2900–3000 cmÀ1 respectively in every composites. However,
Pd(II)-containing POPs demonstrated additional FT-IR peak at
~1600 cmÀ1 arising from the C@O stretching of acetate counter
anions, while absence of any such peak can be clearly seen in par-
ent B3-Azo2 and Pd-B3-Azo2 [Pd(II):Pd(0) = 0:100] (Fig. S8).
2.3. Morphology of the materials
Identical stacked sheet type morphology of all six composites
can be seen from the FESEM images (Figs. S28–S34). TEM analyses
further confirmed this sheet-like appearance of B3-Azo2 as well as
demonstrated apparent homogeneous distributions of the metal
NPs throughout amorphous polymeric bodies for all six Pd-

I. Nath et al. / Journal of Catalysis 385 (2020) 120–128
123
incorporated composites (Figs. S35–S40). Moreover, the lattice
planes observed in HRTEM images of Pd(0)-containing networks
provided additional confirmations of the presence of crystalline
Pd NPs (Figs. 2b and S41–S43).
in absorption maxima manifested by the Pd-loaded materials with
increasing Pd(0)-contents ultimately led to kmax at 430 nm for Pd-
B3-Azo2 [Pd(II):Pd(0) = 0:100].
Inspired from the eccentric photo-absorptivity, valance band
(VB) and conduction band (CB) potentials of the materials were
measured using cyclic voltammogram (vs SCE) to comprehensibly
understand their electrochemical properties (Figs. S44–S56,
Table S3). As depicted in Fig. 3b, compared to 1.77 V and À0.67 V
respective VB and CB potentials of the parent polymer, Pd-loaded
networks showed a successive increase in VB and lowering in CB
potentials. Starting from Pd-B3-Azo2 [Pd(II):Pd(0) = 100:0], this
trend continued till Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] featuring
VB and CB of 1.85 V and À0.84 V respectively, whereas Pd-B3-
Azo2 [Pd(II):Pd(0) = 0:100] possessing purely Pd(0) NPs though
manifested similar CB potential, its VB decreased abruptly to
1.70 V. Accordingly Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] showed a
bandgap of 2.69 eV, the maximum in the Pd-POP series, and Pd-
B3-Azo2 [Pd(II):Pd(0) = 0:100] featured the lowest bandgap of
2.55 eV with respect to 2.44 eV energy gap of the original organic
network B3-Azo2.
2.4. Surface area and porosity pattern of the materials
738 m2gÀ1 Brunauer–Emmett–Teller (BET) surface area and
0.59 cm3gÀ1 total pore volume of B3-Azo2 was obtained from
N2-sorption isotherm at 77 K. A micropore contribution of only
25% of total pore volume was observed indicating an extremely
mesoporous distribution of the network. The wide hysteresis in
the N2-sorption plot (Fig. S14), and a very dense porosity popula-
tion till 5 nm pore diameter followed by a gradual decrease in
the distribution calculated by non-local density functional theory
(NLDFT), further confirmed markedly populated mesopores in
B3-Azo2 (Fig. S15). As the micropores are often too small to allow
an efficient mass transfer, a hierarchical skeleton with substantial
mesopores is generally considered a better choice for higher
catalytic efficiency [28]. The porosity pattern retained after Pd-
incorporation as well, with a significant mesoporous contribution.
Moreover, about 70–110 m2 gÀ1 decreases in BET surface area was
observed after Pd-loading in all six materials maintaining compa-
rable N2-sorption patterns to the pristine POP (Figs. S16–S27,
Table S2).
2.6. Catalytic applications of Pd-B3-Azo2
Detailed characterizations of B3-Azo2 and its all six Pd-
incorporated analogs led us to the realization that post-synthetic
metal loading did not trigger any change in structural backbone
or morphology of the parent organic network. Despite an obvious
drop in BET surface area and overall porosity compared to B3-
Azo2 up to a nominal extent, all the Pd-bearing materials possessed
mutually parallel surface properties with retention of the parent
polymeric porosity pattern. The change in Pd-oxidation states,
however, significantly altered their photophysical and electro-
chemical properties from each other which was anticipated to
affect their photocatalytic abilities as well. To confirm this postu-
late, the six Pd-loaded POPs were subjected to catalyze photo-
mediated hydrogenation of styrene as a model reaction. Methanol
was chosen as the solvent with an idea that the organic environ-
ment provided by our catalyst can solvophobically attract the
hydrophobic olefins from the polar reaction medium [29]. Addi-
tionally, the high mesoporosity of the polymers can accommodate
more substrates at a time, offering a relatively faster reaction rate.
Accordingly, photo-illuminated Pd-B3-Azo2 [Pd(II):Pd(0) = 50:5
0] À styrene mixture having a 1:500 catalyst/styrene ratio afforded
quantitative conversion after 75 min. Conversely, when Pd-B3-Azo2
[Pd(II):Pd(0) = 0:100] and Pd-B3-Azo2 [Pd(II):Pd(0) = 100:0] were
2.5. Photophysical and electrochemical properties
Effect of Pd-incorporation in the POP backbone and the roles
played by different Pd(II)/Pd(0)-ratio in shaping up overall photo-
physical and electrochemical properties of the materials were then
investigated. Accordingly, UV–vis absorption spectra of the Azo2-
monomer and all six polymeric networks were initially analyzed.
Compared to the monomer possessing an absorption maximum
at 347 nm, the organic polymer B3-Azo2 demonstrated a 44 nm
red shift in kmax (Fig. 3a). Such an enormous bathochromic shift
can be ascribed to the significant increase in overall p–conjugation
after efficient polymerization, as anticipated earlier from solid-
state NMR spectroscopy. Interestingly, further Pd-incorporation
in B3-Azo2 resulted in successive red shift in kmax with a sequential
change in the photo-absorption pattern of the materials. As
depicted in Fig. 3a, with increase in Pd(0)-contents, a definite alter-
ation in the photo-absorption pattern was observed compared to
pristine B3-Azo2, with highly increased vis-near IR total absorption
cross-section of the materials. The step-wise bathochromic shifts
Fig. 3. (a) UV–Vis absorption spectra of the materials, colour scheme: Azo2-monomer, purple; B3-Azo2 POP, blue; Pd-B3-Azo2 [Pd(II):Pd(0) = 100:0], cyan; Pd-B3-Azo2 [Pd(II):
Pd(0) = 85:15], green; Pd-B3-Azo2 [Pd(II):Pd(0) = 80:20], yellow; Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50], orange; Pd-B3-Azo2 [Pd(II):Pd(0) = 25:75], wine; Pd-B3-Azo2 [Pd(II):Pd
(0) = 0:100], red; and (b) schematic representation of VB-CB potentials and energy gaps of the materials.

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I. Nath et al. / Journal of Catalysis 385 (2020) 120–128
subjected to identical reaction condition maintaining the same cat-
alyst/styrene ratio, they gave only 24% and 23% conversions
respectively. Pd-B3-Azo2 [Pd(II):Pd(0) = 85:15], Pd-B3-Azo2 [Pd(I
I):Pd(0) = 80:20] and Pd-B3-Azo2 [Pd(II):Pd(0) = 25:75] demon-
strated lower catalytic activity than Pd-B3-Azo2 [Pd(II):Pd(0) = 50
:50] as well with 44, 46, and 62% respective conversions in due
time. The results depicted in Fig. 4 shows that despite having sim-
ilar total Pd-content, gradual increase in Pd(0) concentration i.e.
decrease in Pd(II):Pd(0) ratio led to enhanced substrate conversion
till a certain point, while neither of Pd(II)-POP or Pd(0)-POP was
able to manifest identical reactivity.
Intrigued by this catalytic behaviour, apparent quantum effi-
ciency (AQE) of the six materials were tested against the same
reaction to propose their catalytic activity trend as per IUPAC rec-
ommendation. An identical photocatalytic setup using 420 nm
bandpass filter (k0 = 20 nm) resulted in a maximum AQE of
22.1% for Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50]. Compared to this, the
other materials manifested AQEs ranging from ca. 5–14% (Fig. 4).
Moreover, the AQE values for the catalysts were found to follow
a superficially identical trend as their overall catalytic efficiencies
inferring that Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] is the best per-
former in the batch.
Inspired from this result, Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] was
further employed for photocatalytic hydrogenation of other olefins.
Conventional thermally-induced hydrogenations were targeted as
well to provide a comparative overview of the catalytic propensity.
The results of these catalytic performances of Pd-B3-Azo2 [Pd(II):Pd
(0) = 50:50] are shown in Table 1 and Fig. 5. It is evident from the
conversion profiles that the alkenes showed faster photochemical
conversion compared to the thermal method, while the effect is
reversed for alkynes (Fig. 5a, b). Interestingly, despite the faster-
reducing ability of alkenes, the catalyst mono-hydrogenated the
alkynes selectively and almost quantitatively. A more direct com-
parison can be drawn from the observation that regardless of
almost 3 times higher TOF for hydrogenation of styrene than
phenylacetylene, the later reaction proceeded with utter prefer-
ence (Fig. 5c). A similar trend can be observed for hydrogenation
of cyclopentyl acetylene as well (Fig. 5e).
When norbornadiene bearing two identical C@C double bonds
was subjected to standard catalytic protocol, a very rapid 84% con-
version was achieved within just 15 min under photo-induced con-
dition with 98% selectivity towards mono-reduced norbornene
(Fig. 5d). Compared to the thermal reduction affording far inferior
product selectivity, an unprecedented 2.41 times higher TOF was
recorded for selective photocatalytic mono-reduction of the diene.
In general, step-by-step quantitative reductions were perceived for
all multi-reducible olefins with precise time-dependent control
over product selectivity.
The credibility of the catalytic protocol was tested for relatively
difficult structurally bulky substrates as well. As indicated in Fig. 5-
c-d and Table 1 (Entry 6–8), the catalyst successfully reduced 1-
heptene, cis-cyclooctene, and a-methyl styrene with good to excel-
lent photochemical and thermal conversions. The photocatalytic
protocols remained more superior compared to the thermal ones.
The highly mesoporous nature of the polymer network can be
attributed to its efficient catalytic activity towards sterically con-
gested substrates.
Fig. 4. Photocatalytic conversion of styrene to ethylbenzene with different Pd-B3-
Azo2 [Pd(II):Pd(0)] materials after 75 min Average value for 3 sets of reactions are
given. B3-Azo2 network have been resynthesized and loaded with Pd to produce
fresh Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50], which was used for testing uncertainty in
conversion.
Table 1
Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] catalyzed photo-mediated conversion of the substrates, selectivity of product, reaction time and TOF.a
Entry
1
Substrate
Product
Time (min)
75 (160)
Conv. (%)
100 (100)
Select. (%)
100 (100)
TOFb (hÀ1
)
400 (187.5)
490 (500)
2
60 (60)
98 (100)
100 (100)
3
4
5
190 (150)
180 (60)
30 (60)
89 (94)
99 (100)
90 (98)
98 (97)
98 (90)
96 (73)
137.7 (182.3)
161.7 (450)
864 (357.7)
6
7
600 (600)
90 (180)
56 (39)
98 (94)
100 (100)
100 (100)
28 (19.5)
356.7 (156.7)
8
120 (180)
95 (88)
100 (100)
237.5 (146.7)
9
30
100
100
96
100
100
88
1000
125
10c
240
300
11
84.5
a
1:500 catalyst/substrate ratio and 1 atm H2. Reactions were performed in methanol using 420 nm cut-on filter at room temperature with a stirring speed of 300 rpm. The
data for conventional conditions performed at 35 °C are given in parenthesis. Subsequently, hydrogenated by-products were obtained for entry 3–5 in minor amounts. 1-
phenylethanol was obtained as by-product for entry 8.
b
TOF was calculated as mmol of product formed per mmol of the catalyst at unit time under visible light irradiance.
0.05 mol% potassium tert-butoxide was used.
c

I. Nath et al. / Journal of Catalysis 385 (2020) 120–128
125
Fig. 5. Conversion profile of the olefins over time under (a) and (c) photocatalytic, (b) and (d) thermal condition; change in product selectivity over time for (e)
phenylacetylene, (f) norbornadiene, and (g) cyclopentyl acetylene hydrogenation under photocatalytic and thermal conditions; (h) change in product selectivity over time for
styrene oxide under photocatalytic condition.
The substrate scopes of Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] tested
for other reducible non-olefinic functionalities (Table 1, Entry 9–
11) also showed similar photoactivity and product selectivity.
Accordingly, benzaldehyde and nitrobenzene were reduced to ben-
zyl alcohol and aniline respectively. Moreover, an impressive
regio-selectivity was observed for styrene oxide hydrogenation
towards 2-phenylethanol, which gradually increased by 13% from
its initial value over time (Fig. 5f).
2.7. Mechanism
Control experiments were performed to understand the mech-
anism of the reactions (Table S5). Using hydrogenation of styrene
as the model reaction, only 12% conversion was achieved in dark
at room temperature indicating the necessity of visible light for
the reaction to progress. On the other hand, the reaction did not
proceed at all in the absence of H2 gas. Though under certain pho-
tocatalytic conditions methanol oxidizes to produce H2, this result
suggests that under our catalytic condition the substrates under-
went hydrogenation, not hydrogen transfer from methanol. In
the presence of a proton source without H2 gas, only a trace
amoumt of product was obtained. B3-Azo2 alone without loaded
Pd failed to show any substrate conversion, whereas commercial
Pd/C produced only slight ethylbenzene within 1 h under light irra-
diation. Moreover, a 1:1 physical mixture of Pd-B3-Azo2 [Pd(II):Pd
(0) = 0:100] and Pd-B3-Azo2 [Pd(II):Pd(0) = 100:0] resulted in only
Fig. 6. Schematic illustration of the proposed photocatalytic hydrogenation
18% product in due time. A 1:1 M mixture of Pd-B3-Azo2 [Pd(II):Pd
(0) = 0:100] and Pd(OAc)2 generated 21% product in due time.
These results collectively indicate that the simultaneous presence
of both Pd-species adjacent to each other in the same catalytic
POP body is mandatory for the reactions to progress.
Based on these observations and consulting relevant literature
[30,31], a possible mechanistic explanation for the hydrogenation
via activation of H2 and substrate molecules is proposed. Under
photo-illuminated condition, the excited electrons can travel from
CB of B3-Azo2 to the Fermi level of Pd NPs through Mott-Schottky
junction, which then transfers to the antibonding orbitals of the
unsaturated substrate, and thereby triggers its activation (Fig. 6).
A similar claim has been made by other researchers as well [30–
34]. On the other hand, the electron-deficient holes present in
B3-Azo2 can oxidize H2 to active hydrogen [30,35]. These active
hydrogens can migrate to the Pd NPs following a reverse spill-
mechanism.
over and reduce the activated substrates [36–39]. Apart from the
Pd NPs, we believe that the uniformly distributed Pd(II) simultane-
ously activated the unsaturated substrates through
g-coordination
making them more efficient towards hydrogenation. Similar coor-
dination for homogeneous systems is prevailing [40,41]. The
surface-charge heterogeneity present in Pd-B3-Azo2 can play an
active role in this process. Owing to the high surface area and ultra-
high mesoporosity of the catalyst, a substantial amount of unsatu-
rated substrates can come in the surface contact with the catalyst
at a time. This can possibly be the reason behind a rapid and selec-
tive reduction of the unsaturated functionalities.
This Pd(0)-Pd(II) cooperative performance can explain the dif-
ference in catalytic activities of different Pd-species containing

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I. Nath et al. / Journal of Catalysis 385 (2020) 120–128
B3-Azo2 perceived earlier (Fig. 4). Since 1:1 physical mixture of Pd-
B3-Azo2 [Pd(II):Pd(0) = 0:100] and Pd-B3-Azo2 [Pd(II):Pd(0) = 100:
0] wasn’t able to cater a comparable activity as Pd-B3-Azo2 [Pd(I
I):Pd(0) = 50:50] (Table S5, Entry 6), we reckon that the distribu-
tion of Pd(0) and Pd(II) adjacent to each other on the polymeric
B3Azo2 surface is necessary to manifest such cooperative activity.
Accordingly, we performed an HADDF analysis coupled with EDS
elemental mapping for Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] to visual-
ize the elemental distributions. The acetate counter anions for Pd
(II) being the only source of oxygen (O) in the elemental structure
of the Pd-POP composites, we tracked the distribution of O as a
pseudo-substitute for Pd(II), whereas the overall Pd distribution
accounted for both Pd(II) and Pd(0). Using this technique, we
hoped to ‘‘see” the apparent distribution of both Pd(0) and Pd(II)
on the catalysts surface. As indicated in Fig. 7a, both Pd(II) and
Pd(0) were found to be distributed uniformly and adjacently.
In order to substantiate the role of Pd(II) we further carried
out FTIR analyses of the reaction mixture. The photocatalytic
hydrogenation of styrene were performed in several batches as
the model reaction for varying times. After required reaction span
the reaction mixture from one batch consisting polymeric Pd-B3-
bined spectroscopic results are presented in Fig. S64. The change
in spectral pattern due to the conversion of styrene to ethyl ben-
zene can be percieved. However, two specific observations appre-
hended our attention: (i) a small peak gradually appeared and
dissipated at 473 cmÀ1, and (ii) another small peak emmerged
at 1733 cmÀ1 over time (Fig. 7b and c). Coherent with relevant
literature, the former FTIR peak can be ascribed to the Pd-C
stretching [42–45]. Considering the proposed coordination
between Pd(II) and the olefin in the cooperative mechanism, the
appearance and dissipation of the peak at 473 cmÀ1 is justified.
On the other hand, the emmergence of a new small peak at
1733 cmÀ1 can be ascribed to the free acetate moieties. In line
with the proposed coordination of the olefins with Pd(II), a sub-
stitution of the pre-coordinated acetate units were reasonable.
These two observations accounted as supporting evidence to
our claimed cooperative mechanism.
For better justification of the Pd(0)-Pd(II) cooperation, shutting-
off the Pd(II) centers of the catalyst during reaction was targeted
with an expectation to observe a certain drop in substrate conver-
sion. Use of chelating agents with better Pd-complexation proper-
ties than olefins seemed ideal for this scenario. To this end,
salicylaldehyde was chosen as the chelating partner for Pd(II)
due to well-known complexation properties of the moiety in sim-
ilar homogeneous systems [46,47]. Gladly, the salicylaldehyde-
modified Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] gave only 41% conver-
sion of styrene after 75 min confirming our anticipation.
Azo2 [Pd(II):Pd(0)
= 50:50] catalyst was loaded to a pre-
synthesized KBr pallet, dried and corresponding IR spectroscopy
was analysed. Same amount of the reaction mixtures from differ-
ent batches illuminated for 0, 10, 20, 40, and 60 min respectively
were loaded on different KBr pallets for analysis and the com-
Fig. 7. (a) HAADF image of Pd-B3-Azo2 [Pd(II):Pd(0) = 50:50] and corresponding elemental distribution mapping percieved from EDS experiment; FTIR spectra of the reaction
mixture at different time intervals magnified at (b) 430–515 cmÀ1 range, and (c) 1705–1760 cmÀ1 range.

I. Nath et al. / Journal of Catalysis 385 (2020) 120–128
127
2.8. Adaptability of the catalyst
Declaration of Competing Interest
The catalyst was successfully recycled five times with almost no
change in substrate conversion (Fig. S59). After filtering out the
catalyst, the reaction mixture analyzed by ICP did not demonstrate
any Pd-content confirming that no metal is leaching in the medium
from the catalyst. ICP analysis of the reused catalyst confirmed no
significant change in Pd content either, while TEM and XPS analysis
of the reused catalyst showed no change in oxidation state or over-
all distribution pattern of Pd (Figs. S62 and S63). The FTIR and UV–
Vis pattern of the reused catalyst endorsed the retention of the
original chemical backbone as well (Figs. S60 and S61). Our recent
work also confirmed that Pd(II) coordinated to azobeneze-based
POP retained its original oxidation state after photo-illumination
[48]. Despite some literature reports on the reduction of transition
metal cations by homogeneous azobenzenes under photo-
illumination [49], the retention of original electronic properties
of our catalyst can be attributed to: (i) the use of light having
higher wavelength (low energy) than the excitation wavelength
of the material, and (ii) the extreme structural rigidity of the
POP, that confines azobenzenes in their initial trans conformation.
Moreover, a certain decrease in cis–trans isomerization of azoben-
zene after complexation with metal cations preventing the follow-
up metal reduction was also reported [50].
To check the catalytic behaviour using a shorter wavelength, a
365 nm lamp was used for photocatalytic hydrogenation of styrene
and a 55% conversion was observed after 75 min of reaction time
(Table S5, Entry 9). Moreover, the effect of magnetic stirring speed
on overall product formation was monitored as well. The stirring
speed of the reaction mixture can greatly affect the catalytic per-
formance, especially in the case of suspended heterogeneous cata-
lysts in the presence of gaseous reactants. The standard reduction
of styrene was probed as a model and monitored for 30 min at var-
ious magnetic stirring speed. The product formation rate gradually
increased with increasing the stirring speed from 100 rpm to
400 rpm, and then slightly decreased for further increase in speed
(Fig. S58).
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgment
The authors acknowledge State Key Laboratory of Advanced
Technology for Materials Synthesis and Processing, Wuhan Univer-
sity of Technology for postdoctoral financial support of I.N. and J.C.
F.V. acknowledges the support from the Tomsk Polytechnic Univer-
sity Competitiveness Enhancement Program grant (VIU-69/2019).
Authors also acknowledge Dr. Somboon Chaemcheun Wuhan
University of Technology, for his fruitful analytical help.
Appendix A. Supplementary material
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jcat.2020.03.014.
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