Functional hexanuclear Y(III) cluster-based MOFs supported Pd(II) single site catalysts for aerobic selective oxidation of styrene

Article abstract of DOI:10.1016/j.apcata.2020.117668

Oxidation of styrene is one of the significant reactions in organic synthesis. In this work, a serious of PdCl₂ moiety-decorated Y₆-MOFs adopting post-synthetic strategy were fabricated as applicable single-site catalysts for oxidation of styrene. Specifically, the functional organic linker, H₂bpydc (2,2′-bipyridine-5,5′-dicarboxylic acid), was first incorporated quantitatively into Y₆ clusters-based MOF, [(CH₃)₂NH₂]₂[Y₆(μ₃-OH)₈(bpdc)₆] (bpdc = 4,4′-biphenyl dicarboxylic acid) via post-synthetic ligand exchange. Then post-synthetic Pd(II) metalation was performed to enhance the density of isolated single sites. The obtained Pd(II)-Y-bpydc_x/bpdc_1-x can be used as highly efficient heterogeneous single-site catalysts to promote selective oxidative cleavage of styrene to benzaldehyde using O₂ as a oxidant under solvent-free and mild reaction conditions (1 atm and 80 °C). Importantly, when using Pd(II)-Y-bpydc_0.8/bpdc_0.2 (0.024 mmol Pd) as catalyst, the high conversion of styrene and the selectivity for benzaldehyde can reach 88.7 % and 82.2 %, respectively.

Full text of DOI:10.1016/j.apcata.2020.117668

Journal Pre-proof
Functional hexanuclear Y(III) cluster-based MOFs supported Pd(II)
single site catalysts for aerobic selective oxidation of styrene
Yue Zhang (Writing - original draft) (Software) (Investigation), Na
Wei (Investigation) (Validation) (Formal analysis) (Visualization)
(Software), Zhiqiang Xing (Resources) (Visualization), Zheng-Bo
Han (Conceptualization) (Methodology) (Resources) (Writing -
review and editing) (Supervision)
PII:
S0926-860X(20)30261-1
DOI:
https://doi.org/10.1016/j.apcata.2020.117668
APCATA 117668
Reference:
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Applied Catalysis A, General
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15 February 2020
21 April 2020
27 May 2020
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Functional hexanuclear Y(III) cluster-based MOFs supported
Pd(II) single site catalysts for aerobic selective oxidation of
styrene
Yue Zhanga, Na Weib*, Zhiqiang Xing and Zheng-Bo Hana*
aCollege of Chemistry, Liaoning University, Shenyang 110036, P. R. China
bCollege of Chemistry and Chemical Engineering, Shenyang Normal University,
Shenyang 110034, P. R. China
*N.W.: E-mail: wwweina@126.com.
*Z.-B.H.: E-mail: ceshzb@lnu.edu.cn.
Graphical abstract
A hexanuclear Y(III) clusters-based MOFs was successfully synthesized, then the
functional Pd(bpydc)Cl2 species were quantitatively incorporated into this material
via sequential post-synthetic ligand exchange and metalation. The obtained Pd(II)-Y-
bpydcX/bpdc1-X MOFs can be as highly efficient heterogeneous single-site catalysts to
promote selective oxidative cleavage of styrene to benzaldehyde using O2 as a green
oxidant.

Highlights
 A novel hexanuclear Y(III) clusters-based MOFs was successfully
synthesized.
 The functional Pd(bpydc)Cl2 species were quantitatively
incorporated into this MOF.
 The strategy of post-synthetic ligand exchange and metalation was
employed.
 The Pd(bpydc)Cl2-Y6-MOFs can be as efficient heterogeneous
single-site catalysts.
 The catalysts can promote selective oxidative cleavage of styrene
to benzaldehyde.

ABSTRACT
Oxidation of styrene is one of the significant reactions in organic synthesis. In this
work, a serious of PdCl2 moiety-decorated Y6-MOFs adopting post-synthetic strategy
were fabricated as applicable single-site catalysts for oxidation of styrene. Specifically,
the functional organic linker, H2bpydc (2,2'-bipyridine-5,5'-dicarboxylic acid), was
first
incorporated
quantitatively
into
Y6
clusters-based
MOF,
[(CH3)2NH2]2[Y6(3-OH)8(bpdc)6] (bpdc = 4,4'-biphenyl dicarboxylic acid) via
post-synthetic ligand exchange. Then post-synthetic Pd(II) metalation was performed
to enhance the density of isolated single sites. The obtained Pd(II)-Y-bpydcx/bpdc1-x
can be used as highly efficient heterogeneous single-site catalysts to promote selective
oxidative cleavage of styrene to benzaldehyde using O2 as a oxidant under
o
solvent-free and mild reaction conditions (1 atm and 80 C). Importantly, when using
Pd(II)-Y-bpydc0.8/bpdc0.2 (0.024 mmol Pd) as catalyst, the high conversion of styrene
and the selectivity for benzaldehyde can reach 88.7 % and 82.2 %, respectively.
Key Word: Metal-organic framework, Y6 clusters, heterogeneous single-site catalysts,
oxidation of styrene

1. Introduction
Metal-organic frameworks (MOFs) as a special class of functional porous materials
have been studied extensively in recent decades [1-3]. These materials exhibit high
surface area, porosity and functional tunability, which are promising for applications
in various areas, especially in catalysis [4-7]. MOFs not only combine the advantages
of heterogeneous and homogeneous catalysts, with high catalytic activity and
recyclability, but also have abundant and designable catalytic sites. The MOFs’
inherent metal nodes [8], functional group on the ligand [9] and the guest molecules
in the pore are potential catalytic sites [10]. In addition, catalytic species can also be
incorporated into the frameworks via post-synthetic modification, such as
post-synthetic metalation (PSM) and post-synthetic exchange (PSE) [11-14].
The oxidation of styrene is a very important reaction in the chemical industry since
the products of the oxidation, benzaldehyde and epoxides et al., are important and
useful intermediates in the production of many pharmaceuticals and fine chemicals[15,
16]. In particular, as a versatile chemical raw material, the benzaldehyde is widely
used in medicine, agriculture, and fine chemical industry [17-19]. So, the
development of highly efficient and selective catalysts for the oxidation cleavage of
styrene to benzaldehyde is of vital importance. Many palladium complexes have been
found to be active homogeneous catalysts in the oxidation of styrene reaction in
decades [20]. However, difficulty in separation of catalyst from reaction mixture,
reusing of expensive noble metal and purification of products are still urgent tasks
that need to be addressed. In this regard, many efforts have been made to search some

supports which are used to support homogeneous palladium species to achieve
heterogeneous single-site catalysts. Heterogeneous single-site catalysts are
constitutive of isolated, well-defined and active sites that are spatially separated in a
given solid materials and, ideally, structurally identical [21]. MOFs' unique, periodic
network structure that enables spatial separation of unsaturated metal sites makes
them candidates for single-site catalysts [22-24]. Therefore, immobilized palladium
species into MOFs via PSM is an effective strategy to afford catalytically active and
recyclable material [25-27], and expected to be used as the active sites for the
oxidation of styrene to benzaldehyde.
Herein, a hexanuclear Y(III) clusters-based MOF, [(CH3)2NH2]2[Y6(3-OH)8(L)6]
(L = bpdc (Y-bpdc)), has been constructed by a linear dicarboxylate (H2bpdc =
4,4'-biphenyl dicarboxylic acid) (Fig. 1) under solvothermal condition. The resulting
Y-bpdc was further functionalized through post-synthetic ligand exchange and Pd(II)
metalation process, obtaining the Pd(II) single-site functional MOFs which are highly
crystalline and porous show efficient and heterogeneous catalytic activity during the
oxidation of styrene to benzaldehyde. In addition, the catalytic activity can be
enhanced with the increase of the amount of Pd(II) introducing into the framework
within limits.
2. Experimental
2.1 Materials and methods
All chemicals for synthesis were commercially available reagents of analytical
grade and were used without further purification. The C, H and N microanalyses were

carried out with Perkin-Elmer 240 elemental analyzer. N2 adsorption-desorption
experiments were carried out on an automated gas sorption analyzer (Quantachrome
Instruments ASiQC). The FT-IR spectra were recorded from KBr pellets in the
4000-400 cm-1 range on a Nicolet 5DX spectrometer. Thermogravimetric analyses
(TGA) were performed on a PerkinElmer Pyrisl instrument (25-800 °C, 5 °C min-1,
flowing N2). Powder X-ray diffraction was recorded with a Bruker AXS D8 advanced
automated diffractometer with Cu-K radiation. The products of catalysis reaction are
monitored by Gas chromatography with a SP-2100A Gas chromatograph (GC).
Inductively coupled plasma-optical emission spectroscopy (ICP) was performed using
1
a 700 Series ICPOES (Agilent Technologies). H-NMR spectra were recorded on a
Varian-300 MHz NMR spectrometer. EDX analysis (Genesis Apollo X/XL EDS)
operated at 300 kV. X-ray photoelectron spectroscopy (XPS) measurements were
performed on a Kratos AxisUltra DLD system with a base pressure of 10−9 Torr.
2.2 Preparation of [(CH3)2NH2][Y6(3-OH)8(bpdc)6] (Y-bpdc)
A mixture of H2bpdc (200 mg, 0.83 mmol), Y(NO3)3·6H2O (360 mg, 0.94 mmol),
2-FBA (3.8 g, 27.32 mmol) were added in 100 mL mixed solvents of DMF (80 mL)
and EtOH (20 mL), uniformly utrasonic and evenly divided into scintillation vials.
The resulting mixture was kept in an oven at 105 °C for 36 h. After cooling the vials
to room temperature, the as-synthesized sample was purified through repeated
washings with DMF to yield yellow transparent octahedral crystals. Yield: 55 %
(based on the crystals dried in air). Anal. calcd. for C88H72N2O32Y6: C 47.98, H 3.29,
N 1.27 %. Found: C 48.02, H 3.33, N 1.26 %. IR (KBr, cm-1): 3403 (br), 1668 (s),

1608 (s), 1537 (w), 1412 (s), 1396 (s), 1251 (w), 1175 (w), 1094 (m), 1006 (w), 864
(w), 803 (w), 772 (s), 707 (w), 672 (w).
2.3 Ligand exchange experiments
A mixture of H2bpydc (10 mg) and Y-bpdc (15 mg) were added in 1.5 mL DMF in
a 5 mL scintillation vial and mixed uniformly. The resulting mixture was kept in an
oven at 75 °C for a while. After cooling the vial to room temperature, the
as-synthesized sample was filtered, washed and dried to get hexnuclear yttrium
clusters MOFs (Y-bpydcx/bpdc1-x) with different ligand ratios. The ratio of bpdc and
bpydc ligands in MOFs was controlled quantitatively by adjusting the ligand
exchange time. Ligand exchange progress was determined by 1H-NMR analysis. After
2 days of ligand exchange, the ligand proportion in the MOFs obtained was
bpydc0.2/bpdc0.8,
the
molecular
formula
is
[(CH3)2NH2]2[Y6(μ3-OH)8(bpdc)4.8(bpydc)1.2] (Y-bpydc0.2/bpdc0.8). After 4 days of
ligand exchange, the ligand proportion in the MOFs obtained was bpydc0.5/bpdc0.5, the
molecular formula is [(CH3)2NH2]2[Y6(μ3-OH)8(bpdc)3(bpydc)3] (Y-bpydc0.5/bpdc0.5).
After 7 days of ligand exchange, the ligand proportion in the MOFs obtained was
bpydc0.8/bpdc0.2,
the
molecular
formula
is
[(CH3)2NH2]2[Y6(μ3-OH)8(bpdc)1.2(bpydc)4.8] (Y-bpydc0.8/bpdc0.2).
2.4 Pd (II) metallization experiment
2.4.1 The preparation of PdCl2(CH3CN)2
100 mg PdCl2 was added to a round-bottom flask, which was vacuumed and N2
circulation treatment to remove oxygen in the reaction system. The 40 mL acetonitrile

o
was then injected and heated to 90 C under the N2. At the end of the reaction, the
reaction solution was cooled to room temperature, and the excess solvent was
removed by rotary evaporator to obtain yellow powder, which was the target product
PdCl2(CH3CN)2
2.4.2 Pd (II) metallization of MOFs
30 mg PdCl2(CH3CN)2 (0.12 mmol) was weighed and added into a 5 mL
scintillation vial and dissolved with
2
mL DMF. Y-bpydc0.8/bpdc0.2,
Y-bpydc0.5/bpdc0.5 and Y-bpydc0.2/bpdc0.8 (0.02 mmol) were soaked in the above
o
organic Pd solution and placed in the 65 C oven. The mixture was removed after 3
days of heating, cooled to room temperature, filtered and washed with fresh DMF
several times to remove excess organic Pd solution on solid surface until the filtrate is
colorless. The solid after washing is soaked in methanol solution. The methanol
solvent was changed every day for 3 days. Filtered out the solids after 3 days and
dried in a vacuum.
2.5 X-ray crystallography
Crystallographic data of Y-bpdc was collected at 100 K with a SuperNova
diffractometer equipped with Cu-K radiation (= 1.5418 Å) and graphite
monochromator using the -scan mode. The structures were solved by direct methods
and refined on F2 by full-matrix least squares using SHELXTL [28]. Non-hydrogen
atoms were treated anisotropically. Positions of hydrogen atoms were generated
geometrically. There is a large solvent accessible pore volume in the structure of
Y-bpdc, which is occupied by highly disordered solvent molecules. No satisfactory

disorder model for these solvent molecules could be achieved, and therefore the
SQUEEZE program implemented in PLATON was used to remove these electron
densities of these disordered species [29]. Crystallographic data and experimental
details for structural analyses are summarized in Table S4. The CCDC reference
numbers 1893342 for Y-bpdc.
2.6 Catalytic performance evaluation
Before the catalytic reaction, solvent exchange was treated to remove the residual
guest molecules on the surface and in the hole of the sample. After fully drying under
vacuum, the catalytic experiment was carried out. The styrene (5.0 mmol) and catalyst
(0.005mmol) were introduced into a reaction tube (10 mL), sealed and bubbled into
oxygen under atmospheric pressure. Then reaction mixture was stirred at 80 °C. After
5 h, the reactor was cooled down to room temperature. The catalysts were separated
by filtration, and the samples of the reaction mixture were analyzed by GC to
determine the conversions. Separated catalyst was washed with methanol and dried
under vacuum for reuse.
2.7 Leaching Test
The catalyst Pd(II)-Y-bpydc0.8/bpdc0.2 was separated from the reaction system
right when the reaction lasted for 2 h, and the reaction was continued in the absence
of the catalyst for an additional 3 h.
3. Results and discussion
3.1 Crystal structure
Single-crystal X-ray crystallography showed that Y-bpdc crystallizes in the cubic

̅
space group Fm3m. Each Y(III) ion is eight-coordinated by four 3-OH and four
distinct carboxylate groups from four independent bpdc2- ligands in a bis-monodentate
fashion, showing a square-antiprismatic coordination environment. Six adjacent Y(III)
ions are bridged by 8 μ3-OH and 12 deprotonated carboxylate groups (COO-) in a
bis-monodentate fashion to give a hexanuclear Y(III) cluster secondary building unit
(SBU), [Y6(μ3-OH)8(COO)12] (Fig. 2a), the 12-connected nodes were bridged by bpdc
ligands to form a 3D framework with two polyhedral cages, octahedral and tetrahedral
(Fig. 2b, c).
3.2 Characterization
Accroding to the previously reported literature [30-32], the Pd(II) complexes can
exhibit catalytic activity towards the selective oxidative cleavage of styrene to
benzaldehyde. So, the functional Pd(II) complexes were quantificationally
immobilized into the Y-bpdc by a sequential post-synthetic ligand exchange and
metalation process to investigate the correlation between the amount of Pd(II) sites
and the catalytic capacity.
Post-synthetic ligand exchange of Y-bpdc with H2bpydc ligand (H2bpydc =
2,2'-bipyridine-5,5'-dicarboxylic acid) was achieved by a directly solvothermal
synthesis at 75 oC in DMF for different time. The resulting colorless crystalline solids
were separated and washed with DMF and methanol, then dried in air. As shown in
FT-IR spectra (Fig. S1), the bpydc linkers with bpy chelating sites were introduced
1
into the framework successfully. The H-NMR spectroscopy (Fig. 3a) upon digestion
of these samples with dilute DCl in d6-DMSO indicated that the ratio of linkers varied

according to the reaction time of post-synthetic ligand exchange. As the reaction time
increases, the ratio of bpydc linker linear increases, but once the crystal contains ∼80 %
bpydc, no further exchange occurs, suggesting that there is an upper limit for bpydc
exchange. Powder X-ray diffraction (PXRD) confirmed that the series of obtained
crystal samples, Y-bpydcX/bpdc1-X (0≤x≤1, x indicating the amount of bpydc
incorporation) are isomorphous and maintain high crystallinity (Fig. 3b).
Post-synthetic metalation was performed by treating Y-bpydcX/bpdc1-X with 6.0
o
equiv of PdCl2(CH3CN)2 in DMF at 65 C to afford Pd(II)-Y-bpydcx/bpdc1-x as a
brown solid. The crystallinity of samples were maintained upon metalation, as
evidenced by similar PXRD patterns of Y-bpydcx/bpdc1-x and Y-bpdc. As shown in
Fig. 3b, the PXRD patterns of Pd(II)-metalated Y-bpydcx/bpdc1-x are good agreement
with that of pristine framework, indicating that the crystallinity of these
Y-bpydcx/bpdc1-x are well maintained after the PdCl2 moiety doping. In the case of
Pd(II)-Y-bpydc0.8/bpdc0.2, variable-temperature PXRD and thermogravimetric
analysis (TGA) proved that the Pd(II) metalation of Y-bpydcx/bpdc1-x still had good
thermal stability, reaching 350 oC (Fig. S2 and S3).
To confirm that the PdCl2 moieties were installed onto bpy chelating sites of bpydc
ligands and to determine their coordination environment within the framework, X-ray
photoelectron
spectroscopy
(XPS)
analysis
were
performed
on
Pd(II)-Y-bpydc0.8/bpdc0.2 and Y-bpydc0.8/bpdc0.2. As shown in Fig. 3c, the
appearance of Pd 3d and Cl 2p peaks confirmed the successful immobilization of
PdCl2 moieties on the framework, and the binding energies for Pd 3d3/2 and 3d5/2 are

338 and 343 eV, respectively, indicating that the palladium oxidation state of
metallized Y-bpydcx/bpdc1-x is bivalent. The 1s spectra of
N
Pd(II)-Y-bpydc0.8/bpdc0.2 and Y-bpydc0.8/bpdc0.2 (Fig. 3d) show that with the Pd(II)
doping in, the N 1s peak appears to shift to a higher binding energy, indicating the
coordination between N atom of bpydc group and the Pd(II) site. Moreover, the
absence of multiple features in the N 1s spectrum of the Pd(II)-Y-bpydc0.8/bpdc0.2
seems to further verify the presence of only one environment for N, namely each
Pd(II) is bonded to the uncoordinated bpy group.
The energy-dispersive X-ray spectroscopy (EDX) showed 1:(0.16±0.02):(0.4±0.05),
1:(0.49±0.02):(1.00±0.05) and 1:(0.80±0.04):(1.80±0.06) atomic ratio of Y:Pd:Cl in
Pd(II)-Y-bpydcx/bpdc1-x, respectively (Table S1), indicating that all the bpydc sites
incorporated in these samples are nearly quantitative Pd(II) metallized. Moreover, the
doped Pd(II) amount (1.0 wt %) were also confirmed by inductively coupled plasma
(ICP) analysis, and the ratio of Y:Pd in Pd(II)-Y-bpydc0.2/bpdc0.8 is 1:0.16,
Pd(II)-Y-bpydc0.5/bpdc0.5 is 1:0.49 and Pd(II)-Y-bpydc0.8/bpdc0.2 is 1:0.79 (Table
S2), which are correspond to the EDS analysis. Therefore, the corresponding formula
of these Pd(II)-doping samples can be given as Y6(OH)8(bpydcPdCl2)3(bpdc)3
(
Pd(II)-Y-bpydc0.2/bpdc0.8),
(Pd(II)-Y-bpydc0.5/bpdc0.5)
(Pd(II)-Y-bpydc0.8/bpdc0.2), respectively.
Y6(OH)8(bpydcPdCl2)3(bpdc)3
Y6(OH)8(bpydcPdCl2)4.8(bpdc)1.2
and
It can be inferred from the N2 adsorption experiment of Y-bpydc0.8/bpdc0.2 and
Pd(II)-Y-bpydc0.8/bpdc0.2 at 77 K that all of the Y-bpdcx/bpydc1-x and Pd(II) doping

samples display a fully reversible Type-I isotherm through the investigation of N2
adsorption at 77 K (Fig. S4), confirming that the microporosity of these Y6-MOF is
maintained after post-synthetic ligand exchange and Pd(II)-metalation. Y-bpdc and
Y-bpydc0.8/bpdc0.2 exhibit similar Brunauer-Emmett Teller (BET) surface areas
(1256.55 m² g–1 and 1299.64 m² g–1) and pore volumes (0.48 cm3 g–1 and 0.55 cm3 g–
1). However, with the PdCl2 moieties doping in, both of the BET surface areas and
pore volumes of the framework are decreased obviously due to the steric-hinerance
effect. The BET surface areas and experimental pore volumes of
Pd(II)-Y-bpydc0.8/bpdc0.2 were estimated to 275.73 m² g–1 and 0.096 cm3 g–1,
respectively (Table S3).
3.3 Catalytic oxidation of styrene
As an important industrial transformation process [33-35], the oxidation of styrene
was chosen to demonstrate the catalytic capacity of these Pd(II)-containing catalysts,
which was carried out using O2 as a oxidant under solvent-free and mild reaction
conditions. The product of oxidation of styrene is diversity, such as benzaldehyde,
styrene oxide, benzoic acid, etc., among which the benzaldehyde (a) and styrene oxide
(b) are the main products, and the former is our target product. The conversion and
selectivity for the oxidation of styrene on Y6-MOF and Pd(II)-Y6-MOFs were
confirmed by gas chromatography (GC) analyses and summarized in Table 1. Firstly,
when the reaction was carried out without catalyst, no styrene conversion was
observed (Table 1, entry 1). Then, the catalytic activities of Y6-MOFs with different
Pd(II) loading amounts, Pd(II)-Y-bpydcx/bpdc1-x, were investigated and the results

were shown in Table 1. Thereinto, the Pd(II)-Y-bpydc0.8/bpdc0.2 exhibit higher
catalytic activity. After 5 hours, the conversion of styrene and the selectivity for
benzaldehyde using Pd(II)-Y-bpydc0.8/bpdc0.2 (0.024 mmol Pd) as catalyst can reach
88.7 % and 82.2 % (Table 1, entry 5), respectively, which outperform some
MOF-based catalysts. Under the similar reaction condition, MOF-74(Co) [36]
selectivity (35 %) using approximate 3.5 times amount of catalyst (0.34 mol%) (Table
1, entry 10). Under a non-green reaction condition, MIL-MOFs [37] demonstrated
inferior catalytic activity for oxidation of styrene to benzaldehyde (entries 11-12). For
the Cr-MIL-101 [38] (Table 1, entry 13), the benzaldehyde selectivity exceeded 90 %,
but the conversion was just 46.79 %. TSHM crystals can also give styrene a higher
conversion of ca. 90 %, but need extend reaction times (12 h) and more powerful
oxidant (TBHP), moreover, its benzaldehyde selectivity was just ca. 60 %, lower than
the Pd(II)-loading Y-bpydcx/bpdc1-x (Table 1, entry 14). The conversion of
Pd(II)-Y-bpydc0.5/bpdc0.5 (0.015 mmol Pd) and Pd(II)-Y-bpydc0.2/bpdc0.8 (0.006
mmol Pd) were 61.9 % and 45.0%, the selectivity were 85.9 % and 90.1 %,
respectively. An obvious decline in the conversion of styrene can be noticed with the
amount of Pd(II) sites in the framework decreasing, however, the selectivity of
benzaldehyde exhibit trends of increase (Table 1, entries 6-7). So, this
Pd(II)-Y-bpydcx/bpdc1-x MOFs exhibit tunable catalytic properties for aerobic
selective oxidation of styrene via modulation of the Pd(II) amount incorporated into
the framework. Temperature has always been an important factor affecting catalytic
reactions.
Therefore,
a
model
reaction,
with
5
mmol
styrene,

Pd(II)-Y-bpydc0.8/bpdc0.2 (0.005 mmol, 0.1 mol%), 1 atm O2 and 5 hours, were
performed to explore the effect of temperature on the oxidation of styrene. As shown
in the Fig. S5, the conversion increased significantly as the temperature increases
from 25 °C to 120 °C. However, when the temperature reached 80 °C, the selectivity
reached an inflection point and reaches its maximum. Thus, the best reaction
temperature was 80 °C.
In comparison, Y-bpdc (Table 1, entry 2) and Y-bpydc0.8/bpdc0.2 (Table 1, entry 4)
were also tested. Under the same condition, these two materials show no catalytic
activity. When the reaction time extended to 12 hours, the conversion of styrene just
reached 50.1 % and 85 % selectivity for benzaldehyde (Table 1, entry 6), further
indicating that the introduction of Pd(II) can significantly improve the catalytic
activity of the Y6-MOF, in other words, the Pd(bpydc)Cl2 species immobilized in the
framework play an important role in the catalytic performance. PdCl2 (0.024 mmol) as
a homogeneous catalyst for the oxidation of styrene was also tested, which gave lower
conversion of styrene (54.3 %) and yield of benzaldehyde (78.0 %) under the same
reaction condition. After 7 hours, the conversion increased to ca. 90 %, but a
reduction of benzaldehyde selectivity occur (64.8 %).
To study the general applicability of the catalyst, some aryl- and alkyl- substituted
terminal olefins were investigated and the experimental results were summarized in
Table 2. As observed from Table 2, conversion and selectivity of the aromatic terminal
olefins with electron-withdrawing and electron-donating substituted groups were
63.3-81.6 % and 75.1-80.3 % (Table 2, entries 1-3), respectively. However, for

aliphatic alkene 1-octene, the catalytic effect is relatively poor. Although the
selectivity of the product heptanal was 77.7 %, only 29.1 % conversion was obtained
(Table 2, entry 4).
According to the above experimental results, the proposed reaction mechanism was
inferred in Fig. S6. Firstly, activation of oxygen by the Pd(II) leads to an Pd-superoxo
radical, and the olefin substrates bond to the Pd(II). Next, the superoxo radical is
proposed to undergo radical cyclization with the coordinated olefin, affording a
five-membered peroxometallacycle. The latter collapses via reductive elimination to
give the dioxetane, which is expected to decompose readily to afford benzaldehyde
and styrene epoxide. In addition, benzoic acid may be formed by the deep oxidation
of benzaldehyde [40, 41].
To investigate recyclability and reusability of Pd(II)-Y-bpydc0.8/bpdc0.2, the
recovery experiments were examined under same reaction conditions as mentioned
earlier. After three runs of reactions, Pd(II)-Y-bpydc0.8/bpdc0.2 still remained highly
active (Fig. 4a). The PXRD showed that the structure of the single-site catalyst
remained stable during the catalytic process and was not affected by the reaction
system (Fig. S7). Additionally, ICP-MS analyses showed that the amounts of Pd(II) in
the skeleton of the catalyst did not change significantly before and after the catalytic
reaction (Table S2). Moreover, no further conversion was detected after removal of
Pd(II)-Y-bpydc0.8/bpdc0.2 during the course of the oxidation of styrene reaction (Fig.
4b).
4. Conclusion

In summary, we employed a post-synthetic strategy, including post-synthetic ligand
exchange and metalation operation, to synthesize a series of PdCl2 moiety-decorated
Y6-MOFs. These materials contain high densities of isolated, single active Pd(II) sites
and exhibit excellent conversion and selectivity towards the aerobic oxidation of
styrene to benzaldehyde. More remarkably, their catalytic capacity is tunable via the
regulation of Pd(II) amount. Herein, linker engineering strategy is an effective
approach to synthesize single-site catalysts with tunable catalytic property and can
also be extended to other catalytic system.
Author contributions
Yue Zhang: Writing-Original Draft, Software, Investigation.
Na Wei: Investigation, Validation, Formal analysis, Visualization,
Software.
Zhiqiang Xing: Resources, Visualization.
Zheng-Bo
Han:
Conceptualization,
Methodology,
Resources,
Writing-Review & Editing, Supervision,
Declaration of interests
☐ The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
☒The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
Acknowledgements

This work was granted financial support from the National Natural Science
Foundation of China (Grant 21671090), Liao Ning Revitalization Talents Program
(XLYC1802125) and Liaoning Province Doctor Startup Fund (2019-BS-220).
Appendix A. Supplementary data
Supplementary
data
to
this
article
can
be
found
online
at
https://doi.org/10.1039/x0xx00000x.
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Figure Captions
Fig. 1 Synthesis of Pd(II)-Y-bpydcx/bpdc1-x via sequential post-synthetic ligand exchange
and metalation process.
Fig. 2 Ball-and-stick and schematic representation of the hexanuclear [Y6(μ3-OH)8(COO)12]
SBU (green: Y; gray: C; red: O) (a) and the 12-conncted fcu network of Y-bpdc (b-c),
consisting of tetrahedral (yellow ball) and octahedral (orange ball) cages.

1
Fig. 3 The H-NMR spectra (DCl/D2O/d6-DMSO digested) (a) and PXRD (b) of Y-bpdc,
Y-bpydc and Y-bpydcX/bpdc1-X. XPS survery spectrum, insert: Pd 3d (c) and N 1s (d) of
Y-bpydc0.8/bpdc0.2 and Pd(II)-Y-bpydc0.8/bpdc0.2.
Fig. 4. Reusability experiments (a) and Leaching experiments (b) of
Pd(II)-Y-bpydc0.8/bpdc0.2 for aerobic oxidation of styrene.

Table 1. Aerobic oxidation of styrene over different catalysts
O
Cat.
O
+
O2, 80 oC
(b)
(a)
target product
Conversiona
(%)
Selectivity (%)
Entry
Catalyst
Pd (mmol)
(a)
(b)
1
2
none
Y-bpdc
0
0
0
--
--
--
0
--
3
Y-bpydc0.8/bpdc0.2
Y-bpydc0.8/bpdc0.2
Pd(II)-Y-bpydc0.8/bpdc0.2
Pd(II)-Y-bpydc0.5/bpdc0.5
Pd(II)-Y-bpydc0.2/bpdc0.8
PdCl2
0
0
--
--
4b
0
50.1
88.7
61.9
45.0
54.3
90.4
47.3
72.52
83.72
46.79
90
85.1
82.2
85.8
90.1
78.0
64.8
35
13.3
13.7
12.1
9.2
5
0.024
0.015
0.006
0.024
0.024
--
6
7
8c
9d
10e
11f
12f
13f
14g
19.3
25.3
17
PdCl2
MOF-74(Co) [37]
Mn-MIL-100 [38]
Fe-MIL-101 [38]
Cr-MIL-101 [39]
TSHM [4]
--
81.81
67.94
91.20
~60
12.01
24.61
8.80
--
--
--
--
o
Reaction conditions: styrene (5 mmol), catalyst (0.005 mmol, 0.1 mol%), O2 (1 atm), 80 C
a
b
c
and 5 hours. The conversions were determined by GC. 12 hours. PdCl2 (0.024 mmol), 5
hours. 7 hours. eReaction conditions: styrene (43.6 mmol), catalyst (ca. 0.15 mmol) and O2
d
f
o
balloon at 80 C for 20 h. Reaction conditions: n(70 % TBHP): n(Styrene) = 3:1, catalyst (5
g
wt%), 70 °C and 6 h, solvent: CH3CN; Reaction conditions: catalyst (10 mg, 0.014 mmol),
styrene (2 mmol) and TBHP (6 mmol) in CH3CN (10 mL), 80 oC and 12 hours.

Table 2. Aerobic oxidation of various terminal olefins to aldehydes
Substrate Product Conversion (%) Selectivity (%)
Entry
1
63.3
69.8
75.1
78.0
2
3
4
81.6
29.1
80.3
77.4
Reaction conditions: substrate (5 mmol), Pd(II)-Y-bpydc0.8/bpdc0.2 (0.005 mmol, 0.1 mol%),
O2 (1 atm), 80 oC and 5 hours.