Pd nanoparticles supported on cubic shaped ZIF-based materials and their catalytic activates in organic reactions

Article abstract of DOI:10.1016/j.materresbull.2020.111015

Pd nanoparticles decorated on cubic shaped ZIF-based materials have been successfully prepared. The nanocomposites have been characterized by X-ray diffraction, field emission scanning electron microscopy, N₂ sorption analysis, and transmission

Full text of DOI:10.1016/j.materresbull.2020.111015

Journal Pre-proof
Pd nanoparticles supported on cubic shaped ZIF-based materials and
their catalytic activates in organic reactions
Minoo Dabiri, Hassan Fazli, Neda Salarinejad, Siyavash Kazemi
Movahed
PII:
S0025-5408(20)31496-3
DOI:
https://doi.org/10.1016/j.materresbull.2020.111015
MRB 111015
Reference:
To appear in:
Materials Research Bulletin
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Revised Date:
Accepted Date:
21 March 2020
5 July 2020
27 July 2020
Please cite this article as: Dabiri M, Fazli H, Salarinejad N, Movahed SK, Pd nanoparticles
supported on cubic shaped ZIF-based materials and their catalytic activates in organic
reactions, Materials Research Bulletin (2020),
doi: https://doi.org/10.1016/j.materresbull.2020.111015
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Pd nanoparticles supported on cubic shaped ZIF-based materials and their catalytic activates
in organic reactions
Minoo Dabiri*, Hassan Fazli, Neda Salarinejad and Siyavash Kazemi Movahed
Faculty of Chemistry and Petroleum Sciences, Shahid Beheshti University, Tehran 1983969411,
Islamic Republic of Iran Email: m-dabiri@sbu.ac.ir
Graphical abstract
Highlights

A novel Pd@A-ZIF nanocomposites was prepared by anchoring Pd nanoparticle on the
surfaces of annealed ZIF cubes.
1



The synergistic action between Pd and Zn, and Pd and Co species played a crucial role in
highly active catalytic species.
Pd@A-ZIF nanocomposites show high catalytic activity and reusability.
Abstract
Pd nanoparticles decorated on cubic shaped ZIF-based materials have been successfully
prepared. The nanocomposites have been characterized by X-ray diffraction, field emission
scanning electron microscopy, N2 sorption analysis, and transmission electron microscopy. The
catalytic activities of synthesized materials were evaluated in Suzuki-Miyaura cross-coupling.
The results showed the catalytic activity of Pd@A-ZIF-8 was higher than Pd@A-ZIF-67 and
Pd@A-BMZIF. Additionally, the catalytic activities of as-synthesized nanocomposites were
studied on the reduction of nitroarenes. The catalytic activity of Pd@A-ZIF-67 was higher than
Pd@A-ZIF-8 and Pd@A-BMZIF in the reduction of nitrobenzene. The synergistic action between
Pd and Zn, Pd and Co species may play an important role in highly active catalytic species in
Suzuki-Miyaura cross-coupling reaction and reduction of nitroarenes, respectively. The
reusability of as-synthesized nanocomposites was checked, and the catalytic activity was
retained after six consecutive reaction runs.
KEYWORDS: A. nanostructures; B. chemical synthesis; D. catalytic properties.
Introduction
Suzuki-Miyaura cross-coupling reaction is a palladium-catalysed coupling reaction between
organoboron species and aryl halides. This reaction is a powerful tools for forming
carbon−carbon bonds with broad applications in the preparation of intermediate compounds,
pharmaceuticals, biologically, and naturally products that display the importance of this
transformation for academia and industries [1–3]. Suzuki−Miyaura cross-coupling reaction has
some benefits compared to other coupling reactions: low toxicities, stability, and tolerance
2

toward the thermal and aqueous condition of boron compounds, easy isolation of by-products
from the reaction, and mild reaction conditions [4]. Typically, the Suzuki-Miyaura cross-coupling
reaction is often carried out with organometallic homogeneous Pd complexes such as
Pd(PPh3)4. Despite the relatively high efficiency of these homogeneous systems, the difficulty to
separate and reuse the catalyst from the reaction medium is considered as the pressing issue in
the sustainable development of the chemical industry [5]. To overcome several issues related to
homogeneous Pd catalysts, many efforts have been made to develop heterogeneous Pd
catalysts for Suzuki-Miyaura cross-coupling reaction including the immobilization or
stabilization of Pd species (ion or nanoparticle) on different supports, such as alumina [6],
carbon materials [7], clay [8], metal-organic frameworks [9], metal oxides[10,11], polymers
[12], resins [13], silica [14,15], zeolite [16] and heterogeneous complexes [17].
Recently, the use of metal-organic frameworks (MOFs) gained intense attention as solid
supports for the immobilization of precious metal catalysts. Shu et al. have prepared Pd/MOF-5
as a highly active heterogeneous catalyst [18]. This catalyst applied in ligand- and copper-free
Sonogashira coupling of halobenzenes with phenylacetylene which is applicable to a wide range
of substrates with good electronic and steric tolerance. Islam et al. have synthesized a new
heterogeneous catalyst by grafting Pd(0) nanoparticles at the surface of a the surfaces of a Co-
containing metal–organic framework MCoS-1 [19]. This heterogeneous catalyst showed
excellent catalytic activities in carbon–carbon coupling reactions, namely Suzuki–Miyaura
coupling and Sonogashira coupling. Jiang et al have developed a highly efficient heterogeneous
catalyst system for the water‐mediated Suzuki–Miyaura and Ullmann coupling reactions of aryl
chlorides using porous MIL-101‐supported palladium NPs as the catalyst [20]. Haung et al. have
reported Pd NPs encapsulated in a series of isoreticular mixed-linker bipyridyl MOFs as catalysts
for Suzuki–Miyaura cross-coupling reactions. The modification of MOF linkers serves to unveil
the electronic and steric effects of the linker substitution in varying their catalytic properties
[21].
Zeolitic imidazolate frameworks (ZIFs) are one of the most promising metal-organic
frameworks (MOFs). They are generated by tetrahedral coordination of transition metal cations
3

(typically Zn2+, Co2+, etc.) with the nitrogen atoms of 2-methylimidazole linkers [22–24]. ZIFs
have the advantages of mild synthesis protocols, porosity, crystallinity, controlled structure,
high specific surface area, and exceptional chemical, thermal and hydrothermal stability, which
allow them to deliver excellent performance in the field of catalysis [25], drug delivery [26,27],
gas sorption and storage [28,29], energy storage [30,31], and electrode materials [32,33].
Recently, they are applied as an efficient heterogeneous catalyst support for various reactions
such as Knoevenagel reaction [34], alcohol oxidation [35], trans-esterification [36], 1,3-dipolar
cycloadditions [37], Friedlander and Combes condensations [37], CO2 cycloaddition of epoxides
[38], etc. Additionally, ZIFs provide great potential as hosts for various metal nanoparticles such
as Cu [39], Ru [40], Pd [41], Pt [42], and Au[43] due to their high porosity and high surface area.
The growth of nanoparticles could limit by loading metal nanoparticles into the pores of ZIFs.
Also, they prevent the migration and aggregation of nanoparticles during the reaction, thus led
to an increase in catalytic activity and stability of nanoparticles.
Hong et al. have proposed a strategy based on the capping agent modulator method to
control the shape of ZIF nanocrystals [44]. Among different shapes of ZIFs, the cubic structure
exhibited the highest surface area. Additionally, the surface area to volume ratio for a cube
structure is higher than other shapes. Based on the above introduction, herein, we have
successfully reported the palladium nanoparticles decorated on cubic shaped ZIF-based
materials. In the first step, the ZIF-based cubic materials such as ZIF-67, ZIF-8, and BMZIF were
synthesized with the assist of a capping agent, cetyltrimethylammonium bromide (CTAB). The
ZIF structures were carbonized at low temperature in which the cubic structures were retained.
Finally, the Pd nanoparticles (Pd NPs) anchored on annealed cubic shaped ZIF-base by using
chemical reduction of H2PdCl4 with sodium borohydride (Scheme 1).
4

Scheme 1 Schematic diagram illustrating the synthesis of Pd@A-ZIF nanocomposites.
Experimental
Synthesis of A-ZIF-67 nanocube:
Co(NO3)2·6H2O (580 mg) was dissolved in deionized water (20 mL) containing
cetyltrimethylammonium bromide (CTAB) (20 mg). Then this solution was rapidly injected into
an aqueous solution (140 mL) containing 2-methylimidazole (9.08 g) and stirred at room
temperature for 20 min. The product was collected by centrifugation and washed by ethanol
for several times. ZIF-67 was annealed at 450 °C with a heating rate of 2 °C/min for 2 h in a
furnace under a nitrogen atmosphere.
Synthesis of A-ZIF-8 nanocube:
Zn(NO3)2·6H2O (592 mg) was dissolved in deionized water (20 mL) containing CTAB (20 mg).
Then this solution was rapidly injected into an aqueous solution (140 mL) containing 2-
methylimidazole (9.08 g) and stirred at room temperature for 20 min. The product was
collected by centrifugation and washed by ethanol for several times. ZIF-8 was annealed at 450
°C with a heating rate of 2 °C/min for 2 h in a furnace under a nitrogen atmosphere.
Synthesis of A-BMZIF nanocube:
5

Co(NO2)2·6H2O (474 mg) and Zn(NO3)2·6H2O (116 mg) were dissolved in deionized water (20
mL) containing CTAB (20 mg). Then this solution was rapidly injected into an aqueous solution
(140 mL) containing 2-methylimidazole (9.08 g) and stirred at room temperature for 20 min.
The product was collected by centrifugation and washed by ethanol for several times. BMZIF
was annealed at 450 °C with a heating rate of 2 °C/min for 2 h in a furnace under a nitrogen
atmosphere.
Synthesis of Pd@A-ZIF-8 nanocomposite:
A-ZIF-8 (100 mg) was dispersed in acetonitrile (100 mL). Then a solution of PdCl2 in CH3CN (5
mL, 0.025 M) was added dropwise to the reaction mixture and stirred for 2 h. Then a solution of
NaBH4 in H2O (5 mL) was added dropwise and stirred for 2 h. After that, the mixture was
washed and centrifuged with H2O and ethanol. Finally, it was dried at 70 °C overnight.
Synthesis of Pd@A-ZIF-67 nanocomposite:
A-ZIF-67 (100 mg) was dispersed acetonitrile (100 mL). Then a solution of PdCl2 in CH3CN (5 mL,
0.025 M) was added dropwise to the reaction mixture and stirred for 2 h. Then a solution of
NaBH4 in H2O (5 mL) was added dropwise and stirred for 2 h. After that, the mixture was
washed and centrifuged with H2O and ethanol. Finally, it was dried at 70 °C overnight.
Synthesis of Pd@A-BMZIF nanocomposite:
A-BMZIF (100 mg) was dispersed acetonitrile (100 mL). Then a solution of PdCl2 in CH3CN (5 mL,
0.025 M) was added dropwise to the reaction mixture and stirred for 2 h. Then a solution of
NaBH4 in H2O (5 mL) was added dropwise and stirred for 2 h. After that, the mixture was
washed and centrifuged with H2O and ethanol. Finally, it was dried at 70 °C overnight.
General procedure for the Suzuki-Miyaura cross-coupling reaction
A mixture of aryl halide (0.5 mmol), phenylboronic acid (0.7 mmol), potassium phosphate (1.0
mmol) and catalyst (0.1 mol% of Pd) in EtOH/H2O (1:1, 4 mL) as a solvent was mixed in a round
bottom flask (10 mL). Then, the resulting mixture stirred at 70 °C. The progress of the reaction
was monitored by Thin Layer Chromatography (TLC). After reaction completion, the catalyst
6

was separated by centrifugation and CH2Cl2 was added. The CH2Cl2 layer was separated from
the water layer using a separatory funnel and followed by drying of the organic layer over
Na2SO4. The dried CH2Cl2 layer was concentrated under vacuum, and the product was purified
by thin-layer chromatography (SiO2, n-hexane and ethyl acetate) to isolate the corresponding
products. The catalyst was washed with EtOH (2 × 10 mL) and H2O (2 × 10 mL), and was reused
for the next run.
General procedure for the reduction of nitroarenes
To the stirred mixture of nitroarene compound (0.4 mmol) and sodium borohydride (10 mmol)
in EtOH/H2O (1:1, 5 mL), catalyst (0.05 mol% of Pd) were added. The progress of the reaction
was monitored by TLC. After reaction completion, the catalyst was separated by centrifugation,
and ethyl acetate was added. The ethyl acetate layer was separated from the water layer using
a separatory funnel and followed by drying of the organic layer over Na2SO4. The dried ethyl
acetate layer was concentrated under vacuum, and the product was purified by thin-layer
chromatography (SiO2, n-hexane and ethyl acetate) to isolate the corresponding products.
Result and discussion
The crystal structure of the synthesized materials is characterized by X-ray diffraction (XRD)
(Fig. 1 and Fig. s1). After annealing treatment, the XRD patterns of annealed ZIF materials have
been maintained after the annealed process. However, ZIF diffraction peaks disappeared in A-
ZIF-67, which indicated the degree of carbonization in ZIF-67 and highlighted the fact that
cobalt species would have an impact on catalytic graphitization [45], Diffraction peaks related
to Pd are hardly detected in the XRD patterns of Pd@A-ZIF nanocomposites, maybe due to the
small palladium nanoparticles.
7

Fig. 1. XRD patterns of A-ZIF-8, A-ZIF-67, A-BMZIF, Pd@A-ZIF-8, Pd@A-ZIF-67, and Pd@A-BMZIF
nanocomposites.
The nitrogen sorption analysis is performed for the characterization of the specific
surface area and porous structure of as-synthesized materials. The sorption curves show that all
the as-synthesized materials display similar IV-type isotherms with an H3-type hysteresis loop in
the larger range of 0.45 - 1.0 P/P0, proving the presence of mesoporous structures in the
composites. Also, the sharp increase at a low relative pressure (P/P0<0.05) in A-ZIF-8 and A-
BMZIF, revealed the presence of micropores structures in these composites. However, the
surface area of A-BMZIF was higher than A-ZIF-8 and A-ZIF-67 (Table s1). Additionally, after
immobilization of Pd NPs on A-ZIF materials, the specific surface area and pore volume
decreased, indicating that the Pd NPs are successfully supported on the A-ZIF materials.
8

Fig. 2. N2 sorption isotherms of A-ZIF-8, A-ZIF-67, A-BMZIF, Pd@A-ZIF-8, Pd@A-ZIF-67, and
Pd@A-BMZIF nanocomposites.
Transmission electron microscopy (TEM) and field emission scanning electron
microscopy (FESEM) were used to study the morphology and size of the as-synthesized
materials (Fig. 3 and Fig. S2-S6). The cubic structure was retained after the annealing process in
all of the materials (Fig. 3). However, the TEM and FESEM images of A-ZIF-67 and A-BMZIF
showed a rough surface and shrunken facets cubes that are composed of graphitic carbon
sheets.
9

Fig. 3. FESEM images of a) A-ZIF-8, b) A-ZIF-67, and c) A-BMZIF composites and TEM images of
d) A-ZIF-8, e) A-ZIF-67, and f) A-BMZIF composites.
After anchoring the Pd NPs on cubic shaped ZIF, these were found dispersed in the A-ZIF
materials with an average particle size of 10 nm. Moreover, the aggregated Pd NPs were
observed in the Pd@A-ZIF-67 and Pd@A-BMZIF nanocomposites. The cobalt species in the A-
ZIF-67 and A-BMZIF cause the excessive growth of Pd NPs during reduction and result in the
aggregation of Pd NPs on surface of A-ZIF-67 and A-BMZIF.
10

Fig. 4. TEM images of a-b) Pd@A-ZIF-8, c-d) Pd@A-ZIF-67, and e-f) Pd@A-BMZIF
nanocomposites.
After careful characterization of as-synthesized Pd@A-ZIF, the catalytic activity of these
nanostructures is studied in the Suzuki-Miyaura cross-coupling reaction. The model cross-
coupling reaction of phenyl iodide and phenylboronic acid is investigated to optimize the
reaction parameters (Table 1). The reaction afforded the desired cross-coupling product
(biphenyl) in good yield (84%) using EtOH as a solvent in the presence of Pd@A-ZIF-8 (0.1 mol %
of Pd) as catalyst and K2CO3 (2 eq.) as a base at 70 °C (Table 1, entry 1). Firstly, the effect of
solvent was investigated and the reaction was carried out in other solvent systems such as
MeOH, H2O, dimethylformamide (DMF), (Table 1, entries 2-4). When the mixture solvent
(H2O:EtOH) was used, the yield of biphenyl has obviously increased (Table 1, entry 5). In the
absence of base, the reaction wholly failed (Table 1, entry 6). Thus, K3PO4, NaOH, KOH, NaOAc,
11

and Et3N were evaluated as bases for this transformation (Table 1, entries 7-10). K3PO4 was the
best choice as a base for the Suzuki-Miyaura cross-coupling reaction (Table 1, entry 6). The yield
was reduced upon decreasing of the reaction temperature (Table 1, entry 11).
Table 1. Optimization of reaction for Suzuki-Miyaura cross-coupling reactiona
Entry
Catalyst
Solvent
Base
Yield (%)b
1
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
Pd@A-ZIF-8 (0.1 mol% of Pd)
EtOH
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
NaOH
KOH
84
71
52
49
82
98
71
44
57
42
51
2
MeOH
3
H2O
4
DMF
5
H2O:EtOH (1:1)
H2O:EtOH (1:1)
H2O:EtOH (1:1)
H2O:EtOH (1:1)
H2O:EtOH (1:1)
H2O:EtOH (1:1)
H2O:EtOH (1:1)
6
7
8
9
NaOAc
Et3N
10
11c
K3PO4
a) Phenyl iodide (0.5 mmol), phenylboronic acid (0.7 mmol), Base (2 eq.), Solvent (3 mL),
catalyst, Temp. = 70 °C, Time = 0.5 h.
b) GC yield.
c) Temp. = 50 °C.
12

Other as-synthesized catalysts (Pd@A-ZIF-67, and Pd@A-BMZIF) were used (Fig. 5). The
results showed that the order of the rate of the catalytic activity was Pd@A-ZIF-8 > Pd@A-
BMZIF > Pd@A-ZIF-67. Seemingly, the synergistic action between Pd and Zn species mmay play
a key role in highly active catalytic species in Suzuki-Miyaura cross-coupling reaction [46]. By
decreasing the catalyst loading from 0.1 to 0.05 mol% of Pd, the reaction yield dramatically
reduced from 98 to 46%. However, an increasing in the loading of catalyst to 0.15 mol%, did not
affect on the yield.
Fig. 5. Suzuki–Miyaura cross-coupling reaction of model reaction in the presence of as-
synthesized catalysts.
Using the optimal reaction conditions, the scope of the Suzuki-Miyaura cross-coupling
reaction was investigated (Table 2). In general, aryl iodide, bearing an electron-withdrawing
substituent (p-NO2 and p-COMe), was more reactive and produced higher yield (77% yield)
(Table 2, entries 3 and 4) than those analogs bearing the electron-donating substituents (p−Me)
(Table 2, entry 2). Similar reactivity was observed for aryl bromides (Table 2, entries 7-11).
13

Additionally, the slight influence of the steric effect was observed (Table 2, entry 5).
Unfortunately, the presence of CTAB as a phase transfer reagent was necessary for the Suzuki-
Miyaura cross-coupling reaction of phenyl chloride with phenyl boronic acid and the catalytic
system was less effective in generating corresponding cross-coupled product in moderate yield
(Table 2, entry 11).
Table 2. Pd@A-ZIF-8 nanocomposite-catalyzed Suzuki–Miyaura cross-coupling reactiona
Entry
1
Y
X
I
Yield (%)b
TOF (h-1)
1960
1700
1960
1880
1640
980
H
98
85
98
94
82
98
78
89
91
93
69
2
p-Me
p-NO2
p-COMe
o-Me
H
I
3
I
4
I
5
I
6c
Br
Br
Br
Br
Br
Cl
7c
p-Me
p-OMe
p-COMe
p-NO2
H
780
8c
890
9c
910
10c
11d
930
230
14

a) Aryl halide (0.5 mmol), phenylboronic acid (0.7 mmol), K3PO4 (1.0 mmol), Pd@A-ZIF-8 (0.1
mol% of Pd), H2O:EtOH (1:1, 4 mL), Temp. = 70 °C, Time = 0.5 h.
b) Isolated yield.
c) Time = 1 h.
d) Pd@A-ZIF-8 (0.2 mol% of Pd), CTAB (1 mmol), Temp. = 90 °C, Time = 1.5 h.
The recyclability of the Pd@A-ZIF-8 nanocomposite was examined upon for the cross-
coupling reaction of phenyl iodide and phenylboronic acid. After completion of the reaction,
the mixture was easily centrifuged and the separated the solid catalyst from the reaction
mixture was then subjected to the next catalytic cycle. The catalyst would be recovered and
reused for at least six times without significant loss of catalytic activity (Fig. 6a). Moreover, the
TEM analysis showed that the morphology and size distribution of Pd@A-ZIF-8 did not change
after the 6th cycle. (Fig. 6b). The metal leaching of the catalyst was accomplished by using
inductively coupled plasma-optical emission spectrometry (ICP-OES). The results of the
recovered Pd@A-ZIF-8 nanocomposite showed 2.8 % leaching of palladium after six cycles.
These results demonstrated the heterogeneous character of the Pd@A-ZIF-8 nanocomposite. In
addition, the reused Pd@A-ZIF-8, Pd@A-ZIF-67, and Pd@BMZIF nanocomposites were
characterized by XRD analysis after six consecutive reaction runs (Fig. S7). The results show that
the crystalline structure had no apparent change, indicating that nanocomposites possessed
the relative high stability.
15

Fig. 6. a) Pd@A-ZIF-8 nanocomposite recycling studies and b) TEM image of Pd@A-ZIF-8
nanocomposite after six consecutive reaction runs.
Aromatic amines and their derivates are one of the most important functional groups in
nature, constitute versatile building blocks in synthetic organic chemistry. They play an
important role in antioxidant, dyes, herbicides, polymers, rubbers, and pharmaceuticals [47].
Aromatic amines are mainly produced by catalytic hydrogenation of the corresponding
nitroaromatic compounds [48]. Thus, the catalytic activity of Pd@A-ZIF nanocomposite was
further evaluated for the reduction of nitroarenes. The catalytic activity of the Pd@A-ZIF
nanocomposite was studied by the model reduction of nitrobenzene. Initial studies were
accomplished upon the reduction of nitrobenzene with sodium borohydride as a model
reaction using Pd@A-ZIF-8 (0.05 mol% of Pd) as the catalyst in H2O:EtOH for 1 h (Fig. 7). Then,
the reaction was performed with different as-synthesized catalysts. Among the catalysts tested,
Pd@A-ZIF-67 was found to be the most effective catalyst since it gave the highest yield of the
product. The results showed that the order of the rate of the catalytic activity in the reduction
of nitrobenzene was Pd@A-ZIF-67> Pd@A-BMZIF > Pd@A-ZIF-8. Probably, the synergistic action
between Pd and Co species may play a crucial role in highly active catalytic species in the
reduction of nitroarene [49,50]. Increasing the catalyst loading from 0.05 to 0.025 mol% of Pd
had no effect on the yield, whereas decreasing the loading of catalyst to 0.1 mol% resulted in
lower conversion and yield.
16

Fig. 7. Optimization of the reaction conditions for the reduction of nitrobenzene (nitrobenzene
(0.4 mmol), NaBH4 (10.0 mmol), Catalyst (x mol% of Pd), H2O:EtOH (1:1, 25 mL), Temp. = r.t.,
Time = 1h, under air; Isolated yield.
Under the optimized reaction conditions, the scope of Pd@A-ZIF-67 catalysed reduction
was evaluated over various nitroarenes with structurally divergent functional groups. It was
found that the nitroarenes possessing the hydroxyl substituent (o-OH, m-OH, and p-OH) gave
good yields of corresponding anilines in a very short amount of time (Table 3, entries 2–4). The
nitrophenols (o-NP, m-NP, and p-NP) gave good yields of corresponding anilines. m-NP was
investigated as the most reactive substrate among all the three nitrophenols. The reaction
proceeded via the formation of nitrophenolate and the delocalization of the oxygen’s negative
charge throughout the benzene ring. The p-nitrophenolate ion are the most stable anion
compared to o-nitrophenolate and m-nitrophenolate, which reduces reactivity in p-NP. o-
nitrophenolate is less stable than p-nitrophenolate because of less effective resonance coupled
with the particular degree of steric effect between the substituents in ortho-position. m-NP is
the least stable and the most reactive among all the three nitrophenols. Nitroarenes with
17

electron-releasing substituents such as p-OMe, p-Me, and p-NH2 and electron-withdrawing
substituents such as p-Cl and p-Br also produced the corresponding anilines in good yields
(Table 3, entries 5–9). The halo-substituted nitroarenes could be reduced to the corresponding
anilines with no discernible dehalogenation (Table 3, entries 8–9).
Table 3. The scope of reduction of nitroarenes over Pd@A-ZIF-67 nanocompositea
Entry
Nitroarene
Aniline
Yield(%)b
98
TOF (h-1)
1960
1800
1960
1840
1840
1780
1740
1780
1820
1
2
3
4
5
6
7
8
9
90
98
92
92
89
87
89
91
a) Nitroarene (0.4 mmol), NaBH4 (10.0 mmol), H2O:EtOH (1:1, 25 mL), Pd@A-ZIF-67
nanocomposite (0.05 mol% of Pd), Temp. = r.t., Time = 1h, under air.
b) Isolated yield.
18

Furthermore, the catalytic cycling tests were performed to study the stability of the
Pd@A-ZIF-67 nanocomposite for the reduction of p-nitrophenol. For this purpose, after
completing of reactions, the catalyst was centrifuged for repeated use. After the 6th cycle, the
nanocomposite showed high catalytic activity for the reduction of p-nitrophenol, which
indicated the high stability of the nanocomposite (Table 4).
Table 4. Reusability of the Pd@A-ZIF-67 nanocomposite for the reduction of p-nitrophenola
Reaction cycle
Yield (%)b
1st
95
2nd
94
3rd
90
4th
86
5th
86
6th
82
a) p-nitrophenol (0.4 mmol), NaBH4 (10.0 mmol), Pd@A-ZIF-67 nanocomposite (0.05 mol% of
Pd), H2O:EtOH (1:1, 25 mL), Temp. = r.t., Time = 1h, under air;
b) Isolated yield.
The catalytic performance of our catalyst was compared with those previously reported
palladium-based heterogeneous catalytic methods in terms of catalyst loading, TOF and size of
nanoparticles for the reduction of p-NP (Table 5). The results demonstrate that the Pd@A-ZIF-
67 nanocomposite is highly effective (TOF: 1840 h-1) in comparison to previously reported
catalysts because our catalytic reaction was carried out with a very low loading of the catalyst.
This fact could be attributed to the synergistic action between Pd and Co species which may
play a crucial role in highly active catalytic species in the reduction of nitroarenes.
Table 5. Comparison of the efficiency of various palladium-based heterogonous catalysts in the
reduction of p-nitrophenol
Entry
Catalyst
Size of NPs
(nm)
TOF (h-1)
Catalyst (mol%) Ref.
19

1
2
3
4
5
6
Pd@A-ZIF-67
Fe3O4@N-C@Pd
Pd@NAC-T
Pd/oMWCNT
Pd/ZnO
10
1840
980
99
0.05
1
This work
[51]
25-30
2
0.5
[52]
2-3
-
252
0.9
148
30.5
0.37
[53]
[54]
Pd/SPB-PS
2-4
820
[55]
Conclusions
In conclusion, we have developed the design and synthesis of Pd nanoparticles on cubic
shaped ZIF-based materials. The ZIF-based cubic materials such as ZIF-67, ZIF-8, and BMZIF
were synthesized with the assist of a capping agent (CTAB). The resulted cubic shaped ZIFs were
annealed under inert atmosphere. Finally, the Pd nanoparticles anchored on cubic shaped ZIF
by using chemical reduction of PdCl2(CH3CN)2 with sodium borohydride. The catalytic activities
of synthesized materials were evaluated in Suzuki-Miyaura cross-coupling. The results showed
the catalytic activity of Pd@A-ZIF-8 was higher than Pd@A-ZIF-67 and Pd@A-BMZIF.
Additionally, the catalytic activities of as-synthesized nanocomposites were studied on the
reduction of nitroarenes. The catalytic activity of Pd@A-ZIF-67 was higher than Pd@A-ZIF-8 and
Pd@A-BMZIF in the reduction of nitrobenzene. The synergistic action between Pd and Zn, Pd
and Co species may play an important role in highly active catalytic species in Suzuki-Miyaura
cross-coupling reaction and reduction of nitroarenes, respectively. Also, the reusability of as-
synthesized nanocomposites was checked, and the catalytic activity was retained after six
consecutive reaction runs.
Author Agreement Statement
20

We the undersigned declare that this manuscript is original, has not been published before and is not
currently being considered for publication elsewhere.
We confirm that the manuscript has been read and approved by all named authors and that there are no
other persons who satisfied the criteria for authorship but are not listed. We further confirm that the
order of authors listed in the manuscript has been approved by all of us.
We understand that the Corresponding Author is the sole contact for the Editorial process. She is
responsible for communicating with the other authors about progress, submissions of revisions and final
approval of proofs
Signed by all authors as follows:
Minoo Dabiri
Hassan Fazli
Neda Salarinejad
Siyavash Kazemi Movahed
Conflicts of interest
There are no conflicts to declare.
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.
21

Acknowledgements
We gratefully acknowledge financial assistance from the Research Council of Shahid Beheshti
University and the Iranian National Science Foundation (Proposal No: 96016966).
22

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26

Figure captions
Fig. 1. XRD patterns of A-ZIF-8, A-ZIF-67, A-BMZIF, Pd@A-ZIF-8, Pd@A-ZIF-67, and Pd@A-BMZIF
nanocomposites.
Fig. 2. N2 sorption isotherms of A-ZIF-8, A-ZIF-67, A-BMZIF, Pd@A-ZIF-8, Pd@A-ZIF-67, and
Pd@A-BMZIF nanocomposites.
Fig. 3. FESEM images of a) A-ZIF-8, b) A-ZIF-67, and c) A-BMZIF composites and TEM images of
d) A-ZIF-8, e) A-ZIF-67, and f) A-BMZIF composites.
Fig. 4. TEM images of a-b) Pd@A-ZIF-8, c-d) Pd@A-ZIF-67, and e-f) Pd@A-BMZIF
nanocomposites.
Fig. 5. Suzuki–Miyaura cross-coupling reaction of model reaction in the presence of as-
synthesized catalysts.
Fig. 6. a) Pd@A-ZIF-8 nanocomposite recycling studies and b) TEM image of Pd@A-ZIF-8
nanocomposite after six consecutive reaction runs.
Fig. 7. Optimization of the reaction conditions for the reduction of nitrobenzene (nitrobenzene
(0.4 mmol), NaBH4 (10.0 mmol), Catalyst (x mol% of Pd), H2O:EtOH (1:1, 25 mL), Temp. = r.t.,
Time = 1h, under air; Isolated yield
Table captions
Table 1. Optimization of reaction for Suzuki-Miyaura cross-coupling reactiona
Table 2. Pd@A-ZIF-8 nanocomposite-catalyzed Suzuki–Miyaura cross-coupling reactiona
Table 3. The scope of reduction of nitroarenes over Pd@A-ZIF-67 nanocompositea
Table 4. Reusability of the Pd@A-ZIF-67 nanocomposite for the reduction of p-nitrophenola
Table 5. Comparison of the efficiency of various palladium-based heterogonous catalysts in the
reduction of p-nitrophenol
27

Fig. 1. XRD patterns of A-ZIF-8, A-ZIF-67, A-BMZIF, Pd@A-ZIF-8, Pd@A-ZIF-67, and Pd@A-BMZIF
nanocomposites.
28

Fig. 2. N2 sorption isotherms of A-ZIF-8, A-ZIF-67, A-BMZIF, Pd@A-ZIF-8, Pd@A-ZIF-67, and
Pd@A-BMZIF nanocomposites.
29