Embedded homogeneous ultra-fine Pd nanoparticles within MOF ultra-thin nanosheets for heterogeneous catalysis

Article abstract of DOI:10.1039/d0dt03877f

Metal nanoparticle-incorporated metal-organic framework (MOF) nanosheets can integrate their respective merits to have important applications in heterogeneous catalysis. Herein, we report the loading of ultra-fine Pd nanoparticles within ultra-thin nanosheetsNMOF-Nithrough anin situreduction strategy under mild conditions. Owing to the synergetic effects of ultra-small Pd NPs and the intrinsic characteristics of two-dimensional supports, the obtainedPd@NMOF-Nishowed high catalytic activity and size-selectivity in olefin hydrogenation with easy recovery. Meanwhile,Pd@NMOF-Niis also highly active in the catalytic reduction of aqueous 4-nitrophenol to 4-aminophenol.

Full text of DOI:10.1039/d0dt03877f

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Embedded homogeneous ultra-fine Pd
nanoparticles within MOF ultra-thin nanosheets
for heterogeneous catalysis†
Cite this: Dalton Trans., 2021, 50,
1774
a
a
a
a
b
Taolian Guo,‡ Kaili Mo,‡ Nannan Zhang, Lipengcheng Xiao, Wenlong Liu
and
a
Lili Wen
*
Metal nanoparticle-incorporated metal–organic framework (MOF) nanosheets can integrate their
respective merits to have important applications in heterogeneous catalysis. Herein, we report the loading
of ultra-fine Pd nanoparticles within ultra-thin nanosheets NMOF-Ni through an in situ reduction strategy
under mild conditions. Owing to the synergetic effects of ultra-small Pd NPs and the intrinsic character-
istics of two-dimensional supports, the obtained Pd@NMOF-Ni showed high catalytic activity and size-
selectivity in olefin hydrogenation with easy recovery. Meanwhile, Pd@NMOF-Ni is also highly active in
the catalytic reduction of aqueous 4-nitrophenol to 4-aminophenol.
Received 11th November 2020,
Accepted 4th January 2021
DOI: 10.1039/d0dt03877f
rsc.li/dalton
with nanoscale thickness and minimized distance of mass
transport, the utilization of MOF nanosheets (NMOFs) as
Introduction
2
5
Over the past few decades, Pd nanoparticles (NPs) have been supports, not only offers more exposed accessible active sites,
extensively investigated in a variety of catalytic systems, exhibit- but also renders easy contact between reactants and functional
1
–5
ing high chemical activity and specificity. However, as a con- sites, resulting in significantly enhanced catalytic efficiency of
2
6,27
sequence of high surface energy, thermodynamic stability of Pd@NMOFs.
In particular, stemming from space limit-
Pd NPs is severely decreased, which are prone to aggregate and ations of the highly ordered pores within supports,
2
8
fuse, resulting in the deactivation during continuous reac- Pd@NMOFs are beneficial to the size-selective catalysis.
6
tions. As a particular class of porous materials, metal–organic
Bearing these facts in mind, herein, we fabricate highly dis-
also called porous coordination poly- persed ultra-fine Pd NPs (∼1.0 nm) within ultra-thin NMOF-Ni
7
–9
frameworks (MOFs),
mers, possess large surface areas and uniform pore structures, nanosheets (∼8 nm) through an in situ reduction strategy
enabling them to encapsulate the metal–oxide clusters/ under mild conditions. The Pd@NMOF-Ni composite exhibi-
1
0–12
nanoparticles,
appearing to be an efficient way of prevent- ted high catalytic activity and size-selectivity in olefin hydro-
ing aggregation. Therefore, MOFs can be utilized as promising genation, as well as high catalytic efficiency in the reduction of
host matrices for circumventing the migration and agglomera- aqueous 4-nitrophenol to 4-aminophenol, arising from the
tion of Pd NPs due to confinement and electronic effects synergetic effects of ultra-small Pd NPs and the intrinsic
1
3–22
offered by MOFs.
To date, a great deal of research efforts characteristics of 2D supports.
have mostly been devoted to the encapsulation of Pd NPs
2
3
within bulk MOFs (denoted as Pd@bulk MOFs).
Unfortunately, owing to a lengthy diffusion path and less
Results and discussion
exposed active sites, Pd@bulk MOFs usually exhibit limited
3
,24
mass transfer and modest catalytic activities.
In contrast,
NMOF-Ni nanosheets with a thickness of ∼8 nm were fabri-
cated via a bottom-up method by our group (Fig. S1a and
2
9
a
S1b†).
Considering the available well-organized cavities,
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of
Chemistry, Central China Normal University, Wuhan, 430079, China.
E-mail: wenlili@mail.ccnu.edu.cn
NMOF-Ni nanosheets are desirable for confining and stabiliz-
ing Pd NPs. In brief, the embedding of Pd NPs into an
NMOF-Ni matrix (denoted as Pd@NMOF-Ni) entails inclusion
of a palladium precursor via solution infiltration, and the suc-
cessive reduction of metal precursors to metal atoms. In a
typical synthesis, 150 mg of NMOF-Ni was dispersed in 20 mL
deionized water. Then, 1 mL of PdCl2 aqueous solution
b
College of Chemistry & Chemical Engineering, Yangzhou University, Yangzhou,
225002, China
†
Electronic supplementary information (ESI) available: Details of synthesis,
additional figures, PXRD, SEM and TEM images, XPS spectra, pore size distri-
bution profiles and UV–vis adsorption spectra. See DOI: 10.1039/d0dt03877f
‡
The authors contributed equally.
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2
−1
3
−1
(
8.4 mg, 0.047 M) was added, generating a green crystalline 662 m g and 0.612 cm g of pristine NMOF-Ni (Fig. 2b).
solid, which was collected through filtration, and subsequently As compared to parent NMOF-Ni, the appreciable decrease in
redispersed into 20 mL deionized water. After adding aqueous the surface area and the total pore volume of Pd@NMOF-Ni
NaBH (2 mL, 0.1 M) and further stirring for 0.5 h, the resul- might be attributed to the pore blocking by the deposited Pd
4
tant dark Pd@NMOF-Ni nanocomposite was isolated and NPs. As evident from the pore-distribution study,
washed with water. The Pd NP loading was determined by Pd@NMOF-Ni possesses micropores of diameters ca. 1.16 nm
inductively coupled plasmas (ICP) up to ∼2.5% for the (Fig. S2†). X-ray photoelectron spectroscopy (XPS) was
Pd@NMOF-Ni nanocomposite. For comparison, Pd NPs were employed on Pd@NMOF-Ni (Fig. 2c), which showed two pro-
also loaded on the bulky counterpart via the same path to nounced bands at 341.1 and 335.7 eV (Fig. 2d), assigned to the
0
yield Pd@MOF-Ni, and the content of Pd NPs is ∼2.3%, binding energy of 3d3/2 and 3d5/2 of Pd (metallic palladium),
similar to that of Pd@NMOF-Ni.
Transmission electron microscopy (TEM) (Fig. 1a and b) related to the 3d3/2 and 3d5/2 of Pd (palladium oxide), illumi-
and high-angle annular dark-field scanning transmission elec- nating that the surface of Pd NPs was partially oxidized
tron microscopy (HAADF-STEM) images (Fig. 1c) evidently because of the exposure to air. The ratio of Pd to Pd is
demonstrate a uniform distribution of Pd NPs with a mean about 67 : 33, derived from the peak splitting results. In
size of about 1.0 nm immobilized with the NMOF-Ni matrix, addition, the peaks at 856 and 873.9 eV corresponding to 2p3/2
where no remarkable formation of aggregates can be observed. and 2p of Ni confirmed that no valence state change of
The comparison of PXRD patterns (Fig. 2a) revealed that the Ni was examined for Pd@NMOF-Ni (Fig. S3†).
integrity of the NMOF-Ni structure remained intact after the Comparably, the bulky counterpart MOF-Ni was also used
respectively. The other two peaks at 342.3 and 336.9 eV were
2
+
3
0
0
2+
2
+
1
/2
2
+
31
encapsulation of Pd NPs. Furthermore, the absence of identifi- as a support to incorporate Pd NPs (termed Pd@MOF-Ni).
able Pd characteristic peaks in Pd@NMOF-Ni indicates ultra- Apparently, the TEM images (Fig. 3a and b) disclose that Pd
small size and low content of generated Pd NPs.
Notably NMOF-Ni still retains microporosity upon loading MOF-Ni, instead of evenly spread throughout NMOF-Ni.
of Pd NPs, ascertained by a standard type-I N adsorption iso- Additionally, Pd NPs of mean size 2.82 nm in bulk MOF-Ni
therm of Pd@NMOF-Ni at 77 K. A Brunauer–Emmett–Teller were substantially larger than those in Pd@NMOF-Ni (Fig. 3d).
BET) surface area and microporous volume of Pd@NMOF-Ni Furthermore, a representative HRTEM image (Fig. 3c) presents
NPs were located primarily closer to or at the surface of
2
(
2
−1
3
−1
significantly decreased to 342 m g and 0.421 cm g , from clear lattice fringes with the interplanar spacing of 0.19 and
Fig. 1 TEM images (a, b) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (c) of Pd@NMOF-Ni
nanosheets. (d) Particle size distributions of Pd NPs in Pd@NMOF-Ni nanosheets.
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Fig. 2 (a) PXRD patterns of MOF-Ni, NMOF-Ni, Pd@NMOF-Ni, and simulated form based on single-crystal X-ray diffraction. (b) Nitrogen adsorp-
tion/desorption isotherms at 77 K for NMOF-Ni and Pd@NMOF-Ni. (c) XPS survey spectra of Pd@NMOF-Ni. (d) High-resolution XPS spectrum for Pd
3d of Pd@NMOF-Ni.
Fig. 3 TEM (a, b) and HRTEM (c) images of Pd@MOF-Ni. (d) Particle size distributions of Pd NPs.
1776 | Dalton Trans., 2021, 50, 1774–1779
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0
.22 nm, corresponding to the (200) and (111) planes of face-
3
2–35
centered cubic Pd, respectively.
Given the higher diffusion
barrier within bulk MOF-Ni, it is reasonable to believe that
some Pd nuclei formed first were prone to be near the surface
of the bulk counterpart, which then aggregated into larger Pd
NPs. In contrast, for NMOF-Ni nanosheets, the nucleation and
growth of most Pd(0) tend to be at the same rate upon
exposure to NaBH , resulting in a good uniformity of Pd NPs.
4
As well-known, Pd NPs have been the high interest in the
chemical industry for heterogeneous catalysts for many
organic reactions, for example, the catalytic hydrogenation of
3
6–39
olefins.
Therefore, the catalytic activity of the as-syn-
Fig. 4 Reusability of Pd@NMOF-Ni and Pd@MOF-Ni.
thesized Pd@NMOF-Ni was investigated using the hydrogen-
ation of styrene as a model reaction. After detailed investi-
gations, the optimum conditions were set at 25 °C for 50 min
with styrene (0.8 mmol) and Pd@NMOF-Ni (0.15 mol%) as the conditions, the catalyst was filtered-off after 20 min of reaction
catalyst in ethanol under the atmosphere pressure of H , with and the yield reached 33% for styrene. Apparently, the reaction
2
almost full conversion, giving a turnover frequency (TOF) value was completely terminated after the removal of the catalyst.
of 1230 h− (Table 1, entry 10). The catalytic performance of The leaching tests (Fig. S7†) indicate that any loss of Pd can be
Pd@NMOF-Ni was superior to most of the reported Pd/MOF- neglected during the reaction, which was further confirmed by
1
based catalysts as shown in Table S1.†
the ICP result (no obvious amount of Pd was detected for the
After the reaction, Pd@NMOF-Ni can be easily recovered by filtrate of catalytic reactions after 6 runs). To demonstrate the
centrifugation and washed with ethanol. Of particular note advantages of the Pd@NMOF-Ni composite in catalysis, the
was that Pd@NMOF-Ni could be reused for at least six runs control reaction over Pd@MOF-Ni was also carried out. In
without any distinct loss in activity (Fig. 4). Remarkably, PXRD sharp contrast, the conversion of styrene decreases to only
profiles showed that the crystallinity of the catalyst was intact 20% in the fourth run under otherwise identical conditions
over successive runs (Fig. S4†). Moreover, the original mor- (Fig. 4). Obviously, the outstanding performance of
phology and average particle size of Pd NPs within NMOF-Ni Pd@NMOF-Ni might benefit from the confinement effect
remain well-kept, as revealed by the TEM images of recovered exerted by NMOF-Ni, which can effectively limit the growth
Pd@NMOF-Ni (Fig. S5†), indicative of the high durability of and prohibit the leaching of Pd NPs constrained within pores
the catalyst. In addition, XPS of the used Pd@NMOF-Ni during the catalytic reactions (Fig. S5†). Moreover, the intrinsic
0
2+
(
6
Fig. S6†) revealed that the ratio of Pd to Pd was about characteristics of the nanosheet supports definitely favor more
4 : 36, basically matching with that of the fresh catalyst. accessible active sites and a lower diffusion barrier for the
Confirmation on the heterogeneous nature of Pd@NMOF-Ni reactants.
was provided by leaching experiments. Under the optimum
Furthermore, the well-defined pore structure of
Pd@NMOF-Ni can act as a “molecular sieve” for selective cata-
lysis. Liquid-phase hydrogenations of n-hexene,
a
β-methylstyrene, trans-stilbene, and tetraphenyl ethylene were
selected to investigate the molecular-size selectivity of the
Pd@NMOF-Ni composite. As shown in Table 2, as the mole-
cular size of the substrates gradually increased, the conversion
of the reactants decreased correspondingly. For n-hexene (esti-
Table 1 Catalytic hydrogenation of styrene using the Pd@NMOF-Ni
catalyst
2
mated size: 2.6 × 7.9 Å ), the Pd@NMOF-Ni composite showed
b
high activity with full conversion after 50 min reaction, indicat-
Styrene
(mmol)
T
(°C)
Time
(min)
Yield
(%)
Entry
Solvent
ing that there was no significant diffusion limitation caused by
2
the NMOF-Ni frame. For β-methylstyrene (5.3 × 9.1 Å ), the
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
0.8
0.8
0.8
0.8
EtOH
Ethyl acetate
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
25
25
25
25
35
35
35
25
25
25
25
80
80
60
45
45
50
60
80
60
50
40
97%
87%
80%
50%
80%
93%
100%
100%
100%
corresponding conversion rate dropped to 77.7% under the
same conditions. It needs to be emphasized that the
Pd@NMOF-Ni composite showed no propensity to catalyze the
2
hydrogenation of trans-stilbene (6.5 × 12.5 Å ) after 50 min
(
Table 2, entry 5), whose molecular size was close to the pore
aperture of NMOF-Ni. The conversion of trans-stilbene is only
1
1
0
1
EtOH
EtOH
100% up to 86.8% even after the reaction time was extended to
83%
120 min (Table 2, entry 6). Importantly, with the sterically
a
more demanding tetraphenyl ethylene, whose molecular size
(10.2 × 12.3 Å ) is larger than the pore size of NMOF-Ni, almost
Reaction conditions: Solvent: 3 mL; Pd@NMOF-Ni 0.15 mol%;
b
2
pressure 1 bar H
2
.
Yield was determined by GC.
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a
Table 2 Catalytic hydrogenation of alkenes with different sizes using the Pd@NMOF-Ni catalyst
b
Entry
1
Alkenes
Estimated sizes
Time (min)
50
Yield
n-Hexene
100%
2
3
4
Cyclohexene
Styrene
50
50
50
82.4%
100%
β-Methylstyrene
77.7%
5
trans-Stilbene
50
0%
c
6
trans-Stilbene
120
120
86.8%
c
7
Tetraphenyl ethylene
0%
a
b
Reaction conditions: Alkenes 0.8 mmol; solvent 3 mL EtOH; catalyst 0.15 mol% (5 mg); pressure 1 bar H
2
; time 50 min; T = 25 °C. Yield was
c
determined by GC. Time: 120 min, under otherwise identical conditions.
no conversion was observed even after 120 min (Table 2, entry Based on a pseudo-first-order kinetics, the conversion rate con-
), reflecting the size selectivity of the Pd@NMOF-Ni catalyst. stant of 4-NP is estimated to be 0.495 min− for Pd@NMOF-Ni,
1
7
The above results confirm that Pd NPs are confined inside the which
even
outperforms
that
of
Au-1@NMOF-Ni
NMOF-Ni framework and that the Pd NPs are absent on the (0.404 min− ). Unfortunately, much longer reaction time was
1 29
outer surface of the composite. Notably, both styrene (5.2 × needed over Pd@MOF-Ni (16 min) (based on the same of Pd
2
2
7
.1 Å ) and cyclohexene (5.3 × 5.5 Å ) with small sizes could amount) under identical conditions (Fig. S9†). Apparently, the
diffuse through the aperture of NMOF-Ni without any obstacle. ultrafine Pd NPs and the intrinsic characteristics of 2D
However, the hydrogenation activity of styrene over nanosheets can be responsible for the superior catalytic
Pd@NMOF-Ni is superior to that of cyclohexene, which may be activity of Pd@NMOF-Ni.
ascribed to the regioselectivity of catalyzing terminal alkenes
4
0
not internal alkenes (Table 2, entries 2 and 3).
Further, the catalytic activity of Pd@NMOF-Ni was also eval-
uated by the reduction of 4-nitrophenol (4-NP) to 4-amino-
phenol (4-AP) in NaBH aqueous solution at room tempera-
4
Conclusions
ture. Upon the addition of Pd@NMOF-Ni, as detected by the The Pd@NMOF-Ni composite, characteristic of highly dis-
naked eye, the color of the solution quickly fades from bright persed ultrafine Pd nanoparticles incorporated ultra-thin
yellow to colorless within 6 min, suggesting the complete NMOF-Ni nanosheets, exhibited excellent recyclability and
transformation of 4-NP to 4-AP. As monitored by UV-vis spec- selectivity in olefin hydrogenation, as well as high catalytic
troscopy, the absorption peak of 4-NP at 400 nm rapidly efficiency in reduction of aqueous 4-nitrophenol to 4-amino-
decreased together with a concomitant increase of a 300 nm phenol, arising from the synergetic effects of ultrasmall Pd
characteristic peak of 4-AP, as the reactions proceed (Fig. S8†). NPs and 2D supports.
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1
7 M. T. Zhao, K. Deng, L. C. He, Y. Liu, G. D. Li, H. J. Zhao
and Z. Y. Tang, J. Am. Chem. Soc., 2014, 136, 1738–1741.
Conflicts of interest
There are no conflicts to declare.
18 Q. H. Yang, Q. Xu, S. H. Yu and H. L. Jiang, Angew. Chem.,
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19 Z. Deng, H. J. Yu, L. Wang, J. Y. Liu and K. J. Shea, J. Mater.
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