Highly Stable and Active Palladium Nanoparticles Supported on a Mesoporous UiO66@reduced Graphene Oxide Complex for Practical Catalytic Applications

Article abstract of DOI:10.1002/ejic.201600367

A metal–organic framework-based composite with reduced graphene oxide was successfully designed, prepared and used for the first time as a support for Pd nanoparticles. The as-synthesized UiO66@RGO-Pd composite was characterized by powder X-ray diffractio

Full text of DOI:10.1002/ejic.201600367

DOI: 10.1002/ejic.201600367
Full Paper
Supported Catalysts
Highly Stable and Active Palladium Nanoparticles Supported on
a Mesoporous UiO66@reduced Graphene Oxide Complex for
Practical Catalytic Applications
Xiang Liu,[a] Xiaohua Zhao*[b] Ming Zhou,[b] Yuanyuan Cao,[a] Haiyan Wu,[a] and
Jianjun Zhu*[a]
Abstract: A metal–organic framework-based composite with transmission electron microscopy (TEM) and N2 adsorption. The
reduced graphene oxide was successfully designed, prepared reduction of nitro compounds was used to demonstrate the
and used for the first time as a support for Pd nanoparticles. catalytic efficiency, which showed excellent activity due to the
The as-synthesized UiO66@RGO-Pd composite was character- large surface area and well-defined pore structure. Additionally,
ized by powder X-ray diffraction (XRD), thermal gravimetric the Pd NPs confined in mesoporous cages could be readily re-
analyses (TGA), Fourier transform infrared spectroscopy (FTIR), covered after simple filtration and reused for at least six consec-
Raman spectroscopy, scanning electron microscopy (SEM), utive cycles without loss of activity.
Introduction
ecules to reach the active site in the pores and the large prod-
uct to leave the active sites easily. Among the reported various
MOFs, UiO66-type zirconium dicarboxylates exhibit some re-
markable advantages including high thermal stability, resist-
ance to atmospheric moisture and large accessible pore vol-
ume.[13] Xu et al. synthesized the Pt NPs@UiO66 heterostructure
by encapsulating synthesized Pt NPs into the UiO66 host matrix,
which showed remarkable electro-catalytic performance.[14]
Nevertheless, they suffer from some disadvantages: (1) the
structure of the MOFs is likely to be destroyed after ultrafine
noble metal NPs are introduced; (2) the supported catalyst is
recovered by a complicated method instead of conventional
filtration because of its nanoscale size, which greatly inhibits its
wider practical application in heterogeneous catalysis.
Supported noble metal nanoparticles (NPs), a class of important
heterogeneous catalysts, play a key role in the manufacturing
process in the synthesis of fine chemicals, pharmaceuticals and
so on because of a significant cost advantage and minimum
environmental pollution.[1–3] It is well known that the support is
most crucial in the heterogeneous catalysts, which can improve
specific properties such as mechanical strength, stability, activ-
ity and selectivity of catalysts. Various materials, such as carbon
materials,[4] polymers and metal oxides,[5,6] have been em-
ployed as supports for noble metal catalysts. However, tailoring
the properties of the support for catalysis remains a challenge.
Metal–organic frameworks (MOFs) have been emerging as a
new class of crystalline porous materials and have attracted
great interest in many prospective applications.[7–9] An impor-
tant structural feature of MOFs is the ultrahigh porosity and
incredibly high internal surface areas, which make them very
appropriate candidates for the stabilization of noble metal NPs
(Au, Pt and Pd).[10–12] Furthermore, the mesoporous cages and
large microporous windows may allow the large reactant mol-
An effective approach to overcoming the above problems is
to build MOF-based composites. This is particularly attractive as
it does not only overcome the shortcomings of MOF, but also
brings synergistic properties. For example, Zhou and coworkers
reported the in-situ strategy for preparing ZIF-8 supported
carbon-stabilized Pd NPs (C@Pd/ZIF-8), which showed better ac-
tivity and excellent recycling stability for the hydrogenation of
C=C.[15]
Herein, we report the successful synthesis of highly dis-
persed Pd NPs anchored on Zr-MOFs@RGO (UiO66@RGO) by
a hydrothermal method (Figure 1), and explore the catalytic
performance of the obtained UiO66@RGO-Pd nanocomposite
on the reduction of nitro compounds. Fortunately, the UiO66-
based composite, which combines the merits and mitigates the
shortcomings of each component, exhibits desirable efficiency
and excellent reusability. In addition, the characteristic mor-
phology of UiO66 is perfectly maintained after the Pd NP load-
[a] Institute of Chemical and Materials Engineering, Zhenjiang College,
Zhenjiang 212000, P. R. China
E-mail: liuxiang0222@126.com
http://www.zjc.edu.cn/
[b] School of Materials Science and Engineering, Jiangsu University,
Zhenjiang 212013, P. R. China
E-mail: Zhao12_19@163.com
http://www.ujs.edu.cn/site1/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600367.
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Figure 1. Schematic illustration for the synthesis of the UiO66@RGO-Pd composite.
ing. It is envisaged that this study is a basis for broad applica-
To provide direct insight into the RGO and Pd NP loading
tions in the catalytic field using supported noble metal NPs as onto UiO66, we examined the UiO66@RGO-Pd sample by using
the catalyst.
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). Pristine UiO66 products are octahedral mi-
crocrystals with smooth surfaces, and the sizes of UiO66 were
in the range of 600–800 nm (Figure 3, a). Also, a lot of small
octahedral microcrystals with an average edge length of ca.
400 nm were observed (Figure 3, c). Parts b and d of Figure 3
show that UiO66 and Pd NPs are tightly encapsulated by RGO
layers, hence the hybrid materials can be recycled by simple
filtration. Notably, no significant change in the morphology of
UiO66 was observed after loading the Pd NPs. It is likely that
the GO-based layers present in the composite helped the UiO66
to retain its crystalline structure by dissipating the electrostatic
charges. The ultrafine Pd NPs with an average size of 5 nm were
well encapsulated in the cages of UiO66. It is worth pointing
out that UiO66 effectively inhibited the growth of Pd NPs and
the aggregation phenomenon.
Results and Discussion
The crystallinity of the materials was tested by powder X-ray
diffraction (PXRD) and the results are shown in Figure 2. The
XRD peak at 2θ = 9.76° for GO could be assigned to the (001)
crystal face (Figure 2, a). Peak positions of the pure UiO66 sam-
ple were in good agreement with previous reports, confirming
the formation of the pure crystalline UiO66 phase.[16] The sharp
peaks indicate the excellent crystallinity of the framework. The
XRD patterns of the UiO66@GO and UiO66@RGO-Pd NPs were
almost identical to that of the parent UiO66, implying that the
addition of Pd precursors did not affect the growth of UiO66
after the reduction treatment. The unchanged backbones of
UiO66 in UiO66@RGO-Pd indicate that RGO did not disturb the
crystal structure of UiO66, but the intensity of the peaks in
UiO66@RGO-Pd decreased slightly after loading the Pd NPs.
This might be ascribed to the loss in crystallinity and/or struc-
tural distortion of UiO66 due to the inclusion of RGO and Pd
NPs. The feature peak at around 40° in UiO66@RGO-Pd could
be ascribed to the (111) crystal face of Pd0, which proved a
successful synthesis procedure. Notably, the peak of crystalline
GO was not observed for the UiO-66@GO composite, which
could be attributed to the low GO contents, and the exfoliation/
high dispersion of GO in polar solvents (DMF) upon sonication
during the preparation procedure.
Figure 3. SEM images of (a) UiO66 octahedrons and (b) UiO66@RGO-Pd com-
posites, TEM images of (c) UiO66 octahedrons and (d) UiO66@RGO-Pd com-
posites.
For a deeper understanding of the chemistry and structure
formation of the samples, FTIR and Raman spectra were carried
out. The FTIR spectra of samples are shown in Figure 4 (a), for
the initial GO sample the stretching vibration C=O of the carb-
onyl or carboxyl groups is observed as an intense band present
at 1728 cm–1. The bands at 1614 and 1384 cm–1 are attributed
to deformations of the O–H bond in water and CO–H groups.[17]
Deformation of the C–O bond is observed as the intense band
Figure 2. X-ray diffraction patterns of (a) GO, (b) UiO66, (c) UiO66@GO and
the (d) UiO66@RGO-Pd composite.
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present at 1052 cm–1. These all illustrate the presence of oxy- tization and the average size of the sp2 domains in graphitic
gen-containing functional groups in GO. The IR spectra of the materials. The RGO in UiO66@RGO-Pd showed a higher ID/IG
parent UiO66 in this work are the same as those in the litera- ratio (0.836) than GO in UiO66@GO (0.784), indicating the re-
ture.[18] The out-of-plane vibrations of terephthalic acid (BDC) duction of oxygen-containing groups in GO by NaBH4 and more
are observed in the region 1300–700 cm–1. The asymmetric and defects existing in RGO. The more disordered carbon structure
the symmetric stretching of the carboxylate group of BDC ap- in the UiO66@RGO-Pd may be due to the removal of oxygen-
pear at 1583 cm–1 and 1398 cm–1 in the spectrum. Thus, the Δ containing functional groups leading to a decrease in the aver-
value for the composite was calculated to be about 185 cm–1. age size of the in-plane sp2 domains after the hydrothermal
Generally speaking, the coordination type of the carboxylate treatment. The Raman spectrum of parent UiO66 exhibited
groups and metal cations could be decided according to these many features over the range 400–1700 cm–1, which provided
Δ values. For the Δ values that were in the range of 138– further evidence of the existence of UiO66, except for some
200 cm–1, a bridging ligand was considered.[19] Obviously, for minor shifts in some cases.[21]
UiO66@GO and UiO66@RGO-Pd the asymmetric and symmetric
stretching vibration bands that correspond with UiO66 still re-
main. Although the Δ values of the composites and the UiO66
were slightly different, their Δ values were in the range of 138–
200 cm–1. As a result, it was considered that for the composites
the carboxylate group may bind to the metal cation through
the bridging ligand, which also validates the stability after load-
ing with Pd NPs. Distinct from GO and UiO66@GO, in the spec-
tra of the UiO66@RGO-Pd composite, the FTIR spectrum of RGO
showed a weak C=O peak in addition to a broad O–H stretching
band, along with a significant weakening of the C–OH stretch-
ing after hydrothermal treatment. The above results confirmed
that GO was reduced to RGO. In addition, no sign of a new
phase in the UiO66@RGO-Pd hybrids indicated that no chemical
bonding was produced between UiO66 and RGO.
Porosity is one of the key factors that determine the ability
of porous materials for catalytic applications. N2 sorption iso-
therms collected at 77 K were used to evaluate the surface area
and porosity of UiO66, UiO66@GO and UiO66@RGO-Pd com-
posites. The data for GO are not reported here since its porosity
is negligible.[22] UiO66 and UiO66@GO showed typical type-I
isotherms, and UiO66@RGO-Pd exhibited a hybrid type I/IV iso-
therm with large hysteresis between adsorption and desorption
branches (Figure 5, a). Type I isotherms are indicative of a micro-
porous structure (pore size less than 2 nm), which was antici-
pated from the UiO66 crystal model. Type IV isotherms are
strong evidence of a mesoporous structure (pore size lies be-
tween 2 and 50 nm), which may come from the interstitial voids
between the nanoparticles of UiO66. The steep increase in the
adsorbed volume at low relative pressures revealed the pres-
ence of the micro-pores in the UiO66@RGO-Pd, the small slope
observed at medium relative pressures as well as the desorption
hysteresis denoted the existence of the meso-pores. The general
shape of the N2 adsorption–desorption isotherm of the
UiO66@RGO-Pd confirmed the existence of different pore sizes.
Compared with the bare UiO66, UiO66@RGO-Pd exhibited a
The structural changes of GO and RGO before and after the
reduction of Pd2+ should be traced because of the important
role of RGO in the preparation of UiO66@RGO-Pd, which has
been revealed by Raman spectroscopy (Figure 4, b). The
spectrum presents a G band at 1605 cm–1 and a D band at
1355 cm–1, which is consistent with the results in the litera-
ture.[20] The D band is associated with disordered samples, edge significant decrease in the N2 amount (Figure 5, a, and Table 1).
defects and other defects (sp3 bonded carbon, dangling bonds The Brunauer–Emmett–Teller (BET) surface area of UiO66 syn-
and vacancies) while the G band corresponds to ordered sp2 thesized herein was 705 m2 g–1, which is in the range of the
carbon domains. Therefore, the intensity ratio of the D band reported values for UiO66-type MOFs.[23–25] The BET surface
and G band (ID/IG) is an indication of the number of structural area of UiO66@GO was lower, which could be ascribed to the
defects, which is commonly used to quantify the level of graphi- synergistic effect between UiO66 and GO. Additionally,
Figure 4. (a) FTIR spectra of GO, UiO66, UiO66@GO and the UiO66@RGO-Pd composite; (b) Raman spectra of UiO66, UiO66@GO and the UiO66@RGO-Pd
composite.
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Figure 5. (a) Nitrogen adsorption–desorption isotherms for UiO66, UiO66@GO and UiO66@RGO-Pd composites and (b) pore-size distribution for UiO66@RGO-
Pd composites.
UiO66@RGO-Pd exhibited a lower specific surface area because 56.11 % lost. This figure demonstrates that the UiO66@GO and
the cavities of the host were partially occupied by the Pd NPs UiO66@RGO-Pd composite have exceptionally higher thermal
and/or blocked by palladium NPs that were located at the sur- stabilities than the parent material, UiO66.
face. As indicated in the pore-size distribution curve (Figure 5,
b, and Table 1), UiO66@RGO-Pd composites had a higher poros-
ity as compared to the pristine UiO66. These new pores did not
originate from UiO66 particles or Pd particles alone. Consider-
ing the findings from the XRD and SEM analyses, the UiO66
backbone was unchanged in the composites. This enhanced
porosity was likely due to the creation of new pores at the
interface between the UiO66 and the RGO (Figure S1). This indi-
cates that the large mesoporous cages (2.77 and 3.97 nm) of
the UiO66@RGO-Pd composites also provide a suitable environ-
ment for dispersed nanoparticle growth.
Table 1. Parameters of the porous structure for UiO66, UiO66@GO and
UiO66@RGO-Pd.
Sample
BET surface area
Pore volume
Pore width
[nm]
[m2 g–1
]
[cm3 g–1
]
UiO66
UiO66@GO
UiO66@RGO–Pd
705
408.8
403.6
0.375
0.29
0.327
1.61
1.18
3.97
Figure 6. TGA curves of UiO66, UiO66@GO and the UiO66@RGO-Pd compos-
ites recorded in a N2 atmosphere over the temperature range 25–800 °C.
The reduction of nitroarenes to corresponding arylamines is
one of the most important processes in organic synthesis of
dyes, pesticides and fine chemicals.[26] Hence, we tested the
catalytic property of supported-type Pd NP composites toward
reduction. Many reducing agents have been utilized for the
nitro compound reduction, the most classic being metallic rea-
gents in the presence of an acid. However, this methodology is
messy and environmentally hazardous. Thus, methods involving
cleaner and cheaper alternatives such as NaBH4 in EtOH are
necessary.
Thermal gravimetric analyses (TGA) were performed to inves-
tigate the thermal stability of the parent UiO66, UiO66@GO and
UiO66@RGO-Pd composite (Figure 6). The main turning points
of the three samples were very similar. Based on the TG analy-
ses, the as-synthesized UiO66 showed high thermal stability up
to 540 °C. The first weight loss step over the temperature range
of 25–120 °C was related to the removal of physically adsorbed
water. For UiO66@GO and UiO66@RGO-Pd, the intensity of this
peak decreased greatly, because water was removed by N2
purging. The second weight loss step, which was observed over
the temperature range of 120–540 °C, can be ascribed to the
As shown in Scheme 1, various products were obtained from
evaporation of DMF solvents. The decomposition of UiO66 oc- the reaction between different nitroarenes containing both
curred above 540 °C as a result of the loss of organic-linker electron-rich and electron-deficient groups with NaBH4 cata-
molecules from the frameworks. For this reason, the weight loss lyzed by UiO66@RGO-Pd in good to excellent yields. In general,
of UiO66@GO and UiO66@RGO-Pd was 48.01 % and 48.39 %, the presence of substituents such as OH, CH3 and NH2 in ortho
respectively, while that of UiO66 was higher with approximately and para situations leads to slightly decreased activity because
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of the electron-donating properties. It is interesting to note that stabilize the catalytic UiO66@Pd nanoparticles avoiding loss.
4-nitrobenzonitrile was selectively reduced to 4-aminobenzo- The role of the UiO66 component in the reduction of nitro-
nitrile in good yields and the CN group remained unaffected. arenes was also compared with the RGO-Pd catalyst. The results
1-Nitronaphthalene also afforded excellent yields of the corre- showed that UiO66@RGO-Pd was more efficient with respect to
sponding products in a very short time.
the reaction time and yields of the final product, which was
probably attributed to the unique porous structure of Zr-MOFs.
The UiO66@RGO-Pd composite has a mesoporous structure
(about 15–20 nm, Figure 5, b), allowing the nitroarene mol-
ecules to reach the active site in the pores. On the other hand,
the existing UiO66 enhanced the capacity of electron transfer
mediated by the Pd NP surfaces from the electron donor
–
BH4 to the aromatic nitro compounds.
Conclusions
In summary, the novel heterogeneous catalyst (UiO66@RGO-Pd)
was successfully prepared through encapsulating Pd NPs onto
the support of UiO66@GO. The as-prepared UiO66@RGO-Pd
possessed a good crystalline structure with several well-dis-
persed Pd NPs being encapsulated in octahedron-like UiO66
frameworks. The as-synthesized UiO66@RGO-Pd composite ex-
hibited microporous and mesoporous structures with high po-
rosity. These features made the UiO66@RGO-Pd hybrid an ideal
recyclable catalyst for liquid-phase reactions. It is noteworthy
that the UiO66@RGO-Pd catalyst exhibits remarkable catalytic
activity towards the reduction of nitroarenes and could easily
be recycled six times with no obvious decrease by simple filtra-
tion. Thus, we believe that the as-prepared noble metal nano-
composites could have a great potential for practical applica-
tions in the fields of energy and environment.
Scheme 1. Reduction of nitroarenes catalyzed by UiO66@RGO-Pd.
One of the main advantages of the as-prepared catalyst is
the influence of the reduced graphene oxide structure on keep-
ing the efficiency of the catalyst in the continuous reduction
reaction. In view of this, the compared experiments were per-
formed using UiO66@Pd and RGO@Pd as the catalyst under the
optimal and same reaction conditions.
As illustrated in Figure 7, the UiO66@RGO-Pd catalyst could
be reused six times without any decrease of conversion by a
simple filtration separation. However, the cyclic efficiency
showed a downward trend when UiO66@Pd was used as the
catalyst. Firstly, the UiO66@Pd composite is a nanoscale porous
material and there was difficulty in separating it from the reac-
tion media by simple filtration. At the same time, after six recy-
cling runs of the UiO66@Pd catalyst, almost 18.6 wt.-% of Pd
was leached, which was quantitatively supported by ICP-MS.
Hence, the presence of reduced graphene oxide with outstand-
ing inherent physical and chemical properties was sufficient to
Experimental Section
Materials: All chemical reagents were of analytical grade and were
used as received.
Preparation of Zr-MOFs@Graphene Oxide (UiO66@GO): Dry GO
powder was dispersed into DMF (50 mL) by sonication for 5 h. ZrCl4
(0.232 g, 1.0 mmol), terephthalic acid (0.166 g, 1.0 mmol) and acetic
acid (3 mL) were dissolved in DMF (50 mL), and the solution was
transferred to a 100-mL Teflon-lined stainless steel autoclave. The
autoclave was sealed and heated in an oven at 120 °C for 48 h
under autogenous pressure. After cooling naturally, the sample was
purified with anhydrous methanol several times to make sure that
the occluded DMF molecules were eliminated. Finally, heating the
material at 120 °C for 24 h activated the pores of UiO66. The GO
amount added in the preparation of the composites was 10 wt.-%
of the initial material weight. For comparison, pure UiO66 was ob-
tained using the same reaction conditions.
Preparation of Zr-MOFs@Reduction Graphene Oxide-Stabilized
Palladium NPs (UiO66@RGO-Pd): The as-prepared UiO66@GO
(50 mg) was dispersed in distilled water (10 mL) and placed for
10 min under ultrasonic waves. Subsequently, the self-prepared
PVP-stabilized Pd NP suspension (1 mL) and NaBH4 (5 mg) were
added.[14] After that, the resulting mixture was transferred and
sealed in a 25-mL Teflon-lined stainless steel autoclave, heated at
120 °C for 1.5 h, and then cooled to room temperature naturally.
The product was collected by centrifugation and washed with alco-
hol three times. The content of Pd was estimated to be 5.2 wt.-%
based on ICP-MS.
Figure 7. Reusability of UiO66@RGO-Pd and UiO66@Pd achieved in the reduc-
tion reaction of p-nitrotoluene under the same conditions.
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Received: March 30, 2016
Published Online: ■
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Supported Catalysts
A novel UiO66@RGO-Pd composite
was designed and used as an out-
standing catalyst for the reduction of
nitroarenes with NaBH4 as the reduc-
ing agent in EtOH. The hybrid-material
catalyst could be recycled and reused
after simple filtration, while keeping
similar catalytic activity for six succes-
sive reactions.
X. Liu, X. Zhao,* M. Zhou, Y. Cao,
H. Wu, J. Zhu * .................................... 1–7
Highly Stable and Active Palladium
Nanoparticles Supported on
Mesoporous UiO66@reduced Graph-
ene Oxide Complex for Practical Cat-
alytic Applications
a
DOI: 10.1002/ejic.201600367
Eur. J. Inorg. Chem. 0000, 0–0
www.eurjic.org
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