Cyclodextrin-Assisted Synthesis of Pd/Co/C Nanopolyhedra by ZIF-67 as a Highly Acid Tolerant Catalyst for Hexavalent Chromium Reduction

Article abstract of DOI:10.1021/acs.inorgchem.9b01434

The palladium/cobalt/carbon (Pd/Co/C) nanopolyhedra were synthesized by calcinating the precursor prepared by a cyclodextrin (CD) modified palladium acetylacetonate loaded on ZIF-67, hydroxypropyl-β-cyclodextrin (HP-β-CD)-[Pd(acac)₂]/ZIF-67. On one hand, the HP-β-CD was used to increase the solubility of Pd(acac)₂ by supramolecular interactions, and on the other hand, ZIF-67 was employed to provide the Co/C as the magnetic support. The Pd/Co/C nanopolyhedra obtained with the assistance of HP-β-CD have high activity and reusability for the reduction of the dichromate ion in the presence of formic acid without using any buffer solution, and no significant leaching of metallic Co of the catalyst was observed. This could be ascribed to the fact that the presence of HP-β-CD could lead to a more homogeneous distribution of metallic Pd in the Pd/Co/C nanopolyhedra, which could prevent the dissolution of metallic Co by formic acid.

Full text of DOI:10.1021/acs.inorgchem.9b01434

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Cyclodextrin-Assisted Synthesis of Pd/Co/C Nanopolyhedra by ZIF-
7 as a Highly Acid Tolerant Catalyst for Hexavalent Chromium
Reduction
6
,
†
‡
§
∥
†
†
Lei Bai,* Hongxing Zheng, Jie Ma, Jiao Wang, Zhongping Chen, and Qiuxiang Huang
†
College of Chemistry and Materials Engineering, Anhui Science and Technology University, Bengbu, Anhui 233030, China
School of Chemistry and Chemical Engineering, Liaocheng University, Liaocheng, Shandong 252059, China
College of Mechanical Engineering, Anhui Science and Technology University, Fengyang, Anhui 233100, China
College of Food Engineering, Anhui Science and Technology University, Fengyang, Anhui 233100, China
‡
§
∥
*
S Supporting Information
ABSTRACT: The palladium/cobalt/carbon (Pd/Co/C) nanopolyhe-
dra were synthesized by calcinating the precursor prepared by a
cyclodextrin (CD) modified palladium acetylacetonate loaded on ZIF-
6
7, hydroxypropyl-β-cyclodextrin (HP-β-CD)−[Pd(acac) ]/ZIF-67. On
2
one hand, the HP-β-CD was used to increase the solubility of Pd(acac)
2
by supramolecular interactions, and on the other hand, ZIF-67 was
employed to provide the Co/C as the magnetic support. The Pd/Co/C
nanopolyhedra obtained with the assistance of HP-β-CD have high
activity and reusability for the reduction of the dichromate ion in the
presence of formic acid without using any buffer solution, and no
significant leaching of metallic Co of the catalyst was observed. This
could be ascribed to the fact that the presence of HP-β-CD could lead to
a more homogeneous distribution of metallic Pd in the Pd/Co/C
nanopolyhedra, which could prevent the dissolution of metallic Co by
formic acid.
10−14
1
. INTRODUCTION
acid-resistant component.
Recently, the synthesis of Pd/
ZIF-67 was developed for the reduction of Cr(VI) in the
presence of a buffer solution formed by HCOOH and
HCOONa. It was demonstrated that the Pd/ZIF-67 can be
reused 10 times without a significant loss of its catalytic
Heavy metal pollution is presently a major environmental issue
since metal ions persist in the environment because of their
1
,2
nondegradable property.
The toxicity and biological
accretion tendency of heavy metals in the environment are a
severe health risk to living organisms. Heavy metal ions such as
1
5
activity. On the other side, the metallic cobalt (Co)
nanoparticles display a good magnetic property and can be
easily recovered from the reaction system. If the leaching of
metallic Co in an acid solution could be prevented or slowed
down, it would be of great interest for developing the metallic
Co based composites used under acid conditions. The previous
work confirmed that the calcination of ZIF-67 under an inert
gas could result in the formation of porous carbon
incorporated with metallic Co nanoparticles, which could be
2
+
2+
2+
Pb , Hg , Cd , and Cr(VI) originate from many industries
such as electroplating, mining, fabrication of batteries, and
3
electronics. Among these ions, Cr(VI) has been considered as
the second most harmful inorganic pollutant as well as the
4
third most common pollutant in the hazardous wastes. It has
been revealed that the Cr(VI) compounds are acutely toxic,
and however, the Cr(III) is less toxic to the environment,
which is easy to remove through the formation of hydroxide
16
5
employed as an ideal magnetic support. As mentioned above,
Pd based materials are widely used for the reduction of
species. Therefore, several strategies have been proposed to
6
−9
deal with Cr(VI) compounds.
One possible way is to
6
Cr(VI). For the synthesis of Pd nanomaterials, palladium
reduce the Cr(VI) to Cr(III) by photocatalysis, and another is
to take advantage of the reductants, which is considered to be
the most effective approach and has attracted immense
interest. Particularly, in the process of chemical reduction,
formic acid (HCOOH) is regarded as a promising reductant,
which is used for the reduction of Cr(VI) to Cr(III) due to its
high reducing ability and effectiveness. However, the employ-
ment of HCOOH always requires the presence of noble metal
containing materials such as Pt and Pd based materials as the
acetylacetonate [Pd(acac) ] is a kind of important Pd
2
precursor, and due to the poor solubility in the polar solvents,
the utilization of Pd(acac)2 was limited to some extent.
However, it is worth noting that the cyclodextrins (CDs),
which are a kind of cyclic and nontoxic oligosaccharide and
Received: May 16, 2019
©
XXXX American Chemical Society
A
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Inorganic Chemistry
Article
Figure 1. SEM images of (A) Pd(acac) , (B) ZIF-67-Pd(acac) , and (C) ZIF-67-Pd(acac) -HP-β-CD.
2
2
2
contain a hydrophobic internal cavity and hydrophilic external
spectroscopy (XPS) analysis was performed with a Thermo
ESCALAB 250 instrument with a monochromatic Al Kα (hv =
1486.6 eV) X-ray source. The morphologies of the nanocrystals were
examined by transmission electron microscopy (TEM), using a JEOL
JEM-2010 LaB6 high-resolution transmission electron microscope
operated at 200 kV. The electron diffraction and HRTEM were
performed on JEOL JEM-ARM200F, which was operated at 200 kV.
The content of Pd in the sample was determined by inductively
coupled plasma-atomic emission spectrometry (ICP-AES). The
specific area was calculated from the Brunauer−Emmett−Teller
surface, could interact with different molecules to construct
17−20
composites and increase their solubility.
In addition, the
interactions between platinum acetylacetonate and β-CD have
been proven to increase the solubility of platinum
21
acetylacetonate in the previous work. However, the use of
such a unique characteristic for the synthesis of functional
composites and investigation of the catalytic activity is less
studied.
Taking into account the above fact, we first took advantage
of hydroxypropyl-β-cyclodextrin (HP-β-CD, molar weight:
(BET) equation using P/P values in the 0.05−0.25 range, and the
0
pore size distribution was obtained from the desorption branch using
the Barrett−Joyner−Halenda (BJH) method.
1
514 g/mol) as a supramolecular tool to enhance the solubility
2
.4. Catalytic Reduction of K Cr O by Formic Acid. A 6 mg
2 2 7
of Pd(acac) into the impregnation solution and, on the other
2
portion of Pd/Co/C catalyst was added into a 10 mL aqueous
solution containing 2 mg of K Cr O , and then, 0.2 mL of HCOOH
hand, utilized ZIF-67 to synthesize the magnetic support for
preparing the Pd/Co/C nanocomposites. It was found that the
HP-β-CD obviously increased the solubility of Pd(acac)2,
which then could greatly enhance the dispersion of Pd(acac)2
on the ZIF-67 and Pd nanoparticle in the calcined product.
2
2
7
was added. At a certain time, 0.2 mL of solution was taken at a certain
time by separating the catalyst with a magnet.
After six cycles, the catalyst was washed by distilled water, and then,
10 mL of aqueous solution with 4 mg of K Cr O was added. After
2
2
7
Furthermore, it was demonstrated that the as-synthesized Pd/
Co/C nanocomposite had a high activity for the hydrogenation
of Cr(VI) in the presence of HCOOH and also displayed an
excellent reusability without significant leaching of metallic Co
and activity after 8 cycles, indicating that the Pd/Co/C
nanocomposite obtained with HP-β-CD could be a potential
candidate for the heavy metal treatment applications in an acid
solution. The present work provided a new insight for the use
of a supramolecule as an assistant host together with MOFs to
synthesize other functional nanomaterials as well as offer
applications in contaminant treatment.
adding 0.2 mL of HCOOH, 0.2 mL of solution was taken at a certain
time by separating the catalyst with a magnet. All the reaction
solutions were diluted to 2 mL and characterized by the UV−vis
spectrometer (Shimadzu UV-3600 spectrophotometer).
3. RESULTS AND DISCUSSION
The picture of the impregnation solution of Pd(acac) in the
2
absence and presence of HP-β-CD was displayed in Figure S1.
It was clear that the Pd(acac) could not be totally dissolved in
2
the ethanol, and however, in the presence of HP-β-CD, a
homogeneous solution was obtained, suggesting a good
dispersion of Pd(acac) in the ethanolic solution. In order to
prove the above phenomena, SEM images of the ZIF-67 after
2
2
. EXPERIMENTAL SECTION
2
.1. Synthesis of ZIF-67. Typically, 5 mmol of Co(NO ) ·6H O
3 2 2
impregnation with Pd(acac) and HP-β-CD-Pd(acac) were
2
2
was dissolved in 50 mL of methanol by stirring. Then, a methanolic
solution containing 20 mmol of 2-methylimidazole (50 mL) was
added into the above solution under stirring. After 10 min, the
solution was kept undisturbed for 24 h. The purple precipitate was
obtained by centrifugation and washed with ethanol thoroughly 3
times. Finally, the obtained sample was dried under vacuum overnight
before use.
shown in Figure 1. The Pd(acac) powder alone displayed a
2
salix leaf-like structure. After impregnation of the Pd(acac) on
2
ZIF-67, still the salix leaf-like structure was retained (Figure
1
A,B), which was consistent with the phenomena observed in
Figure S1. Interestingly, for the HP-β-CD-Pd(acac) /ZIF-67,
2
the salix leaf-like structure disappeared, which was possibly due
to the fact that HP-β-CD could effectively interact with
2
.2. Synthesis of Pd(acac) -HP-β-CD/ZIF-67 and Pd/Co/C. In
2
7
.5 mL of absolute ethanol, 12 mg of Pd(acac) and 120 mg of HP-β-
Pd(acac) through a supramolecular interaction to form the
2
2
CD were totally dissolved under vigorous stirring until a clear yellow
solution was obtained. Then, 300 mg ZIF-67 was added, and the
solution was treated under ultrasonic treatment for 20 min before it
was put into an oven to evaporate the solvent. After the solids were
soluble composites, which resulted in a homogeneous
dispersion of Pd(acac) on ZIF-67 (Figure 1C).
2
Next, we examined the acid resistance of Co/C and Pd/Co/
C samples obtained by calcinating ZIF-67, Pd(acac) /ZIF-67,
2
carefully ground, they were further calcined in a N atmosphere at 873
2
and HP-β-CD-Pd(acac) /ZIF-67. Notably, the leaching of Co
K for 2 h by 2 K/min. For comparison, the Pd/Co/C was also
prepared by calcinating the precursor synthesized without HP-β-CD.
2
was observed in the case of Co/C and Pd/Co/C prepared
without HP-β-CD (Figure 2A,B). However, we did not notice
the obvious leaching of the Co for the Pd/Co/C synthesized
2.3. Characterizations. X-ray diffraction (XRD) patterns were
recorded on a Rigaku MiniFlex 600 instrument. X-ray photoelectron
B
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Article
any other carbon residue. The nanoparticles in the concave
polyhedra were confirmed to be metallic Co, as indicated by
the 0.217 nm distance of the (100) crystal face (Figure S3A).
In addition, for Pd/Co/C, similar concave polyhedra were
obtained as shown in Figure 4A,B, and the size of the
polyhedra was about 500 nm. Furthermore, the Pd/Co/C
material prepared by HP-β-CD was investigated by HRTEM,
and the images were displayed in Figure 4C. A polyhedron was
chosen and characterized as displayed in Figure 4B. Clearly,
the polyhedron was connected to the other one, and some
carbon residue was observed on rthe im of the polyhedron,
which was possibly due to the carbon produced from HP-β-
CD. A particle in the red circle on the polyhedron was selected
and analyzed by HRTEM (Figure 4C). The lattice distance on
the edge of particle was calculated to be 0.224 nm, which was
ascribed to the (111) crystal face of metallic Pd. In addition,
the lattice distance with a value of 0.217 nm was confirmed to
be the (100) crystal face of metallic Co. It is reasonable that
the Pd nanoparticle was on the outside of the Co nanoparticle
because ZIF-67 was used as the support and the HP-β-CD-
Pd(acac)2 was impregnated on the ZIF-67. Finally, the
distribution of Pd and Co in Pd/Co/C polyhedron was
studied by HAADF-STEM, and the images were present in
Figure 4D−F. As can be seen, due to the presence of the ZIF-
Figure 2. Photos of Co/C (A) and Pd/Co/C synthesized without (B)
and with (C) HP-β-CD in a mixed solution of 9.8 mL of distilled
water and 0.2 mL of HCOOH.
with HP-β-CD as displayed in Figure 2C. Thus, the attention
in the following was paid to the investigation of the structure
and catalytic performance of the Pd/Co/C obtained with the
assistance of HP-β-CD.
Furthermore, the samples of Co/C and Pd/Co/C prepared
with HP-β-CD were characterized by XRD, and as can be seen
from Figure S2, the two samples exhibited a broad peak
centered at 44.6°, which corresponded to the (002) crystal face
of metallic cobalt. The result indicated that the Co ions of ZIF-
6
7, after calcination, Co was formed and distributed
homogeneously in the Pd/Co/C polyhedron; however, in
the case of Pd, most of the Pd was homogeneously distributed
in the polyhedron, and a few big Pd nanoparticles were
observed. It is worth noting that the large Pd nanoparticles
could, on one hand, result from the decomposition of HP-β-
CD, which provided more heat and led to the aggregation of
Pd nanoparticles, and on the other hand, the interaction of
6
7 were reduced by the products from 2-methylimidazole
during the thermal treatment. Furthermore, for the Pd/Co/C,
no peaks corresponding to metallic Pd were observed, possibly
due to a low amount and the small size of metallic Pd. The
ICP-AES analysis confirmed that 0.34 wt % of Pd was found in
the Pd/Co/C composite.
The morphology of the above two samples was observed by
FESEM, and the images were shown in Figure 3. For the Co/
C, typical concave polyhedra were obtained. However, for the
Pd/Co/C composite, deformed polyhedra connected with
each other were observed, which was possibly due to the fact
that the HP-β-CD adsorbed on the ZIF-67 could also give rise
to the formation of carbon and led to the slight aggregation of
the Co/C to some extent during the calcination.
HP-β-CD and Pd(acac) is not as strong as that of Co ions and
2
2-methylimidazole in ZIF-67. It was suggested that the
coordination interactions could prevent the aggregation of
the Co nanoparticles more effectively than the supramolecular
interaction between Pd(acac)
calcination.
and HP-β-CD during the
2
It was suggested that the pores of the catalysts displayed a
critical role for the accessibility of reactants in contact with
In order to have detailed information about the structure of
Co/C and Pd/Co/C, TEM was performed, and the images of
Co/C are shown in Figure S3. As shown by the TEM image
catalytic active sites; the N
of the ZIF-67 and HP-β-CD-Pd(acac)
2
adsorption−desorption isotherms
/ZIF-67, and the
2
calcined derivatives, were recorded. The corresponding specific
(Figure S3A), regular concave polyhedra can be observed, and
surface areas and total pore volumes are summarized in Table
2
−1
due to the magnetic property of metallic Co, the aggregation of
the regular concave polyhedra of Co/C was noticed. Notably,
the rim of the Co/C concave polyhedra was smooth without
S1. Compared with that of ZIF-67 (1396.2 m g ), the
obvious decrease of surface area for HP-β-CD-Pd(acac) /ZIF-
2
2
−1
67 (897.4 m g ) indicated that some of the pores of the ZIF-
Figure 3. SEM images of Co/C (A) and Pd/Co/C (B).
C
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Figure 4. TEM images of Pd/Co/C at different magnifications (A, B), HRTEM image of a nanoparticle (C), and element distribution of Pd/Co/C
D−F).
(
Figure 5. XPS spectrum of Co 2p (A) and Pd 3d (B) in Pd/Co/C obtained with HP-β-CD.
6
7 were covered by the HP-β-CD-Pd(acac) . After the
Co/C as displayed in Figure 6B. Also, it is worth mentioning
that, during the catalytic reaction, no obvious leaching of Co
2
pyrolysis treatment, the specific surface area and pore size of
the derived Co/C and Pd/Co/C dramatically decreased to
ions was detected from the UV−vis analysis. For the reduction
2
−1
2−
2
56.9 and 232.34 m g as well as to 1.5 and 3.2 nm,
of Cr O
by HCOOH, Yu et al. had proposed that a
2
7
respectively (Figure S4). The results suggested that the loading
of Pd led to a decrease of the specific surface area of Co/C
nanopolyhedra.
The chemical composition and state of the Pd/Co/C were
analyzed by XPS. The Co 2p and Pd 3d spectra were displayed
in Figure 5A,B, respectively. It was suggested that the peak at
reducing agent such as CO and H could be produced by the
2
decomposition of HCOOH in the presence of Pd nano-
2−
particles, which initiated the reduction of Cr O according to
the following equations:
2
7
15
HCOOH → H + CO
(1)
(2)
2
2
7
78.1 eV corresponded to the metallic Co. However, the peak
2
−
+
3+
at 779.1 eV was possibly ascribed to the cobalt oxide, which
was due to the surface oxidation of the metallic cobalt as
pointed out by the previous work. In addition, the peaks
centered on 334.9 and 340.2 eV indicated that the Pd
Cr O
+ 8H + 3H → 2Cr + 7H O
2
7
2
2
22
Furthermore, our catalyst was used under the same condition
for 6 runs to illustrate the reusability, and as shown in Figure
6C, all the runs demonstrated identical catalytic reduction of
23,24
nanoparticles were generally in the metallic state.
2
−
Finally, due to the observation of the leaching of metallic Co
in the Co/C and Pd/Co/C synthesized without HP-β-CD,
only the Pd/Co/C prepared with HP-β-CD was chosen as a
Cr O . Finally, this reused catalyst was employed to reduce
2 7
2−
the Cr O at a higher concentration (4 mg/10 mL H O) as
2
7
2
2
−
shown in Figure 6D. Obviously, the total reduction of Cr O
2
7
2
−
catalyst for the reduction of Cr O7 by employing the
was accomplished in 8 min.
2
HCOOH as a reducing agent. As displayed in Figure 6A, there
Thus, in the catalytic process, the HCOOH in the solution
was dehydrogenated to produce H and CO , and the H
2
−
was no change in the UV−vis spectra of Cr O , indicating
2
7
2
2
2
that HCOOH alone failed in the catalytic reduction of
Cr O . Interestingly, the addition of 6 mg of Pd/Co/C into
the reaction system and the quick reduction of Cr O
would be adsorbed on the surface of Pd nanoparticles and
reduce the Cr(VI) into Cr(III) through a hydrogen transfer
pathway as illustrated in Figure 7. The other reducing process
2−
2
7
2
−
2
7
15
occurred in 5 min, suggesting a good catalytic ability of Pd/
by the CO was eliminated by the previous work.
D
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Inorganic Chemistry
Article
Figure 6. Catalytic reduction performance: (A) 2 mg of K Cr O without catalyst, (B) with 6 mg of Pd/Co/C, (C) the recycle of Pd/Co/C, and
2
2
7
(
D) 4 mg K Cr O with Pd/Co/C after 6 recycles.
2 2 7
analysis that only a slight loss of weight in the catalyst was
observed (Figure S5), and thus, it was suggested that the
leaching of metallic Co was prevented to a large extent. In one
word, a highly active and reusable Pd/Co/C with a high acid
tolerance was obtained with the ZIF-67, HP-β-CD, and
Pd(acac)2.
4. CONCLUSION
In summary, we synthesized Pd/Co/C as a high-performance
catalyst for the reduction of Cr(VI) with high acid tolerance;
this was successfully synthesized by ZIF-67 and HP-β-CD
modified Pd(acac)2 solution. It was suggested that the
presence of the HP-β-CD improved the solubility of Pd(acac)2
and the dispersion of Pd nanoparticles in the calcined
composite. The structural analysis confirmed the existence of
metallic Co and Pd. Finally, the results from the catalytic
2
−
Figure 7. Schematic illustration of the catalytic reduction of Cr O
on Pd/Co/C nanopolyhedra.
2
7
The photos of the solutions before catalytic reaction and
after 8 runs in the presence of the Pd/Co/C were displayed in
3
+
Figure 8, and the light purple confirmed the existence of Cr .
The molar ratio between Pd and Co was slightly changed from
7.6:2.4 to 95.5:4.5 as listed in Table S2, and the Co leaching
in the Pd/Co/C was greatly prevented compared with Co/C
as displayed in Table S3. In addition, it was confirmed by TG
2−
reduction of Cr O by HCOOH suggested that the presence
2
7
2+
9
of Pd, on one hand, acted as catalyst and, on the other hand,
prevented the acid dissolution of metallic Co to a great extent.
Thus, the Pd/Co/C nanopolyhedra synthesized were consid-
ered to be a promising candidate for the potential environ-
mental safety application under the relatively harsh acidic
conditions.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b01434.
Additional characterization details including a photo-
graph, XRD patterns, TEM images, and adsorption−
desorption isotherms (PDF)
Figure 8. Comparison of the solution with Pd/Co/C (A) containing
HCOOH, and the solution left after 8 recycles (B).
E
DOI: 10.1021/acs.inorgchem.9b01434
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Inorganic Chemistry
Article
F. S.; Feng, J. J.; Wei, J.; Zhang, Q. L.; Wang, A. J. Uniform Pt@Pd
nanocrystals supported on N-doped reduced graphene oxide as
catalysts for effective reduction of highly toxic chromium(VI). Mater.
Chem. Phys. 2018, 205, 64−71.
AUTHOR INFORMATION
Corresponding Author
E-mail: baileiwj2014@163.com; leibai@ustc.edu.cn.
■
*
ORCID
(15) Li, H. C.; Liu, W. J.; Han, H. X.; Yu, H. Q. Hydrophilic
swellable metal−organic framework encapsulated Pd nanoparticles as
an efficient catalyst for Cr(VI) reduction. J. Mater. Chem. A 2016, 4,
Lei Bai: 0000-0002-8342-1745
Notes
The authors declare no competing financial interest.
1
1680−11687.
(16) Li, R.; Wang, X.; Dong, Y.; Pan, X.; Liu, X.; Zhao, Z. B.; Qiu, J.
S. Nitrogen-doped carbon nanotubes decorated with cobalt nano-
particles derived from zeolitic imidazolate framework-67 for highly
efficient oxygen reduction reaction electrocatalysis. Carbon 2018, 132,
ACKNOWLEDGMENTS
■
This work is supported by the Anhui Provincial Natural
Science Foundation (1908085QB71). L.B. also wants to
acknowledge the Talent introduction project of Anhui Science
and Technology University (No.830166) for the financial
support.
5
(
80−588.
17) Prochowicz, D.; Kornowicz, A.; Lewinski, J. Interactions of
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DOI: 10.1021/acs.inorgchem.9b01434
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