A Porous Organic Polymer Nanotrap for Efficient Extraction of Palladium

Article abstract of DOI:10.1002/anie.202006596

To offset the environmental impact of platinum-group element (PGE) mining, recycling techniques are being explored. Porous organic polymers (POPs) have shown significant promise owing to their selectivity and ability to withstand harsh conditions. A series of pyridine-based POP nanotraps, POP-Py, POP-pNH₂-Py, and POP-oNH₂-Py, have been designed and systematically explored for the capture of palladium, one of the most utilized PGEs. All of the POP nanotraps demonstrated record uptakes and rapid capture, with the amino group shown to be vital in improving performance. Further testing on the POP nanotrap regeneration and selectivity found that POP-oNH₂-Py outperformed POP-pNH₂-Py. Single-crystal X-ray analysis indicated that POP-oNH₂-Py provided a stronger complex compared to POP-pNH₂-Py owing to the intramolecular hydrogen bonding between the amino group and coordinated chlorine molecules. These results demonstrate how slight modifications to adsorbents can maximize their performance.

Full text of DOI:10.1002/anie.202006596

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Accepted Article
Title: Porous Organic Polymer Nanotrap for Efficient Extraction of
Palladium
Authors: Briana Aguila, Qi Sun, Harper C. Cassady, Chuan Shan,
Zhiqiang Liang, Abdullah M. Al-Enizic, Ayman Nafadyc,
Joshua T. Wright, Robert W. Meulenberg, and Shengqian Ma
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202006596
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Porous Organic Polymer Nanotrap for Efficient Extraction of
Palladium
Briana Aguila, Qi Sun*, Harper C. Cassady, Chuan Shan, Zhiqiang Liang, Abdullah M. Al-Enizic,
Ayman Nafadyc, Joshua T. Wright, Robert W. Meulenberg, and Shengqian Ma*
[*]
Dr. B. Aguila, H. C. Cassady, C. Shan, Prof. S. Ma
Department of Chemistry
University of South Florida
4202 E Fowler Ave., Tampa, FL 33620
E-mail: sqma@usf.edu
Dr. Q. Sun
Key Laboratory of Biomass Chemical Engineering
College of Chemical and Biological Engineering
Zheijang University
Hangzhou, 310027, P. R. China
E-mail: sunqichs@zju.edu.cn
Prof. Z. Liang
State Key Lab of Inorganic Synthesis and Preparative Chemistry
Jilin University
Changchun, 130012, P. R. China
Prof. A. M. Al-Enizic, Prof. A. Nafadyc
Chemistry Department
King Saud University
Riyadh, 11451, Saudi Arabia
Dr. J. T. Wright
Department of Physics
Illinois Institute of Technology
Chicago, IL 60616
Prof. R. W. Meulenberg
Department of Physics and Astronomy and Frontier Institute for Research in Sensor Technologies
University of Maine
Orono, ME 04469
Supporting information for this article is given via a link at the end of the document.
Abstract: To offset the environmental impact of platinum group
element (PGE) mining, recycling techniques are being thoroughly
explored. Porous organic polymers (POPs) have shown significant
promise, due to their selectivity and ability to withstand harsh
conditions. A series of pyridine-based POP nanotraps, POP-Py, POP-
pNH2-Py, and POP-oNH2-Py, have been rationally designed and
systematically explored for the capture of palladium, one of the most
utilized PGEs. All the POP nanotraps demonstrated record uptakes
and rapid capture, with the amino group shown to be vital in improving
performance. Further testing on the POP nanotrap regeneration and
selectivity found that POP-oNH2-Py outperformed POP-pNH2-Py.
Single crystal X-ray analysis indicated that POP-oNH2-Py provided a
stronger complex compared to POP-pNH2-Py due to the
intramolecular hydrogen bonding between the amino group and
coordinated chlorine molecules. These results demonstrate how slight
modifications to adsorbents can maximize their performance.
demands a secondary source of these precious metals is required
to keep up with consumer needs.
Recovery of PGEs from spent processes is a promising
alternative to reduce the dependence on an already finite supply.
Stripping electronic waste of precious metals for recycling has
already been explored to some effectiveness.[3] Harsh chemical
treatments are typically used however, necessitating the search
for an environmentally benign option. Adsorbents, such as metal-
organic frameworks,[4] silica materials,[5] and biosorbents,[6] have
thus been studied to capture PGEs in the aqueous phase. This
method would not only reduce the use of chemical treatments, but
also open up a new avenue of resource recovery through the
capture of precious metals from industrial waste streams. To do
so would require an adsorbent material with exceptional
selectivity and affinity for the analyte of choice, given the diverse
composition of industrial processes with low concentration of
PGEs.[5d,7] Current materials, however, cannot meet all the
requirements of high uptake capacity, fast kinetics, exceptional
selectivity/affinity, facile recyclability despite their preliminary
success thereby urging the development of new types of
adsorbents for PGE capture with superior performance.
Platinum group elements (PGEs) are recognized for their unique
properties such as corrosion resistance, high melting point, and
catalytic qualities. These features have made them economically
invaluable, with widespread use in a variety of industrial sectors.
Elements in this category, including platinum, palladium, rhodium,
ruthenium, iridium, and osmium, are employed in catalytic
processes, and are vital to the electronic and automotive
industries.[1] However, the ubiquitous nature of their application
has put a strain on the already rare terrestrial ores. Currently, 90%
of the PGE supply comes from South Africa and Russia, with the
rest of the world heavily reliant on imports.[2] With increasing
Porous organic polymers (POPs) are a class of adsorbent
material with great promise for such a challenging adsorption
process. POPs are formed from purely organic building blocks
connected by strong covalent bonds.[8] This produces a material
with functionality and exceptional stability, a combination that
most existing adsorbent materials lack. Tunable monomeric units
have resulted in the use of POPs in many applications, such as
1
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catalysis,[9] gas storage,[10] and water treatment,[11] with
encouraging outcomes. From this tunability, structural
modifications can go even further to enhance the performance of
binding sites.
Many strategies have been taken to improve binding with
regards to host-guest interactions. This would include
preorganization, cooperativity, and several non-covalent
interactions, which have been successful for a variety of
compounds.[12] Previous work in our lab used some of these
Following characterization, they were tested to determine their
maximum capacity for palladium adsorption. To obtain adsorption
isotherms, solutions of increasing Pd2+ concentrations (25-800
ppm) were treated with the adsorbents. After stirring overnight to
achieve equilibrium, the solutions were filtered, and analyzed via
ICP to determine the residual Pd2+ concentration. From the
adsorption isotherms (Figure 2A), POP-Py, POP-pNH2-Py, and
POP-oNH2-Py were fit with the Langmuir model with uptake
capacities of 708, 743, and 752 mg/g, respectively. All materials
produced record high palladium uptakes,[4,5a-c,6c,d,16] with the
addition of the amino group further improving the uptake
performance. Typically, a higher surface area results in enhanced
adsorption performance through the availability of more exposed
binding sites, however, in this work we see the opposite trend for
the pyridine-based POPs. The effects of a decreased surface
area for the amino-functionalized POPs is overcome by their
electron donating ability, resulting in a more basic pyridine binding
site and thus enhanced adsorption performance compared to
POP-Py.
methods to design
a
bio-inspired uranium coordination
environment where the de novo introduction of an assistant group
reinforced the interaction between the POP binding site and
uranyl ions.[13] With promising results from this work, we took this
approach to a new model system to determine if this strategy
could be applied for capture and recycling of the rare platinum
group elements.
In this study, we focus on the adsorption of the precious metal
palladium, given its wide industrial use.[14] A series of pyridine-
based POPs were rationally designed, wherein the nitrogen of the
pyridine monomeric unit provides a Lewis base site to coordinate
to palladium.[15] The introduction of an amino group, as well as the
effects from its relative location, was further investigated. Through
analysis of the maximum uptake capacity, kinetic efficiency,
selectivity, and chemical stability, it was determined that the
addition of the amino group increased the nucleophilicity of the
pyridine binding site. Further, by placing the amino group in the
ortho position, a stronger complex with palladium could be
achieved as illustrated by X-ray photoelectron spectroscopy and
single crystal X-ray data. These results guide future design
strategies to enhance adsorbents for precious metal recovery
under real-world conditions.
Figure 2. A) Palladium adsorption isotherms (Inset: linear regression fit with the
Langmuir model) and B) kinetic performance of POP-Py (black), POP-pNH2-Py
(blue), and POP-oNH2-Py (red).
To prepare the POPs, vinyl-based monomers were first
synthesized and denoted as V-Py, V- pNH2-Py, and V-oNH2-Py
(experimental details in SI). The monomers were cross-linked via
radical polymerization to successfully transform to the final
polymeric materials, coined as POP-Py, POP-pNH2-Py, and POP-
oNH2-Py respectively, which were subsequently investigated for
their physical properties through nitrogen sorption measurements
and scanning electron microscopy images (SEM). All POP
materials exhibited a combination of micro- and mesopores as
indicated by the nonlocal density functional theory (NLDFT) pore
size distribution (Figure S1) and N2 sorption isotherms, with a
steep incline at low pressure along with a hysteresis loop during
the desorption process (Figure 1). From the isotherms the surface
areas were calculated using the Brunauer-Emmett-Teller (BET)
method. All materials had moderate to high surface area with
values of 979, 536, and 359 m2/g for POP-Py, POP-pNH2-Py, and
POP-oNH2-Py, respectively. SEM images also show a variation in
the pore size (Figure S2), with the visible larger pores facilitating
mass transfer and the smaller pores serving as a trap to enrich
the materials for greater adsorption performance.
Given the promising results for the uptake of palladium, the
adsorbent materials were studied for their kinetic efficiency. With
20 mg of the materials in 200 mL of a 5 ppm Pd2+ solution, 3 mL
aliquots were collected at increasing time intervals. The filtrates
were analyzed by ICP-MS for the remaining palladium in solution
and the concentration changes were measured over time (Figure
2B). All of the adsorbents reduced the palladium concentration to
ppb level within 10 minutes, and after a 3 hr period the remaining
concentration in solution was 2.23, 1.38, and 0.29 ppb for POP-
Py, POP-pNH2-Py, and POP-oNH2-Py, respectively. The kinetic
data were fit to a pseudo-second order kinetic model (Figure S3),
indicating that the rate-limiting step is a chemisorption process.
Given that all of the adsorbents have a mixture of micro- and
mesopores, the rapid removal is partly attributed to the
hierarchical porosity, creating a nanotrap to efficiently capture
palladium in solution.[17] Additionally, each monomeric unit is
equipped with a pyridine group providing a large number of
binding sites to complex with palladium. POP-oNH2-Py shows the
greatest level of efficacy of all tested adsorbents, reducing to sub-
ppb level after a short treatment time. If these adsorbents are to
be put into practice, not only is a high capacity beneficial, it is also
important to have palladium capture at low concentrations.
Based on the equilibrium results from the kinetic data, the
binding affinity of each adsorbent was quantified. To do so, the
distribution coefficient (Kd) was calculated, with values of 1.5 ×
107, 2.4 × 107, and 1.1 × 108 mL/g for POP-Py, POP-pNH2-Py,
and POP-oNH2-Py, respectively. It is evident that POP-oNH2-Py
demonstrates the strongest binding by an order of magnitude,
followed by POP-pNH2-Py and POP-Py, verifying the
Figure 1. Nitrogen sorption isotherms of A) POP-Py, B) POP-pNH2-Py, and C)
POP-oNH2-Py.
2
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experimental capacity and kinetic data. The electron donating
capability of the additional amino group adds electron density to
the π system, this in turn makes the pyridine more nucleophilic
and is responsible for the enhanced performance of POP-oNH2-
Py and POP-pNH2-Py over POP-Py. Thus, the amino
functionalized POPs were the focus of further studies.
with palladium and is beneficial to selectively capture palladium at
low concentrations (Figure 4).
After palladium is adsorbed, it must be effectively eluted from
the material to recover it for reuse, while also recycling the
adsorbent. To probe this, the palladium contacted adsorbents,
Pd@POP-pNH2-Py and Pd@POP-oNH2-Py, were eluted using a
thiourea:HCl solution. Subsequently, the regenerated adsorbents
were stirred in 200 mL of a 5 ppm Pd2+ solution overnight. The
residual concentration was measured with ICP-MS and quantified
to be 29 and 0.1 ppb for POP-pNH2-Py and POP-oNH2-Py,
respectively. The performance of POP-oNH2-Py is completely
maintained after recycling, while POP-pNH2-Py is noticeably
hindered, resulting in a residual concentration over forty times that
of the pristine sample after overnight treatment (Figure S4). To
illustrate this phenomenon, the binding of palladium to POP-
oNH2-Py was investigated through X-ray photoelectron
spectroscopy (XPS). The Pd 3d spectrum was collected for
Pd@POP-oNH2-Py to verify the adsorption process with two
distinct peaks at 343.3 and 338.2 eV corresponding to 3d3/2 and
3d5/2, respectively.[18] Comparing this to the pristine and the
recycled POP-oNH2-Py sample, which had no discernable Pd 3d
peaks, confirming the ability to elute the palladium from the
adsorbent and the effectiveness of the regeneration process
(Figure 3A). The N 1s spectra further substantiates this with the
pristine sample having a N 1s peak at 399.3 eV however, after
binding with palladium this peak is shifted to 399.6 eV, indicating
an interaction has occurred with the nitrogen groups of POP-
oNH2-Py.[19] After the material was regenerated this peak is
identical to the pristine sample with the N 1s peak at 399.3 eV
(Figure 3B). These results indicate the potential this material has
for multiple adsorption cycles, extending the adsorbent’s lifetime
in practice.
Figure 4. Schematic illustration of Pd2+ selectivity over other common ions for
POP-oNH2-Py.
Owing to the encouraging results for POP-oNH2-Py, it was
used for a final analysis of the stability and performance under
extreme pH conditions. 5 mg of adsorbent was placed in aqueous
solutions of pH 3 and 10, respectively, and allowed to soak over
a 3 day period. The adsorbents were collected and immediately
used for the recovery of a 5 ppm palladium solution (50 mL) at pH
3 and 10. The residual concentration after stirring overnight was
measured with ICP-MS to be 2.08 and 6.14 ppb for pH 3 and 10,
respectively. The structural integrity and palladium uptake after
exposure to extreme conditions demonstrates the full applicability
of POP-oNH2-Py for palladium recovery.
To understand the interaction between palladium and the
amino-functionalized POPs, Fourier-transform infrared (FT-IR)
spectroscopy was conducted on the pristine and palladium-
contacted samples. As seen from Figure S5, Pd@pNH2-Py and
Pd@oNH2-Py both have an additional peak between 568-575 cm-
1
corresponding to Pd-N stretching.[21] With the variation in
intensity and the location in the fingerprint region FT-IR alone is
not sufficient for characterization. Further analysis was done
through the collection of SEM images and energy dispersive X-
ray (EDX) mapping of Pd@pNH2-Py and Pd@oNH2-Py (Figure
S6). From the images we see a homogenous distribution of
palladium throughout the samples, which is in line with the
monolayer adsorption process indicated by the adsorption
isotherms.
Given the amorphous nature of POPs it is challenging to
determine exact structural distinctions, in particular when the
same functionalities are present. Therefore, to elucidate the
differences in the performance of POP-pNH2-Py and POP-oNH2-
Py, monomeric equivalents were complexed with the palladium
salt and single crystals were obtained, denoted as Pd@pNH2-Py
and Pd@oNH2-Py, respectively. As seen in Figure 5, Pd@pNH2-
Py was shown to have preferential binding of four pyridine units
to one palladium, whereas, for Pd@oNH2-Py there is steric
hindrance from the amino group in the ortho position reducing the
coordination of pyridine to two. However, for POP-pNH2-Py the
amorphous polymer form would impede the ability to coordinate
four pyridines to one palladium. This is further evidenced by the
maximum uptake capacity values, wherein both polymers bind to
palladium closer to a 2:1 manner. The EDX mapping images
substantiate this showing the presence of Cl in both samples
(Figure S6), indicating palladium is coordinated to two N and two
Cl groups in Pd@pNH2-Py and Pd@oNH2-Py. Additionally,
through a combined analysis of X-ray absorption near edge
structure and extended X-ray absorption fine structure studies
(Figures S7-S9) it was concluded that the Pd is bound to N atoms
with similar bond lengths and coordination fashion for both POP-
pNH2-Py and POP-oNH2-Py (Table S3). Returning to the single
Figure 3. A) Pd 3d and B) N 1s XPS spectra of pristine (blue), palladium
contacted (green), and regenerated (red) POP-oNH2-Py.
The selectivity of POP-pNH2-Py and POP-oNH2-Py for
palladium in the presence of other ions prevalent in waste streams
was then tested.[7c,20] 5 mg of adsorbent was placed in 50 mL of a
5 ppm mixed metal solution containing Pd2+, Fe2+, Zn2+, Pb2+, Ni2+,
and Cd2+. After overnight treatment, the remaining palladium in
solution was measured with ICP-MS. POP-oNH2-Py exhibited
almost full palladium recovery, reducing the concentration to 1.02
ppb. Meanwhile, POP-pNH2-Py showed severely reduced
capacity, only decreasing the concentration to 188 ppb. Although
at high concentrations the amino group in POP-pNH2-Py is
available as a weaker secondary binding site, it is evident that the
amino group in the ortho position creates a more stable complex
3
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crystal data the difference in performance can be attributed to the
intramolecular hydrogen bonding between the Cl groups from the
palladium salt and the amino group in the ortho position of
Pd@oNH2-Py (Figure 5B). This in turn stabilizes the complex and
allows for POP-oNH2-Py to selectively capture palladium at more
environmentally relevant concentrations compared to POP-pNH2-
Py.
Keywords: enhanced binding affinity • hydrogen bond
stabilization • palladium recovery • platinum group element •
porous organic polymer
[1]
[2]
[3]
a) S. Zhang, Y. Ding, B. Liu, C.-c. Chang, Waste Manag. 2017, 65, 113-
127; b) M. L. Zientek, P. J. Loferski, Platinum-Group Elements—So Many
Excellent Properties: U.S. Geological Survey Fact Sheet 2014, 2014; c)
V. Fernandez, Int. Rev. Financ. Anal. 2017, 52, 333-347; d) T. Bossi, J.
Gediga, Johnson Matthey Technol. Rev. 2017, 61, 111-121.
a) G. M. Mudd, Ore Geol. Rev. 2012, 46, 106-117; b) J. E. Mungall, A. J.
Naldrett, Elements 2008, 4, 253-258; c) D. R. Wilburn, D. I. Bleiwas,
Platinum-Group Metals- World Supply and Demand, US Geological
Survey Open-File Report 2004-1224, US Department of the Interior,
Washington, DC, 2005.
a) E. Hsu, K. Barmak, A. C. West, A.-H. A. Park, Green Chem. 2019, 21,
919-936; b) B. H. Robinson, Sci. Total Environ. 2009, 408, 183-191; c)
P. Tanskanen, Acta Mater. 2013, 61, 1001-1011; d) H.-Y. Kang, J. M.
Schoenung, Resour. Conserv. Recy. 2005, 45, 368-400.
[4]
[5]
a) S. Lin, D. H. K. Reddy, J. K. Bediako, M.-H. Song, W. Wei, J.-A. Kim,
Y.-S. Yun, J. Mater. Chem. A 2017, 5, 13557-13564; b) M. Zha, J. Liu,
Y.-L. Wong, Z. Xu, J. Mater. Chem. A 2015, 3, 3928-3934.
a) T. Kang, Y. Park, K. Choi, J. S. Lee, J. Yi, J. Mater. Chem. 2004, 14,
1043-1049; b) R. K. Sharma, A. Pandey, S. Gulati, A. Adholeya, J.
Hazard. Mater. 2012, 209, 285-292; c) F. Bai, G. Ye, G. Chen, J. Wei, J.
Wang, J. Chen, Sep. Purif. Technol. 2013, 106, 38-46; d) J. Kramer, N.
E. Dhladhla, K. R. Koch, Sep. Purif. Technol. 2006, 49, 181-185.
a) X. Chen, K. F. Lam, S. F. Mak, K. L. Yeung, J. Hazard. Mater. 2011,
186, 902-910; b) A.-E. Rotaru, W. Jiang, K. Finster, T. Skrydstrup, R. L.
Meyer, Biotechnol. Bioeng. 2012, 109, 1889-1897; c) S. W. Won, P.
Kotte, W. Wei, A. Lim, Y.-S. Yun, Bioresour. Technol. 2014, 160, 203-
212; d) J. R. Dodson, H. L. Parker, A. M. García, A. Hicken, K. Asemave,
T. J. Farmer, H. He, J. H. Clark, A. J. Hunt, Green Chem. 2015, 17, 1951-
1965.
[6]
Figure 5. Single crystal structures of A) Pd@pNH2-Py (shown with coordinated
DMF solvent molecules) and B) Pd@oNH2-Py.
In summary, through a comparative study of a series of
pyridine-based POPs, POP-Py, POP-pNH2-Py, and POP-oNH2-
Py, their ability to recover palladium was investigated. All of the
adsorbents had record high uptakes with the ability to reduce the
palladium concentration to ppb level. The addition of the amino
group in POP-pNH2-Py and POP-oNH2-Py was shown to enhance
the palladium recovery performance by increasing the
nucleophilicity of the pyridine binding site. Additionally, the amino
group in the ortho position relative to the pyridine binding site
provided a stronger complex with palladium compared to its para
counterpart due to the hydrogen bonding interaction between the
amino group and coordinated chloride ligand as confirmed by
single crystal data. This resulted in POP-oNH2-Py outperforming
POP-pNH2-Py with regard to regenerative abilities and selectivity,
with additional successful palladium uptake experiments after
exposure to extreme pH conditions. A strong binding affinity for
POP-oNH2-Py was calculated based on experimental equilibrium
data, which was corroborated by XPS analysis of Pd 3d and N 1s.
These results demonstrate how slight modifications to adsorbents
can maximize their performance, which could be extended to a
multitude of applications.
[7]
[8]
a) J. S. Guest, S. J. Skerlos, J. L. Barnard, M. B. Beck, G. T. Daigger, H.
Hilger, S. J. Jackson, K. Karvazy, L. Kelly, L. Macpherson, J. R. Mihelcic,
A. Pramanik, L. Raskin, M. C. M. Van Loosdrecht, D. Yeh, N. G. Love,
Environ. Sci. Technol. 2009, 43, 6126-6130; b) T. Kakoi, N. Horinouchi,
M. Goto, F. Nakashio, J. Membr. Sci. 1996, 118, 63-71; c) I. Matsubara,
Y. Takeda, K. Ishida, Fresenius J. Anal. Chem. 2000, 366, 213-217.
a) Y. H. Xu, S. B. Jin, H. Xu, A. Nagai, D. L. Jiang, Chem. Soc. Rev. 2013,
42, 8012-8031; b) N. Chaoui, M. Trunk, R. Dawson, J. Schmidt, A.
Thomas, Chem. Soc. Rev. 2017, 46, 3302-3321; c) Y. H. Xu, S. B. Jin,
H. Xu, A. Nagai, D. L. Jiang, Chem. Soc. Rev. 2013, 42, 8012-8031; d)
A. G. Slater, A. I. Cooper, Science 2015, 348, aaa8075; e) L. X. Tan, B.
Tan, Chem. Soc. Rev. 2017, 46, 3322-3356; f) S. Das, P. Heasman, T.
Ben, S. L. Qiu, Chem. Rev. 2017, 117, 1515-1563; g) D. Taylor, S. J.
Dalgarno, Z. Xu, F. Vilela, Chem. Soc. Rev. 2020, DOI:
10.1039/C9CS00315K; h) Y.Tian, G. Zhu, Chem. Rev. 2020, DOI:
10.1021/acs.chemrev.9b00687.
[9]
a) Y. Zhang, S. N. Riduan, Chem. Soc. Rev. 2012, 41, 2083-2094; b) Q.
Sun, Z. F. Dai, X. J. Meng, F. S. Xiao, Chem. Soc. Rev. 2015, 44, 6018-
6034; c) S. Kramer, N. R. Bennedsen, S. Kegnaes, ACS Catal. 2018, 8,
6961-6982; d) Q. Sun, B. Aguila, G. Verma, X. Liu, Z. Dai, F. Deng, X.
Meng, F.-S. Xiao, S. Ma, Chem, 2016, 1, 628-639; e) R. Cai, X. Ye, Q.
Sun, Q. He, Y. He, S. Ma, X. Shi, ACS Catal. 2017, 7, 1087-1092; e) L.
Chen,
Y.
Yang,
Z.
Guo,
D.
Jiang*
Acknowledgements
Adv. Mater. 2011, 23, 3149-3154; f) T.-X. Wang, H.-P. Liang, D. A. Anito,
X. Ding, B.-H. Han, J. Mater. Chem. A 2020, 8, 7003-7034.
The authors acknowledge the U.S. National Science Foundation
(CBET-1706025) for financial support of this work. Partial support
from the Distinguished Scientist Fellowship Program (DSFP) at
King Saud University (SM/AMA), the 111 Project (Grant No.
B17020) from National Natural Science Foundation of China (ZL),
and the U.S. National Science Foundation, DMR-1708617
(RWM/JTW) is also acknowledged.
[10] a) C. D. Wood, B. Tan, A. Trewin, F. Su, M. J. Rosseinsky, D. Bradshaw,
Y. Sun, L. Zhou, A. I. Cooper, Adv. Mater. 2008, 20, 1916-1921; b) T.
Ben, H. Ren, S. Ma, D. Cao, J. Lan, X. Jing, W. Wang, J. Xu, F. Deng, J.
M. Simmons, S. Qiu, G. Zhu, Angew Chem. Int. Ed., 2009, 48, 9457-
9460; c) L. Zou, Y. Sun, S. Che, X. Yang, X. Wang, M. Bosch, Q. Wang,
H. Li, M. Smith, S. Yuan, Z. Perry, H.-C. Zhou, Adv. Mater. 2017, 29,
1700229; d) J. L. Wu, F. Xu, S. M. Li, P. W. Ma, X. C. Zhang, Q. H. Liu,
R. W. Fu, D. C. Wu, Adv. Mater. 2019, 31, 1802922.
[11] a) A. Alsbaiee, B. J. Smith, L. Xiao, Y. Ling, D. E. Helbling, W. R. Dichtel,
Nature 2015, 529, 190; b) B. Y. Li, Y. M. Zhang, D. X. Ma, Z. Shi, S. Q.
4
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Ma, Nat. Commun. 2014, 5, 5537; c) B. Aguila, Q. Sun, J. A. Perman, L.
D. Earl, C. W. Abney, R. Elzein, R. Schlaf, S. Ma, Adv. Mater. 2017, 29,
1700665; d) W. C. Wilfong, B. W. Kail, T. L. Bank, B. H. Howard, M. L.
Gray, ACS Appl. Mater. Interfaces 2017, 9, 18283-18294. B. N. Zheng,
X. D. Lin, X. C. Zhang, D. C. Wu, K. Matyjaszewski, Adv.Funct. Mater.
2019, 1907006; e) Q. Sun, L. Zhu, B. Aguila, P. K. Thallapally, C. Xu, J.
Chen, S. Wang, D. Rogers, S. Ma, Nature Commun. 2019, 10, 1646; f)
Q. Sun, B. Aguila, S. Ma, Trends in Chemistry 2019, 1, 292-303.
[12] a) R. W. Gunasekara, Y. Zhao, Chem. Commun. 2016, 52, 4345-4348;
d) H. J. Schneider, Angew. Chem. Int. Ed. 2009, 48, 3924-3977; b) Z.
Zhong, X. Li, Y. Zhao, J. Am. Chem. Soc. 2011, 133, 8862-8865; c) A. S.
Ivanov, B. F. Parker, Z. Zhang, B. Aguila, Q. Sun, S. Ma, S. Jansone-
Popova, J. Arnold, R. T. Mayes, S. Dai, V. S. Bryantsev, L. Rao, I.
Popovs, Nat. Commun. 2019, 10, 819; d) B. Aguila, Q. Sun, H. Cassady,
C. Abney, B. Li, S. Ma, ACS Appl. Mater. Interfaces, 2019, 11, 30919-
30926; e) Q. Sun, B. Aguila, Y. Song, S. Ma, Acc. Chem. Res., 2020, 53,
812-821.
[13] Q. Sun, B. Aguila, J. Perman, A. S. Ivanov, V. S. Bryantsev, L. Earl, C.
Abney, L. Wojtas, S. Ma, Nat. Commun. 2018, 9, 1644.
[14] a) H. Doucet, J.-C. Hierso, Curr. Opin. Drug Discovery Devel. 2007, 10,
672-690; b) G. Ferrier, A. Berzins, N. Davey, Platinum Met. Rev. 1985,
29, 175-179; c) M. Beller, T. H. Riermeier, S. Haber, H.-J. Kleiner, W. A.
Herrmann, Chem. Ber. 1996, 129, 1259-1264; d) M. T. Ravanchi, T.
Kaghazchi, A. Kargari, Desalination 2009, 235, 199-244.
[15] a) G. N. Lewis, Valence and the Structure of Atoms and Molecules,
Chemical Catalog Company, Incorporated, 1923; b) R. G. Pearson, J.
Am. Chem. Soc. 1963, 85, 3533-3539.
[16] a) E. Birinci, M. Gülfen, A. O. Aydın, Hydrometallurgy 2009, 95, 15-21;
b) V. V. Kumar, C. R. Kumar, A. Suresh, S. Jayalakshmi, U. K. Mudali,
N. Sivaraman, Roy. Soc. Open Sci. 2018, 5, 171701-171716.
[17] a) Y. S. Ho, G. McKay, Process Biochem. 1999, 34, 451-465; b) Y.-S.
Ho, J. Hazard. Mater. 2006, 136, 681-689.
[18] a) A. Drelinkiewicz, M. Hasik, M. Choczyński, Mater. Res. Bull. 1998, 33,
739-762; b) M. Hasik, A. Bernasik, A. Drelinkiewicz, K. Kowalski, E.
Wenda, J. Camra, Surf. Sci. 2002, 507, 916-921; c) J. F. Moulder, J.
Chastain, Handbook of X-ray Photoelectron Spectroscopy: A Reference
Book of Standard Spectra for Identification and Interpretation of XPS
Data, Physical Electronics, 1995.
[19] E. T. Kang, K. G. Neoh, K. L. Tan, Mol. Phys. 1990, 70, 1057-1064.
[20] E. Lahtinen, M. M. Hanninen, K. Kinnunen, H. M. Tuononen, A. Vaisanen,
K. Rissanen, M. Haukka, Adv. Sustain. Syst. 2018, 2, 1800048.
[21] J. R. Durig, R. Layton, D. W. Sink, B. R. Mitchell, Spectrochimica Acta.
1965, 21, 1367-1378.
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10.1002/anie.202006596
Angewandte Chemie International Edition
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Entry for the Table of Contents
“Green mining” for platinum group elements. As an alternative to platinum group element (PGE) mining, a series of porous
organic polymer nanotraps were rationally designed for their ability to recover the PGE palladium from aqueous solutions as a
potential recycling method. Through intramolecular hydrogen bonding the adsorbent-palladium complex could be stabilized, allowing
for selective capture of palladium at environmentally relevant conditions.
6
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