Postsynthetically Modified Covalent Organic Frameworks for Efficient and Effective Mercury Removal

Article abstract of DOI:10.1021/jacs.6b12885

A key challenge in environmental remediation is the design of adsorbents bearing an abundance of accessible chelating sites with high affinity, to achieve both rapid uptake and high capacity for the contaminants. Herein, we demonstrate how two-dimensional covalent organic frameworks (COFs) with well-defined mesopore structures display the right combination of properties to serve as a scaffold for decorating coordination sites to create ideal adsorbents. The proof-of-concept design is illustrated by modifying sulfur derivatives on a newly designed vinyl-functionalized mesoporous COF (COF-V) via thiol-ene "click" reaction. Representatively, the material (COF-S-SH) synthesized by treating COF-V with 1,2-ethanedithiol exhibits high efficiency in removing mercury from aqueous solutions and the air, affording Hg²⁺ and Hg⁰ capacities of 1350 and 863 mg g^-1, respectively, surpassing all those of thiol and thioether functionalized materials reported thus far. More significantly, COF-S-SH demonstrates an ultrahigh distribution coefficient value (K_d) of 2.3 × 10⁹ mL g^-1, which allows it to rapidly reduce the Hg²⁺ concentration from 5 ppm to less than 0.1 ppb, well below the acceptable limit in drinking water (2 ppb). We attribute the impressive performance to the synergistic effects arising from densely populated chelating groups with a strong binding ability within ordered mesopores that allow rapid diffusion of mercury species throughout the material. X-ray absorption fine structure (XAFS) spectroscopic studies revealed that each Hg is bound exclusively by two S via intramolecular cooperativity in COF-S-SH, further interpreting its excellent affinity. The results presented here thus reveal the exceptional potential of COFs for high-performance environmental remediation.

Full text of DOI:10.1021/jacs.6b12885

Article
pubs.acs.org/JACS
Postsynthetically Modified Covalent Organic Frameworks for
Efficient and Effective Mercury Removal
Qi Sun,† Briana Aguila,† Jason Perman,† Lyndsey D. Earl,‡ Carter W. Abney,‡ Yuchuan Cheng,§
Hao Wei,∥ Nicholas Nguyen,† Lukasz Wojtas,† and Shengqian Ma*,†
†Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, United States
‡Chemical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States
§Zhejiang Key Laboratory of Additive Manufacturing Materials, Ningbo Institute of Materials Technology and Engineering, Chinese
Academy of Sciences, 1219 Zhongguan West Road, Ningbo, Zhejiang 315201, China
∥School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
S
* Supporting Information
ABSTRACT: A key challenge in environmental remediation is the design of adsorbents
bearing an abundance of accessible chelating sites with high affinity, to achieve both
rapid uptake and high capacity for the contaminants. Herein, we demonstrate how two-
dimensional covalent organic frameworks (COFs) with well-defined mesopore
structures display the right combination of properties to serve as a scaffold for
decorating coordination sites to create ideal adsorbents. The proof-of-concept design is
illustrated by modifying sulfur derivatives on a newly designed vinyl-functionalized
mesoporous COF (COF-V) via thiol−ene “click” reaction. Representatively, the
material (COF-S-SH) synthesized by treating COF-V with 1,2-ethanedithiol exhibits
high efficiency in removing mercury from aqueous solutions and the air, affording Hg2+
and Hg0 capacities of 1350 and 863 mg g−1, respectively, surpassing all those of thiol and
thioether functionalized materials reported thus far. More significantly, COF-S-SH
demonstrates an ultrahigh distribution coefficient value (Kd) of 2.3 × 109 mL g−1, which allows it to rapidly reduce the Hg2+
concentration from 5 ppm to less than 0.1 ppb, well below the acceptable limit in drinking water (2 ppb). We attribute the
impressive performance to the synergistic effects arising from densely populated chelating groups with a strong binding ability
within ordered mesopores that allow rapid diffusion of mercury species throughout the material. X-ray absorption fine structure
(XAFS) spectroscopic studies revealed that each Hg is bound exclusively by two S via intramolecular cooperativity in COF-S-SH,
further interpreting its excellent affinity. The results presented here thus reveal the exceptional potential of COFs for high-
performance environmental remediation.
binding sites.7 However, the limited amount of silanol groups
on the surface inevitably diminishes the population density of
INTRODUCTION
■
Environmental pollution is of growing worldwide concern, and
chelating sites, thus resulting in low saturation capacity and
it is imperative to remove natural and anthropogenic toxic
contaminants from the water and air,1 especially heavy-metal
affinity. Considering these weaknesses associated with existing
species which pose serious health effects.2 Among all the
adsorbents, it is therefore necessary to develop new types of
materials that show greater potential in remediation technology.
techniques trialed for this purpose, adsorption holds consid-
Covalent organic frameworks (COFs)8 have emerged as a
erable promise.3,4 To achieve high uptake capacity, fast kinetics,
type of porous material with significant prospects for addressing
and excellent selectivity, the use of complexing functionalities
has come into play to combat heavy-metal pollution.5
current challenges pertinent to energy and environmental
sustainability, including gas adsorption,9 optoelectronics,10
Numerous adsorbents have been developed based on the
catalysis,11 proton conduction,12 and many more.13 The
covalent grafting of coordination groups to the surface of
tunable nature of COFs may also be beneficial in mitigating
porous materials such as silica gel and clay. Nonetheless, only a
environmental problems caused by toxic heavy metals. We
fraction of the surface-bound ligands is found to interact with
reason that such materials have the right combination of
the targeted metal ions due to their small and irregular pore
properties that give them potential as excellent sorbents for
structures, limiting the access of contaminants to the grafted
ligands and thus compromising metal binding capacity.6
environmental remediation, in particular, two-dimensional
COFs. These advantages, leading to enhanced binding affinity
Mesoporous molecular sieves were alternatively used to
alleviate this issue due to their large uniform pore structures,
which preclude pore blockage by the grafted species and
thereby facilitate the interaction between metal ions and
Received: December 15, 2016
© XXXX American Chemical Society
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Figure 1. (a) Synthetic scheme of COF-V through the condensation of Tab (black) and Dva (blue) and representative channel-wall engineering by
thiol−ene reaction with (COF-S-SH). (b, c) Graphic view of the slipped AA stacking structure of COF-V (blue, N; gray, C; hydrogen is omitted for
clarity). (d) Graphic view of COF-S-SH (blue, N; gray, C; yellow, S; hydrogen is omitted for clarity).
and uptake capacity, include (1) a highly tunable molecular
design, allowing for atomically precise integration of building
blocks into porous structures; (2) crystallinity, offering a
strategy to position functional groups in a highly controlled and
predictable manner; and (3) the nearly eclipsed and ordered π-
columnar structures of most 2D COFs, enabling the functional
groups to be fully accessible and in close proximity with each
other to facilitate their cooperation. Despite the success in
obtaining functionally diverse COFs via direct synthesis, for
example, a thioether based 2D COF developed for sensing
applications,4d the scope of functional groups within the pores
of the COFs has remained relatively limited due to the fact that
not all functional groups are compatible with or stable to the
conditions for the COFs synthesis.
In order to impart specific functionality on the COFs, we
designed a vinyl-functionalized monomer (2,5-divinyltereph-
thalaldehyde) where the vinyl groups can remain intact during
the COF synthesis, thus allowing for further chemical
modifications.14 In addition, it has been proven that COFs
constructed with imine bonds can withstand a wide range of
conditions.15 As such, the newly designed vinyl-functionalized
COF (COF-V), synthesized by the condensation between 2,5-
divinylterephthalaldehyde and 1,3,5-tris(4-aminophenyl)-
benzene, is attractive because of its stability and potential for
versatile modification. In view of toxic metal species, mercury
and other soft heavy-metal species, such as lead, present a
serious environmental concern and constitute a threat to public
health. For example, mercury is inclined to transform to the
potent neurotoxin methylmercury, which can be bioaccumu-
lated and biomagnified in aquatic food chains. In this sense,
effective capture and removal of them is mandatory and has
attracted considerable interest. Considering the extraordinary
binding ability of sulfur derivatives with soft heavy-metal
species, such as mercury, and high throughput of thiol−ene
transformations, a series of thiol compounds with different
flexibility and sulfur species density were chosen to
demonstrate the proof-of-concept. The resultant sulfur-based
chelating-group-laced COFs, with retained crystallinity and
porosity, are capable of effective mercury removal from both
aqueous solutions and the gas phase with outstanding uptake
capacity [up to about 1350 mg g−1 (with an equilibrium
concentration of 110 ppm) and 863 mg g−1 for Hg2+ and Hg0,
respectively] and can rapidly diminish the Hg2+ concentration
down to 0.1 ppb level even with a high concentration of
competing ions present. The synergistic effects of densely
populated yet highly accessible chelating groups on the ordered
one-dimensional meso-channels afford COF-S-SH with the
highest mercury uptake capacity known among thiol and
thioether functionalized materials, placing it within striking
distance of the all-time mercury uptake record.19 Our studies
therefore lay a foundation for developing COFs as a promising
type of host material for the deployment of adsorbents that
circumvent the issues of buried chelating sites and a low degree
of functionalization encountered in conventional materials.
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Figure 2. (a, b) SEM images for COF-V and COF-S-SH, respectively. (c) PXRD profiles. (d) Unit cell of the AA stacking mode of COF-V (N, blue;
C, gray; H, white). (e) Unit cell of the AB stacking mode of COF-V (N, blue; C, gray; H, white; green, a further layer). (f, g) Nitrogen-sorption
isotherm curves measured at 77 K for COF-V and COF-S-SH, respectively.
77 K. DFT fitting of the adsorption branches showed pore size
distributions mainly at 2.8 nm in agreement with that of the
proposed model (Figure S4). The framework of COF-V is
stable in ambient air and retains its crystallinity after soaking in
a variety of organic solvents and water, as well as both acid (1
M HCl) and base (2 M NaOH) aqueous solutions (Figure S5).
Intrigued by its high crystallinity, mesoporous channels, and
chemical stability, we developed COF-V into heavy-metal
adsorbent materials by anchoring sulfur derivatives (thiol and/
or thioether) onto the channel walls using thiol−ene “click”
reaction between thiol compounds and the vinyl groups in
COF-V with the assistance of azobis(isobutyronitrile) (AIBN).
The material obtained by the treatment of COF-V and 1,2-
ethanedithiol (COF-S-SH) was taken as a representative
sample for full illustration. The PXRD pattern of COF-S-SH
exhibited a diffraction pattern comparable to that of COF-V but
with broadened peak widths (Figure 2c). We attributed the
broad peak widths to the large numbers of flexible chains in the
pores of the COF, thus slightly distorting the long-range order
of the crystalline material.16 Nitrogen sorption measurements
revealed that COF-S-SH still gave a BET surface area as high as
546 m2 g−1, which is indicative of pore accessibility after
modification. SEM and TEM images of COF-S-SH also
exhibited uniform nanofibers, indicating that no noticeable
morphological changes occurred following chemical modifica-
tion as compared with that of COF-V (Figures 2b and S6). The
successful grafting of thiol and thioether groups onto the COF-
V was confirmed by X-ray photoelectron spectroscopy (XPS),
FT-IR, solid-state 13C NMR studies, and elemental analysis.
The XPS spectrum of COF-S-SH revealed the sulfur signal at a
binding energy of 163.9 eV, verifying the postsynthetic
modification (Figure S7).17 In addition, the FT-IR spectrum
of the COF-S-SH showed the characteristic band of S−H at
2552 cm−1 compared with that of pristine COF-V, confirming
the existence of free-standing thiol groups (Figure S8).17 The
high throughput of the thio-ene transformation was determined
by the disappearance of the peaks assigned to the vinyl groups
at around 110 ppm in the 13C MAS NMR spectrum of COF-S-
SH and the concomitant emergence of strong peaks at 42.1
RESULTS AND DISCUSSION
■
Material Preparation, Physiochemical Characteriza-
tion, and Local Structure Analysis. Herein, COF-V was
synthesized via an imine condensation reaction between 2,5-
divinylterephthalaldehyde (Dva) and 1,3,5-tris(4-
aminophenyl)benzene (Tab) using 1:1 n-butylalcohol/1,2-
dichlorobenzene as the solvent in the presence of 6 M acetic
acid at 100 °C for 3 days (Figure 1). The Fourier transform
infrared (FT-IR) spectrum of the COF-V was compared with
that of the precursors and showed the appearance of the
characteristic peak for CN at 1606 cm−1 with the
concomitant disappearance of the aldehydic C−H (2875 and
2779 cm−1) and CO (1676 cm−1) stretching vibrations of
Dva and the N−H (3430 and 3355 cm−1) stretching vibrations
of Tab, thus indicating the condensation occurrence (Figure
S1).15 The morphology of the COF-V was examined by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM), which showed a large quantity of uniform
nanofibers with diameter of about 80 nm and lengths up to tens
of micrometers, thus implying its phase purity (Figures 2a and
S2).
To analyze the structure of the obtained material, powder X-
ray diffraction (PXRD) and theoretical simulation experiments
were conducted. In the experimental PXRD profile of COF-V, a
strong peak at 2.72° together with some relatively weaker peaks
at 4.74, 5.48, 7.27, 9.54, and 24.9° were observed, which were
assigned to (100), (110), (200), (210), (220), and (001)
diffractions (Figure 2c). To elucidate the constitution of the
framework, we constructed a structural model using Materials
Studio. Pawley refinements of the PXRD patterns were carried
out for full profile fitting against the proposed model of AA
packing, which resulted in good agreement factors (RWP = 1.7
after convergence) and reasonable profile differences (Figure
S3 and Table S1). We interpreted the refinement data to show
that COF-V has one-dimensional channels, with a diameter of
28 Å, along the c-axis and that the layers stack with an interlayer
distance of 3.6 Å. The porosity and specific surface area were
determined to be 1152 m2 g−1 from N2 sorption isotherms at
C
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Figure 3. (a) Hg2+ adsorption isotherm for COF-S-SH. Inset shows the linear regression by fitting the equilibrium data with the Langmuir
adsorption model. (b) Comparison of Hg2+ saturation uptake amount and Kd value for COF-S-SH with those of other benchmark porous materials:
PAF-1-SH (ref 17); LHMS-1 (ref 19a); FMMS (ref 19b); Chalcogel-1 (ref 19c); S-FMC-900 (ref 19d); Zr-DMBD (ref 19e); W-DR-N-MoS2 (ref
19f); H1.45Na0.45InS2.45 (ref 19g); KMS-1 (ref 19h). (c) Hg2+ sorption kinetics of COF-S-SH with Hg2+ initial concentration of 5 ppm at a V/m ratio
of 46500 mL g−1. (d) Adsorption curve of Hg2+ versus contact time in aqueous solution using COF-S-SH. Inset shows the pseudo-second-order
kinetic plot for the adsorption.
ppm ascribed to the alkyl C species (Figure S9).18
Furthermore, elemental analysis of COF-S-SH revealed sulfur
content of 20.9 wt % that corresponds to 6.53 mmol g−1 sulfur
species. This suggests that more than 90% of the vinyl groups
reacted with 1,2-ethanedithiol. These results are consistent with
the 13C MAS NMR analysis. On the basis of the above, we can
conclude that a high density of sulfur species was incorporated
without significantly altering the crystalline structure of COF-V.
Hg2+ Sorption Studies. After confirming the porosity,
structural integrity, and high density of chelating sites of COF-
S-SH, we examined its ability to capture Hg2+ from aqueous
solutions. To assess the overall capacity of COF-S-SH for Hg2+,
equilibrium values were collected after exposure to Hg2+
aqueous solutions with initial concentrations in the range of
25−700 ppm. As shown in Figure 3a, COF-S-SH exhibits a very
steep adsorption profile for Hg2+, approaching the ideal type-I
isotherm shape, thus indicating that COF-S-SH has a very
strong affinity for Hg2+. The equilibrium adsorption isotherm
data was well-fitted with the Langmuir model, yielding a
correlation coefficient as high as 0.998. Remarkably, COF-S-SH
was determined to have a capacity of about 1350 mg of Hg2+
per gram of adsorbent, surpassing all previously reported thiol
and/or thioether functionalized materials for mercury decon-
tamination and outperforming most of the layered metal sulfide
adsorbents (Figure 3b, Table S2).19 Specifically, it is 2.5 times
better than the best sorption obtained with mesoporous
silica,19b almost doubling the sulfur-functionalized mesoporous
carbon (S-FMC),19d 7 times higher than thiol laced MOF,19e
1.5 times higher than the best MOF (BioMOF),19i and 30%
higher than the benchmark mercury sorbent material PAF-1-
SH.17 Such an outstanding saturation mercury uptake capacity
can be attributed to the tremendous affinity together with a
large number of highly accessible sulfur species that are well-
dispersed throughout the channel surface of COF-S-SH. Based
on the previously determined sulfur content, the calculated Hg
capacity for COF-S-SH would equal 1320 mg Hg per gram of
adsorbent, which means virtually all of the sulfur-based
functionality in the COF-S-SH is available for binding Hg.
The slight deviation between the experimental and calculated
uptake capacities could presumably be attributed to the
conjugated COF host, which is capable of interaction with
Hg2+ from a Hg2+−π interaction.20 Accordingly, it is revealed
that COF-V afforded a mercury saturation adsorption capacity
of 147 mg g−1 treated under the same conditions as Figure 3a
with an initial Hg2+ concentration of 700 ppm. It is noteworthy
that the PXRD pattern and SEM image (Figure S10) suggest
the retention of both crystallinity and the integrity of the
material after the sorption process. More significantly, COF-S-
SH can readily be regenerated by treating it with 1,2-
ethanedithiol, and it was demonstrated to retain its mercury
uptake capacity for at least four consecutive cycles, affording a
value of 1280 mg g−1 (Figure S11).
In addition to the high Hg2+ uptake capacity, time-course
adsorption measurements further indicated that Hg2+ capture
by COF-S-SH is kinetically efficient (Figure 3c). It can attain
99.94% of the adsorption capacity at equilibrium within 10 min
and was able to reduce a heavily contaminated water (Hg2+
concentration of 5 ppm) to 0.73 ppb, far below the acceptable
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limit of 2 ppb for drinking water within 30 min of contact in the
presence of a very small amount of adsorbent (V/m at 46500
mL g−1). Prolonging the contact time to 3 h, the Hg2+
concentration was reduced below the detection limit of ICP-
MS for Hg (0.1 ppb). The adsorption kinetic process was well-
fitted with the pseudo-second-order kinetic model expressed as
t
qt
1
t
qe
=
+
k2q2
e
where k2 (g mg−1 min−1) is the pseudo-second-order rate
constant of adsorption, qt (mg g−1) is the amount of Hg2+
adsorbed at time t (min), and qe (mg g−1) is the amount of
Hg2+ adsorbed at equilibrium (Figure 3d). The initial
adsorption rate, h (k2q2e), has been widely used for evaluation
of the adsorption rates. Accordingly, COF-S-SH gives an h
value as high as 143 mg g−1 min−1, far exceeding that reported
for a series of benchmark materials, for example, PAF-1-SH.17
Such extraordinarily fast kinetics observed for COF-S-SH can
be ascribed to the high concentration of chelating groups
together with well-defined pore channels adequate to facilitate
the diffusion of Hg2+ ions. These results underscore the
superiority of the utilization of COFs as promising candidates
in accomplishing mercury removal from water.
Figure 4. (a) Open structures of the adsorbents and the chelating arms
on the channel walls (S, yellow; N, blue; C, gray). (b) Hg2+ sorption
kinetics over various adsorbents with Hg2+ initial concentration of 5
ppm. (c) Fourier transform of the Hg LIII-edge EXAFS spectrum of
COF-S-SH in R-space. The magnitude of the Fourier transform is fit
by a red line; the real component is fit with a blue line. (d)
Accompanying k3-weighted χ(k) data and fit (black line).
Investigation of Hg2+ Binding Interaction. The out-
standing performance of COF-S-SH in mercury sequestration
can be traced to the remarkable binding interactions between
the Hg and sulfur species in COF-S-SH, which have been
elucidated by XPS, FT-IR, and Raman spectroscopy studies.
Mercury inclusion within COF-S-SH was confirmed by the
appearance of Hg2+ XPS signals at 101.4 and 105.4 eV assigned
to the Hg 4f7/2and Hg 4f5/2, respectively. The strong binding
interactions between the Hg2+ and S species in Hg@COF-S-SH
were observed from the S 2p XPS spectra, which showed a 0.6
eV shift for the S binding energy in Hg@COF-S-SH compared
to that of COF-S-SH (Figure S12). Furthermore, the absence
of the characteristic S−H stretching mode at 2552 cm−1 in the
IR spectrum of COF-S-SH upon saturation with Hg2+ (Hg@
COF-S-SH) verifies the interactions between S−H groups and
Hg2+ (Figure S13). Moreover, the peaks in the Raman
spectrum of Hg@COF-S-SH at 391 and 408 cm−1, attributed
to the Hg−S stretching vibrations (vas = 408 cm−1, vs = 391
cm−1), confirmed the coordination between S and Hg species
(Figure S14).21
It is noteworthy that the mercury-capture performance of
COF-S-SH is superior to that exhibited by state-of-the-art thiol
and/or thioether functionalized adsorbents. Besides the
apparent accessibility of the densely populated chelating
groups, to gain more insight into this unexpected performance,
we gauged the importance of flexibility and type of sulfur
species in the chelating groups. To test flexibility, we modified
COF-V with a more rigid molecule, benzene-1,4-dithiol,
making COF-S-Ph-SH (S content 10.2 wt %, Figures S15−
18). To test the sulfur species, we modified COF-V with
ethanethiol to form COF-S-Et, which only contains the
thioether group (S content 8.9 wt %, Figures S19−S22). As
seen in Figure 4, both COF-S-Ph-SH and COF-S-Et are inferior
to COF-S-SH in terms of removal kinetics and efficiency after
exposure to Hg2+ aqueous solutions with the same initial
concentration of 5 ppm. Specifically, the remaining mercury
concentrations were 1.3 and 22.0 ppb, respectively. The large
disparity in removal efficiency between COF-S-SH and COF-S-
Ph-SH is mainly due to the highly flexible chelating arms of
COF-S-SH that can be arranged in different conformations,
ready for metal ions to adopt a favorable form, thereby
increasing the affinity, which is reminiscent of that seen in
biological systems and protein receptors.22 With respect to
COF-S-Et, due to thioether having a lower affinity for mercury
compared with the thiol group, the removal efficiency is inferior
to that of thiol-contained adsorbents, COF-S-SH and COF-S-
Ph-SH, as demonstrated by its higher residual mercury
concentration (22.0 ppb).
To gain more insight into the coordination environment of
mercury in the three thiol and/or thioether-functionalized COF
materials, we employed X-ray absorption fine structure (XAFS)
spectroscopy collected at the Hg LIII-absorption edge (12.284
keV). Given that the mercury loading in the adsorbents affects
the coordination form, we treated these COFs with different
volumes of mercury aqueous solution (initial concentration of
20 ppm) to ensure that the S/Hg ratio was around 5 in the
resultant mercury included materials.7b A representative fit of
the extended XAFS (EXAFS) data collected for COF-S-SH is
shown in Figure 4c, with other fits provided in Figures S23−
S25. In contrast to previous EXAFS data collected on thiol-
functionalized mesoporous silica adsorbents, high-quality fits
for all adsorbents were only obtainable using Hg−S scattering
paths, and no Hg−O contributions were observed. This reveals
that each Hg was bound exclusively by two S rather than
through −Hg−O−Hg− layering23,24 or formation of terminal
Hg−OH species19b and confirms the adsorption of Hg by the
COF materials rather than precipitation as HgO.
Analysis of the fitted data also reveals for COF-S-Ph-SH and
COF-S-Et that two distinct Hg−S distances are required to
obtain an adequate fit of the data. A more quantitative
assessment reveals near-equal contributions (56 and 43%) from
the two S distances in COF-S-Et, while in COF-S-Ph-SH one S
species, the terminal thiol almost certainly predominates in Hg
binding (78 and 22%). This suggests steric influences from the
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rigidification of the chelating arm or inclusion of the terminal
ethyl restrain the extent of intramolecular cooperativity in Hg
binding, rationalizing the relative decrease in Hg adsorption
performance discussed above. In contrast, COF-S-SH requires
only one Hg−S bond length to fit of the EXAFS spectrum,
suggesting the ethylene bridge ideally colocates two S atoms to
chelate each Hg atom, thereby giving insight of its outstanding
affinity. Additional experimental details and analyses are
provided in the Supporting Information.
To evaluate the affinity of adsorbents for mercury,
distribution coefficient values (Kd) were thus calculated under
conditions identical to those of Figure 4b. Impressively, COF-
S-SH demonstrated an ultrahigh Kd value of approximately 2.3
× 109 mL g−1 (calculated after reaching equilibrium). This
value is the highest for sorbent materials for Hg2+ adsorption,
representing a 6-fold improvement over the best mercury
adsorbent materials reported thus far (3.5 × 108 mL g−1 for W-
DR-N-MoS2)19f and almost 1 and over 2 orders of magnitude
higher than that of COF-S-Ph-SH (1.5 × 108 mL g−1) and
COF-S-Et (9.1 × 106 mL g−1), respectively. More importantly,
even at pH values of 3 and 10, COF-S-SH still gave rise to Kd
values as high as 4.67 × 108 and 1.5 × 108 mL g−1, respectively,
thus indicating its superior affinity toward Hg2+ over a wide
range of pH values. This ability to remove mercury at different
pH values and remain stable under these conditions
demonstrates great promise for real world applications. On
the basis of these results, the following conclusion can be
drawn: The synergetic effect from the flexible ligand,
framework porosity, and concentrated sulfur species with high
affinity for Hg2+ account for the extraordinary performance of
COF-S-SH.
nontoxic competitive ions, such as Ca2+, Mg2+, Zn2+, and Na+.
The unique properties of COF-S-SH stem from its soft sulfur-
based chelating groups selective for soft or relatively soft metal
ions.
The outstanding mercury removal efficiency of COF-S-SH
from solution prompted us to evaluate mercury vapor
remediation, an application useful for industrial flue gas
detoxification. Mercury vapor control has posed a great
challenge for many adsorptive materials, such as activated
carbon, because they display low capture capacity at elevated
temperatures and suffer from interference of adsorption by
other flue gas chemicals.22 Materials containing sulfur show
great promise in terms of adsorption capacity and selectivity;
thus, it would be interesting in this regard to study the Hg0
adsorption properties of COF-S-SH. For the mercury vapor
sorption experiment, we modified the approach reported in the
literature,22 where a vial of elemental mercury (liquid, 300 mg)
was placed inside a larger vial containing COF-S-SH (20 mg) to
create spatial separation. The vial was then immersed in a sand
bath and heated at 140 °C. After 3 days of exposure, the
adsorption equilibrium was reached as indicated by weight
equilibrium. ICP-MS analysis was used to determine the Hg0
adsorption capacity of the COF-S-SH after it was digested by
aqua regia. The equilibrium adsorption capacity value of the
COF-S-SH was determined to be 863 mg Hg0 per gram
adsorbent, ranking among the highest of reported materials
compared with the benchmark adsorbents listed in Table S3,
while under identical conditions, activated carbon (BET = 1011
m2 g−1) and COF-V gave rise to Hg0 uptake capacities of 47
and 106 mg g−1, respectively. The highly accessible and densely
populated sulfur sites within the COF-S-SH channels
contribute to the exceptional mercury vapor adsorption
performance, highlighting its great potential applications in
Hg0 vapor control.
Selectivity Tests and Hg0 Vapor Adsorption. To
evaluate the ability of COF-S-SH to eliminate Hg2+ under
realistic conditions, the adsorption kinetic tests were performed
using a solution containing Hg2+, Pb2+, Cu2+, Ca2+, Mg2+, Zn2+,
and Na+ ions. COF-S-SH can effectively remove toxic ions
Hg2+, Pb2+, and Cu2+, which present a major health hazard. As
depicted in Figure 5, the adsorption rates for the ions of Hg2+,
Cu2+, and Pb2+ were found to be very rapid. Within 10 min, the
COF-S-SH achieved >99% removal efficiency for these ions
which are below the U.S. Environmental Protection Agency
elemental limits for drinking water (Figure S26). Notably,
COF-S-SH exhibited negligible capturing capability for various
CONCLUSION
■
In conclusion, we have demonstrated the deployment of COFs
as an amenable platform for removal of mercury as exemplified
by decorating a 2D mesoporous COF with dense and flexible
thiol and thioether chelating arms, COF-S-SH. The adsorbent
exhibits exceedingly high Hg2+ and Hg0 uptake capacities of
1350 and 863 mg g−1, respectively, a record-high affinity for
Hg2+ with exceptional distribution coefficient values at a wide
pH range from 3 to 10 and extremely fast kinetics for Hg2+
adsorption with an unexpected initial adsorption rate as high as
143 mg g−1 min−1, surpassing those of all previously reported
thiol-functionalized materials. More importantly, COF-S-SH
can effectively reduce the Hg2+ concentration from 5 ppm to
the extremely low level of 0.1 ppb, well below the acceptable
limits in drinking water (2 ppb), even in the presence of a high
concentration of background metal ions Ca2+, Zn2+, Mg2+, and
Na+. The densely populated yet fully accessible and flexible
chelating sites in conjunction with their remarkable affinity for
mercury are responsible for the impressive results. Our work
thus reveals the enormous potential of COFs as an appealing
platform for construction of sorbent materials for environ-
mental remediation, an auspicious function of COFs worthy of
further exploration.
EXPERIMENTAL SECTION
Synthesis of COF-V. A Pyrex tube measuring o.d. × i.d. = 9.5 ×
7.5 mm2 was charged with 2,5-divinylterephthalaldehyde (22.3 mg,
0.12 mmol) and 1,3,5-tris(4-aminophenyl)benzene (28.0 mg, 0.08
■
Figure 5. Time-dependent removal efficiency of various ions in the
presence of COF-S-SH.
F
DOI: 10.1021/jacs.6b12885
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
mmol) in 1.1 mL of a 5:5:1 v/v/v solution of 1,2-dichlorobenzene/n-
butylalcohol/6 M aqueous acetic acid. The tube was flash frozen at 77
K (liquid N2 bath), evacuated, and flame-sealed. Upon sealing, the
length of the tube was reduced to ca. 15 cm. The reaction mixture was
heated at 100 °C for 3 days to afford a yellow-brown precipitate which
was isolated by filtration and washed with anhydrous tetrahydrofuran
using Soxhlet extraction for 2 days. The product was dried under
vacuum at 50 °C to afford COF-V (39.4 mg, 86%).
Synthesis of COF-S-SH. To the mixture of COF-V (100 mg) and
azobis(isobutyronitrile) (AIBN, 10 mg) in a 25 mL Schlenk tube was
introduced 1,2-ethanedithiol (4.0 mL) under N2 atmosphere. After
stirring at 80 °C for 48 h, the title product was isolated by filtration,
washed by acetone, and dried under vacuum at 50 °C.
(3) (a) Mulligan, C. N.; Yong, R. N.; Gibbs, B. F. Eng. Geol. 2001, 60,
193−207. (b) Adewuyi, Y. G. Environ. Sci. Technol. 2005, 39, 3409−
3420. (c) Wang, Q.; Kim, D.; Dionysiou, D. D.; Sorial, G. A.;
Timberlake, D. Environ. Pollut. 2004, 131, 323−336. (d) Aguila, B.;
Banerjee, D.; Nie, Z.; Shin, Y.; Ma, S.; Thallapally, P. K. Chem.
Commun. 2016, 52, 5940−5942.
(4) (a) Lee, S.; Barin, G.; Ackerman, C. M.; Muchenditsi, A.; Xu, J.;
Reimer, J. A.; Lutsenko, S.; Long, J. R.; Chang, C. J. J. Am. Chem. Soc.
2016, 138, 7603−7609. (b) Zhong, L.-S.; Hu, J.-S.; Liang, H.-P.; Cao,
A.-M.; Song, W.-G.; Wan, L.-J. Adv. Mater. 2006, 18, 2426−2431.
(c) Tesh, S. J.; Scott, T. B. Adv. Mater. 2014, 26, 6056−6068.
(d) Ding, S.-Y.; Dong, M.; Wang, Y.-W.; Chen, Y.-T.; Wang, H.-Z.; Su,
C.-Y.; Wang, W. J. Am. Chem. Soc. 2016, 138, 3031−3037.
(5) (a) He, J.; Yee, K.-K.; Xu, Z.; Zeller, M.; Hunter, A. D.; Chui, S.
S.-Y.; Che, C.-M. Chem. Mater. 2011, 23, 2940−2947. (b) Liu, T.;
Che, J.-X.; Hu, Y.-Z.; Dong, X.-W.; Liu, X.-Y.; Che, C.-M. Chem. - Eur.
J. 2014, 20, 14090−14095. (c) Luo, F.; Chen, J. L.; Dang, L. L.; Zhou,
W. N.; Lin, H. L.; Li, J. Q.; Liu, S. J.; Luo, M. B. J. Mater. Chem. A
2015, 3, 9616−9620. (d) Yee, K.-K.; Wong, Y.-L.; Zha, M.; Adhikari,
R. Y.; Tuominen, M. T.; He, J.; Xu, Z. Chem. Commun. 2015, 51,
10941−10944. (e) He, F.; Wang, W.; Moon, J.-W.; Howe, J.; Pierce, E.
M.; Liang, L. ACS Appl. Mater. Interfaces 2012, 4, 4373−4379.
(f) Blanchard, G.; Maunaye, M.; Martin, G. Water Res. 1984, 18,
1501−1507. (g) Benhammou, A.; Yaacoubi, A.; Nibou, B.; Tanouti, B.
J. Colloid Interface Sci. 2005, 282, 320−326. (h) Mon, M.; Ferrando-
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b12885.
■
S
Material synthesis; characterization details; IR, SEM,
XPS, and NMR; XAFS data collection, fits, and analysis
details; supporting figures (PDF)
AUTHOR INFORMATION
Corresponding Author
*sqma@usf.edu
ORCID
Jason Perman: 0000-0003-4894-3561
Lyndsey D. Earl: 0000-0003-4359-0197
Shengqian Ma: 0000-0002-1897-7069
■
́ ́
Soria, J.; Grancha, T.; Fortea-Perez, F. R.; Gascon, J.; Leyva-Perez, A.;
Armentano, D.; Pardo, E. J. Am. Chem. Soc. 2016, 138, 7864−7867.
(6) (a) Mercier, L.; Detellier, C. Environ. Sci. Technol. 1995, 29,
1318−1323. (b) Moreira, W. C.; Gushikem, Y.; Nascimento, O. R. J.
Colloid Interface Sci. 1992, 150, 115−120. (c) Izatt, R. M.; Bradshaw, J.
S.; Bruening, R. L.; Tarbet, B. J.; Bruening, M. L. Pure Appl. Chem.
1995, 67, 1069−1074. (d) Tchinda, A. J.; Ngameni, E.; Kenfack, I. T.;
Walcarius, A. Chem. Mater. 2009, 21, 4111−4121.
Notes
(7) (a) Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500−503.
(b) Crudden, C. M.; Sateesh, M.; Lewis, R. J. Am. Chem. Soc. 2005,
127, 10045−10050. (c) Bibby, A.; Mercier, L. Chem. Mater. 2002, 14,
1591−1597. (d) Billinge, S. J. L.; McKimmy, E. J.; Shatnawi, M.; Kim,
H. J.; Petkov, V.; Wermeille, D.; Pinnavaia, T. J. J. Am. Chem. Soc.
2005, 127, 8492−8498.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the University of South Florida for financial
support of this work. Work by L.D.E. and C.W.A. was
supported financially by the Division of Chemical Sciences,
Geosciences, and Biosciences, Office of Basic Energy Sciences,
U.S. Department of Energy. This manuscript has been authored
by UT-Battelle, LLC under Contract No. DE-AC05-
00OR22725 with the U.S. Department of Energy. Use of the
Stanford Synchrotron Radiation Lightsource, SLAC National
Accelerator Laboratory, is supported by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences
under Contract No. DE-AC02-76SF00515. We acknowledge
the help from Dr. Zachary D. Atlas in the Geochemical Analysis
at USF with the ICP-OES/MS experiments.
̂ ́
(8) (a) Cote, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.;
Matzger, A. J.; Yaghi, O. M. Science 2005, 310, 1166−1170. (b) Feng,
X.; Ding, X.; Jiang, D. Chem. Soc. Rev. 2012, 41, 6010−6022. (c) Ding,
S.-Y.; Wang, W. Chem. Soc. Rev. 2013, 42, 548−568. (d) Ascherl, L.;
Sick, T.; Margraf, J. T.; Lapidus, S. H.; Calik, M.; Hettstedt, C.;
Karaghiosoff, K.; Doblinger, M.; Clark, T.; Chapman, K. W.; Auras, F.;
̈
Bein, T. Nat. Chem. 2016, 8, 310−316. (e) Bunck, D. N.; Dichtel, W.
R. Angew. Chem., Int. Ed. 2012, 51, 1885−1889. (f) Zeng, Y.; Zou, R.;
Luo, Z.; Zhang, H.; Yao, X.; Ma, X.; Zou, R.; Zhao, Y. J. Am. Chem. Soc.
2015, 137, 1020−1023. (g) Pang, Z.-F.; Xu, S.-Q.; Zhou, T.-Y.; Liang,
R.-R.; Zhan, T.-G.; Zhao, X. J. Am. Chem. Soc. 2016, 138, 4710−4713.
(h) Beaudoin, D.; Maris, T.; Wuest, J. D. Nat. Chem. 2013, 5, 830−
834. (i) Chandra, S.; Kandambeth, S.; Biswal, B. P.; Lukose, B.; Kunjir,
S. M.; Chaudhary, M.; Babarao, R.; Heine, T.; Banerjee, R. J. Am.
Chem. Soc. 2013, 135, 17853−17861. (j) Kandambeth, S.; Mallick, A.;
Lukose, B.; Mane, M. V.; Heine, T.; Banerjee, R. J. Am. Chem. Soc.
2012, 134, 19524−19527.
REFERENCES
■
(1) (a) Zhang, Y.; Wei, S.; Liu, F.; Du, Y.; Liu, S.; Ji, Y.; Yokoi, T.;
Tatsumi, T.; Xiao, F.-S. Nano Today 2009, 4, 135−142. (b) Banerjee,
D.; Kim, D.; Schweiger, M. J.; Kruger, A. A.; Thallapally, P. K. Chem.
Soc. Rev. 2016, 45, 2724−2739. (c) Khin, M. M.; Nair, A. S.; Babu, V.
J.; Murugan, R.; Ramakrishna, S. Energy Environ. Sci. 2012, 5, 8075−
8109. (d) Tratnyek, P. G.; Johnson, R. L. Nano Today 2006, 1, 44−48.
(e) Alsbaiee, A.; Smith, B. J.; Xiao, L.; Ling, Y.; Helbling, D. E.;
Dichtel, W. R. Nature 2015, 529, 190−194. (f) Byun, J.; Patel, H. A.;
Thirion, D.; Yavuz, C. T. Nat. Commun. 2016, 7, 13377.
(9) (a) Doonan, C. J.; Tranchemontagne, D. J.; Glover, T. G.; Hunt,
J. R.; Yaghi, O. M. Nat. Chem. 2010, 2, 235−238. (b) Zeng, Y.; Zou,
R.; Zhao, Y. Adv. Mater. 2016, 28, 2855−2873. (c) Stegbauer, L.;
Hahn, M. W.; Jentys, A.; Savasci, G.; Ochsenfeld, C.; Lercher, J. A.;
Lotsch, B. V. Chem. Mater. 2015, 27, 7874−7881. (d) Baldwin, L. A.;
Crowe, J. W.; Pyles, D. A.; McGrier, P. L. J. Am. Chem. Soc. 2016, 138,
15134−15137.
(10) (a) Chen, L.; Furukawa, K.; Gao, J.; Nagai, A.; Nakamura, T.;
Dong, Y.; Jiang, D. J. Am. Chem. Soc. 2014, 136, 9806−9809. (b) Calik,
M.; Auras, F.; Salonen, L. M.; Bader, K.; Grill, I.; Handloser, M.;
(2) (a) Li, X.; Bian, C.; Meng, X.; Xiao, F.-S. J. Mater. Chem. A 2016,
4, 5999−6005. (b) Ponder, S. M.; Darab, J. G.; Mallouk, T. E. Environ.
Sci. Technol. 2000, 34, 2564−2569. (c) Salt, D. E.; Blaylock, M.;
Kumar, N. P. B. A.; Dushenkov, V.; Ensley, B. D.; Chet, I.; Raskin, I.
Bio/Technology 1995, 13, 468−474. (d) Ralston, N. Nat. Nanotechnol.
2008, 3, 527−528. (e) Johnson, N. C.; Manchester, S.; Sarin, L.; Gao,
Y.; Kulaots, I.; Hurt, R. H. Environ. Sci. Technol. 2008, 42, 5772−5778.
Medina, D. D.; Dogru, M.; Lobermann, F.; Trauner, D.; Hartschuh, A.;
̈
Bein, T. J. Am. Chem. Soc. 2014, 136, 17802−17807. (c) Du, Y.; Yang,
H.; Whiteley, J. M.; Wan, S.; Jin, Y.; Lee, S.-H.; Zhang, W. Angew.
G
DOI: 10.1021/jacs.6b12885
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
Chem., Int. Ed. 2016, 55, 1737−1741. (d) Bertrand, G. H. V.;
(24) (a) Oh, Y.; Morris, C. D.; Kanatzidis, M. G. J. Am. Chem. Soc.
2012, 134, 14604−14608. (b) Hsi, H. C.; Rood, M. J.; Rostam-Abadi,
M.; Chen, S. G.; Chang, R. J. Environ. Eng. (Reston, VA, U. S.) 2002,
128, 1080. (c) Meyer, D. E.; Meeks, N.; Sikdar, S.; Hutson, N. D.;
Hua, D.; Bhattacharyya, D. Energy Fuels 2008, 22, 2290−2298. (d) Oh,
Y.; Bag, S.; Malliakas, C. D.; Kanatzidis, M. G. Chem. Mater. 2011, 23,
2447−2456.
̆
Michaelis, V. K.; Ong, T.-C.; Griffin, R. G.; Dinca, M. Proc. Natl. Acad.
Sci. U. S. A. 2013, 110, 4923−4928.
(11) (a) Xu, H.; Gao, J.; Jiang, D. Nat. Chem. 2015, 7, 905−912.
(b) Fang, Q.; Gu, S.; Zheng, J.; Zhuang, Z.; Qiu, S.; Yan, Y. Angew.
Chem., Int. Ed. 2014, 53, 2878−2882. (c) Ding, S.-Y.; Gao, J.; Wang,
Q.; Zhang, Y.; Song, W.-G.; Su, C.-Y.; Wang, W. J. Am. Chem. Soc.
2011, 133, 19816−19822. (d) Peng, Y.; Hu, Z.; Gao, Y.; Yuan, D.;
Kang, Z.; Qian, Y.; Yan, N.; Zhao, D. ChemSusChem 2015, 8, 3208−
3212. (e) Aiyappa, H. B.; Thote, J.; Shinde, D. B.; Banerjee, R.;
Kurungot, S. Chem. Mater. 2016, 28, 4375−4379. (f) Vyas, V. S.;
Haase, F.; Stegbauer, L.; Savasci, G.; Podjaski, F.; Ochsenfeld, C.;
Lotsch, B. V. Nat. Commun. 2015, 6, 8508. (g) Lin, S.; Diercks, C. S.;
Zhang, Y.-B.; Kornienko, N.; Nichols, E. M.; Zhao, Y.; Paris, A. R.;
Kim, D.; Yang, P.; Yaghi, O. M.; Chang, C. J. Science 2015, 349, 1208−
1213. (h) Wang, X.; Han, X.; Zhang, J.; Wu, X.; Liu, Y.; Cui, Y. J. Am.
Chem. Soc. 2016, 138, 12332−12335. (i) Sun, Q.; Aguila, B.; Perman,
J. A.; Nguyen, N.; Ma, S. J. Am. Chem. Soc. 2016, 138, 15790−15796.
(12) (a) Ma, H.; Liu, B.; Li, B.; Zhang, L.; Li, Y.-G.; Tan, H.-Q.;
Zang, H.-Y.; Zhu, G. J. Am. Chem. Soc. 2016, 138, 5897−5903. (b) Xu,
H.; Tao, S.; Jiang, D. Nat. Mater. 2016, 15, 722.
(13) (a) Kandambeth, S.; Biswal, B. P.; Chaudhari, H. D.; Rout, K.
C.; Kunjattu H., S.; Mitra, S.; Karak, S.; Das, A.; Mukherjee, R.; Kharul,
U. K.; Banerjee, R. Adv. Mater. 2017, 29, 1603945. (b) Mitra, S.;
Kandambeth, S.; Biswal, B. P.; Khayum M., A.; Choudhury, C. K.;
Mehta, M.; Kaur, G.; Banerjee, S.; Prabhune, A.; Verma, S.; Roy, S.;
Kharul, U. K.; Banerjee, R. J. Am. Chem. Soc. 2016, 138, 2823−2828.
(c) Huang, N.; Ding, X.; Kim, J.; Ihee, H.; Jiang, D. Angew. Chem.
2015, 127, 8828−8831.
(14) (a) Wang, Y. Q.; Yang, C. M.; Zibrowius, B.; Spliethoff, B.;
́
Linden, M.; Schuth, F. Chem. Mater. 2003, 15, 5029−5035. (b) Sun,
̈
Q.; He, H.; Gao, W.-Y.; Aguila, B.; Wojtas, L.; Dai, Z.; Li, J.; Chen, Y.-
S.; Xiao, F.-S.; Ma, S. Nat. Commun. 2016, 7, 13300.
(15) (a) Pramudya, Y.; Mendoza-Cortes, J. L. J. Am. Chem. Soc. 2016,
138, 15204−15213. (b) Lohse, M. S.; Stassin, T.; Naudin, G.; Wuttke,
S.; Ameloot, R.; De Vos, D.; Medina, D. D.; Bein, T. Chem. Mater.
2016, 28, 626−631. (c) Nagai, A.; Guo, Z.; Feng, X.; Jin, S.; Chen, X.;
Ding, X.; Jiang, D. Nat. Commun. 2011, 2, 536.
(16) Xu, F.; Xu, H.; Chen, X.; Wu, D.; Wu, Y.; Liu, H.; Gu, C.; Fu,
R.; Jiang, D. Angew. Chem., Int. Ed. 2015, 54, 6814−6818.
(17) Li, B.; Zhang, Y.; Ma, D.; Shi, Z.; Ma, S. Nat. Commun. 2014, 5,
5537.
(18) (a) Sun, Q.; Dai, Z.; Liu, X.; Sheng, N.; Deng, F.; Meng, X.;
Xiao, F.-S. J. Am. Chem. Soc. 2015, 137, 5204−5209. (b) Sun, Q.;
Aguila, B.; Verma, G.; Liu, X.; Dai, Z.; Deng, F.; Meng, X.; Xiao, F.-S.;
Ma, S. Chem. 2016, 1, 628−639.
(19) (a) Manos, M. J.; Petkov, V. G.; Kanatzidis, M. G. Adv. Funct.
Mater. 2009, 19, 1087−1092. (b) Feng, X.; Fryxell, G. E.; Wang, L. Q.;
Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923−926.
(c) Bag, S.; Trikalitis, P. N.; Chupas, P. J.; Armatas, G. S.; Kanatzidis,
M. G. Science 2007, 317, 490−493. (d) Shin, Y.; Fryxell, G. E.; Um,
W.; Parker, K.; Mattigod, S. V.; Skaggs, R. Adv. Funct. Mater. 2007, 17,
2897−2901. (e) Yee, K.-K.; Reimer, N.; Liu, J.; Cheng, S.-Y.; Yiu, S.-
M.; Weber, J.; Stock, N.; Xu, Z. J. Am. Chem. Soc. 2013, 135, 7795−
7798. (f) Ai, K.; Ruan, C.; Shen, M.; Lu, L. Adv. Funct. Mater. 2016, 26,
5542−5549. (g) Abney, C. W.; Gilhula, J. C.; Lu, K.; Lin, W. Adv.
Mater. 2014, 26, 7993−7997. (h) Manos, M. J.; Kanatzidis, M. G.
Chem. - Eur. J. 2009, 15, 4779−4784. (i) Mon, M.; Lloret, F.;
Ferrando-Soria, J.; Martí-Gastaldo, C.; Armentano, D.; Pardo, E.
Angew. Chem. 2016, 128, 11333−11338.
(20) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533−3539.
(21) Hoffmann, G. G.; Brockner, W.; Steinfatt, I. Inorg. Chem. 2001,
40, 977−985.
(22) Lian, P.; Guo, H.-B.; Riccardi, D.; Dong, A.; Parks, J. M.; Xu, Q.;
Pai, E. F.; Miller, S. M.; Wei, D.-Q.; Smith, J. C.; Guo, H. Biochemistry
2014, 53, 7211−7222.
(23) Chen, C.-C.; Mckimmy, E. J.; Pinnavaia, T. J.; Hayes, K. F.
Environ. Sci. Technol. 2004, 38, 4758−4762.
H
DOI: 10.1021/jacs.6b12885
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