Preparation of Highly Porous Metal-Organic Framework Beads for Metal Extraction from Liquid Streams

Article abstract of DOI:10.1021/jacs.0c02371

Metal-organic frameworks (MOFs) offer great promise in a variety of gas- and liquid-phase separations. However, the excellent performance on the lab scale hardly translates into pilot- or industrial-scale applications due to the microcrystalline nature of

Full text of DOI:10.1021/jacs.0c02371

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Article
Preparation of Highly Porous Metal-Organic Framework
Beads for Metal Extraction from Liquid Streams
Shuliang Yang, Li Peng, Olga A. Syzgantseva, Olga Trukhina, Ilia V Kochetygov, Anita Justin, Daniel
T. Sun, Hassan Abedini, Maria A. Syzgantseva, Emad Oveisi, Guanchu Lu, and Wendy L. Queen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c02371 • Publication Date (Web): 11 Jul 2020
Downloaded from pubs.acs.org on July 11, 2020
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Preparation of Highly Porous Metal-Organic Framework
Beads for Metal Extraction from Liquid Streams
†
† ‖
⊥,#
Shuliang Yang, Li Peng, Olga A. Syzgantseva, Olga Trukhina,† Ilia Kochetygov,† Anita Justin,†
Daniel T. Sun,† Hassan Abedini,†,‡ Maria A. Syzgantseva,∇ Emad Oveisi,§ Guanchu Lu†,¶ and Wendy L.
Queen*,†
,
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†
Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Rue de l’Industrie 17,
CH-1951 Sion, Switzerland.
‖
College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China.
⊥Laboratory of Molecular Simulation (LSMO), Institut des Sciences et Ingénierie Chimiques, Valais, Ecole Polytechnique
Fédérale de Lausanne (EPFL), Rue de l’Industrie 17, CH-1951 Sion, Switzerland.
#
Laboratory of Quantum Photodynamics, Department of Chemistry, Lomonosov Moscow State University, Moscow 119991,
Russia.
‡
Department of Gas Engineering, Ahvaz Faculty of Petroleum, Petroleum University of Technology, Ahvaz, Iran.
∇
Laboratory of Quantum Mechanics and Molecular Structure, Department of Chemistry, Lomonosov Moscow State
University, Moscow 119991, Russia.
§
Interdiciplinary Center for Electron Microscopy, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne,
Switzerland.
¶
School of Engineering, The University of Edinburgh, The King’s Buildings, Edinburgh EH9 3JL, UK.
ABSTRACT: Metal-organic frameworks (MOFs) offer great promise in a variety of gas and liquid-phase separations. However,
excellent performance on the lab-scale hardly translates into pilot or industrial-scale applications due to their microcrystalline
nature. Therefore, MOF structuring into pellets or beads is a highly solicited and timely requirement. In this work, a general
structuring method is developed for preparing MOF-polymer composite beads based on an easy polymerization strategy. This
method adopts biocompatible, biodegradable polyacrylic acid (PAA) and sodium alginate monomers, which are cross-linked using
Ca2+ ions. Also, the preparation procedure employs water, and hence is nontoxic. Moreover, the universal method has been applied
to twelve different, structurally diverse MOFs and three MOF-based composites. To validate the applicability of the structuring
method, beads consisting of a MOF-polymer composite, namely Fe-BTC/PDA, were subsequently employed for the extraction of
Pb and Pd ions from real-world water samples. For example, we find that only one gram of Fe-BTC/PDA beads is able to
decontaminate > 10 liters of freshwater containing highly toxic lead (Pb) concentrations of 600 ppb while under continuous flow.
Moreover, the beads offer one of the highest Pd capacities to date, 498 mg of Pd per gram of composite bead. Further, large
quantities of Pd, 7.8 wt%, can be readily concentrated inside the bead while under continuous flow, and this value can be readily
increased with regenerative cycling.
INTRODUCTION
is used to shape MOF powders into beads with the assistance
of binders such as sucrose.12 Moreover, inspired by the
preparation methods of ceramic beads, Valizadeh et al.
reported a phase inversion method to prepare MOF beads. For
this, a suspension of a microcrystalline MOF and a polymer
was formed in dimethylformamide (DMF), and then added
dropwise to a coagulation bath containing an ethanol-H2O
mixture; the resulting composite beads reportedly exhibit good
performance for iodine capture from liquid and vapor phase.13
Additionally, Cousin-Saint-Remi et al. used a similar method
but with the precursor suspension consisting of MOF-
polyvinyl formal in DMF.14 While these methods yield high
MOF loadings into composite beads, the reported surface
areas are significantly reduced when compared with the parent
MOF by factors as high as 51%.12 Another alternative method
recently introduced into the literature is to prepare the
polymeric beads first, and then subsequently grow the MOF
crystals onto the beads.15-16 However, prefabricated polymer
Metal-organic frameworks (MOFs) have received extensive
attention because of their potential use in a wide number of
possible applications coupled to the environment, energy, and
future sustainability.1-4 MOFs can be constructed by a diverse
set of building blocks based on inorganic metal ion clusters
and organic linkers, and hence offer a nearly infinite number
of structure types.5 Until now, MOFs have demonstrated
intriguing properties in numerous fields including catalysis,
gas adsorption and separation, sensing, water remediation,
drug delivery and so on.6-10 However, MOFs consist of fine
powders; this makes it difficult to implement MOFs in many
large-scale industrial applications11 or test materials in a
laboratory setting for gas and liquid based separations while
under continuous flow. Structuring MOFs into beads is
considered as an efficient mean to this end.
In the last few years, there have been many different
reported methods to structure MOFs. For example, granulation
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beads can be dense, limiting MOF growth on the bead interior
and leading to low MOF loadings. Even though there are
several other methods for MOF shaping developed,17-18 it is
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Figure 1. MOF-polymer bead preparation. (a) The beads from the gelation of Ca2+ and alginate are fragile due to the lack of sufficient
entanglement. (b) The mechanical strength of PAA-Ca2+-alginate bead is improved by the introduction of PAA as an extra cross-linker.
still a challenge in the field to shape MOFs into composite
beads that offer high surface areas and performance that are
similar to the parent MOF powders, all while using green
methodologies.
(Cu), Cu-TDPAT, Ni-pyrazolate MOF, MIL-101(Cr), ZIF-8,
ZIF-67, UiO-66, NH2-MIL-53(Al), Eu2(BDC)3, Tb2(BDC)3
and Fe3O4/MIL-127-Fe. Further, the properties of the parent
MOF, such as porosity and florescence, is well preserved.
Moreover, beads consisting of a MOF-polymer composite,
namely Fe-BTC/PDA (PDA = polydopamine), previously
developed in our group,24-25 are shown to remove Pb from
surface water with unprecedented performance, and also
concentrate high value Pd from contaminated water, all while
under continuous flow.
Alginate is one of the most widely-used biomaterials as it
forms a hydrogel by simply adding cations such as Ca2+ and
Ba2+ under mild conditions; this results from the dimerization
of monomers via cation coordination.19 However, the rigid
polymer conformations and electrostatic repulsion between
neighboring chains inhibits sufficient entanglement of the
fibers.20 The recent attempts to shape MOFs with alginate
consisted of in-situ growth of the framework on the pre-
fabricated Cu2+-bound alginate hydrogel fibers.21-22 This
approach however did not result in making structured MOFs
with sufficient stability for studies under continuous flow as
well as that based on Ca2+ alginate used for making beads out
of tetraethylenepentamine-functionalized MIL-101(Cr).23
Some existing shortfalls, which limit the utility of present
structuring methods in liquid separations, combined with our
desire to optimize MOF performance in applications such as
water purification and the extraction of valuable commodities
from liquid waste,24-25 inspired us to develop a method for
shaping MOFs to test their performance under continuous flow.
Herein, we describe a simple structuring method for the
preparation of composite beads that is based on double cross-
linked networks consisting of polyacrylic acid (PAA), sodium
alginate, and Ca2+ ions using only H2O as the solvent. We note
that when the beads are prepared using only alginate and Ca2+,
they are fragile and hence prone to crack (Figures 1a and S1).
The addition of PAA leads to cross-linking with alginate
chains and Ca2+, introducing additional hydrogen bonding and
ionic interactions between PAA-PAA, PAA-Ca2+ and PAA-
alginate.26-31 Using PAA, the beads have enhanced stability,
improving their utility in liquid separations (Figure 1b).
This general structuring method was applied to a number of
diverse MOFs, including MIL-127-Fe, Fe-BTC, HKUST-1
RESULTS AND DISCUSSION
Strategy and Characterizations. First, beads based on the
alginate-Ca2+-PAA (ACPs) were prepared. For this, a sodium
alginate solution (20 mg/mL with pH = 7.16) was added
dropwise into the PAA-CaCl2 solution (pH = 2.71), and
transparent ACP beads were immediately formed (Figure S2).
As expected, the pure ACP bead is amorphous in nature
(Figure S3a), and N2 adsorption isotherms reveal a surface
area of ~7 m2/g (Figure S3b). To confirm the presence of PAA
and alginate, FT-IR and Raman spectra were collected. From
the FT-IR, all peaks that are typical for the alginate are clearly
observed. For PAA, it is noticed that the asymmetric C=O
stretching vibration, associated with the free carboxylic acid
groups, at 1694 cm-1, is now much weaker in the ACP bead.
Further, a stronger peak appeared at 1599 cm-1 upon
formation of the bead (Figure S4a). This shift to lower
wavenumbers (1599 cm-1), compared to the free carboxyl in
PAA (1694 cm-1), is indicative of the coordination of the -
COO-1 with Ca2+ ions.32 The strong, broad peak at 3356 cm-
1
corresponds to O-H stretching vibrations and hence
reflects the existence of abundant carboxyl and carbonyl -
OH groups, which are able to provide hydrogen bonding
interactions of varying strengths.33 Such interactions might
include those between the PAA-alginate or PAA-PAA.27, 34
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Further, the Raman spectrum reveals a high- intensity band
centered at 2936 cm-1, which is assigned to
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Figure 2. (a) SEM image of the as-synthesized MIL-127-Fe with a cubic morphology. (b) XRD patterns and (c) N2 adsorption isotherms
of the MIL-127-Fe and the corresponding beads. (d) SEM image and (e) corresponding EDX elemental maps of Ca, Fe and O of the bead
after cutting it in half. (f) Higher magnification SEM image showing the MOFs distributed uniformly in the beads. (g) The continuous-flow
setup for continuous preparation of beads (the inset is the collected beads with weight of 181 mg) and (h) the digital photos of the MIL-
127-Fe MACPs with different size.
XRD suggests that the crystal structure of the MIL-127-Fe is
well maintained during the structuring process (Figure 2b). A
close inspection of the FT-IR spectra in the FIR and MIR
range revealed that the Fe3O-ligand cluster is interacting with
the polymer through the open metal sites; this results in a red
shift of the δsym(Fe-(μ3-O)) vibrations of the cluster (details in
the SI). The polymer further interacts with carboxylate groups
of the MOF and changes their in- and out-of plane bending
modes, as shown in Figures S8-17, further confirming the
interaction between the MOF and the cross-linked polymer
components. ICP-OES analysis reveals ~0.98 wt% calcium in
the MACPs and EDX analysis reveals no residual Na (from
sodium alginate); this confirms the complexation between
Ca2+ and alginate (Figure S18). Further, TGA analysis shows
the polymer loading in the MACPs is ~8.9 wt% (Figure S19),
and their BET surface area is ~1117 m2/g, which is only
slightly lower than the parent MOF structure, 1205 m2/g
(Figure 2c). Pore size distribution analysis of MIL-127-Fe and
the MACPs reveals pore volumes of 0.64 cm3/g and 0.61
cm3/g, respectively (Figure S20). The high surface area and
pore volume of the MIL-127-Fe MACPs is ascribed to the
highly porous polymer network. Further, the molecular weight
of the PAA is quite large, Mw~250,000, and the gelation
process is rapid limiting the polymers ability to diffuse into the
MOF pores. To obtain detailed information of the MOF’s
distribution, a bead was cut in half (Figure 2d) and one piece
was characterized via SEM-EDX mapping. Signals from Ca
and Fe are observed throughout the bead. This implies that the
MIL-127-Fe is dispersed homogeneously throughout the
the v(C-H) stretching vibrations of the PAA chain, which
correspond to the polyethylene chain with grafted
carboxylate groups (Figure S4b).27, 35 Based on the FT-IR
and Raman characterization, it is concluded that the ACP
bead is a cross-linked network consisting of both alginate,
Ca2+ ions, and PAA. Previous reports suggest that PAA based
hydrogels have reversible hydrogen-bonded carboxyl groups,
that are sensitive to high pH, > 9.36 However, we hypothesized
that the inclusion of Ca2+, which promotes Ca2+/COOH and
Ca2+/COO− crosslinking, might help to enhance the stability of
the ACP hydrogel network both in acid and alkaline media.
Indeed, the ACP beads are stable in H2O tested up to 11
months (Figure S5), and are additionally found to be stable in
harsher environments including pH = 0 HCl and pH = 14
NaOH aqueous solution, which was tested for up to three
months (Figure S6).
For the formation of MOF beads, MIL-127-Fe was selected
as the point of ingress as it is a water-stable MOF, which
consists of trimeric Fe (III) octahedra interlinked by 3,3’,5,5’-
azobenzenetetracarboxylate.37 SEM images reveal that the as-
synthesized MIL-127-Fe crystallites have a cubic morphology
with a size of 4.6±1.8 µm (Figures 2a and S7). The preparation
procedure of MIL-127-Fe MOF-Alginate-Ca2+-PAA polymer
composite beads (MACPs) is illustrated in Figure 1b. Firstly,
the orange MIL-127-Fe powder is dispersed in the alginate
solution in water. Afterwards, the well-mixed MOF-alginate
suspension is added dropwise into a PAA-Ca2+ solution. The
MACPs are immediately formed due to the rapid crosslinking.
The resulting spherical orange beads are shown in Figure S7c.
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MACPs (Figure 2e). Further, the cubic MOF crystallites can
two PAA molecules, which is in line with the hydrophobic
nature of PAA.
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be clearly observed inside the porous MACPs (Figure 2f), and
SEM images obtained from different regions in the MACPs
further confirm this point (Figure S21).
For the most energetically favorable complexes having both
PAA and alginate, the complex consists of one Ca2+ cation and
two alginates and two PAA chains (PAA2-Ca-Alg2).
Although, it is noted that PAA-Ca-Alg and PAA2-Ca-Alg
cross-linked complexes are also relatively stable. Additionally,
PAA-PAA, Alg-Alg and PAA-Alg interactions in the absence
of Ca2+ are also energetically favorable, in particular,
hydrogen bonds can be formed between two chains connecting
carboxyl and/or hydroxyl groups. As expected, the hydrogen
bonds are weaker than the Ca-based cross-linking. The
interaction energy between neutral and/or anionic fragments is
within 60-80 kcal/mol, while there is an interaction energy of
approximately 10-20 kcal/mol per hydrogen bond.
When structuring materials, adjusting the MOF content and
bead size is important for practical applications. Thus, we
tested a range of MIL-127-Fe loadings, ranging from 73% to
95% weight percent MOF (Figure S22), and beads of different
size were prepared, ranging from 2.2 to 5.5 mm (Figure 2h). It
is worth mentioning that the crystallinity and the surface area
are well preserved in all the above-mentioned cases (Figures
S22-23). Moreover, the MACPs were prepared under
continuous flow using a peristaltic pump as the driving force
and uniform beads sizes were obtained (Figure 2g, Video S1)
implying that the method can be readily scaled up (Video S2).
For samples prepared using the peristaltic pump, the resulting
surface areas and MOF loadings are the same as the beads
prepared manually, demonstrating high reproducibility of this
method (Figure S24).
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Theoretical Discussion. To assess the interaction between
components in the polyacrylic acid-alginate-Ca2+ blend, as
well as interaction with the MOF, computational modeling
was employed. At first, the interactions between the polymeric
chains were assessed in the presence and then absence of Ca2+
via molecular dynamics (MD) simulations. It should be noted
that for PAA, a structural model containing 7 monomer acrylic
acid units was used. For alginate, it was represented by two
potential oligomers, referred to as MGGM and GGMM, which
are comprised of 4 monomer units of either α-L-guluronic acid
(G) or β-D-mannuronic acid (M). The most stable Ca-alginate
complex reported is MGGM--Ca2+(H2O)2-MGGM-.38-40 It is
noted that this same complex is also found at the lower limit of
formation energies in a series of polymer cross-linked
complexes studied in our work (Ca-Alg, Figure 3c).
The MD simulations reveal a spontaneous binding of
carboxyl and/or hydroxyl groups of PAA and alginate to Ca2+
suggesting that the cations can indeed induce the formation of
cross-links between neighboring chains, as observed
experimentally. The formation energies of the complexes
(Figure 3c), were calculated with respect to neutral PAA
(EPAA), anionic alginate (EAlg) with half-protonated COOH
groups and a hexahydrated Ca2+ cation (E[Ca(H2O)62+]), which
are representative forms of these compounds in the
corresponding solutions:
Figure 3. Chemical interactions between the different bead
components: (a) A representative structure of Ca2+ crosslinking of
PAA and alginate; (b) Hydrogen bonding between PAA and
alginate; (c) Energy distribution of different binding
configurations in the system PAA-Ca-Alginate; (d) Interaction of
alginate with MIL-127-Fe. C, N, O, H, Fe and Ca atoms are
represented by green, purple, red, yellow, brown and blue spheres,
respectively.
2 +
Δ푓퐸 = 퐸푐표푚푝푙푒푥 ― 푛푃퐴퐴퐸푃퐴퐴 ― 푛퐴푙푔퐸퐴푙푔 ― 퐸
(1)
Importantly, the molecular dynamics simulations of the
PAA-Ca-Alg system in water reveal that, upon binding with
Ca2+, the PAA chains orient their hydrophobic side towards
the water layer, repelling neighboring water molecules away
from the PAA chain. On the contrary, the alginate chains
remain hydrated. This observation suggests that Ca-bound
PAA-alginate composites will contain less water than the pure
alginate Ca bead, a factor that could influence the mechanical
stability of the resulting bead.
Finally, to assess the interactions between the MOF and
polymer, MIL-127-(Fe)37 was selected as the representative
system, for which the (001) ligand- and metal-terminated
surfaces41 are considered. Ab initio simulations of interactions
between polymeric chains and MIL-127(Fe) unveil the
interactions between them (Figure S25). In particular, Ca2+
binds to the COO(H) groups of surface ligands and polymers,
while polar carboxyl and hydroxyl groups of PAA and
alginate become attached to the metal sites of MIL-127(Fe).
This is in line with FIR and MIR data, where we observe the
퐶푎(퐻2푂)6
Binding of neutral non-dissociated PAA with Ca2+ is found to
be energetically favorable (Figure 3c). The MD simulations
also indicate that one Ca2+ cation can bind up to three PAA
chains, while the 4th one is repelled from the first coordination
sphere, irrespective of whether the PAA is neutral or charged
(deprotonated). Negatively charged PAA chains do have
higher Ca2+ binding energy, however, it is not the most
probable protonation state in the PAA-Ca2+ solution. Indeed, at
pH=2.71 and concentration of 0.05 mol/L only 1.6% of COOH
groups in PAA are deprotonated. Therefore, complexation
energies for neutral PAA are preferentially considered.
Interestingly, the Ca-alginate complex, MGGM--Ca2+(H2O)2-
MGGM-,38-40 has two coordinated water molecules in the
coordination sphere, while for PAA-Ca2+, one water is
spontaneously replaced by COOH groups upon binding with
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interaction of the polymer with metal sites (Figures S8-S17).
These observations suggest that the physico-chemical
prerequisites for the formation of mechanically stable PAA-
Ca-Alg-MOF composites are fulfilled.
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Figure 4. The digital bead photos of (a) MIL-127-Fe, (b) MIL-101(Cr), (c) HKUST-1, (d) UiO-66, (e) NH2-MIL-53(Al), (f) Cu-TDPAT,
(g) ZIF-67, (h) ZIF-8, (i) Ni-pyrazolate, (j) Fe-BTC, (k) Fe-BTC/PDA, (l) Fe-BTC/PpPDA, (m, n) magnetic Fe3O4/MIL-127-Fe, (o)
Eu2(BDC)3, (p) Tb2(BDC)3 and the (q) Eu2(BDC)3, (r) Tb2(BDC)3 under ultraviolet illumination with wavelength of 254 nm. t: NH2-MIL-
53(Al) is a flexible MOF; u: 100 mg of Fe3O4 and 75 mg of MIL-127-Fe were dispersed in 1 mL 20 mg/mL alginic acid sodium solution; v
and w: Eu2(BDC)3 and Tb2(BDC)3 are non-porous lanthanide MOFs.
Extension of the Current Strategy. To explore the general
applicability of the method, the same structuring process was
extended to a group of structurally diverse MOFs including
MIL-101(Cr), HKUST-1, UiO-66, NH2-MIL-53(Al), Cu-
TDPAT, ZIF-67, ZIF-8, Ni-pyrazolate and Fe-BTC. As can be
seen from Figures 4b to 4j, this facile method works very well
in diverse MOF systems with different metal nodes and
different organic linkers. Further, the method was also adapted
to several MOF-polymer composites such as Fe-BTC/PDA
(PDA = polydopamine) and Fe-BTC/PpPDA (PpPDA =
poly(para-phenylenediamine) (Figures 4k and 4l). Moreover,
the beads can be endowed with magnetic-responsive Fe3O4
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nanoparticles during the shaping process (Figures 4m-4n), and On the other hand, Fe-BTC/PDA@PES beads offer
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last, we demonstrate that lanthanide MOFs, such as
Eu2(BDC)3 and Tb2(BDC)3, can be included into the bead
structures. Despite that their fluorescence can be quenched by
aromatic molecules,42 their fluorescence is well retained after
the structuring process (Figures 4o-4r). Further, using this
shaping method, the form of the MACPs is not limited to be
spherical, the MOFs can be structured into different shapes
such as our EPFL-LFIM logo (École polytechnique fédérale
de Lausanne, Laboratory for Functional Inorganic Materials)
(Figure S26).
significantly lower performance with extraction efficiencies
well below 90% and capacities that are only ~50% that of Fe-
BTC/PDA powder and the MACPs. It is suspected that this
decrease in performance could be due to the lower accessible
surface area of the PES composite bead structure.
Next, experiments were carried out to understand how the
structuring process influences the Pb2+ extraction rates. It is
noted that the Fe-BTC/PDA, is one of the fastest material
reported to date for Pb2+ extraction, reaching an equilibrium in
less than two minutes with over 99% extraction efficiency.24
While it is expected that the structuring process will naturally
lead to lower extraction rates,46-47 due to slower Pb2+ diffusion
throughout the bead, we aimed to assess the magnitude of the
change. Indeed, both Fe-BTC/PDA MACPs and Fe-
BTC/PDA@PES beads have slower Pb2+ extraction, when
compared to the parent powder material in distilled water
(Figure 5d). However, the MACPs are able to reach ~90 %
Pb2+ removal efficiency after 60 minutes; while, Fe-
BTC/PDA@PES beads reach only ~65% extraction in the
same 60-minute window. In addition to this, we also note that
the MACPs are the only sample that is easily separated from
water solutions. As can be seen in Figure 5b inset, the sample
solution that contained the MACPs is clear, and ICP and
MALDI TOF analysis of the solutions indicate no trace of
MOF or polymer, implying that there is minimal leaching
from the formed beads. On the contrary, both Fe-BTC/PDA
and Fe-BTC/PDA@PES beads lead to highly colored
solutions, indicative that the MOF composite is leaching from
the PES beads. And while the Fe-BTC/PDA powder shows
impressive performance, with high extraction rates, some of
the small composite particles suspend in the solution and
hence would also lead to a loss in active material. Further, the
fine particles of Fe-BTC/PDA will lead to pressure drops
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It is also worth mentioning that in addition to achieving a
high MOF loading, the crystallinity and the surface areas of
most of the aforementioned MACPs are also well preserved
(Figures S27-S40 and Figure 4). And it should be noted that
the surface areas reported here are not normalized by the MOF
loading as in some other studies.13 Upon a closer inspection of
the bead surface areas (Figure 4), it is noted that for some
MOFs, such as HKUST-1 and Cu-TDPAT, the decrease in
surface area (29.3 and 45 %, respectively) is larger than
expected when considering the added mass of the polymer (~
9.6 and 10.6 weight percent %, respectively). Given the
acidity of the curing solution, pH ~ 2.7, and soak times of 30
minutes, the decrease in surface areas are not surprising given
that these MOFs are unstable in acidic media.43 Despite this,
the polymerization process reported here is rapid and hence is
still able to render pelletized materials with surface areas that
are competitive to other techniques reported in the literature.13,
17-18
Further, there are a number of surface functionalization
methods that could be used to improve the chemical stability
of MOFs before using the structuring strategy presented
here.43-45
Application for Water Remediation. Given that MOF
structuring is required for their implementation into separation
applications, we wished to demonstrate that the composite
beads can perform well under continuous liquid flow. For this,
a MOF/polymer composite, known as Fe-BTC/PDA was
selected, because it can selectively remove heavy metals, such
as Pb2+ and Hg2+ from water and at record breaking rates.24
However, this material is a fine, dispersible powder, making it
impractical for real applications requiring continuous flow. To
overcome this problem and advance the project, Fe-BTC/PDA
powder was synthesized and then shaped using the
aforementioned hydrogel method. The resulting Fe-BTC/PDA
MACPs had a surface area, 915 m2/g, that was only slightly
reduced when compared to that of the parent MOF composite,
997 m2/g. Next, to better understand how varying the
structuring process might alter performance, Fe-
BTC/PDA@PES beads (PES = polyethersulfone) were also
across
a separation column. These things make the
unstructured material impractical for real applications.
Last, Fe-BTC/PDA MACPs were tested in continuous flow
for the extraction of Pb from Rhone river water (~ 600 ppb).
This allowed us to demonstrate bead performance in a realistic
environment that contains large quantities of competitive ions
and to assess how much water can be purified per gram of
adsorbent. A schematic diagram of this process is shown in
Figure S41. The ion composition of the water was examined
by ion chromatography, before and after the adsorbent column,
assuring that only the Pb concentration is reduced after
passing through the column bed (Figure S41). Also, ICP-OES
showed no fluctuation in the concentration of Ca2+ and Fe3+,
confirming no leaching of these two metals from the Fe-
BTC/PDA MACPs under continuous flow (Figure S42). With
the breakthrough experiment (Figure S43), it was found that
one gram of Fe-BTC/PDA MACPs affords over ten liters of
freshwater, ~10.8 L, reducing the Pb2+ concentrations below
EPA limit of 15 ppb48. We believe this high performance is
owed to the selectivity of the composite for Pb2+ over other
less toxic, competing ions. To the best of our knowledge, Fe-
BTC/PDA MACPs also outperforms existing commercial
adsorbents used in Pb filtration water pitchers, most of which
are not functional at such at Pb concentrations >150 ppb and
also require much larger quantities of active material to purify
much smaller quantities of water.49 It is noted that Fe-
BTC/PDA can also be readily regenerated using previously
established methods.24
produced using
a
previously reported micro-droplet
technique.13 As illustrated in Figures 5a, the surface area of
Fe-BTC/PDA@PES is reduced by over 50%, despite a PES
loading of only 28.4 weight %, a problem that is commonly
encountered when structuring MOFs.13, 18
To assess how the structuring process might change the
properties of Fe-BTC/PDA, the Pb2+ extraction efficiencies
and capacities were studied for the Fe-BTC/PDA powder, Fe-
BTC/PDA MACPs, and Fe-BTC/PDA@PES beads under
soaking conditions first. The three samples were first soaked
in distilled water and Rhone river water that was spiked with
Pb2+ for 24 hours. The removal efficiency (at 1ppm) and the
lead capacity (at 750 ppm) of the MACP beads is comparable
to that observed for the parent MOF powder (Figures 5b-5c).
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Figure 5. (a) N2 adsorption isotherms of Fe-BTC/PDA powder, Fe-BTC/PDA MACPs and the control Fe-BTC/PDA@PES beads. (b)
Concentration of Pb2+ in water before and after treatment with three different samples. (c) Lead adsorption capacity (mg/g) at 750 ppm
Pb2+ with different water source. (d) Lead adsorption kinetics spiked with
1
ppm of Pb2+ in distilled water.
Due to the presence of large numbers of phenolic hydroxyl
groups on the PDA backbone, which are able to coordinate
and reduce metal ions in solution,50-52 we aimed to broaden the
application of the synthesized Fe-BTC/PDA composites,
towards the possible extraction of palladium ions from
complex waste solutions. It should be noted that Pd is
undergoing an acute shortage, driving prices from less than
500 USD per ounce in January of 2016 to almost 2500 USD
per ounce in January of 2020.53
Fe was detected by ICP-OES analysis after the process, which
means the Fe-BTC/PDA beads are stable in the wastewater.
Next, we tested the Fe-BTC/PDA MACPs in Pd
breakthrough tests to see if the material is able to concentrate
Pd inside while under continuous flow. Water, spiked with
8.91 ppm Pd was streamed through a column containing 120
mg of Fe-BTC/PDA MACPs at 0.3 mL/minute (Figure 6c).
After the breakthrough test, the MACPs were digested and the
amount of adsorbed Pd was determined. ICP-OES revealed
that the Pd loading is ~7.8 wt%. It is noted that this value can
be further increased via regeneration with a mild reducing
agent, such as ascorbic acid, and subsequent reuse of the
composite beads. In fact, we have demonstrated that with
cycling, the amount of Pd captured in the composite can be
increase to 19.1 wt % and is likely to be further increased with
continued cycling (Figure S46).
After breakthrough experiments, the Fe-BTC/PDA MACPs
were further characterized. The presence of Pd nanoparticles
inside the MOF is confirmed by the high-angle annular dark-
field scanning transmission electron microscopy images
(HAADF-STEM). The image contrast in HAADF-STEM
mode is strongly correlated to the atomic mass, therefore Pd
nanoparticles show more signal (appear brighter) compared to
the rest of the structure (Figures 6d-i-ii). This is further
confirmed by energy-dispersive X-ray (EDX) STEM
elemental mapping on the selected regions inside the Fe-
BTC/PDA composite (Figures 6d-iii-iv). High-resolution Pd
3d XPS spectra of the recovered Fe-BTC/PDA MACPs
revealed the presence of both coordinated Pd2+ and metallic Pd
species (Figure S47). Moreover, a lattice distance of 2.25 Å
belong to the (1 1 1) facet of metallic Pd can be observed
clearly from the high-resolution HAADF-STEM image
It is noted that the Fe-BTC/PDA MACPS (Figure S44),
which contain 25.7 wt% PDA inside the MOF pores,
demonstrate excellent performance in palladium removal, with
a maximum adsorption capacity of 498 mg of Pd2+ per gram of
MACPs. This is one of the highest Pd removal capacities
reported to date. Also, the capacity and extraction rate are also
comparable to the Fe-BTC/PDA powder itself, which again
confirms the exceptional properties that result from the
developed structuring process (Figures 6a-6b and Table S1).
To test the selectivity, we obtained a small amount of
wastewater from Swiss industry, pH = 8.75, and the
performance of the Fe-BTC/PDA MACPs and Fe-BTC/PDA
powder was evaluated (Figure S45). From the ICP-OES, the
wastewater was found to contain 1.15 ppm Pd, 36.30 ppm Ca,
1.84 ppm Mg, 0.21 ppm Sr, 23.17 ppm B, 0.67 ppm Pt, 2.83
ppm Ag and extremely high salt concentration with 11047
ppm of K and 58740 ppm Na (Figures S45a-c). The parent
MOF and beads were soaked inside the industrial wastewater
for 24 hours. Similar to the MOF powder, Fe-BTC/PDA
MACPs selectively removed 76% of Pd ions from the solution,
while leaving the other inorganics behind. Further, while the
Fe-BTC/PDA powder disperses in the solution, the MACPs
can be readily separated (inset of Figure S45c). Moreover, no
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(Figure S48); these results strongly suggests that the Pd Overall, the high selectivity of the Fe-BTC/PDA MACPs,
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removal is based on
mechanism.
a
combined adsorption/reduction
combined with their ability to concentrate Pd inside the pores
while under continuous flow, indicates that the MACPs could
be employed in real industrial wastewater treatment processes,
which is one of the on-going current endeavors. Further, beads
that are able to selectively recover rare metals from complex
waste streams could be used for the preparation of MOF-
nanoparticle composites having many interesting uses in the
field of catalysis.54
Last, as a control experiment, Fe-BTC MACPs were also
studied for their ability to remove Pd species from water under
continuous flow. We note that without the PDA, over 10% of
the palladium goes through the column almost immediately
indicating significant loss of the valuable metal; however,
when treated with Fe-BTC/PDA MACPs the Pd concentration
is reduced below the detectable limit of 50 ppb (Figure 6c and
S49).
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Figure 6. (a) Palladium removal performance of Fe-BTC/PDA powder and beads from water with different Pd concentration. (b)
Palladium adsorption kinetics spiked with 10.48 ppm of Pd2+ in distilled water. (c) Breakthrough curve of Pd with spiked concentration of
8.91ppm. (d-i, d-ii) HAADF-STEM images and (d-iii, d-iv) EDX-STEM elemental maps of Fe and Pd from the selected region inside the
Fe-BTC/PDA
MACPs
composite
shown
in
(d-ii).
CONCLUSIONS
MOF loading and the bead size can be readily tuned. Further,
the resulting beads can be manufactured under continuous
flow, providing evidence of the method’s potential scalability.
Next, the simple structuring method was also proven to be
compatible with a host of structurally diverse MOFs, including
MIL-127-Fe, Fe-BTC, HKUST-1, Cu-TDPAT, Ni-pyrazolate
MOF, MIL-101(Cr), ZIF-8, ZIF-67, UiO-66, NH2-MIL-53(Al),
Eu2(BDC)3 and Tb2(BDC)3. and several MOF-composites,
including Fe3O4/MIL-127-Fe, Fe-BTC/PDA, and Fe-
BTC/PpPDA. Thanks to the mild shaping method, the
crystallinity and porosity of all MOFs and MOF-composites
are relatively well preserved. Moreover, experimental results,
combined with MD simulations also suggest that the
formation of PAA-Ca-Alg beads is an energetically favorable
process.
To date, the ability to test a MOF’s utility in separations,
which require continuous gas and liquid flow, is limited by the
existence of easy structuring methods that can preserve the
properties of the parent MOF material. Give this, a facile
micro-droplet method, used to structure MOFs into beads, has
been developed. The new method is based on double cross-
linked networks consisting of polyacrylic acid (PAA), sodium
alginate, and Ca2+ ions. Both PAA and alginate are
environmentally friendly macromolecules, and the bead
synthesis is carried out in water, making this structuring
method ecologically attractive.
As a proof of concept, MIL-127-Fe was first used for bead
formation, and it was subsequently demonstrated that the
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Last, to test the applicability of the structuring method for
liquid separations, an Fe-BTC/PDA composite possessing
Corresponding Authors
*wendy.queen@epfl.ch
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record-setting performance for the extraction of heavy metals
from water,24 was structured into beads. The resulting beads,
referred to as Fe-BTC/PDA MACPs, offer exceptional
performance in breakthrough experiments; it is determined
that 1 gram of the beads can purify over 10 liters of surface
water containing highly toxic concentrations of Pb2+ (600 ppb).
In addition, we demonstrate that the Fe-BTC/PDA MACPs
can potentially be extended to other applications, such as the
recovery of Pd from industrial waste.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by the Swiss National Science
Foundation under grant PYAPP2_160581. L.P. is supported by
the National Natural Science Foundation of China (NSFC grant
number 21903066). O.T. is financially supported by SNSF grant
CRSK-2_190823. A. J. is financially supported by the Swiss
Commission for Technology and Innovation (CTI). H. A. was
financially supported by the EPFL Enable Grant. The research is
carried out using the equipment of the shared research facilities of
HPC computing resources at Lomonosov Moscow State
University. The authors gratefully thank Dr. Mounir Driss Mensi
for the help with XPS analysis and Bardiya Valizadeh for the
control sample preparation. The authors also would like to
gratefully acknowledge the referees for their kind input and
valuable suggestions for improving this manuscript and Jie Zhang,
Vikram V. Karve, Till Schertenleib, Dr. Jordi Espín, Dr. Mehrdad
Asgari for their nice help and discussions during this work.
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Overall, the new structuring process is simple, non-toxic,
and carried out under environmentally friendly conditions.
Because of this, combined with the fact that the resulting
beads offer surface areas and other properties that are similar
to those of the parent MOF materials, we believe this method
can be readily extended to many other MOFs and their
corresponding composites. Such work can help us and others
move towards testing MOFs in application relevant conditions
under continuous flow, and aid the eventual implementation of
MOFs in a wide range of large-scale industrial applications or
pilot-scale demonstrations.
EXPERIMENTAL SECTION
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Preparation of MIL-127-Fe MACPs. Generally, 75 mg of
MIL-127-Fe was dispersed in 1 mL of 20 mg/mL sodium
alginate aqueous solution under stirring. Meanwhile, the
curing solution was prepared by dissolving 188 mg of
anhydrous calcium chloride in 30 mL of 0.05 mol/L
polyacrylic acid aqueous solution. Next, the alginate
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spherical MIL-127-Fe MACPs beads were obtained. The
preparation steps of other MOFs and composites, please see
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Computational Details. The calculations were performed
within Density Functional Theory (DFT) using PBE-D3
functional55-56, as implemented in CP2K57, applying DZPV-
MOLOPT basis set58, GTH pseudopotentials59, 450 Ry and
600 Ry charge density cutoffs for MOF-free and MOF-based
systems, respectively. To obtain realistic conformations of Ca
and polymer complexes Born-Oppenheimer molecular
dynamics calculations are performed for 3-5 ps at the constant
temperature of 300K applying velocity rescaling and a time
step of 1 fs, after which the geometry of the obtained
conformations is further optimized. The adopted PAA
structural model contains 7 monomer acrylic acid units, while
alginate is presented by its MGGM and GGMM oligomers,
comprising 4 monomer units of either α-L-guluronic acid (G)
or β-D-mannuronic acid (M). Pure Ca-alginate complex is
represented by an egg-box structure, in line with available
literature data38-40. For MIL-127(Fe) structure (001) ligand and
metal surface terminations are considered, similarly to
previous report on MIL-125.
ASSOCIATED CONTENT
Supporting Information. Supporting figures and tables, SEM
and TEM images, powder X-ray diffraction patterns, TGA
analysis, FT-IR and Raman spectra, N2 adsorption isotherms, pore
size distributions, XPS spectrum, EDX spectrum. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
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