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Photodegradation of Brilliant Green Dye by a Zinc bioMOF and Crystallographic Visualization of Resulting CO2


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We present a novel bio-friendly water-stable Zn-based MOF (1), derived from the natural amino acid L-serine, which was able to efficiently photodegrade water solutions of brilliant green dye in only 120 min. The total degradation was followed by UV-Vis spectroscopy and further confirmed by single-crystal X-ray crystallography, revealing the presence of CO2 within its channels. Reusability studies further demonstrate the structural and performance robustness of 1.
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Photodegradation of Brilliant Green Dye by a Zinc bioMOF and
Crystallographic Visualization of Resulting CO2
Paula Escamilla 1, Marta Viciano-Chumillas 1, Rosaria Bruno 2, Donatella Armentano 2,* , Emilio Pardo 1, *
and Jesús Ferrando-Soria 1, *
Citation: Escamilla, P.; Viciano-
Chumillas, M.; Bruno, R.; Armentano,
D.; Pardo, E.; Ferrando-Soria, J.
Photodegradation of Brilliant Green
Dye by a Zinc bioMOF and
Crystallographic Visualization of
Resulting CO2.Molecules 2021,26,
Academic Editor: Liudmil Antonov
Received: 10 June 2021
Accepted: 2 July 2021
Published: 5 July 2021
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1Departament de Química Inorgànica, Instituto de Ciencia Molecular (ICMOL), Universitat de València,
Paterna, 46980 València, Spain; (P.E.); (M.V.-C.)
2Dipartimento di Chimica e Tecnologie Chimiche, Universitàdella Calabria, 87036 Cosenza, Italy;
*Correspondence: (D.A.); (E.P.); (J.F.-S.)
We present a novel bio-friendly water-stable Zn-based MOF (
), derived from the natural
amino acid L-serine, which was able to efficiently photodegrade water solutions of brilliant green dye
in only 120 min. The total degradation was followed by UV-Vis spectroscopy and further confirmed
by single-crystal X-ray crystallography, revealing the presence of CO
within its channels. Reusability
studies further demonstrate the structural and performance robustness of 1.
metal-organic frameworks; amino acids-derived ligands; water remediation; photocat-
alytic degradation; single-crystal X-ray crystallography
1. Introduction
One of the main challenges that modern society faces is related, undoubtedly, to
the contamination of aquatic environments, which is mainly caused by human/industrial
activities [
]. Among the wide diversity of inorganic/organic chemical pollutants, organic
dyes—waste generated in the cosmetic, textile, tannery, or food industries, among others—
constitute one of the major contaminants in industrial wastewater [3].
Proposed solutions for the removal of such organic contaminants include precipita-
tion, coagulation/flocculation, membrane technology, or biological processes [
]. However,
probably the two most promising technologies for this purpose are based on the straight-
forward capture of the organic dyes by a porous material or their in-situ photocatalytic
degradation [
]. In particular, metal-organic frameworks (MOFs) are porous crystalline
materials that have already been shown to be efficient in the last two approaches [713].
MOFs [
] attract broad attention from many research groups given the great variety
of applications they can exhibit [
]. The reasons for such a variety of applications
are related to a number of unique characteristics such as permanent porosity [
], a
thrilling host-guest chemistry [
]—which can be tuned for fine control over the size,
shape, and functionality of MOF channels—and the possibility of using single-crystal X-ray
diffraction [2631] (SCXRD) to visualize what is going on within their channels [32].
In particular, MOFs have recently been used in water remediation with exceptional
results. For example, the easy of functionalizing, pre- or post-synthetically, MOF channels
has led to the preparation of specifically designed MOFs for the selective and efficient
capture of both organic and inorganic contaminants [
]. Moreover, certain specifically
designed MOFs have already been shown to be effective as photocatalysts to degrade
organic pollutants—i.e., organic dyes– into less toxic intermediates or fully degrade them
into CO
and H
O [
]. In particular, several Zn(II)-based MOFs have recently been
reported, showing, by analogy with zinc oxide based photocatalysts [
], moderately
good efficiencies as photocatalysts towards different organic dyes [4345].
Molecules 2021,26, 4098.
Molecules 2021,26, 4098 2 of 9
2. Results
In this communication, we report the preparation and total characterization of a novel
eco-friendly Zn-based MOF, derived from the natural amino acid L-serine, with formula
O (
) ((S,S)-serimox = [bis[(S)-serine]oxalyl diamide]
(Figure 1a,b). The final material is capable of photodegrading brilliant green (BG) dye
(Figure 1c) in only 120 min with an efficiency of 100% in the absence of any other oxidant or
co-catalyst. In addition, the high robustness and crystallinity of 1 also allowed us to obtain
the crystal structure of 1—with the help of SCXRD—after the photocatalytic process, which
shows, unambiguously, the presence, within the channels, of CO
molecules resulting from
the photodegradation of BG dyes.
Figure 1.
Chemical formulas of the proligand (S,S)-serimox (
) and the secondary building units
(SBU) consisting of Zn
dinuclear units (
). Red oxygen atoms coordinate Zn
atoms from another
SBU, whereas green oxygen atoms belong to neighboring SBUs. (
) Chemical formula of brilliant
green dye.
The crystal structure of 1 was first determined at 100 K. It crystallizes in the chiral P4
space group of the tetragonal system and consists of a chiral 3D pillared square grid where
(S,S)-serimox] moieties are located on the vertices of the edges
(Figures 2and 3a)
. The
robust uni-nodal three-connected srs nets are built up from trans oxamidato-bridged Zn(II)
dimeric units, {Zn
[(S,S)-serimox]} (Figure 2and Figure S1), which are connected to each
other through their carboxylate groups (Figure 2, Figures S1 and S2).
Figure 2.
Perspective view of a fragment of 1 emphasizing the central dinuclear Zn
) unit and
their connection to neighboring Zn
cations (
). Color code: Zn, O, and N atoms are represented as
cyan, red, and deep blue spheres, respectively, whereas C atoms are depicted as grey sticks.
Within the {Zn
[(S,S)-serimox]} moieties, each Zn(II) metal ion results in a highly
distorted pyramidal coordination being linked by nitrogen and oxygen atoms from the
serimox ligand [Zn-N 1.985(4)Å and Zn-O
ranging from 1.958(3) to 2.215(4) Å] and
a terminal water molecule [Zn-O
2.011(4) Å] (Figure 2b). The 3D network features
two types of small pore, different in size and shape, propagating along the band caxes
Figure 3
, Figures S2 and S3). Hydrophilic square sized pores (Figure 3a, left and Figure S3)
and hydrophilic irregular pores of medium size (virtual diameters of ca. 0.30 and 0.40 nm,
respectively) (Figure 3a, right and Figure S2) are decorated by the primary alcohol group
of serine moieties, pointing inwards towards the voids, and accountable for its resulting
host–guest chemistry.
Molecules 2021,26, 4098 3 of 9
Figure 3.
Perspective views of the porous networks of 1 (
) and CO
@1 (
) along the c(
) and
) axes. Color code: Zn atoms and C and O from guest CO
molecules are represented as
cyan, grey, and red spheres, whereas serimox ligands—with the exception of serine residues—are
represented by grey sticks. –CH
OH groups and N atoms are represented as cyan, red, and deep
blue spheres, respectively, whereas C atoms are depicted as grey sticks.
Crystals of 1, left in a sealed glass tube containing an aqueous solution of BG dye for
one week, after irradiation in the range 250–350 nm for 60 min were analyzed by SCXRD at
a temperature of 100 K and the crystal structure of CO
@1 was determined, revealing a
slight deformation of the framework—likely correlated to CO
adsorption process—but
still isomorphous to 1, crystallizing in the P4
2 space group. Adsorption of CO
produced by the photocatalytic process at 298 K—results in linear negative expansion.
Pores that run parallel to the c-axis contract upon CO
inclusion (
b slightly < 0;
c < 0;
V < 0). The largest change in dimensions is observed for the caxis, accounting for a small
decrease in the unit–cell volume (V48 Å3, Table S1).
The crystal structure of the adsorbate CO
@1 clearly evidences the presence of CO
guest molecules hosted in the hydrophilic irregular pores of 1 (Figure 3b,
Figures S4 and S6
A primary adsorption site can be identified on the CO
molecule (Figure 4), where the
plane containing the ligands are situated at only 2.83(1) Å [distance between centroid of
oxamate core and C atom of CO
molecule] from the C atom of CO
molecules occupying
the primary site, suggesting an interaction between the C(
) atom of the CO
molecule and
the O lone pair of the oxamate core of the ligand, as observed before [
]. The described
guest molecule orientation generates a kind of
(oxamate) dimer exhibiting
) and C(CO
O(oxamate) symmetric contacts of 3.08(1) and 3.32(1)
Å, respectively. The secondary adsorption sites identified are situated at the center of the
pores, where CO2molecules are packed with closest contact with the framework through
the H
C- and HC- of the ligand [shortest O-C-O
H-C- at 2.04(3) and 2.09(3) Å, for H
and HC- fragments, respectively] (Figure 4, Figures S4 and S5). The alcoholic fragment
makes a contribution as well, being situated very close to CO
molecules [O-C-O
and HO
O-C-O at 2.11(6) and 2.63(3) Å, respectively]. Despite the supposed flexibility
and structural adaptability that serine residues could offer, no important differences in the
alcoholic chain conformations have been observed in the CO
@1 adsorbate with respect
to 1, where only Zn-O
[ranging from 1.935(3) to 2.191(4) Å] distances show a slight
Molecules 2021,26, 4098 4 of 9
contraction with respect to 1, while Zn-O
[1.992(4) Å], and Zn-N bond lengths [1.959(4)
Å] fall in the range of the expected values for Zn(II) metal ions [47].
Figure 4.
Details of CO
@1 crystal structure showing the adsorption sites identified for CO
molecules. Color code as in Figure 3.
What is worth underlining is the unusual penta-coordination of Zn(II) ions observed
in 1 and CO
@1, reminiscent (in geometry) of that observed for the catalytic metal ions
of the di-zinc aminopeptidase from Aeromonas proteolytica (AAP) [
]. Surveys of the
Cambridge Structural Database (CSD) show zinc ion coordination number frequencies
of ca. 60% and 25% for 4 and 6 coordination numbers, respectively. Interestingly, the
zinc ion coordination prevalence in protein sites depends on whether the zinc plays a
structural or a catalytic role. In structural zinc sites, the occurrence rate for 4, 5, and 6
coordination numbers is 80%, 6%, and 12%, respectively; whereas in catalytic zinc sites,
the occurrence rate for 4, 5, and 6 coordination is 47%, 45%, and 6%, respectively [
Thus, five coordinate, or geometrically strained zinc sites, may represent sites equipped for
catalysis, whereas four coordinate ideal tetrahedral zinc sites may represent stable sites
affiliated with structural support.
Single-crystal X-ray experiment on samples of 1 after irradiation with UV light (with-
out BG dye) do not show the presence of CO
molecules, just coordinated water and small
voids with light diffuse electron density. This ruled out the possibility of CO
tion from air or solvent in 1, and reinforced our hypothesis that CO
@1 comes from the
decomposition of the BG dye.
The experimental powder X-ray diffraction (PXRD) pattern of a polycrystalline sample
of 1 is shown in Figure 5. It is consistent with the theoretical one and confirms the
purity and homogeneity of the bulk sample. The solvent contents were determined by
thermogravimetric analysis (TGA) under a dry N
atmosphere (see Figure S7, Supporting
Information) and helped to established the final chemical formula. Attempts to activate 1
(under different protocols) for measuring N
isotherms proved unsuccessful, most likely
related to its loss of structural stability when all molecule solvents were removed.
The photocatalytic activity of MOF 1 for the degradation of BG dye was then inves-
tigated. For this purpose, 25 mg of 1 were suspended in 50 mL of a 10 ppm aqueous
solution of BG. Prior to irradiation, the mixture was kept in the dark for 30 min to verify
that degradation only occurs under irradiation. After that period, the suspension started to
be irradiated, under mild stirring at 250 nm. At different times (5, 15, 30, 60, and 120 min),
1 mL aliquots were taken, centrifuged, and diluted, and their UV–Vis absorption spectra
were registered (Figure 6a). An identical experiment was performed, under the same
conditions, in the absence of 1 (Figure S8). In order to have a better characterization of 1,
we performed UV-Vis diffuse reflectance spectroscopy, which revealed a strong adsorption
band below 350 nm (Figure S9).
Molecules 2021,26, 4098 5 of 9
Figure 5.
Calculated (
) and experimental (
) PXRD patterns of 1 in the 2
range 2.0–60.0
at R.T.
) Experimental PXRD pattern of 1 after three consecutive cycles of photocatalytic degradation of BG.
Figure 6.
) Evolution with time of the UV-Vis absorption spectra of 10 ppm solutions of brilliant
green in water in the presence of 25 mg of a polycrystalline sample of 1. Blue: t= 0; Red: t= 5 min.;
Green: t= 15 min.; Orange = 30 min.; Yellow: t= 60 min.; Purple: t= 120 min. The inset shows the
BG solutions at t = 0 min. (left) and 120 min. (right) of exposure of the BG solution with MOF 1.
) Kinetic profile of the degradation of brilliant green under irradiation at 250 nm in the presence of
MOF 1 (blue) and with no photocatalyst (red).
Figure 6shows the photodegradation efficiency of 1 towards BG. Such efficiency was
evaluated by measuring the decrease in the characteristic absorption bands of BG dye,
which appear at 420 nm and 625 nm, respectively. Thus, under irradiation in the presence of
1, it a gradual decrease of both peaks with time can be observed, until the vanish completely
after 120 min (Figure 6a), which indicates that 100% of BG dye is eliminated after that time
(Figure 6b). In turn, the same experiment, in the absence of 1, (Figure S8 and Figure 6b),
Molecules 2021,26, 4098 6 of 9
shows a very smooth decrease of the UV–Vis absorption bands, confirming the key role
of 1 as photocatalyst. Tauc plot of the Kubelka–Munk function (Figure S10) allowed us to
obtain an estimation of the optical band of 1 (3.03 eV), which evidenced the suitability of
it to perform the degradation of BG. Moreover, the reuse capability of 1 was stablished
by both UV–Vis and PXRD experiments. On the one hand, two more UV–Vis vs. time
experiments were carried out, using 1 as photocatalyst, with identical results (Figure S11),
confirming that 1 can be employed in at least 3 cycles. On the other hand, PXRD patterns
of 1 after three cycles are identical to those of the synthesized material suggesting that no
degradation occurs during the photocatalytic process (Figure 5c).
The full degradation of the organic dye was also supported by thermogravimetric
analysis coupled with mass spectrometry (TGA-MS) on CO
@1 (Figure S12). The weight
loss in TGA led to two peaks in MS spectra with the mass to charge ratio (m/z) of 18
and 44, which can be attributed to water and carbon dioxide. However, the amount of
desorbed was lower than that obtained from X-ray crystallography. Most likely,
this is related to some losses of CO
during the handling of the measurement itself. The
larger size of brilliant green dye compared to 1 pore size precluded the adsorption of the
organic dye on MOF channels to perform the photocatalytic event. Thus, the photocatalytic
activity of 1 most likely will arise from the crystal surface. With the aim of confirming the
photodegradation’s occurrance at the surface of 1, we measured the evolution of the BG
concentration in solution when it is put in contact with 1 during 30 min in the dark. As
can be observed in Figure S13, there is no appreciable difference in the BG concentration
in solution, which supports our hypothesis. A leaching test ruled out the possibility
of decomposition products of Zn-based MOF being responsible for the photocatalytic
degradation of brilliant green. This was also supported, in an indirect manner, by the
maintenance of both the activity in the recyclability studies (Figure S11) and the high
crystallinity of PXRD diffraction patterns after the photocatalytic process (Figure S5c).
A plausible mechanism for the degradation of BG dye is shown in Scheme S1. With
the aid of UVC light, 1 generates electron and hole pairs. While the electrons jump to the
conducting band, the holes remain in the valence band. The holes were scavenged by water
molecules leading to energetically reactive hydroxyl radicals, and the photogenerated
electrons react with O
to produce the superoxide radical anions of oxygen. From them,
based on literature precedents [
], the OH
is believed to be the dominant oxidizing
agent for the mineralization of BG dye into CO
and H
H NMR of solutions after
photocatalytic experiment do not show any peaks (apart from H
O) that can be attributed
to any known intermediate from a previously reported study.
With the aim of exploring
the formation of intermediate species, we performed liquid chromatography/tandem mass
spectrometry (LC-MS/MS) on solutions after 30 min. of UV irradiation, where we were
able to identify four degradation molecules (Scheme S2) [54].
3. Conclusions
In summary, we have reported a novel, eco-friendly Zn-based MOF with the formula
O (1) and its performance as a photocatalyst of BG aque-
ous solutions. This MOF with good water stability was able to efficiently photodegrade
10 ppm aqueous solutions of BG in 120 min. Indeed, UV-Vis spectroscopy measurements of
irradiated solutions, with and without the presence of 1, clearly revealed the photodegra-
dation role of the framework. Single-crystal X-ray crystallography was applied not only to
structurally characterize the pristine structure of 1, but more interestingly, after the photo-
catalytic process. The resolution of the crystal structure of CO
@1 allowed us to confirm
the total degradation of the dye and the presence of CO
molecules (from the degradation
process) retained within the irregular hydrophilic channels of 1. Reusability tests of 1, with
up to 3 cycles of the photodegradation process, evidenced structural and performance
robustness, which further confirms the viability of 1 as an efficient photocatalyst. Our
current work is focused on extending this study to other Zn-based MOFs derived from
natural amino acids.
Molecules 2021,26, 4098 7 of 9
Supplementary Materials:
The following are available online at, experimental preparation, analytical
and spectroscopic characterization of 1, and additional Figures S1–S13, Table S1, Schemes S1 and S2.
Author Contributions:
Conceptualization, D.A., E.P. and J.F.-S.; methodology, P.E., M.V.-C. and R.B.;
formal analysis, D.A., E.P. and J.F.-S.; investigation, P.E., M.V.-C. and R.B.; writing—original draft
preparation, P.E., M.V.-C. and R.B.; writing—review and editing, D.A., E.P. and J.F.-S.; supervision,
D.A., E.P. and J.F.-S.; project administration, D.A., E.P. and J.F.-S.; funding acquisition, D.A., E.P. and
J.F.-S.. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement:
CCDC 2062289-2062290 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
cif (accessed on 3 July 2021), or by emailing, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-
This work was supported by the Ministero dell’Istruzione, dell’Universitàe
della Ricerca (Italy) and the MINECO (Spain) (Projects PID2019
I00 and Excellence
Unit “Maria de Maeztu” CEX2019
M). R. B. thanks the MIUR (Project PON R&I FSE–FESR
2014–2020) for the predoctoral grant. D.A. acknowledges the financial support of the Fondazione
CARIPLO/“Economia Circolare: ricerca per un futuro sostenibile” 2019, project code: 2019–2090,
MOCA. E.P. acknowledges the financial support of the European Research Council under the Euro-
pean Union’s Horizon 2020 research and innovation programme/ERC Grant Agreement No 814804,
MOF–reactors. Thanks are also extended to the “2019 Post–doctoral Junior Leader–Retaining Fel-
lowship, la Caixa Foundation (ID100010434 and fellowship code LCF/BQ/PR19/11700011” and
“Subvenciones concedidas a la excelencia científica de juniors investigadores, SEJI/2020/034“ (J. F.–
S.). We would like to thank Antonio Leyva-Pérez and Alejandro Vidal-Moya (Instituto de Tecnología
Química, Universidad Politècnica de València-Consejo Superior de Investigaciones Científicas) for
their help with solid-state 13C NMR experiments.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of compounds 1 is available from the authors.
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... From the point of view of supramolecular chemistry, when compared with conventionally used microporous inorganic materials such as zeolites, MOFs have the potential for a more flexible rational design, tuneable structure, large surface area (up to 10,000 m 2 /g), variable pore diameters (from microto mesoporous), and tailorable functionalities [62], which endow MOFs with a variety of tuneable properties such as charge, polarity, chirality, redox potential, photoactivity, hydrophobicity/hydrophilicity, aromatic/lipophilic character, stereochemistry, and so on [63]. Due to all these features, MOF materials have widely been used in a range of potential applications concerning catalysis [64][65][66][67], gas adsorption, storage and release [68], molecular separation [69,70], sensing [71], lighting [72], therapeutics, drug carriers, imaging, and biosensors in biomedicine [73]. ...
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This review focuses on the recent developments in synthesis, properties, and applications of a relatively new family of photoactive porous composites, integrated by metal halide perovskite (MHP) nanocrystals and metal-organic frameworks (MOFs). The synergy between the two systems has led to materials (MHP@MOF composites) with new functionalities along with improved properties and phase stability, thus broadening their applications in multiple areas of research such as sensing, light-harvesting solar cells, light-emitting device technology, encryption, and photocatalysis. The state of the art, recent progress, and most promising routes for future research on these photoactive porous composites are presented in the end.
The use of bimetallic nanoparticles in photocatalysis is one of the prominent techniques for the treatment of wastewater. Pure and N, S-co-doped bimetallic cerium copper oxide nanoparticles were prepared and characterized by FTIR, X-ray diffraction and SEM. Newly synthesized N, S-co-doped CeCuO3 NP’s were used as photocatalysts for the degradation of Brilliant green dye in the presence of ecofriendly LEDs (250 W) irradiations. The values of rate constant for N, S co-doped and undoped CeCuO3 was observed individually. Brilliant green dye was effectively degraded by N, S doped CeCuO3 as compared to undoped CeCuO3 nanoparticles. Effect of doping and operating parameters e.g. pH, Intensity of light and amount of catalyst etc., on the rate of mineralization of dye was studied. Overall degradation of dye was confirmed by the determination of COD, CO2 and UV-Visible investigation of the samples. The mechanism of the mineralization of dye molecules was investigated by the formation of •OH radicals in the aqueous suspension. The reusability of catalyst was tested by different cycles.
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Metal single-atom catalysts (SACs) promise great rewards in terms of metal atom efficiency. However, the requirement of particular conditions and supports for their synthesis, together with the need of solvents and additives for catalytic implementation, often precludes their use under industrially viable conditions. Here, we show that palladium single atoms are spontaneously formed after dissolving tiny amounts of palladium salts in neat benzyl alcohols, to catalyze their direct aerobic oxidation to benzoic acids without ligands, additives, or solvents. With this result in hand, the gram-scale preparation and stabilization of Pd SACs within the functional channels of a novel methyl-cysteine-based metal-organic framework (MOF) was accomplished, to give a robust and crystalline solid catalyst fully characterized with the help of single-crystal X-ray diffraction (SCXRD). These results illustrate the advantages of metal speciation in ligand-free homogeneous organic reactions and the translation into solid catalysts for potential industrial implementation.
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The exact chemical structure of non–crystallising natural products is still one of the main challenges in Natural Sciences. Despite tremendous advances in total synthesis, the absolute structural determination of a myriad of natural products with very sensitive chemical functionalities remains undone. Here, we show that a metal–organic framework (MOF) with alcohol–containing arms and adsorbed water, enables selective hydrolysis of glycosyl bonds, supramolecular order with the so–formed chiral fragments and absolute determination of the organic structure by single–crystal X–ray crystallography in a single operation. This combined strategy based on a biomimetic, cheap, robust and multigram available solid catalyst opens the door to determine the absolute configuration of ketal compounds regardless degradation sensitiveness, and also to design extremely–mild metal–free solid–catalysed processes without formal acid protons.
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Conventionally, composite materials are usually employed as a catalyst in piezo-photocatalytic dye wastewater treatment. Here, we report the synthesis of ZnO nanoparticles, as a single-component catalyst, by surfactant-assisted precipitation in which the size of ZnO nanoparticles (20–100 nm) can be simply controlled by the use of Tween80 as a surfactant. Although, ZnO nanoparticles exhibited appreciable photocatalytic activities for the degradation of methylene blue (MB) dye, upon the addition of a mechanical force, the photocatalytic dye degradation efficiency was substantially improved. Furthermore, we postulated that the surface properties of ZnO play an important role in charge transfer phenomena based on photoluminescence results together with functional groups on the surface of ZnO. In addition, application of single-component ZnO in piezo-promoted photocatalytic degradation of cationic and anionic dyes was accomplished. Our results regarding the behaviour of single-component ZnO nanoparticles under vibrational energy in addition to their conventional solar harvesting can provide a promising strategy for developing photocatalysts for practical wastewater treatment.
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Structural insight into reactive species can be achieved via strategies such as matrix isolation in frozen glasses, whereby species are kinetically trapped, or by confinement within the cavities of host molecules. More recently, Metal-organic Frameworks (MOFs) have been used as molecular scaffolds to isolate reactive metal-based species within their ordered pore networks. These studies have uncovered new reactivity, allowed observation of novel metal-based complexes and clusters, and elucidated the nature of metal-centred reactions responsible for catalysis. This perspective considers strategies by which metal species can be introduced into MOFs and highlights some of the advantages and limitations of each approach. Furthermore, the growing body of work whereby reactive species can be isolated and structurally characterised within a MOF matrix will be reviewed, including discussion of salient examples and the provision of useful guidelines for the design of new systems. Novel approaches that facilitate detailed structural analysis of reactive chemicals moieties are of considerable interest as the knowledge garnered underpins our understanding of reactivity and thus guides the synthesis of materials with unprecedented functionality.
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In this study, we report a surfactant-mediated synthesis of ferrites (MFe2O4: M = Co, Ni, Cu, Zn) using the co-precipitation-oxidation method. The band gap calculated from UV-Visible diffuse reflectance spectra were found in the range of 1.11–1.81 eV. These ferrite nanocatalysts were studied for the photocatalytic degradation of multiple organic dyes in a 32 W UV-C/H2O2 system. All the four ferrites showed an excellent dye degradation rate in the range of 2.065–2.417 min−1 at neutral pH. In the optimized condition, NiF was found to degrade 89%, 92%, 93%, and 78% of methylene blue, methyl orange, bromo green, and methyl red, respectively within 1 min of UV-irradiation. A 40% TOC removal was recorded after 5 min of degradation reaction, which increased to 60% after 50 min. Mechanism elucidated by scavenger studies and fluorescence spectroscopy revealed that •OH and holes were the primary reactive radicals responsible for the degradation process. Ferrite photocatalysts showed an insignificant performance loss in seven consecutive cycles. The photocatalyst was found efficient in the presence of a high concentration of salts. Thus, it was concluded that these photocatalysts are highly suitable for the remediation of dye-contaminated wastewater.
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Since the advent of the first metal-organic frameworks (MOFs), we have witnessed an explosion of captivating architectures with exciting physicochemical properties and applications in a wide range of fields. This, in part, can be understood under the light of their rich host-guest chemistry and the possibility to use single-crystal X-ray diffraction (SC-XRD) as a basic characterization tool. Moreover, chemistry on preformed MOFs, applying recent developments in template-directed synthesis and postsynthetic methodologies (PSMs), has shown to be a powerful synthetic tool to (i) tailor MOFs channels of known topology via single-crystal to single-crystal (SC-SC) processes, (ii) impart higher degrees of complexity and heterogeneity within them, and most importantly, (iii) improve their capabilities toward applications with respect to the parent MOFs. However, the unique properties of MOFs have been, somehow, limited and underestimated. This is clearly reflected on the use of MOFs as chemical nanoreactors, which has been barely uncovered. In this Account, we bring together our recent advances on the construction of MOFs with appealing properties to act as chemical nanoreactors and be used to synthesize and stabilize, within their channels, catalytically active species that otherwise could be hardly accessible. First, through two relevant examples, we present the potential of the metalloligand approach to build highly robust and crystalline oxamato- and oxamidato-MOFs with tailored channels, in terms of size, charge and functionality. These are initial requisites to have a playground where we can develop and fully take advantage of singular properties of MOFs as well as visualize/understand the processes that take place within MOFs pores and somehow make structure-functionalities correlations and develop more performant MOFs nanoreactors. Then, we describe how to exploit the unique and singular features that offer each of these MOFs confined space for (i) the incorporation and stabilization of metals salts and complexes, (ii) the in situ stepwise synthesis of subnanometric metal clusters (SNMCs), and (iii) the confined-space self-assembly of supramolecular coordination complexes (SCCs), by means of PSMs and underpinned by SC-XRD. The strategy outlined here has led to unique rewards such as the highly challenging gram-scale preparation of stable and well-defined ligand-free SNMCs, exhibiting outstanding catalytic activities, and the preparation of unique SCCs, different to those assembled in solution, with enhanced stabilities, catalytic activities, recyclabilities, and selectivities. The results presented in this Accounts are just a few recent examples, but highly encouraging, of the large potential way of MOFs acting as chemical nanoreactors. More work is needed to found the boundaries and fully understand the chemistry in the confined space. In this sense, mastering the synthetic chemistry of discrete organic molecules and inorganic complexes has basically changed our way of live. Thus, achieving the same degree of control on extended hybrid networks will open new frontiers of knowledge with unforeseen possibilities. We aim to stimulate the interest of researchers working in broadly different fields to fully unleash the host-guest chemistry in MOFs as chemical nanoreactors with exclusive functional species.
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Water is essential in all aspects of life, being the defining characteristic of our planet and even our body. Regrettably, water pollution is increasingly becoming a challenge due to novel anthropogenic pollutants. Of particular concern are emerging organic contaminants (EOCs), the term used not only to cover newly developed compounds but also compounds newly discovered as contaminants in the environment. Aside from anthropogenic contamination, higher temperature and more extreme and less predictable weather conditions are projected to affect water availability and distribution. Therefore, wastewater treatment has to become a valuable water resource and its reuse is an important issue that must be carried out efficiently. Among the novel technologies considered in water remediation processes, metal-organic frameworks (MOFs) are regarded as promising materials for the elimination of EOCs since they present many properties that commend them in water treatment: large surface area, easy functionalizable cavities, some are stable in water, and synthesized at large scale, etc. This review highlights the advances in the use of MOFs in the elimination (adsorption and/or degradation) of EOCs from water, classifying them by the nature of the contaminant.
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Industrial processes prominently feature π-acidic gases, and an adsorbent capable of selectively interacting with these molecules could enable important chemical separations1–4. Biological systems use accessible, reducing metal centres to bind and activate weakly π-acidic species, such as N2, through backbonding interactions5–7, and incorporating analogous moieties into a porous material should give rise to a similar adsorption mechanism for these gaseous substrates8. Here, we report a metal–organic framework featuring exposed vanadium(ii) centres capable of back-donating electron density to weak π acids to successfully target π acidity for separation applications. This adsorption mechanism, together with a high concentration of available adsorption sites, results in record N2 capacities and selectivities for the removal of N2 from mixtures with CH4, while further enabling olefin/paraffin separations at elevated temperatures. Ultimately, incorporating such π-basic metal centres into porous materials offers a handle for capturing and activating key molecular species within next-generation adsorbents. Nitrogenases use transition metals to selectively capture weak π acids such as N2 by employing backbonding interactions. Here, a metal–organic framework with exposed vanadium sites is presented that uses this approach for selective capture of N2 from CH4, with impressive selectivity and capacity.
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The size- and shape-dependent characteristics that distinguish nanoscale materials from bulk solids arise from constraining the dimensionality of an inorganic structure1–3. As a consequence, many studies have focused on rationally shaping these materials to influence and enhance their optical, electronic, magnetic and catalytic properties4–6. Although a select number of stable clusters can typically be synthesized within the nanoscale regime for a specific composition, isolating clusters of a predetermined size and shape remains a challenge, especially for those derived from two-dimensional materials. Here we realize a multidentate coordination environment in a metal–organic framework to stabilize discrete inorganic clusters within a porous crystalline support. We show confined growth of atomically-defined nickel(ii) bromide, nickel(ii) chloride, cobalt(ii) chloride, and iron(ii) chloride sheets through the peripheral coordination of six chelating bipyridine linkers. Importantly, confinement within the framework defines the structure and composition of these sheets and facilitates their precise characterization by crystallography. Each metal(ii) halide sheet represents a fragment excised from a single layer of the bulk solid structure, and structures obtained at different precursor loadings enable observation of successive stages of sheet assembly. Finally, the isolated sheets exhibit magnetic behaviours distinct from those of the bulk metal halides, including the isolation of ferromagnetically coupled large-spin ground states through the elimination of long-range, interlayer magnetic ordering. Overall, these results demonstrate that the pore environment of a metal–organic framework can be designed to afford precise control over the size, structure and spatial arrangement of inorganic clusters.
Water is vital for sustenance of all forms of life. Hence, water pollution is a universal crisis of survival for all forms of life and a hurdle for sustainable development. Textile industry is one of the anthropogenic activities that severely pollute water bodies. Inefficient dyeing processes result in ending up of thousands of tons of synthetic dyes every year in the water bodies. Therefore, the efficient removal of synthetic dyes from wastewater has become a challenging research field. Owing to their tuneable structure-property aspects Metal-Organic Frameworks (MOFs) have been emerged as promising adsorbents for adsorptive removal of dyes from the wastewater and textile effluent. In this perspective we highlight recent studies involving application of MOFs for adsorptive removal of hazardous dye molecules. We also classify the developed MOFs in cationic anionic and neutral framework categories to comprehend their suitability for removal of a given class of dyes.