Solar-driven advanced oxidation process catalyzed by metal–organic frameworks for water depollution

Article abstract of DOI:10.1016/j.poly.2019.06.005

Phosphonate-based MOF materials namely, STA-12(M) (M = Mn, Fe, Co, Ni), show photocatalytic activity for the degradation of methylene blue (MB) and methyl Orange (MO) from aqueous solution under natural sunlight irradiation. Thus, photo-Fenton oxidative discoloration of dyes have been surveyed by H₂O₂ catalyzed with the STA-12(M). The degradation of the dyes, appears to be faster with STA-12(Fe) than other synthesized MOFs. The reaction is first order with respect to MO and MB dyes and the synergistic index in the STA-12(Fe)/sunlight/H₂O₂system reached as high as 431%. Mineralization of dyes was discussed by spectroscopic and TOC measurement. Besides, the efficiency of STA-12(Fe) used in photo-Fenton process was attentively investigated through the characterization of reactive radicals, the stability and reusability of the photocatalyst, also the effect of operational parameters such as H₂O₂ dosage, solution pH and initial dye concentration. This work demonstrates the first example of facilitating photo-Fenton excitation of H₂O₂ via phosphonate based metal organic frameworks as photocatalysts and explained a new opportunity for sunlight-induced photocatalytic environmental remediation and protection.

Full text of DOI:10.1016/j.poly.2019.06.005

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6
Solar-driven advanced oxidation process catalyzed by metal-organic frame-
works for water depollution
Alireza Farrokhi, Maasoumeh Jafarpour, Mahbobeh Alipour
PII:
S0277-5387(19)30409-7
https://doi.org/10.1016/j.poly.2019.06.005
POLY 13996
DOI:
Reference:
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Please cite this article as: A. Farrokhi, M. Jafarpour, M. Alipour, 6Solar-driven advanced oxidation process catalyzed
by metal-organic frameworks for water depollution, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.
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Solar-driven advanced oxidation process catalyzed by metal-organic
frameworks for water depollution
Alireza Farrokhi*, Maasoumeh Jafarpour and Mahbobeh Alipour
Department of Chemistry, Faculty of Science, University of Birjand, Birjand, 97179-414 Iran.
*
Corresponding Author
E-mail: afarrokhi@birjand.ac.ir;
Tel: +98 56 32202516; Fax: +98 56 32202515

Abstract: phosphonate-based MOF materials namely, STA-12(M)(M= Mn, Fe, Co, Ni), show
photocatalytic activity for the degradation of methylene blue (MB)and methyl Orange (MO)from
aqueous solution under natural sunlight irradiation. Thus, photo-Fenton oxidative discoloration
of dyes have been surveyed by H O catalyzed with the STA-12(M). The degradation of the
2
2
dyes, appears to be faster with STA-12(Fe) than other synthesized MOFs. The reaction is first
order with respect to MO and MB dyes and the synergistic index in the STA-
1
2(Fe)/sunlight/H O system reached as high as 431 %. Mineralization of dyes was discussed by
2 2
spectroscopic and TOC measurement. Besides, the efficiency of STA-12(Fe) used in photo-
Fenton process was attentively investigated through the characterization of reactive radicals, the
stability and reusability of the photocatalyst, also the effect of operational parameters such as
H O dosage, solution pH and initial dye concentration. This work demonstrates the first
2
2
example of facilitating photo-Fenton excitation of H O via phosphonate based metal organic
2
2
frameworks as photocatalysts and explained a new opportunity for sunlight-induced
photocatalytic environmental remediation and protection.
Keywords: Dye degradation; Hydrogen peroxide; Metal-organic framework; Natural sunlight;
Photocatalysis
1
. Introduction
Nowadays, much of hazardous organic dye wastewater was produced in paper, textile, coloring
and food industries. Dye contaminants are very stable to oxidation and light owing to the stable
aromatic chemical structures, however this causes detriment to the environment and significantly
menaces human health [1]. Consequently, the effective purgation of wastewater is an essential
problem before liberating them into the environment. Meanwhile, many techniques such as
membrane filtration, adsorption, reduction, oxidation and photocatalysis are generally employed
to remove organic contaminants from the industrial sewages [1, 2]. Photocatalysis is an example
of advanced oxidation process that has appeared as a better alternative to the biological method
in overcoming the growth of water and wastewater concerns [3-6]. Due to the infinite and
permanent energy inflow from the solar irradiation, photocatalysis with sunlight has been known
as one of the most profitable strategies to remove organic contaminants for resolving
environmental problems of human societies [7]. For the effective use of sunlight, the
improvement of visible-light-reactive photocatalysts is very necessary. The present metal-oxide
photocatalysts have low quantum yield and inability for utilize the solar light or daylight.
Therefore, the researchers have prompted to develop next descendant solar-responsive
photocatalysts [8]. Studies have shown that metal-organic frameworks (MOFs) could be revealed
as favorable replacements for conventional photocatalysts [9-12].

Compared with the ordinary semiconductor photocatalysts, photo-responsive MOFs have
advantages in more efficient sunlight harvesting, as tunable active sites in their chemical
structure (i.e., organic linkers and metal-oxo clusters), appropriate porosity and extensive surface
area permit not only for sensible design and construction of the photocatalyst structure, but also
for rapid transport and good embracement of reactive molecules. As a result, MOFs could have
considered as an excellent material for light harvesting to accomplish photocatalytic degradation
of organic pollutants [13]. Meanwhile, solar energy is the most abundant of renewable energy
resource. Moreover, solar light, being green and free, is a perfect energy source for resolving
current environmental problems. Also, reports on a sunlight utilizing metal-organic framework
for photocatalytic degradation of water pollutants are limited [14-19]. Thus, the sensible
synthesis of novel photo-active MOFs and the study of solar-driven advanced oxidation process
for dyes destruction are currently required and significant.
Herein, we demonstrate the capability of a series of STA-12 family MOFs for the excitation of
H O to attain high competence in natural sunlight photocatalytic procedure. The photo-
2
2
degradation rates of methyl orang (MO) and methylene blue (MB) over STA-12(Mn), STA-
2(Fe), STA-12(Co) and STA-12(Ni) were increased obviously in the presence of H O . The
1
2
2
Fenton and photo-Fenton mechanism of MOFs was inspected and suggested according to
experimental recognition of hydroxyl radicals which approve the appended catalytic activity
maybe ascended from the collaborative effects of STA-12 and H O under daylight irradiation.
2
2
2
2
. Experimental
.1. Chemicals
All solvents and reagents used in this research are commercially available and consumed without
purification. Manganese(II)acetatetetrahydrate (Mn(OAc) .4H O), Ferrous chloride tetrahydrate
2
2
(FeCl .4H O), Cobalt(II) nitrate hexahydrate(Co(NO ) .6H O), Nickel(II) acetate tetrahydrate
2 2 3 2 2
(
Ni(OAc) .4H O), terephthalic acid (H BDC), Piperazine, Formaldehyde, Methyl Orange(MO),
2
2
2
Methylene blue (MB), H O (30%, v/v), HCl, NaOH and KOH were purchased from Merck
2
2
Company. All the solutions were prepared with deionized water.
2
.2 Synthesis of ligand (H L)
4
The 1,4-piperazinediylbis(methylene) phosphonic acid, H O P-CH -NC H N-CH -PO H , was
2
3
2
4
8
2
3
2
1
31
13
prepared following the previous report [20] and characterized by IR, H NMR, P NMR , C
NMR (Figs. S1-S4) and chemical analysis (CHN).
2
.3. Preparation of MOFs

STA-12(M), (M= Mn, Fe, Co, Ni) were synthesized according to the published procedures with
slight modifications (supporting information) [21].
2
.4. Characterization
Elemental analyses were performed with a Heraeus CHN-O-RAPID elemental analyzer and
inductively coupled plasma atomic emission analyses (ICP-AES, Perkin-Elmer Optima
7
300DV). The IR spectra were recorded on a Shimadzu model 800 FT-IR spectrometer. Powder
X-ray diffraction(PXRD) data were collected on a X’Pert PRO MPD diffractometer using Cu-Kα
radiation. H, 13C, and 31P NMR spectra were carried out on a Bruker (Avance DRS)500
spectrometer. Diffuse reflectance UV/Vis spectra were recorded using an Avantes spectrometer
1
(Avaspec-2048-TEC model). The porosity and textural properties of catalysts were measured
°
using an Belsorp Mini II instrument. The samples were degassed in vacuum at 100 C for 6 h, the
nitrogen adsorption isotherms were measured at 77 K. Thermal stability was measured by TGA
(NETZSCH TG 209 F1 Iris) from 25 to 900 °C with heating rate of 10 °C/min.
2
.5. Photocatalytic degradation experiments
The photocatalytic activities of STA-12(M), (M= Mn, Fe, Co, Ni) were investigated by the
photodegradation of MB and MO under natural sunlight conditions (1 atm and 30 ℃).The photo-
degradation experiments were accomplished in open spaces where it was feasible to collect
appropriately the solar irradiation and in successive sunny days in fall months of October to
November between 10.00 AM and 2.00 PM. Desired amount of as-prepared MOFs was added
-1
into 50 mL of dyes aqueous solution (10 mgL ) in a 80 mL cylindrical vessel reactor. Before
exposed to sunlight, the mixture was stirred in the dark for 3 h to gain the adsorption/desorption
equilibrium, desired volume of known concentration of H O was then added and the reactor
2
2
placed in direct solar radiation. The mixture of the suspension was stirred evenly during the dyes
decomposition reaction. 2 mL of samples were collected at ordered intervals (10 minutes) with
centrifugation to separate catalyst for measure.
2
.6. Analytical methods
Determination of the dye concentration in the reaction mixture was carried out at maximum
absorption wavelength for MO (505 nm) and MB (664 nm) by using a Analytik Jena,
-1
-
SPECORD PLUS UV-vis spectrophotometer. Aqueous solutions of 0.1 molL HCl or 0.1 molL
1
NaOH were added for adjusting the pH of dye solution. Terephthalic acid was used as a probe
•
agent for detection of the hydroxyl radicals (OH ) generated in the interaction of MOF catalyst
and H O [22]. The processes were similar to the measurement of catalytic experiments in the
2
2
presence of MOF and H O under sunlight irradiation however the solution of dye was replaced
2
2
by a solution of 0.01mM sodium terephthalate (NaTA). Sampling was performed with the start

of photocatalytic reaction, and the analysis was carried out at 315 nm of excitation wavelength
by a RF-5301PC Shimadzu fluorescence spectrophotometer.
3
3
. Results and discussion
.1. Characterization of the metal-organic frameworks
The solid state structures of the as-synthesized samples were inspected by powder X-ray
diffraction (PXRD). Fig. 1 shows PXRD patterns of the MOFs. Obviously, the distinct and sharp
peaks in the diffraction patterns exhibited the well-defined crystallinity of the MOFs and are in
good compliance with the simulated pattern of STA-12 MOFs [23]. It should also be noted that
STA-12(Fe) showed broad and low intensity diffraction peaks in its X-ray pattern that can be
attributed to a lower crystallinity and some structural defects in its structure, which can enhance
the catalyst activity.
The IR spectra of MOFs are quite similar and the results were shown in Fig. 2. These spectra
-1
display a number of strong peaks between 1130 and 960 cm correspond to the stretching
vibrations of the phosphonate PO groups which are in agreement with reported data for STA-12
3
MOFs [24]. The existence of lattice water molecules interacting by hydrogen bonding was
-1
confirmed through broad bands around 3400 cm produced by the O-H stretching vibrations.
-1
2-
Bending vibration of water molecules appear at 1630 cm . δ(PO ) bands are found in MOFs at
3
-1
5
76 cm . These samples also exhibit C-H stretching vibrations and bending vibrations of CH
2
-1
attached to nitrogen atoms, near 2900 and 1458 cm , respectively. Therefore, the results of
powder X-ray diffraction analysis and FTIR spectra evidently approve the formation of STA-
1
2(M) structures.
Thermogravimetery analysis of MOFs show water loss below 150 °C, interpreted as due to
physically and chemically absorbed water. (Fig. S5) The total water loss calculations for
synthesized materials corresponding to chemical formulas of Mn L.4.14H O, Fe L.4.5H O,
2
2
2
2
Co L.7.4H O and Ni L.6.6H O. The curves have a plateau in the 150-320 °C range before a
2
2
2
2
progressive weight loss due to structure decomposition. Combustion of the ligand starts at 330
C and continues up to 800 °C. At 800 °C the weight loss trace for MOFs has a slight weight
rise, which can be attributed to the formation of metal pyrophosphate (M P O ) via incorporation
°
2
2
7
of oxygen from the flowing air in the experiment [21].
The adsorptive properties of as-prepared MOFs have been studied with N at 77 K. The samples
2
in Figure S6 exhibited type IV isotherms, with H3 typical hysteresis loop. The isotherms at high
relative pressure (0.3 < P/P0 < 1.0) demonstrated a high adsorption, indicating of the occurrence
of stacking macropores/mesopores in synthesized MOFs [25]. The corresponding textural

parameters of the photocatalysts are presented in Table S1. Among the synthesized materials, the
STA-12(Fe) has higher pore volume and BET surface area.
The diffuse reflectance UV-vis spectroscopy of the MOFs samples was recorded (Fig. S7). In the
visible region, catalysts exhibit many absorption bands, due to ligand field transitions of M(II)
ions, which are in agreement with reported data for STA-12(M) [21]. The characteristic
absorption peaks were observed in the range of 200-400 nm, which can be attributed to charge
transfer (LMCT) of O(II)→M(II) [21]. Clearly, the beginning of the highest absorption edges of
catalysts were located between 307-600 nm. According to the relation Eg = 1240/λ [26], the
optical bandgaps of MOFs are calculated (Fig. S8). Obviously, the outstanding spectral
properties in UV-vis region of STA-12(Fe) could provide an effective photo-activity.
3
.2. Photocatalytic properties of the MOFs
It is desirable to develop photocatalysts that would work in the presence of natural sunlight as
there are not many such catalysts [27]. Photo-degradation of MO and MB was conducted under
natural sunlight to evaluate the catalytic performance of synthesized MOFs. The destruction of
the dyes in the presence of STA-12(Mn), STA-12(Fe), STA-12(Co), STA-12(Ni) were
examined. In the presence of sunlight, the degradation of MO appears to be faster with STA-
1
2(Fe) than other synthesized MOFs (Fig. 3a). So, a number of experiments were intended to
elucidate the nature of photocatalytic degradation reaction of MO over STA-12(Fe) (Fig. 3b).
The adsorption experiment indicates that the dye molecule could not be absorbed by STA-
1
2(Fe), and the dye is rather stable under sunlight irradiation. Whereas the attendance of H O
2 2
could increase the degradation yield of MO up to 20% by producing reactive hydroxyl free
radical owing to the possible H O photolysis prompted by incident light (H O + sunlight →
2
2
2
2
•
¯
OH + OH ) [28]. With STA-12(Fe) alone under sunlight irradiation, about 42% MO removal
was achieved, signifying that the synthesized STA-12(Fe) have a moderate photocatalytic
activity. By the addition of H O , the degradation efficiency of dye over STA-12(Fe) under solar
2
2
irradiation was significantly increased. Given that the iron-based catalyst, after the addition of
H O , the Fenton process could also promote the decomposition of MO. So, we performed the
2
2
control experiment in the dark by mixture of H O and STA-12(Fe). As displayed in Fig. 3b,
2
2
about 12% of MO were degraded within 40 min, which indicates that STA-12(Fe) revealed
ability to the activation of H O and driving the Fenton degradation of MO. Nevertheless, the
2
2
efficiency in Fenton catalytic route was much lower than photo-Fenton STA-
1
1
2(Fe)/sunlight/H O catalytic method. The enhanced photocatalytic activity of STA-
2 2
2(Fe)/sunlight/H O system can be evidently verified by the variation of the UV-vis spectra of
2
2
the reaction mixture during the dye degradation (Fig. 3c). Also, the removal of MO almost
conformed pseudo-first-order kinetics, as demonstrated by the linear plot of ln(C /C) vs. process
0

time t (Fig. 3d). The degradation reaction of MO over STA-12(Fe) was more rapid with the
existence of both H O and sunlight (i.e., the STA-12(Fe)/sunlight/H O system), according to
2
2
2
2
-2
-1
the pseudo-first-order rate constants (kMSH=9.08× 10 min for STA-12(Fe)/sunlight/H O ;
2
2
-2
-1
-3
-1
k =1.37× 10 min for STA-12(Fe)/sunlight; k =3.4× 10 min for STA-12(Fe)/H O ) (Fig.
MS
MH
2
2
3
1
1
d). Besides, the MO removal rate constant in the sum of STA-12(Fe)/sunlight and STA-
-2
-1
2(Fe)/H O catalytic systems (1.71 × 10 min ) was much lower than that of STA-
2
2
-2
-1
2(Fe)/sunlight/H O catalytic system (9.08 × 10 min ), which confirms that the synergetic
2
2
effect of the STA-12(Fe), H O and sunlight in the photocatalytic procedure could provide
2
2
collaboratively to the catalytic activity for the MO decomposition. Also, we calculate the
synergetic index (SI): SI = kMSH/(kMH + k ) for the STA-12(Fe)/sunlight/H O system. The
MS
2
2
magnitude of SI was attained to be 5.31, suggesting this synergistic effect could increase the
catalytic efficiency of STA-12(Fe)by 431%.
For practical application goals, we investigated the effect of various operating parameters such
as H O dosage, initial pH, dyes concentration and the amount of catalyst on the photocatalytic
2
2
performance of STA-12(Fe)/sunlight/H O system for the degradation of MO and MB.
2
2
The effect of solution pH on the removal efficiency of dyes over STA-12(Fe)/sunlight/H O
2
2
system was examined at fixed amount of the catalyst as well as constant concentrations of the
dyes and H O at 30 °C. As presented in Fig. 4, the catalyst could work very efficiently over
2
2
acidic solution and the degradation rate of dyes diminished with the increase of pH and a further
increase of pH until 10 stopped the reaction. This results are similar to the Fenton reaction
usually performed at acidic condition. One probable reason is that for photocatalysts the
potential of the valence-band hole decreases when pH increases [29], thus the oxidizing ability of
a hole became weaker, causing in the reduce of the production rate of hydroxyl radical. Another
cause could be related to the instability and decomposition of H O in basic solution to O and
2
2
2
H O [30].
2
The influence of H O concentration on MO and MB degradations over STA-
2
2
1
2(Fe)/sunlight/H O system was examined and the results are revealed in Fig. 5. When H O
2 2 2 2
dosage increased from 6 to 10 mM for MO degradation and 6 to 12 mM for MB removal, the
degradation yields improved. This augmentation in decomposition rate is owing to an
enhancement in hydroxyl radicals generated by photocatalytic interaction of H O with the MOF.
2
2
•
Meanwhile, OH radicals are very reactive and oxidizing agent, they can attack to the dye
molecules and cause the degradation of the original solution. However, the rate of degradation
reduced with an increase in H O concentration from 10 to 12 mM for MO and 12 to 14 mM for
2
2
MB removal processes. One explain can be that excess H O molecules act as collector of
2
2
hydroxyl radicals to produce less reactive hydroperoxyl radicals, (Eq. 1) [31].

•
•
7
-1 -1
H O + OH → H O + HO , k = 2.7×10 M S (1)
2
2
2
2
1
•
This reaction decreased the possibility of attack of dye molecules by OH radicals, and
subsequently the degradation rate of dye to drop. Also hydroxyl radicals produced at higher
concentration can be dimerize to H O , (Eq. 2) [32].
2
2
•
9
−1 −1
2
OH → H O , k = 5.5×10 M S (2)
2 2 2
Accordingly, there is an optimum hydrogen peroxide dosage for the degradation of dyes.
The effect of original dyes concentration on the removal rates were also evaluated. As displayed
in Fig. 6, the catalytic degradation processes of MB and MO were more proficient at a lower
dyes concentration. The assumed reason is that with the increase in the dye concentration, more
and more dye ions are covered the active sites of catalyst, as a result, the generation of free
radicals on the surface is reduced [33]. Another reason maybe that the large molar extinction
coefficients of MO and MB dyes produce more impermeable solution to irradiation, which
decrease efficiencies of light exploitation and results in the low rate of photocatalytic processes.
The influence of the amount of catalyst on the decomposition of dyes was investigated while all
other effective parameters fixed. The results showed that the degradation rate enhanced with
increasing the dosage of catalyst (Fig. 7). This may be understood in terms of exposing the dye
molecules and hydrogen peroxide to more catalytic active sites encountered form STA-12(Fe)
sample.
3
.3. Recyclability and stability of STA-12(Fe)
The Reusability of STA-12(Fe) was tested by successive reactions in the catalytic degradation of
MO and MB over STA-12(Fe)/sunlight/H O system. As shown in Fig. 8, the catalytic
2
2
degradation efficiency of the STA-12(Fe) remained unchanged after four reaction runs for the
degradation of dyes. It confirms that the catalyst is highly stable and can be used for several
degradation reactions. Furthermore, STA-12(Fe) almost retained its primary color after four
catalytic cycles. Also, the XRD pattern and FTIR spectrum of the reused catalyst were found to
be very similar to that of as-prepared sample (Fig. S9), demonstrating that, the crystal and
chemical structure of the catalyst remain unchanged after four repeated reactions.
3
.4. Determination of Hydroxyl radical
To identify the reactive oxidizing species produced in the catalyst STA-
2(Fe)/sunlight/H O system, sodium terephthalic acid salt (NaTA) was used. sodium
1
2
2
terephthalate could react with hydroxyl radical to give 2-hydroxy terephthalic acid (HTA), which
shows a brilliant stable fluorescence [34]. This reaction is not affected by other reactive species
•¯
•
•
such as H O , O and HO , therefore it could be used as a selective probe in sensing OH
2
2
2
2
radicals [35]. As shown in Fig. 9a, in the fluorescence spectra of the solution containing the
catalytic STA-12(Fe)/sunlight/H O system and NaTA, peak intensity increases sharply to 540
2
2

•
after 10 min, indicating that OH radicals were truly produced in the system. However, when the
dye and NaTA were concurrently added into the reaction, the fluorescence intensity only
•
increasing to 70 after 10 min (Fig. 9b). An explanation may be that part of OH radicals reacted
quickly with dyes, as the dye degradation was detected during fluorescence measurement.
furthermore, the inhibitory effect of methanol, which is an effective trapping material for
hydroxyl radicals [36], was measured. Fig. 9c shows the effect of high concentrations of
•
methanol in inhibition of the photo-degradation reaction, indicating that OH radicals are the
main oxidizing species during the procedure.
Generally, the metal nodes of MOFs can be regarded as isolated semiconductor quantum dots,
which can be excited directly upon light irradiation or activated by the organic linkers acting as
the light absorption antenna [37]. Since the linker in STA-12(Fe) don't absorb visible light, the
fused Fe-NO clusters in this MOF are responsible directly for the sunlight absorption and the
5
related photocatalytic activity, as observed in reported iron-based metal-organic frameworks.
[38] According to this consideration, we suggested the probable mechanism for the activation of
H O with STA-12(Fe) under solar irradiation. As shown in Fig. 10, the STA-12(Fe) structure is
2
2
constructed from bisphosphonate anions and chains of Fe(II) octahedra sharing oxygen atoms of
phosphonate groups to create a 3D porous framework [23]. Under natural sunlight irradiation,
the iron-centered cluster in STA-12(Fe) adsorb received photons and responsible for the visible
light photocatalytic activity. Significantly, in the presence of H O , the positive synergistic
2
2
properties could facilitate the catalytic efficiency of STA-12(Fe). From the one hand, Fe(II) of
STA-12(Fe) can accelerate the decay of H O to generate hydroxyl radicals by the Fenton
2
2
process (Eqs. 3-5) [39].
H O + Fe(II) → Fe(III) + OH + OH (3)
−
•
2
2
+
•
H O + Fe(III) → Fe(II) + H + HO (4)
2
2
2
•
+
HO + Fe(III) → Fe(II) + O + H (5)
2
2
From the other hand, H O could adsorb the photoexcited electrons in the conduct band of STA-
2
2
1
2(Fe) to produce hydroxyl radicals (Eqs. 6-8).
+
−
MOF + hν → h + e (6)
−
•
−
H O + e → OH + OH (7)
[MOF]
2
2
+
•
+
H O + h
2
→ OH + H (8)
[MOF]
As a result, all of the above reactions could participate collaboratively to the activation of H O
2
2
and generate more and more amounts of hydroxyl radicals, hence significantly increasing the
decomposition efficiency of dyes.
3
.5. Analyses of degradation products

The materials in solution after complete degradation of MO by the STA-12(Fe) and H O were
2
2
examined by FTIR spectra and UV-vis spectroscopy. As shown in Fig. S10 the FT-IR spectra of
MO before and after degradation are totally different. Before degradation, methyl orange
-1
displayed the bands of the typical C=C aromatic ring stretching vibrations at 1607 cm , C-C
-1
−1
stretching vibrations at 1520 and 1422 cm , ring stretching vibrations at 1390 cm , C -N
Ar
-1
-1
stretching vibrations at 1368 cm , and H C-N stretching vibrations at 1223 and 1191 cm [40].
3
After degradation process, the loss of the above individual bands identified complete
decomposition of dye aromatic rings besides demethylation. At the same time, three new
-
multiple bands at 1115, 1353 and 1612 cm 1 raised, which must be owing to the stretching
2
−
−
−
vibrations of SO , NO and COO , respectively [40], indicating that the CO and some acids
4
3
2
are the main final products. Moreover, the TOC measurement shows that about 95% of the dye
was degraded into its mineral constituents after about 30 min. UV-vis spectrum of MO shows
one absorbance band at 505 nm and other band located at 326 nm. The conjugation system in the
structure of methyl orange is responsible for the absorption at 505 nm [41]. It was observed that
the absorbance at 505 nm disappeared very fast (Fig. 3c). This suggested a rapid destruction of
the aromatic system and consequently degradation of MO.
4
. Conclusions
A series of phosphonate-based MOFs, namely STA-12(M), has been prepared successfully by a
straight hydrothermal method and utilized for activation of H O to realize the highly efficient
2
2
photocatalytic decomposition of organic dyes under natural sunlight irradiation. The catalytic
behavior was strongly dependent on the various parameters, such as H O dosage, pH of solution
2
2
and initial dye concentration. In the presence of sunlight, the degradation of the dyes, MO and
MB, appears to be faster with STA-12(Fe) than other synthesized MOFs. Considerably,
degradation reaction of MO progressed about 6.6 and 26.7-fold faster in the STA-12(Fe)/sunlight
/
H O system relative to the STA-12(Fe)/sunlight and STA-12(Fe)/H O systems, respectively.
2 2 2 2
This rapid catalytic reaction was considered to be arise from the synergic effect of STA-12(Fe)
•
and H O under solar radiation, which was confirmed by detection of hydroxyl radicals (OH ) in
2
2
the degradation reaction. Additionally, STA-12(Fe) demonstrates excellent reusability and
stability after cyclic usage. It is expected that this study could present a new photo-Fenton
catalytic method for environmental protection and expands the application scope of MOFs in
photocatalysis, sunlight-driven water remediation and organic synthesis.
Acknowledgements
The authors appreciate the financial support from Research Council of the University of Birjand.

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Figure Captions:
Fig. 1. PXRD patterns of as-synthesized STA-12(M) samples: simulated XRD pattern of STA-
1
2 [23] (a), Mn (b), Fe (c), Co (d), Ni (e).
Fig. 2. FTIR spectra of as-synthesized STA-12(M) samples: Mn (a), Fe (b), Co (c), Ni (d).
Fig. 3. (a) Degradation of MO with STA-12(M) MOFs. (b) Degradation of MO under various
conditions. (c) The UV-vis spectral changes of MO. (d) Comparison of the apparent
reaction rate constants of the degradation of MO under different Conditions. Conditions:
-1
-1
MO, 10 mgL ; H O , 8mM; Catalyst, 0.08 gL ; and initial pH= 3.
2
2
-1
Fig. 4. Effect of pH on the degradation of MO and MB. Conditions: dye, 10 mgL ; H O , 8mM
2
2
-1
and Catalyst, 0.06 gL .
Fig. 5. Effect of H O concentration on the degradation of MO and MB. Conditions: dye, 10
2
2
-1
-1
mgL ; pH, 3 for MO, 3.5 for MB and Catalyst, 0.06 gL .
Fig. 6. Effect of initial dye concentration on the degradation of MO and MB. Conditions: H O ,
2
2
-1
1
0 mM for MO, 12 mM for MB; pH, 3 for MO, 3.5 for MB and Catalyst, 0.06 gL .
-1
Fig. 7. Effect of catalyst dosage on the degradation of MO and MB. Conditions: dye, 10 mgL ;
H O , 10 mM for MO, 12 mM for MB; pH, 3 for MO, 3.5 for MB.
2
2
Fig. 8. The photocatalytic degradation of MO (a) and MB (b) during four consecutive runs over
STA-12(Fe)/sunlight/H O system.
2
2
•
Fig. 9. OH-trapping PL spectra of STA-12(Fe)/sunlight/H O catalytic system, (a) in the
2
2
absence dye; (b) in the presence of dye and (c) effect of methanol inhibitor on the
degradation of MO.
Fig. 10. Proposed mechanism for the Fenton and photo-Fenton activation of H O by STA-
2
2
1
2(Fe) under natural sunlight irradiation.

Fig. 1.
Fig. 2.

Fig. 3.
Fig 4.

Fig. 5.
Fig 6.

Fig. 7.
Fig 8.

Fig 9.

Fig. 10.

Graphical abstract
Phosphonate-based MOF materials namely, STA-12(M)(M= Mn, Fe, Co, Ni), show
photocatalytic activity for the degradation of methylene blue (MB) and methyl Orange (MO)
from aqueous solution under natural sunlight irradiation. Thus, photo-Fenton oxidative
discoloration of dyes have been surveyed by H O catalyzed with the STA-12(M).
2
2