Construction of a mixed ligand MOF as “green catalyst” for the photocatalytic degradation of organic dye in aqueous media

Article abstract of DOI:10.1039/d1ra02994k

In the past few years, metal-organic frameworks (MOFs) have emerged as a class of fascinating materials for photocatalysis. Herein, a new MOF formulated as [Zn(bpe)(fdc)]·2DMF (BUT-206, bpe = 1,2-bis(4-pyridyl) ethylene, H₂fdc = 2,5-furan dicarboxylic acid, DMF =N,N-dimethylformamide) is reported, which was synthesized under solvothermal conditions and applied for photocatalytic degradation of dyes (crystal violet and rhodamine B). Noteworthily,BUT-206exhibited high photocatalytic activity toward the degradation of crystal violet without using any photosensitizer or cocatalyst under UV-irradiation. The photocatalytic degradation of crystal violet byBUT-206was effective with a degradation efficiency of 92.5% within 120 minutes. The effects of key parameters including pH, amount of photocatalyst and initial concentration of dye on the dye degradation processes were examined, and the kinetics of dye degradation was established by the pseudo-first order kinetic equation. Furthermore,BUT-206showed good cyclic stability in photocatalytic performance for up to five regeneration cycles, making it a potential green catalyst for dye degradation.

Full text of DOI:10.1039/d1ra02994k

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Construction of a mixed ligand MOF as “green
catalyst” for the photocatalytic degradation of
organic dye in aqueous media†
Cite this: RSC Adv., 2021, 11, 23838
Alamgir,‡a Khalid Talha,‡a Ying-Jie Wang,a Raza Ullah,a Bin Wang,a Lu Wang,a
a
Wei Wu,a Sha Chen,b Lin-Hua Xie
and Jian-Rong Lia
*
In the past few years, metal–organic frameworks (MOFs) have emerged as a class of fascinating materials for
photocatalysis. Herein, a new MOF formulated as [Zn(bpe)(fdc)]$2DMF (BUT-206, bpe ¼ 1,2-bis(4-pyridyl)
ethylene, H2fdc ¼ 2,5-furan dicarboxylic acid, DMF ¼ N,N-dimethylformamide) is reported, which was
synthesized under solvothermal conditions and applied for photocatalytic degradation of dyes (crystal
violet and rhodamine B). Noteworthily, BUT-206 exhibited high photocatalytic activity toward the
degradation of crystal violet without using any photosensitizer or cocatalyst under UV-irradiation. The
photocatalytic degradation of crystal violet by BUT-206 was effective with a degradation efficiency of
92.5% within 120 minutes. The effects of key parameters including pH, amount of photocatalyst and
initial concentration of dye on the dye degradation processes were examined, and the kinetics of dye
degradation was established by the pseudo-first order kinetic equation. Furthermore, BUT-206 showed
good cyclic stability in photocatalytic performance for up to five regeneration cycles, making it
a potential green catalyst for dye degradation.
Received 17th April 2021
Accepted 16th June 2021
DOI: 10.1039/d1ra02994k
rsc.li/rsc-advances
situ generation of highly reactive radical species (e.g. cOH and
cO2ꢀ) for the conversion of organic dyes into CO2 and H2O, has
1. Introduction
been ascertained to be a promising method, and most impor-
tantly it is highly efficient and economical.10–12
With the fast technological development and the growth of
industrialization, environmental protection, particularly water
pollution remediation, has become one of the most critical
Till now, different types of semiconductor photocatalysts,
starting from metal oxides to metal salts and their composites
have been used for dye degradation.13,14 However, these catalysts
suffer from electron–hole pairs recoupling, low energy
consumption efficiency, and easily clustering.15 With the
growing concern about environmental remediation and human
health, green technologies to degrade the contaminants in
wastewater with high efficiency and low cost are in high
demand.16 Particularly, high performance catalysts need to be
developed to efficiently handle these contaminants.17
Metal–organic frameworks (MOFs), as considered to be a class
of attractive and fascinating novel materials, constructed from
organic ligands and metal ions, have publicized great potency for
gas storage, separation, heterogeneous catalysis, sensing, proton
conduction.18–23,47,48 In addition, some MOFs show semi-
conducting behavior under light, suggesting that they can be
possibly used as photocatalysts.24,25 MOFs have many advantages,
such as unsaturated metal sites and the catalytically active
organic ligands as compared to traditional inorganic semi-
conductors.14 The terminated structural deciencies of MOFs are
the source of electron–hole recoupling and the electron–hole
recombination.26 Alvaro et al. rst demonstrated the degradation
of phenol by a MOF-5 based photocatalyst in 2007.27 Liu et al. and
Xia et al. later reported efficient MOF photocatalysts for the
issues in the present century faced by humans worldwide.1,2
A
huge amount of industrial wastewater is produced in many
industrial activities which oen contains a variety of wastes
including inorganic and organic compounds. The insufficient
treatment of these wastes produces discharge of organic
pollutants into surface water reservoirs, which poses a high risk
to the purity of water systems and human health.3,4 In partic-
ular, organic dyes are usually highly toxic, not easily bio-
degraded, stable in water, and even potentially human
mutagenic and carcinogenic.5,6 Therefore, to effectively remove
organic dyes from wastewater, a series of conventional and
advanced methods have been used, including ion exchange,
coagulation/occulation, adsorption, chemical oxidation and
photocatalysis.7–9 Among them, photocatalysis, in which the in
aBeijing Key Laboratory for Green Catalysis and Separation, Department of
Environmental Chemical Engineering, Beijing University of Technology, Beijing
100124, China. E-mail: xielinhua@bjut.edu.cn
bBeijing Key Laboratory on Regional Air Pollution Control, Faculty of Environment and
Life Sciences, Beijing University of Technology, Beijing 100124, P. R. China
† Electronic supplementary information (ESI) available. CCDC 1982845. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1ra02994k
‡ Alamgir and K. Talha contributed equally.
23838 | RSC Adv., 2021, 11, 23838–23845
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
degradation of organic dyes.28,29 A heterogeneous photocatalyst were analyzed by Gas chromatography-mass spectrometry (GC-
for the reduction of aqueous solution of CrVI ions and the MS) analysis. Photocurrent and Mott–Schottky measurements
degradation of organic dyes, was reported by Zhao et al.30 were carried out with a standard three-electrode system on a CHI
Recently, Dong et al. reported a 2D Zn-MOF for the degradation of 660E electrochemical work station (Chenhua Instrument,
organic dyes in water.10 Though these researches have demon- Shanghai, China). The Pt plate and typical Ag/AgCl were used as
strated the great of potency of MOFs for the photocatalytic counter and reference electrode, respectively. The electro-
degradation of environmental contaminants, the study on the chemical impedance spectroscopy (EIS) was measured by Zen-
photocatalytic properties of MOFs is still in early age. For nium electrochemistry work station with a bias potential of
example, most of the reported MOF-based photocatalysts showed ꢀ0.6 V and over the frequency range of 10ꢀ2 to 105 Hz.
high photocatalytic activity towards the degradation of organic
dyes while using cocatalyst such as H2O2. However, using such 2.2 Synthesis of MOF
cocatalyst in a large quantity during the decontamination of
Zn(NO3)2$6H2O (30 mg, 0.1 mmol), 2,5-furan dicarboxylic acid
wastewater is not an environmental-friendly strategy. Therefore,
(H2fdc) (10 mg, 0.061 mmol), and 1,2-bis(4-pyridyl) ethylene
it is in high demand to design a photocatalysts that can produce
(bpe) (6.5 mg, 0.036 mmol) were ultrasonically dissolved in
high photocatalytic efficiency in the absence of any photosensi-
2 mL of DMF in a 5 mL Pyrex vial and sealed. The reaction
system was then heated at 100 ꢁC for 48 hours in an oven. Aer
tizer or cocatalyst.30
Constructing MOFs with mixed ligands is a promising way to
cooling to room temperature, the resulting block shaped crys-
realize the desirable structure and property of MOFs. 1,2-Bis(4-
tals (yield: 80% based on the Zn) were collected by ltration, and
washed with DMF and acetone.
pyridyl) ethylene is a rigid organic ligand, and its conjugated
structure is helpful in electron transfer. Although a large number
of MOFs based on this ligand have been reported, their photo-
2.3 Single-crystal X-ray diffraction
catalytic activities have not been investigated.31–34 In this work, we
The X-ray diffraction data of as-synthesized BUT-206 were
report a new mixed-ligand MOF, [Zn(bpe)(fdc)]$2DMF (denoted as
together by means of an Agilent Supernova CCD diffractometer (a
BUT-206, bpe ¼ 1,2-bis(4-pyridyl) ethylene, H2fdc ¼ 2,5-furan
ꢀ
mirror monochromator, Cu-Ka source, l ¼ 1.54184 A). The
dicarboxylic acid, DMF ¼ N,N-dimethylformamide), which was
datasets were amended by empirical absorption correction using
synthesized under solvothermal conditions. BUT-206 exhibited
spherical harmonics, implemented in the SCALE3 ABSPACK
high photocatalytic activity towards the degradation of crystal
scaling algorithm. The structure was solved using direct methods
violet without using any photosensitizer or cocatalyst. In addition,
and polished by full-matrix least-squares on F2 with anisotropic
the photocatalytic activity of BUT-206 showed good stability up to
displacement using the SHELXTL soware package.35 Hydrogen
atoms of ligands were rened with isotropic displacement
parameters. The disorderly solvents in holes might not be shaped
in terms of atomic sites, but were cured by means of the MASK
routine in the Olex2 soware package.36 The crystal parameters
and structure renement of BUT-206 were summarized in Table
ve consecutive cycles. High photocatalytic activity in the absence
of any photosensitizer or cocatalyst and high recyclability enable
BUT-206 to be a potential green catalyst for the degradation of
organic pollutants in environmental decontamination.
2. Experimental section
S1 (for details, see CCDC 1982845).†
2.1 Materials and instruments
2.4 Photo-catalytic degradation experiments
All general chemicals and solvents (AR grade) were commercially
available and used as received. PXRD patterns were recorded on This experiment was conducted in aqueous media in order to
a BRUKER D8-Focus Bragg–Brentano X-ray powder diffractom- check the photo-catalytic degradation activity of crystal violet
eter equipped with a Cu sealed tube (l ¼ 1.54178) at room dye at room temperature. For this purpose, conditions were
temperature. Simulation of the PXRD pattern from the single- optimized. The solution of 100 mL was added to 250 mL glass
crystal structure was carried out by the diffraction crystal module beaker in which 0.5 g Lꢀ1 catalyst dose was added. The
of Mercury soware. FT-IR data was recorded on SHIMADZU IR concentration of solution was 5 ppm, while the pH ¼ 6. Time
Affinity-1 instrument in the range of 200–800 nm. TGA data was taken for experiment was 120 minutes at 25 ꢁC. Aliquant part of
obtained on a TGA-50 (SHIMADZU) thermogravimetric analyzer used sample was withdrawn for analysis at veritable time
ꢀ1
ꢁ
with a heating rate of 10 C min under nitrogen atmosphere. interval, ltered to eliminate the undissolved particles and then
Gas adsorption isotherms were reported by a volumetric method studied by UV-vis spectrophotometer, in which high pressure
using a Micromeritics ASAP2020 surface area and pore analyzer. UV-Hg source (80 W) was used as light source. To conrm the
UV-vis spectra and band gap were obtained with a UV-2600 accuracy in result, experimentation was performed in replicate
spectrophotometer in the range of 200–800 nm at room in order to obtained average results. Aer completion of single
temperature. Zeta potential (Pzc) of MOF samples at different pH cycle, the used MOF-based catalyst was recycled by ltration,
values was measured on Malvern Zetasizer (Malvern Instruments water-washed and followed by acetone, dried and then studies
Ltd, Malvern, UK). The morphology of MOF was examined their photo-constancy. The percentages, (X%) of the crystal
through SU-3500 scanning electron microscope (SEM) and violet dye degradation were measured by applying the eqn (1),
elemental distributions of MOFs structure were recorded through where C0 ¼ initial concentration of the crystal violet dye and C ¼
EDX analysis. Expected degradation products of crystal violet dye concentration at specied time interval.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23838–23845 | 23839

View Article Online
RSC Advances
Paper
a heating rate of 10 minꢀ1 under nitrogen atmosphere. TGA
curve showed that the guest molecules were removed before
220 ꢁC, and framework started to decompose at around 300 ꢁC
as depicted in Fig. S1b.† The PXRD pattern of the as-synthesized
sample of BUT-206 well matches the simulated one obtained
from its single-crystal data (Fig. 2a), indicating pure phase of
the sample. Aer water treatment at room temperature for 1
week, the PXRD pattern of the BUT-206 sample signicantly
changed (Fig. S6†), indicating a severe structural trans-
formation. Attempt to determine the structure of the sample
aer structural transformation failed. N2 adsorption measure-
ment at 77 K was performed for BUT-206, before the adsorption
experiment the sample was activated by acetone as shown in
Fig. S1(a),† only surface adsorption was observed, suggesting
the porous framework structure of BUT-206 may collapse aer
removal of the guest molecules.
C0 ꢀ Ce
X% ¼
ꢂ 100%
(1)
C0
3. Results and discussion
3.1 Single-crystal structure
Single-crystal X-ray diffraction data showed that BUT-206 crys-
tallizes in the monoclinic space group P21/c with a formula of
[Zn(bpe)(fdc)]$2DMF. In the asymmetric unit of BUT-206, there
are halves of Zn2+ ion, bpe and fdc2ꢀ ligands, and one guest DMF
molecule. Each Zn2+ ion is coordinated with two carboxylate O
atoms from two fdc2ꢀ ligands and two pyridine N atoms from two
bpe ligands in a tetrahedral geometry with the Zn–O and Zn–N
ꢀ
ꢀ
bond lengths ranging from 1.988(3) A to 2.053(4) A (Fig. 1a), and
the other two carboxylate O atoms of fdc2ꢀ ligands are weakly
bonded with the Zn2+ ions with a Zn/O distance of 2.67 A. Each
ꢀ
ligand (bpe or fdc2ꢀ) is bridging two Zn2+ ions. The alternate
connection of the Zn2+ ions and the ligands leads to a 2D
crumpled 44 layer (sql net) (Fig. 1b). The stacking of such 2D
layers along the b axis results in the 3D structure of BUT-206
(Fig. 1c). The stacking of these 2D layers is stabilized by the p–p
3.3 FT-IR spectra
In the FT-IR spectrum of BUT-206, the peaks at 1620–1680 cmꢀ1
are attributed to the stretching of C]C bonds of bpe ligands.
The peaks at 1510–1650 cmꢀ1 are corresponding to the asym-
metrical vibration of carboxylate groups of fdc2ꢀ ligands. The
strong absorption at 1661 cmꢀ1 resulting from the amide bonds
of DMF molecules conrms the existence of the guest mole-
cules. The peaks at 2800–3000 cmꢀ1 are due to the stretching of
C–H bonds in BUT-206, as shown in Fig. 2b.37
ꢀ
interactions between the bpe ligands (interplane distance: 2.9 A)
and C–H/O non-classical hydrogen bonding interactions (C/O
2ꢀ
ꢀ
distance: 3.32 A) between the fdc ligands on neighboring
ꢀ
layers. There are 1D rectangular channels in size of ꢃ9 ꢂ 6 A
along the b axis which are occupied by guest molecules (two DMF
molecules per formula) (Fig. 1d). The total potential solvent area
volume is 43.4% of the crystal volume as estimated by Platon.
3.4 pHpzc and band gap analysis
The surface charge property of the BUT-206 in the aqueous
suspension was determined in the form of zeta potential. The
3.2 TGA, PXRD and N2 sorption
The thermal stability of BUT-206 was accessed by a thermal- zeta potential of BUT-206 as a function of equilibrium pH of the
gravimetric analyzer in a temperature range of 25 to 800 ꢁC with suspension in the range of 2–10 was determined. The results
Fig. 1 (a) Optical photo of single crystals of BUT-206; (b) the coordination environment of Zn2+, (c) a 2D layer, (d) stacking of such 2D layers, (e)
weak interactions between neighboring 2D layers, and (f) the channels in BUT-206.
23840 | RSC Adv., 2021, 11, 23838–23845
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Fig. 2 (a) The simulated PXRD pattern of BUT-206 and PXRD pattern of as-synthesized sample of BUT-206; (b) the FT-IR spectra of BUT-206
before and after photocatalytic reaction; (c) the pHpzc of BUT-206.
showed that the surface of BUT-206 was negatively charged at organic cationic dyes (crystal violet and rhodamine B) as target
pH ¼ 6, 8 and 10, and its surface became positively charged pollutants. The effect of UV-light on the photo-degradation of
when pH ¼ 2 and 4. The pH values of point of zero charge crystal violet and rhodamine B dyes were rst tested by carrying
(pHpzc) for BUT-206 was about 4.9, as shown in Fig. 2c. The out control experiments under UV-irradiation in the absence of
pHpzc is a concept relating to the adsorption phenomena and is MOF. Entirely 3% degradation of both the dyes was observed
dened as the pH at which the surface of the catalyst is within 120 minutes of UV-irradiation (Fig. S4a†). For photo-
uncharged. At pH values higher than the pHpzc the catalyst catalytic degradation experiments with the presence of MOF,
surface is negatively charged and thus the adsorption of cation dye solutions containing the MOFs were agitated continuously
is favored, while at pH values lower than the pHpzc the surface of for 30 minutes in dark to reach the adsorption equilibrium. The
catalyst is positively charged and hence the adsorption of amount of crystal violet adsorbed on the MOF sample was
anions is favored.
found to be 5% was depicted in Fig. S4b.† Their distinctive
To explore the band gap uctuation, UV-vis spectrum of BUT- absorption peaks appearing at 589 nm and 554 nm were
206 was recorded. BaSO4 was used as a reference standard. monitored to check the photocatalytic degradation efficiency of
Band gap of BUT-206 was determined by applying Tauc relation BUT-206 towards crystal violet and rhodamine B. The degra-
(Ahn)n ¼ C(hn ꢀ Eg), where A is absorbance, h is the Planck's dation of crystal violet and rhodamine B were ascertained for
constant, n is frequency and Eg is the band gap energy. The n has 120 minutes under UV-light irradiation, the liquid samples were
values of 2 for the allowed direct and 1/2 for the allowed indirect withdrawn aer different intervals of time from the 5th to 120th
transitions. Therefore, (Ahn)n (y axis) was drawn versus hn (x minutes. The variations in the absorption intensity and
axis), which gave the band gap (Eg) value indicating its semi- concentrations of crystal violet and rhodamine B solutions Ct/C0
conducting property. Finally, the direct and indirect band gaps with the irradiation time were plotted. The photocatalytic
values for BUT-206 were calculated to be 3.35 and 2.74 eV, degradation efficiency of BUT-206 toward crystal violet was
respectively, as shown in Fig. S3a and b.†
92.5% while the degradation of rhodamine B was negligible at
the same conditions, as shown in Fig. 3 and Fig. S5,† respec-
tively. The tendency of degradation observed for rhodamine B
over BUT-206 suggested that there might be some factors which
are responsible for the non-quenching of rhodamine B. The
3.5 Photo-catalytic degradation
To explore the photocatalytic activity of BUT-206 for the degra-
dation of organic dyes in water, we selected two commonly used
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23838–23845 | 23841

View Article Online
RSC Advances
Paper
Fig. 3 (a) Absorption spectrum of crystal violet at different intervals of time; (b) the degradation of crystal violet under optimized conditions:
catalyst dose ¼ 0.5 g Lꢀ1, pH ¼ 6, concentration of crystal violet ¼ 5 ppm, time ¼ 120 minutes and temp. ¼ 25 ꢁC.
factors that affect the degradation process could be surface reactivity than cOH, so the degradation efficiency of crystal
charges of photocatalyst, size of the dye, pH of the solution and violet decreased at pH ¼ 2. Similarly, at lower pH a tough
concentration of dye plays an essential role in deciding the competition between crystal violet and Clꢀ anions with respect
potency of interaction between photocatalyst and dye molecule to cOH starts, which decreases the degradation efficiency of
under analysis, in this way completely affecting the degradation crystal violet. Therefore, it is desirable to increase the pH of
process. Absorption spectra of crystal violet dye at different electrolyte for the enhancement of degradation efficiency. The
intervals of time indicated that BUT-206 could be used as charges on catalyst surface become positive and negative at pHs
a selective catalyst for the photocatalytic degradation of crystal lower and higher than zeta potential, respectively. In the strong
violet in the absence of cocatalyst or photosensitizer as shown acidic condition with pH ¼ 2, the repulsive force between
in Fig. 3a and b. Hence, the effect of several factors on the positively charged catalyst surface and protonated crystal violet
catalytic performance of BUT-206 was systematically studied.
molecules prevent the approaching of crystal violet molecules to
the catalyst surface, where hydroxyl radicals formed. With the
rising of pH, this hurdle decreased and the maximum degra-
dation efficiency achieved at pH ¼ 6 as shown in Fig. 4a.38
The photocatalyst dose also affects the preliminary rate of
photocatalytic degradation of contaminants. Therefore, the effect
of the amount of BUT-206 on the degradation of crystal violet was
also examined in a succession of solutions by changing the
3.6 Effect of pH and dosage of the catalyst
Variations in the pH of electrolyte could alter the surface charge
of photocatalyst and displace the routes of some redox reac-
tions. The pH also disturbs the adsorption of substrates on the
surface of photocatalyst, and thus affects the reactivity and
reaction rate. The effect of pH on the photocatalysis perfor-
mance of BUT-206 was studied at room temperature. The
degradation efficiency of crystal violet increased by raising the
pH of solution from 4 to 6 and decreased beyond this limit
(Fig. 4a). In strong acidic condition, the inorganic radicals ClOc
can be produced via the reaction of Clꢀ ions with hydroxyl
radicals. These inorganic radicals express a much lower
amount of MOF catalyst (0.12, 0.25, 0.5, 0.75 and 1.0 g Lꢀ1
,
respectively) and the results are shown in Fig. 4b. It can be seen
that with the increase in the dose of MOF catalyst from 0.12 to
0.5 g Lꢀ1 the degradation efficiency increases and then decreases
when using relatively higher doses from 0.75 to 1.0 g Lꢀ1. With
the high concentration of catalyst, the non-transparency of
Fig. 4 Experimental parameters investigation for the photocatalytic degradation of crystal violet with BUT-206: (a) effect of pHs (2–10) on the
photocatalytic degradation performance; (b) effect of dose of MOF (0.12–1.0 g Lꢀ1) on the photocatalytic degradation performance.
23842 | RSC Adv., 2021, 11, 23838–23845
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
suspension, light scattering phenomenon and accumulation of
solid particles increases. Consequently, masking effect of excess
particles happens which cover some parts of the photosensitive
surface. Hereaer, the diffusion penetration of the photons
reduces and fewer catalyst particles can be initiated. Conse-
quently, the production of hydroxyl radicals and the degradation
efficiency decreases. The optimum degradation rate was ascer-
tained in the presence of 0.5 g Lꢀ1 catalyst and this optimized
value was used in the succeeding studies.39,40
C0
C
ln
¼ kapp ꢂ t
(2)
3.8 Mechanism of photocatalytic activity
The possible mechanism for the photocatalytic degradation of
crystal violet by BUT-206 is proposed as follows. Once UV-light
strikes the aqueous solutions of crystal violet containing BUT-
206 as photocatalyst, the electrons in the valance band of BUT-206
are excited and absorb photons of energy and moves to the
conduction band. These photons of energy are equal to or larger
than the band gap of the semi-conductor, and aer absorbing
these photons by conduction band, positive holes in the valance
band are formed. The excited electrons can either recoupling with
positive holes or react with dissolved O2 molecules to form reactive
superoxide anion radical (O2cꢀ). Correspondingly, water molecules
can be oxidized by positive charged holes which are adsorbed on
the surface of catalyst to form extremely active hydroxyl radical
(cOH). These produced cOH and O2cꢀ bearing high oxidation
potency and therefore stimulate the degradation of organic
pollutants. These outcomes prove that both the radicals O2cꢀ and
cOH contributed to the photocatalysis processes, but the processes
were mostly depended on the hydroxyl radicals. Consequently, the
following route may show the potential photocatalytic degradation
mechanism of crystal violet in water by the photocatalyst BUT-206
under UV-irradiation.
3.7 Effect of initial concentration of dye
To explore the effect of initial concentration of crystal violet on
the photocatalytic performance of BUT-206, four different
concentrations of the dye solutions, namely 5, 10, 15, and 20 ppm
were used for the control experiments. The results showed that
initial concentration of crystal violet had an obvious inuence on
the photocatalytic degradation rate (Fig. 5a). The degradation
rate using BUT-206 as catalyst declines with the rising of initial
concentration of crystal violet. It might be owing to the reason
that only a certain amount of cOH radicals could be generated
with a certain quantity of photocatalyst and UV-light, which then
reacted with the different initial concentrations of crystal violet.
Additionally, the opaqueness of solution rises with the increasing
of dye concentration and diffusion of UV-light over the solution
turns hard, generating comparatively a smaller amount of the
extremely reactive cOH radicals.41 To investigate the kinetics of
the photo-catalytic degradation process, the values of ln(C0/C)
versus irradiation time was plotted using different initial
concentrations of the crystal violet. The degradation processes
can be well modelled by the pseudo-rst order kinetic equation
as given in eqn (2), where C0 is the initial concentration of crystal
violet and C is the concentration at time t, while kapp is the
apparent rate constant and t is the time. A linear relationship can
be obtained between ln(C0/C) vs. irradiation time for different
initial concentrations of crystal violet dye solutions as shown in
Fig. 5b. The rate constant values kapp (minꢀ1) were estimated
from the linear tting of the plot with the pseudo-rst order
kinetic equation, and the attained results are listed in Table 1.
MOF + hn / eꢀ + h+ + MOF
(3)
The positive holes h+ (charge carrying body in semi-
conducting materials) combining with H2O and the excited
electrons combining with O2 on the surface of MOF can be
shown as follows:
h+ + H2O / OHc + H+
eꢀ + O2 / O2cꢀ
(4)
(5)
The highly reactive OHc and O2cꢀ oxidize the crystal violet
Obviously, the rate constant values decrease as the increasing of molecules (C25H30ClN3), which are adsorbed on the surface of
ꢀ
the initial concentrations of crystal violet dye solutions.
the BUT-206, producing CO2, H2O, NO3 and some other
Fig. 5 (a) Photocatalytic degradation of crystal violet at different initial concentrations; (b) pseudo-first order degradation kinetics using different
initial concentration of crystal violet; experimental conditions: pH ¼ 6, dosage of catalyst ¼ 0.5 g Lꢀ1, time ¼ 50 minutes and temp. ¼ 25 ꢁC.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23838–23845 | 23843

View Article Online
RSC Advances
Paper
Table 1 Observed rate constants and degradation efficiency (%) at
different initial concentrations of crystal violet
MOF sample was used over ve consecutive cycles to examine
their photocatalysis performance, and the results are shown in
Fig. 6. The degradation efficiency declined slightly aer ve
consecutive cycles, which might be due to the irreversible
adsorption of contaminant molecules on active sites of the
catalyst. The PXRD pattern and SEM image were also recorded
for the MOF sample aer 5th regeneration cycles. As shown in
Fig. S6 and S7,† the results suggested that the structure of the
MOF changed during the reactions, although the photocatalytic
activity largely retained.
Conc. of crystal violet Degradation efficiency
(ppm)
(%)
kapp (minꢀ1
)
R2
5
69.9
60.5
31.6
22.0
0.024
0.019
0.008
0.005
0.983
0.984
0.991
0.991
10
15
20
comparatively less toxic products.42–44 The molecular structures
of these expected minor degradation products of crystal violet
dye are listed in Table S2.†45,46 To conrm the formation of OHc
and O2cꢀ radicals, ESR spectra were recorded for the solution
containing BUT-206 under full light irradiation. As shown in
Fig. S8,† typical signal peaks for the hydroxyl radicals and
superoxide anion radicals were observed, although the signal
intensities are relatively low, suggesting that these radicals were
formed in the photocatalytic reactions with BUT-206 as catalyst.
To explore the charge transfer efficiency of BUT-206, the
photocurrent, electrochemical impedance spectra (EIS), and
Mott–Schottky measurements were also carried out (Fig. S9†).
4. Conclusion
In summary, we synthesized a new MOF denoted as BUT-206 by
reacting zinc salt with a dicarboxylate ligand fdc2ꢀ and a neutral
ligand bpe. The structural architecture of MOF suggested that
“one-pot” technique is a controllable method to design and
synthesize functional mixed ligand MOF materials for selective
photocatalytic applications. It has been established that BUT-
206 is a highly efficient photocatalyst for the degradation of an
aqueous solution of dye (crystal violet) without using any
photosensitizer or cocatalyst. In addition, the MOF showed
a good regeneration performance up to ve cycles. This unique
property suggested that BUT-206 is an economical and green
potential candidate for wastewater treatment. It is predicted
that more effective MOF-based photocatalysts can be synthe-
sized toward the objective of green environmental restitution
and photocatalytic degradation of dye-containing wastewater
can be transmitted by using MOF materials as environmentally-
sound and energy efficient photocatalysts.
C25H30ClN3 + (OHc, O2cꢀ) / CO2 + H2O + NO3
ꢀ
+ other simple fragments
(6)
3.9 Recyclability
The recyclability of the BUT-206 photocatalyst is an important
parameter for the economical and green water treatment. To
explore the feasibility of BUT-206, its recyclability was examined
over ve consecutive cycles. Aer each cycle, the sample was
restored by centrifugation and washed consecutively for three
times with deionized water followed by acetone, and ethanol.
The complete process was as follows: the regenerated sample
was immersed into 5 mL of solvent in a vial and sonicated for
a while, and then ltered. The restored sample was dried at high
vacuum overnight, and then use again for the next cycle. The
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors would like to acknowledge the nancial support
from National Key R&D Program of China (Grant No.
2018YFC1903105), and National Natural Science Foundation of
China (Grant No. 22038001, 21771012 and 51621003).
References
1 Z. Wu, X. Yuan, J. Zhang, H. Wang, L. Jiang and G. Zeng,
ChemCatChem, 2017, 9, 41–64.
2 J. Liu and J. Diamond, Nature, 2005, 435, 1179–1186.
3 V. K. Sharma and M. Feng, J. Hazard. Mater., 2019, 372, 3–16.
4 T. Zhang and W. Lin, Chem. Soc. Rev., 2014, 43, 5982–5993.
˜
´
´
5 J. Bedia, V. Muelas-Ramos, M. Penas-Garzon, A. Gomez-
´
´
Aviles, J. J. Rodrıguez and C. Belver, Catalysts, 2019, 9, 52.
6 C.-C. Wang, J.-R. Li, X.-L. Lv, Y.-Q. Zhang and G. Guo, Energy
Environ. Sci., 2014, 7, 2831–2867.
7 G. Velegraki, J. Miao, C. Drivas, B. Liu, S. Kennou and
G. S. Armatas, Appl. Catal., B, 2018, 221, 635–644.
8 A. K. Verma, R. R. Dash and P. Bhunia, J. Environ. Manage.,
2012, 93, 154–168.
Fig. 6 Reproducibility of BUT-206 for the photocatalytic degradation
of crystal violet up to five cycles.
23844 | RSC Adv., 2021, 11, 23838–23845
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
9 Y.-J. Cheng, R. Wang, S. Wang, X.-J. Xi, L.-F. Ma and 30 H. Zhao, Q. Xia, H. Xing, D. Chen and H. Wang, ACS
S.-Q. Zang, Chem. Commun., 2018, 54, 13563–13566. Sustainable Chem. Eng., 2017, 5, 4449–4456.
10 J.-P. Dong, Z.-Z. Shi, B. Li and L.-Y. Wang, Dalton Trans., 31 G. Xiong, Y. Wang, Y. Sun, L. You, B. Ren, Z. Xu, Y. He,
2019, 48, 17626–17632.
11 M.-W. Zhang, K.-Y. A. Lin, C.-F. Huang and S. Tong, Sep.
Purif. Technol., 2019, 227, 115632.
12 Q. Xiang, J. Yu and M. Jaroniec, Chem. Soc. Rev., 2012, 41,
782–796.
13 Z.-a. Zong, C.-b. Fan, X. Zhang, X.-m. Meng, F. Jin and
L. Ruhlmann and F. Ding, Eur. J. Inorg. Chem., 2018, 8,
1038–1046.
32 K. K. Bisht, Y. Rachuri, B. Parmar and E. Suresh, J. Solid State
Chem., 2014, 213, 43–51.
33 D.-M. Chen, N. Xu, X.-H. Qiu and P. Cheng, Cryst. Growth
Des., 2015, 15, 961–965.
Y.-h. Fan, Microporous Mesoporous Mater., 2019, 282, 82–90. 34 Y. Liu, D. Chen, X. Li, Z. Yu, Q. Xia, D. Liang and H. Xing,
14 M. Zhang, L. Wang, T. Zeng, Q. Shang, H. Zhou, Z. Pan and
Q. Cheng, Dalton Trans., 2018, 47, 4251–4258.
15 T. Zeng, L. Wang, L. Feng, H. Xu, Q. Cheng and Z. Pan,
Dalton Trans., 2019, 48, 523–534.
Green Chem., 2016, 18, 1475–1481.
35 J. F. Eubank, L. Wojtas, M. R. Hight, T. Bousquet,
V. C. Kravtsov and M. Eddaoudi, J. Am. Chem. Soc., 2011,
133, 17532–17535.
16 J. Li, X. Wang, G. Zhao, C. Chen, Z. Chai, A. Alsaedi, T. Hayat 36 G. Sheldrick, SADABS, Program for scaling and correction of
¨
and X. Wang, Chem. Soc. Rev., 2018, 47, 2322–2356. area detector data, University of Gottingen, Germany, 1997.
17 X. Liu, B. Liu, G. Li and Y. Liu, J. Mater. Chem. A, 2018, 6, 37 O. V. Dolomanov, A. J. Blake, N. R. Champness and
¨
M. Schroder, J. Appl. Crystallogr., 2003, 36, 1283–1284.
17177–17185.
18 N. C. Burtch, H. Jasuja and K. S. Walton, Chem. Rev., 2014, 38 D. Singh and C. Nagaraja, Dalton Trans., 2014, 43, 17912–
114, 10575–10612. 17915.
19 O. M. Yaghi and A. R. Millward, US Pat., US7799120B2, 2010. 39 K. Talha, B. Wang, J.-H. Liu, R. Ullah, F. Feng, J. Yu, S. Chen
20 T. Yamada, K. Otsubo, R. Makiura and H. Kitagawa, Chem.
Soc. Rev., 2013, 42, 6655–6669.
21 Z. Xie, L. Ma, K. E. deKra, A. Jin and W. Lin, J. Am. Chem.
Soc., 2010, 132, 922–923.
22 J.-R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009,
38, 1477–1504.
23 K. Talha, T. He, L.-H. Xie, B. Wang, M.-J. Zhao, Y.-Z. Zhang,
Q. Chen and J.-R. Li, New J. Chem., 2020, 44, 11829–11834.
and J.-R. Li, J. Environ. Chem. Eng., 2020, 8, 103642.
40 L. Ai, C. Zhang, L. Li and J. Jiang, Appl. Catal., B, 2014, 148,
191–200.
41 R. Ullah, C. Liu, H. Panezai, A. Gul, J. Sun and X. Wu, Arabian
J. Chem., 2020, 13, 4092–4101.
42 Y. Pan, W. Liu, D. Liu, Q. Ding, J. Liu, H. Xu, M. Trivedi and
A. Kumar, Inorg. Chem. Commun., 2019, 100, 92–96.
43 A. Nezamzadeh-Ejhieh and A. Shirzadi, Chemosphere, 2014,
107, 136–144.
´
24 C. G. Silva, A. Corma and H. Garcıa, J. Mater. Chem., 2010, 20,
3141–3156.
44 Y.-H. B. Liao, J. X. Wang, J.-S. Lin, W.-H. Chung, W.-Y. Lin
and C.-C. Chen, Catal. Today, 2011, 174, 148–159.
45 R. Ullah, J. Sun, A. Gul and S. Bai, J. Environ. Chem. Eng.,
2020, 8, 103852.
25 S. N. Habisreutinger, L. Schmidt-Mende and J. K. Stolarczyk,
Angew. Chem., Int. Ed., 2013, 52, 7372–7408.
26 J.-D. Xiao and H.-L. Jiang, Acc. Chem. Res., 2018, 52, 356–366.
´
27 M. Alvaro, E. Carbonell, B. Ferrer, F. X. Llabres i Xamena and 46 G. Parshetti, S. Parshetti, A. Telke, D. Kalyani, R. Doong and
H. Garcia, Chem.–Eur. J., 2007, 13, 5106–5112. S. P. Govindwar, J. Environ. Sci., 2011, 23, 1384–1393.
28 C. X. Liu, W. H. Zhang, N. Wang, P. Guo, M. Muhler, 47 T. He, Z. Huang, S. Yuan, X.-L. Lv, X.-J Kong, X. Zou,
Y. Wang, S. Lin, Z. Chen and G. Yang, Chem.–Eur. J., 2018,
24, 16804–16813.
H.-C. Zhou and J.-R. Li, J. Am. Chem. Soc., 2020, 142,
13491–13499.
29 Q. Xia, X. Yu, H. Zhao, S. Wang, H. Wang, Z. Guo and 48 L.-H. Xie, X.-M. Liu, T. He and J.-R. Li, Chem, 2018, 4, 1911–
H. Xing, Cryst. Growth Des., 2017, 17, 4189–4195.
1927.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 23838–23845 | 23845