A metal-organic framework based multifunctional catalytic platform for organic transformation and environmental remediation

Article abstract of DOI:10.1039/c7dt04057a

Given the need for an efficient reaction platform, a multifunctional material has been developed through the integration of iodine into a Cd²⁺ based MOF as a new catalytic system for organic transformation. Furthermore, the Cd-MOF itself has been successfully used as a potential material for photocatalytic dye degradation. The iodine incorporated MOF showed tremendous efficiency in organic catalysis for the synthesis of a library of benzimidazole derivatives with yield >80%. In addition, the same iodine loaded MOF (I₂@Cd-MOF) exhibits good applicability and reusability for the synthesis of thienyldipyrromethane. Apart from this, the photocatalytic activity of the Cd-MOF was also evaluated under visible light irradiation which revealed excellent photocatalytic activity (82%) for MB degradation without the use of any enhancers. Altogether, this catalytic platform established its efficacy as a multifunctional catalyst in the organic synthesis of biologically active motifs and photocatalytic dye degradation.

Full text of DOI:10.1039/c7dt04057a

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Venkateswarulu, S. Kumar, V. Krishnan and R. R. Koner, Dalton Trans., 2017, DOI: 10.1039/C7DT04057A.
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January 2016 Pages 1–398
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Metal-organic framework based multifunctional catalytic platform
for organic transformation and environmental remediation
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
*b
Harpreet Kaur, M. Venkateswarulu, Suneel Kumar, Venkata Krishnan and Rik Rani Koner
DOI: 10.1039/x0xx00000x
Given the need of efficient reaction platform, a multifunctional material has been developed through
www.rsc.org/
2
+
integration of iodine into a Cd based MOF as a new catalytic system for organic transformation. Further,
the Cd-MOF itself has been successfully used as potential material for photocatalytic dye degradation. The
iodine incorporated MOF showed tremendous efficiency in the organic catalysis for the synthesis of a library
2
of benzimidazole derivatives with yield > 80%. In addition, the same iodine loaded MOF (I @Cd-MOF)
exhibits good applicability and reusability for the synthesis of thienyldipyrromethane. Apart from this, the
photocatalytic activity of Cd-MOF was also evaluated under visible light irradiation which revealed excellent
photocatalytic activity (82%) for MB degradation without the use of any enhancer. Altogether, this catalytic
platform established its efficacy as a multifunctional catalyst in organic synthesis of biologically active motifs
and photocatalytic dye degradation.
.
select an innovatively designed and developed Cd-MOF based on
5
amino functionalized terephthalic acid and 4,4’bipyridine linker.
Introduction
We have explored the functional Cd-MOF for the development of a
new MOF-iodine integrated catalytic system for various organic
transformations as well as for the phtocatalytic degradation of a
model pollutant (Scheme 1 and S1). In order to investigate its
catalytic applicability and to understand the releasing profile of
iodine, we have chosen different organic reactions which resulted
into biologically active products.
In recent times, there is great demand for developing
multifunctional materials for several practical applications. Among
the various kinds of materials, metal organic frameworks (MOFs)
have been found to be promising materials due to their vast
applications such as gas storage and separation, drug delivery,
sensing, magnetism, nonlinear optics, luminescence, proton
1
-7
conduction and energy storage etc.
MOF derived hybrid
After detailed characterization of iodine incorporation, we
materials have also attracted immense attention particularly in
asymmetric catalysis, organic catalysis, electrocatalysis and
employed
2
I @Cd-MOF for the synthesis of benzimidazole
8
-12
derivatives, known for pharmacological activity, catalysis and
photocatalysis in comparison to other porous materials.
The
1
6-18
various other applications.
Although synthetic procedures
as a catalyst have also been reported
open metal centres of MOF materials offer an active reaction sites
as well as tunable porosity with large surface area, which allow the
substrates and guest species to get into the channels and access the
employing simple molecular I
2
1
9
earlier, the drawbacks have been the high reaction temperature,
longer reaction time and low yield. Successful iodine incorporation
enabled us to improve reaction time substantially (4 times lower)
along with considerable enhancement in reaction yield as
compared to reaction in the presence of iodine alone. In addition,
1
3
internal reaction center.
Although, a number of catalytic MOFs and iodine loaded MOFs have
1
4-15
already been developed and reported,
MOFs as
a
b
2
multifunctional catalytic platform has not been explored in detail.
2
we have also employed this I @Cd-MOF for the synthesis of thienyl
The heterogroup functionalized linkers based MOFs have been
preferred for better catalytic activity and particularly MOFs with
amino functionalized linkers already exhibited better catalytic
activity compared to non-functionalized MOF. This inspired us to
dipyrromethane which pronounced to be the important precursor
in the synthesis of porphyrin and boron-dipyrromethanes (BODIPY)
20-22
analogs.
Apart from the organic catalysis, we have also explored the
potential of this Cd-MOF for photocatalytic degradation of
methylene blue (MB) efficiently. Methylene Blue is one of the
widely used well known dye in various fields and, the non-
degradation nature of MB makes serious impact on environment.
Even a small amount of MB causes harmful effects which includes
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haemolytic anemia in infants, vomiting, chest pain, fever, areas were measured by using Quantachrome Autosorb-iQ-MP-XR
2
3-24
hypotension and bluish discoloration of skin.
system at 77K.
Single crystal X-ray diffraction studies
Given the adverse effect of MB, the amino functionalized Cd-MOF
was employed for the degradation of MB to evaluate its
photocatalytic efficiency. This MOF exhibited very high efficiency in
the catalytic degradation of MB without the use of any
Crystals of Cd-MOF were grown by solvothermal method for single
crystal X-ray diffraction analysis. The data were collected using
Crysalis Pro Software (online version) and CrysalisPro Software
(
offline version) was used for data reduction. Olex2 and SHELXS (1,
) direct methods were used to solve the structure of Cd-MOF. Full-
matrix least squares on F2 were used to refine the data.
Synthesis of Cd-MOF(C18 CdN
Cd-MOF was synthesized following the reported procedure with
2 2 3
photocatalytic enhancers such as H O , KBrO , RGO etc.
2
H
8
3 4
O )
5
slight modifications (Scheme 1 and S1).
Cd(NO3)
2 2
.6H O
(
0.156g, 1mmol), 4,4’-bipyridine (0.075g,
1mmol), 2-
aminoterephthalic acid (0.030g, 1mmol) and DMF(3 mL)
were mixed and sealed in a 25 mL Teflon lined acid digestion bomb,
0
then placed inside oven followed by heating at 120 C for 2 days.
After cooling to room temperature, the rod shape crystals were
obtained by filtration, then washed with DMF/Ethanol (1:1) mixture
(
5mL). Finally, crystals were dried under vacuum for 24h. Yield:
1
6
1
8.8%. FT-IR (KBr, ν in cm- ): 3331, 2402, 1660, 1535,1527, 1392,
254, 1218, 1157, 848, 780.
Iodine adsorption and desorption
Iodine adsorption study was performed by immersing 100mg of Cd-
MOF crystals to 5 mL of 0.1 M hexane solution of iodine in a glass
Scheme 1 Schematic pathway of Cd-MOF for photocatalysis and
organic transformation.
25
vial. The colour of the iodine solution gradually started diminishing
which indicated the adsorption of iodine over MOF within 5 h (Fig.
1
). Further, the solution was filtered and the crystals were given
Experimental Section
Materials and Methods
washings with fresh hexane solution (4-5 times) to remove free
iodine present at the crystals surface. Next iodine loaded MOF
Cadmium nitrate, 4,4’- bipyridine, 2-aminoterephthalic acid, DMF
were purchased from Alpha Aesar. All reagents and solvents were
used as such without any further purification. TGA was performed
on Perkin Elimer Analyzer under nitrogen with heating rate of
crystals (I @Cd-MOF) were obtained by air drying at room temp
2
(
Fig. S2). Furthermore, the desorption of iodine loaded in MOF was
also performed by immersing it in ethanol for 24h (Fig.S3).
0
0
1
0 C/min from 30 to 800 C. Powder X- ray diffraction
measurements were done using an Aligent Supernova X-Ray
Diffractometer using Cu-Kα radiations at 45 kV and 40 mA with 2θ
0
0
0
value ranging from 5 to 80 with scan rate of 2 /min and scan step
0
of 0.02 . Perkin Elimer spectrum 2 spectrophotometer was used for
recording FT-IR spectra. Jeol ECX-500MHz spectrometer (internal
1
13
standard: TMS) was used for recording H and C-NMR spectra in
DMSO-d . The absorption spectra was measured using SHIMADZU
6
UV-2450 spectrophotometer. The morphology was measured on FEI
Nova Nano SEM-450, field emission scanning electron microscopy
(
(
FE-SEM) and a high resolution transmission electron microscope
HRTEM) FEI Tecnai G2 20 S-twin microscope with operating voltage
of 200kV. Energy dispersive X-ray spectra (EDAX) were also
obtained using the same instrument. X-ray photoelectron
spectroscopic (XPS) measurements were done with the use of
SCIENTA, R-3000 analyzer with dual anode Al k
source operating at anode voltage of 12 kV and filament current of
3 mA. The data obtained from the instrument was solved with the
help of CASA software. N adsorption isotherms and BET surface
α
(1486.6 eV) as a
Fig.1 UV-vis spectra of 0.1M hexane solution of Iodine at 0 min and
1
80 min respectively.
2
Catalytic performance
2
2
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The I @Cd-MOF was explored as efficient catalyst toward the 3.04(s, 6H) ppm. HRMS: (m/z) calculated for C H N [M+1]
2
15 15 3
synthesis of benzimidazole derivatives from substituted 238.1300 found 238.1355.
2
benzaldehydes and o-phenylenediamines using I @Cd-MOF as well
5
-(2-thienyl)dipyrromethane (2a): Thiophene-2-carboxaldehyde
as the synthesis of thienyl dipyrromethane derivative.
(
0.200g, 1mmol), pyrrole (0.300g, 5mmol) and I @Cd-MOF (0.030g)
2
General Procedure
were mixed and stirred at room temperature. The reaction progress
was monitored by TLC. The reaction color became green after some
2-phenyl-1H-benzo[d]imidazole (1a): Benzaldehyde (0.106g,
1
mmol), o-phenylenediammine (0.108g, 1mmol),
I
2
@Cd-MOF time and changes to somewhat red which indicates the completion
(5mmol) were mixed in DMSO of the reaction. After 5 h, 5 mL of dichloromethane was added to
5mL) and stirred at room temperature. Reaction progress was the reaction mixture and then filtered. The filtrate was treated with
SO
mixture was diluted with DCM and then filtered to separate the The obtained viscous red compound was purified by column
catalyst. To this filtrate, saturated aqueous solution of Na was chromatography (1:1 ratio of hexane and dichloromethane
added to remove the coloration due to iodine. Then the reaction mixture). Yield: 68%.
mixture was extracted with DCM:H O (3 times). The extract was
2 4
(0.150g, 33mmol%) and Na SO
(
monitored by TLC. After the completion of the reaction (6h) the 5 mL of water three times and then dried over anhydrous Na
2
4
.
2 2 3
S O
1
2
H-NMR(500MHz, DMSO-d
H), 6.81(d, 1H), 6.69(m,2H), 6.16(m,2H), 6.04(m,2H), 5.7(s,2H)
ppm. HRMS: (m/z) calculated for C13 S [M-1] 229.0769 found
29.0569.
Photocatalytic Dye Degradation
6
) δ: 8.06(bs,2H), 7.2(m,1H), 6.89-6.95(m,
then washed with brine, dried over Na
2
SO
obtain the product. Yield: 85%. H-NMR (500MHz, DMSO-d
.29(m, 2H), 6.80-6.69(m, 5H), 6.4(m, 2H) ppm. HRMS: (m/z)
calculated for C13 [M+1] 195.0922 found 195.0916. The
recovered catalyst was dried and used further for the next cycle.
-methyl-2-phenyl-1H-benzo[d]imidazole (1b): This was
4
and evaporated to
1
1
6
) δ:
13 2
H N
7
2
11 2
H N
The photocatalytic activity of Cd-MOF was studied by performing
the photocatalytic degradation of MB dye by illuminating under
visible light at room temperature and pressure using a home built
6
synthesized from benzaldehyde (0.106g, 1mmol) and 4-
methylbenzene-1,2-diamine (0.134g, 1mmol) using the same
2
6
photocatalytic reactor equipped with visible light source.
procedure like 1a and purified by column chromatography with
1
Typically, photocatalyst Cd-MOF (40 mg) was dispersed in 100 mL
MeOH/DCM (.5%). Yield: 75%. H-NMR (500MHz, DMSO-d
6
) δ:
-5
-1
of 5 × 10 M (0.0159L ) aqueous solution of MB. Before irradiation,
the resultant solution was stirred magnetically in dark for 30 min to
obtain the adsorption- desorption equilibrium in between the dye
and the photocatalyst. As the equilibrium was set up, the
suspension was illuminated under visible light. After each 15 min, 1
mL of the sample solution was collected, centrifuged and
transferred for UV-analysis. Simultaneously, the same set of
experiment was carried out without any MOF catalyst to monitor
the self-photosensitization of MB. In addition, same experiment
with Cd-MOF catalyst was performed in dark (in the absence of
visible light) to confirm whether there is any adsorption of dye
1
2
2
2.7(bs, 1H), 8.15(d, 2H), 7.56-7.45(m, 4H), 7.37(s, 1H), 7.02(d, 1H),
.42(s, 3H) ppm. HRMS: (m/z) calculated for C H N [M+1]
1
4 12 2
09.1079 found 209.1236.
phenyl(2-phenyl-1H-benzo[d]imidazol-5-yl)methanone (1c): This
was synthesized from benzaldehyde (0.106g, 1mmol) and 3,4-
diaminophenyl(phenyl)methanone (0.233g, 1.1mmol) following
1
procedure of 1b. Yield: 70%. H-NMR (500MHz, DMSO-d
6
) δ:
1
3.2(bs,1H), 8.2(d,1H), 7.7(m,2H), 7.5(m,5H), 7.4(m,2H), 7.07(d,1H),
15 2
.8(m,1H), 6.5(d,1H) ppm. HRMS: (m/z) calculated for C20H N O
6
[M+1] 299.1184 found 299.1276.
2-(4-fluorophenyl)-1H-benzo[d]imidazole
(1d):
This
was molecules by Cd-MOF.
synthesised from 4-fluorobenzaldehyde (0.068g,1.1mmol) and o-
Results and Discussion
phenylenediamine (0.108g,1mmol) following procedure of 1b.
1
Yield: 70%. H-NMR (500MHz, DMSO-d
6
) δ: 12.9(bs,1H), 8.24- Crystal Structure
8
.19(m,2H), 7.53(m,2H), 7.42-7.37(m,2H), 7.23-7.17(m,2H) ppm. The single crystal of Cd-MOF was synthesized from the reported
HRMS: (m/z) calculated for C13
H
8
N
2
F [M-1] 211.0672 found hydrothermal method with slight modifications (Fig S1). The single
crystal of Cd-MOF was grown in an orthorhombic crystal system
211.0664.
1 1
with P2 2 2 space group. The crystallographic data collection
2-(4-bromophenyl)-1H-benzo[d]imidazole
(1e):
This
was
parameters and data refinement details of Cd-MOF are summarized
in Table S1. Fig. 2a showed the ball and stick model view of the
asymmetric unit of Cd-MOF and its ellipsoid model with 50%
probability level. The single crystal data revealed that the
coordination environment of cadmium ion is satisfied by three
different 2-ATC (2-amino terephthalic acid) ligands on the
equatorial positions where as the two axial positions were occupied
by 4,4'-bpy (bipyridine) units. Here, one of the carboxyl unit of 2-
synthesised from 4-bromobenzaldehyde (0.101g,1.1mmol) and o-
phenylenediamine (0.108g,1mmol)following procedure of 1b. Yield:
1
7
0%. H-NMR (500MHz, DMSO-d ) δ: 13.01(bs,1H), 8.12(d,2H),
6
7.76(d,2H), 7.67(d,2H), 7.53(d,1H), 7.24-7.17(m,2H) ppm. HRMS:
(m/z) calculated for C13
H
11
N
2
Br [M+1] 273.0027 found 273.0120.
4-(1H-benzo[d]imidazol-2-yl)-N,N-dimethylbenzenamine (1f): This
was synthesised using 4-(dimethylamino)benzaldehyde (0.164g,
.1mmol) and o-phenylenediamine (0.108g, 1mmol) following
1
2
ATC acts as a chelating unit (η mode) while the other one plays as
1
above procedure of 1b. Yield: 73%. H-NMR (500MHz,DMSO-d
6
) δ:
2
1
27
a chelating bridging unit (µ -η ;η mode). In crystal lattice, the
2
13.01(bs, 1H), 7.69(d, 2H), 6.99(m, 2H), 6.76 (d, 2H), 6.66(d, 2H),
central metal ion, Cd(II) is surrounded by pillar and planar
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connectors like Bpy and 2-ATC units respectively. Finally, these composite. The presence of iodine was further confirmed by energy
connectivity and interactions lead to form the 3D 2-fold dispersive X-ray spectroscopy (Fig. S5b and S7b).
interpenetrated network of Cd-MOF (Fig. 2c).
2
Fig.3 TGA of Cd-MOF and I @ Cd-MOF.
Iodine adsorption studies
The adsorption-desorption studies of iodine revealed the iodine
14a,25
capturing behaviour of the Cd MOF.
The time dependent UV-
vis experiment was studied and found that upon addition of MOF
crystals, the concentration of iodine in hexane solution got
decreased from 1.41 a.u. to 0.11 a.u. within 180 min (Fig. S8). The
slow desorption rate was confirmed by immersing the iodine loaded
MOF crystals in ethanol. The result showed the change in color of
ethanol with time due to iodine release into ethanol solution from
I
2
@Cd-MOF crystals (Fig.S9). The color change was observed upto
4 h and it changed slowly from colorless to brown. The slow
2
Fig.2 (a) Asymmetric unit of Cd-MOF along with numbering scheme.
b) ORTEP modal view of cadmium complex. Thermal ellipsoids are
release of iodine tempted us to check the catalytic behaviour of
@MOF.
(
I
2
scaled to the 50% probability level. (c) View of 3D network of Cd-
MOF along b-axis.
Catalytic transformation study
In an attempt to initiate our study, first reaction was carried out
between benzaldehyde (1 mmol) and o-phenylenediamine (1
Thermal and Morphological studies
mmol), using
benzimidazole derivatives (Scheme 2). Generally, molecular I
as a homogeneous catalyst and is used widely for the catalysis of
I
2
@Cd-MOF as
a
catalyst for synthesizing
The thermal stability of the Cd-MOF was investigated using
thermogravimetric analysis (TGA) and found to be stable upto
2
acts
4
00
°C. After that, continues weight loss was observed upto 550°C
2
8,29,30
various organic transformations,
whereas in the case of
due to ligands decomposition (Fig.3). The iodine uptake capacity of
I
2
@Cd-MOF, the weak host guest interactions are mainly
Cd-MOF was also measured by thermogravimetric analysis which
revealed that MOF is able to uptake 23 wt% of iodine(adsorption
responsible for the stabilization of iodine species in the framework
pores, which can be readily used for the catalysis and gives better
(
Fig.3).The accumulation of large amount of iodine within MOF
yield. The existence of iodine as I
pores was verified by the XPS studies (Fig.4).
supported by N2 adsorption studies. Cd-MOF and I @Cd-MOF
2
(in 0 oxidation state) within the
framework is attributed to the charge transfer complex through
1
4a,31
This was further
weak interactions between iodine and NH group of the linker
2
2
3
2
moiety. The TGA was performed before and after iodine loading.
The amount of iodine released during each cycle of organic
transformation was also determined by TGA study. The scanning
electron microscopy (SEM) and transmission electron microscopy
3
3
showed N
respectively (Fig. S10). The reduced N
MOF may be attributed to the adsorbed iodine which affects the
passage of N gas molecules into the channel pores.
2
uptake value of 17.186 cm /g and 7.549 cm /g at 77 K
2
uptake for iodine loaded
2
(
TEM) images showed the rod shape morphology of MOF with
A series of test reactions were carried out for optimizing the best
reaction conditions. Initially, the reaction was performed with
stacked layers (Fig. S4a, b and S6a), whereas I molecules spread
2
over the surface of MOF (Fig. S4 c,d and S6b) in iodine-MOF
4
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aldehyde and diamine keeping the ratio 1: 1.5 and using t-BuOH as integrated MOF system induced the reactant which led to increase
3
2
a solvent, but the yield turned out to be very low
̴
20%. The low
the reaction rate and yield of the product. After optimizing the
reaction conditions, reaction scope was expanded by performing
Table 2 Synthesis of benzimidazoles from substituted benzaldehyes
and diamines catalysed by I @Cd–MOF.
2
S.
Benzaldehyde
Diamines
Product
No
%
No.
Yield
1
2
3
1a
1b
1c
85
75
70
2
Fig.4 XPS spectrum of I @Cd-MOF
yield of product may be attributed to the excess of diamine and
lesser amount of catalyst. The yield of the product got increased
after several optimizations such as by varying the molar ratio of
aldehyde and diamine and amount of catalyst. But in these cases,
the efficiency in terms of reaction progress time and yield was not
4
5
1d
1e
70
70
2
much higher than that in the case of simple molecular I using
same solvent. To improve the yield, the effect of solvent was also
studied and noticed appreciable improvement in yield in the
presence of DMSO solvent. Further to investigate the required
catalyst amount for better efficiency of the reaction, the reaction
was performed under different catalytic ratio and observed that
best results were obtained with 33mmol % of catalyst (Table 1).
6
1f
73
2 4
Conditions: a. Room temperature, b. Drying agent: Na SO , c.
Solvent: DMSO, d. Time: 6h
the reaction with different aldehyde and diamine derivatives
bearing both electron withdrawing and releasing group. The effect
of electronic parameter was evident from the reaction yield (Table
Scheme2 Synthesis of Benzimidazole derivatives from
benzaldehyde and o-phenylenediamine.
2
) (Fig. S11-S22).
Next, the reusability of the catalyst, the most important part in
catalysis reaction from economic point of view was also examined.
The catalyst was recovered either by centrifugation or filtration
from the reaction mixture and allowed to be used for the next
cycle. However, the yield in the second cycle turned upto 62% due
Table 1 Optimization of catalyst ratio required for better yield.
Catalyst
Catalyst ratio (mmol %)
% Yield
40
10
Iodine loaded Cd-MOF
15
20
33
50
65
70
2
to less amount of iodine available in I @Cd-MOF. The amount of
(
t-BuOH as a solvent)
iodine released in both the cycles was estimated by TGA (Fig. 5). In
the first cycle, 11% of iodine got released into reaction mixture and
in second cycle, the amount of iodine released was 4%.
Finally, we could able to isolate the product with 85% yield within
h using 33 mmol% of catalyst in the presence of DMSO as solvent.
On contrary, the same reaction took 24h to produce 70% yield at
2
In addition to make sure the reusability of this I @MOF, one more
6
organic transformation was performed. We chose a known organic
10b
transformations i.e the synthesis of 5(2-thienyl) dipyrromethane
19
high temperature in the presence of molecular I
2
.
This indicated
(
Scheme 3) (Fig. S23-S24). As our attempt was to explore the
that, I @Cd-MOF played as superior and efficient catalyst than
2
reusability of I @MOF, the reaction was performed under similar
2
molecular I . The accessible functionalized surface area of iodine
2
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2
5
reported
condition.
In
the
synthesis
of
5(2-
thienyl)dipyrromethane, the catalyst was found to be efficient and (Fig. S25a). The reaction time for each cycle was kept constant. The
recyclable. Thermogravimetric analysis revealed the slow release of yield of the product after each cycle got diminished due to the less
iodine during catalytic reactions which led to perform consecutive availability of iodine (Fig.S25a).
three cycles of this reaction. In first cycle about 9% of iodine was This was further being supported by TEM- EDAX data of MOF
released whereas in the next two cycles the amount of iodine recovered after reaction (Fig. S26a and b). Further, to check the
released was 7% and 6%, which can be seen from the TGA curves
2
stability of Cd-MOF, PXRD analysis were carried out for I @Cd-MOF
before and after organic catalysis, which revealed similar pattern as
that of pure Cd-MOF (Fig. S27a,c and d).
Scheme 3 Synthesis of thienyl diyrromethanes.
Photocatalytic Studies
Optical Band Gap
In order to investigate the conductivity of Cd-MOF, the diffuse
reflectance spectroscopy (DRS) was performed in the range of 200-
8
00 nm to determine the band gap (Eg). From DRS spectra, it can be
clearly seen that Cd-MOF has an absorption edge at 442nm (Fig.
S28) which corresponds to a band gap of 2.8eV. Thus the Eg value
clearly imparts for the Cd-MOF absorption in the visible region
which constitutes a major part of sunlight.
Fig. 5 TGA graph for iodine releasing profile during synthesis of 2-
phenyl-1H-benzo[d]imidazole.
Fig. 6(a) Plot showing transformed Kubelka- Munk function vs. energy of light for band gap. Photocatalytic degradation of MB under visible
light irradiation: (b) in the presence of catalyst. (c) in the absence of catalyst and (d) Degradation of MB with catalyst in dark.
6
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The Eg value was calculated from a plot of Kubelka- Munk function it confirmed that there is no adsorption and it was completely a
F versus energy (Fig. 6a). It has been found that the band gap of this degradation process. This was also confirmed by TEM-EDAX data of
particular Cd- MOF is relatively lower than other reported MOFs Cd-MOF recovered after photocatalytic dye degradation, in which
3
3,34,35
and typical inorganic wide band gap semiconductor materials.
sulphur content was found to be zero (Fig. S29a and b). Notably,
the Cd-MOF showed excellent photocatalytic activity with 82% MB
degradation within 90 min in visible region without the addition of
Study of Photocatalytic activity
3
The photocatalytic degradation of MB was studied under visible any photocatalytic enhancers like reduced graphene oxide, KBrO ,
light irradiation to examine the photocatalytic efficiency of Cd-MOF.
The characteristic absorption peak of MB gradually decreased with
H
2
O
2
. Apart from visible light irradiation, the efficiency of Cd-MOF
was also examined under UV light. Under UV light irradiation, the
increase in exposure time in the presence of Cd-MOF catalyst (Fig. catalyst showed only 72% degradation of MB dye in same time
6
b). Whereas, the absorbance intensity of MB was remained
period of 90 minutes which can be clearly justified from the fact
that Cd-MOF is visible light active ( Fig. S30d).
The degradation rate was calculated as the kinetic data of the MB
degradation well fitted with pseudo 1st order kinetics (Fig. 7a and
2
b).
3,36
The degradation rate can be calculated using the following
3
7
equation :
%
Degradation= (1- A / A ) × 100
t o
Where A and Ao are the absorbance at the time t and initial
t
absorbance respectively, which corresponds to relative
concentrations.
The better photocatalytic activity of Cd-MOF may be attributed to
2
the interaction of NH group of linker moiety towards MB, which is
the governing factor for adsorbing MB over MOF surface. The Cd-
MOF established its efficiency towards the photocatalytic
degradation of MB dye than many other reported MOFs (Table
3
8-42
S2).
Next, the stability of Cd-MOF was studied by checking its reusability
for photocatalytic degradation of MB dye. In this case, four cycles of
degradation experiments were performed by recovering the
catalyst each time after completion the process (Fig. S30a to c). The
recyclability results are shown in Fig. 8. It can be clearly seen that
the catalyst showed good photocatalytic efficiency upto four cycles,
which supported the reusability of the Cd-MOF catalyst. Moreover,
the PXRD patterns of Cd-MOF before and after dye degradation
experiments resembled each other (with a slight change in intensity
of peaks) (Fig S27 a and b) which indicated that Cd-MOF retained its
crystalline nature after the photocatalytic activity.
Fig. 7 (a) Kinetic curves for photocatalytic degradation of MB. (b)
Pseudo-first order kinetics plot for photocatalytic degradation of
MB.
Mechanism for the degradation of MB
It is well known that hydroxyl radical plays key role for the
degradation process due to its strong oxidizing power which is able
constant in the absence of Cd-MOF catalyst (Fig. 6c). This clearly
indicated the negligible self-photosensitization of MB. The
degradation experiment in dark was performed to confirm whether
the dye was actually degraded or simply undergo adsorption by Cd-
MOF (Fig. 6d).As the degradation was almost negligible in the dark,
to oxidize wide variety of double bonds containing organic
10,43
compounds.
So, photocatalytic efficiency depends on the
generation of hydroxyl radicals by photocatalyst. Consequently, to
describe the photo degradation process the HOMO-LUMO gap can
be utilized (Fig. 9). In general, the HOMO is mainly contributed by N
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and O bonding electrons and LUMO is contributed by transition The degradation of the dye was further confirmed by the LC-MS
metals. Upon irradiation, the electrons in the HOMO of Cd-MOF got analysis of the degraded aqueous solution of methylene blue. The
mechanistic pathway (Fig. S31) has been proposed for the
degradation of MB by Cd-MOF. First of all, the hydroxyl radical
attacks the ring containing both the heteroatoms N and S which
results into opening of the central aromatic ring. Further the C-S
bond breaks and it results into formation of fragments like 2-amino-
5
-(methylamino)-hydroxybenzenesulphonic acid and 2-amino-5-(N-
methylformamido)-benzenesulphonic acid which were confirmed
by LC-MS peaks at 218 and 230. These fragments got decomposed
into smaller molecules like 2-amino-5-(methylamino)benzenethiol
,
2-amino-5-(methylamino)benzenesulphonic acid
and 2-
aminophenol that corresponds to peaks at 154, 202.9 and 109 (Fig.
S32-S34). At the end all these intermediates will get decomposed
2 2
into CO and H O.
Conclusions
In summary, we report the development of an integrated new
catalytic platform through incorporation of iodine into a porous Cd-
MOF being peripheral amine functionality and performed catalysis
for the various important organic transformations. Especially in case
Fig. 8 Reusability test of Cd-MOF upto four cycles for the
photocatalytic degradation of MB dye.
+
excited and transferred to LUMO, leads to produce holes (h ) in the
of benzimidazole synthesis,
performance at mild reaction conditions and in a short span of time
6h). In contrast, the same reaction in the presence of molecular I
2
I @Cd-MOF showed exceptional
HOMO. These holes having high oxidizing ability either oxidize
water molecules or react with hydroxyl ions to form hydroxyl
radicals. These OH radical could be responsible to furnish the
(
2
4
4,45
requires high reaction temperature and longer reaction time for
completion. Apart from this, thienyldipyrromethane transformation
also established the good efficiency and reusability of this MOF.
photodegradation process.
2
Thus, the detailed study of organic catalysis using I @Cd-MOF
reflects its good catalytic efficiency for clean organic
transformations. Further, the pristine Cd-MOF was investigated to
evaluate its potential to act as photocatalyst for dye degradation
under visible light illumination. Interestingly owing to its low band
gap and strong visible light absorption properties, the MOF was
evaluated to be efficient catalyst for photoinduced dye degradation
without the use of additional oxidizer. The efficiency was evaluated
to be 82% degradation of MB dye which is much higher than many
reported catalytic systems. These studies provide an insight into the
synthesis and applications of multifunctional MOFs.
Conflicts of interest
Fig. 9 Mechanism proposed for the photodegradation of MB under
There are no conflicts to declare.
visible light irradiation.
Acknowledgements
At the same time, molecular oxygen (O
electrons (e-) to produce superoxide radicals (O
possess strong oxidizing ability and hence decompose the organic (IITM/SERB/RRK/173) and IIT Mandi (IITM/SG/NRT-58) are
molecule (MB) into CO , H
O and some other degradation products. thankfully acknowledged. M. V. thanks CSIR for the fellowship. We
2
) captures photogenerated
ͦ-
) which also Financial
support
received
from
DST-SERB,
India
2
2
2
The foremost advantages of this photocatalyst are its high thank Director, IIT Mandi for research facilities. The support of
crystalline nature and large surface area which are responsible for Advanced Materials Research Centre (AMRC), IIT Mandi, for
the enhanced photocatalytic performance as these provide strong sophisticated instrument facility is thankfully acknowledged.
adsorption ability and result into adsorption of large number of
pollutant molecules on catalyst surface.
8
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