Synthesis, characterization and photocatalytic studies of mesoporous silica grafted Ni(ii) and Cu(ii) complexes

Article abstract of DOI:10.1039/c5ra07469j

Mesoporous silica grafted nickel(ii) and copper(ii) complexes of a Schiff base ligand were synthesized. Functionalization of the ligand was achieved by Schiff base condensation of 3-APTES and O,O′-mono methylene bis(salicylidene). The Schiff base moiety w

Full text of DOI:10.1039/c5ra07469j

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Synthesis, characterization and photocatalytic studies of mesoporous silica
grafted Ni(II) and Cu(II) complexes
G. Ramanjaneya Reddy,a and S. Balasubramaniana*
aDepartment of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy
Campus, Chennai-600 025, India. E-mail:bala2010@yahoo.com; Tel: +91 9444016707, 044 22202794,
Fax: +91 4422300488.
Abstract
The mesoporous silica grafted nickel(II) and copper(II) complexes of the Schiff base
ligand were synthesized. The functionalization of the ligand was achieved by the Schiff base
condensation of 3ꢀAPTES and O,Oˈꢀmono methylene bis(salicylidene). The Schiff base moiety
was subsequently grafted with MCM41, followed by complexation with the metal salts. The
compounds were characterized by spectral( Fourier Transform Infrared spectroscopy (FTIR),
diffuse reflectance UV–Vis spectroscopy (DRS/UVꢀVis), Xꢀray diffraction (XRD),
Xꢀray
Photoelectron Spectroscopy (XPS)), thermal (Thermo Gravimetric Analysis (TGA))
microscopic(High Resolution Transmission Electron Microscopy (HRTEM) ) and surface
Brunauer–Emmett–Teller (BET) method, N2ꢀsorption isotherms) analysis. The compounds were
employed in the photo oxidation of Methyl Orange (MO) and Reactive Red198 (RR). The
copper complex exhibits higher activity in the photo degradation of the dyes. The reusable
photocatalysts show comparable activity to that of fresh catalyst without loss of their catalytic
nature
Introduction
Metal organic frameworks (MOFs) are an emerging class of porous crystalline
materials, which can be designed at a molecular level to perform many processes
including catalysis.1 The hybrid materials have good textural properties like high surface
area, porosity, uniform crystalline size and thermal stability.2 Moreover, this class of
materials find applications such as gas storage, separation, proton conductance, sensing,
drug delivery, magnetism, and heterogeneous catalysis.3ꢀ7 The homogeneous catalysts
have been employed in many organic transformations.8 The transition metal complexes
act as homogenous catalysts in organic reactions.9 However, the recovery and reuse of the
catalysts are very challenging tasks in reaction mixture, where as the heterogeneous

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catalysts can overcome these disadvantages.10 The silica based inorganic supports play
major role in catalytic reactions and structurally build up with the SiO4 tetrahedral units
with voids and they can act as hosts for the guest molecules. The hostꢀguest interaction
plays a major role in the field of photocatalysis. The metal activated silica supports were
used as adsorbents. Their porous structures facilitate the diffusion of substrates and
products and make them an ideal platform to incorporate catalytic centres.
Nonꢀcatalytic methods with stoichiometric, toxic, corrosive and expensive oxidants such
as dichromate, permanganate, hydrofluoric acid, sulphuric acid, and peroxy acids under stringent
conditions of high pressure and/or temperature have been widely used for dye molecule
oxidations.11 These reactions are often carried out in high concentration and in environmentally
unfriendly organic solvents. To achieve decolorization of the xanthene class of dyes, initially the
azo bonded moiety should be cleaved catalytically. Aerobic processes utilize oxygen, which can
inhibit azo (N=N) bond cleavage.12 However, enhancement of catalytic activity with nickel(II)
and copper(II) based systems are alternative to highly expensive systems such as platinum,
diamond, ruthenium, gold and palladium.13 The mesoporous silica systems grafted with Schiff
base moiety can be used as efficient catalysts for degradation of dye molecules such as methyl
orange, phenol red, methylene blue, rosebengal, sudanꢀIII and reactive red.14ꢀ18 The development
of inexpensive, stable, efficient and bandꢀgap tunable MOF based photocatalysts is still a major
challenge, since they have absorbing tendency of the dye molecules by electrostatic interactions
with surface containing hydroxyl groups. The high surface area materials have the tendency to
absorb and activate the dye molecules to the excited states. The modified mesoporous silica can
delocalize the band gap excited electrons of HOMO and LUMO levels of metal complexes.
Hence, they minimize the electronꢀhole recombination besides a relative number of active sites
present on the composite surface. Besides, the ability of grafted silica favours photo induced
•
electronꢀtransfer reactions and generate powerful oxidative ionꢀradical species such as OH, ‾O2
+•
and O2H. This supported system improves the rate of reaction and catalytic activity towards
the decolorization of the toxic dyes.19 There are not many reports on the removal of dyes by
silica supported Schiff base Ni(II) and Cu(II) composites and their photocatalytic applications.
Hence, the photo degradation of dyes by mesoporous silica supported N2O2 tetradentate Schiff
base complexes under UV and solar light source has been investigated.

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In the present investigation, the mesoporous silica (MCM41) was functionalized with
Ni(II) and Cu(II) complexes of N2O2 Schiff base ligand by post grafting method. These
complexes were used as photocatalysts in the degradation of Methyl orange (MO) and Reactive
Redꢀ198 (RR) under UV light irradiation. The activity of the solid supported catalysts was
determined by spectrophotometric techniques.
Scheme 1: Grafting of Ni(II) and Cu(II) complexes onto MCM41 by Schiff base
condensation of SL and 3ꢀAPTES.
Experimental
Materials
Salicylaldehyde, diidomethane (Merck, India), copper chloride, nickel chloride
(Qualigens, India), cetyltrimethyl ammonium bromide (CTAB) (LobaChemie. Pvt. Ltd, India),
Tetraethoxyorthosilicate (TEOS), 3ꢀAPTES (SigmaꢀAldrich, India), methyl orange (Merck,
India), reactive redꢀ198 (Textile industry, Tamilnadu) and other analar grade chemicals were
used without further purification. Millipore water was used throughout the work.

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Physicochemical measurements and characterization
The NMR spectra were recorded on a Bruker Avance 400MHz NMR spectrometer. The
FTIR spectra were recorded on a FTIR PerkinꢀElmer 8300 spectrometer with KBr pellets.The
UVꢀVisible Diffuse Reflectance Spectral (UVꢀVis/DRS) studies were carried out on a Perkinꢀ
Elmer lambdaꢀ650 DRS UVꢀVisible spectrophotometer. The EPR spectra of copper(II)
complexes were recorded at LNT on a Varian Eꢀ4 Xꢀband spectrometer using TCNE as the gꢀ
marker (2.0023). The XPS analysis was carried out on a XM1000 Omicron nanotechnology XPS
system using AlꢀKα monochromatic radiation. The samples were made in to pellets and were
used as such for Xꢀray Photoelectron Spectroscopic (XPS) studies. The crystalline nature of the
materials were ascertained by powder Xꢀray diffraction pattern using Bruker D8 advance
diffractometer with monochromatic CuꢀKα1 radiation (λ=1.5418 A°). TGA experiments were
performed with Versa Therm Cahn Thermo balance TGꢀ151 with a sensitivity of 10ꢁg. TGA
experiments were conducted in the temperature range of 300ꢀ1150 K with 20 ± 0.01 mg of the
samples and the analyses were carried out at a heating rate of 10 K/min under static air
atmosphere. The N2 adsorption, desorption isotherms and BrunauerꢀEmmettꢀTeller (BET)
surface area measurements at 77 K were carried out on a Micrometrics ASAP (Model 2020)
surface area analyzer with nitrogen and helium gases with a purity of 99.999%. HRTEM analysis
was carried out on a FEI TECNAI G2 (Tꢀ30) transmission electron microscope with an
accelerating voltage of 250 KV.
Synthesis of O,Oˈꢀmono methylene bis(salicylidene) (SL)
Salicylaldehyde (0.61 g, 5 mmol) was dissolved in 20 mL of DMF to which potassium
carbonate (1.73 g, 12.5 mmol) was added and the mixture was stirred at room temperature. To
this mixture 1,1 diidomethane (0.6 g, 2.5 mmol) was added and then stirred under reflux for 6 h.
The resulting mixture was partitioned between water and ethylacetate. The ethyl acetate layer
was collected, concentrated under reduced pressure and was then subjected to silica gel (100ꢀ200
mesh) column chromatography using hexaneꢀethylacetate (1:9) as eluent to afford the product
(1.1g ; 90%) in pure form. The purified SL was characterized by spectral studies and the results
were compared with those of earlier reports.20 1H NMR (CDCl3, 400 MHz): δ: 2.4 (2H, p,
CH2O), 4.33 (4H, t, J = 7.6 Hz, OCH2O), 7.03 (4H, m, ArꢀH), 7.55 (2H, t, J = 7.7 Hz, ArꢀH),
13
7.83 (2H, d, J = 7.7 Hz, ArꢀH), 10.49 (2H, s, aldehydeꢀH); C NMR (CDCl3, 100 MHz):ppm:
29.2, 64.7, 112.5, 121.0, 124.9, 128.8, 136.2, 161.0, 189.6.

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Synthesis of Schiff base ligand with 3ꢀAPTES (SLTES)
The O,Oˈꢀmono methylene bis(salicylidene) (SL) (0.5g, 2mmol) was dissolved in 50 mL
of ethanol and to this 3ꢀAPTES (1 ml, 4.2 mmol) was added. The solution was refluxed for 6 h
and the resulting dirty white solid was washed with hot ethanol, acetone and diethyl ether, dried
under vacuum for 6 h. Yield: 1 g (80%).
Synthesis of MCMꢀ41 by solꢀgel process
The siliceous mesoporous silica was synthesized according to the reported procedure.21
The typical synthesis involves dissolving 2 g of CTAB in 120 mL of deionised water containing
1 g of NaOH by heating at 323 K. To this solution 6 mL of TEOS was added and the mixture
was vigorously stirred at room temperature for 6 h and aged at 383 K for 24 h in an autoclave
under static air condition. The product was isolated by filtration and washed with deionised
water and methanol. The resulting white powder (SLTES) was dried at 383 K overnight and
calicined at 800 K for12 h, Yield: 0.3 g.
Grafting of Schiff base ligand with SiꢀMCM41 (SiOF)
About 0.1 g of the SiꢀMCM41 was suspended in 30 mL toluene along with 0.2 g
SLTES, and then sonicated for 15 min. The reaction mixture was then heated for 8 h at
363 K.The resulting dirty white solid was washed with ethanol, toluene and dried for 12 h
at 373 K. Yield 0.25 g.
Synthesis of solid supported Schiff base complexes (NiOF&CuOF))
0.5mmol of M(II)[M(II):Ni(II)&Cu(II)] chloride salts were added to an ethanolic
solution of 100 mg of SiOF and was refluxed for 3 h and the resulting solids were dried,
and Soxhlet extracted with ethanol for 6 h and dried again at 393 K.
Photocatalytic degradation of MO under UV light
The photocatalytic oxidation reactions were carried out in a glass reactor with a
diameter of 66 X 98 mm that contains a water jacket at room temperature. A mercury
(Hg) lamp (100 W, 365 nm) was used as a UV light source to trigger photo
decomposition reactions. The catalyst 0.05 g of SiOF/ 0.05 g of NiOF/ 0.05 g of CuOF
and 0.01 mmol of 100 mL aqueous MO dye solution was added to the catalysts
separately. The reaction conditions were optimized in dark at room temperature and
irradiated under UV light (< 360 nm). The removal percentage and consequent spectral
changes at predetermined time intervals were monitored by the UVꢀVisible absorption

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spectra at 464 (± 1) nm for 4.5 h. The percentage conversion was determined by the
following equation.
% Conversion = (A0ꢀAt)/A0*100
(1)
where A0= Initial absorbance, At = Absorbance at time (t).
Photocatalytic degradation of RR under UV light
The photocatalytic oxidation reactions were carried out in a glass reactor with a diameter
of 66 X 98 mm that contains a water jacket at room temperature. A mercury (Hg) lamp (100 W,
365 nm) was used as a UV light source to trigger photo decomposition reactions. The catalyst
0.05 g of SiOF/ 0.05 g of NiOF/ 0.05 g of CuOF and 0.01 mmol of 100 mL aqueous RR dye
solution was added to the catalysts separately. The reaction conditions were optimized in dark at
room temperature and irradiated under UV light (< 360 nm). The removal percentage and
consequent spectral changes at predetermined time intervals were monitored by the UVꢀVisible
absorption spectra at 519 (± 1) nm for 7 h. The percentage conversion is calculated from
equation (1)
Results and discussion
Fourier Transform IR Spectroscopy
The FTIR spectra of the SL, SLTES, SiOF, NiOF and CuOF are provided in Fig. S3
(ESI†). The shift in band positions and their peak assignments in wave number are listed in
Table 1. The disappearance of SL aldehyde carbonyl group at 1720 cm−1 and the formation of a
new peak at 1642 cm−1 due to azomethine (ꢀC=N) group in the Schiff base ligand i.e SLTES
were observed.22, 23 The SLTES shows bands at 2805, 2918, 3012, and 3185 cm−1, for the
aliphatic and aromatic CꢀH vibrations and a peak at 1600 cm−1 is attributed to the C=C phenyl
ring stretching frequencies. SLTES shows typical absorption bands in the region of 2970ꢀ2850
cm−1 associated with the ꢀCH2 group vibrations of the SL ligand,24 which clearly indicates that
the primary amino group of the APTES was involved in the formation of the azomethine with
SL. The FTIR spectrum of the SiOF indicates that a change in the coordination sphere of the
ligand has taken place. In this context, the new broad bands around 1080 cm−1 may be attributed
to the formation of SiꢀOꢀSi linkages with appended SLTES.25
The SiOF also shows peaks at 2968 cmꢀ1 due to the aliphatic and aromatic CꢀH
vibrations. The N=C group frequency is observed at 1642 cm−1 for SLTES, but in the case of

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NiOF and CuOF this peak appears at 1615ꢀ1632 cm−1 due to metal ion coordination with
nitrogen atoms. At lower frequency region, the complexes exhibit medium intensity bands in the
region of 420ꢀ450 cm−1 and 520ꢀ600 cm−1, corresponding to MꢀN and MꢀO vibrations,
respectively,26 indicating that M(II) ion coordinates with two aromatic ether oxygen atom and
two azomethine nitrogens. The broad shoulder at 1630ꢀ1640 cmꢀ1 is due to the bending
frequency of the OꢀH group present on the surface and the azomethine (ꢀN=C) group vibrations
may combine in this region.The structure of SiꢀMCM41 silica frame work has not changed
during anchoring. The FTIR spectral data provide evidence for the anchoring of Schiff base
metal complexes and their stability in the pores of SiꢀMCM41.
Table 1: FTIR spectral frequencies (cm–1)of the free and encapsulated complexes.
Sample
Wave number (cmꢀ1)
ν(C=O)
ν(C=N)
ꢀ
ν(C–O–C)
1253, 1321
1221, 1302
ꢀ
ν(SiꢀO)
ꢀ
1085
ν(CꢀC)
1525
ν(C=C) ν(M–O) ν(M–N)
SL
1706
1601
1599
ꢀ
ꢀ
ꢀ
SLTES
MCM41
SiOF
ꢀ
ꢀ
ꢀ
1638
ꢀ
1560
ꢀ
ꢀ
ꢀ
1086
1086
1083
1090
ꢀ
ꢀ
ꢀ
1645
1613
1620
1248, 1306
1244, 1305
1245, 1310
1563
1542
1565
1606
ꢀ
NiOF
CuOF
ꢀ
1574 556
1542 578
425
441
ꢀ
Diffuse Reflectance Spectra
The DRS UVꢀVisible spectra of the NiOF and CuOF were recorded in the range of 200ꢀ
800 nm and the spectral traces are provided in Fig.1.The NiOF shows two characteristic bands at
278 and 339 nm due to π→π* and n→π* transitions respectively and a broad band centred at 630
nm due to dꢀd transition. The n→π* transition at 278 nm is assignable to the phenyl rings,
whereas the bands in the region of 390–410 nm are assignable to the n→π* transition of the
(C=N) moiety. CuOF also shows the corresponding peak values at 285 nm and 316 nm, which

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are assigned to π→π* and n→π* transitions respectively.27 The bands due to dꢀd transition of
similar energies correspond to the T2g → E2g transition of the Ni(II) centres. The CuOF shows a
broad peak at 622 nm due to dꢀd transition. The NiOF and CuOF samples exhibit several high
intensity CT and intraꢀligand bands in the range of 260ꢀ430 nm. The CT and dꢀd transitions
afforded evidences for the complexation of Ni(II) and Cu(II) ions with the Schiffꢀbase ligand.
The electronic spectra of Cu(II) complexes show two bands near 630 nm (broad) and 400 nm
(shoulder), attributed to 2B1g → 2A1g, 2B1g → 2B2g and 2B1g → 2Eg transitions.28 This is due to the
ligand to metal charge transfer (LMCT) bands in addition to the π→π* of phenolic and imineꢀN
which is coordinated to M(II).
Fig. 1: DRS UVꢀVisible spectra of (a) NiOF and (b) CuOF
EPR spectral studies
The EPR spectrum of CuOF is anisotropic and the nitrogen hyperfine splitting can be
clearly seen in the spectrum(Fig.2) The Cu(II)L.2ClO4 shows values of g||, g⊥ and gav at 2.24,
2.082 and 2.13 respectively. The g||> g⊥> 2.0023 indicates the covalent environment of the metal
ion with the Schiff base and also the hyper fine splitting factor (A||) value at 109.5 G. 27,29

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Fig.2: EPR spectrum of the supported complex CuOF
Powder Xꢀray diffraction
The power XRD studies of the calicined MCM41, SiOF, NiOF and CuOF were recorded
at room temperature( Fig.3). The XRD pattern of the parent MCM41 sample shows three lowꢀ
angle strong reflections at d100, d110 and d200 planes and a weak one at d210 plane. This is
characteristic of hexagonal SiꢀMCM41 structure with a d100 spacing value of 40.83 A°. The SiOF
also shows the same characteristic feature as SiꢀMCM41 with d100 spacing value of 42.1 A°. The
relative peak intensity values of (1 0 0), (1 1 0) and (2 0 0) reflections for NiOF and CuOF
decrease when compared to that of SiꢀMCM41and d210 plane was not observed. The unitꢀcell
parameters calculated using d100 were (d100 = 43.3 A°, 2θ = 2.21°) and (d100 = 43.95 A°, 2θ =
2.28°) for NiOF and CuOF respectively.30,31 The XRD pattern of NiOF and CuOF with
decreased peak intensity and a change in d100 spacing value suggests the presence of Schiff base
complex in the silica matrices. This is attributed to the lowering of the local order, such as wall
thickness or it might be due to the reduction of the scattering contrast between the porous silica
channel and the Schiff base moiety. However, XRD parameters show that the crystalline nature
of SiꢀMCM41 has not changed during the grafting process.
Fig.3: Power XRD pattern of (a) MCM41 (b) SiOF (c) NiOF and (d) CuOF (inset diagram
shows expanded spectra between 2θ =3.5° to 7°)

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Xꢀray photoelectron spectroscopy
Xꢀray photoelectron spectroscopy (XPS) is a powerful technique which is used to
investigate the electronic properties of the species formed on the surface. XPS studies reveal the
electronic environment, i.e., oxidation state and spin multiplicity, which influence the binding
energy of the metal electronic transitions. Fig.4 reveals the presence of C, N, O, Si, Ni(II) and
Cu(II), which confirms the respective oxidation state of the elements. These peaks are related to
carbon in the encapsulated complexes which show medium intensity broad bands. The C1s, N1s
and O1s and Si2p appear at 285.1, 400.0, 531.2 and 103.2 eV respectively.32 The XPS traces of
NiOF indicate that no shakeꢀup satellite peaks are observed below 876 eV for Ni(II)(2p) which
confirms the planar form and diamagnetic nature of Ni(II) (Fig.4c). NiOF exhibits XPS traces at
the binding energy values of 854.13 eV and 874.61 eV for 2p3/2 and 2p1/2 states respectively.33
Fig.4d shows that the compound CuOF has shakeup satellite suggesting that copper is in +2
oxidation state. The Cu 2p level XPS traces of the CuOF indicate the paramagnetic nature of the
Cu(II) ion. These observations confirm the presence of an unpaired electron in dx2–y2 orbital,
and the EPR spectral data also support the same . CuOF exhibits the XPS traces with the binding
energy values of 934.10 eV and 954.91 eV for 2p3/2 and 2p1/2 states respectively.34ꢀ36.
Table 2: Binding energy of the NiOF and CuOF (M(II):Ni&Cu)
Sample
Binding energy (eV)
Si(2p) C(1s) O(1s) N(1s) M(II) (2p)
102.5 284.3 532.0 400.1 854.3, 874.6
102.6 284.2 532.4 400.8 934.9, 954.3
NiOF
CuOF

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Fig. 4: XPS survey spectra of (a) NiOF, (b) CuOF; (c) Ni(II) 2p and (d) Cu(II)2p core level
spectra.
Surface area analysis
The BET surface area, N2 adsorption/desorption isotherms and pore volume of the
materials SiꢀMCM41, SiOF, NiOF and CuOF are shown in Fig.5. The pore size distribution
curves of the samples were evaluated from the sorption branches of the isotherms using Barrettꢀ
JoynerꢀHalenda method and the encapsulated complexes show typeꢀIV isotherm. At relatively
low pressures (P/P0 ≤ 0.3), the N2 adsorption isotherm of the NiOF and CuOF was different from
that of MCM41 due to N2 uptake. A steep increase observed after P/P0 at 0.4 reflects their
narrow pore size distribution due to capillary condensation in the pores by the Schiff base ligand
and its metal complexes. The surface area of SiꢀMCM41 was found to be 900 m2/g. However, in
the case of SiOF, NiOF and CuOF the surface area is drastically reduced to 580 m2/g, 310 m2/g
and 350m2/g, respectively. The average pore width of SiꢀMCM41 is 3.8 nm and it is reduced to
3.2 nm, 0.8 nm and 1.3 nm, for SiOF, NiOF and CuOF respectively. The average pore volumes
are 0.54 cm3/g, 0.35 cm3/ g, 0.18 cm3/ g, and 0.23 cm3/ g for SiꢀMCM41, SiOF, NiOF and
CuOF respectively. The pore volumes get drastically reduced during grafting and it can be
attributed to the increased loading with organics, which occupy void space inside the

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mesopores.37,38 This analysis confirms that NiOF and CuOF have same structural integrity even
during the anchoring process.
Thermal analysis (TGA)
The TG/DTG curves of the MCM41, SiOF, NiOF and CuOF were recorded in the
static air atmosphere and are shown in Fig.6. The DTG curves show a residual mass of
around 64%, clearly suggesting the thermal stability of all the composites. The DTG
curves of the materials show multi step decomposition. The composites show a mass loss
around 6% in the first stage up to 530 K, due to the water molecules present on the
surface. In case of the SiOF, the decomposition during the first step between 343 K and
530 K corresponds to the loss of coordinated water molecules with the mass loss of 5.7%.
In the second step up to 790 K, there is a continuous mass loss of 12.5 % by SL moiety in
Schiff base complexes with a residual mass around 81.5%. The NiOF and CuOF systems
show higher thermal stability with a residual mass around 63%. The second and the third
stage of mass loss around 30% up to 953 K are due to the loss of Schiff base complexes
and the formation of metal oxide residues.39
TEM analysis
The TEM photograph of the parent MCM41, NiOF and CuOF are shown in Fig.7. The
solid support has hexagonal shape and its channels are well ordered. The NiOF and CuOF
systems also exhibit hexagonal geometry and are well ordered with slight change in the
morphology and negligible agglomeration when compared to that of the parent mesoporous
silica.40,41

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Fig. 5: N2 adsorptionꢀdesorption isotherms (a) SiꢀMCM41 (b) SiOF (c) NiOF and (d) CuOF.

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Fig.6: TG /DTG curves of (a) SiꢀMCM41(b) SiOF (c) NiOF and (d) CuOF

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Fig.7: TEM images of (a) SiꢀMCM41 (b) NiOF and (c) CuOF
Catalytic studies
Mechanistic issues involved in photocatalysis
The photo degradation of MO in the presence of MCM41, SiOF, NiOF and CuOF, under
UV light irradiation has been investigated. The efficiency of the catalyst in methyl orange
degradation was calculated from the optical absorption spectral analysis. The aliquot samples of
the reaction medium were collected and the consequent absorption changes were recorded at 464
(± 1) nm.42 The spectral changes in the dye degradation by (a) SiꢀMCM41 (b) SiOF (c) NiOF
and (d) CuOF are given in Fig S4 (ESI†). The MOF exhibits higher degradation efficiency,
when compared to that of SiMCM and SiOF. The removal percentages are 20%, 12%, 83% and
100% for SiꢀMCM41, SiOF, NiOF and CuOF respectively (Fig.8a). The efficiency of the
catalyst in the degradation of RR was calculated by the optical absorption spectral analysis. The
aliquot samples of the reaction medium were collected and the consequent absorption changes
were recorded at 520 (± 1) nm and are given in Fig.S5 (ESI†).43 The removal percentages are
18%, 27%, 87% and 100% for SiꢀMCM41, SiOF, NiOF and CuOF respectively (Fig.8b).
Metal complex enhances the acidity of the composites during grafting process. The
photocatalytic performance and the removal efficiency of the reactive red increases. MOF can
decrease the recombination of electronꢀhole pair and increase the photocatalytic performance.44
The MOF shows higher activity because of its metal loading efficiency which enhances the
photo generated electron pairs (e− and h+). Photogeneratedꢀholes and electrons may react with
•−
•
the surface hydroxyl groups to generate active oxidative ionic radical (O2 and OH) from the
reaction medium. The photo generated electrons exhibit more efficiency towards the hole
•
formation and minimize the band gap energy, meanwhile, the formation of OH, +•O2H, ‾O2
radicals, which are powerful oxidants, starts a cascade of oxidation reactions that can convert the
ꢀ
ꢀ
organic compounds in solution completely into NO2 , NO3 , SO42ꢀ ions H2O and CO2 in reactive
red 198 (SO42ꢀ ions, H2O, CO2 in the case of MO) and other inorganic compounds.45ꢀ47 After the
completion of the reaction, NiOF and CuOF were recovered and their catalytic efficiency was
tested in five cycles and overall percentage is provided in Fig. S6 (ESI†).

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Fig. 8: The percentage degradation of MO (a) and RR (b) by SiꢀMCM41 (b) SiOF (c) NiOF and
(d) CuOF.
Summary
The nickel(II) and copper(II) complexes of the Schiff base ligand O, O'ꢀ mono methylene
bis(salicylidene aptes) were synthesized by the condensation with 3ꢀaminopropyl triethoxysilane
(3ꢀAPTES) which was subsequently grafted with silica. The covalently anchored metal
complexes were investigated by several spectral, thermal, sorption and microscopic techniques.
The solid state catalysts were employed as photocatalysts in the degradation of MO and RR
under UV irradiation. The CuOF showed higher activity than that of NiOF in the dye degradation
reactions. The catalytic activities of the recovered and purified photocatalysts were compared
with that of fresh catalysts

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Fig.9: The proposed radicalꢀion mechanism for the MO and RR dye degradation.
Acknowledgements
The authors are thankful to National Center for Nanoscience and Nanotechnology,
University of Madras, Chennai for XPS, DRS and TEM analysis. One of the authors
G.Ramanjaneya Reddy is grateful to Dr. G. Bhaskar Raju, Chief scientist, CSIRꢀNML Madras
Complex, Chennai, India for his support during the experimental work.
Appendix A. Supplementary data
The NMR, FTIR, absorption plots of the both degradation reactions, overall percentage
conversion of reused catalysts, FTIR and XRD of the recovered MOF are given in the electronic
supplementary information.
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Synthesis, characterization and photocatalytic studies of mesoporous silica
grafted Ni(II) and Cu(II) complexes
G. Ramanjaneya Reddy,a and S. Balasubramaniana*
aDepartment of Inorganic Chemistry, School of Chemical Sciences, University of Madras,
Guindy Campus, Chennai-600 025, India.
E-mail:bala2010@yahoo.com; Tel: +91 9444016707, 044 22202794, Fax: +91 4422300488
Graphical abstract