Synthesis and characterization of 3-[N,N′-bis-3-(salicylidenamino) ethyltriamine] Mo(vi)O2@SBA-15: A highly stable and reusable catalyst for epoxidation and sulfoxidation reactions

Article abstract of DOI:10.1039/c3ra47222a

The efficient and reusable oxidation catalyst 3-[N,N′-bis-3- (salicylidenamino)ethyltriamine] Mo(vi)O₂@SBA-15 has been synthesized by the anchoring of the 3-[N,N-bis-3-(salicylidenamino)ethyltriamine] ligand (L or Salpr) on the inner surfaces of organofunctionalized SBA-15 and subsequent complexation with Mo(vi)O₂(acac)₂. The physico-chemical properties of the functionalized catalysts were analyzed by elemental analysis, ICP-OES, XRD, N₂-sorption measurements, TG & DTA, solid state ¹³C, ²⁹Si NMR spectroscopy, FT-IR, Raman spectroscopy, XPS, DRS UV-Vis spectroscopy, SEM and TEM. XRD and N₂ sorption analyses helped to find out the morphological and textural properties of the synthesized catalysts and confirm that an ordered mesoporous channel structure was retained even after the multistep synthetic procedures. The (100), (110) and (200) reflections in SBA-15 provide hints of a good structural stability, the existence of long range ordering and a high pore wall thickness. TG and DTA results reveal that the thermal stability of (L)Mo(vi)O₂@SBA-15 was maintained up to 300°C. The organic moieties anchored over the surface of the SBA-15 support were determined by solid state ¹³C NMR and FT-IR spectroscopy. Further, solid state ²⁹Si NMR spectroscopy provides the information about the degree of functionalization of the surface silanol groups with the organic moiety. The electronic environment and the oxidation state of the molybdenum site in (L)Mo(vi)O₂@SBA-15 were monitored by Raman spectroscopy, XPS and DRS UV-Vis techniques. Moreover, the morphology and topographic information of the synthesized catalysts were confirmed by SEM and TEM imaging. The synthesized catalysts were evaluated in epoxidation and sulfoxidation reactions, and the results show that (L)Mo(vi)O₂@SBA-15 exhibits high conversion and selectivity towards epoxidation and sulfoxidation reactions in combination with high stability. The anchored solid catalysts can be recycled effectively and reused several times without major loss in activity. In addition, Sheldon's hot filtration test was also carried out.

Full text of DOI:10.1039/c3ra47222a

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Synthesis and characterization of 3-[N,N0-bis-3-
(salicylidenamino)ethyltriamine] Mo(VI)O2@SBA-15:
a highly stable and reusable catalyst for
Cite this: RSC Adv., 2014, 4, 14063
epoxidation and sulfoxidation reactions
a
b
a
*
Anish Lazar, Werner R. Thiel and A. P. Singh
The efficient and reusable oxidation catalyst 3-[N,N0-bis-3-(salicylidenamino)ethyltriamine] Mo(VI)O2@SBA-
15 has been synthesized by the anchoring of the 3-[N,N-bis-3-(salicylidenamino)ethyltriamine] ligand (L or
Salpr) on the inner surfaces of organofunctionalized SBA-15 and subsequent complexation with Mo(VI)
O2(acac)2. The physico-chemical properties of the functionalized catalysts were analyzed by elemental
analysis, ICP-OES, XRD, N2-sorption measurements, TG & DTA, solid state 13C, 29Si NMR spectroscopy,
FT-IR, Raman spectroscopy, XPS, DRS UV-Vis spectroscopy, SEM and TEM. XRD and N2 sorption
analyses helped to find out the morphological and textural properties of the synthesized catalysts and
confirm that an ordered mesoporous channel structure was retained even after the multistep synthetic
procedures. The (100), (110) and (200) reflections in SBA-15 provide hints of a good structural stability,
the existence of long range ordering and a high pore wall thickness. TG and DTA results reveal that the
thermal stability of (L)Mo(VI)O2@SBA-15 was maintained up to 300 ꢀC. The organic moieties anchored
over the surface of the SBA-15 support were determined by solid state 13C NMR and FT-IR spectroscopy.
Further, solid state 29Si NMR spectroscopy provides the information about the degree of
functionalization of the surface silanol groups with the organic moiety. The electronic environment and
the oxidation state of the molybdenum site in (L)Mo(VI)O2@SBA-15 were monitored by Raman
spectroscopy, XPS and DRS UV-Vis techniques. Moreover, the morphology and topographic information
of the synthesized catalysts were confirmed by SEM and TEM imaging. The synthesized catalysts were
evaluated in epoxidation and sulfoxidation reactions, and the results show that (L)Mo(VI)O2@SBA-15
exhibits high conversion and selectivity towards epoxidation and sulfoxidation reactions in combination
with high stability. The anchored solid catalysts can be recycled effectively and reused several times
without major loss in activity. In addition, Sheldon's hot filtration test was also carried out.
Received 2nd December 2013
Accepted 6th March 2014
DOI: 10.1039/c3ra47222a
www.rsc.org/advances
most widely utilized classes of ligands in inorganic chemistry,
which are capable to bind various metal ions to give complexes
1. Introduction
Catalytic oxidations, especially olen epoxidation1 and sul-
foxidation2 reactions, using transition metal complexes,3,4 are
considered to be among the most eco-friendly, most important
reactions in industrial chemistry. Oxidation catalysis has
played as leading role in this era due to the production of huge
quantities of intermediates and monomers for industrial
chemistry. Epoxides are versatile precursors in the synthesis of
a variety compounds such as resins, cosmetics, surface coat-
ings, sweeteners, perfumes, drugs, etc. and sulfoxides are well
known valuable building blocks to produce biologically
important compounds.5,6 Schiff-bases7,8 represent one of the
with tunable properties. They are also used as a ligands in
industrial chemistry due to the following the facts: (i) there are
simple synthetic methods for preparation in large scale (ii)
they provide several binding sites for metals and can be
designed to leave vacant sites for coordination of substrates
(iii) the substitution on the aromatic ring allows a ne-tuning
of their properties.
In recent years, the design and synthesis of catalytically
active supported metal complexes has received increasing
attention due to the some disadvantages of homogeneous
systems compared to heterogeneous catalysts. In the case of
molybdenum based catalysts9–11 derived from Schiff-base
ligands, cis-dioxomolybdenum species have been particularly
investigated because of their good catalytic activity for the
selective oxidation of cycloalkenes and suldes. A variety
of Schiff-base ligands such as bidentate-N-donor ligands
aCatalysis division, CSIR-National Chemical Laboratory, Pune-411008, India. E-mail:
ap.singh@ncl.res.in; Fax: +91-20-2590 2633; Tel: +91-20-2590 2497
bFachbereich Chemie, Technische Universitat Kaiserslautern, 67663 Kaiserslautern,
Germany
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(bipyridines,12 diazabutadienes13 and pyrazolylpyridines14) and
tetradentate-N-ligands15,16 (Salpr) have been used to improve
the catalytic activity of these reactions with suitable peroxides.
Environmentally benign, safer and economically favorable
oxidants such as tert-butyl hydroperoxide (TBHP) and
hydrogen peroxide (H2O2)17–19 have been used for epoxidation
and sulfoxidation reaction with such systems. Out of these
ligands, the tetradentate-N-donor ligand, ‘Salpr’{3-[N,N-bis-
3(salicylidenamino)ethyl triamine]} was used as the ligand for
epoxidation and sulfoxidation reactions due to its high
chelating ability with molybdenum complexes.20 To avoid the
well known limitations from the homogeneous complexes
such as poor recyclability, catalyst contamination in the
products, etc., supported complexes of catalytically active
metals have been synthesized during the recent years. Meso-
porous SBA-15 (ref. 21) shows signicant attraction in this
context due to its high surface area, its uniform pore sizes, its
high wall thickness and its high hydrothermal stability
compared to other mesoporous materials like MCM-41, MCM-
48. To extend the applicability of SBA-15 materials, it is
necessary to modify the surface by organic functional groups
for anchoring metals and metal complexes.
The stability and selectivity of catalysts in epoxidation
and sulfoxidation reactions were challenging tasks in the
last decades. To overcome this limitations, SBA-15 was used as
the support to immobilize of neat (L)Mo(VI)O2 complex than
other mesoporous materials, due to its higher wall thickness.22,23
Further, chelation of molybdenum with Schiff base ligand, Mo(VI)
O2-3-[N,N-bis-3(salicylidenamino)ethyltriamine] complex, over
SBA-15 provides extra stability to the homogeneous complexes. In
this work, we report, the synthesis, characterization and catalytic
applications of highly stable (L)Mo(VI)O2@SBA-15 for efficient
heterogeneous epoxidation and sulfoxidation reactions. The
catalyst is recyclable and exhibits high catalytic activities.
Scheme 1 Organofunctionalization and immobilization of the neat (L)
Mo(VI)O2 complex over SBA-15, 1a: synthesis of homogeneous
complex [(L)Mo(VI)O2 complex], 1-b: chloro functionalization of SBA-
15 [PrCl@SBA-15], 1-c: capping of PrCl@SBA-15 [–OH protected
PrCl@SBA-15], 1-d: heterogenization of homogeneous complex [(L)
Mo(VI)O2@SBA-15].
0.043TEOS–4.4 g P123 Mavg ¼ 5800 ¼ [EO20–PO70–EO20]
2. Experimental
2.1 Synthesis
–8.33H2O–0.24HCl
Tetraethylorthosilicate (TEOS), Pluronic 123 (P123, average
molecular
weight:
5800),
3,5-di-tert-butyl-2-hydroxy-
Typically, 4.4 g of the tri-block co-polymer was dispersed in
benzaldehyde, 3-chloropropyl trimethoxy silane (3-CPTMS), 30 g of distilled water and stirred for 1.5 h. To the resultant
dimethoxydimethyl silane ((MeO)2Me2Si), cyclohexene, cyclo- solution, 120 g of 2 M HCl was added under stirring and the
octene, cyclohexene oxide, cyclooctene oxide and suldes were stirring was continued for 2 h. Finally, 9 g of TEOS was added
purchased from Aldrich. Diethylene triamine and HCl (36.5%) drop wise and the mixture was maintained at 40 ꢀC for 24 h with
were purchased from Merck and Thomas Baker (India), continuous stirring. The mass was submitted to a hydrothermal
respectively. Dry reagent grade solvents were obtained from treatment (100 ꢀC, 48 h) under static condition. The precipitate
Merck (India) and further dried before use according to stan- was ltered, washed with distilled water, dried in an oven
dard methods.
(90 ꢀC, 12 h) and then calcined in the air (500 ꢀC, 6 h) to remove
The highly stable and reusable heterogeneous complex, (L) the template completely. The calcined SBA-15 was characterized
Mo(VI)O2@SBA-15 was synthesized under a nitrogen atmo- by powder XRD.
sphere in a step-by-step manner (Scheme 1). The synthesis of
Surface modication of SBA-15 was achieved by a post
mesoporous SBA-15 (ref. 24) was carried out hydrothermally synthesis graing method (Scheme 1-b). One gram of SBA-15
under the autogeneous pressure in an autoclave. The polymer was suspended in 50 ml of dry toluene and then heated to reux
surfactant P123 was used as a template and hydrochloric acid together with 3.7 mmol of 3-chloropropyltrimethoxysilane
served as a mineralizer. The molecular composition of the (3-CPTMS) for 8 h under a N2 atmosphere. The material was
gelating mixture was the following:
ltered aer cooling to ambient temperature, washed with dry
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toluene and then with dichloro methane. Soxhlet extraction was appropriate cycloalkene in 25 ml of chloroform under a nitrogen
carried out for 24 h with dichloro methane (CH2Cl2) as the atmosphere. The mixture was heated to reux for 18 h.25 Samples
solvent to remove occluded organosilane. The sample was then were withdrawn periodically and analyzed by using a Gas chro-
dried in vacuum for 10 h. The material obtained is designated matograph (Agilent 6890) equipped with a capillary column
as PrCl@SBA-15 (Scheme 1-b). It is evident from elemental (HP-5) and a ame ionization detector (FID).
analysis (C, H, N analysis) that the surface silanol groups on
A 25 ml of round bottom ask was loaded with 5 ml of a
SBA-15 were organofunctionalized by 33% chloropropyl suspension responding solution of the appropriate catalysts
(3-CPTMS) groups as organic modier. The free –OH groups still [25 mg of (L)Mo(VI)O2@SBA-15 or 10 mg (0.015 mmol) of the
present in PrCl@SBA-15 were protected by adding 1.5 mmol of neat (L)Mo(VI)O2 complex] in CH3CN. To this reaction mixture, 1
dimethoxydimethylsilane to a stirred suspension of 1 g of mmol of the suldes (thioanisoles) and 1.1 mmol of 30% H2O2
PrCl@SBA-15 in dry toluene (50 ml), followed by stirring for 12 h were added at room temperature and stirring was continued for
at reux temperature under an inert atmosphere. The resulting 3 h.26 Samples were periodically withdrawn from the reaction
material was ltered off, washed with toluene and was extracted mixture, ltered off, and analyzed with GCMS. Sulde conver-
in a Soxhlet with CH2Cl2 for 24 h. The obtained material was sion (wt%) and selectivity (%) were determined by using a gas
named as –OH protected PrCl@SBA-15 (Scheme 1-c).
chromatograph (Agilent 6890) equipped with a ame ionization
The Salpr ligand 3-[N,N0-bis-3(salicylidenamino)ethyltri- detector (FID) and a capillary column (HP-5).
amine] was prepared according to a reported procedure25
(Scheme 1-a). Diethylene triamine (0.5158 g, 5 mmol) was added
into an ethanolic solution of 3,5-di-tert-butyl-2-hydroxy-
2.3 Analytical procedures and equipment
benzaldehyde (2.3433 g, 10 mmol) and the resulting yellow Elemental analysis (C, H and N) were performed on a Carlo Erba
colored solution was heated to reux for 3 h. Aer that, the (Model EA 1108) elemental analyzer. The molybdenum content
excess of solvent was removed under vacuum and a dark yellow in the material was determined by using an inductively coupled
oily product was obtained aer purication by column chro- plasma-optical emission spectrometer (ICP-OES) on a Therm
matography [silica column, hexane–ethyl acetate (9 : 1)]. 2 g IRIS Intrepid II XSP aer solubilization of the samples in HF/
(0.3732 mmol) of the Salpr ligand were dissolved in 30 ml of HCl solutions. Powder X-ray diffraction (XRD) patterns were
ethanol. This solution was added dropwise into an ethanolic measured on a PAN analytical X'pert Pro dual goniometer
˚
solution of Mo(VI)O2(acac)2 (1.384 g in 120 ml) in a 250 ml two diffractometer using Ni-ltered CuKa radiation (l ¼ 1.5404 A)
neck round bottom ask and the mixture was heated to over the 2q range of 0.5–10ꢀ, with a scan speed of 1ꢀ minꢁ1. A
reux for 24 h. Aer ltration, the product was dried and proportional counter detector was used for the low angle
then extracted in a Soxhlet apparatus with a mixture of experiments and the data collection was carried out using a at
dichloromethane and ethanol (1 : 1) to remove unreacted holder in Bragg–Brentano geometry. N2 adsorption–desorption
Mo(VI)O2(acac)2.
isotherms, pore size distributions as well as the textural prop-
To a suspension of –OH protected PrCl@SBA-15 (1 g) in 30 erties of the hybrid materials were determined by using an
ml of toluene, a solution of the Salpr ligand (L) (1.28 g, 0.0024 Autosorb 1C Quantachrome USA. The program consisting of
mol) in 10 ml of toluene was added, and then the mixture was both an adsorption and desorption branch and typically run at
heated to reux for 24 h under an inert atmosphere. The ꢁ196 ꢀC aer the samples were degassed at 150 ꢀC for 4 h. The
resulting suspension was ltered, the residue was washed with BET method was applied to calculate the total surface area at
toluene and CH2Cl2, and further puried extraction in a Soxhlet relative pressures of P/Po ¼ 0.65–0.45 and the BJH model was
apparatus with a mixture of CH2Cl2 and diethylether (1 : 1) for applied to the adsorption branch of the isotherm to determine
24 h. The nal material was designated as 3-[N,N0-bis-3-(salicy- the total pore volume and average pore diameter at a relative
lidenamino)ethyltriamine]@SBA-15 or (L)PrCl@SBA-15. For pressure of P/Po ¼ 0.99. Pore size distribution (PSD) curves from
complexation, an excess of Mo(VI)O2 (acac)2 (1 g, 3 mmol) in BJH desorption branch were obtained via the NLDFT model
100 ml ethanol was added into 2 g of dried (L)PrCl@SBA-15 assuming cylindrical pore geometry. Thermal analysis (TG-DTA)
and heated to reux for 24 h to get (L)Mo(VI)O2@SBA-15 of the samples was conducted using a Pyriꢁs1Diamond TGA
(Scheme 1-d). Aer ltration, the product was dried and then analyzer with a heating rate of 10 ꢀC min
under an air
extracted in a Soxhlet apparatus with a mixture of dichloro- atmosphere. FTIR spectra of the solid samples were recorded in
methane and ethanol (1 : 1) to remove unreacted Mo(VI)O2 (acac)2. the range of 4000–400 cmꢁ1 on a Bruker alpha T FTIR spec-
trophotometer at room temperatures. Raman spectra of the
complexes were recorded with a Horiba Jobin YVON Lab Ram
HR spectrometer at 633 nm with a He–Ne laser. Magic angle
2.2 Procedure for catalytic reactions
The epoxidation of cycloalkenes such as cyclohexene and cyclo- spinning (MAS) NMR spectra for 29Si and 13C nuclei were
octene were carried out in a 50 ml double necked round bottom recorded on BRUKER DSX300 spectrometer at 7.05 T (resonance
ask tted with a water-cooled condenser and a magnetic stirrer frequencies: 29Si, 59.595 MHz, rotor speed 10 000 Hz, external
by using anhydrous TBHP as the oxidant. In a typical procedure, reference Si(OCH3)4; 13C, 75.43 MHz, rotor speed 10 000 Hz;
anhydrous TBHP (5.0–6.0 M in decane, 1.6 ml, 14.4 mmol) was contact time: 1.5 ms; pulse delay: 5 ms). XPS measurements were
added into a mixture of (L)Mo(VI)O2@SBA-15 (150 mg) or the neat performed on a VG Microtech ESCA 3000 instrument, using
(L)Mo(VI)O2 complex (75 mg, 0.113 mmol) and 8 mmol of the nonmonochromatized Mg-Ka radiation at a pass energy of 50 eV
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and an electron take off angle of 60ꢀ. The correction of the
binding energy was performed by using the C1s peak of carbon
at 284.9 eV, as the reference. DRS UV-Vis spectra of the
complexes were analyzed by using a Perkin-Elmer model
Lambda 650 spectrophotometer. The scanning electron micro-
graphs of the samples were obtained in a dual beam scanning
electron microscope (FEI company, model Quanta 200 3D)
operating at 30 kV. The samples were loaded on stubs and
sputtered with a thin gold lm to prevent surface charging and
also to protect from thermal damaging due to the electron
beam. A JEOL JEM-3010 and Tecnai (Model F30) both operating
at 300 kV were used for HRTEM samples imaging. The samples
were crushed and dispersed in isopropanol with low power
sonication before putting a drop of this suspension over a
carbon coated Cu grid for observation. All synthesized samples
were irradiated before the submission of all characterization
techniques. The epoxidation reaction of cycloalkenes were
performed at 70 ꢀC for 18 h using anhydrous TBHP (5.0–6.0 M in
decane) as oxidant and CHCl3 as solvent. To extend the scope of
the catalytic activity, sulfoxidation reactions of suldes were
ꢀ
carried out at 26 C for 3 h using aqu. H2O2 (70%) as oxidant
and CH3CN as solvent.
3. Results and discussion
3.1 Characterization
The morphological and textural properties of all synthesized
catalysts were investigated by using powder XRD (Fig. 1a–d) and
N2 adsorption–desorption analysis (Fig. 2A and B). Moreover,
these analyses conrmed that the ordered mesoporous channel
structure was retained even aer the organofunctionalization
and the immobilization of the neat (L)Mo(VI)O2 complex over
the SBA-15 support. Fig. 1 exhibits the XRD spectra of (a) as-
synthesized SBA-15 (b) calcined SBA-15 (c) –OH protected
PrCl@SBA-15 (d) (L)Mo(VI)O2@SBA-15. The parent SBA-15
shows three reections at 2q ¼ 0.9ꢀ, 1.5ꢀ and 1.7ꢀ, which are
assigned to the (100), (110) and (200) Miller indices, respec-
tively, indicating the formation of a highly ordered hexagonal
structure.27 In all synthesized samples, the (110) reections are
Fig. 2 N2 adsorption–desorption isotherms and pore size distribu-
tions (inset) of (A) calcined SBA-15 and (B) (L)Mo(VI)O2@SBA-15.
more intense than the (200) reection, which favors a more
complete condensation of the wall structure due to the high
temperatures selected for the hydrothermal synthesis and the
further calcination. The existence of the (100), (110) and (200)
reections (Fig. 1) provides a hint for a high structural stability
of the material, the existence of a long range ordering and a
large pore wall thickness even aer a number of treatments with
organic molecules. The peak intensities of the (100) reections
are decreasing with broadening from calcined SBA-15 to (L)
Mo(VI)O2@SBA-15 due to the proper loading of the organic
modier and the neat (L)Mo(VI)O2 complex in the calcined
SBA-15. Moreover, the decrease in signal intensity may be due to
loss of regularity in the 2D hexagonal structure of mesoporous
SBA-15. The structural stability and ordered mesoporosity of the
samples have retained aer incorporation of (L)Mo(VI)O2
complex in the SBA-15 support.
The N2 adsorption–desorption isotherms and pore size
distributions (inset) of calcined SBA-15 and (L)Mo(VI)O2@SBA-15
are shown in Fig. 2A and B, respectively. (L)Mo(VI)O2@SBA-15
shows type-IV isotherms with a H1 hysteresis related to capillary
condensation steps, a characteristic feature of the highly ordered
mesoporous materials (pore size: 2–50 nm).28 Textural properties
of calcined SBA-15 and (L)Mo(VI)O2@SBA-15 samples are
summarized in Table 1. The mesoporous SBA-15 exhibits a BET
Fig. 1 XRD pattern of (a) as-synthesized SBA-15 (b) calcined SBA-15 (c)
–OH protected PrCl@SBA-15 (d) (L)Mo(VI)O2@SBA-15.
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surface area of 601 m2 gꢁ1, a pore volume of 1.02 cm3 gꢁ1 and
˚
a mean pore diameter of 66 A. As shown in Table 1, compared
to SBA-15, a further reduction of the surface area from 601 to
252 m2 gꢁ1 and of the mean pore size from 66 to 56 A was
˚
observed for (L)Mo(VI)O2@SBA-15.
The SBA-15 samples exhibit a sharp increase in the N2
adsorption at a higher P/Po value (ꢂ0.65) indicating the
uniformity of the mesoporous structure. In the case of (L)Mo(VI)
O2@SBA-15, the P/Po value changed to a lower value of ꢂ0.6,
indicative of a minor structural damage of the material aer the
modications and being consistent with the XRD results. The
signicant decrease in surface area, in pore diameter and in
pore volume of (L)Mo(VI)O2@SBA-15 indicates the successful
anchoring of organic modier group and the further complex-
ation with the molybdenum site occurring on SBA-15, which
reduces a part of the textural qualities of the support.
Thermal analysis is a technique for measuring changes
in the physico-chemical properties of substances as a function
of temperature. The thermal stability of (a) as-synthesized
SBA-15 (b) calcined SBA-15 (c) –OH protected PrCl@SBA-15
(d) (L)Mo(VI)O2@SBA-15 was determined by TGA (Fig. 3A) and
DTA (Fig. 3B) in an atmosphere of air from 30 to 1000 ꢀC with
a temperature ramp of 10 ꢀC minꢁ1. The TGA curve of the
as-synthesized SBA-15 [Fig. 3A(a)] exhibits a 48% weight loss
with a corresponding exothermic peak observed in the DTA
analysis [Fig. 3B(a)] in the region of 170–220 ꢀC which is
assigned to the removal of surfactant from as-synthesized
sample. Calcined SBA-15 [Fig. 3A(b)] shows only 13% weight
loss from TG curve with no other peak observed in DTA
analysis [Fig. 3B(b)]. These evidences support the complete
removal of the surfactants from the calcined SBA-15 and the
successful formation of a pure siliceous SBA-15 material. The
TGA plot of –OH protected PrCl@SBA-15 [Fig. 3A(c)] shows
Fig. 3 TGA (A) and DTA (B) pattern of (a) as-synthesized SBA-15 (b)
calcined SBA-15 (c) –OH protected PrCl@SBA-15 (d) (L)Mo(VI)
O2@SBA-15.
three distinct weight losses corresponding to exothermic
ꢀ
peaks in the DTA [Fig. 3B(c)] (i) between 70 and 150 C cor- PrCl@SBA-15 quantitatively shows 26% of weight loss, being
responding to physisorbed water molecules (ii) between 230 greater than in the case of calcined SBA-15. This strongly
and 310 ꢀC indicating the decomposition of the chloropropyl supports the successful anchoring of 3-CPTMS on the
ꢀ
moiety from PrCl@SBA-15 (iii) between 330 and 450 C rep- calcined SBA-15. Further, (L)Mo(VI)O2@SBA-15 shows 42% of
resenting the combustion of dimethoxydimethylsilane acting weight loss, which is greater than the loss observed for the
as the capping agent. In the case of (L)Mo(VI)O2@SBA-15 –OH protected PrCl@SBA-15, strongly supporting a 16%
[Fig. 3B(d)], one extra peak along with two peaks already loading of neat (L)Mo(VI)O2 sites in the calcined SBA-15. All
observed in the –OH protected PrCl@SBA-15, in the region of these results support that the material was synthesized and
450–510 ꢀC is observed.29 It conrms the high thermal the (L)Mo(VI)O2 sites are directly anchored over the modied
stability of (L)Mo(VI)O2 sites on SBA-15. In some cases the SBA-15.
weight loss between 530 and 570 ꢀC arises due to an addi-
Infrared spectroscopy helped to nd out the nature of the
tional water loss by an ongoing condensation of residual surface functional groups being present in the materials and it
silanol groups.30 The TGA result of the –OH protected is furthermore used for monitoring of the multistep assembly of
Table 1 Textural propertiesa of calcined SBA-15 and (L)Mo(VI)O2@SBA-15
Mo contentb mmol gꢁ1
a0 [A]
BET SA [m2 gꢁ1
]
Dp [A] (BJH)
Vp [cm3 gꢁ1] (BJH)
utd (A)
c
˚
˚
˚
Sample
SBA-15
(L)Mo(VI)O2@SBA-15
—
0.18
106.02
105.46
601
252
66
56
1.02
0.37
40.02
49.46
a
a0, unit cell parameter; SA, surface area; Dp, pore diameter; Vp, pore volume; ut, pore wall thickness. b From ICP-OES analysis. c a0 ¼ 2d100/1.73.
ut ¼ a0 ꢁ Dp.
d
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the catalyst inside the mesoporous SBA-15 material. FT-IR (L)Mo(VI)O2 complex [Fig. 4B(a)] and in (L)Mo(VI)O2@SBA-15
spectra of (a) as-synthesized SBA-15 (b) calcined SBA-15 (c) –OH [Fig. 4B(b)], peaks at 755 cmꢁ1 and 1420 cmꢁ1 are assigned to C–
protected PrCl@SBA-15 and (a) the neat (L)Mo(VI)O2 complex (b) H bending and C]C stretching vibrations, respectively, of the
(L)Mo(VI)O2@SBA-15 are shown in Fig. 4A and B, respectively. In arene groups. Moreover, a strong band at 1640 cmꢁ1 is assigned
the spectrum of the as-synthesized SBA-15 [Fig. 4A(a)], peaks at to the –C]N stretching vibration of the azomethylene group.
2980 cmꢁ1 and 2895 cmꢁ1 indicate the stretching vibrations of Characteristic peaks at 940 cmꢁ1 and 910 cmꢁ1 are assigned to
the –CH2 groups being present in the surfactant molecules. In the symmetric and asymmetric vibrational modes of the cis-
calcined SAB-15 [Fig. 4A(b)], a broad band at 3600–3200 cmꢁ1 coordinated MoO2 moiety of the neat (L)Mo(VI)O2 complex,
and a weak band at 3738 cmꢁ1 are attributed to the n-OH which are not observed for (L)Mo(VI)O2@SBA-15 due to a over-
stretching vibrations of hydrogen bonded and isolated surface lapping of these peaks with n(Si–OH) vibrations. The MoO2
silanol groups being present in the host materials.31,32 Further, a species prefers to form a cis conguration due to a maximized
sharp band at 1628 cmꢁ1 is corresponding to the –OH bending utilization of the dp groups at molybdenum centre.33 In all
vibration of the silanol groups. Moreover, the absence of strong these samples, except the neat (L)Mo(VI)O2 complex, the asym-
absorptions in calcined SBA-15 at 3000–2700 cmꢁ1 indicates the metric and symmetric stretching vibrations of Si–O bonds in the
complete removal of surfactants.
Si–O–Si framework are observed at 798 cmꢁ1 and 1080 cmꢁ1
,
Aer the chlorofunctionalization of the SBA-15 [Fig. 4A(c)], respectively [Fig. 4A(a–c) and B(b)]. A strong band at 954 cmꢁ1 is
the peaks at 3738 cmꢁ1 and 3600–3400 cmꢁ1 had disappeared, attributed to the n(Si–OH) vibration,34,35 indicating the effective
indicating that the silanol groups on the surface of SBA-15 are formation of siliceous materials.
transferred into a Si–O–Si framework. Further, peaks at 2980
In addition to this, the results of Raman spectroscopic inves-
cmꢁ1 and 2895 cmꢁ1 appeared being assigned to the stretching tigations, being technique complementary to IR spectroscopy, of
vibrations of the –CH2 groups in propyl chain of the organic (a) the neat (L)Mo(VI)O2 complex (b) (L)Mo(VI)O2@SBA-15 are
modiers which evidently supports that the chloropropyl plotted in Fig. 5. The characteristic peaks at 821 and 990 cmꢁ1
,
group is attached to the SBA-15. In both compounds, the neat corresponds to asymmetric and symmetric stretching vibrations
of the cis-coordinated MoO2 moiety.36 This indicates that the
oxidation state of molybdenum is VI, which is though retained
even aer immobilization over SBA-15. Further, results obtained
from the Raman spectra are in good agreement with the results of
the IR spectroscopic investigations.
The organic moieties anchored over the surface of SBA-15 are
further conrmed by NMR techniques. The 13C solid state NMR
spectra of (A) PrCl@SBA-15 (B) –OH protected PrCl@SBA-15 and
(C) (L)Mo(VI)O2@SBA-15 are depicted in Fig. 6. The presence of
peaks at 10, 22 and 43 ppm, assigned to the carbon atoms (C1–
C3) of the propyl chain in organic modier, indicate the
successful chlorofunctionalization of SBA-15 (Fig. 6A). Addi-
tionally a sharp peak at ꢁ1.7 ppm (C4) evidences the presence of
–CH3 groups as result of the capping of residual hydroxyl groups
by dimethoxy dimethylsilane in –OH protected SBA-15 (Fig. 6B).
Fig. 4 FT-IR spectroscopy of (A) [(a) as-synthesized SBA-15, (b)
calcined SBA-15, (c) –OH protected PrCl@SBA-15] and (B) [(a) the neat Fig. 5 Raman spectroscopy of (a) the neat (L)Mo(VI)O2 complex (b) (L)
(L)Mo(VI)O2 complex (b) (L)Mo(VI)O2@SBA-15].
Mo(VI)O2@SBA-15.
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The degree of functionalization of surface silanol groups
with organic moieties on the mesostructured materials can be
monitored by means of 29Si CP-MAS NMR spectroscopy. 29Si CP-
MAS NMR spectra of (A) calcined SBA-15 (B) PrCl@SBA-15 (C)
–OH protected PrCl@SBA-15 (D) (L)Mo(VI)O2@SBA-15 are
depicted in Fig. 7. The spectrum of calcined SBA-15 (Fig. 7A)
shows broad resonance peaks from ꢁ90 to ꢁ115 ppm, indica-
tive for a range of Si–O–Si and Si–OH bonds. The bands
centered at ꢁ93 ppm, ꢁ103 ppm and ꢁ113 ppm can be
assigned to the Q2 [germinal silanol, (SiO)2Si(OH)2], Q3 [single
silanol, (SiO)3Si(OH)] and Q4 [siloxane, (SiO)4Si] sites of the
silica framework, respectively.31,32
Compared to the parent SBA-15, PrCl@SBA-15 (Fig. 7B)
shows a decrease in the Q3 and Q2 intensities with a corre-
sponding increase in the percentage of Q4 sites showing that the
3-CPTMS effectively consumes the geminal as well as the single
silanol sites. The appearance of the Q3 signal indicates the
presence of some residual non-condensed OH groups attached
to the silicon atoms. Aer chlorofunctionalization (Fig. 7C), two
additional peaks at ꢁ69 ppm and at ꢁ61 ppm were observed
and assigned to T3 [SiR(OSi)3] and T2 [Si(OH)R(OSi)2] units,
respectively. The existence of the T3 signal conrms that SBA-15
has been modied by organic moieties. Further, a sharp peak at
ꢁ22 ppm in the –OH protected PrCl@SBA-15 material indicates
the Si atoms of the methoxydimethylsilyl units which are
present as capping sites (Fig. 7C). The (Q3 + Q2)/Q4 ratio indi-
cates the presence of silanol groups residing on the support
surface, and a lower (Q3 + Q2)/Q4 value for chlorofunctionalized
samples suggests that the material contains fewer residual
silanols, resulting in a siliceous pore wall structure with a
greater degree of condensation and higher hydrothermal
stability. Changes in the relative intensities of the Q4, Q3 and
Q2 signals can be explained by the redistribution of the
silicon sites during the surface silylation. In the spectrum of
(L)Mo(VI)O2@SBA-15 (Fig. 7D), peak at ꢁ20 ppm decreases
with an increase of the Q3 sites compared to Q4 indicating the
removal of some capping units ongoing with a formation of
free silanol groups under the drastic synthetic conditions
(Fig. 7D). 29Si CP-MAS NMR spectra provide direct evidence for
the formation of a highly condensed siloxane network with
organic group covalently bound to the mesoporous silica and
also revealed that both the synthesis process and surfactant-
extraction treatment did not cause cleavage of the Si–C bonds.
XPS is a surface technique which helps us to nd out the
oxidation state and chemical environment of atoms due to the
shi in binding energies. The XPS spectra of the Mo3d core level
Fig. 6 Solid state 13C CP MAS NMR spectroscopy of (A) PrCl@SBA-15
(B)–OH protected PrCl@SBA-15 (C) (L)Mo(VI)O2@SBA-15.
In the case of (L)Mo(VI)O2@SBA-15 (Fig. 6C), resonances in the of (a) the neat (L)Mo(VI)O2 complex and (b) (L)Mo(VI)O2@SBA-15
region between 110 and 150 ppm correspond to the aromatic are depicted in Fig. 8. The correction of the binding energies
moieties of the Salpr ligand. Moreover, in the region between was performed by using the C1s peak of carbon at 284.9 eV as
0 and 50 ppm, two extra resonances at 28 ppm and 34 ppm the reference. In the neat (L)Mo(VI)O2 complex (Fig. 8a), peaks at
(Fig. 6C), represent the methyl carbon atoms of the tert-butyl 233 and 236 eV correspond to the 3d5/2 and 3d3/2 spin–orbit
groups. The small peak at 165 ppm is ascribed to the carbon component, which are shied to higher binding energy values
atoms of the imine groups in (L)Mo(VI)O2@SBA-15. Further- (234 and 237, respectively) in the case of (L)Mo(VI)O2@SBA-15
more, one shoulder peak at 56 ppm is assigned to –CH2 groups (Fig. 8b). These binding energies correspond to Mo(VI)
of diethylene triamine being present in (L)Mo(VI)O2@SBA-15. species and prove the unchanged oxidation state of the
All the resonance peaks support the successful anchoring of the metal ion even aer heterogenization of the (L)Mo(VI)O2 sites
(L)Mo(VI)O2 sites on SBA-15.
over SBA-15.
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The DRS UV-Vis spectra of (a) calcined SBA-15 (b) the neat
(L)Mo(VI)O2 complex (c) (L)Mo(VI)O2@SBA-15 are presented in
Fig. 9 to get further information about the coordination
environment and the oxidation state of molybdenum in the
neat (L)Mo(VI)O2 complex as well as (L)Mo(VI)O2@SBA-15. The
spectra of calcined SBA-15 (Fig. 9a) and (L)Mo(VI)O2@SBA-15
(Fig. 9c) exhibit a peak at 225 nm, typical for siliceous mate-
rials.37 In both compounds, namely, the neat (L)Mo(VI)O2
complex (Fig. 9b) and (L)Mo(VI)O2@SBA-15, two bands at 240
and 280 nm are attributed to p / p* transitions of the
aromatic ring and the azomethine group, respectively.
Further, a broad peak at 330 nm is assigned to n / p*
transitions of the azomethine group.38 Moreover, a broad
band at 440 nm is attributed to N / Mo(VI) and O / Mo(VI)
ligand to metal charge-transfer transitions (LMCT) due to the
promotion of an electron from the ligand centered highest
occupied molecular orbital (HOMO) to the molybdenum
centered lowest unoccupied molecular orbital (LUMO).39 The
absence of a d–d transition in the visible region (400 nm to
Fig. 8 XPS spectra of the Mo3d core level of (a) the neat (L)Mo(VI)O2
complex (b) (L)Mo(VI)O2@SBA-15.
Fig. 7 Solid state 29Si CP MAS NMR spectroscopy of (A) calcined SBA-
15 (B) PrCl@SBA-15, (C) –OH protected PrCl@SBA-15 (D) (L)Mo(VI)
O2@SBA-15.
Fig. 9 DRS-UV-Vis spectroscopy of (a) calcined SBA-15 (b) the neat (L)
Mo(VI)O2 complex (c) (L)Mo(VI)O2@SBA-15.
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800 nm) conrms the 4d0 electron conguration of the conversion and 95% of selectivity), due to the presence of a
molybdenum sites.40 more electron rich C]C double bond.41 The neat (L)Mo(VI)O2
Scanning electron microscopy (SEM) is an important tool for complex gave 88% and 78% conversion for cyclooctene and
the morphological characterization of mesoporous molecular cyclohexene, respectively. The corresponding selectivities for
sieve materials. Fig. 10(A and B) shows the SEM images of both epoxides were found to be 96%. The blank experiment
calcined SBA-15 and (L)Mo(VI)O2@SBA-15 with a 2D p6mm showed a very low cycloalkene conversion with a poor epoxide
hexagonal type structure. The particle size of both materials was selectivity even aer 18 h of reaction time.
found to be in the range between 1.15 and 1.25 mm. Further-
To nd out the widespread application of the heterogeneous
more, the morphology of (L)Mo(VI)O2@SBA-15 was retained catalyst, (L)Mo(VI)O2@SBA-15, sulfoxidation reactions of various
aer anchoring of the (L)Mo(VI)O2 sites over the functionalized suldes such as thioanisole, 4-chlorothioanisole, 4-bromothio
SBA-15 support. Transmission electron microscopy (TEM) is anisole, ethyl methyl sulde and diethyl sulde were carried out
typically used for high resolution imaging of thin lms of solid with H2O2 as the oxidizing agent and CH3CN as the solvent
samples for microstructural and compositional analysis. It has (Table 3). Normally, suldes like methyl phenyl sulde (thio-
been used to obtain topographic information about the meso- anisole), ethyl phenyl sulde and diethyl sulde exhibit slightly
porous matrices at nearly atomic resolution. The TEM images of higher conversions and comparative selectivities than
calcined SBA-15 and (L)Mo(VI)O2@SBA-15 shown in Fig. 11(A substituted thioanisoles (–Cl & –Br) due to the electronic effects
and B) revealed the formation of a regular hexagonal array of of –chloro and –bromo substituents in the substrates. As can be
uniform channels having a long-range ordering and well seen in Table 3, (L)Mo(VI)O2@SBA-15 catalyst gives slightly
dened 1-D channels. These results are further supported by better conversions (80% to 100%) and selectivities (85% to 91%)
XRD results presented earlier.
for various alkyl aryl suldes than the neat (L)Mo(VI)O2 complex
(conversion, 75% to 100% and selectivity, 84% to 88%). In the
absence of the catalyst, lower conversion (10% to 15%) and
selectivities (82% to 88%) for various alkyl aryl suldes were
obtained.
3.2 Catalytic activity
The catalytic activity and selectivity of the synthesized catalysts
were evaluated in the epoxidation cycloalkenes and in the
oxidation of suldes. The results of the epoxidation reactions
catalysed by (L)Mo(VI)O2@SBA-15 and by the neat (L)Mo(VI)O2
complex together with a blank experiment (absence of catalyst)
are depicted in Table 2. In the epoxidation reaction, cyclooctene
and cyclohexene were used as substrates together with anhy-
drous TBHP (5.0–6.0 M in decane) as the oxidizing agent and
CHCl3 as the solvent. In the case of (L)Mo(VI)O2@SBA-15, results
in Table 2 reveal that cyclooctene exhibits higher conversion
(93%) and selectivity (97%) than cyclohexene (86% of
3.3 Heterogeneity tests
To check the stability and recyclability of the catalyst,
(L)Mo(VI)O2@SBA-15 was recycled four times (fresh + three
cycles) in the epoxidation of cyclooctene and ve times (fresh
+ four cycles) in the sulfoxidation of thioanisole (Table 4).
Aer each cycle, the catalyst was removed by simple centri-
fugation, washed several times with a suitable solvent like
CHCl3 or CH3CN (each time in 5 ml at 26 ꢀC) and dried under
vacuum. In every cycle, it can be seen that there is no
considerable change in the conversion of the cyclooctene or
the thioanisole and for the selectivity of the products. Finally,
aer the last cycle, the ICP-OES analysis of the used catalyst
showed 0.04 mmol gꢁ1 molybdenum metal lost from the
fresh catalyst, which conrms that the stability of the
(L)Mo(VI)O2@SBA-15 was largely retained upto the third (in
the epoxidation) or the fourth (in the sulfoxidation) cycle.
In addition, to conrm the heterogeneity and stability of
(L)Mo(VI)O2@SBA-15 complex, Sheldon's hot ltration test was
carried out, indicating that there is virtually no molybdenum
content leaching into the reaction solution under the applied
reaction conditions. At rst, epoxidation reaction of cyclooctene
(Fig. 12) and sulfoxidation reaction of thioanisole (Fig. 13) were
performed over (L)Mo(VI)O2@SBA-15 complex under appro-
priate reaction conditions (fresh cycle), which are already
mentioned in the bottom of the Table 4. To perform the Shel-
don's hot ltration test in epoxidation of cyclooctene at 70 ꢀC for
18 h (Fig. 12), heterogeneous catalyst like (L)Mo(VI)O2@SBA-15
was ltered out from the reaction mixture aer 6 h during the
reaction, and the ltrate was again charged into round bottom
ask for the continuation of the reaction in the absence of
catalyst upto 18 h. The results show that 53% cyclooctane oxide
Fig. 10 SEM images of (A) calcined SBA-15 (B) (L)Mo(VI)O2@SBA-15.
Fig. 11 TEM images of (A) calcined SBA-15 (B) (L)Mo(VI)O2@SBA-15.
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Table 2 Epoxidation of cycloalkenesa
Cyclooctene
Con. (%)
Cyclohexene
Con. (%)
No
Catalyst
Oxide
Others
Oxide
Others
1
2
3
No catalyst
(L)Mo(VI)O2 complex
(L)Mo(VI)O2@SBA-15
6
88
93
88
96
97
12
4
3
5
78
86
86
96
95
14
4
5
a
Reaction conditions: (L)Mo(VI)O2@SBA-15, 150 mg or (L)Mo(VI)O2 complex, 75 mg; substrate, 8 mmol; TBHP (5.0–6.0 M in decane), 14.4 mmol;
CHCl3, 25 ml; reaction temperature, 70 ꢀC; reaction time, 18 h.
Table 3 Sulfoxidation of various sulfidesa
(L)Mo(VI)O2
complex
(L)Mo(VI)
O2@SBA-15
No catalyst
Substrate
A
B
C
A
B
C
A
B
C
Thioanisole
14 87 13 100 85 15 100 89 11
4-Chloro thioanisole 10 82 18
4-Bromo thioanisole 15 88 12
Ethyl methyl sulde 13 87 13
75 84 16
84 87 13
90 88 12
92 86 14
80 90 10
92 90 10
95 91
9
Diethyl sulde
12 85 15
96 85 15
a
A, conversion; B, sulfoxide; C, sulfone; reaction conditions: (L)Mo(VI)
O2@SBA-15, 25 mg or (L)Mo(VI)O2 complex, 10 mg; substrate, 1 mmol;
H2O2, 1 mmol; CH3CN, 5 ml; reaction temperature, 26 ꢀC; reaction
time, 3 h.
Fig. 12 Sheldon's hot filtration test in epoxidation of cyclooctene.
Table 4 Recycling study of (L)Mo(VI)O2@SBA-15 in epoxidationa and
sulfoxidationb reactions
Epoxidation (cyclooctene)
Sulfoxidation (thioanisole)
No of cycles Conversion
Selectivity
Conversion
Selectivity
Fresh
93
92
88
80
—
97
96
95
93
—
100
99
97
92
85
89
87
86
85
85
I cycle
II cycle
III cycle
IV cycle
a
Reaction conditions: (L)Mo(VI)O2@SBA-15, 150 mg; cyclooctene, 8
mmol, 0.8816 g; TBHP (5.0–6.0 M in decane), 14.4 mmol, 1.6 ml;
CHCl3, 25 ml; reaction temperature, 70 ꢀC; reaction time, 18 h.
b
Reaction conditions: (L)Mo(VI)O2@SBA-15, 25 mg; thioanisole, 1
mmol, 0.1242 g; H2O2, 1.1 mmol, 0.1245 g; CH3CN, 5 ml; reaction
temperature, 26 ꢀC; reaction time, 3 h.
Fig. 13 Sheldon's hot filtration test in sulfoxidation of thioanisole.
was formed (from the GC analysis) aer 6 h and there is no
changes in the conversion of cyclioctene was observed aer 18
h. For the hot ltration test in sulfoxidation reaction of thio-
4. Conclusion
In summary, the heterogenization of the (L)Mo(VI)O2 complex
over SBA-15 was achieved by a multistep synthetic procedure,
using the 3-[N,N-bis-3(salicylidenamino)ethyltriamine] ligand
(L) and Mo(VI)O2(acac)2. The physico-chemical properties of the
functionalized catalyst were analyzed by a series of character-
ization techniques like elemental analysis, ICP-OES, XRD, N2
sorption measurements, TG & DTA, solid state 13C, 29Si NMR
spectroscopy, FT-IR, Raman spectroscopy, XPS, DRS UV-Vis
spectroscopy, SEM and TEM. The integrity and textural
ꢀ
anisole (Fig. 13) at 26 C for 3 h, (L)Mo(VI)O2@SBA-15 catalyst
was ltered from the reaction mixture aer 1 h during the
reaction and the reaction continued uninterrupted in the
absence of catalyst upto 3 h. It was observed that 65% methyl
phenyl sulde was converted aer 1 h (from the GC analysis)
and the conversion of methyl phenyl sulde remained similar
upto 3 h. In both reactions, results show that leaching of the
molybdenum atoms was not occurred.
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¨
properties of mesoporous support and synthesized catalysts 13 F. E. Kuhn, A. M. Santos, A. D. Lopes, I. S. Goncalves,
were obtained from the XRD and N2 adsorption–desorption
analysis. Thermal stabilities of the complexes were conrmed
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Acknowledgements
The authors are grateful to the Department of Science and
Technology (DST), New Delhi and DAAD, Germany for the
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