Post-synthetically modified SBA-15 with NH2-coordinately immobilized iron-oxine: SBA-15/NH2-FeQ3 as a Fenton-like hybrid catalyst for the selective oxidation of organic sulfides

Article abstract of DOI:10.1039/c7nj02742g

A new modified SBA-15 meso-material with NH₂-coordinately immobilized "iron-oxine" was fabricated by stepwise covalent grafting of -NH₂ and then NH₂-coordinate immobilization of tris(8-quinolinolato)iron onto the channels of SBA-15. The structural properties and chemical nature of SBA-15/NH₂-FeQ₃ were characterized by FTIR, scanning electron microscopy (SEM), SEM-EDX/mapping, transmission electron microscopy (TEM), low angle X-ray diffraction (L-XRD), thermal gravimetric analysis (TGA), N₂-adsorption-desorption isotherms (BET) and atomic absorption spectroscopy (AAS). A Fenton-like green and eco-friendly procedure for the catalytic oxidation of aromatic organic sulfides to sulfones using H₂O₂ was investigated for the synthesised Fe/SBA-15 catalyst. This porous catalyst was found to be reusable for ten runs without appreciable change in the activity, or the composition of the catalyst.

Full text of DOI:10.1039/c7nj02742g

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Postsynthetic modified SBA-15 with NH2-coordinately immobilized iron-oxine: SBA-
15/NH2-FeQ3 as a Fenton-like hybrid catalyst for selective oxidation of organic sulfides
Habib Golchin Hosseini, Sadegh Rostamnia *
Organic and Nano Group (ONG), Department of Chemistry, University of Maragheh, P.O. Box
55181-83111, Tel (+98) 4212278001-108; Maragheh, Iran. E-mail: rostamnia@maragheh.ac.ir;
srostamnia@gmail.com.
Abstract:
A new modified SBAꢀ15 mesoꢀmaterial with NH2ꢀcoordinately immobilized “Ironꢀoxine” was
fabricated by a stepwise covalent grafting of -NH2 and then NH2ꢀcoordinately immobilized tris(8ꢀ
quinolinolato)iron onto the channels of SBAꢀ15. The structural properties and chemical nature of
the SBAꢀ15/NH2ꢀFeQ3 were characterized by FTIR, scanning electron microscopy (SEM), SEMꢀ
EDX/mapping, transmission electron microscopy (TEM), low angle Xꢀray diffraction (LꢀXRD),
thermal gravimetric analysis (TGA), N2-adsorptionꢀdesorption isotherms (BET) and atomic
absorption spectroscopy (AAS). A Fentonꢀlike green, and ecoꢀfriendly procedure for the catalytic
oxidation of aromatic organic sulfides to sulfones using H2O2 was investigated for the
synthesised Fe/SBAꢀ15 catalyst. This porous catalyst was found to be reusable for ten runs
without appreciable change in the activity, as well as the composition of the catalyst.
Keywords: Ironꢀoxine, SBAꢀ15, Fentonꢀlike catalyst; sulfide oxidation; tris(8ꢀquinolinolato)iron.
1. Introduction
Postsynthetic modification (PSM) ordered mesoporous silica (OMS) material, constructed by
repeating organic linkers or metalꢀligand functional in the mesochannels, are attractive because
they can be made from readily available and highly tailorable ligand precursors for
multifunctional integration1ꢀ3. The topological structure of OMS is fully ordered in nature and
leads to regular porosity, functional ligand species also could be vastly expanded and thus render
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versatile utility, including catalysis, separation, gas storage, supercapacitance, and
biomolecule and drug delivery1, 4ꢀ10. With this in mind, designing a catalytically active
covalentlyꢀbonded ligand OMS (OMS/L) targeting a specific reaction would be possible. Due to
the strong metalꢀligand bonds, OMS/L acts as a scaffold to support transition metalꢀbased
catalyst and allows for the large interfacial surface area, where the catalytic active sites of
transition metal can be remarkably exposed10. Furthermore, conjugation of OMS/L with other
functional ligands is easily achieved and elaborately modulated. The produced hybrids have wellꢀ
defined organicꢀinorganic hybrid mesostructure expanding their scope of utilization8, 11
.
Due to the importance of sulfoxides in biology and act as intermediates in industrial, the
oxidation of sulfides to corresponding sulfoxides has been the aim of considerable research12, 13
.
In this field, many oxidants such as molecular Oxone, UHP, TBHP, H2O2 are available to
perform this reaction14ꢀ19. The overoxidation of sulfides resulting in sulfone formation is common
problems. In this filed, an intensive effort has been devoted to the development of oxidation
reactions using green reagents and solid catalysts that coꢀproduce only innocuous waste20, 21. For
example, oxidation using aqueous H2O2 that produces only water as the coꢀproduct is a topic of
current interest, and quite a few atomꢀefficient methods using them have been reported.
Previously, we developed synthetic methods for oxidation process and also the synthesis of
interesting organic molecules 22ꢀ30. The challenge in this field was the development of efficient,
green and selective methods30. Moreover, the synthesis of these molecules has usually been
carried out in hazardous and polar solvents such as CCl4, C6H6, and DMSO, leading to complex
isolation and recovery procedures. Recently, we prepared a series of hybrid OMS/L based of
SBAꢀ15 materials by postꢀsynthetic modification method, which displayed comparable activity
30
and selectivity to those of homogeneous system in a wide range of organic reactions
.
Encouraged by these successful efforts and aiming to develop a stable and selective green
2

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catalyst, herein we report the synthesis of SBAꢀ15/NH2ꢀFeQ3 via a stepwise postꢀsynthetic
modification method and then we aimed to examine the catalytic activity of that in the oxidation
of sulfides to sulfoxides. Herein, FeCl3 were chelated with tris(8ꢀquinolinolato), and the formed
tris(8ꢀquinolinolato)iron complexes (FeQ3 or Feꢀoxine) were deposited/coordinated onto the
SBAꢀ15/NH2 to yield SBAꢀ15/NH2ꢀFeQ3 as a new, tunable, highly ordered mesoporous Fe/SBAꢀ
15 silica. However, our proposed catalytic system for the selective oxidation of sulfides has
surpassed these methods and these drawbacks are resolved. This system was accomplished under
wasteꢀfree conditions with high selectivity towards sulfoxides (Figure 1).
Fig. 1. SBAꢀ15/NH2ꢀFeQ3 for green oxidation of sulfides.
2. Experimental
2.1-Materials and apparatus
Triblock copolymer pluronic P123, tetraethoxysilane (TEOS), (3ꢀaminopropyl)triethoxysilane
[(EtO)3SiPrNH2, APTS], 8ꢀhydroxyquinoline (oxyquinoline), FeCl3ꢁ6H2O and other reagents
were obtained from Aldrich, Merck (Germany) and Fluka (Switzerland) and used as received.
Melting points were measured on an Electrothermal 9100 apparatus. The IR spectra were
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determined using a FTꢀIR BrukerꢀVector 22. Homogeneous stirring was done by ultrasonic
(Ultrasonic Homogenizerꢀmodel APU500 Advanced Equipment Engineering CompanyꢀAdeeco,
Iran). Scanning electron microscopy (SEM) and Electron dispersive scanning (EDS) were
recorded on VEGA3 TESCAN. Powder Xꢀray diffraction spectra were recorded on Philips
PW1800. Transmission electron microscopy (TEM) was recorded on JOEL JEM 3010
instrument.
2.2. Synthesis of tris(8-quinolinolato)iron (FeQ3 or iron-oxine)
The tris(8ꢀquinolinolato)iron (FeQ3) was synthesized by taking 8ꢀhydroxyquinoline (436 mg, 3
mmol) and 1 mmol (270 mg) FeCl3. 6H2O in 3:1 molar ratio by stirring it at 65 °C in 10 mL
anhydrous THF for 4 h under a nitrogen atmosphere in threeꢀnecked Schlenk flask. After cooling
the reaction mixture to room temperature, the precipitate was appeared that this solid was filtered
and washed 3 times with cold THF.
2.3. Synthesis of NH2ꢀfunctionalized SBA-15 (SBA-15/NH2)
The mesoporous SBAꢀ15 material was synthesized according to our previouly reported procedure
23, 31, 32. NH2ꢀfunctionalized SBAꢀ15 was synthesised by adding 2000 µL of APTES to a sonicated
o
suspension (2×5 min) of calcinated SBAꢀ15 (1 g) in toluene at 110 C for 24 h. After the stop
reaction, the temperature decreased to room temperature and solid was filtered and washed with
ethanol. Finally, SBAꢀ15/NH2 was dried in vacuum at 60 oC.
2.4. SBA-15/NH2-coordinately immobilized iron-oxine (SBA-15/NH2-FeQ3)
The obtained SBAꢀ15/NH2 was then incorporated with tris(8ꢀquinolinolato)iron. 1 mg of the
obtained FeQ3 was dissolved in 50 mL CHCl3 followed by addition of 1 g of SBAꢀ15/NH2 under
vigorous stirring and then refluxing the suspension for 24 h. Finally, the solid was collected by
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filtering and Soxhlet with EtOH for 72 h and then after two times sonicate around 2 min in
chloroform, it was dried at 60 °C at vacuum. According to atomic absorption analysis (AAS) of
the composite, the amount of loaded Fe3+ was 0.6 mmol per gram of composite.
2.3. General procedure for the oxidation of sulfides
In a roundꢀbottomed flask (25 mL) equipped with a magnetic stirrer, sulfide (0.5 mmol) was
added to reaction flask in the presence of 1.5 mmol H2O2 (30% w/w) and SBAꢀ15/NH2ꢀFeQ3 (5
o
mol% based on FeIII) in 1 mL water and stirred at 25 C for an appropriate time (Table 1). After
completion of the reaction, the product was extracted from the reaction mixture with chloroform.
Then, it was evaporated and analyzed by gas chromatography (GC).
3. Results and discussion
The SBAꢀ15/NH2ꢀFeQ3 was synthesized by taking 8ꢀhydroxyquinoline and ferric chloride in 3:1
molar ratio in THF and mixing of this with amineꢀgrafted SBAꢀ15. Here, the FeQ3 was dissolved
in chloroform followed by addition of SBAꢀ15/NH2 under stirring and then refluxing the
suspension. Finally, the SBAꢀ15/NH2ꢀFeQ3 were collected and dried for next uses (Scheme 1).
Scheme 1. Synthetic route for SBAꢀ15/NH2ꢀFeQ3.
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The physicochemical properties of the SBAꢀ15/NH2ꢀFeQ3 were characterized by FTIR, scanning
electron microscopy (SEM), Electron dispersive scanning (SEMꢀEDS), SEMꢀmapping,
transmission electron microscopy (TEM), low angle Xꢀray diffraction (XRD), thermal
gravimetric analysis (TGA), N2-adsorptionꢀdesorption isotherms (BET) and atomic absorption
spectroscopy (AAS). FTIR spectroscopy was performed for every step of the catalyst synthesis
and changes in vibration modes were carefully monitored (Figure 2). For FeQ3 and SBAꢀ15/NH2ꢀ
FeQ3, the band at aromatic region1650ꢀ1450 cmꢀ1 can be correlated to the split of the
characteristic absorption of FeQ3 at 1643ꢀ1455 cmꢀ1 that shifts to a lower wavenumber. In SBAꢀ
15/NH2ꢀFeQ3, the bands from ~1000 and ~1100 cmꢀ1 belong to the vibrations of (SiꢀOꢀSi) bond,
and the SiOꢀH groups are shown by the very broad IR absorption band in the 3420ꢀ3600 cmꢀ1
region. Some peaks at 2800ꢀ2940 can be attributed to the presence of CꢀH of the alkyl chain of
propyl group and CꢀH of oxine ligand in samples. 1465 cmꢀ1 can probably be attributed to the
presence of quaternary amine. SiOꢀH band is appears around 3300 cmꢀ1.
Fig. 2. FTꢀIR spectra of (red) SBAꢀ15/NH2ꢀFeQ3, and FeQ3 (blue).
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SEM micrograph of the SBAꢀ15/NH2ꢀFeQ3 nanomaterial is shown in Figure 3a and 3b, which
these images show used to examine the morphology and topography of prepared material. The
shape of the magnetic nanomaterial shows that the particles prepared in nanosphericalꢀshaped
morphology in a stockpiled topology with an average size of 60ꢀ80 nm. Then image profile and
plot profile of the selected area of SEM of was measured (Figure 3c and 3d). Line profile of the
stamper surface is given in Figure 3c. It shows the measure of the particle width was around 2
µm of the bit. From similar profile analysis it other parameters such as the pitch and bit height are
measured (Figure 3d).
Fig. 3. SBAꢀ15/NH2ꢀFeQ3: (a) SEM image and (b) BinꢀSEM image, (c) image profile and selected area
highlighted (d) plot profile and selected area highlighted.
Figure 4 shows images of transmission electron microscopy (TEM) of the asꢀsynthesized SBAꢀ
15/NH2ꢀFeQ3 sample. Based on this image, the average size of channels is mainly around 7 nm.
In addition, the TEM images confirmed the presence of a 2D hexagonal network, and no
aggregated Fe particle species was observed in the pore channels. This result, together with the
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elemental mapping images and EDX, indicates homogeneous Fe in the complex format on the
SBAꢀ15. EDX analysis clearly indicates the presence of Fe, C, O, N and Si onto the SBAꢀ
15/NH2ꢀFeQ3 (Figure 4c).
Fig. 4. SBAꢀ15/NH2ꢀFeQ3: (a) TEM, (b) BinꢀTEM, (c) EDX and (d) SEMꢀelemental mapping.
To examine the thermal stability of the SBAꢀ15/NH2ꢀFeQ3, thermal gravimetric (TG) was carried
out between 25 oC and 800 oC in a static atmosphere of nitrogen (Figure 5). TG analysis of SBAꢀ
15/NH2ꢀFeQ3 shows a weight loss due to the desorption of water below 150 °C which is owing to
the loss of inner water. This is finally followed by a set of weight losses centered around 350 °C
corresponding to the elimination of the surface bound organic propyl amine and corresponding
coordinated 8ꢀquinolinolato groups, which indicates that this immobilized hybrid catalyst is
thermally stable up to 380 °C.
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Fig. 5. TG analysis of SBAꢀ15/NH2ꢀFeQ3 and SBAꢀ15/NH2.
Nitrogen adsorptionꢀdesorption isotherm (BET) of SBAꢀ15/NH2ꢀFeQ3 and pure SBAꢀ15 were
compared in Fig. 6a. These isotherms reveal that stepwise postꢀsynthetic modification does not
perturb the isotherm type of the resulting SBAꢀ15, which remain Type IV. However, the
hysteresis loop progressively shifts to lower relative pressures with grafting of propyl amine
coordinated FeQ3 indicative of decreasing pore size. The steep step and narrow hysteresis loops
in the adsorption/desorption branches, evidence retention of the parent SBAꢀ15 cylindrical,
parallel pore channels. The BET surface area of SBAꢀ15 was calculated to be ca. 670 m2·gꢀ1.
Even after grafting of FeQ3 complex, the high surface area, ca. 368 m2·gꢀ1, was still remained.
BJH pore size distribution analysis on the desorption branch (Fig. 6b), shows that pores diameter
of mother SBAꢀ15 is 7 nm and achieved to 4.1 nm in SBAꢀ15/NH2ꢀFeQ3 via our method.
Lowꢀangle Xꢀray diffraction reveals SBAꢀ15 and SBAꢀ15/NH2ꢀFeQ3 materials have good long
range order with characteristic (100) diffraction peaks consistent with the p6mm hexagonal
symmetry (Fig. 6c). According to this comparison, diffraction lines related to Fe3O4 or other
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crystal/particle species were not present or appear in SBAꢀ15/NH2ꢀFeQ3 due to the
nanocomposite formation of silica.
Fig. 6. SBAꢀ15 and SBAꢀ15/NH2ꢀFeQ3: (a) N2 adsorption/desorption isotherm, (b) BJH and (c) XRD.
Oxidation of aromatic sulfides is the freest alternative to supply industrially interest intermediates
of sulfoxides 16, 18. One of the difficulties encountered with this method is overwhelming
overoxidation of generated sulfoxides (R2S=O) to sulfones (R2SO2) in the reaction vessel,
without any selectivity and then lead to undesired compounds. In this project, the catalytic ability
of the SBAꢀ15/NH2ꢀFeQ3 in the oxidation reaction of sulfides to sulfoxides (or sulfones) without
using any salts, stabilization or additives was investigated. Oxidation of methyl phenyl sulfide
(MPS) as model benchmark reaction was first examined with SBAꢀ15/NH2ꢀFeQ3 using H2O2
(Scheme 2).
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Scheme 2. Benchmark model reaction over SBAꢀ15/NH2ꢀFeQ3.
To obtain an optimized condition various parameters such as temperature, amount of catalyst, and
several solvents were taken into consideration. First, the effect of the SBAꢀ15/NH2ꢀFeQ3 amount
o
was examined on the model reaction under, including 25 C, reaction time (2 h), sulfide:H2O2
(1:5) and H2O (1 mL) that reaction scale was 0.5 mmol. Several amounts of catalyst were added
to reaction mixtures and among them 10 mol% of supported FeQ3 was found to be more efficient
from the point view of yield in each reaction (Figure 7).
Fig. 7. Reaction conditions: reaction conditions: 0.5 mmol of MPS, 2.5 mmol of H2O2, in water (1 mL) at
25 oC for 2 h. Violet (conversion), red (sulfoxide) and blue (sulfone). Conversion calculated based on
MPS.
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Amount of H2O2 oxidant was also optimized for the model oxidation reaction. Based on this
study, the best amount of H2O2 for this work was 4 mmol for each mmol of sulfide (room
temperature condition). Figure 8 represents the results for six different amount of H2O2 which
was performed in the similar conditions.
Fig. 8. Optimizing the amount of H2O2 in the model reaction. Reaction conditions: 0.5 mmol of MPS in
H2O (1 mL) at 25 oC for 2 h.
The temperature effect was also monitored in the product yields under the similar conditions. In
o
this regard, temperatures including 10, 25, 60, and 100 C of model reaction were tested. One
o
considerable point in this comparison was that the reaction at 25 C had lower reaction
completion time for selective synthesis of sulfoxide with < 10% of sulfone (Figure 9).
Fig. 9. Reaction conditions: 0.5 mmol of MPS, 2 mmol of H2O2, in 1 mL of H2O for 3 h. Blue
(conversion), red (sulfoxide) and green (sulfone). Conversion calculated based on MPS.
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Several nonꢀpolar and polar solvents were investigated on the selectivity and yield towards
sulfoxide product through the model reaction. Among them, H2O/CH2Cl2 (%87) and H2O (%86)
had better results for selective sulfoxide synthesis. However, due to the fact that water is a
nontoxic, available and therefore a greener solvent and also a bit higher product yield, it was
selected as an optimized solvent towards selective synthesis of sulfoxide. In model reaction, the
turnover number (TON) of reaction system based on moles of sulfoxide as the main product per
moles of “Fe3+” as an active center of the catalyst was 88.3. In same condition turn over
frequency (TOF) was 2.94 (hꢀ1).
Thus, after appropriate studies for the optimization of the selective oxidation of MPS to
corresponding sulfoxide, it was extended to the other derivatives of sulfides and results obtained
from the derivatives are depicted in Table 1. Table 1 shows that the presence of electronꢀ
withdrawing groups on the aromatic ring generally caused a decrease in reaction yield.
Comparably, electronꢀdonating substituents generally provided excellent yields to products at
however slightly extended times of reaction needed for reaction completion.
Table 1. Optimized reaction conditions for the oxidation of aromatic sulfides. a
Entry Ar
R
Time (h)
Yields (%)b
90/4
1
2
3
4
5
6
7
Ph
Ph
4ꢀBrꢀ Ph
Phꢀ
Ph
PhCH2
Ph
PhCH2
CH3
2ꢀPy
3ꢀMeOꢀPh
3ꢀMeOꢀPh
3
9
1
3
4
4
4
44/0
75/12
87/10
65/27
67/9
Ph
3ꢀMeOꢀPh
73/9
8
9
PhCH2ꢀ
4ꢀBrꢀPh
PhCH2ꢀ
2
3
88/6
75/7
4ꢀMeOꢀPh
a Reaction conditions: reaction scale (0.5 mmol).
b Yields based on GC analysis (sulfoxide/sulfone).
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The activity and selectivity of SBAꢀ15/NH2ꢀFeQ3 in oxidation of sulfide was also studied
under the optimized conditions. When we inspected the reaction of mixture of MPS and diphenyl
sulfide under the reaction condition the yields of methyl phenyl sulfoxide and diphenyl sulfoxide
were 64 and 28%, respectively (Scheme 3).
O
O
O
S
S
Ph
Ph
Ph
S
S
+
+
Ph
Ph
(trace)
SBA-15/NH2-FeQ3
(10 mol%)
(0.5 mmol) (0.5 mmol)
(63)
+
H2O- 2 mL, 3 h
25 oC
O
S
H2O2
O
O
(4 mmol)
S
+
Ph
Ph
Ph
(trace)
(28)
Scheme 3. SBAꢀ15/NH2ꢀFeQ3 for the chemoselectivity of sulfoxide vs sulfone.
The probability of recycling the SBAꢀ15/NH2ꢀFeQ3 was also studied. When the reaction was
complete, the catalyst was easily filtered from the reaction mixture. As illustrated in Figure 10a,
the catalyst could be reused over at least 10 runs without significant loss of activity. After the 10th
runs, the yield of sulfoxide decreased to 55%. This indicated that the FeQ3 immobilized SBAꢀ
15/NH2 was robust and recyclable. According to atomic absorption spectroscopy, no Fe was
detected in reaction solution until 10th run, and elemental mapping analysis confirming the good
stability of the catalyst under the investigated reaction conditions even after 10th run (Figure 10b).
Fig. 10. Recyclability of SBAꢀ15/NH2ꢀFeQ3 (a) and its elemental mapping data after 10th run (b).
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In comparison to previously reported methods, our method was milder, greener from
viewpoint of oxidant, support, and metal and costly effective with higher conversion and
selectivity towards production of sulfoxide. This comparison clearly exhibits that how powerful
our proposed catalyst plays the role of catalyst (Table 2).
Table 2. An overview of some methods for oxidation of MPS.
Entry
catalyst
oxidant
H2O2 (2.5 eq.)
TBHP
solvent
H2O
CCl4
T (◦C)/t (h) (%)Yield [reff.]
1
2
3
4
5
6
Na2WO4,
50/1
25/9
0/0.03
25/0.5
ꢀ20/720
25/3
97[14]
28[15]
Ti4+
ꢀ
Oxone (1.3 eq.)
H2O2
MeOH
DCM/MeOH
DCM
84[17]
[WO4][SiO2/PrNH3]2
VO(salen)
SBAꢀ15/NH2ꢀFeQ3
82[18]
CHP a
79[19]
H2O2
H2O
87 [This work]
a 1ꢀmethylꢀ1ꢀphenylethyl hydroperoxide
A possible explanation mechanism for the sulfide oxidation using H2O2 over the SBAꢀ15/NH2ꢀ
FeQ3 catalyst was proposed in Scheme 4. Based on literature reports33ꢀ37, we propose that SBAꢀ
15/NH2ꢀFeQ3 easily reacts with H2O2 to form a FeIIIꢀOOH adduct with a pendant OOH through
opening its longest FeꢀO bond. Then, the FeIIIꢀOOH adduct may undergo a homolytic cleavage of
its OꢀOH bond to generate a [FeIIIꢀO]. and OH. radicals, which MPS can be oxidized to the
corresponding sulfoxide (or sulfone in repeat steps) by OH radicals.
Scheme 4. Plausible mechanisms of FeQ3 mediated sulfide oxidation.
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4. Conclusion
In summary, we have announced a potent supported tris(8ꢀquinolinolato)iron based SBAꢀ
15/NH2ꢀFeQ3 mesoporous material that shows extraordinary catalytic activity and recyclability in
green and selective sulfide oxidation. This separable solid porous catalyst can be prepared by a
very simple procedure using inexpensive and commercially available precursors, and the catalyst
can be reused for ten cycles without any loss in its activity, as well as composition of the catalyst.
Thus, the procedure serves as a valuable alternative in the selective synthesis of sulfoxides.
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New Journal of Chemistry
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DOI: 10.1039/C7NJ02742G
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New Journal of Chemistry
Page 18 of 18
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DOI: 10.1039/C7NJ02742G
Postsynthetic modified SBA-15 with NH2-coordinately immobilized iron-oxine: SBA-
15/NH2-FeQ3 as a Fenton-like hybrid catalyst for selective oxidation of organic sulfides
Habib Golchin Hosseini, Sadegh Rostamnia,
Highlight
We herein report the synthesis and catalysis of SBA-15 grafted iron-oxine. SBA-15/NH2-FeQ3
showed efficient catalytic activity in sulfides oxidation.