Nanolayered manganese–calcium oxide as an efficient catalyst toward organic sulfide oxidation

Abstract

We for the first time report that nanolayered Mn–Ca oxide in the presence of H₂O₂ is an efficient catalyst toward sulfide oxidation to sulfoxide. We characterized the catalyst by DLS, UV-Vis, diffuse reflectance infrared Fourier transform spectroscopy, SEM, TEM and HRTEM. We also considered different parameters of the sulfide-oxidation reaction.

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Organic sulfoxides are among the most important compounds to synthesize various chemically and biologically significant molecules.¹ Among different methods, the oxidation of sulfides is a common method for preparing sulfoxides.² There are several reagents for this conversion but many of them have the problem of over oxidation of sulfoxides to sulfones.^1,2 Different solvents, oxidants and catalysts are used in improving the procedure.^1,2 Mn compounds are among the most attractive inorganic materials not only because of their physical and chemical properties but also because they are low cost and environmentally friendly.³ It was reported that ethyl phenylsulfide (2 mmol) was allowed to react with oxygen/pivalaldehyde (1 eq.) in perfluoro-2-butyltetrahydrofuran (2.5 mL, 25 °C) in the presence of Mn(OAc)₃·2H₂O (2 mol%) and led to a mixture of ethylphenylsulfoxide and ethylphenylsulfone (67 : 33) in 1 h.⁴ Asymmetric oxidation of sulfides was also examined by using (salen)Mn(III) complexes as catalyst and was found to show high asymmetric induction.⁵ Mn oxides are cheap and environmentally friendly compounds and used extensively as bulk, supported, nano-sized with large surface area and colloidal forms. Mn oxides are efficient catalysts for oxidation of both organic⁶ and inorganic compounds such as carbon monoxide,⁷ nitrogen oxides,⁸ and water.⁹ Recently, we used layered Mn oxides as efficient catalysts for oxidation of water,¹⁰ alcohols¹¹ and alkenes.¹² Here, we report nanolayered Mn oxides as efficient catalysts toward organic sulfide oxidation to sulfoxide.

Materials and methods

All reagents and solvents were purchased from commercial sources (Merck or Sigma-Aldrich) and were used without further purification. MIR spectra of KBr pellets of compounds were recorded on a Bruker vector 22 in the range between 400 and 4000 cm⁻¹. Diffuse reflectance infrared Fourier transform spectroscopy were recorded on Perkin Elmer-Spectrum one. TEM and SEM were carried out with JEOL JEM 2010 Fas TEM operating at 200 kV and LEO 1430VP, respectively. The X-ray powder patterns were recorded with a Bruker, D8 ADVANCE (Germany) diffractometer (Cu-Kα radiation). Mn atomic absorption spectroscopy (AAS) was performed on an Atomic Absorption Spectrometer Varian SpectrAA 110. Prior to analysis, the oxide (10.0 mg metal) was added to 1 mL of concentrated nitric acid and H₂O₂, left at room temperature for at least 1 h to ensure that the oxides were completely dissolved. The solutions were then diluted to 25.0 mL and analyzed by AAS.

Synthesis of compound

The compound was synthesized by a previously reported method.¹³ Briefly, it was synthesized by mixing a solution contain CaCl₂·4H₂O (2.70 mmol, 0.494 g) and Mn(CH₃COO)₂·4H₂O (5.6 mmol, 1372 mg) in water (5 mL) and a solution of KMnO₄ (2.4 mmol, 379 mg) and KOH (17.0 g) in 100 mL water. The obtained suspension was filtered and washed with distilled water (3 L) before being allowed to dry for 12 h at 60 °C in an oven. Then the solid was heated to different temperatures (100–400 °C) for 10 h in air to obtain a brown powder. Yield: >%90; at 400 °C the oxide could be formulated as Ca_0.16MnO₂·2H₂O.

Oxidation of sulfides catalyzed by Ca_0.16MnO₂·2H₂O

To a solution of sulfide (0.5 mmol) and catalyst (10.0 mg) in CH₃CN (1.0 mL), H₂O₂ (1.5 mmol) as oxidant was added. For the analysis of products, after stirring at room temperature for 2 h, the solution was subjected to ether extraction (3 × 10 mL), and the extract was also concentrated down to 1.0 mL by distillation in a rotary evaporator at room temperature. Then, a sample (2 μL) was taken from the solution and analyzed by GC. The retention times of the peaks were compared with those of commercial standards, and chloro benzene was used as an internal standard for GC yield calculation.

Results and discussion

The procedure to synthesize of the nanolayered Mn oxides was simple, green, easily and cheap by the reaction of KMnO₄, Mn(II)(OAc)₂ and CaCl₂ in the presence of KOH. In reflectance infrared Fourier transform spectra and FTIR spectra of oxide show a broad band at ∼3200–3500 cm⁻¹ related to antisymmetric and symmetric O–H stretchings and at ∼1630 cm⁻¹ related to H–O–H bending are observed.¹³ The peaks at ∼700 cm⁻¹ are related to Mn–O–Mn bonds (Fig. S1, ESI†).¹³

XRD data for the compound is of very poor resolution and the weak peak in 2θ ∼11 related (00l) in layered Mn oxides, reveals the absence of long-range order of the layer stacking (Fig. S2, ESI†).¹³

Using extended-range X-ray absorption spectroscopy (XAS) at the K-edges of both Ca and Mn, two different Ca-containing motifs were identified. One of the motifs was the formation of Mn₃CaO₄ cubes while the other Ca(II) ions probably interconnected oxide-layer fragments (Fig. 1).¹⁴

Fig. 1 A motif for Mn–Ca cubic in layered Mn oxide (a). Surface area of layered Mn oxide (Ca(II) ions are omitted for clarification) (b).

SEM images from the catalyst show aggregated nanoparticles from <100 nm. In TEM images, layers of Mn oxide are clearly detected and showed that the nano-compounds observed by SEM contain nanosheets (Fig. 2 and S3, ESI†). Only a few short-range orders among layers can be observed in the sample by HRTEM. Such layered Mn oxides without any long-range order among layers are rare. DLS shows that particles are in the range of 100–200 nm (Fig. S4, ESI†). Comparing with SEM (<100 nm), we found aggregation of particles in solution. Peak at ∼400 nm in solid state UV-Vis spectrum of oxide is related to Mn(III)/Mn(IV) ions in the structure (Fig. S5, ESI†).

Fig. 2 SEM micrographs (a), TEM (b) and HRTEM (c and d) images of nano-size Mn–Ca oxide prepared at 400 °C.

Catalytic performance of Ca_0.16MnO₂·2H₂O

In order to evaluate the catalytic activities of Ca_0.16MnO₂·2H₂O for oxidation reactions, the reactions were optimized according to the oxidation of methyl phenyl sulfide (MPS) through the investigation of the influence factors of oxidation, such as the various solvents and oxidants, and the amount of catalyst and oxidant.

In order to understand the effect of catalyst on the catalytic reactivity, the first, MPS oxidation reaction was carried out without catalyst. Results indicated that in the absence of catalyst, trace amount of product was generated in presence of H₂O₂ as an oxidant. Various oxidants including TBHP, H₂O₂, NaClO and molecular oxygen were tested for the oxidation of MPS over Ca_0.16MnO₂·2H₂O at room temperature. Among these oxidants, molecular oxygen and H₂O₂ were much more efficient for the oxidation of MPS. The amount of H₂O₂ could also significantly affect the conversion and selectivity for MPS oxidation to sulfoxide (selectivity: sulfoxide/(sulfoxide + sulfone) × 100) (Table 1). When the amount of H₂O₂ was increased from 0.5 to 1.5 mmol, the conversion of MPS increased drastically from 47 to 85%. With a further increasing of H₂O₂ to 2.0 mmol, the selectivity for MPS oxidation to sulfoxide decreases from 92 to 70%. The conversion of MPS monotonously increases with the addition of catalyst from 5 to 10 mg. When the amount of catalyst is increased to 20 mg, the selectivity for MPS oxidation to sulfoxide reduces from 95 to 70%. Hence, the amount of catalyst can enhance the reaction rate for the selectivity for MPS oxidation to sulfoxide. In the next step, a series of reactions was investigated by performing the model reaction in different solvents such as dichloromethane, methanol, acetonitrile, benzene, acetone and n-hexane. Among the solvents examined, acetonitrile was found to be the best for this protocol (Table 1).

Table 1 The effect of various conditions in the oxidation of MPS by Ca_0.16MnO₂·2H₂O

Entry	Catalyst amount (mg)	Oxidant	Oxidant amount (mmol)	Solvent	Conversion^a (%)	Selectivity^b (%)
a The GC conversion (%) are measured relative to the starting substrate.b Selectivity to sulfoxide = (sulfoxide/(sulfoxide + sulfone)) × 100.
1	—	H2O2	0.5	CH3CN	Trace	—
2	20	H2O2	0.5	CH3CN	47	100
3	20	TBHP	0.5	CH3CN	45	94
4	20	NaClO	0.5	CH3CN	28	100
5	20	O2	Bubbled	CH3CN	97	41
6	20	H2O2	1.0	CH3CN	63	99
7	20	H2O2	1.5	CH3CN	85	92
8	20	H2O2	2.0	CH3CN	88	70
9	15	H2O2	1.5	CH3CN	83	93
10	10	H2O2	1.5	CH3CN	83	95
11	5	H2O2	1.5	CH3CN	62	99
12	10	H2O2	1.5	CH3OH	75	96
13	10	H2O2	1.5	CH2Cl2	21	100
14	10	H2O2	1.5	C6H6	29	100
15	10	H2O2	1.5	CH3COCH3	33	100
16	10	H2O2	1.5	n-Hexane	8	100

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A series of various types of structurally diverse sulfides were subjected to the oxidation reaction using Ca_0.16MnO₂·2H₂O as catalyst and H₂O₂ as oxidant (Table 2). Arylalkyl (Table 2, entries 1 and 2), arylbenzyl (Table 2, entry 3), dibenzyl (Table 2, entry 4), diaryl (Table 2, entry 5) and dialkyl (Table 2, entries 6–8) sulfides underwent clean and the selectivity for MPS oxidation to sulfoxide under air, is high (89–100%). Significantly, the very good conversions of substrates depending on the nature of the sulfide, in the range of 51–100% were obtained for all cases. It was observed that aromatic sulfides undergo oxidation reactions can be oxidized to sulfoxide more easily than aliphatic substrates. These results may indicate that the Mn^V=O intermediate can efficiently transfer oxygen atom to electron-rich sulfides than electron-poor one. The highest and the lowest conversions were obtained for dibenzyl sulfide (100%) and diethyl sulfide (51%), respectively (Table 2, entries 4 and 6).

Table 2 Oxidation of sulfides catalyzed by Ca_0.16MnO₂·2H₂O/H₂O₂^a

Entry	Substrate	Conversion^b (%)	Selectivity^c (%)
a The molar ratios for substrate : oxidant are 1 : 3. The reactions were performed in CH3CN (1 mL) under air at room temperature within 2 h.b The GC yield (%) are measured relative to the starting sulfide.c Selectivity to sulfoxide = (sulfoxide/(sulfoxide + sulfone)) × 100.
1		83	95
2		85	91
3		99	91
4		100	89
5		68	98
6	CH3CH2–S–CH2CH3	51	100
7	CH3(CH2)2–S–(CH2)2CH3	55	100
8	CH3(CH2)7–S–(CH2)7CH3	52	100

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The recovery and recyclability of Ca_0.16MnO₂·2H₂O have been also examined for the oxidation of MPS at room temperature for 2 h and the results have been shown in Fig. 3. The catalyst was separated after each run by filtration, washed 2–3 times with water and diethyl ether repeatedly, and then dried at 50 °C for half an hour. The catalyst could be recycled five times with no significant loss in activity. During reusability studies fifth run provided ∼79% conversion and 95% selectivity.

Fig. 3 Recycling studies of the catalyst Ca_0.16MnO₂·2H₂O in the oxidation of MPS. Condition is similar to Table 2.

In order to investigate catalyst leaching in the oxidation of MPS as a model reaction, the reaction was stopped at half the reaction time (1 h) and the solid catalyst was completely separated from solution. The rest of the reaction mixture (without catalyst) was allowed to stir for another period of half the reaction time. As seen in Fig. 4, no amount of MPS was produced after catalyst separation. After separation of the catalyst, the reaction mixture was analyzed by AAS and no Mn ions were detected. These results show that Ca_0.16MnO₂·2H₂O is truly heterogeneous and catalyst leaching is negligible under these conditions.

Fig. 4 Leaching experiment; the blue line shows the reaction without Ca_0.16MnO₂·2H₂O and the red line shows the reaction with Ca_0.16MnO₂·2H₂O.

The probable factors of Mn–Ca oxide for activity toward oxidation reactions are:

• The catalyst has a layered structure with considerable thermodynamic stability.¹⁵

• The oxide has high surface area and many of Mn ions are active sites regarding layered structure. In other words, in contrast to other Mn oxides, in layered Mn oxides many Mn ions are on the surface where can act as active sites.

• Both Mn(III)/Mn(IV) ions are stable in the structure that the factor helps to conversion of Mn(III) and Mn(IV) in the catalytic cycle.¹⁵

Conclusions

In conclusions, we introduced layered Mn–Ca oxide as efficient catalyst with high selectivity for MPS oxidation to sulfoxide in the presence of H₂O₂. The catalyst is stable and could be recycled five times with no significant loss in activity.

Acknowledgements

This article is dedicated to the memory of Dr Warwick Hillier who died on January 10, 2014. The authors are grateful to Institute for Advanced Studies in Basic Sciences, the National Elite Foundation and University of Maragheh for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra00008k

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