Dioxomolybdenum(VI) complex immobilized on ascorbic acid coated TiO₂ nanoparticles catalyzed heterogeneous oxidation of olefins and sulfides

Abstract

Addition of MoO₂ (acac)₂ to TiO₂ coated with ascorbic acid (AA) under ultrasonic agitation resulted in a nanohybrid (TiO₂/AA/MoO₂) with size ranging between 20–25 nm. The structural and morphological characterization of the as-prepared nanohybrid was carried out by different techniques such as XRD, FT-IR, TGA and transmission electron microscopy (TEM). The catalytic performance of the TiO₂/AA/MoO₂ nanocomplex in the heterogeneous oxidation of olefins and sulfides using H₂O₂ in ethanol as a safe solvent was exploited. Our results clearly demonstrated the efficiency, selectivity and oxidative stability of the heterogeneous nanocatalyst providing its effective reusability and removing by-products. The catalytic activity of the TiO₂/AA/MoO₂ nanocomplex was strikingly different from other nanometer sized metal oxides such as m-ZrO₂, MoO₃, Fe₃O₄ and TiO₂, as well as their nanocomposites such as TiO₂/AA, ZrO₂/AA/MoO₂, MoO₃/AA/MoO₂ and Fe₃O₄/AA/MoO₂.

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Introduction

In recent years, heterogenization of homogeneous catalysts has attracted much attention because they combine the best properties of both homogeneous and heterogeneous catalysts.¹ Among the various approaches for immobilizing soluble catalysts, covalent attachment has been the most frequently used strategy, as the resulting heterogeneous catalysts have good stability during the course of catalytic reactions. The stable covalent bonds experience no leaching from the support and thus provide an efficient approach to prepare site isolated catalysts.

Epoxides, as precious precursors in organic synthesis, can be easily obtained from alkenes by the use of strong organic oxidants (m-CPBA, NaClO),² or smoother oxidants (THBP, H₂O₂),³ with the assistance of metal-based catalysts (Fe, Mn, Re, Mo, V, and W).^3,4 Among these metals, molybdenum attracts most attention.

The enthusiasm shown in the coordination chemistry of molybdenum followed the discovery of molybdenum in a number of redox enzymes, such as aldehyde oxidase, sulphite oxidase, xanthine oxidase, nitrate reductase and nitrogenase.⁵ Dioxomolybdenum(VI)-complexes are important catalysts as well as catalyst-precursors for oxygen-transfer-reactions in chemical and biological systems.⁶ Considerable efforts devoted to the preparation and investigation of the catalytic properties of dioxomolybdenum complexes under both homogeneous and heterogeneous conditions are a testimony that they are valuable catalysts.⁷ Most of the investigated processes are performed in chlorinated solvents with catalyst loadings as high as 1% Mo. Solvent recovery requires extra efforts and costs that could be eliminated by the application of greener processes.

With the aim of developing cleaner processes and in continuation of our ongoing research on the catalytic activity of Mo catalysts,⁸ in this work, we incorporated simple and easily available MoO₂ (acac)₂ into ascorbic acid-coated TiO₂ nanoparticles under ultrasonic agitation to obtain a novel heterogeneous molybdenum nanocatalyst for the oxidation of olefins and sulfides using H₂O₂ in ethanol as a safe solvent. The catalyst showed high thermal and oxidation stability and desired activity and selectivity in the oxidation of olefins and sulfides using hydrogen peroxide as an industrially and environmentally important oxidant (Scheme 1). The additional advantage of this catalytic system is facile and efficient reusability of the solid catalyst at the end of the reaction.

Scheme 1 Catalytic activity of the TiO₂/AA/MoO₂ nanocomplex in oxidation of olefins and sulfides.

Results and discussion

Synthesis and characterization of the catalyst

In the first step of this investigation, TiO₂ nanoparticles of approximately 18–20 nm diameter were prepared by the PC sol–gel method (Scheme 2).^8f The particles were subsequently coated with an ascorbic acid layer followed by complexation with MoO₂ (acac)₂ to yield the corresponding supported cis-dioxomolybdenum nanocomplex (Scheme 3). Inductively coupled plasma atomic emission (ICP) analysis revealed that the content of the supported Mo was 2.78 wt%. Therefore, each gram of the heterogeneous catalyst contains 29 μmol Mo or MoO₂ group.

Scheme 2 The flowchart of the preparation of nanoTiO₂ by the sol–gel process.

Scheme 3 Schematic representation of the procedure for the fabrication of the TiO₂/AA/MoO₂ nanocomplex.

The spectral and analysis data confirmed successful synthesis of the title nanocatalyst. The XRD pattern of TiO₂ nanoparticles exhibited two different phases of TiO₂ such as anatase (tetragonal, a = b = 3.782 Å, c = 9.502 Å) and rutile (tetragonal, a = b = 4.584 Å, c = 2.953 Å)⁹ (Fig. 1). The strong diffraction peaks observed at 2θ = 25, 38, 48, 54, 55, 63, 69, 70, 75 were assigned to the (101), (004), (200), (105), (211), (204), (116), (220), (215) reflection planes of tetragonal crystals of anatase TiO₂, respectively (JCPDS no. 21-1272).¹⁰ The other diffraction peaks observed at 2θ = 27, 35, 41, 56 were assigned to the (110), (101), (111), (220) reflection planes of tetragonal crystals of rutile TiO₂ respectively (JCPDS no. 21-1276).¹¹ The size of particles was estimated to be 20 nm according to the Debye Sherrer formula (D = Kλ/β cos θ). It is well known that TiO₂ containing both the rutile and anatase phases exhibit higher photocatalytic activity in visible light than either pure phase alone.¹²

Fig. 1 XRD pattern of nanoTiO₂: (anatase), (rutile).

The FT-IR spectrum of TiO₂ nanoparticles (Fig. 2a) reveals the presence of major bands at 450–775 cm⁻¹ which are attributed to the stretching vibrations of the Ti–O group. It can be observed that there are broad peaks at 3400 and 1638 cm⁻¹, which correspond to the surface adsorbed water and hydroxyl groups.¹³Fig. 2b confirms the successful fabrication of TiO₂/AA composite particles. ortho-substituted hydroxyl groups of the furan ring binding to AA act as bidentate ligands to attach to surface Ti atoms forming chelate-type coordination in the desired complexes.¹⁴ The appearance of peaks in the range of 927 and 1010 cm⁻¹ rationalized to C–O stretching vibration of Ti–O–C.¹⁵ The strong peak at 1320 cm⁻¹ corresponding to the (O–H) enediol for the free acid disappeared in the spectra of the complexes showing the coordination of the O_C3 and O_C2 atoms to the Ti atoms.¹⁶

Fig. 2 FT-IR spectra of (a) nanoTiO₂, (b) the TiO₂/AA nanocomposite and (c) the TiO₂/AA/MoO₂ nanocomplex.

Apparently, AA coating on the surface of TiO₂ particles contributes to the decrease of the number of –OH on the TiO₂ surface. The vibration absorption at low frequencies, such as that observed at 720 cm⁻¹, shows the existence of the nanoTiO₂ core in TiO₂/AA nanocomposites.

Comparison of the FT-IR spectra of the TiO₂/AA/MoO₂ nanocomplex (Fig. 2c) with those of TiO₂ and the TiO₂/AA composite supports the formation of the respective composite since significant spectral changes are observed. It revealed the presence of major bands at 500–750 cm⁻¹ which are attributed to the stretching vibrations of the Ti–O group and the symmetric and asymmetric stretching vibrations of the cis-MoO₂ at 800–950 (Fig. 2c).

Transmission electron microscopy (TEM) observations clearly revealed spherical nanoparticles of TiO₂ and the TiO₂/AA/MoO₂ composite with size ranging between 18–20 nm and 20–25 nm, respectively (Fig. 3).

Fig. 3 TEM of (A) nanoTiO₂ and (B) the TiO₂/AA/MoO₂ nanocomplex.

Thermal behavior of the synthesized amorphous powder dried at 150 °C was analyzed through TGA (thermogravimetric analysis) from ambient temperature to 800 °C. The results showed that 600 °C was an optimum calcination temperature for the synthesis of TiO₂ nanopowders (Fig. 4).

Fig. 4 The TGA of TiO₂ powder.

The TGA curve of the TiO₂/AA/MoO₂ nanocomplex (Fig. 5) demonstrates its degradation at 817 °C, which indicates the high thermostability of the catalyst. The organic parts decomposed completely at 877 °C.

Fig. 5 The TGA of the TiO₂/AA/MoO₂ nanocomplex.

Catalytic activity of the TiO₂/AA/MoO₂ nanocomplex

Epoxidation of olefins with H₂O₂. Preliminary experiments were initiated with oxidation of cyclooctene (1 mmol) using H₂O₂ (2 mmol) as a model reaction under neat conditions which did not proceed in the absence of the catalyst under different conditions. To find optimum reaction conditions, the influence of different factors that may affect the conversion and selectivity of the cyclooctene epoxidation was investigated.

A systematic examination of the solvent nature was performed in various solvents such as chloroform, dichloroethane (DCE), acetonitrile, methanol, ethanol and water using 0.03 mol% of the TiO₂/AA/MoO₂ nanocomposite catalyst (Fig. 6) at different temperatures (Fig. 7). The best yield and conversion rate were obtained in ethanol at 70 °C.

Fig. 6 The screening of the solvent nature (1 mL) on oxidation of cyclooctene (1 mmol) using H₂O₂ (2 mmol) catalyzed by the TiO₂/AA/MoO₂ nanocomplex (0.03 mol%) at 70 °C except for solvents having lower b.p. which were studied at reflux temperature.

Fig. 7 The screening of temperature on oxidation of cyclooctene (1 mmol) using H₂O₂ (2 mmol)–EtOH (1 mL) catalyzed by the TiO₂/AA/MoO₂ nanocomplex (0.03 mol%) after 6 h.

The reaction was further optimized with respect to the catalyst (Fig. 8) and oxidant amounts (Fig. 9). It was observed that full conversion of cyclooctene required 0.03 mol% of the nanocatalyst and two equivalents of H₂O₂ within 6 h, and an increase in any of these ratios did not affect noticeably the reaction rate.

Fig. 8 The screening of the catalyst amount on the epoxidation of cyclooctene (1 mmol) using H₂O₂ (2 mmol)–EtOH (1 mL) catalyzed by the TiO₂/AA/MoO₂ nanocomplex at 70 °C.

Fig. 9 The screening of H₂O₂ amount on the epoxidation of cyclooctene (1 mmol) in EtOH (1 mL) catalyzed by the TiO₂/AA/MoO₂ nanocomplex (0.03 mol%) at 70 °C after 6 h.

To evaluate the oxidizing potential of other common oxidants, cyclooctene was subjected to the oxidation protocol using TBHP, NaIO₄ and Oxone® under the catalytic influence of TiO₂/AA/MoO₂ nanocomposites in ethanol at 70 °C (Fig. 10). Only small amounts of the oxidation products were observed.

Fig. 10 The screening of different oxidants (2 mmol) on oxidation of cyclooctene (1 mmol) in EtOH (1 mL) catalyzed by the TiO₂/AA/MoO₂ nanocomposite (0.03 mol%) at 70 °C after 6 h.

Under the optimized conditions (10 000 : 20 000 : 3 molar ratio for olefin–H₂O₂–catalyst in ethanol at 70 °C), cyclooctene converted completely within 6 h and 95% of the corresponding epoxide was secured as the sole product.

Moreover, prolonged reaction resulted for cyclooctene oxidation when the TiO₂/AA/MoO₂ nanocomplex was replaced by other nanooxometals such as MoO₃,^8gm-ZrO₂, Fe₃O₄ and TiO₂, as well as their nanocomposites such as TiO₂/AA, ZrO₂/AA/MoO₂, MoO₃/AA/MoO₂ and Fe₃O₄/AA/MoO₂ under the same conditions. Photocatalytic properties of TiO₂ at the core of the title nanocomplex may be an acceptable reason for the great improvement of catalytic activity (Fig. 11). To support this claim, oxidation of α-methyl styrene and diphenyl sulfide under UV light, room light lamps as well as in the dark was investigated (Table 1). Accelerated reactions under light radiation, particularly UV light, demonstrated the photocatalytic activity of the TiO₂ core on the oxidation efficiency of the TiO₂/AA/MoO₂ nanocomplex.

Fig. 11 The comparison of the catalytic activity of the TiO₂/AA/MoO₂ nanocomplex with other nanooxometals and nanocomposites in the epoxidation of cyclooctene (1 mmol) in EtOH (1 mL) at 70 °C after 6 h.

Table 1 The screening of photocatalytic activity of the TiO₂/AA/MoO₂ nanocomplex in the oxidation of α-methyl styrene and diphenyl sulfide using H₂O₂ in EtOH^a

Substrate	Condition	Product (selectivity %)	Conversion % (time)
a The molar ratio of the substrate–oxidant–catalyst was 10 000 : 20 000 : 3. The reactions were run under air at 70 °C.
	Room light lamps	a-Methyl styrene oxide (75)	100 (10 h)
	UV light	a-Methyl styrene oxide (70)	100 (8 h)
	Darkness	a-Methyl styrene oxide (60)	100 (12 h)
	Room light lamps	Diphenyl sulfoxide (90)	80 (30 min)
	UV light	Diphenyl sulfoxide (90)	80 (20 min)
	Darkness	Diphenyl sulfoxide (90)	50 (45 min)

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Considering the results presented in Fig. 11 and Table 1 the catalytic activity of the TiO₂/AA/MoO₂ nanocomplex can be related to the Lewis acid catalyst activity of Mo(VI) centers^7c,17 combined with photocatalytic activity of the TiO₂ core which also acts as a support. It is noteworthy that one of the most widely used methods to provide the selectivity is encapsulation of TiO₂ particles into substances.¹⁸

In order to establish the general applicability of the method, various olefins were subjected to the oxidation protocol under the catalytic influence of the TiO₂/AA/MoO₂ nanocomplex (Table 2).

Table 2 Oxidation of alkenes using H₂O₂ catalyzed by the TiO₂/AA/MoO₂ nanocomplex in EtOH^a

Entry	Alkene	Conversion % (isolated yield %)	Product^b	Selectivity^b %	Time (h)
a The molar ratio of the substrate–H2O2–catalyst was 10 000 : 20 000 : 3. The reactions were run under air at 70 °C. b The products were identified by ^1H NMR or by comparison with authentic samples retention times of GC analysis.^19
1		90 (82)		100	6
2		95 (85)		100	6
3		100 (95)		100	6
4		90 (83)		100	12
5		15		100	24
6		100 (93)		75	16
				25
7		100 (94)		75	10
				25
8		80 (72)		85	20
				15
9		100		100	12

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Several useful features of this catalytic method can be seen in Table 2. Different olefins were generally good substrates for this catalyst. It led to high conversions of cyclooctene, norbornene and cyclohexenes with the formation of the corresponding epoxides as sole products (entries 1–4).

It is worth mentioning that cyclic olefin conjugated with a phenyl ring produced exclusively the pertinent epoxide, albeit with low yield (entry 5).

When the terminal C=C double bond was conjugated with an aromatic ring, the major product was the corresponding epoxide and the related carbonyl compound was achieved as a byproduct resulting from a ring-opening reaction of styrene oxide derivatives (entries 6–8).

The chemoselectivity of the procedure was notable. The catalyst was able to oxidize 2-cyclohexene-1-ol as an allylic alcohol to the related unsaturated carbonyl compound in high yield and selectivity (entry 9).

Oxidation of sulfides to sulfoxides

To extend the generality and scope of the present methodology, oxidation of sulfides was investigated. Oxidation of thioanisole using 2.0 equivalents of H₂O₂ in the presence of 0.03 mol% of the TiO₂/AA/MoO₂ nanocomposite in ethanol produced the related sulfoxide as the sole product in excellent yield within 30 min. Accordingly, different sulfides were subjected to the reaction system in the presence of the title nanocatalyst (Table 3). The results given in Table 3 illustrate the high efficiency of this protocol for the oxidation of structurally different sulfides. All substrates could be smoothly converted into sulfoxides with excellent conversion rates, and excellent selectivities were obtained under mild conditions.

Table 3 Oxidation of sulfides using H₂O₂ in ethanol catalyzed by the TiO₂/AA/MoO₂ nanocomplex^a

Entry	Substrate	Conversion % (isolated yield %)^b	Sulfoxide selectivity %
a The reactions were run at 70 °C and the molar ratio of the sulfide–H2O2–catalyst was 10 000 : 20 000 : 3 in 1 mL ethanol within 30 min. b The products were identified by comparison with authentic samples.^20 c The remainder is the related sulfone.
1		100 (96)	100
2		100 (95)	90^c
3		100 (96)	100
4		80 (72)	95^c
5		80 (73)	90^c
6		100 (96)	100
7		90 (80)	95^c
8		100 (96)	100

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The chemoselectivity of the method is noteworthy, as exemplified by the sulfide containing hydroxyl group (Table 3, entry 3) or C=C double bond (Table 3, entries 2 and 6). While the sulfide oxidized completely, the alcohol and olefin moieties remained intact. In addition, dibenzyl sulfide (Table 3, entry 4) was selectively oxidized to its corresponding sulfoxide without the formation of any benzylic oxidation byproducts.

Recovery of the catalyst

The good/excellent yields of epoxides and sulfoxides obtained using these new catalytic methods demonstrated the high stability and catalytic activity of the prepared TiO₂/AA/MoO₂ nanocomplex. This was further supported by the evaluation of the recovery potential of the catalyst in the oxidation of both olefins and sulfides under different conditions mentioned in Tables 2 and 3.

Recovery of the TiO₂/AA/MoO₂ nanocomplex catalyst was easy and efficient. The catalyst was recovered by centrifuging and decantation of the reaction mixture. It was then washed with ethanol as safe solvents, dried under vacuum, and used directly for the next round of reaction without further purification. The ease of recovery, combined with the intrinsic stability of the TiO₂/AA/MoO₂ nanocomplex, allows the catalyst to be recovered efficiently over at least five times in the oxidation of olefins and sulfides under different conditions used in this study (Fig. 12).

Fig. 12 Recycling of the catalytic system for oxidation of cyclooctene (left) and thioanisole (right) using H₂O₂ by the TiO₂/AA/MoO₂ nanocomplex at 70 °C according to procedures mentioned in the Experimental section.

The comparison of FT-IR and TEM images of the used TiO₂/AA/MoO₂ nanocomplex (Fig. 13) with the fresh one showed that the structure, size and morphology of the catalyst remained almost intact after five times recovering.

Fig. 13 (Left) FT-IR spectra of the fresh TiO₂/AA/MoO₂ nanocomplex (A) and after 5 times reuse (B) in the oxidation of cyclooctene; (right) TEM image of the TiO₂/AA/MoO₂ nanocomplex after 5 times reuse in the oxidation of cyclooctene.

Table 4 shows the merit of this operationally simple catalytic protocol for epoxidation of cyclooctene as a model substrate in comparison with the other catalysts in terms of oxygen source, conversion rate, catalyst loading and especially the conditions used in the reactions.

Table 4 The comparison of catalytic activity of the TiO₂/AA/MoO₂ nanocomplex with other catalysts in the epoxidation of cyclooctene

Entry	Catalyst	Catalyst amount	Conditions	Time (h)	Conversion (%)	Ref.
a This work. b Unpublished results. The experiments were performed under the same conditions in our laboratory. c Manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane, in the presence of trichloroacetic acid. d Diol selectivity. e The Keggin-type di-vanadium-substituted silicotungstate [γ-1,2-H2SiV2W10O40]^4− with the {VO-(μ-H)2-VO} core. f Host (nanocavity of zeolite-X)–guest (manganese(III)tetrakis[4-N-methylpyridinium]porphyrin).
1	TiO2/AA/MoO2 nanocomplex	0.03 mol%	Ethanol–H2O2 (70 °C)	6	100
2	m-ZrO2	0.03 mol%	Ethanol–H2O2 (70 °C)	6	0
3	TiO2	0.03 mol%	Ethanol–H2O2 (70 °C)	6	45
4	Fe3O4	0.03 mol%	Ethanol–H2O2 (70 °C)	6	53
5	MoO3	0.03 mol%	Ethanol–H2O2 (70 °C)	6	71
6	TiO2/AA	0.03 mol%	Ethanol–H2O2 (70 °C)	6	62
7	Fe3O4/AA/MoO2	0.03 mol%	Ethanol–H2O2 (70 °C)	6	64
8	MoO3/AA/MoO2	0.03 mol%	Ethanol–H2O2 (70 °C)	6	71
9	ZrO2/AA/MoO2	0.03 mol%	Ethanol–H2O2 (70 °C)	6	0
10	Dititanium-containing 19-tungstodiarsenate(III)	0.002 M	CH3CN–H2O2 (50 °C)	5	70	21
11	Mn(II)/pyridine-2-carboxylic Acid	0.01 mol%	CH3CN–H2O2 (0–25 °C)	0.5–2	95	22
12	[Ga(phen)2Cl2]Cl	5 mM	CH3CN–PAA (0 °C)	1	77	23
13	Mn–TMTACN^c [Mn^IV,IV2(μ-O)(μ-RCO2)2(TMTACN)2]^2+	0.1 mol%	CH3CN–H2O2 (0 °C)	1	71	24
14	[p-C5H5N(CH2)15CH3]3[PW4O32]	30 mg	EtOAc–H2O2 (65 °C)	1	98	25
15	Ti–Fe3O4@MCM-41	0.1 g	Toluene–TBHP (80 °C)	6	91	26
16	Ti–Fe3O4@MCM-41	0.1 g	CH3CN–H2O2 (80 °C)	6	24	27
17	[Fe(BMIP^nPr)2](OTf)2	0.1 mol%	CH3CN–H2O2 (r.t.)	24	25	27
18	Manganese complexes of (tmtacn)	0.3 mM	CH3CN–H2O2 (0 °C)	5	54^d	28
19	Bis(μ-hydroxo) bridged di-vanadium^e	1.67 mM	CH3CN–^tBuOH–H2O2 (20 °C)	24	93	29
20	Polyoxotungstate Na9[SbW9O33] [MTCA]^+Cl^−	0.09 mmol	S.F–H2O2 (60 °C)	6	97	30
21	Mn (TMPyP)-NaX^f	0.007 mmol	CH3CN–H2O–NaIO4 (r.t.)	10	94	31
22	Mn-beta-1 Mn^2+-exchanged zeolites	100 mg	DMF–NaHCO3–H2O2 (r.t)	4	13.5	32
23	NH TiO2-SiO2	50 mg	CH3CN–H2O2 (60 °C)	6	67	33
24	[MnL(OTf)2]	0.5 mol%	CH3CN–PAA(0 °C)	Over 3 min	89	34

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Therefore, title methodologies are cost effective and industrially important because of reusing the catalyst and using H₂O₂ as an environmental oxidant, especially in ethanol as a safe reaction media. These advantages for high yielding oxidation methods offered ready scalability. For example, the use of a semi-scale-up procedure (10 mmol) for epoxidation of cyclooctene and oxidation of thioanisole in the presence of the TiO₂/AA/MoO₂ nanocomplex led to the isolation of the related epoxide and sulfoxide in 95 and 96% yield, respectively.

Conclusions

In conclusion, ascorbic acid coated TiO₂ nanoparticles provided a favored solid support for heterogenization of dioxomolybdenum(VI) centers under ultrasonic agitation producing a novel nanohybrid (TiO₂/AA/MoO₂) with size ranging between 20–25 nm. The TiO₂/AA/MoO₂ nanocomplex demonstrated the desired activity and selectivity in the epoxidation of olefins and oxidation of sulfides to sulfoxides using H₂O₂ as an environmentally benign oxidant in ethanol as a green solvent. These favorable characteristics for a heterogeneous catalytic system, along with the easy and safe workup procedure and excellent reusability of the catalyst, are environmentally benign and cost-effective. Thus, our method possesses high generality which makes it suitable for industrial goals.

Experimental

General remarks

All chemicals were purchased from Merck and Fluka Chemical Companies. Powder X-ray diffraction (XRD) was performed on a Bruker D8-advance X-ray diffractometer with Cu Kα (λ = 1.54178 Å) radiation. The FT-IR spectra were recorded on a NICOLET system. Raman spectra were recorded on a Bruker senterra (2009) model with the spectral range 200–3500 cm⁻¹ and a laser wavenumber of 785 nm. Thermogravimetric analysis (TGA) of nanopowders was performed in air using Shimadzu 50. TEM images were obtained by TEM instrumentation (Philips CM 10). Progress of the reactions was monitored by TLC using silica-gel SIL G/UV 254 plates and by GC-FID on a Shimadzu GC-16A instrument using a 25 m CBP1-S25 (0.32 mm ID, 0.5 μm coating) capillary column. NMR spectra were recorded on Bruker Avance DPX 250 and 400 MHz instruments.

Fabrication of TiO₂ nanoparticles

To a solution of TiCl₃ (0.05 mol) in a mixture of double-distilled water and absolute ethanol (1 : 1) was added citric acid (0.15 mol) and ethylene glycol (0.15 mol). The resulting mixture was dissolved at 45 °C under ultrasound for 15 min to give a clear violet solution. The solution was refluxed at 120 °C for 8 h which turned into a metal-citrate homogeneous complex with a little color change from clear violet to black-violet. After cooling down, in order to bring about the required chemical reactions for the development of polymerization and evaporation of the solvent, the sol was further slowly heated at 90 °C for 6 h in an open bath until a beige wet gel was obtained. During continuous heating at this temperature, the polymerization between citric acid, ethylene glycol and complexes was developed and ultimately the sol became more viscous as a wet gel. In the final step of the sol–gel process, the wet gel was fully dried by direct heating on the hot plate at 150 °C for 6 h. The resulting product was a black powder. Then it was calcined in a furnace at 600 °C for 3 h at a rate of 5 °C min⁻¹. Finally TiO₂ nanoparticles with white color were obtained (Scheme 2).

Preparation of TiO₂/AA nanocomposite particles

To 1.0 g TiO₂ particles was gradually added 10.0 mL of 0.01 M ascorbic acid in water over a period of 20 min at room temperature under ultrasonic agitation. Then, the reaction mixture was stirred at room temperature for 8 h. Afterwards, the product was centrifuged and washed with distilled water. Finally, TiO₂/AA nanocomposites were obtained after drying for 4 h at 100 °C.

Preparation of the TiO₂/AA/MoO₂ nanocomplex

To 0.2 g of the TiO₂/AA nanocomposite was gradually added 1.0 mmol MoO₂(acac)₂ dissolved in ethanol over a period of 20 min at room temperature under ultrasonic agitation. Then, the as-obtained mixture was refluxed for 12 h. Afterwards, the product was centrifuged and washed with ethanol. Finally, the TiO₂/AA/MoO₂ nanocomplex was obtained after drying for 12 h at 80 °C (Scheme 3).

General procedure for catalytic oxidation of olefins

To a mixture of olefin (1.0 mmol) and the TiO₂/AA/MoO₂ nanocomplex (0.03 mol%, 0.01 g) in ethanol (1.0 mL) was added 30% H₂O₂ (2.0 mmol), and the reaction mixture was stirred in air at 70 °C for the required time. The reaction progress was monitored by GC, and the yields of the products were determined by GC and NMR analyses. After the completion of the reaction, the TiO₂/AA/MoO₂ nanocomplex (solid phase) was separated by centrifugation followed by decantation (3 × 5 mL ethanol). Then, the desired product (liquid phase) was extracted by plate chromatography and eluted with n-hexane–EtOAc (10/1). Assignment of the products was done by IR, ¹H NMR and MS spectral data in comparison with authentic samples.

General procedure for the oxidation of sulfides

To a mixture of the sulfide (1.0 mmol) and the TiO₂/AA/MoO₂ nanocomplex (0.03 mol%, 0.01 g) in ethanol (1.0 mL) was added H₂O₂ (2.0 mmol), and the reaction mixture was stirred in air at 70 °C for 30 min. The reaction progress was monitored by TLC, and the yields of the products were determined by GC analysis. After the completion of the reaction, the TiO₂/AA/MoO₂ nanocomplex (solid phase) was separated by centrifugation followed by decantation (3 × 5 mL ethanol). Then the desired product (liquid phase) was extracted by plate chromatography and eluted with n-hexane–EtOAc (10/3). Assignment of the products was done by IR, ¹H NMR and MS spectral data in comparison with authentic samples.

Reusability of the catalyst

To a mixture of the cyclooctene (10.0 mmol) and the TiO₂/AA/MoO₂ nanocomplex (0.3 mol%, 0.1 g) in ethanol (10.0 mL) was added H₂O₂ (20.0 mmol), and the reaction mixture was stirred in air at 70 °C for 4 h. After completion of the reaction, the TiO₂/AA/MoO₂ nanocomplex was separated by centrifugation followed by decantation (3 × 5 mL ethanol). The isolated solid phase (TiO₂/AA/MoO₂ nanocomplex) was dried under reduced pressure and reused for the next runs. Catalyst recovery was also investigated in the oxidation of sulfides with H₂O₂ in ethanol according to the above mentioned procedure.

Acknowledgements

Support for this work by the Research Council of University of Birjand is highly appreciated.

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