Maasoumeh
Jafarpour
*,
Mahboube
Ghahramaninezhad
and
Abdolreza
Rezaeifard
*
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand, Birjand, 97179-414 Iran. E-mail: mjafarpour@birjand.ac.ir; rrezaeifard@birjand.ac.ir; rrezaeifard@gmail.com; Fax: +98 561 2502515; Tel: +98 561 2502516
First published on 4th October 2013
The novel catalytic activity of α-MoO3 nanobelts prepared by a new and safe sol–gel method for the epoxidation of olefins and oxidation of sulfides to sulfoxides using H2O2 in ethanol as a safe solvent has been exploited. The reactions also proceeded efficiently in the presence of tert-butyl hydroperoxide (TBHP). Good/high yields and excellent selectivity resulted. The ammonia TPD profile demonstrated strong acidic sites in synthesized α-MoO3 nanobelts, which generated different catalytic activity than the bulk material. The separation and reuse of this heterogeneous nanocatalyst was simple, effective and economical in the presented oxidation methods.
This is of particular importance, considering that the conservation and management of resources should be the main focus of interest when novel chemical processes are developed. Thus, the innovation and improvement of catalytic epoxidation methods where molecular oxygen or economical peroxides, namely hydrogen peroxide and tert-butyl hydroperoxide (TBHP), are employed as terminal oxidants is highly desirable.2–5
Metal oxides represent one of the most important and widely employed categories of solid catalyst, either as active phases or as supports. They are utilized for both their acid–base and redox properties and constitute the largest family of catalysts in heterogeneous catalysis. Recently, the use of nanomaterials has increased, as their activity is very high under mild conditions due to their very large surface area.6–9
Molybdenum catalyzed oxygen atom transfer (OAT) chemistry is a very important chemical reaction, both in biological systems and on an industrial scale. In biology, molybdenum is found in a large family of enzymes that catalyze important OAT reactions in the living cell.10,11 Several research groups,12,13 including ours,14–18 have developed functional and structural model molybdenum complexes and investigated their OAT reactivity for a better understanding of this important reaction.
Molybdenum oxide is widely used in sensors, lubricants, and fuel cell materials.19–22 Catalysis using MoO3 greatly relies on the crystal phase, surface structure, and particle size.23 Bulk MoO3 alone presented low/moderate catalytic oxidation activity24–26 and a ligand/additive or Lewis acid should be added to reach the desired activity.27–30
The synthesis and structural properties of nanostructured MoO3 have been widely investigated, however, there is only limited work on its catalytic activity.31 Just recently, we have developed a new and safe route to synthesize α-MoO3 nanobelts by a simple sol–gel method.32 An orthorhombic lattice system was established by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and Raman analyses. The HRTEM (high resolution transmission electron microscopy) images revealed that the nanobelt-form mostly ranged from 20–70 nm in width and 200–400 nm in length. The prepared nanostructured MoO3 exhibited a high efficiency in catalyzing the condensation reaction of various 1,2-diamine and carbonyl compounds for synthesis of heterocyclic compounds.32 We would now like to describe its catalytic potential in the epoxidation of olefins and oxygenation of sulfides to sulfoxides using hydrogen peroxide and TBHP, both of which are industrially and environmentally important oxidants.
Temperature-programmed desorption of NH3 adsorbed on a catalyst (NH3-TPD) was carried out in a conventional flow system equipped with a thermal conductivity detector (TCD).
The sample was pretreated in nitrogen at 550 °C for 1 h before adsorbing NH3. The NH3-TPD experiments were carried out at 100–600 °C in a flow of dry He (19 mL min−1). The rate of heating was 10 °C min−1.
The progress of the reactions was monitored by thin layer chromatography (TLC) using silica-gel SIL G/UV 254 plates and also by gas chromatography (GC) 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 a Brucker Avance DPX 250 and 400 MHz instruments.
Assignment of the products was made by IR, 1H NMR and MS spectral data in comparison with control samples. [Note: oxidation of olefins using TBHP in dichloroethane (DCE) was performed by the same procedure].
Assignment of the products was made by IR, 1H NMR and MS spectral data in comparison with control samples. [Note: oxidation of sulfides using TBHP in DCE was performed with the same procedure].
A systematic examination of the solvent nature was performed in various solvents, namely chloroform, DCE, acetonitrile, methanol, ethanol and water, using 1 mol% of the nano-MoO3 catalyst (Fig. 1) at different temperatures (Fig. 2). The best yield and conversion rate were obtained in ethanol at 70 °C.
Fig. 2 The effect of temperature on oxidation of cyclooctene (1 mmol) using H2O2(2 mmol)–EtOH (1 mL) catalyzed by nanobelts of α-MoO3 (0.01 mmol). |
The reaction was further optimized with respect to catalyst loading (Fig. 3) and oxidant amount (Fig. 4). It was observed that full conversion of cyclooctene required 1 mol% of nanocatalyst and two equivalents of H2O2 over 10 h and an increase in any of these ratios did not noticeably affect the reaction rate.
Fig. 3 Effect of quantity of catalyst on the oxidation of cyclooctene (1 mmol) using H2O2 (2 mmol)–EtOH (1 mL) catalyzed by nanobelts of α-MoO3 at 70 °C. |
Fig. 4 Effect of H2O2–olefin molar ratios on oxidation of cyclooctene (1 mmol) in EtOH (1 mL) catalyzed by nanobelts of α-MoO3 (0.01 mmol) at 70 °C. |
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 α-MoO3 in ethanol at 70 °C (Fig. 5). Only small amounts of the oxidation products were observed.
Fig. 5 Effect of different oxidants (2 mmol) on oxidation of cyclooctene (1 mmol) in EtOH (1 mL) catalyzed by nanobelts of α-MoO3 (0.01 mmol) at 70 °C. |
Under the optimized conditions (100:200:1 molar ratio for olefin–H2O2–catalyst in ethanol at 70 °C), cyclooctene converted completely within 10 h and 95% of the corresponding epoxide was secured as the sole product.
It should be mentioned that, when the nano-MoO3 was replaced by its bulk counterpart, cyclooctene remained completely intact under the same conditions. Enhanced acid strength and surface area of nanostructured MoO3 (Fig. S6†) may be a plausible reason for this great improvement in catalytic activity.
In order to establish the general applicability of the method, various olefins were subjected to the oxidation protocol under the catalytic influence of α-MoO3 nanobelts (Table 1).
Entry | Olefin | Conversion % (isolated yield) | Productb | Time (h) |
---|---|---|---|---|
a The molar ratio of substrate–H2O2–catalyst was 100:200:1. The reactions were run under air at 70 °C. b The products were identified by 1H NMR or by comparison with control sample retention times using GC analysis.35–39 The selectivity of the products was >99%. | ||||
1 | 100 (92) | 12 | ||
2 | 100 (95) | 10 | ||
3 | 100 (95) | 10 | ||
4 | 100 (93) | 22 | ||
5 | 10 | 24 | ||
6 | 50 | 24 | ||
90 (85) | 48 | |||
7 | 100 (96) | 20 | ||
8 | 73 (65) | 48 | ||
9 | 30 | 48 | ||
10 | 50 (43) | 22 | ||
11 | 45 (38) | 22 | ||
12 | 100 | 12 |
Several useful features of this catalytic method can be seen in Table 1. Different olefins were generally good substrates for this catalyst. It led to complete conversion of cyclooctene, norbornene and cyclohexenes with the formation of the corresponding epoxides as sole products (Table 1, entries 1–4).
The epoxidation of 1-octene as the least reactive terminal olefin proceeded sluggishly (10% yield, entry 5). When the terminal CC double bond was conjugated with aromatic ring, the reaction rate was enhanced, however, the epoxide ring was completely opened, producing the related carbonyl compound (Table 1, entries 6–8). It seems that benzaldehyde and acetophenone are favorably formed at high temperature in the oxidation of styrene and α-methylstyrene, respectively, because high temperature will supply enough energy to break the CC bond.33,34 These results were further supported by the oxidation of styrene oxide as substrate under the same conditions. Benzaldehyde was formed quantitatively as the sole product within 48 h. It should be noted that, in all the above mentioned reactions, no phenylacetaldehyde product was detected in the oxidation of either styrene or styrene oxide. Inspection of these results demonstrates that the reaction is significantly affected by electronic factors. While α-methyl styrene was converted completely to acetophenone within 20 h, the oxidation of electron-deficient 4-Cl-styrene proceeded slowly and moderate yield of the related benzaldehyde was obtained (73% after 48 h). It is worth mentioning that a cyclic olefin conjugated with a phenyl ring produced exclusively the pertinent epoxide, albeit with low yield (Table 1, entry 9). The chemoselectivity of the procedure was notable. While primary alcohols containing CC double bonds (allylic and homoallylic alcohols) oxidized completely to the corresponding epoxides (Table 1, entries 10 and 11), a secondary one converted to the related α, β-unsaturated ketone as the only product (entry 12).
Entry | Olefin | Conversion % (isolated yield) | Productb | Time (h) | |
---|---|---|---|---|---|
a The molar ratio of substrate–TBHP–catalyst was 100:200:1. The reactions were run under air at 70 °C. b The products were identified by 1H NMR or by by comparison with control sample retention times using GC analysis.35–39 The selectivity of the products was >99%, except for entry 9 which was 85%. | |||||
1 | 100 (94) | 6 | |||
2 | 100 (93) | 6 | |||
3 | 10 | 24 | |||
4 | 56 | 12 | |||
100 (94) | 24 | ||||
5 | 100 (96) | 14 | |||
6 | 100 (94) | 24 | |||
7 | 55 (43) | 14 | |||
8 | 50 (45) | 14 | |||
9 | 100 | 85 | 10 | ||
15 |
Entry | Substrate | Conversion % (isolated yield) | Sulfoxide selectivity % | |
---|---|---|---|---|
TBHP–DCEb | H2O2–ethanola | |||
a The reactions were run at 70 °C and completed within 45 min. The molar ratio of sulfide:H2O2:catalyst was 100:200:1 in 1 mL ethanol. b The reactions were run at 70 °C and completed within 15 min. The molar ratio of sulfide:TBHP:catalyst was 100:200:1 in 1 mL DCE. The products were identified by comparison with control samples.40–43 | ||||
1 | 100 (96) | 100 (97) | 96 | |
2 | 95 (90) | 95 (88) | 95 | |
3 | 100 (94) | 100 (95) | 90 | |
4 | 100 (96) | 100 (96) | 100 | |
5 | 90 (85) | 90 (87) | 95 | |
6 | 80 (75) | 80 (75) | 90 | |
7 | 100 (94) | 100 (96) | 100 | |
8 | 95 (89) | 95 (88) | 95 | |
9 | 100 (95) | 100 (96) | 100 |
Oxidation of diphenylsulfide (entry 6) under the same conditions produced a lower yield of the desired sulfoxide than other substrates, indicating the influence of steric effects on the reaction rate.
The chemoselectivity of the method is noteworthy, as exemplified by sulfides containing a hydroxyl group (Table 3, entry 4) or CC double bond (Table 3, entry 2, 3, 7). While the sulfide oxidized completely, the alcohol and olefin moieties remained intact. In addition, benzyl phenyl sulfide (Table 3, entry 5) was selectively oxidized to its corresponding sulfoxide without formation of any benzylic oxidation by-products.
As observed in olefin epoxidation, an accelerated oxidation of sulfide occurred when H2O2 was replaced with TBHP (in DCE), albeit with the same yields and selectivity (Table 3).
Recovery of the nanoMoO3 catalyst was easy and efficient. The catalyst was recovered by centrifuging and decantation of the reaction mixture. It was then washed with ethanol as a safe solvent, 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 MoO3 nanoparticles, allows the catalyst to be recovered efficiently at least four times in the oxidation of olefins and sulfides under the different conditions used in this study (Fig. 6 and 7).
The comparison of Raman spectra (Fig. 8) and TEM images of used MoO3 nanobelts (Fig. 9) with fresh catalyst showed that the structure, size and morphology of the catalyst remained almost intact after four times recovering.
Fig. 8 Raman spectra of fresh nano α-MoO3 (A) and after being reused 4 times (B) in the oxidation of olefins and sulfides under different conditions. |
Fig. 9 TEM images of fresh nano α-MoO3 (A) and after being reused 4 times (B) in the oxidation of olefins and sulfides under different conditions of nanobelts of α-MoO3. |
The presented methodologies are therefore cost effective and industrially important, as they allow recycling of the catalyst and use of H2O2 and TBHP as environmentally-friendly oxidants, particularly in ethanol as a safe reaction media. In addition to these advantages, the high yielding oxidation methods also 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 nanobelts of α-MoO3 led to isolation of the related epoxide and sulfoxide in 93 and 95% yield, respectively.
Footnote |
† Electronic supplementary information (ESI) available: Synthesis and characterization of the nanobelts of α-MoO3. See DOI: 10.1039/c3ra44404j |
This journal is © The Royal Society of Chemistry 2014 |