A Nano-Hybrid of Molybdenum Oxide Intercalated by Dithiocarbamate as an Oxidation Catalyst
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- Afsharpour, M., Mahjoub, A. & Amini, M.M. J Inorg Organomet Polym (2008) 18: 472. doi:10.1007/s10904-008-9223-y
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A novel nano-layered material based on molybdenum oxide has been synthesized by hydrothermal method using dithiocarbamate. On the basis of the X-ray diffraction, scanning electron microscopy, thermal analysis and infrared spectroscopy results, a possible arrangement of organic ligands in the interlayer space of molybdenum oxide has been proposed. Moreover, the catalytic activity of the synthesized nanohybrid of molybdenum oxide was investigated in oxygen transfer reactions. This reagent can oxidize alkenes, alcohols, sulfides, and amines in the presence of hydrogen peroxide with high yield and selectivity.
KeywordsOxidation catalystHybrid materialsMolybdenum oxideDithiocarbamate
The design and synthesis of organic–inorganic hybrid compounds has received a considerable amount of interest due to their potential applications in electronics, magnetisms and optics [1–5]. These materials also have attracted much attention because of their potential application as catalysts [6–9]. In hybrid materials, organic components act as the ligand and incorporate into the metal oxide backbone; consequently, it is expected that the organic component will influence the structure of the inorganic oxides and produce materials with new properties. There are various methods for the preparation of organic–inorganic hybrid materials; among them, the hydrothermal method is an impressive approach for their synthesis. In the hydrothermal method, one can overcome the solubility problems of inorganic solids and organic ligands to avoid phase separation. Therefore, this method provides opportunity for the fabrication of unique materials with new features and special properties.
Transition metal oxo complexes are involved in oxygen transfer chemistry in both biological and industrial processes [6–9]. Molybdenum complexes have been extensively studied, especially as models for oxidation catalysts and as the active site of oxo transfer in the molybdenum-containing enzymes [10–15]. To understand oxo transfer properties, numerous oxo molybdenum complexes involving a wide range of ligands (S,S-, N,N-, O,O- and N,O-donor ligands) have been prepared and characterized [16–30]. Among the investigated complexes, the ones that received the most attention are those with ligands that contain sulfur atoms. Molybdenum complexes with S,S-donor ligands are well interested, especially as models for oxidation catalysts and as the active site of oxo transfer, molybdenum-containing enzymes. The enzymes that contain molybdenum and molybdenopterin (MoCo) are diverse and broadly distributed [31–33]. As a contribution to these interesting class of compounds, molybdenum hybrid material containing an anionic S,S ligand (pyrrolidine dithiocarbamate) has been chosen to demonstrate its effectiveness as catalyst in oxidation of different organic substrates. Here, we describe the synthesis and characterization of new trioxomolybdenum(VI) complexes with the general formula of MoO3(S,S). In general, these hybrid materials consist of two-dimensional layers of corner-sharing MoO6 octahedra with a ligand molecules directly bound to the molybdenum. The intercalation of organic ligands into the inorganic backbone could yield a hybrid with the properties of both the inorganic and organic components. Furthermore, such organic–inorganic hybrid materials may have new properties arising from the interplay of the two components. Our study was particularly focused on the catalytic application of this nano-hybrid material in selected oxygen transfer reactions (oxidation of alkenes, alcohols, sulfides and amines). An important property of this catalyst is its high selectivity at a reasonable yield.
2.1 Materials and Methods
All reagents purchased from Merck and used without further purification. Infrared spectra were recorded on a Bruker Equniox-55 spectrometer. Scanning electron microscopy was performed on a Philips XL-300 instrument. Thermal analysis was carried out using a PL STA-1500 system with a heating rate of 10 °C min−1 in air. The X-ray diffraction patterns were recorded on a Philips X’-Pert diffractometer using CuKα radiation (λ = 1.54060 Å). A HP 5,890 gas chromatograph equipped with a FID detector and a Rtx-5 capillary column was used to monitor reactions products.
2.2 Synthesis of MoO3(S–S)
A mixture of yellow molybdic acid (2 mmol, 120 mg), and ammonium salt of pyrrolidin-1-dithiocarbamate (1 mmol, 345 mg) along with 8 mL of a solution containing ethanol and water in a 1:3 ratio, was placed in a 200 mL Teflon-lined stainless steel autoclave reactor and heated for 24 h at 120 °C under autogenous pressure. After allowing the reaction mixture to cool for 10 h, the precipitate was collected by filtration, washed with water and dried at room temperature.
2.3 Catalytic Test
The catalytic activity was examined by suspending 3% molar ratio (cat./substrate) (0.03 mmol, 19.8 mg) catalyst in 1 mL CH2Cl2 with 1 mmol of pre-selected organic substrate. Hydrogen peroxide (30%; 1 mL) was introduced into the reaction mixture. The two phase reaction system was stirred at room temperature. The substrate and reaction products were dissolved in the organic phase, and the catalyst is appeared in the aqueous phase. The extent of alkene conversion was monitored by sampling aliquots of the organic phase of the reaction mixture every half hour and analyzing by gas chromatography. After the reaction completed, the aqueous phase was filtered to remove the catalyst, and the recovered material was repeatedly used as a recyclable catalyst.
3 Results and Discussion
3.1 Characterization of Catalyst
The layered molybdic acid (MoO3 · 2H2O) is used as starting material in this synthesis protocol. Yellow molybdic acid has a monoclinic crystal structure (P21/n) with cell dimensions of a = 10.476, b = 13.833, c = 10.606 Å and β = 91.63° (JCPDS 39-0363). The layers are stacked along the b axis and interconnected by interlayer water molecules. By replacing the interlayer water molecules with organic ligands under mild conditions, molybdenum oxide hybrids are obtained.
3.2 Catalytic Performance
Oxygen transfer reactions that catalyzed by synthesized hybrid material
The oxidation reactions were carried out with different amounts of catalyst, reagent, and oxidant, and the best conditions for the catalytic reactions were investigated. The influences of solvent and oxidant were evaluated for the catalyst. Dichloromethane and hydrogen peroxide worked well, and the latter was used for the following experiments. In addition, the optimum temperatures for each reaction were selected. Based on the catalytic data, we reported here the optimal conditions of the systems that led to the oxidation of various organic substrates in the highest yields.
All of the reactions showed a high selectivity in the presence of this catalyst. This high selectivity of the catalyst in the oxidation of alkenes, alcohols, sulfides and amines to corresponding epoxides, aldehydes, sulfoxides and nitroso compounds can be attributed to the binding of organic ligand to the molybdenum, which reduces the Lewis acidity of the metal center. This catalyst efficiently converted both cyclic and linear alkenes at room temperature. The linear alkene 1-heptene was completely oxidized to the corresponding epoxides by this catalyst. Cyclooctene and cyclohexene were also converted to epoxide at 63–72% yield. No formation of diols from the epoxides was observed during the course of the reactions. Also, high conversion of alcohols to the corresponding aldehydes was observed at higher temperature (50 °C). Oxidation of primary and secondary alcohols proceeded well at high conversion. This means that the catalyst is effective for the activation of alcohols, and that it is active for the selective oxidation to aldehydes. Amines were converted to the corresponding nitroso compounds rapidly at room temperature, and sulfoxides were exclusively produced by the oxidation of sulfides, without overoxidation to sulfones.
We present here the synthesis and characterization of a novel organic–inorganic hybrid material of molybdenum oxide using dithiocarbamate. This research indicates that organic ligands can influence the structure and properties of the intercalated hybrid. Furthermore, the catalytic properties of this hybrid were examined in selected oxygen atom transfer reactions. The results show a good performance of the hybrid material as a recyclable catalyst in the oxidation of various organic substrates.
Support of this investigation by Tarbiat Modares University is gratefully acknowledged.