Journal of Inorganic and Organometallic Polymers and Materials

, 18:472

A Nano-Hybrid of Molybdenum Oxide Intercalated by Dithiocarbamate as an Oxidation Catalyst

Authors

  • Maryam Afsharpour
    • Department of Chemistry, Faculty of ScienceTarbiat Modares University
    • Department of Chemistry, Faculty of ScienceTarbiat Modares University
  • Mostafa M. Amini
    • Department of ChemistryShahid Beheshti University
Article

DOI: 10.1007/s10904-008-9223-y

Cite this article as:
Afsharpour, M., Mahjoub, A. & Amini, M.M. J Inorg Organomet Polym (2008) 18: 472. doi:10.1007/s10904-008-9223-y

Abstract

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.

Keywords

Oxidation catalystHybrid materialsMolybdenum oxideDithiocarbamate

1 Introduction

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 [15]. These materials also have attracted much attention because of their potential application as catalysts [69]. 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 [69]. 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 [1015]. 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 [1630]. 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 [3133]. 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 Experimental

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.

The X-ray diffraction pattern of this intercalation compound is shown in Fig. 1. The presence of ligands in the interlayer of the hybrid can be seen in the XRD pattern. The pattern exhibits sharp and intense reflections at lower angles compared with molybdic acid. The hybrid has a monoclinic crystal structure with cell dimensions of a = 13.872, b = 9.509, c = 6.812 Å and β = 118.828°. A gradual increase in the d spacing of the (001) reflection is observed by adding ligands. The interlayer distance of 0.7 nm can be calculated from a d spacing value of 12.6º, which is associated with the 001 plane of the molybdic acid diffraction pattern. Indeed, the X-ray powder diffraction pattern of this hybrid material shows the 001 reflection at 9.36°, with a calculated d spacing value of 0.9 nm.
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Fig. 1

Powder XRD diffraction patterns of molybdic acid and the synthesized hybrid material

The infrared spectroscopy studies on molybdenum oxide hybrids identified the intercalation reaction (Fig. 2). In MoO3(S–S) hybrid material, characteristic peaks were present at 927 cm−1 that could be assigned to the stretching mode of terminal Mo=O groups. The peaks in the 857 and 678 cm−1 region are characteristic of O–Mo–O group signals. A comparison of the frequencies of hybrid materials and the molybdenum oxide hydrate show that after ligand insertion, υ(Mo=O) is shifted to a lower frequency and the second absorption becomes relatively weaker and broader. According to the relationship between frequency and Mo–O bond length, a band lengthening effect may be accompanied by shifts of some vibrations to lower frequencies. Generally, it seems, the intercalation reaction induces Mo–O bond distortions while preserving the Mo–O skeleton. Such a weak Mo=O bond should have an interesting behavior in the catalytic oxidation of organic substrates.
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Fig. 2

FT-IR spectra of the synthesized hybrid material

Figure 3 shows the TG/DTA curves of MoO3(S–S), which was carried in air up to 700 °C. According to the thermal analysis data, the structure is stable up to only 202 °C, and at higher temperatures decomposition occurs. The TG curve shows four weights losses that are associated with the endothermic peaks at about 202, 335, 364 and 436 °C. The first weight loss (4%) depends on the drying condition, and it corresponds to the departure of water. Thermal analysis indicated that some interlayer water molecules remained in the molybdenum oxide structure. The other weight loss (31.5%) is corresponds to the release of the S–S ligand, which is in good agreement with the calculated 31.9% value. The remaining weight, 64.5%, is in good agreement with the MoO3 (calc. 63.6%). Thermal analysis results confirm the formation of a 2.3:1:1 complex between MoO3, the S–S ligand and interlayer water molecules.
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Fig. 3

TGA/DTA curves of the synthesized hybrid material

The morphology of this molybdenum oxide hybrid was studied by SEM. The SEM images (Fig. 4) show plates with a thickness of about 20–30 nm for the hybrid material, MoO3(S–S), and confirmed the formation of nanoscale material.
https://static-content.springer.com/image/art%3A10.1007%2Fs10904-008-9223-y/MediaObjects/10904_2008_9223_Fig4_HTML.jpg
Fig. 4

SEM images of the synthesized hybrid material

3.2 Catalytic Performance

As shown in Table 1, the intercalated nano-hybrid material exhibited general catalytic oxidizing activity towards alkenes, alcohols, sulfides and amines, converting organic substrate into their oxidized products with good conversion and high selectivity.
Table 1

Oxygen transfer reactions that catalyzed by synthesized hybrid material

Entry

Substrate

Product

Time

Conversion (%)

Selectivity (%)

1

Cyclohexene

Cyclohexene oxide

5 h

72

100

2

Cyclooctene

Cyclooctene oxide

7.5 h

63

100

3

1-Heptene

Heptene oxide

8.5 h

99

100

4

Propanol

Propanal

2.5 h

98

100

5

Butanol

Butanal

2.5 h

94

100

6

Pentanol

Pentanal

1.5 h

82

100

7

Aniline

Nitrosobenzene

5 min

94

89

8

Ethanolamine

2-Nitrosoethanol

5 min

96

81

10

Dibutyl sulfide

Dibutyl sulfoxide

1 h

91

100

11

Dibenzyl sulfide

Dibenzyl sulfoxide

1 h

90

100

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.

Oxygen transfer reactions involving d0 transition metal peroxo species are assumed to proceed through an attack of the electrophilic oxygen center, by V, Mo, and W peroxo complexes (Scheme 1) [34]. So, the oxidation activity of the catalyst is mainly controlled by the charges on the peroxo oxygen centers and reagents as a measure of the electrophilicity of oxygen and nucleophilicity of reagents and interaction between the peroxo σ*(O–O) orbital in the LUMO group of the metal catalyst and HOMO of the reagents [35]. A comparison between the yields obtained for the different substrates reveals that more nuclophilic substrates accelerate the epoxidation reaction. A more nucleophilic nature of the reagent causes a higher energy of the HOMO reagent orbital, which decreases the gap between the HOMO-LUMO orbitals, decreasing the oxidation barrier accordingly.
https://static-content.springer.com/image/art%3A10.1007%2Fs10904-008-9223-y/MediaObjects/10904_2008_9223_Sch1_HTML.gif
Scheme 1

Mechanisms of epoxidation of alkenes by oxomolybdenum compounds

In addition, the recyclability of this catalyst was monitored by using multiple sequential epoxidations of cyclooctene with H2O2 (Fig. 5). The results show only a slight decrease in activity of the catalyst during several successive recycling experiments. After each run, no molybdenum was detected in the filtrates and the catalytic activity remained constant. The recyclability of this catalyst is beneficial for commercial applications. Finally, the simplicity, “greenness”, stability, high activity, and selectivity of this nano-hybrid material make it a good oxidation catalyst in biological and industrial processes. Furthermore, because of its neutrality in the reaction medium; it is also suitable for acid-sensitive compounds.
https://static-content.springer.com/image/art%3A10.1007%2Fs10904-008-9223-y/MediaObjects/10904_2008_9223_Fig5_HTML.gif
Fig. 5

Comparison of the recyclability of the catalyst in two consecutive runs

4 Conclusions

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.

Acknowledgement

Support of this investigation by Tarbiat Modares University is gratefully acknowledged.

Copyright information

© Springer Science+Business Media, LLC 2008