Epoxidation of Cyclohexene on Heterogenized Molybdenum Compounds

 

E.F.M. Barradas, A.R. Cestari, C. Airoldi and R. Buffon^*
Instituto de Química - Unicamp, P.O. Box 6154, 13083-970 Campinas - SP, Brazil
fax: 019 788-3023 e-mail: rbuffon@iqm.unicamp.br

(Received: ; Accepted: March 4, 1998)

 

Abstract - Heterogenization of molybdenum species, starting either with Mo(CO)₆ or MoO₂(acac)₂, on the surface of functionalized silicas bearing one (Si-Et1) or two (Si-Et2) ethylenediamine ligands was studied. The resulting systems are active in the catalytic epoxidation of cyclohexene by tert-butyl hydroperoxide. Using fresh catalysts, Si-Et2 results in higher selectivity, regardless of the Mo precursor. However, MoO₂(acac)₂-based systems are far more active. Formation of diols was never detected. Although XPS analyses point mainly to Mo(VI) species on the surface of all systems, UV-vis spectra suggest different ligands in their coordination sphere.
Keywords: Epoxidation, molybdenum, functionalized silica.

 

 

INTRODUCTION

Soluble Mo(VI) compounds are the most versatile catalysts for the epoxidation of olefins (Parshall and Ittel, 1992). Homogeneous catalysis, however, is plagued with several drawbacks, warranting a search for supported molybdenum catalysts. Research done so far has focused on the use of organic polymers as supports. Preparation of such polymer-supported catalysts has usually been based on anion exchange (Sobczak and Ziólkowski, 1978), cation exchange (Ivanov et al. 1979; Boeva et al. 1984), and chelating ion exchange resins (Bhaduri and Khwaja, 1983; Yokoyama et al. 1985; Sherrington and Simpson, 1991). Of these catalytic systems, the one based on polybenzoimidazol (PBI) showed the best performance and stability (Miller and Sherrington, 1995,a,b). Nevertheless, several drawbacks such as the following have limited the utilization of these systems: i) the instability of the catalyst due to molybdenum leaching; ii) the thermo-oxidative instability of the polymer under reaction conditions; and iii) the poor mechanical properties of the organic support. Therefore, utilization of an organic functionalized silica might be a means of by-passing such problems, since functionalized silicas bearing organic groups such as amine (Allum et al. 1975; Gonçalves and Airoldi, 1989), acetylacetone (Espínola et al. 1992), etc. may be used as coordinating supports for molybdenum complexes, resulting in systems suitable for the epoxidation of olefins.

Interestingly, both zero- and hexavalent molybdenum compounds, viz. Mo(CO)₆ and MoO₂(acac)₂, show similar catalytic activity and are considered to be precursors of the actual catalyst. In view of these facts, the aim of this work was to study heterogenization of Mo(CO)₆ and MoO₂(acac)₂ on functionalized silicas bearing ethylenediamine ligands. Cychohexene was used as the substrate in the catalytic tests since internal olefins are more reactive towards epoxidation (Parshall and Ittel, 1992).

 

EXPERIMENTAL

The organofunctionalized silicas, bearing one (Si-Et1) or two (Si-Et2) ethylenediamine groups, were prepared via a sol-gel process, according to the literature (Cestari et al. 1996), from tetraethoxysilane and 3-trimethoxysilylpropylethylenediamine in an acidic medium. Reaction of Si-Et1 with glutaraldehyde, followed by reduction, produced Si-Et2. Nitrogen loading was determined by the Kjeldahl method.

Catalyst Preparation

The Mo/support catalysts were prepared by addition of a toluene solution of the molybdenum compound to the support, followed by heating under reflux for 72 h (MoO₂(acac)₂) or irradiation (Hg lamp, 125 W) for 30 min (Mo(CO)₆). After the solution was filtered off, the catalysts were washed with toluene and dried in vacuo. Molybdenum loadings were determined using X-ray fluorescence.

Characterization

Diffuse reflectance infrared spectra were recorded on a Perkin-Elmer FT-IR spectrometer. UV-vis were obtained on a CARY 5 G spectrometer using pressed samples. XPS spectra were obtained with a Perkin-Elmer (PHI-1257) spectrometer, using a standard X-ray source with Mg as the anode (hn = 1253.6 eV), operating at 15 kV and 200 W. The binding energy values were corrected by fixing the Si 2p band at 103.5 eV.

Catalytic Tests

Epoxidation of cyclohexene (Riedel) was carried out in argon, in the liquid phase in a glass batch reactor, using tert-butyl hydroperoxide (TBHP) as the oxidant. In a typical experiment, 275 mg of catalyst (0.06 mmol of Mo), 7.0 ml of cyclohexene, 1.4 ml of a solution of tert-butyl hydroperoxide (86 wt.% in cyclohexane - 11.5 mmol), 0.5 ml bromobenzene (internal standard), and 0.2 ml 1,2-dichloroethane were stirred under reflux. The reaction was monitored by GC analysis of the liquid phase (HP 5890 gas chromatograph, equipped with an HP 1 capillary column and a flame ionization detector). Conversions are given as a function of the tert-butyl hydroperoxide consumption.

 

RESULTS AND DISCUSSION

The organofunctionalized silicas employed in this work can be formulated as

When these materials were allowed to react with the molybdenum compounds, a color change from light yellow to a greenish tonality was observed. The maximum molybdenum loading, 0.25 mmol/g of support, might be due to the low specific area of these systems (ca. 30 m²/g). It should be noted that the increase in the ethylenediamine/silica ratio does not correspond to a net increase in the nitrogen content. Therefore, the molybdenum loadings on both silicas are nearly the same (Table 1).

FT-IR Spectroscopic Studies

According to the IR spectra of Mo(CO)₆/support, all CO ligands were released during the surface reaction. In the case of MoO₂(acac)₂, it appeared that acetylacetone had also been replaced. Since all impregnation reactions were performed in air, the formation of oxo species was to be expected. However, in the region between 1000 and 850 cm^-1, characteristic of n _Mo=O, all spectra were similar (Si-Et1, SiEt-2, Si-Et1/Mo(CO)₆, Si-Et1/MoO₂(acac)₂, Si-Et2/Mo(CO)₆, and Si-Et2/MoO₂(acac)₂). The only exception was a sample of Si-Et1/MoO₂(acac)₂ that showed a shoulder at ca. 900 cm^-1.

XPS Analyses

Analyses of the Mo 3d_5/2 orbital energy (~ 232.4 eV) of both Si-Et1/Mo(CO)₆ and Si-Et2/MoO₂(acac)₂ clearly show the presence of Mo(VI) on the surface. However, the line widths of these spectra (Fig. 1a), higher than 1.5 eV, suggest the existence of molybdenum species in oxidation states other than Mo(VI) and/or in different chemical environments. Interestingly, when the MoO₂(acac)₂ sample was treated with TBHP and 1,2-dichloroethane, its spectrum did not change; in the case of the Mo(CO)₆-based sample only Mo(VI) species were observed (Fig. 1b).

 

Table 1: Catalyst composition

a Kjeldahl method (N) and X-ray fluorescence (Mo); ^b not determined; ^c poor spectrum.

Binding energy (eV)	Binding energy (eV)

Figure 1: XPS spectra of Si-Et1/Mo(CO)₆: a) fresh sample; b) sample after interaction with tert-butyl hydroperoxide and 1,2-dichloroethane.

 

UV-Vis Spectroscopic Studies

The UV-vis spectrum of Si-Et1/MoO₂(acac)₂ (Fig. 2b) shows an absorption band at ca. 330 nm, which can not be assigned either to the support (Fig. 2a) or to MoO₂(acac)₂. The lack of this band (which is probably due to a ligand-metal charge transfer) in the spectrum of Si-Et1/Mo(CO)₆ is the only spectroscopic evidence of the existence of different species when the molybdenum precursor is changed.

A band between 500 and 600 nm, as attributed to the Mo(V) species by Garbowski and Praliaud (1988), was not observed in any of our systems.

Catalytic Activity

Preliminary experiments were performed under the standard conditions described in the literature (Miller and Sherrington, 1995,a) in order to evaluate the effects of 1,2-dichloroethane and/or of previous activation (treatment with TBHP and 1,2-dichloroethane under reflux) on the catalytic activity of Si-Et1/MoO₂(acac)₂ in the epoxidation of cyclohexene. It turned out that previous activation did not lead to an increase in catalytic activity (Table 2). However, the addition of 1,2-dichloroethane (DCE), in very small amounts ([DCE]:[Mo] = 0.04) appeared to substantially affect the reaction rate. Therefore, all subsequent catalytic experiments were carried out with DCE and without previous activation.

Table 3 presents the results obtained with the Mo(CO)₆ and MoO₂(acac)₂ precursors, either supported on Si-Et1 and Si-Et2 or in homogeneous solution, along with some data from the literature. It appeared that the heterogenization of Mo(CO)₆ and MoO₂(acac)₂ on the organofunctionalized silicas caused a decrease in the cyclohexene epoxidation rate: within 4 h reaction time, TBHP reached a maximum conversion of ca. 72%, corresponding to a turnover number ([cyclohexene epoxide]:[Mo]) of ca. 140.

In preliminary recycling experiments with Si-Et1/MoO₂(acac)₂, Mo leaching was not observed. Catalytic activity, however, decreased slightly.

Epoxidation of cyclohexene over molybdenum species heterogenized on functionalized silicas, as studied in this work, can be considered to be highly selective. The only by-products observed were cyclohexene and chlorocyclohexanol. The latter probably results from a reaction between epoxide and DCE.

Figure 2: UV-vis spectra of Si-Et1: a) support; b) sample a after reaction with MoO₂(acac)₂ ; c) sample a after reaction with Mo(CO)₆.

 

Table 2: Preliminary catalytic tests with the Si-Et1/MoO₂(acac)₂ system in the epoxidation of cyclohexene^a

a Using fresh catalyst samples; [TBHP]:[Mo] = 192.

 

Table 3: Effects of the support on the catalytic activity of MoO₂(acac)₂ and Mo(CO)₆

Catalyst precursora	conversion %
	20 min	30 min	60 min	90 min	120 min	240 min
Si-Et1/MoO2(acac)2		15.9		45.2		71.5
Si-Et1/Mo(CO)6		20.0		28.0		34.7
Si-Et2/MoO2(acac)2		24.4		59.5		72.1
Si-Et2/Mo(CO)6		13.1		n.d.		32.1
MoO2(acac)2	73.9		88.7		93.1	100
MoO2(acac)2 b	90.2		92.2		95.9
Mo(CO)6	64.8		93.8		97.5	100
PBI/MoO2(acac)2 b	91.0		98.5		100

 

Although the MoO₂(acac)₂-based systems were far more active (Table 3), the least and the most selective systems were based on Mo(CO)₆: Si-Et1/Mo(CO)₆ was the least selective of the systems studied, yielding 9% by-products; Si-Et2/Mo(CO)₆ showed the highest selectivity (~100% epoxide). Formation of diols was never observed.

 

CONCLUSIONS

Heterogenization of molybdenum compounds on the surface of organofunctionalized silicas bearing one or two ethylenediamine ligands results in catalysts which are active in the epoxidation of cyclohexene with tert-butyl hydroperoxide. Although these systems show lower catalytic activity than those reported for analogous heterogeneous systems, molybdenum does not leach from the surface, allowing the catalyst to be recycled. The differences in activity and selectivity observed for the different systems analysed in this work can be ascribed to the existence of molybdenum in diverse chemical environments which originate from the different molybdenum precursors used, as detected by UV-vis spectroscopy.

 

ACKOWLEDGEMENTS

The financial support received from FAPESP and CNPq is gratefully acknowledged. The authors also thank Carlos André Perez (NUCAT-PEQ-COPPE-UFRJ) for the XPS analyses.

 

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* To whom correspondence should be addressed.

a Using fresh catalyst samples; [TBHP]:[Mo] = 192. ^b Miller and Sherrington (1995,a); PBI = polybenzimidazole.