Epoxidation of Alkenes with Molecular Oxygen Catalyzed by Immobilized Co(acac) 2 and Co(bpy) 2 Cl 2 Complexes within Nanoreactors of Al-MCM-41

: Cobalt complexes with different ligands such as bipyridine, and acetylacetonate were immobilized within nanoreactors of Al-MCM-41, designated as Co(acac) 2 /Al-MCM-41 and Co(bipy) 22+ /Al-MCM41. The immobilized complexes were characterized by XRD, N 2 -adsorption desorption, FT-IR and UV-Vis techniques. It was found that Co(bipy) 22+ /Al-MCM41 and Co(acac) 2 /Al-MCM-41 successfully catalyze the oxidation of norbornene, styrene, cis-stilbene, trans-stilbene, and cyclohexene with 68% to 100% conversion and 41% to 90% selectivity toward the corresponding epoxides. No desorption was observed during the course of reactions. The same results are observed for immobilized Co(bpy) 2 Cl 2 complex.


Introduction
Catalytic oxidation is a key technology for converting petroleum feedstocks to useful chemicals such as alcohols, carbonyl compounds and epoxides. Epoxides are important synthetic intermediates for the synthesis of oxygen containing natural or unnatural compounds 1-2 . Many transition metal complexes such as Co, Ti, Mn, V and Mo have been used as catalyst for high selectivity epoxidation of cyclic olefins 2 . Cobalt complexes as the efficient oxyfunctionalization catalysts of alkenes and alkanes have been the subject of intense research in the last two decades. Different types of cobalt complexes such as Cobalt (II) Schiff base complexes [3][4] , cobalt(II) porphyrines 5 and metallophthalocyanines of cobalt complexes [6][7] , have been prepared and used for epoxidation reactions. Methods including incorporation of cobalt species within microporous or mesoporous frameworks [8][9] and fixation of active cobalt complexes on appropriate supports could provide selective and stable catalysts with facile recovery and recycling [10][11][12][13] . Immobilization of Co (salen) and Co perflurophthalocyanine on MCM-41 and or on modified MCM-41 have been studied for oxidation of alkenes and alkanes [14][15][16][17] .
In this study, attempts have been made to prepare the alkenene epoxidation catalyst via immobilization of bipyridine and acetylacetonate cobalt(II) complexes within Al-MCM-41 as anionic based lattice with positive charge, would be a good candidate for immobilization of different cobalt complexes.

Experimental
FT-IR spectra were recorded on a Bruker Tensor 27 FT-IR Spectrometer using KBr pellts. The UV-Vis measurements were performed on a double beam UV-Vis Perkin Elmer Lambda 35 spectrophotometer. X-ray powder diffraction (XRD) data were recorded on a Seifert XRD 3003 PTS diffractometer with Cu kα 1 radiation (λ=1.5406 Å). Nitrogen sorption studies were performed at liquid nitrogen temperature (77 K) using a Quanta chrome Nova Win 2, version 2.2. Oxidation products were analyzed by GC and GC-MS using an Agilent 6890 series with FID detector, HP-5, 5% phenylmethylsiloxane capillary and an Agilent 5973 Network, mass selective detector, HP-5 MS6989 Network GC system, respectively. Atomic absorption data obtained by double beam GBC (909).

Preparation of catalysts
The Al-MCM-41 (Si/Al=50) was prepared according to previously reported method 18 . Co(bpy) 2 Cl 2 .3H 2 O and Co(acac) 2 complexes were prepared according to the previously described procedure [19][20] . Al-MCM-41 (0.5 g in 5 mL methanol) was added to the desired complex (0.1 g in 5 mL methanol). The mixture was refluxed for 24 h while stirring. The solid was filtered, washed with hot methanol and the resultant Co(acac) 2 /Al-MCM-41 or Co(bipy) 2 2+ /Al-MCM-41 were dried in air at room temperature. The percentage of cobalt determined by AAS, were 0.15% and 0.49% respectively.
Catalytic epoxidation, general procedure All epoxidation reactions of the alkenes were carried out in a round bottom flask equipped with a magnetic stirrer and a water-cooled condenser under atmospheric pressure. Typically, a mixture of catalyst (0.2 g) and substrate (20 mmol, dissolved in 10 mL CH 3 CN) was added to the reaction flask with slow stirring. After a few minutes, isobutyraldehyde (24 mmol) was added and the mixture refluxed for 12 h under O 2 atmosphere. The solid was then filtered and washed with fresh solvent. The filtrate was subjected to GC and GC mass analysis.

Spectral characteristics
The XRD patterns of calcined Al-MCM-41, Co(acac) 2 /Al-MCM-41 and Co(bpy) 2 complex/Al-MCM-41 are shown in Figure 1. The XRD pattern of Al-MCM-41 was consistent with that reported before 18 . As seen, the peaks d 100 of Co(acac) 2 /Al-MCM-41 and Co(bpy) 2 complex/Al-MCM-41 shift to higher angles with lower intensities. These changes indicate that the pore surface silanol groups of Al-MCM-41 have reacted with Co(acac) 2  The type IV isotherm indicates that at low pressure P/P 0 , adsorption take places as a thin layer on the walls (monolayer coverage). In addition, the height of inflection in nitrogen adsorption isotherm plots of Co(acac) 2 /Al-MCM-41 and Co(bpy) 2 2+ /Al-MCM-41 are smaller than that of Al-MCM-41. It is attributed to the reduced surface area from 1343 to 1247 and 1279 m 2 /g respectively (Table 1). Thus, it can be concluded that the complexes are included into the Al-MCM-41 pores 23 .

Epoxidation of alkenes catalyzed by Cobalt complexes/Al-MCM-41
Optimization of the epoxidation reaction times were carried out in the presence of 0.1 g of the cobalt complex/Al-MCM-41 as catalyst using trans-stilbene as the representing substrate. The results are presented in Figure 3. As seen, trans-stilbene is mostly oxidized during 8 h, beyond which no further oxidation occurs during the next 4 h.
Since the epoxidation of alkenes by O 2 is an important research direction, we described to examine this reaction with the combination O 2 and isobutyraldehyde as oxidant and co-reductant in the prescence of Co(acac) 2/ Al-MCM-41 and Co(bpy) 2 2+ /Al-MCM-41 as catalysts. Identification of the products was carried out by comparison of product mass patterns with those of the authentic samples.  As seen in Table 3, the oxidation reactions of norbornene, styrene, cis-stilbene, transstilbene, cyclohexene have proceeded with 68% to 100% conversion and 90% to 41% selectivity toward the corresponding epoxides using Co(acac) 2 /Al-MCM-41 as catalyst. The oxidation result carried out in the present of Co(bipy) 2 2+ /Al-MCM-41 as catalyst are given in Table 4. The last worthwhile point to be emphasized is the highly heterogeneous character of the cobalt complex/Al-MCM-41 catalysts used in this work. Experimental evidence ruled out the effect of the participation of any active species, leached from the solid catalysts in the epoxidation reactions. Oxidation reactions of trans-stilbene under the effect of the recovered Co(acac) 2 /Al-MCM-41 and Al-MCM-41 void of complexes as catalysts were found to proceed with 85% and 5% conversion toward the corresponding epoxide. We believe that epoxidation mechanism under our catalysis system is similar to that studied by Nam et al 26 for the epoxidation of olefins using cobalt(II) porphyrins or cobalt (II) cyclams complexes as catalysts (Scheme 1). As seen in Scheme 1, path a, isopropylacyl radical generated in situ via electron transfer from aldehyde to L n Co 2+ affords the corresponding peroxide radical in reaction with O 2 . Subsequent addition of this radical to alkenes such as stilbene, styrene, cyclohexene and norbornene followed by back electron transfer from complex radical anion to peroxy radical intermediates regenerates the catalyst with the formation of the corresponding epoxide and isobutyric acid. Several points need to be elaborated with respect to the results depicted in Tables 3 and 4: (a): Either cis or trans-stilbene affords the corresponding trans-epoxide. This seems to be due to the higher thermodynamic stability of trans-epoxide in which the phenyl groups are located anti with respect to each other. (b): Oxidation of styrene gives a mixture of styrene epoxide and benzaldehyde. As depicted in Scheme 1, path b, styrene epoxide seems to have partly undergone further oxidation to benzaldehyde as the main byproduct. Such process has been observed previously by us and others [27][28] . (c): Since cyclohexene contains active allylic hydrogens, there seem to be a competition reaction available to it, in which it behaves as an acyl radical scavenger, regenerating the isopropyl aldehyde by hydrogen atom transfer. The in situ generated cyclohexene radical in turn in reaction with O 2 affords 2-cyclohexene-1-ol and 2-cyclohexe-1-one as the byproducts (Scheme 1, path c). (d): Although norbornene contains allylic hydrogenes, it cannot have a competition reaction similar to cyclohexene, due to the formation of a highly unstable bridgehead radical intermediate (Scheme 1, path d) 29 . Therefore, norbornene epoxidion proceeds conclusively, affording the corresponding epoxide in 90 to 97% yields (Tables 3 and 4).

Scheme 1.
Mechanism evoked for the oxidation reaction of alkenes.

Conclusion
It was found that the Al-MCM-41 mesoporous molecular sieve is a suitable support for immobilization of cobalt complexes with bipyridine and acetylactonate ligands. Oxidation of cis-stilbene, trans-stilbene, styrene, cyclohexene and norbornene were carried out under mild conditions in the presence of co(acac) 2 /Al-MCM-41and Co(bpy) 2 2+ /Al-MCM-41 as catalysts with O 2 and isobutyraldehyde with moderate to high activity and selectivity towards the formation of the corresponding epoxides. In contrast, cyclohexene undervent allylic oxidation to afford 2-cyclohexene-1-ol and 2-cyclohexe-1-one. No desorption was observed during the course of reactions.