The coordinatively saturated vanadium MIL-47 as a low leaching heterogeneous catalyst in the oxidation of cyclohexene

Abstract

A Metal Organic Framework, containing coordinatively saturated V +IV sites linked together by terephthalic linkers (V-MIL-47) is evaluated as a catalyst in the epoxidation of cyclohexene. Different solvents and conditions are tested and compared. If the oxidant TBHP is dissolved in water, a significant leaching of V-species into the solution is observed and also radical pathways are 2 prominently operative leading to the formation of an adduct between the peroxide and cyclohexene. If however the oxidant is dissolved in decane, leaching is negligible and the structural integrity of the V-MIL-47 is maintained during successive runs. The selectivity towards the epoxide is very high in these circumstances. Extensive computational modelling is performed to show that several reaction cycles are possible. EPR and NMR measurements confirm that at least two parallel catalytic cycles are co-existing: one with V +IV sites and one with pre-oxidized V +V sites, and this in complete agreement with the theoretical predictions.

Mechanistic insight into the cyclohexene epoxidation with VO(acac)2 and tert-butyl hydroperoxide

“…These results are in agreement with our earlier published data, where a vanadium cluster with two terephthalate linkers was used to simulate the MIL-47 framework [11]. In that case a reaction free energy was calculated of -34.7 kJ/mol.…”

“…In Leus and Vandichel et al [11], it has been found that the reaction free energy for this step is competitive (around 37 kJ/mol) with epoxidation reactions. In Table 5 we also report the reaction energies, but in contrast to the MIL-47 cluster these "recycling" reactions become less probable when bidentate ligands are replaced by monodentate ligands.…”

“…In view of these findings this specific epoxidation path can be seen as a minor route toward cyclohexene oxide. The interested reader is referred to our earlier work [11] for more details on this mechanism. …”

“…The reaction chemistry of vanadyl acetylacetonate (VO(acac) 2 ) as catalyst for the oxidation of cyclohexene with hydrogen peroxide or tert-butyl hydroperoxide (TBHP) as oxidant has been the subject of several experimental studies [4][5][6][7][8]. Recently, the homogeneous VO(acac) 2 /TBHP system and the porous metal organic framework type catalyst MIL-47 [9] have been compared for their cyclohexene epoxidation activity and they showed similar conversion patterns [10,11]. In view of our interest in both the homogeneous (VO(acac) 2 ) system and the heterogeneous epoxidation cycles on MIL-47, it is important to obtain mechanistic insight into the plethora of reactions possible for the VO(acac) 2 /TBHP system.…”

Insight in the activity and diastereoselectivity of various Lewis acid catalysts for the citronellal cyclization

“…To account for the rigidity of this hypothetical framework, the low level oxygen atoms of the carboxyl groups were kept fixed preventing the deformation of the cluster. A similar procedure was also applied previously for the description of active sites within the Metal Organic Framework MIL-47 [60]. …”