The remarkable catalytic activity of the saturated metal organic framework V-MIL-47 in the cyclohexene oxidation

Abstract

The remarkable catalytic activity of the saturated metal organic framework MIL-47 in the epoxidation of cyclohexene is elucidated by means of both experimental results and theoretical calculations.Metal organic frameworks (MOFs) are crystalline porous solids composed of a three-dimensional network of metal ions held in place by multidentate organic molecules. 1,2 Recently, the catalytic applications of MOFs have been reviewed by Hupp et al. and Farrusseng et al. 3,4 However, catalytic reaction mechanisms in metal organic frameworks and in particular at saturated coordinated centres are still poorly understood.In this study, we describe the high catalytic activity of a coordinative saturated V-MOF in the liquid phase oxidation of cyclohexene. The metal organic framework MIL-47 consists of a porous terephthalate framework built from infinite chains of V 4+ O 6 octahedra and has a three-dimensional orthorhombic structure that exhibits large pores. 5 Every structure unit in MIL-47 contains an O-V +IV -O bridge. The linkage of the V +IV to dicarboxylate groups of the terephthalate linkers results in an octahedral structure unit. Hence each vanadium(+IV) centre is saturated.The selective oxidation of cyclohexene is particularly interesting as it can yield, depending on the catalyst and the reaction conditions, several products as shown in Scheme 1. In this study, the reaction was carried out with tert-butyl hydroperoxide (70 wt% TBHP in water) as oxidant and chloroform as solvent. The internal standard used was 1,2,4-trichlorobenzene. The molar ratios were chloroform : cyclohexene : TBHP : 1,2,4-trichlorobenzene = 462 : 1 : 2 : 1. The reaction temperature was kept constant at 323 K. Samples were taken out from the reaction mixture at certain times and analyzed by gas chromatography. Next, MIL-47 was compared with a homogeneous vanadium catalyst vanadyl acetylacetonate VO(acac) 2 . Prior to the reaction, all the catalysts were dried under vacuum. Blank tests were performed without catalyst and no cyclohexene conversion was observed. Fig. 1 depicts the cyclohexene conversion and the detailed product distributions for MIL-47 and the homogeneous VO(acac) 2 catalyst. In both reactions, we used an identical amount of V-sites (0.4 mmol, 0.095 g MIL-47 and 0.106 g VO(acac) 2 ). The four main reaction products formed are cyclohexene oxide 2, cyclohexane-1,2-diol 3, tert-butyl-2-cyclohexenyl-1-peroxide 4 and 2-cyclohexen-1-one 5. From these figures it can be seen that the homogeneous catalyst VO(acac) 2 and the MIL-47 show high reaction rates for the conversion of cyclohexene. The initial turnover frequencies considered at the first 20 min are 57 h À1 and 43 h À1 for the VO(acac) 2 and the MIL-47 respectively. Both catalysts showed an almost linear conversion rate during the first 2 h of reaction and finally topped out at 60% conversion. The turnover numbers calculated at the end of the reaction were 62 and 68 for VO(acac) 2 and MIL-47 respectively. Moreover, considering the product distributions, truly interesting similaritie...

Aerobic Epoxidation of Olefins Catalyzed by the Cobalt‐Based Metal–Organic Framework STA‐12(Co)

“…MOFs usually have high porosity and specific surface areas that make them adequate candidates for catalytic applications both as high-surface-area supports and as intrinsic catalysts. [26,27] MOFs have been used as catalysts for various types of oxidation reactions in the liquid phase, for example, alcohol oxidation, [28] epoxidation, [29][30][31][32] hydrocarbon oxidation, [33][34][35][36] hydroquinone oxidation, [37] the oxidation of organic sulfides, [38] and the oxidation of aromatics. [39] For these oxidation reactions especially, a certain degree of deactivation is frequently observed.…”

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

“…This was supported by nitrogen adsorption measurements that showed that the surface area of MIL-47 after a first run dropped from 1225 m 2 /g to approximately 500 m 2 /g, this will surely lead to a decrease of the reaction rate in the additional runs. We have shown previously 30 that even in conditions of severe leaching (TBHP in water), still these leached out species are relatively inert for the epoxidation mechanism as the building of epoxide stops immediately after the hot filtration of the catalysts; whereas the radical formation of the adduct 4 continues. …”

“…But in both cases also a significant amount of leached V is observed. Our earlier work 30 showed that after 1 h of catalysis 12.8 % of the V is leached out of the structure with 0.11 mmol of V-sites. In order to reduce this leaching, the catalytic performance of MIL-47 in the oxidation of cyclohexene was analyzed with TBHP/decane as oxidant (see Fig.…”

“…Recently, we published the first catalytic evaluation of V-MIL-47 in the liquid phase oxidation of cyclohexene, using TBHP in water as the oxidant and chloroform as the solvent 30 . In general, the liquid phase oxidation of cyclohexene can produce one or more of following products as shown in Scheme 1.…”

“…Garcia et al recently published a review on the use of MOFs as oxidation catalysts 29 . Recent studies by us 30 have focused on the catalytic performance of a saturated vanadium MOF, namely MIL-47 (V IV O(CO 2 -C 6 H 4 -CO 2 ) 31 . As can be seen from Fig.…”