Grafting of vanadyl acetylacetonate onto organo-hexagonal mesoporous silica and catalytic activity in the allylic epoxidation of geraniol

Abstract

Vanadyl(IV) acetylacetonate ([VO(acac) 2]) was grafted onto a hexagonal mesoporous silica (HMS) using three different methodologies: method A – direct complex immobilisation; method B – functionalisation of the HMS with 3-aminopropyltriethoxysilane (APTES) followed by the complex immobilisation; and method C – treatment of the APTES functionalised support prepared by method B with trimethylethoxysilane (TMS) to deactivate eventually unreacted surface silanol groups, followed by complex grafting. All the materials were characterised by nitrogen elemental analysis, XPS, FTIR, N 2 adsorption isotherms at −196 °C and the [VO(acac) 2] based materials were also characterised by vanadium ICP-AES analysis. The results indicated that, in method B, APTES was successfully grafted onto the HMS with 90% of efficiency and allowed the covalent attachment of [VO(acac) 2] complex mainly in the inner pores with an efficiency of 65%. In method C, a lower complex immobilisation efficiency was obtained, c.a. 25%, but the complex was covalently bonded throughout the functionalised material. In the case of method A, the parent HMS material immobilised a very low quantity of vanadium complex (2% of efficiency), mainly in the external surface through non-covalent interactions. The catalytic activity of [VO(acac) 2] based materials in the epoxidation of geraniol using tert-butyl hydroperoxide ( t-BuOOH) as oxygen source was assessed. The selectivities of the two epoxides formed in the heterogeneous phase reactions were similar to those observed in the homogeneous phase and the major reaction product was always 2,3-epoxygeraniol. The catalyst which allowed higher substrate conversion was obtained by method B but, when considering the leaching of the active phase, method C produced the most efficient catalyst.