Vanadyl acetylacetonate anchored onto amine-functionalised clays and catalytic activity in the epoxidation of geraniol

Abstract

The parent clays laponite (Lap) and K10-montmorillonite (K10) and those functionalised with (3-aminopropyl)triethoxysilane (APTES) were used as supports to immobilise vanadyl(IV) acetylacetonate ([VO(acac) 2]). All the materials were characterised by elemental analysis, XRD, TG-DSC, nitrogen adsorption isotherms at −196 °C and IR spectroscopy. The K10-based materials were also characterised by XPS. In the case of K10-based materials, higher vanadium content was obtained for the APTES-functionalised K10, showing that the clay functionalisation enhanced the complex anchorage. In Lap-based materials the opposite tendency was observed, with a higher vanadium loading being obtained for direct complex immobilisation onto the parent clay. This is probably due to clay particles aggregation (resultant from the delaminated nature of Lap) provoked by functionalisation with APTES. Furthermore, FTIR data pointed out that in the amine-functionalised clays, the VO(IV) complex was anchored by Schiff condensation between the carbonyl groups of the acetylacetonate ligand and the amine groups from the grafted APTES, whereas the direct immobilisation of the complex onto the parents clays took place mostly through covalent bonding between the metal centre and the clay surface hydroxyl groups. The [VO(acac) 2]-based materials were tested in the epoxidation of geraniol using t-BuOOH as oxygen source and reused several times repeatedly. The [VO(acac) 2]APTES@K10 material was the most efficient and stable catalyst upon reuse (5 cycles), among the four materials tested, with a substrate conversion and 2,3-epoxygeraniol regioselectivity comparable to the homogeneous phase reaction; the organofunctionalisation of K10 was also quite advantageous in the catalytic reaction since it passivated some active sites of the K10 support. In the case of Lap-based materials, the [VO(acac) 2]@Lap was more catalytically efficient than [VO(acac) 2]APTES@Lap.