Selective catalytic oxidation of olefins by novel oxovanadium(iv) complexes having different donor ligands covalently anchored on SBA-15: a comparative study

Abstract

Using a post-synthetic grafting method, mesoporous SBA-15 was functionalized first with aminopropyl silane reagents such as (3-aminopropyl)trimethoxysilane (APTMS) or bis[3-(trimethoxysilyl)propyl]amine (BTMSPA) and then with chlorinated ligands such as 4-(2-chloroacetyl)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one (H-AP) or 5-chloromethyl-8-quinolinol hydrochloride (H-HQ). Vanadyl cations [(VO)²⁺] were then immobilized over functionalized silica samples in order to prepare new oxovanadium(IV) based “quasi-homogeneous” catalysts, namely Ia (Ia′), Ib (Ib′), and IIa. Alternatively, a preformed oxovanadium(IV) complex such as [VO(AP)₂(H₂O)] has been immobilized over a previously functionalized SBA-15-NH₂ silica support, affording catalyst Ic. Elemental analysis (C, H, N), N₂ adsorption–desorption isotherms, FT-IR, ²⁹Si and ¹³C CP-MAS NMR, XPS, ICP-AES, SEM-EDX and TG-DTG techniques have been used for the full characterization of these materials. Their catalytic properties in the H₂O₂ promoted oxidation of conjugated olefins like styrene, α-methyl and β-methylstyrene were investigated, in terms of activity, selectivity and recyclability, and compared with that shown by more simple systems prepared by the direct grafting of the vanadyl cation onto SBA-15 (catalyst A) or SBA-15-NH₂ (catalyst B). Generally, high activity, selectivity and recyclability (over the first three runs) were shown by all anchored catalysts, with the only exception of A and B systems. A full account of the obtained results, along with insights into the effects due to the different strategies employed for the functionalization of SBA-15 on the properties of final anchored catalysts, are reported.