Organic Radical Funcionalized SBA-15 as a Heterogeneous Catalyst for Facile Oxidation of 5-Hydroxymethylfufural to 2,5-Diformylfuran

Abstract

Organic radical functionalized mesoporous silica was prepared by the immobilization of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) on ordered mesoporous silica (SBA-15) and applied as a heterogeneous catalyst (TEMPO-SBA-15) for the synthesis of 2,5-diformylfuran (2,5-DFF) from 5-hydroxymethylfurfural (5-HMF). A 73 % 2,5-DFF yield was obtained from 5-HMF oxidation under mild reaction conditions in ethyl acetate with the addition of [bis(acetoxy) iodo]benzene (BAIB) and acetic acid as co-oxidants in an oxygen-rich environment. The presence of BAIB and oxygen facilitated the in-situ regeneration of TEMPO (the hydroxylamine form) whereas acetic acid addition aided in the initial formation of oxoammonium form of TEMPO, which was responsible for facile 5-HMF oxidation. All reaction components were found critical in maximizing the 2,5-DFF yield. The pore size distribution and architecture of SBA-15 played essential roles for the diffusive transport of reactants and products and resulted in improved reaction selectivity. Aside from these advantages, TEMPO-SBA-15 catalyst was easily separated and re-used several times with negligible loss in its catalytic activity. The prepared heterogeneous TEMPO-SBA-15 is an efficient catalyst that offers a sustainable route for 2,5-DFF production.