Covalent triazine-based frameworks – switching selectivity in HMF photooxidation

Abstract

Covalent triazine-based frameworks (CTFs) – a class of porous organic polymers – excel as photocatalysts due to their chemical and thermal robustness, their highly conjugated and nitrogen rich nature, facile syntheses, and most important unprecedented structural variability. This allows the design of tailor-made photocatalysts. Here, we present how a controlled modification of the CTF allows us to switch the selectivity of 5-hydroxymethylfurfural (HMF) oxidation into two different valuable biomass-based molecules which can substitute oil-based products in existing value chains. Two reaction mechanisms can be chosen by the choice of the CTF's building block depending on the activation mechanism of molecular oxygen: either oxygen is reduced and HMF is directly oxidized to 2,5-diformylfuran (DFF) or molecular oxygen is photoactivated to singlet oxygen which reacts in a cycloaddition with HMF to yield 5-hydroxy-5-hydroxymethyl-furan-2-one (H²MF). Extensive characterization of the optoelectronic properties combined with quantum chemical calculations and detailed investigation of the reaction mechanism allowed us to elucidate the origin of this switch in selectivity.