Biomass-to-biomass strategy: sulphonic acid functionalized covalent organic frameworks for metal-free esterification of biomass-derived acids.

A sulphonic acid functionalized Covalent Organic Framework (COF-SO3H) was synthesized via an ultrasound-assisted aqueous-phase Schiff base condensation, providing a green and efficient route to incorporate Brønsted acidic sites directly into the framework. Characterisation techniques showed that the resulting material exhibits a partially crystalline, hierarchically porous structure with a surface area of 185 m2 g-1. DFT calculations performed using B3LYP reveal a well-defined hexagonal pore topology with uniformly distributed -SO3H groups along the framework edges. The catalytic activity of COF-SO3H was evaluated for the esterification of biomass-derived molecules, levulinic acid (LA) and succinic acid (SA), with butyl alcohol. Under optimized conditions, the metal-free catalyst afforded excellent conversion (96%) of LA with 99.7% selectivity toward n-butyl levulinate, while SA esterification showed tunable selectivity between mono- and diesters depending on reaction parameters. The catalytic performance is attributed to the accessible -SO3H sites and efficient mass transport within the porous framework. The catalyst demonstrated good stability and recyclability over multiple cycles with minimal loss of activity. The scope of the esterification catalysis could also be extended to various other alcohols, including secondary alcohols and cyclohexanol. It was observed that the chain length and structure of alcohols have significant influence on conversion and selectivity. This study establishes the potential utilization of -SO3H functionalized COFs as sustainable metal-free heterogeneous acid catalysts for the valorization of biomass-derived acids into value-added esters.