Upcycling of spent functional biocarbon adsorbents to catalysts for the conversion of C5/C6 carbohydrates into platform chemicals

Abstract

Upcycling of spent heavy metal functional carbon adsorbents for use in catalytic applications must be considered to avoid hazardous wastes and to develop circular economies. In this study, amino-Brønsted acid precursor functional biocarbons were prepared by the hydrothermal-ammonia carbonization of biomass and subsequent mechanochemical modification with L-cysteine. Mix ball-milling of hydrothermal-ammonia-carbonized solids with L-cysteine afforded biocarbon materials with three kinds of sites, namely amino-functional, Brønsted acid precursor, and metal ion binding. The adsorption of Ni²⁺ metal onto biocarbons was high (Q_m > 313 mg g⁻¹) and the spent adsorbents could be upcycled by the oxidation of Brønsted acid precursor sites (–SH groups) with H₂O₂ and applied as catalysts for C5/C6 carbohydrate dehydration–oxidation reactions. In an ionic liquid [BMIM]Cl reaction phase, the spent adsorbent catalyzed the conversion of C5 carbohydrate substrate xylose to furoic acid and furfural in yields of 46.2% and 24.7%, respectively, while C6 carbohydrate substrates, fructose and glucose, afforded 5-hydroxymethylfurfural in yields of 71% and 24%, respectively. The preparation procedure for amino-functional biocarbons allows heavy metal recovery from wastewater and upcycling of spent materials to catalysts for carbohydrate dehydration–oxidation reactions.