Characterization and Valorization of Humins Produced by HMF Degradation in Ionic Liquids: A Valuable Carbonaceous Material for Antimony Removal

The processing of biomass in ionic liquids has demonstrated many benefits compared to organic solvents. This includes the maximization of 5-hydroxymethylfurfural (HMF) yield from sugars through the suppression of byproducts, such as formic acid and levulinic acid. Inefficiencies still exist due to the low stability of HMF at high temperature, leading to side reactions which ultimately result in the undesirable formation of humins. Valorization of this polymeric side product is thus needed to improve the economics of the biorefinery and could lead to humins being viewed as valuable materials for various applications. However, a much better understanding is needed of how humins form from HMF in the various ionic liquids proposed for the biorefinery. In this contribution, humin formation is probed by a range of analytical techniques, including FT-IR, SEM, solid-state ¹³C NMR, MS, GPC, and XPS analyses. This reveals that the structure and morphology of the humins formed does not resemble those reported in the literature and that the material displays a number of unique aspects. The hydrogen bonding proprieties of the ionic liquids employed exert a strong influence on the chemical functionality of the humins, and this is used to demonstrate their potential as functional materials. To demonstrate this, the humins produced in various ionic liquid environments are applied to metal extraction and compared with commercial activated carbon. This reveals that humins are superior for the extraction of antimony ions from wastewater, showing promise as an adsorbent additive for water purification.