Advances in humins formation mechanism, inhibition strategies, and value-added applications

Abstract

Humins, as a group of by-products formed through the condensation and coupling of fragment intermediates during lignocellulosic biomass refining, can cause numerous negative effects such as the wastage of carbon resources, clogging of reactor piping, deactivation of catalyst, and barriers to product separation. Elucidating the generation mechanism of humins, developing efficient inhibitors, and even utilizing them as a resource, both from the perspective of atom economy and safe production, constitutes a research endeavor replete with challenges and opportunities. Orbiting the critical issue of humins structure and its generation mechanism from cellulose and hemicellulose resources, the random condensation between intermediates such as 5-hydroxymethylfurfural, furfural, 2,5-dioxo-6-hydroxyhexanal, and 1,2,4-benzenetriol etc. were systematically summarized. Additionally, the presence of lignin in real biorefining processes further promotes the formation of a special type of humins known as “pseudo-lignin”. The influences of various factors, including raw materials, reaction temperature and time, acid-base environment, as well as solvent systems and catalysts, on the formation of humins were comprehensively analyzed. To minimize the formation of humins, the design of efficient solvent systems and catalysts is crucial. Furthermore, this review investigates the approaches to value-added applications of humins. The corresponding summary could provide guidance for the development of the humins chemistry.