Identification of Crucial Intermediates in the Formation of Humins from Cellulose‐Derived Platform Chemicals Under Brønsted Acid Catalyzed Reaction Conditions

Abstract

Humins are one of the undesirable products formed during the dehydration of sugars as well as the conversion of 5‐hydroxymethylfurfural (HMF) to value‐added products. Thus, reducing the formation of humins is an important strategy for improving the yield of the aforementioned reactions. Even after a plethora of studies, the mechanism of formation and the structure of humins are still elusive. In this regard, we have employed density functional theory‐based mechanistic studies and microkinetic analysis to identify crucial intermediates formed from glucose, fructose, and HMF that can initiate the polymerization reactions resulting in humins under Brønsted acid‐catalyzed reaction conditions. This study brings light into crucial elementary reaction steps that can be targeted for controlling humins formation. Moreover, this work provides a rationale for the experimentally observed aliphatic chains and HMF condensation products in the humins structure. Different possible polymerization routes that could contribute to the structure of humins are also suggested based on the results. Importantly, the findings of this work indicate that increasing the rate of isomerization of glucose to fructose and reducing the rate of reaction between HMF molecules could be an efficient strategy for reducing humins formation.

Investigation of the Formation, Characterization, and Oxidative Catalytic Valorization of Humins

“…This confirmed van Zandvoort’s results [ 8 ]. The different conversions of glucose and fructose can be explained by the fact that fructose directly forms HMF and DHH, since it possesses a more stable ring structure, while glucose must first isomerize to fructose before reacting further to HMF and DHH [ 5 , 7 , 43 , 44 ]. Accordingly, sucrose also has a lower conversion, being a dimer of glucose and fructose, which must first be hydrolyzed into the monomeric sugars during the conversion process, with subsequent isomerization of glucose to fructose and conversion of the latter.…”

Tailored sulfonated carbons: unraveling enhanced catalytic dynamics for fructose dehydration under conventional and microwave heating

“…12 Humin polymers are formed during the acid-catalyzed conversion of fructose to 5-hydroxymethyl furfural (5-HMF). [70][71][72] These humins can be impregnated in sulfonated carbon porous materials. 73 The formation of humins involves the condensation of key intermediate molecules such as fructose, 5-HMF, and other derived species.…”

Molecular Views on Mechanisms of Brønsted Acid-Catalyzed Reactions in Zeolites

“…This intermediate can undergo the first dehydration, yielding another key intermediate which is a dione B called 3-deoxyglucosone. This intermediate can then undergo a second dehydration on this open form, which can lead, depending on the regioselectivity, to either an intermediate C that cyclizes ( D ) and finally undergoes the third dehydration to 5-HMF, or to another intermediate E called 2,5-dioxo-6-hydroxyhexanal (DHH) that is suspected to be a precursor of humins in a number of studies. , Alternatively, intermediate B can cyclize and undergo two successive dehydrations (intermediates F and Z ) further to 5-HMF.…”