Computational survey of humin formation from 5-(hydroxymethyl)furfural under basic conditions

Area of Science:

• Chemical Engineering
• Physical Chemistry
• Organic Chemistry

Background:

• Humin formation is a significant byproduct during the oxidation of 5-(hydroxymethyl)furfural (HMF) to furan-2,5-dicarboxylic acid (FDCA).
• Understanding humin formation mechanisms is crucial for optimizing FDCA production and minimizing waste.
• Previous studies suggested humin formation occurs even with high-purity HMF under storage conditions.

Purpose of the Study:

• To elucidate the reaction pathways responsible for humin formation during HMF oxidation.
• To investigate the role of impurities and reaction conditions in the oligomerization of HMF.
• To computationally validate methods for suppressing humin formation.

Main Methods:

• Utilized quantum chemical calculations and the multi-component artificial-force-induced reaction (MC-AFIR) method.
• Performed comprehensive reaction-path searches for HMF oligomerization under various conditions (HMF + H2O, HMF + OH-, HMF + O2).
• Investigated the effect of acetal protection on HMF reactivity.

Main Results:

• No direct HMF dimerization pathway with a barrier below 185 kJ mol⁻¹ was identified, suggesting impurities initiate humin formation.
• Identified three reaction paths with barriers <65 kJ mol⁻¹ for HMF reacting with hydroxide ions (OH⁻).
• Computationally confirmed that acetal protection of HMF effectively suppresses humin formation.

Conclusions:

• Humin formation in HMF oxidation is primarily driven by reactions with impurities, particularly hydroxide ions, rather than HMF self-condensation.
• The reaction of HMF with hydroxide ions presents low-energy pathways for oligomerization.
• Acetal protection offers a viable strategy to mitigate humin byproduct formation during FDCA synthesis.