Comparison of Structural Features of Humins Formed Catalytically from Glucose, Fructose, and 5-Hydroxymethylfurfuraldehyde

Abstract

The IR spectra of humins formed during the acid-catalyzed conversion of glucose, fructose, and 5hydroxymethylfurfuraldehyde were compared. The spectra are quite similar except for three groups of features that can be attributed to furan rings and carbonyl groups conjugated with carbon−carbon double bonds. IR spectroscopy further revealed that benzyl groups could be added to the humins as they formed or in a separate aldol addition/condensation reaction after they had been recovered. The IR spectra are consistent with a model where each of the three reactants must first be converted to 2,5dioxo-6-hydroxyhexanal (DHH) before humins can form via subsequent aldol addition and condensation. The differences in the IR spectral features can then be explained by variations in the concentrations of other aldehydes and ketones that can react with DHH.

Kinetic Study of Acid Hydrolysis of Corncobs to Levulinic Acid

“…Both the model in this study and the model proposed by Girisuta and co-workers (2013) could fit the experimental data very well by using the modified parameter fitting method. However, according to the studies of Patil et al (2012) and Van et al (2013) about humins, the model in the present study should be more reasonable.…”

“…The maximum yield of LA was obtained at 150 °C and 0.8 mol/L H2SO4 at 58.0 mol%. Based on the study of Patil (2012) and Van (2013), the mechanism of the formation of humins, the authors proposed the kinetic model that contained four consecutive irreversible first-order reactions under the assumption that humins could be only converted by 5-HMF. A very good fit was observed between the experimental data and the kinetic model.…”

“…Thus it is worthy of further study. Patil et al (2012) put forward a pathway for humins formation from cellulose. They proposed that humins were mainly derived from the aldol addition/condensation of 2,5-dioxo-6-hydroxyhexanal, which was converted from 5-HMF with aldehydes and ketones.…”

“…Based on the studies of Patil et al (2012) and Van et al (2013), the authors researched the simplified kinetic model of the hydrolysis reaction and made the following assumptions: (1) the model consisted of four consecutive reactions, all of which were defined as first-order reactions; (2) cellulose was hydrolyzed to glucose, and then glucose was decomposed to form 5-HMF; once 5-HMF was generated, it quickly converted into the final product levulinic acid and by-product humins; (3) humins were only converted by 5-HMF, and cellulose and glucose were not directly involved in the conversion of humins; (4) all of the unknown products were humins. The humins were generated from 5-HMF and it would be further polymerized to form macromolecular humins.…”

Conversion of Fructose into Levulinic Acid by Catalysis of Ionic Liquid

“…For hydrothermal treatment of fructose in the absence of any additional catalysts at 120-140 ℃, Yao et al [5] proposed that fructose first undergoes intramolecular dehydration to HMF, and HMF further polymerizes to form carbon spheres, which contain a dense hydrophobic carbon core and a hydrophilic shell according to field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) images. For H 2 SO 4 -catalyzed fructose conversion at 135 ℃, Patil et al [6] suggested that fructose must first be converted to 2,5-dioxo-6-hydroxyhexanal (DHH), which subsequently undergoes aldol addition and condensation to form humins. For hexose derived humins catalyzed by H 2 SO 4 at higher temperature of 180 ℃, van Zandvoort et al [7] inferred a polyfuranic molecular structure, wherein the inclusion of DHH and limited LA occurs through aldol condensations with the aldehyde groups of HMF or DHH itself.…”