Local Overheating Explains the Rate Enhancement of Xylose Dehydration under Microwave Heating

Abstract

The NH4Cl-assisted dehydration of xylose to furfural was studied using traditional and microwave heating. Significant differences in rate, pH profiles, and selectivity profiles were observed between both heating systems. A comparative kinetic analysis showed 7–13 times higher first-order rate constants for the xylose dehydration reaction under microwave heating. Dedicated experiments with varying irradiation power and liquid mixing intensity suggested that the differences are due to the development of overheated areas under microwave heating. This hypothesis was supported by modeling the rate enhancement observed under microwave heating using a simple kinetic model that assumes a minor overheated fraction amid a bulk liquid at target temperature.

Selectivity switch by phase switch – the key to a high-yield furfural process

“…20 mol% higher than traditional industrial approaches, thus providing less waste. 1,5 A preliminary economic analysis, which is detailed in the SI and follows the approach presented in the literature, 33 suggests that the increase from 70 to 95 mol% selectivity in furfural provides sufficient economic room to pay for the increased complexity of the process proposed here. Based on premises detailed in the SI, particularly biomass and furfural market prices of $80 (± 20) and $1400 (±200) /t, the selectivity increase more than doubles the room for conversion cost -i.e.…”

“…[1][2][3][4] The xylose-tofurfural selectivity is, however, severely limited by sidereactions, in particular the formation of solid humic byproducts, observed consistently over several reaction conditions. [1][2][3][4][5][6] To address these problems and enhance the production of furfural, various strategies have introduced an organic solvent in the process. [6][7][8][9][10][11][12][13][14] A common strategy to improve the efficiency of xylose dehydration is to perform the reaction in a biphasic system, [6][7][8][9][10][11][12] with the aim to extract furfural as soon as it is formed and thereby protect it from degradation.…”

Production of furans from C5 and C6 sugars in the presence of polar organic solvents

“…33,34 Non-traditional heating methods, such as microwave heating, have also been used for the conversion of sugars to furans, both in heterogeneous and homogeneous catalysis. 1,2,8,35,[44][45][46][47][48][49] The presence of specific, non-thermal microwave effects has been excluded, and the rate enhancement observed at microwave heating conditions has been attributed to local overheating. 48,50…”

“…1,2,8,35,[44][45][46][47][48][49] The presence of specific, non-thermal microwave effects has been excluded, and the rate enhancement observed at microwave heating conditions has been attributed to local overheating. 48,50…”

“…25,60 Conversely, humic by-products obtained from the same sugar feed, but from reactions performed in different solvent systems (e.g., aqueous monophasic vs aqueous-organic biphasic) do not show differences in structure and composition. 48,49 When performing the acid-catalyzed dehydration of sugars in the presence of water, rehydration of the newly formed carboncarbon double bonds leads to the formation of low-molecular weight by-products (e.g., carboxylic acids and lactones). 55,56 Nimlos et al propose, based on quantum-mechanical modelling, the significance of the hydroxyl group of xylose, which is protonated first, in steering the selectivity.…”