Abstract
In this study, the effect of microwaves on the production of 5-hydroxymethylfurfural (HMF) in a biphasic system was evaluated via a kinetic analysis. The reaction system consisted of an acidified aqueous phase and methyl isobutyl ketone (MIBK) as an organic phase, in which HMF is extracted directly upon formation during the reaction. Two identically shaped reactors were used to assess the influence of microwaves on the production of HMF. A borosilicate glass reactor was used to heat the reaction mixture via microwaves directly, whereas the silicon carbide (SiC) wall of the second reactor absorbed all microwaves and hence the reactor content was heated via convective heat transfer. An identical temperature profile was imposed on both reactors. Cellulose, glucose and fructose were chosen as feedstocks for the conversion to HMF. It was observed that microwaves have a significant effect on the reactions. The hydrolysis of cellulose to glucose was a 2.3 folds faster in the presence of microwaves at the process conditions (0.046 M HCl, 177 °C). The isomerization of glucose to fructose showed a similar increase (factor 2.5). The required energy input for the reaction was systematically higher for the SiC reactor.
Highlights
5-hydroxymethylfurfural (HMF) is a key bio-based platform chemical for the production of a broad spectrum of fine chemicals
The borosilicate glass reactor allows microwave irradiation to penetrate through the reactor wall into the reaction mixture, the heating is caused by dipole rotation and ionic conduction
The results showed that both heating profiles were almost identical and there was no temperature gradient between the reaction mixture and the surface of the reaction vessel (IR sensor)
Summary
5-hydroxymethylfurfural (HMF) is a key bio-based platform chemical for the production of a broad spectrum of fine chemicals. It is more difficult to convert cellulose to HMF, mainly because of two reasons: (i) the hydrolysis of the polymeric chain is required, which takes longer reaction times under harsh reaction conditions, (ii) the hydrolysis step requires the presence of water but this induces the unwanted ring opening and further conversion of HMF to levulinic acid. These unwanted side reactions can be suppressed with the use of the water immiscible organic solvent methyl isobutyl ketone (MIBK) in the reaction mixture to create a biphasic reaction system. These mechanisms are called microwave-specific mechanisms and include the following phenomena: (i) the superheating effect of solvents, (ii) the formation of so-called molecular radiators, (iii) selective heating by using a strongly absorbing microwave catalyst or reagent in a less polar reaction medium and (iv) elimination of wall effects found in convective heating[14,16,17,19]
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