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Abstract
The increase in energy demand and depletion of fossil fuels are among major issues of modern society. Valorization and transformation of raw materials in products of industrial value represent a challenge. This justifies the growing interest of scientific research toward the identification of suitable media and methodologies able to pursue above goals, paying attention to matter of sustainability. On this subject, we studied sulfonic-acid functionalized diimidazolium salts as catalysts for the conversion of fructose and sucrose to 5-hydroxymethylfurfural (5-HMF) in an ionic liquid mixture. In general, using these salts allowed us to obtain 5-HMF in good yields from both substrates in mild conditions. Indeed, at 60°C and in the presence of 20 mol% of catalyst, 5-HMF yields of 60 and 30% were obtained from fructose and sucrose, respectively. The catalytic system was recycled and used up to six times observing no appreciable loss in yield for the first four cycles. Moreover, we gathered mechanistic information by in situ 1H NMR monitoring the dehydration of fructose. To dissect the role of acidity on the reaction, we determined the Hammett acidity function of each salt. Comparison of these results with yields and reactivity observed in the presence of related monocationic salts and with a dicationic salt bearing only one sulfonic acid group, allowed stating that the reactivity observed is the result of the combined action of acidity and structural features of the catalysts. Overall, the approach proposed here could contribute to pave the way to increase sustainability in the raw material valorization processes.
Highlights
The steady increase in energy demand in modern society has led researchers to tackle pressing challenges like global warming and the depletion of petroleum feedstocks together with the urgent need to reduce the generation of waste from industrial processes
5hydroxymethylfurfural (5-HMF) and its derivatives are among the most important chemical platforms obtained from the conversion of the carbohydrate rich fraction of biomasses (Rout et al, 2016). 5-HMF is a key intermediate to the production of solvents, biofuels and polymers (Zhang and Dumont, 2017) and is in principle obtained from the acid-catalyzed dehydration of fructose
The neutral diimidazole precursor (4-6) was reacted with excess of 1,4-butanesultone (2) to obtain the relevant sulfonate appended diimidazolium zwitterion, which in turn was treated with the stoichiometric amount of hydrochloric acid to yield the relevant sulfonic acid functionalized diimidazolium salt
Summary
The steady increase in energy demand in modern society has led researchers to tackle pressing challenges like global warming and the depletion of petroleum feedstocks together with the urgent need to reduce the generation of waste from industrial processes. Efficient conversion of biomass gives access to a wide range of chemical platforms, i.e., key intermediates for the synthesis of industrially relevant products. These include ethanol, succinic and 3hydroxypropionic acids, levulinic acid, isoprene and so on (Chinnappan et al, 2016; Mika et al, 2018). In this context, 5hydroxymethylfurfural (5-HMF) and its derivatives are among the most important chemical platforms obtained from the conversion of the carbohydrate rich fraction of biomasses (Rout et al, 2016). The conventional methods for the production of 5-HMF require harsh reaction conditions with high temperatures and strong mineral acids like H2SO4 or metal based Lewis acids like CrCl3 (Chheda et al, 2007)
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