Tim Ståhlberg

Synthesis of 5-(Hydroxymethyl)furfural in Ionic Liquids: Paving the Way to Renewable Chemicals

The synthesis of 5-(hydroxymethyl)furfural (HMF) in ionic liquids is a field that has grown rapidly in recent years. Unique dissolving properties for crude biomass in combination with a high selectivity for HMF formation from hexose sugars make ionic liquids attractive reaction media for the production of chemicals from renewable resources. A wide range of new catalytic systems that are unique for the transformation of glucose and fructose to HMF in ionic liquids has been found. However, literature examples of scale-up and process development are still scarce, and future research needs to complement the new chemistry with studies on larger scales in order to find economically and environmentally feasible processes for HMF production in ionic liquids. This Minireview surveys important progress made in catalyst development for the synthesis of HMF in ionic liquids, and proposes future research directions in process technology.

Metal-Free Dehydration of Glucose to 5-(Hydroxymethyl)furfural in Ionic Liquids with Boric Acid as a Promoter

The dehydration of glucose and other hexose carbohydrates to 5-(hydroxymethyl)furfural (HMF) was investigated in imidazolium-based ionic liquids with boric acid as a promoter. A yield of up to 42% from glucose and as much as 66% from sucrose was obtained. The yield of HMF decreased as the concentration of boric acid exceeded one equivalent, most likely as a consequence of stronger fructose-borate chelate complexes being formed. Computational modeling with DFT calculations confirmed that the formation of 1:1 glucose-borate complexes facilitated the conversion pathway from glucose to fructose. Deuterium-labeling studies elucidated that the isomerization proceeded via an ene-diol mechanism, which is different to that of the enzyme-catalyzed isomerization of glucose to fructose. The introduced non-metal system containing boric acid provides a new direction in the search for catalyst systems allowing efficient HMF formation from biorenewable sources.

Enzymatic isomerization of glucose and xylose in ionic liquids

Glucose isomerase has been found for the first time to catalyze the isomerization of glucose to fructose in the ionic liquid N,N-dibutylethanolammonium octanoate (DBAO). Isomerization was achieved at temperatures of 60–80 °C although a substantial amount of mannose was formed at elevated temperatures via the Lobry-de Bruyn–van Ekenstein transformation. Complete recovery of the sugars after reaction was achieved by extraction with aqueous HCl, thus making the protocol attractive for continuous operation.

Dependency of the hydrogen bonding capacity of the solvent anion on the thermal stability of feruloyl esterases in ionic liquid systems

Three feruloyl esterases, EC 3.1.1.73, (FAEs), namely FAE A from Aspergillus niger (AnFaeA), FAE C from Aspergillus nidulans (AndFaeC), and the FAE activity in a commercial β-glucanase mixture from Humicola insolens (Ultraflo L) were tested for their ability to catalyse esterification of sinapic acid with glycerol in four ionic liquid (IL) systems. The IL systems were systematically composed of two selected pairs of cations and anions, respectively: [BMIm][PF6], [C2OHMIm][PF6], [BMIm][BF4], and [C2OHMIm][BF4]. AnFaeA had activity in [PF6]−-based ILs, whereas the AndFaeC and the FAE in Ultraflo L had no appreciable activities and were generally unstable in the IL systems. FAE stability in the IL systems was apparently highly dependent on enzyme structure, and notably AnFaeAs similarity to IL-compatible lipases may explain its stability. The thermal stability of AnFaeA was higher in buffer than in the IL systems, but at 40 °C and below there was no significant difference in AnFaeA stability between the buffer and the [PF6]−-based systems: AnFaeA was stable in the [BMIm][PF6] and [C2OHMIm][PF6] systems for 2 h at 40 °C. However, the IL anion had a major effect on stability: [BF4]− caused rapid inactivation of AnFaeA, while [PF6]− did not. The cation did not have a similar effect. These observations could be explained in terms of the hydrogen bonding capacity of IL cations and anions via COSMO-RS simulations.