The Catalytic Conversion of Glucose to 5-Hydroxymethylfurfural in Nanostructured Ionic Liquids and Ionic Liquid Mixtures

Abstract

As we aim to reduce our worldwide carbon footprint associated with chemical and energy usage derived from fossil-based sources, bio-based chemicals and biofuel production becomes increasingly important. Lignocellulosic biomass sources offer an alternative to the likes of starch and sugar-based feedstocks due to their abundance and ability to be used without competition from food. One of the key proposed transformations to achieve bio-based chemicals and fuels from lignocellulosic biomass involves the isomerisation of glucose and the dehydration of fructose into 5-hydroxymethylfurfural (HMF). Dehydration reactions are important fundamental reactions. They are primarily used in bio-based chemical and biofuel production as means to reduce the oxygen content of the biomass feedstocks to convert them into useful alternatives for various applications. Though, they are currently limited by their efficiency, selectivity, the use of harsh reaction conditions, their environmental impact, and the generation of waste products. Ionic liquids (ILs), low melting salts, are being increasingly explored for some of these biomass conversions, and in many cases offer less harsh and more selective routes for these transformations. The ILs explored for these transformations tend to be limited to those with short alkyl chains and strong hydrogen bond accepting anions due to the high solubility of biopolymers such as cellulose in these ILs. This has meant that little is known about the effect of some key structural features of ILs on the conversion of glucose into HMF. This includes the ability of ILs with sufficiently long alkyl chains to form amphiphilic nanostructures with well-defined polar and non-polar domains, or the impact of using less strongly hydrogen bond accepting anions. The aim of this project was to explore and understand the effect that both amphiphilic nanostructure and anion selection in ILs had on the conversion of glucose to afford the desired biofuel and bio-based chemical intermediate HMF. The purpose was to determine how these features affected the overall reaction rate, HMF yields, and the rate of glucose isomerisation, as this is known to be the rate limiting step. A known issue for the formation of HMF is the poor stability of HMF under the reaction conditions. Initial studies focused on the stability of HMF within a series of ILs to determine their suitability for the conversion of glucose to HMF. The conversion of glucose was initially explored using HCl as a catalyst. It was found that HCl did not sufficiently catalyse the rate limiting glucose isomerisation step resulting in low yields. CrCl3 was then explored as an alternative catalyst, with further HMF stability studies completed. The conversion of glucose into HMF using CrCl3 was explored for a series of 1-alkyl-3-methylimidazolium ([CnC1im]) ILs. This included the simple [CnC1im]Cl, and [CnC1im]Br ILs (n = 4, 10), with the addition of [CnC1im][OTf] to create the mixtures [CnC1im]Brx[OTf]1-x and [CnC1im]Clx[OTf]1-x. This was done to determine the effect that the [OTf]− anion has on glucose isomerisation or HMF yield given it has been linked to increased rates of other alcohol dehydration reactions. The effect of [OTf]− on the conversion of glucose was not uniform throughout the mixtures. Within [C4C1im]Br the addition of [OTf]− at a ratio of 0.67:0.33 Br:[OTf] leads to an apparent increase in HMF yields. Within [C4C1im]Cl the addition of the minimal amount of [OTf]− leads to a faster rate of reaction, but also appears to reduce the stability of HMF. Within [C10C1im]Br the addition of [OTf]− has no significant effect. Finally, within [C10C1im]Cl the addition of [OTf]− appears to eliminate an initiation period for HMF production seen in the simple [C10C1im]Cl IL and leads to higher yields. The effect of the alkyl chain length also varied depending on the nature of the anion. For both the simple Br− and Cl− ILs the increased alkyl chain length appeared to have a detrimental effect on the HMF yield although the relationship between alkyl chain length and the behaviour of the mixtures was more complex. The results of this project demonstrate that anion selection and the existence of amphiphilic nanostructures have differing effects depending on the catalyst selection and the interplay between other IL components. There is evidence to suggest that the isomerisation of glucose to fructose can be affected independently of the conversion of fructose to HMF which is an effect that might be able to be exploited to achieve previously unattainable yields of HMF.