Separation of sugar cane bagasse monosaccharide components in an ionic liquid medium
Creators: name: BANDARA, Hetti
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400409
subject: Separation technologies

description: Sugar cane bagasse (SCB), a byproduct of sugar production, is an environmentally sustainable source of lignocellulosic biomass that can be used in the production of biofuels such as ethanol. The major steps to producing ethanol from biomass are pretreatment, hydrolysis of hemicellulose/cellulose to monosaccharides and fermentation of the monosaccharides to ethanol. Lignocellulosic biomass are resistant to hydrolysis and ionic liquid (IL) pretreatment of biomass has gained popularity because it can dissolve both hemicellulose and cellulose, which makes them more easily hydrolysed. The resulting monosaccharides need to be separated from the IL and other constituents, such as lignin, which may inhibit fermentation.

Sugars can be selectively transported across a supported liquid membrane (SLM) to an aqueous receiving phase to yield high purity sugar solutions amenable to fermentation. The transportation of sugars in an IL medium via a SLM has not been reported. The aim of this study is to understand the mechanism of transport via a SLM of sugars in an IL medium and how it is affected by process variables e.g. pH and extractant composition. Experiments were designed to selectively transport glucose and xylose in a synthetic aqueous IL solution (the departure phase) via a SLM. The IL used was 1-ethyl-3-methylimidazolium acetate ([EMIM]Ac). The monosaccharides were transported to an aqueous neutral receiving phase. The membrane separating the departure and receiving phases was impregnated with a boronic acid (phenylboronic acid -PBA) and/or a quaternary amine (Aliquat 336) extractant(s) and a diluent (2-nitrophenyl octyl ether).

The transport flux was found to be higher when the IL medium in the departure phase was alkaline. Xylose was transported at greater rates and has a greater extent of extraction than glucose from equi-molar solutions in the departure phase. The transport rate of xylose was approximately 2.5 times that of glucose. However, a PBA/Aliquat 336 combination decreased extraction when compared to extraction using Aliquat 336 alone. This inhibitory effect was unexpected because the extraction and transport of monosaccharides from aqueous solutions is enhanced when PBA is added to Aliquat 336. When Aliquat 336 was used alone in the membrane phase, increasing its concentration linearly increased the extraction of both sugars and their transport rates. The effective diffusivity, of the glucose/Aliquat 336 complex and the xylose/Aliquat 336 complex in the organic membrane were determined to be 2.41 x 10-8 m2, h-1and 2.97 x 10-8 m2.h-1 respectively. The transport of the solute species was significantly lower than that expected in the absence of a membrane showing that the SLM used in the experiments of this study was stable.

The transport of undesired lignin, lignin degradation products and IL across the SLM was measured for a model hydrolysate solution in which the departure phase contained soluble lignin as well as the monosaccharides in an IL/aqueous solution. Nuclear Magnetic Resonance (NMR) showed that lignin or lignin degradation products were not transported across the membrane to the receiving phase at detectable levels. Both ions of the IL were
transported at greater rates for the PBA/Aliquat 336 extractant combination compared to Aliquat 336 alone. Thus, using only Aliquat 336 increases the potential for recycling the IL present in the departure phase. Reduced transport rates of acetate to the receiving phase were observed in the presence of lignin in the departure phase. Approximately, 0.20% of IL cation from the departure phase was found to be transported to the receiving phase. However, in the absence of lignin, IL cation derivatives in the receiving phases were detected, showing that the IL had degraded into a series of closely related structural analogues.

Hydrolysis trials of SCB immersed in [EMIM]Ac were conducted using an acid catalyst (HCl) to complement the work on separation of monosaccharides from an IL medium. [EMIM]Ac was seen to inhibit acid hydrolysis of SCB. It was found that this was caused by the acetate ion in [EMIM]Ac. To overcome the inhibitory effect of the acetate ion, trials were conducted with similar ILs where the chloride ion was substituted for the acetate ion. The hydrolysis of SCB was conducted using HCl as a catalyst in the IL 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) or 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl) at temperatures between 100˚C and 140˚C over 5 hours. In all experiments, the yield of sugars increased over time to a maximum and then declined as the sugars were degraded. The maximum yield was affected by the temperature, the acid concentration and
the IL used.

In conclusion, it was found that it is feasible to selectively transport sugars from an IL medium via a SLM, thus potentially producing purified sugar solutions enabling both IL recovery and subsequent fermentation to produce ethanol. Conditions that influence the transport rate (via a SLM) and the extent of extraction of monosaccharides from an IL medium were identified.