Lignin depolymerisation in acidic ionic liquids

• Main Author: De Gregorio, Gilbert Francis
• Other Authors: Welton, Tom ; Hallett, Jason
• Published: Imperial College London 2016
• Subjects: 546
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.726917

In this thesis, mechanistic insights into lignin depolymerisation and more specifically, the hydrolysis of the abundant β-O-4 ether linkage was gained. This was accomplished by exploring the reactivity of different lignin model compounds in acidic ionic liquids. Reported herein are investigations into the reactivity of 2-phenoxyethanol in 1-butylimidazolium hydrogensulfate with varying amounts of sulfuric acid. Ring sulfonation was observed with the ether bond remaining intact. A similar study was then conducted using 1-ethyl-3-methylimidazolium bromide in the presence of hydrobromic acid. A Hammett plot confirmed the mechanism of ether cleavage proceeding via an A2 mechanism, yielding the phenolic product and 2-bromoethanol. The preparation of an array of lignin model compounds bearing the full β-O-4 linkage was also accomplished. Mechanistic studies indicated that the rate determining step involves substrate dehydration followed by hydrolysis in ionic liquid media. This is analogous to the mechanism reported in acidic aqueous media, however was found to proceed 25 times faster. The reactivity of guaiacolglycerol-β-guaiacol ether was compared between hydrogensulfate ionic liquids. An Eyring investigation showed a consistent mechanism in all ionic liquids with full isokinetic compensation observed between the ΔH‡ and ΔS‡ components. The differences in the absolute values of ΔH‡ and ΔS‡ were deduced to be due to the interaction of the hydrogensulfate anion with the protonated substrate, influenced primarily by the associative forces between the cation and anion, namely hydrogen bonding and sterics. A final study explored the oxidative depolymerisation of ionosolv lignin using a polyoxometalate catalyst in 1-butylimidazolium hydrogensulfate. Vanillin and syringaldehyde were identified to be the products of highest abundance and it was observed that shorter pretreatment times led to higher aldehyde yields. Preliminary studies identified that a 5 wt % POM loading with hydrogen peroxide at 100 ° C provided the optimal conditions to yield the aldehydes.