New insights into the catalytic cleavage of the lignin β-O-4 linkage in multifunctional ionic liquid media

Abstract

a Ionic liquids are attractive reaction media for the solubilisation and depolymerisation of lignin into valueadded products. However, mechanistic insight related to the cleavage of specific linkages relevant to efficient lignin depolymerisation in such solvents is still lacking. This study presents important insight into the scission of the most abundant lignin β-O-4 motif in Brønsted acidic ionic liquids. Using relevant model compounds, cleavage products were identified and undesired side reactions examined carefully. Stabilization of reactive intermediates was achieved in ionic liquids comprising both Brønsted acidic function and stabilized nanoparticles that comprise hydrogenation activity in order to suppress undesired side reactions. Especially, the in situ hydrogenation of the aldehyde intermediate originating from the acid-catalysed cleavage of lignin beta-O-4 model compounds into more stable alcohols was investigated. This is the first time that such products have been systematically targeted in these multifunctional reaction media in relation to lignin depolymerization.

An Introduction to Model Compounds of Lignin Linking Motifs; Synthesis and Selection Considerations for Reactivity Studies

“…An example of ar elatively easy to access model compound is the b-O-4 Type Am odel 82.I ns uch studies, the choice for ar elevant aromatic substitution pattern is typically also considered less critical as the focus is primarily on activation of the b-O-4 CÀO bond. [216][217][218][219][220][221] Alternatively,ab-O-4 Type Bm odel such as 83 can be used if more information on the reaction product is desired [42,[222][223][224][225][226][227][228][229][230] or if the breaking of the b-O-4 CÀOb ond is envisioned to be facilitated by the oxidation of the benzylic alcohol in the a-position. [17,[231][232][233][234][235][236] For the breaking of CÀOb onds in general,s ometimes even more simple commerciallya vailable modelc ompounds such as benzyl phenylether are used for initial catalystdevelopment.…”

Nucleophilic Cleavage of Lignin Model Compounds under Acidic Conditions in an Ionic Liquid: A Mechanistic Study

“…Finally, cleavage at the phenyl ether was observed; the complete reaction pathway is shown in Scheme for 3 . This is an interesting result because previous studies of acid‐catalysed hydrolysis in a range of different solvents with varied temperature, acids (Lewis and Brønsted acids) and reaction time, demonstrated cleavage of the β‐O4 ether but not cleavage of the methyl ether.…”

Tandem Acid/Pd‐Catalyzed Reductive Rearrangement of Glycol Derivatives

“…[2] As imilarm echanism was found with Pd/C as catalyst [3] andu nder photocatalytic conditions. [4] Another redox-neutral transformation provides phenylacetaldehyde under catalysis by aB rønsted acid [5] or methyltrioxorhenium (MTO) [6] in ionic liquids. For both catalysts, a mechanism via an enol ether was proposed.A na lternative pathway via as emi-pinacol rearrangementw as found for the organocatalytic reduction with Et 3 SiH.…”