A Metal‐Free, Carbon‐Based Catalytic System for the Oxidation of Lignin Model Compounds and Lignin

Abstract

Nitrogen‐containing graphene material (LCN) has been identified as an effective catalyst for the oxidation of β‐O‐4 and α‐O‐4 types of lignin model compounds in the presence of tert‐butyl hydroperoxide, to provide aromatic aldehydes, acids and other organic chemicals in high yield. The transformations of five lignin model compounds over LCN were investigated systematically. Instrumentation analysis, kinetic study and radical trapping experiments highlight the mechanistic features of the reaction, including: 1) the reaction pathway starts by benzylic CH or COH bond activation, followed by CαCβ or CαO bond cleavage, and finally further oxidation of intermediate aromatics; and 2) the reaction follows a free‐radical mechanism with all the key steps involving radical species. In addition, the LCN proved to be a highly stable catalyst; no significant activity decrease was observed for four consecutive runs, and X‐ray photoelectron spectroscopy analysis indicates negligible decrease in the content of the active nitrogen species in the catalyst. Notably, this new catalytic system can be extended to the oxidative depolymerisation of real lignin, to produce a significant portion of liquefied, low‐molecular‐mass products.

Base promoted hydrogenolysis of lignin model compounds and organosolv lignin over metal catalysts in water

“…New methods that can process lignin into value-added aromatic chemicals are highly desirable. (Sales et al, 2007;Marshall and Alaimo, 2010;Zakzeski et al, 2010;Murat Sen et al, 2012; Ben et al, 2013;Gao et al, 2014) Lignin consists of interlinked arylpropane units with various functional groups like ethers, methoxy and hydroxyl groups, as well as C-C linkages. The most abundant lignin linkage is the β-O-4 linkage with 45-50% occurrence in softwood and even more than 60% in hardwood.…”

Transient absorption spectroscopy and photochemical reactivity of CAU-8

“…One possibility for this transformation consists in the oxidative depolymerization (OD) of lignin using oxygen to assist the rupture of the ether bonds. [2,3] Although organic peroxides have been used for lignin OD, [4] oxygen is the preferable oxidizing chemical from the point of view of sustainability and affordability. [5] Typical homogeneous catalysts reported for lignin OD are transition metals ions, such as vanadium, [6,7] rhenium [8] and cobalt .…”

“…[9] In this context, one current line of research in catalysis is the evaluation of metal-free catalysts and OD of lignin can offer an opportunity to develop this type of catalytic process. [3][4][5] Graphene oxide (GO) and other doped graphenes (G) have been reported to act as metal-free catalysts for the aerobic oxidation of benzylic hydrocarbons, [10] benzylic alcohols [11] and cycloalkanes. [12] Therefore, it would be interest to check if carbocatalysts based on G can also promote OD of lignin.…”

“…In a recent precedent, it has been found that N-containing G [(N)G] is able to catalyze the oxidation of simple lignin models using tert-butyl hydroperoxide (TBPH) as terminal oxidant. [4] Continuing with this line of research it would be of interest to determine if G-based carbocatalysts can also promote the cleavage of ether bonds in lignin models using molecular oxygen as the only oxidation reagent. For the present study -guaiacylglycerol-β-guaiacyl ether (1), (see structure in Scheme 1) was selected as lignin model to evaluate the potential of some Gs as catalysts for lignin OD by cleavage of the guaiacyl ether bond.…”

NiAg Catalysts for Selective Hydrogenolysis of the Lignin C-O Bond

“…[29][30][31][32] The catalytic cleavage of β-O-4 linkage has been studied in oxidative, [33][34][35][36][37] reductive, [38,39] and redox-neutral [40][41][42] manners. C O bond hydrogenolysis, which transforms lignin into a library of phenolic platform chemicals in the presence of hydrogen, is attracting increasing interests.…”