Metal Triflates for the Production of Aromatics from Lignin

Abstract

Abstract:The depolymerization of lignin into valuable aromatic chemicals is one of the key goals towards establishing economically viable biorefineries. In this contribution we present a simple approach for converting lignin to aromatic monomers in high yields, under mild reaction conditions. The methodology relies on the use of catalytic amounts of easy to handle metal triflates. Initially, we evaluated the reactivity of a broad range of metal triflates using simple lignin model compounds. More advanced lignin model compounds were also used to study the reactivity of different lignin linkages. The product aromatic monomers were either phenolic C2 acetals obtained by stabilization of the aldehyde cleavage products by reaction with ethylene glycol, or methyl aromatics obtained by catalytic decarbonylation. Notably, when the former method was ultimately tested on lignin, especially Fe(OTf)3 proved very effective and the phenolic C2 acetal products were obtained in an excellent, 19.3 ±3.2 Wt% yield.

Preparation and Reactivity of Biomass‐Derived Dihydro‐Dioxins

“…The reactions of 5 a and its acetylated derivative 5 b with EG and Fe(OTf) 3 at 140 °C for 15 minutes led to a significant difference in the C 3 /C 2 product ratios (Table , entries 1 and 2). In accordance with prior reports, 5 a was fully converted into the C 2 acetal 6 (Table , entry 1, and Figure S1) with trace amounts of C 3 dioxene 7 also formed. In contrast, 5 b was fully converted to 6 and 7 in a C 3 7 /C 2 6 ratio of 1:1 (Table , entry 2).…”

“…[b] Reaction conditions based on ref. : Fe(OTf) 3 (3 mol %), EG (2.7 equiv), 140 °C, 1,4‐dioxane, 15 min. [c] Repeated and products isolated to give a C 3 7 /C 2 6 ratio of 1.4.…”

“…Given the relative instability of 3 , the formation of degradation products of 3 in the mixture of lignin‐derived monomers (Figure S14), and the number of reactive groups (e.g., alcohols) present in lignin that 3 or degradation products of 3 could react with, it seems reasonable to assume that degradation of 3 has an impact on the observed yields from lignin. Rather than fight against the instability of 3 , we decided to embrace its reactivity as a means of preparing more stable products –…”

State-of-the-art catalytic hydrogenolysis of lignin for the production of aromatic chemicals

“…The reactivity of a broad range of metal triflates in catalytic reactions with ethylene glycol was evaluated together with catalytic decarbonylation of different lignin-related model compounds. 146 Product distribution from the model compound depolymerization was significantly related to the type of metal triflate used. As for lignin, Fe(OTf) 3 was proved to be very active and the phenolic C2acetal products were obtained in a high yield as 19.3±3.2 wt%.…”

Selective catalytic transformation of lignin with guaiacol as the only liquid product

“…As we know, ether linkages can be efficiently cleaved with Lewis acid catalysts. 12,[32][33][34][35][36][37] However, guaiacol could not be effectively obtained from lignin using weak Lewis acid catalysts, such as La(NO 3 ) 3 , Al(OTf) 3 and LaCl 3 ( Table 1, entries 2-4). Unlike the above acids, the triate salts with stronger Lewis acidity could effectively catalyze the production of guaiacol from lignin with various yields (Table 1, entries 5-8 and Fig.…”