Selective hydrogenolysis of catechyl lignin into propenylcatechol over an atomically dispersed ruthenium catalyst

Shuizhong Wang,Guoyong Song,Helong Li,Ling-Ping Xiao,Kaili Zhang

doi:10.1038/s41467-020-20684-1

Shuizhong Wang, Guoyong Song + Show 3 more

Open Access

https://doi.org/10.1038/s41467-020-20684-1

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Abstract

C-lignin is a homo-biopolymer, being made up of caffeyl alcohol exclusively. There is significant interest in developing efficient and selective catalyst for depolymerization of C-lignin, as it represents an ideal feedstock for producing catechol derivatives. Here we report an atomically dispersed Ru catalyst, which can serve as an efficient catalyst for the hydrogenolysis of C-lignin via the cleavage of C−O bonds in benzodioxane linkages, giving catechols in high yields with TONs up to 345. A unique selectivity to propenylcatechol (77%) is obtained, which is otherwise hard to achieve, because this catalyst is capable of hydrogenolysis rather than hydrogenation. This catalyst also demonstrates good reusability in C-lignin depolymerization. Detailed investigations by model compounds concluded that the pathways involving dehydration and/or dehydrogenation reactions are incompatible routes; we deduced that caffeyl alcohol generated via concurrent C−O bonds cleavage of benzodioxane unit may act as an intermediate in the C-lignin hydrogenolysis. Current demonstration validates that atomically dispersed metals can not only catalyze small molecules reactions, but also drive the transformation of abundant and renewable biopolymer.

Highlights

C-lignin is a homo-biopolymer, being made up of caffeyl alcohol exclusively
The weight percentage of Ru element was determined as 0.2 wt% according to the analysis of inductively coupled plasma-atomic emission (ICP-AES) (Supplementary Table 1)
The X-ray diffraction (XRD) patterns of Ru/ZnO/ C showed typical peaks for wurtzite ZnO phase (JCPSD No 361451);[46] no characteristic peaks for Ru were observed in XRD, probably because of low loading and/or high dispersion of Ru element (Supplementary Fig. 1)

Summary

Introduction

C-lignin is a homo-biopolymer, being made up of caffeyl alcohol exclusively. There is significant interest in developing efficient and selective catalyst for depolymerization of C-lignin, as it represents an ideal feedstock for producing catechol derivatives. We report an atomically dispersed Ru catalyst, which can serve as an efficient catalyst for the hydrogenolysis of C-lignin via the cleavage of C−O bonds in benzodioxane linkages, giving catechols in high yields with TONs up to 345. An unusual catechyl lignin (C-lignin) was found in a series of seed coats of Vanilla planifolia[20,21], Melocactus obtusipetalus[22], and Ricinus communis (castor) (Fig. 1)[21] Such lignin is shown to be essentially a homopolymer biosynthesized from caffeyl alcohol owing to the lack of O-methyltransferase[21,23]. Lignin displays stability toward harsh (i.e., Klason and acidic LiBr procedures) or mild (i.e., enzymic treatment) biomass pretreatment conditions, so that it can be fractionated from raw seed coats without condensation[24,25] These features of C-lignin render it an ideal lignin archetype for valorization[25].

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