Phenolic acetals from lignins of varying compositions via iron(III) triflate catalysed depolymerisation

TLDR: In this article, the authors demonstrate that lignins obtained from a range of different biomass sources and pretreatment methods can be successfully depolymerized using iron(III) triflate in the presence of ethylene glycol to give p-(1,3-dioxolan-2-yl)methyl substituted phenols.

About: This article is published in Green Chemistry.The article was published on 2017-06-19 and is currently open access. It has received 125 citations till now. The article focuses on the topics: Organosolv & Lignin.

Figures

Fig. 4 Correlation between the G to S ratio determined by 2D HSQC and molar yield ratio between P2 and P3 as determined by GC-FID analysis of the low molecular weight fraction.

Fig. 5 a) Comparison between yields of product P1-3 and the amount of total β-aryl ether linkages relative to aromatic signals in the lignin starting material. b) Showing only lignins originating from extractions from walnut shells. c) Showing only lignins obtained by dioxosolv procedure 2 from different biomass sources.

Fig. 6 Additionally, identified low molecular weight minor products from lignin depolymerisation reactions. P4-P9 arising from cleavage of the β-O-4 through a different pathway and P10 and P11 from modification of β-5 and β-β linkages respectively. 5b,16

Table 1. Overview of lignin extraction yields by weight using different lignin sources and extraction conditions.

Fig. 1 A) aromatic region of the 2D-HSQC NMR spectrum of pinewood dioxasolv lignin L13 and B) alkyl region. C) The aromatic region of 2D-HSQC NMR spectrum 12 of the 1,4- dioxane soluble/DCM insoluble residue obtained from the depolymerisation of L13, D) alkyl region showing 1,3-dioxolane end-groups.

Fig. 3 Yields of P1-P3 determined in the low-molecular weight fractions of product mixtures obtained after Fe(OTf)3 catalysed depolymerisation of 50 mg of lignins L1-L27 (Conditions in Scheme 2, averages for 2-4 experiments, error bars indicate the standard deviation of the total yield P1-P3).

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "University of groningen phenolic acetals from lignins of varying compositions via iron(iii) triflate catalysed depolymerisation" ?

In this paper, a mild acid-catalysed depolymerization of lignin with and without ethylene glycol trapping is presented. 

Q2. What is the way to depolymerize lignins?

The authors and others have recently highlighted the importance of stabilizing reactive intermediates formed during the acid catalysed depolymerisation of lignin, for example using HOTf, metal triflates and other acids. 

Q3. What is the cleavage pathway of Hibbert-ketones?

5b,16 These Hibbert-ketones are formed via a cleavage pathway that, unlike theformation of P1-3, does not involve the loss of formaldehydeand which was previously established to be a minor pathway when Fe(OTf)3 was used for β-aryl ether cleavage. 

Q4. What is the -aryl ether content of the lignins?

4f This leads to fragments containing other linkages becoming short oligomers with endgroups that are formed from the cleavage of a β-aryl ether linkage. 

Q5. What is the role of phenolic acetals in the development of lign?

Such phenolic acetals have recently been shown to allow access to lignin based epoxy resins with high glass transition temperatures, highlighting their potential utility as renewable aromatic building blocks. 

Q6. What is the main diastereomeric form of P11?

The two main diastereomeric forms of P11 resultsfrom the epimerization of a β-β linkage (for the original stereochemistry of the β-β motif see Fig. 1). 

Q7. What is the -aryl ether cleavage pathway?

17 They were observed in relativelysmall amounts (up to 2.6 wt%) and typically in 1-20%compared to the yield of parent major compounds P1-3demonstrating that these are indeed likely side-products from β-aryl ether cleavage via a minor cleavage pathway. 

Q8. What is the amount of -aryl ether in lignins?

This product wasespecially observed in relatively high amounts (up to 2.5 wt%) in G-rich lignins, which contain a significant amount of β-5 units such asL5, L6, L8, L12, L13, L14 and L21. 

Q9. How much is the molar ratio of P2 and P3 in the lign?

According to HSQC NMR analysis these are present in a 32 to 68 ratio respectively, a slightly higher than expected molar ratio of P2 and P3 (41 : 59 respectively) indicating relatively favourable formation of P2 compared to P3 from L19. 

Q10. What is the effect of the capture of aldehydes on the yield of phenol?

In particular, the capture of the released aldehydes as acetals was successful in providing good yields of phenolic monomersfrom both model compounds and lignin.