Preserving Both Lignin and Cellulose Chemical Structures: Flow-Through Acid Hydrotropic Fractionation at Atmospheric Pressure for Complete Wood Valorization

Abstract

Poplar wood was rapidly fractionated via a flow-through reaction using aqueous solutions of an acid hydrotrope (AH), p-toluenesulfonic acid (p-TsOH), at temperatures below 98 °C. 13C–1H two-dimensional nuclear magnetic resonance (NMR) spectroscopic analyses demonstrated that the AH-solubilized lignins (AHLs) from a range of fractionation conditions with yields up to approximately 80% had a very high content of β-aryl-ether linkages compared to milled wood lignin (MWL) with a low enough condensation to facilitate subsequent reductive catalytic depolymerization resulting in a lignin monomer yield of over 30%. Gel-permeation chromatographic (GPC) and differential scanning calorimetric (DSC) analyses showed that the AHLs have high molecular weights and low glass transition temperatures T g. These AHLs also have a pinkish color suitable for applications such as cosmetics and dye dispersants. AH fractionation (AHF) preserved the cellulose fraction as solid fibers also with a light pinkish color for the materials market and solubilized up to approximately 90% of xylan which can be converted to furfural using p-TsOH in the spent liquor without additional catalysts. The advantages herein are the use of one recyclable industrial chemical such as p-TsOH in an aqueous system below water boiling temperature to valorize all three major fractions of lignocelluloses in a short time frame, with very promising yields and well-preserved lignin and cellulose structure.

Biomass Fractionation and Lignin Fractionation towards Lignin Valorization

“…[57] The β-O-4 linkages were well preserved (43 % retention) even under harsh conditions (67 wt% p-TsOH, 70°C for 30 min). [58] Recently, Zhu and his co-workers used maleic acid hydrotropic fractionation (MAHF) to treat birch wood at mild conditions (atmospheric pressure, � 100°C), giving a dissolved lignin with a low degree of condensation and very light color that is dispersible due to carboxylation, which makes it amenable to a variety of applications. [28] To improve the reactivity of lignin for the following valorization, a novel method, the sequential organosolv fragmentation approach (SOFA) using ethanol plus different-stage catalysts, was developed by Liu et al to selectively dissolve lignin for producing multiple uniform lignin streams, and they fabricated lignin nanoparticles (LNPs) with desired quality using fractionated lignin by tailing its chemistry and reactivity.…”

“…[26] The acid-hydrotrope-dissolved lignin (AHL) yield was exceptionally good at 81.9 % with a high number of β-O-4 linkages (80 % retention) when using p-TsOH (2.5 mol L À 1 ) in flow-through conditions at 98°C for 40 min. [58] In addition, sequential extraction (SF) of hemicellulose and lignin to avoid excessive degradation of them have been described. [94] The second step of SF was performed at mild conditions (100°C, 40 wt% p-TsOH) for 1 h, and achieved 83 % lignin dissolution with a well-preserved structure.…”

Comparison of Two Acid Hydrotropes for Sustainable Fractionation of Birch Wood

“…L‐P T85 also has a much darker color than MWL, L ‐M T110 , and L ‐M T120 (Figure ) because of condensation and the formation of chromophore groups. The pinkish color of the AHL is attributed to the conjugated double bond and aldehyde auxochrome group in coniferyl aldehyde and the ketone auxochrome group in 1‐hydroxy‐3‐(4‐hydroxy‐3‐methoxyphenyl)‐2‐propanone (HHMP), both formed by depolymerization of Hibbert's ketones (HK), which are observed at δ C / δ H =67.1/4.2 (γ‐position) in Figure B–D …”

“…L-M T110 and L-M T120 retained more than 60 %and 40 %ofthe b-O-4 linkages, respectively,whereas AHL L-P T85 lost almost 95 % b-O-4 ( Figure 2a nd Table 2). L-P T85 also has am uch darkerc olor than MWL, L-M T110 ,a nd L-M T120 ( Figure 2) because of condensation and the formation of chromophore groups.T he pinkish color of the AHL is attributed to the conjugatedd ouble bond and aldehyde auxochrome group in coniferyl aldehyde and the ketone auxochrome group in 1-hydroxy-3-(4-hydroxy-3-methoxyphenyl)-2-propanone (HHMP) [31] ,b oth formed by depolymerization of Hibbert's ketones( HK), which are observed at Figure 2B-D. [32] The S/G ratio increased after AHF (Table 2) but much more significantly with the use of p-TsOH mainly because of the de-creasedGsignal upon condensation. The decrease in the amount of Gs ubunits in L-P T85 compared with MWL, as measured by G 2 signal, was 90 %, in contrast to the decrease of 56 %and 33 %inL-M T110 and L-M T120 ,respectively.This suggests that MAHF more effectively dissolves Gs ubunits than does p-Ts OH fractionation at equivalent levels of delignification.…”

“…AHF substantially depolymerized lignin as well as repolymerized lignin through forming CÀCb onds to condense lignin. [21,31] L-M T110 and L-M T120 had much lower molecular weight and more narrow (or more uniform) distribution than MWL because of increasedd epolymerization ( Figure 3a nd Table 2). The fact that the M w distribution of L-M T120 almostc ompletely overlaps that of L-M T110 ,w ith only as light decrease in average M w ,s uggests that repolymerization starts becoming important as MAHF temperature increases above 110 8C, approachingt he boiling point of approximately 130 8C.…”