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. 2017 Sep 15;3(9):e1701735.
doi: 10.1126/sciadv.1701735. eCollection 2017 Sep.

Rapid and near-complete dissolution of wood lignin at ≤80°C by a recyclable acid hydrotrope

Affiliations

Rapid and near-complete dissolution of wood lignin at ≤80°C by a recyclable acid hydrotrope

Liheng Chen et al. Sci Adv. .

Abstract

We report the discovery of the hydrotropic properties of a recyclable aromatic acid, p-toluenesulfonic acid (p-TsOH), for potentially low-cost and efficient fractionation of wood through rapid and near-complete dissolution of lignin. Approximately 90% of poplar wood (NE222) lignin can be dissolved at 80°C in 20 min. Equivalent delignification using known hydrotropes, such as aromatic salts, can be achieved only at 150°C or higher for more than 10 hours or at 150°C for 2 hours with alkaline pulping. p-TsOH fractionated wood into two fractions: (i) a primarily cellulose-rich water-insoluble solid fraction that can be used for the production of high-value building blocks, such as dissolving pulp fibers, lignocellulosic nanomaterials, and/or sugars through subsequent enzymatic hydrolysis; and (ii) a spent acid liquor stream containing mainly dissolved lignin that can be easily precipitated as lignin nanoparticles by diluting the spent acid liquor to below the minimal hydrotrope concentration. Our nuclear magnetic resonance analyses of the dissolved lignin revealed that p-TsOH can depolymerize lignin via ether bond cleavage and can separate carbohydrate-free lignin from the wood. p-TsOH has a relatively low water solubility, which can facilitate efficient recovery using commercially proven crystallization technology by cooling the concentrated spent acid solution to ambient temperatures to achieve environmental sustainability through recycling of p-TsOH.

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Figures

Fig. 1
Fig. 1. Description of the present wood fractionation experimental study and lignin solubilization mechanism.
(A) Schematic flow diagram shows wood fractionation using p-TsOH for the production of fibers, lignocellulosic nanomaterials, sugars, and LNPs. Processes with dashed lines were not carried out in this study. (B) Schematics of carbohydrate and lignin solubilization by p-TsOH (Cp-TsOH > MHC) and precipitation (Cp-TsOH ≤ MHC).
Fig. 2
Fig. 2. Effects of reaction conditions on dissolution (1 − R) of lignin (●) and carbohydrates (glucan, ⬢; xylan, ⬟; mannan, ◆).
R is based on the component in untreated poplar wood. (A) p-TsOH concentration effects for 20 min at 80°C. (B) Temperature effects at p-TsOH concentration P = 75 wt % for 20 min. (C and D) Time effects at P = 75 wt % and 80°C (C) and at P = 80 wt % and 80°C (D).
Fig. 3
Fig. 3
Determining the p-TsOH MHC by (A) measuring the conductivities of aqueous p-TsOH solutions of different concentrations, showing discontinuity at the p-TsOH MHC of 11.5 wt % and by (B) measuring the effective size of lignin particles in diluted spent p-TsOH liquors using DLS [inset shows images of the diluted spent p-TsOH liquor with (bottom) and without (top) centrifugation].
Fig. 4
Fig. 4. Demonstration of centrifugation for fractionating solubilized LNPs.
(A) AFM images of LNPs in a diluted spent p-TsOH liquor of 10 wt % that is produced using Wiley-milled NE222 at P75T80t20. (B) AFM-measured topographical height profiles corresponding to lines 1 and 2 in (A). (C to E) AFM images of LNPs in the supernatant from the centrifuged sample in (A) at different speeds for 10 min, at 3000g (C), at 10,000g (D), and at 15,000g (E). (F) AFM-measured topographical height distributions of samples shown in (C) to (E).
Fig. 5
Fig. 5. Aromatic and side-chain (δC/δH 48–140/2.5–8.0) regions in the 2D HSQC NMR spectra of the WCW and three dissolved lignin samples from poplar wood.
Fig. 6
Fig. 6. Time-dependent enzymatic digestibility of fractionated NE222 WISs.
(A) Different substrates from various fractionation conditions under constant cellulase loading of CTec3 = 20 FPU/g glucan. (B) Effects of cellulase CTec3 loading.

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