Impact of formic/acetic acid and ammonia pre-treatments on chemical structure and physico-chemical properties of Miscanthus x giganteus lignins

https://doi.org/10.1016/j.polymdegradstab.2011.07.022Get rights and content

Abstract

Miscanthus x giganteus was treated with formic acid/acetic acid/water (30/50/20 v/v) for 3 h at 107 °C and 80 °C, and soaking in aqueous ammonia (25% w/w) for 6 h at 60 °C. The effects of these fractionation processes on chemical structure, physico-chemical properties and antioxidant activity of extracted lignins were investigated. Lignins were characterized by their purity, carbohydrate composition, thermal stability, molecular weight and by Fourier transform infrared (FTIR), 1H and quantitative 13C nuclear magnetic resonance (NMR), adiabatic broadband {13C–1H} 2D heteronuclear (multiplicity edited) single quantum coherence (g-HSQCAD). The radical scavenging activity towards 2,2-diphenyl-1-picrylhydrazyl (DPPH) was also investigated. Formic/acetic acid pretreatment performed in milder conditions (80 °C for 3 h) gave a delignification percentage of 44.7% and soaking in aqueous ammonia 36.3%.

Formic/acetic acid pretreatment performed in harsh conditions (107 °C for 3 h) was more effective for extensive delignification (86.5%) and delivered the most pure lignin (80%). The three lignin fractions contained carbohydrate in different extent: 3% for the lignin obtained after the formic/acetic acid pretreatment performed at 107 °C (FAL-107), 5.8% for the formic/acetic acid performed at 80 °C (FAL-80) and 13.7% for the ammonia lignin (AL). The acid pretreatment in harsh conditions (FAL-107) resulted in cleavage of β–O-4′ bonds and aromatic C–C. Repolymerisation was thought to originate from formation of new aromatic C–O linkages. Under milder conditions (FAL-80) less β–O-4′ linkages were broken and repolymerisation took place to a lesser extent. Ammonia lignin was not degraded to a significant extent and resulted in the highest weight average 3140 g mol−1. Despite the fact of FAL-107 repolymerisation, significant phenolic hydroxyls remained free, explaining the greater antioxidant activity.

Introduction

Lignin constitutes 15–30% by weight of the lignocellulosic biomass [1], and after cellulose and hemicellulose it is the third most abundant biopolymer on earth. This major cell wall component fills the spaces between cellulose, hemicellulose and pectin and crosslinks polysaccharides conferring the strength and rigidity of the lignocellulose matrix. In addition, lignin polymer conducts water and nutrients in stems and protects the cell wall from microorganism attacks [2].

This complex three-dimensional amorphous polymer is composed of methoxylated phenylpropane structures and their precursors are three aromatic alcohols namely p-coumaryl, coniferyl and sinapyl alcohols. These molignols are incorporated into lignin in the form of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) respectively [3]. Depending on their origin (hardwoods, softwoods, grasses), the type of monolignols units differs. Lignins from woods are mainly composed of guaiacyl and syringyl, whereas herbaceous plants contain all the three units (H, G, S) [2]. Different types of linkages connect the phenylpropane units, where the most common is the β–O-4′ linkage consisting of more than half of the lignin linkages [4]. Lignin is linked to the other polymers through covalent bonds: α-ether, α-ester [5], [6] and phenyl glycosidic linkages [7].

Despite the fact that lignin is the major aromatic bio-resource, it has received less attention with regards to value enhancement compared to cellulose. From the pulp and paper industry, only 2% of the lignins are used for commercial purposes [8]. With its aromatic structure and chemical properties a wide variety of bulk and fine chemicals can be produced in a sustainable way and may substitute petroleum based compounds. Biofuels or bulk aromatic and phenolic compounds can be produced by catalytic lignin conversion, (hydro)cracking or lignin reduction reactions. Lignin oxidation reactions are used to make functionalized aromatics for the production of fine chemicals. High molecular weight lignin can be used to produce carbon fibres, polymer modifiers, adhesives, and resins [9]. In addition, lignins have antioxidant activity due to their diverse functional groups (phenolic groups, hydroxyl groups, …) [10].

A growing number of environmental friendly fractionation technologies to remove lignin from the lignocellulose are being developed, and each method changes the lignin form and chemical properties to a greater or lesser extent. No one method is ideal and the advantages and limitations of each method have to be evaluated in terms of the overall valorization of the lignocellulose material.

Miscanthus x giganteus, which can grows 4 m high and gives high yields of biomass per hectare, was identified as a potential high-yielding bioenergy crop in Europe and the USA number of treatments have been applied on Miscanthus x giganteus in order to delignify it for pulp production, before enzymatic saccharification or for lignin isolation [11], [12], [13], [14], [15].

In this study, the effect of different fractionation processes on chemical structure, physico-chemical properties and antioxidant activity of Miscanthus x giganteus lignin were investigated. Three different lignin preparations were obtained by: (i) formic/acetic acid pretreatment under harsh conditions, (ii) formic/acetic acid pretreatment under mild conditions, (iii) soaking in aqueous ammonia. Extracted lignins were subjected to characterisation by FTIR, TGA, 1H NMR, quantitative 13C NMR, g-HSQCAD NMR and gel permeation chromatography in order to investigate the effect of the pre-treatments on the chemical structure and physico-chemical characteristic. The radical scavenging activity towards 2,2-diphenyl-1-picrylhydrazyl (DPPH) was also investigated.

Section snippets

Raw material

Miscanthus x giganteus comes from a crop cultivated in spring 2007, and harvested after two years (Tournai, Belgium). Stems were stripped of leaves, air dried and ensiled. The material was ground to particles around 1–2 mm. The untreated miscanthus was mainly composed of 24.5% lignin, 48.4% glucose, 15.7% xylose, 1.9% arabinose, 1.2% galactose, 0.2% mannose, 6.4% extractives and 2.4% ash.

Formic/acetic acid pretreatment

Miscanthus x giganteus was pre-treated by the protocol described in Lam et al. [16] with some modifications.

Composition of the isolated lignins

Depending on the conditions of the pre-treatments, different delignification degrees were obtained on the pulps. The pretreatment that gave an extensive delignification (86.5%) was the formic/acetic acid performed at 107 °C for 3 h. The second formic/acetic acid pretreatment achieved in milder conditions (80 °C for 3 h) gave almost the half delignification value (44.7%). A smaller delignification value was obtained with the ammonia pretreatment (36.3%). The largest delignification value can be

Conclusions

In the acid pretreatment in harsh conditions (FAL-107) depolymerisation and repolymerisation took place. Cleavage of β–O-4′ bonds and aromatic C–C were the major mechanisms of lignin breakdown. Repolymerisation was thought to originate from formation of new ester substructures.

Under milder conditions (FAL-80) fewer β–O-4′ linkages were broken and repolymerisation took place to a lesser extent. Ammonia lignin was “contaminated” by residual polysaccharides but was not degraded to a significant

Acknowledgements

This study was financially supported by the Walloon Region (TECHNOSE project number 716757). Ms. Virginie Byttebier is acknowledged for excellent assistance. Mr. Mario Aguedo is acknowledged for his help in molecular weight analysis. Mrs. Lynn Doran is acknowledged for the thermogravimetric analysis.

References (33)

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