Research Article pubs.acs.org/journal/ascecg
Fractionation of Organosolv Lignin Using Acetone:Water and Properties of the Obtained Fractions Hasan Sadeghifar,†,‡ Tyrone Wells,‡ Rosemary Khuu Le,‡ Fatemeh Sadeghifar,§ Joshua S. Yuan,∥ and Arthur Jonas Ragauskas*,‡,⊥,# †
Department of Wood and Paper Science, Sari Branch, Islamic Azad University, P.O. Box 48161-19318, Sari, Iran Department of Chemical and Bimolecular Engineering, and ⊥Department of Forestry, Wildlife, and Fisheries; Center for Renewable Carbon, University of Tennessee, 323-B Dougherty Engineering Building, Knoxville, Tennessee 37996, United States § Department of Biology Science, North Carolina State University, Dan Allen Street, Raleigh, North Carolina 27695, United States ∥ Texas A&M Agrilife Synthetic and Systems Biology Innovation Hub, Texas A&M University, College Station, Texas 77843, United States # Joint Institute for Biological Sciences, Biosciences Division, Oak Ridge National Laboratory (ORNL), Oak Ridge, Tennessee 37831, United States
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ABSTRACT: Lignin fractions with different molecular weight were prepared using a simple and almost green method from switchgrass and pine organosolv lignin. Different proportions of acetone in water, ranging from 30 to 60%, were used for lignin fractionation. A higher concentration of acetone dissolved higher molecular weight fractions of the lignin. Fractionated organosolv lignin showed different molecular weight and functional groups. Higher molecular weight fractions exhibited more aliphatic and less phenolic OH than lower molecular weight fractions. Lower molecular weight fractions lead to more homogeneous structure compared to samples with a higher molecular weight. All fractions showed strong antioxidant activity.
KEYWORDS: Lignin fractionation, Organosolv lignin, Antioxidant activity, Molecular weight, Acetone:water mixture
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INTRODUCTION Lignin is the second most abundant biopolymer after cellulose and can be derived from wood and nonwood plants via pulping and biorefinery processing. It is an amorphous, highly branched polyphenolic macromolecule with a complex structure.1 Its physical and chemical properties can vary depending on the wood species and its isolation process. The isolation process will change the native structure of lignin and can make it unsuitable for many value added applications. Despite its polyaromaticity,2,3 lignin has a variety of functional groups, namely hydroxyl, methoxyl, carbonyl, and carboxyl groups. Phenolic hydroxyl groups in the aromatic rings are the most reactive functional group in lignin and can significantly affect the chemical reactivity of the material.2 Biomass is one of the main sources for sustainable energy, biofuel, and chemical production using the biorefining process.4 Lignocellulosic feedstock (LCF) including poplar, switchgrass, miscanthus, corn stover urban waste, and forest residues streams are promising resources for biorefining biomass sources.5 The biotechnology platform for converting biomass to biofuels most often yields a “waste” stream, rich in lignin that currently is most often burned as a low-value fuel. Lignin constitutes ∼20−35 wt % of most common bioresources. As © 2016 American Chemical Society
such this would yield substantial amounts of lignin during cellulosic ethanol and alternative biofuels production. Since most cellulosic bioethanol plants utilize only ∼50% of the lignin for internal power requirements, their residual fraction remains potential opportunity for lignin valorization.4 As advanced biofuels production facilities are developed, massive amounts of lignin will be generated from the biorefineries utilizing the biological conversion platform, which could become a valuable resource for biobased chemicals and materials. Although the use of lignin for high-value applications has had some minimal successes, such as its use as an expander for leadacid storage batteries,1 many other potential applications have not been achieved, due in part, to lignin’s structural complexity, augmented reactivity, and thermal instability. During chemical pulping, native lignin structures can be altered significantly by fragmentation and condensation reactions.6 Therefore, depending on the pulping process, recovered lignins are generally heterogeneous, of high polydispersity, and have complex and variable functional group distributions.7 In order to solve the Received: August 15, 2016 Revised: November 6, 2016 Published: November 7, 2016 580
DOI: 10.1021/acssuschemeng.6b01955 ACS Sustainable Chem. Eng. 2017, 5, 580−587
Research Article
ACS Sustainable Chemistry & Engineering technical issues associated with the heterogeneity of lignin, a variety of lignin fractionation methodologies have been proposed in the literature. The proposed methods result in relatively homogeneous lignin fractions and a better understanding of its composition,8−11 manufacturing processes,12−14 and utility as a phenolic compound;15,16 these methods also facilitate subsequent melt spinning efforts.17,18 In this study, we investigated a simple and environmentally friendly method for lignin fractionation and examined the lignin molecular weight homogeneity, functional groups, and physical and antioxidant properties.
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Scheme 1. Lignin Fractionation Procedures
MATERIALS AND METHODS
Materials. The switchgrass (Panicumvirgatum L.) was acquired from University of Georgia, and pine chips were obtained from a kraft pulp mill located in GA. All samples were air-dried and milled with a Wiley mill equipped with a 0.85 mm screen and subsequently stored at 0 °C prior to use. All of the chemicals and reagents used in this work were purchased from VWR International or Sigma-Aldrich and used as received. Ethanol Organosolv Pretreatment. The pretreatment of extractive-free switchgrass and pine were carried out in a 2.0 L glasslined pressure Parr reactor equipped with a 4842 temperature controller (Parr Instrument Company, Moline, IL). In brief, switchgrass and pine samples were first Soxhlet-extracted with acetone (8 h) followed by hot water (2 h), and then washed and air-dried. The switchgrass or pine extractive-free sample (200.00 g, oven-dried) was charged in the Parr reactor. The extractive-free sample was treated with aqueous ethanol (65:35, v/v) with sulfuric acid (0.9%, w/w, on the basis of sample dry weight) as the catalyst in the glass liner of the Parr reactor. The solid/liquid ratio was 1:8, and the pretreatment was performed at 180 °C for 1 h.5 The pretreated sample was filtered and washed 3 times using aqueous 65% ethanol (150 mL, 60 °C). The washes were combined with the filtrate. Three volumes of deionized water were added to the combined filtrate to precipitate the lignin. The precipitated ethanol organosolv lignin (EOL) was then filtered through a Whatman No. 1 filter paper, thoroughly washed with deionized water, and dried under vacuum at 40 °C overnight before analysis. Lignin Solubility in Acetone and Acetone:Water. Switchgrass and pine organosolv lignin were mixed with pure acetone as well as mixtures of acetone:water, ranging from 30% to 98% acetone. The lignin solution was stirred for 6 h at room temperature and then centrifuged at 5500 rpm to separate the dissolved fraction from undissolved one. Fractional Precipitation of Switchgrass and Pine Organosolv Lignin. 100 g of lignin sample was dispersed into 60% acetone (1000 mL) under stirring, and the resulting suspension was kept under moderate stirring for another 12 h at room temperature. Both switchgrass and pine organosolv lignin dissolved completely in 60% acetone. Water was then added to the acetone solution to reduce the acetone concentration from 60% to 55%. After 60 min of mixing, the sample was centrifuged at 5500 rpm and the insoluble fraction were separated (Fraction F55% from the soluble portion). The acetone concentration of the supernatant, soluble lignin fraction was then further decreased to 52.5% (referred to as fraction two or F52.55%) with the addition of water and the insoluble fraction was separated using centrifugation. The reduction of acetone concentration was continued to 50%, 45%, and 30%. The final remaining soluble lignin in 30% acetone was separated using solvent evaporation (Fraction F
