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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01955 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 11, 2016
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ACS Sustainable Chemistry & Engineering
Fractionation of Organosolv Lignin Using Acetone:Water and Properties of the Obtained Fractions Hasan Sadeghifar1,2, Tyrone Wells2, Rosemary Khuu Le2, Fatemeh Sadeghifar3, Joshua S. Yuan4, Arthure Jonas Ragauskas2,5,6 1
Department of Wood and Paper Science, Sari Branch, Islamic Azad University, P.O. Box 48161-19318, Sari, Iran Department of Chemical and Bimolecular Engineering, 323-B Dougherty Engineering Bldg University of Tennessee, TN 37996 USA 3 Department of Biology Science, Dan Allen st. North Carolina State University, Raleigh NC USA 4 Texas A&M Agrilife Synthetic and Systems Biology Innovation Hub, Texas A&M University, College Station, TX 77843 5 Department of Forestry, Wildlife, and Fisheries; Center for Renewable Carbon, University of Tennessee, Knoxville, TN, 37996, USA. 6 BioEnergy Science Center, Oak Ridge National Laboratory, Oak Ridge, TN, 37830, USA 2
Corresponding Author: Arthure Jonas Ragauskas
[email protected] Abstract Lignin fractions with different molecular weight were prepared using a simple and almost green method from switchgrass and pine organosolv lignin. Different proportion of acetone in water, ranging from 30-60% were used for lignin fractionation. 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
Introduction Lignin is the second most abundant biopolymer after cellulose and can be derived from wood and non-wood plants via pulping and bio-refinery processing. It is an amorphous, highly branched polyphenolic macromolecule with a complex structure1. Its physical and chemical properties can vary depending on the wood species and its isolation process. The isolation 1
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process will change the native structure of lignin and can make it unsuitable for many value added applications. Despite its ployaromaticity2,3, 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 material2. Biomass is one of the main sources for sustainable energy, biofuel, and chemicals production using the biorefining process4. 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 lowvalue fuel. Lignin constitutes ∼20-35 wt% of most common bio-resources. As such this would yield substantial amounts of lignin during cellulosic ethanol and alternative biofuels production. Since most cellulosic bio-ethanol plants utilize only ∼ 50% of the lignin for internal power requirements, their residual fraction remains potential opportunity for lignin valorization4. 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 bio-based 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 lead-acid 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 reactions6. Therefore, depending on the pulping process, recovered lignins are generally heterogeneous, of high polydispersity, and have complex and variable functional group distributions7. In order to solve the 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, a better understanding of its composition8-11, manufacturing processes12-14, its utility as phenolic compound15,16 and facilitate subsequent melt spinning efforts17,18.
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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. Materials and Methods Materials The switchgrass (PanicumvirgatumL.) 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 0oC prior to use. All of the chemicals and reagents used in this work were purchased from VWR International or SigmaAldrich and used as received. Ethanol Organosolv Pretreatment The pretreatment of extractive-free switchgrass and pine were carried out in a 2.0 L glass-lined 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 (8h) 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
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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
