Characteristion of Bagasse Soda Lignin
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QUT Digital Repository: http://eprints.qut.edu.au/ This is the author version published as: This is the accepted version of this article. To be published This is the author version published as: Mousavioun, Payam and Doherty, William O.S. (2010) Chemical and thermal properties of fractionated bagasse soda lignin. Industrial Crops and Products, 31(1). pp. 52-58. Catalogue from Homo Faber 2007 Copyright 2009 Elsevier B.V. 1 Chemical and Thermal Properties of Bagasse Soda Lignin 2 by 3 P. Mousavioun and W.O.S. Doherty 4 Centre for Tropical Crops and Biocommodities, Queensland University of 5 Technology, GPO Box 2343, Brisbane, Australia 6 7 Abstract 8 A major challenge of the 21st century will be to generate transportation fuels using 9 feedstocks such as lignocellulosic waste materials as a substitute for existing fossil 10 and nuclear fuels. The advantages of lignocellulosics as a feedstock material are that 11 they are abundant, sustainable and carbon-neutral. To improve the economics of 12 producing liquid transportation fuels from lignocellulosic biomass, the development 13 of value-added products from lignin, a major component of lignocellulosics, is 14 necessary. Lignins produced from black liquor through the fractionation of sugarcane 15 bagasse with soda and organic solvents have been characterised by physical and 16 chemical means. The soda lignin fractions have different physico-chemical properties 17 from each other, and have been compared to bagasse lignin extracted with aqueous 18 ethanol. 19 1 Introduction 20 In the last century, energy sources have been derived from petroleum (30 %), natural 21 gas (23 %), coal (22 %), renewable (19 %) and nuclear (6 %) (Song., 2002). In the 22 chemical industry, 4 % of crude oil and 31 % of natural gas are used in the 23 manufacture of platform chemicals and composite materials. The state of the oil 24 market ($US 40-$US100 per barrel) is unpredictable because of economic and 25 political pressures, and the ever increasing oil demand from developing Asian 26 countries will probably maintain the current high price of crude oil. There is the 27 ongoing debate among geologists as to the time frame when oil reserves will be 28 depleted. There is also the strong push towards reduced greenhouse gas (GHG) 29 emission. Fossil fuels used in transportation contribute over 25 % of GHG. It has 30 been estimated that the utilisation of plant/crop-based feedstock for the production of 31 chemicals in the European Union could deliver GHG reductions of over 6 M tonnes 32 per annum in the next decade. As a consequence of these events, there has been 33 coordinated R&D strategy across the globe for the utilisation of plant/crop-based 34 products. 35 The International Energy Agency, Energy Outlook in November 2006 said, “Rising 36 food demand, which competes with biofuels for existing arable and pasture land, will 37 constrain the potential for biofuels production using current technology”. Such a 38 constraint causes feedstock price increases for both food and fuel. So the challenge 39 we now have is to be able to produce transportation fuels from non-food sources e.g., 40 bagasse, wheat straw, rice stalk, cotton linters, agricultural wastes, and forest thinning, 41 at an economically competitive price without government subsidies. To bring down 42 production cost of biofuels, developing a market for lignin products which have 43 equivalent properties as the petroleum-based products is necessary. 44 In 2006, the world produced 1.4 billion tonnes of sugarcane (FAO, 2008). This 45 equates to ~400 million tonnes of bagasse. Currently, a vast majority of bagasse is 46 used to produce low value co-generation of power, manufacture of pulp and paper 47 products and furfural production. However, with continuing improvements in energy 48 efficiencies of sugar factories, more and more bagasse will be available for other 49 applications, such the production of cellulosic ethanol. The advantage bagasse has 50 over other non-food sources is that it is located centrally due to existing transportation 51 infrastructure. So, co-location of a cellulosic ethanol plant to a sugar factory gives the 52 opportunity to share production systems but also to share processing facilities. 53 Bagasse, an non-wood consist mainly of cellulose (50 %), hemicellulose (30%) and 54 lignin (20%). Lignin is an amorphous large, cross-linked, macromolecule with 55 molecular masses in the range 1000 g/mol to 20,000 g/mol. The degree of 56 polymerisation in nature is difficult to measure, since it is fragmented during 57 extraction and the molecule consists of various types of substructures which appear to 58 repeat in a haphazard manner. There are three monolignol monomers, methoxylated to 59 various degrees: p-coumaryl alcohol, coniferyl alcohol, and sinapyl. Despite 60 improvements in structure elucidation, the exact structure of lignin is still unknown. 61 The consensus by a number of workers (Adler, 1980; Sjostrom, 1981; Chen, 1991; 62 Ede and Kilpelaeinen, 1995; Karhunren et al., 1995a; Karhunen et al., 1995b) is that 63 the two commonest linkages is ether and carbon-carbon bonds. The phenylpropane β- 64 aryl ether linkages constitute the largest proportion of the different linkages 65 connecting the monomeric units. These linkages need to be broken for effective 66 lignin fractionation from biomass. 67 The processes used in the extraction of lignin from woody plants are conducted under 68 conditions were lignin is progressively broken down to lower molecular weight 69 fragments resulting to changes to its physico-chemical properties. Thus, apart from 70 the source of the lignin, the solvating ability of the solvent to either lignin or the 71 cellulose, or both, the properties of the solvent to inhibit to C-C bond formation, the 72 solution pH and the method of extraction influences the chemical and functional 73 group composition of lignin. Gosselink et al. (2004) have reported that lignin 74 composition is different not only among plants but also different between parts of the 75 same plant. The structural heterogeneity of lignin has also been studied by various 76 methods in a number of investigations. In several of these studies the lignin was 77 subjected to fractionation prior to the analysis (Robert et al., 1984; Moerck et al., 78 1986; Vanderlann and Thring, 1998; Wallberg et al., 2003). These fractionations 79 were analysed for functional groups, elemental composition and molecular weight. 80 The results of these investigations showed that the fractionation process separated the 81 lignin into distinct molecular weights and that there were differences in the carboxylic 82 acids, phenolic hydroxyl and methoxyl contents The properties of the materials 83 produced were dependent on these structural properties 84 As our research goals are to produce cellulosic ethanol from bagasse via pretreatment 85 with soda and add value to soda lignin, we have undertaken a characterisation 86 exercise on soda lignin and examined its heterogeneity through sequential extraction 87 in order to target products based on structure-property relationships. Where possible 88 we have compared the results to that of bagasse lignin obtained through aqueous 89 ethanol extraction, as the lignin obtained by this process is generally regarded to be of 90 good quality. 91 2 Materials and methods 92 2.1 Lignin extraction 93 Bagasse was obtained from a Mackay Sugar Mill, Queensland Australia. It was wet 94 depithed and then air dried. Lignin was extracted from bagasse by the soda process 95 using a 20 L Parr reactor. In this method, 1 kg of bagasse is cooked with about 10.5 L 96 of 0.7 - 1 M NaOH. Once the reactor reaches the operating temperature of 170˚C, it is 97 maintained for 1.5 h. After cooling, the liquid (black liquor) was removed from the 98 bottom of the reactor and sieved to remove fibrous material. To the black liquor, 99 dilute sulphuric acid was slowly added with stirring to pH 5.5. Near pH 5.5 an 100 obvious change in the appearance of the solution occurs; from black to murky brown. 101 This change is due to the initial stages of lignin precipitation. The mixture was stirred 102 for 10-15 min after which acidification is continued to pH 3. It was then transferred 103 to a 65 °C water bath and stirred using an overhead stirrer for 30 – 45 min. The 104 mixture was then vacuum filtered to recover the lignin. The lignin was repeatedly 105 washed with hot water until all signs of foaming have subsided. It was then left to air- 106 dry before being further dried in a vacuum oven at 45 °C overnight. This procedure 107 increases the purity of lignin by reducing the inclusion of ash and carbohydrate 108 components. It is different from other procedures reported in the literature because it 109 is based on a two-stage acid precipitation process. The initial precipitation process at 110 pH 5.5 produces lignin particles of high purity which are then allowed to grow to 111 larger sizes before proceeding to the second precipitation stage where the proportion 112 of impurities is highest. 113 Organosolvo lignin (OL) was obtained through precipitation of the black liquor into a 114 dilute H2SO4 solution. The black liquor was obtained by the delignification of 115 bagasse with 50 % aqueous ethanol solution in the 20 L Parr reactor. Crude lignin 116 was dissolved in 0.1 M NaOH equilibrated and precipitated with H2SO4 at pH 3. The 117 slurry was filtered hot and the residue was washed with water until the filtrate became 118 colourless. The lignin was air dried and further dried at 45°C and 100°C, 119 consecutively. 120 2.2 Lignin fractionation 121 Soda lignin is a complex and heterogeneous mixture with a rather broad molecular 122 weight distribution.