Physical–chemical characteristics of lignins separated from biomasses for second-generation ethanol
Introduction
Plant biomasses rich in lignocellulose represent the largest renewable source of hexose and pentose sugars for potential conversion in either ethanol for fuels or various chemicals for industry [1]. First-generation ethanol has been massively obtained from food crops such as wheat and maize [2], [3], with consequent reduction of food for animal and human nutrition [1] and overexploitation of valuable productive soil [4]. In order to correct such an unsustainable approach, a second-generation ethanol is now planned to be obtained from non-food biomasses rich in lignocellulose [5], [6], and easily grown on marginal soils [7]. Examples of such biomasses are the perennial rhizomatous herbaceous plants, such as miscanthus (Miscanthus × Giganteus, Greef et Deuter) in cool and humid areas, and giant reed (Arundo donax, L.) in warm and dry zones. Both these grasses exhibit high productivity and favorable energy balance even at low nutrient and energy inputs [8], [9], [10]. Since no sowing or tillage is required, cultivation of these biomasses appears to be more ecologically sustainable than annual food crops, thereby contributing to reduce soil erosion risk [11] and improve soil carbon storage and biodiversity.
Different methods have been devised to obtain cellulose from biomasses rich in lignocellulose and, thus, ethanol and other chemicals [12]. Once cellulose is separated, the lignin-rich residue is usually burnt or discarded [1], disregarding a more profitable exploitation of precious aromatic photosynthates. One technique separates cellulose from the bulk of biomass lignocellulose at acidic pH by strong inorganic acids such as sulfuric (H2SO4) and hydrochloric (HCl) acids [13]. While this method enables extraction of easily fermentable sugars, it also produces furfural and hydroxymethylfurfural, which are well-known inhibitors of microbial growth and may impair full fermentation of carbohydrates [14]. Even though this harsh acidic cellulose extraction and degradation employs hazardous and rather expensive chemicals, it is still of practical interest and widely applied [15].
A method of lignin separation from cellulose, originally developed in the paper-making industry as an alternative to environmentally hazardous chlorine-based reagents, consists in treating biomasses with alkaline solutions [16]. Strong bases, such as NaOH or Ca(OH)2, are both environmentally compatible and efficient agents for delignification of cellulose and solubilization of separated lignin Ref. [17]. Furthermore, lignin hydrolysis is significantly improved by adding hydrogen peroxide (H2O2) to the reaction mixture [18]. In fact, this alkaline oxidative solution easily disrupts cell walls, efficiently dissolves hemicellulose and lignin by hydrolyzing uranic and acetic acid esters, and yields poorly crystalline cellulose [19]. Moreover, α-aryl ether linkages in lignin phenolic units are readily cleaved and the conversion of the resulting phenolic units into, first, quinone methide intermediates, and, then, fragmented hydroxyl–phenolic groups, contributes to lignin solubilization in alkaline solution [20]. Lignin is then easily recovered by lowering the pH [21]. The resulting material has been reported to be lower in molecular weight and richer in carbonyl and carboxyl functional groups than lignin obtained by Kraft or sulfite processes [22]. However, no detailed information on the physical–chemical characteristics of the different lignin separates has been ever described, up to our knowledge.
The objective of this work was to compare the physical–chemical characteristics of lignin separated by either a sulfuric acid or an oxidative alkaline extraction, from three different plant sources rich in lignocellulose and used for second-generation ethanol: miscanthus, giant reed, and a microbially pre-treated giant reed. The isolated lignin was characterized by Attenuated Total Reflectance Infrared Fourier Transform spectroscopy (ATR-IR), solid state NMR spectroscopy (13C-CPMAS spectra and derived T1ρH relaxation times), thermogravimetric analysis (TGA), and high performance size exclusion chromatography (HPSEC).
Section snippets
Biomasses
Miscanthus (Miscanthus × Giganteus, Greef et Deuter) sample (MG) was provided by the Institute of Biological, Environmental & Rural Sciences (IBERS) of the University of Birmingham, in collaboration with Phytatec Ltd (UK). MG biomasses were grown in open field trials in Aberystwyth, Wales (UK) and harvested in February 2007. Giant reed (Arundo donax, L) sample (AD) was cropped at the experimental farm of the University of Naples Federico II (Bellizzi, IT) and harvested in January 2009. A third
Gravimetric analyses
The gravimetric results for the isolated lignins are reported in Table 1. The percent of Klason lignin in MG (26.5%) was comparable to that reported in literature [28], [29], whereas the percent yield obtained from AD (28.72%) was slightly greater than usually reported [30], [31], [32]. This difference may be explained with the delayed harvesting season for the AD of this work, that may have affected the relative composition of plant biomasses. In fact, delay in harvest is reported to reduce
Conclusions
We characterized the physical–chemical properties of lignin extracted from three different biomasses for energy (miscanthus, giant reed and a lignocellulosic residue from a microbially treated giant reed) by either hydrolysis in concentrated H2SO4 or alkaline oxidation with H2O2. Thermal analyses and both ATR-IR spectrometry and solid-state NMR spectroscopy indicated that lignin isolated by sulfuric acid contained a lower amount of residual cell wall carbohydrates than the Ox-lignins. Moreover,
Acknowledgment
This work was conducted in partial fulfillment of the first author's PhD work and was partially funded by the MIUR project PON01_01966/2 “ENERBIOCHEM”.
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