Physical–chemical characteristics of lignins separated from biomasses for second-generation ethanol

Highlights

• Lignin extracted from energy crops by either acidic or alkaline treatment.

• Lignin chemical features were assessed by analytical and spectroscopic techniques.

• Acidic extraction gave poorly water-soluble and conformationally rigid lignins.

• Alkaline oxidation produced depolymerized and water-soluble lignins.

• Water-soluble lignins may be environmentally and industrially useful.

Abstract

Lignin was extracted by two extraction methods from two biomasses for energy (Mischantus and Giant Reed) and a lignocellulosic material resulting from a microbial treatment of giant reed. One method of extraction involved the use of H₂SO₄ (SA), providing a highly aromatic water-insoluble material, while a second method employed H₂O₂ at alkaline pH (Ox), resulting in a water-soluble lignin. Extraction yields were related to the total Klason lignin measured for the three materials. We compared the physical–chemical features of the isolated lignins, by employing solid-state nuclear magnetic resonance spectroscopy (¹³C-CPMAS spectra and derived T1ρH relaxation times), thermogravimetric analyses, infrared spectrometry and high performance size exclusion chromatography (HPSEC). We found that lignin separated by the Ox method owned a more mobile molecular conformation, and was largely more water-soluble and fragmented than the lignin obtained by the SA treatment. In line with T1ρH-NMR and thermogravimetric results, the HPSEC of Ox lignins showed nominal molecular weights less than 3 kDa, indicating well depolymerized materials. Such low-molecular weight and fragmented lignin obtained from biomasses for energy may become useful for application of recycled products in agriculture and in green chemistry reactions, thereby promoting an increase in the economic sustainability of biorefineries.

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).