Article pubs.acs.org/EF
Quantitative Molecular Structure−Pyrolytic Energy Correlation for Hardwood Lignins Teresa Cristina Fonseca Silva,†,‡ Ricardo Baillerini Santos,§ Hasan Jameel,§ Jorge Luiz Colodette,† and Lucian A. Lucia*,‡ †
Departamento de Química, Universidade Federal de Viçosa, Viçosa, Minas Gerais 36570-000, Brazil Laboratory of Soft Materials and Green Chemistry, Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695-8005, United States § Department of Forest Biomaterials, North Carolina State University, Raleigh, North Carolina 27695-8005, United States ‡
ABSTRACT: The molecular structures of the milled wood lignins (MWLs) and technical lignins (TLs) obtained from four hardwood species (Eucalyptus urograndis, Eucalyptus nitens, Eucalyptus globulus, and Populus trichocarpa) were quantified by 13C nuclear magnetic resonance (NMR) in tandem with their thermal responses, as obtained by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Both MWLs and TLs showed similar DSC curves with two exothermic peaks (referred to as peaks 1 and 2). Also, maximum temperatures for MWL peaks were higher than for TL peaks, with an average of 20 and 10 °C for the first peak and second peak, respectively. Even though calculated enthalpies for MWLs were higher because of their purities, TLs had reasonable relationships between heat values and lignin substructures. TL had a positive correlation between condensed structures and the enthalpy value between 413 and 428 °C, but negative relationships were found at the latter temperature window for lignin substructures, such as methoxyl groups, syringyl/guaiacyl ratio, and aliphatic OHs.
1. INTRODUCTION Lignin is among one of the most prevalent biological materials on the planet and comprises 20−30% of the dry weight of woody plants, displaying high structural complexity. Indeed, the lignin structure exhibits a seemingly random (highly polydisperse), highly cross-linked polymeric network characterized by monomeric phenylpropane units cross-linked through ether linkages and carbon−carbon bonds. In addition to the multiplicity of lignin forms potentially available based on their origin, lignin structures vary within the same species and specimen according to the isolation method. The most commonly used lignin isolation method requires milling of the plant material followed by extraction with dioxane−water to provide what is referred to as milled wood lignin (MWL).1 Although MWL is considered the most representative type of lignin to study because of its alleged similarity with native lignin in the wood cell wall, it has been demonstrated that chemical changes during the isolation process must be considered for structural veracity, especially with respect to lignin purity. Another method of extracting lignin is by precipitating the alkali-dissolved lignin from the viscous chemical waste medium produced during the kraft wood pulping process (the nominal “black liquor”) with mineral acid. The chemical structure of this lignin, called technical lignin (TL), is extensively modified along the process, and it is largely recovered as a byproduct in the pulp industry, to be used either as a solid fuel or additive in fuel oil. The use of lignin has attracted considerable interest from the “bio”-energy perspective. The use of renewable resources has emerged as a powerful alternative to fossil fuels because not only does it provide a local solution to energy dependence but it also helps to minimize environmental impacts by closing the greenhouse gas (CO2) loop. © 2012 American Chemical Society
Recent bioenergy engineering advances have increased the desirability of the use of biomass products to provide energyrich solutions.2 For example, because pyrolysis is considered one of the promising thermochemical conversion routes, a more thorough understanding of the thermochemistry of biomass may expedite its use for large-scale transformations.3 It has been shown that biomass pyrolysis can be divided into several stages: moisture evolution, hemicellulose decomposition, cellulose decomposition, and lignin decomposition.4 Thus, knowledge of the thermochemical response (pyrolysis characteristics) of each component is crucial for optimizing biomass conversion. Results of the pyrolysis of lignin and biomass have led to information on reaction kinetics of thermal decomposition,5,6 product formation during lignin pyrolysis,7 and the production of chemicals from lignin by pyrolysis.4,8 Moreover, there have been a number of biomass types, pretreatment studies, and therefore, new lignin preparations that would be interesting to investigate their pyrolytic properties.9−11 Developing a better understanding of the lignin structures and correlating the results with the pyrolytic properties of it would be extremely helpful to exploit the potential of lignin as a renewable and readily available feedstock. The objective of this work was to compare the results of the pyrolysis behavior of the lignin obtained from thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) and correlate them to the quality and quantity of lignin substructures determined by 13C nuclear magnetic resonance (NMR). For that, MWL and TL from four hardwood species Received: September 29, 2011 Revised: January 13, 2012 Published: January 17, 2012 1315
dx.doi.org/10.1021/ef2014869 | Energy Fuels 2012, 26, 1315−1322
Energy & Fuels
Article
Figure 1. Lignin substructures determined by 13C NMR. reference, assuming that it includes 6 aromatic carbons and 0.12 vinylic carbons. In this way, the integral values divided by 612 gave results equivalent to 100 aromatic rings (Ar). Carbohydrate composition was determined by standard chromatographic methods after acid hydrolysis. In a typical experiment, 0.1 g of sample was hydrolyzed with 1.5 mL of 72% (w/v) sulfuric acid at room temperature for 2 h, followed by sulfuric acid (3%, w/v)catalyzed hydrolysis at 120 °C for 1.5 h. The resulting suspensions were diluted with ultrapure water to the desired concentration and filtered through a 0.2 μm nylon filter (Milipore, Billerica, MA) before chromatography. They were then analyzed using a Dionex ICS 3000 IC system equipped with a CarboPac PA1 cartridge, an eluent generator (EG50), and an electrochemical detector (ED50). Water was used as the eluent at the flow rate of 1.0 mL/min, and the column temperature was 18 °C. A post-column base containing 40 mM NaOH was added to improve the detection by pulsed amperometry. 2.3. Thermal Analysis of Lignin. The pyrolysis of the MWL and TL obtained from all species was carried out in a thermogravimetric analyzer (Q500, TA Instruments). The samples masses were 10−15 mg and were heated to 600 °C at a linear temperature ramp of 10 °C min−1 under both purified nitrogen and air. A differential scanning calorimeter (Q100, TA Instruments) equipped with a cooling unit [refrigerated cooling system (RCS)] was used to determine enthalpies (1−5 mg sample masses). These samples were placed in sealed aluminum pans, and a linear temperature ramp of 10 °C min−1 was used from room temperature and 500 °C. Data were analyzed using TA Instruments Universal Analysis Software. Experiments were performed in duplicate for TGA and triplicate for DSC (error less than 2% on TGA and 10% on DSC).
(Eucalyptus urograndis, Eucalyptus nitens, Eucalyptus globulus, and Populus trichocarpa) were isolated and analyzed.
2. MATERIALS AND METHODS 2.1. Materials. Hardwood species cottonwood (P. trichocarpa) and three eucalyptus varieties (E. urograndis, E. nitens, and E. globulus) were used as the raw material to obtain isolated MWL and related TL from black liquor. Klason lignin and acid-soluble lignin were determined for each wood species using the extractive-free wood meal.12 The sum of Klason lignin and acid-soluble lignin was reported as the total lignin content. MWL was isolated by a slightly modified Björkman procedure1 described as follows: 40−60 mesh wood sawdust fraction from appropriate wood chips was extracted with 0.3% NaOH solution over 1 h (liquid/wood ratio = 50:1) under reflux. The extracted wood was thoroughly washed with hot distilled water until the filtrate reached neutral pH, and it was dried at 30 °C under vacuum. Alkaline-extracted wood sawdust was ball-milled using a planetary ball mill (Pulverisette 7, Fritsch). After the necessary milling time achieved a 30% yield, the wood sawdust was suspended in 96:4 dioxane/water (v/v) and agitated for 12 h at 40 °C. The mixture was centrifuged, and the liquid phase was collected. This operation was performed in triplicate. Then, the filtrates were combined, rotoevaporated to remove the solvents, and finally, dried in a vacuum oven. Lignin isolated from the alkalineextracted wood had a low sugar content (
