Lignin Profiling: A Guide for Selecting Appropriate Lignins as

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Lignin Profiling: A Guide for Selecting Appropriate Lignin as Precursors in Biomaterials Development Behzad Ahvazi, Eric Cloutier, Olivia Wojciechowicz, and Tri Dung Ngo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00873 • Publication Date (Web): 11 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016

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ACS Sustainable Chemistry & Engineering

Lignin Profiling: A Guide for Selecting Appropriate Lignin as Precursors in Biomaterials Development

Behzad Ahvazi*1,3, Éric Cloutier2, Olivia Wojciechowicz1,† and Tri-Dung Ngo2,3

1. Biotechnology Research Institute, National Research Council of Canada, 6100 Royalmount Avenue, Montréal, Québec, Canada H4P 2R2 2. Industrial Materials Institute, National Research Council of Canada, 75 Mortagne Blvd., Boucherville, Québec, Canada J4B 6Y4 3. Alberta Innovates - Technology Futures, 250 Karl Clark Road, Edmonton, Alberta, Canada, T6N 1E4

*

To whom correspondence should be addressed:

Telephone: +1 780 450 5488, Fax: +1 780 450 5397 Email: ([email protected])

†

Present Address :

Enerlab 2000 Inc. 1895 de l’Industrie, St-Mathieu-de-Beloeil, Québec, Canada J3G 4S5 1 ACS Paragon Plus Environment


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ABSTRACT A number of industrial and technical lignins from forestry and agricultural residues extracted by different chemical pulping processes were characterized by evaluating their physical and chemical properties. Several qualitative and quantitative methods were performed to elucidate lignin profiles and their potentials as substitute in view of biobased products. The morphology, molecular weight distributions, elemental compositions, glass transition temperature, and several important functional groups containing hydroxyl units such as phenolic, aliphatic, and carboxylic acid were classified and their contents were determined quantitatively by employing

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P NMR spectroscopy. The emerging information aims at

addressing a number of pressing issues relevant to the scientific development for value added applications from lignins during industrial production of biomaterials.

KEYWORDS:

Lignins; Characterization;

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P NMR Spectroscopy; FTIR; SEM; Tg;

Elemental analysis; Biomaterials.

2 ACS Paragon Plus Environment

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INTRODUCTION Polymers play an essential and ubiquitous role in our daily lives due to their extraordinary wide variety of properties. With the instability of petroleum prices and the high energy intensity1 in production of chemicals and synthetic polymers, the conversion of renewable lignocellulosic resources into chemicals, liquids fuels and feed supplement has gained considerable attention in recent years.2 More specifically, the development of value-added products from lignin as feed stock has been scrutinized for production of higher valued green chemicals and novel bio-based materials.3-6 Lignin is an amorphous, highly branched polyphenolic macromolecule of complex structure with high molecular weight. The chemical structure of lignin is highly irregular and extremely challenging. Lignin polymer consists primarily of phenylpropanoid monomers cross-linked together in three dimensions via a radical coupling process during its biosynthesis.7,8 Lignin is available in large quantities as a byproduct of the pulp and paper industries. The annual production of lignin is exceeding 70 million tons worldwide.9 The most abundant industrial lignins are from kraft and sulfite pulping processes.10 Lignins obtained under these processes undergo significant structural changes and are no longer identical to their original native structures.11-13 Due to its complex nature and undefined chemical structure,14-16 the industrial applications of lignin are rather limited. Lignin is utilized almost exclusively as fuel to power the evaporators of the chemical recovery processes and liquor concentration system of pulp mills.17 However, based on its interesting functionalities and properties,18 lignin offers perspective for higher added value applications in renewable products.19 The significant factor for integrating lignins more efficiently into polymeric materials is to 3 ACS Paragon Plus Environment


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establish compatibility between lignin and its petroleum-based counterparts. This can be achieved only by elucidating their diverse physical and chemical characteristics and properties. Scheme 1 The physical and chemical properties of lignin are highly dependent on the wood species, botanic region and most importantly, extraction and the recovery processes.20 These factors impact the chemical structural units21 of lignin and its various functional groups22 altering its overall reactivity. Softwoods are known to contain almost exclusively guaiacylpropane units (G) with R1=H, or OCH3 and R2=H whereas hardwoods contain syringylpropane (S) with R1=OCH3 and R2=H in addition to guaiacylpropane units. Lignin contains several other important functional groups including para-hydroxyphenyl (H) with R1=R2=H, aliphatic hydroxyl, and carboxylic acid groups interconnected via aryl, alkyl, or ester linkages. Different lignin samples contain different types and quantities of these functional groups. Based on the delignification process which is aimed at the fragmentation of the lignin macromolecule in wood matrix causing their dissolution, lignin units undergo condensation reactions causing considerable structural rearrangements. The mechanism which are involved in these reactions are governed by the nature of the C-O and C-C bonds of the C9 units as well as several classes of interconnected arylpropanes18 bearing ether and C-C linkages in lignin.23 Reactions involving cleavage of carbon-carbon bonds are of relatively minor importance when compared to the extensive lignin fragmentation from the rupture of the ether linkages in lignin.24 The physical and chemical properties of lignin isolated from these processes are significantly different even within the same wood species. These variations in lignin composition and the 4 ACS Paragon Plus Environment

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uniqueness of its structural features make the lignin that much specific as a substitute for majority of petroleum-based products during the manufacturing of industrial biomaterials and bio-composites.25-27 Amongst various delignification processes, the type of lignin is determined by four dominant chemical pulping processes namely kraft, sulfite, soda, and organosolv. The kraft or sulfate pulping process accounts for more than 90% of the chemical pulp production worldwide. Delignification during kraft pulping proceeds in three distinct phases of initial, bulk and final in an aqueous solution of sodium hydroxide and sodium sulfide.28 Lignin is recovered from this process through precipitation by lowering the pH of the black liquor with either sulfuric acid or carbon dioxide.29 The isolated kraft lignin is hydrophobic and contains 1-2% sulfur by weight as an aliphatic thiol groups. Sulfite process is the second largest chemical pulping process operating on the opposite end of pH-scale of the kraft process. The lignin isolated from the acid sulfite process is called lignin sulfonate or lignosulfonate. The physicochemical properties of lignosulfonates are affected by presence of cationic base such as calcium, magnesium, sodium, and ammonium.30 Lignosulfonates are typically highly cross-linked. The lignin contains about 5% sulfur by weigh as sulfonate groups. Due to low pKa of sulfonates (pKa≤2), lignosulfonates are water soluble lignins. Lignin recovered through alkaline hydrolysis or soda process is called soda lignin. In this process, the lignin is extracted only with sodium hydroxide and is somewhat sulfur free. Such a cost effective process is often implemented for non-woody agricultural residues. The lignin isolated from soda is often enriched with carboxylic acids and condensed units.31 The former



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is due to the oxidation of aliphatic hydroxyl units while the latter is caused by the absence of sulphide ions allowing the intermediate quinonemethides to lose formaldehyde, causing the formation of undesirable alkaline stable enol ethers.23,24 The term organosolv refers to the most alternative pulping process which utilizes mixtures of organic solvents with water as the pulping media at relatively low pH and elevated temperature. Ethanol is the most commonly used organic solvent in this process. Lignin dissolution results from solvolytic degradation which follows the rapid initial alkoxylation reaction.32,33 The lignin contains considerable amounts of free phenolic hydroxyl groups as well as condensed units. Lignin recovered from this process is often free of carbohydrate contaminants. The sulfur-free lignin has higher purity with lower ash content over other lignin samples. Lignin incorporation in polymers for application of biomaterials25-27 is not limited only by the reactivity of its various chemical composition and functional groups but rather by other complex factors such as solubility, molecular weight, rheology, dispersity, and morphology. For example, it is well known that many polymer properties such as Tg, modulus, and tensile strength are directly dependent upon their molecular weights. Molecular weight or size distribution of different polymeric fragments in lignin impacts their performance during copolymerization.34 The objective of this study was to characterize a number of industrial, technical, and commercially available lignins isolated from kraft, sulfite, soda, and organosolv processes and elucidate their profile by several qualitative and quantitative methods. This was carried out in order to compare their physical and chemical properties aimed at identifying the most


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suitable lignin(s) with similar or better properties in view of partial and/or complete substitution of petroleum-based polymers during industrial production of biomaterials.



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EXPERIMENTAL Materials and Methods Lignin Samples and Reagents. Seven different lignin samples were selected for this study. The first and the second samples were hardwood lignin (L1 & L2). The L1 was provided by a Canadian kraft mill isolated under the conventional kraft pulping process while the L2 was purchased from Aldrich (Catalog No., 371017). The next two samples were lignosulfonates. The sodium lignosulfonate (L3) sample was received from Tembec (Témiscaming, Québec) and the fourth (L4) was purchased from Aldrich (Catalog No., 471038). Indulin AT (L5), was obtained from MeadWestvaco (South Carolina, USA) and the sixth sample (L6) was also purchased from Aldrich (Catalog No., 471003). The last sample (L7) was extracted from wheat straw by the soda process and provided by GreenValue (Lausanne, Switzerland) for this study. All the chemicals and reagents utilized in this study were purchased from SigmaAldrich Chemicals (Oakville, ON, Canada). Methods Characterization of Lignin. Lignin moisture and ash contents were determined gravimetrically according to the ASTM D 2974-87 Standards Test Methods. The elemental analyses of lignin samples were performed by Galbraith Laboratories, Inc., Knoxville, TN/USA. 31

P NMR Spectroscopy. Quantitative

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P NMR spectra of all lignin preparations were

obtained using published procedures.35-37 Approximately 30-40 mg of dry lignin was dissolved in 500 µL of anhydrous pyridine and deuterated chloroform (1.6:1, v/v) under constant stirring. This was followed by addition of 100 µL of cyclohexanol (23.76 mg/mL in pyridine and deuterated chloroform 1.6:1, v/v) used as an internal standard and 50 µL of 8 ACS Paragon Plus Environment

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chromium(III)acetylacetonate solution (5.5 mg/mL in anhydrous pyridine and deuterated chloroform 1.6:1, v/v) used as relaxation reagent. Finally, 100 µL of phosphitylating reagent (2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane, TMDP) was added and the vial was sealed and shaken to ensure thorough mixing. The mixture was transferred into a 5 mm NMR tube for subsequent NMR analysis. In the case of lignosulfonate, the sample was dissolved in 300 µL of dimethylformamide (DMF) followed by addition of 200 µL of anhydrous pyridine and 100 µL of cyclohexanol (23.76 mg/mL in pyridine and deuterated chloroform 1.6:1, v/v) and 50 µL of chromium(III)acetylacetonate solution (5.5 mg/mL in anhydrous pyridine and deuterated chloroform 1.6:1, v/v). Then, 100 µL of phosphitylating reagent (2-chloro-4,4,5,5tetramethyl-1,3,2-dioxaphospholane, TMDP) was added followed by 125 µL of deuterated chloroform.38 All NMR experiments were carried out at 298 K on a Bruker Avance 500 NMR spectrometer operated at 1H frequency of 500.13 MHz and equipped with a 5 mm broadband inverse probe.

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P NMR spectra were recorded with 32768 data points and a spectral width of

60606.06 Hz. A relaxation delay of 5 s was used and the number of scan was 512. The

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P

chemical shifts were referenced with respect to water signal at 132.2 ppm. Spectra were processed and analyzed using Bruker TOPSPIN 1.3 software package. All chemical shifts are reported relative to the product of TMDP with cyclohexanol, which has been observed to give a sharp signal at 145.15 ppm referenced from the hydrolysis product of the phosphorylation agent39 at 132.2 ppm. The content of hydroxyl groups was obtained by integration of the following spectral regions: aliphatic hydroxyls (150.4-145.5 ppm), condensed phenolic units (DPM: 144.4-143.1; 4-O-5': 143.1-141.7 & 5-5': 141.7-140.8 ppm), syringyl phenolic units (143.1-141.7 ppm), guaiacyl phenolic hydroxyls (140.3-138.3 ppm),



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p-hydroxyphenyl phenolics (138.3-137.3 ppm & 136.8-136.5 ppm), carboxylic acids (135.9134.0 ppm). Fourier Transform Infrared. Fourier transform infrared (FT-IR) spectra were obtained for powdered solid lignin on KBr discs using a Bruker Tensor Series FT-IR Spectrometer in the Transmittance (TR) analysis mode. Spectra were collected from 4000 to 400 cm-1 with 64 scans and a 2 cm-1 resolution. Glass Transition Temperature (Tg) and Differential Scanning Calorimetry (DSC). Glass transition temperature of the lignin samples was determined using a Perkin-Elmer Pyris 1 DSC instrument (Woodland, CA/USA) under nitrogen atmosphere. Prior to determination of glass transition temperature trials, the lignins were dried overnight under vacuum at 60-70 °C. During each trial, approximately 10 mg of dry lignin was used in an aluminum pan. The temperature program used for this study is reported as follows: The samples were cooled from room temperature to -40 °C at cooling rate of 20 °C/min, isothermal for 1 minute, then heated to 200 °C at heating rate of 20 °C/min (first measurement cycle), isothermal for 1 minute, cooled to -40 °C at cooling rate of 20 °C/min, isothermal for 1 minute, then reheated to 200 °C at heating rate of 20 °C/min (second measurement cycle). The Tg analysis was duplicated for each lignin sample and reported as the average. Tg is defined as one-half the change in heat capacity occurring over the transition of the second heating run.40,41 Size Exclusion Chromatography. Gel permeation chromatography (GPC) analysis was performed according to the published procedures for samples L1, L2, L5, and L7.42 Gel permeation chromatography (GPC) analysis was performed using a multi-detection system from Agilent Technologies (Model 1200, 76337 Waldbronn, Germany) consisting of a high


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performance liquid chromatography system equipped with a solvent tray, degasser, quaternary pump, autosampler, column heating module, UV diode-array detector and LC 3D software. The additional detectors were a Wyatt (Santa-Barbara, CA/USA) Dawn Heleos II multi angle laser light scattering (MALLS) detector equipped with fluorescence filters on even number detectors, and a Wyatt Optilab rEX refractive index detector. The Astra 5.3.4 software from Wyatt was used for data collection and calibration. Separation was performed with dry BHT-stabilized tetrahydrofuran (THF) by injecting 75 µL of acetobrominated lignin solutions in THF. Each sample was prepared by dissolving 1.0 mg/mL and filtering through a 0.2 µm membrane before injection into the thermostatically (25 °C) controlled columns (300 mm × 7.8 mm) of Styragel HR4, HR4E, HR1, Waters (MA/USA). The flow rate was 1 mL/min. Molar masses of the samples were determined with the MALLS even number detector signals using the specific refractive index increments (dn/dc) of each sample from the refractive index detector. A calibration curve was based on polystyrene standards from Polymer Laboratories (Amherst, MA/USA) and Sigma-Aldrich (Oakville, ON/Canada) using the molar masses determined by the manufacturers for the determination of molar mass based on the elution time from the Agilent UV detector signal at 254 nm. The GPC analyses for samples L3, L4, and L6 were carried out in aqueous medium with an electrolyte (0.1 M NaNO3) and a Waters system: 1525 pump with a flow rate of 1.0 mL/min, a 717 plus auto injector, a column heater maintained at 35 °C containing a Polysep-3000 column (Phenomenex, 30 cm, 8 m), a Polysep-5000 column (Phenomenex, 30 cm, 8 m), a Ultrahydrogel 500 column (Waters, 30 cm, 10 microns) and an index 2410 refractive detector maintained at 35 °C. Breeze software v.3.20 was used for acquisition and data processing. The samples were diluted to a concentration of 10 mg/mL. The samples were then filtered



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through a 0.45 µm nylon membrane of pore size. The molecular weights were determined from a calibration curve with standards, of different molecular weights. The standards used were pullulan; a polysaccharide. The retention time for each chromatogram were corrected with an internal standard ethylene glycol (EG) added to sample and standards. The number of theoretical plate determined with an injection of EG was 15000 for the system. The data for each sample was processed using the software in triplicate including the chromatogram, the calibration curve and the distribution curve. Scanning Electron Microscopy (SEM). SEM images were obtained using JEOL scanning electronic microscope (JSM-6100, Tokyo, Japan) operating at a voltage of 15 kV. Prior to the SEM analysis, a 10-20 nm thin layer of Pd/Au was deposited on the sample using Ted Pella 208 Sputter Coater (CA, USA) in order to minimize the charge effect due to the insulating properties of the samples.


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RESULTS AND DISCUSSION A number of industrial, technical and commercially available lignin samples were selected and characterized by employing different qualitative and quantitative techniques. In this study, seven lignin samples, namely L1-L7 were examined to elucidate their physical and chemical profiles in view of biomaterials applications. The L1 and L2 samples were hardwood lignins extracted by kraft and organosolv pulping processes, respectively. The subsequent four samples were all recovered from softwoods. The L3 and L4 were extracted by sulfite while L5 and L6 were isolated by kraft process. However, the L6 lignin sample was further modified with low sulfonated sodium salt. Detailed information regarding their suppliers, as well as their moisture and ash contents corresponding to each lignin sample is reported in the Table 1. Table 1 Moisture and Ash Content Moisture and ash content analysis of lignin are simple techniques which provide crucial information for absolute determination of lignin weight and its inorganic impurities. In this study, the moisture and ash contents were determined gravimetrically for all the lignin samples. The experimental data shows the L2 sample has the lowest, while the L4 and L6 have the highest moisture and ash content. This is not surprising since the L2 was extracted by organosolv process. The process is known to yield lignin which is hydrophobic and has the higher purity with lowest ash content among other lignin samples. Meanwhile, the L4 and L6 were isolated by sulfite process where the resulting lignosulfonates are known to be hydrophilic lignin and prone to absorb moisture more readily. More specifically, in the case of L6, the percent moisture content was found to increase by more than two folds from 3.70



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± 0.09% to 8.20 ± 0.08% during two and a half years of storage time. However, the most striking feature was its high concentration of inorganic contaminants. Repeated measurements showed that almost two thirds of L6 contains ash and non-organic materials. In general, the lignosulfonates and sulfonated kraft lignin were found to have the highest ash content. Another important factor for elucidating lignin profile is the characterization of its elemental compositions. Since the elemental compositions for various types of lignins are exclusively different, the percent elemental composition for each lignin sample in terms of carbon, hydrogen, nitrogen, oxygen, sulfur, and methoxy content were analyzed. Based on this data, the empirical formulae and the molecular weight of each lignin per C9 unit was calculated and reported in the Table 2. Closer examination shows that lignosulfonate samples (L3 & L4) have the lowest carbon, hydrogen, and methoxy content followed by the sulfonated kraft lignin (L6). The oxygen, sulfur, and the molecular weights for these samples were found to be at the highest. This is somewhat to be expected since during the sulfite pulping process, the substituted sulfonate groups mostly at Cα and Cγ of lignin propanoic side-chains bear three oxygen atoms, leading to higher molecular weight. The organosolv lignin (L2) sample was found to contain the highest percentage of carbon and methoxy contents even when compared to its hardwood counterpart kraft lignin (L1) sample. Considering the monomeric units of both L1 and L2 samples bear two methoxy groups, the L2 was found to have the lowest oxygen content with the lowest molecular weight among other lignins. Apart from lignosulfonates and sulfonated kraft lignin (L6), the sulfur content for kraft lignins (L1) and Indulin AT (L5) were found to be 1.93% and 1.32%, respectively. Determination of nitrogen content in lignin is also important since its quantity varies for different types of industrial


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lignins. The percent elemental nitrogen in order of descending was found for the L3, L5, and L7 samples to be 1.09%, 0.59% and 0.31%, respectively. However, the nitrogen content was calculated in the order of L7, L3, and L5 to contain 0.43, 0.22, and 0.08 nitrogen element per C9 unit, respectively. Table 2 31

P NMR Spectroscopic Analysis

In this study,

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P NMR spectroscopy35-37 was employed for classification and quantitative

determination of several different classes of hydroxyl groups in lignin. The technique has been successfully implemented for the absolute determination of various phenolic and nonphenolic structures in several native and technical lignins.43-45 The availability of a phosphitylating reagent35 that permits excellent resolution and the quantitative spectroscopic detection of several phenolic moieties with varying substitution patterns provide an excellent opportunity for systematically exploring the complex reactions that occurs in lignin during several different pulping processes. The phosphitylating reagent (2-chloro-4,4,5,5tetramethyl-1,3,2-dioxaphospholane) or TMDP is a powerful analytical tool for distinguishing between different phenolic condensed units such as diphenylmethanes (DPM), diaryl ethers (4-O-5'), and biphenolics (5-5'), and as well as benzylic and terminal carboxylic acids in lignin. Figure 1 The

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P NMR spectrum of two hardwood lignin samples isolated by kraft (L1) and

organosolv (L2) processes and their signal assignments is presented in the Figure 1. At a glance, both spectra seem to be alike in terms of their chemical compositions. However,



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closer examination shows that the nature and the content of carboxylic acids, guaiacyl, syringyl, and aliphatic hydroxyl units between the two sets of samples are quite different. The aliphatic hydroxyl and the syringyl units were found to be higher and more uniform in the L1 than the L2 lignin sample while the condensed phenolic hydroxyl units were higher in the L2. This was also true for the guaiacyl units. Although the guaiacyl contents were found to be higher in L2, the S:G ratio for L1 (2.61) exceeded the L2 (1.85). Organosolv lignin was found to contain more than two folds para-hydroxyphenyl units over the kraft lignin. In addition, the presence of catechols recorded at 138.98 ppm in 31P NMR spectrum was more pronounced in the L2 sample than the L1. The content of terminal carboxylic acids for both samples was almost identical, but the kraft lignin contained almost three folds more benzylic carboxylic acids than the organosolv lignin sample. Figure 2 Figure 2 represents the

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P NMR spectrum of two sodium lignosulfonate samples extracted

from softwoods. The spectra of both lignins have unique features over their lignin counterparts isolated by other processes. The aliphatic hydroxyl groups were recorded to be significantly branched and lopsided. One may suspect the presence of carbohydrate impurities in these samples. The content of both condensed phenolics and parahydroxyphenyl structures were determined to be insignificant. The guaiacyl units were also found to be highly non-uniform and entirely different from typical guaiacyl units of softwoods isolated by kraft, soda, and organosolv processes. The benzylic type of the carboxylic acids was found to constitute a small fraction of the overall carboxylic groups. However, the presence of terminal types was determined to be more dominant. This is highly suspected to be caused by the formation of organic acids such as acetic acid during the


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delignification process. The comparison between two sets of data shows that the L3 sample has a higher alphatic hydroxyl content, condensed phenolics, guaiacyl, para-hydroxyphenyl, and carboxylic acids than the L4 lignin sample. Figure 3 The

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P NMR spectrum of two softwood lignin samples is presented in the Figure 3. The

high resolution of 31P NMR spectra and well recorded signals allows for accurate integration and subsequent quantification of their content with TMDP. The L5 spectrum of the Indulin AT lignin is known to be recovered through extraction with kraft process from the pine tree. The spectrum is a typical representation of a kraft softwood lignin, where aliphatic hydroxyl units are depicted by a smooth and uniform hump followed by a well-resolved condensed phenolic units such as diphenylmethanes (DPM), diaryl ethers, namely 4-O-5', and biphenolic 5-5' structures, respectively. In this spectrum, the guaiacyl units were found to be more homogeneous in which the catechols could be easily distinguished for their quantification. This was also true for para-hydroxyphenyl structures with chemical shift of 138.06 ppm. However, this was not the case for the L6 lignin sample. Although, the L6 lignin was isolated by the kraft process from softwood, its 31P NMR spectrum was entirely different from the L5 and somewhat had similar features to the spectra of L3 and the L4 samples. This was solely due to its further modification with low sulfonated sodium salt. The 31P NMR chemical shift of the sulfonate groups is usually recorded at 131.06 ppm. However, in the case of L6 the chemical shift was detected at 131.09 ppm. The aliphatic hydroxyl groups seemed to be less uniform. The condensed phenolic units were less resolved for integration and quantification of their content. In particular, between 143.1-141.7 ppm of 31P NMR region where syringyl 17 ACS Paragon Plus Environment


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units of hardwoods overlaps with the condensed diaryl ethers such as 4-O-5' type structures, an unusual signal was visibly recorded. The –OH content of this signal was almost in parity to its guaiacyl units misleading the softwood lignin for a hardwood. The guaiacyl units and the carboxylic acids were also affected by such a modification and their 31P NMR signals did not resemble or correspond to typical softwood kraft lignins. Griggs46 had strongly suggested that oxidative sulfonation of kraft lignin introduces sulfonic acid groups in the phenolic rings and side chains. He also proposed that such modification would lead to the formation of carboxyl groups by ring opening reactions. The 31P NMR spectral analysis of L6 had lent a hand to similar observations where sulfonate groups seem to be substituted on the ortho position of guaiacyl units transforming them to syringyl-like structures. The reduction of guaiacyl content and the formation of this signal which is assigned to syringyl units of hardwoods confirmed the previous observations of Griggs. Furthermore, the analysis of the spectral region assigned to the benzylic type of the carboxylic acids displayed signal indicative of formation of carboxylate groups during such modification. The quantification of several hydroxyl groups for both lignin samples is reported in Table 3. The comparison between two sets of data strongly suggests that sulfonation process of kraft lignin may lead to reduction of hydroxyl contents in several different lignin moieties, except for syringyl-like structures and terminal carboxylic acids. Table 3 Figure 4 The 31P NMR spectroscopic analysis was not limited to the forestry lignins but rather lignin sample from agricultural residues feedstock was selected as the last sample for this investigation. Wheat straw lignin isolated by soda processes was characterized by employing 18 ACS Paragon Plus Environment

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31

P NMR spectroscopy. The

31

P NMR spectrum of this lignin which is presented in the

Figure 4 displayed distinctive features. Unlike softwood lignin, which is exclusively guaiacyl and hardwood lignin that contains both guaiacyl and syringyl units, the lignin from agroresidues exhibited several distinct signals corresponding to all three different (G:S:H) monomeric components of lignin. The second feature was the presence of several spike-like signals in different regions of the 31P NMR spectra such as free phenolic, guaiacyl and even in the syringyl region. The presence of these signals, which generally are not detected in a typical lignin, may be attributed by either some type of lignin-moieties exclusively from the feedstock or may be created during the isolation processes of the lignin. Finally, high concentration of carboxylic acids, typical of soda lignin47,48 with a similar ratio of the benzylic to the terminal type was detected. In addition to quantitative determination of several different classes of hydroxyl groups in several lignin samples, their quantities were also normalized. This is a recommended approach for determining the contribution of lignin hydroxyl units toward the reaction with other polymeric precursors in consideration of biomaterial fabrications. Table 4 represents the percent distribution of four major lignin functional groups by quantitative

31

P NMR

spectroscopy. The percent aliphatic hydroxyl units were found to be the highest for both the L4 and L3 sample of lignosulfonates, respectively, even though the L3 contained 4.1 mmole/g more aliphatic hydroxyl units than L4 sample. Both hardwood lignin samples (L1 and L2) contained the most non-condensed hydroxyl units due to the presence of both guaiacyl and syringyl units while the most condensed units were present in the Indulin AT. Soda lignin from the agricultural residues was found to have most carboxylic acids. Fourier Transform Infrared (FTIR)



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Infrared spectroscopy is a relatively old technique which has been dynamically useful in the comparison of lignin isolated by different techniques as well as lignin isolated by the same procedure from different species. Assignments of infrared absorption bands in lignin49 are rather empirical due to the amorphous nature of lignin and the complexity of its chemical components and their random linkages. However, general structural assignments to a certain lignin functional groups has been made possible based on a number of lignin-like model compounds and lignin derivatization techniques via acetylation, methylation, deuteration, reduction, sulfonation, catalytic hydrogenation, ethanolysis, etc. The general assignments for some lignin functional groups and their IR absorption bands are summarized in Table 4.50 Table 4 Figure 5 The infrared spectra of L1 to L7 are illustrated in Figure 5. The FTIR spectral analyses of these lignins display common features in several region of IR spectrum corresponding to the presence of different functional groups such as hydroxyl groups, methyl and methylene groups, carbonyl groups, aromatic skeletal bands, as well as ethylenic double bonds in lignin. However, closer examinations provide evidence of some structural rearrangements specific to the nature of the lignin and its recovery during the isolation or modification procedure. For example, softwood lignins generally have bands at 560 and 470 cm-1 and differ from hardwood lignins which absorb at 535 cm-1. Furthermore, softwood lignins show two absorption bands at 855 and 815 cm-1, which is typical characteristic of the guaiacyl ring. In addition, the band intensity for guaiacyl ring breathing with C-O stretching at 1270 cm-1 and the aromatic C-H in-plane deformation band at 1035 cm-1 are more pronounced in softwoods than their hardwood lignins counterparts at 1230 and 1140 cm-1, repectively.49 20 ACS Paragon Plus Environment

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Lignosulfonate acids have, in addition, bands at 655 cm-1, and 540-520 cm-1 due to sulfonic acid groups while kraft or thiolignins exhibit a very weak band at 630 cm-1 which is assigned to a C-S vibration and an absorption band at 2600 cm-1 due to S-H stretching of thiol groups. The IR spectra of L1 and L2 represent hardwood lignins due to the absorption bands at 538 and 534 cm-1, respectively. Furthermore, they can be distinguished from softwood lignins on the basis of their lower relative intensity at 1230 and 1140 cm-1 band which is originated from both guaiacyl and syringyl nuclei. The comparison between L1 and L2 of spectra shows notable differences due to the different methods of lignin recovery. The L1 spectrum shows two sets of splitting band at 1558-1547 cm-1 and 1402-1387 cm-1 due to the aromatic skeletal vibrations. In addition, a number of week bands at 1726, 1717, 1697, 1686, 1661, 1643 cm-1 due to the presence of different classes of carbonyl groups were observed. The presence of carbonyl groups as saturated open-chain ketones, aliphatic carboxyl, α-β unsaturated esters, and aryl aldehyde may be caused by oxidative cleavage and subsequent hydrolysis of their precursors during the alkaline treatment. The IR spectrum of L2 was obtained at much higher intensity than L1. The only significant observation in L2 was the absorption band at 1707 cm1

which is somewhat stronger than its counterpart in L1 lignin. This band is attributed to a β-

ketone carbonyl group51 which is generated upon cleavage of β-aryl ether groups and subsequent acid-catalyzed rearrangement during the organosolv pulping process. L3 and L4 samples are easily identified as lignin sulfonates due to the presence of sulfonic acid groups. Both L3 and L4 display distinct bands at 532 and 656 cm-1 indicative of such a group.49 These bands were also recorded in IR spectrum of low sulfonated kraft lignin (L6). The infrared spectroscopy of L3 and L4 lignin samples reveal similar bands at 12101220 cm-1, 1045 cm-1 which are characteristic of sulfonates. Sodium lignosulfonates show 21 ACS Paragon Plus Environment


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little or no absorption in the carbonyl stretching region, thus indicating no free carbonyl groups. However, a strong absorption band at 1611 cm-1 was recorded for both samples that may be due to carboxylate C=O stretching during their conversion to a sodium salt. This band was also detected in L6 sample, where the softwood kraft lignin was further modified by low sulfonation with sodium salt. The infrared spectra of both L5 and L6 exhibit bands typical of softwood lignins at 560 and 470 cm-1 with lower intensity in L6. Similar phenomenon was also observed for two absorption bands at 822 and 859 cm-1 which are characteristic of guaiacyl ring. The intensity of these bands is directly dependent on the degree of substitution in the 5-position and this may explain their lower intensity in L6. Although both samples were isolated from softwood by kraft process, the IR spectra of these lignins are quite different due to further modification of L6 by sulfonation. In fact, the IR spectrum of L6 resembles both L3 and L5 spectra and may be considered as a hybrid of both kraft and sulfonic acids lignins. Evidence may be provided in the C-O stretching bands in the 1150-1000 cm-1 region which are frequently of value in distinguishing primary (1070-1010 cm-1), secondary (1120-1075 cm-1), and tertiary (1145-1125 cm-1) hydroxyl groups. The absence of any absorption bands for secondary hydroxyl groups in L6 strongly suggests the substitution of sulfonate groups at Cα during the modification of kraft lignin with low sulfonated sodium salt. Meanwhile, the presence of the secondary hydroxyl groups was unequivocally detected in the L5 sample at 1096 cm-1. Furthermore, the carbonyl groups which are known52 as a reaction site during sulfonation process were not detected in L6. However, L5 exhibited an absorption band at 1682 cm-1 due to the α,β-unsaturated carbonyl groups.


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L7 represents agro-residues lignin. The lignin sample was extracted from wheat straw by alkaline soda process. This becomes more evident since its IR spectrum shows a weak absorption band at 621 cm-1. This band is often found to be slightly weaker in soda than kraft lignin indicating the possibility of a weak superimposed C-S stretching band in kraft lignin. No absorption band was observed at 2600 cm-1 that could be ascribed to S-H stretching of thiol groups. However, the IR spectrum shows the presence of high concentration of carboxylic acids at 2650-2460 cm-1 typical of soda lignin. In the region where the stretching frequency of carbonyl groups is observed, a number of weak shoulders were recorded at 1680, 1653, and 1638 cm-1 corresponding to the presence of aromatic acid, ketones, namely, acetoguaiacone, and sodium phenolate of acetoguaiacone, respectively. However, the most strong absorption band was observed at 1605 cm-1 which is due to the carboxylate C=O stretching. Other absorption bands at 1427 cm-1 were assigned to aromatic skeletal vibrations, 1369 cm-1 to C-H deformation (symmetric), 1333 cm-1 to syringyl ring breathing with COstretching, and 1155 cm-1 to aromatic C-H in-plane deformation of guaiacyl type. Infrared spectroscopy is a facile and versatile technique capable of identifying various functional groups and classes of compounds either on its own or in support of other analysis for validation of modified or untreated lignin samples. Scanning Electron Microscopy (SEM) The lignin properties are related not only to the nature of the hydrophilic groups, which depends upon the pulping process, but also to the size of the polymeric pieces, which in turn depends on the recovery technique used to process the black liquors. The grain diameters and relative proportions of the various size distributions of lignin fragments for differently isolated samples plays an important role in view of biomaterial applications. 23 ACS Paragon Plus Environment


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In this investigation, scanning electron micrographs of L1 to L7 lignin samples were obtained at ×1000 magnification with the scale bar of 10 µm and presented in the Figure 6. The L1 and L2 are representing hardwood lignins isolated by kraft and organosolv processes, respectively. The SEM image of L1 shows uniform round-shaped particles sizing from 1.3 µm to agglomerates of 12.7 µm. The image also shows agglomerates of various shaped granulate combined together in different sizes varying up to 22.7 µm. The overall morphology seems homogeneous with particles having smooth surfaces. However, this was not the case for the hardwood organosolv lignin. The SEM image of L2 shows different morphology, platelet-like forms, porous particles that can be grouped in three size categories. The diameter for the smallest particles was measured between 1.3-8 µm. The medium particles were averaging at about 15 µm, and the largest were found to be between 20-50 µm. All the particles were found to have rough surface with sharp edges containing indentations and hallow pores of different diameters and depths. Figure 6 The SEM image of Arbo SO1 lignin (L3) shows spherical-shaped granules with different size and shapes in which some have been collapsed during the isolation process displaying distinct round-shaped indentations. The smaller and well-rounded granules were found to have diameters averaging between 3-9 µm. The structural integrity of these particles was very much intact. The medium-sized granules had diameters ranging from 20 to 30 µm with uniform and smooth surfaces. The SEM image of L4 lignin sample was dominantly composed of medium to large granules and contained little or no round-spherical small granules. The average diameter was estimated between 20 to 40 µm. Much like the L3 sample, collapsed granules with circular indentations of several different diameters and


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depths were identified. However, unlike the L3 sample, the surfaces of all the particles, including the smaller ones were found to be rougher due to the further modification process. The SEM image of Indulin AT (L5) shows a diverse morphology for both the size distribution and the shape of granules. The majority of the particles was not smooth and found to have diameters ranging from 4 µm up to 25 µm. However, a number of large and smoother spherical particles with diameters up to 45 µm were detected. The comparison of SEM images for the L5 and the L6 shows some significant differences between two sets of samples. The L6 granules were mainly composed of large spherical particles with smooth surfaces. Its SEM image clearly shows the cross-section of two collapsed granules having several indentations with various diameters and depths. Although the softwood lignin (L6) was recovered by similar process as the L5 sample, its granules were found to be much similar to the L3 sample due to the postsulfonation of kraft lignin. The SEM image of wheat straw soda lignin shows small, medium, and fairly larger granules with average diameters corresponding to 3 µm, 15 µm, and 30 µm, respectively. The smaller and medium granules seem to be randomly attached together in a non-uniform structures making up a larger mass. The granules do not have a well-defined or spherical shape. However, the surface of all the granules seems to be coarse and porous. The SEM images unequivocally confirm the difference in the morphology of these lignin samples. Considering that the morphology of lignin particles depends primarily on the type of lignin including their extraction and recovery processes, profiling its morphology should help to establish the relationship between its reactivity, processability within particular biomaterials application.



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Size Exclusion Chromatography The molecular weight distribution for L1-L7 was investigated. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) distributions were determined. The emerging information coupled with the molecular weight per C9 lignin unit paved the way to calculate the polydispersity and the degree of polymerization (DP) for each individual lignin. The data as presented in Table 5 shows lignin from agro-residues (L7) has the highest DP, Mw and Mn with polydispersity value closer to unity than any other lignin samples. This was followed by the hardwood organosolv lignin (L2) where similar trend was also observed. However, amongst the remaining forestry lignin samples, hardwood kraft lignin (L1) and the postsulfonated softwood kraft lignin (L6) had the highest and the lowest polydispersity values, respectively. The DP for the L6 sample was determined to be the lowest among other samples suggesting oxidative degradation of lignin macromolecule due to the postsulfonation process. This becomes more evident when comparing the DP of both softwood lignosulfonates (L3 and L4) to the softwood kraft lignins. Table 5 Glass Transition Temperature (Tg) and Differential Scanning Calorimetry (DSC) Glass transition temperature (Tg) of lignin is another important physicochemical parameter which requires special attention when considering lignin utilization in the manufacturing of various bioproducts. Differential scanning calorimetry (DSC) is an established method for determination of glass transition temperature (Tg) of different lignin samples and their derivatives.53 The magnitude of lignin’s Tg will depend on several important factors such as the molecular weight,54,55 amount of water,56-58 extent of hydrogen bonding, presence of carbohydrate impurities,58,59 chemical functionalization, the degree of branching and 26 ACS Paragon Plus Environment

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condensation.60 The presence of moisture in lignin will act as a plasticizer. Plasticization increases the mobility of the lignin polymer and consequently reduces its Tg. Therefore, prior to determination of Tg, the lignin samples were dried overnight under vacuum at 60-70°C to remove as much moisture as possible. In contrast, lignin Tg will increase significantly with increasing the molecular weight of lignin. Glass transition temperature analysis for several different types of lignin samples were reported by Doherty et al.61 Organosolv lignin was found to have the lowest Tg between 9197 °C, followed by steam-explosion and kraft lignins corresponding to 113-139 °C, and 124174 °C, respectively. Furthermore, the Tg for softwood milled wood lignin (138-160 °C) was reported to be higher than its hardwood lignin (110-130 °C) counterpart. Glass transition temperature (Tg) measurements for the L1-L7 lignin samples were carried out under similar conditions and the data is reported in Table 6. Conventionally, Tg of the materials during the DSC trials is obtained and reported from the second heating cycle since the first heating cycle is often employed to eliminate the thermal history stored within the materials. All the samples displayed a single transition temperature except for the L7 sample in which two distinct Tg were detected. This is may be due to the presence of impurities or other lignin fractions from the lignin feedstock. The magnitude of both Tg’s were found to be the lowest among others suggesting that the wheat straw soda lignin from agro-residues has the highest mobility. Table 6 The comparison of the Tg data and its magnitude for several forestry lignin samples shows that hardwood lignins have generally lower Tg values than the softwood lignins. This is in



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agreement with the previous observation where softwood lignins are found to have higher cross-linked chemical structures than hardwoods. In fact, amongst the two hardwood samples,

organosolv lignin was found to have the lowest Tg than its kraft hardwood lignin. Softwood lignins were found to have the highest glass transition temperatures. Within differently isolated softwood lignins, Indulin AT (L5) and Arbo SO1 (L3) were found to have the highest and lowest Tg’s corresponding to the least and the most lignin mobility. Inspite of repeated measurements, a meaningful Tg curve for postsulfonated kraft lignin (L6) sample could not be obtained. Lignin Profiling and Potential Applications Development The lignin properties are related not only to the nature of the hydrophilic groups, which depends upon the pulping process, but also to the size of the polymeric pieces, which in turn depends on the recovery techniques used to process the black liquors. The molecular weight and molecular weight distribution play an important role in determining their compatibilities during the process of chemical polymerization. Many polymer properties such as Tg, modulus, tensile strength, etc. are directly dependent upon their molecular weights. Since lignin is considered as a biopolymer, molecular weight determinations would provide essential information on the molecular size or size distribution of different polymeric fragments in lignin and their physical and chemical properties during copolymerization.62 The application of lignin in biomaterials has been long considered for manufacturing of polyurethanes (PU),26,63-67 phenol formaldehyde resins (PF),68-71 rubber compounds,72-75 and carbon fibers.76-78 In addition, lignin has been considered as reinforcement agent in preparing 28 ACS Paragon Plus Environment

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biocomposites79,80 to improve mechanical properties, thermal and light stability and to provide biodegradability. Particularly, lignin incorporation in PU production63,64,81,82 and lignin-based PF resins (LPF) has been the two most intensively investigated applications in recent years. Lignin substitution in polyurethane modifies the cure rate by contribution of its aromatic groups and improving the degree of cross-linking.64 Several studies have suggested the solubility and the uniformity of the lignin are the two important key factors affecting its role as a substitute for polyols in polyurethane productions.67,83 These factors are governed directly by the presence of different classes of hydroxyl groups causing both electronic and steric effects.84 Among three major types of hydroxyl groups in lignin, namely phenolic hydroxyl, αhydroxyl, and γ-carbinol, phenolic hydroxyl units are the most acidic and reactive groups. However, despite their higher reactivity,85 the benzylic hydroxyl groups were shown to be more reactive than phenolic hydroxyl groups toward diisocynates.86 Lignin extracted under soda cook was reported to contain less phenolic hydroxyl groups with higher condensed structures and carboxylic acids than kraft lignin. On the other hand, kraft lignin, as compared to MWL, was characterized by Gierer et al. to contain higher phenolic hydroxyl group, α-carbonyl, and stilbene structure with lower aliphatic hydroxyl group.87 In several studies, steam explosion lignin and acid hydrolysis have been suggested to be the most and the least compatible in preparation of polyurethane network films. Lignin isolated from other processes such as organosolv, kraft and MWL were reported to decrease in the order of their compatibility during polyurethane copolymerization.88 29 ACS Paragon Plus Environment


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These studies have all concluded that the most suitable lignin for partial polyols substitution should contain high benzylic hydroxyl content such as α- and γ-carbinol units and have little or no phenolic hydroxyl groups. However, this is in direct violation of lignin application for development of thermosetting resins for wood adhesives. In contrast to the lignin requirements for PU synthesis, LPF processes afford lignin whose structural components is essentially abundant with phenolic hydroxyl groups such as para-hydroxyphenyl, catechol, and guaiacyl units. Lignin has been successfully substituted up to 35% to replace petroleum-based phenol in phenol-formaldehyde resin formulations with performance adequate to the commercial resins68,89 in manufacturing particleboard, fiberboard, waferboard, oriented strandboard (OSB), plywood and other adhesives and board products.69-71,90-92 LPF can be prepared by i) reaction of lignin with phenol (phenolation) followed by formaldehyde,88,93 ii) reaction of lignin with formaldehyde (methylolation) followed by phenol,94 iii) reaction of lignin with phenol-formaldehyde,95 or iv) reaction of lignin with phenol-formaldehyde followed by either formaldehyde or phenol-formaldehyde.96,97 The extent of cross-linking between lignin and phenol-formaldehyde is the key factor for preparing lignin-based adhesive formulation with good glueability. The phenolation of lignin and the extent of cross-linking depend on the availability of specific sites on benzene ring and on the propanic units.98 The condensation reaction occurs between o- and p- position of phenols and the Cα of propanic units containing hydroxyl unit, carbonyl group, aryl, or alkyl linkages.99-103


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The introduction of hydroxymethyl groups into the lignin macromolecule by reaction of formaldehyde increase the reactivity of lignin and promote cross-linking.104,105 The crosslinking of the lignin polymer units mainly occurs via a Lederer-Manasse type reaction,94 with side chains being linked by a Tollens reaction.106 Amongst several

different lignins such

as steam-explosion,91 organosolv,96,107-109

lignosulfonates,110-116 sugar cane bagasse,91,117 and others,118-123 kraft lignin from pine has been suggested to be the most amenable to chemical modification with formaldehyde and phenol due to the availability of its reactive sites.27 The lignin profiling in this investigation with regards to the above applications shows that L5 and L7 samples are the most suitable lignin samples as a major component in preparation of biobased LPF resins, respectively. These samples have identified to offer the most available reactive sites for substitution of hydroxymethyl groups on to the o-position of aromatic rings which occurs via a Lederer-Manasse reaction. The hydrophilic L3 sample with the most aliphatic hydroxyl groups was found to be most applicable as a substitute for petroleumbased polyol in polyurethane formulation. However, the large ash content associated with this or other lignosulfonates often hinders their ultimate performance during the PU formulation and processing. The lignosulfonates are often found as fillers posing threat of leaching out when in contact with water. The post-sulfonated softwood kraft lignin or L6 contained the most ash content and may be used as filler or as a partial substitute for applications that requires hydrophilic and hydrophobic interface. The sulfur-free L2 sample contained the highest purity with low ash content, including lower molecular weight per C9 units and lower Tg. These criteria warrant L2 as a major precursor in preparation of carbon fiber which requires similar physical and chemical properties. 31 ACS Paragon Plus Environment


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CONCLUSION Lignin profiling is unequivocally essential step for elucidating different chemical and physical properties when biomass lignin is considered as a component for biomaterials application development. The emerging data from this study further validates the diverse lignin characteristics and confirms that each individual lignin has a unique profile which can be utilized as a substitute for specific biobased applications when selected accordingly. Our investigation shows that L5 and L7 samples were the most suitable lignins as a major component in preparation of biobased LPF resins, respectively. The hydrophilic L3 sample with the most aliphatic hydroxyl groups was found to be most applicable as a substitute for petroleum-based polyol in polyurethane formulation. The sulfur-free L2 sample contained highest purity with low ash content, including lower molecular weight per C9 units and lower Tg. These criteria warrant L2 as a major precursor in preparation of carbon fiber which requires similar physical and chemical properties.


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ACKNOWLEDGMENTS This study was carried out with the support of the National Bioproduct Programs at the National Research Council of Canada. The authors gratefully acknowledge Dr. Jairo H. Lora at

Green

Value

for

providing

the

lignin

samples,

Dr.

Lamfeddal

Kouisni,

Dr. Michael Paleologou, and Alain Gagné at FPInnovations, for providing lignin samples and performing GPC analysis and Sabahudin Hrapovic for useful discussion on SEM. We would also like to acknowledge Mrs. Kathy Gates for helping prepare this manuscript.



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REFERENCES 1. Song, C. Fuel Processing for Low-Temperature and High-Pressure Fuel CellsChallenges, and Opportunities for Sustainable Development in the 21st Century. Catal. Today. 2002, 77, 17-49. 2. Kleinert, M.; Barth, T. Towards a Lignincellulosic Biorefinery: Direct One-Step Conversion of Lignin to Hydrogen-Enriched Biofuel. Energy & Fuels. 2008, 22, 1371-1379. 3. Glasser, W. G.; Barnett, C. A.; Muller, P.C.; Sarkanen, K.V. The Chemistry of Several Novel Bioconversion Lignins. J. Agric. Food Chem. 1983, 31 (5), 921-930. 4. Borges da Silva, E. A.; Zabkova, M.; Araújo, J. D.; Cateto, C. A.; Barreiro, M. F.; Belgacem, M. N.; Rodrigues, A. E. An Integrated Process to Produce Vanillin and LigninBased Polyurethanes from Kraft Lignin. Chem. Eng. Res. Des. 2009, 87, 1276-1292. 5. Wise,

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Benjamin/Cummings: Menlo Park, California, 1983. 6. Argyropoulos, D. S. Materials, Chemicals and Energy from Forest Biomass; ACS Symposium Series 954; American Chemical Society: Washington, DC. 2007. 7. Freudenberg, K. The Constitution and Biosynthesis of Lignin. In Molecular Biology Biochemistry and Biophysics; Kleinszeller, A.; Springer, G. F.; Wittmann, H. G.; Eds.; Springer-Verlag: Berlin-Heidelberg Vol. 2; 1968, pp 47-122. 8. Wardrop, A. B. Occurrence and Formation in Plants. In Lignins. Occurrence, Formation, Structure and Reactions; Sarkanen, K. V., Ludwig, C. H., Eds.; Wiley Interscience: New York, 1971, pp 19-41. 9. Satheesh Kumar, M. N.; Mohanty, A. K.; Erickson, L.; Misra, M. Lignin and Its Applications with Polymers. J. Biobased Mater. Bioenergy. 2009, 3 (1), 1-24.


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Figure Captions Scheme 1.

The elementary monomeric phenylpropanoid structure in lignin.

Scheme 2.

Schematic diagram of phenylpropane building blocks of lignin from various chemical pulping processes.

Figure 1.

31

P NMR spectra and signal assignments of hardwood: kraft (L1) and

organosolv (L2) lignins phosphitylated with TMDP. Figure 2.

31

P NMR spectra and signal assignments of softwood: Tembec (L3) and

Aldrich (L4) sodium lignosulfonates phosphitylated with TMDP. Figure 3.

31

P NMR spectra and signal assignments of softwood: Indulin AT (L5) and

Aldrich low sulfonated kraft (L6) lignins phosphitylated with TMDP. Figure 4.

31

P NMR spectra and signal assignments of wheat straw soda lignin (L7)

phosphitylated with TMDP. Figure 5.

Infrared spectra of differently isolated lignins: (L1) kraft and (L2) organosolv hardwoods, (L3) sodium and (L4) modified sodium lignosulfonates softwoods, (L5) Indulin AT and (L6) low sulfonated kraft softwoods, and (L7) wheat straw soda lignin.

Figure 6.

SEM images of differently isolated lignins: (L1) kraft and (L2) organosolv hardwoods, (L3) sodium and (L4) modified sodium lignosulfonates softwoods, (L5) Indulin AT and (L6) low sulfonated kraft softwoods, and (L7) wheat straw soda lignin.


Page 49 of 66

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List of Tables Table 1.

Specification of various types of lignin and their moisture and ash content.

Table 2.

Elemental analysis, OCH3 content, empirical formulae and molecular weight per C9 lignin unit for different types of lignin.

Table 3.

Quantitative

31

P NMR determination of carboxylic, phenolic, condensed

structures and aliphatic hydroxyl groups present in the lignins. Table 4.

General assignments for some lignin functional groups and their IR absorption bands.

Table 5.

Molecular weight distribution and degree of polymerization for different types of lignin.

Table 6.

Determination of glass transition temperature (Tg) for different types of lignin.



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Page 50 of 66

Where: R = Another phenylpropane unit R1 = H, or OCH3, or OR R2 = H, or R R3 = OH, or R

Scheme 1.

The elementary monomeric phenylpropanoid structure in lignin.


Page 51 of 66

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Internal Standard

Condensed Phenolic Units

Syringyl -OH

Guaiacyl -OH Aliphatic -OH

Carboxylic -OH

p -Hydroxyphenyl -OH

L1

L2

ppm

149

Figure 1.

147

145

143

141

139

137

135

133

31

P NMR spectra and signal assignments of hardwood: kraft (L1) and organosolv (L2) lignins phosphitylated with TMDP.



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Page 52 of 66

Internal Standard

Aliphatic -OH

Carboxylic -OH Water Sulfonate Guaiacyl -OH


L3

≈

p-Hydroxyphenyl -OH

≈ L4

ppm

148.5

Figure 2.

146.5

144.5

142.5

140.5

138.5

136.5

134.5

132.5

130.5

31

P NMR spectra and signal assignments of softwood: Tembec (L3) and Aldrich (L4) sodium lignosulfonates

phosphitylated with TMDP.


Page 53 of 66

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Internal Standard Guaiacyl -OH

Aliphatic -OH


p -Hydroxyphenyl -OH

Carboxylic -OH

L5

L6

ppm

Figure 3.

149

147

145

143

141

139

137

135

133

31

P NMR spectra and signal assignments of softwood: Indulin AT (L5) and Aldrich low sulfonated kraft (L6) lignins

phosphitylated with TMDP.



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Page 54 of 66

Internal Standard

Carboxylic -OH Condensed Phenolic Units Guaiacyl -OH Aliphatic -OH

Syringyl -OH p -Hydroxyphenyl -OH

L7

ppm

Figure 4.

149

147

145

143

141

139

137

135

133

31

P NMR spectra and signal assignments of wheat straw soda lignin (L7) phosphitylated with TMDP.


Page 55 of 66

L7

L6

L5

L4 Transmittance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


L3

L2

L1

4000

Figure 5.

3600

3200

2800

2400 2000 -1 Wavelength, cm

1600

1200

800

400

Infrared spectra of differently isolated lignins: (L1) kraft and (L2) organosolv hardwoods, (L3) sodium and (L4) modified sodium lignosulfonates softwoods, (L5) Indulin AT and (L6) low sulfonated kraft softwoods, and (L7) wheat straw soda lignin.



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L1

L2

L3

L4

L5

L6

Page 56 of 66

L7

Figure 6.

SEM images of differently isolated lignins: L1, L2, L3, L4, L5, L6 and L7 56 ACS Paragon Plus Environment

Page 57 of 66

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Table 1. Sample

Specification of various types of lignin and their moisture and ash content. Lignin Source

Feedstock

Isolation Process

Moisture, %

Ash, %

L1

Canadian Mill

Hardwoods

Kraft

6.05 ± 0.09

0.45 ± 0.04

L2

Aldrich, Catalog No. 371017

Hardwoods

Organosolv

2.40 ± 0.00

0.11 ± 0.01

L3

Tembec, Arbo SO1

Softwoods

Sulfite

4.66 ± 0.03

24.15 ± 0.27

L4

Aldrich, Catalog No. 471038a

Softwoods

Sulfite

6.81 ± 0.04

20.02 ± 0.08

L5

Indulin AT, MeadWestvaco

Softwoods

Kraft

4.66 ± 0.12

3.06 ± 0.30

L6

Aldrich, Catalog No. 471003b

Softwoods

Kraft, low sulfonated

3.70 ± 0.09

66.19 ± 0.18

L7

Protobind 3000, Green Value

Wheat straw

Soda

6.45 ± 0.02

2.12 ± 0.03

a

Modified sodium lignosulfonate. b Sodium salt of low sulfonated kraft lignin.



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Table 2.

Page 58 of 66

Elemental analysis, OCH3 content, empirical formulae and molecular weight per C9 lignin unit for different types of lignin. Elemental Analysis

Lignin Sample

%C

%H

%N

%O

%S

%OCH3

L1

58.12

6.18

Lignin Profiling: A Guide for Selecting Appropriate Lignins as

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