Preparation of Submicron Lignin Particles from Different Lignocellulosic Biomass and Their Modification

Abstract

A very straightforward and technologically and economically viable Material-driven Phase III Lignocellulosic Feedstock Biorefinery (LCF Biorefinery) system has been developed for separating alkali-lignin that complies with the principles of green chemistry. The system has been designed based on a sequential acid-base pretreatment on extractive-free lignocellulosic biomass at the temperature range 150 to 170 oC and successfully applied to fractionate non-woods, e.g., Coconut fibre and Bagasse and woody biomasses, e.g., Trema orientalis and Dipterocarpus turbinatus. Simple operation process and easy separation of the main structural components, i.e., hemicelluloses (22.66 to 27.06% w/w), cellulosic pulps (29.80 to 40.06% w/w), lignin (17.46 to 26.00% w/w) and the recovery of used chemicals, i.e., acid and base (about 100%) along with the degraded organic and inorganic residues from black liquors as valuable nitrogen, phosphorus and potassium (NPK) containing materials in crystalline and liquid forms have made the process as green. Phosphoric acid and potassium hydroxide were used as the acid and base. Isolation and subsequent recoveries of phosphoric acid precipitated alkali-lignin (ALs) from the black liquors were 84.48% for Coconut fibre (CF_AL), 79.56% for Bagasse (BG_AL), 79.34% for Trema orientalis (TO_AL) and 75.22% for Dipterocarpus turbinatus (GW_AL) based on their Klason lignin contents. These amounts were higher than lignin isolated using different ionic liquids, e.g., pyrrolidinium acetate, 1-ethyl-3-methylimidazolium acetate and 1-butyl-3-methylimidazolium chloride. The particle sizes of ball milled alkali-lignin were in submicron range, with medium dispersities (D) ranging from 0.299 to 0.594, as determined by dynamic light scattering (DLS) experiments. Intrinsic viscosities of alkali-lignin in dimethylsulfoxide at 25 oC were 22.7 mL/g for CF_AL; 4.0 mL/g for BG_AL; 16.8 mL/g for TO_AL and 13.1 mL/g for GW_AL. C9-formula of four alkali-lignin samples were C9H7.45O5.88(OCH3)1.06 for Coconut fibre; C9H8.09O3.23(OCH3)1.03 for Bagasse; C9H7.88O3.50(OCH3)1.13 for Trema orientalis and C9H7.80O3.60(OCH3)1.02 for Dipterocarpus turbinatus. X-ray photoelectron spectroscopic (XPS) study showed the alkali-lignin samples were pure with oxygen to carbon ratios ranging from 0.177 to 0.291. Sulfur was absent in all lignin samples as the characteristics of alkali-lignin, but a trace amounts of nitrogen and silicon were present. Based on the high resolution deconvoluted XPS spectra, percentages of carbon and oxygen in C1s and O1s components were estimated using corresponding peak areas and assigned accordingly. Fundamental, overtone and combination bands of the alkali-lignin samples were examined ii | P a g e by Fourier transform mid and near infrared (FT-MIR-NIR) spectroscopic technique. Some agglomeration in the milled lignin samples were observed from Field emission-scanning electron microscopic (FE-SEM) study. All lignin samples showed a weight loss about 15% upto 200 oC and about 30% upto 375 oC. About 20 to 42% carbon materials, i.e., char obtained at 850 oC from the degradation of alkali-lignin samples under inert atmosphere. All lignin samples showed a glass transition temperature of approximately 75 oC, which was suggestive of degraded lignin. Ultraviolet diffuse reflectance spectroscopic (UV-DRS) analysis showed that the powdered lignin samples absorbed radiation in the ultraviolet region. Kubelka-Munk function and apparent absorbance of alkali-lignin samples showed a good agreement with the reference values and exhibited higher absorption coefficient in UV regions, i.e., UV-A 315 to 400 nm; UV-B 280 to 315 nm and UV-C 200 to 280 nm. Surprisingly, CF_AL showed the highest Kubelka-Munk function with the highest apparent absorbance value and absorbed strongly in UV regions, i.e., 224 to 428 nm. Antioxidant activity of all lignin samples were measured by 2,2-diphenyl-1-picrylhydrazyl (DPPH) method and half-maximal inhibitory concentration (IC50) values ranging from 6.02 to 19.57 μg/mL were obtained, whereas ascorbic acid showed IC50 value of 9.52 μg/mL. Overall evaluation of thermal characteristics, antioxidant activity, and ultraviolet light absorption properties of alkali-lignin samples also produced encouraging findings for high-value applications. Depending on the intended applications, lignin's structure could be transformed using a variety of reagents and different conditions, e.g., chemical modifications of lignin. It is known that acetyl bromide (AcBr) has been used to acetobrominate lignin, analytically, in acetic acid medium and subsequently quantify and elucidate the structural features of lignin, including molar mass determination and compositions of monolignols applying derivatization followed by reductive cleavage (DFRC) methodology. Acetyl bromide can modify the lignin structure in mainly three ways, e.g., acetylation, benzylic bromination and potential cleavage of α-ether bonds. No report yet published on the lignin acetobromination, where bromination in the lignin’s aromatic ring occurred concurrently with acetylation and benzylic bromination. On the other hand, in-depth research into the physicochemical properties of acetobrominated lignin has not yet been performed, nor has there been any straightforward method for synthesizing acetobrominated lignin with higher bromine contents. In this investigation we have described a new synthetic route (utilizing a flow chemistry approach) for producing acetobrominated alkali-lignin with higher bromine contents by acetylation, benzylic bromination and the bromination in iii | P a g e aromatic ring of the monolignols, concurrently, using acetic anhydride and bromine in the presence or absence of acetic acid. Bromine levels in the synthesized products were substantially higher (17 to 22% w/w) in comparison to acetobrominated lignin made using conventional method, e.g., reaction with acetyl bromide in acetic acid medium (about 5% w/w). In contrast to conventional method, the new method dramatically increased the percentage of C-Br bonds in lignin, while the amounts of the aromatic rings' C-H bonds reduced, indicating the aromatic ring bromination. Increased amounts of acetyl group in the lignin structure were also observed. High resolution deconvoluted X-ray photoelectron spectroscopic (XPS) analysis supported these facts. 1H-13C Heteronuclear single quantum coherence (HSQC) Nuclear magnetic resonance (NMR) spectroscopic study also showed no significant correlation in the aromatic range. Fourier transform mid and near infrared (FT-MIR-NIR) spectroscopic studies revealed that bromine and acetyl groups replaced the hydroxyl groups from lignin structure and thus the polarity of lignin macromolecule decreased. The acetobrominated samples were highly soluble in acetone, dioxane, tetrahydrofuran and dimethylsulfoxide, but partially soluble in ethanol and insoluble in water. Dilute solution viscometry (DSV), Scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), X-ray diffraction analysis, Simultaneous thermal analysis (STA), UV-Vis spectroscopic analysis, etc., were applied to evaluate and compare the molecular properties of acetobrominated products. Mechanism of the new acetobromination reaction of lignin has also been elucidated. Overall studies demonstrated the potential of new acetobromination method as a lignin modification tool for the commercialization of technical lignin. As the sustainability of lignocellulosic biomass conversion processes largely depend on the minimization of environmental pollution in any forms, thus an efficient conversion of lignocellulosic biomasses into their main structural components, i.e., cellulose, hemicelluloses and lignin with no waste generation is of great importance. To ensure the sustainability of the Material-driven Phase III LCF Biorefinery System, the used pretreatment chemicals, i.e., phosphoric acid and potassium hydroxide along with degraded organics from waste liquors have been recovered as valuable nitrogen, phosphorus and potassium (NPK) containing materials in crystalline and liquid forms. The recovery steps were performed after the separation of alkali-lignin from black liquors by adding ammonium hydroxide and crystallized the acid and bases as salts at room temperature. The salts were characterized by XRD, XPS and SEM-EDX techniques. Analyses showed that the salts crystallized from black liquors were K(NH4)HPO4, iv | P a g e (NH4)2HPO4, KH2PO4 and (K,NH4)H2PO4. The spent liquors obtained after the crystallization step might be used as liquid-fertilizer containing potassium, ammonium and phosphate ions and hydrolyzed biomass constituents. Properties of the liquid fertilizer were also evaluated by different physicochemical methods, including viscosity measurement, moisture content determination, etc. SEM-EDX analysis of the solid contents of liquid fertilizers confirmed the presence of nitrogen, potassium, phosphorus, carbon and oxygen in the materials. Thus, all solids and liquids involved in the biomass conversion process were recovered to achieve higher atom economy (greater than 96%) and ensure economic viability of the biorefinery system. Principles of green chemistry were obeyed in every step of the work. The developed material-driven phase III LCF biorefinery system might add a new dimension in the conversion of lignocellulosic biomass to structural components, i.e., hemicelluloses, cellulose, and alkali-lignin along with the recovery of pretreatment chemicals from waste liquor as valuable NPK containing materials/fertilizers for agricultural applications.