Isolation and Characterization of Polyethylene Glycol (PEG)-Modified Glycol Lignin via PEG Solvolysis of Softwood Biomass in a Large-Scale Batch Reactor

Abstract

We have developed an environmentally benign large-scale (50 kg wood meal per batch) lignin production plant, operating based on acid-catalyzed polyethylene glycol (PEG) solvolysis of softwood biomass. The motivation for the proposed process was to promote technological innovation in biomass utilization systems in Japanese rural areas based on widely abundant Japanese cedar (sugi) biomass. In this study, the process was evaluated by investigating the effects of the source sugi wood meal size and the solvent PEG molecular mass on the yield, chemical structure, molecular mass, and thermal properties of the resultant PEG-modified lignin derivatives, glycol lignins (GLs). Reducing the source wood meal size and PEG solvent molecular mass not only promoted lignin PEGylation but also the subsequent acid-induced chemical rearrangements of the GLs as demonstrated by chemical analyses, 2D NMR, and size exclusion chromatography (SEC). Reducing the source wood meal size and/or increasing the solvent PEG molecular mass enhanced the thermal properties of GLs as determined by thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). We considered that the proposed process can efficiently produce lignin derivatives with substantial control over the chemical structure and thermal properties to meet commercial and industrial needs for lignin-based advanced material production.

Effect of Heat Treatment on the Chemical Structure and Thermal Properties of Softwood-Derived Glycol Lignin

“…Supporting this contention, analytical thioacidolysis, which quantifies lignin monomers released from β-O-4 units upon chemical degradation [31,32], of the heat-treated GL400 samples indicated that the amount of β-O-4 units per Klason lignin (Table 1) decreased with different degrees depending on the treatment temperature and the size of the source wood meal. In consistent with our previous work [16], the monomer yield of control GL400 samples decreased with decreasing source wood meal size (GL400L > GL400M > GL400S). For the same treatment temperature range of 100-220 • C, the GL400S series showed little or relatively minor change, whereas the GL400M and GL400L series showed a sharp decrease in the thioacidolysis monomer yield at treatment temperatures of ≥ 200 • and ≥ 160 • C, respectively (Table 1).…”

“…As reported in our previous study, the particle size distribution of the source wood meal (JC) strongly affects the molecular mass, chemical structure, and thermal properties of the resultant GLs. For example, a greater source wood meal size (an average size of 0.4-1.6 mm was applied in the present study range) gives rise to GLs with a higher average molecular mass, higher yield of β-O-4-linked units, greater degree of PEGylation, and slightly reduced thermal transition temperatures, such as glass transition temperature (T g ), viscous thermal flow temperature (T f ), decomposition starting temperature (T dst ), and maximum degradation temperature (T dmax ) [16]. Hence, three types of control GL samples, denoted as GL400S, GL400M, and GL400L (Table 1), produced from PEG400 solvolysis of JC-S, JC-M, and JC-L, respectively, were selected for thermal treatment.…”

“…For the same treatment temperature range of 100-220 • C, the GL400S series showed little or relatively minor change, whereas the GL400M and GL400L series showed a sharp decrease in the thioacidolysis monomer yield at treatment temperatures of ≥ 200 • and ≥ 160 • C, respectively (Table 1). Thioacidolysis data therefore suggest that the source wood meal size used for GL production affect not only the amount of β-O-4 units in GL400 [16] but also their chemical rearrangements upon the heat treatment. (Table 1) decreased with different degrees depending on the treatment temperature and the size of the source wood meal.…”

“…Utilizing the abundance of Japan's softwood plantation forests, 50% of which are occupied by Japanese cedar (Cryptomeria japonica; JC), we have developed a large-scale (50 kg wood meal per batch) lignin production process based on technically feasible and environmentally benign acid-catalyzed polyethylene glycol (PEG) solvolysis of JC. A test plant was developed in 2015 at the Forestry and Forest Products Research Institute (FFPRI), Tsukuba, Japan, under a project of the Strategic Innovation Promotion (SIP) Lignin Research Consortium [16]. JC wood meals with various particle size distributions (JC-S < JC-M < JC-L) and liquid PEG with various molecular masses (PEG200 < PEG400 < PEG600) are typically used as starting raw materials to produce PEG-modified lignin, namely, glycol lignin (GL).…”

“…It has been found that the chemical structure of GLs contains three main intermonomeric linkage types, i.e., α-PEG-β-O-4, β-5, and β-β linkages, indicating that the substitution of PEG moieties occurs primarily at the benzyl (α) carbon in α-OH-β-O-4 units in the source JC lignins during PEG solvolysis, giving rise to α-PEG-β-O-4 units. The PEGylation of lignin exhibits a viscous thermal flow at temperature above the glass transition state, providing suitable space for the molecular motion of rigid lignin side chains [16,17]. This type of thermal fusibility is crucial for thermal processing in material fabrication.…”