Isolation and Characterization of Polyethylene Glycol (PEG)-Modified

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Isolation and Characterization of Polyethylene Glycol (PEG)-modified Glycol Lignin via PEG Solvolysis of Softwood Biomass in a Large-scale Batch Reactor Thi Thi Nge, Yuki Tobimatsu, Shiho Takahashi, Eri Takata, Masaomi Yamamura, Yasuyuki Miyagawa, Tsutomu Ikeda, Toshiaki Umezawa, and Tatsuhiko Yamada ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00965 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018

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ACS Sustainable Chemistry & Engineering

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TITLE

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Isolation and Characterization of Polyethylene Glycol (PEG)-

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modified Glycol Lignin via PEG Solvolysis of Softwood Biomass in a

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Large-scale Batch Reactor

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AUTHORSHIP

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Thi Thi Nge,†* Yuki Tobimatsu,‡ Shiho Takahashi,† Eri Takata,† Masaomi Yamamura,‡

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Yasuyuki Miyagawa,‡ Tsutomu Ikeda,† Toshiaki Umezawa,‡§ Tatsuhiko Yamada†*

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†

Center for Advanced Materials, Forestry and Forest Products Research Institute, 1 Matsunosato,

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Tsukuba, Ibaraki 305-8687, Japan

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‡

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0011, Japan

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§

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Kyoto 611-0011, Japan

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*Corresponding Authors: [email protected]; [email protected]

Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto 611-

Research Unit for Development and Global Sustainability, Kyoto University, Gokasho, Uji,

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ABSTRACT

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We have developed an environmentally benign large-scale (50 kg wood meal per batch) lignin

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production plant, operating based on acid-catalyzed polyethylene glycol (PEG) solvolysis of

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softwood biomass. The motivation for the proposed process was to promote technological

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innovation in biomass utilization systems in Japanese rural areas based on widely abundant

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Japanese cedar (sugi) biomass. In this study, the process was evaluated by investigating the

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effects of the source sugi wood meal size and the solvent PEG molecular mass on the yield,

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chemical structure, molecular mass, and thermal properties of the resultant PEG-modified lignin

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derivatives, glycol lignins (GLs). Reducing the source wood meal size and PEG solvent

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molecular mass not only promoted lignin PEGylation but also the subsequent acid-induced

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chemical rearrangements of the GLs as demonstrated by chemical analyses, 2D NMR, and size

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exclusion chromatography (SEC). Reducing the source wood meal size and/or increasing the

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solvent PEG molecular mass enhanced the thermal properties of GLs as determined by

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thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). We considered that

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the proposed process can efficiently produce lignin derivatives with substantial control over the

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chemical structure and thermal properties to meet commercial and industrial needs for lignin-

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based advanced material production.

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KEYWORD

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Glycol lignin, Polyethylene glycol (PEG) solvolysis, Wood meal size, 2D HSQC NMR,

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Thioacidolysis, Thermal flow property

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INTRODUCTION

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Lignin is an amorphous polyphenolic macromolecule with a range of complex chemical

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structures and accounts for 20%–35% of lignocellulosic biomass, representing the most abundant

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aromatic biopolymer on earth.1 In nature, lignin plays a vital role in providing plant cell walls

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with essential mechanical support, waterproofing, and resistance to pathogen attacks.2 In our

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society, lignin is regarded as a major byproduct in chemical wood pulping and other industrial


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biomass processing plants, where the polymer is typically used as an internal energy supply to

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recover the inorganic chemicals used in the pulping process and /or to cover heat and power

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demands for bioethanol production.3 Technical lignins recovered from the downstream processes

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show a diverse range of physicochemical properties depending on the biomass sources,

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processing conditions, and method of isolation.4,5 Mainly because of these heterogeneous

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features, high-value applications of technical lignins are still limited despite massive quantities

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being potentially available for further use; the annual production of technical lignins from pulp

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and paper industries exceeds 70 million tons worldwide.6 The development of biomass

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processing methods to isolate lignin in high yields and with low contamination and substantial

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structural homogeneity is thus a pressing issue for realizing lignin as a commercially relevant

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feedstock for a range of high-value material applications. Numerous reports on various solvent

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extraction and fractionation strategies to isolate lignin from biomass have been reported,7-11 and

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some of them, for example, LignoBoost,7 LignoForce,8 and CIMVBiolignin processes9 all of

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which are integrated in chemical pulping processes, have been realized in large-scale plants.

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However, there have been few attempts to directly isolate lignin from woody biomass as a major

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product on a large-scale level.

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In hardwood and grass biomass, lignins are typically mixtures of three different types of

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phenyl propane units, i.e., p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, with

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different ratios depending not only on plant species, but also on the climate and geographical

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region of plant growth as well as the location within the plant body.12-13 Conversely, softwoods

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are known to contain lignins almost exclusively composed of G units, which makes it possible to

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produce lignin products with less structural heterogeneity in terms of the aromatic unit

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composition. In light of this, we recently reported a lab-scale (50 g wood meal) isolation method


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for lignin derivatives directly from softwood Japanese cedar (Cryptomeria japonica).14 Japanese

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cedar, or sugi, represents approximately 50% of softwood plantation forest in Japan and is thus

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the most promising biomass feedstock for industrial-scale lignin production in the country. Here,

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we introduce a large-scale (50 kg wood meal/batch) lignin production plant through a technically

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feasible and environmentally benign acid-catalyzed polyethylene glycol (PEG) solvolysis

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process, which has been developed under the Strategic Innovation Promotion (SIP) Lignin

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Research Consortium.15 The objective of the SIP-Lignin research consortium is to develop an

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alternative new lignin business platform that uses abundant lignin biomass resources to promote

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rural area economic development.

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The proposed acid-catalyzed PEG solvolysis process is a re-evaluation of the known

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organosolv delignification process, which uses either low or high boiling point solvents.16-17 The

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high boiling solvents, such as glycerol/PEG and ethylene glycol, with an acid catalyst (3 wt%

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based on solvent) were reported as a suitable method for liquefaction of a wide variety of wood

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species.18-19 The choice of a non-volatile PEG solvent not only enable processing under

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atmospheric pressure but also promotes concurrent introduction of PEG into the isolated lignin

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polymers resulting in PEG-modified lignin derivatives, here named as glycol lignins (GLs). We

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previously demonstrated that GLs prepared by our preliminary lab-scale PEG solvolysis process

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exhibited viscous thermal flow properties.14 These features are not generally shown by

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conventional industrial softwood lignins without further processing steps; grafting soft PEG

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chain segments enables viscous thermal flow above the glass transition state by providing

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suitable space for molecular motion of rigid lignin side-chains. Potential applications of PEG-

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modified thermoplastic lignins have been pursued for fabrication of carbon fibers20 and

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organic/inorganic composites21-22 with high thermal stability.


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In the present study, we developed a large-scale (50 kg wood meal/batch) lignin

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production plant based on the one-step PEG solvolysis process as a key step toward its potential

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industrial implementation. The effectiveness of the plant process was demonstrated through

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investigation of the effects of the source sugi wood meal size and the solvent PEG molecular

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mass on lignin isolation yield as well as the polymer structure and properties of GLs. Detail of

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the chemical structure of GLs was investigated with the use of 2D HSQC NMR approaches. The

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knowledge obtained in this study contributes to potential industrial implementation of the one-

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step PEG solvolysis process as an alternative biorefinery platform that isolates high-quality

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lignin derivatives as a main product.

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EXPERIMENTAL SECTION

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Production of GLs. A large-scale batch reactor with a helical blade ribbon impeller installed at

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the Forestry and Forest Products Research Institute (FFPRI, Tsukuba) was used for the PEG

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solvolysis reactions (Scheme 1). Air-dried sugi wood meal obtained from different processing

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sources; Sugi-L, Sugi-M, and Sugi-S (average meal size, Sugi-L > Sugi-M > Sugi-S) with

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moisture contents of 13 ± 0.6 %, 6 ± 1.8 %, and 9 ± 2.8 %, respectively, were used (Figure 1A

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and Table S1); these source wood meals were byproducts of local timber producers and used as

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received. Liquid PEG with various molecular masses (PEG200, PEG400, and PEG600) and 0.3

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wt% sulfuric acid (based on solvent) were used as the solvolysis reagent and catalyst,

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respectively. The mixing ratio of wood meal (46 Kg, dry basis) to solvent PEG was 1 to 5 by

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weight. The acid-catalyzed PEG solvolysis was conducted at 140 °C for 90 min based on the

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predetermined results of solvolysis reaction time on Sugi-L. After addition of an appropriate


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volume of aqueous NaOH (0.2 M) as a diluent to enhance the lignin solubilization, the solvolysis

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product was subjected to a solid-liquid separation process. The solid residue was mainly

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composed of polysaccharide components, while the liquid part, which was transferred to a pH

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adjustment tank, was lignin-solubilized PEG solvolysis liquor. The liquid part was then acidified

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by slow addition of 3 N sulfuric acid under constant stirring. The precipitated GLs were collected

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by centrifugation followed by washing and vacuum-drying for further analyses.

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Chemical Analyses. Klason lignin and neutral sugar compositional analyses were performed

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according to the standard NREL procedures23 with minor modifications (further described in

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Supporting Information). Analytical thioacidolysis was performed to quantify β–O–4-linked

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lignin units in GLs as described previously.24-26 For each of the above chemical analysis,

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duplicate or triplicated runs were performed, and the average values were reported. The lignin

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yield based on the percent composition of Klason lignin in GL and PEG/lignin composition ratio

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calculations were performed on a weight basis with the following equations: % =

× 100 1

/ !" # % =

100 − × 100 2

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where KLGL and KLSWM represent the Klason lignin contents of GL and source wood meals,

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respectively. Trace amounts of acid-soluble lignin and residual carbohydrates in GL samples

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(Table S2) were excluded for the composition ratio calculation.

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2D NMR. Extractive-free cell wall residue27 and purified milled-wood lignin (MWL) samples28-

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For NMR analysis, cell wall residue samples (ca. 60 mg) were ball-milled and swelled in

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dimethylsulfoxide (DMSO)-d6/pyridine-d5 [4:1 (v/v), 600 µL], and GL and MWL samples (ca.

were prepared from Japanese cedar wood meal (Sugi-L) by previously described procedures.


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15 mg) were dissolved in DMSO-d6 (600 µL). NMR spectra were acquired on a Bruker Biospin

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Avance III 800US spectrometer fitted with a cryogenically cooled 5-mm TCI gradient probe.

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Adiabatic 2D 1H–13C short-range correlation (HSQC) experiments were performed using the

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standard Bruker implementation (“hsqcetgpsp.3”) with parameters described by Mansfield et

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al.30 Data processing used Bruker TopSpin software and the central DMSO solvent peaks (δC/δH:

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39.5/2.49 ppm) were used as an internal reference. HSQC plots were obtained with typical

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matched Gaussian apodization in F2 and squared cosine-bell apodization and one level of linear

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prediction (16 coefficients) in F1. For contour integration analysis (Figure 2), well-resolved Cα–

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Hα contours from I, II, III, IV, V, and I´, and OMe and P2 contours were integrated.  

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Size Exclusion Chromatography (SEC). The SEC analysis of GLs and a sugi milled-wood

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lignin (MWL) sample was conducted with a Shimadzu Prominence LC-20AD system equipped

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with UV (280 nm) and refractive index (RI) detectors, and a two-column sequence of Shodex

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KD-802 and KD-804. The HPLC grade N, N-dimethylformamide (DMF) with 10 mM LiBr was

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used as eluent (1.0 mL min−1 at 40 °C). The Asahipack GF-210HQ×2 column with RI detector

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was used for SEC analysis of solvent PEGs, where an aqueous 40 mM LiBr/DMF (50/50, v/v)

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was used as eluent (0.5 mL min-1 at 50 °C). The molecular mass calibration was performed using

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Fluka polyethylene glycol/poly(ethylene oxide) standard ReadyCal sets (Sigma-Aldrich Buchs-

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Switzerland). GLs and standard samples were completely dissolved in the eluent and filtered

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through 0.45 µm PTFE syringe filter (ADVENTEC) prior to injection to the system.

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Thermomechanical Analysis (TMA). A Q400 TMA (TA Instruments, U.S.A.) was used to

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determine the glass transition temperature (Tg) and the viscous thermal flow temperature (Tf)

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under a nitrogen environment (100 mL min−1). A vacuum-dried finely ground powder sample

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(7–8 mg) was loaded in a TGA platinum sample pan (ϕ 6 × 2.5 mm). A flat aluminum plate (ϕ 4


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mm) was placed on top of the sample. The assemble was set between a quartz stage equipped

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with a thermal sensor and a movable probe with a contact diameter of 2.54 mm. The sample was

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heated from room temperature to 240 °C at a heating rate of 5 °C min−1 under an applied load of

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0.05 N. The volume changes occurred within GL samples while heating was recorded as the

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distance moved by the probe. The duplicate data set were analyzed using TA universal analysis

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software, where Tg and Tf are identified as onset temperature.

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Thermogravimetric Analysis (TGA). A Q500 TGA, (TA Instruments, USA.) was used to

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determine the thermal stability of GLs under a nitrogen atmosphere (60 mL min−1 for sample

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chamber and 40 mL min−1 for balance chamber). Approximately 7–8 mg of GL was loaded on

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100 µL aluminum pan. The heating program was set as initially heated to 105 °C with 10 °C

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min−1 and held isothermally for 20 min, followed by heating to 850 °C at a rate of 10 °C min−1.

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The decomposition starting temperature (Tdst) and the maximum decomposition temperature

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(Tdmax), temperature at 5 wt % and maximum weight loss, respectively, were determined from

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duplicate data set by the TA universal analysis software. Tdst was recalculated after fixing the

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weight at 105 °C.

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RESULTS AND DISCUSSION

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As described in Scheme 1, there are three main subprojects that work jointly together under the

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SIP-Lignin research consortium: subproject 1: the development of the GL production process

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and supply system; subproject 2: GL-based materials fabrication; subproject 3: effective

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utilization of the cellulose-rich solid residues. The focal point of this study is the subproject 1

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with regard to the development of the GL production system. The PEG-modified GL production


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via acid-catalyzed PEG solvolysis of softwood biomass in a large-scale batch reactor was

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designed based on our previous process trials under lab-scale conditions.14 In light of

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environmental and economic benefits, the criteria used for our proposed process are (a) to

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produce lignin in a high yield with low contamination, a substantial structural homogeneity and

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controllable thermal properties, (b) to design a simple processing system using safe chemical

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reagents (such as PEG) within an unpressurized reactor that can be performed under atmospheric

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condition, and (c) to enable chemical recovery and recycling systems to be used for follow GL

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production processes (Scheme 1). In this study, the criteria (a) and (b) were explored in terms of

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the effects of the two main starting materials, i.e., source softwood biomass and solvent PEG.

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For the former, we used Japanese cedar sugi wood meals, Sugi-S, Sugi-M, and Sugi-L, with

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different particles size distributions - [average meal size, Sugi-L (1.6 mm) > Sugi-M (0.8 mm) >

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Sugi-S (0.4 mm)] (Figure 1A); we confirmed that lignin and polysaccharide composition of these

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three sugi wood meals were overall similar (Table S1). For the solvolysis media, we used liquid

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PEG with different molecular masses, i.e., PEG200, PEG400, and PEG600. The other major

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processing parameters, e.g., sulfuric acid catalyst concentration, solvolysis temperature and

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duration, lignin isolation regimes, etc., were determined based on our previous process trials at

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lab-scale conditions.14 As for recycling of solvent PEG, we recently reported a lab-scale GL

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production process using recycled PEG, where the solvent was recovered by removal of water

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via distillation and mixed with a small amount of fresh PEG for being re-used in the subsequent

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GL production process; 31 a large-scale PEG recycling process to be integrated into the present

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batch-scale GL production system is currently under investigation. The GL samples obtained in

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this study were named GL200L, GL400L, and GL600L prepared from Sugi-L, GL200M,


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GL400M, and GL600M prepared from Sugi-M, and GL200S, GL400S, and GL600S prepared

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from Sugi-S, with PEG200, PEG400 and PEG600, as a solvolysis media, respectively.

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Lignin Yield. Lignin yield (dry wood basis) was determined based on Klason lignin analysis of

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the isolated GL samples and the source wood meals (see experimental). The yield varied

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between 40–70%, depending on the source wood meal size as well as the solvent PEG molecular

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mass (Figure 1B). Overall, the yield tended to increase with decreasing source wood meal size

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and with decreasing PEG solvent molecular mass. Consequently, the highest lignin isolation

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yield (~70%) was achieved when with Sugi-S and PEG200 as the source wood meal and the

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solvolysis media, respectively. Based on Mn values of PEG200 (197), PEG400 (396), and

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PEG600 (583) determined by SEC analysis, 1168, 581, and 395 moles of PEG were charged

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together with 46 Kg of the source wood meals in PEG200, PEG400, and PEG600 solvolysis

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reaction, respectively. Therefore, PEG200 has substantially higher mole counts of reactive

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hydroxyl groups than those of PEG400 and PEG600. Our data indicated that reducing the source

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wood meal size and/or PEG molecular mass improved the conversion of the softwood lignin into

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GL, which could be attributed to the increased reactivity of the wood particle substrates and PEG

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in the lignin solvolysis reactions. Consequently, the lignin contents (dry wood basis) in GLs and

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the cellulose-rich solid residues were 19-22% and 5-14%, respectively, while acid-soluble lignin

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in the supernatant fraction was 3-5% depending on the source wood meal size (Figure 1C).

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Neutral sugar analysis detected only trace amounts (< 3%) of residual carbohydrates in all the

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GL samples tested in this study, suggesting that they were mainly composed of derivatized lignin

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and substituted PEG moieties (Table S2). Thus, we considered that the GLs produced from the

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current large-scale batch reactor solvolysis process were suitable to be used as lignin feedstock

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for industrial applications, whereas the cellulose-rich solid residues may be either subjected to


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the saccharification and subsequent bioprocessing for sugar-derived chemicals, or used directly

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in lignocellulosic-thermoplastic composites fabrication (Scheme 1).

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Chemical Structure of GLs. The PEG/lignin composition ratio (wt/wt) calculated based on

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Klason lignin analysis (see experimental) showed a tendency to decrease with decreasing source

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wood meal size (Table 1). The PEG/lignin composition ratio of GL200M was higher than that of

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GL200L, which could be due to the enhanced reactivity of the wood particle substrates with

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decreased source wood meal size as discussed above. However, the PEG/lignin composition ratio

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of GL200S substantially lower than those of the other two GLs (Table 1). As further discussed

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below, acid-induced structural rearrangements, including a partial liberation of PEG moieties

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from GL, might prevail in the solvolysis with decreased source wood meal size. The PEG/lignin

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composition ratio increased with elevated PEG molecular mass (Table 1). For further in-depth

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structural characterization of GLs, 2D HSQC NMR and analytical thioacidolysis were performed

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(Figure 2 and Table 1). The HSQC NMR spectrum of the purified source sugi lignin (milled-

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wood lignin, MWL, prepared from Sugi-L) displayed signals from the major lignin inter-

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monomeric units and the polymer chain end-units typical in softwood lignins. Volume

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integration analysis determined that the source sugi lignin is most rich in α–OH-β–O–4 (I),

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which accounted for approximately 70% of the total identifiable inter-monomeric linkages

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(based on their contour signal intensities; see experimental). There were also more modest

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amounts of β–5 (II) and β–β (III) present and minor amounts of β–O–4/5-5 (IV) and β–1 (V)

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units (Figure 2F) as is typical for softwood lignins.27, 32 In contrast, the α–OH-β–O–4 (I) signals

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abundant in the original sugi lignins were depleted to undetectable levels in all the spectra of the

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GLs obtained in this study (Figure 2A-E). Instead, new and intense signals assigned to α–PEG-

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β–O–4 units (I´) and substituted PEG chains (P) were clearly seen; the chemical shifts of I´


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signals [Cα–Hα correlations (I´α) at δC/δH ~80/~4.5; Cβ–Hβ correlations (I´β) at δC/δH ~83/~4.3]

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were consistent with the reported values for those of β–O–4 lignin model compounds bearing α-

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alkyl-ethers.33-35 In addition, intact II and III signals were clearly visible in all the GL spectra,

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while IV and V along with I signals were no longer detectable. These NMR data suggested that

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α-OH-β–O–4 units in the source sugi lignins were completely processed during the PEG

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solvolysis, giving rise to GLs containing α-PEG-β–O–4 as well as intact β–5 and β–β linkages as

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the major inter-monomeric linkage types. Scheme 2 shows a proposed reaction scheme for acid-

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catalyzed α-PEGylations that convert α–OH-β–O–4 to α-PEG-β–O–4 linkages. Earlier studies on

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lignin organosolv processes proposed that acid-catalyzed alkylations may also occur partially on

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the γ-positions;36 within our investigation with 2D NMR, however, we are not entirely convinced

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of the presence of γ-PEGylated linkages in GLs at this moment. The unannotated correlation

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peaks appearing in the HSQC spectra of GLs (colored in grey) could be unresolved γ-

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correlations and also possibly from new substructures which had been generated through acid-

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induced chemical rearrangements (Scheme 2).

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The signal ratio of the PEG contours per lignin methoxyl contours (P2/OMe), which

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reflects the degree of PEGylations of the GLs, showed a tendency to decrease with decreasing

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source wood meal size and also with increasing PEG solvent molecular mass. In addition, we

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observed notable depletion of the three major inter-monomeric linkage signals (I´, II, and III) in

267

the spectrum of the GL obtained with decreased source wood meal size (Figure 2A-C) as well as

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in the spectrum of GL with increased PEG molecular mass (Figure 2A, 2D and 2E). Consistent

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with this, analytical thioacidolysis, which quantifies lignin monomers released from β–O–4 units

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upon chemical degradation,24, 37 determined that the amounts of β–O–4 units per Klason lignin in

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GL considerably decreased with decreasing source wood meal size and also with increasing PEG


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solvent molecular mass (Table 1). These data collectively suggested that, besides PEGylations of

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lignin side-chains, substantial structural rearrangements of GL, possibly lignin linkage cleavages

274

and condensation reactions that typically occur under acidic conditions (Scheme 2),38 proceeded

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with decreasing source wood meal size and increasing PEG molecular mass. The relative

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proportions of α-PEG-β–O–4 signals (I´) as a fraction of all the detectable inter-monomeric

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linkage signals were notably decreased in the spectrum of GL200S (Figure 2C) compared with

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those in the spectra of GL200L and GL200M and also in the spectrum GL600L compared with

279

those in the spectra of GL200L and GL400L (Figure 2). Hence, α-PEG-β–O–4 linkages were

280

likely more susceptible to such acid-induced chemical rearrangements compared with the β–5

281

and β–β linkages.

282

Molecular Mass Distributions of GLs. SEC analysis determined that both weight average

283

molecular weight (Mw) and number average molecular weight (Mn) of GLs were markedly

284

decreased in the solvolysis with smaller source wood meal size, where Mw values determined for

285

GL200S, GL400S, and GL600S decreased to approximately 32%, 40%, and 47% of those

286

determined for GL200L, GL400L, and GL600L, respectively (Table 1). The molecular mass

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values of GLs prepared from Sugi-L and Sugi-M generally decreased with increased solvent

288

PEG molecular mass (Table 1). These results are in line with the structure analysis data above

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showing that decreasing source wood meal size and/or increasing solvent PEG molecular mass

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promoted acid-induced lignin linkage cleavage (Scheme 2). However, the molecular mass values

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determined for GLs prepared from Sugi-S, slightly increased as the PEG molecular mass

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increased (Table 1), although their molecular mass values were all considerably lower than those

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of GLs prepared from Sugi-L and Sugi-M; we confirmed that the weight-average molecular mass

294

values of these GLs were still higher than that of the sugi MWL control (Mw = 4120; Mn = 1880;


13


Page 14 of 32

295

PD = 2.2). SEC profiles of all the GL samples tested in this study displayed bimodal distribution

296

curves indicating the presence of two major polymer fractions with different molecular mass

297

distributions (Figure 3A); similar SEC profiles were also recorded for other organosolv lignin

298

preparations.39

299

Thermal Properties of GLs. The thermal behavior of lignin derivatives is important for their

300

industrial applications because it greatly affects their processability and thermal stability.40-41

301

Here, the thermal behavior of GLs produced by the current large-scale batch reactor solvolysis

302

process with different source wood meal size and solvent PEG molecular mass was investigated

303

with the use of TMA and TGA measurements. All the TMA profiles of GLs obtained in this

304

study, except that recorded for GL200L, displayed two distinct inflection points corresponding to

305

the glass transition temperature (Tg) and viscous thermal flow temperature (Tf), as shown in

306

Figure 3B. The Tg corresponding to the first inflection point was determined to be in the range

307

from 100 to 144 °C and the Tf corresponding to the second inflection points to be from 138 to

308

174 °C (Table 2). Apparently, both Tg and Tf tended to decrease with increased PEG solvent

309

molecular mass with no apparent correlation to the source wood meal size. Within the chemical

310

structure and molecular mass parameters determined in the present study (Table 1), both Tg and

311

Tf showed positive and negative correlations with the Klason lignin and PEG/lignin composition

312

ratio, respectively. These results suggest that the PEG moieties play an important role in

313

promoting chain segmental motion of GL upon heating; the higher the degree of PEGylation or

314

the higher the molecular mass of the PEG moieties introduced to lignins, the lower are the

315

thermal flow temperatures of the resultant GLs. Notably, no inflection point corresponding to Tf

316

was detected in the TMA profile of GL200L (Figure 3B and Table 2). The viscous thermal flow


14

Page 15 of 32 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 49 50 51 52 53 54 55 56 57 58 59 60


317

of GL200L might be hindered by large molecular mass fractions (> 104) present in greater

318

amount in this sample than in others as determined by SEC (Figure 3A and Table 1).

319

Finally, TGA was conducted to determine the thermal stabilities and compositional

320

properties of GLs. The TGA-derived degradation starting temperature (Tdst), temperature at the

321

maximum weight loss (Tdmax), and char residue mass remaining at 800 °C of GLs are listed in

322

Table 2. Apparently, both Tdst and Tdmax showed a tendency to increase with decreasing source

323

wood meal size and also with increased solvent PEG molecular mass (Table 2). As

324

aforementioned, decreasing source wood meal size and/or increased solvent PEG molecular mass

325

promote(s) acid-induced chemical rearrangements of lignin linkages that possibly lead to GL

326

with thermally stable condensed lignin substructures (Scheme 2). In addition, increasing PEG

327

solvent molecular mass leads a higher thermal stability of PEG moieties introduced into GLs;

328

reported Tdmax values for PEG600 and PEG200 are 374 and 202 °C, respectively,14 Overall,

329

promoting the PEG solvolysis process by reducing the source wood meal size and/or by

330

increasing the solvent PEG molecular mass can enhance the thermal stability of GLs.

331

Importantly, the Tdst observed for all the GLs obtained in this study (219–299 °C) indicated that

332

their high thermal stability was sufficient to withstand typical polymer thermal processing

333

temperature used for molding and blending.40-42 As one such example, we have recently reported

334

the production of flexible GL/clay nanocomposite films with high vapor moisture and oxygen

335

barrier properties comparable with those of synthetic polyimides, commonly used as plastic

336

substrates for printed electronics (Scheme 1).21- 22

337

In summary, as demonstrated by the large-scale batch reactor solvolysis process

338

developed in the current study, high-quality lignin derivatives with attractive thermal properties

339

can be produced directly from abundant softwood biomass, such as sugi in Japan, through a


15


Page 16 of 32

340

technically feasible and environmentally benign PEG solvolysis process. The source wood meal

341

size and PEG solvent molecular mass critically affect the yield, chemical structure, molecular

342

mass, and thermal properties of the resultant GLs. Reducing the source wood meal size and PEG

343

solvent molecular mass promoted lignin PEGylation and further acid-induce chemical

344

rearrangements of GLs as demonstrated by 2D NMR and thioacidolysis, eventually leading to

345

enhanced lignin isolation yield. The GL chemical profiles inflicted by reducing the source wood

346

meal size and/or by increasing the solvent PEG molecular mass can enhance the viscous thermal

347

flow temperature as well as the thermal stability of GLs. We considered that, by manipulating

348

the solvolysis conditions, such as the source wood meal size and PEG solvent molecular mass,

349

the proposed process can produce lignin derivatives with substantially controlled chemical

350

structural and thermal properties to meet commercial and industrial needs for lignin-based

351

advanced material production. Besides, the development of large-scale PEG recycling system,

352

search for a more environmentally-benign (but yet economically feasible) green catalyst that may

353

be replaced with the sulfuric acid catalyst used in this study, as well as effective utilization of the

354

cellulose-rich solid residues byproduct along with GL are crucial to realize the full potential of

355

the current biomass solvolysis process for future biorefinery.

356 357

ASSOCIATED CONTENT

358 359

Supporting Information

360

The Supporting Information is available free of charge on the ACS Publications website at DOI:

361

XXXX. Compositions of source sugi wood meals and residual carbohydrate contents in GLs

362

(PDF).


16

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363 364


AUTHOR INFORMATION

365 366

Corresponding Author

367

*Email: [email protected] * [email protected]

368

Tel.: +81-29-829-8348. Fax: +81-29-874-3720.

369

Author Contributions

370

TTN and YT contributed equally to this work.

371

Notes

372

The authors declare no competing financial interest.

373 374

ACKNOWLEDGMENTS

375 376

This work was supported by SIP-Lignin Project, Technologies for Creating Next-Generation

377

Agriculture, Forestry and Fisheries under the Cross-Ministerial Strategic Innovation Promotion

378

Program (SIP) administered by Council for Science Technology and Innovation (CSTI), Japan.

379

We thank Ms. Ayano Kitasaka and Ms. Hinako Kunita for their assistance in GL samples

380

preparation and Klason lignin analysis, Ms. Keiko Tsuchida, Ms. Naoko Tsue and Ms. Megumi

381

Ozaki for their assistance in thioacidolysis, Dr. Hironori Kaji and Ms. Ayaka Maeno for their

382

assistance in NMR analysis. A part of this study was conducted using the facilities in the

383

DASH/FBAS at the Research Institute for Sustainable Humanosphere, Kyoto University, and the

384

NMR spectrometer in the JURC at the Institute for Chemical Research, Kyoto University.

385 386

REFERENCES


17


Page 18 of 32

387 388 389

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Flexible clay glycol lignin nanocomposite film with heat durability and high moisture-barrier

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a lower termite, Coptotermes formosanus. J Wood Sci. 2017, 63, 464.

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Ralph, J. CCoAOMT suppression modifies lignin composition in Pinus radiata. Plant J. 2011, 67,

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O-(α-l-arabiofuranosyl)-β-d-xylopyranoside and methyl β-d-xylopyranoside towards a β-O-4-

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methyl β-D-Glucoside. Holzforschung 1996, 50, 156-160.


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(35) Sano, Y.; Kishimoto, T. Delignification mechanism during high-boiling solvent pulping.

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V. Reaction of nonphenolic β-O-4 model compounds in the presence and absence of glucose. J.

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Wood Chem. Technol. 2003. 23, 279-292.

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(36) Kubo, S.; Kadla, J. F. Poly (ethylene oxide)/organosolv lignin blends: Relationship

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(37) Lapierre, C.; Monties, B.; Roland, C. Preparative thioacidolysis of spruce lignin: isolation and identification of main monomeric products. Holzforschung 1986, 40, 47-50. (38) Rinaldi, R.; Jastrzebski, R.; Clough, M. T.; Ralph, J.; Kennema, M.; Bruijnincx, P. C. A.;

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Weckhuysen, B. M. Paving the way for lignin valorisation: Recent advances in bioengineering,

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(39) El Mansouri, N. E.; Salvado, J. Structural characterization of technical lignins for the

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ethanol process lignins. Ind. Crop. Prod. 2006, 24, 8-16.

489 490 491 492 493

(40) Li, Y.; Sarkanen, S. Alkylated kraft lignin-based thermoplastic blends with aliphatic polyesters. Macromolecules 2002, 35, 9707-9715. (41) Sen, S.; Patil, S.; Argyropoulos, D. S. Thermal properties of lignin in copolymers, blends, and composites: A Review. Green Chem. 2015, 17, 4862-4887. (42) Prime, R. B.; Bair, H. E.; Vyazovkin, S.; Gallagher, K.; Riga, A. Thermogravimetric

494

Analysis (TGA). In Thermal Analysis of Polymers: Fundamentals and Applications; Menczel, J.

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D.; Prime, B. R., Eds.; John Wiley: Hoboken, NJ, 2009; pp 241-317.

496 497


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498


(Scheme and Figure Captions)

499 500

Scheme 1.

Schematic illustration of the acid-catalyzed PEG solvolysis of softwood Japanese

501

cedar (sugi) in a large-scale batch reactor process being developed in this study

502

and under the SIP-Lignin research consortium.

503 504

Scheme 2.

Proposed scheme for the generation of α-PEGylated β–O–4 in PEG solvolysis of

505

softwood lignins. Acid-induced chemical rearrangements including condensation

506

and side-chain cleavage reactions may also proceed via reactive benzyl cations.

507 508

Figure 1.

Particle size compositions of the source sugi wood meal used for PEG solvolysis

509

(A) and lignin yield (dry wood basis) of GLs based on Klason lignin content

510

analysis (B). GL200L, GL400L, and GL600L prepared from Sugi-L, GL200M,

511

GL400M, and GL600M prepared from Sugi-M, and GL200S, GL400S, and

512

GL600S prepared from Sugi-S, via sulfuric acid-catalyzed solvolysis reactions

513

with PEG200, PEG400 and PEG600 solvents, respectively. Solvolysis products of

514

source sugi wood meal (dry wood basis) using PEG200 solvent (C). The

515

composition ranges indicated are the ranges determined for solvolysis products of

516

Sugi-L, Sugi-M, and Sugi-S.

517 518

Figure 2.

2D HSQC NMR spectra of GLs. GL200L (A), GL200M (B) and GL200S (C)

519

were obtained via solvolysis reactions with PEG200 from Sugi-L, Sui-M, and

520

Sugi-S, respectively. GL400L (D) and GL600L (E) were obtained via solvolysis


23


Page 24 of 32

521

of Sugi-L with PEG400 and PEG600, respectively. The spectrum of purified sugi

522

lignin (MWL) prepared from Sugi-L is displayed for comparison (F). Boxes

523

labeled x2 indicate regions that are vertically scaled 2-fold. Contour coloration

524

matches that of the lignin substructure units shown. Normalized contour integrals

525

of the major lignin side-chain signals are listed in each spectrum. Values are

526

expressed as a percentage of the total of I, I´, II, III, IV and V.

527 528

Figure 3.

SEC molecular weight distribution profiles (A) and TMA thermal phase transition

529

profiles (B) of glycol lignins, GL200L, GL200M, and GL200S obtained via

530

solvolysis reactions with PEG200 solvent from Sugi-L, Sui-M, and Sugi-S,

531

respectively.


24

Page 25 of 32 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


(Table 1)

Table 1. Chemical structure and molecular mass data of softwood-derived GLs

Klason lignin (wt%)

PEG / ligninb (wt%)

P2/OMe signal ratio by NMRc

GL200L

81.7

22.3

GL200M

79.2

GL200S

a

GL samples

thioacidolysis monomer yield (µmol/g)d

molecular masse

per GL

per Klason lignin

Mw

Mn

PD

0.51

178

217

15,200

2,660

5.7

26.2

0.62

145

183

9,180

2,300

4.0

86.3

15.9

0.39

22.9

26.5

4,800

1,730

2.8

GL400L

77.8

28.5

0.34

168

215

14,200

2,460

5.8

GL400M

79.3

26.1

-

-

-

7,970

2,200

3.6

GL400S

79.9

25.2

0.28

17.1

21.4

5,720

1,990

2.9

GL600L

76.1

31.4

0.30

102

134

13,900

2,580

5.4

GL600M

78.3

27.7

-

-

-

7,360

2,080

3.5

GL600S

78.8

26.9

0.21

27.9

35.3

6,480

2,020

3.2

a

GL200L, GL400L, and GL600L prepared from Sugi-L, GL200M, GL400M, and GL600M prepared from Sugi-M, and GL200S, GL400S, and GL600S prepared from Sugi-S, via solvolysis reactions with PEG200, PEG400 and PEG600, respectively. b Calculated based on Klason lignin content. c Relative contour integration ratio in HSQC NMR (Figure 2). P2, C2–H2 contours in PEG moieties. OMe, lignin methoxyl contours. d Yield of guaiacyl-type trithioethylphenylpropanes. eDetermined by SEC using polyethylene glycol/poly(ethylene oxide) standards. PD, polydispersity (Mw/Mn). -, not determined.


25


Page 26 of 32

(Table 2)

Table 2. Thermal Properties of Softwood-Derived Glycol Lignins (GLs) TMA

TGA

Tg (°C)

Tf (°C)

Tdst (°C)

Tdmax (°C)

char residue at 800 °C (wt %)

GL200L

126.5

NAb

219

360.5

32

GL200M

121.2

167.8

242.8

368.2

34.3

GL200S

144.0

174.4

279.2

368.4

38.2

GL400L

106.5

145.2

251.9

359.7

32.4

GL400M

112.7

149

277.3

362

33.4

GL400S

102.9

147.8

279.7

371.1

34

GL600L

100.0

143.2

276.4

361.5

32.1

GL600M

102.2

138

277.6

370.1

31.2

GL600S

103.3

146.7

299.4

373

34.1

GL samplesa

a

GL200L, GL400L, and GL600L prepared from Sugi-L, GL200M, GL400M, and GL600M prepared from Sugi-M, and GL200S, GL400S, and GL600S prepared from Sugi-S, via sulfuric acid-catalyzed solvolysis reactions with PEG200, PEG400, and PEG600 solvents, respectively. b NA, not available as the second volume changes related to Tf was not detected.


26

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(Scheme 1)

(Designed in double column size) Scheme 1. Schematic illustration of the acid-catalyzed PEG solvolysis of softwood Japanese cedar (sugi) in a large-scale batch reactor process being developed in this study and under the SIP-Lignin research consortium.


27


Page 28 of 32

(Scheme 2)

(Designed in single column size) Scheme 2. Proposed scheme for the generation of α-PEGylated β–O–4 in PEG solvolysis of softwood lignins. Acid-induced chemical rearrangements including condensation and side-chain cleavage reactions may also proceed via reactive benzyl cations.


28

Page 29 of 32 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 49 50 51 52 53 54 55 56 57 58 59 60


(Figure 1)

(Designed in single column size) Figure 1. Particle size compositions of the source sugi wood meal used for PEG solvolysis (A) and lignin yield (dry wood basis) of GLs based on Klason lignin content analysis (B). GL200L, GL400L, and GL600L prepared from Sugi-L, GL200M, GL400M, and GL600M prepared from Sugi-M, and GL200S, GL400S, and GL600S prepared from Sugi-S, via sulfuric acid-catalyzed solvolysis reactions with PEG200, PEG400 and PEG600 solvents, respectively. Solvolysis products of source sugi wood meal (dry wood basis) using PEG200 solvent (C). The composition ranges indicated are the ranges determined for solvolysis products of Sugi-L, Sugi-M, and SugiS.


29


Page 30 of 32

(Figure 2)

(Designed in double column size) Figure 2. 2D HSQC NMR spectra of GLs. GL200L (A), GL200M (B) and GL200S (C) were obtained via solvolysis reactions with PEG200 from Sugi-L, Sui-M, and Sugi-S, respectively. GL400L (D) and GL600L (E) were obtained via solvolysis of Sugi-L with PEG400 and PEG600, respectively. The spectrum of purified sugi lignin (MWL) prepared from Sugi-L is displayed for comparison (F). Boxes labeled x2 indicate regions that are vertically scaled 2-fold. Contour coloration matches that of the lignin substructure units shown. Normalized contour integrals of the major lignin side-chain signals are listed in each spectrum. Values are expressed as a percentage of the total of I, I´, II, III, IV and V.


30

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(Figure 3)

(Designed in single column size) Figure 3. SEC molecular weight distribution profiles (A) and TMA thermal phase transition profiles (B) of glycol lignins, GL200L, GL200M, and GL200S obtained via solvolysis reactions with PEG200 solvent from Sugi-L, Sui-M, and Sugi-S, respectively.


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TOC/ABSTRACT GRAPHIC

SYNOPSIS Glycol lignin production from softwood biomass via acid-catalyzed polyethylene glycol solvolysis in a large-scale batch reactor was developed.


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Isolation and Characterization of Polyethylene Glycol (PEG)-Modified

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