Investigation of structural modification and thermal characteristics of lignin after heat treatment

https://doi.org/10.1016/j.ijbiomac.2014.02.013Get rights and content

Abstract

Milled wood lignin was subjected to heat treatment between 150 and 300 °C to understand the pattern of its structural modification and thermal properties. When the temperature was elevated with interval of 50 °C, the color of the lignin became dark brown and the lignin released various forms of phenols from terminal phenolic groups in the lignin, leading to two physical phenomena: (1) gradual weight loss of the lignin, up to 19% based on dry weight and (2) increase in the carbon content and decrease in the oxygen content. Nitrobenzene oxidation and 13C NMR analyses confirmed a cleavage of β-O-4 linkage (depolymerization) and reduction of methoxyl as well as phenolic hydroxyl group were also characteristic in the lignin structure during heat treatment. Simultaneously with lignin depolymerization, GPC analysis provided a possibility that condensation between lignin fragments could also occur during heat treatment. TGA/DTG/DSC data revealed that thermal stability of lignin obviously increased after heat treatment, implicating the structural rearrangement of lignin to reduction of β-O-4 linkage as well as accumulation of Csingle bondC bonds.

Introduction

Lignin is a natural polymeric material composed of three different C6C3 types (phenyl-propane) of monolignols (p-coumaryl, coniferyl and sinapyl alcohol) with various interunit linkages such as β-O-4 (40–60%), β-5 (4–10%) and biphenyl (3.5–25%) [1]. Native lignin in biomass has a complex, three dimensional amorphous structures with a high degree of condensation, which allows the lignin to resist external physical, chemical, and/or biological forces [2]. This resistance is one reason why utilization of lignin in high value-added products is so difficult. For this reason, less than 2% of lignin byproduct produced from the paper industry, called technical lignin, is used for value-added products [3]. Most lignin from the pulp and paper industry is burned to generate heat [4]. In addition, as lignocellulosic biomass begins to be used in monomeric sugar or bioethanol platforms, efficient lignin utilization is economically necessary in biorefinery processes [5]. Lignin utilization has been studied widely. Technical lignin produced by the paper industry has traditionally been utilized as a stabilizer for plastics and rubber, phenolic resins, dispersants, automotive brakes and wood panel products [6]. Production research on aromatic chemicals, carbon fibers and thermoplastic or fusible materials from various lignins has also been actively carried out [7], [8], [9].

All the lignin uses mentioned so far require proper conversion process conditions, but it is also important to select suitable raw lignin materials because their structural features significantly influence the yield and characteristics of the target products. During the production of aromatic chemicals, lignin containing a greater number of ether linkages is desirable because these linkages degrade more easily with lower energy consumption [10]. Kubo and Kadla [11] reported that organosolv lignin thermally blended with polyethylene oxide (PEO) had better plastic behavior than kraft lignin because organosolv lignin contained more phenolic hydroxyl groups and a more highly oxidized structure. Saraf and Glasser [12] found that the tensile strength of lignin-based polyurethane films could be attributed to differences in phenolic hydroxyl content and the molecular weight of lignins.

The diverse chemical/structural properties of lignin such as the molecular weight (Mw), polydispersity index (PDI), the frequency of β-O-4 linkages, the thermal stability, the content of functional groups (methoxyl and phenolic hydroxyl) and the degree of condensation vary considerably with the extraction condition [13], [14]. However, it is hard to change process conditions to obtain lignin with particular of properties because it is only a byproduct, not the main product of many biorefinery processes. Therefore, appropriate treatments, which can control the chemical/structural characteristics of lignin byproducts, should be indispensable for efficient and versatile utilization of lignin byproducts.

In this study, unlike previous research, in which an entire biomass was heat treated, only the lignin fraction extracted from poplar wood (milled wood lignin; MWL) was heat treated under inert conditions to obtain chemically/structurally modified lignins [15], [16]. For in-depth elucidation of the modified lignin structure, several analyses such as GC/MS (released volatiles during heat treatment), GPC, TGA, DSC, FT-IR, 13C NMR and wet chemistry were used to determine functional group contents (methoxyl and phenolic hydroxyl groups) and the frequency of ether linkages (nitrobenzene oxidation). Based on these results, a plausible reaction occurring in lignin during heat treatment was suggested.

Section snippets

Preparation of milled wood lignin (MWL) and heat treatment of the lignin

Milled wood lignin (MWL) was extracted from poplar wood xylem (P. albaglandulosa) according to the Bjorkman method [17]. The yield of MWL was ca. 5.5% based on the dry weight of poplar wood xylem. For lignin heat treatment, 100 ± 5 mg of MWL was introduced in a cylindrical glass tube (4 × 730 mm) with glass wool and subjected to a CDS Pyroprobe 1500 interface (CDS Analytical, Oxford, PA, USA) with sufficient helium gas (>99.9% purity). The temperature of the interface was maintained at 150, 200, 250

Weight loss of lignin during heat treatment

The visible color change of lignin during heat treatment is depicted in Fig. 1. The color of the native lignin (control) was originally pale yellow. The lignin gradually turned dark brown as the temperature increased. This physical change could be attributed to the structural change in the lignin. In fact, it was well known that thermochemical reactions such depolymerization, side chain cleavage, recondensation and carbonization occur during exposure to heat [22], [23]. Fig. 2 shows the weight

Conclusion

In this study, MWL from poplar wood was treated at various temperatures (150, 200, 250 and 300 °C) under inert conditions and these lignins were subjected to various chemical/thermal and spectroscopic analyses. With increasing treatment temperature, gradual weight loss of the lignin was occurred up to 19% (300 °C) due to loss of various phenols from terminal phenolic groups in the lignin. GPC analysis revealed concurrent reaction involving cleavage of the β-O-4 linkage (depolymerization) and

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation (NRF) of Korea, funded by the Ministry of Education, Science and Technology (MEST), Republic of Korea (NRF-2012R1A1A2038676).

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