Evaluation of optical properties and chemical structure changes in enzymatic hydrolysis lignin during heat treatment

Abstract

Heat treatment is an excellent method of improving the properties of wood. Study of the physical and chemical changes of lignin during heat treatment is essential for expanding the application areas based on new properties. In this study, we investigated the degradation behavior of enzymatic hydrolysis lignin (EHL) heat treated at different temperatures (180 °C, 200 °C, and 220 °C) and for different durations (2 h and 4 h). The optical property changes of EHL before and after heat treatment were investigated by measuring color and UV-Vis absorbance changes. Fourier transform infrared spectroscopy (FTIR) was used to characterize the chemical modifications resulting from heat treatment. Mass spectrometry (MS) was used to elucidate the variation of the mass-to-charge (m/z) ratios of ions produced by EHL during heat treatment. These experimental results indicate that both the EHL mass loss percentages and chromatic aberration exhibited an increase with increasing temperature and duration of heat treatment. In contrast, the UV-Vis absorbance, most of the FTIR absorption peaks, and the m/z values of EHL showed a decreasing trend. It was found that the color change of lignin was not the major cause of wood discoloration during heat treatment. The better resistance of the heat treated wood toward solar radiation was attributed to the lower UV absorption of post-treatment lignin.