How does ultrasound influence the thermal stability of wood?

This study elucidates the mechanism of the influence of ultrasonic treatment on wood pyrolysis and the details of the thermal stability changes of samples treated with ultrasonic alkali are explained. Four groups of samples were designed to clarify this scientific question, including the control group, the ultrasonic water-treated group, the alkali-treated group and the ultrasonic alkali-treated group. The extractives contents (organic extractives and hot water extractives) were determined, the crystal region, the functional groups of chemical components, the distribution of chemical components, and the thermal stability of wood were investigated using X-ray diffraction, Fourier-transform infrared spectroscopy, confocal Raman microscopy, and thermogravimetric analysis, respectively. The results suggested that all the treatments reduced the extractives contents, increased the relative crystallinity, destroyed the structure of hemicellulose and lignin, and affected the thermal stability of wood, to different degrees. Ultrasonic time and alkali concentration were shown to be two important indexes. Ultrasound can enhance the treatment effects of low concentration alkali (0.1 %), but in a highly concentrated alkali environment, the auxiliary effect is not obvious. Ultrasonic treatment reduced the intensity of lignin in the whole cell wall region, while alkali treatment followed a more systematic process as the solvent permeated from the S layer to CCML. The initial degradation temperature T0 was not only related to the extractives content but also to the concentrations of hemicellulose and lignin in the sample. The hemicellulose degradation temperature T1 and cellulose degradation temperature T2 changed with changes in T0. A detailed understanding of the spatial and temporal variations of the major components in the rigid cell wall during the pretreatment process will help to develop more effective pretreatment strategies, which is crucial for the efficient optimization of the process.