Objective The introduction of exogenous acid can solve the problem of high temperature and long time during heat treatment and improve thermal efficiency. It is necessary to clarify how exogenous acid affects wood components and thermal degradation characteristics of different woods.

Method Two types of softwood (larch, Chinese fir) and two types of hardwood (eucalyptus, poplar) were studied, and AlCl₃ in Lewis acid was taken as exogenous acid. The thermal degradation characteristics of wood treated with different concentrations of AlCl₃ solution (0, 0.1, 0.3 mol/L) were studied by Fourier transform infrared spectroscopy, thermogravimetric analysis and Flynm-Wall-Ozawa method.

Result AlCl₃ treatment had significant effects on chemical structure of wood, which showed that the characteristic peak of hemicelluloses decreased significantly, the redshift occurred, the intensity of lignin characteristic peak increased, the total crystallinity index (TCI) decreased, the OH/CH₂ ratio decreased, and the hydrogen bond strength (HBI) increased. After 0.3 mol/L AlCl₃ solution treatment, TCI of the samples decreased by 29.17%−52.67%, OH/CH₂ ratio decreased by 23.18%−51.01%, HBI value increased by 2.86%−13.27%, and the lateral order intensity of fiber was reduced by less than 8%. During the pyrolysis process, the degradation temperature of samples decreased, the degradation rate increased, and the activation energy decreased after AlCl₃ treatment. When the treatment concentration was 0.3 mol/L, the initial degradation temperature of wood was reduced to below 110 ℃, a obvious shoulder peak appeared in softwood, and the activation energy of softwood was significantly lower than that of hardwood at the rapid thermal degradation stage.

Conclusion The introduction of AlCl₃ can significantly reduce the thermal degradation temperature of wood, thereby effectively improve the thermal modification efficiency of wood. Its catalytic effect on softwood wood is more significant than that on hardwood. The research can provide theoretical support for the application of Lewis acid in the field of low temperature heat treatment of wood.