Comparative study on the properties of wood vinegar prepared from the pretreated Chinese fir: Effect of hydrothermal parameters and hydrogen peroxide

Abstract

Hydrothermal (HT) pretreatment and hydrothermal oxidative (HO) pretreatment are efficient in upgrading the quality of wood vinegar (WV), while little research pays attention to the systemic comparison between the effects of HT and HO pretreatment on the resulted WV. Therefore, in the present study, the response surface method is adopted first to analyze the effect of HT temperature, solid-to-liquid ratio (S/L), and retention time on the physicochemical properties of WV prepared from Chinese fir (CF). Then, the main compositions (hemicellulose, cellulose, and lignin) of the pretreated CF are quantitatively measured, and the comparative study between the effects of HT pretreatment and HO pretreatment on the yield, density, and compositions of the resulted WV is carried out based on CF composition. The results show that HT temperature is the main parameter that affects the physicochemical properties of the resulted WV. With the increase of HT temperature, the cellulose content of CF samples increases first at 180–230 °C and then decreases at 260–280 °C, the hemicellulose content decreases, and the lignin enriches. The cellulose enrichment gives rise to the increasing WV yield at 180–200 °C and the increasing WV density at 180–230 °C. While the dramatic cellulose decomposition at 230–280 °C results in the decreasing WV yield at 230–280 °C and the decreasing WV density at 260–280 °C. The cellulose is the main component that contributes to the yield and density of WV. Moreover, due to the cellulose enrichment, the contents of the organic compounds, especially the phenols, ketones, and aldehydes, increase dramatically at 180–230 °C, contributing to the lower pH value of the resulted WV. However, HO pretreatment aggravates the decomposition of CF components. During HO pretreatment, the decreases in the hemicellulose content and cellulose content are advanced to 160–200 °C and 180–230 °C respectively, and the lignin content increases. Thus, WV with the comparable yield and density to the unpretreated WV can be prepared at HO pretreatment temperature of 160 °C. Moreover, the pH value of WV decreases significantly after HO pretreatment of 160–200 °C because of the existence of the acid compounds with higher acidity in the resulted WV. Meanwhile, the content of the aldehydes contained in WV prepared from HO pretreated CS increases dramatically.