Study of kinetics of reaction of lignin model compounds with propylene oxide

摘要

The etherification of phenolic groups has been found to inhibit photodegradation in wood and lignin rich pulps. The precise understanding of kinetics of chemical reac-tion between lignins or their model compounds and the etherifying agent is the first step for developing a viable modification procedure. In this study, we have invest-igated the reaction of lignin model compounds (namely, phenol and guaiacol) with propylene oxide in aqueous media. The kinetics of etherification reaction was studied under varying pH conditions in the temperature range 30-60℃. The etherified reaction products were charac-terized by gas chromatogram-mass spectrum (GC-MS). The extent of etherification of phenols and the rate of chemical reaction was followed by UV-Visible absorption spectroscopy. The reaction between lignin model com-pounds and propylene oxide was indicated by a rapid reduction in the absorbance accompanied by the devel-opment of a new band corresponding to etherified prod-ucts. The reaction kinetics was investigated at pH ～ 12 under the condition of excess concentration of propylene oxide. The reaction followed first order kinetics and rate constants increased linearly with an increase in the tem-perature and concentration of propylene oxide. The MS fragment data of reaction product support the proposed reaction scheme. The activation energy of the reaction of propylene oxide with phenol and guaiacol, calculated with the Arrhenius equation, was 56.2 kJ mol~(-1) and 45.4 kJ mol~(-1), respectively.