Kinetics and mechanism of monomeric product from methanolysis of poly (3-hydroxybutyrate) catalyzed by acidic functionalized ionic liquids

Poly (3-hydroxybutyrate) (PHB) from an industrial manufacturer was depolymerized by methanol in the presence of acidic functionalized ionic liquids (ILs). It was demonstrated that 1-methyl-3-(3-sulfopropyl)-immidazolium hydrogen sulfate ([HSO3-pmim][HSO4]) exhibited higher catalytic activity than other acidic functionalized ILs and traditional acids such as H2SO4 and H3PO4. The main product of methanolysis was confirmed to be methyl 3-hydroxybutyrate monomer by FT-IR and 1H NMR. The influences of experimental parameters, such as the amount of catalyst, reaction temperature, methanolysis time, and dosage of methanol on the conversion of PHB and yield of methyl 3-hydroxybutyrate were investigated. The results showed that the reaction temperature was a critical factor for depolymerization of PHB. Under the optimum conditions of n(CH3OH):n(PHB) = 5:1, m(IL):m(PHB) = 0.03:1, reaction temperature 140 °C and time 3 h, the conversion of PHB and the yield of product were over 90% and 83%, respectively. The IL could be reused up to 6 times without apparent decrease in the conversion of PHB and yield of product. Kinetics of the reaction was also investigated, it was indicated that methanolysis of PHB catalyzed by acidic functionalized [HSO3-pmim][HSO4] was a first-order kinetic reaction with an activation energy of 16.02 kJ/mol. In addition, a detailed reaction mechanism of the methanolysis was proposed.