Methanolysis of Poly(lactic Acid) Using Catalyst Mixtures and the Kinetics of Methyl Lactate Production.

Polylactic acid (PLA) is a leading bioplastic of which the market share is predicted to increase in the future; its growing production capacity means its end-of-life treatment is becoming increasingly important. One beneficial disposal route for PLA is its chemical recycling via alcoholysis. The alcoholysis of PLA leads to the generation of value-added products alkyl lactates; this route also has potential for a circular economy. In this work, PLA was chemically recycled via methanolysis to generate methyl lactate (MeLa). Four commercially available catalysts were investigated: zinc acetate dihydrate (Zn(OAc) 2 ), magnesium acetate tetrahydrate (Mg(OAc) 2 ), 4-(dimethylamino)pyridine (DMAP), and triazabicyclodecene (TBD). Dual catalyst experiments displayed an increase in reactivity when Zn(OAc) 2 was paired with TBD or DMAP, or when Mg(OAc) 2 was paired with TBD. Zn(OAc) 2 coupled with TBD displayed the greatest reactivity. Out of the single catalyst reactions, Zn(OAc) 2 exhibited the highest activity: a higher mol% was found to increase reaction rate but plateaued at 4 mol%, and a higher equivalent of methanol was found to increase the reaction rate, but plateaued at 17 equivalents. PLA methanolysis was modelled as a two-step reversible reaction; the activation energies were estimated at: Ea 1 = 25.23 kJ∙mol -1 , Ea 2 = 34.16 kJ∙mol -1 and Ea -2 = 47.93 kJ∙mol -1 .