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

Abstract

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: Ea1 = 25.23 kJ∙mol−1, Ea2 = 34.16 kJ∙mol−1 and Ea-2 = 47.93 kJ∙mol−1.

Lactone Ring-Opening Equilibria in Methanol by 1H NMR Analysis: An Assessment of the Ring-Opening Polymerizability of Lactone Monomers

“…The progress of the ring opening of 4 in methanol over time is shown in Figure 6. The initial formation of diester 4′ was observed, although, as mentioned, the rate of that reaction was 21 a process that simultaneously produces two molecules of methyl lactate (4″). This became the overall thermodynamic sink for the system given that methanol was the bulk solvent.…”

“…The onset of a second, slower event ( t 1/2 ca. 2 days) was also observed, namely the transesterification of the internal ester in 4′ by additional methanol, a process that simultaneously produces two molecules of methyl lactate ( 4″ ). This became the overall thermodynamic sink for the system given that methanol was the bulk solvent.…”

A Multitool for Circular Economy: Fast Ring‐Opening Polymerization and Chemical Recycling of (Bio)polyesters Using a Single Aliphatic Guanidine Carboxy Zinc Catalyst

“…Nonetheless, the general trends for the PLA depolymerization to oligomers are in line with the findings of Wood and co-workers. [30] Therefore, to our knowledge this is the first study to present an in-depth kinetic analysis for the PLA methanolysis at mild reaction conditions in THF, taking into account both catalyst and alcohol loading.…”

“…[16f] Scheme adapted from. [30] ChemSusChem degradation to oligomers and MeLa was performed. Using the obtained data, the observable reaction rate constant k obs was determined via the slope of a semilogarithmic plot of ln(PLA 0 / PLA t ) vs. time (Figure 3).…”

La(iii)-Catalysed degradation of polyesters to monomersviatransesterifications

“…Wood and co-workers recently reported zinc acetate-catalysed methanolysis of poly(lactic acid). 11 Tanaka, Nakajima and co-workers reported the degradation of poly(ethylene terephthalate) ( PET ) catalysed by LiOMe (5 mol%) under ambient conditions. However, this method required a considerable amount of dimethyl carbonate to capture the ethylene glycol formed.…”