Lactide, a crucial precursor of the bioplastic polylactic acid, was obtained through the gas-phase transesterification of alkyl lactates using a 5 wt% TiO2/SiO2 catalyst as an alternative to the conventional liquid-phase process. Lactide selectivity above 80% was achieved at 220 degrees C but conversion was thermodynamically limited at around 50% for all alkyl lactates. The nature of the ester alkyl chain had minimal impact of the thermodynamics of the reaction, but significantly influenced its kinetics. The Taft equation indicated that this kinetic effect was due to the polarity of the alkyl chain. Mechanistic studies indicated that lactide formed via a Langmuir-Hinshelwood mechanism involving two lactate molecules. The derived kinetic expression was fitted to experimental data of methyl lactate transesterification, resulting in a reduced chi-squared of 0.99 and an activation energy of 89.8 kJ.mol(-1). Initial rate kinetics confirmed that the proposed mechanism was valid for other alkyl lactate species.
NSTL
Elsevier