Alkyl lactate formation
from the depolymerization of post-consumer polylactic acid by metal complex
catalysts

Alkyl lactates are green solvents due to their
biodegradability and low toxicity, which have applications in several
industries such as the pharmaceutical, agricultural and polymer.^1,2
They can be obtained from the transesterification reaction of polylactic acid
(PLA) with the corresponding alcohol. Although PLA is a promising alternative
to replace traditional synthetic polymers, its higher cost and still low
natural degradation rates have limited its commercial growth. Chemical
recycling of PLA is an alternative to mechanical recycling and composting of
post-consumer PLA, resulting in lower production costs and value-added
products.³
It can enable products to be recycled in line with circular economy.

A recent study by our research group has shown the feasibility
of producing of methyl lactate from PLA by employing a Zn(II) complex catalyst Zn(1)₂
previously used for the polymerization of lactide.⁴
It was also shown that the main parameters affecting the reaction were
temperature and catalyst concentration when studying the effects of these and other
parameters (stirring speed, particle size and PLA molecular weight).

In the present work, we report the use of this Zn(II)
catalyst for the production of other lactates (ethyl lactate, propyl lactate
and butyl lactate) from post-consumer PLA. Additionally, two new complexes were
simply synthesised. The PLA conversions and alkyl lactate yields and selectivities
with the new catalysts were compared with those of Zn(1)₂ to
determine their activity.

Reactions were performed in a 300 mL stirred batch reactor
with temperatures varying from 50 to 100 °C. Samples were withdrawn a different
times and analysed by gas-chromatography and ¹H NMR spectroscopy to
follow the progress of the reactions. PLA samples included virgin PLA (Ingeo™) with
varying molecular weight, and post-consumer PLA (cups, 3D-printing, films and
toys). GPC was used to characterise the molecular weight and dispersity of the
PLA samples. The reactions were first carried out with virgin PLA in order to study
the effects of catalyst type on reaction rate and to obtain the kinetic parameters
at the different temperatures. The experimental data was fitted to a reaction
mechanism consisting of two consecutive reactions.⁵
In a first step, PLA degrades irreversibly into oligomers, followed by a second
step where alkyl lactate is formed from the oligomers through a reversible
reaction. The kinetic model with fitted parameters was then used to compare the
results of the degradation of post-consumer PLA.

The new catalysts showed higher activities compared with Zn(1)₂.
For instance, at 50 °C with Zn(1)₂ the reaction reached
completion after 4 days, whereas with one of the new catalysts it was completed
in less than 3 hours (Figure 1). Contrary to the previous investigations with Zn(1)₂,
the new catalysts showed mass transfer limitations. Higher activities were
observed at higher speeds when testing 300, 700 and 870 rpm. Despite the
molecular weight variability and presence of unknown additives in the
post-consumer PLA samples, the kinetic model with fitted parameters was able to
predict the final concentration of alkyl lactate regardless of the sample. The
results of this work can be used for the design of process equipment for the
chemical recycling of post-consumer PLA.

Figure 1. Methyl lactate concentration profiles with Zn(1)₂
and new catalyst.

References

(1) Calvo-Flores, F. G.; Monteagudo-Arrebola, M. J.; Dobado, J. A.; Isac-Garcia,
J. Green and Bio-Based Solvents. Top. Curr. Chem. 2018, 376.

(2) NREL Chemicals from Biomass: A Market Assessment of
Bioproducts with near-Term Potential; National Renewable Energy Laboratory:
March 2016, 2016.

(3) Hong, M.; Chen, E. Y. X. Chemically Recyclable Polymers: A
Circular Economy Approach to Sustainability. Green Chem. 2017, 19,
3692-3706.

(4) McKeown, P.; McCormick, S.; Mahon, M. F.; Jones, M. Highly
Active Mg(II) and Zn(II) Complexes for the Ring Opening Polymerisation of
Lactide. Polym. Chem. 2018, 9, 5339-5347.

(5) Román-Ramírez, L. A.;
McKeown, P.; Jones, M. D.; Wood, J. Poly(Lactic Acid) Degradation into Methyl
Lactate Catalyzed by a Well-Defined Zn(II) Complex. ACS Catal. 2019,
1, 409-416.

Acknowledgments

The authors gratefully
acknowledge NatureWorks LLC for their donation of PLA samples.