Kinetics of Alkyl Lactate Formation from the Alcoholysis of Poly(Lactic Acid)

Abstract

Alkyl lactates are green solvents that are successfully employed in several industries such as pharmaceutical, food and agricultural. They are considered prospective renewable substitutes for petroleum-derived solvents and the opportunity exists to obtain these valuable chemicals from the chemical recycling of waste poly(lactic acid). Alkyl lactates (ethyl lactate, propyl lactate and butyl lactate) were obtained from the catalysed alcoholysis reaction of poly(lactic acid) with the corresponding linear alcohol. Reactions were catalysed by a Zn complex synthesised from an ethylenediamine Schiff base. The reactions were studied in the 50–130 °C range depending on the alcohol, at autogenous pressure. Arrhenius temperature-dependent parameters (activation energies and pre-exponential factors) were estimated for the formation of the lactates. The activation energies (Ea1, Ea2 and Ea−2) for alcoholysis in ethanol were 62.58, 55.61 and 54.11 kJ/mol, respectively. Alcoholysis proceeded fastest in ethanol in comparison to propanol and butanol and reasonable rates can be achieved in temperatures as low as 50 °C. This is a promising reaction that could be used to recycle end-of-life poly(lactic acid) and could help create a circular production economy.

Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

“…94 Indeed, the ethyl lactate market is projected to reach $ 92 million by 2024 and currently trades at £ 2.54-3.49 per kg relative to £ 1.69 per kg for virgin PLA. 91,95 PLA recycling processes include hydrolysis [96][97][98][99][100][101][102][103] and alcoholysis. [104][105][106][107][108][109][110][111][112][113][114][115] Hydrogenation [116][117][118] and hydrosilylation 119 processes exploiting ruthenium and iridium have also been reported.…”

Synergistic Dual Catalytic System and Kinetics for the Alcoholysis of Poly(Lactic Acid)

“…The experimental data were modelled using the simplified two-step reaction mechanism shown below in Equation ( 1). The model is simplified from the previous paper by not assuming an equilibrium for the second step [22]. The equilibrium only becomes significant at temperatures below 90 • C, as lower temperatures favor the reverse equilibrium step.…”

“…The chemical recycling of PLA is a promising alternative to limited mechanical recycling and biodegradation [20]. One of the most promising chemical recycling methods for depolymerising PLA is via alcoholysis; this route can be carried out at milder operating conditions than other chemical methods and has the additional benefit of generating a value-added product alkyl lactate (AL) [21][22][23][24]. ALs are employed in several industries and are considered green solvents due to their biodegradability and low toxicity [25].…”

“…Lactide is the precursor for ring opening polymerisation, which generates virgin grade PLA. Zn(II) complexes have previously been shown to be highly active for both the polymerisation and depolymerisation of PLA [22,29]. It is worth noting that it is possible to chemically recycle PLA directly to lactide; this is a more useful route if the regeneration of virgin PLA is desired over obtaining the value-added ALs [30].…”

“…Likewise, 4-(dimethylamino)pyridine (DMAP) has been shown to be an effective transesterification catalyst and has been employed in PLA alcoholysis [35][36][37]. As both ZnAc and DMAP are commercially available, they can be bought instead of synthesised, which is an advantage over the Zn(II) catalysts used in our previous paper [22]. Both catalysts were tested individually and in different mixtures at the same total weight percentage relative to PLA weight.…”