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

Abstract

Plastic pollution is a global issue that is approaching crisis levels as plastic production is projected to reach 1.1 GT annually by 2050. The bioplastic industry along with a circular production economy are solutions to this problem. One promising bioplastic polylactic acid (PLA) has mechanical properties comparable to polystyrene (PS), so it could replace PS in its applications as a more environmentally sustainable material. However, since the bioplastic PLA also suffers from long biodegradation times in the environment, to ensure that it does not add to the current pollution problem, it should instead be chemically recycled. In this work, PLA was chemically recycled via alcoholysis, using either methanol or ethanol to generate the value-added products methyl lactate and ethyl lactate respectively. Two catalysts, zinc acetate dihydrate (ZnAc) and 4-(dimethylamino)pyridine (DMAP), were tested both individually and in mixtures. A synergistic effect was exhibited on the reaction rate when both catalysts were used in an equal ratio. The methanolysis reaction was determined to be two-step, with the activation energy estimated to be 73 kJ mol−1 for the first step and 40.16 kJ mol−1 for the second step. Both catalysts are cheap and commercially available, their synergistic effect could be exploited for large-scale PLA recycling.

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

“…It is also well reported in the literature that the metal acetates zinc acetate dihydrate (Zn(OAc) 2 ), and magnesium acetate tetrahydrate (Mg(OAc) 2 ), as well as the organocatalysts 4-(dimethylamino)pyridine (DMAP), and triazabicyclodecene (TBD), which are all effective catalysts for transesterification [31][32][33][34][35][36][37]. Furthermore, the recent literature investigated it using dual catalysts (Lewis acid-base pairs) for polyester recycling and found that they outperformed single catalysis [38][39][40]. A synergistic effect has been reported for Zn(OAc) 2 coupled with DMAP, resulting in an increased polyester depolymerization rate [39,40].…”

Guanidine functionalized porous SiO2 as heterogeneous catalysts for microwave depolymerization of PET and PLA

“…Therefore, sealed glass vessels were used to achieve higher temperatures for the reaction. Lamberti et al 48 reported that the mechanism is a two-step reversible reaction in which the PLA chains are quickly converted to chain end oligomers, which then slowly form the product MeLa. As the reaction proceeds, the amount of internal oligomers decreased as the polymer chains were degraded, producing more chain end oligomers and subsequently the MeLa monomer.…”

“…There have also been reports of using an autoclave to perform the degradation of PLA to MeLa, where the higher pressure can also increase MeLa yield. 48 On carrying out the reaction in an autoclave, it was possible to achieve full conversion of PLA and further increase the yield of MeLa to 42% aer 72 h (Fig. 7(c)).…”

Ionic Liquid‐Assisted Depolymerization of Condensation Polymers: A Review

“…Alcoholysis (glycolysis and methanolysis), hydrolysis, aminolysis, and ammonolysis are polymers′ most common chemical recycling mechanisms (Figure 5). [79–83] …”

“…[72][73][74][75][76][77][78] Alcoholysis (glycolysis and methanolysis), hydrolysis, aminolysis, and ammonolysis are polymers' most common chemical recycling mechanisms (Figure 5). [79][80][81][82][83] The depolymerization of plastics can be catalyzed by different catalysts, including homogeneous, heterogeneous and biocatalysts. Most homogeneous systems employ transition metal complexes; [84][85][86] but this system pose the problem because of the difficult recovery and separation of the catalysts from the products.…”