A silica-supported organocatalyst for polycarbonate methanolysis under mild and economic conditions

In the pursuit of circular economies aimed at eliminating waste and pollution, chemical recycling emerges as a promising avenue for transforming plastics into monomers. This study addresses the need for economically viable and mild depolymerisation methods, focusing on poly(bisphenol A carbonate) (BPA-PC), an engineering plastic with the monomer bisphenol A (BPA), a known xenoestrogen. Improving BPA-PC recycling is crucial to prevent the release of BPA into the environment. Our investigation centres on three commercially available organocatalyst-modified silica gels in polycarbonate (PC) methanolysis, alongside one functionalised with 1,5,7-triazabicyclo-[4.4.0]-dec-5-ene (TBD), previously reported for polyethylene terephthalate (PET) glycolysis. The latter Si-TBD, among these catalysts, exhibits superior catalytic activity in PC methanolysis. The PC methanolysis process was optimised economically by experimental design. Under optimal reaction conditions (PC: 2.00 g, methanol (n(MeOH):n(PC) = 13:1), Si-TBD: 5 mol%, 65 °C, 2 h), 96 % non-isolated BPA monomer yield was obtained. The kinetics of the reaction reveals that the Si-TBD-catalysed PC methanolysis is a pseudo-first-order reaction with an exceptionally low activation energy of 44.19 kJ mol−1, the lowest reported to date. Si-TBD was recycled in ten reaction cycles after regeneration, and even though the regenerated catalyst has slightly lower activity than the native catalyst, good BPA yields (72 ± 4 %) were achieved consistently. Investigations into the necessity of an inert atmosphere during catalyst recycling indicate that it is not required. Impressively, Si-TBD demonstrates the ability to depolymerise PC even at room temperature, without stirring, in 2 days, with an excellent BPA yield (94 %). Notably, this catalyst offers similar performance at lower temperature than others reported in the literature for PC methanolysis.