Zinc-Oxide-Supported Zeolite Subcrystal Catalyst: Enhancing Chemical Recycling of Poly(lactic acid) via Optimized Active Site Dispersion and Polymer Diffusion

Chemical recycling of poly(lactic acid) (PLA) to lactide (LD) monomers offers a dual benefit in terms of production and waste management. However, the efficiency of this recycling process is often hindered by harsh reaction conditions, high catalyst loadings, and product separation issues. In this study, ZSM-5 zeolite subcrystal (SC) with size smaller than 10 nm and ultrahigh external surface area was employed as a support to disperse zinc oxide (ZnOx) species. Different from nanosized ZSM-5 (nano-Z5) zeolite, various characterization results demonstrated that ZnOx nanoparticles on SC exhibited exceptionally high dispersion and small size, even at ultrahigh loadings. As a result, rapid depolymerization of PLA (∼97 molPLA molZn–1 h–1) and high recovery of LD (65%) can be achieved at a low Zn(II)/PLA mass ratio of 0.008 under optimum condition. However, it was found that it is preferable to minimize the use of the carrier (SC) on the basis of achieving a high dispersion of ZnOx. This can be assigned to the fact that excessive use of the carrier can hinder the accessibility of PLA to active sites due to diffusion limitations. This work contributes to advancing heterogeneous catalysis for the closed loop recycling of biodegradable plastic, thus promoting a sustainable circular economy.