Sustainable Methanolysis of PLA Enabled by a Biochar-Supported Catalyst: Toward PET Purification in Mixed Polymer Waste

The development of selective and sustainable catalysts is essential to enable the chemical recycling of mixed plastic waste. In this work, calcium-modified biochars derived from cocoa pod husk (CPH) and palm kernel shell (PKS) were prepared for treating a mixture of poly(ethylene terephthalate) (PET) and poly(lactic acid) (PLA). The aim was to separate the mixture through the PLA methanolysis, while maintaining the PET unreacted for a potential physical recycling. Biochar was ex situ modified with calcium precursor using a value-added concentrate recovered from the hydrothermal treatment of Jatropha fruit husk. Subsequently, a pyrolysis step was further applied to convert the calcium species into CaO, which is the active phase for the methanolysis reaction. Structural, microscopic, and spectroscopic analyses revealed that the carbon matrix strongly influences the evolution and stabilization of calcium phases during pyrolysis and post-treatment. CPH-derived biochars promoted the formation of highly dispersed CaO, whereas PKS favored the growth of larger, less reactive Ca(OH)₂ domains. As a result, the CPH_Ca10 (i.e., 10% desired calcium loading based on CPH-biochar mass) catalyst exhibited superior basicity and catalytic activity, achieving near-complete PLA conversion under mild conditions (90–110 °C) depending on the system with only 2 wt.% catalyst. Importantly, under these mild conditions, PET remained chemically intact, demonstrating the process’s high selectivity and applicability to mixed bioplastic–fossil plastic streams. This study highlights a circular, low-carbon route to producing effective Ca-based catalysts from agricultural residues. It establishes a promising strategy for selective depolymerization and separation in complex plastic waste systems.