Magnetically recoverable calcium-biochar catalysts for PET methanolysis: effects of magnetic-phase incorporation on catalytic performance and recyclability

This study demonstrates the successful impregnation of small magnetic clusters within a calcium-biochar matrix to develop magnetic recoverability while preserving the catalytic performance associated with calcium-containing species. Calcium-based biochars derived from rice husk and cocoa pod husk were functionalized with controlled amounts of iron or cobalt to impart magnetic recoverability while preserving the high catalytic activity of calcium sites during PET methanolysis. Structural and morphological analyses confirmed the homogeneous dispersion of Ca clusters within the carbon matrix and the effective incorporation of finely distributed magnetic phases without severely obstructing the Ca-containing surface domains potentially involved in catalysis. Catalytic screening revealed PET conversions exceeding 85% for Ca-biochar catalysts compared to 7% of the thermal reaction, while the introduction of magnetic functionality enabled efficient post-reaction recovery. Among the evaluated materials, cobalt-modified catalysts exhibited superior magnetic response and maintained high depolymerization activity, outperforming their iron-based counterparts. Kinetic studies using the optimal cobalt-containing catalyst demonstrated zero-order behavior and a significant reduction in activation energy, from 110 to 73 kJ mol −1 , enabling efficient PET methanolysis at temperatures as low as 140-170 °C. Reusability tests further confirmed enhanced catalyst stability and ease of separation through magnetic-assisted recovery. Overall, the developed magnetic calcium-biochar composites combine high catalytic efficiency with practical recyclability, offering a low-energy and sustainable pathway for the chemical recycling of PET waste.