Magnetic Mesoporous γ-Fe2O3 Supports from Meso-MIL-88A via Controlled Oxidation–Reduction Pyrolysis: Enabling Recyclable Lipase Catalysis in Phospholipid Synthesis

Enzyme immobilization on solid supports enhances stability and reusability, yet nanoscale carriers such as metal–organic frameworks (MOFs) still face challenges in efficient recovery. While pyrolysis can magnetize Fe-MOFs, conventional methods often compromise either enzyme activity or structural integrity. This study presents a rational two-step oxidation–reduction (O–R) pyrolysis strategy to convert Meso-MIL-88A into a magnetically recyclable, mesoporous biocatalyst support (O-R500). Unlike one-step carbonization, which generates enzyme-incompatible Fe3O4, or carbonization-oxidation routes that collapse the framework, our approach first transforms the MOF into a robust α-Fe2O3 template while preserving its morphology. Citric acid then acts as a mild, slow-releasing reductant, selectively producing a γ-Fe2O3-rich phase without damaging the mesostructure. The resulting O-R500 exhibits well-defined mesopores (∼13 nm), sufficient magnetization (16 emu/g) for rapid separation, and a biocompatible surface that maintains the native conformation of immobilized Candida antarctica lipase B (CalB). In the synthesis of phosphatidyl EPA/DHA, CalB@O-R500 achieved 84.5% incorporation and retained 90.3% activity over five cycles, outperforming nonmagnetic counterparts. This work not only provides a high-performance magnetic biocatalyst but also establishes a generalizable design principle for converting Fe-MOFs into structured, biocompatible, and functionally integrated carriers.