Which modification for which feedstock? a strategy-by-strategy review of Zr-MOF catalysts for biodiesel production.

Catalyst design is central to producing sustainable biodiesel from waste feedstocks. Homogeneous bases such as NaOH and KOH are highly active but cause corrosion, saponification, and difficult separation, while conventional heterogeneous catalysts suffer from low surface area and poor tunability. Zirconium-based metal-organic frameworks (Zr-MOFs) such as UiO-66 have emerged as attractive alternatives, combining high porosity, exceptional chemical and thermal stability, and tunable Lewis and Brønsted acidity. Pristine Zr-MOFs, however, are limited by low acid site density and by mass transfer resistance toward bulky triglycerides. This review assesses five structural modification strategies reported: metal and node defect engineering, pore engineering, linker and ligand functionalization, composite formation, and enzyme immobilization and, unlike previous surveys, compares them directly against one another rather than in isolation. Each strategy is evaluated on yield, reaction severity, reusability, and tolerance to water and free fatty acids, and matched to the feedstock class it is best suited to address. We show that the central constraint on catalyst design is a trade-off between acid site density and pore accessibility, and that the highest-performing catalysts are those that decouple the two. We further show that only about a third of the studies surveyed test genuine triglyceride transesterification; the remainder use isolated fatty acids or non-lipid model reactions such as benzaldehyde acetalization, levulinic acid conversion, and short-chain ester synthesis, leading to a systematic overstatement of catalytic performance in such literature. Future directions and the challenges of zirconium cost, long-term stability, and scalable green synthesis are discussed.