ZIF-8 and UiO-66 Enzyme Biocomposites for Waste Cooking Oil Valorization

Introduction

Metal-organic frameworks (MOFs) are porous materials with high surface area, tunable structures, and significant potential for enzyme immobilization. ZIF-8 is particularly attractive due to its mild synthesis conditions, suitability for one-pot encapsulation, and the biocompatibility of Zn²⁺ ions, whereas zirconium-based UiO-66 offers a more rigid and chemically stable structure for post-synthetic immobilization. In this work, lipases, peroxidase, and laccase were immobilized using both approaches. The activity and stability of the resulting biocomposites were evaluated, and the most promising systems were applied in waste cooking oil (WCO) epoxidation and transesterification.

Methods

For one-pot immobilization, the linker, metal precursor, and enzyme were mixed under optimized synthesis conditions. In the post-synthetic approach, the MOF was first synthesized and subsequently subjected to enzyme immobilization by adsorption. The resulting biocomposites (enzyme@ZIF-8 and enzyme@UiO-66) were characterized by FTIR-ATR, powder X-ray diffraction (XRD), and thermogravimetric analysis (TGA-DSC). Catalytic activity was assessed using p-nitrophenyl butyrate for lipases and ABTS for oxidoreductases. Reusability was evaluated through repeated catalytic cycles.

Results

Immobilization efficiency, activity, and stability depended on the enzyme type, immobilization method, and MOF structure. Bare MOFs exhibited intrinsic catalytic activity in both epoxidation and transesterification; however, enzyme immobilization generally enhanced catalytic performance. enzyme@UiO-66 biocatalysts showed superior activity in epoxidation (>50% conversion), whereas enzyme@ZIF-8 systems performed better in transesterification (>60% conversion). Moreover, the immobilized systems retained most of their activity after several reaction cycles, demonstrating good recyclability.

Conclusions

MOF-based biocatalysts represent promising platforms for enzyme immobilization and sustainable WCO valorization. The comparative evaluation of different enzymes, MOF structures, and immobilization strategies provides useful insights for the rational design of efficient and recyclable biocatalysts.