Enzyme–MOF Composites: Quantitative Benchmarking and Evaluation of Biocatalyst Performance

The integration of enzymes into crystalline framework materials offers a promising route to combine the selectivity of biocatalysis with the practical advantages of heterogeneous catalysts. In this work, we present a quantitative benchmarking study of enzyme immobilization within the calcium benzene‐1,4‐dicarboxylate (CaBDC) metal–organic framework (MOF) using two structurally and functionally distinct model enzymes: phenolic acid decarboxylase (PAD) and cytochrome c (CC). An in situ synthesis route enabled the formation of enzyme@CaBDC composites under mild aqueous conditions. We establish rigorous and transparent evaluation practices for enzyme@MOF systems, with particular emphasis on reliable determination of enzyme loading and activity. Quantitative analyses show that encapsulation preserves biocatalytic activity but that apparent reaction rates are frequently limited by substrate accessibility and mass‐transport effects. Immobilization in CaBDC does not improve the intrinsic thermal or solvent stability of the already robust enzymes studied here. However, it successfully converts soluble biocatalysts into recoverable solid formulations that facilitate reuse. Immobilization of pre‐reduced CC yielded catalytically active ^red CC@CaBDC composites enabling carbene‐transfer reactions in non‐aqueous media. Overall, this work provides a methodological template for the quantitative assessment of enzyme@MOF composites and highlights key limitations that must be considered when interpreting immobilization effects and comparing different systems.