A Hierarchical Enzyme-Mimetic Supramolecular Catalyst Derived from Folic Acid-KOH/Cu Hydrogel: Self-Assembly Study and Mechanism for Enhanced Oxidation of Phenolic Compounds.

The rational design of enzyme-mimetic catalysts through supramolecular self-assembly offers a promising avenue for robust, cost-effective, and tunable green catalysis avoiding biological limitations. In this work, we demonstrate the development of a hierarchical supramolecular catalyst derived from folic acid (FA), a natural low-molecular-weight gelator (LMWG), through a two-step process involving pH-triggered self-assembly and copper-(II)-induced hydrogelation. FA was first solubilized in aqueous KOH (pH ≈ 8.5) to form folate ions, which underwent time-dependent self-assembly into fibrillar structures. Upon addition of Cu-(II), these self-assembled structures cross-linked into a stable hydrogel matrix (FA-KOH/Cu). The resulting xerogels, derived from both fresh and aged FA-KOH solutions, displayed distinct morphologies and catalytic performances. Aged hydrogels (FA-KOH/Cu-Aged) exhibited higher mechanical strength and catalytic activity than their freshly prepared counterparts (FA-KOH/Cu-Fresh), owing to the development of a flake-like microstructure with enhanced surface area and redox accessibility. These xerogels efficiently catalyzed the oxidative coupling of 2,4-dichlorophenol and 4-aminoantipyrine, mimicking laccase activity. Kinetic studies revealed enzyme-like Michaelis-Menten behavior, with the FA-KOH/Cu-Aged xerogel showing superior catalytic efficiency (k cat/K m = 235 × 10-5 (g·L-1)-1·s-1) compared to the FA-KOH/Cu-Fresh xerogel (k cat/K m = 399 × 10-5 (g·L-1)-1·s-1). This work highlights how aging-guided hierarchical nanostructure formation critically modulates catalytic efficiency and substrate accessibility. Overall, our findings demonstrate a biomimetic design framework that integrates molecular self-assembly, metal coordination, and supramolecular material engineering to develop sustainable and responsive catalysts for environmental applications.