Copper-based metal–organic frameworks as highly efficient catalysts for the biomimetic catalytic synthesis of theaflavins

Abstract

Theaflavins (TFs) are the principal quality and active compounds present in fermented tea, generated during the fermentation process. Due to the exceedingly low concentration of TFs in black tea, the in vitro synthesis of TFs has emerged as a predominant trend in industrial production. Despite significant advancements in the in vitro research of TFs in recent years, several challenges remain in the preparation of TFs according to established literature methods. These challenges include the high cost of natural enzyme preparation, difficulties in preservation, and poor thermal and chemical stability. Consequently, the development of efficient and stable catalysts has emerged as a critical issue for the industrial preparation of TFs. This study investigated the feasibility of utilizing Cu-BTC (copper(II) benzene-1,3,5-tricarboxylate) metal–organic frameworks (MOFs) as biomimetic catalysts for the synthesis of TFs from catechins. Cu-BTC was synthesized using a solvothermal method and characterized. The synthesized Cu-BTC exhibited an octahedral structure and demonstrated commendable thermal stability. The enzyme-like activity of Cu-BTC was evaluated using catechol as the substrate, yielding kinetic parameters of V_max = 0.0338 mM s⁻¹ and K_m = 14.19 mM, which indicated substantial polyphenol oxidase-like catalytic activity. Cu-BTC was employed to catalyze the oxidation of catechins to synthesize TFs. The results indicated that the total yield of catechins using Cu-BTC as a biomimetic catalyst was 30% higher than that achieved through chemical oxidation methods and 50% higher than that of tyrosinase. The optimal catalytic reaction conditions were determined as follows: a reaction temperature of 80 °C, a reaction time of 60 minutes, a pH of 5.0, and a Cu-BTC dosage of 0.05 g mL⁻¹, resulting in a total yield of TFs of 800 μg mL⁻¹. This study verified that the copper-based MOF Cu-BTC was not only facile to prepare, but also exhibited excellent catalytic activity and thermal stability, which opens promising prospects for the development of new biomimetic catalysts for the synthesis of TFs.