Construction of a boric acid-functionalized MOF via metal-ligand-fragment coassembly for Stevia glycoside separation

Abstract

Rebaudioside A is a promising natural sweetener, but its separation from steviol glycoside (sGs) mixtures remains challenging. The rational design of crystalline porous adsorbents with abundant specific binding sites presents a promising solution. A boronic acid-functionalized metal-organic framework (BA-MIL) was synthesized via metal-ligand-fragment co-assembly (MLFC) strategy, using terephthalic acid as the parent ligand and incorporating ligands (p-phenylenediboronic acid, 4-carboxyphenylboronic acid, 3-carboxyphenylboronic acid and 4-formylphenylboronic acid). By adjusting the type and proportion of these fragment ligands, the pore size and the density of boronic acid groups in the adsorbent were accurately controlled. Results demonstrated that BA-MIL(4-CPB)-70 featured a 2.60 nm pore size, high surface area (1360 m²/g), and dense boronic acid groups. By combining molecular sieving and boronate affinity, BA-MIL(4-CPB)-70 efficiently separated stevioside and Rebaudioside A. When applied to a crude sGs extract, it achieved high adsorption capacities (Q_STV = 145.2 mg/g, Q_RA = 126.1 mg/g), selectivity (3.04), and excellent recyclability.