Boronic acid-functionalized hollow COFs with hierarchical porosity for selective separation of stevioside and rebaudioside A

Stevioside (STV) and rebaudioside A (RA), which dominate the composition of steviol glycosides (SGs), pose great challenges to efficient isolation owing to the resemblance in their molecular structure and physicochemical characteristics. Herein, hollow architecture was introduced into COFs to lower mass transfer resistance and further expose carboxyl binding sites dedicated to boronic acid post-functionalization, which in turn enables selective adsorption. TAPTA-DMTP was first synthesized, followed by hollow structure construction through a Fenton reaction. The resulting H-TAPTA-DMTP preserved a high specific surface area of 1484 m 2 /g and exhibited a hierarchical pore structure with pore sizes centered at 1.8, 2.3, and 3.9 nm, which improved the total equilibrium adsorption capacity toward STV and RA from 742.2 to 948.8 mg/g. Subsequently, 3-aminophenylboronic acid was grafted onto H-TAPTA-DMTP to obtain a series of H-TAPTA-BA (0.5-2.0) adsorbents. Among them, H-TAPTA-BA (1.5) delivered the best separation performance, achieving an adsorption selectivity (α STV/RA = 3.55) by combining hierarchical pore and boronic acid affinity interactions. Furthermore, it exhibited excellent recyclability after five consecutive adsorption-desorption cycles in crude SGs solution. This work provides an effective strategy for integrating structural engineering with functional group modification in COFs, enhancing the adsorption and separation performance of high-value natural products SGs.