Pore engineering of phenylboronic covalent organic framework membranes for Steviol glycosides separation

The adsorption-diffusion sites in the channels of the COF membrane markedly enhance SGs diffusion. • Its phenylboronic acid groups selectively form dynamic boronate ester bonds with the vicinal diol moieties of SGs. • This specific interaction enables a hopping-diffusion mechanism of SGs in the TAPB-BuPa COF membrane. • The TAPB-BuPa COF membrane exhibited the SGs transmission of 98.1% and the permeance of 175 L m −2  h −1  bar −1 . Membrane separation offers high efficiency, environmental compatibility, and precise molecular-level discrimination, making it highly promising for the purification of steviol glycosides (SGs). However, the directional construction of high-permeance membranes that allow efficient SGs transmission remains challenging. To address these issues, we constructed adsorption-diffusion functional sites within the well-defined nanochannels of the covalent organic framework (COF) membrane. The hydrophobic TAPB-TFB COF membrane from 1,3,5-tris(4-aminophenyl)benzene (TAPB) and 1,3,5-triformylbenzene (TFB), and the hydrophilic boronic-acid-functionalized TAPB-BuPa COF membrane from TAPB and 3,5-diformylphenylboronic acid (BuPa) were fabricated via liquid-liquid interfacial polymerization. The TAPB-TFB COF membrane exhibited SGs transmission of 45.4% and the permeance of 48 L m −2  h −1  bar −1 . The TAPB-BuPa COF membrane created a hydrophilic affinity microenvironment within its ordered nanochannels. Its pendant phenylboronic acid groups selectively form dynamic, reversible boronate ester bonds with the vicinal diol moieties of SGs. This specific interaction enabled a hopping diffusion mechanism. As a result, the permeance increased 3.6-fold to 175 L m −2  h −1  bar −1 , and achieved a high SGs transmission of 98.1%. Elucidating this microenvironment-controlled switch from adsorption to diffusion provides a design strategy for high-permeance membranes tailored for natural product concentration. In the TAPB-BuPa COF membrane, PBA specifically forms bonds with the adjacent dihydroxy groups on the SGs molecules, causing the bound SGs to undergo hopping-diffusion along the phenylboronic acid sites lining the pore walls. This established pore microenvironment determines the adsorption-to-diffusion switching paradigm, opening a new pathway for engineering COF-based membranes that use dynamic covalent bonds as the recognition mechanism, and providing a theoretical basis and material design strategy for efficient separation of natural products. Download: Download high-res image (168KB) Download: Download full-size image