Synthesis of a Stable Benzoxazole Gel from an Imine Gel for Adsorption and Catalysis

Developing stable gel materials for adsorption and catalysis is one of the major themes of gel materials. However, it has been proven to be challenging to achieve them from small molecules. Herein, an imine gel is developed from tetra-aldehyde 4-{2,2-bis­[(4-formylphenoxy)­methyl]-3-(4-formylphenoxy)­propoxy}­benzaldehyde (A4) and 3,3′-dihydroxybenzidine (B2) based on dynamic covalent chemistry. The unstable A4B2-imine gel is further converted into a stable aromatic benzoxazole-linked A4B2-benzoxazole gel via oxidative cyclization, which has significantly improved chemical stability under acidic and basic conditions. Benefiting from the stability under acidic conditions, the A4B2-benzoxazole gel is used for Pd­(II) adsorption and the adsorption capacity is 250 mg g–1. After PdCl2 immersion and reduction, palladium nanoparticles with a size distribution of 1.3–14.7 nm are encapsulated by the network structure of the stable porous benzoxazole gel matrix. The Pd@A4B2-benzoxazole gel exhibits high catalytic activity toward the reduction of toxic hexavalent chromium Cr­(VI) (reaction rate constant = 0.0377 min–1), while there is no significant decrease in the catalytic efficiency after five cycles.