Adsorptive removal of cationic methylene blue dye in a robust metal-azolate framework

Organic pollutants found in river streams necessitate the development of effective adsorbents for decontamination. The presence of dyes such as methylene blue (MB), among others, remains the leading challenge. Herein, we report the solvothermal synthesis of a double-walled zinc-azolate framework (ZnBDP) (1) using benzene-1,4-bis(pyrazole) (BDP) and zinc nitrate hexahydrate in dimethylformamide and a modulator. Single crystal X-ray diffraction (SCXRD) analysis showed that the framework (1) contains one-dimensional channels with two distinct large pore apertures. An adsorption capacity of 604.5 mg g^-1 at an equilibrium concentration of 2.9 mg L^-1 was obtained in aqueous solution during MB adsorption experiments, indicating that it is a better performing adsorbent. The experimental data fitted the pseudo-second order kinetics and the Freundlich isotherm model suggesting the interplay between MB and ZnBDP, consistent with a heterogeneous surface adsorption phenomenon. The adsorption mechanism of cationic (MB⁺) dye could be ascribed to the available pore channels and the electrostatic interactions. Our findings offer structural insight to guide the future design of robust porous materials for outstanding adsorption capabilities.