Investigation of Congo Red Adsorption Behavior by Metallo-Hydrogen-Bonded Organic Networks Based on Tripodal Ligands

Metallo-Hydrogen-Bonded Organic Networks (MHONs), a novel category of organic–inorganic hybrid crystalline materials, have demonstrated broad applications in gas adsorption, selective metal ion separation, and solid-state dehydrohalogenation reactions. However, their excellent solubility in protonic solvents, such as methanol, has left their potential as an adsorbent largely underexplored. In this work, a series of novel MHONs have been synthesized based on two tripodal ligands L1 and L2 (L1 = 1,3,5-tri(3′-(1′-methyl)imidazoliummethyl)-2,4,6-trimethylbenzene, L2 = 2,4,6-trimethyl-1,3,5-tris(morpholinomethyl)benzene) with SnX4 (X = Cl, Br). These novel networks exhibited poor solubility in the protic solvent methanol and demonstrated good adsorption performance for the negatively charged dye Congo Red (CR), achieving removal rates of 92.4, 94.2, and 88.4%. Adsorption kinetic and isotherm studies indicated that the adsorption process followed pseudo-second-order kinetics and the Freundlich isotherm model. The MHONs showed great selective adsorption for CR in mixed dyes, which is attributed to the strong electrostatic interactions between the anionic dyes and the MHONs, as proved by the Fourier transform infrared spectroscopy (FT-IR) and ζ-potential.