Bakelite-type anionic microporous organic polymers with high capacity for selective adsorption of cationic dyes from water

A new family of bakelite-type anionic microporous organic polymers (MOPs) is described in this work, which was assembled by the condensation reaction between anionic unit tri(protocatechuic aldehyde)-silicate and phloroglucinol or resorcinol by reference to the synthetic method of bakelite. The new materials exhibit uniform and solid microspheres, remarkably high BET specific surface areas of 1846.5 and 2206.7 m²/g, and a narrow pore size distribution around 1 nm region. The adsorption capacities of MOP-1 and MOP-2 toward cationic Methylene blue (MB) and Malachite green (MG) run up to 712.2, 593.6, 233.8 and 324.7 mg/g at 25 °C, respectively, but little adsorption was found for the anionic dye Methyl Orange (MO) and large size cationic dye basic blue 7 (BB7), exhibiting charge and size- selectivity. The adsorption thermodynamics and kinetics were investigated and a pseudo-second-order kinetic model and Langmuir isotherm model was found to be suitable for the adsorption process of MB and MG on MOP-1 and MOP-2. The negative values of the Gibbs free energy and the positive values of enthalpy changes indicated that adsorptions were spontaneous and endothermic. Moreover, the absorbents MOP-1 and MOP-2 can be re-used at least four times in the adsorption of MB and MG with less than 5% loss of adsorption capacity. Overall, MOP-1 and MOP-2 were found to be inexpensive, feasible and efficient materials for removal dyes, presenting future prospects in research and applications in the purification of water contaminated by organic dyes.