One-Pot Synthesis of Carboxyl-Functionalized Hyper-Cross-Linked Microporous Polymer Based on the Dual-Purpose Reagent Strategy for the Efficient Removal of Methylene Blue from Water

Methylene blue (MB) is extensively used as a coloring agent in the textile and pharmaceutical industries, causing ecological risk and physical defunctions. Unfortunately, the adsorbents commonly used to remove MB from the environment are also limited by disadvantages, such as harsh synthesis conditions and poor adsorption capacity. In this paper, we present a simple and low-cost method for preparing carboxyl-functionalized hyper-cross-linked microporous polymers (HCP_TPB-COOH) capable of efficiently removing MB from water. To reduce chemical reagent consumption and improve productive efficiency, aluminum trichloride was employed as a catalyst for both Friedel-Craft alkylation and Friedel-Craft acylation reactions; meanwhile, dichloromethane served the dual function of solvent and cross-linker reagent. The successful synthesis of HCP_TPB-COOH was confirmed by clearly characterizing its chemical structure, morphology, and pore distribution. Due to its rich pore structure (Brunauer–Emmett–Teller (BET) = 720.2 m² g^–1) and multiple interactions, including hydrophobic interactions, π–π stacking, hydrogen bonding, and electrostatic interactions, the developed HCP_TPB-COOH exhibited excellent adsorption capacity for MB compared to most traditional adsorbents. In particular, the organic contaminants were efficiently adsorbed from the aqueous solution despite the small amount of adsorbent and the presence of other ionic species, and they also exhibited excellent reusability with a minimal decline in adsorption performance after at least six adsorption cycles. Adsorption isotherms demonstrated that HCP_TPB-COOH was more consistent with the Langmuir adsorption model, with a maximum adsorption of 371 mg g^–1 for MB. The adsorption kinetics indicated that the pseudo-second-order dynamics model was more compatible with MB. The presence of negative Gibbs free energy change indicated that the adsorption process was spontaneous.