Porphyrin-Based Conjugated Microporous Polymers for Highly Efficient Adsorption of Metal Ions

Abstract

The capture and elimination of anions and cations from water have attracted a great deal of attention and are quite vital for clean production and environmental remediation. In this work, we present the synthesis of four porphyrin (Por)-based conjugated microporous polymers (CMPs, namely, Por-CMP-1-4), which were produced through a Sonogashira-Hagihara linked response using porphyrin and acetylene aromatic compounds as building blocks and used as absorbents to eliminate metal ions from water. The as-synthesized Por-CMP-1-4 exhibit an amorphous porous structure and outstanding caloric and physicochemical properties. Taking advantage of their larger specific surface areas, i.e., 541.47, 614.58, 382.38, and 677.90 m² g^-1 for Por-CMP-1-4, respectively, and their chelating active site that originated from the porphyrin ring, Por-CMP-1-4 show better Zn²⁺, Cu²⁺, and Pb²⁺ adsorption ability. Among them, Por-CMP-3 has the greatest adsorbability of 640 mg g^-1 for Zn²⁺, with an adsorption efficiency of 80%, whereas its adsorption capacities for Cu²⁺ and Pb²⁺ ions were both 334 mg g^-1, with an adsorption efficiency of 42% for Cu²⁺ and Pb²⁺. Employing Por-CMP-3 as a representative example, its adsorption kinetics has been systematically investigated. The adsorption behavior of Por-CMP-3 with respect to the Zn²⁺ ion is shown to exhibit pseudo-first-order kinetics and Langmuir isotherm modes. Meanwhile, the adsorption mechanism is discussed in detail, and it was thought it might be chelation, in which the nitrogen atoms with a single pair of electrons on the porphyrin ring interacted with metal ions to form stable chelation coordination bonds, thus removing metal ions selectively and effectively. Furthermore, Por-CMP-3 exhibited good reusability, retaining 60% of its Zn²⁺ removal rate after four continuous adsorptions.