Adsorption behavior of ZIF composites with different loading methods of lacunary polyoxometalates towards cationic dyes

Abstract

In this study, two novel composite materials, SiW₁₁@ZIF-67 and ZIF-67@SiW₁₁, were synthesized via in situ encapsulation and post-loading methods, respectively. By leveraging the high negative charge characteristics of the vacancy polyoxometalate (SiW₁₁) in synergy with the stability of the ZIF-67 framework, the adsorption performance for the cationic dye neutral red (NR) was significantly enhanced. Experimental results demonstrated that under alkaline conditions (pH 8–11), the adsorption capacities of both materials for NR exceeded 35.5 mg g⁻¹. SiW₁₁@ZIF-67 exhibited excellent structural stability and regeneration performance because the polyoxometalate was embedded in the cavities of ZIF-67 (the adsorption efficiency remained >90% after multiple methanol elutions). In contrast, ZIF-67@SiW₁₁ showed significantly accelerated adsorption kinetics (reaching 90% saturation within 1 min) due to the surface-exposed SiW₁₁. The combination of electrostatic interactions and hydrogen bonds synergistically enhanced the selective adsorption of NR (40% higher than that of the anionic dye methyl orange). This study provides an innovative strategy for the design of vacancy polyoxometalate/MOF composite materials. It lays a theoretical and technical foundation for the development of efficient, renewable adsorbents suitable for the treatment of industrial alkaline dye wastewater and paves the way for their extended applications in complex dye systems.