Incorporating Polyoxometalates into a Porous MOF Greatly Improves Its Selective Adsorption of Cationic Dyes

• Published in: Chemistry : a European journal 2014-06, Vol.20 (23), p.6927-6933
• Main Authors: Yan, Ai-Xue, Yao, Shuang, Li, Yang-Guang, Zhang, Zhi-Ming, Lu, Ying, Chen, Wei-Lin, Wang, En-Bo
• Format: Article
• Language: English
• Subjects: Acetal resins; Adsorption; Cationic dyes; Chemistry; Composite materials; Dyes; Inductively coupled plasma; Materials selection; metal-organic frameworks; polymers; polyoxometalates; Polyoxometallates; Selective adsorption
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201400175

Various polyoxometalates (POMs) were successfully immobilized to the mesoporous coordination polymer MIL‐101 resulting in a series of POM–MOF composite materials POM@MIL‐101 (POM=K4PW11VO40, H3PW12O40, K4SiW12O40). These materials were synthesized by a simple one‐pot reaction of Keggin POMs, tetramethylammonium hydroxide (TMAH), terephthalic acid (H2bdc), and Cr3+ ions. XRD, FTIR, thermogravimetric analyses (TG), inductively coupled plasma (ICP) spectrometry, and energy‐dispersive X‐ray spectroscopy (EDX) collectively confirmed the successful combination of POMs and the porous framework. Further, these composites POM@MIL‐101 with different loading of POMs were achieved by variation of the POM dosage. Notably, the uptake capacity of MIL‐101 towards organic pollutants in aqueous solution was significantly improved by immobilization of hydrophilic POMs into cages of MIL‐101. An uptake capacity of 371 mg g−1, comparable to that of the graphene oxide sponges, and much higher than that of the commercial activated carbon, was achieved at room temperature in 5 min when dipping 20 mg PW11V@MIL‐101 in the methylene blue (MB) solution (100 mL of 100 mg L−1 MB solution). Further study revealed that the POM@MIL‐101 composite materials not only exhibited a fast adsorption rate towards dye molecules, but also possessed of selective adsorption ability of the cationic dyes in wastewater. For example, the adsorption efficiency of PW11V@MIL‐101 (10 mg) towards MB (100 mL of 10 mg L−1) could reach 98 % in the initial 5 min, and it could capture MB dye molecules from the binary mixture of the MB and MO with similar size. Also, the POM@MIL‐101 materials could be readily recycled and reused, and no POM leached in the dye adsorption process. The adsorption efficiency and the selective adsorption ability of MIL‐101 towards organic dyes in aqueous solution were significantly improved by immobilizing polyoxometalates (POMs) into cages of MIL‐101. The POM@MIL‐101 composites could be readily recycled and reused, and no POM leached during the dye adsorption process (see figure; MB=methylene blue).