Leaching-free encapsulation of cobalt-polyoxotungstates in MIL-100 (Fe) for highly reproducible photocatalytic water oxidation

Abstract Leaching of polyoxometalate (POM) anions from metal-organic frameworks MOFs during catalytic performance is an important issue towards stability and activity of POM-MOF composite materials. For this purpose, a careful and judicious choice of POM and MOF along with the correct synthetic procedure is crucial. Herein, two different cobalt-functionalized polyoxometalate anions, [CoIICoIIIW11O39(H2O)]7− (Co2) and [Co4(PW9O34)2(H2O)2]10− (Co4), were successfully encapsulated in the MIL-100 (Fe) metal-organic framework to form Co2@MIL-100 (Fe), (1) and Co4@MIL-100 (Fe), (2) composites. Maximum loading of 48% and 54% was achieved for Co2 and Co4, respectively. The synthesized composites were characterized by PXRD, FTIR, TGA, elemental analysis, and nitrogen adsorption studies. The total surface area of pure MIL-100 (2800 m2/g) decreases to 1035 m2/g and 1189 m2/g after encapsulation of Co2 and Co4, respectively. Photocatalytic water oxidation ability of the synthesized materials was investigated under the visible light irradiation. The results revealed that the composite materials showed higher oxygen yield (i.e. 41% for 1 and 72% for 2) and turnover frequencies (i.e. 0.53 for 1 and 9.2 × 10−3 s−1 for 2) as compared to virgin Co2 and Co4. The used photocatalysts can be easily recovered via centrifugation and recycled.