A Marriage of POM and MOF

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Science  04 Nov 2011:
Vol. 334, Issue 6056, pp. 569
DOI: 10.1126/science.334.6056.569-c
CREDIT: SONG ET AL., J. AM. CHEM. SOC. 133, 10.1021/JA203695H (2011)

Polyoxometalates (POMs) are clusters, typically bearing a net negative charge, that assemble from multiple metal oxide units sharing oxygens and have shown useful catalytic activity for a range of organic oxidation reactions. Metal organic framework (MOF) materials are three-dimensional porous structures that can be designed with specific pore size and composition. As such, they have found use for gas adsorption, separation, and in some cases catalysis. Song et al. show that the Keggin-type POM [CuPW11O39]5− can fit snugly into the pores of MOF-199, leading to a strong enhancement of catalytic properties for the oxidation of thiols to disulfi des and the removal of hydrogen sulfide. Analysis of the unit cells shows that the MOF remains intact after inclusion of the POM and associated (tetramethyl)ammonium counter-ion into adjacent large and small pores, and that 50% of the large pores remain empty, allowing for easy movement of reactants and products. Aerobic H2S oxidation was achieved from aqueous solutions at a rate of 4000 turnovers within 20 hours and was also observed under gas-phase conditions but with lower turnover as the pores were blocked by sulfur accumulation. Tests with a non–copper-containing POM did not show catalytic activity, indicating that the Cu centers in the POM unit are probably the active sites. Air-based oxidations of thiols to disulfides showed excellent selectivity, although decreasing yield as the size of the thiol substituents increased, leading to steric hindrance within the MOF.

J. Am. Chem. Soc. 133, 10.1021/ja203695h (2011).

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