Basicity of isostructural porous ionic crystals composed of Nb/Ta-substituted Keggin-type polyoxotungstates

Abstract

Three isostructural porous ionic crystals (PICs) based on Keggin-type POMs with different compositions but equal negative charge ([BW12O40]5– (BW12), [SiW11NbO40]5– (SiW11Nb), and [SiW11TaO40]5– (SiW11Ta)) are synthesized. Experimental and theoretical characterizations...

CO2 Activation on Lindqvist-Type Polyoxotantalate: Structural Analysis by In Situ HERFD-XANES

“…In recent years, the base catalysis of POMs has also attracted attention. Mizuno and co-workers reported that mono-oxo tungstate [WO 4 ] 2– , which is one of the building units of POMs and defective Keggin-type POMs like [γ-H 2 GeW 10 O 36 ] 6– , acted as a homogeneous base catalyst. − Our group and Ge et al found that group V (Nb, Ta) POMs such as [Nb 6 O 19 ] 8– , [Ta 6 O 19 ] 8– , [Nb 10 O 28 ] 6– , and [SiNb 12 O 40 ] 16– exhibit superior base catalysis to group VI metal polyoxometalates for Knoevenagel condensation and CO 2 fixation reactions. − Those base properties come from the highly negatively charged surface oxygen atoms in {MO 6 } units. , Since the cluster size and valence affect the charge density of oxygen atoms in {MO 6 }, the [Nb 6 O 19 ] 8– with highly negatively charged oxygen exhibits higher base catalytic activity for Knoevenagel condensation reactions than [Nb 10 O 28 ] 6– . , In addition, Keggin-type [SiW 11 NbO 40 ] 5– and [SiW 11 TaO 40 ] 5– that were substituted with {NbO 6 } and {TaO 6 } units show base catalysis for Knoevenagel condensation reactions . Thus, the base catalysis of Nb- and Ta-based POMs is strongly related to the negative charges on oxygen atoms of the {NbO 6 } and {TaO 6 } units, respectively, owing to their electronic and geometric structures.…”

“…20,22 In addition, Keggin-type [SiW 11 NbO 40 ] 5− and [SiW 11 TaO 40 ] 5− that were substituted with {NbO 6 } and {TaO 6 } units show base catalysis for Knoevenagel condensation reactions. 26 Thus, the base catalysis of Nb-and Ta-based POMs is strongly related to the negative charges on oxygen atoms of the {NbO 6 } and {TaO 6 } units, respectively, owing to their electronic and geometric structures. We recently reported the controlled product selectivity for the CO 2 fixation reaction into styrene oxide over Nb-and Ta-mixed metal oxide clusters; single Ta substitution into [Nb 6 O 19 ] 8− improved the selectivity toward styrene carbonate while suppressing the polymerization of styrene oxide.…”