Effect of Counter Cations on Base Catalysis of [Nb10O28]6−

Abstract

The tetraalkylammonium salts of decaniobate ((CnH2n+1)4N+)6[Nb10O28]6− (TAAnNb10; n = 1–4) were applied as homogeneous catalysts for the Knoevenagel condensation reaction between benzaldehyde and phenylacetonitrile. The base catalytic activity of TAAnNb10 was dependent on the alkyl chain length n of TAAn; the initial reaction rate (rn) increased monotonously with n and reached a plateau at n = 3. The catalytic activity of TAA4Nb10 decreased with the addition of tetraalkylammonium bromide (CnH2n+1)4N+Br− (TAAnBr; n = 1, 2). These results indicate that TAAn (n = 1, 2) acts as a poison for the base catalyst Nb10. The adsorption–desorption equilibrium constants of TAAn (Kn) estimated using a simple kinetic model decreased monotonously with n and became negligible at n ≥ 3. The anticorrelation between Kn and rn led us to conclude that the catalytic poisoning by TAAn (n = 1, 2) is caused by the blocking of the base sites of Nb10 due to the electrostatic interaction. These findings suggest that a counter cation with a small charge density is suitable for the application of polyoxometalates as base catalysts.