Base Catalytic Activity of [Nb₁₀O₂₈]^6–: Effect of Countercations

Tetraalkylammonium salts of decaniobate ((C n H 2 n +1 ) 4 N + ) 6 [Nb 10 O 28 ] 6– ( TAA n Nb 10 ) ( n = 1–4) were applied as homogeneous catalysts for the Knoevenagel condensation reaction between benzaldehyde and phenylacetonitrile. The base catalytic activity of TAA n Nb 10 was found to be dependent on the alkyl chain length n of TAA n; the initial reaction rate ( r n ) increased monotonously with n and reached a plateau at n = 3. The catalytic activity of TAA 4 Nb 10 decreased with the addition of tetraalkylammonium bromide (C n H 2 n +1 ) 4 N + Br – ( TAA n Br ) ( n = 1, 2). These results indicated that TAA n ( n = 1, 2) acts as a poison of the base catalyst of Nb 10 . The adsorption–desorption equilibrium constants of TAA n ( K n ) estimated by a simple kinetic model decreased monotonously with n and became negligible at n ≥ 3. The anticorrelation between K n and r n led us to conclude that the catalytic poisoning by TAA n ( n = 1, 2) is caused by the blocking of the base sites of Nb 10 because of the electrostatic interaction. The findings suggest that a countercation with a small charge density is suitable for the base catalytic application of polyoxometalates.