Base Catalytic Activity of [Nb<sub>10</sub>O<sub>28</sub>]<sup>6–</sup>: Effect of Countercations
Shun Hayashi, Seiji Yamazoe, Tatsuya Tsukuda
Abstract
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 found to be 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 indicated that TAAn (n = 1, 2) acts as a poison of the base catalyst of Nb10. The adsorption–desorption equilibrium constants of TAAn (Kn) estimated by 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 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.