Base Catalytic Activity of [Nb_{10}O_{28}]~{6-}: Effect of Counter Cation
S. Hayashi, S. Yamazoe, T. Tsukuda
J. Phys. Chem. C, in press.
Tetraalkylammonium salts of decaniobate ((C_{n}H_{2n+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 (rn) 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_{2n+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} (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 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.