Base Catalytic Activity of [Nb10O28]6-: Effect of Countercations
Base Catalytic Activity of [Nb10O28]6-: Effect of Countercations
复制标题
DOI：
10.1021/acs.jpcc.0c01488
复制
发表时间：
2020-05-21
影响因子：
3.7
通讯作者：
Tsukuda, Tatsuya
中科院分区：
文献类型：
--
作者：
Hayashi, Shun;Yamazoe, Seiji;Tsukuda, Tatsuya
癸酸四烷基铵盐 ((CnH2n+1)(4)N+)(6)[Nb10O28](6-) (TAA(n)Nb(10)) (n = 1-4) 用作苯甲醛和苯乙腈之间 Knoevenagel 缩合反应的均相催化剂。发现TAA(n)Nb(10)的碱催化活性取决于TAA(n)的烷基链长度n；初始反应速率(r(n))随n单调增加，并在n = 3时达到稳定水平。TAA(4)Nb(10)的催化活性随着四烷基溴化铵(CnH2n+1)(4)N+Br- (TAA(n)Br) (n = 1, 2)的添加而降低。这些结果表明 TAA(n) (n = 1, 2) 作为 Nb-10 碱催化剂的毒物。通过简单的动力学模型估计的 TAA(n) (K-n) 的吸附-解吸平衡常数随着 n 单调下降，并且在 n >= 3 时变得可以忽略不计。K-n 和 r(n) 之间的反相关使我们得出结论，TAA(n) (n = 1, 2) 的催化中毒是由于静电相互作用而阻塞 Nb-10 的碱基位点引起的。研究结果表明，电荷密度小的抗衡阳离子适合多金属氧酸盐的碱催化应用。
Tetraalkylammonium salts of decaniobate ((CnH2n+1)(4)N+)(6)[Nb10O28](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 (CnH2n+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.