摘要
本文研究了聚苯乙烯系强碱性阴离子交换树脂负载的钯催化剂对取代硝基苯催化加氢的性能,结果表明,取代基的电子性质及空间位置对加氢速率影响很大;反应活化能与取代常数σ之间呈线性关系:E_1=-20.7σ+47.0在该反应体系中存在补偿效应:log A_(red)=0.14E_a-6.
The polymer-supported palladium catalyst (Pd/ D290) was prepared by anchoring palladium on strong basic anion-exchange resin ((P)-N+(CH3) 3D290). Its catalytic activity for the hydrogenation of nitro-compounds was investigated. Experimental results show that the rate of catalytic hydrogenation is significantly influenced by the electronic property and the position of substituent on nitrobenzene. The hydrogenation rates of all substituted nitrobenzenes are lower than that of nitrobenzene itself, i. e. all substituents retard the hydrogenation of -NO2. For different substituents on the same position of nitrobenzene, the hydrogenation rate decreases in the order:
-H> -NH2> -CH3> -OCH3> -Cl. For the same substituent on different position, the hydrogenation rate is p- > o-substituted nitrobenzene. We also studied the influence of substituents on the activation energy ( Ea) of hydrogenation of substituted nitrobenzenes. The results show that there is a linear relationship between Eaand Hammett substituent constant (σ): Ea = -20.7σ+47.0, which demonstrates that the Ea decreases with the increase of the withdrawing -electron ability of substituent from benzene ring.
Furthermore, we discussed the compensation effect in this hydrogenation system. There is a linear relationship between Ea and the logarithm of relative pre-exponential factor (Ared ):
logAred=0.14Ea-6.86
From the above two equations, we can see that Eadecreases linearly with the increase of substituent constant (α), but the pre-exponential factor decreases exponentially with the decrease of Ea. So the co-effect of Ea and Ared on hydrogenation rate (R) result in that there is no linear relationship between R and a. Therefore, it is not reasonable to conclude that the Hammett relationship can't be applied to heterogeneous catalytic system.
出处
《分子催化》
EI
CAS
CSCD
1989年第1期36-39,共4页
Journal of Molecular Catalysis(China)
