烯烃在Pd/C催化剂上加氢中的取代基效应和含氮添加剂的影响

SUBSTITUENT EFFECT AND N-CONTAINING ADDITIVE INFLUENCE ON THE CATALYTIC HYDROGENATION OF OLEFINS ON PALLADIUM

本文研究了Pd/活性炭催化剂对烯烃催化加氢的性能,考察了双键碳原子上取代基与加氢速率的关系,探讨了添加吡啶系列化合物对加氢速率的影响,添加吡啶类化合物可提高烯烃的加氢速率;烯烃催化加氢速率随吡啶类化合物N原子上的电子密度Q_M增大而增大。并证明取代基团从双键碳原子上吸电子或由氮原子向金属钯供给孤对电子,都有利于Pd对>C—C<π~*轨道反馈电子而活化了双键。

The active carbon supported palladium catalyst (Pd/C) was prepared by impregnating active carbon with PdCl_2 and reducing with HCHO. The substituent effect and the N-containing additive influence on the hydrogenation of olefins on Pd/C catalyst was studied, which shows that: 1. The initial rate R_0 of catalytic hydrogenation of olefins is influenced largely by the substituent on (?). There is a linear relationship between log Re and the Taft substituent constant ∑σ~*: log R_0:=0.27 ∑σ~*-2.11, which shows that the electron withdrawing substituent on (?) increase the hydrogenation rate of olefins. In otherword, lowering the electron density on (?) may facilate the d-electron of Pd transfering to π~* bond of olefins,thus weaken the π bond. 2. When the pyridine is added to the catalytic hydrogenation system, the hydrogenation rate increases with the increasing of pyridine concentration (Cpy) and then decreases after Cpy over 6.21×10^(-3) mol/L. It can be explained as following: when the Cpy is low, the adsorbed (coordinated) pyridine donate electron to metallic Pd and then trahsfer to the π~* orbital of (?),which activate the (?) and increase the hydrogenation rate. Howener, when the Cpy increase, the competive adsorption of pyridine with olefin on Catalyst surface becomes significant, which decreases the hydrogenation rate. 3. With HMO method, the electron density (Q_N) of nitrogen atom in substimed pyridine has been calculated, there is a linear relationship between the Q_N and the hydrogenation rate (log R_0 ) of hexene-1: log R_0=15.41Q_N-19.06. Equations shows that either substituent on (?) withdrawing electron from double bond or the nitrogen atom in pyridine derivative donating electron to Pd will weaken the (?), thus increase the hydrogenation rate.