Zn对非晶态Co-B催化剂巴豆醛选择加氢性能的影响

考察了Zn的两种不同修饰方式对非晶态Co-B催化剂上巴豆醛液相加氢性能的影响, 并采用ICP, XRD, DSC, XPS等手段对催化剂进行了系统的表征. 实验表明, 当反应液中加入适量的ZnCl₂时, Zn^2＋离子一方面通过在活性位上的选择性吸附抑制C＝C键的加氢; 另一方面, Zn^2＋离子可通过极化作用使羰基活化, 令解离吸附氢对羰基的亲电进攻更容易, 从而提高羰基的加氢活性. 当ZnCl₂/Co摩尔比为5%时, 羰基加氢产物巴豆醇的最高得率为36 mol%, 达到文献较高水平. 催化剂制备时加入的ZnCl₂在催化剂中以Zn(OH)₂的形式存在, 并使催化剂中的Co, B氧化物增多. 这些氧化物无选择性地覆盖了活性位, 使羰基和C＝C键的加氢速率以相似的程度下降, 这样不仅不能有效提高巴豆醇的选择性, 还使催化剂活性急剧降低.

Selective Hydrogenation of Crotonaldehyde over Amorphous Co-B Catalysts: The Effect of Zn

The promoting effect of Zn on liquid phase hydrogenation of crotonaldehyde over amorphous Co-B catalysts has been investigated. It was found that when ZnCl2 was introduced to the reactant, zinc ions could selectively block the active sites for C=C bond hydrogenation. Moreover, zinc ions might polarize the carbonyl group as Lewis acid, making the nucleophilic attack of adjacent dissociatively adsorbed hydrogen atom much easier, thus leading to a higher yield of crotyl alcohol. On the other hand, in amorphous Co-Zn-B catalysts, zinc was in the form of Zn(OH)2 and the amount of CoO and B2O3 increased with more zinc in- corporated. These oxides were expected to nonselectively cover the active sites and depress the hydrogena- tion rates of the C=C bond and the carbonyl group to a similar extent, thus marginally improving the selec- tivity towards crotyl alcohol.