Zn对非晶态Co-B催化剂巴豆醛选择加氢性能的影响

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

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,Zn2+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,Zn2+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36mol%,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.

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 incorporated. These oxides were expected to nonselectively cover the active sites and depress the hydrogenation rates of the C=C bond and the carbonyl group to a similar extent, thus marginally improving the selectivity towards crotyl alcohol.