纳米金催化肉桂醛选择加氢制肉桂醇

α,β-不饱和醛/酮选择加氢生成不饱和醇是化学工业中一类重要反应,在精细化工生产中具有广泛应用,近年来吸引了研究者的广泛关注.该类反应因涉及不饱和官能团和碳氧双键的选择加氢而颇具挑战性:以肉桂醛选择加氢生成肉桂醇反应为例,肉桂醛分子中同时含有共轭的C=C双键和C=O双键,从热力学角度上看, C=O双键键能比C=C双键键能大,因而碳碳双键比碳氧双键更容易被活化从而加氢得到饱和醛;从动力学角度上看, C=C双键也比C=O双键更容易加氢.对于传统的铂族贵金属催化剂,其应用于该类反应时往往存在选择性低,容易深度加氢等问题.负载型金催化剂此前被报道在该类反应中表现出高选择性,然而在反应物接近完全转化时,目标产物也容易发生过度加氢生成饱和醇.前期的研究结果发现用锌铝水滑石作载体,硫醇稳定的金原子团簇(Au25)作为金的前驱体制备负载型金催化剂时,其在不饱和芳香硝基化合物的选择加氢反应中表现出很高的选择性.考虑到在肉桂醛分子中C=O双键的加氢相比于C=C双键更加困难,因此,本工作尝试将上述催化剂应用于以肉桂醛为代表的不饱和醛/酮选择加氢反应中.考察了反应温度、氢气压力以及溶剂效应对反应活性的影响,结果发现升高温度或提高压力都能明显提升反应速率,然而不同的溶剂对催化性能影响很大,当以具备氢转移能力的异丙醇和乙醇作为反应溶剂时,催化活性和选择性最优,在反应温度为130 ℃,氢气压力为15 atm,异丙醇为溶剂时反应5 h,肉桂醛的转化率和肉桂醇的选择性可以达到98.3%和95.4%,并且延长反应时间至15h,目标产物也不会发生过度加氢生成苯丙醇,其选择性可以维持在95%以上.为了研究该催化剂高活性和高选择性的原因,制备了不同粒径大小和不同载体负载的金催化剂,结果发现相比于其它负载型金催化剂,以锌铝水滑石负载的Au25团簇作为催化剂前体制得的催化剂在肉桂醛选择加氢制肉桂醇反应中表现出最优的活性和选择性.对照实验和原位漫反射红外光谱测试表明上述催化剂对碳碳双键的加氢表现为惰性,对目标产物的吸附也相对较弱.27Al固体核磁共振结果表明配位不饱和的五配位Alp物种可能为C=O双键的优先吸附提供所需的氧空位,这可能是该催化剂具有较高选择性的原因.综上,推测小尺寸的金颗粒具有较多低配位的金原子,可以活化氢气,而反应物和产物的吸脱附性质与载体密切相关,在以锌铝水滑石为前驱体制备的金催化剂表面, C=C双键吸附较弱, C=O双键优先吸附,产物较容易脱附,不容易发生过度加氢反应,因此该催化剂在肉桂醛选择加氢反应中表现出高活性和高选择性.上述工作可以为设计制备高选择性的负载型金催化剂提供参考.

Producing of cinnamyl alcohol from cinnamaldehyde oversupported gold nanocatalyst

Chemoselective hydrogenation of unsaturated aldehyde to unsaturated alcohol has attracted grow-ing interests in recent years due to its widespread applications in fine chemicals. However, the hy-drogenation of the C=O bond was thermodynamically and kinetically unfavorable over the hydro-genation of the C=C bond. Thus, to obtain the unsaturated alcohol from the unsaturated aldehyde is very difficult in most of the catalytic systems. In this work, ZnAl-hydrotalcite-supported cyste-ine-capped Au25 nanoclusters were used as the precatalysts for chemoselective hydrogenation of cinnamaldehyde to cinnamyl alcohol. The catalyst showed stable high selectivity (～95％) at pro-longed reaction time and complete conversion of the substrate. According to the results of the con-trol experiments, the in-situ DRIFTS of the substrate under high pressure of hydrogen and the 27Al MAS-NMR spectroscopy, we proposed that the difference of the preferential adsorption of the C=O bond to that of the C=C bond was derived from the nature of the support of the gold catalysts.