碱性钠添加剂在苯酚选择性加氢中的作用

doi:10.1016/S1872-2067(19)63386-3

摘要/Abstract

摘要： 环己酮是重要的大宗化工原料，其下游产品广泛应用于纺织品、电子电器、塑料工业、石油化工和医药等领域.苯酚选择性加氢副产物少，制备得到的环己酮纯度高，在工业上被认为是最具潜力的高品质环己酮生产工艺.然而，由于环己酮容易过加氢到环己醇，因此如何在维持催化剂高活性的前提下，提高催化剂对环己酮的选择性是苯酚选择性加氢催化剂开发的重点和难点.相关工作表明，碱助剂的添加有利于提高过渡金属催化剂的催化性能，但这些助剂在催化反应中的作用机理尚不明确，在苯酚加氢中的作用机制亦存在争议.本文使用不同碱性钠助剂（Na₂CO₃，NaHCO₃和NaOH），通过分步浸渍法制备了一系列的Na-Pd/Al₂O₃催化剂，并利用一氧化碳（CO）化学吸附，X射线光电子能谱（XPS），密度泛函理论计算（DFT）等手段研究了钠助剂的加入如何提高苯酚气相选择性加氢的催化活性.
对系列Na-Pd/Al₂O₃催化剂进行苯酚气相加氢测试后发现，由于碱性钠助剂的添加，苯酚转化率从8.3%提高到> 99%，环己酮选择性从89%提高到> 97%，连续1200 h的反应后未观察到性能衰减，表明该催化体系具有优异的稳定性.CO化学吸附结果表明，添加Na₂CO₃，NaHCO₃和NaOH后，Pd分散度分别从31.8%增加到36.6%、40.2%和41.5%.这可归因于钠助剂和Pd前驱体离子之间反应形成的碱性Pd物质，如PdCO₃，Pd（HCO₃）₂，Pd（OH）₂等与Al₂O₃的酸性位点结合，促进金属颗粒的分散.XPS结果显示，添加Na₂CO₃，NaHCO₃和NaOH后，金属态Pd的含量从55.2%分别增加到92.6%，75.0%和82.3%.DFT结果也表明，吸附在Pd表面的钠助剂会向Pd转移0.34-0.80个电子，与XPS结果相符，均说明了钠助剂可以调控Pd的电子结构，使Pd处于富电子状态，并促进H₂在Pd表面的活化.进一步的DFT研究表明，不同的钠助剂均可以将苯酚的解离吸附能从-0.48eV降至-0.80eV左右，说明钠助剂可以促进苯酚的酚羟基解离形成酚氧基.H₂在高分散且富电子的Pd纳米颗粒表面被快速活化形成H原子，与酚氧基反应形成环己酮.另一方面，环己酮的羰基和钠助剂之间形成的"-C=O-Na-"结构能有效地抑制了环己酮的过加氢和偶联反应.

关键词: 环己酮, 苯酚, 碱助剂, 机理, 选择性加氢

Abstract: The selective hydrogenation of phenol to cyclohexanone is an important process in the chemical industry. However, achieving high selectivity at high conversion rates is highly challenging, particularly under continuous reaction conditions. Here, we found that the presence of Na alkaline additives (NaX, X=CO₃^2-, HCO₃^-, or OH^-) on Pd/Al₂O₃ not only promoted the phenol conversion from 8.3% to >99% but also increased the cyclohexanone selectivity from 89% to >97% during the continuous hydrogenation of phenol on a fixed bed reactor. After 1200 h of continuous reaction, no activity or selectivity attenuation was observed and the turnover number was approximately 2.9×10⁵. Density functional theory calculations, spectroscopic, and dynamics studies demonstrated that the addition of NaX greatly promoted phenol adsorption and hydrogen activation, thereby improving catalytic activity. Simultaneously, the formation of a "-C=O-Na-" intermediate inhibited the excessive hydrogenation and intermolecular coupling of cyclohexanone, leading to high selectivity.

Key words: Cyclohexanone, Phenol, Alkaline additive, Mechanism, Selective hydrogenation

陈宇卓, 孔祥千, 毛善俊, 王哲, 巩玉同, 王勇. 碱性钠添加剂在苯酚选择性加氢中的作用[J]. 催化学报, 2019, 40(10): 1516-1524.

Yuzhuo Chen, Xiangqian Kong, Shanjun Mao, Zhe Wang, Yutong Gong, Yong Wang. Study of the role of alkaline sodium additive in selective hydrogenation of phenol[J]. Chinese Journal of Catalysis, 2019, 40(10): 1516-1524.

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