On the Origin of the Rate Enhancement by a R‐(+)‐1‐(1‐Naphthyl)‐Ethylamine‐Modified Pd(111) Model Catalyst for Methyl Pyruvate Hydrogenation
On the Origin of the Rate Enhancement by a R‐(+)‐1‐(1‐Naphthyl)‐Ethylamine‐Modified Pd(111) Model Catalyst for Methyl Pyruvate Hydrogenation
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R-( )-1-(1-萘基)-乙胺-修饰Pd(111)模型催化剂丙酮酸甲酯加氢提速的根源
DOI：
10.1002/cctc.202201078
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发表时间：
2023
期刊：
影响因子：
4.5
通讯作者：
Tysoe, Wilfred T.
中科院分区：
文献类型：
--
作者：
Bavisotto, Robert;Roy, Sree Pradipta;Tysoe, Wilfred T.
多相催化剂的手性修饰剂可以作为活性促进剂，以最大限度地减少未修饰位点对对映体过量的影响，从而获得高对映体选择性的催化剂。然而，这种效应的起源并不清楚。研究了R-（+）-1-（1-萘基）-乙胺（R-1-NEA）/Pd（111）模型催化剂催化丙酮酸甲酯（MP）加氢合成乳酸甲酯（ML）的反应。模型催化剂的活性在多次周转中保持恒定。对于R-1-NEA覆盖率低于0.5单层（ML），未发现速率增强，但在R-1-NEA覆盖率为0.75 ML时发现显著增加，速率约为未改性催化剂的两倍。 这是使用红外光谱法进行研究，以区分MP单体和二聚体。用氢气进行的MP滴定实验显示出半级氢气压力依赖性，单体的反应速率是二聚体的两倍。发现二聚体是清洁Pd（111）上最丰富的物种，但由于表面拥挤，单体与二聚体的比率随着R-1-NEA覆盖度的增加而增加。单体物种也被发现是更稳定的拥挤的表面上比干净的Pd（111），手性改性剂也起到稳定反应物。最后，这个模型很好地解释了反应性的不寻常的1-NEA-覆盖依赖性。
Chiral modifiers of heterogeneous catalysts can function as activity promotors to minimize the influence of unmodified sites on the enantiomeric excess to obtain highly enantioselective catalysts. However, the origin on this effect is not well understood. It is investigated using a model catalyst of R‐(+)‐1‐(1‐naphthyl)‐ethylamine (R‐1‐NEA)/Pd(111) for the hydrogenation of methyl pyruvate (MP) to methyl lactate (ML). The activity of the model catalyst remains constant for multiple turnovers. No rate enhancement is found for R‐1‐NEA coverages below ∼0.5 monolayer (ML), but a significant increase is found at R‐1‐NEA coverages of ∼0.75 ML, with a rate approximately twice that of the unmodified catalyst. This is investigated using infrared spectroscopy to distinguish between MP monomers and dimers. MP titration experiments with hydrogen show a half‐order hydrogen pressure dependence, with the monomer reacting at twice the rate as the dimer. It is found that the dimer is the most abundant species on clean Pd(111), but the ratio of monomers to dimers increases as the R‐1‐NEA coverage increases due to surface crowding. The monomeric species is also found to be more stable on the crowded surface than on clean Pd(111); the chiral modifier also serves to stabilize the reactant. Finally, this model nicely explains the unusual 1‐NEA‐covergae dependence of the reactivity.