The Mechanism of the Wacker Reaction: A Tale of Two Hydroxypalladations

We present a concise review on the most pertinent investigations that illuminate the complicated and elusive mechanism for the Wacker process, homogeneous olefin oxidation by palladium(II) catalysts. For more than four decades, multitudes of creative and elegant studies detailing the nucleophilic addition and other steps of the Wacker process have appeared contradictory, while in fact modern perspective has shown an intricate and colorful picture of the “textbook” organometallic reaction. A summary and critical analysis of previous studies is of great importance to explain resolved and highlight unresolved questions about this frequently misunderstood reaction.     

Cooperative Catalysis of Noncompatible Catalysts through Compartmentalization: Wacker Oxidation and Enzymatic Reduction in a One‐Pot Process in Aqueous Media

A Wacker oxidation using CuCl/PdCl₂ as a catalyst system was successfully combined with an enzymatic ketone reduction to convert styrene enantioselectively into 1‐phenylethanol in a one‐pot process, although the two reactions conducted in aqueous media are not compatible due to enzyme deactivation by Cu ions. The one‐pot feasibility was achieved via compartmentalization of the reactions. Conducting the Wacker oxidation in the interior of a polydimethylsiloxane thimble enables diffusion of only the organic substrate and product into the exterior where the biotransformation takes place. Thus, the Cu ions detrimental to the enzyme are withheld from the reaction media of the biotransformation. In this one‐pot process, which formally corresponds to an asymmetric hydration of alkenes, a range of 1‐arylethanols were formed with high conversions and 98–99 % ee. In addition, the catalyst system of the Wacker oxidation was recycled 15 times without significant decrease in conversion.     

Recent progress in Wacker oxidations: moving toward molecular oxygen as the sole oxidant

Wacker oxidations, the conversion of terminal olefins to methyl ketones with Pd(II) catalysts, have seen widespread use in synthetic applications. Standard synthetic Wacker conditions use catalytic PdCl2 with stoichiometric CuCl under an aerobic atmosphere in a mixed-solvent system of N,N-dimethylformamide and H2O. Though much attention has been directed toward elucidating the rate-determining step and the mechanism of nucleopalladation, the assumption that Cu does not participate in this portion of the catalytic cycle has recently been called into question based on an isolated Pd/Cu bimetallic species and the influence of Cu on product selectivity. Fortunately, recent advancements have been made toward the elimination of Cu additives, thereby alleviating these issues. Success in this area has come from the application of information gained in studying other direct-O2-coupled Pd(II) oxidation systems, including ligand modulation, something which could not be achieved in the presence of Cu. The developments in peroxide-mediated and direct-O2-coupled Wacker oxidations are highlighted herein.     

Hydroformylation of Olefins by a Rhodium Single‐Atom Catalyst with Activity Comparable to RhCl(PPh3)3

Homogeneous catalysts generally possess superior catalytic performance compared to heterogeneous catalysts. However, the issue of catalyst separation and recycling severely limits their use in practical applications. Single‐atom catalysts have the advantages of both homogeneous catalysts, such as “isolated sites”, and heterogeneous catalysts, such as stability and reusability, and thus would be a promising alternative to traditional homogeneous catalysts. In the hydroformylation of olefins, single‐atom Rh catalysts supported on ZnO nanowires demonstrate similar efficiency (TON≈40000) compared to that of homogeneous Wilkinson's catalyst (TON≈19000). HAADF‐STEM and infrared CO chemisorption experiments identified isolated Rh atoms on the support. XPS and XANES spectra indicate that the electronic state of Rh is almost metallic. The catalysts are about one or two orders of magnitude more active than most reported heterogeneous catalysts and can be reused four times without an obvious decline in activity.     

高活性铜催化剂无受体乙醇催化脱氢制乙醛研究

相较于Wacker工艺进行乙醛工业化生产,发展多相催化体系实现乙醇直接无氧催化脱氢制乙醛和副产氢气,从生产工艺和经济价值方面无疑是一条更加安全高效的路线.在此,我们发展了一种高效、稳固的Cu/SiO₂催化剂,用于乙醇的无受体催化脱氢.通过氨蒸发法制备得到高度分散的Cu颗粒,在没有任何平衡气体的纯乙醇进料条件下,显示出超强的热稳定性.活性组分Cu和载体SiO₂之间的强相互作用,使其具有优异的催化性能.通过反应条件优化,在250℃下实现了较高的乙醇转化率(>40%)和乙醛选择性(>95%),且催化剂在固定床连续反应过程中可稳定运行超过400 h.     

沸石和活性炭为载体的Fe3+和Cu2+型催化剂催化氧化苯酚的比较

以沸石和活性炭为载体，制备了Fe3 和Cu2 型沸石和活性炭催化荆，研究了非均相Fe和Cu催化剂催化氧化高浓度含酚废水．在Fenton反应机理的基础上，探讨了Cu2 的均相和非均相催化氧化机理，以人造沸石和活性炭为栽体制备了相应的4种非均相催化剂，进行了均相、非均相Fe和Cu催化剂催化氧化高浓度含酚废水的对比试验，分析了这两种载体的比表面积、孔径分布和中孔孔容，比较了4种非均相催化剂对苯酚降解率的影响．结果发现Cu2 不仅能大大提高反应速率，而且其均相、非均相反应体系的苯酚降解率均可达到约97％．     

金单原子催化剂上一氧化碳低温氧化

CO低温氧化对于基础研究和实际应用均具有重要意义.自上世纪八十年代日本的 Haruta教授发现氧化物负载金催化剂对 CO氧化的超高活性以来,负载金催化剂受到了广泛关注与深入研究,被认为是目前活性最高的 CO氧化催化剂.在诸多影响 CO氧化活性的因素中,纳米金的粒子尺寸是最重要因素之一.目前主流观点认为对于 CO氧化,纳米金有一个最优尺寸范围,在0.5–5 nm,而 Au原子/离子(Au3+, Au+)的活性则低一到两个数量级.因此,一般认为负载金单原子催
　　化剂对于 CO氧化反应的活性要比金纳米粒子和团簇低很多.然而,最近几年的理论与实验研究均表明,金单原子负载于合适的载体上可以显示出与金纳米粒子和团簇相当的活性.本文对这些新进展进行综述,阐述金单原子催化剂对 CO氧化的独特反应性能. Gates教授研究组进行了大量关于正价金对 CO氧化影响的研究,其中包括孤立的金原子(Au+).他们的研究发现, CO氧化活性随价态降低而降低,表明正价金对 CO氧化至关重要.此外,他们的研究也表明,孤立金原子对 CO氧化的活性(TOF)比金纳米粒子低一到两个数量级.然而,在他们的研究中,有几个因素可能导致催化剂的低活性.首先,他们一般采用非或弱还原性的载体.而载体的还原性对金催化剂上 CO氧化活性影响非常巨大.另外,他们所用的金原子前驱体为配合物,在催化剂制备与反应过程中配体并没有去除,可能也是导致催化剂活性低的原因之一.与此相反,张涛课题组近期采用氯金酸为前驱体,通过简单的吸附浸渍法制备了一系列负载金单原子催化剂.同时用相同的载体制备了负载金纳米粒子催化剂进行对比,可以排除载体等其它影响因素.对比结果显示,单原子催化剂均显示出与纳米粒子相当的 TOF(单位表面 Au原子)和更高的反应速率(单位重量金).首先制备了氧化铁负载金单原子催化剂,该催化剂在室温即展现出可观活性, TOF值与2–3 nm金粒子 TOF值相当(~0.5 s–1).更有趣也更重要的是,该催化剂在高温(200oC以上)展现出非常高的反应稳定性,在200oC反应100 h无失活.在300和400oC反应50 h也无失活发生,为开发高温稳定的金催化剂提供了新途径.其次制备了氧化钴负载金单原子催化剂,该催化剂以0.05%金负载量即可实现室温全转化,其 TOF值高达1.4 s–1.然而该催化剂在达到高活性之前必须首先在反应气氛中进行高温处理,这限制了其实用性.此外,催化剂需经反应气氛活化的原因尚待进一步研究.随之又制备了氧化铈负载金单原子催化剂,对富氢条件下 CO选择氧化不仅具有高活性,而且具有极高的 CO选择性.进一步研究结合理论计算表明,高选择性来自氧化铈负载的金单原子不能解离活化氢,对于氢气氧化活性极低,从而导致 CO氧化的高选择性.理论研究方面也有进展. Camellone等计算发现金原子可以取代 CeO2(111)面上的 Ce原子形成 Au+并促进 CO氧化.然而该金原子会扩散至氧空位形成带负电荷的 Auδ-,阻止 CO和 O2吸附,因而使催化剂失活.李隽课题组利用从头算分子动力学模拟首次发现氧化铈和氧化钛负载的 Au纳米粒子在 CO氧化过程中可以形成单原子的现象,并将之称为动态单原子催化剂. Yang等则计算了二维材料 BN负载 Au单原子催化 CO氧化并发现反应优先通过三原子 E-R机理进行.     

PdCl2-CuCl2/Al2O3催化剂低温催化CO氧化的失活机理

采用X射线衍射、N₂吸-脱附、X射线光电子能谱分析、氢气-程序升温还原和原位红外漫反射等方法对新鲜和失活的PdCl₂-CuCl₂/Al₂O₃低温催化CO氧化催化剂进行表征,研究了高相对湿度(100%)下催化剂的失活机理.结果表明,催化剂表面沉积的水使得活性铜物种容易从催化剂表面向载体孔道内部迁移,由于Pd、Cu相互作用弱化从而减弱了Pd与Cu物种间的相互作用,使得催化剂的氧化还原性能受到影响,抑制了Pd⁰再氧化为Pd²⁺的过程,从而因CO氧化反应中催化剂氧化还原循环受阻而导致失活.     

Choline chloride/CuCl as an effective homogeneous catalyst for palladium-free Sonogashira cross-coupling reactions

Choline chloride–CuCl (ChCl–CuCl) has been employed as an efficient catalytic system in the palladium-free Sonogashira-type cross-coupling reactions of phenylacetylene with a variety of aryl halides. Choline chloride, as an effective ligand and a quaternary ammonium salt, demonstrated an efficient stabilizing effect on the Cu(I) species during the reaction. This homogeneous catalytic system is attractive in view of its ease of application and low cost.     

基于立方烷结构的分子催化剂在光催化水氧化中的研究进展

随着化石燃料大量使用带来的气候变化和环境污染问题日趋严重,寻找清洁高效的可再生能源用做传统化石燃料的替代品,已经成为当前的研究热点。光驱动的水分解反应被认为是太阳能制氢的可行途径。水的全分解包括两个半反应-水的氧化和质子还原。其中水的氧化反应是一个涉及四个电子和四个质子转移的复杂过程,需要很高的活化能,被认为是全分解水反应的瓶颈步骤。因此,开发高效、稳定、廉价丰产的水氧化催化剂是人工光合作用突破的关键因素。立方烷具有类似自然界光合作用酶光系统II(PSII)活性中心Mn_4CaO_5簇的结构,世界各国的科学家受自然界光合作用的启发,开发出了许多基于过渡金属的立方烷结构的催化剂,常见的有锰、钴和铜等立方烷催化剂。本文简要地综述了近年来立方烷分子催化剂在光催化水氧化中的研究进展。首先介绍了立方烷基光催化水氧化反应历程,继而详细介绍了基于有机配体的立方烷配合物和全无机的多金属氧酸盐立方烷水氧化催化剂,其次是半导体(BiVO4或聚合的氮化碳(PCN))为捕光材料复合立方烷分子催化剂的水氧化体系最新研究进展。最后总结并展望了该领域所面临的挑战及其前景。     

单原子催化剂合成方法

单原子催化剂作为一种原子尺度的催化剂,在制氢、CO氧化及光催化等领域均具有广阔的应用前景。大量实验结果和理论计算证实了金属单原子和载体之间的相互作用,及由两者之间电荷转移引起的电子结构改变是单原子催化剂具有高的选择性和催化活性的主要原因。本文着重综述了近年来共沉淀法、化学还原法及浸渍法所制备单原子催化剂的催化性能,并进行展望。     

Formal Aza‐Wacker Cyclization by Tandem Electrochemical Oxidation and Copper Catalysis

In oxidative electrochemical organic synthesis, radical intermediates are often oxidized to cations on the way to final product formation. Herein, we describe an approach to transform electrochemically generated organic radical intermediates into neutral products by reaction with a metal catalyst. This approach combines electrochemical oxidation with Cu catalysis to effect formal aza‐Wacker cyclization of internal alkenes. The Cu catalyst is essential for transforming secondary and primary alkyl radical intermediates into alkenes. A wide range of 5‐membered N‐heterocycles including oxazolidinone, imidazolidinone, thiazolidinone, pyrrolidinone, and isoindolinone can be prepared under mild conditions.     

金属单原子催化剂增强硫正极动力学的研究进展

单质硫具有理论能量密度高(2600 Wh·kg-1)、放电比容量高(1672mAh·g-1)、成本低等优势,是锂硫电池的理想正极材料。然而,在充放电过程中硫正极迟缓的反应动力学显著地限制了锂硫电池的性能。金属单原子催化剂(SMACs)具有独特的电子结构、金属含量低、理论上100%的原子利用率、催化活性高等优势,其不仅有效地促进了不同中间相的转化反应,而且可为含硫物质提供丰富的锚定位点,从而显著优化硫正极氧化还原反应动力学、多硫化物的穿梭行为和锂硫电池电化学性能。本文以剖析金属单原子催化剂与硫正极间的相互作用为出发点,结合其催化效应表征技术,重点解析了不同类型单原子催化剂的构筑策略、活性调控及其优化硫正极氧化还原行为的机制,展望了金属单原子催化剂在锂硫电池领域面临的挑战和未来发展方向。     

Catalytic properties of monomeric and oligomeric Cu(II) phenylhydrazono imine chelates immobilized on zeolite with a topological anchor

Heterogeneous catalysts for liquid-phase oxidation of cyclohexene with molecular oxygen were prepared by topological-anchor immobilization on CaA zeolite of a monomeric Cu(II) chelate with N,N′-trimethylenebis(2-methyl-1-benzoylglyoxal 1-phenylhydrazone 2-imine) and of an oligomeric product of its condensation with ethylenediamine. The samples prepared contain surface monomeric and oligomeric Cu(II) chelates oriented toward the reaction medium, which makes them accessible to reactant molecules. The catalytic properties of the above monomeric chelate are preserved in going to the oligomers. The apparent rate constants per metal center for both homogeneous (soluble monomeric chelate) and heterogeneous modes coincide, suggesting the pseudohomogeneous behavior of the supported samples. The topological-anchor immobilization of insoluble oligomeric Cu(II) polychelates on zeolite leads to their more efficient utilization as heterogeneous catalysts and can be considered as one of examples of combining advantages of homogeneous and heterogeneous catalysis.     

Oxidation of Carbon Monoxide over Cu/CeO_2 Catalysts Prepared by SMAI

Introduction Carbon monoxide is ubiquitous air pollutants emitted by many sources. Total oxidation of carbon monoxide to carbon dioxide is employed to meet environmental regulations in an economic way. Precious metals (Pt, Pd) are well-known oxidation catalysts with high activity and stability, and are widely used for exhaust gas emission control. However, the high cost of precious metals and their sensitivity to sulfur poisoning have long motivated the search for substitute catalysts. …     

Cs_yP_（1．33）－kMo_（12）SblCu_mV_nAs_kO_x催化剂的研究

以共沉淀方法制备了杂多酸盐催化剂ＣｓｙＰ１．３３－ｋＭｏ１２ＳｂｌＣｕｍＶｎＡｓｋＯｘ．利用ＩＲ及ＳＥＭ研究了催化剂的结构，ＤＳＣ研究了催化剂的稳定性，ＥＳＲ研究了催化剂的氧化还原性，及ＮＨ３－ＴＰＤ研究了催化剂的酸性．同时考察了催化剂对异丁醛一步氧化制甲基丙烯酸的催化性能，并与催化剂的氧化还原性进行了关联．发现Ｃｓ１．５Ｐ１．１３Ｍｏ１２Ｓｂ０．２５Ｃｕ０．２５Ｖ０．２５Ａｓ０．２０Ｏｘ催化剂使异丁醛氧化的目的产物甲基丙烯酸收率最高，达７０．１％．     

Ordered Porous Nitrogen‐Doped Carbon Matrix with Atomically Dispersed Cobalt Sites as an Efficient Catalyst for Dehydrogenation and Transfer Hydrogenation of N‐Heterocycles

Single‐atom catalysts (SACs) have been explored widely as potential substitutes for homogeneous catalysts. Isolated cobalt single‐atom sites were stabilized on an ordered porous nitrogen‐doped carbon matrix (ISAS‐Co/OPNC). ISAS‐Co/OPNC is a highly efficient catalyst for acceptorless dehydrogenation of N‐heterocycles to release H₂. ISAS‐Co/OPNC also exhibits excellent catalytic activity for the reverse transfer hydrogenation (or hydrogenation) of N‐heterocycles to store H₂, using formic acid or external hydrogen as a hydrogen source. The catalytic performance of ISAS‐Co/OPNC in both reactions surpasses previously reported homogeneous and heterogeneous precious‐metal catalysts. The reaction mechanisms are systematically investigated using first‐principles calculations and it is suggested that the Eley–Rideal mechanism is dominant.     

Pd（OAc）2／FePc催化环己烯氧化合成环己酮的研究

考察了几种Ｆｅ－大环配合物与Ｐｄ（ＯＡｃ）２组成的双组分催化体系，在乙腈酸性水溶液中环己烯经合成环己酮的催化活性，实验结果表明，其中以酞菁失（ＦｅＰｃ）与Ｐｄ（ＯＡｃ）２组成的催化体系活性最高，而ＦｅＴＰＰＣｌ与Ｐｄ（ＯＡｃ）２催化体系，虽然催化活性较高，但催化剂的稳定性较低，各种因素对Ｐｄ（ＯＡｃ）２／ＦｅＰｃ催化活催化影响的研究结果指出，在无水和酸存在的非水溶液中，Ｐｄ（ＯＡｃ）２／ＦｅＰｃ对     

Synthesis,characterization and catalytic activity of polymer anchored transition metal complexes toward oxidation reactions

The polymer-anchored Schiff base complexes of Cu(II), Co(II), Ni(II), Mn(II) and Fe(III) were prepared by reacting polystyrene amine with 2-pyridinecarbaldehyde followed by loading of metal atom in methanol. These complexes were characterized by using different physico-chemical and spectroscopic methods. The catalytic activity of these polymer-supported metal catalysts was tested for the oxidation of various olefins and alcohols. Influence of various reaction parameters, such as reaction temperature, reaction time, oxidant, substrate-to-oxidant mole ratio and nature of solvent, was studied for the oxidation of cyclohexene with these catalysts. Among the catalysts studied, Cu-Cat showed higher catalytic activity toward oxidation reactions than the other catalysts. Moreover, hot filtration experiments proved that these catalysts are truly heterogeneous and can be reused a number of times without significant loss of activity.     

Noble‐Metal‐Free Single‐Atom Catalysts CuAl4O7–9− for CO Oxidation by O2

The single copper atom doped clusters CuAl₄O_7–9^? can catalyze CO oxidation by O₂. The CuAl₄O_7–9^? clusters are the first group of experimentally identified noble‐metal free single atom catalysts for such a prototypical reaction. The reactions were characterized by mass spectrometry and density functional theory calculations. The CuAl₄O₉CO^? is much more reactive than CuAl₄O₉^? in the reaction with CO to generate CO₂. One adsorbed CO is crucial to stabilize Cu of CuAl₄O₉^? around +I oxidation state and promote the oxidation of another CO. The widely emphasized correlation between the catalytic reactivity of CO oxidation and Cu oxidation state can be understood at the strictly molecular level. The remarkable difference between Cu catalysis and noble‐metal catalysis was discussed.