天然光合放氧反应的理论研究进展

光合放氧复合物是位于光系统II中氧化水的催化中心。对其结构、性质以及催化机理的研究不仅能够深入了解植物光合作用机制,也是人工合成稳定、高效、廉价的水氧化催化剂的基础。由于光合放氧复合物结构的复杂性,现代计算量子化学方法在对其研究中起着极为重要的作用。本文综述了天然水氧化催化剂的理论研究进展,总结了理论研究工作的重点,并展望了未来的发展方向。     

The possible role of proton-coupled electron transfer (PCET) in water oxidation by photosystem II

All higher life forms use oxygen and respiration as their primary energy source. The oxygen comes from water by solar-energy conversion in photosynthetic membranes. In green plants, light absorption in photosystem II (PSII) drives electron-transfer activation of the oxygen-evolving complex (OEC). The mechanism of water oxidation by the OEC has long been a subject of great interest to biologists and chemists. With the availability of new molecular-level protein structures from X-ray crystallography and EXAFS, as well as the accumulated results from numerous experiments and theoretical studies, it is possible to suggest how water may be oxidized at the OEC. An integrated sequence of light-driven reactions that exploit coupled electron-proton transfer (EPT) could be the key to water oxidation. When these reactions are combined with long-range proton transfer (by sequential local proton transfers), it may be possible to view the OEC as an intricate structure that is "wired for protons".     

Visible‐Light‐Driven Water Oxidation by a Molecular Manganese Vanadium Oxide Cluster

Photosynthetic water oxidation in plants occurs at an inorganic calcium manganese oxo cluster, which is known as the oxygen evolving complex (OEC), in photosystem II. Herein, we report a synthetic OEC model based on a molecular manganese vanadium oxide cluster, [Mn₄V₄O₁₇(OAc)₃]^3?. The compound is based on a [Mn₄O₄]⁶⁺ cubane core, which catalyzes the homogeneous, visible‐light‐driven oxidation of water to molecular oxygen and is stabilized by a tripodal [V₄O₁₃]^6? polyoxovanadate and three acetate ligands. When combined with the photosensitizer [Ru(bpy)₃]²⁺ and the oxidant persulfate, visible‐light‐driven water oxidation with turnover numbers of approximately 1150 and turnover frequencies of about 1.75 s^?1 is observed. Electrochemical, mass‐spectrometric, and spectroscopic studies provide insight into the cluster stability and reactivity. This compound could serve as a model for the molecular structure and reactivity of the OEC and for heterogeneous metal oxide water‐oxidation catalysts.     

单核第一过渡周期金属水氧化催化剂（英文）

由于传统化石能源的不可再生性,其储量日益减少.同时,传统化石能源的使用对环境产生了巨大影响,给人类社会带来了一系列问题,包括温室效应、酸雨等.因此,进入二十一世纪以后,人类面临着日益严峻的能源危机和环境问题,寻找清洁、高效的替代能源已经迫在眉睫.太阳能被认为是一种洁净的可再生能源.自然界通过光合作用将太阳能转化为化学能,在这一过程中,水被氧化产生氧气,同时释放出的电子和质子通过和二氧化碳作用生成碳水化合物.为了模拟这一过程,人工光合作用可以直接将电子和质子结合形成氢气.由此生成的氢气也被认为是洁净的可再生能源,因为在其燃烧过程中只产生水.因此,通过光致水分解析氢析氧的人工光合作用受到了越来越广泛的重视.水分解可以分为两个独立的半反应,即水的氧化析氧和水的还原析氢.水的氧化无论在热力学还是动力学方面,都存在着非常大的阻碍.在热力学上,两分子的水氧化生成一分子氧气需要提供很多能量(ΔE=1.23 V vs NHE).在动力学上,由于涉及到四个氢原子和两个氧原子的重组,并且涉及到氧氧键形成并释放出一分子氧气,因此水氧化是一个非常缓慢的过程.在自然界,水的氧化主要发生在光合作用中,在绿色植物的叶绿体中完成.通过对光合作用的研究,科学家们发现氧气的产生由光系统Ⅱ(PSII)中的释氧中心来完成.释氧中心是一个钙锰簇合物,由四个锰和一个钙组成(Mn_4CaO_x).自然界水分解产生氧气的过程给了我们很大启示,对设计和研究高效稳定的水氧化催化剂具有一定的指导意义.目前水氧化催化剂主要有两大类.第一类是基于材料的水氧化催化剂.该类催化剂的催化效率高,过电势小,但是对水氧化催化过程的机理缺乏深入研究.第二类是基于金属配合物的分子催化剂.相比基于材料的催化剂,分子催化剂具有以下特点:(1)分子催化剂的结构可以通过实验手段表征清楚;(2)可以结合光谱对水氧化的机理进行深入研究,可以对催化过程中间体进行表征;(3)催化剂的结构可以从分子水平上进行修饰,因此可以更好地研究催化效率与结构之间的关系,为设计高效、稳定的催化剂提供必要信息;(4)比较容易组装成分子器件从而应用到实际的水氧化装置中;(5)通过实验与理论的结合,对氧氧成键提出新的认识与理解.近几年来,一些单核的金属配合物逐渐被发现可以高效、稳定地催化水氧化.研究表明,一些基于钌和铱的催化剂具有良好的催化活性,但由于金属钌和铱储量少、价格昂贵等因素,限制了该类催化剂的大量使用.由于第一过渡系金属元素具有储量丰富、安全无毒、廉价易得等优势,第一过渡周期金属化合物逐渐成为科学家们研究的热点.近几年来,基于第一过渡系金属的水氧化催化剂已经有大量报道.本文主要总结了近几年来基于第一过渡系金属的单核水氧化分子催化剂.通过对催化机理进行深入的讨论,特别是对氧氧成键的总结,本文将对设计合成结构新颖、具有高催化效率和良好稳定性的水氧化分子催化剂提供理论依据.     

分子催化剂催化水氧化过程及其O–O成键机理

随着化石燃料的不断消耗和生存环境的日益恶化,可再生、清洁且环境友好的新能源逐渐受到广泛关注与利用.太阳能作为一种洁净的可再生能源,在自然界中,植物可以通过光合作用将太阳能转换成化学能.在该过程中,水分子在光系统II中被氧化而释放出氧气,伴随生成的质子和电子进一步将二氧化碳转化为蕴含生物质能的碳水化合物.在光系统II中,叶绿素P680被光照激发生成阳离子自由基P680^·+,其具有很强的氧化能力,可以从附近的析氧中心中夺取电子.析氧中心通过这一过程失去4个电子,可以将两分子水氧化生成一分子氧气和4个质子.作为水裂解的半反应之一,水氧化在热力学方面需要很多能量来断裂4个O-H键(ΔE=1.23 V vs.NHE),在动力学方面涉及4个氢原子与2个氧原子的重组以及氧气的释放,因而水氧化析氧是一个非常缓慢的过程,如何高效稳定地催化水氧化一直是人们研究的热点和难点.研究发现,自然界中存在的析氧中心为Mn₄CaO(x)的钙锰簇合物,在水氧化过程中生成的Mn=O物种可以被游离的水分子亲核进攻形成O-O键,也可以与桥连μ-O(H)反应生成O-O键.通过对析氧中心持续的研究,在过去几十年中设计合成了一系列具有水氧化催化活性的基于金属配合物的分子催化剂.分子催化剂催化水氧化一般主要分为金属-氧物种的演化过程以及O-O成键过程.通常,金属-氧物种可以通过失电子或质子耦合的失电子过程逐步生成高价态的金属-氧物种,其引发的O-O成键过程通常是水氧化催化循环的决速步骤.基于之前的研究成果,目前主要报道了五种不同的O-O成键机理:(1)水亲核进攻金属-氧物种的WNA机理,(2)金属-氧自由基耦合的I2M机理,(3)金属-羟基自由基耦合的HC机理,(4)分子内进攻桥连氧的IOC机理以及(5)氧化还原异构的RI机理.本文综述了过去几十年水氧化分子催化剂的发展,总结了贵金属钌和铱配合物到第一过渡金属锰、铁、钴、镍和铜配合物催化水氧化过程中金属-氧物种的生成与演化,重点阐述了引发O-O成键过程的高价态金属-氧物种的种类及其不同的O-O成键机理.重点总结了O-O成键中WNA机理与I2M机理的异同,并阐述了催化剂设计对WNA与I2M机理选择性的影响.通过对金属-氧物种种类和O-O成键机理的总结,将有助于进一步设计合成高效稳定的水氧化分子催化剂.     

Recent Progress on Nickel-Based Oxide/(Oxy)Hydroxide Electrocatalysts for the Oxygen Evolution Reaction

Developing clean and sustainable energies as alternatives to fossil fuels is in strong demand within modern society. The oxygen evolution reaction (OER) is the efficiency-limiting process in plenty of key renewable energy systems, such as electrochemical water splitting and rechargeable metal–air batteries. In this regard, ongoing efforts have been devoted to seeking high-performance electrocatalysts for enhanced energy conversion efficiency. Apart from traditional precious-metal-based catalysts, nickel-based compounds are the most promising earth-abundant OER catalysts, attracting ever-increasing interest due to high activity and stability. In this review, the recent progress on nickel-based oxide and (oxy)hydroxide composites for water oxidation catalysis in terms of materials design/synthesis and electrochemical performance is summarized. Some underlying mechanisms to profoundly understand the catalytic active sites are also highlighted. In addition, the future research trends and perspectives on the development of Ni-based OER electrocatalysts are discussed.     

The Schenck Ene Reaction: Diastereoselective Oxyfunctionalization with Singlet Oxygen in Synthetic Applications

Oxyfunctionalized molecules are principal building blocks in organic synthesis. In cellular processes highly efficient enzymes serve as selective catalysts for the formation of such synthetic units, for example the oxygenases oxyfunctionalize substrates by activating molecular oxygen. To date no comparable effective chemical oxidation system has been found. A useful photochemical process is the oxyfunctionalization of allylic substrates by sensitized photooxygenation, for which molecular oxygen and light serve as natural sources. This allylic oxidation of olefins by the ene reaction with singlet oxygen (Schenck reaction) figures as a highly versatile synthetic method. While the regioselectivity of this transformation has been studied for decades, only during the last years has attention focused on stereocontrol. Through these recent efforts it has become possible to control high stereoselectivity in the photooxygenation of organic substrates. This breakthrough has enhanced substantially the utility of singlet oxygen in diastereoselective synthesis.     

W具有μ-OH,μ-O2双钴核结构的基于多吡啶配体的配合物催化水氧化反应的能力

化石燃料的大量使用已经带来了一系列的环境问题, 开发和使用可再生的清洁能源十分有必要. 氢能可以作为传统化石燃料的理想替代品, 因为它不但清洁而且热值高. 受光合作用启发的水裂解反应被认为是一种将太阳能转化为氢能的理想途径. 水裂解包括两个半反应, 即水的氧化(2H2O → 4H++ O2+ 4e-)和质子的还原(4H++ 4e-→ 2H2). 水氧化反应需要高的活化能, 因此它也被认为是水裂解反应的瓶颈步骤. 为了提高水氧化反应的效率, 已经有很多关于水氧化催化剂的研究工作被开展. 然而, 迄今为止, 寻找高效的水氧化催化剂仍然是巨大挑战. 考虑到成本以及丰度的因素, 基于第一过渡系金属的水氧化催化剂日益受人关注. 相比于多相水氧化催化剂, 均相的水氧化催化剂, 特别是基于有机配合物的均相催化剂, 在结构调变, 机理研究方面更具有优势. 均相的水氧化催化剂主要分两类: 无机的多金属氧酸盐和基于有机配体的配合物.在所有的均相的水氧化催化剂中, 含钴的配合物被广泛研究, 因为在光驱动水氧化反应中它们通常能表现出来较好的活性. 很多研究工作都集中于研究多核的含钴的均相催化剂, 特别是具有Co4O4框架立方烷结构的配合物, 因为它们具有类似于自然界光合作用光系统II活性中心Mn4CaO5簇的结构. 例如, Co4O4(Ac)4(py)4簇以及相关衍生物曾被报道过用于水氧化反应,然而Nocera等人发现该化合物本身没有活性(J. Am. Chem. Soc., 2014, 136, 17681-17688),表观的活性来源于催化 剂 合 成 过 程 中 引 入 的 二 价 钴 离 子. 2014年, 一 个 具 有 双 核 钴 核 心 结 构 的 多 吡 啶 配 合 物[(TPA)CoⅢ(μ-OH)(μ-O2)CoⅢ(TPA)](ClO4)3被报道具有催化光驱动水氧化反应的能力. 然而随后的研究工作(ACS Catal., 2016, 6, 5062-5068)表明其表观活性也是来自于自由钴离子杂质, 纯的化合物是没有活性的.在检查一个均相分子水氧化催化剂的时候, 应当进行充分的实验, 特别是对于钴基的水氧化催化剂. 因为在合成含钴配合物的过程中可能引入杂质钴离子, 杂质钴离子在反应过程中会转化为CoOx, 它本身就是很常见的高效的水氧化催化剂. 在定性一个真正的均相的钴基水氧化催化剂之前, 这一可能性必须要被排除. 在这里我们报道了另外一个同样以双三价钴离子为核心的, 具有μ-OH, μ-O2结构的基于多吡啶配体的均相配合物. 我们通过一系列的实验验证了它催化光驱动水氧化反应的能力. 实验证明, 该化合物没有催化活性, 表观活性依然是来自于合成过程中引入的杂质钴离子. 这一结果与之前的报道相比, 既是进一步的探索, 也是一个很好的补充. 结合前人的工作, 我们发现并总结了一个规律: 以双核三价钴为核心的, 拥有μ-OH, μ-O2核心结构的基于多吡啶配体的配合物不适合被选用于催化光驱动水氧化反应. 这一发现能为高效水氧化催化剂的开发设计提供见解与指导.     

Photocatalyzed sulfide oxygenation with water as the unique oxygen atom source

In our research program aiming to develop new ruthenium-based polypyridine catalysts for oxidation we were interested in combining a photosensitizer and a catalytic fragment within the same complex to achieve catalytic light-driven oxidation. To respond to the lack of such conjugates, we report here a new catalytic system capable of using light to activate water molecules in order to perform selective sulfide oxygenation into sulfoxide via an oxygen atom transfer from H(2)O to the substrate with a TON of up to 197 ± 6. On the basis of electrochemical and photophysical studies, a proton-coupled electron-transfer process yielding to an oxidant Ru(IV)-oxo species was proposed. In particular, the synergistic effect between both partners in the dyad yielding a more efficient catalyst compared to the bimolecular system is highlighted.     

基于多金属氧酸盐催化剂的光驱动产氢研究进展

能源和环境问题是21世纪人类面临的两个巨大挑战.鉴于此,为了实现人类社会的可持续发展,寻求能够替代化石能源的安全无污染可再生能源已迫在眉睫.太阳光驱动水分解是实现太阳能转化生产清洁可再生氢能的理想方法,其分解产物氢气和氧气在燃烧释放能量的同时生成洁净无污染的可饮用水,实现了完美的可持续能量循环,对于解决当今全球面临的能源危机与环境污染问题具有巨大的应用价值.然而,长期以来光驱动水分解所面临的巨大难题是半反应动力学非常缓慢,通常需要克服较高的能量势垒,导致整体能量转化效率低.利用非贵金属制备高催化效能、低成本的水分解催化材料成为该领域的研究热点和难点.目前,已报道的光驱动产氢催化剂可以被归纳为两大类:均相催化剂和异相催化剂.均相催化剂通常具备高催化活性、高选择性以及易于进行机理研究等优点,而异相催化剂则具备廉价、易得和高稳定性等优点;然而它们也存在一些不容忽视的问题,如均相催化剂的低稳定性、易分解失活,异相催化剂表面易被毒化失活、低催化转化数及转化频率等.如何设计合成兼具二者优点的产氢催化剂吸引了领域内研究者的广泛关注.作为一类新兴的多电子转移催化剂,多金属氧酸盐因其丰富多样的合成策略以及高度可调的物理化学及光化学性质,已被广泛用于催化水分解制氢气研究.该类多金属氧酸盐催化剂具备了介于均相分子化合物和异相金属氧化物之间的结构,这种独特的结构赋予它们同时具备均相分子催化剂的高活性、高选择性、高可控性、易于进行机理性研究等优点,又具备异相金属氧化物催化剂的廉价易得及稳定性高等优势.随着研究的开展,基于多金属氧酸盐的光催化产氢体系已由当初的贵金属辅助逐渐转变为丰产元素参与,光源的选择方面也从与太阳光谱匹配度低的紫外光转变为可见光.本文对30多年来基于多金属氧酸盐催化剂的光驱动产氢成果进行了综述,主要包括有/无贵金属辅助的多金属氧酸盐,多酸@金属有机框架复合物,多酸-半导体复合材料在紫外光或可见光条件下的光催化产氢研究;同时讨论总结了不同类型催化体系的反应机理;并对该领域的未来发展趋势及研究方向进行了展望.     

Efficient Homogeneous Electrocatalytic Water Oxidation by a Manganese Cluster with an Overpotential of Only 74 mV

Water electrolysis is among the simplest method for generating hydrogen as an alternative renewable fuel. A major challenge associated with this process is the development of cheap, simple, and environmentally benign catalysts that lead to a minimum overpotential for water oxidation. Inspired by the Mn₄CaO_x cluster that catalyzes water oxidation in photosystem II, described here is the synthesis and characterization of the manganese cluster [Mn₁₂O₁₂(O₂CC₆H₂(OH)₃)₁₆(H₂O)₄] (Mn₁₂TH) along with its electrocatalytic activity at pH 6. Electrochemical, spectroscopic, and electron microscopy studies show that Mn₁₂TH is a homogeneous electrocatalyst for water oxidation and enables oxygen evolution with a reaction rate of 22 s^?1, high Faradic efficiency (93 %), and an overpotential of only 74 mV, the lowest reported to date. Based on the electrochemical data, the organic ligands, which can be described as the second coordination sphere of the catalytic manganese core, play a key role in facilitating the oxidation process and accelerating the reaction.     

氧化镁上乙酸乙酯和乙酸丁酯氧化反应（英文）

采用新型无溶剂反应和回流的方法制得锰钾矿型氧化镁(K-OMS-2),同时采用常规方法制得氧化镁,并测试不同催化剂对工业排放气中有机挥发性物质(VOCs)中的模型化合物––乙酸乙酯和乙酸丁酯的催化氧性能.采用N2吸附-脱附、X射线衍射、扫描电镜、程序升温还原和X射线光电子能谱等技术对催化剂进行了表征.所有氧化镁样品均表现出很高的催化乙酸乙酯和乙酸丁酯氧化生成CO2的活性,且制备方法对催化剂性能起着重要作用.新型无溶剂法制得的K-OMS-2纳米棒样品比常规的回流法制得样品表现出更好的催化性能,含锰钾矿型氧化镁的样品比常规方法制得样品表现出更高的活性.性能最好的催化剂也表现出较高的稳定性,在213和202 oC条件下,可分别使90%的乙酸乙酯和乙酸丁酯转化为CO2.催化剂性能的显著差异清楚地表明,对于所选VOCs氧化反应,采用新型无溶剂法制得的K-OMS-2纳米棒样品比常规法制备的氧化镁混合物更好,这可能与样品结构中含有更高的Mn平均氧化态有关.本文表明了催化剂性能与其表面化学性质间存在显著的关联,显示了K-OMS-2内在性质决定了其高的催化性能.     

Cu(II) 2-quinoxalinol salen catalyzed oxidation of propargylic,benzylic, and allylic alcohols using tert-butyl hydroperoxide in aqueous solutions

The synthesis of carbonyl compounds by oxidation of alcohols is a key reaction in organic synthesis. Such oxidations are typically conducted using catalysts featuring toxic metals and hazardous organic solvents. Considering green and sustainable chemistry, a copper(II) complex of sulfonated 2-quinoxalinol salen (sulfosalqu) has been characterized as an efficient catalyst for the selective oxidation of propargylic, benzylic, and allylic alcohols to the corresponding carbonyl compounds in water when in combination with the oxidant tert-butyl hydroperoxide. The reactions proceed under mild conditions (70 °C in water) to produce yields up to 99% with only 1 mol % of catalyst loading. This reaction constitutes of a rare example of propargylic alcohol oxidation in water, and it makes this process greener by eliminating the use of hazardous organic solvents. Excellent selectivity was achieved with this catalytic protocol for the oxidation of propargylic, benzylic, and allylic alcohols over aliphatic alcohols. The alcohol oxidation is thought to go through a radical pathway.     

Hydroxylation of cyclohexane with molecular oxygen catalyzed by highly efficient heterogeneous Mn(III) porphyrin catalysts prepared by special synthesis and immobilization method

A novel method to synthesize and immobilize porphyrins as well as manganese porphyrins on crosslinked polystyrene (CPS) microspheres was designed. The chloromethyl groups of chloromethylated CPS microspheres (CMCPS microspheres) were first oxidized to aldehyde groups via Kornblum oxidation reaction, obtaining aldehyde group-functionalized microspheres, and then, the synchronous synthesis and immobilization of porphyrins on CPS microspheres were carried out via the Adler reaction between solid–liquid phases, obtaining three kinds of functional microspheres, on which phenyl porphyrin (PP), p-chlorophenyl porphyrin (CPP) and p-nitrophenyl porphyrin (NPP) were immobilized. Finally, three manganese porphyrin-immobilized microspheres, MnPP–CPS, MnCPP–CPS and MnNPP–CPS, were prepared, these solid catalysts were used in the catalytic hydroxylation reaction of cyclohexane with molecular oxygen as oxidant, and their catalytic performances were mainly investigated in this work. Some surprising experimental results were obtained. The prepared immobilized manganese porphyrin catalysts display amazing catalytic activity and selectivity, and cyclohexane conversion can get up to 45?% and cyclohexanol selectivity in the reaction product can be up to 90–100?%.     

Supramolecular Water Oxidation with Ru–bda‐Based Catalysts

Extremely slow and extremely fast new water oxidation catalysts based on the Ru–bda (bda=2,2′‐bipyridine‐6,6′‐dicarboxylate) systems are reported with turnover frequencies in the range of 1 and 900 cycles s^?1, respectively. Detailed analyses of the main factors involved in the water oxidation reaction have been carried out and are based on a combination of reactivity tests, electrochemical experiments, and DFT calculations. These analyses give a convergent interpretation that generates a solid understanding of the main factors involved in the water oxidation reaction, which in turn allows the design of catalysts with very low energy barriers in all the steps involved in the water oxidation catalytic cycle. We show that for this type of system π‐stacking interactions are the key factors that influence reactivity and by adequately controlling them we can generate exceptionally fast water oxidation catalysts.     

Calcium manganese oxides as oxygen evolution catalysts: O2 formation pathways indicated by 18O-labelling studies

Oxygen evolution catalysed by calcium manganese and manganese-only oxides was studied in (18)O-enriched water. Using membrane-inlet mass spectrometry, we monitored the formation of the different O(2) isotopologues (16)O(2), (16)O(18)O and (18)O(2) in such reactions simultaneously with good time resolution. From the analysis of the data, we conclude that entirely different pathways of dioxygen formation catalysis exist for reactions involving hydrogen peroxide (H(2)O(2)), hydrogen persulfate (HSO(5)(-)) or single-electron oxidants such as Ce(IV) and [Ru(III) (bipy)(3)](3+) . Like the studied oxide catalysts, the active sites of manganese catalase and the oxygen-evolving complex (OEC) of photosystem II (PSII) consist of μ-oxido manganese or μ-oxido calcium manganese sites. The studied processes show very similar (18)O-labelling behaviour to the natural enzymes and are therefore interesting model systems for in vivo oxygen formation by manganese metalloenzymes such as PSII.     

Catalytic Electron‐Transfer Oxygenation of Substrates with Water as an Oxygen Source Using Manganese Porphyrins

Manganese(V)–oxo–porphyrins are produced by the electron‐transfer oxidation of manganese–porphyrins with tris(2,2′‐bipyridine)ruthenium(III) ([Ru(bpy)₃]³⁺; 2 equiv) in acetonitrile (CH₃CN) containing water. The rate constants of the electron‐transfer oxidation of manganese–porphyrins have been determined and evaluated in light of the Marcus theory of electron transfer. Addition of [Ru(bpy)₃]³⁺ to a solution of olefins (styrene and cyclohexene) in CH₃CN containing water in the presence of a catalytic amount of manganese–porphyrins afforded epoxides, diols, and aldehydes efficiently. Epoxides were converted to the corresponding diols by hydrolysis, and were further oxidized to the corresponding aldehydes. The turnover numbers vary significantly depending on the type of manganese–porphyrin used owing to the difference in their oxidation potentials and the steric bulkiness of the ligand. Ethylbenzene was also oxidized to 1‐phenylethanol using manganese–porphyrins as electron‐transfer catalysts. The oxygen source in the substrate oxygenation was confirmed to be water by using ¹⁸O‐labeled water. The rate constant of the reaction of the manganese(V)–oxo species with cyclohexene was determined directly under single‐turnover conditions by monitoring the increase in absorbance attributable to the manganese(III) species produced in the reaction with cyclohexene. It has been shown that the rate‐determining step in the catalytic electron‐transfer oxygenation of cyclohexene is electron transfer from [Ru(bpy)₃]³⁺ to the manganese–porphyrins.     

Artificial photosynthesis: from molecular catalysts for light-driven water splitting to photoelectrochemical cells

Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light-to-chemical energetic transduction. This field of research, called "artificial photosynthesis," is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon-free fuel hydrogen through the light-driven water splitting. In this review, we highlight the recent achievements on light-driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.     

制备方法对Cr/Si-2催化剂在CO2氧化乙烷脱氢制乙烯反应中的催化性能的影响

采用直接水热合成法和浸渍法制备了相同Cr含量的Cr/Si-2催化剂,并在常压固定床微反应器上,考察了它们在CO2或者N2气氛下的乙烷脱氢制乙烯反应中的催化性能及稳定性.由于存在逆水煤气反应和Boudouard反应,CO2能显著促进乙烷的脱氢反应.不论是在CO2还是在N2气氛下,直接水热法制备的催化剂均比浸渍法制备的催化剂显示出更好的催化性能.高价态的Cr物种被认为是催化剂具有高活性的关键.在CO2气氛下的乙烷脱氢制乙烯反应中,浸渍法制备的催化剂比水热法制备的催化剂失活更快,催化剂失活速率的差异可能与它们的氧化还原性质有关.然而在N2气氛下的乙烷脱氢制乙烯反应中,这两种方法制备的催化剂失活速率差异不大.     

Review of Acetic Acid Synthesis from Various Feedstocks Through Different Catalytic Processes

Acetic acid (AA) has been largely used with a wide range of applications such as a raw material for a synthesis of vinyl acetate monomer, cellulose acetate or acetate anhydrate, acetate ester and a solvent for a synthesis of terephthalic acid and so on. The present paper briefly summarizes the commercialized chemical processes with their Rh or Ir-based catalytic systems in a liquid-phase carbonylation reaction such as Monsanto, Cativa and Acetica processes. In addition, some alternative catalytic systems such as heterogeneous catalysts to produce AA by direct oxidation or indirect carbonylation of dimethyl ether through BP-SaaBre process in a gas-phase reaction to solve some problems such as a difficult separation of homogeneous catalysts in a corrosive reaction medium. Some home-made heterogeneous catalysts such as a rhodium incorporated graphitic carbon nitride (Rh-g-C₃N₄) and some heterogenized homogeneous catalysts using the supports of tungsten carbide, iron oxide or graphitic carbon nitride containing rhodium complexes were also introduced for the synthesis of AA through a liquid-phase methanol carbonylation reaction to effectively solve the leaching problem of active rhodium metal as well as to mitigate the separation problem of homogeneous catalysts.