Influence of Process Parameters on the Reaction Kinetics of the Chromium‐Catalyzed Trimerization of Ethylene

In this paper we report the results of an extensive experimental kinetic study carried out on the novel ethylene trimerization catalyst system, comprising the chromium source [CrCl₃(thf)₃] (thf=tetrahydrofuran), a Ph₂P‐N(iPr)‐P(Ph)‐N(iPr)H (PNPNH) ligand (Ph=phenyl, iPr=isopropyl), and triethylaluminum (AlEt₃) as activator. It could be shown that the initial activity shows a first‐order dependency on the ethylene concentration. Also, a first‐order dependency was found for the catalyst concentration. The initial activity follows a typical Arrhenius behavior with an experimentally determined activation energy of 52.6 kJ mol^?1. At elevated temperatures (ca. 80 °C), a significant deactivation was observed, which can be tentatively traced back to a ligand rearrangement in the presence of AlEt₃. After a fast initial phase, a pronounced ‘kink’ in the ethylene‐uptake curve is observed, followed by a slow, almost linear, further increase of the total ethylene consumption. The catalyst composition, in particular the ligand/chromium and the cocatalyst/chromium molar ratio, has a strong impact on the catalytic performance of the trimerization of ethylene.     

Oligomerization of 1,2‐Ethanedithiol: An Expedient Approach to Oligothiaethylenethioglycols

Reactions of ethylenedithioglycol (ETG) with Na₂CO₃, K₂CO₃, and Cs₂CO₃ provided the oligothiaethylenethioglycols (nETG): di‐ (DETG), tri‐ (TrETG), tetra‐ (TETG), and pentathiaethylenethioglycol (PETG), along with higher polymers. The most efficient carbonate was K₂CO₃ and reactions using DETG and TrETG as starting materials—or their mixtures—were also found to afford similar species. This largely unknown oligomerization process was thoroughly explored and potential pathways were put forward. A convenient conversion of ETG to laboratory quantities of the otherwise scarce and/or expensive DETG, TrETG, TETG, and PETG oligomers, in organic or aqueous media was achieved. Notably, this straightforward reaction takes place without the addition of expensive or toxic metal catalysts and with pure water as the solvent, thereby highlighting its potential as a green chemical reaction. Moreover, the process opens up new approaches to dynamic combinatorial libraries (DCLs) of oligomers and macrocycles with manifolded nETG [(SCH₂CH₂)_nS] bridges.     

Highly Selective Iron‐Catalyzed Synthesis of Alkenes by the Reduction of Alkynes

Herein, the iron‐catalyzed reduction of a variety of alkynes with silanes as a reductant has been examined. With a straightforward catalyst system composed of diiron nonacarbonyl and tributyl phosphane, excellent yields and chemoselectivities (>99 %) were obtained for the formation of the corresponding alkenes. After studying the reaction conditions, and the scope and limitations of the reaction, several attempts were undertaken to shed light on the reaction mechanism.     

Photo‐induced iron‐catalyzed allylic amination of unfunctionalized olefins with nitroarenes

Photochemical reactions between unfuctionalized olefins and nitroarenes under CO pressure, catalyzed by [Cp*Fe(CO)₂]₂, form the corresponding allyl amines. The use of mechanistic probes containing 2‐nitrobiphenyl, 2,3‐dimethylbutadiene and pentafluoronitrobenzene suggest that neither aryl nitrene nor nitrosoarene is the active aminating agent in these transformations. Copyright © 2006 John Wiley & Sons, Ltd.     

Palladium‐Catalyzed Asymmetric Reductive Heck Reaction of Aryl Halides

Asymmetric reductive Heck reaction of aryl halides is realized in high stereoselectivity. Hydrogen‐bond donors, trialkylammonium salts in a glycol solvent, were used to promote halide dissociation from neutral arylpalladium complexes to access cationic, stereoselective pathways.     

碳纳米管封装铁纳米粒子催化剂上CO加氢制低碳烯烃

由于石油资源的逐步枯竭,近年来费托(F-T)反应因其可以高效将煤、天然气和生物质等转化成液体燃料和高值化学品而越来越受到人们的关注。相比于Co, Ni和Ru等F-T催化剂, Fe基催化剂因其价格低廉,产物分布广而被广泛研究。以合成气直接制备低碳烯烃的F-T过程为例,铁基催化剂通常会因积碳和烧结的问题,而导致失活。因此,人们通常使用一些氧化物载体,比如氧化硅,氧化铝或者分子筛来分散并稳定铁粒子。但是这类氧化物载体通常与铁有非常强的相互作用,特别是在铁粒子较小的情况下,容易生成一些难于还原的硅酸铁和铝酸铁。而活性炭、碳纤维等惰性载体与铁的相互作用较弱,不足以稳定小的铁粒子在而反应过程中聚集。近来,我们组提出了利用石墨烯碳层封装过渡金属粒子作为催化剂,利用“穿透”的金属电子来催化反应,从而可以使活性中心和反应介质隔离,有效地增强了非贵金属催化剂的活性和稳定性。在此基础上,我们组和其他课题组的研究表明,一系列石墨烯碳层封装的非贵金属催化剂在燃料电池阴极氧还原反应,电催化析氢反应,染料敏化太阳能电池中的I3–还原反应以及催化氧化还原反应中都有着广泛的应用前景。这种材料中碳层不仅能在氧化气氛、酸性介质中保护包覆的金属,防止其被氧化或者腐蚀,还与包覆的金属有着较强的相互作用,可以促进非贵金属的电子向碳层表面的转移,有望在一些苛刻的反应条件下实现对贵金属催化剂的替代。本文进一步拓展了其在高温反应中的应用,发现豆荚状碳纳米管封装的金属铁纳米粒子在合成气制备低碳烯烃中可以有效防止金属铁纳米粒子的烧结和聚集,因此表现出优异的低碳烯烃选择性和催化稳定性。我们利用一步化学反应法合成了豆荚状碳纳米管封装的铁纳米粒子催化剂(Pod-Fe),并通过酸洗除去碳管外面裸露的铁粒子。透射电镜(TEM)和X射线衍射(XRD)表明酸洗后铁粒子被包覆在碳管内,并且呈金属态,而酸洗前,则还有大量的氧化铁粒子分布于碳管外部(FeOx/Pod-Fe)。将酸洗前后的两个催化剂用于固定床气相F-T反应中。通过调节空速和温度考察了它们的催化反应性能,结果表明两个催化剂在不同的反应条件下都有着良好的低碳烯烃选择性。不同反应温度下,它们表现出不同的变化趋势：Pod-Fe活性随着温度的升高而缓慢增长,至380 oC都没有明显的失活现象;而对于FeOx/Pod-Fe催化剂,随着温度的升高, CO的转化率先升高,在300 oC时达最高,但随着温度进一步升高,活性迅速降低,呈现一个火山型曲线。 TEM结果发现,反应后FeOx/Pod-Fe催化剂粒子上产生了很多杂乱的碳丝,并且铁粒子有着明显的聚集长大。而Pod–Fe催化剂即使在380 oC反应后,其形貌仍然保持完好,没有积碳产生,粒子也没有发生聚集和长大。进一步在320 oC下120 h的寿命试验发现, Pod-Fe催化剂的初始活性较低,但经20 h的活化阶段,活性会先增加后略有下降,20 h后趋于稳定。而FeOx/Pod-Fe催化剂在反应初始虽然表现出较高的活性,但是随着时间进行,活性迅速下降一半以上,最后趋于稳定。同时结合反应后TEM和XRD的结果发现碳管外部裸露的铁粒子会在反应过程中形成碳化铁物种,并随着反应进行产生聚集,并伴有大量积碳,导致活性迅速下降;而碳层的包覆对于铁粒子有着很好的稳定作用,使得铁粒子能够在高温反应中保持稳定,并且没有积碳的产生。由此可见石墨烯碳层可以有效保护其包覆的金属粒子,并且能够提高其在高温反应下的低碳烯烃选择性和稳定性。此类催化剂有望在一些苛刻条件下的多相催化反应中得到广泛应用。