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941.
942.
The ring-opening reactions of methyloxirane and ethyloxirane on ZSM-5 catalysts are dramatically different in spite of otherwise identical reaction conditions. Even though both reactants undergo single ring opening providing isomers of the starting compound, the major transformation route of methyloxirane is dimerization resulting in dioxolane and dioxane derivatives of various kinds. This reaction pathway, however, is completely lacking in the case of ethyloxirane. Molecular modeling revealed that possible dimers of ethyloxirane are too bulky to be formed and diffuse through the channel system of the ZSM-5 structure. 相似文献
943.
Ana Vidiš Ernst Küsters Gottfried Sedelmeier Paul J. Dyson 《Journal of Physical Organic Chemistry》2008,21(4):264-270
The Diels–Alder reaction has been examined in room temperature ionic liquids with high molar concentrations of Lewis acids under various conditions. A molar ratio of 10% catalyst gave a large increase in the selectivity and the yield of the reaction. The effect of catalysts on reaction rates was also examined under 100 MPa of pressure which leads to modest improvements in reaction rates. Ultrasound and microwave dielectric heating were also shown to improve the rate and, to a minor extent, selectivity of the examined reactions. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
944.
A. N. Panin T. A. Sukhova N. M. Bravaya 《Journal of polymer science. Part A, Polymer chemistry》2001,39(11):1901-1914
Ethylene polymerizations with catalytic systems Me2SiCp*NtBuZrX2 ( 1 ) [Cp* = C5(CH3)4; X = Cl ( 1Cl ), Me ( 1Me )], triisobutylaluminum (TIBA), perfluorophenylborate CatB(C6F5)4 [Cat = CPh3 ( 3 ), Me2NHPh ( 4 )], or Me2SiCp2ZrX2 [X = Cl ( 2Cl ), Me ( 2Me )]/TIBA/ 3 ( 4 ) were performed within a wide range of ethylene pressures of different Al/Zr ratios, and Zr/B = 1. Catalytic systems 1Cl ( 2Cl )/TIBA/ 3 led to the formation of very high linear molecular weight polyethylene (PE) of Mη ∼2,000,000 with low activity. The replacement of both chlorine ligands in the precatalyst for the methyl ones led to the formation of active species producing low molecular weight PE with high activity. Chain transfer to ethylene was shown to be the main reaction controlling PE chain propagation: kp/ktr ∼20–30 for 1Me /TIBA/ 3 and kp/ktr ∼350–500 for 2Me /TIBA/ 3 . It was suggested that TIBA was present in the active center first in the form of a neutral heterobimetallic Zr–Al bridged complex followed by the formation of a partially polarized Zr–Al(Cl)R2 (R = iBu) or an unreactive Zr–AlR3 cationic complex by abstraction of the alkyl ligand under the action of borate. It was concluded that AlR3 from the latter cationic complex may be easily reversibly replaced under the specific coordination of ethylene or accumulated α-olefin, giving rise to highly labile and sterically accessible cationic species. Experiments on ethylene polymerization with the catalytic systems 1Cl ( 1Me )/TIBA/ 3 /Ph2NH, 1Cl ( 1Me )/TIBA/ 4, 2Cl ( 2Me )/TIBA/ 3 /Ph2NH, and 2Cl ( 2Me )/TIBA/ 4 were performed to confirm the suggestion. Catalytic systems derived from dichloride complexes in the presence of a σ-donor substrate also produced low molecular weight PEs with molecular weight characteristics similar to those of products obtained with the dimethylated precatalysts. The specific feature of active species derived from 2Me complexes to isomerize coordinated α-olefin into trans-vinylene polymer chains was also revealed. The catalytic behavior of the ternary catalytic system based on 2Me relative to 2Me or 2Cl precatalysts activated with polymethylaluminoxane at different Al/Zr ratios was compared. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1901–1914, 2001 相似文献
945.
Dr. Karl P. J. Gustafson Dr. Tamás Görbe Dr. Gonzalo de Gonzalo-Calvo Ning Yuan Cynthia L. Schreiber Dr. Andrey Shchukarev Dr. Cheuk-Wai Tai Prof. Dr. Ingmar Persson Prof. Dr. Xiaodong Zou Prof. Dr. Jan-E. Bäckvall 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(39):9174-9179
Herein, we report on the use a biohybrid catalyst consisting of palladium nanoparticles immobilized on cross-linked enzyme aggregates of lipase B of Candida antarctica (CalB CLEA) for the dynamic kinetic resolution (DKR) of benzylic amines. A set of amines were demonstrated to undergo an efficient DKR and the recyclability of the catalysts was studied. Extensive efforts to further elucidate the structure of the catalyst are presented. 相似文献
946.
Molecular catalysts (metal complexes), with molecularly defined uniform active sites and atomically precise structural tailorability allowing for regulating catalytic performance through metal- and ligand-centered engineering and elucidating reaction mechanisms via routine photoelectrochemical characterizations, have been increasingly explored for electrocatalytic CO2 reduction (ECR). However, their poor stability and low catalytic current density are undesirable for practical applications. Heterogenizing discrete molecular catalysts can potentially surmount these issues, and the resulting integrated catalysts largely share catalytical properties with their discrete molecular counterparts, which bridge the gap between heterogeneous and homogeneous catalysis and combine their advantages. This minireview surveys advances in design and regulation of molecular catalysts such as porphyrin, phthalocyanine, and bipyridine-based metal complexes and their integrated catalytic materials for selective ECR. 相似文献
947.
948.
Ju Hyoung Jo Sunghan Choi Ha-Yeon Cheong Prof. Jae Yoon Shin Prof. Chul Hoon Kim Prof. Dae Won Cho Prof. Ho-Jin Son Prof. Chyongjin Pac Prof. Sang Ook Kang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(70):16733-16754
Herein, we report the synthesis, and photochemical and -physical properties, as well as the catalytic performance, of a series of heteroleptic IrIII photosensitizers (IrPSs), [Ir(C^N)2(N^NAryl)]+, possessing ancillary ligands that are varied with aryl-substituents on bipyridyl unit [C^N=(2-pyridyl)benzo[b]thiophen-3-yl (btp); N^NAryl=4,4′-Y2-bpy (Y=−Ph or −PhSi(Ph)3]. We found that the π-extension of bipyridyl ligand by aryl-substitution put bipyridyl ligand in use as an electron relay unit that performed charge accumulation before delivering to the catalytic center, greatly improving the overall CO2-to-CO conversion activities. In a typical run, the aryl-substituted IrPS ( t Bu IrP-PhSi )-sensitized homogeneous systems (IrPS+ReI catalyst) gave a turnover number of 1340 (ΦCO=24.2 %) at the early stage of photolysis (<5 h). This study demonstrates that the π-character modulation on the ancillary bipyridyl ligand is critical for forthcoming catalytic performance. 相似文献
949.
Prof. Dr. Xinjiang Cui Dr. Antoine P. van Muyden Prof. Dr. Paul J. Dyson 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(1):12-19
In recent years, core–shell nano-catalysts have received increasing attention due to their tunable properties and broad applications in catalysis. Control of the two components of these materials allows their catalytic properties to be tuned to various sustainable processes in synthetic and energy-related applications. This Concept article describes recent state-of-the-art core–shell materials and their application as heterogeneous catalysts for a range of sustainable catalytic transformations, focusing on two important classes of renewable substrates, CO2 and biomass. In the discussion, emphasis is directed to the role of the constituent parts of the core–shell structure and how they can be manipulated to enhance activity. 相似文献
950.
Mustafa alayan Alessandra Lucini Paioni Edy Abou‐Hamad Genrikh Shterk Alexey Pustovarenko Marc Baldus Abhishek Dutta Chowdhury Jorge Gascon 《Angewandte Chemie (International ed. in English)》2020,59(38):16741-16746
Methane dehydroaromatization (MDA) is among the most challenging processes in catalysis science owing to the inherent harsh reaction conditions and fast catalyst deactivation. To improve this process, understanding the mechanism of the initial C?C bond formation is essential. However, consensus about the actual reaction mechanism is still to be achieved. In this work, using advanced magic‐angle spinning (MAS) solid‐state NMR spectroscopy, we study in detail the early stages of the reaction over a well‐dispersed Mo/H‐ZSM‐5 catalyst. Simultaneous detection of acetylene (i.e., presumably the direct C?C bond‐forming product from methane), methylidene, allenes, acetal, and surface‐formate species, along with the typical olefinic/aromatic species, allow us to conclude the existence of at least two independent C?H activation pathways. Moreover, this study emphasizes the significance of mobility‐dependent host–guest chemistry between an inorganic zeolite and its trapped organic species during heterogeneous catalysis. 相似文献