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991.
The successful application of zeolite A membranes in the industrial market has thus far been restricted to the pervaporative dehydration of solvent streams in the pharmaceutical and engineering industries. Their application in gas separation processes remains elusive, largely due to the existence of uncontrollable, intercrystalline diffusion pathways in the boundary regions of neighbouring crystals. 相似文献
992.
The chemical state of particles of 5wt% Fe in α-Fe2O3 and the subsequently reduced iron particles supported on different particle size (50–200 mesh) of silica (SiO2), alumina (Al2O3), magnesium oxide (MgO) and carbon (C) was examined by Mössbauer spectroscopy at various stages of calcination and reduction. The particle size of the α-Fe2O3 supported on different mesh sizes (50, 100, 140, 200 mesh) of SiO2 has been determined. The strength of metal-support interaction with respect to the kind of support was found to be MgO>SiO2>Al2O3>C. 相似文献
993.
Frederick J. Karol Sun-Chueh Kao Kevin J. Cann 《Journal of polymer science. Part A, Polymer chemistry》1993,31(10):2541-2553
High-activity titanium- and vanadium-based catalysts for ethylene polymerization frequently show an increase in reaction rate in the presence of an α-olefin. The magnitude of this increase depends on the specific α-olefin. The results show propylene > 1-butene > 1-hexene in increasing initial reaction rates. Addition of certain electron-donor compounds to these catalysts can lower the magnitude of the comonomer effect and, in some cases, totally eliminate such an effect. Among the classes of electron-donor compounds examined were ether-alcohols, ether-esters, amino-alcohols, alkoxysilanes, siloxanes, and phosphine oxides. Reaction kinetics show that the presence of a comonomer can influence the kinetic order of the reaction. These results can be interpreted using a mechanistic model involving two vacant coordination positions at the active sites. In this model electron donors and comonomers are viewed as Lewis-base ligands which influence features of chain propagation and chain termination. As Lewis-base ligands, the comonomers can also increase the number of active sites available for polymerization. Catalyst deactivation following the initial comonomer rate increase is believed to be caused by reaction with the Lewis bases (α-olefin included) in the system and by possible reduction in the oxidation state of the metal centers. The most acidic metal centers activated by the comonomer are most reactive to Lewis bases and deactivate most rapidly. Veratrole (1,2-dimethoxybenzene) can be employed as a probe for estimating the number of bis-vacant coordination sites in vanadium-based catalysts. Addition of low levels of veratrole led to significant deactivation of the vanadium-based catalyst. © 1993 John Wiley & Sons, Inc. 相似文献
994.
The characteristic studies, by means of LR, UV-Vis and XPS spectroscopies, of the preparation process of Mo-Co-S and Mo-Fe-S catalysts for HDS and HDN, derived from (NH4)2MoS4-CoCl2 and (NH4)2MoS4-FeCl2 complexes supported on γ-Al2O3, respectively, indicate that the catalytically essential moiety on the surface of the catalysts is dominantly some sulfido-bimetallic species with such a structural unit (M' =Co or Fe), and both Co and Fe, served as promoters, can donate electrons to Mo probably via bridging-S. The nature of active-sites and the mechanism of promotion are discussed according to the results. 相似文献
995.
Belokon" Yu. N. Petrosyan A. A. Maleev V. I. Savel"eva T. F. Grachev A. V. Ikonnikov N. S. Sagiyan A. S. 《Russian Chemical Bulletin》2002,51(11):2086-2089
The effect of the nature of the central metal atom in chiral salen type complexes on the stereodifferentiating capacity of these complexes as catalysts in phase transfer asymmetric alkylation of Schiff"s base derived from alanine isopropyl ester and benzaldehyde by benzyl bromide. The nature of the central metal atom in the complex has a pronounced influence on the stereochemistry of alkylation; copper(ii) complexes exhibit the highest activity combined with a high stereoselectivity. 相似文献
996.
997.
钠改型天然斜发沸石的铵离子交换平衡 总被引:3,自引:0,他引:3
对分别来自国内三个产地和澳大利亚的天然斜发沸石样品进行了铵离子交换性能的研究,在25℃下测定了样品的总交换容量和平衡等温线,根据测定的平衡等温线,提出了表示沸石平衡情况的双孔道组合模型,并用文献报道的结果对模型进行检验,此模型不仅与实验结果较好地符合而且也适用于文献报道的结果。 相似文献
998.
999.
The effects of Lewis acid catalysts based on sulfur containing boron heterocycles on the Diels-Alder reactions of two methyl acroleins with cyclopentadiene have been studied using DFT methods. These reactions take place along highly asynchronous concerted processes. While the reaction with crotonaldehyde leads to the expected endo adduct, the reaction with methacrolein leads to exo one in agreement with the experiments. The catalytic effect can be explained through the analysis of the electrophilicity index (ω) of the reagents, and the molecular structure of the corresponding transition structures. 相似文献
1000.
过渡金属杂原子Y型分子筛的合成及表征 总被引:1,自引:0,他引:1
采用水热法合成了含过渡金属(Ti,Fe)的Y型分子筛,通过XRD、FT-IR、UV-vis-DRS和TG等对其进行了表征.结果表明:少量杂原子Ti、Fe进入骨架后,不影响Y分子筛的晶型及结晶度,但晶胞常数变大,骨架振动IR吸收峰向低频移动;杂原子Y分子筛的UV-vis-DRS谱图上不存在非骨架Ti、Fe的特征峰;TPR曲线表明,在600℃下通氢处理1h后,FeY分子筛仍无还原峰出现,这些均表明杂原子Ti、Fe已进入Y分子筛骨架. 相似文献