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21.
22.
A conceptually new strategy for asymmetric catalysis, namely asymmetric activation, in which a chiral activator selectively activates one enantiomer of a racemic chiral catalyst, and a highly efficient screening system for finding the most effective catalysts, namely super high throughput screening (SHTS), by which the reaction can be conducted in parallel and the ee% of the product is allowed to determine within minutes, are summarized in the present account. It is reasonable to believe that SHTS technique combined with asymmetric activation or deactivation principle will provide a very powerful methodology for finding the new catalysts and the best catalyst tuning for asymmetric reactions. 相似文献
23.
Deactivation studies of jack bean urease immobilized on porous alumina beads in the hydrolysis of urea were conducted in a
continuously stirred tank reactor (CSTR) at a temperature of 25°C and pH 7.0. Though the mechanism of poisoning of urease
by product ammonia is fairly well understood from the literature, the nature of the poisoning of urease by urea is presented
in this article. These studies were conducted by adsorbing the ammonia formed in the hydrolysis reaction. The results indicate
that, in the presence of the adsorbent Zeolite W, the deactivation rate is reduced by a factor of almost two, and thus provide
a technique for prolonging the life of the enzyme. The deactivation model suggests that the free form of the enzyme is most
susceptible to attack by the substrate urea. The experimental data suggest that deactivation by combined ammonia and urea
is fairly complex. 相似文献
24.
We simulated models of diffusion and aggregation in long pores of small widths in order to represent the basic mechanisms of coke deposition in catalysts’ pores. Coke precursors are represented by particles injected at the pore entrance. Knudsen diffusion, which is usually expected inside the pores, is modeled by ballistic motion of those particles. The regime of molecular diffusion is also analyzed via models of lattice random walks biased along the pores. The aggregation at the surface or near previously aggregated particles was modeled by different probabilistic rules, accounting for the possibilities of more compact or more ramified deposits. In the model of Knudsen diffusion and in some cases of molecular diffusion, there is an initial regime of uniform deposition along the pore, after which the deposits acquire an approximately wedge shape, with the pore plugging near its entrance. After the regime of uniform deposition and before that of critical pore plugging, the average aggregation position slowly decreases with the number N of deposited particles approximately as N-0.25. The apparently universal features of deposits generated by microscopic models are compared with those currently adopted in continuum models. 相似文献
25.
26.
This article outlines some of the factors influencing the choice of a suitable reactor for using immobilized biocatalysts.
We have concentrated on biochemical engineering parameters of immobilized biocatalysts, which are important with respect to
their application in industrial processes. 相似文献
27.
采用丙烯气相进料,在预混段中先与双氧水的甲醇溶液混合后进入固定床反应器,考察了进料液中pH值、丙二醇单甲醚、乙硫醇以及铁锈等对丙烯环氧化连续反应中TS-1分子筛催化剂性能的影响。结果表明,进料液体pH值对催化剂的催化性能有影响,适宜的进料液体pH值在7左右;副产物丙二醇单甲醚含量的增加不会对催化剂性能产生影响;乙硫醇量的增加使环氧丙烷选择性下降,但不会引起催化剂的失活;而进料液体中铁锈的引入会导致催化剂中部分孔堵塞,使催化剂部分失活。当进料液中的pH值用0.1mol/L的氨水调节为7左右,在反应温度55℃,反应压力0.7MPa,TS-1催化剂具有较好的稳定性,经130h的连续试验考察,双氧水的转化率和环氧丙烷的选择性约为90%。 相似文献
28.
催化燃烧是目前最有效的处理挥发性有机物(VOCs)技术之一. 本文从催化剂活性组分、催化剂载体、有效组分颗粒大小、水蒸汽的影响及催化燃烧反应中的积碳等几个方面, 对近年来催化燃烧处理VOCs的研究进行了总结. 分析表明: 贵金属催化剂的研究主要着重于选择有效的载体和双组分贵金属催化剂; 非贵金属催化剂的研究主要集中在高活性的过渡金属复合氧化物、钙钛矿和尖晶石型等催化剂的研制, 还有这些活性组分粒径大小及载体对催化燃烧VOCs反应活性的影响;此外, 在实际应用中,水蒸汽和催化剂积碳失活等问题对催化燃烧VOCs的反应也有很大影响. 本文的评述将为选择合适的催化燃烧技术处理VOCs污染物提供一定参考. 相似文献
29.
Three photocatalytic oxidation (PCO) systems: C7H16-O2, SO2-O2 and C7H16-SO2-O2 were carried out with the aid of UV-illuminated TiO2 nanoparticles at room temperature in a batch reactor. In C7H16-O2-TiO2 system, no catalyst deactivation was observed, while for SO2-O2-TiO2 and C7H16-SO2-O2-TiO2 systems, the photocatalytic activity of used TiO2 powder showed decreasing and eventually no activity after used consecutively. The reaction products such as sulfur trioxide or sulfuric acid adsorbed onto the surface of TiO2 catalyst were poisoning species. Photocatalytic activity of the deactivated TiO2 powder could be regenerated by sonicating treatment with water and methanol for the two systems, respectively. 相似文献
30.
利用X-射线衍射,扫描电镜,拉曼光谱,等离子发射光谱和比表面测定等方法研究了不同状态下失活及新鲜催化剂的结构,杂种类及含量。结果表明,超温使催化剂晶化明显,制备方法对催化剂结构有直接影响,再生使其强度降低,条形催化剂比较容易粉化,无机杂质是次要的失活原因。 相似文献