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31.
The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and -isocinchonine-modified Pt/Al2O3 catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmoldm–3, 1 bar H2, 23°C, 6% AcOH in toluene), and 76% ee in the case of -isocinchonine (0.14 mmoldm–3, 1 bar H2, –10°C, 6% AcOH in toluene). Since -isocinchonine of rigid structure exists only in anti-open conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of anti-open conformation in these cinchona-modified enantioselective hydrogenations. 相似文献
32.
The porosity of -alumina-based materials is an important parameter affecting the extent of fluorination (aerogels > commercial -Al2O3 > xerogels) and, consequently, also the textural, acidic and catalytic properties of the final fluorinated materials. Only the highly fiuorinated aluminas having strong Lewis acidic sites catalyse the isomerisation of CHF2CHF2 to CF3CH2F. 相似文献
33.
Tetsuya Tanigami Kanna Kai Koji Tanaka Shuji Matsuzawa 《Journal of Inorganic and Organometallic Polymers》2003,13(4):237-253
Poly[bis(-phenoxyethoxy)phosphazene] [PBPEP] had been shown in our previous paper to be a very useful polymer for investigating the crystallization mechanism of polymers, as the crystallization rate of PBPEP is extraordinarily small when isothermally crystallized from the melt. The crystallization of the low molecular weight oligomers of PBPEP was first studied in comparison to the high molecular weight polymers. The oligomer-rich fraction was obtained by fractionation of the as-polymerized sample, which had a broad molecular weight distribution. The fractions thus obtained were characterized by solution viscometry and size exclusion chromatography. The melting temperature and the growth rate of the spherulite from the melt were investigated by differential scanning calorimetry and optical microscopy. The growth rate was one or two orders of magnitude smaller in the oligomer-rich fraction than in the other high molecular weight fractions. A collapsed spherulite appeared in the oligomer-rich fraction at high crystallization temperatures. It is speculated that in the oligomer-rich fraction there is an excess free energy due to defects in the crystal phase. This defect is considerably larger in the oligomer-rich fraction than in the other fractions because a large quantity of short length chains is present. 相似文献
34.
In view of the importance for CoxOy,-MoO3/-Al2O3 hydrodesulphurization (HDS) catalysts, the reactivity of cobalt oxide layers towards cobalt aluminate formation was investigated on both MoO3-covered and bare -Al2O3 substrates. Co3O4/MoO3/-Al2O3 and Co3O4/-Al2O3 systems were prepared by vapour-deposition of MoO3 (12 × 1015 Mo atoms/cm2) and Co (400 × 1015 Co atoms/cm2) layers onto a -Al2O3 substrate, followed by oxidation of the Co layer to Co3O4. After annealing at 800°C for 40 h, the interfacial reaction to cobalt aluminate was assessed using Rutherford backscattering spectrometry. The presence of molybdenum oxide appeared to enhance cobalt aluminate formation. The Mo atoms, which spread out over the entire cobalt-containing layer, presumably caused a high defect density, which explains the observed higher reaction rate. The amount of MoO3 was much too low to stabilize all cobalt atoms by cobalt molybdate formation. 相似文献
35.
主要针对7.63m特大型焦炉砌筑施工技术,从工程实践效果分析其施工技术在质量、工期、成本等方面收到良好的经济效益和社会效益,现对7.63m特大型焦炉施工技术作简要介绍。 相似文献
36.
钱永江 《延边大学学报(自然科学版)》2009,35(3):207-210
讨论了缺失数据在随机补充方式下均值的估计,证明了估计的相合性和渐进正态性,并给出了经验似然比统计量的渐进分布. 相似文献
37.
刘劲松 《佳木斯工学院学报》2009,(4):535-537
提出一种多步自适应组合预测方法.预测由多个并行不同的多步自适应预测模型组成,最终预测结果,由多个预测模型预测值的线性加权组合决定.本法适用于随机时变动态系统过程的多步预测问题,在某省粮食产量趋势预测中,应用此法获得了良好的效果. 相似文献
38.
从提高中部企业核心竞争力的角度出发,对中部企业人才发展通道不畅的现状和原因进行分析,提出建立管理人才、技术人才和技能人才“多梯制”发展通道的构想,试图通过充分发挥人才资源的作用来有效整合其内部和外部的各种资源,不断推陈出新,提升企业的核心竞争力,使中部企业在日益激烈的市场竞争中始终立于不败之地,实现中部地区的快速崛起。 相似文献
39.
居住类建筑一直是建筑设计人员关注的重点,关于"住"的思考人们也从未停歇过,将结合一些流行的住宅设计观点展开对于"住"的思考. 相似文献
40.
The effect of alumina pretreatment on the performance of alumina supported nickel catalysts was demonstrated in gas phase hydrogenation of toluene to methylcyclohexane. The state of the alumina was changed from pure to pure phase through various heat treatments in air. The catalysts were prepared from vapor phase by saturating the accessible binding sites on the pretreated alumina with the nickel precursor. The highest number of active sites for hydrogenation was observed for catalysts prepared on alumina having an incomplete phase transition and a / alumina phase ratio between 0.5 and 10. Results from temperature programmed desorption (TPD) studies revealed that a maximum in weakly chemisorbed hydrogen as well as in total amount of desorbed hydrogen was found for the same catalysts. By hydrogen chemisorption studies the total hydrogen uptake was found to correlate with the observed hydrogenation maximum. It is suggested that both the chemical and physical properties of the alumina influence the activity. An optimal metal-support interaction and structural defects on the alumina due to the phase transition can explain the observed maximum in the number of active sites and in hydrogen uptake. 相似文献