Transient studies on methane oxidative coupling over alkali-metal promoted titanate catalysts

A transient method to study the role of the different oxygen species involved in oxidative coupling of methane is discussed. The method is applied to two new active catalysts for this process, which are based on alkali metal loaded titanates of lanthanum and nickel and also to a lithium/titania catalyst used in previous works. The results show that the participation of lattice oxygen in the formation of C2 products depends on the Li loading and the type of titanate used.     

阳极氧化铝的表面参数对硅酮结构的装锒密封胶长期黏结性的影响(连载一)

硅酮密封胶对阳极氧化铝的黏结质量千差万别.影响黏结性的关键因素是铝材表面的封孔程度、氧化层的着色情况、清洁剂的特性以及基材表面清洁与打胶之间允许的时间间隔.由于清洁剂将有机污染物从阳极氧化铝表面去除的效果不同,所以此效果并不和黏结质量有必然联系.据猜测,吸附在阳极氧化铝表面的清洁溶剂会改善基材表面从而提高硅酮胶的黏结性,但这种改善效果会随着清洁溶剂随时间的的挥发而降低.对于给定的溶剂,最佳的黏结效果取决于材料表面的封孔程度.依照ISO 2143酸刻蚀方法测量材料表面的封孔程度,可以预测未着色的阳极氧化铝基材的黏结性.对于着色的阳极氧化铝表面,依照ISO 2931的测试标准,用电相位漂移方法可以用来预测黏结质量.一种控制阳极氧化铝表面的方法被提了出来,此方法是测量不同频率下基材的电阻抗并将它成功地和硅酮密封胶与该材料表面的长期黏接性联系起来.     

采用有限元分析法量化不同中空玻璃边缘密封的热位移

间隔条和玻璃板片之间的相对热位移是中空玻璃边缘密封乃至中空玻璃(IGU)本身老化的主要原因之一.作者采用有限元分析法(FEA),建立了发生在大尺寸(1.5 m×2.1 m)的中空玻璃边缘密封的热位移与温差变化之间的模型,这是三种商业化的、不同材质的、不同设计的间隔条,在温度变化为-30～ 60℃范围内的模型.此模型是基于铝间隔条和镀锌钢间隔条与尼龙插角的搭配,不锈钢间隔条和弯曲插角的搭配.尼龙插角假定为实心的并稳固地固定于间隔条上;弯曲插角被建模成实心弯曲的金属插角,并同样稳固地固定于间隔条上.由于实际的弯曲插角是空心的,模型可能高估了这种转角设计的应力.正如预期那样,在低温时,插角会被向内拉,导致弯曲角度大于90°;在高温下,插角会被向外推,导致弯曲角度小于90°.通过检测发生在中空玻璃周边第一道密封的聚异丁烯密封胶厚度的变化,作者发现,到目前为止,不锈钢间隔条对于截面积的变化影响最小;铝制间隔条的影响最大.这个发现与基于间隔条材料与浮法玻璃之间热膨胀系数差异而预期的性能一致.因此,热位移引起的第一道密封的有效扩散横截面变化,可以解释为何具有不同间隔条材料的中空玻璃之间的性能差异.     

Optimized cooling and gauge tolerances in blown film extrusion

Numerical calculations of the air flow and heat transport processes in film blowing have been made, considering the turbulence of the air flow. The parameters air mass flow, air temperature, air velocity, and angle-of-attack have been varied. The results have shown that the air massflow has the strongest influence on cooling in the processing window examined. So the massflow is a very efficient actuator of a film thickness control system. Moreover, the angle-of-attack together with the bubble shape is responsible for the occurrence of vortices. Such vortices can be the reason for unstable behavior in the bubble forming zone and hence for increased thickness tolerances of the film.     

Pheromone biosynthesis in lepidoptera

Pheromone components for many lepidopteran species are produced by the use of unique chain-shortening and 9, 10, and 11 desaturase systems. Correlations in the Tortricidae indicate that the pheromone components derived from 9 and 01 desaturases are found in the more primitive species (those possessing morphological plesiomorphies). The precise blend ofZ andE acetates in a number of species is regulated in the final reduction sequence from acyl intermediates. Preliminary research has been conducted on the characterization of the various desaturase enzymes used and on the important blend regulating sequence. Initial purification work on the 11 desaturase enzyme found in the cabbage looper moth is reported.     

Off-line optimization and control for real time integration of a three-phase hydrogenation catalytic reactor

In order to develop and test the integration procedure, in this paper a real time process integration involving the optimization and control of the process is presented, in this case, with the two-layer approach. The used optimization algorithms were Levenberg–Marquardt and SQP that solve a non-linear least square problem subject to bounds on the variables. The two-layer approach is a hierarchical control structure where an optimization layer calculates the set points and manipulated variables to the advanced controller, which is based on the dynamic matrix control with constraints (QDMC). The non-isothermal dynamic model of the three-phase slurry catalytic reactor with appropriate solution procedure was utilized in this work (Vasco de Toledo, E. C., Santana, P. L., Maciel, M. R. W., & Maciel Filho, R. (2001). Dynamic modeling of a three-phase catalytic slurry reactor. Chemical Engineering Science, 56, 6055–6061). The model consists on mass and heat balance equations for the catalyst particles as well as for the bulk phases of gas and liquid. The model was used to describe the dynamic behavior of hydrogenation reaction of o-cresol to obtain 2-methil-cyclohexanol, in the presence of a catalyst Ni/SiO₂.     

Transient and isotopic studies of the oxygen transport and exchange during oxidative coupling of methane on Sr promoted La2O3

Isotopic transient techniques were applied to study oxidative coupling of methane over lanthanum oxide and strontium promoted La₂O₃ catalysts. Results of the¹⁸O₂/¹⁶O₂ isotopic exchange experiments indicate that Sr promotion increases oxygen uptake from the lattice of the catalyst. Oxygen self diffusion coefficients, which were determined for the series of lanthana catalysts, reach a maximum for the 1% Sr/La₂O₃. Steps of¹⁸O₂ in the presence of a steady flow of methane over Sr/La₂O₃ catalysts, indicate that surface and bulk oxygen appear in the reaction products before gas-phase¹⁸O². Steps of CO₂ over catalysts in which lattice oxygen has been exchanged with¹⁸O₂, show that gas/solid exchange involves over 50% of the lattice oxygen. Under reaction conditions, methane pulses with no gas-phase oxygen yield negligible amounts of products which indicates that methane interacts with lattice oxygen only in the presence of the gas-phase oxygen.     

A predicted three-dimensional structure of human cytochrome P450: implications for substrate specificity

A three-dimensional structure for human cytochrome P450IA1 waspredicted based on the crystal coordinates of cytochrome P450camfrom Pseudomonas putida. As there was only 15% residue identitybetween the two enzymes, additional information was used toestablish an accurate sequence alignment that is a prerequisitefor model building. Twelve representative eukaryotic sequenceswere aligned and a net prediction of secondary structure wasmatched against the known -helices and ß-sheets ofP450cam. The cam secondary structure provided a fixed main-chainframework onto which loops of appropriate length from the humanP450IA1 structure were added. The model-built structure of thehuman cytochrome conformed to the requirements for the segregationof polar and nonpolar residues between the core and the surface.The first 44 residues of human cytochrome P450 could not bebuilt into the model and sequence analysis suggested that residues1–26 formed a single membrane-spanning segment. Examinationof the sequences of cytochrome P450s from distinct gene familiessuggested specific residues that could account for the differencesin substrate specificity. A major substrate for P450IA1, 3-methyl-cholanthrene,was fitted into the proposed active site and this planar aromaticmolecule could be accommodated into the available cavity. Residuesthat are likely to interact with the haem were identified. Thesequence similarity between 59 eukaryotic enzymes was representedas a dendrogram that in general clustered according to genefamily. Until a crystallographic structure is available, thismodel-building study identifies potential residues in cytochromeP450s important in the function of these enzymes and these residuesare candidates for site-directed mutagenesis.     

Quantitation of the human basal ganglia with positron emission tomography: a phantom study of the effect of contrast and axial positioning

The accurate measurement of the concentration of a radioisotope in small structures with positron emission tomography (PET) requires a correction for quantitation loss due to the partial volume effect and the effect of scattered radiation. To evaluate errors associated with measures in the human basal ganglia (BG), a unilateral model of the BG has been inserted in a 20-cm cylinder. The recovery coefficient (RC=measured activity/true activity) for the BG phantom has been measured on a CTI tomograph with different background concentrations (contrast) and at different axial locations in the gantry. The BG was visualized on four or five slices, depending on its position in the gantry and on the contrast used. The RC was 0.75 with no background (contrast equal to 1.0). Increasing the relative radioactivity concentration in the background increased the RC from 0.75 to 2.00 when the contrast was -0.7 (BG相似文献   

Computing Minimum Area Homologies

Calculating and categorizing the similarity of curves is a fundamental problem which has generated much recent interest. However, to date there are no implementations of these algorithms for curves on surfaces with provable guarantees on the quality of the measure. In this paper, we present a similarity measure for any two cycles that are homologous, where we calculate the minimum area of any homology (or connected bounding chain) between the two cycles. The minimum area homology exists for broader classes of cycles than previous measures which are based on homotopy. It is also much easier to compute than previously defined measures, yielding an efficient implementation that is based on linear algebra tools. We demonstrate our algorithm on a range of inputs, showing examples which highlight the feasibility of this similarity measure.