浆态床二甲醚合成中V2O5、Sm2O3对脱水组分γ-Al2O3的修饰作用

采用等容浸渍法制备改性脱水催化剂,通过H2-TPR、Pyridine-IR、还原态NH3-TPD、XRD等表征手段,以及目标反应浆态床CO+H2合成二甲醚,研究了催化剂的还原性能以及酸中心分布与反应性能之间的关系。H2-TPR结果表明,在脱水催化剂γ-Al2O3、V2O5/γ-Al2O3和Sm2O3/γ-Al2O3上不出现还原峰,V2O5、Sm2O3的加入改善了复合催化剂中Cu的还原性能,促进了甲醇催化剂的还原。Pyridine-IR表明,V2O5和Sm2O3的加入对L酸、B酸的量影响不大。还原态NH3-TPD说明V2O5和Sm2O3的加入改变了酸中心的分布,增加了弱酸中心的比率。XRD结果发现,V2O5和Sm2O3均匀分散在γ-Al2O3上,没有新的物种生成。二甲醚合成目标反应的结果表明,改性后催化剂的反应活性增强,合成反应中CO转化率、二甲醚的选择性都得到提高。V2O5和Sm2O3的添加增加了弱酸中心数量,促进了脱水活性,从而提高了复合催化剂合成二甲醚的活性和选择性。     

本刊论文按SCI被引频次排序（前10名）表

据美国 ISI(科技信息研究所 )网络版刊载的 1985～ 2000年的文章引用情况,我们对在我刊发表的论文的有关情况进行了统计,并按作者总被引频次顺序整理出来 (见表 )。是否可以说,这个结果从一个侧面反映出,这些作者对我刊走向世界,扩大影响作出了突出的贡献。同时,我们也要对每一位向本刊投稿的作者、稿件评审人,以及关心、帮助过我们的各方人士表示衷心的感谢。我刊刊出文章按作者被引频次排序（前 10名）表(根据 1985～ 2000年 SCI网络版提供的数据 ) 作者总被引频次在我刊发表论文数名次作者总被引频次在我刊发表论文数名次赵新…     

国际学校学业水平"校本评估"的质量保障体系研究——以SACE国际课程为例

国内的国际高中在开展"以校为本"的学生学业评价方面已经积累了比较成熟的经验,其中一些做法,例如"以校为本的学生评估理念、内部评估与外部考核相结合的评价机制、校本评估的质量保障体系等",对于破解"新高考"模式下学生评价工作难题有一定的借鉴意义.国内部分国际高中引进的SACE国际课程"校本评估"的质量保障体系直接目的 指向...     

光学元件损伤在线检测中的图像处理

利用在传统的边缘提取方法上加入灰度抑制,并在梯度计算中考虑每个像素与周围8个邻域的关系的图像处理方法,解决了在线检测中被检测元件以布儒斯特角放置、元件片数较多、CCD所采集的损伤图像噪声成分复杂等造成的像质较差的图像处理问题,并系统地进行了光学元件疵点分析和计算。分析计算结果表明:此图像处理方法得到的损伤疵点尺寸与实际尺寸相符,误差在检测的范围内;为大型光学系统中光学元件损伤的在线、自动化检测提供了一种有用的技术途径。     

小问题? (36)

117.梯子AB靠在墙上,其重P=20千克。梯长为l,并与水平面交角θ=60°。已知接触面间的摩擦系数均为0.25。今有一重65千克的人沿梯上爬,问人所能达到的最高点C到A点的距离s应为多少?(地矿部张家口技工学校力学教研组杨贺田,选自王铎主 ...     

Quasiperiodic events in an earthquake model

We introduce a modification of the Olami-Feder-Christensen earthquake model [Phys. Rev. Lett. 68, 1244 (1992)10.1103/PhysRevLett.68.1244] in order to improve the resemblence with the Burridge-Knopoff mechanical model and with possible laboratory experiments. A constant and finite force continually drives the system, resulting in instantaneous relaxations. Dynamical disorder is added to the thresholds following a narrow distribution. We find quasiperiodic behavior in the avalanche time series with a period proportional to the degree of dissipation of the system. Periodicity is not as robust as criticality when the threshold force distribution widens, or when an increasing noise is introduced in the values of the dissipation.     

Directed supramolecular assembly of Cu(II)-based "paddlewheels" into infinite 1-D chains using structurally bifunctional ligands

The construction of Cu(II)-containing supramolecular chains is achieved by combining suitable anionic ligands (for controlling the coordination geometry and for creating a neutral building block) with four new bifunctional ligands containing a metal-coordinating pyridyl site and a self-complementary hydrogen-bonding moiety. Seven crystal structures are presented and in each case, the copper(II) complex displays a "paddlewheel" arrangement, with four carboxylate ligands occupying the equatorial sites, leaving room for the bifunctional ligand to coordinate in the axial positions. The supramolecular chemistry, which organizes the coordination-complexes into the desired infinite 1-D chains, is driven by a combination of N-H...N and N-H...O hydrogen-bonds in five of the seven structures.     

High-pressure accelerated asymmetric organocatalytic Friedel-Crafts alkylation of indoles with enones: application to quaternary stereogenic centers construction

An organocatalytic Friedel-Crafts alkylation of indoles with α,β-unsaturated ketones was found to be efficiently accelerated under high-pressure conditions with a low loading of chiral primary amine salts with good yield and enantioselectivity up to 90%. This approach also allows, for the first time, selected indole derivatives containing quaternary stereogenic centers to be obtained from prochiral β,β-disubstituted enones with an enantioselectivity up to 80%.