One-step synthesis of Ti-MSU and its catalytic performance on phenol hydroxylation

In this study, high-quality Ti-MSU was synthesized by a convenient one-step hydrothermal synthesis procedure. This is the first detailed study on the role of addition time of a Ti source. We found that the addition time of the Ti source can effectively influence the structure and morphology of the product by adjusting the balance of hydrolysis-polymerization and nucleation-aggregation in different self-assembly steps. The optimum synthesized Ti-SMU sample showed a good catalytic performance on the phenol hydroxylation.     

单个多元正态总体均值向量联合置信区间的比较

单个多元正态总体均值向量的Bonferroni和Scheff\'{e}联合置信区间在实际中经常用到,本文主要采用解析的办法比较这两个置信区间的长短, 证明了当均值向量的维数$2\le p\le 12$时, Bonferroni联合置信区间比Scheff\'{e}联合置信区间短.     

Stereoselective synthesis of polysubstituted 2,5-dihydrofurans from reaction of 1,4-dilithio-1,3-dienes with aldehydes

[reaction: see text] Reaction of 1,4-dilithio-1,3-diene derivatives with 2 equiv of aldehydes affords polysubstituted 2,5-dihydrofurans in good to high yields with perfect regio- and stereoselectivities. Hexa-2,4-diene-1,6-dialcoholates are proposed as the first intermediates, which undergo a cyclization and subsequent elimination of Li(2)O to generate the 2,5-dihydrofuran derivatives.     

Evolution of a Synthetic Strategy for Garsubellin A

Garsubellin A is a thirty-carbon meroterpenoid capable of enhancing the enzyme choline acetyltransferase whose decreased level is associated with the symptoms of Alzheimer's disease. Due to the potentially useful biological activity along with the novel molecular architecture, this plant metabolite has remained a popular synthetic target. Herein we report a full account of our synthetic investigations that have led to the enantioselective total synthesis of garsubellin A, establishing its absolute stereostructure. The protecting group-free, twelve-step synthetic route has enabled the syntheses of the natural (−)-garsubellin A and its unnatural (+)-antipode.     

Positive solutions for infinite semipositone problems with falling zeros

We consider the positive solutions to the singular problem

(P)     

具有碳化硅纳米线编织结构的氧化铝泡沫陶瓷的制备及性能研究

本文利用简单、高效的浆料直接发泡法制备气孔率高达96%的Al₂O₃/Si泡沫陶瓷,并选用简便、易行的焦炭埋烧工艺在Al₂O₃/Si泡沫陶瓷坯体中生长出大量SiC纳米线。通过控制烧结温度来观察分析SiC纳米线的生长形貌变化。采用扫描电子显微镜(SEM)、X射线衍射仪、BET比表面积测试仪、电子万能试验机等对泡沫陶瓷的微观结构、物相组成、比表面积、气孔率、抗压强度、热导率进行分析与表征。结果表明,1 450 ℃烧结时得到的SiC纳米线最多,纳米线在泡沫陶瓷孔壁交织缠绕。同时观察到SiC纳米线的存在改变了氧化铝泡沫陶瓷固有的脆性断裂模式,SiC纳米线可有效促进泡沫陶瓷在压缩过程中的裂纹偏转。本实验制备了一种新型的纳米线缠绕在孔壁上的三维网络结构的泡沫陶瓷,为在泡沫陶瓷内部原位生长SiC纳米线提供了新的方法,更好地拓展了泡沫陶瓷在环境过滤、催化剂载体等领域中的应用。     

超声辅助浊点萃取-原子荧光光谱法测定水中痕量汞

为了适应水中痕量汞的分析要求,降低汞的检出限,建立了一种超声辅助浊点萃取预富集联合原子荧光光谱法测定水样中痕量汞的方法。该方法采用硫代米氏酮为络合剂,以非离子型表面活性剂Triton X-114为萃取剂,将水样中的痕量汞富集于表面活性剂相。在原子荧光光谱分析前,加入磷酸三丁酯作为消泡剂,用5%盐酸溶液定容,富集倍数可以达到10倍。优化萃取体系pH、络合剂用量、萃取剂用量、超声平衡条件等浊点萃取的条件,在最优条件下,该方法检出限为0.003 μg/L,相对标准偏差为4.2%~8.6%,加标回收率为85%~106%。结果表明,超声辅助浊点萃取-原子荧光法能够适用于对地表水、自来水等清洁水样中痕量汞的分析,且具有更低的检出限,良好的准确度和精密度,能够适应于更高的样品分析要求。     

Acyl-lithiation of olefins: formation of cyclopentenones from 1-lithio-butadienes and CO

[reaction: see text] Intramolecular acyl-lithiation of C=C double bonds proceeds following carbonylation of 1-lithio-1,3-dienes with CO to afford 2- or 3-cyclopentenone derivatives in good to excellent yields after hydrolysis. Addition of electrophiles to the carbonylation reaction mixtures affords various multiply substituted cyclopentenones.