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排序方式: 共有157条查询结果,搜索用时 546 毫秒
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Rongbin Qi Yujun Wang Jiding Li Changwei Zhao Shenlin Zhu 《Journal of membrane science》2006,280(1-2):545-552
Worldwide concerns over environment have stimulated increasing interest both in academic and industry for deep desulfurization of gasoline. Polydimethylsiloxane (PDMS) composite membrane was used to separate the binary and multicomponent alkane/thiophene mixtures by pervaporation. Effect of carbon number and concentration of alkane, and of feed temperature, on the separation efficiency of alkane/thiophene mixtures was investigated experimentally. Experimental results of binary mixtures indicated that the total fluxes for different alkane/thiophene mixtures decrease with increase of carbon number in the alkanes. Corresponding activation energies of permeation for alkanes in PDMS membrane increase with increase of carbon number in the alkanes. Differences of molecular size and structure of the alkanes lead to various selectivities thereof within PDMS membrane. In addition, the permeability and activation energy of thiophene in various systems differ from each other due to coupling effect which should be taken into consideration when dealing with multicomponent systems. Pervaporation results of ternary systems indicated that, the increase of content of lighter alkane in feed would result in a larger total flux, but a smaller selectivity to thiophene simultaneously. A quaternary system, the mixture of n-heptane, n-octane, n-nonane and thiophene, was employed to simulate the desulfurization process of gasoline. With the membrane having a PDMS layer of 11 μm, the total flux was measured to be about 1.65 kg/m2 h, with the corresponding enrichment factor of thiophene 3.9 at 30 °C. 相似文献
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A novel BF3-promoted [3+2] annulation of azonaphthalenes and ynamides is described.This protocol provides a modular and efficient entry to functionalized amino benzo[e]indole derivatives smoothly. 相似文献
5.
Haijing Liang Lixia Ma Changwei Li Qiang Peng Zhaoyan Wang Zhan-xin Zhang Lan Yu Huanxiang Liu Fengli An Weihua Xue 《Tetrahedron letters》2019,60(1):84-87
Glycosylation reactions are significant as they provide access to model compounds that are useful for elucidating biochemical pathways. Herein, we describe the development of glycosyl ortho-alkynylbenzoates as novel, bench-top stable, and readily available glycosyl donors. Glycosylation is promoted by inexpensive trimethylsilyl triflate (TMSOTf) in combination with N-iodosuccinimide (NIS) under mild reaction conditions; hence, the novel glycosyl donors are promising reagents for the synthesis of glycosides. 相似文献
6.
Dr. Weiyi Li Zhishan Su Prof. Dr. Changwei Hu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(1):124-134
The mechanism of the allylation reaction between 4‐chloroacetophenone and pinacol allylboronates catalyzed by ZnEt2 with alcohols was investigated using density functional theory (DFT) at the M05‐2X/6‐311++G(d,p) level. The calculations reveal that the reaction prefers to proceed through a double γ‐addition stepwise reaction mechanism rather than a Lewis acid‐catalyzed concerted one. The intermediate with a four‐coordinated boron center, which is formed through proton transfer from EtOH to the ethyl group of ZnEt2 mediated by the boron center, is the active species and an entrance for the catalytic cycle. The latter is composed of three elementary steps: 1) boron to zinc transmetalation leading to the formation of allylzincate species, 2) electrophilic addition of ketone to allylzincate species, and 3) generation of the final product with recovery of the catalyst. The boron to zinc transmetalation step has the largest energy barrier of 61.0 kJ mol?1 and is predicted to be the rate‐determining step. The calculations indicate that the additive EtOH plays important roles both in lowering the activation free energy for the formation of the four‐coordinated boron active intermediate and in transforming the low catalytic activity ZnEt2 into high activity zinc alkoxide species. The alcohols with a less sterically encumbering R group might be the effective additives. The substituted groups on the allylboronates might primarily affect the boron to zinc transmetalation, and the allylboronates with substituents on the Cγ atom is poor in reactivity. The comparison of the catalytic effect between the zinc compounds investigated suggest that Zn(OEt)2, Zn(OH)2, and ZnF2 exhibit higher catalytic efficiency for the boron to zinc transmetalation due to the activation of the B? Cα bond through orbital interactions between the p orbitals of the EtO, OH, F groups and the empty p orbital of the boron center. 相似文献
7.
Changwei Wang Dr. Zhenhua Chen Prof. Dr. Wei Wu Prof. Dr. Yirong Mo 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(4):1436-1444
The generalized anomeric effect refers to the conformational preference of a gauche structure over an anti structure for molecules with a R‐X‐C‐Y moiety. Whereas there are conflicting reports regarding the origin of this ubiquitous effect, a general consensus is that both the steric (more specifically electrostatic) and hyperconjugative interactions contribute. Here we employed the block‐localized wavefunction (BLW) method, which is the simplest variant of ab initio valence bond (VB) theory and can define reference electron‐localized states self‐consistently, to evaluate the magnitude of the hyperconjugation effect in a number of acyclic molecules exhibiting the generalized anomeric effect. The BLW‐based energy decomposition analysis revealed that both the steric and hyperconjugation effects contribute to the conformational preferences of methoxymethyl fluoride and methoxymethyl chlorides. But for the other systems under investigation, including methanediol, methanediamine, aminomethanol and dimethoxymethane, the hyperconjugative interactions play a negative role in the conformational preferences and the steric effect is solely responsible for the generalized anomeric effect. 相似文献
8.
Lewis acid-catalyzed tandem acylation–Nazarov cyclization for the syntheses of fused cyclopentenones
Ertong Li Changwei Li Jing Wang Jinqian Wang Lihong Dong Xiaohe Guo Chuanjun Song Junbiao Chang 《Tetrahedron》2014
Treatment of N-tosylpyrroles or N-tosylindoles with α-unsubstituted α,β-unsaturated carboxylic acids or unsaturated carboxylic acids having an α electron-withdrawing substituent in the presence of TFAA and a Lewis acid catalyst resulted in the formation of fused cyclopentenones via a tandem acylation–Nazarov cyclization sequence, while either the acylation product obtained or no reaction occurred in the absence of the Lewis acid catalyst. 相似文献
9.
Mechanistic Study of the Role of Primary Amines in Precursor Conversions to Semiconductor Nanocrystals at Low Temperature 下载免费PDF全文
Dr. Kui Yu Dr. Xiangyang Liu Queena Y. Chen Prof. Huaqing Yang Prof. Mingli Yang Xinqin Wang Prof. Xin Wang Dr. Hong Cao Dr. Dennis M. Whitfield Prof. Changwei Hu Dr. Ye Tao 《Angewandte Chemie (International ed. in English)》2014,53(27):6898-6904
Primary alkyl amines (RNH2) have been empirically used to engineer various colloidal semiconductor nanocrystals (NCs). Here, we present a general mechanism in which the amine acts as a hydrogen/proton donor in the precursor conversion to nanocrystals at low temperature, which was assisted by the presence of a secondary phosphine. Our findings introduce the strategy of using a secondary phosphine together with a primary amine as new routes to prepare high‐quality NCs at low reaction temperatures but with high particle yields and reproducibility and thus, potentially, low production costs. 相似文献
10.
Fractional pyrolysis and one-step pyrolysis of natural algae Cyanobacteria from Taihu Lake were comparatively studied from 200 to 500 ℃. One-step pyrolysis produced bio-oil with complex composition and low high heating value (HHV〈30.9 MJ/kg). Fractional pyrolysis separated the degradation of different components in Cyanobacteria and improved the selectivity to products in bio-oil. That is, acids at 200 ℃, amides and acids at 300 ℃, phenols and nitriles at 400 ℃, and phenols at 500 ℃, were got as main products, respectively. HZSM-5 could promote the dehydration, cracking and aromatization of pyrolytic intermediates in fractional pyrolysis. At optimal HZSM-5 catalyst dosage of 1.0 g, the selectivity to products and the quality of bio-oil were improved obviously. The main products in bio-oil changed to nitriles (47.2%) at 300 ℃, indoles (51.3%) and phenols (36.3%) at 400 ℃. The oxygen content was reduced to 7.2 wt% and 9.4 wt%, and the HHV was raised to 38.1 and 37.3 MJ/kg at 300 and 400 ℃, respectively. Fractional catalytic pyrolysis was proposed to be an efficient method not only to provide a potential solution for alleviating environmental pressure from water blooms, but also to improve the selectivity to products and obtain high quality bio-oil. 相似文献