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131.
132.
Layered double hydroxide supported nanopalladium catalyst for Heck-, Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes 总被引:1,自引:0,他引:1
Choudary BM Madhi S Chowdari NS Kantam ML Sreedhar B 《Journal of the American Chemical Society》2002,124(47):14127-14136
Layered double hydroxide and Merrifield resin supported nanopalladium(0) catalysts are prepared by an exchange of PdCl(4)(2-) followed by reduction and well characterized for the first time. The ligand-free heterogeneous layered double hydroxide supported nanopalladium (LDH-Pd(0)) catalyst using the basic LDH in place of basic ligands indeed exhibits higher activity and selectivity in the Heck olefination of electron-poor and electron-rich chloroarenes in nonaqueous ionic liquids (NAIL) over the homogeneous PdCl(2) system. Using microwave irradiation, the rate of the Heck olefination reaction is accelerated, manifold with the highest turnover frequency ever recorded in the case of both electron-poor and electron-rich chloroarenes. The basic LDH-Pd(0) shows a superior activity over a range of supported catalysts, from acidic to weakly basic Pd/C, Pd/SiO(2,) Pd/Al(2)O(3), and resin-PdCl(4)(2-) in the Heck olefination of deactivated electron-rich 4-chloroanisole. The use of LDH-Pd(0) is extended to the Suzuki-, Sonogashira-, and Stille-type coupling reactions of chloroarenes in an effort to understand the scope and utility of the reaction. The catalyst is quantitatively recovered from the reaction by a simple filtration and reused for a number of cycles with almost consistent activity in all the coupling reactions. The heterogeneity studies provide an insight into mechanistic aspects of the Heck olefination reaction and evidence that the reaction proceeds on the surface of the nanopalladium particles of the heterogeneous catalyst. TEM images of the fresh and used catalyst indeed show that the nanostructured palladium supported on LDH remains unchanged at the end of the reaction, while the XPS and evolved gas detection by TGA-MS of the used catalyst identify ArPdX species on the heterogeneous surface. Thus, the ligand-free nanopalladium supported on LDH, synthesized by the simple protocol, displays superior activity over the other heterogeneous catalysts inclusive of nanopalladium in the C-C coupling reactions of chloroarenes. 相似文献
133.
Polymers that are biodegradable currently achieve high interest in material science since they offer reductions of landfill space during waste management as well as new end-user benefits in various fields of applications. In this work, cellulose esters such as cellulose benzoate, cellulose succinate and cellulose cinnamate were prepared using dimethylaminopyridine along with dimethylaminopyridine-p-toluene sulfonic acid catalyst. Films of cellulose esters were cast from solution. Cellulose esters were characterized by spectral methods such as infrared, nuclear magnetic resonance, thermal method such as thermogravimetric analysis. Various methods of kinetic analysis were compared in the case of thermal degradation of the cellulose and cellulose esters. Copyright© 2003 John Wiley & Sons, Ltd. 相似文献
134.
Vijay Reddy Anand T. Krishnan Andrew Marshall John Rodriguez Sreedhar Natarajan Tim Rost Srikanth Krishnan 《Microelectronics Reliability》2005,45(1):31-38
We have examined the impact of NBTI degradation on digital circuits through the stressing of ring oscillator circuits. By subjecting the circuit to pMOS NBTI stress, we have unambiguously determined the circuit reliability impact of NBTI. We demonstrate that the relative frequency degradation of the NBTI stressed ring oscillator increases as the voltage at operation decreases. This behavior can be explained by reduced transistor gate overdrive and reduced voltage headroom at the circuit level. We present evidence that donor interface state generation during NBTI stress is a significant component of the transistor degradation. Further, we show that the static noise margin of a SRAM memory cell is degraded by NBTI and the relative degradation increases as the operating voltage decreases. 相似文献
135.
The oxide responsible for high-temperature superconductivity (onset ∼100 K, zero resistance above liquid N2 temperature) is found to be YBa2Cu3O7±δ.
Contribution No. 432 from the Solid State and Structural Chemistry Unit 相似文献
136.
Oxides of the Y-Ba-Cu-O system are found to show onset of superconductivity in the 100–120 K region. 相似文献
137.
Vinod VT Saravanan P Sreedhar B Devi DK Sashidhar RB 《Colloids and surfaces. B, Biointerfaces》2011,83(2):351-298
An environmentally benign method for the synthesis of noble metal nanoparticles has been reported using aqueous solution of gum kondagogu (Cochlospermum gossypium). Both the synthesis, as well as stabilization of colloidal Ag, Au and Pt nanoparticles has been accomplished in an aqueous medium containing gum kondagogu. The colloidal suspensions so obtained were found to be highly stable for prolonged period, without undergoing any oxidation. SEM-EDXA, UV-vis spectroscopy, XRD, FTIR and TEM techniques were used to characterize the Ag, Au and Pt nanoparticles. FTIR analysis indicates that -OH groups present in the gum matrix were responsible for the reduction of metal cations into nanoparticles. UV-vis studies showed a distinct surface plasmon resonance at 412 and 525 nm due to the formation of Au and Ag nanoparticles, respectively, within the gum network. XRD studies indicated that the nanoparticles were crystalline in nature with face centered cubic geometry. The noble metal nanoparticles prepared in the present study appears to be homogeneous with the particle size ranging between 2 and 10 nm, as evidenced by TEM analysis. The Ag and Au nanoparticles formed were in the average size range of 5.5±2.5 nm and 7.8±2.3 nm; while Pt nanoparticles were in the size range of 2.4±0.7 nm, which were considerably smaller than Ag and Au nanoparticles. The present approach exemplifies a totally green synthesis using the plant derived natural product (gum kondagogu) for the production of noble metal nanoparticles and the process can also be extended to the synthesis of other metal oxide nanoparticles. 相似文献
138.
Mallipattu Sreedhar Madhusudana T. Reddy Kodigutta Balaji 《International journal of environmental analytical chemistry》2013,93(10):757-765
The polarographic behaviour of cyfluthrin (CY), an α-cynoester pyrethroid, was studied using a dropping mercury electrode and hanging mercury drop electrode in methanolic Britton–Robinson (B–R) buffer of pH 2.0–12.0 with different ionic media. The nature of the electrode process was examined, the number of electrons was evaluated, and the reduction mechanism was proposed. Quantitative determination was achieved in the concentration range of 6.0?×?10?8 to 1.15?×?10?5?mol?dm?3 using a differential pulse polarographic method with a lower detection limit of 2.4?×?10?8?mol?dm?3. The proposed method was successfully applied in the determination of CY in formulations, grains, soils, and spiked water samples. 相似文献
139.
Isophlorins and its analogues bridge the structural features between porphyrins and annulenes. Invariably they are known to be unstable but can be stabilized by appropriate substituents in the core and periphery of a macrocycle. Solid-state characterization of these 20pi systems displays planarity for each macrocycle and even in their supramolecular arrangement due to C-H...F-C interactions. 相似文献
140.
First successful design of semi-IPN hydrogel-silver nanocomposites: a facile approach for antibacterial application 总被引:4,自引:0,他引:4
Murthy PS Murali Mohan Y Varaprasad K Sreedhar B Mohana Raju K 《Journal of colloid and interface science》2008,318(2):217-224
Semi-IPN hydrogels in which poly(vinyl pyrrolidone) (PVP) chains were physically dispersed throughout poly(acrylamide) (PAM) gel networks were synthesized. These semi-IPN hydrogel networks can act as excellent nanoreactors for producing and stabilizing metal nanoparticles. The current methodology allows us to entrap metal nanoparticles throughout hydrogel networks via PVP chains. An optimized semi-IPN hydrogel formulation was found to produce silver nanoparticles, ca. 3-5 nm. The synthesized semi-IPN hydrogel-silver nanocomposites were fully characterized by using UV-vis, X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The developed semi-IPN hydrogel-silver nanocomposite (SHSNC) was evaluated for preliminary antibacterial applications. 相似文献