A new proline–threonine (H-Pro-Thr-OH) dipeptide has been demonstrated as an efficient organocatalyst for a direct asymmetric aldol reaction. It was found that this new peptide-based catalyst efficiently catalyzed the reaction between an aldehyde and acetone to provide β-hydroxy ketones in good yields with good enantioselectivities. 相似文献
Zeolite-β and mordenite were impregnated with 0.1 wt% Pd and varying the amount (0.1–0.5 wt%) of Ni. The prepared nanoporous catalyst were characterized by various physico-chemical techniques such as XRD, nitrogen adsorption–desorption isotherm (BET), NH3-TPD and TPR, XPS and TEM. Hydroisomerisation of n-octane was carried out in the temperature range from 200 to 450 °C in the presence of flowing H2 gas under 1 atm. Finally we found that Ni addition up to 0.3 and 0.2 wt% over 0.1 wt% Pd/H-β and H-mordenite enhances the n-octane conversion and isomerisation selectivity. As the Ni amount exceeds the threshold values, the conversion decreases with increase in cracked products, and also, the selectivity of mono and dibranched isomers were improved suggestion operation of PCP intermediate mechanism. The bimetallic catalysts were more selective to the formation of dibranched isomers with higher octane number, when compared with monometallic catalysts. Ni-Pd loaded zeolite-β supports always show higher activity and selectivity than mordenite supports. Moreover, we achieved higher conversion (74.9 %) and isomerisation selectivity (92.5 %) at low Ni loading (0.3 Ni wt% over 0.1 Pd wt%/Hβ) for the first time. 相似文献
Mercury(II) halides, HgX2 (X = Cl–, Br–, I–) react with 1,3-benzothiazole-2-thione (btztH) in methanol solutions giving the HgX2(btztH) and HgX2(btztH)2 types of compounds. Mercury(II) acetate gives the thiolato compound Hg(btzt)2 because of the deprotonation of btztH. Hg(btzt)2 reacts with 2,2-bipyridine (bipy) giving a 1:1 complex. IR, 1H, and 13C NMR spectral studies indicate that btztH acts as a monodenatate ligand through the S thione donor atom in all complexes. The X-ray crystal structure determinations of [HgI2(btztH)]2, HgBr2(btztH)2, Hg(btzt)2, and Hg(btzt)2(bipy) have been carried out revealing tetrahedrally coordinated mercury atom in [HgI2(btztH)]2 and HgBr2(btztH)2, while in Hg(btzt)2(bipy) 2 + 2 coordination is achieved through strong Hg (N(bipy) contacts. A linear coordination in Hg(btzt)2 is not affected by the Hg N contacts, which are longer than in Hg(btzt)2(bipy), but still shorter than the van der Waals sum of mercury and nitrogen covalent radii. [HgI2(btztH)]2 exists as centrosymmetrical dimer with a Hg2I2 bridging core. The dimeric molecules are linked by the intermolecular hydrogen bonds between the terminal iodine atom and the NH group [3.63(1) Å] into infinite chains along the z-axis. There are N–H Br(bridging) intermolecular hydrogen bonds in HgBr2(btztH)2 joining molecules into endless chains along the x-axis. The Br(bridging) atom acts as double proton acceptor and two NH groups as proton donors [NH Br(bridging) 3.278(9) and 3.338(7) Å]. The mercury to sulfur and mercury to halogen bond distances in [HgI2(btztH)]2 and HgBr2(btztH)2 are discussed in relation to the analogous compounds, revealing strong influence of hydrogen bonds on their relative strengths as well as crystal packing requirements of the ligand. The sulfur and halogen atoms are more tightly bound to mercury implicating severe distortion of the coordination polyhedron in the structures in which they do not take part in hydrogen bonds formation. The influence of steric requirements of the ligands in Hg(btzt)2 and Hg(btzt)2(bipy) on the distortion of the mercury coordination polyhedra accompanied with the relative strength of Hg N contacts is considered. 相似文献
Summary Reactions between graphite and magnesium, silicon, vanadium and aluminium oxides in graphite electrodes have been investigated by spectrochemical and X-ray diffraction methods. Samples were ignited by means of an evaporator up to controlled temperatures. In the range of 1000–1900°C magnesium oxide does not react with graphite. Aluminium trioxide first forms -Al2O3 and at 1900°C Al4C3. Silicon dioxide forms silicon carbide at about 1400°C. Vanadium pentoxide is first reduced to VO2 and than at higher temperatures (1200° C) forms -VC. At about 1400° C MgO and SiO2 mixed with graphite powder form magnesium silicate, Mg2SiO4, and this silicate was stable at higher temperatures (up to 2000°C).
Hochtemperaturreaktionen in spektralanalytisch wichtigen SystemenII. Reaktionen von Graphit mit den Oxiden von Magnesium, Silicium, Vanadium und Aluminium
Zusammenfassung Die Reaktionen wurden durch Spektralanalyse und Röntgendiffraktometrie untersucht. Die Proben wurden in einem Evaporator auf kontrollierte Temperaturen erhitzt. Im Bereich von 1000–1900° C reagiert MgO nicht mit Graphit. Al2O3 bildet zunächst -Al2O3 und bei 1900°C Al4C3. SiO2 bildet bei etwa 1400°C SiC. V2O5 wird zunächst zu VO2 reduziert und geht dann bei höheren Temperaturen (1200°C) in -VC über. MgO und SiO2 im Gemisch mit Graphit bilden Mg2SiO4, das bei hohen Temperaturen (bis 2000°C) noch beständig ist.
Subnanometer size cluster precursors of uncapped CdS quantum dots were produced via the electroporation of synthetic dioleoylphosphatidylcholine (DOPC) unilamellar bilayer vesicles of mean hydrodynamic diameter Dh = 175 nm. During electroporation, Cd2+ ions are ejected from the interior compartments of the vesicles into the bulk solution where they react with S(2-) ions to form CdS monomers. The monomers adsorb on the exterior surface of the vesicles, where their spontaneous self-aggregation to (CdS)n clusters occurs on the hour and day time scale. The stepwise growth of the clusters was monitored through the time evolution of the UV absorption spectrum of the solution. The process is characterized by initial stepwise blue shifts of the absorption maxima: 285 nm --> 269 nm --> 245/275 nm --> 240 nm --> 236 nm, followed by a red shift to 494 nm. Nonlocal density functional theory (DFT) calculations of the optimized geometry and HOMO-LUMO gap of (CdS)n particles with n = 1-6 were carried out. The optimized structures are characterized by strong Cd-Cd bonds, with the S atoms bridging those bonds or capping the faces of the Cd polyhedra. The structure of such clusters bears no resemblance to fragments of the bulk crystal. The trend of the calculated HOMO-LUMO gaps facilitates the attribution of aggregation numbers (n) to particular clusters responsible for the observed absorption bands: n = 1 (285 nm), n = 2 (269 nm), n = 4 (245/275 nm --> 240 nm), n = 5 (236 nm), and larger quantum dots absorbing around 494 nm. The multiple bands assigned to the tetramer reflect the existence of its two distinct structures with similar stability. 相似文献
We prove that for two elements x, y in a Hilbert C*-module V over a C*-algebra the C*-valued triangle equality |x + y| = |x| + |y| holds if and only if 〈x, y〉 = |x| |y|. In addition, if has a unit e, then for every x, y ∊ V and every ɛ > 0 there are contractions u, υ ∊ such that |x + y| ≦ u|x|u* + υ|y|υ* + ɛe.
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Heteroatom functionalization on a graphene surface can endow the physical and structural properties of graphene. Here, a one-step in situ polymerization method was used for the noncovalent functionalization of a graphene surface with poly-N-vinyl-2-pyrrolidone (PNVP) and the exfoliation of graphite into graphene sheets. The obtained graphene/poly-N-vinyl pyrrolidone (GPNVP) composite was thoroughly characterized. The surface morphology of GPNVP was observed using field emission scanning electron microscopy and high-resolution transmission electron microscopy. Raman spectroscopy and X-ray diffraction studies were carried out to check for the exfoliation of graphite into graphene sheets. Thermogravimetric analysis was performed to calculate the amount of PNVP on the graphene surface in the GPNVP composite. The successful formation of the GPNVP composite and functionalization of the graphene surface was confirmed by various studies. The cyclic voltammetry measurement at different scan rates (5–500 mV/s) and electrochemical impedance spectroscopy study of the GPNVP composite were performed in the typical three-electrode system. The GPNVP composite has excellent rate capability with the capacitive property. This study demonstrates the one-pot preparation of exfoliation and functionalization of a graphene surface with the heterocyclic polymer PNVP; the resulting GPNVP composite will be an ideal candidate for various electrochemical applications. 相似文献
The objective of this work is the use of cellulose fibers extracted from coir fibers as Janus nanocylinders to suppress the phase retraction and coalescence in poly(lactic) acid/polypropylene bio-blend polymers via prompting the selective localization of cellulose fibers at the interface using chemical modification. The untreated and modified cellulose fibers extracted from coir fibers using a silane molecule (tetraethoxysilane) were used as reinforcement and as Janus nanocylinder at two weight contents (2.5 wt% and 5 wt%) to manipulate the morphology of the bio-blends. Their bio-composites with PLA-PP matrix were prepared via melt compounding (at PLA/PP: 50/50). The treatment effect on component interaction and the bio-composites properties have been studied via Scanning electron microscopy, infrared spectroscopy, and differential calorimetry analysis. The mechanical and rheological properties of nanocomposites were similarly assessed. Young's modulus and tensile strength of PLA-PP nanocomposites reinforced by silanized cellulose fibers show a great enhancement as compared to a neat matrix. In particular, there was a gain of 18.5% in Young's modulus and 11.21% in tensile strength for silanized cellulose fiber-based bio-blend composites at 5 wt%. From the rheological point of view, it was found that the silanized cellulose fibers in PLA-PP at both fibers loading enhances the adhesion between both polymers leading to tuning their morphology from sea-island to the continuous structures with the appearance of PLA microfibrillar inside of bio-composites. This change was reflected in the relaxation of the chain mobility of the bio-blend composites.
Zinc(II) and mercury(II) thiocyanate complexes with nicotinamide, bis(nicotinamide-N)-bis(thiocyanato-N)zinc(II) (1) and catena-[nicotinamide-N-(μ-thiocyanato-S,N)(thiocyanato-S)mercury(II)] (2), have been prepared and characterized by spectroscopic, thermal and X-ray crystallographic methods. The vibrational bands of diagnostic value are compared to the values of the free ligand and the data are in good correlation with the X-ray results. Centrosymmetrical hydrogen bonded dimers are found, R22(10) in 1 and R22(8) in 2. 相似文献