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11.
A. Sartbaeva S. A. Wells M. Sommariva M. J. T. Lodge M. O. Jones A. J. Ramirez-Cuesta G. Li P. P. Edwards 《Journal of Cluster Science》2010,21(3):543-549
A major research theme to emerge in the science and technology of materials is the incorporation of nanostructure into the
functionality of properties. Such nanostructured materials can offer distinct advantages over bulk materials, partly because
the physical properties of the material itself can vary in a tunable, size-dependent fashion. Of course, in addition, nanoparticles
offer a greatly increased surface area for chemical reaction. Typical methods for nanoparticle synthesis include: reaction
in the liquid phase using the sol–gel approach and mechanical ball-milling of the bulk material; both of these approaches
are somewhat problematic for the preparation of reactive nanostructured materials which are sensitive to air and/or moisture.
We report here the formation of crystalline nanoparticles of sodium hydride encapsulated in a host amorphous silica gel matrix.
These nanoparticles are formed by in situ hydrogenation of a precursor material—Na loaded silica gel—under mild conditions.
The resulting material is considerably less pyrophoric and less air-sensitive than the bulk hydride. We anticipate that this
formation method of in situ modification of reactive precursor material may have wide applications. 相似文献
12.
In the presence of onium salts, at 140-170 degrees C, methyl alkyl carbonates [1a-c, ROCO2Me, R = MeO(CH2)2[O(CH2)2]n; n = 2-0, respectively] react with primary aromatic amines (XC6H4NH2, X= p-OMe, p-Me, H, p-Cl, p-CO2Me, o-Et, and 2,3-Me2C6H3NH2) to yield the corresponding N,N-dimethyl derivatives (ArNMe2) with high selectivity (up to 96%) and good isolated yields (78-95%). Phosphonium salts (e.g., Ph3PEtI and n-Bu4PBr) are particularly efficient catalysts. Overall, a solvent-free reaction is coupled with safe methylating agents (1a-c) made from nontoxic dimethyl carbonate. 相似文献
13.
[reaction: see text] At 130 degrees C, in the presence of CO2 (5-200 bar), primary aliphatic amines react with dimethyl carbonate (MeOCO2Me, DMC) to yield methyl carbamates (RNHCO2Me) and N-methylation side-products (RNHMe and RNMe2). The pressure of CO2 largely influences both the reaction conversion and the selectivity toward urethanes: in general, conversion goes through a maximum (70-80%) in the midrange (40 bar) and drops at lower and higher pressures, whereas selectivity is continuously improved (from 50% up to 90%) by an increase of the pressure. This is explained by the multiple role of CO2 in (i) the acid/base equilibrium with aliphatic amines, (ii) the reactivity/solubility of RNHCO2- nucleophiles with/in DMC, and (iii) the inhibition of competitive N-methylation reaction of the substrates. Cosolvents also affect the reaction: in particular, a drop in selectivity is observed with polar protic media (i.e., MeOH), plausibly because of solvation effects (through H-bonds) of RNHCO2- moieties. The reaction shows also a good chemoselectivity: bifunctional aliphatic amines bearing either aromatic NH2 or OH substituents [XC6H4(CH2)n NH2, X = NH2, OH; n = 1, 2], undergo methoxycarbonylation reactions exclusively at aliphatic amino groups and give the corresponding methyl carbamates [XC6H4(CH2)n NHCO2Me] in 39-65% isolated yields. 相似文献
14.
We extend Clenshaw–Curtis quadrature to the square in a nontensorial way, by using Sloan’s hyperinterpolation theory and two families of points recently studied in the framework of bivariate (hyper)interpolation, namely the Morrow–Patterson–Xu points and the Padua points. The construction is an application of a general approach to product-type cubature, where we prove also a relevant stability theorem. The resulting cubature formulas turn out to be competitive on nonentire integrands with tensor-product Clenshaw–Curtis and Gauss–Legendre formulas, and even with the few known minimal formulas. 相似文献
15.
Adaptive bivariate Chebyshev approximation 总被引:1,自引:0,他引:1
We propose an adaptive algorithm which extends Chebyshev series approximation to bivariate functions, on domains which are
smooth transformations of a square. The method is tested on functions with different degrees of regularity and on domains
with various geometries. We show also an application to the fast evaluation of linear and nonlinear bivariate integral transforms.
Work supported by the research project CPDA028291 “Efficient approximation methods for nonlocal discrete transforms” of the
University of Padova, and by the GNCS-INdAM. 相似文献
16.
In this paper, we investigate variants of the well-known Golub and Welsch algorithm for computing nodes and weights of Gaussian quadrature rules for symmetric weights w in intervals (?a, a) (not necessarily bounded). The purpose is to reduce the complexity of the Jacobi eigenvalue problem stemming from Wilf’s theorem and show the effectiveness of Matlab implementations of our variants for reducing the computer times compared to some other methods. Numerical examples on three test problems show the benefits of these variants. 相似文献
17.
Alvise Perosa Pietro Tundo Maurizio Selva 《Journal of molecular catalysis. A, Chemical》2002,180(1-2):169-175
The multiphase heterogeneous enantioselective hydrogenation of acetophenone in the presence of cinchona-modified Pt/C was investigated. The system demonstrated the feasibility of this reaction on non-activated ketones. The reaction proceeded selectively, at room temperature and atmospheric pressure, towards the formation of 1-phenylethanol, with up to 20% ee (enantiomeric excess) of either enantiomer depending on the modifier used. A mode of action of the modifier is proposed to account for the mechanism. A comparison with other systems indicates that the investigated system likely acts by a different mechanism, and that it is quite specific for acetophenone. 相似文献
18.
Quantitative chemical analysis is a powerful tool for the classification and discrimination of small glass fragments in forensic
and archaeometric analytical issues. In the choice of the analytical method the small dimensions and the irregular shape of
the fragments must be accounted for. Several accurate and highly sensitive instrumental techniques successfully employed for
the chemical analysis of small glass samples (AAS, ICP-AES and FAAS) are destructive, and therefore unacceptable for these
applications. X-ray analytical techniques such as WDXRF, EPMA and SEM/EDX provide means for rapid, non destructive and sensitive
multi-elemental analysis of glass fragments. In this study the analytical sensitivity and accuracy of WDXRF, EPMA and SEM/EDX
for the analysis of small commercial glass fragments are investigated. The results are compared and discussed in order to
establish a relation between the minimum size of glass samples and the accuracy and sensitivity of the analytical methods. 相似文献
19.
20.
Alvise Parma Isidora Freris Pietro Riello Francesco Enrichi Davide Cristofori Alvise Benedetti 《Journal of luminescence》2010,130(12):2429-2436
The synthesis, morphology and luminescence properties of two systems comprising luminescent Eu3+-doped zirconium oxide nanocrystals embedded in an amorphous silica matrix are reported. The two systems, prepared with the same overall wt% composition of silica (75%) and EuxZr(1−x)O(2−x/2) solid solution (25%), have been annealed in the range 135–1000 °C and subsequently functionalized with (3-aminopropyl)triethoxysilane. Detailed X-ray diffraction analyses and transmission electron micrographs, combined with infrared spectroscopy and luminescence spectroscopy data, have been used to demonstrate the influence of annealing temperature on the: (i) nanostructure, (ii) luminescence properties and (iii) availability of superficial –OH groups for efficient surface functionalization. The optimum calcination temperature was found to be 700 °C for each series in terms of luminescence lifetime efficiency and post-functionalization efficiency with (3-aminopropyl)triethoxysilane. 相似文献