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51.
52.
The acid properties of pure and modified silica surfaces were studied by 2-phenylethylamine (PEA) desorption in a thermogravimetric (TGA) apparatus, carrying out the experiments at different heating rates (5 < beta/(degrees C.min(-1)) < 30). The samples, containing about 13 wt % alumina, titania, and zirconia, were prepared by the sol-gel route from molecular precursors. The textural, structural, and surface properties of the materials were studied by complementary techniques (ICP, XRD, N(2) physisorption, SEM-EDS, and XPS). The chemical modification of the silica surface by enrichment with Al, Ti, or Zr, in amounts of about 90, 50, and 60% of that introduced in the preparation as determined by XPS, justified the increase of acidity of the modified silica surfaces compared with that of pure silica. The total number of strong acid sites was found to be in the order of SZ > SA > ST > S. Two different kinetic approaches were applied to the thermogravimetric data to kinetically interpret the PEA desorption from the different types of acid sites. The classical differential Kissinger model was found to be inadequate in representing the very complex situation of the acid surfaces. A more complex model is proposed by simultaneously taking into account PEA desorption from the different acid sites by a set of parallel and independent desorption reactions following Arrhenius's kinetic law. The fraction of each type of acid site on each surface and the relevant activation parameters were optimized through a computational procedure. Very good fitting of the experimental-calculated desorption profiles corroborated the validity of the model. For each surface, the acid-site energy distribution is presented and discussed in relationship to the surface composition of the oxides. 相似文献
53.
Bianco A Guiso M Ballero M Foddai S Nicoletti M Piccin A Serafini M Tomassini L 《Natural product research》2004,18(3):241-246
During our systematic study on the species of genus Linaria (Scrophulariaceae) present in Italy, we examined the glycosidic fraction of Linaria capraria Moris et De Not., a species endemic of Tuscany archipelago. This fraction is particularly complex and we considered in this article only the medium polarity components. In accordance with previous studies, L. capraria shows acyl derivatives of antirrhinoside 1 as specific chemotaxonomic iridoidic markers. L. capraria exhibits a complex composition, with regard to iridoidic constituents, with several chromatographic problems to be resolved. We then isolated, besides the known antirrhinoside 1, two acyl derivatives of antirrhinoside, the 6'-O-senecioyl derivative, 2, and the 6'-O-angeloyl derivative, 3. In addition a glucoside of an acyclic monoterpene, 4, was also isolated, which may be correlated to the other monoterpenic glycosides isolated from other species of Scrophulariaceae. 相似文献
54.
Pollutants removal in two vertical flow experimental wetlands receiving pre-treated municipal wastewater was investigated. Laboratory investigations using microcosms were also carried out in order to identify key processes regulating pollutants removal. Results obtained from experimental wetlands seem to confirm the ability of a single stage installation, where nitrification and denitrification processes are jointly optimised using a gravel medium, always wet but not saturated with water. Results obtained from microcosm investigations show the prominent role of O2 supply, of microbial activity and of bio-available C from vegetal metabolism in the nitrogen removal. 相似文献
55.
van Der Voort M Rella CW van Der Meer LF Akimov AV Dijkhuis JI 《Physical review letters》2000,84(6):1236-1239
We present results of the first vibrational photon-echo, transient-grating, and temperature dependent transient-bleaching experiments on a-Si:H. Using these techniques, and the infrared light of a free electron laser, the vibrational population decay and phase relaxation of the Si-H stretching mode were investigated. Careful analysis of the data indicates that the vibrational energy relaxes directly into Si-H bending modes and Si phonons, with a distribution of rates determined by the amorphous host. Conversely, the pure dephasing appears to be single exponential, and can be modeled by dephasing via two-phonon interactions. 相似文献
56.
Vincenzo Benessere Raffaella Del Litto Antonella De Roma Francesco Ruffo 《Coordination chemistry reviews》2010,254(5-6):390-401
“Privileged ligands” are chiral auxiliaries of wide applicability in asymmetric catalysis. In the previous decades, their effective three-dimensional structures have often been reproduced by using building blocks from a “chiral pool”, such as the carbohydrates. This strategy has provided unique ligand moieties which combine the performance of “privileged ligands” with increased flexibility and accessibility. This review gives an overview of the research within this field, giving emphasis to the best results obtained with each ligand type. 相似文献
57.
The present paper is focused on the ability of aluminium phosphate (ALPC), magnesium ammonium phosphate (MGPC), magnesium hydrogen phosphate (MGHPC), and calcium hydrogenphosphate (CAHPC), adsorbed onto charcoal, to immobilise actinides by adsorption from natural waters. The objective of this process is to evaluate the environmental pollution due to the actinides. Europium, thorium, protactinium, neptunyl, and uranyl ions were chosen to simulate actinides in the +3, +4, +5 and +6 oxidation state. The adsorbers were tested using natural waters samples. The adsorption trends and capacities were analysed. ALPC and MGPC exhibited a similar behaviour and adsorbed demonstrating that the +5, +4 and +3 actinide ions can be easily immobilised from natural waters and may be successfully used at pH 7-8. MGHPC may be used at a higher pH, whereas CAHPC is effective in the whole pH range. In all cases, thorium, protactinium and europium were strongly 相似文献
58.
Carlo Costa Antonella Bertazzo Graziella Allegri Silvia Catinella Pietro Traldi 《Journal of mass spectrometry : JMS》1991,26(11):972-976
The electron impact-induced fragmentation processes of pyrrole-2-, -3-, -2,3-, -3,4-, -2,5- and -2,3,4-carboxylic acids were investigated with the aid of metastable ion, collisional and deuterium labelling experiments. The general behaviour of these compounds is discussed in detail, together with the reasonable structures of the more relevant fragment ions. 相似文献
59.
Dalla Cort A Mandolini L Schiaffino L 《Chemical communications (Cambridge, England)》2005,(30):3867-3869
Whereas the parent uranyl salophen is catalytically inactive, its phenyl derivative effectively catalyses with turnover the reaction of benzoquinone with 1,3-cyclohexadiene, while showing no appreciable affinity towards reactants and product. 相似文献
60.
Multi-component, regio-selective aldol addition of β-ketoesters to aldehydes: scope and applications
More V Di Mola A Croce G Tedesco C Petronzi C Peduto A De Caprariis P Filosa R Massa A 《Organic & biomolecular chemistry》2011,9(24):8483-8488
Simple and effective multi-component one-pot aldol addition/protection reactions of β-ketoesters to a series of aldehydes in the presence Me(3)SiCl and i-Pr(2)EtN have been described. The analysis of the scope of the reaction revealed a dramatic dependence of the reactivity on the substrates used. Thus the effect of a catalytic amount of DMF and different reaction conditions was widely investigated. Further transformations of the aldol adducts were particularly useful to give valuable diols and compounds with quaternary stereocenters, while X-ray structural analysis gave also important stereochemical information about this challenging reaction. 相似文献