The oligomerization reaction of ethene and propene on H-ZSM-5 has been studied by fast FTIR spectroscopy. Oligomerization proceeds through : (i) formation of short-lived hydrogen-bonded precursors by interaction of the alkene with the internal acidic Brsnsted sites, (ii) a protonation step and (iii) a chain-growth step. The relative strength of the hydrogen bonds in the e t h e n M H and p r o p e n M H n-complexes (precursors) is estimated on the basis of the downward shift of both the v(0H) and v(C=C) frequencies (-389 and -11 cm-' for ethene and -539 and -19 cm-' for propene). For both molecules, the protonation of the precursors is the rate-determining step of the oligomerization process. The chain-growth mechanism and the structure of the entrapped oligomers are discussed on the basis of computer graphic and molecular dynamics simulations. Mainly linear or low branched products are formed whose length and structure is essentially determined by the steric hindrance imposed by the zeolitic framework.
The recently synthesized Engelhard titanosilicate (ETS-10) represents a material which contains in the structure well defined atomic ⋯O–Ti–O–Ti–O⋯ quantum wires embedded in a highly insulating siliceous matrix. We report and discuss the UV-Vis spectrum of this material and compare the experimentally determined optical band gap with the results predicted by simple modeling of a titanium oxide semiconductor wire unidimensionally confined by an infinite potential barrier.
Publication costs assisted by Italian CNRA definite compound is formed between CrOs and SiOg, the predominant surface structures being dichromates. By reduction in CO of the surface phase, Cr11 is mainly obtained. The activity of the reduced samples toward CO and ethylene polymerization is affected by chromium concentration and thermal treatments, both high Cr contents and high reduction temperatures decreasing the specific activity. The most active catalysts are characterized by Cr loading <0.5% and reduction temperature equal to 350°. Reflectance spectra on reduced diluted samples show the presence of a small Cr111 percentage and of two Cr11 species differing in coordination.
Receiued 2 I st June, 1973 An i.r. study of nitric oxide adsorption on high area MgO (previously degassed at 800°C) at room temperature is reported. The adsorption of NO onto MgO produces i.r. bands at 800-900 cm-I, 110@1250crn-', and 1350-1450cm-'. These absorption bands were assigned to several surface species : NO; ions with various configurations and NOand N202-ions, co-ordinated to Mg2+ ions. The adsorption (3.3 x l O I 3 molecule cm-2, at 10.4 x lo3 N m-2 of NO) occurs in limited surface zones.
An investigation has been made of Aerosil surface, determining hydroxyl concentration and surface area for various activation temperatures. Strong adsorption of methanol on Aerosil occurs through the formation of two species. The first one is methanol linked by more than one hydrogen bond. The second one originates from the reopening of some siloxane bridges on the surface and is predominant on samples activated at high temperatures.
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