Jean Bandiera

Catalytic investigation of the dehydrogenation properties of pentasil type zeolites as compared with their cracking properties

Abstract The kinetics of propane conversion on acidic zeolites were investigated. The kinetic measurements were conducted under conditions that ensure the true kinetic rates for dehydrogenation and cracking to be determined. The ratio of dehydrogenation to cracking rates was shown to be independent of temperature. It was not affected by the acid strength or topology. These observations were rationalized in terms of a common rate-determining step involving the formation and decomposition of the carbonium ion to propene on the one hand and ethylene and methane on the other to restore the initial proton.

An electron spin resonance study of cation radicals on the surface of MoO3Al2O3 and MoO3SiO2 catalysts

Abstract ESR spectra of polynuclear aromatic hydrocarbons adsorbed on MoO 3 Al 2 O 3 and MoO 3 SiO 2 have been recorded. Formation of the corresponding cation radicals indicates the presence of strong electron-deficient sites on the surface of these catalysts. The effect of activation temperature on radical forming ability of these solids has been studied. In the electron transfer reaction, Mo 6+ is reduced to Mo 5+ . Molybdenum is therefore directly involved in the electron transfer process. The radical concentration was dependent upon the molybdenum content and its oxidation state. The effect of oxygen on radical-ion formation has also been studied.

On the enhanced dehydrogenation versus cracking ability of an acidic gallium MFI: A tentative acid base interaction model

Abstract The hydrogenation and cracking properties of an authentic H(Ga)-MFI are investigated in a true kinetic regime. The cracking of propane over the H(Ga)-MFI decreased dramatically in comparison to results obtained over a comparable H(Al)-MFI. Conversely, the dehydrogenation of propane was very much increased. The Bronsted acid sites associated with the framework gallium seem to operate in a similar manner to those associated with framework aluminium in H(Al)-MFI. However, since their strength is attenuated, their greater dehydrogenation activity is ascribed to their preferred reaction with a C-H σ bond of propane to form the carbonium ion with respect to a C-C σ bond, while the reverse situation prevails with the stronger acid sites of H(Al)-MFI. A kind of soft-soft and hard-hard acid-base interaction is thought to govern the formation of carbonium ions having distinct structures which lead, respectively, to cracking or dehydrogenation.

Ethane conversion: kinetic evidence for the competition of consecutive steps for the same active centre

Abstract The kinetics of ethane dehydrogenation over catalysts with distinct aromatization properties unveiled some peculiar features of site “bifunctionnality”. It was shown that the apparent activation energy of ethane dehydrogenation decreased as the aromatizing ability of the catalyst increased. The kinetic data were interpreted in terms of a homographic variation of the dehydrogenation rate with the selectivity to aromatics. The data fit well a scheme where dehydrogenation of ethane and aromatization of intermediate higher hydrocarbons take place competitively over the same active centre.

Calorimetric and catalytic investigation of alkanes reactivity over a variety of MFI structures

Abstract Various substituted MFI zeolites have been investigated using adsorption microcalorimetry of different alkanes. The acid strength of the zeolite protons decreased following the sequence H-Al-MFI > H-Ga-MFI > H-Fe-MFI. The heats of adsorption decreased with the basicity of the alkane in the order n-butane > isobutane > propane. The variation of cracking selectivity in the conversion of alkanes was correlated with the basicity of the C-C bond. The selectivity towards dehydrogenation was related to the increased basicity of the C-H bond or to the attenuation of the acid strength of the zeolite.

Elementary reactions and thermodynamic effects in the conversion of propene over an acidic A1MFI

Abstract The conversion of propene over H-A1MFI zeolites was investigated at various temperatures and subatmospheric pressure. The overall conversion scheme was cracked down into a set of well known reactions: namely a primary dimerization step, two secondary or tertiary disproportionation reactions involving intermediate carbenium ions within the C 6 –C 9 range and, in addition, a number of hydrogen transfer reactions. Quantitative analysis of the product distributions provided an evaluation of the relative weight of each step depending on the temperature. The variation of the overall conversion with temperature was interpreted in terms of a thermodynamically limited process above 573 K.