Studies on the decomposition of alcohols on metal oxide catalysts
Ghosh, Chanchal Kumar
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The catalytic decomposition of 2-propanol has been investigated by a micropulse reactor technique in the temperature range from 461 to 547 K on nickel oxide and alumina doped nickel oxide. Nickel oxide is active for the dehydrogenation of 2-propanol, yielding acetone, but the introduction of A1[superscript 3+] ions leads to the development of some dehydration activity; nickel alumlnate is purely a dehydrating catalyst. X-ray diffraction data indicate the formation of the nickel aluminate at the higher dopant concentrations. The variations in catalyst activity and selectivity as a function of catalyst compositions have been roughly divided into three regions: region I, in which there is a significant change in activity and selectivity with very small concentrations of Al[superscript 3+] ions; region II, in which the activity and selectivity remains virtually constant with changes in composition; region III, in which there is a marked increase in activity and dehydration selectivity with Increasing concentrations of Al[superscript 3+] ions The behavior in region I is believed to be associated with electronic changes in nickel oxide brought about by the substitutional incorporation of Al[superscript 3+] ion, while the behaviour in region III is ascribed to increasing amounts of nickel aluminate in the surface zones of the samples. Self-poisoning in the dehydrogenation reaction has been observed and an equation accounting for the self-poisoning kinetics has been proposed. It is suggested that in the dehydrogenation reaction each alcohol molecule uses one active site on the catalyst, while self-poisoning causes elimination of three sites per deactivating event. Strong chemisorption of acetone on the surface is considered to be responsible for the self-poisonlng of the dehydrogenation reaction. The conclusions are supported by conductivity studies of the catalysts under reaction conditions. A characteristic feature of the dehydration reaction is the deviation from linearity in the Arrhenius plots at higher reaction temperatures. Isotope effect measurements show that this is not due to a diffusion controlled reaction. The deviation is believed to be associated with the decomposition of hydroxyl groups on the catalyst surface and investigations of the thermal dehydroxylation of mixed nickel-aluminium hydroxides support this proposal.
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