Chromium oxide catalysts in the dehydrogenation of alkanes

Abstract

Light alkenes, such as propene and butenes, are important intermediates in the manufacture of fuel components and chemicals. The direct catalytic dehydrogenation of the corresponding alkanes is a selective way to produce these alkenes and is frequently carried out using chromia/alumina catalysts. The aim of this work was to obtain structure–activity information, which could be utilised in the optimisation of this catalytic system. The properties of chromia/alumina catalysts were investigated by advanced in situ and ex situ spectroscopic methods, and the activities were measured in the dehydrogenation of isobutane.

The dehydrogenation activity of chromia/alumina was attributed to coordinatively unsaturated redox and non-redox Cr3+ ions at all chromium loadings. In addition, the oxygen ions in the catalyst appeared to participate in the reaction. The reduction of chromia/alumina resulted in formation of adsorbed surface species: hydroxyl groups bonded to chromia and alumina were formed in reduction by hydrogen and alkanes, and carbon-containing species in reduction by carbon monoxide and alkanes. Prereduction with hydrogen or carbon monoxide decreased the dehydrogenation activity. The effect by hydrogen was suggested to be related to the amount of OH/H species on the reduced surface affecting the number of coordinatively unsaturated chromium sites, and the effect by carbon monoxide to the formation of unselective chromium sites and carbon-containing species.

The chromia/alumina catalysts were deactivated with time on stream and in cycles of (pre)reduction–dehydrogenation–regeneration. The deactivation with time on stream was caused mainly by coke formation. The nature of the coke species changed during dehydrogenation. Carboxylates and aliphatic hydrocarbon species formed at the beginning of the reaction and unsaturated/aromatic hydrocarbons and graphite-like species with increasing time on stream. The deactivation in several dehydrogenation–regeneration cycles was attributed to a decrease in the number of actives sites, which was possibly caused by clustering of the active phase into more three-dimensional structures.

Acidic hydroxyl species of exposed alumina support may have contributed to the side reactions observed during dehydrogenation. Chromium catalysts prepared on unmodified alumina and on alumina modified with basic aluminium nitride-type species were compared in an attempt to increase the activity and selectivity in dehydrogenation. However, the presence of nitrogen in the catalyst was not beneficial for the dehydrogenation activity.

A kinetic model was derived for the rate of dehydrogenation of isobutane on chromia/alumina. The dehydrogenation results were best described by a model with isobutane adsorption, possibly on a pair of chromium and oxygen ions, as the rate-determining step. Satisfactory description of the reaction rate depended upon inclusion of the isobutene and hydrogen adsorption parameters in the mathematical model. The activation energy of the rate-determining step was estimated to be 137±5 kJ/mol.