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The effect of hydrogen and carbon monoxide prereduction on the initial activity and deactivation
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of chromia/alumina was investigated in isobutane dehydrogenation. Measurements were done
at 580 °C in a fixed bed reactor and by in situ diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) combined with mass spectrometry. Prereduction with hydrogen decreased
the dehydrogenation activity compared to an isobutane-reduced catalyst, and prereduction by
carbon monoxide increased the cracking activity. The catalysts deactivated with time on stream
due to formation of carbon-containing deposits: carboxylates and aliphatic and unsaturated/
aromatic hydrocarbon species. Prereduction affected the rate of coke deposition but not the nature
of the species formed. The observed effects were attributed mainly to the hydroxyl groups formed
during hydrogen prereduction and to the carbonate and formate species formed during carbon
monoxide prereduction. Furthermore, the carbon monoxide-prereduced surface possibly contained
a higher number of active chromium sites unselective for dehydrogenation.
Introduction 3789
Chromia supported on alumina is an active catalyst
in the dehydrogenation of light alkanes to alkenes.
1
Oxidized chromia/alumina contains chromium mainly
as Cr3 and Cr6 and in trace amounts as Cr5 . The
relative amounts and the structures of the Cr3 and
Cr6 oxides depend on the chromium content of the
catalyst.
1 4 At low chromium loadings below about 5
atCr/nm2 (4 8 wt % depending on the catalyst surface
area) Cr6 dominates and forms mono- and polychro-
mates. With increasing chromium content, the amount
of Cr6 stabilizes whereas the amount of Cr3 increases.
The Cr3 oxide phase is first amorphous; crystalline
Cr2O3 has been detected, for example, by X-ray diffrac-
tion above about 8 10 atCr/nm2 (6 16 wt %). Under
dehydrogenation conditions the high oxidation state
chromium species are reduced by the alkane with
release of carbon oxides and water. Thereafter, dehy-
drogenation products start to form. Coordinatively
unsaturated (cus) Cr3 ions formed in the reduction or
present already in the oxidized catalyst are generally
considered to be the active sites in dehydrogenation.
1 3,5
The initial unselective combustion period can be
avoided by prereducing the catalyst for example with
hydrogen or carbon monoxide. However, these gases
have been observed to affect the activity in dehydroge-
nation compared to reduction with the alkane feed.
5 9
Prereduction with hydrogen decreases the dehydroge-
nation activity, and prereduction by carbon monoxide
increases side reactionsscracking and coke formations
during dehydrogenation. These effects have been specu-
lated to be caused by the formation of different adsorbed
surface species or chromium oxidation states during
reduction with different gases.
1,5,6 Earlier, we investi-
gated by in situ diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) the nature of the
surface species formed during reduction of chromia/
alumina by hydrogen,
10 12 carbon monoxide,
10 pro-
pane,
11 and isobutane.
12 Hydroxyl groups were formed
in the reduction by hydrogen or the alkanes, and
oxygen-containing carbon species were formed in reduc-
tion by carbon monoxide or the alkanes. The purpose
of the present study was to evaluate the effect of these
species on the initial activity of chromia/alumina in
isobutane dehydrogenation.
Another aim in the study was to determine the effect
of carbon monoxide prereduction on the deactivation of
chromia/alumina. Carbon-containing deposits are formed
during dehydrogenation that decrease the catalytic