Catalytic Oxidative dehydrogenation
The new technology of catalytic oxidative dehydrogenation (ODH) may completely change the
way some of the most important organic chemicals are manufactured. The conversion of alkanes like
ethane (a by-product of petroleum processing and present in natural gas) to olefins (ethylene,
propylene, the butenes, and butadiene) is in great demand in worldwide chemical industry. High
operational costs and environmental issues have made this conversion profitable only on a very large
scale. With successful development of ODH, high yields of olefins will be possible through the
conversion of much smaller volumes of alkanes.
Compared with the conventional steam-cracking method of dehydrogenating alkanes to olefins and
current catalytic dehydrogenation processes, ODH could reduce costs, lower greenhouse gas emissions,
and save energy. Capital and operational efficiencies are gained by eliminating the need for a furnace
and for decoking shutdowns, lowering operating temperatures, lessening material demands,
conducting fewer maintenance operations, and using a greater proportion of the alkanes in the olefin
conversion process.
Low molecular weight alkenes, such as ethene and propene, can be formed via non-oxidative
dehydrogenation of the corresponding alkane. Non-oxidative dehydrogenation reactions are
endothermic and lead to the concurrent formation of carbon and of lower molecular weight alkanes,
both of which decrease alkene yields. Oxidative dehydrogenation (ODH) of light alkanes offers a
potentially attractive route to alkenes, since the reaction is exothermic and avoids the thermodynamic
constraints of non-oxidative routes by forming water as a byproduct. In addition, carbon deposition
during ODH is eliminated, leading to stable catalytic activity. However, the yield of alkenes obtained
by ODH on most catalysts is limited by alkene combustion to CO and CO2 (COx
A relevant example concerns catalytic conversion of n-butane to butenes by oxidative
dehydrogenation (ODH) as an alternative process to direct dehydrogenation. It is well known that
supported vanadium pentoxide is a promising catalyst for the ODH of n-butane. MgO supported
vanadium was reported as a very selective catalyst in the oxidative dehydrogenation of propane and
n-butane, while γ-A12O3 supported vanadium catalyst was found to present a good selectivity to olefin
products for ethane ODH but a poor selectivity in the ODH of n-butane. The acid-base character of the
support explained this different behavior. On MgO, a support with basic properties, the interaction
between vanadium species and the supports leads to the formation of vanadate compounds. In the case
of more acid supports, such as SiO2 or A12O3 a weak interaction is expected leading to less dispersed
vanadium species on the surface which, in turns favors the formation of V2O5 crystallites
Catalytic cracking gives many by-products and is not necessarily optimized with respect to propene
yield. Propane dehydrogenation, on the other hand, yields propene as the main product. The problem is
that the dehydrogenation equilibrium favors propene only at high temperature or low pressure, adding
to the overall cost of the process. The need for cryogenic separation of the unconverted propane and
produced propene also adds to the process costs. Improvements of the dehydrogenation process to
make it commercially more attractive focus on increasing the yield by shifting the equilibrium through
removal of one of the reaction products. Thus, in situ removal of hydrogen will shift the
dehydrogenation equilibrium to the product side. This can be done either physically by means of a
membrane, or chemically by in situ catalytic oxidation using a (post) transition metal or its oxide. The
latter approach has the additional advantage of energy release by exothermic oxidation where it is
needed to aid the dehydrogenation. However, mixing oxygen, hydrogen, and hydrocarbons at high
temperatures creates a highly dangerous mixture. The risk of explosion may be reduced by separating
the reactants in space using oxygen-selective membranes.