Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1992-04-21
1993-11-02
Shine, W. J.
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
502330, B01J 2110, B01J 2378
Patent
active
052583481
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a supported catalyst, to a method for the preparation thereof and to the use thereof in the dehydrogenation of hydrocarbons.
The dehydrogenation of compounds like hydrocarbons is a widely used and large scale type of process. Examples of such dehydrogenation processes include: the dehydrogenation of alkanes to alkenes, such as propane to propene and butane to butene, the dehydrogenation of alkenes to corresponding alkadienes, such as from butene to 1,3-butadiene, the dehydrogenation of alkyl-aromatic hydrocarbons to alkenyl-aromatic hydrocarbons, such as from ethylbenzene to styrene. Hereinafter the invention will be further explained by reference to the dehydrogenation of ethylbenzene to styrene, unless mentioned otherwise.
The catalysts used for such a large scale process are based on iron oxide as active component. U.S. Pat. No. 2,395,875 discloses a dehydrogenation catalyst consisting of magnesium oxide as base material (support material), iron oxide as active component and a small quantity of an alkali or alkaline earth metal oxide as promoter. Optionally the catalyst further contains a small quantity of a transition metal oxide as stabilizer. The alkali- or alkaline earth metal oxide promoter also serves to reduce to the minimums together with steam supplied to the process, the precipitation on the catalyst of the carbon-containing byproducts. In U.S. Pat. No. 2,395,875 the catalyst is prepared by mixing powdered magnesium oxide with a solution of iron(III) nitrate, after which complete precipitation is achieved by addition of lye. Then the mixture is treated with solutions of the stabilizer and of the promoter. The resulting mixture is dried, heated and subsequently formed into pills or pellets of the desired dimensions and shape. However, these dehydrogenation catalysts deactivate quickly, which also becomes evident from the article "Catalytic Dehydrogenation of Butenes", K. K. Kearby, Industrial and Engineering Chemistry Vol. 42, No. 2 (1950), pages 295-300.
At present, the most widely used type of catalyst for the dehydrogenation of hydrocarbons and especially of ethylbenzene to styrene is an unsupported catalyst based on iron(III) oxide, chromium(III) oxide and potassium oxide, as described in U.S. Pat. No. 2,461,147. The use of such unsupported catalysts in large fixed-bed reactors, such as in the dehydrogenation of ethylbenzene to styrene where in the reactors catalyst beds of for example 10 to 200 tons are used has, however, some disadvantages. Under reaction conditions the major catalyst component, a-iron(III) oxide (hematite, Fe.sub.2 O.sub.3) usually undergoes a reduction to Fe.sub.3 O.sub.4 (magnetite). As a consequence, the hexagonal lattice structure of hematite is converted to the cubic lattice structure of magnetite. The mechanical strength of the catalyst bodies is reduced considerably by this conversion or recrystallization, as iron oxide is the main catalyst component. Because of the high mechanical forces in the catalyst bed, on the long run this recrystallization contributes to the disintegration or pulverization of the catalyst bodies. This in its turn results in an increase of the pressure drop over the catalyst bed, which adversely affects the selectivity and yield of the catalytic dehydrogenation process. When the pressure drop becomes too high the catalyst bed should be replaced, which is a time consuming and costly operation. Further, during the reduction of the iron oxide also metallic iron may be formed, which is known to stimulate the formation of carbon.
A further disadvantage associated with this type of unsupported catalyst is the migration of the promoter, usually a potassium compound such as potassium (hydr)oxide or carbonate. Under reaction conditions the potassium compound is slightly volatile. As the catalytic reaction is endothermal, thereby requiring heat to be supplied to the reaction mixture, and as the thermal conductivity of the catalyst particles is limited, a temperature gradient may develop in the catalyst bed. This tempe
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K. K. Kearby, "Catalystic Dehydrogenetion of Butenes", Industrial and Engineering Chemistry, vol. 42, No. 2, (1950), pp. 295-300.
Geus John W.
Stobbe Dick E.
van Buren Frederik R.
van Dillen Adrianus J.
Shine W. J.
The Dow Chemical Company
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