Process for the catalytic dehydrogenation of a light alkane

Details Process for the catalytic dehydrogenation of a light alkane Process for the catalytic dehydrogenation of a light alkane

2001-11-06

2003-06-24

Bell, Mark L. (Department: 1755)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Reforming

C208S135000, C208S137000, C585S379000, C585S616000, C585S627000, C585S654000, C585S660000

Reexamination Certificate

active

06582589

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to the preparation of highly stable, high surface area catalyst carrier materials derived from hydrotalcite-type materials by calcination at an elevated temperature.
BACKGROUND OF THE INVENTION
The dehydrogenation of paraffins to olefins is of considerable commercial importance due to the need for olefins for the manufacture of products such as high octane gasolines, synthetic elastomers, detergents, plastics, ion exchange resins and pharmaceutical products. For a dehydrogenation process to be commercially useful, it must utilize catalysts exhibiting a high activity, a high rate of conversion, a high selectivity for the formation of olefins, and a high stability.
A large number of catalysts are previously known for the dehydrogenation of paraffins. These catalysts comprise a solid carrier material on an inorganic oxide base and various catalytic metals and promoter metals deposited on the carrier material or incorporated into the carrier material by other means. Carrier materials on an alumina base have been widely used in such dehydrogenation catalysts.
U.S. Pat. No. 4,788,371 discloses such catalyst and a process for the steam dehydrogenation of dehydrogenatable hydrocarbons with oxidative reheating. A dehydrogenatable C
2-30
hydrocarbon, steam and an oxygen-containing gas are contacted in a reaction zone with a catalyst comprising a Group VIII noble metal, one or more components selected from lithium, potassium, rubidium, cesium and francium, and a component selected from boron, gallium, indium, germanium, tin and lead, deposited on an inorganic oxide carrier material. The preferred carrier material is alumina having a surface area of 1-500 m
2
/g, preferably 5-120 m
2
/g. Alumina is employed as the catalyst carrier in all the working examples of the patent. A preferred catalyst according to said U.S. patent contains about 0.70 wt. % of platinum, about 0.50 wt. % of tin and about 3.86 wt. % of cesium, and has a surface area of about 85 m
2
/g.
Mixtures of magnesium oxide MgO and alumina Al
2
O
3
and mixed oxides of Mg and Al have also been utilized as catalysts, and as carrier materials for catalysts. International Patent Application No. PCT/JP89/00053 discloses an alkoxylation catalyst comprising a magnesium oxide that has been modified by adding thereto at least one trivalent metal ion, preferably selected from Al
3+
and Ga
3+
. British Patent Application GB 2,225,731 discloses a catalyst for hydrotreatment, e.g. hydrodemetallization or hydrodesulphurization, comprising in a substantially homogenous phase magnesia and alumina wherein the molar ratio of Mg to Al is preferably from 3:1 to 10:1, together with a Group VI metal and/or at least one Group VIII metal.
Hydrotalcite is a layered mineral of formula: Mg
6
Al
2
(OH)
16
CO
3
4H
2
O. Over the years, a large number of hydrotalcite-like compounds, of general formula: [M(II)
1−x
M(III)
x
(OH)
2
]
x+
(A
n−
x

) mH
2
O, where A=anions, have been prepared. Cavani, F. et al.,
Cat. Today
, vol.11, no.2, 173 (1991). These compounds are characterized by a sheet-like structure, in which the anions are located in the interlayer between two brucite-like sheets containing the metal ions. M
II
1
M
III
metal ions having an ionic radius which is not too different from Mg
2+
can form hydrotalcite-like compounds. Cavani, F. et al., supra.
Upon calcination at 400-700° C., a high surface area (typically 160-220 m
2
/g) material with an XRD pattern typical for MgO is formed, without separation of the two metal ions into separate oxide phases. Schaper, H., et al.,
Appl. Cat.
, vol. 54, 79 (1989). Upon calcination at even higher temperatures, the mixed oxide is gradually transformed into a spinel structure, i.e., M
II
M
III
2
O
41
with a much lower surface area. McKenzie, A. L., et al.,
J. Catal.
, vol. 138, 347 (1992); Bellotto, M., et al.,
Phys. Chem.
, vol. 100, 8535 (1996). One major use for these materials is as support materials for catalysts, (see, Cavani, F. et al., supra) for instance for the catalytic dehydrogenation of lower alkanes. Akporiaye, D. et al., Norwegian Patent No. 179131 (1993). It has been reported that certain materials formed by calcination of a Mg—Al-containing hydrotalcite at 300-700° C. exhibit a high stability towards sintering in a humid atmosphere. See, Schaper, H., et al.,
Appl. Cat.
, supra.; Schaper, H., European Patent No. 0 251 351 (1988).
SUMMARY OF THE INVENTION
The present invention provides a catalyst which has improved catalytic performance compared to prior art catalysts with regard to catalyst activity, and at the same time exhibits an increased catalyst life time by preventing irreversible deactivation like sintering of the support.
In one early embodiment of the invention described in co-pending U.S. patent application Ser. No. 08/569,185, it had been found that if a mixed oxide of Mg and Al is used in combination with a Group VIII noble metal and certain promoters of the kind disclosed in the above-mentioned U.S. Pat. No. 4,788,371, a catalyst can be obtained which exhibits improved activity and stability when used for dehydrogenating dehydrogenatable hydrocarbons.
The carrier for that embodiment of the catalyst may be prepared by adding a solution of sodium hydroxide and sodium carbonate to a solution of magnesium nitrate and aluminum nitrate according to the method described in
Journal of Catalysis
, vol. 94, pp.547-557, (1985), incorporated herein by reference. Instead of sodium hydroxide and sodium carbonate, potassium hydroxide and potassium carbonate can be used, see
Applied Catalysis
, vol. 55, pp. 79-90 (1989), incorporated herein by reference. A hydrotalcite-like compound Mg
6
Al
2
(OH)
16
CO
3
-4H
2
O is formed by evaporation (drying) of the above-mentioned mixtures. The hydrotalcite is then calcinated at a temperature 500-800° C. to give Mg(Al)O. The molar ratio of Mg to Al typically ranges from 1:1 to 10:1, and the surface area is typically ranging from 100 to 300 m
2
per gram, preferably from 140 to 210 m
2
per gram, and the particle size can be in the range of 100 &mgr;m to 20 mm.
The calcination temperature for that embodiment of the catalyst was within the range of about 500 to about 800° C. A calcination temperature that had been shown to produce good results was about 700° C. In some of the examples set forth herein, this temperature was held for about 15 hours.
It has now been found, however, that the stability of the catalyst described herein could be further improved.
Thus, the present invention provides for a catalyst support material comprising a mixed oxide consisting essentially of a divalent metal and a trivalent metal in a substantially homogeneous phase, which is a calcination product of a hydrotalcite-like phase calcinated at a temperature of about 700-1200° C., wherein the divalent metal/trivalent metal molar ratio is equal to, or higher than 2.
Tests of the effect of the calcination temperature of hydrotalcite and hydrotalcite-like materials at different temperatures from 700° C. to 1200° C. were therefore investigated.
By performing these investigations, it has been surprisingly found that by raising the calcination temperature of the catalyst support precursor hydrotalcite to 700° C. to 1200° C., preferably to the range of 750 to 950° C., an improvement of the catalyst stability could be achieved with an acceptable reduction in the surface of the catalyst carrier compared to the gain in stability at use. In a further aspect, the present invention thus relates to a catalyst support material comprising a mixed oxide consisting essentially of Mg and Al in a substantially homogenous phase, which is a calcination product of a hydrotalcite phase, preferably calcinated at a temperature of 750 to 950° C., wherein the Mg/Al molar ratio is equal 2 or higher than 2. A most preferred range for the calcination has been found to be at 770 to 850° C., and within that range, the preferred temperature is at about 800° C.
Preferably the Mg/Al molar ratio is in the

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