Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reissue Patent
1998-12-07
2002-04-16
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S325000, C502S349000, C502S352000
Reissue Patent
active
RE037663
ABSTRACT:
This application and Ser. No.
09
/
240
,
150
are copending reissue applications of the same original patent.
The present invention concerns improvements in catalysts and in catalytic processes. More especially it concerns catalysts and processes for dehydrogenation of alkanes.
It is known to dehydrogenate isobutane to isobutene using direct dehydrogenation at low space velocity (GHSV=100-1000 hr
−1
). The conventional industrial process has several inherent disadvantages:
(a) it is an endothermic reaction, requiring high thermal input;
(b) the yield of isobutene is equilibrium limited; and
(c) at temperatures favouring high yields of isobutene, the rate of catalyst de-activation is also high.
Improvements to the conventional process have included the addition of either steam (eg U.S. Pat. No. 4,926,005 and 4,788,371) or hydrogen (eg U.S. Pat. No. 4,032,589) to the gas feed. The function of the hydrogen is as a diluent, and to reduce the deposition of carbon on the catalyst. The steam improves thermal conduction through the catalyst bed and reduces the deposition of carbon on the catalyst, and hence it too has been used as a diluent. The catalysts used in industry include platinum on alumina, platinum on tin oxide and chromium oxide-based catalysts. There remains a need for an improved process for the dehydrogenation of alkanes, especially for the dehydrogenation of isobutane, which is a starting material for MTBE (methyl-tert-butyl-ether) production. The conventional processes require high inputs of energy and the capital cost of a catalytic reactor designed to supply large amounts of heat is particularly high. Moreover, conventional processes demonstrate rapid catalyst deactivation, so that expensive and complex catalyst regeneration has to be designed into the equipment and the process.
The present invention provides an improved process and novel catalyst for alkane dehydrogenation.
Accordingly, the invention provides a process for the dehydrogenation of an alkane to form an alkene, comprising passing a feedstock comprising said alkane in the gas phase in admixture with oxygen and in the absence of added steam over a dehydration and oxidation catalyst comprising a platinum group metal deposited upon a support.
The invention provides also a catalyst for alkane dehydrogenation, comprising platinum deposited upon a support which is a mixture of tin oxide and zirconium oxide. The invention also provides a process for the dehydrogenation of an alkane to form an alkene, comprising passing a feedstock comprising said alkane in the gas phase over this catalyst.
The present processes and catalyst are advantageous over the known processes and catalyst by reason of one or more of such features as higher yield of the alkene, higher selectivity to the alkene, lower operating temperature, lower heat input, a simpler system and lower catalyst deactivation.
There is much prior
an
art
on the dehydrogenation of alkanes to alkenes, (though a scant amount on the oxidative dehydrogenation of alkanes to alkenes), yet the present improvements were not realised before. As explained in the U.S. specification 4,788,371 mentioned above, the dehydrogenation of hydrocarbons is endothermic. In a system employing a dehydrogenation catalyst only, it is typically necessary to add superheated steam at various points in the process or to intermittently remove and reheat the reaction stream between catalyst beds. In an improvement, processes were developed which utilised a two-catalyst system with distinct beds or reactors of dehydrogenation or selective oxidation catalysts. The purpose of the selective oxidation catalysts was to selectively oxidise the hydrogen produced as a result of the dehydrogenation with oxygen that had been added to the oxidation zone to generate heat internally in the process. The heat generated would typically be sufficient to cause the reaction mixture to reach desired dehydrogenation temperatures for the next dehydrogenation step. The US specification explains that in its invention one specific catalyst can be used to accomplish both the dehydrogenation and oxidation reactions. It discloses a process for the steam dehydrogenation of a dehydrogenatable hydrocarbon with oxidative reheating which comprises contacting a dehydrogenatable hydrocarbon comprising C
2
-C
15
paraffins and steam at a steam to hydrocarbon molar ratio of from 0.1:1 to 40:1, at a pressure from 0.1 to 10 atmospheres, a temperature of from 400° to 900° C., and a liquid hourly space velocity of from 0.1 to 100 hr
−1
with a catalyst in the first reaction zone of a reactor containing a plurality of reaction zones and introducing an oxygen-containing gas into the second, and every other reaction zone of the plurality of reaction zones such that the total rate of the oxygen-containing gas introduced into the reaction zone ranges from 0.01 to 2 moles of oxygen per mole of C
2
-C
15
paraffin feed wherein the catalyst is comprised of from 0.1 to 5 weight % platinum, and from 0.01 to 5 weight% potassium or cesium or mixtures thereof on an alumina support having a surface area of from 5 to 120 m
2
/g and recovering the products of the reaction. Though the specification mentions the possibility of a single reaction zone within a single reactor with single inlet and outlet parts, all co-feeds entering the inlet of the reactor and products and by-products leaving the system through the reactor outlet part, there is no Example illustrating this concept. Moreover, the present broad process involving passing an alkane in admixture with oxygen over a dehydrogenation and oxidation catalyst is not a steam dehydrogenation; instead, it is carried out in the absence of added steam (though some steam is formed by reaction of the oxygen with hydrogen which is present).
FIG. 3
shows a schematic diagram of a reactor
10
for performing this process. As shown in
FIG. 3
, the present broad process involves passing an alkane stream
10
in admixture with oxygen over the dehydrogenation and oxidation catalyst in the reactor
20
to result in a product stream
30
.
In this aspect of the present invention, we have discovered that oxygen in the absence of added steam is advantageously admixed with the alkane and passed over the catalyst, so that heat produced by the exothermic reaction of the oxygen with hydrogen which is present provides, partially or fully, the heat required by the endothermic dehydrogenation. The hydrogen required for the reaction with the oxygen can be introduced into the reaction zone, but this is not preferred. Advantageously, the hydrogen is hydrogen produced by the dehydrogenation of the alkane to alkene, so as to shift the equilibrium in favour of the alkene. Preferably, the amount of oxygen is such that the dehydrogenation is
carded
carried
out under adiabatic conditions, so that no heat is supplied (or removed) from the reaction. Especially preferred is the amount of oxygen being such that the endothermic dehydrogenation is balanced by the exothermic reaction of the oxygen with hydrogen which is present so that the temperature remains constant (this situation is referred to herein as thermally neutral conditions). Thus, the optimum temperature for yield, life of catalyst etc can be maintained, eg so that at least 95% selectivity to the alkene is obtained.
The amount of oxygen is desirably less than the amount of the alkane, on a molar basis, and preferably less than half the amount of the alkane on this basis. For example, employing isobutane as the alkane, it is preferred that the amount of oxygen be below that indicated by the stoichiometry of the reaction equation:
C
4
H
10
+0.5O
2
→C
4
H
8
+H
2
O.
The optimum amount of oxygen will vary with the desired operating temperature, and as a guide we would predict that the maximum amount of oxygen for highly selective, thermally neutral, dehydrogenation of isobutane be 5% at 450° C., 7.5% at 500° C. and 9% at 550° C., based on the combined volumes of isobutane and oxygen.
The present oxidative dehydrogenation is usually c
Golunski Stanislaw E.
Hayes John W.
Johnson Matthey Public Limited Company
Stevens Davis Miller & Mosher LLP
Wood Elizabeth D.
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