Dehydrogenation catalyst and process for preparing same

A calcined dehydrogenation catalyst comprising at least one sodium compound and one calcium compound is disclosed. The catalyst exhibits improved moisture stability as evidenced by improved crush strength and resistance to swelling and cracking.

BACKGROUND OF THE INVENTION 
This invention relates to improved catalysts for the dehydrogenation of 
hydrocarbons and to a method of making such catalyst compositions which 
exhibit improved moisture stability when subjected to contact with 
hydrogen and steam at elevated temperatures during the dehydrogenation 
process. This improved stability is evidenced by improved crush strength 
and resistance to swelling and cracking. 
TECHNICAL BACKGROUND 
Catalytic dehydrogenation of hydrocarbons using various catalyst 
compositions has been known from just prior to World War II. Commercial 
examples are the manufacture of styrene and butadiene from ethylbenzene 
and butylene. Promoted iron oxide catalysts have been found to be 
especially useful in the dehydrogenation of alkyl aromatic hydrocarbons to 
vinyl aromatic hydrocarbons. Most commercial iron oxide dehydrogenation 
catalysts include minor amounts of promoters, e.g., salts or oxides of 
chromium, manganese, bismuth, tungsten, or molybdenum, with chromium being 
preferred, together with a compound of potassium, e.g., potassium oxide or 
carbonate. The potassium compound gives the catalyst a self-regenerative 
property that prolongs its useful life for long periods of time without 
significant loss of activity. Recent improvements include the 
incorporation of minor amounts of vanadium and modifiers, such as carbon 
black or graphite and methyl cellulose, which can beneficially affect the 
pore structures of the catalysts. 
The catalyst life of dehydrogenation catalysts is often dictated by the 
pressure drop across a reactor. An increase in the pressure drop lowers 
both the yield and conversion to the desired product. Physical degradation 
of the catalyst typically increases the pressure drop across the reactor. 
For this reason, the physical integrity of the catalyst is of major 
importance. Dehydrogenation catalysts containing iron oxide can undergo 
substantial changes under process conditions which decrease their physical 
integrity. For example, in the dehydrogenation of ethylbenzene to styrene, 
the catalyst is subjected to contact with hydrogen and steam at high 
temperatures and, under these conditions, Fe.sub.2 O.sub.3, the preferred 
source of iron for the production of styrene catalysts, is reduced to 
Fe.sub.3 O.sub.4. This reduction causes a transformation in the lattice 
structure of the iron oxide, resulting in catalyst bodies which have 
poorer physical integrity and are very susceptible to degradation by 
contact with water at temperatures below 100.degree. C. This degradation 
by contact with water is characterized by the catalyst bodies (e.g., 
pellets or granules) becoming soft and/or swollen and/or cracked. The 
water which contacts the catalysts may be in the form of liquid or a wet 
gas, such as air with a high humidity. "High humidity" refers to a 
relative humidity above about 50%. 
The catalysts in styrene production plants are often exposed to 
temperatures below 100.degree. C. during start-ups, shutdowns, and upsets. 
Because large amounts of steam are used in styrene production, there is 
significant potential for exposing the catalyst to moisture at low 
temperatures. 
As previously discussed, this exposure causes physical degradation of the 
catalyst, which increases pressure drop across the reactor, resulting in 
decreased catalyst life. 
In recent years, catalysts with higher amounts of potassium have been used: 
in U.S. Pat. No. 4,503,163 assigned to Mobil Oil Company, for example, 
catalysts are disclosed which contain 13-48% and preferably 27-41% by 
weight of a potassium promoter compound, calculated as potassium oxide. 
Such catalysts are self regenerative catalysts which perform well at lower 
steam to oil ratios; e.g., ratios of&lt;2:1 (by weight). The economic 
advantages of using less steam are obvious. The problem with using higher 
concentrations of potassium is that the vulnerability of the used iron 
oxide catalyst to moisture increases with increasing potassium 
concentration. 
A need exists for a dehydrogenation catalyst that has both high activity, 
selectivity and resistance to moisture. A way has now been discovered to 
enhance moisture resistance of these catalysts without any significant 
detrimental effects to catalyst performance. 
SUMMARY OF THE INVENTION 
In one embodiment the invention is a novel calcined dehydrogenation 
catalyst comprising (a) an iron oxide; (b) a potassium and/or cesium 
compound; (c) at least one sodium compound in an amount from about 0.2 to 
about 10 weight percent sodium, calculated as sodium oxide; and (d) at 
least one calcium compound in an amount from 1.5 to 20 weight percent 
calcium calculated as calcium oxide. 
In a second embodiment, the invention is a process for preparing such 
improved dehydrogenation catalysts. This process comprises the steps of 
(a) preparing an extrudate by admixing iron oxide, a potassium and/or 
cesium compound, at least one sodium compound in an amount from about 0.2 
to about 10 weight percent sodium calculated as the oxide, at least one 
calcium compound in an amount from 1.5 to 20 weight percent calcium 
calculated as calcium oxide and sufficient water to form an extrudable 
mixture; (b) forming said extrudable mixture into pellets; and (c) 
calcining said pellets into a finished catalyst. 
The new catalyst compositions are useful for the dehydrogenation of an 
alkyl aromatic compound to form a vinyl aromatic compound by contacting 
the alkyl aromatic compound with the dehydrogenation catalyst under 
dehydrogenating conditions. The new catalysts have improved moisture 
stability, as evidenced by improved crush strength, which decreases 
catalyst degradation during process upsets. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention resides in the discovery that the addition of sodium and 
calcium compounds to known dehydrogenation catalysts produce new 
dehydrogenation catalysts having improved stability. Thus, any of the 
known class of dehydrogenation catalyst compositions containing red or 
yellow iron oxides and various catalyst promoters (as disclosed, for 
example, in U.S. Pat. Nos. 4,503,163, 3,703,593, 4,684,619, all of which 
are assigned to The Dow Chemical Company, and are incorporated by 
reference) may be used herein. Iron is generally added to the catalyst 
compositions of the invention as red iron oxide, Fe.sub.2 O.sub.3, or 
yellow iron oxide, Fe.sub.2 O.sub.3.H.sub.2 O. Particularly suited are 
pigment grades of red and yellow iron oxides. Likewise the catalyst 
promoter can be any material taught by the art, for example, an alkali 
metal compound(s) that is converted to an alkali metal oxide under 
calcination conditions. Potassium compounds are the preferred promoters. 
The promoter can be added to the catalyst in various forms. The alkali 
metal oxides, hydroxides, carbonates, bicarbonates, and the like, and 
mixtures thereof are preferred, and potassium carbonate or a mixture of 
potassium carbonate with potassium oxide is most preferred. 
The catalyst compositions of the present invention also may contain, and 
preferably do contain, cerium to enhance selectivity. Cerium, if used in 
the catalyst compositions of the present invention, can be added to the 
catalyst in the form of cerium oxide or in the form of other cerium 
compounds that decompose upon calcination to form cerium oxide, as for 
example, cerium carbonate, cerium nitrate, cerium hydroxide or any 
combination thereof. 
Other known catalyst additives can be included in the catalysts of the 
invention, but are not essential. A chromium compound which can serve as a 
stabilizer for the active catalytic components is illustrative of an 
optional but preferred additive. Chromium compounds have previously been 
added to alkali-promoted iron oxide catalysts to extend their life. 
Chromium, as used in the compositions of this invention, can be added to 
the catalyst in the form of a chromium oxide or in the form of chromium 
compounds which decompose upon calcination to chromium oxides. 
Another optional component, used to improve the selectivity of the 
catalyst, is molybdenum which can be added as its oxide or as a molybdate. 
Other metal compounds that may be added as promoters include compounds of 
aluminum, vanadium, cobalt, cadmium, copper, magnesium, manganese, and 
nickel, providing they can be calcined to the corresponding metal oxide. 
The physical strength, activity and selectivity of the catalyst 
compositions of the present invention can be improved by adding certain 
binding agents. Binding agents can include and consist of hydraulic 
cements, for example, calcium aluminate or Portland cement. These cements 
can be added individually or in combination. 
The density of the catalyst composition can be modified by the addition of 
various filler substances, for example, combustible materials such as 
graphite and methyl cellulose. Such materials can be added to the 
compositions during preparation, but are burned out after the catalyst 
pellets have been formed during the calcining step. Porosity promoting 
aids can also facilitate extrusion of catalyst pellets. 
The catalyst components can be mixed in various ways known to the art. One 
method comprises ballmilling together a mixture of desired compounds, 
adding a small amount of water, and extruding the composite to produce 
small pellets, which are then dried and calcined. Another method is mixing 
the components together with water, drying them to form a powder and 
tabletizing and calcining the tablets. Another procedure involves mixing 
the components together with an excess of water, partially drying, and 
then subsequently extruding, drying, and calcining the resulting pellets. 
The choice of the mixing method depends on the preference of the skilled 
artisan. 
A preferred method of preparing the catalysts is to blend the catalyst 
ingredients together, including the ingredients of the present invention, 
in the presence of sufficient water to make a moist extrudable mixture. 
This mixture is then extruded to produce extrudates between 1/8-inch and 
1/4-inch in diameter. The extrudates are then calcined under conventional 
calcining conditions. Calcination temperatures can range from about 
500.degree. C. to about 900.degree. C., preferably from about 600.degree. 
C. to about 800.degree. C. After calcination the extrudates are ready for 
use as catalysts. 
The amount of sodium included in the new catalyst, measured as sodium oxide 
and based on the weight of the calcined catalyst may range from about 0.2% 
to about 10%. Preferably sodium is present in the catalyst in the range of 
about 0.5% to 5%. Most preferably, sodium is present in amounts of from 
0.8 to 3.0%. The sodium may be added to the catalyst mixture as sodium 
hydroxide or carbonate or bicarbonate or other salts such as acetate, 
oxalate, nitrate, or sulfate. 
The amount of calcium in the new catalyst, measured as calcium oxide and 
based on the weight of the calcined catalyst, may range from 1.5% to 20%. 
Preferably calcium is present in the range of about 3% to 15%. Most 
preferably calcium is present in amounts from 4% to 12%. Calcium can be 
added to the catalyst mixture in the form of calcium carbonate, calcium 
sulfate, calcium hydroxide, or other salts.

EXPERIMENTAL 
All catalysts prepared in the following examples are prepared from 
commercially available chemicals. LUMNITE is the trade name for calcium 
aluminate cement manufactured by Lehigh Cement Company. 
Examples 1-5 
A catalyst formulation is made by blending in a heated steam jacketed blade 
mixer 135 grams(g) red iron oxide (Fe.sub.2 O.sub.3), 153.4 g yellow iron 
oxide (Fe.sub.2 O.sub.3.H2O), 120 g calcium aluminate cement (LUMNITE), 38 
g gypsum (CaSO.sub.4.2H.sub.2 O), 120 g calcium carbonate (CaCO.sub.3), 10 
g molybdenum oxide (MoO.sub.3), 190.5 g hydrated cerium carbonate 
(Ce.sub.2 (CO.sub.3).sub.3.5H.sub.2 O), 450 g potassium carbonate (K.sub.2 
CO.sub.3), 10 g potassium dichromate (K.sub.2 Cr.sub.2 O.sub.7), and 41.5 
g of a aqueous 50% solution of sodium hydroxide (NaOH). About 15% (wt) of 
water is then blended into the formulation. The mixture is mixed and dried 
in the heated steam jacketed mixer until the formulation reaches a 
consistency suitable for extrusion. The hot catalyst mixture is 
transferred to a California Model CL-3 Laboratory Pellet Mill and extruded 
(5/32" in diameter and about 10/32" in length). 
Four additional catalysts formulations are prepared according to the above 
procedure, but with different amounts of ingredients. Table 1 lists the 
formulations of Examples 1-5. The extrudates of Examples 1-5 are then 
calcined by slowly ramping the temperature to 775.degree. C. over a period 
of about two hours, and maintaining at 775.degree. C. for 30 minutes. The 
finished compositions of the calcined catalysts, expressed as shown, are 
given in Table 2. 
TABLE 1 
______________________________________ 
Catalyst Formulation 
Ex. Ex. Ex. Ex. Ex. 
Component* 
1 2 3 4 5 
______________________________________ 
Fe.sub.2 O.sub.3 
135 135 135 135 135 
Fe.sub.2 O.sub.3.H.sub.2 O 
153.4 153.4 153.4 
153.4 
153.4 
LUMNITE 120 120 120 120 120 
MoO.sub.3 10 10 10 10 10 
Ce.sub.2 (CO.sub.3).sub.3.5H.sub.2 O 
190.5 190.5 190.5 
190.5 
190.5 
K.sub.2 CO.sub.3 
450 450 450 450 450 
K.sub.2 Cr.sub.2 O.sub.7 
10 10 10 10 10 
CaSO.sub.4.2H.sub.2 O 
38 38 38 38 38 
CaCO.sub.3 
120 240 60 60 60 
NaOH, 50% 41.5 45.8 19.5 39.3 
66.2 
______________________________________ 
*All weights in grams. 
TABLE 2 
______________________________________ 
Components of Calcined Catalyst 
Ex. Ex. Ex. Ex. Ex. 
Component* 
1 2 3 4 5 
______________________________________ 
Fe.sub.2 O.sub.3 
23.46 21.21 25.0 24.78 24.48 
LUMNITE 10.43 9.43 11.11 11.01 10.88 
MoO.sub.3 
0.87 0.79 0.93 0.92 0.91 
Ce.sub.2 O.sub.3 
10.43 9.43 11.11 11.01 10.88 
K.sub.2 CO.sub.3 
39.10 35.35 41.68 41.30 40.79 
K.sub.2 Cr.sub.2 O.sub.7 
0.87 0.79 0.93 0.92 0.91 
CaSO.sub.4 
2.61 2.36 2.78 2.75 2.72 
CaCO.sub.3 
10.43 18.85 5.56 5.51 5.44 
NaOH 1.80 1.80 0.90 1.80 3.00 
______________________________________ 
*Weight percent 
COMATIVE EXPERIMENTS 1to 4 
The same procedure employed in Examples 1-5 was used to make all the 
comparative catalysts. Table 3 lists the formulations of the comparative 
catalysts. 
TABLE 3 
______________________________________ 
Comparative Catalyst Formulation 
Comp. Comp. Comp. Comp. 
Component* Ex. 1 Ex. 2 Ex. 3 Ex. 4 
______________________________________ 
Fe.sub.2 O.sub.3 
135 135 135 135 
Fe.sub.2 O.sub.3.H.sub.2 O 
153.4 153.4 153.4 
153.4 
LUMNITE 120 120 120 120 
MoO.sub.3 10 10 10 10 
Ce.sub.2 (CO.sub.3).sub.3.5H.sub.2 O 
190.5 190.5 190.5 
190.5 
K.sub.2 CO.sub.3 
450 450 450 450 
K.sub.2 Cr.sub.2 O.sub.7 
10 10 10 10 
CaSO.sub.4.2H.sub.2 O 
38 38 -- 38 
CaCO.sub.3 -- 60 -- -- 
NaOH, 50% -- -- 36 39.3 
______________________________________ 
All weights in grams. 
The finished compositions of the calcined comparative catalysts, expressed 
as shown, are given in Table 4. 
TABLE 4 
______________________________________ 
Components of Calcined Comparative Catalysts 
Comp. Comp. Comp. Comp. 
Component* Ex. 1 Ex. 2 Ex. 3 Ex. 4 
______________________________________ 
Fe.sub.2 O.sub.3 
26.74 25.24 27.06 26.22 
LUMNITE 11.88 11.21 12.02 11.65 
MoO.sub.3 0.99 0.93 1.00 0.97 
Ce.sub.2 O.sub.3 
11.88 11.22 12.03 11.66 
K.sub.2 CO.sub.3 
44.55 42.05 45.09 43.70 
K.sub.2 Cr.sub.2 O.sub.7 
0.99 0.93 1.00 0.97 
CaSO.sub.4 2.97 2.81 -- 2.92 
CaCO.sub.3 -- 5.62 -- -- 
NaOH -- -- 1.80 1.91 
______________________________________ 
*Weight percent 
The catalysts of the invention and those prepared for comparison are tested 
for activity and selectivity in the reaction for dehydrogenating 
ethylbenzene to styrene by placing 70 or 100 milliliters (mL) of the above 
calcined catalyst extrudates in a fixed bed reactor and passing a 
preheated mixture of steam and ethylbenzene at a weight ratio of 1.5:1 
(called the steam to oil ratio) through the bed which is maintained at a 
temperature of 580.degree.-590.degree. C. The LHSV (liquid hourly space 
velocity) is 1.0 and the pressure is maintained at atmospheric. The liquid 
hourly space velocity is a number denoting residence time in a reactor 
commonly used by those skilled in the art. After a minimum of 5 days, the 
weight ratio of steam to ethylbenzene is reduced to 1.2 and the bed 
temperature adjusted so that an ethylbenzene conversion of 50% is 
achieved. This temperature adjustment is continued each day until a 
constant conversion of about 50% is achieved at a fixed bed temperature, 
that temperature being an indication of the activity of the particular 
catalyst; i.e., the lower the temperature, the higher the activity. The 
results of the dehydrogenation reaction for Examples 1-5 and Comparative 
Experiments 1-4 are shown in Table 5. 
TABLE 5 
______________________________________ 
Catalyst Temp, .degree.C. 
Conversion 
Selectivity 
______________________________________ 
Example 1 589 50.0 97.2 
Example 2 597 49.8 97.1 
Example 3 589 49.7 96.9 
Example 4 596 50.3 96.5 
Example 5 600 50.5 97.2 
Comp. Ex. 1 
598 50.2 95.9 
Comp. Ex. 2 
595 50.4 96.6 
Comp. Ex. 3 
590 50.4 97.1 
Comp. Ex. 4 
587 49.7 96.8 
______________________________________ 
The moisture resistance of the used catalyst is measured by the following 
method. After approximately two to three weeks of operation, the catalyst 
is unloaded from the reactor and twenty randomly chosen extrudates are 
placed in a glass dish with a fiat bottom. The extrudates are separated so 
that they do not touch each other. The glass dish is then placed in a 
controlled relative humidity chamber (Vapor-Temp Model No. VP-100AT-1) 
made by Blue M, a unit of General Signal, adjusted to 30.degree. C. and 
70% relative humidity. After 20 hours the glass dish is removed from the 
humidity chamber. The excess water in the dish is removed, and the 
extrudates are placed in a drying oven at 150.degree. C. for 24 hours. The 
extrudates are removed and the average crush strength of the extrudates 
determined. 
Crush strength is a common physical measurement that indicates the strength 
of the catalyst body; i.e., tablet, pellet, or extrudate. In this test, 
catalyst bodies are compressed between two fiat metal surfaces or blocks, 
and the pressure required to crush the body is measured. A Comten crush 
strength machine, model no. 922T-10-OP, serial no. 830202, is used 
according to the following procedure. The length of the extrudate is first 
measured, then the extrudate is crushed between the two blocks of the 
Comten unit and the pressure required to crush the extrudate is recorded. 
The crush strength per one-quarter inch length of extrudate is then 
calculated. This procedure is done on twenty randomly chosen extrudates 
from each catalyst sample which are all preconditioned as noted in the 
preceding paragraph. Averaged crush strength is expressed as "PSIG/1/4 
inch". The average of these twenty measurements is shown in Table 6. 
TABLE 6 
______________________________________ 
Average Crush 
Strength 
Catalyst PSIG/1/4 inch. 
Physical Appearance 
______________________________________ 
Example 1 
18.5 Hard, No Cracking or Swelling 
Example 2 
18.0 Hard, No Cracking or Swelling 
Example 3 
13.5 Hard, No Cracking or Swelling 
Example 4 
15.5 Hard, No Cracking or Swelling 
Example 5 
18.8 Hard, No Cracking or Swelling 
Comp. Ex. 1 
&lt;5 Soft, Cracked, Swollen 
Comp. Ex. 2 
&lt;5 Soft, Cracked, Swollen 
Comp. Ex. 3 
&lt;5 Soft, Cracked, Swollen 
Comp. Ex. 4 
6 1/2 Pellets Soft, Slightly Swollen, 
Other 1/2 Hard, No Cracking or 
Swelling 
______________________________________