Iron oxide-chromium oxide catalyst and process for high temperature water-gas shift reaction

Iron oxide-chromium oxide catalyst of increased mechanical strength (lateral crushing strength) for high-temperature water-gas shift reaction. The catalyst contains as an additional component magnesium oxide and/or magnesium spinels in the form of discrete particles.

FIELD OF THE INVENTION 
The invention relates to iron oxide-chromium oxide catalysts of increased 
mechanical strength for high-temperature water-gas shift reaction. 
Hydrogen production from carbon monoxide and steam has for decades been one 
of the most important processes of the chemical industry. As is generally 
known, catalysts containing in particular iron oxide and chromium oxide 
have proved successful for high-temperature water-gas shift reaction. They 
are normally applied in the practice of the art at 350.degree. to 
390.degree. C. under atmospheric pressure. 
BACKGROUND OF THE INVENTION 
Recently, however, units are frequently constructed which, for economic 
reasons, operate at elevated pressures, e.g. up to 25 or 50 bar. As 
experience with such units has shown, commercial iron oxide-chromium oxide 
catalysts lose their original mechanical strength relatively fast during 
the running time. This effect often causes the formation of fracture or 
dust and at the same time a definite increase in flow resistance. 
Mechanical strength is generally understood to mean the lateral crushing 
strength (LSC), which is measured on catalyst tablets. With commercial 
iron oxide-chromium oxide catalysts, LCS losses of about 54 to 63% have 
been found after use in a high-temperature water-gas shift reaction plant 
at a pressure of 50 bar after a running time of 2000 hours. This applies 
also to catalysts with originally high LCS values. 
Since under technical conditions the CO conversion takes place in the areas 
of the internal diffusion, both the porosity and the pore distribution in 
the catalyst compacts (in particular tablets) are of importance. The 
production of a catalyst with increased lateral compressive strength (15 
kg/tablet) of the compacts requires the application of higher crushing 
pressures. This leads to a reduction of the pore volume as well as to an 
alteration of the pore distribution in the produced tablets and hence also 
to a considerable loss of apparent activity. 
DESCRIPTION OF THE PRIOR ART 
Another solution of the strength problem offers itself in the addition of 
various components which are able to increase the lateral crushing 
strength of the iron oxide-chromium oxide catalysts or respectively to 
prevent its rapid decline. 
Thus it is known for example from DE-AS No. 12 52 184 that the lateral 
crushing strength of Co--MoO.sub.3 --Al.sub.3 O.sub.3 conversion catalysts 
can be increased by addition of inorganic binders, such as portland 
cement, alumina cement, or calcium aluminate, whereby the use of these 
catalysts in pressure systems becomes possible. When employing this method 
for iron oxide-chromium oxide catalysts, one does indeed obtain an 
increase in lateral crushing strength; but this is at the expense of the 
apparent activities of the catalyst compacts. 
Further, from DE-OS No. 18 12 813 teaches iron oxide-chromium oxide 
catalysts for water-gas shift reaction which contain as active metals 
iron, chromium and cobalt in the form of the oxides on an aluminum oxide 
support, where the cobalt oxide may be partly or wholly replaced by nickel 
oxide. These catalysts may further contain aluminum oxide or a mixture or 
compound of aluminum oxide and magnesium oxide, whereby a longer life and 
less sensitivity to catalyst poisons is said to be achieved. But, the use 
of aluminum oxide does not improve the mechanical strength of the 
catalysts, even when it is used in mixture with magnesium oxide. Besides, 
these catalysts are not suitable for use at high pressures, as the 
presence of cobalt and/or nickel leads to an undesired methanization or 
Fischer-Tropsch synthesis. 
SUMMARY OF THE INVENTION 
It is the object of the invention to make available iron oxide-chromium 
oxide catalysts of the above-defined kind which excel by their high 
mechanical strength, which decreases little even when the catalysts are 
used in water-gas shift reaction under elevated pressure, and in which the 
original activity of the catalysts is maintained without substantial 
undesirable methanization. The problem underlying the invention is solved 
in that the catalyst contains nickel and/or cobalt in quantities of at 
most 200 ppm (preferably 0 to 100 ppm) and as an additional component 
magnesium oxide and/or magnesium spinels (MgFe.sub.2 O.sub.4, MgCr.sub.2 
O.sub.4) formed by reaction of magnesium oxide with iron oxide and/or 
chromium oxide in the form of discrete particles. Said compounds and their 
crystallite size can be determined by x-ray diffraction analysis. 
Preferably the discrete particles have an average crystallite size of 100 
to 180 .ANG., the average crystallite size of magnesium oxide being 
normally 100 to 120 .ANG. and that of the spinels normally 130 to 180 
.ANG.. Such crystallite sizes can be obtained, e.g. by using in the 
production of the catalysts magnesium oxide (or a precursor of magnesium 
oxide, such as magnesium carbonate) of which more than 70 wt.% have a 
particle size in the range of about 5 to 15 microns. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The analytical composition of the catalysts according to the invention is 
preferably as follows: 
80 to 90 wt.% iron oxides (Fe.sup.+2 and/or Fe.sup.+3); 
7 to 11 wt.% Cr.sub.2 O.sub.3 ; 
2 to 10 wt.%, preferably 4 to 6 wt.% MgO. 
Part of the magnesium is present, as mentioned above, in the form of the 
magnesium spinels of iron and/or chromium. Therefore, the analytical 
results determined for iron oxide or chromium oxide content are higher 
than is indicated by the concentration of the corresponding free oxides of 
iron or chromium, for the iron-chromium mixed oxide systems. The content 
of magnesium spinels depends on the intensity of the thermal treatment to 
which the catalyst is exposed. 
The catalyst according to the invention is preferably obtainable by adding 
magnesium oxide or a precursor stage transformable into magnesium oxide 
during calcining to the iron oxide-chromium oxide component. Alternately, 
the magnesium component may be added to a precursor stage transformable 
into the latter during calcining, prior to calcination of the mixture. 
A precursor stage of the iron oxide-chromium oxide component is, e.g. a 
mixture of the hydroxides and/or oxide hydrates of iron and of chromium, 
in which the iron is present in bi and/or trivalent form and the chromium 
generally in trivalent form. Suitable as precursor stage of magnesium 
oxide are, for example, magnesium hydroxide, carbonate, acetate, formate 
and/or oxalate. 
The magnesium oxide, or respectively the precursor thereof, can be added to 
an aqueous suspension or to a precipitated mass of the iron oxide-chromium 
oxide component or the precursor thereof, the obtained mixture being 
calcined (in the first case after removal of the aqueous phase). The 
magnesium oxide or its precursor (e.g., magnesium hydroxide or magnesium 
carbonate) is then preferably added to a filter cake which had been 
obtained from freshly precipitated iron hydroxide-chromium hydroxide after 
removal of the aqueous phase. The subsequent processing of the 
Mg-containing filter cake normally includes also the following steps: 
Addition of lubricants such as graphite, drying at temperatures of about 
150.degree. to 220.degree. C., shaping to tablets, and calcining. During 
this thermal treatment, the iron and chromium hydroxides on the one hand 
are transformed into the corresponding oxides, and magnesium hydroxide, 
magnesium carbonate or respectively hydroxycarbonate into magnesium oxide 
and partially by reaction with the iron oxide and the chromium oxide into 
the corresponding magnesium spinels (MgFe.sub.2 O.sub.4 and MgCr.sub.2 
O.sub.4). 
But also one can impregnate the dried filter cake of iron and chromium 
hydroxide or respectively the corresponding oxides with water-soluble and 
easily thermodecomposable magnesium salts, such as magnesium acetate, 
formate or oxalate, and thermally treat the mixture, whereby again 
magnesium oxide forms, which, if desired, can react with iron and chromium 
oxide to the corresponding spinels. 
According to a less preferred form of realization, in which smaller 
magnesium oxide or magnesium spinel particles are formed, the catalyst of 
the invention can be obtained by co-precipitation of the iron 
oxide-chromium oxide component and of the magnesium oxide component or 
respectively precursors of these components, from the corresponding 
water-soluble metal salts with alkali and subsequent calcining of the 
precipitated mass. Suitable water-soluble metal salts are, e.g. the 
nitrates and sulfates of iron, chromium and magnesium, which are 
precipitated preferably with sodium carbonate or sodium hydroxide. 
Regardless of how the magnesium component is applied on the iron 
oxide-chromium oxide component, the calcining is usually done at about 
450.degree. to 520.degree. C., preferably at about 470.degree. and 
490.degree. C. 
The catalyst according to the invention is preferably in the form of shaped 
bodies, such as tablets or rings. For this purpose, the iron 
oxide-chromium oxide component and the magnesium oxide component or 
respectively their precursors are pressed to corresponding compacts before 
calcination. Normally, graphite is added as a lubricant. 
The invention also relates to the use of the catalyst according to the 
invention for water-gas shift reaction with steam at temperatures of 
300.degree. to 400.degree. C. at atmospheric or elevated pressure. In the 
latter case, one operates preferably in the range from about 10 to 100 
bar. 
The invention is explained by the following examples. The chemical and 
physical-chemical data of the catalysts thus produced are stated in the 
table. The decrease in loss of lateral crushing strength caused by the 
addition of magnesium oxide is clearly evident from the table. In 
addition, a positive effect on the thermoresistance of the catalysts was 
observed. This effect is generally reflected in a reduction in loss of 
surface area of the catalysts as measured by BET. 
COMISON EXAMPLE 1 
Production of the standard catalyst without MgO. 
3200 ml deionized water were heated to 50.degree. to 55.degree. C. and 
therein 1500 g FeSO.sub.4.7H.sub.2 O were dissolved; then 85.5 g Na.sub.2 
Cr.sub.2 O.sub.7.H.sub.2 O were added, and the solution was maintained at 
55.degree. C. to precipitation. 
3400 ml deionized water and 680 ml 50% NaOH (D=1.525) were mixed and heated 
to 40.degree. C. Then air was injected (400 liter/h) into the sodium 
hydroxide solution, and the Fe-Cr solution was added within 30 minutes 
while stirring. With further stirring and introduction of air, the 
suspension was heated to 60.degree. C. and maintained at this temperature 
for 3 hours. 
The precipitate was suction filtered, and the filter cake was washed by 
repeated suspension with 4000 ml deionized water of 60.degree. C., until a 
resistance of &gt;700 Ohm/cm was reached in the filtrate. In the last 
suspension, 21 g of natural graphite was added. The washed filter cake was 
dried for 15 hours at 220.degree. C. The dried product was granulated 
through a 1.5 mm screen and pressed to cylindrical tablets of a diameter 
of 6 mm and a height of 6 mm. The tablets thus obtained were calcined for 
one hour at 480.degree. C. 
COMISON EXAMPLE 2 
Production of a catalyst with cement as binder. 
550 g of the filter cake obtained according to Example 1 from the 
precipitation (loss on ignition at 480.degree. C.=55.5%) were mixed with 
13.1 g portland cement for 15 minutes in a mix muller. The resulting mass 
was then dried for 15 hours at 220.degree. C. and subsequently further 
treated as stated in Example 1.

EXAMPLE 1 
Production of a magnesium oxide-containing catalyst. 
Following the procedure of Example 1, the precipitation was carried out 
having 1500 g FeSO.sub.4.7H.sub.2 O and 85.5 g Na.sub.2 Cr.sub.2 
O.sub.7.H.sub.2 O. 550 g of the filter cake obtained (loss on ignition at 
480.degree. C.=55.6%) were mixed with 19.6 g Mg(OH).sub.2 (MgO 
content=65.7%) in the mix muller for 15 minutes. The mixture was dried at 
220.degree. C. for 15 hours and processed as described in Comparison 
Example 1. 
EXAMPLES 2 AND 3 
Production of magnesium oxide-containing catalysts 
The production of these catalysts occurred as according to Example 1, but 
admixing 29.4 g Mg(OH).sub.2 according to Example 2 and 39.2 g 
Mg(OH).sub.2 according to Example 3. 
EXAMPLE 4 
Production of a magnesium oxide-containing catalyst. 
The production of this catalyst followed the procedure used in the 
production of the catalyst of Comparison Example 1, except that instead of 
Mg(OH).sub.2, 30.7 g MgCO.sub.3 (MgO content=42.0 wt.%) were admixed. 
EXAMPLE 5 
Production of a magnesium oxide-containing catalyst. 
The production of this catalyst was identical to the method described in 
Comparison Example 1, adding to the filter cake washed and suspended in 
deionized water (corresponds to 244.2 g anhydrous substance) 30.7 g 
MgCO.sub.3 (MgO content=42.0 wt.%). The further treatment occurred as 
according to Comparison Example 1. 
EXAMPLE 6 
Production of a magnesium oxide-containing catalyst. 
The production of this catalyst was by the same method as that of Example 
5, but admixing 46.1 g MgCO.sub.3 rather than 30.7 g. 
EXAMPLE 7 
Production of a magnesium oxide-containing catalyst by co-precipitation of 
the components. 
3200 ml deionized water were heated to 50.degree. to 55.degree. C. and 1500 
g FeSO.sub.4.7H.sub.2 O and 152.7 g MgSO.sub.4 were dissolved therein. 
Then 85.5 g of Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O was added and the 
solution was maintained at 55.degree. C. to precipitation. 
3400 ml deionized water and 745 ml 50% NaOH (D=1.525) were mixed and heated 
to 40.degree. C. 
The precipitation and subsequent further treatment was identical to the 
method described in Comparison Example 1. 
EXAMPLE 8 
Production of a magnesium oxide-containing catalyst. 
The precipitation and production of the filter cake was the same as that 
for Example 3. 
550 g of the obtained filter cake (heat loss at 480.degree. C.=55.6%), 
dissolved in 150 ml deionized water, were impregnated with 46 g 
Mg(CH.sub.3 COO).sub.2, and the obtained mass was dried at 220.degree. C. 
for 15 hours and thereafter processed as stated in the Comparison Example 
1. 
__________________________________________________________________________ 
Chemical and Physical-Catalytic Data of the Catalysts Produced 
Produc- 
BET Sur- Loss 
Catalyst MgO tion face area (3) 
LCS (4) 
of CO Conver- 
(Tablets 
Additions 
Content 
Method 
(m.sup.2 /g) 
(kg) LCS 
sion (5) 
6 .times. 6 mm) 
(wt. %) (1) (2) f g f g (%) 
(%) 
__________________________________________________________________________ 
Comp. 
1 -- -- S 80 40 13.8 
8.4 
39.0 
78.0 
Ex. 2 Cement (5.0) 
-- A 73 48 16.2 
11.6 
28.4 
65.8 
Ex. 1 Mg(OH).sub.2 
5.0 A 78 50 12.0 
11.0 
8.3 
70.1 
2 Mg(OH).sub.2 
7.5 A 82 44 12.0 
10.7 
10.8 
61.2 
3 Mg(OH).sub.2 
10.0 A 93 44 12.4 
9.9 
20.2 
61.7 
4 MgCO.sub.3 
5.0 A 48 43 13.2 
11.3 
15.2 
66.2 
5 MgCO.sub.3 
5.0 B 46 43 13.9 
11.0 
21.0 
67.9 
6 MgCO.sub.3 
7.5 B 47 38 15.0 
10.9 
27.4 
63.2 
7 MgSO.sub.4 
5.0 C 78 50 11.2 
8.3 
26.0 
77.2 
8 Mg(OOCCH.sub.3).sub.2 
5.0 D 47 40 10.5 
9.3 
11.4 
61.9 
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Explanations concerning the table: 
(1) MgO content in the finished catalyst. 
(2) Production method: S = Standard, A = Admixing the magnesium compound 
to the filter cake, B = Admixing the magnesium compound to the aqueous 
suspension of the precipitated Fe and Cr hydroxide, C = Joint 
precipitation, D = Impregnation of the dried filter cake with magnesium 
acetate. 
(3) Determined was the BET surface area of the fresh (f) and of the used 
(g) catalyst after the test. 
(4) The lateral crushing strength (LCS) of the fresh (f) and of the used 
(g) catalyst after the test was determined, namely with 6 .times. 6 mm 
tablets. The loss of lateral crushing strength was determined after 
performance of the activity test (running time 8 hours). For this purpose 
the catalyst tablets were removed from the test reactor under nitrogen an 
measured with a commercial instrument for crushing strength determination 
(5) The CO conversion was determined under the following reaction 
conditions: T = 370.degree. C., P = 50 bar, ratio H.sub.2 O/gas = 1.0; 
gas composition (%): CO = 49.6, CO.sub.2 = 4.7, H.sub.2 = 45.7; space 
velocity (HSV) = 3000 vol. gas per vol. of catalyst per hour; 
thermodynamically possible conversion = 94.2%.