Process for the selective preparation of acetic acid using a molybdenum, palladium, and rhenium catalyst

A process for the selective preparation of acetic acid from a gaseous feed comprising ethane, ethylene or mixtures thereof plus oxygen at elevated temperature, which comprises bringing the gaseous feed into contact with a catalyst comprising the elements Mo, Pd, Re, X and Y in gram atom ratios a:b:c:d:e in combination with oxygen EQU Mo.sub.a Pd.sub.b Re.sub.c X.sub.d Y.sub.e (I) where the symbols X, Y have the following meanings: PA1 X=Cr, Mn, Nb, B, Ta, Ti, V and/or W PA1 Y=Bi, Ce, Co, Cu, Te, Fe, Li, K, Na, Rb, Be, Mg, Ca, Sr, Ba, Ni, P, Pb, Sb, Si, Sn, Tl and/or U; PA1 the indices a, b, c, d and e are the gram atom ratios of the corresponding elements, where PA1 a=1, b>0, c>0, d=0.05-2, e=0-3.

The present invention relates to a process for the selective preparation of 
acetic acid by catalytic gas-phase oxidation of ethane and/or ethylene in 
the presence of a palladium-containing catalyst. 
The oxidative dehydrogenation of ethane to ethylene in the gas phase at 
temperatures of &gt;500.degree. C. is known, for example from U.S. Pat. Nos. 
4,250,346, 4,524,236 and 4,568,790. 
Thus, U.S. Pat. No. 4,250,346 describes the use of a catalyst composition 
comprising the elements molybdenum, X and Y in the ratio a:b:c for 
converting ethane into ethylene, where X is Cr, Mn, Nb, Ta, Ti, V, and/or 
W and Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, Tl and/or U 
and a is 1, b is from 0.05 to 1 and c is from 0 to 2. The total value of c 
for Co, Ni and/or Fe must here by less than 0.5. 
The reaction is preferably carried out in the presence of added water. The 
disclosed catalysts can likewise be used for the oxidation of ethane to 
give acetic acid, with the efficiency of the conversion to acetic acid 
being about 18%, at an ethane conversion of 7.5%. 
The abovementioned documents are concerned mainly with the preparation of 
ethylene, less with the target preparation of acetic acid. 
In contrast, EP-B-0 294 845 describes a process for the selective 
preparation of acetic acid from ethane, ethylene or mixtures thereof using 
oxygen in the presence of a catalyst mixture comprising at least A.) a 
calcined catalyst of the formula Mo.sub.x V.sub.y or Mo.sub.x V.sub.y 
Z.sub.y, where Z is one or more of the metals Li, Na, Be, Mg, Ca, Sr, Ba, 
Zn, Cd, Hg, Sc, Y, La, Ce, Al, Tl, Ti, Zr, Hf, Pb, Nb, Ta, As, Sb, Bi, Cr, 
W, U, Te, Fe, Co and Ni, and x is from 0.5 to 0.9, y is from 0.1 to 0.4 
and z is from 0.001 to 1, and B.) an ethylene hydration catalyst and/or 
ethylene oxidation catalyst. The second catalyst component B is, in 
particular, a molecular sieve catalyst or a palladium-containing oxidation 
catalyst. When the catalyst mixture described is used and a gas mixture 
comprising ethane, oxygen, nitrogen and water vapor is passed through the 
catalyst-containing reactor, the maximum selectivity is 27% at an ethane 
conversion of 7%. 
A further process for preparing a product comprising ethylene and/or acetic 
acid is described in EP-B-0 407 091. Here, ethane and/or ethylene and a 
gas comprising molecular oxygen is brought into contact at elevated 
temperature with a catalyst composition comprising the elements A, X and 
Y. A is here Mo.sub.d Re.sub.e W.sub.f, X is Cr, Mn, Nb, Ta, Ti, V and/or 
W and Y is Bi, Ce, Co, Cu, Fe, K, Mg, Ni, P, Pb, Sb, Si, Sn, Tl and/or U. 
The maximum selectivities which were able to be achieved when using the 
catalyst described in the oxidation of ethane to acetic acid are 78%. 
Further by-products formed are carbon dioxide, carbon monoxide and 
ethylene. 
However, none of the publications listed above describes the use of a 
catalyst comprising the elements rhenium, palladium and molybdenum for the 
selective oxidation of ethane and/or ethylene to give acetic acid. 
Furthermore, the selectivities achieved up to now in the prior art for the 
oxidation to acetic acid are still not satisfactory. 
It is therefore an object of the invention to provide a process which 
allows ethane and/or ethylene to be oxidized in a simple and targeted 
manner and with high selectivity to give acetic acid. 
It has now surprisingly been found that use of a catalyst comprising the 
elements molybdenum, rhenium and palladium and one or more elements 
selected from the group consisting of chromium, manganese, niobium, 
tantalum, titanium, vanadium and/or tungsten makes it possible to oxidize 
ethane and/or ethylene under relatively mild conditions, in a simple 
manner with high selectivity to give acetic acid. 
The present invention accordingly provides a process for the selective 
preparation of acetic acid from a gaseous feed comprising ethane, ethylene 
or mixtures thereof plus oxygen at elevated temperature, which comprises 
bringing the gaseous feed into contact with a catalyst comprising the 
elements Mo, Pd, Re, X and Y in gram atom ratios a:b:c:d:e in combination 
with oxygen 
EQU Mo.sub.a Pd.sub.b Re.sub.c X.sub.d Y.sub.e (I) 
where the symbols X, Y have the following meanings: 
X=Cr, Mn, Nb, B, Ta, Ti, V and/or W, in particular Nb, V and W 
Y=Bi, Ce, Co, Cu, Te, Fe, Li, K, Na, Rb, Be, Mg, Ca, Sr, Ba, Ni, P, Pb, 
Sb, Si, Sn, Tl and/or U, in particular Ca, Sb, Te and Li. 
The indices a, b, c, d and e are the gram atom ratios of the corresponding 
elements, where 
a=1, b&gt;0, c&gt;0, d=0.05-2 and e=0-3. 
If X and Y are a plurality of different elements, the indices d and e can 
likewise assume a plurality of different values. 
The present invention further provides a catalyst for the selective 
preparation of acetic acid comprising the elements Mo, Pd, Re, X and Y in 
the gram atom ratios a:b:c:d:e in combination with oxygen. 
The gram atom ratios a:b:c:d:e are preferably within the following ranges: 
a=1; b=0.0001-0.5; c=0.25-1.0; d=0.1-1.0; e=0-1.0. 
Palladium contents in the catalyst which are above the upper limit 
specified promote the formation of carbon dioxide in the process of the 
invention. Furthermore, higher palladium contents are generally also 
avoided because they make the catalyst unnecessarily expensive. On the 
other hand, palladium contents below the limiting value specified favor 
ethylene formation. 
Rhenium contents below the limiting value specified likewise lead to 
preferential formation of ethylene at the expense of the selectivity to 
acetic acid. On the other hand, rhenium contents which are higher than the 
limiting value specified give no further improvement in the catalytic 
properties and would therefore also just make the catalyst unnecessarily 
expensive. 
The catalyst used according to the invention preferably comprises not only 
the elements molybdenum, palladium and rhenium but also vanadium, niobium, 
antimony and calcium in combination with oxygen. The gram atom ratios 
a:b:c:d.sup.1 :d.sup.2 :e.sup.1 :e.sup.2 of the elements 
Mo:Pd:Re:V:Nb:Sb:Ca are preferably as follows: 
a(Mo)=1; b(Pd)=0.0001-0.5, in particular 0.001-0.05; 
c(Re)=0.25-1.0; d.sup.1 (V)=0.2-1.0; d.sup.2 (Nb)=0.1-0.5; 
e.sup.1 (Sb)=0-0.5; e.sup.2 (Ca)=0-0.2; 
Examples of such catalyst compositions which are preferably used in the 
process of the invention are: 
Mo.sub.1.0 Pd.sub.0.01 Re.sub.0.7 V.sub.0.7 Nb.sub.0.2 Sb.sub.0.1 
Ca.sub.0.05 
Mo.sub.1.0 Pd.sub.0.02 Re.sub.0.7 V.sub.0.7 Nb.sub.0.2 Sb.sub.0.1 
Ca.sub.0.05 
Mo.sub.1.0 Pd.sub.0.02 Re.sub.0.5 V.sub.0.5 Nb.sub.0.5 Sb.sub.0.1 
Mo.sub.1.0 Pd.sub.0.02 Re.sub.0.7 V.sub.0.5 Te.sub.0.5 
Mo.sub.1.0 Pd.sub.0.02 Re.sub.0.7 V.sub.0.7 Nb.sub.0.2 Sb.sub.0.1 
Ca.sub.0.05 
Mo.sub.1.0 Pd.sub.0.02 Re.sub.0.7 W.sub.0.2 V.sub.0.7 Nb.sub.0.2 Sb.sub.0.1 
The catalysts used according to the invention can be prepared by 
conventional methods. These start out from a slurry, in particular an 
aqueous solution, comprising the individual starting components of the 
elements in accordance with their proportions. 
The starting materials for the individual components in the preparation of 
the catalyst of the invention are, apart from the oxides, preferably 
water-soluble substances such as ammonium salts, nitrates, sulfates, 
halides, hydroxides and salts of organic acids which can be converted into 
the corresponding oxides by heating. To mix the components, aqueous 
solutions or suspensions of the metal salts are prepared and mixed. 
In the case of molybdenum, it is advisable to use the corresponding 
molybdates e.g. ammonium molybdate, as starting compounds because of their 
commercial availability. 
Suitable palladium compounds are, for example, palladium(II) chloride, 
palladium(II) sulfate, tetramminepalladium(II) nitrate, palladium(II) 
nitrate and also palladium(II) acetylacetonate. 
In the case of rhenium, it is possible to use, for example, perrhenic acid, 
ammonium perrhenate and also rhenium(III) and rhenium(V) chlorides, to 
name only a few, as starting compound. 
The reaction mixture obtained is then stirred at from 50 to 100.degree. C. 
for from 5 minutes to 5 hours. The water is subsequently removed and the 
remaining catalyst is dried at a temperature of from 50 to 150.degree. C., 
in particular from 80 to 120.degree. C. 
If the catalyst obtained is subsequently subjected to a calcination 
process, it is advisable to calcine the dried and pulverized catalyst at a 
temperature in the range from 100.degree. C. to 800.degree. C., in 
particular from 200 to 500.degree. C., in the presence of nitrogen, oxygen 
or an oxygen-containing gas. The duration is from 2 to 24 hours. 
The catalyst can be used without a support material or be mixed with an 
appropriate support material or applied thereto. Suitable support 
materials are the customary materials such as porous silicon dioxide, 
ignited silicon dioxide, kieselguhr, silica gel, porous or nonporous 
aluminum oxide, titanium dioxide, zirconium dioxide, thorium dioxide, 
lanthanum oxide, magnesium oxide, calcium oxide, barium oxide, tin oxide, 
cerium dioxide, zinc oxide, boron oxide, boron nitride, boron carbide, 
boron phosphate, zirconium phosphate, aluminum silicate, silicon nitride 
or silicon carbide, but also meshes made of glass or metals. 
Preferred support materials have a surface area of less than 100 m.sup.2 
/g. Preferred support materials are silicon dioxide and aluminum oxide 
having a low specific surface area. The catalyst can, after shaping, be 
used as a regularly or irregularly shaped support body or else in powder 
form as a heterogeneous oxidation catalyst. 
The reaction can be carried out in a fluidized bed or in a fixed-bed 
reactor. For use in a fluidized bed, the catalyst is milled to a particle 
size in the range from 10 to 200 .mu.m. 
The gaseous feed comprises ethane and/or ethylene which are fed to the 
reactor as pure gases or in admixture with one or more other gases. 
Suitable additional or carrier gases of this type are, for example, 
nitrogen, methane, carbon monoxide, carbon dioxide, air and/or water 
vapor. The gas containing molecular oxygen can be air or a gas containing 
more or less molecular oxygen than air, e.g. oxygen. Preference is given 
to adding water vapor to the gas comprising ethane and molecular oxygen 
since this promotes the selectivity to acetic acid. The proportion of 
water vapor is in the range from 5 to 30% by volume, preferably from 0 to 
20% by volume. Lower water vapor contents lead to a loss of selectivity in 
respect of acetic acid formation, while higher water vapor concentrations 
would make the work-up of the resulting aqueous acetic acid unnecessarily 
more expensive for technical process reasons. The addition of oxygen or 
the gas comprising molecular oxygen depends on the explosive limits under 
the reaction conditions. Relatively high oxygen contents are preferred, 
since the achievable ethane conversion and thus the yield of acetic acid 
is higher. The maximum oxygen concentration is, however, limited by the 
explosive limits. The ratio of ethane to oxygen is advantageously in the 
range between 1:1 and 10:1, preferably 2:1 and 8:1. 
The reaction is carried out at temperatures between 200 and 500.degree. C., 
preferably from 200 to 400.degree. C. The pressure can be atmospheric or 
superatmospheric, e.g. in the range between 1 and 50 bar, preferably from 
1 to 30 bar. 
The reaction can be carried out in a fixed-bed or fluidized-bed reactor. 
Advantageously, ethane is first mixed with the inert gases such as 
nitrogen or water vapor before oxygen or the gas containing molecular 
oxygen is fed in. The mixed gases are preferably preheated to the reaction 
temperature in a preheating zone before the gas mixture is brought into 
contact with the catalyst. Acetic acid is separated from the gas leaving 
the reactor by condensation. The remaining gases are recirculated to the 
reactor inlet where oxygen or the gas containing molecular oxygen plus 
ethane and/or ethylene are metered in. 
When using the catalyst of the invention, the selectivity in the oxidation 
of ethane and/or ethylene to acetic acid is &gt;75 mol %, preferably &gt;80 mol 
%, in particular &gt;85 mol %, at an ethane conversion of &gt;3%, preferably 
&gt;4%, in particular &gt;5%, so that, in comparison with the prior art, the 
process of the invention enables an increase in the acetic acid yields to 
be achieved in a simple manner while simultaneously reducing the formation 
of undesired by-products.

EXAMPLES 
The catalyst compositions specified in the examples are given in relative 
atom ratios. 
Catalyst Preparation 
Catalyst (I) 
A catalyst comprising the elements in the following composition (in 
combination with oxygen) was prepared: 
EQU Mo.sub.1.00 Re.sub.0.67 V.sub.0.70 Nb.sub.0.19 Sb.sub.0.08 Ca.sub.0.05 
Pd.sub.0.01 
Solution 1 
10.0 g of ammonium perrhenate, 0.12 g of palladium acetate and 9.7 g of 
ammonium molybdate in 50 ml of water. 
Solution 2 
4.5 g of ammonium metavanadate in 50 ml of water. 
Solution 3 
6.5 g of niobium oxalate, 1.34 g of antimony oxalate, 0.58 g of calcium 
nitrate in 180 ml of water. 
The solutions are stirred separately at 70.degree. C. for 15 minutes. The 
third solution is then added to the second. The combined mixtures are 
stirred at 70.degree. C. for 15 minutes before they are added to the 
first. The resulting mixture is stirred at 70.degree. C. for 15 minutes. 
The water is subsequently removed on a hot plate until a thick paste is 
formed. This is dried at 120.degree. C. overnight. The solid is crushed 
(sieve fraction: 0.35-2 mm) and subsequently calcined in static air at 
300.degree. C. for 5 hours. The catalyst is then sieved in order to obtain 
a sieve fraction between 0.35 and 1 mm. 
Catalyst (II) 
A catalyst comprising the elements in the following composition (in 
combination with oxygen) was prepared: 
EQU Mo.sub.1.00 Re.sub.0.67 V.sub.0.70 Nb.sub.0.19 Sb.sub.0.08 Ca.sub.0.05 
Pd.sub.0.02 
The preparation was carried out as described in Catalyst Example (I) except 
that 0.24 g instead of 0.12 g of palladium acetate was used. 
Comparative Example 
Catalyst (III) 
For comparison, a catalyst corresponding to EP 0 407 091 and having the 
following composition was prepared: 
EQU Mo.sub.1.00 Re.sub.0.67 V.sub.0.70 Nb.sub.0.19 Sb.sub.0.08 Ca.sub.0.05 
The preparation was carried out as described in Catalyst Example (I) except 
that no palladium acetate was used. 
The conversion of 14.3% reported in EP-B-0 407 091, Table 2, cannot be 
achieved for stoichiometric reasons even with complete conversion of the 
oxygen. At the selectivities indicated and the composition of the feed 
gas, the conversion can be at most 5.9%. In this calculation, it was 
assumed that only carbon monoxide is formed in addition to acetic acid and 
ethylene. If carbon dioxide is formed instead of carbon monoxide, the 
maximum achievable ethane conversion is only 5.5%. It may be assumed that, 
owing to the experimental procedure, ethane was condensed in the cold trap 
located downstream of the reactor, which led to the incorrect calculation 
of an excessively high conversion. To compare the catalytic properties of 
this catalyst with the catalyst of the invention, both catalysts were 
tested under identical reaction conditions (see comparative example). 
Method of Catalyst Testing 
A steel reactor having an internal diameter of 10 mm was charged with 10 ml 
of the catalyst. The catalyst was heated to 250.degree. C. under a stream 
of air. The pressure was subsequently set by means of an admission 
pressure regulator. The desired ethane: oxygen: nitrogen mixture was 
metered together with water into a vaporizer zone where water was 
vaporized and mixed with the gases. The reaction temperature was measured 
using a thermocouple in the catalyst bed. The reaction gas was analyzed 
on-line by gas chromatography. 
In the examples, the following terms are defined as: 
ethane conversion (%)=100.times.([CO]/2+[CO.sub.2 ]/2+[C.sub.2 H.sub.4 
]+[CH.sub.3 COOH])/([CO]/2+[CO.sub.2 ]/2+[C.sub.2 H.sub.4 ]+[C.sub.2 
H.sub.6 ]+[CH.sub.3 COOH]) 
Ethylene selectivity (%)=100.times.([C.sub.2 H.sub.4 ])/([CO]/.sub.2 
+[CO.sub.2 ]/2+[C.sub.2 H.sub.4 ]+[CH.sub.3 COOH]) 
Acetic acid selectivity (%)=100.times.([CH.sub.3 COOH])/([CO]/.sub.2 
+[CO.sub.2 ]/2+[C.sub.2 H.sub.4 ]+[CH.sub.3 COOH]) 
where 
[]=concentrations in mol % and 
[C.sub.2 H.sub.6 ]=concentration of the unreacted ethane. 
The residence time is defined as: 
t(s)=bed volume of the catalyst (ml)/volume flow of the gas through the 
reactor based on the reaction conditions (ml/s). 
Reaction Procedure 
The reaction was carried out at 280.degree. C. and 15 bar. The feed gas to 
the reactor consisted of 40% by volume of ethane, 8% by volume of oxygen, 
32% by volume of nitrogen and 20% by volume of water vapor. The results 
are summarized in the following table. 
______________________________________ 
Residence 
Ethane Acetic acid 
Ethylene 
CO + CO.sub.2 
time conversion selectivity selectivity selectivity 
Catalyst (s) (%) (%) (%) (%) 
______________________________________ 
(I) 30 3 91 0 9 
(II) 30 4 91 0 9 
(II) 60 8 90 2 8 
(III) 30 5 61 29 10 
______________________________________ 
Compared to Catalyst (III), Catalysts (I) and (II) give higher 
selectivities to acetic acid without the CO+CO.sub.2 selectivities being 
increased. This leads to an improved acetic acid yield based on the amount 
of catalyst used and the ethane feed stream.