Process and catalyst for preparing acetic acid by catalytic...

Details Process and catalyst for preparing acetic acid by catalytic... Process and catalyst for preparing acetic acid by catalytic...

1999-11-10

2001-08-14

Killos, Paul J. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

C562S548000, C562S607000, C502S305000

Reexamination Certificate

active

06274765

ABSTRACT:

The present invention relates to a process for the selective preparation of acetic acid by catalytic gas-phase oxidation of ethane and/or of ethylene in the presence of a tungsten-containing catalyst, and to the catalyst.
The oxidative dehydrogenation of ethane to ethylene in the gas phase at temperatures >500° C. is disclosed, for example in U.S. Pat. Nos. 4,250,346, 4,524,236 and 4,568,790. Thus, U.S. Pat. No. 4,250,346 discloses the use of a catalyst composition which contains the elements molybdenum, X and Y in the ratio a:b:c for converting ethane into ethylene, in which 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 0.05 to 1 and c is 0 to 2. The total value of c for Co, Ni and/or Fe must in this case be less than 0.5. The reaction is preferably carried out in the presence of added water. The disclosed catalysts may likewise be used for oxidizing ethane to acetic acid, in which case the efficiency of the conversion to acetic acid is about 18%, with an ethane conversion of 7.5%.
EP-A-0 294 845 discloses a process for the selective preparation of acetic acid from ethane, ethylene or mixtures thereof with oxygen in the presence of a catalyst mixture containing
A.) a calcined catalyst of the formula Mo
x
V
y
or Mo
x
V
y
Z
y
in which Z can be 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, x is 0.5 to 0.9, y is 0.1 to 0.4 and z is 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. The maximum selectivity which can be achieved is 27% with an ethane conversion of 7%. The high ethane conversion rates are, according to EP-A-0 294 845, achieved only with the catalyst mixture described, but not with a single catalyst containing the components A and B.
EP-A-0 407 091 discloses a process for preparing a mixture of ethylene and/or acetic acid. In this case, ethane and/or ethylene and a gas containing molecular oxygen are brought into contact at elevated temperature with a catalyst composition which contains the elements A, X and Y. A in this case is Mo/Re/W, 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 it was possible to achieve on use of the described catalyst for the oxidation of ethane to acetic acid are 78%. Other byproducts formed are carbon dioxide, carbon monoxide and ethylene.
Said publications disclose catalysts which comprise molybdenum as main component. Catalysts which comprise molydenum are, however, disadvantageous because, under the prevailing reaction conditions, molybdenum forms volatile molybdenum compounds which lead to a decrease in the activity and selectivity of the catalyst.
None of the publications listed above discloses the use of a catalyst which comprises tungsten and a noble metal for the selective oxidation of ethane and/or ethylene to acetic acid. Furthermore, the selectivities achieved for this oxidation to date in the prior art are unsatisfactory.
The object therefore was to provide a process with which ethane and/or ethylene can be oxidized in a simple manner, specifically and with high selectivity under reaction conditions which are as mild as possible to acetic acid.
It has been found, surprisingly, that it is possible on use of a catalyst which comprises tungsten in combination with a noble metal (for example Pd, Pt, Ag and Au) and one or more elements from the group of vanadium, niobium, tantalum to oxidize ethane and/or ethylene under relatively mild conditions in a simple manner with high selectivity to acetic acid. Tungsten oxide is far less volatile than molybdenum oxide. Thus, the catalysts according to the invention which comprise tungsten in place of molybdenum prove to be stable in respect of their activity and selectivity over a long period.
The present invention thus relates to a process for the selective preparation of acetic acid from a gaseous feedstock of ethane, ethylene or mixtures thereof, and oxygen or oxygen-containing gases, at elevated temperature on a tungsten-containing catalyst which comprises the elements W, X, Y and Z in the gram-atom ratios a:b:c:d in combination with oxygen
W
a
X
b
Y
c
Z
d
  (I)
in which
X is one or more elements selected from the group of Pd, Pt, Ag and/or Au,
Y is one or more elements selected from the group of V, Nb, Cr, Mn, Fe, Sn, Sb, Cu, Zn, U, Ni and/or Bi,
Z is one or more elements selected from the group of Li, Na, K, Rb, Cs, Be, Mg, Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf, Ru, Os, Co, Rh, Ir, B, Al, Ga, In, Tl, Si, Ge, Pb, P, As and/or Te,
a is 1,
b is a number greater than 0,
c is a number greater than 0, and
d is a number from 0 to 2.
X is preferably Pd, Y is preferably V, Nb, Sb and/or Cu, and Z is preferably K, Ca, Si and/or P.
Where X, Y and Z are several different elements, the indices b, c and d can likewise assume several different values.
The present invention further relates to a catalyst for the selective preparation of acetic acid comprising the elements W, X, Y and Z in the gram-atom ratios a:b:c:d in combination with oxygen.
The stoichiometric indices b, c and d are preferably:
b 0.0001 to 0.5;
c 0.1 to 1.0, and
d 0.001 to 1.0.
Values of b above the preferred range may lead to a favoring of the formation of carbon dioxide in the process according to the invention. By contrast, with contents below the stated preferred range there is observed to be a preference for the formation of ethylene. The preferred values for b additionally permit the invention to be carried out particularly economically.
In another preferred embodiment, the catalyst according to the invention comprises, apart from the elements tungsten and palladium, also vanadium, niobium and/or antimony and calcium in combination with oxygen. The gram-atom ratios a:b:c
1
:c
2
:c
3
:d
1
of the elements W:Pd:V:Nb:Sb:Ca are preferably as follows:
a(W)=1;
b(Pd)=0.0001 to 0.5, in particular 0.0002 to 0.05;
c
1
(V)=0.1 to 1.0;
c
2
(Nb)=0.1 to 0.5;
c
3
(Sb)=0 to 0.5;
d
1
(Ca)=0 to 0.2.
Examples of catalysts which are particularly preferably employed in the process according to the invention are:
W
1.00
Pd
0.0005
V
0.50
Nb
0.12
W
1.00
Pd
0.0005
V
0.75
Nb
0.20
W
1.00
Pd
0.0004
V
0.50
Nb
0.20
Cu
0.10
P
0.05
W
1.00
Pd
0.0005
V
0.50
Nb
0.12
Sb
0.10
Ca
0.02
W
1.00
Pd
0.0004
Au
0.0001
V
0.75
Nb
0.25
Te
0.002
W
1.00
Pd
0.0005
Ag
0.0001
V
0.75
Nb
0.12
Si
0.01
The catalysts according to the invention can be prepared by processes described in the prior art. These start from a suspension, in particular an aqueous solution, which comprises the individual starting components of the elements appropriate for their proportions.
The starting materials of the individual components for preparing the catalyst according to the invention are, besides the oxides, preferably water-soluble substances such as ammonium salts, nitrates, sulfates, halides, hydroxides and salts of organic acids which can be converted by heating into the corresponding oxides. To mix the components, aqueous solutions or suspensions of the metal compounds are prepared and mixed.
Advisable starting materials for tungsten are, because of the commercial availability, the corresponding tungstates such as, for example, ammonium tungstate.
The resulting reaction mixture is then stirred at 50 to 100° C. for 5 minutes to 5 hours. The water is subsequently removed, and the remaining catalyst is dried at a temperature of 50 to 150° C., in particular 80 to 120° C.
In the case where the resulting catalyst is subsequently subjected to a calcination process, it is advisable to calcine the dried and powdered catalyst at a temperature in the range from 100° C. to 800° C., in particular 200 to 500° C., in the presence of nitrogen, oxygen or an oxygen-containing gas. The duration of the calcination is prefer

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