Shell catalyst for producing acetic acid by gas phase...

Details Shell catalyst for producing acetic acid by gas phase... Shell catalyst for producing acetic acid by gas phase...

1999-05-14

2001-08-14

Killos, Paul J. (Department: 1623)

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

Organic compounds

Carboxylic acids and salts thereof

C562S607000, C502S303000, C502S304000, C502S349000, C502S353000

Reexamination Certificate

active

06274763

ABSTRACT:

The invention relates to a coated catalyst for preparing acetic acid by gas-phase oxidation of unsaturated C
4
-hydrocarbons, and also a process for preparing acetic acid by gas-phase oxidation of unsaturated C
4
-hydrocarbons using the coated catalyst.
It is known that acetic acid can be prepared by gas-phase oxidation of C
2
-, C
3
- and C
4
-molecules by means of a catalyst. However, up to now it has not been possible to find a process which is economical and fully satisfactory in terms of operability.
DE-B 1279011 describes a process for preparing acetic acid by catalytic gas-phase oxidation of butene with oxygen using aluminium vanadate and titanium vanadate catalysts. These catalysts are prepared by precipitating the mixed oxides from the corresponding solutions, with the mixed oxides being able to be mixed, if desired, with inert materials such as silica. The catalyst is used as a finely divided powder in fluidized-bed reactors. A disadvantage of such uniform-composition catalysts is the high degree of total oxidation.
To improve the yield of such catalysts, DE-A 2016681 proposes pretreating the catalysts with an oxidizing agent prior to calcination. DE-A 2354425 (U.S. Pat. No. 3,954,857) proposes treating the calcined titanium-vanadium mixed catalyst with hydrochloric acid to improve the selectivity. The catalysts are used as uniform-composition catalysts, if desired in admixture with inert support materials such as silica.
A further starting point known from the prior art for improving the activity of titanium-vanadium mixed catalysts in the gas-phase oxidation of butenes to acetic acid is the use of TiO
2
in a defined crystal form or with a defined surface area. DE-A 2026744 (U.S. Pat. No. 3,917,682) describes Ti-V mixed catalysts whose TiO
2
component is predominantly present as rutile. The catalysts can be used in powder form or after pressing into shaped bodies. U.S. Pat. No. 4,448,897 discloses Ti-V catalysts for butene oxidation which contain TiO
2
having a BET surface area of greater than 40 m
2
/g. The catalysts are likewise used in powder form or as pressed compacts.
It is also known from the prior art that the selectivity of Ti-V catalysts in butene oxidation can be improved by completely or partially replacing the titanium dioxide by other metal oxides. DE-A 2110876 (GB-A 1333306), for example, describes catalysts which contain oxides of molybdenum, tin and vanadium as active components. The catalysts are used in powder form and the mixed oxide catalyst can, if desired, also be applied to finely divided support materials such as silicon dioxide. U.S. Pat. No. 4,146,734 discloses the use of vanadium mixed oxides which are doped with cerium and further transition metal oxides. The catalyst is used as a finely divided granular material, but can also be applied as precipitate to finely divided, inert supports.
DE-A 2235103 discloses Ti-V mixed oxide catalysts for the gas-phase oxidation of butenes in the form of supported catalysts prepared by impregnating a preformed porous support with the mixed solution of the catalyst components.
In all these processes, the catalysts used are uniform-composition catalysts in which the active components themselves are used as powder or compacts, or diluted with finely divided support materials as powder or compacts. Porous supports impregnated throughout with active component as described in DE-A 2235103 are also to be regarded as uniform-composition catalysts since here too the entire catalyst volume is catalytically active. Disadvantages of uniform-composition catalysts are the high degree of total oxidation and the difficulty of controlling the butene oxidation at high conversions.
It is therefore an object of the invention to provide a catalyst and a process for preparing acetic acid by gas-phase oxidation of unsaturated C
4
-hydrocarbons, which catalyst and process lead to a better yield and better operability in the oxidation reaction.
It has been found that coated catalysts in which the active composition is applied as a thin layer to a nonporous support body are particularly suitable for preparing acetic acid by gas-phase oxidation of unsaturated hydrocarbons having four carbon atoms.
The invention provides a coated catalyst for preparing acetic acid by gas-phase oxidation of unsaturated C
4
-hydrocarbons which consists of an inert nonporous support body and a catalytically active mixed oxide composition which is applied to the outer surface of the support body and contains
a) one or more oxides selected from the group consisting of titanium dioxide, zirconium dioxide, tin dioxide, aluminium oxide and
b) from 0.1 to 1.5% by weight, based on the weight of the component a) and per m
2
/g of specific surface area of the component a), of vanadium pentoxide.
Here, % by weight based on the weight of the component a) and per m
2
/g of specific surface area of the component a) means that the proportion by weight of the component b) to be used depends on the specific surface area of the component a). Thus, for example, at a specific surface area of the component a) of 10 m
2
/g the proportion of the component b) is from 1 to 15% by weight, based on the weight of the component a).
TiO
2
is suitable in both the rutile and anatase forms and mixtures thereof. As component a), preference is given to titanium dioxide having a BET surface area of from 20 to 400 m
2
/g, preferably from 70 to 300 m
2
/g. If mixtures of titanium dioxide with zirconium dioxide or tin dioxide are used, from 5 to 95% by weight, preferably from 5 to 50% by weight, of the titanium dioxide can be replaced by zirconium dioxide, aluminium oxide or tin dioxide.
As additional component a), one or more oxides of the metals selected from the group consisting of hafnium, niobium, tungsten, lanthanum and cerium may also be present. If the component a) is doped with the oxides mentioned, they are generally present in an amount of from 1 to 15% by weight, based on the total weight of the component a).
The proportion of the component b) is preferably from 0.1 to 0.5% by weight, particularly preferably from 0.1 to 0.2% by weight, in each case based on the weight of the component a) and per m
2
/g of specific surface area of the component a).
In the component b), part of the vanadium pentoxide, preferably from 10 to 90%, can, if desired, be replaced by one or more oxides of molybdenum, chromium and antimony. If desired, one or more oxides of alkali metals, elements of main groups V and VI of the Periodic Table of the Elements and the transition metals may also be present as additional component b). Examples are the oxides of lithium, sodium, potassium, rubidium, caesium, phosphorus, bismuth, sulphur, selenium, tellurium, manganese, iron, cobalt, palladium, copper, silver, gold, zinc and cadmium. In general, the amount of these dopants is from 0.05 to 15% by weight, calculated as oxides and based on the total weight of the component b). The proportion of alkali metal oxides and noble metal oxides is preferably from 0.05 to 1.0% by weight.
Preference is given to compositions having a high surface area of the component a) of from 70 to 300 m
2
/g, in which, if desired, tin oxide or tungsten oxide may also be present, and containing a component b) which is doped with Mo, Cr, Sb and/or Au.
The catalytically active mixed oxide composition may, if desired, additionally contain from 10 to 50% by weight, based on the total weight of the catalytically active mixed oxide composition, of inert diluents such as silicon dioxide and silicon carbide.
The catalytically active mixed oxide composition is applied as a shell to the outer surface of the support body in an amount of from 1 to 40% by weight, preferably from 5 to 25% by weight, in each case based on the total weight of support body and active composition. The thickness of the layer is generally from 10 to 1000 &mgr;m, preferably from 100 to 500 &mgr;m. The coated catalyst can also contain a plurality of layers which differ in composition. One or more constituents of the active components a) and b) can also be present in dif

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