Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
1999-08-26
2002-10-01
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C502S209000
Reexamination Certificate
active
06458970
ABSTRACT:
The present invention relates to a process for preparing phthalic anhydride by catalytic gas-phase oxidation of o-xylene or naphthalene or o-xylene
aphthalene mixtures using a gas comprising molecular oxygen and a coated catalyst comprising an inert, nonporous support material on which a catalytically active composition comprising titanium dioxide and vanadium pentoxide is applied in layer form, and also a catalyst for this purpose.
In the prior art, many catalysts have been proposed for preparing PA from o-xylene or naphthalene or o-xylene
aphthalene mixtures by oxidation with gases comprising molecular oxygen in the gas phase over a fixed-bed catalyst. The PA catalysts generally comprise an inert support material on which a thin layer of the catalytically active composition is applied in the form of a shell, which is why these catalysts are generally also referred to as coated catalysts.
The composition of the catalytically active composition plays a decisive role for the catalytic properties of these PA catalysts. Virtually all PA catalysts employed nowadays have a catalytically active composition comprising the components titanium dioxide, generally in the anatase modification, and vanadium pentoxide. Since PA catalysts whose catalytically active composition consists of these base components alone give economically unsatisfactory results in respect of conversion, yield and selectivity owing to secondary reactions such as the formation of phthalide of the formula
maleic anhydride, benzoic acid and citraconic anhydride of the formula
or total combustion and are also not satisfactory in respect of long-term activity and selectivity, the efforts made in the prior art have been directed at improving these catalysts in respect of their activity, selectivity, yield and the quality of the PA product produced therewith by doping the catalytically active composition with a wide variety of additives and ever more additives, ie. increasingly complicated catalyst formulations have been developed over time to solve these problems. Such additives are, for example, antimony, boron, cesium, calcium, cobalt, iron, potassium, lithium, molybdenum, sodium, niobium, phosphorus, rubidium, silver, thalium, bismuth, tungsten and tin.
In DE-A 24 36 009 and DE-A 24 21 406, PA is prepared using coated catalysts which comprise a steatite support and a catalytically active composition comprising from 60 to 99% by weight of titanium dioxide in the anatase modification, from 1 to 40% by weight of vanadium pentoxide and, based on the titanium dioxide, from 0.15 to 1.5% by weight of rubidium and/or cesium.
DE-A 25 10 994 relates to vanadium- and titanium-containing supported catalysts having a particular external shape and whose catalytically active composition can comprise from 70 to 99% by weight of titanium dioxide in the anatase modification having a specific internal surface area of from 5 to 20 m
2
/g, from 1 to 30% by weight of vanadium pentoxide and up to 5% by weight of other materials such as the oxides of the elements cesium, rubidium, thallium, phosphorus or antimony. No information is given as regards the precise content of these other materials, in particular their ratios to one another. The support material used is steatite (magnesium silicate).
DE-A 25 47 624 describes catalysts for preparing PA, whose catalytically active composition comprises from 60 to 99% by weight of titanium dioxide (anatase), from 1 to 40% by weight of vanadium pentoxide and from 0.1 to 10% by weight of rubidium and antimony in an atomic ratio Rb:Sb of from 1:2.5 to 1:30. The anatase used can, according to this document, have an internal surface area of from 5 to 50 m
2
/g, preferably from 5 to 20 m
2
/g; according to an example, anatase having an internal surface area of 11 m
2
/g is used.
EP-A 21 325 relates to coated catalysts for preparing PA, whose catalytically active composition comprises from 60 to 99% by weight of anatase, from 1 to 40% by weight of vanadium pentoxide and, based on the total amount of TiO
2
and V
2
O
5
, up to 2% by weight of phosphorus and up to 1.5% by weight of rubidium and/or cesium where the catalytically active composition is applied to the support in two layers, the inner layer of which contains from 0.2 to 2% by weight of phosphorus but no rubidium or cesium and the outer layer of which contains from 0 to 0.2% by weight of phosphorus and from 0.02 to 1.5% by weight of rubidium and/or cesium. The catalytically active composition of these catalysts can, apart from the constituents mentioned, contain small amounts, eg. up to 10% by weight, of an oxide of the metals niobium, tin, silicon, antimony, hafnium, molybdenum or tungsten. The titanium dioxide used for producing these catalysts has an internal surface area of from 5 to 30 m
2
/g. The support material used is steatite.
EP-A 286 448 relates to a process for preparing PA in which use is made of two types of catalyst in a combined catalyst bed, both of which have similar contents of titanium dioxide and vanadium pentoxide and differ essentially in that one catalyst additionally contains from 2 to 5% by weight of a cesium compound, in particular cesium sulfate, but no phosphorus, tin, antimony, bismuth, tungsten or molybdenum compounds and the second catalyst additionally contains from 0.1 to 3% by weight of a phosphorus, tin, antimony, bismuth, tungsten or molybdenum compound, but virtually no alkali metal.
EP-A 522 871, EP-A 539 878, EP-A 447 267, DE-A 29 48 163 and DE-A 30 45 624 all relate to catalysts for preparing PA which are composed of a porous support material, preferably silicon carbide, and a catalytically active composition comprising, apart from titanium dioxide and vanadium pentoxide, many further catalytically active elements such as phosphorus, alkali metals, antimony (in EP-A 522 871 pentavalent antimony), niobium and/or silver.
Despite progress in catalyst development, the catalysts and catalyst systems known and available at present for the preparation of phthalic anhydride still have a series of disadvantages. The initial PA yield achievable with a fresh catalyst is about 80 mol %, but even in the first year of operation a significant drop in yield has to be accepted. For quality reasons, the phthalic anhydride initially obtained as crude product has to be subjected to a chemical treatment before it can be processed by distillation to give a pure product of the quality required nowadays. Owing to their great sensitivity to temperature, pressure and loading fluctuations, reliable use of these catalysts in large-scale operation requires a high outlay for monitoring and control. A further disadvantage of these catalysts is that, owing to their incomplete o-xylene conversion and the formation of under-oxidized or over-oxidized by-products, they result in odor problems and o-xylene and benzene emissions which, for environmental reasons, make costly waste gas incineration necessary.
These disadvantages are increased at high o-xylene and/or naphthalene loadings of the feed gas stream, in particular at loadings of 80 g of o-xylene per standard m
3
of gas and more.
It is an object of the present invention to find a process for preparing PA which does not have the abovementioned disadvantages and to provide a suitable catalyst for this purpose. The process of the invention should, particularly in long-term operation using high o-xylene loadings, continue to give good yields of PA having a high degree of purity and thus make superfluous a chemical work-up of the crude PA to remove disadvantageous by-products which are virtually impossible to remove by distillation, eg. phthalide.
We have found that this object is achieved by a process for preparing phthalic anhydride by catalytic gas-phase oxidation of o-xylene or naphthalene or o-xylene
aphthalene mixtures with a gas comprising molecular oxygen over a coated catalyst comprising an inert, nonporous support material on which a tatalytically active composition comprising titanium dioxide and vanadium pentoxide is applied in layer form, wherein a catalyst whose catalyti
Hefele Gerhard
Kratzer Otto
Scheidmeir Walter
Ulrich Bernhard
BASF - Aktiengesellschaft
Keil & Weinkauf
Killos Paul J.
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