Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2000-11-15
2002-03-26
Higel, Floyd D. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
active
06362345
ABSTRACT:
The present invention relates to a process for preparing phthalic anhydride by catalytic gas-phase oxidation of o-xylene
aphthalene mixtures by molecular oxygen using 2 catalyst beds comprising coated catalysts which comprise cesium, where the cesium content of the catalyst of the second zone is less than 15% by weight of the cesium content of the first zone.
The gas-phase oxidation of o-xylene and/or naphthalene to give phthalic anhydride is well known and has been widely described in the literature. It is generally carried out by passing a mixture of a gas comprising molecular oxygen, for example air, and the starting material to be oxidized through a multiplicity of tubes arranged in a reactor, with a bed of at least one catalyst being located in the tubes. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt. Despite this thermostating, it is possible for “hot spots” in which the temperature is higher than in the remainder of the catalyst bed to occur. These hot spots give rise to secondary reactions such as the total combustion of the starting material or they lead to the formation of undesirable by-products which can be separated from the reaction product only with great difficulty, if at all, for example the formation of phthalide or naphthoquinone. To weaken these hot spots, it has already been proposed that catalysts of different activity be arranged in zones in the catalyst bed, with the less active catalyst generally being arranged toward the gas inlet end and the more active catalyst being arranged toward the gas outlet from the catalyst bed. The catalysts of different activity in the catalyst bed can be exposed to the reaction gas at the same temperature, but the two zones of catalysts of different activity can also be thermostated to different reaction temperatures for contact with the reaction gas.
Catalysts which have been proven useful for the preparation of phthalic anhydride are coated catalysts (see below).
The oxidation of naphthalene or o-xylene is, owing to the different reactivities of the starting materials, generally carried out using different catalysts, so that when o-xylene
aphthalene mixtures are employed, it is necessary to use specifically adapted-catalysts which are suitable both for the oxidation of o-xylene and the oxidation of naphthalene, as is described, for example, in BE 893521, EP 286448 and DE 2238067.
A specific solution to this problem is proposed in EP 539878, which states that, when using catalysts of the inlet stage in a bed height of 15-85%, based on the volume of the total catalyst bed, and a catalyst of the subsequent stage in a bed height of 85-15%, based on the volume of the total catalyst bed, in the direction of the gas outlet, in the form of superposed zones, very good results are achieved when, in the catalyst of the inlet stage, a catalyst substance is applied to an inactive support in a ratio in the range from 5 to 20 g/100 ml, where the catalytically active substance comprises from 1 to 20% by weight of V
2
O
5
and from 99 to 80% by weight of titanium dioxide of the anatase type having a specific surface area of from 10 to 60 m
2
/g, with incorporation of, based on 100 parts by weight of the total amount of the abovementioned two components, from 0.01 to 1% by weight of Nb
2
O
5
, from 0.2 to 1.2% by weight of P
2
O
5
, from 0.5 to 5% by weight of Sb
2
O
3
and from 0.3 to 1.2% by weight of at least one compound selected from the group consisting of oxides of potassium, cesium, rubidium and thallium, where in the catalyst of the second zone the amount of a compound selected from the group consisting of compounds of potassium, cesium, rubidium and thallium is from 17 to 63% by weight of the amount of the corresponding compound in the catalyst of the inlet stage. Outside this “window” (variation of the length ratio of the second catalyst to the total length within the range 85-15% and the alkali metal ratio within the range 17-63%), the reaction is, according to this patent, “difficult” or associated with relatively high yield losses.
It is an object of the present invention to remedy these deficiencies.
We have found that this object is achieved by using catalysts of virtually the above composition in which the alkali metal present is cesium and the cesium content of the second zone is less than 15% by weight of the cesium content of the first zone. Such catalysts have particularly advantageous properties compared to the above-described catalysts in the specified “window”.
Specifically, we have found an improved process for preparing phthalic anhydride by catalytic gas-phase oxidation of o-xylene
aphthalene mixtures by molecular oxygen using
a catalyst I in a first zone on the gas inlet side which makes up from 25 to 75 percent by volume of the total catalyst volume, comprising, in each case based on the catalytically active composition, from 1 to 10% by weight of vanadium oxide (calculated as V
2
O
5
), from 1 to 10% by weight of antimony oxide (calculated as Sb
2
O
3
) and from 80 to 98% by weight of titanium dioxide of the anatase type having a BET surface area of from 13 to 28 m
2
/g and also from 0.05 to 1% by weight of cesium (calculated as Cs) applied to a steatite support and
a catalyst II in a second zone which makes up the remaining 75 to 25 percent by volume of the total catalyst volume, comprising, in each case based on the catalytically active composition, from 1 to 10% by weight of vanadium oxide (calculated as V
2
O
5
), from 1 to 10% by weight of antimony oxide (calculated as Sb
2
O
3
) and from 80 to 98% by weight of titanium dioxide of the anatase type having a BET surface area (cf. J. Amer. Chem. Soc. 60 (1938), 309 et seq.) of from 13 to 28 m
2
/g and from 0.01 to 1% by weight of phosphorus oxide (calculated as P) and also from 0.01 to 0.2% by weight of cesium (calculated as Cs) applied to a steatite support, wherein the cesium content of the catalyst II is less than 15% by weight of the cesium content of the catalyst I and the catalyst I and the catalyst II have been prepared without addition of compounds of niobium.
The reduction of the cesium content of the catalyst II to less than 15% by weight of the cesium content of the catalyst I enables the product quality to be significantly increased by reducing the content of the undesirable by-products phthalide and naphthoquinone without significantly influencing the phthalic anhydride yield. This is especially pronounced at the relatively low bath temperatures which are necessary to achieve space velocities over the catalyst of more than 60 g/standard m
3
. This effect is particularly noticeable when the cesium content of the catalyst II is 10-13% by weight of the cesium content of catalyst I and the proportion of the total catalyst bed occupied by the zone of the catalyst II is 40-60%.
The catalysts used can, even without doping, further comprise small amounts of metal oxides such as those of niobium, tungsten and/or lead. These arise via possible impurities in the commercial anatase used and should thus not be regarded as doping of active compositions. Further impurities, in particular alkali metal impurities, are generally less than 0.01% by weight.
Catalysts which can be used in the process of the present invention are, within the limits defined in the claims, coated catalysts known per se, as are described in the specialist literature for preparing phthalic anhydride. The composition and production of such “standard catalysts” can be summarized as follows:
in general, catalysts which have found to be useful are coated catalysts in which the catalytically active composition (“active composition”) is applied in the form of a shell to a support material which is generally inert under the reaction conditions, e.g. quartz (SiO
2
), porcelain, magnesium oxide, tin dioxide, silicon carbide, rutile, alumina (Al
2
O
3
), aluminum silicate, magnesium silicate (steatite), zirconium silicate or cerium silicate or mixtures thereof. The catalytically active constituents of the catalytically active composi
Arnold Heiko
Hefele Gerhard
Heidemann Thomas
Wanjek Herbert
BASF - Aktiengesellschaft
Keil & Weinkauf
Small Andrea D'{umlaut over (S)}ouza
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