Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-11-30
2003-03-04
Parsa, J. (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S412000, C562S888000, C502S104000, C502S240000, C428S522000
Reexamination Certificate
active
06528683
ABSTRACT:
This application is a 371 of PCT/EP99/03828 claiming priority of Jun. 3, 1998.
Process for the preparation of coated catalysts for the catalytic gas-phase oxidation of aromatic hydrocarbons and catalysts obtainable in this way
The invention relates to a process for the preparation of coated catalysts for the catalytic gas-phase oxidation of aromatic hydrocarbons to carboxylic acids and/or carboxylic anhydrides, to the carrier material of which is applied a layer of catalytically active metal oxides in layer form using certain binders, to catalysts obtainable in this way, and to a process for the catalytic gas-phase oxidation of aromatic hydrocarbons to carboxylic acids and/or carboxylic anhydrides with a gas comprising molecular oxygen in a fixed bed using these catalysts.
As is known, a large number of carboxylic acids and/or carboxylic anhydrides are prepared industrially by the catalytic gas-phase oxidation of aromatic hydrocarbons, such as benzene, the xylenes, naphthalene, toluene or durene, in fixed bed reactors, preferably tube bundle reactors. In this way, for example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellithic [sic] anhydride are obtained. To do this, in general a mixture of a gas comprising molecular oxygen, for example air, and the starting material to be oxidized is passed through a plurality of tubes arranged in a reactor in which is located a packing of at least one catalyst. For temperature control, the tubes are surrounded by a heat exchanger medium, for example a salt melt. In spite of this thermostatting, the formation of so-called “hot spots” can occur in the catalyst packing, in which a higher temperature prevails than in the other part of the catalyst packing. These “hot spots” give rise to secondary reactions, such as the total combustion of the starting material or lead to the formation of undesirable secondary products which cannot be separated from the reaction product or can only be separated with great difficulty, for example to the formation of phthalide or benzoic acid, in the preparation of phthalic anhydride (PA) from o-xylene. In addition, the formation of a marked hot spot prevents rapid starting-up of the reactor since from a certain hot spot temperature the catalyst can be irreversibly damaged, so that the loading increase can only be carried out in small steps and has to be controlled very carefully.
For the reduction of this hot spot, a change was made in the art to arranging catalysts of differing activity in layers in the catalyst packing, where as a rule the less active catalyst is arranged in the fixed bed such that the reaction gas mixture comes into contact with it first, i.e. it is in the packing toward the gas inlet, whereas the more active catalyst is located toward the gas outlet of the catalyst packing. Thus either the catalysts of differing activity in the catalyst packing can be exposed to the reaction gas at the same temperature, or else the two layers of catalysts of differing activity can also be brought into contact with the reaction gas at different reaction temperatures under thermostatted conditions, as is described in DE-A 40 130 51.
Catalysts which have proven suitable are so-called coated catalysts, in which the catalytically active material is applied in layer form to a core of carrier material which is in general inert under the reaction conditions, such as 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 of these carrier materials. In general, in addition to titanium dioxide in the form of its anatase modification, vanadium pentoxide serves as the catalytically active constituent of the catalytically active material of these coated catalysts. In addition, a multiplicity of other oxidic compounds, which as promoters affect the activity and selectivity of the catalyst, for example in that they decrease or increase its activity, can be contained in small amounts in the catalytically active material. As promoters of this type, mention may be made by way of example of the alkali metal oxides, in particular lithium, potassium, rubidium and cesium oxide, thallium(I) oxide, aluminum oxide, zirconium oxide, iron oxide, nickel oxide, cobalt oxide, manganese oxide, tin oxide, silver oxide, copper oxide, chromium oxide, molybdenum oxide, tungsten oxide, iridium oxide, tantalum oxide, niobium oxide, arsenic oxide, antimony oxide, cerium oxide and phosphorus pentoxide. For example, the alkali metal oxides act as promoters which decrease the activity and increase the selectivity, whereas oxidic phosphorus compounds, in particular phosphorus pentoxide, increase the activity of the catalyst, but decrease its selectivity. These constituents are all known from the relevant technical literature.
For the preparation of coated catalysts of this type, for example, according to the process of DE-A 16 42 938 and DE-A 17 69 998 an aqueous solution or suspension, and/or solution or suspension comprising an organic solvent, of the active material constituents and/or of their precursor compounds, which in the following is referred to as a “mix”, is sprayed onto the carrier material in a heated coating pan drum at elevated temperature until the desired active material content in the catalyst total weight is achieved. According to DE 21 06 796, the coating can also be carried out in fluidized bed coaters, such as are described, for example, in DE 12 80 756. On spraying in the coating pan drum and on coating in the fluidized bed, however, high losses occur, since considerable amounts of the mix are atomized or, due to abrasion, parts of the already stratified active material are abraded again and carried away by the off gas. Since the active material content in the total catalyst should in general have only a slight difference from the required value, as activity and selectivity of the catalyst are strongly affected by the amount of active material applied and the layer thickness of the shell, for the determination of the amount of active material applied, the catalyst in the outlined preparation procedure must be frequently colled, removed from the coating pan drum or the fluidized bed and reweighed. If too much active material has been deposited on the catalyst support, in general a subsequent, careful removal of the excessively high amount of active material applied is not possible without an adverse effect on the stability of the shell, in particular without crack formation in the catalyst shell.
In order to decrease these problems, a change was made in the art to adding organic binders, preferably copolymers, advantageously in the form of an aqueous dispersion, of vinyl acetate/vinyl laurate, vinyl acetate/acrylate, vinyl acetate/maleate and vinyl acetate/ethylene to the mix, according to EP-A 07 442 14 amounts of binder of 10-20% by weight, based on the solids content of the mix, being employed. If the mix is applied to the support without organic binders, coating temperatures of over 150° C. are advantageous. In the case of addition of the binders indicated, according to DE 210 67 96 the coating temperatures utilizable, at 70-130° C., are markedly lower. The binders applied burn off within a short time after the filling of the catalyst and putting into operation of the reactor. The addition of binder moreover has the advantage that the active material adheres well to the support, so that transport and filling of the catalyst are facilitated.
During the combustion, however, a loosening of the adhesion of the active material layer on the support occurs. This is normally not critical, since the catalyst in the reactor tube is no longer exposed to strong mechanical stresses. In particular in the case of relatively large amounts of binder additive, however, it cannot be ruled out that the active material layer is loosened so much that it is slowly removed under reaction conditions by the gas mixture flowing through. This
Hefele Gerhard
Heidemann Thomas
Linden Gerd
Lorz Peter Michael
Rosowski Frank
BASF - Aktiengesellschaft
Keil & Weinkauf
Parsa J.
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