Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-02-26
2003-07-01
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S209000, C502S344000, C502S350000
Reexamination Certificate
active
06586361
ABSTRACT:
The present invention relates to a coated catalyst for the catalytic gas-phase oxidation of aromatic hydrocarbons, comprising, on an inert nonporous support, a catalytically active composition comprising, in each case based on the total amount of catalytically active composition, from 1 to 40% by weight of vanadium oxide, calculated as V
2
O
5
, from 60 to 99% by weight of titanium dioxide, calculated as TiO
2
, up to 1% by weight of a cesium compound, calculated as Cs, up to 1% by weight of a phosphorus compound, calculated as P, and up to a total of 10% by weight of antimony oxide, calculated as Sb
2
O
3
. In addition, it relates to a production process for these catalysts and to a process using these catalysts for preparing carboxylic acids and/or anhydrides and especially phthalic anhydride.
It is known that many 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 multitube reactors. These processes are used to obtain, for example, benzoic acid, maleic anhydride, phthalic anhydride isophthalic acid, terephthalic acid or pyromellitic anhydride. The usual procedure is to pass a mixture of a gas comprising molecular oxygen, for example air, and the starting material to be oxidized through a plurality of tubes arranged in a reactor, with a bed of at least one catalyst being present in each tube. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt. Despite this thermostatting, it is possible for hotspots in which the temperature is higher than in the remainder of the catalyst bed to occur. These hotspots give rise to secondary reactions such as the total combustion of the starting material or lead to formation of undesirable by-products which can be separated from the reaction product only with difficulty, if at all, for example the formation of phthalide or benzoic acid in the preparation of phthalic anhydride (PA) from o-xylene. Furthermore, the formation of a pronounced hotspot prevents a rapid running-up of the reactor to the reaction temperature of the reaction since the catalyst can be irreversibly damaged above a certain hotspot temperature, so that the loading can be increased only in small steps and has to be monitored very carefully.
To reduce this hotspot, it has become customary in industry to arrange catalysts having different activities in zones in the catalyst bed, with the less active catalyst generally being arranged in the fixed bed so that the reaction gas mixture comes into contact with it first, i.e. it is located toward the gas inlet end of the bed, while the more active catalyst is located toward the gas outlet end of the catalyst bed. The catalysts of differing activity in the catalyst bed can be exposed to the reaction gas at the same temperature, but the two zones of catalysts having differing activities can also be thermostatted to different reaction temperatures for contact with the reaction gas (DE-A 40 13 051).
Catalysts which have proven useful for these oxidation reactions are coated catalysts in which the catalytically 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 a mixture of these support materials. Catalytically active constituents of the catalytically active composition of these coated catalysts are generally titanium dioxide in the form of its anatase modification plus vanadium pentoxide. In addition, the catalytically active composition may further comprise small amounts of many other oxidic compounds which, as promoters, influence the activity and selectivity of the catalyst, for example by lowering or increasing its activity. Examples of such promoters are the alkali metal oxides, in particular lithium, potassium, rubidium and cesium oxides, 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. Promoters which reduce the activity and increase the selectivity are, for example, the alkali metal oxides, while oxidic phosphorus compounds, in particular phosphorus pentoxide, increase the activity of the catalyst but reduce its selectivity.
According to the processes of DE-A 16 42 938 and DE-A 17 69 998, such coated catalysts are produced by spraying an aqueous and/or organic solvent-containing solution or suspension of the constituents of the active composition and/or their precursor compounds, which is hereinafter referred to as a “slurry”, onto the support material in a heated coating drum at elevated temperature until the amount of active composition as a proportion of the total weight of the catalyst has reached the desired value. According to DE 21 06 796, the coating procedure can also be carried out in fluidized-bed coaters as are described, for example, in DE 1280756. However, spraying in a coating drum and coating in a fluidized bed result in high losses since considerable amounts of the slurry are converted into a mist or parts of the active composition which has already been applied are rubbed off again by abrasion and are carried out by the waste gas. Since the proportion of active composition in the total catalyst should generally have only a small deviation from the prescribed value because the amount of active composition applied and the thickness of the shell strongly influence the activity and selectivity of the catalyst, the production methods indicated require the catalyst to be cooled, taken from the coating drum or the fluidized bed and weighed at frequent intervals to determine the amount of active composition applied. If too much active composition is deposited on the catalyst support, it is generally not possible to carry out a subsequent, careful removal of the excess active composition without adversely affecting the strength of the shell, in particular without crack formation in the catalyst shell.
To reduce these problems, it has become customary in industry to add organic binders, preferably copolymers, advantageously in the form of an aqueous dispersion, of vinyl acetate/vinyl laurate, vinyl acetate/acrylate, styrene/acrylate, vinyl acetate/maleate and vinyl acetate/ethylene, to the slurry. The amounts of binder used are 10-20% by weight, based on the solids content of the slurry (EP-A 744 214). If the slurry is applied to the support without using organic binders, coating temperatures above 150° C. are advantageous. When the abovementioned binders are added, the usable coating temperatures are, depending on the binder used, from 50 to 450° C. (DE 21 06 796). The binders applied burn off within a short time after introduction of the catalyst into the reactor and start-up of the reactor. The addition of binder has the additional advantage that the active composition adheres well to the support so that transport and charging of the catalyst are made easier.
Gas-phase oxidations over the abovementioned coated catalysts do not take place only on the outer surface of the shell. To achieve the catalyst activity and selectivity required for complete conversion of the high loadings of the reaction gas with starting material employed in industrial processes, it is necessary for the total active composition shell of the catalyst to be utilized efficiently and thus for the reaction centers located in this shell to be readily accessible to the reaction gas. Since the oxidation of aromatic compounds to give carboxylic acids and/or carboxylic anhydrides proceeds via many intermediates and the desired product can be further oxidized over the catalyst to form carbon d
Bauer Stefan
Heidemann Thomas
Linden Gerd
Petersen Hermann
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