Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-08-26
2001-09-11
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S412000, C562S888000, C502S104000, C502S113000, C502S240000
Reexamination Certificate
active
06288273
ABSTRACT:
The present invention relates to a process for producing coated catalysts for the catalytic gas-phase oxidation of aromatic hydrocarbons to give carboxylic acids and/or carboxylic anhydrides, on the support material of which a layer of catalytically active metal oxides is applied in the form of a shell, and also a process for the catalytic gas-phase oxidation of aromatic hydrocarbons to give carboxylic acids and/or carboxylic anhydrides using a gas comprising molecular oxygen in a fixed bed at elevated temperature and by means of one or more coated catalysts arranged in layers, where the support material of the coated catalysts has a layer of catalytically active metal oxides applied to it in the form of a shell.
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 tube-bundle reactors. For example, benzoic acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid or pyromellitic anhydride are obtained by such processes. For this purpose, in general, a mixture of a gas comprising molecular oxygen, for example air, and the starting material to be oxidized is passed through a multiplicity of tubes arranged in a reactor, in which tubes there is located a bed of. at least one catalyst. To regulate the temperature, the tubes are surrounded by a heat transfer medium, for example a salt melt. Despite this thermostatting, the formation of hot spots in which the temperature is higher than in the remaining part of the catalyst bed can occur. These hot spots give rise to secondary reactions such as the total combustion of the starting material or lead to the formation of undesired by-products which can be separated from the reaction product only with great difficulty, if at all, for example the formation of phthalimide or benzoic acid in the preparation of phthalic anhydride (PA) from o-xylene. Furthermore, the formation of a pronounced hot spot when running up a plant obstructs a prompt increase of the loading of the gas fed to the reactor with xylene, since above a certain hot spot temperature the catalyst can be irreversibly damaged. For this reason, the xylene loading of the gas feed when running up the reactor can be increased only slowly, in small steps, to the xylene loading intended in the process, and this stepwise increase in loading has to be monitored very carefully.
To reduce the extent of the secondary reactions caused by hot spots, industrial practice has changed to arranging different active catalysts in layers in the catalyst bed. In such an arrangement, the less active catalyst is generally arranged in the fixed bed such that the reaction gas mixture comes into contact with it first, ie. it is located on the gas inlet side of the bed, whereas the more active catalyst is located toward the gas outlet from the catalyst bed. The different active catalysts in the catalyst bed can be exposed to the reaction gas at the same temperature, but the two layers comprising different active catalysts can also be thermostatted to different reaction temperatures for contact with the reaction gas.
Catalysts which have been found to be 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, for example 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. The catalytically active constituent of the catalytically active composition of these coated catalysts is generally titanium dioxide in the form of its anatase modification plus vanadium pentoxide. In addition, the catalytically active composition can comprise small amounts of many other oxidic compounds which as promoters influence the activity and selectivity of the catalyst, for example by decreasing or increasing its activity. Examples of such promoters are 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. Promoters which act to reduce the activity and increase the selectivity are, for example, the alkali metal oxides, whereas oxidic phosphorus compounds, in particular phosphorus pentoxide, increase the activity of the catalyst but reduce its selectivity.
According to the processes of DE-A 1642938 and DE-A 1769998, such coated catalysts are produced by spraying an aqueous and/or organic solvent-containing solution or suspension of the active composition constituents and/or their precursor compounds, which is hereinafter referred to as the “mix”, 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 figure. However, spraying results in high losses since considerable amounts of the mix are atomized and carried from the coating drum by the off-gas. Since the proportion of active composition in the total catalyst should generally deviate only slightly from the desired value because the amount of active composition applied and the thickness of the shell strongly influence the activity and selectivity of the catalyst, in the production method indicated the catalyst frequently has to be cooled, taken from the coating drum and weighed to determine the amount of active composition applied. If too much active composition is deposited on the catalyst support, a subsequent, gentle removal of the excess catalyst applied is generally not possible without impairing the strength of the shell, in particular without crack formation in the catalyst shell.
Further catalyst parameters which are important for oxidation reactions, namely the macroscopic structure of the active composition, for example its porosity and pore radius distribution, and also the chemical composition within the shell of active composition, can be influenced only with great difficulty in the abovementioned processes. The method of sequential spraying of mixes having different chemical compositions, as described in DE-A 2212964, does not lead to defined chemical compositions within the shell of active composition since chromatographic effects which occur during application in the heated coating drum lead to uncontrollable formation of concentration gradients of the active substances in the catalyst shell which result in a nonuniform chemical composition of the active composition in the catalyst shell and in the individual catalyst particles.
Gas-phase oxidations over the above-described coated catalysts take place not only on the outer surface of the shell. To achieve the catalyst activity and selectivity required for the complete reaction of the high starting material loadings in the reaction gas which are employed in industrial processes, efficient utilization of the total active composition shell of the catalyst and thus good accessibility for the reaction gas to the reaction centers located in this shell is necessary. Since the oxidation of aromatic compounds to give carboxylic acids and/or carboxylic anhydrides proceeds via many intermediate stages and the desired product can be further oxidized over the catalyst to give carbon dioxide and water, achievement of a high conversion of starting material with simultaneous suppression of the oxidative degradation of the desired product requires optimal matching of the residence time of the reaction gas in the active composition by generating a suitable, macroscopic active composition structure in the catalyst shell.
Furtherm
Cimniak Thomas
Heidemann Thomas
Ulrich Bernhard
BASF - Aktiengesellschaft
Keil & Weinkauf
Killos Paul J.
LandOfFree
Method for producing shell catalysts for catalytic gas-phase... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for producing shell catalysts for catalytic gas-phase..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for producing shell catalysts for catalytic gas-phase... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2453019