LAYERED DEHYDROGENATION CATALYST PARTICLES

Abstract: A novel hydrocarbon conversion catalyst system comprises a catalyst particle containing a platinum group metal component, a modifier metal component selected from tin, germanium, rhenium and mixtures thereof, an optional alkali metal or alkaline earth metal component, and an optional halogen component, on a solid refractory oxide support having an equivalent nominal diameter of at least 850 microns. The platinum group metal component and the modifier metal component are surface-impregnated so that the average concentration of the surface impregnated platinum group component and the modifier metal component in the outside 100 micron layer of the particle are each at least twice the concentration of the respective components in the 200 micron diameter centre core of the catalyst particle. The novel catalyst system has particular utility as a hydrocarbon dehydrogenation catalyst.

Description: This invention relates to the conversion of hydrocarbons, especially the dehydrogenation of dehydrogenatable hydrocarbons, in the presence of particles of a catalyst composite having a novel layered construction. This invention also pertains to a new layered catalyst particle and a method of making it. The dehydrogenation of hydrocarbons is an important commercial process because of the great demand for dehydrogenated hydrocarbons for the manufacture of various chemical products, such as detergents, high octane gasolines, pharmaceutical products, plastics, synthetic rubbers, and other products well known to those skilled in the art. One example of this process is dehydrogenating isobutane to produce isobutylene, which can be polymerized to provide tackifying agents for adhesives, viscosity-index additives for motor oils and impact-resistant and anti-oxidant additives for plastics. Numerous catalyst composites comprising a platinum group component and a modifier metal component selected from tin, germanium, rhenium, and mixtures thereof, are known. For example US-A-3632503, -3755481, and -3878131 disclose catalysts comprising a platinum group component, a tin component, and a germanium component on a porous carrier material. These references disclose that best results are achieved when the germanium component is uniformly distributed throughout the carrier material, and are silent as to any advantage to be achieved by limiting the location of substantially all of the catalyst metals to an outer layer of the catalyst particle. In fact, it is indicated in these references that it is preferred that the catalyst composite thereof be subjected to a substantially water-free reduction step before its use in the conversion of hydrocarbons. It is further disclosed that the step is designed to ensure a uniform and fine divided dispersion of metal components throughout the carrier material. US-A-3682838 and -3761531 disclose catalyst composites comprising a platinum group component, and a Group IVA metal component. As with the previous references, it is disclosed therein that it is preferred that the catalysts are subjected to a reduction step before use. It is further disclosed in these references that the reduction step is designed to assure uniform distribution of the metallic particles throughout the catalysts. US-A-3558477, -3562147, -3584060, and -3649566 all disclose catalyst composites comprising a platinum group component, and a rhenium component on a refractory oxide support. As before, however, these references disclose that the best results are achieved when the platinum group component and rhenium component are uniformly distributed throughout the surface and pores of the catalyst. US-A-3897368 describes a method for the production of a noble metal catalyst where the noble metal is platinum and the platinum is deposited selectively upon the external surface of the catalyst. This disclosure describes the advantages of impregnating only platinum on the exterior layer and utilizes a specific type of surfactant to achieve the surface impregnation of the noble metal. Surprisingly, it has now been discovered that by incorporating into the outer layer of the catalyst support substantially all of both a platinum group metal component and a metal component selected from tin and germanium, a catalyst displaying improved performance in a dehydrogenation process is obtained. The present invention is concerned with the problems of providing an improved catalyst composite and a method of making the same. A further problem is that of providing an improved process for the conversion of hydrocarbons, especially for the dehydrogenation of hydrocarbons. Accordingly, in a broad embodiment, the present invention provides a catalyst particle comprising a combination of catalytically effective amounts of a platinum group metal component; a modifier metal component selected from tin, germanium, and mixtures thereof; and an optional alkali metal or alkaline earth metal component, with a solid refractory oxide support having a nominal equivalent diameter of at least 850 micrometres (µm) wherein the platinum group and modifier metal components are impregnated upon the surface of the refractory oxide support in such a manner that the average concentration of these components in the outside 100 µm layer of the catalyst particle is at least twice the concentration of the respective components in the 200 µm diameter centre core of the catalyst particle. In yet another embodiment, the invention is a hydrocarbon conversion process comprising the steps of contacting a convertible hydrocarbon at hydrocarbon conversion conditions with a catalyst system comprising catalyst particles having the layered metal structure defined above. In the most preferred embodiment, the convertible hydrocarbon is a dehydrogenatable hydrocarbon, and the conversion conditions utilized are dehydrogenation conditions selected to produce the corresponding dehydrogenated hydrocarbon. The expression "surface-impregnated" is hereinafter used to mean that the respective metal component is impregnated upon the surface of the catalyst particles in the manner defined above. An essential feature of the present invention is that a platinum group metal component and a modifier metal component selected from tin, germanium, and mixtures thereof, are surface-impregnated upon said support material having a nominal equivalent diameter of at least 850 µm with substantially all of the platinum group component and modifier metal located within at most a 400 µm exterior layer of the catalyst support. Put another way, it is a characteristic feature of the instant invention that the platinum group component and the modifier metal component, are surface-impregnated into the solid support particle in such a manner that the average concentrations thereof in the outside 100 µm layer of the support are at least twice the respective concentrations of these components in the 200 µm diameter centre core of the support particles. It is to be understood that the term "exterior" is defined as the outermost layer of the catalyst particle. By "layer", it is meant a stratum of substantially uniform thickness. The term "nominal equivalent diameter" means, for particles having a shape other than spherical, the smallest diameter of the particle, that is to say, the minimum thickness of the particle measured along a straight line passing through the centre of the particle. A metal component is considered to be surface-impregnated when its average concentration within the 100 µm exterior layer of the catalyst particle is at least twice the average concentration of the same metal component in the 200 µm diameter centre core of the particle. In addition to the platinum group component, and optionally the modifier metal component, being surface-impregnated, it is also an aspect of this invention that substantially all of the platinum group component, and modifier metal component, when it is surface-impregnated, are located within at most the 400 µm exterior layer of the catalyst support. "Substantially all", means at least 75% of the surface-impregnated component in question. This characterization of the catalyst composite is intended to describe a platinum group metal concentration, and an optionally surface-impregnated modifier metal concentration, gradient upon and within the catalyst support. The concentration of the platinum group component, or optionally surface-impregnated modifier component, within the 100 µm layer exterior surface of the support is as stated at least twice that of the platinum group component or the modifier metal component concentration within the 200 µm diameter centre core of the catalyst. The surface-impregnated metals concentration then decreases as the centre of the support is approached. The actual gradient of the platinum group metal, or the optionally surface-impregnated modifier metal component, within the catalyst support varies depending upon the equivalent nominal diameter of the support and the exact manufacturing method employed to fabricate the catalyst. Therefore, distribution of the platinum group metal component, is best defined as being both surface-impregnated and substantially all located within at most the 400 µm exterior layer of the support where "substantially all" means at least 75% of the component. As previously stated, the support material has a nominal equivalent diameter of at least 850 µm. For a catalyst support material which approaches this diameter, the exterior layer wherein 75% of the surface-impregnated components are located will approach 100 µm. The exterior layer wherein 75% of the surface-impregnated components are located will approach a maximum value of 400 µm as the equivalent diameter of the catalyst support increases beyond 2000 µm. Although it is not understood completely, it is believed that by restricting substantially all of the surface-impregnated components to at most a 400 µm wide exterior layer of the catalyst support, more facile access to these catalytic sites is achieved, allowing the hydrocarbon reactants and products much shorter diffusion paths. By decreasing the length of the diffusion paths, the reactants and products have a shorter residence time in the catalyst particle, thereby lessening the likelihood of undesirable side reactions due to secondary product. For example, in the dehydrogenation of a paraffin to a monoolefin, reducing the length of the diffusion path decreases the chances of dehydrogenating the desired monoolefin consecutively to undesirable skeletal isomers, cracked products, and aromatics which could result from readsorption onto a catalytic site before the desired monoolefin can leave the catalyst particle. Another essential feature of the present invention is the presence of a modifier metal component in the catalyst system. This component may be tin, germanium, rhenium, and mixtures thereof. As heretofore indicated, the catalytically effective amount of the modifier metal component is preferably surface-impregnated in the same manner as the platinum group component. When the modifier metal component is substantially all located within at most a 400 µm wide exterior layer of the catalyst support particle. Generally the catalyst will comprise from 0.01 to 10 wt. % of modifier metal component, calculated on an elemental basis on the weight of the final composite. Preferably, the catalyst will comprise from 0.1 to 5 wt. % of the modifier metal component. When the modifier metal component of the present invention comprises germanium, all the germanium may be present in the catalyst in an oxidation state above that of the elemental metal. This component may exist within the composite as a compound, such as the oxide, sulphide, halide, oxychloride, aluminate, etc., or in combination with the carrier material or other ingredients of the composite. Preferably, the germanium component is used in an amount sufficient to result in the final catalytic composite containing, on an elemental basis, 0.01 to 10 wt. % of germanium, with best results typically obtained with 0.1 to 5 wt. % of germanium. Alternatively and preferably, the modifier metal component may comprise a tin component. This component may be present as the elemental metal, as a chemical compound, such as the oxide, sulphide, halide, oxychloride, etc., or as a physical or chemical combination with the porous carrier material and/or other components of the catalytic composite. The tin component is preferably utilized in an amount sufficient to result in a final catalyst composite containing 0.01 to 10 wt. % of tin, calculated on an elemental basis, with best results obtained at a level of 0.1 to 5 wt. %. Regarding the preferred amounts of the various modifier metal components of the catalyst, according to the invention, it has been found it to be a good practice to specify the amounts of the germanium component, and the tin component as a function of the amount of the platinum group component. On this basis, the amount of the tin component is ordinarily selected to produce a composite containing an atomic ratio of the platinum group metal to tin of 0.1:1 to 5:1, with the preferred range being 0.5:1 to 3:1. The amount of the germanium component is ordinarily selected to produce a composite containing an atomic ratio of germanium to platinum group metal of 0.25:1 to 5:1. The platinum group metal and modifier metal components may be incorporated into the catalytic composite of the invention by any means suitable to result in surface-impregnation of the platinum group metal, and the surface-impregnation of the modifier metal components, wherein substantially all of the surface-impregnated modifier metal components may be located within at most a 400 µm wide exterior layer of the catalyst support particle. A preferred method of surface-impregnating the platinum group component, and optionally the modifier metal component, is by means of a low acid impregnation, utilizing a solution of a soluble, decomposable complex compound of the platinum group component and the desired modifier metal component. In general, the solvent used in this impregnation step is selected on the basis of its ability to dissolve the desired decomposable complex compound and is a low acid, preferably aqueous solution. "Low acid" means that the impregnation solution generally has a normality of 2 or less. An HCl solution is preferred. Nitric acid and the like also can be used. Typical of some of the water soluble, decomposable platinum group compounds which may be employed in preparing the catalyst of the invention are chloroplatinic acid, ammonium chloroplatinate, bromoplatinic acid, platinum dichloride, platinum tetrachloride hydrate, platinum dichlorocarbonyl dichloride, dinitrodiaminoplatinum, palladium chloride, palladium chloride dihydrate, palladium nitrate. Chloroplatinic acid is preferred as a source of platinum. Germanium compounds which may be used include germanium oxide, germanium tetraethoxide, germanium tetrapropoxide, germanium tetrachloride, germanium difluoride, germanium tetrapropoxide, germanium diiodide, germanium monosulphide. One particularly preferred impregnation solution comprises nascent germanium metal dissolved in chlorine water to yield germanium oxychloride. A second preferred impregnation solution comprises germanium tetrachloride dissolved in anhydrous ethanol. Suitable tin salts or water-soluble compounds of tin which may be used include stannous bromide, stannous chloride, stannic chloride, stannic chloride pentahydrate, stannic chloride tetrahydrate, stannic chloride trihydrate, stannic chloride diamine, stannic trichloride bromide, stannic chromate, stannous fluoride, stannic fluoride, stannic iodide, stannic sulphate, stannic tartrate. The use of a tin chloride compound, such as stannous or stannic chloride is particularly preferred. The platinum group component and modifier metal component may be composited with the support in any sequence. Thus, the platinum group component may be surface impregnated on the support followed by sequential surface impregnation of one or more modifier metal components. Alternatively, the modifier metal component or components may be surface impregnated on the support followed by surface impregnation with the platinum group component. It is also contemplated that the platinum group component and modifier metal component may be simultaneously surface-impregnated into a solid refractory oxide support having a nominal equivalent diameter of at least 850 µm. As indicated above, the present invention involves use of a catalyst composite containing an optional alkali metal component, or an optional alkaline earth metal component, or mixtures thereof. More specifically, this component is selected from the group consisting of the compounds of the alkali metals (casium, rubidium, potassium, sodium, and lithium) and of the alkaline earth metals (calcium, strontium, barium, magnesium and beryllium). This component may exist within the catalytic composite as a relatively stable compound, such as the oxide or sulphide, or in combination with one or more of the other components of the composite, or in combination with one or more of the other components of the composite, or in combination with an alumina carrier material, such as in the form of a metal aluminate. Since, as is explained hereinafter, the composite containing the alkali metal or alkaline earth metal component is always calcined in an air atmosphere before use in the conversion of hydrocarbons, the most likely state of this component during use in dehydrogenation is the metallic oxide. Regardless of the precise form in which it exists in the composite, the amount of this component is preferably selected to provide a composite containing 0.01 to 10 wt. % of the alkali metal or alkaline earth metal, and more preferably 0.05 to 5 wt. %. The alkali metal or alkaline earth component is preferably but not necessarily uniformly distributed throughout the catalyst particle. Best results are ordinarily achieved when this component is a compound of lithium, potassium, caesium, or mixtures thereof. This alkali metal or alkaline earth metal component may be combined with the porous carrier material in any manner known to those skilled in the art, such as by impregnation, coprecipitation, physical admixture, ion exchange, etc. The preferred procedure involves impregnation of the carrier material, either before or after it is calcined, and either before, during, or after the other components are added to the carrier material. Best results are ordinarily obtained when this component is added after the platinum group component and modifier metal component, because it serves to neutralize the acid used in the preferred impregnation procedure for incorporation of these components. Typically, the impregnation of the carrier material is performed by contacting it with a solution of a suitable, decomposable compound or salt of the desired alkali metal or alkaline earth metal. Hence, suitable compounds include the halides, sulphates, nitrates, acetates, carbonates, and the like compounds. For example, excellent results are obtained by impregnating the carrier material, after the platinum group component has been combined therewith, with an aqueous solution of lithium nitrate or potassium nitrate. The catalytic composite of the present invention may also optionally contain a halogen component. The halogen component may be either fluoride, chloride, bromide or iodine or mixtures thereof. Chlorine and bromine are the preferred halogen components. The halogen component is generally present in a combined state with the porous carrier material and alkali component. Although not essential to the invention, the halogen component is preferably well dispersed throughout the catalyst composite. The halogen component may comprise from 0.2 to 15 wt. %, calculated on an elemental basis, of the final catalyst composite. The halogen component may be incorporated in the catalyst composite in any suitable manner, either during the preparation of the carrier material, or before, while or after other catalytic components are incorporated. For example, the alumina sol used to form the preferred alumina carrier material may contain halogen, and this contribute at least some of the halogen content in the final catalyst composite. Also, the halogen component or a portion thereof may be added to the catalyst composite during the incorporation of other catalyst components into the carrier material, for example, by using chloroplatinic acid in impregnating a platinum component. Also, the halogen component or a portion thereof may be added to the catalyst composite by contacting the catalyst with the halogen, or with a compound, solution, suspension or dispersion containing the halogen before or after other catalyst components are combined with the carrier material. Suitable compounds containing the halogen include acids containing the halogen, for example, hydrochloric acid. Alternatively, the halogen component or a portion thereof may be incorporated by contacting the catalyst with a compound, solution, suspension or dispersion containing the halogen in a subsequent catalyst-regeneration step. Optionally, the catalyst of the present invention may also contain a sulphur component. Generally, the sulphur component may comprise 0.01 to 2 wt. %, calculated on an elemental basis, of the final catalyst composite. The sulphur component may be incorporated into the catalyst composite in any suitable manner. Preferably, sulphur or a compound containing sulphur such as hydrogen sulphide or a lower molecular weight mercaptan, for example, is contacted with the catalyst composite in the presence of hydrogen at a hydrogen to sulphur molar ratio of about 100 and a temperature of from 10 to 540°C, preferably under water-free conditions, to incorporate the sulphur component. Optionally, the catalyst may also contain other, additional components or mixtures thereof which act alone or in concert as catalyst modifiers to improve catalyst activity, selectivity or stability. The catalyst modifiers are preferably, but not necessarily, dispersed uniformly throughout the catalyst particle. Some well-known catalyst modifiers include antimony, arsenic, bismuth, cadmium, chromium, cobalt, copper, gallium, gold, indium, iron, manganese, nickel, scandium, silver, tantalum, thallium, titanium, tungsten, uranium, zinc, and zirconium. A thallium, gallium or indium component is preferred. These additional components may be added in any suitable manner to the carrier material during or after its preparation, or they may be added in any suitable manner to the catalyst composite either before, while or after other catalytic components are incorporated. Preferably, the catalyst of the present invention is nonacidic. "Nonacidic" in this context means that the catalyst has very little skeletal isomerization activity, that is, the catalyst converts less than 10 mole % of butene-1 into isobutylene when tested under dehydrogenation conditions and, preferably, converts less than 1 mole %. The acidity of the catalyst can be decreased, if necessary, to make the catalyst nonacidic by increasing the amount of the alkali component within the claimed range, or by treating the catalyst with steam to remove some of the halogen component. After the catalyst components have been combined with the porous carrier material, the resulting catalyst composite will generally be dried at a temperature of from 100 to 320°C for a period of typically 1 to 24 hours, or more, and thereafter calcined at a temperature of 320 to 600°C for a period of 0.5 to 10 hours or more. Finally, the calcined catalyst composite is typically subjected to a reduction step before use in the hydrocarbon conversion process. This reduction step can be effected at a temperature of 100 to 650°C for a period of 0.5 to 10 hours, or more, in a reducing environment, preferably dry hydrogen, the temperature and time being selected to be sufficient to reduce substantially all of the platinum group component to the elemental metallic state. Regarding the solid refractory oxide support or carrier material, it is preferably a porous, adsorptive support with high surface area of from 5 to 500 m²/g. The porous carrier material should be relatively refractory under the conditions utilized in the hydrocarbon conversion process. It is intended to include within the scope of the instant invention the use of carrier materials which have traditionally been utilized in hydrocarbon conversion catalysts, for example (1) activated carbon, coke, or charcoal; (2) silica or silica gel, silicon carbide, clays, and silicates, including synthetically prepared and naturally occurring ones, which may or may not be acid treated, for example, attapulgus clay, china clay, diatomaceous earth, fuller's earth, kaolin, kieselguhr; (3) ceramics, porcelain, crushed firebrick, bauxite; (4) refractory inorganic oxides such as alumina, titanium dioxide, zirconium dioxide, chromium oxide, beryllium oxide, vanadium oxide, cerium oxide, hafnium oxide, zinc oxide, magnesia, boria, thoria, silica-alumina, silica-magnesia, chromia-alumina, alumina-boria, silica-zirconia; (5) crystalline zeolitic aluminosilicates, such as naturally occurring or synthetically prepared mordenite and/or faujasite, for example, either in the hydrogen form or in a form which has been exchanged with metal cations, (6) spinels such as MgAl₂O₄, Feal₂O₄, ZnAl₂O₄, CaAl₂O, and other like compounds having the formula MO-Al₂O₃, where M is a metal having a valence of 2; and (7) combinations of materials from one or more of these groups. The preferred carrier material for the instant catalyst is alumina, especially gamma- or eta-alumina. As indicated above, the solid oxide support employed according to the present invention has a nominal equivalent diameter of at least 850 µm. Nominal equivalent diameter means that the particle must be no less than 850 µm thick at its narrowest diameter. Thus if the particle is spherical, it must have a diameter of at least 850 µm, while if it is cylindrical (as might be in the case of an extrudate), the diameter of the circular cross section must be at least 850 µm and the length of the cylinder must be at least 850 µm. Likewise if the particle shape of the extrudate is a cube or elongated box, no side or height may be less than 850 µm. As set forth above, it is believed that by limiting the surface-impregnated platinum group component and the modifier metal component to at most a 400 µm exterior layer, regardless of equivalent nominal diameter of the support, diffusion paths are minimized and selectivity to the desired products is maximized. It would thus seem logical to decrease the diffusion paths further by reducing the nominal equivalent diameter of the support particle to less than 850 µm. However, a reduction in particle size leads to an increase in pressure drop through the catalyst bed and an increase in the problems associated therewith. Accordingly, the limiting of substantially all of the surface-impregnated catalytic species to at most the 400 µm exterior layer, while limiting the catalyst particle to a nominal equivalent diameter of at least 850 µm, and preferably about 1500 µm, enables the minimization of diffusion paths while maintaining commercially practicable catalyst bed pressure drop and catalyst loading. The preferred alumina carrier material may be prepared in any suitable manner from synthetic or naturally occurring raw materials. The carrier may be formed in any desired shape such as spheres, pills, cakes, extrudates. A preferred shape of alumina is the sphere. It is essential that the particle size be such that it has a nominal diameter of at least 850 µm, though particles having a nominal diameter of about 1600 µm are preferred. To make alumina spheres, aluminium metal is converted into an alumina sol by dissolving it in a suitable acid, and then dropping a mixture of the resulting sol and a gelling agent, such as urea, into an oil bath to form spherical particles of an alumina gel which are easily converted into the preferred gamma- or eta-alumina carrier material by known methods including aging, drying and calcining. To make alumina cylindars, an alumina powder is mixed with water and enough of a suitable peptizing agent such as nitric acid, until an extrudable dough is formed. The dough is then extruded through a suitably sized die and cut to form extrudate particles. Other shapes of the alumina carrier material may also be prepared by conventional methods. After the alumina particles are shaped, generally they are dried and calcined. The alumina carrier may be subjected to intermediate treatments during its preparation, including washing with water or a solution of ammonium hydroxide, for example, which treatments are well known in the art. As indicated above, the catalyst of the present invention has particular utility as a hydrocarbon conversion catalyst. The hydrocarbon which is to be converted is contacted with the catalyst at hydrocarbon conversion conditions. These conditions include a temperature of from 200° to 1000°C, a pressure of from atmospheric to 25 atmospheres gauge (101 to 2634 kPa) and liquid hourly space velocities of from 0.1 to 200 hr⁻¹. According to one embodiment of the invention, dehydrogenatable hydrocarbons are contacted with the catalyst composite of the present invention in a dehydrogenation zone maintained under dehydrogenation conditions. This contacting may be accomplished in a fixed catalyst bed system, a moving catalyst bed system, a fluidized bed system, etc., or in a batch-type operation. A fixed bed system is preferred. In this fixed bed system, the hydrocarbon feed stream is preheated to the desired reaction temperature and then passed into the dehydrogenation zone containing a fixed bed of the catalyst. The dehydrogenation zone may itself comprise one or more separate reaction zones, with heating means therebetween to ensure that the desired reaction temperature can be maintained at the entrance to each reaction zone. The hydrocarbon may be contacted with the catalyst bed in either upward, downward or radial flow fashion. Radial flow of the hydrocarbon through the catalyst bed is preferred for commercial scale reactors. The hydrocarbon may be in the liquid phase, a mixed vapour-liquid phase or the vapour phase, when it contacts the catalyst. Preferably, it is in the vapour phase. Hydrocarbons which may be dehydrogenated include dehydrogenatable hydrocarbons having from 2 to 30, or even more, carbon atoms including paraffins, alkylaromatics, naphthenes and olefins. One group of hydrocarbons which can be dehydrogenated with the catalyst is the group of normal paraffins having from 2 to 30, or more, carbon atoms. The catalyst is particularly useful for dehydrogenating paraffins having from 2 to 15, or more, carbon atoms to the corresponding monoolefins, or for dehydrogenating monoolefins having from 3 to 15, or more, carbon atoms to the corresponding diolefins. Dehydrogenation conditions include a temperature of from 400 to 900°C, a pressure of from 0.01 to 10 atmospheres (1 to 1013 kPa) and a liquid hourly space velocity (LHSV) of from 0.1 to 100 hr⁻¹. Generally, for normal paraffins the lower the molecular weight the higher the temperature required for comparable conversion. The pressure in the dehydrogenation zone is maintained as low as practicable, consistent with equipment limitations, to maximize the chemical equilibrium advantages. The effluent stream from the dehydrogenation zone generally will contain unconverted dehydrogenatable hydrocarbons, hydrogen, and the products of dehydrogenation reactions. This effluent stream is typically cooled and passed to a hydrogen separation zone to separate a hydrogen-rich vapour phase from a hydrogen-rich liquid phase. Generally, the hydrocarbon-rich liquid phase is further separated by means of either a suitable selective adsorbent, a selective solvent, a selective reaction or reactions, or by means of a suitable fractionation scheme. Unconverted dehydrogenatable hydrocarbons are recovered and may be recycled to the dehydrogenation zone. Products of the dehydrogenation reactions are recovered as final products, or as intermediate products in the preparation of other compounds. The dehydrogenatable hydrocarbons may be mixed with a diluent material before, while or after being passed to the dehydrogenation zone. The diluent material may be hydrogen, steam, methane, ethane, carbon dioxide, nitrogen, argon and the like, or a mixture thereof. Hydrogen is the preferred diluent. Ordinarily, when hydrogen is utilized as the diluent it is utilized in amounts sufficient to ensure a hydrogen to hydrocarbon mole ratio of 0.1:1 to 40:1, with best results being obtained when the mole ratio is 1:1 to 10:1. The diluent hydrogen stream passed to the dehydrogenation zone will typically be recycled hydrogen, separated from the effluent from the dehydrogenation zone in the hydrogen separation zone. Water, or a material which decomposes at dehydrogenation conditions to form water, such as an alcohol, aldehyde, ether or ketone, for example, may be added to the dehydrogenation zone, either continuously or intermittently, in an amount to provide, calculated on the basis of equivalent water, 1 to 20,000 weight ppm of the hydrocarbon feed stream. 1 to 10,000 weight ppm of water addition gives best results when dehydrogenating paraffins having from 6 to 30, or more, carbon atoms. To be commercially successful, a dehydrogenation catalyst should exhibit three characteristics, namely high activity, high selectivity and good stability. Activity is a measure of the ability of the catalyst to convert reactants into products at a specific set of reaction conditions, that is, at a specified temperature, pressure, contact time and concentration of diluent such as hydrogen, if any. For dehydrogenation catalyst activity, the percentage conversion or disappearance of paraffins relative to the amount of paraffins in the feedstock was measured. Selectivity is a measure of the ability of the catalyst to convert reactants into the desired product or products relative to the amount of reactants converted. For catalyst selectivity, the amount of olefins in the product, in mole percent, relative to the total moles of the paraffins converted was measured. Stability is a measure of the rate of change with time on stream of the activity and selectivity parameters - smaller rates implying more stable catalysts. Since dehydrogenation of hydrocarbons is an endothermic reaction, and conversion levels are limited by chemical equilibrium, it is desirable, in order to achieve high conversion, to operate at high temperatures and low hydrogen partial pressures. At such severe conditions, it is difficult to maintain high activity and selectivity for long periods of time, because undesirable side reactions such as aromatization, cracking, isomerization and coke-formation increase. Therefore, it is advantageous to have a new hydrocarbon dehydrogenation catalyst with improved activity, selectivity and stability characteristics. The following examples are introduced to further describe the catalyst and process of the invention. Results obtained in the following Examples are shown in the accompanying Drawings, in which: Figures 1 and 2 are graphical representations of the performance in the paraffin dehydrogenation process of Catalyst A, different from the invention, and Catalyst B, in accordance with the invention. Figure 1 is a graph of the normal paraffin conversion in weight percent as a function of hours on stream of the test. Figure 2 is a plot of the selectivities of the catalysts in weight percent for producing normal olefins as a function of the normal paraffin conversion in weight percent. Figures 3, 4, and 5 are graphical representations of the platinum group metal and modifier metal gradient across the radius of catalysts of Examples 2, 4, and 5, respectively. The metals distribution is presented as a ratio of a special metal concentration to the alumina concentration, plotted against the distance in µm from the surface of the particle. The distributions were determined by scanning electron microscopy (SEM). It should be noted that the plots do not represent actual metal content, but only the relative concentration of the metal across the radius of a catalyst pill. EXAMPLE 1 (Control) In order to demonstrate the advantages to be achieved by the present invention, a first control catalyst, different from the invention, was prepared. Firstly, a spherical alumina support was prepared by the well-known oil-drop method. A tin component was incorporated in the support by mixing a tin component precursor with the alumina hydrosol and thereafter gelling the hydrosol. The tin component in this control example was uniformly distributed throughout the catalyst particle. The spherical alumina support containing a tin component had a diameter of about 1500 µm. The tin-containing support was then impregnated with an aqueous solution comprising H₂PtCl₆+LiNO₃ and 3.4% of HNO₃. After impregnation, the catalyst was oven-dried at 150°C for 2 hours and calcined at 540°C for 2.5 hours. Thereafter the catalyst was reduced with hydrogen for 2 hours at 500°C. This catalyst was designated Catalyst A, and comprised 0.38% of platinum, 0.5% of tin, and 0.6% of lithium, uniformly distributed throughout the catalyst particle. EXAMPLE 2 In this example, a second catalyst was prepared in accordance with the invention. 250 cm³ of a spherical alumina support having a diameter of about 1500 µm was impregnated with an aqueous solution of a Pt(II)-Sn(II) complex prepared by combining H₂PtCl₆+SnCl₂ and 1% of HCl. After impregnation, the catalyst was oven-dried at 150°C for 2 hours and calcined at 540°C for 2.5 hours. Thereafter the catalyst was impregnated with an aqueous solution of LiNO₃. The catalyst was then again oven-dried at 150°C for 2 hours and calcined at 540°C for a further 2.5 hours. The calcined catalyst was then reduced in hydrogen for 2 hours at a temperature of 500°C. This catalyst, made in accordance with the invention, was designated Catalyst B. Catalyst B had a final composition of 0.38% of platinum, 0.4% of tin, and 0.6% of lithium. EXAMPLE 3 In this example, Catalysts A and B were evaluated as catalysts for the dehydrogenation of normal paraffins. These tests were carried out in a pilot plant comprising a reactor and product separation facilities. The charge stock was passed into the reaction zone wherein it was contacted with the catalyst. The effluent from the reaction zone was thereafter separated and analyzed. The charge stock comprised on a normalized basis, 11.8 wt. % of normal C₁₀ paraffins, 26.7 wt. % of normal C₁₁ paraffins, 34 wt. % of normal C₁₂ paraffins and 27.5 wt. % of normal C₁₃ paraffins. The reaction zone was maintained at a pressure of 20 psig (138 kPa gauge). The charge stock was passed to the reaction zone at a rate sufficient to produce a liquid hourly space velocity of 70 hr⁻¹. Hydrogen diluent was fed to the reaction zone at a rate sufficient to provide a hydrogen to hydrocarbon molar ratio of 4:1. The feedstock was heated to a temperature of 495°C before contact with the catalyst. The water content of the reaction zone was maintained at 2000 ppm by weight. The results of these tests are set forth in Figures 1 and 2. Figure 1 is a graphical depiction of the normal paraffin conversion in weight percent as a function of the number of hours on stream. The normal paraffin conversion is defined as the weight of the components in the fresh feed which actually underwent some reaction, divided by the total weight of the feed. From Figure 1, it can be seen that Catalysts A and B exhibit substantially the same normal paraffin conversion. Figure 2 is a graphical depiction of the total normal olefin (TNO) selectivity in weight percent, as a function of the normal paraffin conversion in weight percent. The total normal olefin selectivity in weight percent is defined as the weight of charge stock components converted into the desired normal olefin product, divided by the total number of charge stock components undergoing some reaction. A review of the data depicted in Figure 2 shows that Catalyst B, according to the invention, showed higher selectivity for the production of the desired normal olefins than did Catalyst A, different from the invention. It is believed that the higher selectivity exhibited by the catalyst according to the invention is a direct result of the platinum and tin being surface-impregnated and also is a result of substantially all of the platinum and tin components being located within at most a 400 µm wide exterior layer of the catalyst particle. With this distribution of the catalytic metal components, it is believed that the reactant and product diffusion paths are minimized, while allowing commercially practicable pressure drops through the reaction zone. EXAMPLE 4 The catalyst of this example was prepared according to the method set forth in US-A-3584060. This catalyst is different from that of the present invention. To produce the catalyst of this example, a spherical alumina support having a diameter of approximately 1500 µm was prepared by the oil drop method. An impregnation solution was next prepared by dissolving a small amount of stannic chloride in nitric acid. To this solution were added small amounts of a 3.26 wt. % Re solution of perrhenic acid, and a 2.543 wt. % Pt solution of chloroplatinic acid. Water was added to bring the solution weight to 100 grams. The solution was combined with the spherical alumina support mentioned above. The impregnated spheres were dried at 150°C for 2 hours. The dried spheres were next calcined at 540°C for 2 hours in air with 10 wt. % of steam added followed by calcination for 0.5 hour at 540°C under an air atmosphere only. This impregnation and finishing procedure resulted in a catalyst composite comprising the following levels of catalytically active metals; 0.388 wt. % of platinum, 0.423 wt. % of rhenium, and 0.366 wt. % of tin. This catalyst will hereinafter be referred to as Catalyst C. EXAMPLE 5 In this example, a catalyst was prepared according to the example set forth in US-A-3878131. This catalyst is not according to the present invention. A spherical alumina support approximately 1500 µm in diameter was prepared by the oil drop method. An impregnation solution was prepared by dissolving a small amount of stannous chloride in hydrochloric acid and diluting the solution further with water. Germanium tetrachloride dissolved in ethanol was next added to the solution, followed by the addition to the solution of a small amount of chloroplatinic acid. The resulting impregnation solution was combined with the catalyst base mentioned above. The impregnated base was next dried for 2 hours at 150°C. The dried, impregnated base was then calcined for 2 hours at 540°C under an air atmosphere with 10 wt. % of steam added, followed by a 0.5 hour calcination under the same conditions without steam addition. The resulting catalyst composite comprised the following amounts of catalytically active metals: 0.389 wt. % of platinum, 0.263 wt. % of tin; and 0.245 wt. % of germanium. This catalyst will hereinafter be referred to as Catalyst D. EXAMPLE 6 In this example, Catalyst B, according to the present invention, and Catalysts C and D, according to the prior art were evaluated by Scanning Electron Microscopy (SEM). The purpose of this analysis was to identify the relative distribution of impregnated metals across the radius of a catalyst pill in each of the catalysts. Before proceeding, it should be noted that the SEM data does not show the actual level of metals at any one point within the catalyst pill, but indicates only the relative metals-distribution profile in relation to the alumina level. In other words, the results of each analysis may not be based upon a zero point, and attempting to integrate a distribution curve is not possible and could lead to interpretation errors as the entire curve could be shifted either up or down. Therefore the curves are useful for making relative comparisons of metal distributions only. As examination of the platinum levels across the catalyst pills of Catalysts B, C, and D, as indicated in Figures 3, 4, and 5 respectively, shows the platinum distribution of Catalyst B of the instant invention to be contained essentially within the exterior layer of the catalyst particle. The platinum level begins at a (Pt K-ratio/Al K-ratio)*1000 value of 11 to 12 on the surface of the particle and drops to a value of 3 at a distance of 100 µm from the surface of the catalyst support. Catalyst B can therefore be characterized as being surface-impregnated with platinum, as the concentration of the platinum in the 100 µm surface layer is over twice the relative platinum concentration in the 200 µm centre core of the catalyst support. This is in contrast to Catalyst C of the prior art, which exhibits a constant (pt K-ratio/Al K-ratio)*1000 value of 10 across the entire radius of the catalyst pill, as seen from Figure 4, and Catalyst D of the prior art as seen in Figure 5, with a constant value of about 12 across the catalyst pill. These two catalysts therefore can be described as being uniformly impregnated with platinum. The tin component of the catalysts according to the present invention is also surface-impregnated. The tin level as seen from Figure 3 begins at a (Ti K-ratio/Al K-ratio)*1000 value of greater than 40, and drops to a value of almost zero at a distance of 100 µm from the surface of the catalyst particle. The tin component of Catalyst C of the prior art, as seen from Figure 4 is also surface impregnated. As shown earlier however, the platinum component is not surface-impregnated, and therefore is different from the catalyst of the instant invention. Finally, the tin component distribution of Catalyst D of the prior art is uniformly impregnated, as shown in Figure 5.

Claims: A particulate catalyst comprising a combination of catalytically effective amounts of platinum group metal component and a modifier metal component selected from tin, germanium, and mixtures thereof, with a solid refractory oxide support characterized in that said solid support has a nominal equivalent diameter of at least 850 micrometres, and the platinum group metal and modifier metal components are impregnated on the surface of the catalyst particles, the average concentration of the platinum group component and modifier metal component in the outside 100 micrometre layer of the catalyst particle are each at least twice the concentration of the respective component in the 200 micrometre diameter centre core of the catalyst particle. A catalyst according to Claim 1 characterized in that it also contains a catalytically effective amount of an alkali metal or alkaline earth metal component. A catalyst according the Claims 2 characterized in that the alkali metal component is a lithium component. A catalyst according to any one of Claims 1 to 3 characterized in that it also contains a catalytically effective amount of a halogen component. A catalyst according to claim 4 characterized in that the halogen is chlorine. A catalyst according to any one of claims 1 to 5 characterized in that it additionally contains a sulphur component. A catalyst according to any one of Claims 1 to 5 characterized in that the solid support has a nominal equivalent diameter of 1500 micrometres. A hydrocarbon conversion process comprising the steps of contacting a convertible hydrocarbon at hydrocarbon conversion conditions with a catalyst system, characterized in that the catalyst is as defined in any one of Claims 1 to 7. A hydrocarbon conversion process according to Claim 8 characterized in that the convertible hydrocarbon is a dehydrogenatible hydrocarbon and the hydrocarbon conversion conditions are dehydrogenation conditions.

Inventors: IMAI, TAMOTSU
Applicants: UOP (A NEW YORK GENERAL PARTNERSHIP)
Application year: 1987
Main classification: B01J23
IPC codes: B01J35, B01J23, C10G45