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Title: Catalyst and method of preparing same
Description: Description:
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention pertains to the field of catalytic compositions and methods for preparation and use thereof. More particularly, this invention pertains to a novel crystalline alumino-silicate zeolite catalyst and to a method of preparing such catalyst. 2. Description of the Prior Art One of the recent major advances in catalyst technology was the discovery that catalytic compositions possessing both high activity and selectivity as well as superior attrition resistance in hydrocarbon conversion processing could be obtained by dispersing a crystalline aluminosilicate zeolite in an inorganic oxide matrix. Such compositions have been described, e.g., in U.S. Pat. Nos. 3,140,249 and 3,140,253 of C. J. Plank and E. J. Rosinski. It has further been found that certain desirable properties of such catalysts, including stability and activity, could be improved by replacing the alkali metals contained in the zeolites with other metals, particularly those of the rare earth group, and also by various pretempering treatments, e.g., steaming and dry thermal calcining. SUMMARY OF THE INVENTION I have discovered a new catalytic composition for use in the catalytic cracking of hydrocarbon oils and a method for preparation and use thereof, which composition exhibits decreased carbon deposition during use. My catalyst comprises a composite of a crystalline aluminosilicate carrying rare earth metal cations (hereinafter sometimes referred to as a rare earth zeolite), dispersed in an inorganic oxide matrix, wherein at least 50 weight percent of the inorganic oxide is silica and/or alumina. The matrix preferably is made up of silica, silica-alumina, silica-zirconia, or silica-zirconia-alumina, desirably along with a weighting agent, preferably clay. The aluminosilicate may have previously been ion exchanged with rare earth cations. The composite contains rare earth introduced by impregnation in an amount from about 1 to 6 percent by weight expressed as RE.sub.2 O.sub.3. That is to say, foregoing 1 to 6 percent by weight of RE.sub.2 O.sub.3 is in excess of the maximum rare earth content which can be achieved by ion exchange alone. In accordance with one preferred aspect of my invention, the additional rare earth is incorporated in the composite by contacting the rare earth-exchanged composite with a rare earth salt solution, desirably removing excess salt solution, and drying. In accordance with another aspect of my invention, the additional rare earth is incorporated in the composite by treating the rare earth crystalline aluminosilicate with a solution of rare earth cations and then admixing the resulting rare earth-wetted crystalline aluminosilicate with the matrix. In accordance with another preferred aspect of my invention, after the rare earth impregnation, subsequent drying is carried out over a period of time of at least 5 minutes under such conditions as to reduce the water content to below 20 percent by weight at the completion of said drying, thereby attaining enhanced performance of the resultant catalyst. The composite catalysts of my invention exhibit superior selectivity and are particularly desirable because of their ability to crack hydrocarbons to relatively high yields of gasoline while having low coking tendencies (hereinafter sometimes referred to as "coke make"). This is of great value when dealing with "dirty" feedstocks, e.g., heavy gas oils and "recycle" stocks, which ordinarily give off appreciable coke yields when subjected to cracking. DESCRIPTION OF THE PREFERRED EMBODIMENTS The composite catalyst of my invention comprises crystalline aluminosilicate particles, having rare earth cations therein, these particles being contained in a porous inorganic oxide matrix, the matrix comprising an inorganic oxide, e.g., silica, silica-alumina, silica-zirconia, or silica-zirconia-alumina, desirably along with a weighting agent. As noted, the inorganic oxide should be made up of at least 50 percent be weight of silica and/or alumina. The inorganic oxide should have a pore volume of at least 0.4 cc per gram. The weighting agent employed as a component of the matrix should be present in such an amount as to yield a resulting catalytic composition having a packed density of at least 0.3 gram per cc. (It is to be understood that when reference is made herein to properties of the composite such as, e.g., packed density, or to properties of the silica, silica-alumina, silica-zirconia or silica-zirconia-alumina matrix such as, e.g., pore volume, these references are to the fresh catalyst composite, i.e., to the composite prior to its actual use in catalytic conversion, but subsequent to the removal of water therefrom, as by heating to a temperature of 1200.degree.F for three hours in substantially dry air.) Referring to the synthetic amorphous inorganic oxide, e.g., silica, silica-alumina, silica-zirconia or silica-zirconia-alumina of the catalyst matrix, as previously noted, such synthetic amorphous inorganic oxide should desirably have a pore volume of at least 0.4 cc per gram. In general, the higher the pore volume, the more desirable is the overall composite catalyst, of course, provided that the pore volume is not so high as to adversely affect the attrition resistance of the catalyst. Thus, the pore volume of the synthetic amorphous inorganic oxide should be at least about 0.4 cc per gram. Generally it is from about 0.6 to 1.5 cc per gram, with a more preferred range being from about 0.8 to 1.3 cc per gram. The most preferable pore volume range is from about 1 to 1.2 cc per gram. The matrix for my composite catalyst desirably also includes a weighting agent. Preferred weighting agents are clay and/or alumina. Representative clays are attapulgite, montmorillinite, hectorite and halloysite, with Kaolin being preferred. If alumina is employed, alpha alumina is preferred. Where a weighting agent is employed, the amount of agent employed desirable should be such that the final composite catalyst has packed density of at least 0.3 gram per cc. Generally the packed density of the composite catalyst will be from about 0.3 to 1 gram per cc, a more preferred range being from about 0.4 to 0.6 gram per cc. The mean particle size of the weighting agent which may be incorporated as one component of the matrix is desirably less than about 40 microns. Preferably the particle size is from about 0.1 to 20, and most preferably from about 2 to 10 microns. In the make up of the matrix, the relative proportions as between the synthetic amorphous inorganic oxide and weighting agent are advantageously from about 20 to 95% by weight of synthetic amorphous material and from about 5 to 80% by weight of weighting agent. A more preferred range is one wherein the synthetic amorphous inorganic oxide is from about 50 to 70 weight percent of the matrix and the weighting agent is from about 30 to 50 weight percent of the matrix. Crystalline aluminosilicate particles are dispersed in the foregoing matrix, generally in such quantity that the overall composite contains from about 1 to 80 percent by weight of such crystalline aluminosilicate particles. Preferably, the composite will contain from about 2 to 25 percent by weight of crystalline alumino-silicate particles, the most preferred range being from about 5 to 15 percent by weight. One means of preparing my composite catalyst involves dispersing the particulate weighting agent in a liquid medium, preferably water, to form a dispersion. Advantageously the concentration of weighting agent in the dispersion is from about 0.5 to 10 percent by weight, and most preferably from about 1 to 3 percent by weight. The foregoing dispersion is intimately admixed with an alkali metal silicate. Thus, aqueous alkali metal silicate may be slowly added to the weighting agent dispersion with thorough mixing. The mixing is conveniently carried out at room temperature, although if desired, lower or higher temperatures may be employed. The relative proportions as between the weighting agent dispersion and alkali metal silicate solution are not critical, and merely require that there be present sufficient alkali metal silicate to assure that the particles of weighting agent are coated therewith. Hence, the mixing is thorough so as to insure that the clay is uniformly dispersed and coated with alkali metal silicate. After mixing, the admixture is heated to a temperature from about 70.degree. to 150.degree.F and then a strong acid, preferably H.sub.2 SO.sub.4, is added to the admixture with mixing. Preferably, the acid is added at a uniform rate over a given period, e.g., from about one half hour up to about six hours. The admixture is then heated to a temperature of from about 90.degree. to about 200.degree.F and maintained at this temperature for about 0.5 to 6 hours. Longer ageing times may be employed, but to no particular advantage. As will be apparent, in general, the higher the temperature, the less the time required at that temperature to effect ageing. Thus, the ageing could be carried out at temperatures as low as, e.g., room temperature, but then the time requirements for such ageing would be considerable so that the process would be uneconomical. If silica-alumina, silica-zirconia, or silica-zirconia-alumina is to be employed rather than silica, suitable sources of aluminum and/or zirconium ions are added after the ageing step. In one embodiment of the present invention, a source of aluminum ions is added to the aged admixture, generally in amounts sufficient to give from about 0.3 to 1.0 percent by weight Al.sub.2 O.sub.3 in the final catalyst, on a dry basis. The alumina is typically added in the form of an aluminum salt, preferably aluminum sulfate. Neither the concentration nor the amount of aluminum salt solution employed is critical. Thus, each may be adjusted so as to achieve the desired level of alumina in the overall amorphous inorganic oxide-weighting agent matrix. By way of illustration, the concentration of the aluminum salt solution may be of the order of 1 percent by weight to 30 percent by weight or even higher, a preferred range being from about 5 to 20 percent by weight, the most preferred range being from about 10 to 15 percent by weight. Likewise, the temperature of the aluminum salt solution is not at all critical. It is generally most convenient to make up the solution at ambient temperature conditions and then add it to the aged admixture, although higher or lower temperatures may, of course, be employed. Of course, the catalyst of my invention may also comprise a catalyst wherein the matrix is silica-zironia or silica-zirconia-alumina rather than silica-alumina or silica alone. In preparing such catalysts, a source of zirconium ions is added to the admixture after the foregoing aging step. If the matrix is also to contain alumina, the source of aluminum ions also may be added, as described hereinabove. The source of zirconium ions desirably is a zirconium salt, zirconium sulfate or acidified sodium zirconium silicate being preferred. An aqueous solution of the zirconium salt is advantageously employed. Neither the concentration nor the amount of salt solution employed is critical. Thus, each may be adjusted so as to achieve the desired level of zirconia in the overall matrix. By way of illustration, the concentration of the zirconium salt solution may be of the order of 1 percent by weight to 30 percent by weight or even higher, a preferred range being from about 5 to 20 percent by weight, the most preferred range being from about 10 to 15 percent by weight. Likewise, the temperature of the zirconium salt solution is not at all critical. It is generally most convenient to make up the solution at ambient temperature conditions and then add it to the aged admixture, although higher or lower temperatures may, of course, be employed. Where zirconia is to be present as a component of the matrix, it is desirable that the zirconia level of the synthetic amorphous inorganic oxide be from about 0.5 to 25 % by wt on a dry basis. A more preferred range is from about 1 to 10 percent, with the most preferred range being from about 2 to 5 percent. As previously pointed out, the desired zirconia level is readily obtained by appropriate selection of concentration and/or amount of zirconium salt solution employed. After the heat-ageing step and the addition of any aluminum or zirconium salts, sufficient acid (desirably sulfuric) is added to the slurry, with agitation, to reduce the pH from a higher value, such as in the range of 9 to 10.5, to a pH in the approximate range of 4 to 7. Preferably the pH is reduced to from about 4.0 to 5.0, with from about 4.4 to 4.6 being the most preferred range. This addition of acid at this point results in the formation of a synthetic amorphous inorganic oxide-weighting agent matrix slurry wherein the inorganic oxide is characterized on a dry basis, by a pore volume of at least 0.4 cc/gram. To the foregoing synthetic amorphous oxide-weighting agent matrix slurry a catalytically active component is added, this component comprising a crystalline aluminosilicate. While it frequently is advantageous to employ a rare earth crystalline aluminosilicate, this is not at all essential, as is demonstrated by Examples 17 and 18 hereinafter. Thus, other crystalline aluminosilicates, e.g., sodium Y, may readily be employed. Various suitable crystalline aluminosilicates for use in the composite catalysts of my invention are described in U.S. Pat. Nos. 3,140,249 and 3,140,253, both of which are incorporated herein by reference. Representative crystalline aluminosilicates suitable for the present invention include those natural and synthetic crystalline aluminosilicates having uniform pores of a diameter preferably between about 3 and 15 Angstrom units. Such crystalline aluminosilicates include a wide variety of aluminosilcates both natural and synthetic which have a crystalline or combination of crystalline and amorphous structure. However, it has been found that exceptionally superior catalysts can be obtained when the starting aluminosilicate has either a crystalline or a combination of crystalline and amorphous structure and possesses at least 0.4 and preferably 0.6 to 1.0 equivalent of metal cations per gram atom of aluminum. The aluminosilicates can be described as a three-dimensional framework of SiO.sub.4 and AlO.sub.4 tetrahedra in which the tetrahedra are cross linked by the sharing of oxygen atoms whereby the ratio of total aluminum and and silicon atoms to oxygen atoms is 1:2. In their hydrated form, the alumino-silicates may be represented by the formula: ##EQU1## wherein M represents at least one cation which balances the electrovalence of the tetrahedra, n represents the valence of the cation, w the moles of SiO.sub.2 and Y the moles of H.sub.2 O. The cation can be any or more of a number of metal ions, depending upon whether the aluminosilicate is synthesized or occurs naturally. Typical cations include rare earths, sodium, lithium, potassium, silver, magnesium, calcium, zinc, barium, iron, nickel, cobalt and manganese. Although the proportions of inorganic oxides in the silicates and their spatial arrangements may vary affecting distinct properties in the aluminosilicate, the main characteristic of these materials is their ability to undergo dehydration without substantially affecting the SiO.sub.4 and A10.sub.4 framework. Aluminosilicates falling within the above formula are well known and include synthesized aluminosilicates or natural occuring aluminosilicates. Among the aluminosilicates are included Zeolites A, Y, L, D, T, X, ZK-4, ZK-5, levynite, erionite, faujasite, analcite, noselite, phillipsite, brewsterite, natolite, chabazite, gmelinite, leucite, scapolite, and mordenite. The preferred aluminosilicates are those having pore diameters of at least about 4 Angstroms. Particularly preferred rare earth zeolites for use in this invention may be made by base exchange of sodium zeolite X with rare earth ions to form rare earth zeolite X (see, e.g., Plank et al., U.S. Pat. No. 3,140,249, Example 26), and by base exchange of sodium zeolite Y with rare earth ions to form rare earth zeolite Y (see, e.g., Plank et al., U.S. Pat. No. 3,436,357, Example 1), as described hereinafter. It is to be understood that mixtures of the various aluminosilicates previously set forth can be employed as well as individual aluminosilicates. Crystalline aluminosilicates having pore diameters between about 3 and 5 Angstrom units may be suitable for size-selective conversion catalysis, while crystalline aluminosilicates having pore diameters between about 6 and 15 Angstrom units are preferred for hydrocarbon conversion such as catalytic cracking and the like. The crystalline aluminosilicate particles employed as a component in the catalyst compositions of the present invention are essentially characterized by a high catalytic activity. This high catalytic activity is imparted to the particles by base exchanging, as by base exchanging alkali metal aluminosilicate particles before dispersion thereof in the matrix with a base-exchange solution containing rare earth cations. Suitable methods of base exchange are described in the aforenoted U.S. Pat. Nos. 3,140,249 and 3,140,253. Where an alkali metal aluminosilicate is employed initially, it is frequently advantageous to base exchange the aluminosilicate particles before compositing with the matrix to reduce the sodium content of the final product to less than about 4% by weight and preferably less than 1% by weight. Such base exchange can also be performed after compositing. The sodium content of the final composite is essentially less than 4% by weight. In no instance should there be any more than 0.25 equivalents of alkali metal per gram atom of aluminum associated with the aluminosilicate. Such compositions provide high catalytic activity when Zeolite Y is the crystalline aluminosilicate component. Preferably, however, and particularly when Zeolite X is the crystalline aluminosilicate component, the sodium content of the final composite should be less than 1% by weight. As noted, the rare earth crystalline aluminosilicate is obtained by treating a crystalline aluminosilicate with a fluid medium, preferably a liquid medium, containing cations of at least one rare earth. Rare earth metal salts represent the source of rare earth cation. The product resulting from treatment with a fluid medium is an activated crystalline and/or crystalline-amorphous aluminosilicate in which the structure thereof has been modified primarily to the extent of having the rare earth cations chemisorbed or ionically bonded thereto. As described hereinafter, the incorporation of the rare earth cations is preferably carried out on the zeolite prior to dispersion in the matrix, although alternatively, the entire composite may be subjected to ion exchange. In either event, the ion exchange is carried out in such manner to insure essentially complete substitution of the rare earth cation for the alkali. Water is the preferred solvent for the cationic salt, e.g., rare earth metal salt, for reasons of economy and ease of preparation in large scale operations involving continuous or batchwise treatment. Similarly, for this reason, organic solvents are less preferred but can be employed providing the solvent permits ionization of the cationic salt. Typical solvents include cyclic and acyclic ethers such as dioxane, tetrahydrofuran, ethyl ether, diethyl ether, diisopropyl ether, and the like; ketones, such as acetone and methyl ethyl ketone; esters such as ethyl acetate; alcohols such as ethanol, propanol, butanol, etc; and miscellaneous solvents such as dimethylformamide, and the like. In carrying out the treatment with the fluid medium, the procedure employed varies depending upon the particular alumino-silicate which is treated. If the aluminosilicate which is treated has alkali metal cations associated therewith, then the treatment with fluid medium or media should be carried out until such time as the alkali metal cations originally present are substantially exhausted. Alkali metal cations, if present in the treated aluminosilicate, tend to suppress or limit catalytic properties, the activity of which, as a general rule, decreases with increasing content of these metallic cations. Effective treatment with the fluid medium to obtain a modified aluminosilicate having high catalytic activity will vary, of course, with the duration of the treatment and the temperature at which the treatment is carried out. Elevated temperatures tend to hasten the speed of treatment whereas the duration thereof varies inversely with the general concentration of ions in the fluid medium. In general, the temperatures employed range from below ambient room temperature of 24.degree.C. up to temperatures below the decomposition temperature of the alumino-silicate. Following the fluid treatment, the treated alumino-silicate is washed with water, preferably distilled water, until the effluent wash water has a pH value of wash water, i.e., between 5 and 8. The aluminosilicate material is thereafter analyzed for metallic content by methods well known in the art. Analysis also involves analyzing the effluent wash for anions obtained in the wash as a result of the treatment, as well as determination of and correction for anions that pass into the effluent wash from soluble substances, or decomposition products of insoluble substances, which are otherwise present in the aluminosilicate as impurities. The treatment of the aluminosilicate with the fluid medium or media may be accomplished in a batchwise or continuous method under atmospheric, superatmospheric or subatmospheric pressures. A solution of rare earth metal cations in the form of a molten material, vapor, aqueous, or non-aqueous solution may be passed slowly through a fixed bed of aluminosilicate. If desired, hydrothermal treatment or corresponding non-aqueous treatment with polar solvents may be effected by introducing the aluminosilicate and fluid medium into a closed vessel maintained under autogeneous pressure. Similarly, treatments involving fusion or vapor phase contact may be employed. A wide variety of rare earth compounds can be employed with facility as a source of rare earth ions. Operable compounds include rare earth chlorides, bromides, iodides, carbonates, bicarbonates, sulfates, sulfides, thiocyanates, peroxysulfates, acetates, benzoates, citrates, fluorides, nitrates, formates, propionates, butyrates, valecates, lactates, malanates, oxalates, palmitates, hydroxides, tartrates, and the like. The only limitation on the particular rare earth metal salt or salts employed is that it is sufficiently soluble in the fluid medium in which it is used to give the necessary rare earth ion transfer. The preferred rare earth salts are the chlorides, nitrates and sulfates. Representative of the rare earth metals are cerium, lanthanum, praseodymium, neodymium, illinium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, scandium, yttrium, and lutecium. The rare earth metal salts employed can either be the salt of a single rare earth metal or mixtures of rare earth metals, such as rare earth chlorides or didymium chlorides. As hereinafter referred to, unless otherwise indicated, a rare earth chloride solution is a mixture of rare earth chlorides consisting essentially of the chlorides of lanthanum, cerium, neodymium and praseodymium with minor amounts of samarium, gadolinium and yttrium. Rare earth chloride solutions are commercially available and the ones specifically referred to in the examples contain the chlorides of the rare earth mixture having the relative composition cerium (as CeO.sub.2) 48% by weight, lanthanum (as La.sub.2 O.sub.3) 24% by weight, praseodymium (as Pr.sub.6 O.sub.11) 5% by weight, neodymium (as Nd.sub.2 O.sub.3) 17% by weight, samarium (as Sm.sub.2 O.sub.3) 3% by weight, gadolinium (as Gd.sub.2 O.sub.3) 2% by weight, and other rare earth oxides 0.8% by weight. Didymium chloride is also a mixture of rare earth chlorides but having a lower cerium content. It consists of the following rare earths determined as oxides: lanthanum 45-56% by weight, cerium 1-2% by weight, praseodymium 9-10% by weight, neodymium 32-33% by weight, samarium 5-7% by weight, gadolinium 3-4% by weight, yttrium 0.4% by weight, and other rare earths 1-2% by weight. It is to be understood that other mixtures of rare earths are also applicable for the preparation of the novel compositions of this invention, although lanthanum, neodymium, praseodymium, samerium and gadolinium as well as mixtures of rare earth cations containing a predominant amount of one or more of the above cations are preferred since these metals provide optimum activity for hydrocarbon conversion, including catalytic cracking. It is preferred that the novel compositions of the present invention have at least 0.4 and more desirably 0.6 to 1.0 equivalent of rare earth metal cations per gram atom of aluminum. A more preferred embodiment of this invention uses rare earth zeolite compositions which have from 0.5 to 1.0 equivalent per gram atom of aluminum of rare earth metal cations. Thus, as noted hereinabove, rare earth metal cations are substantially the only metallic cations associated with the aluminosilicate. While not wishing to be bound by any theory of operation, it nevertheless appears that the rare earth cations tend to impart stability to the aluminosilicate compositions, thereby rendering them far more useful for catalytic purposes, particularly in hydrocarbon conversion processes such as cracking. The mean particle size of the crystalline aluminosilicate incorporated into the matrix is advantageously less than about 40 microns. Preferably the particle size is in the range of about 0.1 to 20 microns, and most preferably from about 2 to 10. As previously noted, the matrix into which the crystalline aluminosilicate is dispersed is prepared in such a manner that, as charged to the cracking unit, the synthetic amorphous oxide desirably has a pore volume of at least about 0.4 cc/g. The porosity of the matrix can be adjusted so as to obtain the desired pore volume. Thus, increased porosity may be obtained, for example, by increasing the time and temperature of aging the silica gel. For a more detailed discussion of such prior art techniques for adjusting porosity, see "Control of Physical Structure of Silica-Alumina Catalyst" by Ashley et al., Vol. 44, Industrial and Engineering Chemistry, at pages 2861-2863 (December 1952). The aluminosilicate is incorporated into the matrix by preparing a slurry of the fine particles of the crystalline aluminosilicate, preferably in an aqueous medium. Its concentration in its slurry is preferably in the range from about 1 to 40%. The concentration of the matrix in its slurry is preferably in the range of about 1 to 15%. The two slurries are then thoroughly mixed. In one embodiment, after mixing, the blend is then filtered to removed water from the slurry and thus improve control of the solids concentration in the slurry going to the spray dryer. This, in turn, provides greater control over the particle size distribution of the particles coming from the spray dryer. Filtration normally increases the total solids concentration of the blend to over 8%, e.g., typically from about 10 to 12%. Filtration also removes some dissolved salts. Without such filtration, and without the improved control of solids content of the slurry obtained thereby, the particle distribution of the catalyst coming from the spray dryer would vary over a wide range. The rate of filtration of the slurry is also important inasmuch as the faster the slurry filters, the better is the control over the solid content of the slurry going to the spray dryer. A significant factor affecting the filtration rate of the slurry is the size of the particles in the slurry. The larger the particles, the faster the slurry filters. The smaller the gel particles in the slurry, the slower the filtration time. If the particles are too small, the filtration operation is virtually impossible due to plugging of the filter. The filtered material is then subdivided and dried to form the desired particles. A particularly good method of making microspherical particles (e.g. of particle size of about 1 to 200 microns, the bulk of which are in the range of about 40 to 80 microns) especially suitable for use in fluidized catalytic cracking, is spray drying, perferably under high pressures, e.g., of the order of from about 200 to 2000 psig, and preferably from about 1000 to 1500 psig. The spray drying temperature is ordinarily within the range of 200.degree.F to 1000.degree.F. The temperature used will depend upon such factors as the quantity of material to be dried and the quantity of air used in the drying. The evaporation rate will vary depending on the quantity of air used in the drying. The temperature of the particles which are being dried is preferably within the range of 150.degree.F to 300.degree.F at the completion of the drying. The drying is preferably effected by a process in which the particles to be dried and a hot air stream are moving in the same direction for the entire drying period (concurrent drying), or where the hot air stream flows in the opposite direction (countercurrent drying), or by semi-counter current drying. After the dried particles have been formed they are preferably given a wet treatment to further remove alkali metal (which may, for example, be present, at this stage in amount of about 1 to 5%, and more usually from about 1 to 3%, based on the zeolite), by further base exchange with materials capable of providing hydrogen ions. One suitable technique for this purpose is to treat the particles with a solution of ammonium salt, e.g., with water containing about 1 - 5%, of ammonium sulfate to remove sodium ions, and then to wash the particles with water. A series of alternating ammonium sulfate and water treatments may be used, ending with a water wash to remove sulfate ions. In accordance with one aspect of the invention, the foregoing ammonium-exchanged, water-washed composite, is then impregnated with rare earth ions under such conditions as to acheive a rare earth content, expressed as RE.sub.2 O.sub.3, of from about 1 to 6 percent by weight (dry basis) in excess of the maxiumum RE.sub.2 O.sub.3 content which can be achieved by ion exchange alone. Preferably, the said excess rare earth content by impregnation, expressed as RE.sub.2 O.sub.3, should be from about 2 to 4 weight percent. The impregnation is advantageously effected by contacting the foregoing composite with an aqueous solution of a rare earth salt. The residence time is not critical, provided that it is sufficient to assure good contact between the foregoing product and the rare earth salt solution. The impregnation may be carried out at atmospheric pressure or at a pressure of several atmospheres. The temperature of the impregnating solution should, of course, be sufficiently high to assure dissolution of the rare earth salt. The concentration of the impregnating solution is not critical. Thus, the concentration will vary, depending upon such factors as the amount of rare earth pickup desired, the solids content of the composite being impregnated, etc. Generally, the concentration of the impregnating solution will be from about 5 to 75 grams of rare earth salt (expressed as RECl.sub.3.6 H.sub.2 O) per liter, although either higher or lower concentrations may be employed. The impregnation step may be carried out by any number of known contracting procedures. The procedure that is particularly advantageous involves slurrying the rare earth salt in a quantity of water that is in excess of that required to fill the catalyst pore volume, e.g., from one and one half to several times such required amount. The ammonium-exchanged water-washed composite is then added to the rare earth solution. The contacting may be carried out at any convenient temperature and pressure, e.g., ambient temperature and atmospheric pressure, such conditions not being critical. The contact time may vary depending on the pore volume of the catalyst, but should be sufficient to achieve thorough blending. Desirably, the mixture is subsequently filtered, e.g., to approximately 50% solids, and is then dried, e.g., at about 250.degree.F. The concentration of rare earth salt in the slurry is selected so as to give the desired take-up of either soluble rare earth salt or rare earth oxide. Alternatively, the ammonium-exchanged, water-washed composite may be dried. Thereafter, the dried composite may be contacted with a solution of rare earth salt, the amount of solution being sufficient to fill the pores of the catalyst but leave the external surface of the catalyst essentially dry. Here too, the concentration of the rare earth salt in the solution will depend on the amount of rare earth desired to be impregnated in the catalyst; the contacting may be carried out at any convenient temperature and pressure; and the contact time may vary. The impregnated catalyst is then dried, generally at about 250.degree.F. A wide variety of soluble rare earth salts can be employed with facility for the foregoing impregnation step. Suitable compounds include rare earth chlorides, bromides, iodides, carbonates, bicarbonates, sulfates, sulfides, thiocyanates, peroxysulfates, acetates, benzoates, citrates, fluorides, nitrates, formates, propionates, butyrates, valerates, lactates, malanates, oxalates, palmitates, hydroxides, tartrates, and the like. The preferred rare earth salts are the chlorides, nitrates and acetates. In accordance with another aspect of the invention, the ammonium-exchanged water-washed composite previously referred to is first subjected to a post-exchange with rare earth ions, and thereafter the post-exchanged composite is impregnated with rear earth. Thus, the ammonium-exchange water-washed composite is treated with a solution containing rare earth ions so as to replace ammonium and residual alkali metal with rare earth ions and to insure substantially complete substituion of rare earth metal for other cations. Desirably, the rare earth ions are used as aqueous solutions of water soluble salts thereof, e.g., as rare earth chloride hexahydrate. The foregoing post-exchange is desirably carried out using an equivalent amount of rare earth cation equal to at least 50% of the equivalents of alkali metal, e.g., sodium, present in the crystalline zeolite prior to the wet processing treatment with ammonium ions. Preferably, the equivalent amount of rare earth cation employed is equal to 100% of sodium present, i.e., the full stoichiometric amount required to replace all of the sodium present, or is in excess of the stoichiometric amount required. The rare earth cation may be supplied from a solution having a concentration of about 0.1 to 1% by weight of the soluble salts thereof, for example, a rare earth chloride, although higher concentrations may, of course, be employed. Desirably, the exchange is conducted at a temperature of from about 60.degree. to 120.degree.F for a time between about 1 and 60 minutes. The foregoing is followed with one or more water washes to minimize the chloride content of the finished catalyst. The particles are then dried in any suitable manner, as by flash drying. By virtue of the foregoing wet treatment of the dried particles with aqueous ammonium sulfate (and optionally with aqueous rare earth chloride if a post-exchange is employed) to further remove alkali metal from the zeolite and matrix, ammonium ions (and, if there has been a post-exchange, additional rare earth cations) are introduced. Upon subsequent drying, ammonia is liberated leaving hydrogen ions, so that the zeolite may contain both rare earth metal cations and hydrogen ions, thus resulting in a catalyst having highly desirable characteristics. The efficiency of this subsequent treatment is greatly improved if the rare earth zeolite, in finely divided condition, has previously been pretempered by subjecting it to dehydrating conditions, as by calcination, to lower its residual moisture content to a value within the range of 0.3 to 6%, more preferably within the range of 1.5 to 6%, such pretempering having been effected before the rare earth zeolite is brought into contact with the matrix. As a result of this pretempering the rare earth zeolite can be later exchanged to a lower sodium content much more easily, it becomes more resistant to loss of crystallinity on contact with acidic media and the relative crystallinity of the final product is higher. In addition, the rare earth component becomes more fixed in the crystalline aluminosilicate and more resistant to removal on subsequent base exchanges. Suitable pretempering conditions are, for example, a temperature of about 650.degree.F in air for about 60 minutes or a temperature of about 1500.degree.F in air for about 10 minutes, or a treatment with superheated steam at about 1100.degree.-1200.degree.F at 15 psig for from about 10 to 60 minutes; a preferred treatment is at atmospheric pressure at a temperature of about 1050.degree.-1250.degree.F in steam, air, or a steam-air mixture for from about 10 to 60 minutes. (This pretempering technique is described more fully in U.S. application Ser. No. 459,687, filed May 28, 1965, entitled "Improved Crystalline Zeolites and Method of Preparing Same".) Where the crystalline aluminosilicate initially employed is a rare earth crystalline aluminosilicate, then the catalyst composite will, prior to impregnation with rare earth salt solution, generally have a earth oxide (RE.sub.2 O.sub.3) content in the range from about 1.0 to 6.0% by weight depending on the total amount of rare earth zeolite in the catalyst. This rare earth has been incorporated in the catalyst by ion exchange that is, by the exchange carried out on the zeolite itself, and optionally also by post-exchange of the composite, each of which have been described in detail previously. Thus, the maximum RE.sub.2 O.sub.3 content of the catalyst obtainable by ion exchange is limited by the total number of sites in the zeolite available for such exchange. In accordance with said second aspect of the invention, the dried catalyst is then subjected to a rare earth impregnation step in order to increase the RE.sub.2 O.sub.3 content of the catalyst an additional 1 to 6 percent by weight, i.e., 1 to 6 percent by weight over and above the RE.sub.2 O.sub.3 content resulting from ion exchange. Preferably, the increase in RE.sub.2 O.sub.3 content by impregnation is from about 1 to 3 percent by weight. The impregnation step is carried out as described previously, the preferred methods being either (1) contacting the dried catalyst with a solution of rare earth salt, the amount of solution being sufficient to fill the pores of the catalyst but leave the external surface of the catalyst substantially dry, and thereafter drying, or (2) slurrying the rare earth salt in a quantity of water that is in excess of that required to fill the catalyst pore volume, adding the dried catalyst to this rare earth solution, filtering the resulting mixture, and drying. It is important to note that, following the rare earth impregnation step, subsequent processing, such as drying to fix the impregnated rare earth as RE.sub.2 O.sub.3, etc., is preferably carried out in the absence of any intermediate washing of the rare earth-impregnated product. Thus, washing of the rare earth-impregnated product prior to further processing, e.g., drying, would tend to remove excess rare earth ions from the product, thereby tending to eliminate the desired excess rare earth of impregnation. Of course, in some instances it may be desirable to subject the rare earth impregnated product to slight or moderate washing prior to drying, provided that care is taken not to remove all of the excess of rare earth ions, so that sufficient excess rare earth ions remain to form the desired level of impregnated RE.sub.2 O.sub.3 upon drying. As noted previously, rather than subject the overall composite to rare earth impregnation, an alternative method of making the rare earth-impregnated composite is to treat the matrix with a rare earth salt solution and thereafter to admix the so-treated matrix with the rare earth crystalline alumino-silicate. Alternatively, the rare earth-treated matrix can first be filtered and the filter cake then be admixed with the rare earth crystalline aluminosilicate. Note that such admixing is effected without an intermediate washing step. The resultant mixture is then dried without any additional wet processing, so as to avoid removal of rare earth. This technique is illustrated in Example 32 hereinafter. In accordance with yet another embodiment of the invention, the rare-earth impregnated composite may be obtained by treating a rare earth crystalline aluminosilicate with a solution of rare earth ions to form a rare earth-wetted crystalline aluminosilicate, and thereafter admixing such wetted crystalline aluminosilicate with the matrix. See, e.g., Example 19 hereinafter. This technique, while practicable, is not as desirable as the procedure wherein the entire catalyst composite (matrix plus particulate rare earth zeolite) is subjected to rare earth impregnation. Compare, e.g., Examples 20 and 21 with Example 19. In accordance with still another aspect of my invention, I have found that particularly good results are obtained when, following the rare earth impregnation step, the subsequent drying step is carried out over a period of time of at least 5 minutes, and preferably at least 10 minutes, under such conditions as to reduce the water content to below 20 percent by weight at the completion of said drying. See, e.g., Examples 34 and 35 hereinafter. As a result of this relatively slow drying procedure, the performance of the resultant catalyst is further enhanced. The finished catalyst is characterized by a residual sodium content not in excess of about 1.0 weight percent, expressed as Na.sub.2 O, based upon the weight of the dried catalyst. Indeed, a catalyst having a residual sodium content not in excess of about 0.2 weight percent Na.sub.2 O may readily be attained, and where the dispersed rare earth zeolite is of the X form (as contrasted to rare earth zeolite Y) the residual sodium level is preferred to be not in excess of about 0.1 weight percent Na.sub.2 O. The catalysts of this invention can, by a relatively mild heat treatment, be put in a highly active condition in which they are suitable for direct use in fluid catalytic cracking and in which they exhibit the desired selectivity for producing gasolines, mainly at the expense of the undesirable products of cracking, e.g., dry gas and coke. This heat treatment can take place during regular cracking-regeneration cycles. Thus, when the catalysts are added, as makeup, in an operating fluid catalytic cracking installation they will soon attain their desired selectivity after a few cracking-regeneration cycles, without the need of a preliminary steam-activating step. Alternatively, the catalysts may be given a preliminary heat treatment in air (and in fluidized condition) at a temperature of 1100-1400.degree.F for from about 3 to 16 hours. The following examples will further illustrate my invention. All parts are by weight unless otherwise indicated. EXAMPLES 1-8 A series of eight catalysts was prepared, each having the following composition: 10% rare earth Y crystalline aluminosilicate zeolite (REY) and 90% matrix, the matrix being made up of 40% clay, 57.4% silica, 2% zirconia, and 0.6% alumina. The procedure employed in preparing the eight catalysts was as follows: 2125 pounds of Georgia Kaolin clay on a dry weight basis were mixed with 54,400 pounds (6550 gallons) of deionized water. 11,100 pounds (960 gallons) of Q-brand sodium silicate [Na.sub.2 O(SiO.sub.2).sub.3.3 ] containing 3200 pounds of SiO.sub.2 were added to the water-clay slurry with stirring over a period of one half hour. The clay was uniformly dispersed and coated with sodium silicate. The admixture was then heated to 120.degree.F, and sulfuric acid (35%) was added at a uniform rate, while mixing, to adjust the pH to 9.8. After the foregoing acid addition, in each instance, the admixture was heat aged for one hour at a temperature sufficient to produce a catalyst with a pore volume of 0.65-0.71 cc/g. An aqueous solution of aluminum sulfate (20 weight percent aluminum sulfate) was then added to the aged admixture at a uniform rate over a period of one half hour in such amount so as to provide a final alumina content of 0.6 weight percent, based on the total dry catalyst weight. A slurry was prepared by mixing 19 gallons of 66.degree. Baume sulfuric acid into a dispersion of 240 pounds of sodium zirconium silicate (Na.sub.2 ZrSiO.sub.5) in 270 gallons of deionized water. This slurry, which had a pH less than 0.4, was added at a uniform rate over a period of 30 to 45 minutes in such amount as to provide a final concentration of zirconia (ZrO.sub.2) of 2.0 weight percent, based on the weight of the dry catalyst. The pH of the mixture was then adjusted to between 4.5 and 4.6 by the addition of 35% sulfuric acid over a one half hour period. 600 pounds of REY that previously had been 68% exchanged, i.e., 68% of the sodium content had been replaced with rare earth cations, which previously had been pretempered by calcining at about 1200.degree.F. for about ten minutes, were slurried in about 210 gallons of deionized water. (The REY had the following composition: Al.sub.2 O.sub.3 = 19.9%; SiO.sub.2 = 60.3%; RE.sub.2 O.sub.3 = 15.5%; Na.sub.2 O = 4.3%). This slurry was added to the silica-alumina-zirconia-clay slurry while mixing, in such amount as to provide a final REY concentration, based on the dry weight of the catalyst, of 10% by weight. The blend was filtered. The filter cake was spray dried at 1400 - 1500 psig to yield a coarse grade catalyst. The spray dried product was ion exchanged with ammonium sulfate solution and then washed with deionized water until substantially free of sulfate ions. The resultant product was exchanged with an aqueous rare earth chloride solution (RECl.sub.3.sup.. 6H.sub.2 O) in such proportions that about 20 parts of rare earth chloride contacted about 2000 parts of catalyst (bone dry basis). After the exchange, the catalyst was water washed until essentially chloride free and the sulfate content was 0.5% by weight or less on a dry basis. The thus treated catalyst had an Na.sub.2 O content of from about 0.11 to 0.16% by weight on a dry basis. After washing, the catalyst was flash dried. A sample of the foregoing flash dried catalyst was withdrawn and held as a "control". Hence, it was not subjected to impregnation. This control catalyst is designated as Example 1. Seven samples of the flash dried catalyst were each subjected to an impregnation treatment. This treatment comprised mixing each sample with a solution of a rare earth salt. The concentration and amount of the rare earth salt in the solution varied from sample to sample in order to produce differing amounts of impregnated RE.sub.2 O.sub.3 in each sample. The resulting impregnated catalysts are designated as Examples 2-8. The impregnation treatments for each of the samples were as follows. EXAMPLE 2 2,000 grams of the flash dried catalyst were slurried in a mixture of 50 grams of rare earth chloride in 2,000 cc. of deionized water to provide an additional 1.0% by weight of RE.sub.2 O.sub.3 in the finished catalyst. EXAMPLE 3 2,000 grams of the flash dried catalyst were slurried with 100 grams of rare earth chloride in 2,000 cc. of deionized water to provide an additional 2.0% by weight of RE.sub.2 O.sub.3 in the finished catalyst. EXAMPLE 4 2,000 grams of the flash dried catalyst were slurried with 77.3 grams of rare earth acetate in 2,000 cc. of deionized water to provide an additional 1.2 weight percent of RE.sub.2 O.sub.3 in the finished catalyst. EXAMPLE 5 2,000 grams of the flash dried catalyst were slurried with 79.5 grams of rare earth nitrate in 2,000 cc. of deionized water to provide an additional 1.3 weight percent of RE.sub.2 O.sub.3 in the finished catalyst. For each of the catalysts of Examples 2-5, the amount of impregnating solution used was just sufficient to fill the pores of the catalyst, but to allow the surface of the catalyst to remain essentially dry. After thorough blending of the catalyst and the solution, each mixture was dried in an oven at 250.degree.F. for approximately 6 hours. EXAMPLE 6 2,000 grams of the flash dried catalyst were slurried with 250 grams of rare earth chloride in 7,000 cc. of deionized water to provide an additional 1.4 weight percent of RE.sub.2 O.sub.3 in the finished catalyst. EXAMPLE 7 2,000 grams of the flash dried catalyst were slurried with 104.4 grams of rare earth chloride in 3,000 cc. of deionized water to provide an additional 1.7 weight percent of RE.sub.2 O.sub.3 in the finished catalyst. EXAMPLE 8 2,000 grams of the flash dried catalyst were slurried with 550 grams of rare earth chloride in 7,000 cc. of deionized water to provide an additional 3.3 weight percent of RE.sub.2 O.sub.3 in the finished catalyst. For each of the catalysts of Example 7, the amount of impregnating solution used was about 1.5 times the volume required to fill the pores of the catalyst; in Examples 6 & 8, the impregnating solution was 3.5 times the volume required. After mixing the samples with the impregnating solution, the mixtures were filtered to a solids content of about 50 %. Each of the filtered mixtures was then dried in an oven at 250.degree.F. for approximately 16 hours. The samples of the catalysts of Examples 1-8 were subjected to thermal treatments of varying degrees of severity. These treatments were: Mild thermal Calcining for 3 hours at 1,200.degree.F in air; Mild steaming Steaming in 100% steam for 4 hours at 1,400.degree.F. and 0 psig; and Severe Steaming Steaming in 100% steam for 5 hours at 1,400.degree.F. and 15 psig. After the foregoing thermal treatments, the samples were evaluated for catalytic performance using FCC Bench Tests. The results of these Tests are set out in Tables 1, 2 and 3.
TABLE 1
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CATALYST IMPREGNATED WITH RARE EARTH:
AFTER MILD THERMAL TREATMENT (CALCINED 3 Hr/1200.degree.F IN AIR)
BENCH FCC TESTS: 910.degree.F, 2 C/O, 12.5 WHSV, WCMCGO
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Run No. 1 2 3 4
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Catalyst of Example
1 2 7 3
Impregnation Salt None RECl.sub.3
RECl.sub.3
RECl.sub.3
RE.sub.2 O.sub.3 Added by Impregnation, % wt
0 1.0 1.7 2.0
Conversion, % vol 69.3 69.6 67.5 65.5
C.sub.5 + Gasoline, % vol
55.3 59.4 57.4 56.4
Total C.sub.4 's, % vol
15.0 13.5 12.6 11.9
Dry Gas, % wt 7.1 5.9 6.2 5.9
Coke, % wt 3.3 2.7 2.6 2.1
Carbon on Cat., % wt
1.43 1.16 1.09 0.91
Selectivity (C.sub.5 + Gasoline,
% vol/Coke, % wt) 16.8 22.0 22.0 26.8
Hydrogen Factor* 37 35 35 39
Physical Properties
Pore Volume, cc/g 0.62 0.61 0.63 0.59
Packed Density, g/cc
0.62 0.62 0.58 0.60
Surface Area, m.sup.2 /g
345 330 347 326
Chemical Properties
Na.sub.2 O, % wt 0.15 0.12 0.07 0.12
RE.sub.2 O.sub.3, % wt
2.20 3.20 3.90 4.20
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Moles H.sub.2
*Hydrogen Factor = .times. 100
Moles C.sub.1 + Moles C.sub.2
Referring to the data in Table 1, the markedly lower coke make, after mild thermal treatment obtained in Runs 2, 3 and 4, namely, weight percents of 2.7, 2.6 and 2.1, respectively, wherein the catalysts were impregnated in accordance with the present invention, as compared to that in Run 1, namely, 3.3, wherein the control catalyst was used, are readily apparent. Similarly, Runs 2-4 resulted in less dry gas than did Run 1. Also the yield of C.sub.5 + Gasoline for the catalysts of Runs 2, 3 and 4 are substantially higher than that for the control. The thermal treatment of the catalyst drives off about 10-15% weight of water. The condition of the resulting catalyst simulates the condition of makeup catalyst just after such catalyst has been added to an equilibrium catalyst in a regenerator. The coke make of the catalyst at this stage of the process is very important, inasmuch as the lower the coke make, the longer life the catalyst will have, due to a decrease in heat damage during the initial regeneration cycles. The data in Table 2 shows the FCC Test results of the catalysts of Examples 1-8 after mild steam treatment. The decrease in coke make for the impregnated catalysts of the present invention, i.e., Runs 7, 8 and 9, as compared with control, i.e., Run 1, is readily apparent. Note also that in Run 10, wherein the RE.sub.2 O.sub.3 added by impregnation is above 3.0 weight percent, although the coke make decreases to a value of 1.6, the conversion level, i.e., activity of the catalyst, is substantially decreased, that is, 64.1% vol. for Run 10 as compared with the 67-68% vol. range for Runs 7-9. Runs 11 and 12 illustrate the catalysts of the present invention wherein the impregnation salt is rare earth acetate and rare earth nitrate, respectively. Here too improvements in coke make and selectivity are obtained that are similar to those observed with catalysts wherein the impregnation salt was a rare earth chloride. Referring to Table 3, it is apparent that after severe steaming the activity (conversion) and the selectivity of those catalysts which have been impregnated with rare earth, namely, the catalysts used in Runs 14, 15 and 16, are superior to the conversion and the selectivity obtained using the control catalyst (Run 13). Mild steam pretreatment produces a catalyst which possesses activity similar to that possessed by the equilibrium catalyst in an actual process. The severe steam treatment produces a catalyst possessing an activity similar to that of the aged, less active fraction of an equilibrium catalyst. The difference in the loss of activity between the mild steam treatment and the severe steam treatment is indicative of the steam stability and the resistance to ageing of the catalyst. The superior steam stability of the catalysts of this invention is shown by comparing the decrease in conversion for the catalysts of Examples 6, 4 and 5. That is, for Runs 7 and 14, 11 and 15 and 12 and 16, the decrease is only 12.0, 16.3 and 12.9% vol., respectively, as compared with a conversion loss of 20.5% vol. for the control catalyst (Runs 5 and 13). EXAMPLES 9 AND 10 A second series of two catalysts was prepared. This series was prepared in essentially the same manner as the catalysts of Examples 1-8, with the exception that the catalysts of the second series contained 15 weight percent rare earth zeolite rather than 10%. These catalysts were prepared as follows: 2125 lbs. of Georgia Kaolin clay on a dry weight basis were mixed with 54,400 lbs. (6550 gallons) of deionized water. 11,100 lbs. (960 gallons) of Q-brand sodium silicate [Na.sub.2 O(SiO.sub.2).sub.3.3 ] containing 3200 lbs. of SiO.sub.2 were added to the water-clay slurry with stirring over a period of one-half hour. The clay was uniformly dispersed and coated with sodium silicate. The admixture was then heated to 120.degree.F. and sulfuric acid (35%) was added at a uniform rate, while mixing, to adjust the pH to 9.8. After the foregoing acid addition, in each instance the admixture was heat aged for one hour at a temperature sufficient to produce a catalyst with a pore volume of 0.65-0.71 cc/g. An aqueous solution of aluminum sulfate (20 weight percent aluminum sulfate) was then added to the aged admixture at a uniform rate over a period of one-half hour in such amount as to provide a final alumina content of 1.0 weight percent based on the total weight of dry catalyst. A slurry was prepared by mixing 19 gallons of 66.degree. Baume sulfuric acid into a dispersion of 240 lbs. of sodium zirconium silicate (Na.sub.2 ZrSiO.sub.5) in 270 gallons of deionized water. This slurry, which had a pH less than 0.4, was added at a uniform rate over a period of one-half hour in such amount as to provide a final concentration of zirconia (ZrO.sub.2) of about 2 percent by weight, based on the weight of the dry catalyst. The pH of the mixture was then adjusted to between 4.5 and 4.6 by the addition of 35% sulfuric acid over a one-half hour period. 953 lbs. of REY that previously had been 68% exchanged, i.e., 68% of the sodium content had been replaced with rare earth cations, and pretempered by calcining at about 1200.degree.F. for about ten minutes, were slurried in about 335 gallons of deionized water. (The REY had the following composition: Al.sub.2 O.sub.3 = 19.9%; SiO.sub.2 = 60.3%; RE.sub.2 O.sub.3 = 15.5%; Na.sub.2 O = 4.3%). This slurry was added to the silica-alumina-zirconia-clay slurry while mixing, in such amount as to provide a final REY concentration, based on the dry weight of the catalyst, of 15% by weight. The blend was filtered. The filter cake was spray dried at 1400-1500 psig to yield a course grade catalyst. The spray dried product was ion exchanged with ammonium sulfate solution and then washed with deionized water until substantially free of sulfate ions. The resultant product was exchanged with an aqueous rare earth chloride solution (RECl.sub.3.sup.. 6H.sub.2 O) in such proportions that about 20 parts of rare earth chloride contacted about 2000 parts of catalysts (bone dry basis). After the exchange, the catalyst was water washed until essentially chloride-free and the sulfate content was 0.5% by weight or less on a dry basis. The thus treated catalyst had a Na.sub.2 O content of from about 0.15 to 0.22% by weight on a dry basis. After washing, the catalyst was flash dried. A sample of the foregoing flash dried catalyst was withdrawn and held as a "control". Hence, it was not subjected to impregnation. This control catalyst is designated as Example 9. The flash dried catalyst was then contacted with a rare earth nitrate salt solution, the amount of solution being sufficient to fill the pores of the catalyst but to leave the surface of the catalyst dry. The solution contained 133 grams of rare earth nitrate in 2000 cc. of deionized water to give an additional 2.0% by weight of RE.sub.2 O.sub.3 in the finished catalyst. The impregnated catalyst was then dried in an oven at about 250.degree.F for about 10 hours. This catalyst was designated Example 10. The catalysts of Examples 9 and 10 were then subjected to a mild thermal treatment for three hours at 1200.degree.F. in air. Each of the catalysts were then evaluated for catalytic performance using FCC Bench Tests. The results of these Tests are set out in Table 4.
TABLE 4
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Catalyst (15% REY) Impregnated With Rare Earth
(as RE(NO.sub.3).sub.3): After Mild Thermal Treatment
(Calcined 3 Hr/1200.degree.F in Air)
Bench FCC Tests: 910.degree.F, 2C/0, 12.5 WHSV, WCMCGO
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Run No. 17 18
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Catalyst of Example 9 10
RE.sub.2 O.sub.3 Added by Impregnatio
Other Refs: Primary Examiner: Dees, Carl F. Assistant Examiner: Attorney: Huggett; Charles A., Barclay; Raymond W. |
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