CATALYTIC CRACKING PROCESS

Abstract: A process for catalytically cracking a hydrocarbon feed in a reaction zone under catalytic cracking conditions in the absence of added hydrogen with a catalyst comprising a conventional cracking catalyst. The improvement comprises adding to the conventional cracking catalyst a thermally treated zeolite having a mole ratio of SiO₂:Al₂O₃ of about 10:1 to 100:1 and a constraint index of about 1 to 12. Thermal treatment reduces the additive zeolite's alpha activity to about 1 to 10 prior to use.

Description: This invention relates to a catalytic; for example, a fluidized catalytic cracking process in the presence of a conventional catalyst and a zeolite which has been given a thermal treatment prior to use. Hydrocarbon conversion processes utilizing crystalline zeolites have been the subject of extensive investigation during recent years, as is obvious from both the patent and scientific literature. Crystalline zeolites have been found to be particularly effective for a wide variety of hydrocarbon conversion processes including the catalytic cracking of a gas oil to produce motor fuels and have been described and claimed in many patents, including U. S. Patent Nos. 3,140,249; 3,140,251; 3,140,252; 3,140,253; and 3,271,418. It is also known in the prior art to incorporate the crystalline zeolite into a matrix for catalytic cracking and such disclosure appears in one or more of the above-identified U. S. patents. Preferred conventional cracking catalyst comprises a crystalline aluminosilicate having a pore size greater than 0.7 nm (7 Angstroms), such as Y type zeolite, REY zeolite, or ultrastable Y, in a matrix. It is also known that improved results will be obtained with regard to the catalytic cracking of gas oils if a crystalline zeolite having a pore size of less than 7 Angstrom units is included with a crystalline zeolite having a pore size greater than 8 Angstrom units, either with or without a matrix. A disclosure of this type is found in U. S. Patent No. 3,769,202. Improved results in catalytic cracking with respect to both octane number and overall yield were achieved in U. S. Patent No. 3,758,403. In that patent, the cracking catalyst was comprised of a large pore size crystalline zeolite (pore size greater than 7 Angstrom units) in admixture with ZSM-5 type zeolite wherein the ratio of ZSM-5 zeolite to large pore size crystalline zeolite was in the range of from 1:10 to 3:1. The use of ZSM-5 in conjunction with a zeolite cracking catalyst of the X or Y faujasite variety is described in U. S. Patent Nos. 3,894,931; 3,894,933; and 3,894,934. The two former patents disclose the use of ZSM-5 in amounts up to and about 5 to 10 weight percent; the latter patent discloses the weight ratio of ZSM-5 to large pore size crystalline zeolite in the range of from 1:10 to 3:1. The addition of very small amounts of pure, finely divided shape selective catalyst to a conventional fluid catalytic cracking (FCC) catalyst, was taught in U.S. Patent No. 4,309,280. This patent taught the advantage of using, as the powdered additive catalyst, a ZSM-5 zeolite with very high silica-alumina ratios. Use of a 1500:1 SiO₂/Al₂O₃ mole ratio ZSM-5 catalyst in conjunction with a fluid cracking process was disclosed in Example 8 and Example 13. Use of ZSM-5 with an even higher ratio was disclosed in Example 9, which added ZSM-5 containing only 15 ppm Al₂O₃. The patentees commented that the preferred additives, shape selective zeolites such as ZSM-5, were very active even with high silica to alumina mole ratios. This activity was considered surprising, since catalytic activity of zeolites is generally attributed to cations associated with framework aluminum atoms. U.S. Patent No. 4,309,279 disclosed the addition of very small amounts of a special class of zeolites characterized by a silica to alumina mole ratio greater than 12 and a constraint index of from 1 to 12, to conventional cracking catalyst. The patentees included a teaching, but no examples, to the addition of shape selective zeolites, e.g., ZSM-5, with very high silica to alumina mole ratios, up to 30,000 and greater. In U.S. Patent No. 4,340,465 the patentees taught use of ZSM-5 with very high silica to alumina mole ratios for catalytic cracking. The examples in this patent show that as the silica to alumina mole ratio increased, the activity of the catalyst, as measured by the weight percent conversion, decreased. In going from a sieve containing 2.3 weight percent Al, to 0.45 to 0.04 weight percent, the weight percent conversion declined from 34 to 12 to 7, respectively. This indicated a significant loss in cracking activity when using ZSM-5 with a relatively low aluminum content. Some work has been done on removal of alumina from relatively small pore zeolites such as mordenite. In U.K. Patent Specification No. 1,151,653, the patentees disclosed that the hydrogen form of a zeolite was preferred for many hydrocarbon conversion processes and taught a combination treatment of the zeolite with acid and an ammonium compound to achieve the desired hydrogen form. The treatment included boiling with acid, which would extract some aluminum. U.K. Patent Specification No. 1,261,616 taught a method of making acid extracted mordenite. Some work has also been reported on preparation of aluminum-deficient faujasites in "Chemistry of Crystalline Aluminosilicates" , G.T. Kerr, the Journal of Physical Chemistry , Vol. 72, 1968, pages 2594-2596 and in U.S. Patent 3,442,795. Aluminum was removed directly from sodium zeolite Y, using ethylenediaminetetraacetic acid, EDTA. This reference taught that, as most of the Al was removed from the NaY, the crystallinity of the material changed, and indeed was lost when less than 20 percent of the original aluminum framework content remained. This reference reported increased sorptive capacity, based on the number of grams of SiO₂ in the samples, up to about 70 percent aluminum removal, after which sorptive capacity decreased. The art has recognized the desirability of adding ZSM-5 material, preferably with a low aluminum content in the ZSM-5 framework, to conventional FCC catalyst. Most of the art's efforts at achieving low aluminum content have been directed towards either producing a material with a very low aluminum content, by, e.g., making ZSM-5 material with a silica sol that contained only a very small amount of aluminum. Another way of achieving this low aluminum content is to use conventional aluminum extraction methods, such as treating the ZSM-5 with a strong mineral acid or a chelating agent such as etnylenediaminetetraacetic acid. It is also known that the alpha activity of ZSM-5 catalyst can be modified by thermal treatment. In U.S. Patent No. 4,016,218 a process for the alkylation of aromatic hydrocarbons using thermally treated ZSM-5 catalyst is disclosed. Thermal treatment is used to reduce the alpha activity to less than 250, and preferably less than about 200 but greater than 10. It has now been found that it is possible to achieve the benefits of adding low alumina shape selective material, e.g., ZSM-5, to a conventional FCC process without the expense and inconvenience of making a zeolite with an inherently low alumina content or acid extracting a conventional zeolite. ZSM-5 with, e.g., 40:1 to 90:1 silica to alumina mole ratio can be converted into material that acts in the FCC process as if it had a relatively low aluminum content equivalent to a 500:1 to 1500:1 silica to alumina ratio. Thermal treatment of conventional ZSM-5 has been found to significantly alter the characteristics of this shape selective catalyst so that it responded in the FCC process, as a shape selective catalyst that had been made by a special manufacturing techniques or acid extracted to contain a relatively low aluminum content. The process of the present invention used to alter the catalytic activity of the shape selective material is a simple one - steaming or other thermal treatment. The simplicity and efficacy of steaming to achieve these results is surprising because the art has been steaming FCC catalysts for years without recognizing the benefits that could be obtained by controlled steaming of these shape selective materials. Accordingly, the present invention provides a process for catalytically cracking a hydrocarbon feed having an initial boiling point of at least 200°C, a 50% point range of at least 260°C and an end point of at least 320°C to improve the octane number of the cracked product, which process comprises contacting the feed in a reaction zone under catalytic cracking conditions including a temperature from 150°C to 750°C in the absence of added hydrogen with a catalyst comprising a conventional cracking catalyst to produce a cracked hydrocarbon product which is recovered as a product of the process wherein there is added to the conventional cracking catalyst a thermally treated zeolite having a mole ratio of SiO₂:Al₂O₃ from 10:1 to 100:1 and a Constraint Index from 1 to 12, wherein prior to use in the process the zeolite has been subjected to thermal treatment at a temperature from 250°C to 1000°C at a pressure from 0 to 980.7 kPa (0 to 10 atmospheres) for a time from 0.01 to 1000 hours to reduce its alpha activity to a value from 1 to 10. Conventional Cracking Process Any conventional cracking, preferably a fluidized catalytic cracking or FCC, process and catalyst for use therein can be used in the present invention. Catalytic cracking units which are amenable to the process of this invention operate within the temperature range of from 150 to 750°C and under reduced atmospheric or superatmospheric pressure. The catalytic cracking process may be operated batchwise but preferably operated continuously. The catalytic cracking process can be either fixed bed, moving bed or fluidized bed and the hydrocarbon charge stock flow may be either concurrent or countercurrent to the conventional catalyst flow. The process of this invention is particularly applicable to the fluid catalytic cracking (FCC) process, wherein the catalytic cracking conditions include a temperature from 400 to 700°C, a pressure from 0 to 980.7 kPa (0 to 10 atmospheres), a catalyst to hydrocarbon weight ration from 1:1 to 20:1, and a residence time of the hydrocarbon in the reaction zone from 0.1 to 20 seconds. A recent advance in the art of catalytic cracking is disclosed in U.S. Patent No. 4,072,600. One embodiment of this patent teaches that trace amounts of a metal selected from the group consisting of platinum, palladium, iridium, osmium, rhodium, ruthenium, and rhenium when added to cracking catalysts significantly enhance the conversion of carbon monoxide during the catalyst regeneration operation. In employing this recent advance to the present invention, the amount of the metal added to the conventional cracking catalyst can vary from between 0.1 ppm and 100 ppm based on total catalyst inventory. The aforesaid metals can also be introduced into the process with or as part of the zeolite in amounts such that the overall content of such metal based on total catalyst in the cracking unit is within the aforenoted range. After cracking, the resulting product gas may be compressed and the resulting products may suitably be separated from the remaining components by conventional means such as adsorption or distillation. Hydrocarbon charge stocks undergoing cracking in accordance with this invention comprise hydrocarbons generally and, in particular, petroleum fractions having an initial boiling point of at least 200 o C, a 50% point range of at least 260 o C and an end point of at least 320 o C. Such hydrocarbon fractions include gas oils, residual oils, cycle stocks, whole top crudes and heavy hydrocarbon fractions derived by the destructive hydrogenation of coal, tar, pitches, asphalts and the like. As will be recognized, the distillation of higher boiling petroleum fractions above about 400 o C must be carried out under vacuum in order to avoid thermal cracking. The boiling temperatures utilized herein are expressed in terms of convenience of the boiling point corrected to atmospheric pressure. Suitable Shape Selective Zeolites The members of the class of zeolites to be thermally treated for use in this invention are characterized by a pore dimension greater than about 5 Angstroms, i.e., the zeolite is capable of sorbing paraffins having a single methyl branch as well as normal paraffins, and it has a silica to alumina mole ratio of at least 12. Zeolite A, for example, with a silica to alumina ratio of 2.0, is not useful in this invention, and, moreover, it has no pore dimension greater than about 5 Angstroms. These zeolites exhibit some unusual properties. These zeolites induce profound transformations of aliphatic hydrocarbons to aromatic hydrocarbons in commercially desirable yields and are generally highly effective in alkylation, isomerization, disproportionation and other reactions involving aromatic hydrocarbons. Although they have relatively low alumina contents, i.e., high silica to alumina mole ratios, they are very active even with silica to alumina mole ratios of, e.g., 30 to 80. This activity is surprising, since catalytic activity of zeolites is generally attributed to framework, aluminum atoms and cations associated with these aluminum atoms. These zeolites retain their crystallinity for long periods in spite of the presence of steam even at high temperatures which induce irreversible collapse of the crystal framework of other zeolites, e.g. of the X and A type. Furthermore, carbonaceous deposits, when formed, may be removed by burning at higher than usual temperatures to restore activity. In many environments, the zeolites of this class exhibit very low coke forming capability, conducive to very long times on stream between burning regenerations. The high cracking activity, or alpha activity, of these zeolites is good from the standpoint of high conversions, but the high alpha activity translates, in the FCC process, into excessive cracking of paraffins, and loss of gasoline yield. Thus the shape selective catalyst's ability to promote aromatic formation is overshadowed by its ability to fragment paraffins. An important characteristic of the crystal structure of this class of zeolites is that it provides constrained access to, and egress from, the intracrystalline free space by virtue of having a pore dimension greater than about 5 Angstroms and pore windows of about a size such as would be provided by 10-membered rings of oxygen atoms. It is to be understood, of course, that these rings are those formed by the regular disposition of the tetrahedra making up the anionic framework of the crystalline zeolite, the oxygen atoms themselves being bonded to the silicon or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred zeolites useful as starting materials in this invention, possess, in combination: a Constraint Index, as hereinafter defined, of from 1 to 12, a silica to alumina mole ratio of from 10:1 to 100:1, and a structure providing constrained access to the intracrystalline free space. The silica to alumina mole ratio referred to may be determined by conventional analysis. This ratio is meant to represent, as closely as possible, the ratio in the rigid anionic framework of the zeolite crystal and to exclude aluminum in the binder or in cationic or other form within the channels. In some zeolites, the upper limit Of silica to alumina mole ratio is unbounded, with values of 30,000 and greater. Such zeolites may not be good starting materials for the present invention since they already have such low alpha values that little or nothing would be gained by the thermal treatment of the present invention. Suitable starting materials have a 10:1 to 100:1 silica to alumina mole ratios. Preferred starting materials will have silica to alumina mole ratios from 12:1 to 80:1; for example from 20:1 to 70:1. These shape selective materials are useful in a variety of processes, such as methanol to gasoline and catalytic dewaxing, and a lot of these materials are made for such uses. It is also inherent that these materials will have a high alpha activity, usually well in excess of 100. These shape selective zeolites, after activation, acquire an intracrystalline sorption capacity for normal hexane which is greater than that for water, i.e., they exhibit "hydrophobic" properties. It is believed that this hydrophobic character is advantageous in the present invention. The zeolites used as starting materials in this invention freely sorb normal hexane and have a pore dimension greater than about 5 Angstroms. In addition, their structure must provide constrained access to some larger molecules. It is sometimes possible to judge from a known crystal structure whether such constrained access exists. For example, if the only pore windows in a crystal are formed by 8-membered rings of oxygen atoms, then access by molecules of larger cross-section than normal hexane is substantially excluded and the zeolite is not of the desired type. Zeolites with windows of 10-member rings are preferred, although excessive puckering or pore blockage may render these zeolites substantially ineffective. Zeolites with windows of 12-membered rings do not generally appear to offer sufficient constraint to produce the advantageous conversions desired in the instant invention, although structures can be conceived, due to pore blockage or other cause, that may be operative. Rather than attempt to judge from crystal structure whether or not a zeolite possesses the necessary constraint access, a simple determination of the "Constraint Index" may be made by continuously passing a mixture of equal weight of normal hexane and 3-methylpentane over a small sample, approximately 1 gram or less, of zeolite at atmospheric pressure according to the following procedure. A sample of the zeolite, in the form of pellets or extrudate, is crushed to a particle size about that of coarse sand and mounted in a glass tube. Prior to testing, the zeolite is treated with a stream of air at 538°C for at least 15 minutes. The zeolite is then flushed with helium and the temperature adjusted between 288°C and 510°C to give an overall conversion between 10% and 60%. The mixture of hydrocarbons is passed at 1 liquid hourly space velocity (i.e., 1 volume of liquid hydrocarbon per volume of catalyst per hour) over the zeolite with a helium dilution to give a helium to total hydrocarbon mole ratio of 4:1. After 20 minutes on stream, a sample of the effluent is taken and analyzed, most conveniently by gas chromatography, to determine the fraction remaining unchanged for each of the two hydrocarbons. The "Constraint Index" is calculated as follows: The constraint index approximates the ratio of the cracking rate constants for the two hydrocarbons. Zeolites suitable for the present invention are those which employ a zeolite having a constraint index from 1.0 to 12.0. Constraint Index (C.I.) values for some typical zeolites, including some not within the scope of this invention, are: Zeolite C.I. Erionite 38 ZSM-5 8.3 ZSM-11 8.7 ZSM-35 6.0 TMA Offretite 3.7 ZSM-38 2.0 ZSM-12 2 ZSM-4 0.5 Acid Mordenite 0.5 REY 0.4 Amorphous Silica-Alumina 0.6 ZSM-23 9.1 The above-described Constraint Index is an important and even critical definition of those zeolites which are useful in the instant invention. The very nature of this parameter and the recited technique by which it is determined, however, admit of the possibility that a given zeolite can be tested under somewhat different conditions and thereby have different constraint indexes. Constraint Index seems to vary somewhat with severity of operation (conversion). Therefore, it will be appreciated that it may be possible to so select test conditions to establish multiple constraint indexes for a particular given zeolite which may be both inside and outside the above-defined range of 1 to 12. Thus, it should be understood that the parameter and property "Constraint Index" as such value is used herein is an inclusive rather than an exclusive value. That is, a zeolite when tested by any combination of conditions within the testing definition set forth hereinabove to have a constraint index of 1 to 12 is intended to be included in the instant catalyst definition regardless that the same identical zeolite tested under other defined conditions may give a constraint index value outside of 1 to 12. A useful measure of the acid activity of a material is the alpha activity or alpha value. The alpha value reflects the relative activity of the catalyst with respect to a high activity silica-alumina cracking catalyst. To determine the alpha value as such term is used here in, n-hexane conversion is determined at about 427°C. Conversion is varied by variation in space velocity such that a conversion level of 10 to 60 percent of n-hexane is obtained and converted to a rate constant per unit volume of zeolite and compared with that of silica-alumina catalyst which is normalized to a reference activity of 538°C. Catalytic activity of the catalysts are expressed as multiple of this standard, i.e., the silica-alumina standard. The silica-alumina reference catalyst contains about 10 weight percent Al₂O₃ and the remainder SiO₂. This method of determining alpha, modified as described above, is more fully described in the Journal of Catalysis, Vol. VI, pages 278-287, 1966. The zeolites utilized as promoters herein constitute an unusual class of natural and synthetic materials. They are characterized by having a rigid crystalline framework structure composed generally of an assembly of atoms other than those of oxygen, each surrounded by a tetrahedron of shared oxygen atoms, and a precisely defined pore structure. Two of the crystalline porous zeolites which may be used are ZSM-5 and ZSM-11, which have in common some characteristic X-ray diffraction pattern lines. Four strong lines common to ZSM-5 and ZSM-11 are set forth in Table 1 below: These values were determined by standard techniques. The radiation was the K-alpha doublet of copper, and a scintillation counter spectrometer with a strip chart pen recorder was used. The peak heights, I, and the positions as a function of 2 times theta, where theta as the Bragg angle were read from the spectrometer chart. From these, the relative intensities, 100 I/Io where Io is the intensity of the strongest line or peak and d(obs.), the interplanar spacing in A, corresponding to the recorded lines, were calculated. The members of the class of zeolites defined herein of the catalyst are exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and other similar materials. ZSM-5 is described in U. S. Patent No. 3,702,886 and U.S. Reissue Patent No. 29,948, ZSM-11 in U. S. Patent No. 3,709,979, ZSM-12 in U. S. Patent No. 3,832,449 ZSM-23 in U. S. Patent No. 4,076,842, ZSM-35 in U. S. Patent No. 4,016,245, ZSM-38 in U. S. Patent No. 4,046,859, and ZSM-48 in U. S. Patent No. 4,377,497. Natural zeolites may sometimes be converted to this class of zeolites by various activation procedures and other treatments such as base exchange, steaming, alumina extraction and calcination, alone or in combinations. Natural minerals which may be so treated include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite and clinoptilolite. The preferred zeolites of the catalyst are ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38, with ZSM-5 and ZSM-11 particularly preferred. Preferably, the thermally treated zeolite comprises from 0.025 to 25, especially from 1 to 15, weight percent of the total amount of conventional cracking catalyst plus the thermally treated zeolite plus matrix, if any. The zeolites used in this invention may be in the hydrogen form or they may be base exchanged or impregnated to contain a rare earth cation complement. Such rare earth cations comprise Sm, Nd, Pr, Ce and La. It is desirable to calcine the zeolite after base exchange. In a preferred aspect of this invention, the zeolites comprising the catalysts herein are selected as those having a crystal framework density, in the dry hydrogen form, of not substantially below about 1.6 grams per cubic centimeter. It has been found that zeolites which satisfy all three of these criteria are most desired. Therefore, the preferred catalysts of this invention are those comprising a zeolite having a constraint index as defined above of about 1 to 12, a silica to alumina mole ratio of from 10:1 to 100:1 and a dried crystal density of not substantially less than about 1.6 grams per cubic centimeter. The dry density for known structures may be calculated from the number of silicon plus aluminum atoms per 1000 cubic Angstroms, as given, e.g., on page 19 of the article on Zeolite Structure by W. M. Meier. This paper is included in Proceedings of the Conference on Molecular Sieves, London, April 1967 , published by the Society of Chemical Industry, London, 1968. When the crystal structure is unknown, the crystal framework density may be determined by classical pycnometer techniques. For example, it may be determined by immersing the dry hydrogen form of the zeolite in an organic solvent which is not sorbed by the crystal. It is possible that the unusual sustained activity and stability of this class of zeolites are associated with its high crystal anionic framework density of not less than about 1.6 grams per cubic centimeter. This high density, of course, must be associated with a relatively small amount of free space within the crystal, which might be expected to result in more stable structures. This free space, however, seems to be important as the locus of catalytic activity. Crystal framework densities of some typical zeolites, including some which are not within the purview of this invention, are: The catalysts of this invention may be in a matrix, used alone or mixed with other conventional catalysts. The particle size of the individual component particles may be quite small, for example from 20 to 150 microns, when intended for use in fluid bed operation, or they may be as large as up to about 1 cm for fixed bed operation. Or the components may be mixed as powders and formed into pellets or extrudate, each pellet containing both components in substantially the required proportions. In one embodiment, the thermally treated zeolite, e.g., ZSM-5, and a conventional cracking catalyst, e.g., RENaY, are mixed together in a common clay or amorphous silica/alumina binder and spray dried, pelleted, pilled to the desired size and shape. As is the case of many catalysts, it is desirable to incorporate the zeolite component in a matrix. Such matrix is useful as a binder and imparts greater resistance to the catalyst for the severe temperature, pressure and velocity conditions encountered in many cracking processes. Matrix materials include both synthetic and natural substances. Such substances include clays, silica and/or metal oxides. The latter nay be either naturally occurring or in the form of gelatinous precipitates, sols or gels including mixtures of silica and metal oxides. Frequently, zeolite materials have been incorporated into naturally occurring clays, e.g. bentonite and kaolin. In addition to the foregoing materials, the zeolite for use herein can be composited with a porous matrix material such as silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well as ternary compositions such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix can be in the form of a cogel. A mixture of clay in combination with silica or any of the above specified cogels to form a matrix is highly preferred. Conventional cracking catalysts contain active components which may be zeolitic or non-zeolitic. The non-zeolitic active components are generally amorphous silica-alumina and crystalline silica-alumina. However, the major conventional cracking catalysts presently in use generally comprise a crystalline zeolite (active component) in a suitable matrix. Representative crystalline zeolite active component constituents of conventional cracking catalysts include zeolite A (U. S. Patent No. 2,882,243), zeolite X (U. S. Patent No. 2,882,244), zeolite Y (U. S. Patent No. 3,130,007), zeolite ZK-5 (U. S. Patent No. 3,247,195), zeolite ZK-4 (U. S. Patent No. 3,314,752), synthetic mordenite and dealuminized synthetic mordenite, merely to name a few, as well as naturally occurring zeolites, including chabazite, faujasite, mordenite, and the like. Preferred crystalline zeolites include the synthetic faujasite zeolites X and Y, with particular preference being accorded zeolite Y, more preferably, a rare earth exchanged Y or an ultrastable Y zeolite. In general, the crystalline zeolites are ordinarily ion exchanged either separately or in the final catalyst with a desired cation to replace alkali metal present in the zeolite as found naturally or as synthetically prepared. The exchange treatment is such as to reduce the alkali metal content of the final catalyst to less than 1.5 weight percent and preferably less than 0.5 weight percent. The purpose of ion exchange is to substantially remove alkali metal cations which are known to be deleterious to cracking, as well as to introduce particularly desired catalytic activity by means of the various cations used in the exchange medium. For the cracking operation described herein, preferred cations are hydrogen, ammonium, rare earth and mixtures thereof, with particular preference being accorded rare earth. Ion exchange is suitably accomplished by conventional contact of the zeolite with a suitable salt solution of the desired cation such as, for example, the sulfate, chloride or nitrate. It is preferred to have the crystalline zeolite of the cracking catalyst in a suitable matrix, since this catalyst form is generally characterized by a high resistance to attrition, high activity and exceptional steam stability. Such catalysts are readily prepared by dispersing the crystalline zeolite in a suitable siliceous sol and gelling the sol by various means. The inorganic oxide which serves as the matrix in which the above crystalline zeolite is distributed includes silica gel or a cogel of silica and a suitable metal oxide. Representative cogels include silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-beryllia, silica-titania, as well as ternary combinations such as silica-alumina-magnesia, silica-alumina-zirconia and silica-magnesia-zirconia. Preferred cogels include silica-alumina, silica-zirconia or silica-alumina-zirconia. The above gels and cogels will generally comprise a major proportion of silica and a minor proportion of the other aforementioned oxide or oxides. Thus, the silica content of the siliceous gel or cogel matrix will generally fall within the range of 55 to 100 weight percent, preferably 60 to 95 weight percent, and the other metal oxide or oxides content will generally be within the range of 0 to 45 weight percent and preferably 5 to 40 weight percent. In addition to the above, the matrix may also comprise natural clays, synthetic clays, or chemically treated clays such as kaolin type clays, montmorillonite, bentonite or halloysite. These clays may be used either alone or in combination with silica or any of the above specified cogels in matrix formulation. Where a matrix is used, content of crystalline zeolite, i.e. the amount of the zeolite Y component, is generally between 5 and 50 weight percent. Ion exchange of the zeolite to replace its initial alkali metal content can be accomplished either prior to or subsequent to incorporation of the zeolite into the matrix. The above compositions may be readily processed so as to provide fluid cracking catalysts by spray drying the composite to form microspheroidal particles of suitable size. Alternatively, the composition may be adjusted to suitable concentration and temperature to form bead type catalyst particles suitable for use in moving bed type cracking systems. The catalyst may also be used in various other forms such as those obtained by tabletting, balling or extruding. Thermal Treatment In the present invention, it is not necessary to either make shape selective zeolites with very high silica/alumina ratios, nor to use conventional aluminum extraction techniques on shape selective zeolites having relatively high aluminum contents. In the present invention, a relatively simple thermal treatment profoundly transforms the catalytic activity of ZSM-5 and other shape selective zeolites with silica/alumina ratios of 10:1 to about 100:1. After the controlled thermal treatment disclosed herein, these shape selective materials behave, catalytically, as if they had much higher silica/alumina ratios, while still retaining a significant amount of acid activity. There is nothing magical about a certain time or temperature or gas used in thermal treatment, although a steam containing atmosphere is preferred. The art has long recognized that steaming has a deactivating effect on FCC catalysts in general, and has used this deactivating effect to avoid undesirable hyperactivity when starting up a unit containing fresh catalyst. The prior workers in this field realized the benefits of controlled thermal treatment. We use a thermal treatment which is relatively severe, to profoundly change the catalytic characteristics of the shape selective material. The broad general parameters of steaming to deactivate catalyst are well known in the art. What is not known is to continue steaming the shape selective catalyst, with a relatively high aluminum content, until a catalyst with an alpha activity of 1 to 10 is obtained, and use of this catalyst in a cracking process. In general, thermal treatment conditions include temperatures of 250 to 1000°C, and, preferably, 500 to 800°C, pressures of subatmospheric to superatmospheric, although operation at atmospheric pressure gives good results. Thermal treatment time will depend on temperature and presence of steam. Higher temperatures and steam will shorten treatment time required. Treatment times, of 0.01 to 1,000 hours, preferably 0.1 to 100 hours, are satisfactory. The treatment may be conducted in either fixed bed, fluid bed, or moving bed operation, with fluid bed steaming being preferred. The fluid bed treatment eliminates dead spots and channeling within the catalyst bed and the presence of steam reduces the temperature required for reduction of the alpha value. Operation with pure steam or steam mixed with inert gases or with air or other gases gives acceptable results. Operation with pure steam or steam mixed with air is preferred. Small amounts of reacting gases could also be beneficial, such as acidic gases, Cl, F, HCl and organo halogens. Steaming should be conducted until the cracking activity, or alpha activity or value as measured by the alpha test is reduced to about 1 to 10. When starting with a ZSM-5 material with a silica/alumina ratio of 70:1, the alpha activity of the virgin material is about 200. Steaming this material for 8 hours at 690°C temperature in a fluidized bed, fluidized by a live steam, should reduce the alpha activity to about one. A ZSM-5 material with a silica:alumina mole ratio of about 35:1 will have a virgin alpha value of 750. Steaming of this material in a fluidized bed of live steam for 6 hours at 600°C should give an alpha value of about 10. A similar procedure is used when steaming other shape selective materials, such as ZSM-11, or any other shape selective material with a constraint index of from 1 to 12 and a silica to alumina mole ratio of from 10:1 to 100:1. Although we are not positive why thermal treatment, preferably steaming, brings about such a profound change in the catalyst activity, we believe that the effect of steaming is to change the distribution of aluminum atoms within the zeolite framework. We are confident that the aluminum is not volatilized by the treatment, it is not removed as a vapor, nor does it plate out on the walls of the vessel used to contain the catalyst during thermal treatment. An elemental analysis of the shape selective additive both before and after steaming would show essentially the same aluminum content. The aluminum is still in the shape selective material, though probably not occupying some of its original positions in the framework of the zeolite. Because the thermally treated shape selective material of the present invention exhibits catalytic behavior similar to that of shape selective material made with very low aluminum contents, it is possible that steaming simply displaces aluminum atoms to some non functional portion of the zeolite framework so that the resulting skeletal structure is aluminum-deficient. It is also possible that some much more complex or entirely different phenomenon is happening within the zeolite to account for the profound change in catalytic activity which is observed. EXAMPLES Example 1 - Prior Art This example is copied from U.S. Patent No. 4,340,465 to show that simple steaming of a ZSM-5 with a 39:1 SiO₂:Al₂O₃ mole ratio did not make it behave like a ZSM-5 material with a 2200:1 SiO₂:Al₂O₃ ratio given a similar steam treatment. These catalysts were severely steamed, resulting in significant catalyst deactivation. We estimated that these catalysts would have the following alpha activity after steaming for 6 hours at 760°C. SiO₂/Al₂O₃ Alpha Activity 2200 0.4 200 0.5 39 0.5 39 0.5 Example 2 - Prior Art Example 2 shows the effect of mixing conventional ZSM-5 material, with varying silica to alumina mole ratios in with a conventional FCC catalyst in a conventional FCC process. All the additive catalysts used in this study were prepared in fluid particle size by dispersing the ZSM-5 shape selective component, in a freshly prepared silica-alumina-clay matrix or a conventional processed silica alumina matrix, followed by spray drying. The ZSM-5 level in the additive was 25% by weight. The freshly prepared silica-alumina-clay matrix required a base exchange in conventional manner with solutions of (NH₄)₂SO₄ followed by RECl₃. The resulting fluid particles, containing the octane promoting ZSM-5, were added to a Filtrol FS30, cracking catalyst, steamed 4 hours at 760°C. In most cases 0.25 weight percent ZSM-5 based upon total cracking catalyst was used. The alpha activity data reported were obtained at 538°C. for the additive. The alpha activity was determined using particles which contained the 25 weight percent ZSM-5 component. The catalytic cracking data summarized in Table 4 were obtained in a bench scale fluidized bed unit with the steamed Filtrol FS30 catalyst as the base. To this base was added the various promoting fluid catalyst particles containing the ZSM-5 octane promoter. These combination catalysts were evaluated with the Joliet Sour Heavy Gas Oil (JSHGO) or with a hydrotreated Arab Light Resid (HDT ALR) as described in Tables 1 and 2. Properties of these feeds are presented in Table 3. Example 3 - Present Invention This example shows that catalytically, steamed ZSM-5 with a silica:alumina ratio of about 70 acts as if it had a relatively high silica:alumina mole ratio, after steaming to reduce its alpha activity to 0.6 to 3. The same test apparatus, and feedstock used in Example 2 was used for Example 3. Results are reported in Table 5. Using the process of the present invention, it is possible to take ZSM-5, with relatively low silica:alumina mole ratio, subject this to a controlled thermal treatment, and obtain ZSM-5 which behaves as if it had a significantly higher silica to alumina mole ratio. The process results obtained using the steamed catalyst show that use of the steamed catalyst with a SiO₂:Al₂O₃ ratio of, e.g., 40:1 to 90:1 is about as good as the use of, e.g., 1500:1 silica to alumina ZSM-5. Some data indicate that the steamed material may give slightly higher yields of gasoline than will addition of, e.g., 1500:1 ZSM-5 material. Even if the results of using the steamed catalyst are deemed to be no better than those achieved using catalyst manufactured to have a very high silica:alumina mole ratio, the present invention provides another way of obtaining these benefits without making a special batcn of ZSM-5 catalyst with a high SiO₂:Al₂O₃ ratio or subjecting the conventional material to an aluminum extraction step. It is also possible to simplify manufacturing and inventory procedures, whereby ZSM-5 with a 70:1 silica to alumina mole ratio, can, after thermal treatment, be made to take the place of ZSM-5 with a 1500:1 silica:alumina mole ratio. Using the process of this invention, petroleum refiners can add relatively small amounts of thermally treated shape selective catalyst to their conventional FCC units containing conventional catalysts and obtain an immediate increase in octane number with only a slight, or perhaps no, yield penalty. The final alpha number of the thermally treated shape selective catalyst which is the subject of the present invention can be determined based on the relative cost of the shape selective material to conventional catalyst, the need for octane, and the aging characteristics of the added shape selective material in that particular unit. In general, higher initial alpha activities are needed to compensate for rapid aging of, e.g., ZSM-5 within the FCC unit. Today we do not know the optimum amount and alpha activity of ZSM-5 to be added, although we suspect that in commercial practice it will be beneficial to operate with an initial alpha activity, on the order of 5 to 10, to compensate for aging of the material in the cracking unit. It is believed that the steam treated ZSM-5 having the alumina redistributed, will be more stable in use and have less activity decline.

Claims: A process for catalytically cracking a hydrocarbon feed having an initial boiling point of at least 200 o C, a 50% point range of at least 260 o C and an end point of at least 320 o C to improve the octane number of the cracked product, which process comprises contacting the feed in a reaction zone under catalytic cracking conditions including a temperature from 150 o C to 750 o C in the absence of added hydrogen with a catalyst comprising a conventional cracking catalyst to produce a cracked hydrocarbon product which is recovered as a product of the process wherein there is added to the conventional cracking catalyst a thermally treated zeolite having a mole ratio of SiO₂:Al₂O₃ from 10:1 to 100:1 and a Constraint Index from 1 to 12, wherein prior to use in the process the zeolite has been subjected to thermal treatment at a temperature from 250 o C to 1000 o C at a pressure from 0 to 980.7 KPa (0 to 10 atmospheres) for a time from 0.01 to 1000 hours to reduce its alpha activity to a value from 1 to 10. A process accords to Claim 1 wherein the ratio of SiO₂:Al₂O₃ is from 12:1 to 80:1. A process according to Claim 1 to 2 wherein the conventional catalyst comprises a crystalline aluminosilicate cracking component having a pore size greater than 0.7 nm (7 Angstrom units) in a matrix. A process according to Claim 3 wherein the crystalline aluminosilicate cracking component comprises a Y type zeolite. A process according to Claim 3 wherein the crystalline aluminosilicate cracking component comprises an REY zeolite. A process according to Claim 3 wherein the crystalline aluminosilicate cracking component comprises ultrastable Y. A process according to any of Claims 1 to 6 wherein the catalytic cracking conditions include a temperature from 400 to 700 o C, a pressure from o to 980.7 KPa (0 to 10 atmospheres), a catalyst to hydrocarbon weight ratio from 1:1 to 20:1, and a residence time of the hydrocarbon in the reaction zone from 0.1 to 20 seconds. A process according to any of Claims 1 to 7 wherein the thermal treatment conditions include the presence of steam with a steam partial pressure of 98.07 KPa (one atmosphere) at a temperature from 400 to 800 o C for a time from 0.1 to 100 hours. A process according to any of Claims 1 to 8 wherein the thermally treated zeolite is selected from ZSM-5 and ZSM-11, with SiO₂:Al₂O₃ ratios from 12:1 to 80:1, and the zeolites have an alpha activity, before treatment, exceeding 100. A process according to Claim 9 wherein the thermal treatment of the zeolite reduces its alpha value from 5 to 10 and is conducted with steam at atmospheric pressure and a temperature from 500 to 800 o C for a time from 0.1 to 100 hours. A process according to any of Claims 1 to 10 wherein the zeolite and the conventional cracking catalyst are in a common matrix. A process according to claim 9 wherein the zeolite is NaZM-5. A process according to Claim 12 wherein the silica to alumina mole ratio is 70:1. A process according to any of Claims 1 to 10 wherein the zeolite is added as a separate catalyst. A process according to any of Claims 1 to 14 wherein the thermally treated zeolite comprises from 0.025 to 25 weight percent of the total amount of conventional cracking catalyst plus the thermally treated zeolite plus matrix, if any. A process according to Claim 15 wherein the thermally treated zeolite comprises from 1 to 15 weight percent of the total amount of catalyst. A process according to any of Claims 1 to 16 wherein the catalytic cracking process is a fluid catalytic cracking (FCC) process.

Inventors: DWYER, FRANCIS GERARD, ROSINSKI, EDWARD JOSEPH, SCHWARTZ, ALBERT B.
Applicants: MOBIL OIL CORPORATION
Application year: 1985
Main classification: C10G11
IPC codes: C10G11, B01J29