CA1050956A - Catalytic cracking - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A novel particulate material for promoting combustion of carbon monoxide to carbon dioxide in the regeneration zone of a cyclic fluid cracking process without substantially affecting the ability of separate fluid cracking catalyst particles containing an active crystalline zeolitic aluminosilicate component to catalyze the hydrocarbon conversion reaction in the conversion zone. The novel promoter particles comprise coherent, catalytically inert microspheres of calcined kaolin clay having a SiO2/Al2O3 molar ration of about 2/1, a surface area (B.E.T.) in the range of about 10 to 15 m2g., a pore volume (as determined by nitrogen absorption) in the range of about 0.02 to 0.04 cc./gm., the calcined microspheres being impregnated with a trace amount of a platinum compound and being free from a component capable of cracking hydrocarbons in the absence of added hydrogen.

Description

105~)956 BACKGROUND OF THE INVENTION
1. Pield of the Invention This invention relates to the well-known continuous cyclic fluid catalytic cracking (FCC) of hydrocarbons wi-th a catalyst, generally a catalyst containing a crystalline aluminosilicate zeolite component, in the absence of added hydrogen to produce gasoline, which cracking results in the for~ation on the catalyst particles of a deposit of combustible hydrocarbons known as coke, and the spent catalyst particles from the catalytic reactor are regenerated in a separate zone by burning off sufficient coke to place the catalyst particles in a condition suitable for recycling to the hydrocarbon conversion zone.
In particular the invention is concerned with a solid additive capable of promoting combustion of carbon monoxide to carbon dioxide in FCC
regenerators wlthout appreciably affecting the ability of the catalyst particles to catalyze the hydrocarbon conversion reaction in the conversion zone.
, Present-day continuous cyclic FCC processes utilize j fluidizable catalyst particles containing a crystalline zeolitic aluminosilicate component (usually an ion-exchanged form of a synthetic faujasite such as zeolite X or Y) and a porous inorganic oxide matrix.
I This type of catalyst must be regenerated to low carbon levels, typically 0.5% or less, to assure required activity and selectivity beEore the catalyst particles can be recycled to a conversion zone. In most regenerators, the combustible solids deposited on the spend solid catalyst ~; 25 - particles from the cracking zone are burned in a con*ined regeneration one in the form of a fluidized bed which has a relatively high concentration of catalyst particles (dense phase). A region of lower solids concentration (light phase) ls maintained above the dense phase.
~A typical regeneration cycle is described in U. S. Patent 3,944,482 to Mitchell.

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High residual concentrations of carbon monoxide in flue gases from regenerators have been a problem since the inception of catalytic cracking processes. The evolution of FCC has resulted in the use of increasingly high temperatures in FCC regenerators in order to achieve the required low carbon levels in the regenerated crystalline aluminosilicate catalysts. Typically regenerators now operate at temperatures in the range of 1100 to 1350F. and result in flue gases having a CO2/CO ratio in the range of 1.5 to 0.8. The oxidation of carbon monoxide is highly exothermic and can result in so-called "carbon monoxide afterburning" which can take place in the dilute catalyst phase, in the cyclones or in the f~ue gas lines. ~fterburning has caused significant damage to plant equlpment. On the other hand, unburned carbon monoxlde in atmosphere~vented flue gases represents a loss of fuel value and ls ecologically undesirable.
Restrictions on the amount of carbon monoxide which can be exhausted into the atmosphere and the need for efficient coke removal from spent catalyst particles have stimulated several approaches to the provlsion of means for achieving a balance between afterburning and incomplete regeneration of spent fluid zeolitic catalysts.
It is well known that metal such as iron, nickel, vanadium and copper can promote carbon monoxide combustion when present as contamlnants in cracking feedstocks. ~arly in the development of catalytic cracking and long prior to the introduction of crystalline zeolitic aluminosilicate catalysts, it was proposed (U. S. 2,436,927 25- to Kassel) to prevent afterburning in fluldized catalytic cracking processes by introducing a small amount of a carbon monoxide oxidiæing catalyst. ~he proposed oxidant was an oxide of metals from the first transition series. It was suggested to introduce such material either as a component of the cracking catalyst or, preferably, as separate 3Q particles supported "on a suitable carrier." Such carrier was not gs6 described in the patent. Chromium oxide was proposed as an impregnant for gel-type moving bed cracking catalysts in U. S. 3,647,860 to Plank et al. This was also prior to the introduction of crystalline zeolitic catalysts. Subsequently it was suggested to incorporate titanium in ; 5 cracking catalysts for improved carbon monoxide conversion but this approach was directed to achieve only partial combustion of carbon monoxide since regenerators available at that time were not capable of withstanding the heat release resulting from full combustion.
U. S. 3,36~,136 to Chen suggested the use of a noble metal - 10 such as platinum to promote carbon monoxide oxidation in a regenerator of a FCC unit operated with a zeolitic aluminosilicate catalyst.
According to the teachings of the patent, the noble metal had to be held wlthin the inner pore structure of a so-called "shape selective"
zeol:Lte, speciflcally a zeolite having pores large enough to allow penetration of oxygen, carbon monoxide and carbon dioxide but too small for molecules of gas-oil. In one preferred embodiment, the particles of shape selective zeolite containing the oxygen promoter within the pores were contained in the same particles which included both the larger pore catalytically active zeolite and a conventional inorganic oxide matrix component. For example, the two different zeolites, one including a promoter such as platinum within the pores, were composited into unitary particles with an lnorganic oxide matrix material. An alternative I disclosed in the Chen patent invol~ed mixing the particles of sieve ; containing the oxidation promoter with particles of the zeolitic catalyst.
In a preferred embodiment of this alternative, the individual components were of different particle size so that the oxidation component could be withdrawn as well as added to the circulating catalyst mass to alter the degree of carbon monoxide conversion. In all variations of this technology, preparation of a costly small pore zeolitic component is required and the oxidant will be present on a high surface a:rea support.

' According to the teachings of West German Application DT 2444911, small amounts of metal or metallic elements of Period 5 and 6 of Group VIII of the Periodic Table or rhenium or compounds thereof are simply added in amounts up to 50 p.p.m. to conventional FCC (or TCC) catalysts to decrease the carbon monoxide content of flue gases, as e~idenced by the improved CO2/C0 ra-tio of such gases, without appreciably affecting the cracking properties of the catalysts. The metal component, preferably a platinum compound, is introducecl into the catalyst by impregnation or by ion exchange during any stage of catalyst manufacture, or even after the catalyst particles are formed. According to the teachings of the German patent application, the active cracking catalyst component (zeolitic aluminosilicate) is preferably ion-exchanged with the metal and the ion-exchanged material is composi~ed with the porous matrix to produce catalyst partlcles. The German appllcatlon also dlscloses that a sillcon-containing support or clay can be ion-exchanged or lmpregnated wlth the metal but there is no explanation as to how this is accompllshed.
Based on illustrative examples, a reasonable interpretation is that the exchanged support or clay is mixed with the catalytically active zeolite component to form composite catalyst particles in which the metal promoter and actlve æeollte are present in the same partlcles.
The patented techniques for preparlng a platlnum metal promoted cracking catalyst leave something to be desired. Impregnation or ion-exchange of the zeollte or the porous matrlx beEore composltlng - the constituents can be used only in the production of those catalysts in which the zeolite is formed separately from the matrix; for example, catalysts prepared as described in U. S. 2,140,249 and U.S. 2,140,253 to Plank et al. When a finished catalyst is trea-ted, the entire tonnage of catalyst must be processed. Similarly, the entire catalyst must be treated with a metal when the catalyst particles are produced in SitU
from preforms, such as catalysts produced in accordance with the teachings ' .

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of U. S. 3,647,718 to Haden et al. By way of example, in Example 10 of the DT 2444911 application, a promoted FCC catalyst was prepared containing 3 p.p.m. platinum by impregnating HFZ~20 zeolitic molecular sieve catalyst with a solution of platinum-tris (ethylenediamine) tetrachloride Eollowed by washing and drying. Using this technique on a commercial basis, the production of 10,000 tons of metal-promoted catalyst would require the use of about 35,000 tons of platinum solution to add only 100 tons of platinum. This would necessitate a substantial capital investment for equipment for impregnation, washing and drying.
Prior to our invention, the suggestion was made that the platinum oxidation might be incorporated on a solid support material. Presumably, a conventional high surface area gel-type catalyst was intended as the support.
A general object of the lnvention is to provide -lmprovements in prior art means for achieving controlled oxidation of carbon monoxide in the regeneration zone of a cyclic FCC process.
THE INVENTION
The essence of the present invention resides in promoting the combustion of carbon monoxide in a ~CC regenerator by the use of an additive obtained by uniformly impregnating a small amount of solution of a platinum compound on coherent fluidi.zable particles of kaolin clay calclned to a substantially anhydrous condition and having a low surface area (in the range of about 10 to 15 m2/g. as determ:Lned by the B.E.T.
nitrogen absorption method) and a total pore volume as determined by nitrogen absorption in the range of about 0.02 to 0.04 cc./g., said particles being free from a component having appreciable ability to crack hydrocarbons. The amount of platinum compound present in the particles is generally in the range of about 5 -to 150 p.p.m., most ` usually in the range of 50 to 100 p.p.m., expressed as platinum metal.
Another aspect of the invention comprises a cracking catalyst ~s~gs6 for use in a cyclic FCC cracking process, the catalyst being a mixture of a major weight percentage, preferably at least 90% by weight, of particles of a conventional zeolitic aluminosilicate FCC catalyst and a minor amount of separate particles of said platinum impregnated microspheres of calcined clay, the latter being present in amount such that the platinum content of the mixture is in the range of 1 to 50 p.p.m., preferably in the range of about 1 to 5 p.p.m.
Still another aspect of the invention comprises an improvement in a conventional cyclic FCC process carried out in the absence of added hydrogen. The improvement comprises the use of a catalyst which is a mixture of fluidizable particles of a conventional zeolitic catalyst and separate particles of the novel oxidation promoter of the lnventlon, the mlxture being introduced into a cracking zone and subsequently regenerated ln a separate regeneration zone by burning and recycled into a crack-Lng zone. This embodiment of the invention is especially adapted for use in cracking units in which essentially complete combustion of carbon monoxide to carbon dioxlde is feasible. However, the catalyst mixture may be useful in achieving partial controlled combustion in units in which complete combustion is not feasible; for example, in regenerators not capable af withstanding the high temperatures resulting ~rom complete combustion.
The novel particulate promoter oE the invention has the desirable properties o mechanical hardness (generally comparable to that of quality FCC zeolitic catalyst particles) and it is readily fluidized in conversion zones and in the regenerator. The base material ; for the promoter (calcined microspheres of kaolin clay) is relatively inexpensive. The apparent bulk density of such promoter (0.9) is similar to that of conventional equilibrium FCC catalysts and undesirable segregation of the promoter during storing, shipment or use of a mixture of the promoter and active FCC catalyst particles is minimized.

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Processing advantages over pri~r art methods involving impregnation or ion-exchange of the entire catalyst tonnage are sel~-evident. Only a fraction of -the catalyst requires treatment and risks of platinum contamin-ation and 105s of platinum are minimized. In marked contrast to the separate promoter particles o~ the Chen patent (supra) which contain a high surface area zeolitic component, the promoter particles of this in-vention do not contain zeolite and they have a relatively low surface area.
An unexpected benefit of providing platinum-containing promoter and cracking catalyst in di~feren-t particles, the promoter being impregnated on calcined microspheres of kaolin clay, is that the mixture is frequently more effective in promoting the oxidation of carbon monoxide to carbon dioxide in a regenerator than would be the case if the same quantity of platinum were impregnated on the particles of the active cracking catalyst.
D~SCRIPTION OF PREFERRED EMBODIMENTS
The microspheres of calcined kaolin clay used in the production of the promoter particles are known in the art and are employed as a chemical reactant wi~h a sodium hydroxide in the manufacture of ~luid zeolitic cracking catalysts as described ln U. S. 3,647,718 to ~aden et al. In practice of the instant invention, in contrast, the micr~spheres of calcined kaolin clay are not used as a chemical reactant. Thus the chemical composition of the microspheres of calcined clay used in practice of this invention corresponds to that of a dehydrated kaolin clay. Typically, the calcined microspheres analyze about 51% to 53% (wt.) SiO2, ~1 to ~5% A12O3, and from O to 1% H20, the balance being minor amounts of indigenous i~purities, notably iron, titanium and alkaline earth metals. Generally, iron content (expressed as ~e2O3~ is about 1/2% by weight and titanium (expressed as TiO2) is approximately 2%. It is reasonable to believe that the metallic impurities in kaolin clay which are present in the microspheres may contribute to the outstanding effectiveness of the platlnum impregnated microspheres as a promoter for carbon monoxide oxidation.

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9~6 The microspheres are preferably produced by spray during an aqueous suspension of kaolin clay. The term "kaolin clay" as used herein embraces clays, the predominating mineral constituent of which is kaolinite, halloyslte, nacrite, dickite, anauxite and mixtures thereof. Preferably a fine particle size plastic hydrated clay, i.e!., a clay containing a substantial amount of submicron size particles, is used in order to produce microspheres having adequate mechanical strength.
To facilitate spray drying, the powdered hydrated clay is preferably dispersed in water in the presence of a defloccuLIting agent exemplified by sodium silicate of a sodium condensed phosphate salt such as tetrasodium pyrophosphate. By employing a deflocculating agent, spray drying may be carried out at higher solids levels and h æder products are usually obtained. When a deElocculating agent is employed, slurries containlng about 55 to 60% sollds may be prepared and these hlgh sollds slurries are preferred to the ~O to 50% slurries which do not contain a deflocculating agent.
Several procedures can be followed in mixing the ingredients to form the slurry. One procedure, by way of example, is to dry blend the finely divlded solids, add the water and then incorporate the deflocculating agent. The components can be mechanically worked together or individually to produce slurrles or deslred vlscosity characteristics.
Spray dryers wlth countercurrent, cocurrent or mixecl counter-current and cocurrent flow of slurry and hot alr can be employed to produce the~microspheres. The air may be heated electrically or by other indirect means. Combustlon gases obtained by burning hydrocarbon fuel in alr can be used.
Using a cocurrent dryer, air lnlet temperatures ~o 1200F.
ma~ be used when the clay feed is charged at a rate sufficient to produce an air outlet temperature within the range of 250F. to 600F. At these temperatures, free moisture is removed from ~he slurry ~ithout removing _~_ ., . . ~ ~ . . . .

water of hydration (water of crystallization) from the raw clay ln~redient.
; Dehydration of some or all of the r~w clay during spray drying is, however, within the scope of the invention. The spray dryer discharge may be fractionated to recover microspheres of desired parti,rle size. Typically S particles having a diameter in the range of 20 to 150 microns are preferably recovered for use in preparing the support for the platinum promoter.
While it is preferable in some cases to calcine the microspheres at temperatures in the range of about 1600F. to 21C'~0F. in order to produce particles of maximum hardness, it is possible to dehydrate the microspheres by calcination at lower temperatures; for example, temperatures in the range of 100F. to 1600F. 9 thereby converting the clay into the material known as "metakaolin." After calcination the micro-spheres should be cooled and fractlonated, if necessary, to recover the portion which is in desired size range.
Pore volume of the microspheres will vary slightly with the calcination temperature and duration of calcination. Pore size distribution analysis of a representative sam~le obtained by nitrogen desorption indicates that most of the pores have diameters in the range of 150 to 600 Angstrom ' units.
I 20 The surface area of the calcined microspheres is usually within the range of lQ to 15 m2/g. as measured by the well-known B.E.T.
method using nitrogen ab~orptlon. It is noted that the surface areas of commercial fluid zeolitic catalysts i,~ considerably higher, generally exceeding values of 100 m2/g. as measured by ~he B.E.T. method.
Simple impregnation of the calcined microspheres with an aqueous solution of a soluble platinum compound will su~fice to achieve uniform deposition of the trace platlnum compound on the spray dried calcined microspheres since these microspheres have adequate porosity for uniform depositio~n of trace amoun~s of an i~ regnant. However, the porosity of the calcined microspheres is sufficiently low to minimize coke deposition ln '.' .
.! _9_ ''i i the cracking zone of a FCC unit.
The platinum compound may be one in which the platinum is in the anion, such as for exa~ple chloroplatinic acid, or the platinum may be in the cation, such as for example Pt (ethylene diamine) C14. During impregnation, the microspheres should be agit:ated. Preferably the solution of platinum compound is applied by means of a spray. Provided the platinum compound is applied as an aqueous solution oi- sufficiently high concentration, a drying step will be optional after impregnation. ~efore use or during use, the platinum impregnated microspheres are contacted with hot air or steam, possibly converting the platinum compound to an oxide. Any conventional method for impre~nating platinum on inorganic support material may be used and sources of platinum other than the specific materials mentioned above may be ernployed. DT2444911 (supra), U. S. 3,840,514 to Haensel and ~. S.

2,971,904 to Gladrow et al set Eorth procedures that can be used. Such procedures are modified when necessary to reduce the amount of impregnated platinum to levels suitable for practice of this inventlon.
The amount of platinum deposited on the microspheres will depend inter alia on the proportion of impregnated microspheres to be blended with separate particles of active cracking catalyst and whether complete or partial combustion of carbon monoxide is desi~ea. Generally, ; from 70 to 95 part~ by weight of catalytically active cracking catalyst particles are mixed with 30 to 5 parts by weight o~ th~ platinum impregnated microspheres. Preferably the platinum impregnated microspheres constitute 10% by weight or less of the total mixture since the presence of more than 10% of the promoter particles may result in an appreciable ascrease in ~he cracking activity of the catalyst. Use of less than about 3 ~ 5% platinum impregnated microspheres can result in difficulties in securing ~iform blends. In general, the use of about 4 to 7% impregnated microspheres is especially preferable.
The level of platinum in a blend of promoter particles and .

separate catalyst particles is usually in the range of 3 to 10 p.p.m.
(based on the total mixture) when full combustion is desired~ From 0.5 to

3 p.p.m. may be used for partial combustion. A suitab]e level of platinum will vary with the design of a particular regeneration system.
In an illustrative example, microspheres of calcined kaolin clay were produced using a fine particle si~e uncalcined paper coating grade of hydrated Georgla kaolin clay as a starting material. The clay was formed into a slurry of about 60% solids using tetrasodium pryophosphate in amount of 0.5% of the clay weight as a deflocculating agent. The slurry was spray dried and calcined at a temperature of about 1900F. ~o an essentially - anhydrous condition. The calcined spray dried microspheres were screened to recover a desired fraction which had the following particle size dlstribution:
Tyler Screen Wt. %
15 ~100 1-2 -100 ~200 35-50 -200 +325 30-48 Surface area was 12.8 m2/g. (B.E.T. method, using nitrogen as an absorbate).
Pore volume (nitrogen absorption) was 0.026 cc./gm.
j~ A 600 gram charge oE the microspheres was placed in a Teflon-; coated 1-1/2 gallon can provided wfth flights. The can was ro~ated slowly ~35 r.p.m.) while an aqueous solution of chloroplatinum acid containing 400 p.p.m. Pt was sprayed as a fine mlst into the open drum. The concen-tratlon and amount of impregnating solutlon were calculated to incorporate 60 p.p.m. of platinum on the support without increasing the L.O.I. (loss on ignition as determined at 1800~F.) above 13.7%.
A sample of the impregnated microspheres calcined clay (5 parts by weight~ was blended with particles o HFZ-2Q cracking catalyst ~95 parts by weight). The ml~ture (identified as Sample A) had a platinum . .

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content of 3 p.p.m.
For purposes of comparison another sample of HFZ-20 cracking catalyst was impregnated with the solution of chloroplatinic acid in generally the same manner to provide a catalyst containing 3 p.p.m. Pt except that the amount and concentration of chloroplatinic acid were increased. The catalyst sample is identified as Sample B.
A sample of HFZ-20 without a promoter was identified as Sample C. A typical sample of H~Z-20 analyzes 0.9% Na20, 37.0% SiO2, 59.3% SiO2, 2.4% TiO2, 0.61% Fe203 and 13.0% L.O.I. Surface area is above 300 m2/g. before steaming.
Catalysts A, B and C were activated and aged by calcination at 1400F. and 1500F. for 4 hours in an atmosphere of 100% steam and the steamed catalysts were used in cracking gas-oil feedstock in a micro-activity test unit. It was found that with the exception of a slight increase in hydrogen make, catalysts A and B had substantially the same activity and selectivity as catalyst C. Thus, the presence of platinum did not materially affect the activity and selectivity of the HFZ-20 catalyst.
In order to determine whether the catalysts containing added platinum (A and B) were capable of promoting the oxidation of carbon monoxide to carbon dioxide, the following carbon monoxide conversion test was carried out with samples of catalysts A, B, a~d C steamed at 1400F. for 4 hours in an atmosphere of 100% steam.
To carry out the test, a fluidized bed of the sample was brought to a temperature of 1215F. in the presence of helium and a gas containing carbon dioxide (8%), carbon monoxide (4%) and oxygen (4%) was iniected through the catalyst. A~ter a steady state was established, a chromotograph was used to determine the CO2/C0 ratio in the effluent gas.
Catalysts A and B, both containing impregnated platinum, converted essentially all of the carbon monoxide to carbon clioxide, while the control (catalyst C) converted 25% of the carbon mono~ide. Thus, the carbon monoxide ~LOS0956 conversion test indicated that uncoked catalysts A and B were capable of catalyzing carbon monoxide burning.
; To compare the effectiveness of platinum promoters during regeneration, spent catalyst A was ~ixed with fresh catalyst A (steamed at 1400F.? to provide a blend containing 0.65% coke. The same was done wi~h catalysts B and C. To stimulate regeneration, a 3 to 4 gram sample of each spent (coked) catalyst was fluidized and heated to 1215F. in a hslium atmosphere. Air was then passed through the fluidized bed at-a - constant flow rate of 215 cc./min. for 5 minutes to burn off the coke. The gas was collected and the C02/CO ratio was determined by gas chromotography.
Results are summarized below in table form.
EFFECT OF PLATINUM ON REGEN~RATION
OF SPENT FCC CATALYST

C02/CO Ratio Upon Sample Regeneration i A - 95% HFZ-20 & 5% Pt impregnated 63 calcined microspheres of kaolin clay, 3 p.p.m. Pt B - HFZ-20 impregnated with 3 p.p.m. Pt 49 C - HPZ-20 - no Pt 1.3 Data for the regeneration test show that when catalysts A
and B were used to promote oxidation of carbon monoxide during conditions stimulating regeneration of a coked catalyst, the catalyst of the invention (catalyst A) was s-lgnificantly more effective than the catalyst containing the same amount of platinum impregnated directly on the catalyst particles (catatlys~ B~. As mentioned above, the data for the CO conversion test ; show that prior to coking, catalysts A and B were both capable of ` ~ catab zing fully the oxidation of carbon monoxide at 1215F. Since the surface area of the calcined kaolin support for the platinum in catalyst A
is only about 13-~m2kg.while the surface area of the support for the ,~ 3Q platlnum in catalyst B is over 300 m2/ g. a reasonable explanation for .;, ~ ~ -13-il :
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the superiority of catalyst A is that less coke is present on the support particles of catalyst A during regeneration with the result that the platinum is accessible for a longer period during regeneration to burn carbon monoxide. On the other hand, it is conceivable that the porous microstructure (minus lO0 Angstrom pores) oE a æeolitiæed HFZ-20 microsphere or conventional cracking catalyst is such that when the platinum is ion-exchanged or impregnated thereon (in~o a zeolitiæed microsphere) the platinum becomes less readily accessible and a diffusion controlled mechanism may prevail, thus hindering burn-off of CO to CO2.
Results similar to those detailed above were realized when the platinum impregnated microspheres were blended wlth other zeolitic cracking catalysts containing a type Y zeolite component, lncluding catalysts containing rare earth metals.
It is intended that the invention should not be limited to the details of the examples but broadly as deflned in the appended claims.

Claims (5)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An article of manufacture consisting of calcined spray dried microspheres of kaolin clay impregnated with a minor amount of a platinum compound, the platinum compound being present in amount sufficient to promote the oxidation of carbon monoxide to carbon dioxide in a regenerator for a fluid cracking catalyst without appreciably affecting the ability of a crystalline alumino-silicate cracking catalyst to catalyze the cracking of a hydrocarbon to produce gasoline in the absence of added hydrogen, said impregnated spray dried microspheres being free from a zeolite component having appreciable ability to crack hydrocarbons.

2. Spray dried microspheres of kaolin clay which have been calcined to a substantially anhydrous condition at a temperature in the range of about 1000 to 2100°F. and then impregnated with a platinum compound in amount such that the content of platinum, expressed as the metal, is within the range of 5 to 150 p.p.m., said particles having a SiO2/Al2O3 mole ratio about 2/1, and a surface area in the range of 10 to 15 m2/g.

3. As an article of manufacture particles of spray dried kaolin clay which have been calcined at a temperature in the range of about 1800 to 2100°F., said microspheres having a surface area, as measured by the B.E.T. nitrogen absorption method, in the range of about 10 to 15 m2/g., a pore volume, as measured by nitrogen absorption, in the range of 0.02 to 0.04 cc./gm. and a particle size distribution such that the particles are predominantly in the size range of 20 to 150 microns, said calcined microspheres having uniformly impregnated thereon a compound of platinum in amount such that the microspheres contain from 50 to 100 p.p.m. platinum, expressed as the metal.

4. The article of manufacture of claim 3 wherein said calcined microspheres have a pore size distribution such that most of the pores have diameters in the range of 150 to 600 Angstrom units.

5. A cracking catalyst composition consisting essentially of a physical mixture of 70 to 95 parts by weight of particles of a zeolitic aluminosilicate fluid cracking catalyst free from a noble metal and from 30 to 5 parts by weight of particles of spray dried kaolin clay in the form of microspheres, said microspheres having been calcined at a temperature in the range of about 1000 to 2100°F., having a surface area, as measured by the B.E.T. nitrogen absorption method, in the range of about 10 to 15 m2/g., a pore volume, as measured by nitrogen absorption, in the range of about 0.02 to 0.04 cc./gm. and a particle size distribution such that the particles are predominantly in the size range of 20 to 150 microns, said calcined microspheres having thereon a compound of platinum in amount such that the microspheres contain from about 50 to 100 p.p.m. platinum, expressed as the metal, said catalyst composition containing from 1 to 10 p.p.m. platinum, expressed as the metal.
6. The catalyst composition of claim 5 in which said particles of zeolitic fluid cracking catalyst and said particles of impregnated calcined spray dried microspheres both have about the same specific gravity and particle size distribution.
7. The composition of claim 5 which contains not more than 10% by weight of said platinum impregnated microspheres.
8. In a process for the continuous cyclic fluid catalytic cracking of hydrocarbons with a zeolitic cracking catalyst in the absence of hydrogen in a reactor to produce lower boiling hydrocarbons wherein cracking results in the deposition on the fluid cracking catalyst particles of a solid deposit of combustible hydrocarbons, the catalyst particles containing said deposit are regenerated by oxidation in the presence of air at elevated temperature to burn off said deposit, and the catalyst is recycled to a reactor where it is used to crack hydrocarbons in the absence of added hydrogen, the improvement which comprises using as the cracking catalyst the catalyst composition of

claim 5.