CA1037455A - Hydrocracking catalyst and process - Google Patents

Abstract

Abstract of the Disclosure The invention disclosed is for a process of preparing a catalyst specific for hydrocracking hydrocarbons boiling in the gas oil range and contaminated with N containing and S containing compounds, which comprises directly ex-changing highly crystalline alkali metal type Y zeolite with a solution containing dysprosium ions at a pH of 3.0 to 3.5, followed by washing, drying and calcining the dys-prosium exchanged zeolite, and thereafter compositing the calcined Dy exchanged zeolite with an effective amount of the hydrogenation metal or compound thereof. Catalysts pre-pared by this process typically exhibit improved activity, good thermal stability and high specificity in converting such f??d stocks to lower boiling hydrocarbo?s, such ??
gasoline cuts.

Description

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Field of the Invention The present invention relates to a process for preparing a catalyst specific for hydrocracking hydro-carbon feeds boiling,in the gas oil range and contamin-ated with nitrogen and sulphur containing compounds to provide lower boiling hydrocarbon cuts, to the catalyst thus prepared, and to methods for hydrocracking using the catalyst thus prepared or further composited with an inorganic oxide matrix.
Background of the Invention As a result of the increasing demand for light motor fuels and the decreasing demand for heavier petroleum products such as fuel oil and the like there is much current interest in more eE~icient metho~s for convert-ing the hea~ier products of refinincJ :into yaso~ine. The conventional methods of accomplishing this, e.g. catalytic cracking, coking, thermal cracking and the like,typically result in the production of a more highly reEractory un-converted oil or cycle oil which heretofore could not be .:
' converted to gasoline without great difficulty.
-~ It is known that such refractory materials can be converted to gasoline by catalytic hydrocracking. How-ever, the application of hydrocracking techniques has ~; heretofore been substantially limited due to the high ., ',,' costs thereof especially with respect to heavy nitrogen `~ containing and sulphur containing hydrocarbon feed s-toc]cs. I

A major problem in hydrocracking heavy nitrogen con- ¦

taining and sulphur containing hydrocarbon feeds results ,~ from the failure of the prior art to provide effective ' ' :'"'"
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catalysts and efficient preparation methods therefor in order that high conversions per unit of catalyst could be realized without undergoing rapid deactivation and without resorting to expensive separate prehydrofining to remove nitrogen. Prior art attemtps to apply hydro-cracking to such feed stocks have been complex, inefficient and expensive.
U. S. Patent 3,499,835 (Hansford) proposes use of rare earth metal X type zeolites for hydrocracking heavy nitrogen containing feed stocks and teaches superior activity therefor relative to corresponding rare earth Y type zeolites. This patent also illustrates the widely held belief that direct ion excharlcJe of sodium r~eolites with rare earth metal salts :is disadvantayeous in that it is difficult to remove more than about 75%
of the sodium by direct exchange with polyvalent metals and proposes avoidance of the problem by Eirst exchanging the sodium zeolite with ammonium salt solutions and subsequently exchanging the ammonium exchanged zeolite with rare earth to achieve low sodium content.
Hamner et al, U. S. Patent 3,385,781, proposes a two stage hydrocracking process utilizing crystalline alumino-silicate zeolite catalyst in each stage and teaches successful hydrocracking of gas oils using the hydrogen and/or magnesium form of these zeolites in up~
grading nitrogen containing naphtha fractions to mator gasolines. While such catalysts, typically Eurther in-cluding a hydrocracking metal of the platinum group type, have heretofore enjoyed limited commercial success, these ... . .

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catalysts have not been entirely satisfactory in convert-ing nitrogen containing and sulphur containing feedstocks with high activity and thermal stability.
Scherzer et al, U. S. Patent 3,676,368, discloses rare earth hydrogen exchanged zeolites, preferably of the faujasite crystalline zeolite type containing 6 to 14 percent by we.ight rare earth ions. While the hydro-cracking catalysts of the Scherzer et al.patent provided a substantial advance in the art, significant problems remain, especially regarding nitrogen containing and sulpher containing feedstock conversion. U. S. Patent 3,534,115 (Bushick) yenerally discloses Dy alumino-sil-icate zeolites in combination with a hydrocJen catalyst for conversions involv.ing catalytic contactincJ o~ hydro-carbons in the presence of hydrogen such as aromatization of naphthenes or olefins, cyclizations, etc.

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However,the prior art has failed to recognize that ' crystalline alumino-silicate Y type zeolite can be directly exchanged under carefully controlled conditions of p~I with .
, dysprosium ions to prepare a catalyst selective for hydro-cracking nitrogen containing and sulphur containing fee~
` stocks boiling in the gas oil range to useful lower boiling hydrocarbon mixtures.
Brief Description of the Invention It has now been found that a catalyst specific for hydrocracking nitrogen containing and sulphur containing hydrocarbonaceous :Eeeds boili.ng in the gas oil ran~e can be prepared by a process which comprises ,' (a) directly exchanging highly crystalline ;`'', , ...... ~ ~ .

37~55i alkali metal type Y zeolite, preferably characterized with an atomic ratio of silica to alumina from about 3 to about 6, with a solution containing dysprosium ions at a pH of 3.0 to 3.5 to reduce the alkali metal oxide content of the . zeolite to less than about 0.5 percent by weight and pro-vide dysprosium ions on the zeolite in an amount from about S to about 15 percent hy weight calculated as Dy;
washing, drying, and calcining the directly exchanged zeo-. lite and thereafter compositing with a hydrogen active metal.
~~ Catalysts thus prepared are typically found to exhibit good activity and thermal stability in hydrocracking N and S
`; contaminated feedstocks.
Detailed Descr ption of -the Inventlon The d~sprosium hyd.rocJen cxcll~cJed Y,eolit~ ~ produced by the present process is found to possess a dysprosium hydrogen ion distribution within the zeolite crystalline ' structure which provides an exceptionally high degree of thermal stability and catalytic hydrocarbon cracking : activity. Accordingly the present dysprosium hydrogen zeolite finds utility as a hydrocarbon cracking catalyst . or a hydrocarbon cracking catalyst ingredient which may ~,.
j~ be composited with conventional catalyst matrix components.

.. The above catalyst is preferably composited with a .,~ .
hydrogenation metal for use in the present hydrocracking process.

The crystalline zeolite Y may be represented chemically ' by the following general formula:

WM2/nO:Al2o3:xsio2 yH2o :.- wherein W is from about 0.7 to about l.1, M is a cation ;'' , :

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~37~5 having a valence of n, x is from about 3 to about 6 and preferably about 4 to about 5O5 and y is from 0 to about 10.
Type Y zeolite and its method of preparation is described in U. S. Patent No. 3,130,007 to Breck. The zeolite used in the preparation of the present improved dysprosium exchanged product is commercially available in the form of sodium and other alkali metal synthetic faujasites. The initial alkali metal faujasite will typically contain from about 12 to about 18 percent by weight alkali metal measured as alkali metal oxides. As is well known to those skilled in the art, the precise alkali metal content varies according to the silica to alumina ratio of the zeolite and varies inver~ly to the silica-alumina ratio thereo~.
The in:itial step in the preparation oE the present dysprosium exchanged hydrogen exchanged zeolite Y involves dire~tly exchanging the initial alkali metal zeolite Y
with dysprosium ions under precisely controlled conditions of pH. It is critically important to conduct the dysprosium ion exchange step within the pH range of 3.0 to 3.5, inclusive.
The aqueous slurries of sodium zeolite Y typically possess a pH of about 10 to 12. Dysprosium ion solutions, e.g. aqueous dysprosium chloride solutions, typically have an initial pH
of about 2.5 to 3Ø The dysprosium solution is combined with the zeolite slurry and the pH is adjusted as may be . . .
required to the desired 3.0 to 3.5 pH range, as by the addition oE mineral acid.
It is found that use of an exchange pH below 3.0 results in substantial destruction of zeolitic crystallinity ~nd results in poor dysprosium ion and hydrogen ion distribution.

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~hen extraction is carried out above pH 3.5 the efficiency of the reaction is poor and distribution of dysprosium ion and hydrogen ion is ineffective for the intended purpose hereof.
The dysprosium ion utilized in the present process is pre-ferably derived ~rom dysprosium chloride, while dysprosium ni-trate may be utilized if desired. In gèneral the amount of dysprosium ion necessary to conduct the required exchange in a single step will vary according to the p~ of the exchange media. For example, at a pH of about 3.5 an excess of up to 5 percent by weight dysprosium ion is desired to obtain the necessary deyree of exchange. When a pE~ of 3.0 i5 utilized the excess dysprosium ion will increase to about ~0 percent of the theoretical amount recluired. The present exchange is carried out a temperature from about 150 to 212 F, and preferably at least 180 F, over a period of ahout 1 to 3 hours.
After the exchange is completed it is found that the dysprosium ion concentration in the faujasite typically is in the range from about 5 to about 15 percent by weight measured as Dy 3 and the initial alkali concentration, e.g. sodium, - will be reduced to not more than 0.5 percent by weight.
Preferably the dysprosium ion concentration in the directly exchanged zeolite is in the range from about 9 to about 13 percent by weight measured as Dy 3. ~fter the novel direct dysprosium exchange step, the e~changed zeol:ite :is washed, preferably with deionize~ water, to remove all soluble anions which may be for example chloride or nitrate. Thereafter it is generally preferred to dry the exchanged zeolite to a mois-ture content o~ not more than about 20 percent by weight. ~fter ..

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~379L55 washing and drying the exchanged zeolite is calcined, that is heated, to a temperature of about 800 to 1400 F for a period from about 1 to about 4 hours. The calcining step is conducted in the atmosphere or in the presence of inert gas as desired. A suitable calcining temperature is about 1000 E` and typical calclning periods may be on the order of 2 hours.
It is especially surprising that dysprosium ion may not only be conveniently exchanged in the direct exchange step of the present process without substantial difficulty as noted in the rior art (see U. S. Patent 3,~99,835 Cited in the above description) but also that thc al]~ali metal oxide, e.g., Na2O, content of the ~colite can be dir~ctly reduced to substantia:Lly low le~eLs e.g., not more than about 0.5 percent as the oxide. The present process thus precludes the practical necessity of many of the prior art processes to exchange in two steps with rare earth oxide initially followed by ammonium ion exchange with either ammonium ion removal by way of subsequent calcina-tion in an elevated temperature environment or "back exchange'7 with additional rare earth salts. While the reason for this substantial efficiency is not completely understood, data indicates that such efficiency is especially observed when the direct exchange is carried out with a boiling solution of dysprosium ions and accordingly such 7 condition is preEerred.
It is found that the dysprosium hydrogen zeolite Y
prepared by the process of the present invention contains from about 5 to about 15 percent by weight dysprosium ,"

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as Dy and considerable amounts of hydrogen ion (i.e. protons).
The concentration of hydrogen ion present in the finished zeolite will be that concentration equivalent to the differenee between the theoretical cation eoncentration of the zeolite Y in question and the amount of cation present in the form of dysprosium and residual sodium ion or other alkali metal ion. In other words when a typical sodium zeolite Y having a silica to alumina ratio of about S is exchanged as herein taught to lmpart a 13 per-cent dysprosium oxide concentration and a resldual soda concentration of about 0.5 percent by weight, approximatly 54 percent of the theoretical equivalent cat:ion concentra-tion will be provided by dysprosium ion, about ~ percent of the theo~etical cation concentration b~ socl~um ion ancl the remaining ~2 percent ecluivalent cation requirement by hydrogen ion.
The dysprosium cations deposited on the zeolite by the above described direct exchange step may appear as Dy+3, Dy(OH)+2, and Dy((OH)2+ and mixtures thereof. The present catalyst ls preferably hydrated to an extent of 10 percent by weight H20, which may be provided as may be required by heating in a dry environment or by exposure to steam in air or nitrogen.
~.. , , i After caleination of the dysprosium exchanged hydrogen exchanged zeolite the resulting product is desirably composi-ted with a hydrogenation catalyst component containing a hydrogen active metal such as paladium, platinum, rhodium, rhenium, ruthenium, eobalt, nickel, and molybdenum. These hydrogenation metals or eompounds thereof such as oxides, and sufides are typically added in an amount from about ,. _g _ ',.,':
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~37~5~i 0.05 to about 10 percent and preferably from about 0.1 to about 3 percent by weight of the resulting composite. The hydrogenation metal values may be incorporated using well known techniques such as for example impregnation,ion exchange and the like. To incorporate the hydrogenation metals by ion exchange the dysprosium hydrogen zeolite, while pre~erably still in anhydrous form,is digested with an aqueous solution of a suitable compound of the desired metal wherein the metal is present in a cationic ~orm.
Suitable noble metal compounds include for e~ample palladium tetrammine chloride, platinurn tetrammine ch].oride and the like.
Catalysts prepared in accordance with the descr.iption above typically appear as powders which may be compr~ssed or extruded into pellets, pills and other nodular Eorms o~ a : ,.
desired size, for example, ~rom about 1/16 inch to about 3/8 inch. Suitable binders or lubricants such as hydrogenated corn oil, graphite and the like may also be added. In addi-tion the catalyst powders may also be admixed with various inorganic porous powdered adjuvants which (1) are chemically stable at temperatures up to abcut 1200 F;

(2) are inert with respect to the catalyst component; and

(3) have an average pore diameter greater than about 20 angstroms,preferably 50 to 150 angstroms.
These adjuvants may be used in any desired proportion ranging between about 10 percent and 90 percent by weight o~ the total composition. The optimum proportion o~ adjuvant de-pends upon several ~actors,principally the relative activity o~ the zeolite component and its particle size. Small micro-i .',' i .~ -10-. I
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crystalline zeolites having high intrinsic ca-talytic acti-vity tend to produce difrusion limited pellets and in these cases a substantially greater efficiency of hydrocracking is obtained by copelleting the zeolite catalyst with the relatively inert adjuvants.
In a preferred embodiment the catalyst is ~urther com-posited with Erom about 5 to about 95 percent by weight of an inorganic oxide matrix. Suitable inorganic oxides use~ul as such matrix include for example silica, alumina, silica-alumina, hydrogel, clay and mixtures thereof. Especially good rèsults have been observed using alurnina as matrix or binder ~or the composite catalyst comprising the dyspros:ium hyclroyen zeolite Y composited with a h~drogenat:ion cata:l~st Wh:i'll may be pallad:ium. The :inoryanic oxidc may itse~l~ be compos:ited with a hydrogenation metal. Pre~erably the inorganic oxide matrix is amorphous, thereby operating as a binder for com-posites ~ormed there~rom.
The catalyst of this invention is conslderably more active than commercial molecular sieve based hydrocracking catalysts such as paladium composited with magnesium exchanged zeolite Y and zeolite Y catalysts containing rare earths other than -' dysprosium. This superior activity is -typically demonstrated by lower temperature of operation provided in hydrocracking with the catalyst of this invention. Lower temperatures are . . , particurally desirable in commercial hydrocracking operations to maintain substantially constant conversion and through put and to increase the range between initial and terminal temper-atures thereby increasing the active life of the catalyst with resulting economies in operation.

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Catalysts prepared by the process of the present inven-tion are especially useful in hydrocracking feedstocks such as catalytic or thermally cracked cycleoils, atmospheric gas oils, coker gas oils, deasphalted crude oils, straight - run gas oils, etc., and especially where the hydrocarbon feedstocks boil between 400~ and 800 F and have API gravity of 20 to 35.
: The hydrocracking process of the present invention may be carried out by contacting the above described catalyst composites with an N-containing and S-containing hydro~
carbon feed in therJresence of added hydrogen using hydro-cracking conditions including a reaction zone temperature from about ~50 to about 850 F, and preeexabl~ ~rom about 500 to about 750F; a pressure Erom ~bout 500 to about 3,000 p.s.i.y., and preferab:Ly from about 750 to about 2,000 p.s.i.g.; a liquid hourly space velocity (LHSV) of from about 0.5 to about 5 volumes of feed per volurne of cata-lyst per hour, and preferably from about 1 -to about 4 v./v./hr;
~, with a hydrogen feed rate of from about 1,000 to about , 20,000 standard cubic feet hydrogen per barrel of feed and preferably from about 3000 to about 15,000 s.c.f./bbl. of feed. For even greater efficiency the initial hydrocracking . . , temperature is varied depending on the nitrogen content in the hydrocarbon feed as follows: At 1 to 10 P.P.M. nitrogen the feed temperature may desirably be from about 450to 540~F:

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at 10 to 50 P.P.M. nitrogen the initial hydrocracking temper-ature may desirably be from about 540 to about 640F; and at nitrogen concentrations of 50 to about 2000 P.P M. in the feed the initial hydrocracking temperature may desirably be from about 640 to about 700F.
The present hydrocrackiny process is especially effective for hydrocracking a hydrocarbon feed boiling in the gas oil range and contaminated with at least one nitrogen-containing compound present in an amount of from about 900 to about 10,000 p.p.m., calculated as elemental N, and at least one sulfur-containing compound present in an amount of from about 0.5 to about 5 weiyht percent, calculated as elemental 5.
Practice o~ the present invention will b~ furkher illustrated by the ~ollow:iny speciEic but nonLimitincJ
examples. All percentages and parts yiven throughout this description are by weight unless otherwise noted.
Example 1 3,200 grams (dry basis) of primarily crystalline sodium Y zeolite molecular sieves was mixed wi-th 16,140 grams of deionized water in a reaction flask. To the resulting slurry was added with stirring gno ml o~ a solution of 826 grams of dysprosium chloride hexahydrate (DyC13-6H2O) in water. The resulting exchange reaction mixture was heated to boiling and boiling was continued for one hour through-out which time the exchange pH was maintained at 3.4 to 3.5 (as measured at 25C) by addition of lN hyclrochloric acid as required. The ~eolite product was found to be highly crys-talline partially protonated dysprosium exchanged zeolite Y
molecular sieves containiny 11.3 percent dysprosium and ,., haviny a low soda content (0.48 percent Na~O). The thus di-rectly dy9prosiu~ exchanged sieves were thereafter filtered and washed with 8,000 ml of deionized water acid-.. , . . . . ; . , .

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ified with 160 ml of 5N HCl, followed by washing with de-ionized water until the sieves were essentially free of chloride ion. The washed sieves were dried and calcined at 1000F for 3 hours. The resulting dysprosium exchanged acidic zeolite Y ~Dy-H -Y hereinafter) was ~ound to have 6.75 percent total volatles (%T.V.).
Example 2 85.5 grams of the Dy-H -Y zeolite prepared by the procedure of Example 1 was exchanged with palladium by adding ammoniated aqueous palladium nitrate at 10 pH with stirring to a dilute aqueous suspension of the Dy ex-changed ~eolite. The exchange was continued at 80 F for one hour. The resul~incJ Dy-exchangecl Pcl-exchanged acld:ic zeolite Y was highly crysta:L:L:irle and had a pa:LLadLum con-tent of 0.6 percent as Pd. The product was dried without washing and will be referred to hereinafter as Pd-Dy-H -Y.
Example 3 29.4 of Boehmite alumina having 32% T.V. and commer-cially available from the Davison Chemical Div. of W.R.
Grace & Co. was impregnated with a solution of palladium nitrate (Pd(NO3)2~ ammoniated to 6 pH. The resulting Pd-exchanged alumina was washed with deionized water and dried in air at 70 F. This product was designated Pd-A12O3.
Example The Pd-Dy-H -Y of Example 2 was mixed with the Pd-A12O3 of Example 3 for one hour in a ball mixer using a graphite lubricant. The thus mixed Pd-containing ma-terial was formed into 1/8 inch x 1/8 inch pills using a conventional pilling procedure. The pilled composite of 80% crystalline Pd-Dy-H~-Y and 20~ Pd-A12O3 binder was calcined in air at 500 F for 2 hours followed by calcin-ation at 1000 F for 3 hours.

' ~ , Example 5 This example illustrates the improved hydrocracking `process of this invention.
To a light catalytic cycle oil, 35.7API gravity, were added n-butylamine and thiophene in amounts suffi-cient to provide lO00 ppm of amille nitrogen and one per-cent thiophene sulfur. Such additions effec-tively sim-ulate ammonia and hydrogen sulfide partial pressures in catalyst zones where LCCO (i.e., light catalytic cycle oils) are hydrocracked. Analysis of the thus contaminated LCCO in distillation tests using the ASTM D-86 procedure gave the following results:
Dlsti.llate '['empe ~tur~ (F) 0 (Initial l3O:i:Lirlg Point) 36~
~13 ; 10 ~32 50~

;lO0 (Flnal Boiling Point) 640 Effectiveness of the composite Pd containing, directly , Dy exchanged zeolite Y containing catalyst prepared in Example

4 in hydrocracking nitrogen and sulfur containing mineral oils , which boil in middle distillate ranges was tested using the above ; feed under hydrocracking conditions as follows. The feed was contacted with the catalyst using a hydrogen pressure , of 1500 p.s.i.g. and a hydrogen ~eed rate oE 8000 SCF
(standard cubic feet) per barrel of hyclrocarbonaceous Eeed.
The LHSV (liquid hour space velocity) was l.0 V/V/hour. The catalyst prepared using the procedure of Example 4 was ound ! ; 15 .' , ."''' ~
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to be highly active for hydrocracking the N and S contaminated feed.
The foregoing hydrocracking test was repeated except that a typical prior art RE (i.e., rare earth) promoted pro-tonated zeolite Y was substituted for the Dy-exchan~ed zeolite in the catalyst composite. The test was again re-peated except that a typical prior art magnesium promoted protonated zeolite Y was substituted for the Dy-exchanged zeolite in the catalyst composite. In all three cases, the composite catalysts were aged about 100 hours ending at the start of the tests. The results were as follows:
Promoted Zeolite Dy-H-Y Mg-H-Y RE-~I-Y
~ Dy, RE or Mg11.3 Dy3.0 McJ :L:L.0 ~;, Temp~ (F) eor 50~
conversion to 430F 646 6~6 656 end product Hy naphtha yield 84.6 82.8 84.4 (180-430F) (vol.%) C-5+gasoline yield 98.3 98.4 99.5 (vol.%) The above data clearly indicates that the performance ;~ of the catalyst prepared by the process of the present in-vention is substantially better than each of the typical prior art catalysts. ~his superiority is especially evi-denced by the significantly lower temperature at which 50%
conversion to lower boiling products was effected.
It is to be understood that the foregoing detailed description is given merely by way of illustration and that numerous modifications may be made therein without departing from the spirit or scope of the present inven-t~ion.

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Claims (10)

WHAT IS CLAIMED IS:

1. A process for preparing a catalyst specific for hydrocracking hydrocarbonaceous feedstocks which boil in the gas oil range and are contaminated with nitrogenous compounds and sulfurous compounds, which comprises (a) directly exchanging highly crystalline alkali metal Type Y zeolite characterized with an atomic ratio of silica to alumina of from about 3 to about 6 with a solution containing dysprosium ions at a pH of 3.0 to 3.5 to reduce the alkali metal oxide content of said zeolite to less than about 0.5 percent by weight and provide dysprosium ions on the zeolite in an amount of from about 5 to about 15 percent by weight calculated as Dy, (b) washing and drying the dysprosium exchanged zeolite, (c) calcining the washed and dried zeolite at a temperature of about 800° F to about 1400° F for a period of about 1 to about 4 hours; and (d) compositing the calcined dysprosium-exchanged zeolite withfrom about 0.05 to about 5 per-cent by weight of a hydrogenation metal or a compound thereof, calculated as said metal.

2. The process of Claim 1 wherein said dysprosium ion solution is an aqueous solution of dysprosium chloride.

3. The process of Claim 1 wherein said dysprosium exchange is effected at a temperature of at least 180°F.

4. The process of Claim 1 wherein said metal is selected from the group consisting of palladium, platinum, rhodium, rhenium, ruthenium, cobalt, nickel and molybdenum.

5. The hydrocracking catalyst prepared by the process of Claim 1.

6. The catalyst of Claim 5 composited with from about 5 to about 95 percent by weight of an inorganic oxide matrix.

7. The catalyst of Claim 6 wherein said inorganic oxide is alumina.

8. A method for hydrocracking a hydrocarbon feed boil-ing in the gas oil range and contaminated with at least one nitrogen-containing compound and at least one sulfur-containing compound, which comprises contacting said feed at hydrocracking conditions in the presentce of added hydrogen with the catalyst of Claim 5.

9. The method of Claim 8 wherein said hydrocarbon is contacted with said catalyst in a reaction zone maintained at a temperature of from about 450° F to about 850° F, a pressure of from about 500 to about 3000 p.s.i.g., a liquid hour space velocity of from about 0.5 to about 5 volumes of said feed per volume of catalyst per hour, with a hydrogen feed rate of from about 1000 to about 20,000 standard cubic feet per barrel of said feed.

10. The method of Claim 8 wherein said at least one N-containing compound is present in an amount of from about 900 to about 10,000 p.p.m. as N and said at least one S-containing compound is present in an amount of from about 0.5 to about 5 percent as S.