Hydrotreating catalyst and process for its preparation
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates to a hydrotreating catalyst and a method for its preparation. More particularly, the invention relates to a hydrotreating catalyst prepared from gamma alumina precalcined at a temperature above 1375° F. 
     In the refining of liquid hydrocarbons derived from mineral oils and other sources, it is often necessary to subject the liquid hydrocarbon or fraction thereof to hydrotreating. Hydrotreating is a process for reducing the concentration of nitrogen and sulfur components in a hydrocarbon feedstock so that, when the product hydrocarbon is eventually combusted, less SO 2  and NO will form, and hence present less of a pollution problem. In addition, it is often desirable to remove nitrogen from such feedstocks in order to protect hydrocracking catalysts and the like which deactivate in the presence of nitrogen. 
     In general, hydrotreating is accomplished by contacting the feedstock containing nitrogen and/or sulfur with a catalyst in the presence of hydrogen and under conditions, including elevated temperature and pressure, such that the sulfur components are converted to H 2  S and the nitrogen components to NH 3 , both of which are separated from the desulfurized and denitrogenated liquid product. 
     A typical hydrotreating catalyst comprises particles containing a Group VIII active metal component and a Group VI active metal component supported on a refractory oxide such as alumina. Oftentimes, phosphorus components are also present in the catalyst to improve its activity by increasing its acidity. One catalyst which has been successfully employed on a commercial basis consists essentially of molybdenum, nickel, and phosphorus components supported on gamma alumina. A typical preparation procedure for such a catalyst is as follows: particles of hydrated alumina are firstly formed into a desired size and shape by extruding the hydrated alumina through a die having circular or clover leaf-shaped openings therein and cutting the extruded matter into particles (or extrudates) of 1/16-1/2 inch lengths. After calcining at a temperature of about 1150°-1250° F., the resulting gamma alumina extrudates are in a condition to be contacted with an impregnating solution comprising dissolved salts of molybdenum and nickel in phosphoric acid. The impregnated extrudates (or composites) are then subjected to a final calcination at a temperature around 900° F. to convert the impregnated metals to their oxide forms. 
     Quite unexpectedly, it has now been found that, if the above procedure is only slightly altered, the resulting catalyst is substantially more active for hydrotreating purposes. More specifically, it has been found that, if the first calcination, usually termed the precalcination, is conducted at a temperature of at least 1375° F., and preferably at 1450°-1600° F., then the final catalyst will have increased activity for removing sulfur and nitrogen components from liquid hydrocarbons under typical hydrotreating conditions. 
     In one embodiment, therefore, the invention comprises a hydrotreating catalyst composition comprising molybdenum, nickel, and phosphorus components supported on gamma alumina prepared by a method including the step of impregnating a support comprising gamma alumina precalcined at a temperature of at least 1375° F. with components of nickel, phosphorus, and molybdenum. In an alternative embodiment, the invention comprises a method for preparing a hydrotreating catalyst wherein particles of gamma alumina precalcined at at least 1375° F. are impregnated with a solution comprising phosphoric acid containing dissolved nickel and molybdenum components, and the resulting composite is calcined at an elevated temperature. In yet another embodiment, the invention comprises an improved hydrotreating process wherein the improvement resides in the use of a catalyst prepared by calcining a composite comprising molybdenum, nickel, and phosphorus components impregnated into gamma alumina precalcined at at least 1375° F. Usual precalcination temperatures herein are above 1400° F. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with this invention, a catalyst suitable for hydrotreating purposes is prepared by contacting gamma alumina particles precalcined at a temperature of at least 1375° F., preferably at least 1450° F., with a suitable impregnating solution or solutions known in the art as useful for compositing molybdenum, nickel, and phosphorus components with gamma alumina. A preferred impregnating solution consists of an aqueous solution of phosphoric acid, ammonium heptamolybdate, and nickel nitrate. Depending upon the proportion of nickel, molybdenum, and phosphorus desired in the final catalyst, the impregnating solution contains molybdenum in a concentration of 10-30 wt.% as MoO 3 , nickel in a concentration of 1-10 wt.% as NiO, and phosphorus in a concentration of 1-10 wt.% P. After the gamma alumina particles have been contacted for a sufficient time with the impregnating solution to insure full impregnation of the desired amount of nickel, molybdenum, and phosphorus, it is dried and subjected to a final calcination, usually at a temperature in the 800°-1000° F. range. The catalyst is then presulfided, either by contact at an elevated temperature with an H 2  and H 2  S-- containing gas stream or in situ by allowing the sulfur in the hydrocarbon feedstock to gradually convert the oxide components to the sulfide form. The final catalyst preferably comprises 14-25 wt.% molybdenum components (calculated as MoO 3 ), 2.0-6.0 wt.% nickel components (calculated as NiO), and 2.0-6.0 wt.% phosphorus components (calculated as P). Most preferably, the ratio of nickel components as NiO to molybdenum components as MoO 3  is between about 0.18:1and 0.19:1, and the ratio of phosphorus components as P to nickel components as NiO is about 1:1. Suitable catalysts comprise 12-30 wt.% molybdenum components (calculated as MoO 3 ). 
     It is a critical feature of the invention herein that the gamma alumina support material be precalcined at at least 1375° F., preferably at at least 1450° F., prior to contact with one or more impregnating solutions. In one embodiment of the invention, the precalcination is carried out by subjecting a hydrated alumina (Al 2  O 3 .xH 2  O) to a temperature in the range of 1375°-1800° F., preferably 1450°-1600° F., for an extended time period, usually 1/4-5 hours, preferably 1/2-2 hours, in the presence of air or other environment consisting of gases (e.g., N 2 , CO 2 , Ar), which do not adversely affect the catalyst under high temperature conditions. Alternatively, the same calcination procedure may be applied to any gamma alumina that has never been subjected to temperatures above 1375° for a significant length of time. For example, there are many commercially available gamma alumina catalyst supports which, during the manufacture thereof, are calcined in the 800°-1300° F. range, and it is a specific embodiment of the invention that such catalyst supports be calcined again at a temperature above 1375° F., the second calcination being considered herein as the precalcination required prior to contact with an impregnating solution. 
     Catalysts prepared in accordance with the invention may be used to hydrotreat any hydrocarbon feedstock or fraction thereof containing sulfur and/or nitrogen components. Typical hydrocarbon feedstocks suitable for treatment herein are light and heavy gas oils, cycle oils, naphthas, kerosene, turbine fuels, diesel fuels and syncrudes such as shale oils. The preferred feedstocks are gas oils, and in particular gas oils or vacuum gas oils having at least 50% of the components thereof boiling at temperatures less than 700° F., preferably less than 650° F. The typical gas oil to be treated by contact with the catalyst described herein contains at least 2 ppmw of nitrogen components (calculated as nitrogen), usually 10-5000 ppmw of nitrogen components, and at least 0.02% by weight of sulfur components (calculated as sulfur), usually 1.0-3.0% by weight. 
     Hydrotreating with the catalysts herein is accomplished under conditions known in the art for denitrogenating and/or desulfurizing hydrocarbon feedstocks in the presence of hydrogen. In the usual instance, the feedstock is passed at an elevated temperature and pressure through a catalytic reactor containing a stationary bed of catalyst. Hydrogen is also passed through the reactor with the feedstock, and the hydrogen not consumed in converting the sulfur components to H 2  S and the nitrogen components to NH 3  is separated from the denitrogenated and/or desulfurized product oil and recycled to the inlet of the reactor. The conditions employed vary from feedstock to feedstock, but the range of conditions set forth in the following table will be those typically employed: 
     
                       TABLE I                                                     
______________________________________                                    
                                   Most                                   
Operating Conditions                                                      
               Suitable  Preferred Preferred                              
______________________________________                                    
Temperature, °F.                                                   
               400-1000  600-850   650-800                                
Pressure, PSIG 100-5000  400-3000  500-2000                               
Space Velocity, LHSV                                                      
               0.1-15    1-10      2-7                                    
Hydrogen Recycle Rate,                                                    
cf/bbl.sup.1   400-20000 1000-15000                                       
                                   4000-10000                             
______________________________________                                    
 .sup.1 Measured at 60° F. and 1 atmosphere                        
 
    
     Although the conditions chosen for any given feedstock will depend in large measure upon the quality of the product desired and the concentrations of sulfur and nitrogen in the feedstock, conditions are usually selected to remove a substantial proportion of both nitrogen and sulfur components, usually at least 50% of each and preferably at least 80% of the sulfur components and 90% of the nitrogen components. Most preferably, conditions are chosen to reduce the nitrogen compounds concentration to less than 10 ppmw (as N) and the sulfur compounds concentration to less than 200 ppmw (as S). 
    
    
     The following comparative example is provided to illustrate the invention; it is not intended to be limiting. 
     COMPARATIVE EXAMPLE 
     Several catalysts were prepared and tested under typical hydrotreating conditions against a reference catalyst consisting of particles of a commercially available catalyst sold under the designation HCF by the American Cyanamid Company. The HCF catalyst had a colver leaf cross-sectional shape and was of nominal composition: 18 wt.% MoO 3 , 2.9 wt.% NiO, 3.2 wt.% P, and the balance gamma alumina. The five catalysts compared against the performance of this commercial catalysts were prepared as follows: 
     Catalyst No. 1 
     To 30 g of ammonium heptamolybdate ((NH 4 ) 6  Mo 7  O 24 .4H 2  O) were slowly added 7 ml of 85% phosphoric acid (H 3  PO 4 ) in the presence of sufficient water that the resulting liquid volume was about 50 ml. The ammonium heptamolybdate was allowed to fully dissolve in the liquid, and then 17 g of nickel nitrate (Ni(NO 3 ) 2 .6H 2  O) was added to the solution and dissolved. The resulting deeply green impregnating solution had a total volume of 73 ml. 
     One hundred grams of hydrated alumina extrudates having a size and shape essentially identical to that of the HCF catalyst particles were precalcined for 1/2 hour at 1250° F. and then contacted with the impregnating solution by pouring the solution into a beaker containing the precalcined gamma alumina particles. After brief stirring to fully saturate the pores, the alumina particles were aged for about 1 hour, oven dried overnight at 110° C., and calcined in flowing air at 900° F. for about one-half hour. Catalyst No. 1 had the following composition: 18.0 wt.% MoO 3 , 3.3 wt.% NiO, 2.6 wt.% P, gamma alumina the balance. 
     Catalysts Nos. 2 and 3 
     Catalyst Nos. 2 and 3 were prepared in a manner identical to No. 1 except that the precalcination temperature was 1500° F. rather than 1250° F. Catalysts No. 2 and 3 had the same composition as Catalyst No. 1. 
     Catalyst No. 4 
     Catalyst No. 4 was prepared in a manner identical to that of Catalysts Nos. 2 and 3 except that impregnation was accomplished as follows: 100 ml of impregnating solution of the same composition as that described above for Catalysts Nos. 1-3 was prepared. It was heated to 40° C. and contacted with the alumina particles for 1 minute, after which time the excess liquid was separated on a Buchner funnel. The catalyst was aged less than 1 hour, and then oven dried and calcined as Catalysts Nos. 1 through 3. Catalyst No. 4 had the following composition: 19.4 wt.% MoO 3 , 3.5 wt.% NiO, 3.8% P, gamma alumina the remainder. 
     Catalyst No. 5 
     This catalyst was prepared identically to Catalyst No. 4 except that the impregnating solution consisted of 62 ml of the impregnating composition used in preparing Catalyst No. 4 plus sufficient added water to produce a total solution of 100 ml. Catalyst No. 5 had a composition as follows: 15.1 wt.% MoO 3 , 2.7 wt.% NiO, 3.0 wt.% P, gamma alumina the remainder 
     Catalysts Nos. 1-5 and the HCF catalyst were then each presulfided by contact with a gas stream consisting of 10 vol.% H 2  S and 90 vol.% H 2 . The temperature during the presulfiding was initially at room temperature and then was gradually increased hourly by 50° F. until 700° F. was reached. The 700° F. temperature was held for two hours. 
     Catalysts Nos. 1-5 were then each tested to determine their individual activities for hydrodenitrogenation and hydrodesulfurization in comparison to the reference HCF catalyst. The catalysts were each charged to a reactor and twice (once at 700° F. and again at 720° F.) utilized to hydrotreat a gas oil feedstock having the characteristics shown in Table I under the following conditions: 1400 psig total pressure, 4.0 LHSV, and hydrogen rate of 6000 CFH/bbl (measured at 60° F. and 1 atmosphere). Giving the reference HCF catalyst an arbitrary activity of 100, the relative activities of Catalysts Nos. 1 through 5 compared to the HCF catalyst were determined by calculation and tabulated in Table II. These determinations were based on a comparison of the reaction rates for denitrogenation and desulfurization obtained from the data of the experiment according to the following standard equations which assume first order kinetics for denitrogenation and one and a half order kinetics for desulfurization: ##EQU1## where N fr  and N pr  are the respective concentrations of nitrogen in the feed and product obtained with the reference catalyst and N f  and N p  are the respective concentrations of nitrogen in the feed and product obtained with a catalyst being compared to the reference, and ##EQU2## where S fr  and S pr  are the respective concentrations of sulfur in the feed and product obtained with the reference catalyst and S f  and S p  are the respective concentrations of sulfur in the feed and product obtained with a catalyst being compared to the reference. 
     
                       TABLE II                                                    
______________________________________                                    
FEEDSTOCK CHARACTERISTICS                                                 
______________________________________                                    
Boiling                                                                   
Range, °F.                                                         
IBP/5      250/460  Gravity, D287, °API                            
                                    24.6                                  
10/20      512/545  Sulfur, wt. %   1.35                                  
30/40      577/600  Nitrogen:                                             
50/60      626/664  Basic, wt. %    0.0738                                
70/80      691/725  Total, wt. %    0.1810                                
90/95      767/804  Pour Point, D97, °F.                           
                                    +40                                   
EP/Rec., Vol. %                                                           
           833/99.3 Carbon Residue on                                     
                    10% Botts, D-524, wt. %                               
                                    0.32                                  
______________________________________                                    
 
    
     
                                           TABLE III                               
__________________________________________________________________________
                        Relative Activities                               
                        Basic   Total                                     
Compos. wt. %  Precalcination                                             
                        Nitrogen                                          
                                Nitrogen                                  
                                        Sulfur                            
Catalyst                                                                  
     MoO.sub.3                                                            
          NiO                                                             
             P Temperature, °F.                                    
                        700° F.                                    
                            720° F.                                
                                700° F.                            
                                    720° F.                        
                                        700° F.                    
                                            720° F.                
__________________________________________________________________________
Ref  18   2.9                                                             
             3.2                                                          
                 1200° F.*                                         
                        100 100 100 100 100 100                           
No. 1                                                                     
     18   3.3                                                             
             2.6                                                          
               1250     102 112 104 106 100  96                           
No. 2                                                                     
     18   3.3                                                             
             2.6                                                          
               1500     119 125 113 114 117 121                           
No. 3                                                                     
     18   3.3                                                             
             2.6                                                          
               1500     121 126 118 120 118 124                           
No. 4                                                                     
     19.4 3.5                                                             
             3.8                                                          
               1500     120 138 108 118 112 120                           
No. 5                                                                     
     15.1 2.7                                                             
             3.0                                                          
               1500     110 119 105 108 117 114                           
__________________________________________________________________________
 *The support precalcination temperature of 1200° F. is believed to
 be accurate; 1200° F. is typical precalcination temperature for   
 commercial hydrotreating catalysts.                                      
 
    
     As shown by the results tabulated in Table II, Catalysts Nos. 2 through 5, all of which were precalcined at 1500° F., evidenced higher activity in every category than both the HCF catalyst believed to have been precalcined at 1200° F. and Catalyst No. 1 which was known to have been precalcined at 1250° F. Even Catalyst No. 5, which had a low MoO 3  content, exhibited higher activity than HCF or Catalyst No. 1. 
     In view of the foregoing, it should be apparent that the invention is capable of many modifications, alterations, and variations. Accordingly, it is intended to embrace all such alterations, modifications, and variations as fall within the spirit and scope of the appended claims.