Patent Publication Number: US-2009223866-A1

Title: Process for the selective hydrodesulfurization of a gasoline feedstock containing high levels of olefins

Description:
This application is a continuation-in-part application of prior application Ser. No. 12/043,847, filed Mar. 6, 2008, and this application is a continuation-in-part application of prior application Ser. No. 12/043,841, filed Mar. 6, 2008, and this application claims the benefit of U.S. Provisional Application No. 61/051,300, filed May 7, 2008, and this application claims the benefit of U.S. Provisional Application No. 61/051,278, filed May 7, 2008, and this applications claims the benefit of U.S. Provisional Application No. 61/051,294, filed May 7, 2008. 
    
    
     FIELD OF INVENTION 
     This invention relates to the selective hydrodesulfurization of an olefin-containing hydrocarbon feedstock boiling in the naphtha or gasoline boiling range. 
     BACKGROUND OF INVENTION 
     Gasoline regulations are increasingly creating a need to treat various refinery streams and products, for example, cracked gasoline blending material, including coker naphtha and gasoline from a catalytic cracking unit, to remove undesirable sulfur that is contained in such refinery streams and products. 
     One means by which sulfur may be removed from hydrocarbon streams that contain olefin compounds is through the use of various of the known catalytic hydroprocessing methods. A problem with the use of such catalytic hydroprocessing methods is that they typically tend to hydrogenate the olefin compounds as well as the sulfur compounds contained in the hydrocarbon feed stream that is being treated, When the treated hydrocarbon feed stream is used as a gasoline-blending component, usually, the presence of the olefins therein is desirable because of their relatively high-octane values and octane contribution to the gasoline pool. 
     Cracked gasoline blending material typically contains high concentrations of high-octane olefin compounds as well as concentrations of sulfur compounds It is desirable to be able to catalytically desulfurize the cracked gasoline blending materials with a minimum of hydrogenation of the olefins contained in them, or, in other words, to selectively hydrodesulfurize the cracked gasoline blending material. 
     Disclosed in the prior art are many types of hydroprocessing catalysts and processes, and the prior art even discloses processes for the selective hydrodesulfurization of olefin containing hydrocarbon feedstock. For instance, in the recent patent application publication US 2006/0237345 is disclosed a process for the selective hydrodesulfurization of an olefin-containing hydrocarbon feedstock. This process uses a catalyst composition having a high content of a nickel component and an effective but small amount of a molybdenum component that are supported on a porous refractory oxide. The catalyst provides for the selective hydrogenation of sulfur compounds contained in the hydrocarbon feedstock with a minimal amount of olefin hydrogenation as compared to certain other hydrogenation catalysts. 
     U.S. Pat. No. 5,266,188 is one patent that discloses a process for the selective hydrotreating of a cracked naphtha using a catalyst comprising a Group VIB metal component (molybdenum is preferred), a Group VIII metal component (cobalt is preferred), a magnesium component, and an alkali metal component. The Group VIB metal is present in the catalyst in an amount in the range of from about 4.0 wt % to about 20.0 wt %, and the Group VIII metal component is present in the range of from about 0.5 wt % to about 10.0 wt % The support is a refractory inorganic oxide that comprises magnesium and an alkali metal, and the refractory inorganic oxide can be alumina. The alumina will have an average pore diameter in the range of from about 30 to about 120 Angstroms and a surface area of at least 150 m 2 /g. There is no disclosure of a co-impregnation of the support with cobalt, molybdenum and phosphorus. 
     U. S. Patent Publication No. 2003/0183556 discloses a process for the selective hydrodesulfurization of naphtha which process uses a preferred catalyst that comprises a MoO 3  concentration of about 1 to 10 wt. % and a CoO concentration of about 0.1 to 5 wt. %. The median pore diameter of the catalyst is from about 60 Å to about 200 Å. The catalyst may be supported on an inorganic oxide that is preferably a high surface area alumina, but the form of the alumina is not disclosed (e.g., there is no teaching that the alumina is in the gamma form or any other transitional form). The support is preferably substantially free of contaminants, but the support may have an additive selected from the group consisting of phosphorus and metals or metal oxides from Group IA (alkali metals) of the Periodic Table. 
     U.S. Pat. No. 5,686,375 discloses a hydroprocessing catalyst that contains an overlayer of a Group VIB metal (preferably molybdenum) component on a support comprising an underbedded Group VIII metal (preferably nickel) component combined with a porous refractory oxide. A preferred catalyst is essentially free of supported metal components other than molybdenum and underbedded nickel. A most highly preferred embodiment of the catalyst contains above 3 weight percent of nickel components, including underbedded nickel components encompassing at least 4.5 weight percent of the support. Ihe median pore diameter of the catalyst usually lies in the range of from about 60 to about 120 angstroms, and, typically, the surface area is greater than about 100 m 2 /gram. While the catalyst is used in hydroprocessing methods such as desulfurization and denitrogenation, there is no indication that the process is selective to desulfurization. The catalyst has a relatively high surface area with a small median pore size and requires an underbedded metal component that is calcined with the support material. 
     As may be seen from the above review of some of the prior art, there is great interest in the development of processes that provide for the selective catalytic hydrodesulfurization of sulfur-containing naphtha or hydrocarbon feedstocks that boil in the gasoline range and contain high olefin contents. By the selective hydrodesulfurization of the sulfur without significant simultaneous saturation of the olefins, the loss in octane of the feedstock may be minimized. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved process for the selective hydrodesulfurization of a gasoline feedstock having a relatively high olefin concentration and a relatively high feed sulfur concentration, e.g., sulfur concentrations exceeding 0.01 weight percent. What is meant when referring herein to the selective hydrodesulfurization of a feedstock is that sulfur is removed from the feedstock by the catalytic hydrogenation of the organic sulfur compounds contained therein but with a minimum of simultaneous hydrogenation of the olefin compounds contained in the olefin-containing feedstock to yield a hydrotreated product having a reduced sulfur content and, preferably, a minimally reduced olefin concentration relative to the olefin concentration of the feedstock. 
     In one embodiment of the inventive selective hydrodesulfurization process, an olefin-containing gasoline feedstock having a feed sulfur concentration exceeding 0.01 wt % is contacted, under suitable hydrodesulfurization conditions, with a high activity catalyst composition comprising a support having a high surface area of at least 280 m 2 /g, a cobalt component in the amount of from 3 to 10 wt %, a molybdenum component in an amount of 12 to 30 wt %, and from 0 to 0.5 wt % of a phosphorous component, with the wt % of each component being based on the total weight of the catalyst composition and being calculated assuming each component is in its elemental form. In addition to having high surface area, it is important that the support used to prepare the high activity catalyst consist essentially of alumina and a low concentration of silica, e.g., from 0.3 to 10 wt % silica, preferably 0.5 to 6 wt % silica, based on the total weight of said support. 
     Catalysts prepared using a high surface area, silica-containing alumina support described above and having the requisite chemical composition, including a relatively low phosphorus content, or no phosphorus at all, have been found to have highly advantageous hydrodesulfurization activity and high selectivity. 
     A second embodiment of the inventive process provides for the selective hydrodesulfurization of an olefin-containing gasoline feedstock having a feed sulfur concentration exceeding 0.01 wt %, which comprises the steps of: separating the olefin-containing feedstock into a light fraction and a heavy fraction; contacting, under suitable heavy fraction hydrodesulfurization conditions, the heavy fraction with the high activity catalyst as described in first embodiment, contacting under suitable light fraction hydrodesulfurization conditions, the light fraction with a high selectivity catalyst composition having a low surface area of less than 125 m 2 /g and high mean pore diameter of greater than 200 Å, wherein the high selectivity catalyst composition comprises a cobalt component, a molybdenum component, a phosphorous component and a support consisting essentially of alumina; and yielding a light hydrotreated product having a light hydrotreated product sulfur concentration below the feed sulfur concentration and a heavy hydrotreated product having a heavy hydrotreated product sulfur concentration below the feed sulfur concentration. 
     A third embodiment of the inventive process involves a selective hydrodesulfurization process comprising the steps of: providing a distillation column reactor which comprises a lower distillation reaction zone containing a lower catalytic distillation structure formed with the high activity catalyst described in the first embodiment, and an upper distillation reaction zone containing an upper catalytic distillation structure formed with the high selectivity catalyst described in the second embodiment, wherein the upper distillation reaction zone is in a juxtaposed vertical relationship above the lower distillation reaction zone; utilizing said distillation column reactor to separate an olefin-containing gasoline feedstock having a feed sulfur concentration exceeding 0.01 wt % and a feed olefin concentration into a light fraction and a heavy fraction and contacting the light fraction with at least a portion of said upper catalytic distillation structure and contacting the heavy fraction with at least a portion of said lower catalytic distillation structure; and yielding as an overhead from the distillation column reactor a light hydrotreated product having a light hydrotreated product sulfur concentration below the feed sulfur concentration, and as a bottoms from the distillation column reactor a heavy hydrotreated product having a heavy hydrotreated product sulfur concentration below the feed sulfur concentration. 
     An important aspect of each of the foregoing embodiments is the use of a particularly defined high activity catalyst, alone or in combination with a particularly defined high selectivity catalyst, to affect the selective hydrodesulfurizations. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a flow diagram in schematic form representing one embodiment of the inventive process wherein an olefin and sulfur-containing hydrocarbon feedstock is separated into a light and heavy fraction prior to being contacted with a high selectivity hydrodesulfurization catalyst and a high activity hydrodesulfurization catalyst, respectively. 
         FIG. 2  is a flow diagram in schematic form of another embodiment of the inventive process involving a distillation column reactor containing an upper and a lower catalytic hydrodesulfurization reaction zones, 
         FIG. 3  presents comparative plots of the desulfurization activity of a high activity catalyst composition useful in various embodiments of the inventive process and a comparison catalyst composition versus reactor temperature. 
         FIG. 4  presents comparative plots of the selectivity performance of a high activity catalyst composition useful in various embodiments of the inventive process and a comparison catalyst composition versus the negative log of the fraction of feed sulfur not converted (i.e., −log (1−x), where x is equal to the difference of the inlet feed sulfur concentration less outlet product sulfur concentration with this difference being divided by the inlet feed sulfur concentration). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Refinery cracked feedstocks typically contain high concentrations of sulfur as well as olefins, and it is desirable to be able to selectively desulfurize such cracked feedstocks with a minimum of olefin saturation The various embodiments of the inventive process provides for such selective desulfurization. 
     The feedstocks contemplated for use in the inventive process can be a hydrocarbon feedstock that typically boils in the naphtha or gasoline boiling range, which is typically from about 10° C. (50° F.) as the initial boiling temperature to about 245° C. (473° F.) as the endpoint temperature, and, preferably from about 21 or (70° F.) to about 225° C. (437° F.), More preferably, the hydrocarbon feedstock predominantly boils in the range of from 32° C. (90° F.) to 210° C. (410° F.). It is desirable for the feedstock to have the distillation characteristics as specified by the ASIM specifications for gasoline. These specifications vary depending upon the particular season and geographic area in which the gasoline is to be marketed. In general, the hydrocarbon feedstock of the inventive process can have a distillation characteristic, as determined by the ASTM DS 6  method, wherein the temperature at which 10% of the feedstock is evaporated (i.e., T 10 ) is at least 50° C., the temperature at which 50% of the feedstock is evaporated (i.e., T 50 ) is in the range of from 77 to 121° C., the temperature at which 90% of the feedstock is evaporated (i.e., T 90 ) is no more than 190° C., and the endpoint temperature (i.e., EP) is no more than 225° C. 
     The hydrocarbon feedstock (also herein referred to as a gasoline feedstock) of the inventive process contains both olefin compounds and sulfur compounds. The olefin content or concentration of the olefin-containing hydrocarbon feedstock of the inventive process can be in the range of upwardly to about 60 weight percent of the total weight of the hydrocarbon feedstock and usually at least 5 weight percent of the total weight of the olefin-containing hydrocarbon feedstock comprises olefin compounds. A typical olefin content of the olefin-containing hydrocarbon feedstock is in the range of from 5 weight percent to 55 weight percent of the total weight of the olefin-containing hydrocarbon feedstock, and, more typically, the range is from 8 weight percent to 50 weight percent, It is contemplated, however, that the olefin-containing hydrocarbon feedstock of the inventive selective hydrodesulfurization process can have concentrations of olefin compounds exceeding  10  weight percent and even exceeding 15 or even 20 weight percent. 
     Generally, the olefin-containing hydrocarbon feedstock can be a cracked naphtha product such as products from catalytic or thermal cracking units including, for example, an FCC cracked naphtha product from a conventional fluid catalytic cracking unit, a coker naphtha from either a delayed coker unit or a fluid coker unit, a hydrocracker naphtha and any combination of cracked naphtha products. The cracked naphtha product typically has a high concentration of olefin compounds and may have an undesirably high concentration of sulfur compounds. 
     The olefin-containing gasoline feedstock to the inventive selective hydrodesulfurization process will normally have a significant sulfur content or sulfur concentration exceeding 0.01 weight percent (wt %), e.g., from 0.1 wt % to about 3 wt %, i.e., from 1,000 to about 30,000 ppmw. More typically, the sulfur content is in the range of from 1,000 ppmw to 7,000 ppmw, and, most typically, from 1,000 ppmw to 5,000 ppmw. 
     The sulfur compounds of the olefin-containing gasoline feedstock include organic sulfur compounds, such as, for example, mercaptan compounds, disulfide compounds, thiol compounds, thiophene compounds and benzothiophene compounds (including alkylbenzothiophenes and other substituted benzothiophenes). The olefin-containing hydrocarbon feedstock may also contain other hydrocarbon compounds besides paraffin compounds and olefin compounds. The olefin-containing hydrocarbon feedstock may further comprise naphthenes, and, further, comprise aromatics, and, further, comprise other unsaturated compounds, such as, open-chain and cyclic olefins, dienes, and cyclic hydrocarbons with olefinic side chains. 
     The olefin-containing gasoline feedstock may also contain nitrogen compounds, if nitrogen compounds are present, at a nitrogen concentration in the range of from about 5 ppmw to about 150 ppmw, and, more typically, in the range of from 20 ppmw to 100 ppmw. 
     The various embodiments of the inventive process provide for the selective removal of sulfur from an olefin-containing gasoline feedstock, having a sulfur concentration, by catalytic hydrodesulfurization. It is understood herein that the references to hydrodesulfurization means that the sulfur compounds of a feedstock are converted by the catalytic hydrogenation of the sulfur compounds to hydrogen sulfide which may then be removed to provide a hydrotreated product having a reduced sulfur concentration. 
     It has been discovered that the beneficial selective hydrodesulfurization of a gasoline feedstock containing relatively high levels of both sulfur and olefins can very effectively be accomplished by use of certain specifically defined catalysts in three different embodiments of the inventive process as described in greater detail below. 
     The first embodiment of the inventive process uses a specifically defined high activity catalyst which comprises a support having a high surface area of from 280 to 360 m 2 /g, a cobalt component in the amount of from 3 to 10 wt %, a molybdenum component in an amount of 12 to 30 wt %, and from 0 to 0.5 wt % of a phosphorous component, with the wt % of each component being based on the total weight of the catalyst composition and calculated as the element regardless of what form the component is actually present in the catalyst composition. The high surface area support for the catalyst consists essentially of alumina and from 0.3 to 10 wt % silica based on the total weight of said support. It has been found that the combination of the use of a high surface area support consisting essentially of alumina and silica, and a very low amount, or no, phosphorus, and relatively high concentrations of cobalt and molybdenum provides excellent hydrodesulfurization activity and good selectivity as compared to processes employing conventional gasoline hydrodesulfurization catalysts. It should be understood that while the afore-described catalyst is referred to herein as a “high activity” catalyst, it is, in fact, also selective in that it provides excellent sulfur reduction without significant olefin saturation. 
     In the first embodiment of the inventive process the high activity catalyst described above is contacted with the sulfur and olefin-containing gasoline feedstock under suitable hydrodesulfurization conditions yielding a hydrotreated product having a significantly reduced sulfur concentration without significant olefin saturation. It has been found that because of the high sulfur removal activity of these catalysts utilizing high surface area supports and very low or no phosphorus, the reactor can be operated at lower temperature, which results in a significant gain in the reduction of olefin saturation. 
     It has been determined that in order to provide the desired level of desulfurization activity and selectivity relative to olefin saturation, it is important that the high activity catalyst composition employed in the inventive process contain at least 3 wt % cobalt, e.g., from 3 to 10 wt % cobalt, and at least 12 wt % molybdenum, e.g., from 12 to 30 wt % molybdenum, based on the total weight of the catalyst composition and calculated as the metal The preferred concentration range for the cobalt component is from 3 wt % to 8 wt % cobalt, calculated as the metal (i.e., from 3.8 wt % to 10.2 wt % cobalt when calculated as CoO), while the preferred concentration range for the molybdenum component is from 12 wt % to 20 wt % molybdenum, calculated as the metal (i.e., from 18 wt % to 30 wt % molybdenum when calculated as MoO 3 ) The is most preferred concentration range for the cobalt component is from 3.4 wt % to 5 wt % cobalt, calculated as the metal (i.e., from 4.3 wt % to 6.4 wt % cobalt when calculated as CoO), while the most preferred concentration range for the molybdenum component is from 14 wt % to 16 wt % molybdenum, calculated as the metal (i.e., from 21 wt % to 24 wt % molybdenum when calculated as MoO 3 ). 
     It has also been determined that in order to achieve the desired level of desulfurization activity and selectivity, it is important, to limit the concentration of phosphorus component. Therefore, if the phosphorus component is present at all, it should be present in an amount no greater than 0.5 wt %, erg. from 0 wt % up to 0.5 wt %, preferably from 0 wt % to 0.1 wt %, based on the total weight of the catalyst composition and calculated as elemental phosphorus. It is most preferred that the high activity catalyst contain no phosphorus at all. 
     It has further been determined that in order to achieve the desired level of desulfurization activity and selectivity, it is important that the support used to prepare the high activity catalyst composition should have a relatively high surface area of at least 280 m 2 /g, preferably at least 300 m 2 /g, as measured using the B.E.T. method. All surface area values or measurements mentioned herein refer to the total surface area as measured by the B.E.T method A practical upper limit for the surface area of the support is no more than 500 m 2 /g, even no more than 450 m 2 g. Thus, the surface area for the support used to prepare the high activity catalyst employed in the inventive process can be within the general range of from 280 to 450 m 2 /g, but preferably will be in the range from 280 to 360 m 2 /g, most preferably will be in the range from 300 to 340 m 2 /g. 
     It is important that the support used to prepare the high activity catalyst consist essentially of alumina with a low concentration of silica, e.g., from 0.3 wt % to 10 wt % silica based on the total weight of the support. A preferred support consists essentially of alumina and from 0.5 wt % to 6 wt % silica, while a particularly preferred support consists essentially of alumina and from 1.5 wt % to 2.5 wt % silica based on the total weight of the support. 
     It is desirable that the alumina in the finished support (ire., the support after calcination), be present predominantly in the form of gamma-alumina. Thus, generally less than 5 wt % of the alumina in the support for the high activity catalyst will be present in a form other than gamma-alumina, preferably, less than 1 wt % of the alumina is present in a form other than gamma-alumina, and, most preferably, less than 0.5 wt % of the alumina is present in a form other than gamma-alumina. 
     The high activity catalyst composition used in the process of the invention may be prepared by any suitable method known to those skilled in the art that will suitably provide the catalyst composition having the properties and composition as described herein The preferred method of preparing the catalyst composition includes the several steps of preparing the support having the specific composition and high surface area as described herein and incorporating into the support the metal components of molybdenum, cobalt, and optionally phosphorus, with the resulting metal-incorporated support being calcined under suitable calcination conditions. 
     In the preferred method of making the high activity catalyst composition, the support particle is first prepared by mixing the starting alumina and silica, or silica-alumina precursor powder with water by any suitable means or method for providing a substantially homogeneous mixture of the alumina with silica and water Many of the possible mixing means that may suitably be used in preparing the mixture are described in detail in Perry&#39;s Chemical Engineers&#39; Handbook, Sixth Edition, published by McGraw-Hill, Inc. at pages 19-14 through 19-24, which pages are incorporated herein by reference Thus, possible suitable mixing means can include, but are not limited to, such devices as tumblers, stationary shells or troughs, Muller mixers, which are either batch type or continuous type, impact mixers, and any other mixer or device known to those skilled in the art and that will suitably provide the homogeneous mixture of alumina and silica or silica-alumina and water. 
     The amount of water mixed with the alumina and silica or silica-alumina should be such that a paste mixture is formed that can then be formed into agglomerated particles. Typically, the amount of water present in the mixture is in the range of from 30 wt % to 85 wt %, and, preferably, it is in the range of from 40 wt % to 75 wt % of the mixture. A peptizing agent, such as nitric acid or other acid, may be added to the mixture of alumina and water to assist in the dispersion of the alumina and silica or alumina-silica and the formation of the paste. It is particularly desirable for the paste to have the plasticity required for extrusion thereof. 
     While the formation of the agglomerate is preferably done by any of the standard extrusion methods known to those skilled in the art, other possible suitable means or methods for forming the agglomerate may include, for example, molding, tableting, pressing, pelletizing, tumbling and densifying. 
     The agglomerate is then dried, preferably, at a temperature in the range of from 40° C. (104° F.) to 260° C. (500° F.), and calcined under conditions which will result in support particles having the requisite high surface area of at least 280 m 2 /g, preferably at least 300 m 2 /g. Suitable conditions for calcining the support will generally include temperatures in the range of from 399° C. (750° F.) to 593° C. (1,100° F.). The calcination time period can be in the range of from 0.5 hours to 72 hours, or even longer, if required. 
     The catalytic components of the high activity catalyst may be incorporated into the support particle using one or more impregnation solutions containing one or more of the catalytic components. The preferred impregnation solution is an aqueous solution of the desired catalytic component or a precursor thereof. 
     Potential cobalt compounds that may be used in the formation of the aqueous solution include the cobalt hydroxides, acetates, carbonates, nitrates, and sulfates or mixtures of two or more thereof. 
     Potential molybdenum compounds that may be used in the formation of the aqueous solution include molybdenum oxide and the ammonium salts of molybdenum, such as ammonium heptamolybdate and ammonium dimolybdate. 
     Any suitable phosphorus containing compound may be used in the aqueous solution to provide for the phosphorus component of the catalyst composition. One such phosphorus compound is phosphoric acid. If a phosphorus compound is used at all, it should be used in very low concentrations, e.g., from 0.1 to 0.5 wt %, as discussed above. 
     The cobalt compound, molybdenum compound, and phosphorus compound (if used), are dissolved in water to form an aqueous solution in such amounts as to provide, when incorporated into the support particles, the desired metal concentrations in the final catalyst composition as defined earlier herein. Typically, the concentration of the metal compounds in the impregnation solution is in the range of from 0.01 to 100 moles per liter. 
     The impregnation may be conducted by any procedure or method or means that suitably incorporates the desired metal components in the desired amounts into the support particles. Such impregnation methods include, for example, spray impregnation, soaking, multi-dip procedures, and incipient wetness impregnation methods. 
     The impregnated support particles are then dried, preferably, at a temperature in the range of from 40° C. (104° F.) to 260° C. (500° F.), and calcined to form the final catalyst composition. The calcination is conducted in the presence of oxygen or an oxygen-containing inert gas or air. The temperature at which the impregnated support particles are calcined should be in the range of from 427° C. (800° F.) to 649° C. (1200° F.), But, it is preferred to carefully control the calcination temperature to within the range of from 482° C. (900° F.) to 538° C. (1000° F.), with a calcination temperature within the range of 482° C. (900° F.) to 510° C. (950° F.) being particularly preferred. 
     The selective hydrodesulfurization reaction temperature used in the first embodiment of the inventive process is generally in the range of from about 150° C. to 420° C. The preferred selective hydrodesulfurization reaction temperature is in the range of from 175° C. to 400° C., and, most preferred, from 200° C. to 380° C. 
     The inventive process generally operates at a selective hydrodesulfurization reaction pressure in the range of from 50 psia to about 1000 psia, preferably, from 60 psia to 800 psia, and, most preferably, from 150 psia to 700 psia. 
     The flow rate at which the olefin-containing hydrocarbon feedstock is charged to the reaction zone in the first embodiment of the inventive process is generally such as to provide a weight hourly space velocity (WHSV) in the range exceeding 0 hr −1  such as from 0.1 hr −1  upwardly to 10 hr −1 . The term “weight hourly space velocity,” as used herein, means the numerical ratio of the rate at which the hydrocarbon feedstock is charge to the reaction zone of the process in pounds per hour divided by the pounds of catalyst composition contained in the reaction zone to which the olefin-containing hydrocarbon feedstock is charged The preferred WHSV is in the range of from 0.1 hr −1  to 250 hr −1 , and, most preferred, from 0.5 hr −1  to 5 hr −1 . 
     The hydrogen treat gas rate is the amount of hydrogen charged to the reaction zone with the olefin-containing hydrocarbon feedstock. The amount of hydrogen relative to the amount of olefin-containing gasoline feedstock charged to the reaction zone is in the range upwardly to about 10,000 cubic meters (at standard conditions) hydrogen per cubic meter of olefin-containing hydrocarbon feedstock, but, typically, it is in the range of from 10 to 10,000 m 3  hydrogen per m 3  of olefin-containing gasoline feedstock. The preferred range for the hydrogen-to-olefin-containing hydrocarbon feedstock ratio is from 20 to 400 m 3  hydrogen per m 3  of olefin-containing gasoline feedstock, and, most preferred, from 20 to 200 m 3  hydrogen per m 3  of olefin-containing gasoline feedstock. 
     The second embodiment of the inventive process is based on the discovery that the beneficial selective hydrodesulfurization of the olefin-containing gasoline feedstock having a relatively high concentration of sulfur, can be achieved more effectively by separating the olefin-containing feedstock into a light fraction and a heavy fraction, and hydodesulfurizing the respective fractions with different particularly defined hydrodesulfurization catalysts as hereafter described. 
     In the second embodiment of the inventive process, the olefin-containing gasoline feedstock is divided into a light fraction and heavy fraction using a distillation column, fractionator, or any other suitable means of separating hydrocarbon fractions. 
     The term “light fraction” as used herein refers to a fraction of the olefin-containing hydrocarbon feedstock generally boiling in the range from 10° C. (50° F.) to about 135° C. (275° F.). 
     The term “heavy fraction as used herein refers to a fraction of the olefin-containing hydrocarbon feedstock generally boiling in the range from 135° C. (275° F.) to about 245° C. (473° F.) 
     Generally, the light fraction will have a lower sulfur content and more of desirable high octane olefins than the heavy fraction. 
     Accordingly, it has been found advantageous to hydrodesulfurize the light fraction under milder conditions with a catalyst having a high selectivity to desulfurization relative to olefin saturation. Since the heavy fraction generally has a higher content or more difficult to convert sulfur compounds such as heavier mercaptans, thiophenes, sulfides and disulfides, it has been found advantageous to desulfurize the heavy fraction under more severe conditions with a catalyst having high activity, while still having relatively good selectivity. 
     In accordance with the second embodiment of the inventive process, after separation, the separated light fraction and heavy fraction are contacted with different hydrodesulfurization catalysts under appropriate hydrodesulfurization conditions, yielding a light hydrotreated product having a light hydrotreated product sulfur concentration below the feed sulfur concentration, and a heavy hydrotreated product having a heavy hydrotreated product sulfur concentration below the feed sulfur concentration. 
     In the second embodiment, the heavy fraction, after separation, is contacted, under suitable heavy fraction hydrodesulfurization conditions, with a high activity hydrodesulfurization catalyst of the type described above in connection with the first embodiment. This high activity catalyst comprises a support having a relatively high surface area of from 280 to 360 m 2 /g, a cobalt component in the amount of from 3 to 10 wt %, a molybdenum component in an amount of 12 to 30 wt %, and from 0 to 0.5 wt % of a phosphorous component, with the wt % of each component being based on the total weight of said catalyst composition and being calculated assuming each component is in its elemental form. Ihe high surface area support consists essentially of alumina and from 0.3 to 10 wt % silica, preferably 0.5 to 6 wt % silica, and most preferably 1.5 to 2.5 wt % silica, based on the total weight of the support. 
     By “suitable heavy fraction hydrodesulfurization conditions” is meant a temperature generally in the range of from 200° C. to 420° C., a pressure generally in the range of 100 psia to 1000 psia, a WHSV generally in the range of 0.1 hr −1  to 250 hr −1  and a hydrogen rate generally in the range of from 10 m 3  to 10,000 m 3  hydrogen per m 3  of the sulfur and olefin-containing heavy fraction, Preferred heavy fraction hydrodesulfurization conditions include a temperature in the range of from 240° C. to 350° C., a pressure in the range of 150 psia to 600 psia, a WHSV in the range of 0.1 hr −1  to 50 hr −1  and a hydrogen rate in the range of from 20 m 3  to 1000 m 3  hydrogen per m 3  of the heavy fraction. Most preferred heavy fraction hydrodesulfurization conditions include a temperature in the range of from 260° C. to 300° C., a pressure in the range of 200 psia to 400 psia, a WHSV in the range of 0.1 hr −1  to 30 hr −1  and a hydrogen rate in the range of from 30 m 3  to 200 m 3  hydrogen per m 3  of the heavy fraction. 
     The light fraction is contacted, under suitable light fraction hydrodesulfurization conditions, with a particularly defined high selectivity hydrodesulfurization catalyst, characterized by having a relatively low surface area of less than 125 m 2 /g and a high mean pore diameter of greater than 200 Å. 
     The conditions employed to selectively desulfurize the light fraction will generally be milder than the conditions used to selectively desulfurize the heavy fraction. Suitable conditions to selectively hydrodesulfurize the light fraction include a temperature in the range of from 200° C. to 400° C., a pressure in the range of 100 psia to 1000 psia, a WHSV in the range of 0.1 hr −1  to 250 hr −1  and a hydrogen rate in the range of from 10 m 3  to 10,000 m 3  hydrogen per m 3  of the light fraction. Preferred light fraction hydrodesulfurization conditions include a temperature in the range of from 220° C. to 330° C., a pressure in the range of 150 psia to 600 psia, a WHSV in the range of 0.1 hr −1  to 50 hr −1  and a hydrogen rate in the range of from 30 m 3  to 200 m 3  hydrogen per m 3  of the light fraction. Most preferred light fraction hydrodesulfurization conditions include a temperature in the range of from 240° C. to 280° C., a pressure in the range of 200 psia to 400 psia, a WHSV in the range of 0.1 hr −1  to 30 hr −1  and a hydrogen rate in the range of from 30 m 3  to 200 m 3  hydrogen per m 3  of the light fraction. 
     The high selectivity catalyst used to selectively desulfurize the light fraction comprises a cobalt component, a molybdenum component, and a phosphorus component that are incorporated onto or into a support that comprises alumina. The alumina support, before the incorporation therein of the cobalt, molybdenum and phosphorus components, preferably has a material absence of components that materially affect the finished catalyst properties of having a significant and reasonably selective desulfurization activity. 
     It is important that the support used to prepare the high selectivity catalyst used to selectively desulfurize the light fraction consist essentially of alumina and, further, for the alumina to be predominantly in the form of theta-alumina and/or delta-alumina. This is in contrast to the alumina supports used in many prior art HDS catalysts, which are predominantly in the form of gamma alumina or other transitional form. Thus, the finished support, i.e., the support after calcining, used to prepare the high selectivity catalyst will be predominantly in the form of theta alumina and delta alumina with less than 50 wt % of the support in the form of gamma-alumina, preferably less than 30 wt % of the support in the form of gamma-alumina, most preferably less than 20 wt % of the support in the form of gamma-alumina. 
     The metal components of the high selectivity catalyst used to desulfurize the light fraction may be present therein in their elemental form or as their oxides, sulfides or mixtures of each. The form of the metal components depends upon whether or not the catalyst composition has been calcined or sulfided or reduced or some combination thereof. 
     The amount of the cobalt component present in the high selectivity catalyst can typically be in the range of from 0.1 wt % to 12 wt %. This weight percent is based on the total weight of the catalyst composition and is calculated assuming the cobalt component is in the oxidic form (CoO) regardless of the form (e.g., elemental form, oxide form, or sulfide form) in which it is actually present in the catalyst composition. It is preferred for the cobalt component to be present in the high selectivity catalyst in an amount in the range of from 2 wt % to 9 wt %, and, most preferred, the cobalt component is present in an amount in the range of from 4 wt % to 8 wt % of the total catalyst composition calculated as CoO. 
     The amount of the molybdenum component present in the high selectivity catalyst used to desulfurize the light fraction can typically be in the range of from 3 wt % to 45 wt %, This weight percent is based on the total weight of the catalyst composition and is calculated assuming the molybdenum component is in the oxidic form (MoO 3 ) regardless of the form (e.g., elemental form, oxide form, or sulfide form) in which it is actually present in the catalyst composition. It is preferred for the molybdenum component to be present in the high selectivity catalyst in an amount in the range of from 6 wt % to 30 wt %, and, most preferred, the molybdenum component is present in an amount in the range of from 20 wt % to 25 wt % of the total catalyst composition calculated as MoO 3 . 
     It is important to the control of the desulfurization activity of the high selectivity catalyst that the concentration of the phosphorus component to be relatively small, e.g., in the range of from 0.1 wt % to 5 wt %. This weight percent is based on the total weight of the catalyst composition and is calculated as assuming the phosphorus component is in the oxidic form (P 2 O 5 ) regardless of the form in which it is actually present in the catalyst. It is preferred for the phosphorus component to be present in the high selectivity catalyst in an amount in the range of from 0.3 wt % to 3 wt %, and, most preferred, to be from 0.5 wt % to 2 wt %, based on the total weight of the catalyst composition and calculated assuming the phosphorus component is present as P 2 O 5 . 
     It has been determined that, in order to provide the optimum level of desulfurization activity and selectivity of desulfurization relative to olefin saturation, it is desirable to control the amounts of the cobalt and phosphorus components contained in the high selectivity catalyst relative to the amount of the molybdenum component. Thus, the amounts of cobalt and molybdenum present in the high selectivity catalyst should be such that the atomic ratio of molybdenum-to-cobalt (Mo/Co) is in the range of from 1:1 to 20:1. The preferred atomic ratio of molybdenum-to-cobalt in the high selectivity catalyst is in the range of from 1.25:1 to 15:1, and, most preferred, the Mo/Co is in the range of from 2:1 to 10:1. 
     The amount of molybdenum relative to phosphorus present in the high selectivity catalyst should be such that the atomic ratio of molybdenum-to-phosphorus (Mo/P) is in the range of from 5:1 to 150:1. The preferred atomic ratio of molybdenum-to-phosphorus in the high selectivity catalyst is in the range of from 10:1 to 100:1, and, most preferred, the Mo/P is in the range of from 15:1 to 50:1. 
     It is particularly important for the high selectivity catalyst used to hydrodesulfurize the light fraction to have certain physical properties in order to provide for high sulfur removal activity while being selective against olefin saturation It is significant and unexpected that a catalyst composition having the composition and components as described above but with a low surface area and a high mean pore diameter is particularly useful in the selective desulfurization of a light fraction of an olefin-containing hydrocarbon feedstock that has a sulfur concentration and an olefin concentration. 
     In general, the low surface area of the catalyst composition, as measured by the B.E.T. method, is less than 125 m 2 /gram, but it is particularly desirable for the low surface area to be less than 110 m 2 /gram. It also can be beneficial for the surface area of the catalyst composition to be no more than 105 m 2 /gram, and, even, no more than 103 m 2 /gram. A lower limit for the surface area of the catalyst composition can be no less than 25 m 2 /gram, preferably, no less than 30 m 2 /gram, and, most preferably, no less than 35 m 2 /gram. 
     The mean pore diameter of the catalyst composition should be significantly large as to, in combination with the other features of the catalyst composition, provide for the desired selective desulfurization activity, As measured using standard mercury porosimetry, the mean pore diameter of the catalyst composition is, in general, greater than 190 angstroms (Å), but it is particularly desirable for the mean pore diameter of the catalyst composition to be greater than 200 Å. It is preferred for the mean pore diameter of the catalyst composition to exceed 225 Å, and, most preferred, it can exceed 250 Å. In certain instances, the mean pore diameter of the catalyst composition can exceed 275 Å or even 300 Å. An upper limit for the mean pore diameter is less than 500 Å, or, less than 475 Å, and, evenr less than 450 Å. 
     The high selectivity catalyst used to desulfurize the light fraction may be prepared by any suitable method known to those skilled in the art that will suitably provide a catalyst composition having the properties and composition as described herein. The preferred method of preparing the catalyst composition of the invention includes preparing a support particle that consists essentially of alumina that is impregnated with a cobalt component, a molybdenum component and a phosphorus component with the resulting impregnated support particle being calcined under suitable calcination conditions. 
     In the preferred method of making the high selectivity catalyst composition, the support particle is first prepared by mixing the starting alumina or alumina precursor powder with water by any suitable means or method for providing a substantially homogeneous mixture of the alumina and water. Many of the possible mixing means that may suitably be used in preparing the mixture are described in detail in Perry&#39;s Chemical Engineers&#39; Handbook, Sixth Edition, published by McGraw-Hill, Inc. at pages 19-14 through 19-24, which pages are incorporated herein by reference. Thus, possible suitable mixing means can include, but are not limited to, such devices as tumblers, stationary shells or troughs, Muller mixers, which are either batch type or continuous type, impact mixers, and any other mixer or device known to those skilled in the art and that will suitably provide the homogeneous mixture of alumina and water. 
     The amount of water mixed with the alumina should be such that a paste mixture is formed that can then be formed into agglomerated particles. Typically, the amount of water present in the mixture is in the range of from 30 wt % to 85 wt %, and, preferably, it is in the range of from 40 wt % to 75 wt %. A peptizing agent, such as nitric acid or other acid, may be added to the mixture of alumina and water to assist in the dispersion of the alumina and the formation of the paste. It is particularly desirable for the paste to have the plasticity required for extrusion thereof. 
     While the formation of the agglomerate is preferably done by any of the standard extrusion methods known to those skilled in the art, other possible suitable means or methods for forming the agglomerate may include, for example, molding, tableting, pressing, palletizing, tumbling and densifying. 
     The agglomerate is then dried, preferably, at a temperature in the range of from 40° C. (104° F.) to 260° C. (500° F.), and calcined to form the support particles into which is incorporated the catalytic components The calcination is conducted in the presence of oxygen or an oxygen-containing inert gas or air. In order to obtain a support having the preferred theta-alumina and/or delta alumina form, it is important to calcine the support particles at a relatively high temperature, e.g., from 760° C. (1400° F.) to 1,093° C. (2000° F.). A particularly preferred calcination temperature range is from 871° C. (1,600° F.) to 982° C. (1,800° F.). The calcination time period can be in the range of from 0.5 hours to 72 hours, or even longer, if required. However, it is preferred to control the calcination temperature and time within a range that produces a support having surface area of less than 150 m 2 /g, as measured using the B.E.T. method, preferably less than 140 m 2 /g. For example, calcination of the support at a temperature of 1650° F. for 2 hours will produce a support having a surface area of about 133 m 2 /g, and which will be predominantly in the form of theta-alumina and delta-alumina. 
     The catalytic components of the high selectivity catalyst may be incorporated into the support particles using one or more impregnation solutions containing one or more of the catalytic components. The preferred impregnation solution is an aqueous solution of the desired catalytic component or a precursor thereof. Potential cobalt compounds that may be used in the formation of the aqueous solution include the cobalt hydroxides, acetates, carbonates, nitrates, and sulfates or mixtures of two or more thereof. Potential molybdenum compounds that may be used in the formation of the aqueous solution include molybdenum oxide and the ammonium salts of molybdenum, such as ammonium heptamolybdate and ammonium dimolybdate. Any suitable phosphorus containing compound may be used in the aqueous solution to provide for the phosphorus component of the catalyst composition. One such phosphorus compound is phosphoric acid. The cobalt compound, molybdenum compound, and phosphorus compound are dissolved in water to form an aqueous solution in such amounts as to provide, when incorporated into the support particles, the desired metal concentrations in the final catalyst composition as defined earlier herein. Typically, the concentration of the metal compounds in the impregnation solution is in the range of from 0.01 to 100 moles per liter. 
     The impregnation may be conducted by any procedure or method or means that suitably incorporates the desired metal components in the desired amounts into the support particles. Such impregnation methods include, for example, spray impregnation, soaking, multi-dip procedures, and incipient wetness impregnation methods. 
     The impregnated support particles are then dried, preferably, at a temperature in the range of from 40° C. (104° F.) to 260° C. (500° F.), and calcined to form the final catalyst composition. The calcination is conducted in the presence of oxygen or an oxygen-containing inert gas or air. The temperature at which the impregnated support particles are calcined should be in the range of from 482° C. (900° F.) to 649° C. (1200° F.). But, it is particularly preferred to carefully control the calcination temperature to within the range of from 510° C. (950° F.) to 593° C. (1100° F.). 
     The high selectivity catalyst composition described above is especially useful in the selective hydrodesulfurization of the light fraction of an olefin-containing hydrocarbon feedstock having a feed sulfur concentration and an olefin concentration. As noted above, it has been found that the particularly described catalyst compositions can provide for improved selectivity toward the hydrodesulfurization of the olefin-containing light fraction as compared to the use of other hydrotreating-type catalysts. It is the use of a catalyst composition specifically having the physical features and composition as described herein that provides for the advantageous selective hydrodesulfurization of the light fraction in the second embodiment of the inventive process. 
     In the third embodiment of the inventive process an olefin-containing gasoline feedstock having a feed sulfur concentration exceeding 0.01 wt % is fed into a distillation column reactor which comprises a lower distillation reaction zone containing a lower catalytic distillation structure formed with high activity catalyst, and an upper distillation reaction zone containing an upper catalytic distillation structure formed with a high selectivity catalyst, wherein the upper distillation reaction zone is in a juxtaposed vertical relationship above the lower distillation reaction zone. 
     The distillation column reactor is utilized to separate the olefin-containing gasoline feedstock into a light fraction and a heavy fraction, and to concurrently hydodesulfurize both the light and heavy fractions. The light fraction is desulfurized by contacting the light fraction with at least a portion of the upper catalytic distillation structure containing the high selectivity catalyst of the type described above in connection with the second embodiment, while the heavy fraction is desulfurized by contacting the heavy fraction with at least a portion of the lower catalytic distillation structure containing a high activity catalyst of the type described above in connection with the first and second embodiments. 
     The yield from the distillation column reactor is a light hydrotreated product having a light hydrotreated product sulfur concentration below the feed sulfur concentration and a heavy hydrotreated product having a heavy hydrotreated product sulfur concentration below the feed sulfur concentration. 
     As used herein the term “distillation column reactor” means a distillation column which also contains one or more catalysts, such that reaction and distillation are occurring simultaneously in the column. Preferably, the catalyst is prepared as a distillation structure so that it can facilitate both distillation and the desired reaction(s). Suitable catalytic distillation structures include those described in U.S Pat. Nos. 5,266,546, 4,731,229, 5,073,236, 5,431,890, and 5,730,843, which are incorporated herein by reference. 
     Typical conditions for desulfurization of the gasoline feedstock in the distillation column reactor having an upper distillation reaction zone containing the above described high selectivity catalyst and lower catalytic distillation zone containing the above described high activity catalyst, include a temperature in the range of from 150° C. (302° F.) to 420° C. (788° F.), a pressure in the range of 25 psia to 500 psia, a LHSV in the range of 1 to 5 and a hydrogen rate in the range of from 10 to 1000 SCF per barrel of the gasoline feedstock. Preferred distillation column reactor conditions include a temperature in the range of from 175° C. (347° F.) to 400° C. (752° F.), a pressure in the range of 50 psia to 300 psia, a LHSV in the range of 2 to 5 and a hydrogen rate in the range of from 20 to 800 SCF hydrogen per barrel of feedstock. 
     An advantage of conducting the selective hydrodesulfurization process in a distillation column reactor is that lower pressures can be used than in standard trickle bed reactors. As in any distillation there is a temperature gradient in the distillation column reactor. Thus, the temperature in the bottom section of the distillation column reactor containing lower catalytic distillation reaction zoner will be at a higher temperature than upper section of the distillation column reactor containing the upper catalytic distillation zone. Because the more readily removable sulfur compounds will be in the light fraction which is desulfurized in the upper catalytic distillation reaction zone, the upper catalytic distillation zone can be operated at lower temperatures, which results in the greater selectivity with less loss of olefins due to saturation. 
     Each of the three embodiments of the inventive process described above provide for the selective hydrodesulfurization of an olefin-containing hydrocarbon feedstock, and each of the three embodiments yield a hydrotreated product having a significantly reduced sulfur concentration that is much reduced below the feed sulfur concentration of the olefin-containing hydrocarbon feedstock. The various embodiments of the inventive process can provide for a sulfur reduction in an amount greater than 15 weight percent of the sulfur contained in the olefin-containing hydrocarbon feedstock while causing less than a 25 weight percent olefin reduction by the catalytic hydrogenation of the olefin compounds contained in the olefin-containing hydrocarbon feedstock to yield the hydrotreated product. 
     While the sulfur reduction of at least 15 weight percent with less than a 25 weight percent olefin compound reduction is a reasonably selective hydrodesulfurization of an olefin-containing feedstock, it is desirable for the process to be more selective in the hydodesulfurization of the feedstock by providing for a higher percentage of sulfur reduction but with a lower percentage of olefin reduction. Thus, with the various embodiments of the inventive process it is desirable to provide for a sulfur reduction of at least 18 weight percent and even at least 20 weight percent. Preferably, the sulfur reduction is at least 25 weight percent, and, more preferably, the sulfur reduction is at least 30 weight percent. Most preferably, the sulfur reduction is greater than 32 weight percent. 
     It is desirable for the hydrotreated product(s) from the three embodiments of the inventive process to have a reduced sulfur concentration that is low enough so that when it is combined (after removal of the hydrogen sulfide therefrom) with other gasoline blending stocks the combination meets a significantly low sulfur target. The hydrodesulfurized product(s), thus, will generally have a reduced sulfur concentration of less than 100 ppmw organic sulfur, preferably less than 75 ppmw organic sulfur, and more preferably less than 50 ppmw organic sulfur. Most preferably the reduced sulfur concentration of the hydrotreated product(s) will be less than 30 ppmw organic sulfur. 
     Because the highly selective nature of the desulfurization process of the present invention, the high percentage of sulfur removal described above is accompanied with a relatively low percentage of olefin removal through hydrogenation of the olefin compounds in the feedstock to saturated compounds. In each of the instances noted above with respect to the sulfur reduction it is desirable that there not be a substantial reduction in olefin concentration, i.e., that the reduction in olefin concentration be minimized. By “no substantial reduction in olefin concentration” is meant that the reduction in olefin concentration is less than 25 weight percent. Preferably, the weight percent olefin reduction is less than 20 weight percent, and, most preferably, the weight percent olefin reduction is less than 15 weight percent. 
     When referring herein to the weight percent sulfur reduction of the sulfur contained in the olefin-containing hydrocarbon feedstock, what is meant is that the weight percent sulfur reduction is the ratio of the difference between the weight of organic sulfur in the olefin-containing feedstock and the weight of organic sulfur in the yielded hydrotreated product divided by the weight of organic sulfur in the olefin-containing feedstock with the ratio being multiplied by the number one-hundred (100). It is understood that the concentrations of hydrogen sulfide in the olefin-containing feedstock and yielded hydrotreated product are ignored in this computation. 
     When referring herein to the weight percent olefin reduction of the olefin compounds contained in the olefin-containing hydrocarbon feedstock, what is meant is that weight percent olefin reduction is the ratio of the weight of the olefin compounds in the feedstock that are hydrogenated to saturated compounds divided by the weight of olefin compounds in the feedstock with the ratio being multiplied by the number one-hundred (100). The olefin compounds hydrogenated to saturated compounds is defined as being the difference between the weight of olefin compounds in the feedstock and the olefin compounds in the yielded product. 
     The high activity catalyst employed in the first embodiment of inventive process, and the high activity catalyst and high selectivity catalyst employed in the second embodiment of the inventive process, may be employed as a part of any suitable reactor system that provides for the contacting of the catalyst composition with the hydrocarbon feedstock under suitable selective hydrodesulfurization reaction conditions that can include the presence of hydrogen and an elevated temperature and total pressure. Such suitable reactor systems can include fixed catalyst bed systems, ebullating catalyst bed systems, slurried catalyst systems, and fluidized catalyst bed systems. In the case of the third embodiment, the preferred reactor system is a distillation column reactor containing an upper and a lower catalytic distillation structure. The preferred reactor system for the first and second embodiments is one that includes a fixed bed of the catalyst contained within a reactor vessel equipped with a reactor feed inlet means, such as a feed inlet nozzle, for introducing the hydrocarbon feedstock into the reactor vessel, and a reactor effluent outlet means, such as an effluent outlet nozzle, for withdrawing the reactor effluent or low sulfur product from the reactor vessel. 
     Referring now to  FIG. 1 , which is a simplified flow diagram of the second embodiment of the inventive process. An olefin containing gasoline feedstock is fed through flow line  1  into fractionator  2  wherein it is separated into a light fraction and a heavy fraction. The light fraction passes overhead through flow line  3  into reactor  5  containing catalyst bed  6  comprising a high selectivity desulfurization catalyst as described above having a surface area of less than 125 m 2 /g and mean pore diameter of greater than 200 Å. Hydrogen is fed into reactor  5  via line  7  and is contacted with the light fraction in the presence of the high selectivity catalyst resulting in the conversion of the sulfur compounds contained therein to H 2 S with minimal saturation of the high octane olefins in the light fraction A light hydrotreated product, e.g., light naphtha containing a substantially reduced sulfur content is withdrawn from reactor  5  via flow line  8 . 
     The heavy fraction separated in fractionator  2  is withdrawn through flow line  4  and is passed to reactor  10  containing a bed  11  of high activity catalyst of the type described above prepared from an alumina support having a low concentration of silica and a high surface area exceeding 280 m 2 /g and little or no phosphorus. The feedstock is contacted with hydrogen which is fed into reactor  10  via line  12  in the presence of the high activity catalyst resulting in the substantial conversion of sulfur compounds to H 2 S, with minimal saturation of olefins, most of which will have been separated and form part of the light fraction. 
     A simplified flow diagram of the third embodiment of the inventive process is shown in  FIG. 2 , the main difference between it and the second embodiment being that the separation of the feedstock into light and heavy fractions and the catalytic desulfurization of the respective fractions is accomplished in a single distillation column reactor  20  containing an upper distillation reaction zone  22  and a lower distillation reaction zone  23 . 
     In this embodiment, a gasoline feedstock is fed into the distillation column reactor through flow line  21  and hydrogen is fed into the distillation column via lines  24  and  25 . Alternatively, hydrogen can be combined with the gasoline feedstock prior to introduction into the distillation column reactor The gasoline feedstock is separated in the distillation column reactor into a light fraction, which comes into contact with at least a portion of the high selectivity catalyst in upper distillation reaction zone  22 , and a heavy fraction, which comes into contact with at least a portion of the high activity catalyst in the lower distillation reaction zone  23 . The resulting light hydrotreated product having a substantially reduced organic sulfur concentration, with minimal loss of high octane olefins, is taken overhead through flow line  26  and can be suitably employed for gasoline blending. The heavy hydrotreated product having a substantially reduced heavy organic sulfur content is taken as bottoms from the distillation column reactor via flow line  27  and is also suitable for gasoline blending. 
     H 2 S formed as a result of conversion of organic sulfur in the gasoline feedstock can be removed from the hydrotreated streams by any suitable means such as caustic wash, stripping, or the like, Any unreacted hydrogen may be recycled. 
     The following examples are presented to further illustrate the invention, but they are not to be construed as limiting the scope of the invention. 
     EXAMPLE I 
     This Example describes the preparation of Catalyst A, which is a high activity catalyst suitable for use in the various embodiments of the inventive process. Also described in this example is a comparison catalyst, which is a commercially available catalyst, that was compared to Catalyst A in the selective hydrodesulfurization experiments described in Example II. 
     Catalyst A 
     In accordance with the invention, Catalyst A was prepared from an alumina support having a relatively high surface of at least 280 m 2 /g, and which contained a low concentration of silica. The support for Catalyst A was prepared using a commercially available silica-alumina powder material, which contained 2 wt % silica with the remaining balance being substantially entirely alumina. The powder was mixed with deionized water in an amount to provide an approximate loss on ignition for the mixture of around 64 wt %. Also added to the mixture was approximately 1 wt % of a 69.8% nitric acid solution and approximately 1 wt % Superfloc 16. The mixture was extruded using 1.2 mm trilobe extrusion die inserts. The extrudate was dried at a is temperature of 150° C. and then calcined at a temperature of about 538° C. (1000° F.) for two hours to provide the support. The support had a relatively high surface area of 300 m 2 /g, a water pore volume of 0.85 ml/gram, and contained 2 wt % silica. 
     The impregnation solution for impregnating the above-described support was prepared by separately preparing a molybdenum solution and a cobalt solution and then mixing the two to provide the impregnation solution. The molybdenum solution was prepared by mixing 25.65 parts (NH 4 ) 2 Mo 2 O 7 , 7.06 parts MoO 3 , 5.03 parts 30% hydrogen peroxide, and 20.91 deionized water. To this mixture 1.24 parts monoethanlamine was added in a controlled fashion in order to control the resultant exotherm. The resultant molybdenum solution was cooled. The cobalt solution was prepared by dissolving 22.52 parts Co(NO 3 ) 2 .6H2O in 9.84 parts deionized water. The molybdenum solution and cobalt solution were combined and diluted with 82.5 parts deionized water. This final solution was used to impregnate the support. The impregnated support was dried at a temperature of 125° C. and then calcined at a temperature of 482° C. (900° F.) for two hours. The composition of the final catalyst contained 3.6 wt % cobalt, calculated as the metal (i.e., 4.58 wt % cobalt, calculated as CoO), 14.2 wt % molybdenum, calculated as the metal (i.e., 21.30 wt % molybdenum, calculated as MoO 3 ) with the balance being the alumina support containing the low concentration of silica. The final catalyst had a nitrogen surface area of 250 m 2 /g, a total pore volume of 0.53 cc/g. and a median pore diameter of 112 Å, as measured by mercury porosimetry. 
     Comparison Catalyst 
     The comparison catalyst was a commercially available catalyst having 3.4 wt % cobalt and 13.6 wt % molybdenum on an alumina support and further having a surface area of 235 m 2 /gram and a water pore volume of 0.53 cc/gram. Like Catalyst A, the comparison catalyst contained no phosphorus. However, the comparison catalyst was prepared from a support which had a surface area less than 300 m 2 /g, and which did not contain silica, both of which are important features of the high activity catalyst employed in the various embodiments of the inventive process. 
     EXAMPLE II 
     This Example summarizes the experiment used to measure the performance of Catalyst A and the comparison catalyst in the selective hydrodesulfurization of an olefin-containing hydrocarbon feedstock having a concentration of sulfur. 
     The testing was performed using high throughput nanoreactors. Approximately 1 ml of crushed catalyst was used in each reactor. The feed to the reactors was a synthetically prepared gasoline feedstock that included a range of hydrocarbon components that are typically found in cracked gasoline (e.g., heptane, hexane, octane, octane, butylenes, and toluene), and it was spiked with organic nitrogen and sulfur compounds to provide concentrations thereof. The reactors were operated under suitable selective hydrodesulfurization temperature, pressure and space velocity process conditions. 
     Summaries of the results from the aforedescribed testing are presented for illustrative purposes in the comparative plots presented in  FIG. 3  and  FIG. 4 . 
       FIG. 3  presents comparative plots, with a linear fit of the data, of the performance results for Catalyst A and the Comparison Catalyst in terms of their catalytic activity toward sulfur conversion (i.e., the k value) as a function of the reaction temperature. As may be observed from the plots of  FIG. 3 , the activity value exhibited by Catalyst A for a given reactor temperature is greater than that exhibited by the comparison catalyst for the same reactor temperature  FIG. 3  shows that at low reactor temperatures Catalyst A is about 35% more active than the comparison catalyst. Also, the data summarized in  FIG. 3  show that the activity slope for Catalyst A is greater than the activity slope for the comparison catalyst, thus, indicating that Catalyst A provides for a better improvement in desulfurization activity for a given reactor temperature increase. 
       FIG. 4  presents comparative plots of the selectivity of Catalyst A and the comparison catalyst. The data summarized in  FIG. 4  presents the performance results for Catalyst A and the comparison catalyst in terms of the percentage of olefins contained in the feed that is converted as a function of the negative log of the fraction of the sulfur contained in the feed that is not converted, i.e , 1- (feed sulfur concentration product sulfur concentration)/(feed sulfur concentration). As may be observed from the plots of  FIG. 4 , for a given sulfur conversion, Catalyst A provides for a lower percentage of feed olefins conversion than that provided by the comparison catalyst. Thus, Catalyst A is more selective than the comparison catalyst in that it provides for a smaller percentage of feed olefins that is converted for a particular feed sulfur conversion. 
     The data summarized in this Example show that a particular catalyst composition prepared from a high surface area alumina support containing a low concentration of silica can be very effectively used for the selective desulfurization of a sulfur and olefin-containing gasoline feedstock.