Abstract:
A platinum-containing aluminum oxide reforming catalyst is disclosed which is modified by incorporating silica therein. The resulting catalyst is used to reform naphtha yielding a reformate having a higher C5 and aromatic content than that achieved using a similar catalyst which contains no added silica.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 60/348,790, filed Jan. 14, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to a novel catalyst for use in reforming of naphtha and a reforming process using that catalyst.  
           [0004]    2. Brief Description of Related Art  
           [0005]    Catalytic reforming is a major petroleum refining process used to raise the octane rate of naphthas (C5 to C11 hydrocarbons) for gasoline blending. Catalytic reforming is also a principle source of aromatic chemicals (benzene, toluene, and xylenes) via conversion of paraffins and naphthenes to aromatics. The principle chemical reactions which occur during catalytic reforming include dehydrogenation of cyclohexanes to aromatics, dehydrocyclization of paraffins to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, isomerization of normal paraffins to branched paraffins, dealkylation of alkylbenzenes and hydrocracking of paraffins to light hydrocarbons, i.e. methane, ethane, propane, and butane. The latter reaction is undesirable and should be minimized since it produces light hydrocarbons not suitable for gasoline blending which have less value than gasoline fractions.  
           [0006]    Reforming is carried out at temperatures of 800° F. to 1100° F., pressures of 50 to 300 psi, weight hourly space velocities of 0.5 to 3.0 and in the presence of hydrogen at hydrogen to hydrocarbon molar ratios of 1 to 10.  
           [0007]    Reforming catalysts currently widely used in commercial reformers are platinum on an alumina substrate, and platinum plus a second promoting metal such as rhenium, tin, or indium on alumina. These catalysts are bifunctional, i.e., the dehydrogenation reactions required in the reforming process are accomplished on the catalytic metal in the catalysts and the isomerization and cyclization reactions also required in reforming are accomplished on acid sites on the alumina catalyst support. Undesirable hydrocracking reactions which break C6+ paraffins down to lower molecular weight hydrocarbons and reduce selectivity to aromatics occur primarily on the acid catalytic sites.  
           [0008]    Alumina based reforming catalysts demonstrate reasonably high selectivities for converting C8+ paraffins and naphthenes to aromatics but are less satisfactory for aromatizing C 6  to C 8  paraffins because they tend to hydrocrack more of the lower paraffins to low value fuel gas rather than they convert to aromatics.  
         SUMMARY OF THE INVENTION  
         [0009]    This invention provides a modified refractory aluminum oxide catalyst for use as a naphtha reforming catalyst which produces a reformate having enhanced C 5 + yields and particularly enhanced yields of aromatic compounds as compared to a similar unmodified catalyst. More specifically, the invention provides a modified platinum-containing refractory aluminum oxide reforming catalyst containing from about 0.1 to about 10% by weight silica.  
           [0010]    The invention also provides a process for reforming naphtha comprising contacting a naphtha stream under reforming conditions with the modified catalyst and recovering a reformate having an enhanced content of C 5 + and aromatic compounds.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0011]    The catalyst support used in the present invention is a porous refractory aluminum oxide material such as alumina, alumina-titania, alumina-chromia and the like, in combination with a Group VIII noble metal such as platinum and at least one other metal, such as indium, rhenium, tin, gallium, palladium, lead, iron, or tungsten, and, for certain uses, a halogen component. The support component of the catalyst is preferably a porous, adsorptive material having a surface area, as determined by the Brunauer-Emmett-Teller (BET) method, of about 20 to 800, preferably 100-300 square meters per gram. This support material should be substantially refractory at the temperature and pressure conditions utilized in any given hydrocarbon conversion process.  
           [0012]    Alumina in its gamma or eta forms is the preferred catalyst support. Typically, the support materials are prepared in the form of spheres, granules, powders, extrudates or pellets. The precise size or shape of the support material used is dependent upon many engineering factors not within the purview of the instant invention. It is also within the scope of this invention to have all the metals of the multi-metallic platinum-containing catalyst on the same support in one particle, e.g., platinum and iridium on alumina, or as a mixture of separate particles, e.g., platinum on alumina mixed with indium on alumina.  
           [0013]    The multi-metallic platinum-containing catalyst may be prepared employing simple impregnation techniques. Such a catalyst may be prepared by impregnating the support material with a solution of a soluble platinum compound and soluble compounds of any additional metals to be incorporated in the catalyst. Generally, an aqueous solution of the metal compounds is used. The support material may be impregnated with the various metal-containing compounds either sequentially or simultaneously. The carrier material is impregnated with solutions of appropriate concentration to provide the desired quantity of metals in the finished catalyst. In the case of indium, compounds suitable for the impregnation onto the carrier include, among others, chloroiridic acid, indium tribromide, indium trichloride, and ammonium chloroiridate. In the case of platinum, compounds such as chloroplatinic acid, ammonium chloroplatinate, and platinum amine salts can be used. Additional catalyst metals may be incorporated onto the support by employing compounds such as perrhenic acid, ruthenium trichloride, rhodium trichloride, rhodium nitrate, palladium chloride, palladium amine salts, stanous chloride, silver nitrate, cobalt nitrate, nickel nitrate, and the like. The preferred catalyst manufacturing technique involves contacting a previously prepared support, such as alumina with an aqueous solution of indium and platinum compounds, alone or in combination with a compound of at least one additional catalyst metal.  
           [0014]    After impregnation of the carrier, the composite catalyst is dried at a temperature varying from about 220° to 250° F. The catalyst may be dried in air at the above stated temperatures or may be dried by treating the catalyst in a flowing stream of inert gas or hydrogen. The drying step may be followed by an additional calcination step at temperatures of about 500° to 700° F. Care must be taken to avoid contacting the catalyst at temperatures in excess of about 700° F. with air or other gases of high oxygen concentration. If the catalyst is contacted with oxygen at too high a temperature, at least a portion of the non-platinum component, such as indium, will be oxidized, with loss of surface area, to crystallites of indium oxide.  
           [0015]    Additional materials may be added to the platinum-containing catalyst composites to assist in the promotion of various types of hydrocarbon conversion reactions for which the catalyst might be employed. For example, the naphtha reforming activity of the catalyst is enhanced markedly by the addition of a halogen moiety, particularly a chlorine or fluorine moiety, to the catalyst. The halogen should be present in the catalyst in amounts varying from about 0.1 to about 3.0 weight percent, based on the total dry weight of the catalyst. The halogen may be incorporated into the catalyst at any suitable stage in the catalyst manufacturing operation, i.e. before, during or after incorporation of the active metal component onto the support material. Halogen is often incorporated into the catalyst by impregnating the support with halogen-bearing metal compounds such as chloroiridic acid. Further amounts of halogen may be incorporated in the catalyst by contacting it with hydrogen fluoride, ammonium fluoride, hydrogen chloride or ammonium chloride, either prior to or subsequent to the impregnation step. Other components may also be added to the catalyst composite. For example, the catalyst may be sulfided before or during use.  
           [0016]    In accordance with this invention, the bifunctional aluminum oxide catalysts as described above are modified by ex-situ treatment with silica or an organosilicon compound, followed by calcination to convert the organosilicon compound to silica. The bifunctional aluminum oxide catalysts contain both metal sites and acid sites and it is believed that the presence of silica in the catalyst reduces the acidity in the catalyst thereby decreasing its cracking activity with a consequent enhancement of the content of C 5 + and aromatic compounds present in the reformate.  
           [0017]    The incorporation of silica (SiO 2 ) into the catalyst (hereafter referred to as silylation) may be accomplished by mixing the catalyst with an aqueous dispersion of colloidal silica to impregnate the aluminum oxide with silica, followed by drying the resulting mixture at a temperature of from about 100 to about 400° C. for a period of from about 0.5 to 10 hours.  
           [0018]    Yet another silylation technique is analogous to methods used to selectivate zeolite catalysts used in isomerization and separation processes such as described in U.S. Pat. Nos. 5,476,823 and 5,365,003. This method involves contacting the catalyst particles with an organo silicon compound or solvent solution or aqueous emulsion thereof to form a coating on the surface of the particles of catalyst, followed by calcination to convert the organic compound to SiO 2 .  
           [0019]    Representative silicone compounds include dimethyl silicone, diethyl silicone, phenylmethyl silicone, methylhydrogen silicone, ethylhydrogen silicone, phenylhydrogen silicone, methylethyl silicone, phenylethyl silicone, diphenyl silicone, methyltrifluoropropyl silicone, ethyltrifluoropropyl silicone, polydimethyl silicone, tetrachlorophenylmethyl silicone, tetrachlorophenylethyl silicone, tetrachlorophenylhydrogen silicon, tetrachlorophenylphenyl silicon, methylvinyl silicone and ethylvinyl silicone. The silicone compound need not be linear, but may be cyclic, for example, hexamethyl cyclotrisiloxane, octamethyl cyclortetrasiloxane, hexaphenyl cyclotrisiloxane and octaphenyl cyclotetrasiloxane. Mixtures of these compounds may also be used, as may silicones with other functional groups.  
           [0020]    Other silicon compounds, including silanes and alkoxy silanes, such as tetramethoxy silane, may also be utilized.  
           [0021]    Preferred silicon-containing silylation agents include dimethylphenylmethyl polysiloxane (e.g., Dow-550) and phenylmethyl polysiloxane (e.g., Dow 710). Dow-550 and Dow-710 are available from Dow Chemical Co.  
           [0022]    The silicon compound may be applied to the aluminum oxide powder in neat form or as a solvent solution or aqueous emulsion. After application of the silicon compound to the surface of the aluminum oxide catalyst, the catalyst is calcined at 350-550° C. in air from a period of 1-24 hours to convert the organosilicon compound to silica. The process may be repeated one or more times to achieve the desired content of silica in the catalyst.  
           [0023]    The final content of silica present on the surfaces of the aluminum oxide catalyst powder may range from about 0.1 to 10 wt %, more preferably from about 1 to 6 wt %.  
           [0024]    The resulting silica-modified catalyst may then be subjected to an oxychlorination treatment to re-disperse the platinum metal and establish the appropriate halogen level in the catalyst.  
           [0025]    Such a process involves treating the catalyst with a mixture of oxygen-containing and chlorine-containing gases at a temperature of up to about 1250° F. to bring about a reduction in size of the platinum crystallites and reducing the treated catalyst in the presence of hydrogen at a temperature in the range of about 400°-1000° F. Such an oxychlorination process is described in U.S. Pat. No. 3,134,732, the complete disclosure of which is incorporated herein by reference.  
           [0026]    The catalysts of the invention are particularly useful in promoting the dehydrogenation, isomerization, dehydrocyclization and hydrocracking reactions that occur in a naphtha hydroforming process.  
           [0027]    In a naphtha hydroforming process (reforming) a substantially sulfur-free naphtha stream that typically contains about 15 to 80 volume percent paraffins, 15 to 80 volume percent naphhthenes and about 2 to 20 volume percent aromatics and boiling at atmospheric pressure substantially between about 80° and 450° F., preferably between about 150° and 375° F., is contacted with the platinum-containing catalyst composite in the presence of hydrogen. The reactions typically occur in a vapor phase at a temperature varying from about 650° to 1100° F., preferably about 750° to 1000° F. Reaction zone pressures may vary from about 1 to 50 preferably from about 5 to 30 atmospheres. The naphtha feed stream is passed over the catalyst composite at space velocities varying from about 0.5 to 20 parts by weight of naphtha per hour per part by weight of catalyst (W/hrW) preferably from about 1 to 10 W/hr/W. The hydrogen to hydrocarbon mole ratio within the reaction zone is maintained between about 0.5 to 20, preferably between about 1 and 10. During the reforming process, the hydrogen used may be in admixture with light gaseous hydrocarbons. In a typical operation, the catalyst is maintained as a fixed bed within a series of adiabatically operated reactors. The product stream from each reactor (except the last) in the reactor train is reheated prior to passage to the following reactor. As an alternate to the above-described process, the catalyst may be used in a moving bed in which the naphtha charge stock, hydrogen and catalyst are passed in parallel through the reactor or in a fluidized system wherein the naphtha feed stock is passed upwardly through a turbulent bed of finely divided catalyst particles. Finally, if desired, the catalyst may be simply slurried with the charge stock and the resulting mixture conveyed to the reaction zone for further reaction.  
           [0028]    The following examples are illustrative of the invention. The base catalyst before silica treatment is a bimetallic CCR reforming catalyst with Pt/Sn loading. Catalyst properties are shown in Table 1.  
                                       TABLE 1                           Catalyst Properties                Property                            Pt/Sn, wt. %   0.3/0.27           Surface Area, m 2 /g   200           Support   Al 2 O 3             Chloride, wt %   1.23                      
 
       
    
    
     EXAMPLE 1  
     Silica Modification of the Catalyst Base  
       [0029]    The catalyst described above was impregnated with Ludox LS 30 colloidal silica (30 wt % suspension in water) as follows. 50 g Ludox LS 30 was combined with 110 g water. This suspension was slowly added to 200 g of the catalyst at room temperature to impregnate the catalyst with silica. The material was then dried for three hours at 650F in 5 v/vol air. Four 200 g batches of material were prepared in this manner and combined for further experiments.  
       EXAMPLE 2  
     Oxychlorination of Catalyst of Example 1  
       [0030]    Prior to experiments, the catalyst of Example 1 was treated in a Moving Bed Regenerator to re-disperse platinum and establish the appropriate chloride level. A chloride level of about 1.1 wt. % was targeted. During this procedure, excess silica was removed from the catalyst.  
       EXAMPLE 3  
       [0031]    The base catalyst and the catalyst of Example 2 were evaluated using a moving bed, four reactor pilot plant.  
         [0032]    Tables 2 and 3 respectively show the operating conditions and the composition of the naphtha used.  
                             TABLE 2                           Operating Conditions                Process Variable   Value                       Feed   Light naphtha           Average Reactor Pressure, psia   135           High Pressure Separator Pressure,   115           psia           Weight Hourly Space Velocity   1.5           (WHSV), hr −1           Hydrogen Recycle Ratio (HRR),   2.2           mol/mol           C5+ RON   100.25           Catalyst Circulation, g/hr   5                      
 
         [0033]    [0033]                             TABLE 3                       Feed Composition                                    Paraffins   55.9           Naphthenes   31.1           Aromatics   13.0                        
         [0034]    The modified catalyst of Example 2 was placed in the reactor and the naphtha feed was introduced. Reactor inlet temperature was maintained at about 990° F. so as to target C 5 + octane number of 100.25. After about 12 days of service, the reformate yield was evaluated.  
       EXAMPLE 4  
       [0035]    The unmodified base catalyst was introduced into the reactor and reforming was conducted under the same process conditions as described in Example 3. Temperature was adjusted to obtain the desired octane. Table 4 below shows the yield summaries for the catalysts of examples 3 and 4 using the light naphtha described in Table 3 as the feed.  
                                                           TABLE 4                           Yield Summaries                    Base Case   Si-modified               Yields, # on   (Catalyst of   (Catalyst of               feed   Example 4)   Example 3)   Difference                            C5+ vol %   71.45   72.87   1.42           C6+ vol %   61.80   63.64   1.84           H2 wt %   2.55   2.72   0.17           C1 + C2 wt %   4.51   4.38   (0.13)           C3 + C4 wt %   14.67   13.00   (1.67)           NC5 vol %   3.57   3.40   (0.17)           IC5 vol %   5.49   5.25   (0.24)           NC6 vol %   3.65   3.81   0.16           IC6 vol %   10.11   10.64   0.53           NC7 vol %   0.26   0.30   0.04           IC7 vol %   1.60   1.63   0.03           Benzene vol %   8.04   8.32   0.28           Toluene vol %   18.23   18.64   0.41           Xylene vol %   12.76   13.22   0.46           EB vol %   2.15   2.19   0.04           C9+ vol %   4.87   4.80   (0.08)           BTX wt %   46.91   48.29   1.38           WAIT, ° F.   989.83   992.07   2.24           Coke, wt %   ≈5.00   ≈4.50                      
 
         [0036]    It is to be noted from Table 4 that the Si-modified catalyst of the invention results in increased yields of benzene, toluene and xylenes, as well as an increased yield of C5+ compounds.  
         [0037]    It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.