Alkene oligomerization process
This application is a continuation-in-part of application Ser. No. 219,279, filed Dec. 22, 1980, now U.S. Pat. No. 4,335,022 which is a continuation-in-part of application Ser. No. 061,205, filed July 27, 1979, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a process for oligomerizing alkenes to higher alkenes using as catalysts novel siliceous compositions prepared by reacting porous silica with aluminum hydride. 
     BACKGROUND OF THE INVENTION 
     Aluminum hydride has been decomposed to produce aluminum metal coatings on glass objects in U.S. Pat. No. 3,501,333, issued Mar. 17, 1970. These processes are typically low temperature processes, below 250° C. The instant compositions contain no aluminum metal. 
     U.S. Pat. No. 3,146,209, issued Aug. 25, 1964, teaches a method for producing an aluminum hydride containing silica in which bound water molecules on the silica surface are replaced with aluminum hydride molecules to provide a solid composition basically comprising silica and a hydride source attached thereto. There is no teaching in this reference of any subsequent treatment being applied to their gel material to produce an oxide material comparable to the instant compositions. 
     U.S. Pat. No. 3,305,590, issued Feb. 21, 1967, discloses a general process for making alumino-silicates. This reference generally teaches impregnation of silica with any decomposable aluminum salt. The aluminum hydride used to prepare the instant composition is not considered a salt but rather is a covalent compound (see Metal Hydrides, Mueller et al, p. 545, Academic Press, 1968). 
     SUMMARY OF THE INVENTION 
     Alkenes are oligomerized to higher alkenes by contacting the alkenes with catalysts comprising unique siliceous materials which are prepared by impregnating porous silica with solutions of aluminum hydride and subsequently heating the impregnated silica in a non-oxidizing environment to temperatures of from about 300° to about 900° C. The oligomerization reaction is typically carried out at temperatures ranging from about 0° C. to about 300° C. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The porous silicas used in the preparation of the instant composition are readily available commercially and are known as silica gels which are essentially substantially dehydrated amorphorous silica. These materials are available in various density grades, from low density with surface areas ranging from about 100-200 m 2  /g, to regular density with surface areas up to about 800 m 2  /g. These commercially available materials are used as desiccants, selective adsorbents, catalysts and catalyst supports. The porous silica may contain minor proportions of other materials without departing from the scope of the invention such as for example alumina and carbon. Prior to use the porous silica should be substantially free of adsorbed water, i.e. &#34;substantially dehydrated&#34;. The residuum of chemically bound water, i.e. water of hydration, is not harmful to the process of this invention. The adsorbed or free water is removed by heating the porous silica at temperatures from about 200° to about 900° C. prior to contact with the aluminum hydride impregnating solution. Any environment that will provide for drying is suitable, such as air, vacuum, inert gas such as nitrogen, etc. The dried porous silica should be kept away from a humid atmosphere after drying. A convenient drying atmosphere is that used to heat the impregnated silica, such as nitrogen. 
     The aluminum hydride suitable for use in preparing the catalysts used in this invention is prepared commercially by reacting at room temperature lithium aluminum hydride and aluminum chloride in diethyl ether (Et 2  O). The product is isolated in high yield by decanting and drying at room temperature. The product is analyzed as AlH 3 .1/3Et 2  O. For purposes of this invention the aluminum hydride is dissolved in a suitable organic solvent. The prime requirement on the solvent is that it be anhydrous and non-hydroxyl containing since water and alcohol react with aluminum hydride. Suitable solvents are for example, ethers, such as diethyl ether, tetrahydrofuran, pyridine, benzene, toluene, chloroform and the like. 
     To prepare the compositions used in the instant invention, porous silica suitably dried of adsorbed water is contacted with a solution of aluminum hydride in appropriate proportions as to provide the desired amount of aluminum hydride per unit weight of silica. A suitable method of impregnation is described in U.S. Pat. No. 3,146,209, issued Aug. 25, 1964. The impregnated silica is dried of solvent and then heated (activated) in a non-oxidizing atmosphere at temperatures from about 300° C. to about 900° C., more preferably at temperatures of from about 450° C. to about 750° C. The drying step is preferably carried out in the initial stages of the heating step. Suitable non-oxidizing atmospheres are inert atmospheres such as nitrogen, helium, argon, vacuum, etc; and reducing atmospheres such as hydrogen, carbon monoxide, etc. Drying temperatures are not critical and depend on the particular solvent and will range from about 60 to about 100% of the boiling point (absolute). Drying and heating times are not critical and depend upon temperatures. They are readily determined by simple experimentation. Five minutes to one hour are usually sufficient. Typically the amount of aluminum hydride (measured as aluminum metal) added will range from about 0.01 to about 35, preferably from about 0.1 to about 25 and more preferably from about 1 to about 10 percent by weight of the total composition. Different reactions will require different optimum amounts of aluminum hydride added. For example, for dehydrocoupling of isobutenes to aromatics the aluminum hydride added will range from about 2 to about 10 percent by weight of aluminum per total weight of composition. 
     The instant compositions find use for catalyzing acid catalyzed reactions. The instant compositions are acidic as compared to the essentially neutral silicas. The instant composition are similar in acidity to the acidic binary alumina-silica gels. While the exact physical structure of the instant composition is not known, it is speculated that the decomposition of the aluminum hydride on the silica surface produces localized Lewis acid sites having an atomic ratio of oxygen to metal ratios lower than the normal oxygen to silica ratio. Analysis of the instant composition indicated no aluminum metal had been deposited on the surface and an insignificant amount of residual aluminum hydride remained. These findings are consistent with the above theory of aluminum hydride reacting with the silica. 
     The instant compounds prepared using the covalent aluminum hydride compounds differ significantly in their physical characteristic when compared to compounds prepared using decomposable salts. For example, materials prepared using aluminum hydride were compared to those prepared using aluminum nitrate by using X-Ray Photoelectric Spectroscopy (XPS or ESCA). This analysis allowed the relative number of silicon, aluminum and oxygen atoms on the surface to be determined. The composition originating from the AlH 3  treatment exhibited a very high ratio Al/Si atoms on the surface compared to a composition prepared via Al(NO 3 ) 3  wet impregnation. These data suggest concentration of the Al atoms on the external surface when AlH 3  is the reactant but not when Al(NO 3 ) 3  is the impregnating material. This concept was confirmed by re-analysis of the compositions after they were ground to expose their interiors. The ratio of Al/Si was substantially less after grinding in the AlH 3  /SiO 3  case but about the same or a little higher in the Al(NO 3 ) 3  /Al 2  O 3  case. The results are summarized below in Table I. 
     
                       TABLE 1                                                     
______________________________________                                    
Relative Number of Atoms Detected on Surface by XPS (ESCA)                
       AlH.sub.3 /SiO.sub.2.sup.b                                         
                     Al(NO.sub.3).sub.3 /SiO.sub.2.sup.b                  
         20-30               20-30                                        
Sample   Mesh     Ground.sup.a                                            
                             Mesh   Ground.sup.a                          
Treatment                                                                 
         Particles                                                        
                  to Powder  Particles                                    
                                    to Powder                             
______________________________________                                    
Si.sup.c 100      100        100    100                                   
Al       25       9.9        5.0    6.5                                   
O        184      164        168    164                                   
______________________________________                                    
 
    
     Alkenes are oligomerized to higher alkenes according to the following general equation ##EQU1## where z is an integer. The reactant alkene typically has a carbon number ranging from two to about twelve and may be straight-chain or branched-chain. 
     The catalyst compositions used in the instant process are used in typical fashion, for example, in packed beds, in batch reactors or in fluidized beds. Reaction temperatures typically range from about 0° C. to about 300° C., preferably from about 50° C. to about 200° C. The reaction product is subjected to standard separation techniques and product oligomers are separated from unreacted alkene. Unreacted alkene may be conveniently recycled. Suitable inert solvents may also be utilized in the instant process, such as for example aliphatic compounds. Excess starting material can be utilized for its solvent effect. Reaction pressures are not critical and they may be atmospheric, superatmospheric or subatmospheric. 
     The preparation of the compositions used as catalysts in the process of the instant invention and their utilization as catalysts will be further described below by the following illustrative embodiments which are provided for illustration and are not to be construed as limiting the invention. 
    
    
     ILLUSTRATIVE EMBODIMENTS 
     Composition Preparation 
     A porous silica gel (Davison Grade 57, surface area of 300 m 2  /g, pore volume of 1.0 cc/gm and density of 0.4 gm/cc) 20-30 mesh was pretreated in dry nitrogen at 700° C. for one half an hour. Exposure of the dried support to air was avoided. In a glove box with a dry nitrogen atmosphere 5 grams of the dried support was impregnated with a solution prepared by dissolving 0.4 grams of AlH 3 .1/3(Et 2  O) in 8 ml of tetrahydrofuran. The impregnated material was placed in a vycor tube and dry nitrogen passed over the catalyst as the temperature was increased in 50° C. intervals (15 min. at each temp.) to 700° C. and held at this temperature for 15 minutes. The finished composition was cooled with a nitrogen flow of 40 ml/min. 
     Similar compositions were prepared using high surface area silica gels (e.g. Davison Grade 03, surface area of 750 m 2  /g, pore volume of 0.43 gm/cc and density of 0.7 g/cc). 
     Similar compositions were made by activating at 500° to 550° C. 
     Oligomerization of Isobutene 
     A catalyst prepared as described above (3.8% wt added aluminum; Davison Grade 57 20-30 mesh activated at 550° C.) was tested for oligomerization activity. Fourteen milliliters of catalyst was used in a vycor tube. Isobutene was fed at a GHSV of 600, at atmospheric pressure. The results are shown in the table below. As can be seen, lower temperatures promote the oligomerization to higher carbon numbers. Regenerated catalyst behaved similarly to fresh catalysts. 
     
                       TABLE II                                                    
______________________________________                                    
OLIGOMERIZATION OF ISOBUTENE                                              
                  Re-     Conv. Selectivity, Mole %                       
                  action  of    (Based on C.sub.4                         
Ex-               Temp.   i-C.sub.4 H.sub.8                               
                                equivalents)                              
ample Catalyst    °C.                                              
                          %     C.sub.8 ═                             
                                     C.sub.12 ═                       
                                           C.sub.16 ═                 
______________________________________                                    
1     AlH.sub.3 /Davison                                                  
                  150     77.0  57   43    --                             
      SiO.sub.2 Grade 57                                                  
      3.8% Al                                                             
2     AlH.sub.3 /Davison                                                  
                   65     98.0   3   81    16                             
      SiO.sub.2 Grade 57                                                  
      3.8% Al                                                             
______________________________________