Abstract:
A process for the oligomerization of propylene is disclosed wherein a tungstated zirconia catalyst prepared as a distillation structure is used in a reaction distillation zone under conditions of temperature and pressure to concurrently react the propylene to produce oligomers thereof and separate the oligomer products from unreacted propylene by fractional distillation in a distillation column reactor. Compared to the prior art tubular or plug flow reactors, lower temperatures and pressures are used to produce higher conversions and selectivities to preferred isomeric forms useful for preparing neo acids.

Description:
BACKGROUND OF DISCLOSURE 
     1. Field of the Disclosure 
     Embodiments disclosed herein relate generally to a process for converting propylene over a tungstated zirconia catalyst to provide higher molecular weight hydrocarbons, particularly C 6 , C 9  and C 12  olefins with high conversion and high selectivity to the C 6  and C 9  olefins having good branching type isomers for producing neo acids. More particularly the conversion is carried out simultaneously with distillation in a distillation column reactor. 
     2. Background 
     In the present state of the art the catalysts are used in tubular reactors at severe conditions, i.e., 330-482° F. and 1000 to 1215 psig pressures. Prior catalysts which have been used for the oligomerization of propylene include supported phosphoric acid (sPa), metal complexes (U.S. Pat. Nos. 5,510,555; 4,695,664 and 6,501,001) and various zeolites, especially ZSM-22 and ZSM-57 (U.S. Pat. No. 6,143,942). These reaction systems have undesirable qualities characterized as one or more of: severe reaction conditions, short catalyst life and poor selectivity. 
     The reaction requires high temperature (330-482° F.) and high pressure (1000 to 1215 psig). The sPa system has a life of less than 1000 tons of product per ton of catalyst and then must be removed and discarded. The zeolites have shown increased life, e.g., 1500 to 3000 tons product per ton of catalyst, but lose activity and must be regenerated at considerable expense. U.S. Pat. No. 6,072,093 teaches that the catalyst life may be extended by recycling cycloparaffins through the tubular reactor, which requires additional separation and recycling apparatus and an inventory of the non associated cycloparaffins. The metal complexes are homogeneous catalysts wherein the catalyst and the products must be separated with continuous catalyst makeup required. The selectivity of the sPa is toward the C 9  and heavier while the preferred oligomers are the C 6  and C 9  which are converted to alcohols. The selectivities of the zeolites and metal complexes are somewhat better. 
     U.S. Pat. No. 4,956,514 discloses zeolite MCM-22 which has been shown to have favorable characteristics for the oligomerization of propylene at lower pressures and temperatures than the other catalyst. 
     U.S. Pat. No. 4,242,430 discloses the dimerization of isobutylene in a distillation column reactor using an acidic cation exchange resin as the catalyst which avoided the formation of higher oligomers. 
     U.S. Pat. Nos. 5,113,034 and 5,608,133 disclose that tungsten/zirconia catalysts may be used in fixed bed tubular reactors to dimerize C 3  and C 4  olefins. 
     SUMMARY OF THE DISCLOSURE 
     In one aspect, embodiments disclosed herein relate to a process for the oligomerization of propylene comprising: contacting propylene with a tungstated zirconia catalyst in a reaction distillation zone under conditions of temperature and pressure to concurrently react the propylene to produce oligomers thereof and separate the oligomer products from unreacted propylene by fractional distillation. 
     It has been found that the oligomerization of propylene over tungstated zirconia in a distillation column reactor may be carried out at lower temperatures, below 300° F., preferably less than 200° F., and pressures below about 500 psig, than in the prior art tubular reactors to produce a higher conversion to more desirable oligomeric isomer forms. The conditions for the present reaction are much less severe than that required by earlier zeolite oligomerization processes. The distillation column reactor preferably operates at a pressure in the range from about 200 to 450 psig, such as about 400 psig, and temperatures in the range of about 140 to 200° F., such as between 140° F. and 165° F. In a family of embodiments, the temperature may be in the range from about 143 to 147° F.; from about 158° to 165° F. in yet other embodiments. Conversions of about 70 to 75% have been achieved yielding about 30% hexene and 40% nonene. The branched type of product is particularly suited for producing neoacids. 
     As used herein the term “distillation column reactor” means a distillation column which also contains catalyst such that reaction and distillation are going on concurrently in the column. In a preferred embodiment the tungstated zirconia catalyst is prepared as a distillation structure and serves as both the catalyst support and distillation structure. 
     Other aspects and advantages will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified flow diagram of the invention with the distillation column reactor operated in the up flow mode. 
         FIG. 2  is a simplified flow diagram of the invention with the distillation column reactor operated in the down flow mode. 
     
    
    
     DETAILED DESCRIPTION 
     The normal feed for the oligomerization is a C 3  cut, which contains 20 to 100 mole % propylene. The balance is predominately propane, with minor amounts of ethylene, ethane and the lighter C 4 &#39;s. 
     The column may be operated in up flow mode or down flow mode. In up flow mode, the feed (propane and propylene) is placed below the catalyst bed. The reactants are boiled up into the catalyst where they react and the heavier oligomer product is removed out the bottom of the distillation column reactor. Unreacted propylene and inert propane are removed for the top of the distillation column reactor and may be recycled back into the reactor after adjusting for the propane/propylene content. 
     In down flow mode the column is operated such that the feed (propane and propylene) enters the top of the column, while oligomer product and inert propane are removed from the bottom of the distillation column reactor. The reactive component, propylene, is the lighter component and becomes concentrated in the top of the column by distillation. The catalyst bed is placed in the top of the column where the propylene concentration bulges. Overhead distillate flow may be minimized such that the propylene is refluxed to exhaustion. 
     Catalyst life is improved when using the tungstated zirconia catalyst as packing in a distillation column reactor. The unique hydraulic action in a distillation column washes out the heavy oligomers as they are produced and prevents fouling. Tungstated zirconia catalyst is described in detail in U.S. Pat. No. 4,956,514 which is incorporated herein. 
     In accordance with the present invention, anion modified zirconium are impregnated with a tungstate precursor to form the catalyst used in the present invention. Suitable sources of the oxide include salt solutions, such as zirconium oxychlorides, nitrates and tetrachlorides. The salt solution is preferably water soluble and capable of forming a hydroxide precipitate upon the addition of a base. Suitable bases include, but are not limited to ammonium hydroxide and alkylammonium hydroxide, which are added in order to adjust the pH of the solution in the range from about 9 to about 11, thereby facilitating the formation of the hydroxide precipitate. Alkoxides may also be employed for preparing the catalyst, e.g., zirconium propoxide which is hydrolyzed with water to form the hydroxide precipitate. 
     Any material capable of forming tungstate when calcined with the zirconia oxide may be used to provide the tungstate, such as ammonium meta-tungstate. 
     The anion can be incorporated with the hydroxide or oxide by any one of several known methods. For example, a zirconium hydroxide or oxide can be immersed in an aqueous solution containing the tungstate followed by drying at about 100° C. to 150° C. After the tungstate source has been incorporated with the zirconium hydroxide or oxide and dried, calcination is carried out, preferably in an oxidizing atmosphere or one that will allow conversion to the tungstate, at temperatures of about 450° C. to about 800° C., preferably about 500° C. to about 600° C. for about 0.5-30 hours, preferably about 1-24 hours. In the most preferred embodiment calcination is carried out at about 600EC for about 0.5 to about 10 hours in air. The concentration of the tungstate remaining on the catalyst after calcination preferably ranges for about 3.0 wt % to about 5 wt % based on the weight of the zirconia metal oxide. 
     Alternatively, the hydroxide can first be calcined at temperatures ranging from 450° C. to 650° C. to convert the hydroxide, the tungstate being incorporated as previously mentioned. 
     The supported catalyst may be formed by heating at 60° C. to 90° C. a water slurry of silica added to an aqueous solution of zirconium oxynitrate and urea. The zirconium salt deposits on the silica and is then dried and calcined as previously mentioned. Catalyst loading for supported catalyst may range from about 1% to 25% by weight of catalyst. 
     The tungstated zirconia catalyst, as provided, is much too fine to function as catalytic distillation structures in a distillation column reactor as required by the present invention. The catalytic distillation structure must be able to function as catalyst and as mass transfer medium. The catalyst is preferably supported and spaced within the column to act as a catalytic distillation structure. The catalytic distillation process employs a catalyst system (See U.S. Pat. Nos. 4,215,011 and 4,302,356) which provides for both reaction and distillation concurrently in the same reactor, at least in part within the catalyst system. The method involved is briefly described as one where concurrent reaction and distillation occur in a combination reactor-distillation structures. Catalytic distillation structures useful for this purpose are disclosed in U.S. Pat. Nos. 4,731,229, 5,073,236, 5,431,890, 5,266,546 and 5,730,843 which are incorporated by reference. A preferred catalytic distillation structure embodiment is described in U.S. Pat. No. 5,431,890. 
     Referring now to  FIG. 1 , the operation of the distillation column reactor in the up flow mode is shown. Fresh feed which includes propylene in flow line  101  is combined with recycle from flow line  110  in flow line  112  and fed to distillation column reactor  10  below a bed  12  of tungstated zirconia catalyst prepared as a distillation structure. The reactants are boiled up into the bed where the propylene reacts with itself and dimers of itself to produce the oligomer products, mainly C 6 , C 9  and C 12  oligomers. The oligomer products, being higher boiling, are removed from the distillation column reactor as bottoms via flow line  106 . A portion of the bottoms are cycled through reboiler  40  via flow lines  107  and  108 , and the remainder of the bottoms is recovered via flow line  109 . Unreacted propylene and inert propane are removed from the distillation column reactor  10  as overheads via flow line  102 , condensed in condenser  20  and collected in receiver  30 . The condensed liquid is removed from the receiver  30  via flow line  104  with a portion being returned to distillation column  10  as reflux via flow line  105 . The remainder of the liquid distillate is passed via flow line  113  to distillation column  50  where the propane is separated from the mixture and removed as bottoms via flow line  111 . The propylene, along with some propane, is taken as overheads and may be recycled to distillation column reactor  10  via flow line  110 . 
     Referring now to  FIG. 2 , the operation of the distillation column reactor in the down flow mode is shown. Feed containing propylene in flow line  201  is fed to the top of the distillation column  10  having a bed  12  of the tungstated zirconia catalyst prepared as a distillation structure. The reactive propylene is the lighter component and is concentrated in the upper part of the column containing the tungstated zirconia catalyst. Some unreacted propylene is taken as overheads via flow line  202 , condensed in condenser  20  and thence to receiver  30  via flow line  203  where all of the liquid is returned as reflux to the column  10  via flow line  205  assuring essentially complete conversion. A purge via flow line  204  is provided to prevent build up. The propylene reacts with itself and dimers of itself in the catalyst bed  12  to produce the desired oligomer product, mostly C 6 , C 9  and C 12  oligomers. The oligomer product and inert propane are removed as bottoms from the distillation column reactor  10  via flow line  206 , of which a portion may be fed via flow line  207  to reboiler  40  and thence returned to column  10  via flow line  208 . The remaining portion of the bottoms may be fed to distillation column  50  via flow line  209  where the propane is separated as overheads via flow line  210  from the oligomer product which is taken as bottoms via flow line  211 . 
     As used herein the description “feeding at the top of the bed” includes feed above the catalyst bed and the description “feeding at the bottom of the bed” includes feed below the catalyst bed. 
     TABLE I below presents comparative data showing results using various processes including the present invention. In the MODE section CD=catalytic distillation or the use of a catalytic distillation column. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE I 
               
             
             
               
                   
                   
               
               
                   
                 Catalyst 
               
             
          
           
               
                   
                 sPa 
                 ZSM-22 
                 ZSM-27 
                 WZrO 
               
               
                   
                   
               
             
          
           
               
                 Reactor Mode 
                 Tubular 
                 Tubular 
                 Tubular 
                 CD 
               
               
                 Propylene Feed 
                 — 
                 — 
                 — 
                 Downflow* 
               
               
                 Temperature, ° F. 
                 330-482 
                 330-482 
                 330-482 
                 143-147 
               
               
                 Pressure, psig 
                 1000-1215 
                 1000-1215 
                 1000-1215 
                 400 
               
               
                 Catalyst Life (tons/ton) 
                 &lt;1000 
                 1500-2000 
                 2000-3000 
                 To Be 
               
               
                   
                   
                   
                   
                 Determined 
               
               
                 Conversion, wt. % 
                 — 
                 — 
                 — 
                 70 
               
               
                 Selectivity, wt. % 
               
               
                 C 6   
                 4 
                 36 
                 3.5 
                 31.3 
               
               
                 C 7   
                 5 
                 &lt;1 
                 2 
                 2.3 
               
               
                 C 8   
                 9 
                 &lt;1 
                 2.5 
                 0.0 
               
               
                 C 9   
                 52 
                 36 
                 71 
                 38.9 
               
               
                 C 10/11   
                 10 
                 1.5 
                 1.5 
                 2.1 
               
               
                 C 12   
                 15 
                 17 
                 13 
                 15.2 
               
               
                 C 12+   
                 4 
                 6 
                 6 
                 10.2 
               
               
                   
               
               
                 *Downflow = fed at the top of the catalyst bed 
               
             
          
         
       
     
     The skeletal arrangement of the oligomer affects the reactions that the oligomers can undergo. There are five types of branching about the double bond: Type I (vinyl) is bad and Type II (1,2-disubstituted) is undesirable for producing neoacids. Type III (vinylidene) and Type IV (tri-substituted) are good and Type V (tetra-substituted) is the best for producing neoacids. As shown below in TABLE II the tungstated zirconia (WZrO) catalyst produced more skeletal typed in the hexene product which are particularly suited for producing neoacids, a primary use of oligomer olefins. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE II 
               
             
             
               
                   
                   
               
               
                   
                 Catalyst 
               
             
          
           
               
                   
                 sPa 
                 ZSM-22 
                 ZSM-27 
                 WZrO 
               
               
                   
                   
               
             
          
           
               
                 Reactor Mode 
                 Tubular 
                 Tubular 
                 Tubular 
                 CD 
               
               
                 Branching Type (Hexenes) 
               
               
                 Type I 
                 1.3 
                 2.4 
                 NA 
                 6.17 
               
               
                 Type II 
                 19.4 
                 17.6 
                 NA 
                 33.4 
               
               
                 Type III 
                 6.7 
                 10.1 
                 NA 
                 7.16 
               
               
                 Type IV 
                 39.4 
                 61.2 
                 NA 
                 33.5 
               
               
                 Type V 
                 5.6 
                 0.6 
                 NA 
                 17.1 
               
               
                   
               
             
          
         
       
     
     In the nonene product the tungstated zirconia catalyst again produced more skeletal types suitable for neoacid production. See TABLE III below. 
     
       
         
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE III 
               
             
             
               
                   
                   
               
               
                   
                 Catalyst 
               
             
          
           
               
                   
                 sPa 
                 ZSM-22 
                 ZSM-27 
                 WZrO 
               
               
                   
                   
               
             
          
           
               
                 Reactor Mode 
                 Tubular 
                 Tubular 
                 Tubular 
                 CD 
               
               
                 Branching Type (Nonenes) 
               
               
                 Type I 
                 1.7 
                 2.0 
                 1.0 
                 5.6 
               
               
                 Type II 
                 14.2 
                 19.8 
                 13.9 
                 21.0 
               
               
                 Type III 
                 8.2 
                 7.6 
                 7.2 
                 14.5 
               
               
                 Type IV 
                 64.2 
                 61.1 
                 56.7 
                 36.2 
               
               
                 Type V 
                 11.8 
                 10.4 
                 21.2 
                 22.8 
               
               
                   
               
             
          
         
       
     
     While the disclosure includes a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments may be devised which do not depart from the scope of the present disclosure. Accordingly, the scope should be limited only by the attached claims.