Abstract:
The present development is a catalyst for use in water gas shift processes, a method for making the catalyst and a method of using the catalyst. The catalyst is composed of iron oxide, copper oxide, zinc oxide, alumina, and optionally, potassium oxide, and is produced using a hydrothermal synthesis process. The catalyst demonstrates surprising activity for conversion of carbon monoxide under high to moderate temperature shift reaction conditions.

Description:
BACKGROUND 
     The present development is a catalyst for use in water gas shift processes. The catalyst comprises iron oxide, copper oxide, zinc oxide and alumina, and optionally, potassium oxide. 
     The water-gas-shift reaction is a well-known reaction suitable for production of hydrogen. Large volumes of hydrogen gas are needed for a number of important chemical reactions, and since the early 1940&#39;s, the water-gas-shift (WGS) reaction has represented an important step in the industrial production of hydrogen. For example, the industrial scale water-gas-shift reaction is used to increase the production of hydrogen for refinery hydro-processes and for use in the production of bulk chemicals such as ammonia, methanol, and alternative hydrocarbon fuels. 
     Typically, the catalysts used in the industrial scale water-gas-shift reaction include either an iron-chromium (Fe—Cr) metal combination or a copper-zinc (Cu—Zn) metal combination. The Fe—Cr oxide catalyst is typically used in industrial high temperature shift (HTS) converters. The industrial HTS converters—which have reactor inlet temperatures of from about 300° C. to about 380° C.—exclusively use the Fe-based catalysts because of their excellent thermal and physical stability, poison resistance and good selectivity. These attributes are especially beneficial when low steam to CO ratios are used and the formation of hydrocarbons is favored. Typically, the commercial catalysts are supplied in the form of pellets containing 8-12% Cr 2 O 3  and a small amount of copper as an activity and selectivity enhancer. However, because of governmental regulations that pertain to chromium-comprising catalysts, it would be advantageous to have a water gas shift catalyst that demonstrates all the benefits of the prior art Fe—Cr catalyst but which does not require chromium in the composition. 
     The copper-based catalysts function well in systems where the CO 2  partial pressure can affect the catalyst performance, so the copper-based catalysts tend to demonstrate more favorable CO conversion at lower temperatures. However, unsupported metallic copper catalysts or copper supported on Al 2 O 3 , SiO 2 , MgO, pumice or Cr 2 O 3  tend to have relatively short lifespan (six to nine months) and low space velocity operation (400 to 1000 h −1 ). The addition of ZnO or ZnO—Al 2 O 3  can increase the lifetime of the copper-based catalysts, but the resultant Cu—Zn catalysts generally function in a limited temperature range of from about 200° C. to about 300° C. Further, Cu—Zn catalysts tend to be susceptible to poisoning by sulfur-containing compounds. Thus, merely using a Cu-based catalyst or a Cu—Zn catalyst in water gas shift processes is not a viable commercial option. 
     Thus, there is a need for a stable chromium-free iron-based water gas shift catalyst that performs as well as or better than the prior art iron-chromium catalysts. 
     SUMMARY OF THE PRESENT INVENTION 
     The present development is a catalyst for use in water gas shift processes, a method for making the catalyst and a method of using the catalyst. The catalyst comprises iron oxide, copper oxide, zinc oxide and alumina, and optionally, potassium oxide. The catalyst demonstrates surprising activity for conversion of carbon monoxide under high to moderate temperature shift reaction conditions, particularly when the reactor inlet temperatures ranges from about 250° C. to about 370° C. 
     The invention further comprises a process for the production of an Fe 2 O 3 /CuO/ZnO/Al 2 O 3  catalyst. The catalyst is produced using a hydrothermal synthesis process. The process comprises combining salts of iron, copper, zinc and aluminum with an aqueous base solution while maintaining tight control of the reaction temperatures and pH. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present development is an Fe 2 O 3 /CuO/ZnO/Al 2 O 3  catalyst intended to be used in water gas shift processes. Specifically, the catalyst comprises from 0.5 wt % to 15 wt % CuO, from 0.1 wt % to 15 wt % ZnO, from 0.5 wt % to 10 wt % Al 2 O 3 , and Fe 2 O 3  to balance. (For the purposes of this writing, wt % is based on total weight of the finished catalyst.) In a preferred embodiment, the catalyst comprises from 1.5 wt % to 10 wt % CuO, from 1.5 wt % to 10 wt % ZnO, from 2.0 wt % to 8 wt % Al 2 O 3 , and Fe 2 O 3  to balance. In a most preferred embodiment, the catalyst comprises from 3.5 wt % to 5.0 wt % CuO, from 3.0 wt % to 5.0 wt % ZnO, from 4.0 wt % to 6.0 wt % Al 2 O 3 , and Fe 2 O 3  to balance. Optionally, potassium oxide may be included at concentrations up to 5 wt %, and preferably at a concentration of 0.1 wt % to 2.0 wt %, and most preferably at a concentration of 0.2 wt % to 1.0 wt %. 
     The method for preparing the catalyst does not provide for the addition of any chromium-containing materials, but chromium traces may be present, for example, in iron salts of technical purity and thus must be tolerated in the production of the catalysts according to the invention on a technical scale. Further, the method for preparing the catalyst does not provide for the addition of a zinc-aluminum spinel, nor does the method of preparation allow for a zinc-aluminum spinel to form. Thus, the finished catalyst is essentially free of zinc-aluminum spinel as determined by X-Ray Diffraction (XRD). 
     Generally, the catalyst of the present development is prepared by first preparing a first solution comprising soluble iron, copper, zinc and aluminum salts. A second basic solution is also prepared. The two solutions are then combined with deionized water in a mix tank at a predetermined temperature of less than about 65° C. while maintaining the pH of 4.0-12.0. After the solutions are combined, the mixture is held at a predetermined temperature up to about 100° C. for a predetermined time period. The mixture is then heated and held at the elevated temperature for several hours. The solids are washed, dried, and then calcined at 400° C.±100° C. for several hours. In an alternative embodiment, the second basic solution may be added to the water in the mix tank before the addition of the iron solution. In general, the precipitation step may be performed using co-precipitation, acid to base precipitation, or base to acid precipitation methods as are known in the art. 
     The soluble metal salts may include nitrate salts, sulfate salts, acetate salts, amine complexes, chloride salts, and other organic and inorganic salts known in the art to deliver iron, copper, zinc and/or aluminum to catalysts. The aluminum may further be added in the form of aluminum oxide, aluminum hydroxide, sodium aluminate and other aluminum compounds that are known in the art for addition of aluminum to catalysts. The basic solution may be prepared from sodium hydroxide, sodium carbonate, potassium hydroxide, potassium carbonate, ammonium hydroxide, ammonium carbonates, and other compounds known to produce basic aqueous solutions. 
     The resulting catalyst has a BET surface area of 50-150 m 2 /g and a pore volume of 0.15-0.45 cc/g. 
     The following are representative examples for making the catalyst of the present development. These examples are presented to further explain the invention and are not intended, or to be taken, to limit the scope of the invention. 
     EXAMPLE 1 
     A catalyst is prepared according to the invention by the following procedure: Ferrous sulfate heptahydrate (145 g) is dissolved in de-ionized (D.I.) water (365 ml). Aluminum nitrate nonahydrate (10.5 g) is dissolved in D.I. water (40 ml). The ferrous sulfate heptahydrate solution and the aluminum nitrate nonahydrate solution are combined, and 15.5% Cu (11.8 g) and 15.5% Zn (5.2 g) nitrate solutions are mixed with the ferrous sulfate and aluminum nitrate solutions. The mixed solution is then added into 10% NaOH solution (550 ml) over a 30 minute period, with stirring. The solution is heated to about 65° C. and held for at least two hours. The resulting suspension is filtered. The precipitate is washed 4 times with D.I. water followed by drying at 110° C. for about four hours. The dried powder is calcined at about 400° C. for approximately three hours and the powder is then mixed with 1.5 g graphite. The mixed powders are tabbed. 
     EXAMPLE 2 
     A catalyst is prepared according to the invention by the following procedure: 10.2% ferrous nitrate solution (2190 g), 15.5% Cu nitrate solution (96 g), 15.5% Zn nitrate solution (32 g) and 4.5% Al nitrate solution (236 g) are mixed in a 3000 ml beaker. The mixed solution is added into 20% NH 4 OH solution (350 ml) over about a 10 minute period, with stirring, followed by heating to about 85° C. and holding at temperature for about two hours. The resulting suspension is filtered. The precipitate is washed three times with D.I. water. 4.7 g potassium nitrate solution (43%) is added into the cake. The cake is then dried at 110° C. for about four hours followed by calcination at about 300° C. for approximately four hours. The calcined powder is mixed with 20 g graphite and tabbed. 
     EXAMPLE 3 
     A catalyst is prepared according to the invention by the following procedure: 357 ml Cu amine (0.14 g/1), 143 ml Zn amine (0.14 g/l) and 83 g alumina powder are added into about 1900 ml NH 4 OH solution (0.2 g/ml). After stirring thoroughly, 6154 ml ferrous nitrate solution (0.13 g/ml) is added to the NH 4 OH solution. The suspension is then heated to about 80° C. and held for six hours. After washing and filtrating four times, 6.44 g potassium nitrate and 30 g graphite are mixed into the slurry. The slurry then is spray-dried followed by calcination with rotation furnace (zone-1, 260° C. and zone-2, 350° C.). The calcined powder is tabbed. 
     EXAMPLE 4 
     A catalyst is prepared according to the invention by the following procedure: Ferric nitrate nonahydrate (221 g) is dissolved in 720 ml D.I. water. 15.5% Cu nitrate solution (12.9 g), 15.5% Zn nitrate solution (6.5 g) and aluminum nitrate powder (18.3 g) are added into the ferric nitrate solution. Then the mixed solution is pumped into a precipitation tank containing 250 ml NaOH solution (25%). The precipitation tank is heated to 60° C. and held for 5 hours. The resulting suspension is filtered and washed four times. The cake is dried at 110° C. for 3 hours followed by calcination at 400° C. for four hours. The calcined powder is then tabbed. 
     EXAMPLE 5 
     A catalyst is prepared according to the invention by the following procedure: Ferric sulfate (75.4 g) is dissolved in 500 ml D.I. water. Ferrous sulfate (41.8 g) is dissolved in 122 ml D.I. water. 10.5 g aluminum nitrate is dissolved in 35 ml D.I. water. The ferric sulfate, ferrous sulfate and aluminum nitrate solutions are mixed in a first vessel, and a 15.5% Cu nitrate (13 g) solution and 15.5% Zn nitrate (5 g) solution are also mixed into the first vessel. 10% NaOH (500 ml) is prepared in second vessel. The solutions from the first vessel and second vessel are added simultaneously to a precipitation tank over about a 30 minute period. The temperature is increased to 65° C. and held for about two hours. The resulting suspension is filtered and washed four times with D.I. water followed by drying at 110° C. for about four hours. The dried powder is calcined at 400° C. for three hours before mixed with 1.5 g graphite. 
     EXAMPLE 6 
     A catalyst is prepared according to the invention by the following procedure: 55 g aluminum nitrate is dissolved in 180 ml D.I. water. 34 g Cu nitrate solution (15.5%) and 20 g Zn nitrate solution (15.5%) along with the above aluminum nitrate solution are added into 962 g ferrous nitrate solution (9.5%). The mixed solution and 300 ml NaOH solution (25%) are simultaneously added into a precipitation tank with stirring over a 40 minute period. The temperature is then increased to 65° C. and held for five hours. The resulting suspension is filtered and washed four times with D.I. water followed by drying at 110° C. for four hours. The dried powder is calcined at 400° C. for three hours before mixed with 4.5 g graphite. The mixed powders are tabbed. 
     EXAMPLE 7 
     A catalyst is prepared according to the invention by essentially the same procedure as in Example 6 except 82.5 g aluminum nitrate and no Zn nitrate solution are used. 
     EXAMPLE 8 
     A catalyst is prepared according to the invention by essentially the same procedure as in Example 6 except 51 g Cu nitrate solution (15.5%) and no Zn nitrate solution are used. 
     The catalyst is preferably used in a typical water gas shift process. Typical reaction conditions for the process would include inlet temperatures equal to or greater than 200° C., feed pressure equal to or greater than 1 atm, and a space velocity greater than about 1000/hr, and a steam to gas ratio of greater than 0.2. The concentration of the carbon monoxide in the feed gas can vary depending on the technical process of producing the gas to be converted. The exit carbon monoxide concentration is expected to be at equilibrium. 
     In the water gas shift process, the chromium-free iron-based catalyst of the present invention demonstrates surprisingly improved activity and CO conversion as compared to prior art Fe—Cr catalysts designed for the same purpose. In an exemplary embodiment of the present invention, the catalyst of Example 6 is prepared. The resulting catalyst has a BET surface area of 67 m 2 /g and a pore volume of 0.28 cc/g. A prior art catalyst is prepared according to U.S. Pat. No. 7,037,876 Example 1, and comprising about 8 wt % Cr 2 O 3  and 2.5 wt % CuO and 89.5 wt % Fe 2 O 3 . The resulting catalyst has a BET surface area of 75 m 2 /g and a pore volume of 0.26 cc/g. (U.S. Pat. No. 7,037,876 is incorporated herein in its entirety by reference.) 
     Each catalyst is packed in a fixed bed reactor having an inlet temperature that can be adjusted from 250° C. to 370° C. A dry feed comprising 8% CO, 13% CO 2 , 24% N 2  and balanced with H 2  is fed through the catalyst bed at a pressure of 150 psig and a steam to gas ratio of 0.6. The CO conversion is measured for each catalyst at varying inlet temperatures ranging from 250° C. to 370° C. As indicated in Table I, the inventive catalyst demonstrates significantly greater CO conversion at every inlet temperature tested. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Inlet Temp 
                 Prior Art Catalyst 
                 Inventive Catalyst 
               
               
                 (° C.) 
                 % CO Conversion 
                 % CO Conversion 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 250 
                 3 
                 28 
               
               
                 275 
                 3 
                 35 
               
               
                 300 
                 9 
                 58 
               
               
                 325 
                 23 
                 69 
               
               
                 370 
                 42 
                 70 
               
               
                   
               
             
          
         
       
     
     Using the same catalyst compositions, the activity is measured at 370° C. for the fresh catalyst and for the same catalysts after the catalyst has been heat treated for 24 hours at 530° C. The fresh prior art catalyst has a CO conversion of 42%, which decreases to 29% after heat treatment. Even with heat treatment the inventive catalyst retains greater activity: the fresh inventive catalyst has a CO conversion of 70%, which decreases to 43% after heat treatment. 
     Although the invention has been described in detail in the foregoing for the purpose of illustration, it is to be understood that such detail is solely for that purpose and that variations can be made therein by those skilled in the art without departing from the spirit and scope of the invention except as it may be limited by the claims.