Abstract:
Process for the preparation of ammonia from ammonia synthesis gas by contacting the synthesis gas with a bimetallic catalyst comprising a Group VIII metal combined with a Group VIB metal at conditions being effective in the formation of ammonia, wherein the bimetallic catalyst is in its nitride form during contact with the synthesis gas.

Description:
FIELD OF INVENTION 
     The present invention relates to synthesis of ammonia from ammonia synthesis gas and in particular to a catalyst being highly active in the ammonia synthesis. 
     DISCUSSION OF THE RELATED ART 
     Industrial preparation of ammonia is most usually performed by contacting hydrogen and nitrogen containing synthesis gas with catalyst at a pressure in the range of 100-400 bar and temperatures between 300° C. and 600° C. Widely used catalysts contain iron, typically promoted with oxides of aluminum and magnesium, plus oxides of calcium and potassium to increase heat resistance and synthesis activity. Furthermore, ammonia synthesis catalysts containing other Group VIII metal compounds are known in the art. 
     WO 96/38222 discloses a catalyst for synthesis of ammonia in presence of a catalyst comprising Group VIII metal clusters supported on basic zeolite support. 
     Use of ruthenium containing catalysts in the synthesis of ammonia from synthesis gas is disclosed in U.S. Pat. No. 4,600,571, JP patent publication No. 9168739 and GB patent No. 2,033,776. 
     Furthermore, molybdenum oxycarbonitride as active catalyst in the ammonia synthesis is mentioned in U.S. Pat. Nos. 4,271,041 and 4,337,232. 
     Bimetallic ammonia synthesis catalysts consisting of nickel-molybdenum are described in Z. Elektrochem. Vol. 36, Pages 690-692, 1930. 
     It has now been found that bimetallic catalysts comprising a combination of a Group VIII metal and a Group VIB (CAS version) metal improve activity in the synthesis of ammonia from ammonia synthesis gas, when in the nitride form. 
     SUMMARY OF THE INVENTION 
     In accordance with the above observation, this invention provides a process for the preparation of ammonia from ammonia synthesis gas by contacting the synthesis gas with a bimetallic catalyst comprising ternary compounds of a Group VIII metal and a Group VIB metal at conditions being effective in the formation of ammonia, wherein the bimetallic catalyst is in its nitride form at contact with the synthesis gas. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     When operating the inventive process the catalyst will typically be arranged in a fixed bed in an ammonia converter. The catalyst may thereby be loaded in its oxidised form which, by contact with hydrogen and nitrogen and ammonia in the reacting synthesis gas, is converted to the active nitride form according to the following reaction: 
     
       
         M′xM″y Oz+H2M′xM″y 
       
     
     
       
         M′xM″y+H2+NH3M′xM″yN 
       
     
     In the above general formula of the catalyst, M′ represents a group VIB metal, M″ a group VIII metal and x and y are mixed numbers between 1 and 10. 
     Still improved catalytic activity has been observed when promoting the inventive catalyst with one or more metals selected from Group IA and IIA. 
     At present, preferred metals for use in the above process include iron, cobalt and nickel as Group VIII metals. Molybdenum is a preferred Group VIB metal. Caesium and barium are preferred as Group IA and Group IIA promoter metals, respectively. 
     Suitable promoters are additionally lanthanide metals. Furthermore, the invention provides a catalyst being active in the synthesis of ammonia from ammonia synthesis gas, which catalyst is a ternary nitride having the general formula 
     
       
         M′xM″yN, 
       
     
     wherein 
     M′ represents a group VIB metal, 
     M″ a group VIII metal and 
     x and y are mixed numbers between 1 and 10, the catalyst is optionally promoted with one or more metals selected from Group IA, IIA and the lanthanides. 
     Operation conditions of the catalyst and the above ammonia synthesis process are conventional and known to those skilled in the art. 
    
    
     The invention will become more apparent from the following examples explaining in more detail preparation and operation of the catalyst and ammonia synthesis process according to specific embodiments of the invention. 
     Example 1 
     Preparation of oxidic CoMo precursor. 
     An aqueous solution containing Co(NO 3 ) 2  (0.1 mole) is added dropwise to an aqueous solution containing 0.1 mole Mo as (NH 4 ) 6 Mo 7 O 24 .4H 2 O. After completion of the addition, the reaction mixture is evaporated to dryness and dried at 110° C. The product is calcined for 4 h at 600° C. under air and analysed by XRPD to be pure CoMoO 4 . 
     Example 2 
     Preparation of oxidic NiMo precursor. 
     Preparation of NiMoO 4  according to Example 1 using Ni(NO 3 ) 2  instead of Co(NO 3 ) 2 . 
     Example 3 
     Preparation of oxidic NiMo precursor. 
     Preparation of Ni 2 Mo 3 O 11 , according to Example 2 using 0.066 mole Ni(NO 3 ) 2  instead of 0.1 mole Ni(NO 3 ) 2 . 
     Example 4 
     Preparation of oxidic CoMo precursor. 
     Preparation of Co 4 MoO 7  according to Example 1 using 0.4 mole Co(NO 3 ) 2  instead of 0.1 mole Co(NO 3 ) 2 . 
     Example 5 
     Preparation of oxidic FeMo precursor. 
     Preparation of Fe 2 Mo 3 O 12  according to Example 3 using 0.066 mole Fe(NO 3 ) 3  instead of 0.066 mole Ni(NO 3 ) 2 . 
     Example 6 
     Preparation of oxidic NiW precursor. 
     Preparation of NiWO 4  according to Example 2 using 0.1 mole W as (NH 4 ) 10 [H 2 W 12 O 40 ].7H 2 O instead of 0.1 mole Mo as (NH 4 ) 6 Mo 7 O 24 .4H 2 O. 
     Example 7 
     Preparation of Cs promoted oxidic CoMo precursor. 
     Preparation of Co 0.95 Cs 0.10 MoO 4  according to Example 1 using 0.095 mole Co(NO 3 ) 2  and 0.010 mole CsNO 3  instead of 0.10 mole Co(NO 3 ) 2 . 
     Example 8 
     Preparation of barium promoted oxidic CoMo precursor. 
     Preparation of Co 0.90 Ba 0.10 MoO 4  according to Example 1 using 0.09 mole Co(NO 3 ) 2  and 0.010 mole Ba(NO 3 ) 2  instead of 0.10 mole Co(NO 3 ) 2 . 
     Example 9 
     Preparation of lanthanum promoted oxidic NiMo precursor. 
     Preparation of Ni 0.85 La 0.05 MoO 4  according to Example 1 using 0.085 mole Ni(NO 3 ) 2  and 0.005 mole La(NO 3 ) 3  instead of 0.10 mole Co(NO 3 ) 2 . 
     Example 10 
     Nitridation of oxidic CoMo precursor in pure ammonia. 
     CoMoO 4  from Example 1 is pressed into pellets, crushed and sieved to a particle size of 0.3-0.8 mm 5.0 g of material and placed in the reactor described in A. Nielsen: An Investigation on Promoted Iron Catalysts for the Synthesis of Ammonia, Gjellerup 1968. The catalyst is heated in a 50 l/h stream of gaseous ammonia at 0.1° C./min to 650° C. It is kept at 650° C. for 24 h and cooled to room-temperature. 
     Example 11 
     Nitridation of oxidic CoMo precursor in diluted ammonia. 
     This experiment is conducted as in the above experiment 10 except that the nitridation is performed in a 50 l/h flow of 4.5% ammonia in 71.6% hydrogen and 23.9% nitrogen. 
     Example 12 
     Nitridation of oxidic NiMo precursor in diluted ammonia. 
     This experiment is conducted as Example 11 except that 3.1 g of NiMoO 4  from Example 2 is used as in a starting material. 
     Example 13 
     Nitridation of non-oxidic CoMo precursor in diluted ammonia. 
     This experiment is conducted as in Example 11 except that 4.3 g of Co(NH 3 ) 6 Mo(CN) 8  is used as a starting material. 
     Example 14 
     Promotion of CoMo nitride catalyst with Cs. 
     Two and one-half grams of product from Example 10 is impregnated to incipient wetness with a solution of CsNO 3  in water. The resulting product is dried at 110° C. and contains 6.2% Cs as found by chemical analysis. 
     Example 15 
     Promotion of NiMo nitride catalyst with Ba. 
     One and two-fifths gram of product from Example 12 is impregnated to incipient wetness with an aqueous solution of Ba(OH) 2 . The product obtained after drying at 110° C. is found by chemical analysis to contain 5.4% Ba. 
     Example 16 
     Supported oxidic CoMo catalyst. 
     A spinel carrier MgO·xAl 2 O 3 , wherein the spinel has a specific surface area A sp [m 2 /g] higher than 400·exp (-Tc/400° C.) obtained by calcination at a temperature Tc [° C.], disclosed in U.S. patent application Ser. No. 09/467,242 filed on Dec. 22, 1998, was calcined at 700° C. and impregnated to contain 10.5 wt % Co and 16.2% Mo after calcination at 500° C. The impregnation was conducted by multiple incipient wetness impregnations with Co(NO 3 ) 2 .6H 2 O and (NH 4 ) 6 Mo 7 O 24 .4H 2 O. 
     Example 17 
     Supported Cs promoted CoMo catalyst. 
     The impregnation was conducted as in Example 16 except that a final impregnation with aqueous CsOH was performed to obtain 4.1% Cs on the catalyst precursor. 
     Example 18 
     Testing of catalytic ammonia synthesis activity. 
     The testing was performed in the equipment used for the nitridation studies mentioned in Example 10. In all test experiments, between 3-8 g of catalyst was loaded into the reactor. All catalysts were nitrided using the procedures of Example 10-12 prior to testing. The catalysts were tested at 400° C. and 100 bar total pressure. The inlet gas contained 4.5% ammonia in a 3:1 hydrogen-nitrogen mixture. The flow rate was adjusted to obtain 12% ammonia in the exit. Typical flow rates were between 2000 ml/h and 50 l/h. 
     
       
         
               
               
             
               
               
             
           
               
                   
               
               
                 Catalyst 1   
                   2 Ammonia Production Nm1/h · g 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 CoMo according to 
                 120 
               
               
                 Ex. 1 and 10 
               
               
                 CoMo according to 
                 210 
               
               
                 Ex. 1 and 11 
               
               
                 CoMo according to 
                 120 
               
               
                 Ex. 13 
               
               
                 CsCoMo according to 
                 650 
               
               
                 Ex. 7 and 11 
               
               
                 CsCoMo according to 
                 680 
               
               
                 Ex. 14 
               
               
                 Ba/CoMo according to 
                 520 
               
               
                 Ex. 8 and 11 
               
               
                 NiMo according to 
                 80 
               
               
                 Ex. 2 and 12 
               
               
                 La/NiMo according to 
                 310 
               
               
                 Ex. 9 and 10 
               
               
                 FeMo according to 
                 85 
               
               
                 Ex. 5 and 10 
               
               
                 Cs/FeMo according to 
                 540 
               
               
                 Ex. 5, 14 and 10 
               
               
                 NiW according to 
                 50 
               
               
                 Ex. 6 and 11 
               
               
                 CoMo/MgAl 2 O 4  according to 
                 70 
               
               
                 Ex. 16 and 11 
               
               
                 CsCoMo/MgAl 2 O 4  according to 
                 210 
               
               
                 Ex. 17 and 11 
               
               
                   
               
               
                   1 The examples refer to the method of the precursor preparation and the activation procedure, respectively.  
               
               
                   2 The activity is based on the mass of catalyst loaded into the reactor.  
               
             
          
         
       
     
     Example 19 
     The testing was performed as in Example 18, but with a 4.5% ammonia in a 1:1 hydrogen-nitrogen mixture. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Catalyst 1   
                   2 Ammonia Production Nm1/g · h 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 CoMo according to 
                 230 
               
               
                   
                 Ex. 1 and 10 
               
               
                   
                 Cs/CoMo according to 
                 1040 
               
               
                   
                 Ex. 7 and 11 
               
               
                   
                   
               
               
                   
                   1 The examples refer to the method of the precursor preparation and the activation procedure, respectively.  
               
               
                   
                   2 The activity is based on the mass of catalyst loaded into the reactor.  
               
             
          
         
       
     
     Comparison Example 20 
     High surface area molybdenum nitride MO 2 N was prepared according to Boudart et al. (Stud. Surf. Sci. Catal. Vol 16 page 147) resulting in a surface area of 130 m 2 /g. A test of this catalyst by a procedure as described in the above Example 18 revealed an ammonia production activity of 20 Nml/g·h.