Patent Publication Number: US-2002010087-A1

Title: Skeletal iron catalyst and its preparation for fischer-tropsch synthesis processes

Description:
BACKGROUND OF INVENTION  
       [0001] This invention pertains to skeletal iron catalysts, and particularly pertains to catalyst preparation methods and process for use of such catalysts in Fischer-Tropsch synthesis processes for CO and H 2  feeds to produce hydrocarbon products.  
       [0002] As a basic technology for producing synthetic liquid fuels from CO+H 2  feedstreams, the Fischer-Tropsch catalytic synthesis process has undergone worldwide development and use since the 1920s. Iron-based catalysts have been widely investigated and used and precipitated iron catalyst and fused iron catalyst have been the commonly used catalysts in such F-T synthesis processes. However, the preparation procedure for the precipitated iron catalyst is undesirably complicated and includes several steps of precipitation, washing, filtration, drying, formation, calcination, pulverization, and reduction. Also, the precipitated iron catalyst is significantly influenced by various parameters, including precipitating agent, solution concentration, precipitation temperature, solution pH value, pretreatment temperature, and atmosphere, and such catalyst is undesirably expensive. Furthermore, the fused iron catalyst has undesirably low active surface area (˜10 m 2 /g) that is difficult to increase, and it also has low catalytic activity and minimal economic advantage. Both of these two known iron catalysts provide only conventional hydrocarbon product selectivity. On the other hand, since development of the known skeletal nickel catalyst systems, such skeletal type catalysts have been used in organic reactions, particularly in liquid phase hydrogenation systems. Some previous development work has been focused on skeletal nickel catalyst and worked only on some simple hydrogenation reaction systems. Because of the desirability for improving the activity and selectivity of iron catalyst for Fischer-Tropsch synthesis processes, development of an improved skeletal iron catalyst was initiated.  
       SUMMARY OF INVENTION  
       [0003] The present invention provides a skeletal iron catalyst, which is uniquely suitable for Fischer-Tropsch synthesis of CO and H 2  feedstreams, for producing desired hydrocarbon products. This unique skeletal iron catalyst material is made using a preparation method including, providing an iron powder mixed with a suitable non-ferrous metal powder selected from aluminum, antimony, silicon, tin, or zinc sufficient to provide 20-80 wt %, iron together with 0.01-5 wt % of a promotor metal including calcium, copper, chromium, magnesium, or potassium. The mixed metal powders are heated and melted together so as to form an iron alloy precursor material, followed by pulverizing the iron alloy to 0.1-10 mm particle size, and extracting the non-ferrous metal portion using a suitable caustic solution such as NaOH or KOH and leaving the iron particles as a skeletal iron catalyst material.  
       [0004] The resulting skeletal iron catalyst has high activity and provides good selectivity towards the formation of desirable low-molecular-weight hydrocarbon products from the CO and H 2  feedstreams. This catalyst has catalytic activity equivalent to that of precipitated iron catalyst and product selectivity exceeding that of either precipitated or fused iron catalysts,and can be utilized in either fixed bed or slurry bed type reactors for Fischer-Tropsch synthesis reaction processes.  
       [0005] The skeletal iron catalyst of this invention has various advantages compared with the conventional precipitated iron or fused iron catalysts for Fischer-Tropsch synthesis processes, which include: 
       [0006] (a) The preparation method and pretreatment procedures for skeletal iron catalyst are relatively simple and inexpensive.  
       [0007] (b) Specific surface area of skeletal iron catalyst (&gt;60 m 2 /g) can approach that of precipitated iron catalyst and exceeds that of fused iron catalyst.  
       [0008] (c) Synthesis feed gas conversion using skeletal iron catalyst (CO conversion &gt;90%) is equivalent to that achieved by precipitated iron catalyst and exceeds that achieved by fused iron catalyst.  
       [0009] (d) For slurry-phase Fischer-Tropsch synthesis processes, the skeletal iron catalyst has stable activity and significant selectivity for low molecular weight hydrocarbons (C 4  selectivity&gt;10%).  
     
    
    
     DESCRIPTION OF INVENTION  
     [0010] The skeletal iron catalyst of this invention is prepared by using a preparation method which includes the following three basic steps: 
     [0011] 1. Preparation of Catalyst Precursor: Provide iron powder and a non-ferrous metal powder selected from aluminum, antimony, silicon, tin, and zinc in weight proportion having iron content of 20-80 wt %, and 0.01-5.0 wt % of a promotor metal selected from calcium, copper, chromium, potassium or magnesium and add the metals powders into an electric arc induction furnace. Then ignite an electric arc with suitable high current and low voltage under an inert gas protection of argon or nitrogen;and stir the metal powders uniformly under a magnetic field to heat and melt the powder. Then cool the molten iron alloy material to room temperature and mechanically pulverize the resulting metal alloy to provide iron alloy precursor particles having 0.1-10 mm particle size.  
     [0012] 2. Preparation of skeletal iron catalyst: The skeletal iron catalyst is prepared from the iron alloy precursor particles under hydrogen atmosphere protection, using either one of the following three procedures:  
     [0013] (a) Add a sufficient volume of NaOH or KOH solution caustic (10-50% concentration) into a stirred container, heat the solution to a temperature of 30°-95° C., add the iron alloy particles (0.1-10 mm) into the caustic solution and maintain the reaction condition for 2-150 minutes after the metal alloy particle addition to extract or leach out a major portion of the non-ferrous metal portion from the iron. Then wash the treated iron particles with deionized water to pH=7, replace water with water-free ethanol, and temporarily store the resulting skeletal iron catalyst particles in ethanol.  
     [0014] (b) Mechanically mix uniformly the fine iron alloy precursor catalyst particles, and solid sodium hydroxide powder at weigh ratio to the iron alloy powder of 5-10:1, add deionized water drop-wise to wet the mixture into a paste form but not in a fluid state, while stirring to allow the reaction to proceed in desired moisture content. After the reaction has proceeded for 5-30 minutes and gas release has gradually decreased, add fresh NaOH or KOH solution (10-50% concentration), maintain for 2-60 minutes at 50-950° C., to extract and remove a major portion of the non-ferrous metal portion from the iron. Then wash the iron particles with deionized water to pH=7, displace water with water-free ethanol, and store the skeletal iron particles in ethanol.  
     [0015] (c) Place fine size iron alloy particles in a well-stirred container, spray (preferably in the mist form) high-concentration (40-60%) NaOH or KOH solution onto the powder while stirring, maintain reaction in a wet state but not fluid state for 5-30 minutes, then add additional NaOH or KOH solution of 10-50% concentration, maintain for a period of 2-60 minutes at 50-95° C. to extract and remove a major portion of the non-ferrous metal portion from the iron particles. Then wash particles with deionized water to pH=7, displace water with water-free ethanol, and store the skeletal iron particles in ethanol.  
     [0016] 3. Pretreatment of skeletal iron catalyst Before reaction evaluation of the skeletal iron catalyst prepared in Step 2, the catalyst stored in water-free ethanol is screened to remove undesired fine particles before pretreatment step. This pretreatment can be done via either of two routes. The first route is to place the catalyst in a porcelain boat inside a tubular reactor and dry the catalyst with high space velocity hydrogen at 200°-500° C. for 2-12 hours, then screen, weigh, and transfer the catalyst under high-purity nitrogen into water-free ethanol for evaluation in either a fixed-bed reactor, or into a liquid paraffin for evaluation in slurry-phase reactor. The second pretreatment route is to screen the catalyst in water-free ethanol, then dry catalyst in a curved quartz-tube under hydrogen atmosphere at 100-200° C., then transfer catalyst into either water-free ethanol or liquid paraffin as before.  
     [0017] 4. Process utilizing skeletal iron catalyst The skeletal iron catalyst of this invention is uniquely useful in processes for Fischer-Tropsch synthesis of CO and H 2  feedstreams to produce desired hydrocarbon products. Useful reaction conditions for slurry bed reactor include 0.5-2.5 H 2 /CO molar ratio feedstreams and 4-20 wt % catalyst loading relative to liquid paraffin, catalyst particle size of 0.062-2.00 mm, 200-350° C. temperature, 1.0-3.0 mPa pressure, and gas hourly space velocity of 0.5-3 L/g Fe/h.  
     [0018] The catalyst preparation method and use for this invention will be further disclosed by the following examples, which should not be construed as limiting in their scope.  
     EXAMPLE I  
     [0019] 1. Add metal iron and aluminum powders together with small amount of copper oxide promoter in weight ratio 49:50:1 into an electric-arc induction furnace, evacuate the air and fill the furnace with argon inert blanketing gas, then apply 450 A, 25 V electric current to induce an electric arc for heating and melting the powder materials uniformly during magnetic stirring. Then cool the iron-aluminum alloy material to room temperature and mechanically pulverize the alloy material to 0.1-1 mm particle size to produce metal alloy precursor material particles.  
     [0020] 2. Under hydrogen atmosphere, add a volume of 25% concentration NaOH in a stirred container, heat to temperature of 85° C., then add the precursor iron alloy particles into the caustic solution at certain time intervals, and maintain reaction condition for 30 minutes to dissolve or extract the aluminum from the iron particles. Then wash the particles with deionized water to pH=7, displace water with water-free ethanol and temporarily store the resulting skeletal iron catalyst in ethanol under refrigeration.  
     [0021] 3. Before evaluating the stored skeletal iron catalyst in a reaction system, dry the catalyst with ethanol in a porcelain boat inside a tubular reactor, at 300° C. for two hours with hydrogen at high space velocity, then screen and weigh in a container under high-purity nitrogen, and transfer the dried catalyst to slurry-phase reaction medium for evaluation.  
     [0022] 4. Fischer-Tropsch synthesis conditions used for catalyst evaluation in a CO/H 2  feedstream within ranges of traditional research include catalyst loading of 6 wt %, catalyst particle size of 0.062-0.075 mm, 2.0 H 2 /CO molar ratio, 270° C. temperature, 1.6 mPa pressure and reaction duration of 72 hours. Evaluation results are listed below in Table 1.  
               TABLE 1                          Fischer-Tropsch Synthesis Results Using Skeletal Iron Catalyst       and Traditional Iron Catalysts under Identical Reaction Conditions*                                     Precipi-   Fused               tated   Iron           Skeletal Iron Catalyst   Iron   Cata-                                                 Example No.   1   2   3   4   5   6   Catalyst   lyst                                                         Catalyst   25.0   37.8   42.5   45.6   58.6   64.5   ˜70   8.6       Surface       Area, M2/g       CO Con-   94.25   94.53   95.78   94.36   89.59   91.22   92.59   83.77       version, %       C4 Selec-   9.2   17.5   15.5   20.2   19.7   16.4   6.5   11.4       tivity, %**                                  
 
     EXAMPLE II  
     [0023] 1. Add metal iron and aluminum power and a small amount of copper oxide at weight proportion of 25:74:1 into an electric-arc induction furnace, evacuate the air, and fill with argon gas for protection. Use 450 A, 25 V electric current to ignite electric arc for heating and melting the metal powders uniformly during magnetic stirring. Then quench the molten iron-aluminum alloy to room temperature, and mechanically pulverize the metal alloy material to 0.1-1 mm particle size to provide precursory metal particles. Steps 2, 3 and 4 were the same as for Example I.  
     EXAMPLE III  
     [0024] Add metal iron and aluminum powder and a small amount of copper oxide promoter at weight ratios of 33:66:1 in an electric-arc induction furnace, evacuate the air, and fill with argon gas for protection. Use 450 A, 25 V current to ignite electric arc to heat the metal powders and melt uniformly during magnetic stirring. Then cool metal alloy to room temperature, and mechanically pulverize to 0.1-1 mm precursor particles. Steps 2, 3, and 4 were the same as for Example I.  
     EXAMPLE IV  
     [0025] Provide iron-aluminum-copper alloy proportions the same as in Example III, and provide the heating, melting, cooling and pulverizing steps the same as in Example I. Before the catalyst stored in water-free ethanol is evaluated in a reaction system, screen the catalyst in water-free ethanol, dry the catalyst containing ethanol in a curved quartz tube at 200° C. under hydrogen flow, transfer catalyst into a slurry-phase reaction medium (liquid paraffin). Catalyst evaluation was the same as Step 4 in Example I and results are listed in Table 1.  
     EXAMPLE V  
     [0026] Provide same iron-aluminum powder mixture proportions and heat, melt, cool and pulverize the metal alloy as in Example L Then mechanically mix uniformly the fine alloy catalyst precursor particles with solid sodium hydroxide (NaOH) powder, add deionized water drop-wise to wet the mixture into a paste but not in a fluid state, while stirring to allow reaction to proceed at desired moisture level. After gas released from reaction gradually decreases, transfer mixture into fresh 20% concentration NaOH solution, maintain for a time of 2-60 minutes, then wash with deionized water to pH=7, displace water with water-free ethanol, and store catalyst in ethanol temporarily under refrigeration. Then pre-treat the catalyst the same as in Example IV and evaluate it the same as in Example I.  
     EXAMPLE VI  
     [0027] Use an iron-aluminum-copper mixture as in Example III Place the fine size iron alloy precursor particles in a well-stirred container, spray (preferably in the mist form) 48% concentration NaOH or KOH solution onto the alloy particles while stirring, allow reaction to proceed in wet state but not fluid state for 20 minutes, then carefully add NaOH or KOH solution of 20% concentration and maintain for 30 minutes period of time at 55° C. Then wash with deionized water until pH value reaches 7, displace water with water-free ethanol, and store catalyst in ethanol. Pretreat the catalyst the same as in Example IV, and evaluate it the same as in Example I.  
     [0028] The six iron-aluminum alloy catalyst samples, which were prepared in Examples I through VI, were evaluated by F-T synthesis reaction using a CO and H 2  feedstream having H 2 /CO molar ratio of 2.0 as for Example I. Results of the catalyst evaluations are provided in Table 1 above.  
     [0029] From these results, it is noted that surface area for the skeletal iron catalyst approach that of the known precipitated iron catalyst and significantly exceeds that of the fused catalyst Also, CO conversion is generally equivalent to that of the precipitated iron catalyst and exceeds that of the fused iron catalyst Furthermore, the selectivity for C 4  product generally exceeds that for both the precipitated and fused iron catalyst.