Skeletal iron catalyst and its preparation for...

Details Skeletal iron catalyst and its preparation for... Skeletal iron catalyst and its preparation for...

1999-09-21

2001-07-24

Richter, Johann (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Liquid phase fischer-tropsch reaction

C502S314000, C502S327000, C502S301000, C502S331000, C502S332000, C502S336000, C502S338000, C420S077000, C420S590000, C420S089000, C420S090000, C420S091000

Reexamination Certificate

active

06265451

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention pertains to skeletal iron catalysts, and particularly pertains to catalyst composition and preparation methods and processes for use of such skeletal iron catalysts in Fischer-Tropsch synthesis processes for CO and H
2
feeds to produce hydrocarbon products.
As a basic technology for producing synthetic liquid fuels from CO+H
2
feedstreams, the Fischer-Tropsch (F-T) 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 THE INVENTION
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 contains 40-90 wt % iron with the remainder being smaller percentage of non-ferrous metals selected from aluminum, antimony, silicon, tin, or zinc together with less than 5 wt % of promotor metal selected from calcium, copper, chromium, magnesium or potassium, and has surface area of 25-65 m
2
/gm. This skeletal iron catalyst 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 non-ferrous promotor metal powder 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 then extracting and/or leaching a major portion of the non-ferrous metal portion from the iron alloy using a suitable caustic solution such as NaOH or KOH and after drying and reducing with hydrogen leaving the iron particles as a skeletal iron alloy catalyst material. The catalyst final particle size can be within a 20-10,000 micron range, with the larger size particles within this range being intended for F-T fixed bed reactor usage and the smaller size particles up to about 2000 micron within this range intended for slurry phase bed reactor usage.
The resulting skeletal iron catalyst which contains 50-90 wt % iron 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 skeletal iron 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 for producing the desired hydrocarbon products.
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 advantages include:
(a) The preparation method and pretreatment procedures for skeletal iron catalyst are relatively simple and inexpensive.
(b) Specific surface area of skeletal iron catalyst (25-65 m
2
/g) can approach that of precipitated iron catalyst and exceeds that of fused iron catalyst.
(c) Synthesis feed gas conversion using skeletal iron catalyst (CO conversion>90%) is equivalent to that achieved by precipitated iron catalyst and exceeds that achieved by fused iron catalyst for equivalent space velocities.
(d) For slurry-phase Fischer-Tropsch synthesis processes, the skeletal iron catalyst has stable activity and significant selectivity for low molecular weight hydrocarbon products (C
4
selectivity>10%).
DESCRIPTION OF INVENTION
The skeletal iron catalyst of this invention is prepared by using a preparation method which includes the following three basic steps:
1. Preparation of Catalyst Precursor Particles
Provide and mix together iron powder and a non-ferrous metal powder selected from aluminum, antimony, silicon, tin, or zinc in weight proportion having iron content of 20-80 wt. %, and 0.01-5.0 wt. % of a promotor metal powder selected from calcium, copper, chromium, potassium or magnesium, and place the mixed metals powders into a suitable furnace such as 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 powders. 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.
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:
(a) Add a sufficient volume of NaOH or KOH caustic solution (10-50% concentration) into a stirred container, heat the solution to a temperature of 30°-95° C., add the iron alloy precursor particles (0.1-10 mm size) into the caustic solution, and maintain the reaction condition for 2-150 minutes after the metal alloy particle addition to extract and/or leach out a major portion of the non-ferrous metal portion from the treated iron alloy particles which now contain 40-90 wt. % 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.
(b) Mechanically mix uniformly the iron alloy precursor catalyst particles and solid sodium hydroxide powder at weight 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 a 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 50-95° C., to extract and remove a major portion of the non-ferrous metal portion from the iron alloy particles. Then wash the resulting skeletal iron particles with deionized water to pH=7, displace water with water-free ethanol, and store the resulting skeletal iron catalyst particles in ethanol.
(c) Place iron alloy precursor particles in a well-stirred container, spray (preferably in the mist form) high-concentration (40-60%) NaOH or KOH solution onto the particles while stirring, maintain reaction in a wet state but not fluid state for 5-3

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