(701e) Solvent Deficient Precipitation (SDP): New Method To Prepare Active, Stable Iron Fischer-Tropsch Catalysts

Introduction: The Fischer-Tropsch process has been a valuable means of producing synthetic fuels since its invention over 90 years ago. This process continues to draw significant interest in light of current environmental and energy concerns. The development of new and more cost effective catalysts is an area of ongoing research.

The work presented in this paper will discuss solvent deficient precipitation (SDP), a novel catalyst preparation method for iron Fischer-Tropsch (FT) catalysts. SDP is a simple, dry-mix method that produces the precursor catalyst (pre-calcination and reduction) in about one-quarter the time that it takes to prepare standard precipitated catalysts. The objective of this paper is to demonstrate that the SDP preparation method may be effectively used to prepare active and stable unsupported Fe-Cu-K-SiO₂ FT catalysts comparable to some of the most active catalysts reported in the literature [1].

Materials and Methods: Catalysts were nominally prepared from nitrate salts of iron and copper along with potassium (KHCO₃) and silica (Cab-O-Sil) promoters by SDP, a proprietary co-precipitation method developed by Cosmas, Inc. [2]. Nominal catalyst composition was 100 Fe/ 5Cu/ 4K/ 16SiO₂ by mass. Catalysts were dried quickly at 100°C for 48 hours. All catalysts were calcined at 300°C for 10 hours in air with GHSV = 2000 h^-1. After calcination, catalysts were reduced in H₂ at 300°C, GHSV of 2000 h^-1. Following reduction, the catalysts were cooled to room temperature and then carefully passivated in air such that the bed temperature during passivation was 25-30°C.

Activity studies were performed in a 3/8 inch ID fixed-bed reactor. Activation and reaction conditions were 300 psig, 31% H₂, 31% CO, 4% Ar, 34% He, GHSV = 7,000-10,000 h^-1. Activation was at 250°C for 48-100 hours. Reaction temperatures were varied from 220° to 260°C. Reactor effluent was analyzed online by an HP 6890 GC.

Results and Discussion: The Fe-Cu-K-SiO₂ catalysts prepared by the SDP method compare favorably with most active reported catalysts in literature. First order activities (mmolH₂CO/g Fe/h/MPaH₂) at 260°C, 21 atm, and H₂:CO = 0.7-1 are 215-414 compared with 155-450 in literature. Selectivity to CO₂ of the catalysts is consistent with active iron FT catalysts with high K loading (4 parts K per 100 part Fe) at a H₂ to CO feed ratio of 1. Selectivity to methane is 5.1-7.8 percent compared with 2.7-6.4 percent in literature. One catalyst in this study was tested for more than 500 hours and showed little or no downward trend with time, suggesting that the SDP method produces catalysts with excellent stability.

Significance: Solvent deficient precipitation is a promising, new preparation method for FT catalysts. The catalysts presented compare favorably in activity, stability, and productivity to the most active catalysts reported in the literature. It is anticipated that an optimized catalyst prepared by SDP will compare even more favorably. That the catalysts can be prepared using the SDP method with much less time and equipment will have a significant impact on the cost of production. Increased catalyst life and performance will also improve the economics of FT plants.

References

1. Bukur, D.B. and Lang, X. Ind. Eng. Chem. Res.38, 3270 (1999).  
2. Liu, S., Liu, Q., Boerio-Goates, J. and Woodfield, B. F. J. Adv. Mater. 38, 18 (2007).