Preparation and adsorption, chemical and catalytic properties of iron boride fischer-tropsch catalysts

Abstract

Sodium-free and sodium-promoted, iron borides were prepared by reducing anhydrous iron acetate with diborane/THF or NaBH 4/diglyme solution. These new preparation techniques provide straightforward pathways to preparation of high surface area iron borides without oxidation of the catalysts. The catalysts were characterized by hydrogen and carbon monoxide adsorptions, Moessbauer Spectroscopy, and X-ray diffraction (XRD). CO hydrogenation activities and selectivities of these catalysts were determined at 493 K to 577 K and 10 or 20 atm with a H 2/CO reactant ratio of 2. An in situ Moessbauer study indicates that the sodium-free, chemically reduced, iron boride interstitial consists mainly of a boron-rich FeB phase which transforms to a mixture of Fe 0, FeB and Fe 2B phases upon treating with hydrogen at 400°C; the corresponding sodium-containing catalyst contains mainly Fe 2B. Laboratory reactor data indicate that unpromoted iron borides have activity/selectivity properties similar to unpromoted iron catalysts for the Fischer-Tropsch Synthesis (FTS). Sodium significantly enhances activity and alkene selectivity of iron boride for FTS. Unpromoted iron borides are not stable under FTS conditions over long reaction times, however their stability is enhanced by sodium promotion. The rate of carbon monoxide hydrogenation and the average molecular weight of products obtained on iron boride catalysts increase with increasing pressure.