(181b) Fischer-Tropsch Synthesis with Ultrafine Iron-Based Catalyst: Nano-Scale Growth of Particles and Associated Effects on Wax/Catalyst Separation

Fischer-Tropsch synthesis (FTS) using nano-catalyst in slurry bubble column reactor (SBCR) is advantageous for efficient control and utilization of high reaction exotherm and enhanced mass transfer. The Fischer-Tropsch wax can be refined to produce premium quality fuels and a wide variety of chemical feed-stock. Catalyst/wax separation is necessary for wax upgrading and to avoid catalyst loss. The advantages of SBCR for FTS can be realized commercially only when a continuous and cost-effective technique for wax separation is developed. Iron-based catalysts are preferred for their low cost, high wax selectivity and excellent water-gas shift activity; although they have the tendency to undergo attrition making wax separation extremely difficult. The phase-transformations during the activation and reaction play an important role in determining the attrition resistance of the particles. The commonly accepted mechanism for particle attrition is transformation of single-crystal hematite into iron carbide crystallites formed as nodules at surface which split off to nano-range particles. Since the oxide/carbide conversion is dynamic and reversible, a detailed knowledge of particle size is therefore necessary for design of any mechanical catalyst/wax separation system.

The morphological and chemical nature of ultrafine iron catalyst particles (3-5 nm diameters) during activation/FTS was studies by HRTEM, EELS, and Mössbauer spectroscopy. The composition of CO activated catalyst was 85% iron carbide (Hägg carbide) and rest magnetite. With the progress of FTS, the carbide re-oxidized to magnetite and catalyst activity gradually decreased. The growth of oxide phase continued and average particle size also increased simultaneously. The average particle size was 61.5 nm after 600 h of FTS. Some particles having maximum dimension larger than 130 nm was also observed. Amorphous carbon rims of 3-5 nm thickness around some of the particles and some hexagonal shaped particles were also observed. The thickness of carbon rims did not grow above 5-6 nm. Hence, the growth of particles was not due to carbon deposition on surface. The phase transformation occurred in a ?shrinking core? manner with different nano-zones. The nano-range carbide particles did not show fragmentation or attrition as generally observed in micrometer range particles. Nevertheless, when the dimension of particles reached the micrometer range, the crystalline carbide phase appeared to be sprouted on the surface of magnetite single crystal.