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Please cite this article in press as: B. Todic, et al., Fischer-Tropsch synthesis product selectivity over an industrial iron-based catalyst: Effect of process conditions, Catal. Today (2015), http://dx. a b s t r a c tThe effect of process conditions on product selectivity of Fischer-Tropsch synthesis (FTS) over industrial iron-based catalyst (100 Fe/5 Cu/4.2 K/25 SiO 2 ) was studied in a 1-L stirred tank slurry reactor. Experiments were performed over a range of different reaction conditions, including three temperatures (T = 493, 513 and 533 K), four pressures (P = 0.8, 1.5, 2.25 and 2.5 MPa), two synthesis gas feed molar ratios (H 2 /CO = 0.67 and 2) and gas space velocity from 0.52 to 23.5 Ndm 3 /g-Fe/h. The effect of process conditions on reaction pathways of FTS and secondary 1-olefin reactions was analyzed by comparing product selectivities, chain growth probabilities and ratios of main products (n-paraffin, 1-and 2-olefin). Reduction of methane production and increase of C 5+ products was achieved by decreasing temperature, inlet H 2 /CO ratio and/or increasing pressure. Overall selectivity toward methane and C 5+ did not show significant changes with variations in residence time. All of the product selectivity variations were shown to be related to changes in chain length dependent growth probabilities.

A precipitated iron catalyst (100 Fe/5 Cu/4.2 K/25 SiO2 on a mass basis) was tested in a fixed bed reactor and a stirred tank slurry reactor under the same process conditions (250°C, 1.48 MPa, 2 L (STP)/gcat · h, H2 : CO = 2:3). Two different pretreatment procedures were employed (hydrogen reduction at 220°C and carbon monoxide activation at 280°C) in each of the two reactor types. In the stirred tank slurry reactor tests the activity (based on an apparent first order reaction rate constant) of the carbon monoxide pretreated catalyst was about 25% higher than that of hydrogen reduced catalyst, due to incomplete reduction of the latter. In all tests the catalyst selectivity changed slowly with time on stream. Hydrocarbon distribution shifted toward lower molar mass products, and secondary reactions (l‐olefin hydrogenation, isomerization and readsorption) increased with time. The secondary reactions were the most pronounced on the hydrogen reduced catalyst in the fixed bed reactor.

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Conradson et al. have analyzed X-ray absorption fine-structure spectra of the UO2-U4O9 system and concluded that oxygen atoms are incorporated in U4O9 as oxo groups with U-O distances in the range 1.72-1.76 A. They also found that the uranium sublattice consists of an ordered portion and an additional 'spectroscopically silent' glassy portion. We have carried out studies of powdered U4O9 by neutron diffraction which contradict these conclusions from EXAFS measurements. Our analysis shows that there are no U-O bonds shorter than 2.2 A and that U4O9 is crystallographically ordered with no evidence of a glassy structure.