Fischer–Tropsch Synthesis DOI: 10.1002/ange.201200280 Suppression of Carbon Deposition in the Iron-Catalyzed Production of Lower Olefins from Synthesis Gas** Ard C. J. Koeken,* Hirsa M. Torres Galvis, Thomas Davidian, Matthijs Ruitenbeek, and Krijn P. de Jong* Utilizing alternative feedstocks to crude oil for the production of olefins is of eminent importance to the chemical industry. Mixtures of carbon monoxide and hydrogen (syngas) pro- duced from natural gas, coal, or biomass can be directly converted into hydrocarbons by the Fischer–Tropsch syn- thesis (FTS). [1–4] Cobalt- and iron-based catalysts are applied commercially in FTS processes. While cobalt-based catalysts are preferred for the production of long-chain linear alkanes (transportation fuels), [5] iron-based catalysts are attractive for the production of olefins. [2, 6–9] For industrial application of iron catalysts in FTS to produce lower olefins (hereafter designated as Fischer– Tropsch to olefins, FTO) increasing the selectivity to lower olefins (the C 2 –C 4 fraction) while suppressing the selectivity to methane remains one of the main challenges. [9] Indeed, the hydrocarbon distribution generally follows the Anderson– Schulz–Flory distribution, which states that at the optimal chain growth probability factor a to obtain the highest proportion of C 2 –C 4 olefins (ca. 50 wt %), about 30 wt % CH 4 will also be produced. [7] Generally, methane production can be reduced through increasing a by addition of promoters, such as alkali metals, [10] or lowering the operating temper- ature. [7] On the other hand, improving catalyst stability poses an even greater challenge than optimizing selectivity. Several deactivation mechanisms can occur, such as poisoning, sintering, iron phase changes, and carbon deposition. [4, 10–12] In particular, at the high temperatures required for FTO, carbon formation by the Boudouard reaction, 2 CO(g)! C(s) + CO 2 (g), can be significant. [9] If these carbon species are not removed by hydrogenation to hydrocarbons they can block active catalytic sites and eventually cause breakup of catalyst particles when they form carbon filaments. [10, 11, 13, 14] Data on carbon deposition rates obtained under actual FTS conditions are relatively few and are mostly based on ex situ analysis of catalyst samples. [10, 15] Obtaining in situ carbon deposition rates under FTS conditions poses a great practical challenge, as the use of a microbalance is usually restricted to atmospheric conditions. [16] Herein, we describe a method to assess the extent of carbon formation for an iron-based FTO catalyst in situ in combination with online hydrocarbon product analysis. For a novel a-Al 2 O 3 supported iron catalyst, [17] we have found conditions where carbon formation was essentially undetect- able, while methane production was kept at a low level. Herein, we present the application of the tapered-element oscillating microbalance (TEOM) in FTS research. Previ- ously, the TEOM has been applied for coking studies involving gas-phase reactants and products over heteroge- neous catalysts, such as methanol-to-olefins, and the kinetics of carbon nanofiber formation. [18, 19] The main advantages of the TEOM over conventional microbalances are the wide range of operating conditions and the plug-flow hydrody- namics, [18] which resemble that of a conventional packed-bed reactor often used in FTS research. This makes a comparison with results from other studies possible and meaningful. In the Supporting Information (section S1) a description of the TEOM setup is given. The 10 wt% Fe supported on a-Al 2 O 3 (10 %Fe/a-Al 2 O 3 ) catalyst was prepared by incipient wetness impregnation with an aqueous solution containing ammonium iron citrate as metal precursor ; sodium and sulfur were used as promoters to enhance olefin formation and decrease selectivity to methane, as has been explained previously in more detail. [17] Further details on preparation and characterization are given in the Supporting Information, S2. Prior to FTO conditions, the catalyst was reduced using a 21 vol % H 2 in Ar mixture at 350 8C at 1.7 bar (Supporting Information, S3). In Figure 1, TEOM results, that is, the mass increase of the catalyst bed with time, are presented for FTO carried out at 20 bar and 350 8C. Time zero indicates the start of the feed of the synthesis gas. From calibration measurements, it is known that from t = 0 to t = 0.2 h, the mass will change as a result of the replacement of Ar gas by synthesis gas and build-up of carbonaceous species in the catalyst bed. From t = 0.2 h to 2 h, mass increase is only caused by buildup of carbonaceous materials in the catalyst bed.At t  2 h, the gas feed is switched from synthesis gas to argon again, which caused an increase in the mass as a result of the increase in mass density of the gas flowing through the catalyst bed. There was no significant decrease in mass from t = 2.3 to 4 h, which [*] Dr. A. C. J. Koeken, H. M. Torres Galvis, Prof. Dr. K. P. de Jong Inorganic Chemistry and Catalysis, Department of Chemistry, Utrecht University Universiteitsweg 99, 3584 CG Utrecht (The Netherlands) E-mail: k.p.dejong@uu.nl Dr. T. Davidian, Dr. M. Ruitenbeek Hydrocarbons R&D, Dow Benelux B.V. P.O. Box 48, 4530 AA, Terneuzen (The Netherlands) [**] A.C.J.K. and K.P.d.J. acknowledge support from Dow Benelux. A. van Laak, F. Broersma, and R. van Zwienen are acknowledged for assistance with the oscillating microbalance studies. G. Bonte, A. Chojecki, and C. van Dijk are acknowledged for assistance with the plug-flow reactor experiments. C. van der Spek is acknowledged for the electron microscopy measurements and B. Dickie and K. Scieranka for TGA-MS studies. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201200280. . Angewandte Zuschriften 7302  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2012, 124, 7302 –7305