Elucidating the Nature of Fe Species during Pyrolysis of the Fe-BTC MOF into Highly Active and Stable Fischer–Tropsch Catalysts

Abstract

In this combined in situ XAFS, DRIFTS, and Mössbauer study, we elucidate the changes in structural, electronic, and local environments of Fe during pyrolysis of the metal organic framework Fe-BTC toward highly active and stable Fischer–Tropsch synthesis (FTS) catalysts (Fe@C). Fe-BTC framework decomposition is characterized by decarboxylation of its trimesic acid linker, generating a carbon matrix around Fe nanoparticles. Pyrolysis of Fe-BTC at 400 °C (Fe@C-400) favors the formation of highly dispersed epsilon carbides (ε′-Fe2.2C, d p = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly Hägg carbides are formed (χ-Fe5C2, d p = 6.0 nm). Extensive carburization and sintering occur above these temperatures, as at 900 °C the predominant phase is cementite (θ-Fe3C, d p = 28.4 nm). Thus, the loading, average particle size, and degree of carburization of Fe@C catalysts can be tuned by varying the pyrolysis temperature. Performance testing in high-temperature FTS (HT-FTS) showed that the initial turnover frequency (TOF) of Fe@C catalysts does not change significantly for pyrolysis temperatures up to 600 °C. However, methane formation is minimized when higher pyrolysis temperatures are applied. The material pyrolyzed at 900 °C showed longer induction periods and did not reach steady state conversion under the conditions studied. None of the catalysts showed deactivation during 80 h time on stream, while maintaining high Fe time yield (FTY) in the range of 0.19–0.38 mmolCO gFe –1 s–1, confirming the outstanding activity and stability of this family of Fe-based FTS catalysts.

Structural and elemental influence from various MOFs on the performance of Fe@C catalysts for Fischer–Tropsch synthesis

“…For the unpromoted samples, the particle size effect on the activity was less pronounced than for the promoted samples, in line with previously reported FTY values for a similar range of particle sizes produced by pyrolysis of F300 Fe-BTC at different temperatures 16 . These findings indicate that particle size is the key factor in Fe@C catalyst design, and this effect is even more pronounced in the K-promoted samples.…”

“…The similar pyrolysis behaviour for the Fe@C catalysts was confirmed by analysing the degree of carbonization, calculated from C/O ratios of the C matrix, indicating one molecule of CO 2 is removed from the framework next to an additional O molecule, assumed to be in the form of a water molecule. In previous work, we showed that the structure collapse by initial decarboxylation is followed by carboxylate side reactions, supposedly forming anhydride species as intermediates towards more graphite-like structures 16 . Additionally, higher C/O ratios were found on the surface than in the bulk, suggesting that surface C is more prone to releasing O from its structure.…”

“…Recent investigations into MOF-derived catalysts for FTS by the group of Wang confirmed the high potential of the Fe@C system using the MIL-88B topology as catalyst precursor 15 . Recently, we reported an in-depth study of the nature of the Fe species during pyrolysis and subsequent FTS reaction, elucidating the carbonization mechanism of the commercial Fe-BTC MOF with the MIL-100 topology 16 . In situ DRIFTS-MS and XAFS applied during the pyrolysis process revealed the decarboxylation of the framework and auto-reduction of the Fe 3+ species towards active Fe carbide.…”

Influence of precursor porosity on sodium and sulfur promoted iron/carbon Fischer–Tropsch catalysts derived from metal–organic frameworks

“…16,17 In particular, MOF-mediated synthesis of iron-based FTO catalysts leads to exceptional performance in the synthesis of lower olefins from synthesis gas. [18][19][20] The effects of the addition of potassium promoters/additional carbon loading, 18 pyrolysis temperature, 19 and linker molecule structure/heteroatoms 20 on the catalysts properties have been investigated. However, the influence of the porosity of the MOF precursor on the iron/carbon catalysts has not been investigated but is likely to be crucial for the performance of the resulting catalysts.…”

MOF‐Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo‐Like N‐Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions

“…Very few studies have reported the synthesis of Fe 3 C‐based catalysts from MOF precursors, probably because of two reasons. Firstly, carbonization of the Fe‐MOF mostly leads to a mixed‐phase product such as Fe 3 C, iron oxide, or metallic iron; this complicates unambiguous identification of the true active sites for the ORR. Secondly, the prepared catalysts do not have well‐defined nanostructures .…”