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

Tim A. Wezendonk, Vera P. Santos, Maxim A. Nasalevich, Quirinus S.E. Warringa, A. Iulian Dugulan, Adam Chojecki, Ard C.J. Koeken, Matthijs Ruitenbeek, Garry Meima, Husn Ubayda Islam, Gopinathan Sankar, Michiel Makkee, Freek Kapteijn*, Jorge Gascon Sabate

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

105 Scopus citations

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 (?′-Fe2.2C, dp = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly Hägg carbides are formed (?-Fe5C2, dp = 6.0 nm). Extensive carburization and sintering occur above these temperatures, as at 900 °C the predominant phase is cementite (?-Fe3C, dp = 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.

Original languageEnglish (US)
Pages (from-to)3236-3247
Number of pages12
JournalACS Catalysis
Volume6
Issue number5
DOIs
StatePublished - May 6 2016

Keywords

  • Fischer?Tropsch synthesis
  • MOF mediated synthesis
  • dispersion
  • iron
  • iron carbide phases
  • metal organic framework
  • pyrolysis
  • structure?activity relations

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)

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