Highly Active and Stable Fischer-Tropsch Catalysts Obtained through Unconventional Metal-Organic Framework Mediated Synthesis

Highly active and stable Fischer-Tropsch catalysts obtained through unconventional metal-organic framework mediated synthesis [1]

T.A. Wezendonk, V.P. Santos, J. J. Delgado Jaen, A. I. Dugulan, A. Chojecki, S. Sartipi, A. A. Hakeem, A. Koeken, M. Ruitenbeek, G. R. Meima, X. Sun, M.A. Nasalevich, S. Gopinathan, H. Islam, F. Kapteijn, M. Makkee, J. Gascon

Catalysis Engineering, Chemical Engineering Department, Delft University of Technology, Julianalaan 136, 2628 BL Delft, and Core R&D, Hydrocarbons R&D, Dow Benelux B.V., P.O. Box 48, 4530 AA, Terneuzen, The Netherlands

In the search for efficiently converting non-fossil feedstock, many catalytic processes are evaluated for improving their performance. Prime examples of such processes are found in the production of fuels and chemicals from synthesis gas over heterogeneous catalysts: Fischer-Tropsch synthesis (FTS) on supported Fe and Co catalysts.

Traditional catalyst preparation methods present several drawbacks: often, supported catalysts show lower specific activity with increased loading. Additionally, agglomeration of nano-particles results in catalyst deactivation over time-on-stream. Therefore, the challenge is to create catalysts with high loading that maintain the highly dispersed active phase and thus, their catalytic performance during FTS operation.

In this work, metal-organic frameworks (MOFs) were selected for an unconventional synthesis method to produce Fe nanoparticles encapsulated in a carbon matrix through pyrolysis of the metal-organic compound. Both the active site and the support are created simultaneously in this one-step synthesis, much contrary to impregnating a catalyst support with metal precursor, followed by additional heat treatment.

Characterization of such pyrolyzed Fe-MOFs (Fe@C) shows that very small metal nanoparticles can be formed on a porous carbon support, where most of the metal is encapsulated in a few layers of graphitic carbon. In situ Mossbauer characterization confirms the large accessibility of reactants, transforming nearly all the Fe into highly active Hagg carbide species. In situ XAFS studies reveal the reduction of the Fe³⁺ metal nodes to Fe²⁺ species during pyrolysis, forming Fe carbides upon exposure to FTS.

The results of the catalytic testing of the Fe@C compounds in FTS display outstanding activity and stability in both the high- and low temperature regime: the novel Fe@C catalysts outperform commercial benchmark catalysts. These promising results open the way for many different metal on carbon-supported catalysts, given the astonishing amount of available MOFs and their exceptional versatility.

[1] Santos, V. P. et al., Nat. Commun. 6:6451, doi: 10.1038/ncomms7451 (2015).