Efficient hierarchically structured composites containing cobalt catalyst for clean synthetic fuel production from Fischer–Tropsch synthesis

• TiO2-coated CNTs decorated on α-Al2O3 hierarchical structure is synthesized.
• The as-prepared hierarchical catalyst presents high Fischer–Tropsch (FTS) activity.
• Excellent FTS rate up to 0.80 gC5+ gcat−1 h−1 along with high stability is obtained.
• Zero field 59Co NMR is used to understand catalyst structure–activity relationship.
• Cobalt particles with size <10 nm are the most abundant in hierarchical catalyst.

We report a straightforward preparation method to synthesize hierarchical composite consisting of TiO2-coated multi-walled carbon nanotubes (CNTs) decorating a macroscopic host structure of α-Al2O3. The obtained composite possesses moderate specific surface area and very open porous structure, as well as moderate interaction with active sites, which significantly improve the cobalt nanoparticles dispersion and the mass diffusion during the reaction. The Co/TiO2/CNT-α-Al2O3 (CoTiCNTA) catalyst is then used in the Fischer–Tropsch synthesis (FTS) process. This hierarchical catalyst achieves a FTS rate to C5+ of 0.80 gC5+ gcat−1 h−1 along with a long-chain hydrocarbons (C5+) selectivity of 85%, which can be pointed out as the most outstanding noble promoter-free catalyst for the FTS process. The as-synthesized catalyst also exhibits an extremely high stability as a function of time on stream which is also one of the prerequisites for the development of future FTS catalysts, especially for the Biomass-to-Liquids process where trace amount of impurities and/or moisture could have an impact on the catalyst stability. The present work also introduces a new investigation methodology based on the use of zero field 59Co NMR, which allows one to map in a precise manner the cobalt active phase distribution and to correlate it with the FTS performance. It is expected that such technique would be extremely helpful for the understanding of the catalyst structure–performance relationship and for future optimization in the FTS process as well as in other fields of investigation where cobalt particles are involved.

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