MOF-Mediated Synthesis of Highly Active Fe- and Co-Based FTS Catalysts

Metal Organic Frameworks are well-defined crystalline porous hybrid structures with an excellent distribution of inorganic and organic building blocks. Many examples contain elements that are active in catalysis, including Fe and Co, well-known high and low temperature Fischer-Tropsch synthesis (FTS) catalysts, respectively.

Subjecting these specific MOFs to a thermal treatment in inert atmosphere, called MOF-mediated synthesis (MOFMS), causes decomposition, pyrolysis, reduction and particle growth phenomena resulting in a composite of nanosized oxidic or metallic particles covered by graphenic shells. The metal loadings may reach up to 60 wt.% [1]. Depending on the inorganic element the thermal decomposition, followed by in-situ XRD and TGA, can be described by a single- or a two-step solid conversion process, well captured by the Avrami-Erofeev nucleation models with as most relevant parameters heating rate and final temperature.

Fe-MOF precursors yield nanoparticles of Fe-suboxide encapsulated by a porous carbon shell. When exposed to the reducing FTS environment Fe-carbide is formed. Temperature and gas composition can be used to form the Hägg and/or epsilon carbide.

The Fe-loading (36–46 wt%) and nanoparticle size (3.6–6.8 nm) of the obtained Fe@C catalysts are directly related to the elementary composition and porosity of the initial MOFs. Furthermore, the carbonization leads to similar surface areas for the C matrix (S_BETbetween 570 and 670 m²g^-1), whereas the pore width distribution is completely different for the various MOFs. Comparison of different Fe-MOF precursors shows that the high temperature FTS activity is proportional to the external particle surface. Promotion with alkali elements boosts the activity and increases the olefin/paraffin selectivity considerably. The catalytic performance @ 340 ^oC, stable >600 h, results in iron time yields (FTY) in the range of 1.9–4.6·10^-4mol_COg_Fe^-1s^-1. In combination with the high Fe-loading these materials represent extreme highly active catalysts on a mass or volume basis [2-5].

This material, however, is not easy to pelletize, and these pellets disintegrate quickly. A procedure based on phase-inversion of a slurry of the MOF precursor and a selected polymer, followed by pyrolysis results in robust catalyst particles. Interestingly, the nitrogen stemming from the polymer already exerts a promotion influence on the selectivity of the catalyst.

A similar approach using a Co-MOF precursor (ZIF-67), yielded metallic Co nanoparticles covered with an impermeable graphene coating. This was nearly inactive for low temperature FTS. Adaptation of the synthesis procedure towards an oxidative treatment after impregnation with a silicon precursor and pyrolysis, yielded an extremely active LT-FTS catalyst consisting of highly loaded cobalt nanocomposites (~ 50 wt.% Co, ~12 nm Co), with cobalt oxide reducibility in the order of 80% and a good particle dispersion, that exhibit high activity, C₅₊selectivity and stability in Fischer–Tropsch synthesis [6]. The structure of this material resembles that envisaged as optimal for this process, while the activity is about 30% of the maximum reachable for a pure Co catalyst of the optimal particle size [7].

References

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7. Guttel, T. Turek, Energy Technology 2016, 4, 44-54.