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

Abstract

The structure and elementary composition of various commercial Fe-based MOFs used as precursors for Fischer–Tropsch synthesis (FTS) catalysts have a large influence on the high-temperature FTS activity and selectivity of the resulting Fe on carbon composites. The selected Fe-MOF topologies (MIL-68, MIL-88A, MIL-100, MIL-101, MIL-127, and Fe-BTC) differ from each other in terms of porosity, surface area, Fe and heteroatom content, crystal density and thermal stability. They are re-engineered towards FTS catalysts by means of simple pyrolysis at 500 °C under a N2 atmosphere and afterwards characterized in terms of porosity, crystallite phase, bulk and surface Fe content, Fe nanoparticle size and oxidation state. We discovered that the Fe loading (36–46 wt%) and nanoparticle size (3.6–6.8 nm) of the obtained 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 (SBET between 570 and 670 m2 g−1), whereas the pore width distribution is completely different for the various MOFs. The high catalytic performance (FTY in the range of 1.9–4.6 × 10−4 molCO gFe−1 s−1) of the resulting materials could be correlated to the Fe particle size and corresponding surface area, and only minor deactivation was found for the N-containing catalysts. Elemental analysis of the catalysts containing deliberately added promoters and inherent impurities from the commercial MOFs revealed the subtle interplay between Fe particle size and complex catalyst composition in order to obtain high activity and stability next to a low CH4 selectivity.

Metal organic frameworks as precursors for the manufacture of advanced catalytic materials

“…Interestingly, a control experiment employing pyrolyzed molecular Fe-oxo clusters showed little activity, underlining the importance of a framework-type precursor for pyrolysis and generating dispersed Fe particles. Wezendonk et al 129 further investigated the effect of the MOF precursor using different MOFs while employing fixed pyrolysis conditions. It was found that under the same temperature, the different MOFs undergo a similar degree of carbonization of the framework by decarboxylation.…”

Formulation and catalytic performance of MOF-derived Fe@C/Al composites for high temperature Fischer–Tropsch synthesis

“…After pyrolysis, none of the materials show diffraction patterns of their precursors, evidencing the decomposition of the framework in the case of the MOF and full transformation of the AlOOH binder into γ-Al2O3. In situ measurements in our group have proven that the iron phases present in the 38Fe@C composite after pyrolysis at 500 °C consist of a mixture of metallic iron (Fe), iron (II) oxide (FeO) and iron carbides (FeC) 8 . The highly dispersed Fe particles are very pyrophoric after preparation, and the required passivation step at room temperature readily converts significant amounts of the Fe phase into Fe3O4.…”

“…Altogether, these results point out that the lower activity for higher amount of binder is not only linked to the Fe particle size variation in the composites but also to possible Fe-Al interactions created during the pyrolysis, which although not detected by PXRD may still be present. The Fe-Al interactions may hinder carburization of Fe towards its transformation into Hägg carbide (χ-Fe5C2) during the activation period in HT-FTS 8 , leading to a lower overall activity.…”

“…Moreover, when promoted with potassium, these catalysts show a higher activity due to an enhancement of the water gas shift functionality and a higher olefin to paraffin ratio due to modulation of the hydrogenation ability. This effect also leads to a significant decrease in methane selectivity 8 .…”

Highly active cluster structure manganese‐based metal organic frameworks (Mn‐MOFs) like corals in the sea synthesized by facile solvothermal method as gas electrode catalyst for lithium‐oxygen batteries

“…Metal organic frameworks (MOFs) are functional porous metal compound material with Metallic nodes interlinked by electron‐donating organic linkers . For the moment, MOFs has been widely used in catalysis , drug delivery and energy storage owing to its outstanding porous, crystal structure and special fictionalization. For this reason, as the gas electrode catalyst for lithium‐oxygen batteries has started to be researched right now.…”