(342bd) Clean, High Quality Low Emission Transportation Fuels with Fischer-Tropsch Synthesis: A Mesoscale Study of Transport Processes in Confined Systems

The Fischer-Tropsch synthesis (FTS) is a polymerization reaction used extensively in the Gas-to-Liquids (GTL) process to transform synthesis gas (H₂ and CO)¹ into clean, high quality low emission transportation fuels. The main FT reaction products, namely water, wax and small amounts of oxygenates (e.g. alcohols < 10 wt %), form a mixture through which the dissolved reactants diffuse, reach the catalytic nanoparticles and react. The transport properties of these mixtures, particularly inside catalyst pores, is a topic of significant interest for the petrochemical industry. Key factors in ensuring FT reactor’s activity and stability is the selection of the catalyst (e.g. Co) and the support material. Among the typical catalyst carriers (eg. TiO₂, Al₂O₃), graphene and its derivatives have emerged as alternative supports in recent years.² Unfortunately, FTS catalysts deactivate over time. Among the factors contributing to deactivation, excess water reaction conditions are of particular interest, since – under this regime – sintering of catalytic nanoparticles is observed, which is responsible for FT reactor’s reduced lifetime and increased operational cost. The degree of catalyst deactivation increases with increasing water partial pressure caused by high water loads.³ Oxygenates in low concentrations were found to be responsible for increased active metal aggregation, especially under the conditions of high-water concentration with high CO conversion.⁴ Studies remain inconclusive on the role of oxygenates and excess water in the loss of commercial catalyst activity and these issues remain as open questions.⁵

In order to gain a better understanding of the phase behavior of confined wax – water – alcohol mixtures in hydrophilic or hydrophobic environments at reaction conditions, our efforts concentrated on the n-C₂₈ – H₂O mixture at 473.15 K inside pristine graphene (G) and graphene oxide (GO) pores by means of Coarse Grained Molecular Dynamics (CGMD) simulations with MARTINI.⁶ Dodecan-1-ol was used to study the effect of long-chain alcohols on the wax – water mixture inside mesopores. Our simulations show that CG approaches capture the mixture’s phase separation, the component’s diffusivity as a function of the distance from the pore center and that dodecan-1-ol’s preference towards the wax being mostly located at the n-C₂₈ – H₂O interface. CGMD simulations with DFT derived parameters for Co, show that the Co NP does not affect phase separation of the mixture inside the pores. However, extensive coverage by water on the Co NP is observed. Given the experimental difficulties in probing the relevant mechanisms at this scale, our results showcase that CGMD using the MARTINI force field can be employed to study FTS related processes at this level^7,8 and are expected to open new pathways in the investigation of the effect of NPs on catalyst support interfaces in the presence of FTS relevant mixtures.

References

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8. Papavasileiou, K.D., Peristeras, L. D., Bick, A., Economou, I. G., Energy Fuels, , 35, 4313–4332, 2021.