Catalysts for Hydrocarbons
The main objective of the research within the “Catalysis for Hydrocarbons” group is to make contributions to the development of renewable carbon resources for societies energy needs in the transportation sector. Our investigations target the understanding of these processes on the atomic-scale through periodic density functional theory (DFT) and ab initio calculations on cluster models. Here we use higher-level methods (MP2 or DLPNO-CCSD(T)) since common DFT approaches typically underestimate barriers significantly1. The main program packages used are VASP, TURBOMOLE and ORCA. The main topics within the group are the conversion of methanol to hydrocarbons and the targeted functionalization of biomass as well as catalyst deactivation.
Methanol to hydrocarbons
The methanol-to-olefins (MTO) and related processes allow the synthesis of hydrocarbons from biomass via the syngas-methanol route. There is thus an increased interest in a mechanistic understanding of the reactions that control selectivity and deactivation of the employed acidic zeotype catalysts. Another intriguing questions concerns the initiation of the MTO process. While it is accepted that the reactivity is at later stages determined by catalytically active hydrocarbons that constitute the so call hydrocarbon pool (HCP), it remains unclear, how the HCP is established. We tackle this problem by exploring a large number of potential initiation reactions, using accurate MP2 calculations on cluster models in addition to periodic DFT calculations1-2. Our results show that direct C-C bond formation from methanol is indeed a viable initiation mechanism and the most favorable reaction path proceeds via formation of CO from methanol followed by methylation.
We are currently working on kinetic models and are extending our research to investigate the role of olefins3 and aromatic molecules, again employing accurate MP2:DFT calculations.
Alternative approaches to fuels from renewable feed stocks
Another interesting class of products are oxymethylene dimethyl ethers (OMEs), which can be derived from methanol and formaldehyde. OMEs have shown great potential for the use as diesel fuel additives, but challenges for the catalytic process to produce targeted OMEs with the right chain-lengths remain. Most commonly, solid acid catalysts like zeolites are employed for these processes. Here we focus on the reaction of trioxane (TOX) and OME1 as alternative precursors. We find that direct incorporation of TOX is more favorable than its prior decomposition to formaldehyde, thus explaining the observed initial selectivity for OME44.
Another project of the group is catalyst deactivation, in particular through sintering. Here, we are currently focusing on the development of improved models for sintering5. Apart from classical, surface-mediated Ostwald ripening, we are particularly interested in ripening via the vapor phase. This mechanism is believed to be relevant for platinum, where the volatile species PtO2 may be formed under high temperatures and oxidizing conditions. Using mean-field models, we find that this mechanism is indeed realistic and can explain experimental sintering kinetics6. We are also studying particle migration7 as an additional mechanism, as well as the combined effect of these two deactivation mechanisms8.
- Plessow, P. N.; Studt, F., ACS Catal. 2017, 7, 7987-7994.
- Plessow, P. N.; Studt, F., Catal. Lett. 2018, 148, 1246-1253.
- Plessow, P. N.; Studt, F., Catal. Sci. Technol. 2018, 8, 4420-4429.
- Goncalves, T. J.; Arnold, U.; Plessow, P. N.; Studt, F., ACS Catal. 2017, 7, 3615-3621.
- Dietze, E. M.; Plessow, P. N., J. Phys. Chem. C 2018, 122, 11524-11531.
- Plessow, P. N.; Abild-Pedersen, F., ACS Catal. 2016, 6, 7098-7108.
- Li, L.; Plessow, P. N.; Rieger, M.; Sauer, S.; Sanchez-Carrera, R. S.; Schaefer, A.; Abild-Pedersen, F., J. Phys. Chem. C 2017, 121, 4261-4269.
- Dietze, E. M.; Abild-Pedersen, F.; Plessow, P. N., J. Phys. Chem. C 2018, 122, 26563−26569.