Philipp N. Plessow

Theoretical Insights into the Effect of the Framework on the Initiation Mechanism of the MTO Process

In this contribution, we investigate the initiation mechanism of the methanol-to-olefins process for the different zeotype materials, H-SSZ-13, H-ZSM-5, H-BEA and H-SAPO-34 using density functional theory. While the energetics differ between these materials, variations are systematic so that the relative ordering of the barriers remains the same. We hence predict that the initiation mechanism follows an identical path in all materials with similar rate-limiting steps. We show that the observed trends that have been found for the reaction barriers can be explained by differences in acidity and van-der-Waals interactions of the materials.Graphical Abstract

Kinetic Monte Carlo Model for Gas Phase Diffusion in Nanoscopic Systems

Transport of atoms and molecules via the gas phase plays an important role in many processes in heterogeneous catalysis. Macroscopic diffusion, for example, in reactors, is typically modeled with continuum models. Much smaller length scales are involved if diffusion occurs between nanoparticles. One such example is a sintering mechanism, where volatile PtO2 mediates mass transfer between Pt particles. We developed a kinetic Monte Carlo model that explicitly simulates the kinetics of single atoms or molecules in the gas phase that result from collisions with a background gas. This model accurately reproduces ideal gas properties such as the diffusion constant. In model applications, we study gas-phase-mediated mass transfer as a function of the distance between the involved surfaces. If these distances are within the mean free path, typically a micrometer or lower, continuum models based on Fick’s laws deviate from the explicit simulation. This can be explained by the low number of collisions that occur if...

Olefin methylation and cracking reactions in H-SSZ-13 investigated with ab initio and DFT calculations

The olefin cycle of the methanol-to-olefins process is investigated for the zeolite H-SSZ-13 using periodic, van-der-Waals corrected DFT calculations, together with MP2 corrections derived from cluster models, which are essential for accurate barriers. The two main reactions, olefin methylation and cracking are systematically investigated for different olefin isomers up to C9. The barrier for cracking depends sensitively on the involved cationic intermediates. The most favorable cracking reactions involve tertiary cations, in particular the t-butyl cation that leads to the formation of isobutene along with another olefin. The transition state for olefin methylation is mainly influenced by van-der-Waals interactions and is therefore stabilized for larger olefins.