Converting methanol to longer hydrocarbons using zeolite catalysts is a great way to selectively make hydrocarbons for a range of applications. Understanding this selective conversion and the catalysis process more generally has therefore been a popular area of study for scientists in both academia and industry.
The diffusion of the gas molecules through the zeolite pores is crucial to the catalyst's function. By understanding this diffusion, and what impacts it, researchers can gain an insight into how to make a catalyst more efficient and last longer without degrading.
As part of a long-standing collaboration with the chemicals company Johnson Matthey, scientists from ISIS, the universities of Glasgow and Aberdeen and the UK Catalysis Hub (based at the Research Complex at Harwell) carried out neutron experiments to understand a common degradation process in a zeolite catalyst. In collaboration with STFC Scientific Computing, they also ran molecular dynamics (MD) simulations of the system that support their experimental results.
Scientific Computing supported the researchers in developing a robust software infrastructure to facilitate setting up complex structures as models for MD simulations. As a result they were able to create a defect-free model to compare with their experimental observations and measurements.
The work demonstrates the use of molecular simulation techniques to assess information that is otherwise difficult to derive experimentally – namely, to elucidate the underlying atomic structures and motions within the complex zeolite network structures.
In their study, published in the Journal of Chemical Physics, they focused on the zeolite ZSM‑5. As this catalyst is so good at attracting methanol and converting it, it is difficult to study the movement of methanol molecules themselves between the pores. To get around this this, the researchers used methane instead, as it does not have the -OH group that causes the methanol to strongly interact with the catalyst.
They studied how the molecules' movements changed when the catalyst had succumbed to a common form of degradation: the build-up of carbon deposits, known as coke. Using quasielastic neutron scattering, they were able to look at the motions of the methane molecules inside the catalyst pores for both fresh samples and those that had coke built up on their surface.
Despite the challenge of distinguishing the movement of the methane molecules from the movement of the coke, they were able to determine how the coke formation impacts the diffusion of the methane. They found that the methane molecules were so mobile, that the measurements could only take place at relatively low temperatures. Therefore, they suggest the next step would be to extend their study to use ethane, as its larger size would make it less mobile and therefore enable experiments to take place at catalytically relevant temperatures.
Further information
The full paper can be found online at DOI: 10.1063/5.0123434
