Catalysis: Locating coke

Angew. Chem. Int. Ed. http://doi.org/f3qtc5 (2016)

Converting methanol into hydrocarbons is an important step in pathways that use non-crude oil carbon sources such as natural gas, coal, biomass and CO₂ as feedstocks for synthesis of liquid transportation fuels. While zeolites — porous aluminosilicate materials — such as ZSM-5 are effective catalysts for the reaction, build-up of carbonaceous deposits (coke) as the reaction proceeds leads to their deactivation. Therefore, a deeper understanding of the properties at the nanoscale that give rise to the deposition of coke is needed to design improved catalysts. Now, Simon Bare, Bert Weckhuysen and colleagues in the US and the Netherlands use atom probe tomography (APT) with subnanometre resolution to investigate how fine-scale variations in catalyst composition affect coke deposition in ZSM-5.

The researchers first run the methanol-to-hydrocarbons reaction for 90 minutes at 350 °C, with ¹³C-labelled methanol, to partially deactivate a single crystal of ZSM-5 before using APT to spatially resolve in 3D the carbon distribution in the catalyst. Using cluster analysis, they find that on average the carbon clusters are composed of about 30–60 atoms. The amount of coke correlates with small fluctuations in the Al concentration, and clusters form preferentially in regions of increased Al content and, therefore, regions of increased Brønsted acid site density. This work highlights that the Si/Al ratio in the catalyst must be closely controlled on the nanoscale to regulate coke production.