Feature ArticleRelating coke formation and characteristics to deactivation of ZSM-5 zeolite in methanol to gasoline conversion

- Coke characteristics and structure differ over nanocrystal and microcrystal ZSM-5.
- The rate of internal coke formation determines catalyst resistance to deactivation.
- Microcrystal ZSM-5 rapidly deactivates due to dominantly internal coke formation.
- Porous structured external coke has little adverse impact on catalyst performance.
- Nanocrystal ZSM-5 deactivates slowly due to absence of significant internal coke.

Two ZSM-5 catalysts, differing only in their crystal size, viz, nanocrystal at ∼ 100 nm and microcrystal at 13 μm, respectively, were synthesised and tested in methanol to gasoline (MTG) conversion, with a focus on the formation and characteristics of coke deposits. Over time periods when methanol conversion decreased to 50%, herein termed as the service lifespan of the catalyst, the nanocrystal catalyst incurred 31.1 wt% coke deposition, while the microcrystal counterpart had 14.1 wt% coke. The nanocrystal catalyst showed a service lifespan almost seven times longer than the microcrystal catalyst. The difference in the catalytic service lifespans was examined in terms of the rate of formation of internal coke and structural properties of external coke, as determined using nitrogen physisorption, TGA and TEM. It was found that the internal coke was quickly formed in the microcrystal catalyst leading to rapid coverage of the active sites and blockage of the pores, resulting in fast deactivation. In contrast, coke formed preferentially on the external surface in the case of the nanocrystal catalyst. This external coke was of porous graphitic structures, and thus was not detrimental to the catalytic performance. The coke fouled nanocrystal catalyst was regenerated and the activity of the regenerated catalyst was evaluated under the same reaction conditions. An increase in catalytic service lifespan compared to the pristine nanocrystal catalyst was observed, due to the effect of decreased Al concentration on the catalytic performance.

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