H-ZSM-5 zeolite model crystals: Structure-diffusion-activity relationship in methanol-to-olefins catalysis

- Defect-free large ZSM-5 model crystals were synthesized.
- These large ZSM-5 crystals proved to be highly competitive MTP-catalysts.
- PFG-NMR studies highlighted peculiar diffusion phenomena over these crystals.
- The impact of molecular diffusion during MTP-catalysis has been modelled.

Large ZSM-5 zeolite crystals synthesized in fluoride medium show an astonishing activity, stability as well as selectivity towards light olefins in the Methanol-To-Olefins (MTO) reaction. By proper control of the synthesis parameters, ZSM-5 single crystals of unprecedented high quality are produced. The absence of usually uncontrollable variables such as structural defects, external non selective surface acid sites and extra-framework aluminium (EFAl) species was evidenced by SEM, HRTEM, CO-FTIR, 27Al and 19F MAS-NMR, Rietveld structure refinement and N2- and Ar-gas sorption measurements. Interestingly, diffusivity evaluation of different probe molecules (toluene, benzene and neopentane) has been carried out with PFG-NMR, allowing casting light on an interesting structure-diffusion-activity relationship. A “levitation” effect could be experimentally demonstrated and its impact on catalysis is highlighted in a rationalization attempt: Maxwell-Boltzmann based diffusion models properly predict product distributions for this counter-intuitively outstanding Methanol-To-Propylene (MTP) catalyst.

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