Effect of the Methanol-to-Olefin Conversion on the PFG NMR Self-Diffusivities of Ethane and Ethene in Large-Crystalline SAPO-34

SAPO-34 catalysts obtained after MTO conversion times of 0-30 min were investigated by PFG NMR spectroscopy for studying the self-diffusivities of ethane and ethene. Furthermore, the catalysts were investigated by in situ UV/vis, C-13 MAS NMR, and H-1 MAS NMR spectroscopy, giving insights into the organic deposits and acid sites in different periods of the catalyst lifetime. The ratio of the self-diffusivities of ethene and ethane, D-ethene/D-ethane, used as a measure of the diffusion selectivity, was found to increase to the benefit of the olefin compared to the alkane for higher adsorbate loadings and increasing MTO conversion times. This change of the diffusion selectivity is one of the reasons for the variation of the product selectivity during the catalyst lifetime. Variable-temperature PFG NMR studies gave a significantly larger apparent activation energy for the self-diffusivity of ethane (E-a = 7.6 kJ/mol) compared with ethene (E-a = 4.2 kJ/mol).