(10d) A Mechanistic Basis for the Effects of Crystallite Size on Light Olefin Selectivity in Methanol to Hydrocarbons Conversion on MFI

The change in selectivity of methanol to hydrocarbons conversion (MTH) with change in MFI crystallite size has been mechanistically linked to the transport restrictions within the crystallite. Three MFI samples with varying diffusion lengths were investigated: SPP MFI (~1 nm), 3DOm-i MFI (~20 nm), and 17-micron MFI (~8500 nm). The light olefin (ethene + propene) selectivity, for the reaction of dimethyl ether at 623 K, increased from 21.8 C% in SPP MFI to 45.8 C% in the 17-micron MFI at 51-59% dimethyl ether conversion. We postulate that as the crystallite size of MFI increases, the intra-crystalline residence time of bulkier hydrocarbons like polymethylbenzenes increases, enabling these molecules to undergo multiple methylation/dealkylation cycles before exiting the crystal. Transport restrictions were also introduced externally in a conventional ZSM-5 sample by silylation using tetraethyl orthosilicate. Silylation treatment partially or completely blocks the pore openings of MFI and increases the residence time of bulkier hydrocarbons inside the crystal, similar to a large crystallite MFI. Three silylated MFI samples were obtained by repeating the treatment 1-3 times. Transport restrictions in these silylated MFI samples were described by adsorption uptake measurements of 2,2-dimethylbutane at 293 K and 20 kPa 2,2-dimethylbutane pressure. The light olefin selectivity, for the reaction of dimethyl ether at 623 K and at 50-56% dimethyl ether conversion, increased from 27.5 C% in conventional MFI, which had lower transport restrictions, to 49.7 C% in the sample that was silylated three times and therefore had higher transport restrictions.