Light olefin selectivity in methanol-to-hydrocarbons conversion on MFI increases with an increase in crystallite size because intra-crystalline residence time of methylbenzenes increases as a consequence of increased transport restrictions, which enables these methylbenzenes to undergo multiple methylation/dealkylation reactions before exiting the crystal. Selectivity toward light olefins, for the reaction of dimethyl ether (DME) at 623 K, increased monotonically from 22% in 2 nm-MFI (∼2 nm crystallites) to 47% in 17 μm-MFI (∼17 μm crystallites) at 46-59% net DME conversion. Transport restrictions were introduced externally in a conventional MFI sample (500 nm-MFI) by single-/multi-cycle silylation using tetraethyl orthosilicate. Light olefin selectivity, for the reaction of DME at 623 K and at 46-59% net DME conversion, increased from 33% in the conventional MFI sample to 49% in a sample that had undergone three silylation treatments. Adsorption uptake measurements of 2,2-dimethylbutane were used to estimate the "effective" crystallite size of the silylated MFI samples. Total light olefin selectivity and ethene/(2-methyl-2-butene + 2-methylbutane) increased monotonically with the effective crystallite size for all zeolite samples used in this study, irrespective of their provenance, thereby suggesting that the mechanistic basis for increase in light olefin selectivity with increasing crystallite size is the enhanced propagation of aromatics-based catalytic cycle.
Bibliographical noteFunding Information:
The authors acknowledge financial support from The Dow Chemical Company and National Science Foundation (CBET 1055846). The authors also acknowledge Prof. Dongxia Liu, University of Maryland, for the synthesis and characterization of 40 nm-MFI and 17 μm-MFI samples, and Ms. Dandan Xu, University of Minnesota, for the synthesis of 2 nm-MFI sample. The authors also acknowledge Mr. Neel Rangnekar, University of Minnesota, for help with the SEM analysis of zeolites, which was carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program.
- Crystallite size
- Ethene selectivity
- Light olefin selectivity