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Published February 5, 2016 | Supplemental Material + Published
Journal Article Open

Effect of Heteroatom Concentration in SSZ-13 on the Methanol-to-Olefins Reaction

Abstract

SSZ-13 materials have been synthesized with varying amounts of Al to produce samples with different concentrations of Brønsted acid sites, and consequently, these SSZ-13 materials contain increasing numbers of paired Al heteroatoms with increasing Al content. These materials were then characterized and tested as catalysts for the methanol-to-olefins (MTO) reaction at 400 °C and 100% methanol conversion under atmospheric pressure. A SAPO-34 sample was also synthesized and tested for comparison. SSZ-13 materials exhibited significant differences in MTO reactivity as Si/Al ratios varied. Reduced Al content (higher Si/Al ratio) and, consequently, fewer paired Al sites led to more stable light olefin selectivities, with a reduced initial transient period, lower initial propane selectivities, and longer catalyst lifetime. To further support the importance of paired Al sites in the formation of propane during this initial transient period, a series of experiments was conducted wherein an H-SSZ-13 sample was exchanged with Cu^(2+), steamed, and then back-exchanged to the H form. The H-SSZ-13 sample exhibited high initial propane selectivity, while the steamed H-SSZ-13, the Cu^(2+)-exchanged SSZ-13 sample, and the steamed Cu-SSZ-13 sample did not, as expected since steaming selectively removes paired Al sites and Cu^(2+) exchanges onto these sites. However, when it was back-exchanged to the proton form, the steamed Cu-SSZ-13 sample still exhibited the high initial alkane selectivity and transient period typical of the higher Al content materials. This is attributed to protection of paired Al sites during steaming via the Cu^(2+) cation. Post-reaction coke analyses reveal that the degree of methylation for each aromatic species increases with increasing Si/Al in SSZ-13. Further, SAPO-34 produces more polycyclic species than SSZ-13 samples. From these data, the paired Al site content appears to be correlated with both MTO reaction behavior and coke species formation in SSZ-13 samples.

Additional Information

© 2015 American Chemical Society. This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Received: July 10, 2015; Revised: December 11, 2015; Published: December 22, 2015. The authors thank The Dow Chemical Company for financial support of this project. The authors also thank Cassie Fhaner for analytical work associated with the GC-MS data. The authors declare no competing financial interest.

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Published - acscatal_2E5b01450.pdf

Supplemental Material - cs5b01450_si_001.pdf

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Additional details

Created:
August 22, 2023
Modified:
October 17, 2023