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<sup>2+</sup>, 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<sup>2+</sup>-exchanged SSZ-13 sample,
and the steamed Cu-SSZ-13 sample did not, as expected since steaming
selectively removes paired Al sites and Cu<sup>2+</sup> 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<sup>2+</sup> 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
