Trends in the Reactivity of Proximate Aluminum Sites in H‑SSZ-13

The reactivity of acidic zeolites
with close Al-pairs was investigated
using density functional theory. Different spatial distances and relative
orientations of the two Al atoms were considered. Adsorption of methanol
and ammonia was computed and shown to be correlated. Additionally,
reaction barriers for the stepwise and concerted mechanism of the
dehydration of methanol to dimethyl ether were computed. These barriers
were found to correlate well with the adsorption energy of ammonia
as a descriptor. This correlation reduces some of the stronger deviations
observed in apparent activation barriers, when computing intrinsic
barriers referenced to an adsorbed species. Excluding nearest neighbors
Al-pairs that violate Löwenstein’s rule, the effect
of different Al-pair distributions is found to influence apparent
activation barriers typically by less than 20 kJ/mol with a mean absolute
deviation of 7 kJ/mol.