(406f) Acceleration of Methanol Dehydration in H-SSZ-13 By Acid Site Proximity

CHA framework zeolites with distinct Al site distribution are synthesized without altering the Si/Al ratio by varying the ratio of organic (TMAda⁺) and inorganic (Na⁺) structure directing agents, confirmed by an increase in Co²⁺ uptakes which selectively titrate Al pairs sharing 6-member rings (6-MR) (‘paired’ sites) [1]. Methanol dehydration to form dimethyl ether (DME) and water on these paired sites occurs more rapidly (normalized per proton) than on isolated sites [2]. The observed first- and zero-order rate constants increase with the fraction of Al in paired arrangements (as counted by Co²⁺ titration), suggesting that paired sites should react methanol with effective free energy barriers that are 5–7 kJ mol^−1 lower than isolated sites. Paired sites are stronger acids than isolated sites, as measured by deprotonation energy, a theoretical metric of acid strength which correlates with Brønsted acid reactivity [3]. Methanol dehydration can occur by two routes: an associative route and a dissociative route. In the associative route, two methanol molecules react in one step to form DME and water; in the dissociative route, methanol reacts with the Brønsted acid site to form water and a surface methyl group, the latter of which then reacts with a second methanol to form DME [4,5]. Density functional theory (DFT) calculations on CHA framework zeolites indicate that the associative route is preferred over the dissociate route on isolated sites, with overall barriers of 118 and 122 kJ mol^−1, respectively. These reactions were also examined on Al site pairs arranged across the 6-MR of CHA; this showed that site-pairs interact during catalysis via adsorbed methanol to reduce overall reaction barriers for DME formation. A proximal acid site positioned two Si T-sites away from the reacting site, for example, reduces the free energy barrier for the dissociative route by 8 kJ mol^−1 (to 114 kJ mol^−1) while increasing the barrier for the associative route by 7 kJ mol^−1 (to 125 kJ mol^−1) when a methanol is present on the proximal site. The decrease in the barrier of the dissociative route is caused by methanol on the second site stabilizing the conjugate base of the reacting site through H-bonding between both sites, distributing charge and increasing acid strength. For the associative route, this interaction of sites via methanol still occurs, but is offset by crowding at the reactive site (as there are now three proximal methanol molecules) resulting in a barrier increase. These data indicate a shift from the associative to dissociative mechanism on paired sites and a decrease in the overall DME formation barrier of 4 kJ mol^−1, both of which are consistent with previous kinetic studies and in situ IR studies which show that surface methyls are present and proportional to the fraction of paired Al sites [2]. These dipole-charge interactions between sites may explain effects of Si/Al ratio on kinetic rates and can inform future synthesis procedures to optimize catalyst design by providing an additional method for tuning zeolite reactivity.

References

[1] Di Iorio, J. R.; Gounder, R., Chem. Mater., 2016, 28, 2236–2247

[2] Di Iorio, J. R.; Nimlos, C. T.; Gounder, R., ACS Catal., 2017, 7, 6663–6674

[3] Nystrom, S.; Hoffman, A.; Hibbitts, D., ACS Catal., 2018, 8, 7842–7860

[4] Jones, A. J.; Iglesia, E., Angew. Chem. Int. Ed., 2014, 53, 12177–12181

[5] Carr, R. T.; Neurock, M.; Iglesia, E., J. Catal., 2011, 278, 78–93