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

Abstract

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.

Reactivity of Surface Lewis and Brønsted Acid Sites in Zeolite Catalysis: A Computational Case Study of DME Synthesis Using H-SSZ-13

“…Depending on the temperature and partial pressures, oxygenates such as water, methanol, and dimethyl ether (DME) adsorb at the BAS through the formation of a hydrogen bond. Recent studies also focus on aluminum siting − as well as the effect of different Si/Al ratios. − The reactivity of surface BAS, however, has rarely been studied, although with a crystal size of a few nanometers to micrometers, , zeolite particles as well as 2D zeolites − offer a large surface area. At the surface, substitution of a silicon atom at a single silanol group followed by dehydration leads to a threefold coordinated aluminum atom (a structural motif not present in BAS), which generally acts as a Lewis acid site (LAS, see Scheme ).…”

Ordered Heteroatom Siting Preserved by B/Al Exchange in Zeolites

“…Typically, an occupancy distribution of Al over various T sites is reported. − This inherent disorder evokes potential ambiguities in model assumptions on the local structure and environment of the active site. Both, the Al distribution and the local pore confinement, determine the reactivity and selectivity in zeolite-catalyzed reactions. − A particular focus has been dedicated to confinement effects. − , Additionally, a local Al pairing can alter the catalytic function. , Gounder and coauthors have previously noted that “... higher precision can result from more reliable control of structure...” and that this would transform “... zeolite active site design from an empirical endeavor into a more predictable science founded on validated models.” Davis also emphasized that “... it would be advantageous to be able to not only alter the distribution between pore/cage size areas but create crystallographic site specific synthetic methods” . Here, we present four zeolites, which have Al on defined crystallographic positions.…”

Effect of Aluminum Siting in H-ZSM-5 on Reaction Barriers

“…However, one should also be aware that barriers can vary by 20 kJ/mol between the various T-sites, which translates to differences in rate constants of more than 2 orders of magnitude at relevant temperatures. While this study assumes a high Si/Al ratio and thus only one acid site per unit cell, we note that lower Si/Al ratios will also influence transition-state geometries and energies with a similar magnitude. , …”

“…Computational modeling, mostly based on density functional theory (DFT), has become an integral part of mechanistic studies shedding light on how an acid site and its confinement facilitate specific reactions at the atomic level. , Furthermore, with DFT, reaction barriers can be computed, allowing the search for rate-determining steps and construction of kinetic models. Obviously, the outcome of these computational studies depends on the limitations of DFT and the accuracy with which free energies are obtained − but also on the structural model of the active site employed in the calculations. − …”

“…16,17 Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen D-76344, Germany Felix Studt − Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen D-76344, Germany; Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology, Karlsruhe 76131, Germany; orcid.org/0000-0001-6841-4232 Complete contact information is available at: https://pubs.acs.org/10.1021/acs.jpcc.1c06670…”