(504b) First Principles Modeling of Extended Solvent Structures in Defected Microporous Materials and Their Influence on the Kinetics of Lewis Acid Site Speciation

Authors: 
Bukowski, B. C., Purdue University
Bates, J. S., Purdue University
Gounder, R., Purdue University
Greeley, J., Purdue University
Microporous zeolites have found wide success in the petroleum industry as selective catalysts for refining fuels. In addition to Brønsted acid zeolites, which mediate chemical reactions through protons, Lewis acid zeolites have been synthesized with framework-substituted metal cations such as Sn substituted zeolite Beta (Sn-Beta). Sn-Beta has been found to selectively catalyze oxygenate chemistries such as glucose isomerization,1 in which Sn atoms within the framework generate catalytically active sites. Precise determination of the structure and speciation of the active Sn site is, however, complicated by the propensity for Sn atoms to adopt one of many possible “open” configurations that are characterized by the presence of a hydroxy ligand and framework silanol. Further, in addition to imparting catalytic active sites, Sn substitution changes the microporous voids in Beta from hydrophobic to hydrophilic in character by addition of polar binding sites for solvent. This transition introduces an additional parameter for catalyst design based on solvent interactions with reactive intermediates. While the structure of water has been described previously in defect-free zeolites2,3, we provide the first detailed analysis using ab-initio­ molecular dynamics (AIMD) of water structure and thermodynamics at defects relevant to Lewis acid zeolites. The size and degree of hydrogen bonding of water structures at different chemical potentials and different defect types is considered and used to construct rational solvent models.

Experimental IR spectroscopy is used to observe the changes in hydrogen bond networks in low-defect and high-defect materials compared to those calculated with AIMD. We find that defect-free materials have low water uptakes at ambient conditions, but Sn sites nucleate water clusters. Silanol nests, formed through silicon vacancies introduced during synthesis, generate large extended water networks. The degree of clustering and transience of hydrogen bonding is analyzed using a graph theory approach to define membership rules for water clusters and extended networks. These results are then used to study the dissociation of water to form open Sn sites and study the role of liquid water on the relative populations of open and closed sites. Control of the relative populations of open and closed Sn sites, as well as the structure of the surrounding water network, has direct implications on the selectivity and rate of reactions catalyzed in microporous materials.

References:

(1) Bermejo-Deval, R., et al. PNAS. 109, 9727-9732 (2012).

(2) Demontis, P., et al. Phys. Chem. B. 107, 4426-4436 (2003).

(3) Zhou, T., et al. Phys. Chem. C. 121, 22015-22024 (2017).