The ability to vary the atomic arrangement of Al atoms in zeolite frameworks enables influencing the catalytic behavior of acid and metal-exchanged zeolites, and one of the only options to do so for materials of fixed bulk structure and composition. Framework Al arrangements can be varied in a systematic manner using mixtures of structure-directing agent (SDA) cations of different molecular structure and charge density to guide the assembly of amorphous heteroatom sources into crystalline zeolite phases. The dominant SDA-framework interactions that influence Al arrangement are more straightforward to deduce from studies of chabazite (CHA) zeolites, because the presence of a single crystallographically-unique tetrahedral site (T-site) avoids the added complexities of needing to account for Al siting among distinct T-site locations and enables more faithful comparison of experimental data to theoretical models. Experiment and theory are combined to investigate how different organic and inorganic SDA cations either cooperate or compete for occupancy within the microporous voids and rings of CHA zeolites, and the resulting consequences for biasing arrangements of Al-Al pairs in different ensembles (Al−O(−Si−O)x−Al, x = 1-3) within 4-, 6- and 8-membered rings. These different Al-Al pair ensembles are identified and quantified using a complementary suite of experimental titration and spectroscopic methods and DFT calculations. We will also discuss how this combined experimental and theoretical approach can be extended to synthesize and interrogate lower symmetry zeolite frameworks (e.g., MFI) with varying Al arrangements, and the resulting consequences for the acid-catalyzed dehydration of methanol to dimethyl ether. These findings also identify opportunities to exploit using mixtures of SDAs to more efficiently utilize organic SDAs during the crystallization of zeolites with different Al arrangements.

Influence of structure-directing agents on aluminum siting and proximity in zeolites and consequences for acid catalysis

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.