(107c) Synthetic and Post-Synthetic Methods to Alter Al-Al Site Pair Arrangements in Zeolites

The arrangement of Al heteroatoms in zeolite frameworks is becoming increasingly recognized to influence the structure and stability of reactive intermediates and transition states in Brønsted acid catalysis, and the speciation of transition metal ions and complexes that are active sites for redox catalysis. Here, we will discuss recent progress using both synthetic and post-synthetic methods to systematically alter the arrangement of different Al-Al site pair ensembles in zeolites, with a focus on the CHA, MFI and MEL frameworks. Distinct populations of Al-Al site pair ensembles are quantified by complementary experimental characterization techniques, including divalent cation titration [1,2] and infrared spectra of H-form and cation-exchanged zeolites [3], and corroborated by density functional theory (DFT) assessments. For example, Co²⁺ is used to titrate Al-Al site pairs in second-nearest-neighbor (2NN) and third-nearest-neighbor (3NN) configurations in 6-membered rings (6-MR) of CHA [1], and we will discuss recent progress to quantify different Al-Al site pair ensembles in CHA and other frameworks. Finally, we will discuss the influence of Al-Al site proximity on turnover rates for Brønsted acid-catalyzed reactions, including methanol dehydration and alkane activation. This presentation will highlight how developing methods to more precisely quantify distinct Al-Al site pair ensembles in zeolites helps to clarify their consequences for catalysis, and to design synthetic and post-synthetic routes to predictably alter active site arrangements, in order to more fully realize the potential catalytic diversity among aluminosilicate zeolites.

[1] J. Am. Chem. Soc. 142 (2020) 4807.

[2] Chem. Mater. 32 (2020) 9277.

[3] J. Catal. 395 (2021) 210.