Tuning Brønsted Acid Strength by Altering Site Proximity in CHA Framework Zeolites

This study examines how Bronsted acid strengths—as predicted by dispersion-corrected periodic DFT calculations of deprotonation energy (DPE), dehydrogenation energy (DHE), and NH3 binding energy (NH3 BE)—are affected by site proximity in proton-form zeolites and how adsorbates on one acid site alter the strength of nearby acids. Protons can bind to four distinct O atoms around the single crystallographically unique T-site of CHA, and all such locations were examined as bare and NH3-occupied sites. Protons prefer to bind to O1 atoms and orient within the plane of six-membered-ring (6MR) structures of CHA. NH4+ cations show a strong preference for binding in 8MR windows; 6MR structures are too small to solvate them. These preferences govern proximity effects on acid strength, studied here by probing the strength of a Bronsted acid site while a second site is placed in 23 locations separated by 1–3 T-sites. Placing a second acid in the 6MR of CHA decreased DPE and NH3 BE values for the first site by >10 kJ m...