Tuning Bronsted 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 (NH3BE)-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 mol(-1) because the proton of the second site stabilized the deprotonated site across the 6MR. Acid site-pairs across 8MR structures interact very little when the second acid is bare, as residual protons do not prefer to orient within 8MR One location of the second acid stabilized the adsorbed proton without stabilizing the deprotonated state, resulting in a significantly weaker acid. All of these effects are altered when the second site is instead occupied by an adsorbed NH3, which acts as a proxy for strongly bound reactive intermediates and cationic transition states. The strength of the first site is significantly weakened (DPE and NH3 BE increases of >20 kJ mol(-1)) when a second site is NH3-occupied and placed in the 6MR because such structures are too small to effectively solvate NH4+ cations. Acid sites are strengthened, however, when second sites are NH3-occupied and placed across 8MR windows, because they are appropriately sized to solvate the NH4+ cations that simultaneously interact with both deprotonated sites. The alteration of acid strength by acid site proximity therefore depends on the specific arrangement (not merely Al-Al distances), the structural motifs present (such as 6MR structures which allow protons, but not NH4+, to stabilize proximal conjugate base anions), and the status of proximal sites as vacant or occupied, which determines the distances over which cationic-anionic stabilizations of deprotonated sites can take place.