(52e) Investigating the Effects of Extra-Framework Aluminum Species in Acidic Chabazite Zeolites Using Protolytic Alkane Activation Kinetics

Brønsted acid (H⁺) sites in zeolites catalyze alkane activation through mechanisms involving carbocationic transition states.¹ The stability of these transition states, and consequently the rates of alkane activation, is influenced by the secondary environment that surrounds H⁺ sites, which comprises the confining inorganic pore² and other H⁺ sites³ or extra-framework aluminum (Al_ex) species^4,5 located nearby. In H-MFI, Al_ex species are proposed to modify H⁺ site environments through Lewis acid-base or dispersive interactions, leading to contrasting proposals on their effects on alkane activation transition state entropic and enthalpic stability.^4,5 Moreover, H⁺ sites in MFI are located in diverse void environments (e.g., smaller straight and sinusoidal channels, and larger channel intersections)⁶ and a variety of proximal Al-Al site configurations⁷, complicating interpretation of kinetic data. Thus, the role of Al_ex species on the intrinsic reactivity of H⁺ sites and its consequences for alkane activation catalysis remain imprecisely understood. Herein, H-CHA zeolites with isolated H⁺ sites are used as model materials (single T-site) to interrogate the influence of Al_ex species on the kinetics of protolytic alkane activation. Measured first-order rate constants (per H⁺) for alkane activation increased with Al_ex fraction, determined from site balances considering total and framework Al content. Measured activation enthalpies and entropies for propane and n-butane activation were lower on samples with higher Al_ex fractions. Furthermore, entropy-enthalpy compensation trends for alkane activation transition states in CHA samples with varying Al_ex fractions resemble those for alkane adsorption⁸, which are predominantly governed by dispersive interactions. This suggests that Al_ex species located near H⁺ sites stabilize alkane activation transition states through dispersive interactions by occluding void space, leading to lower activation barriers and higher rate constants. Our findings reveal effects of Al_ex species on the reactivity of zeolite H⁺ sites and develop mechanistic explanations that predict chemical properties and catalytic reactivity of acid zeolites.