Siting of Brønsted acid sites (BAS) in microporous zeolites has significant ramifications on reactivity, selectivity, and stability, especially in systems involving diffusionally hindered molecules due to competing reactions on external and internal BAS. These kinetic phenomena can be deconvoluted in part by quantifying fractions of BAS that reside within micropores and on crystallite surfaces to enable separate treatments of the sites. However, techniques commonly utilized to interrogate BAS siting such as in situ 2,6-di-tert-butylpyridine dosing with FTIR are only applicable to small- and medium-pore zeolites and are further precluded by low external BAS densities that approach instrument sensitivity limits. Here, reaction-based techniques are utilized to extract densities of surface BAS (SBAS) in microporous MFI zeolites. Specifically, Al-normalized zero-order rate constants for alkylation reactions with 1,3,5-trimethylbenzene and dibenzyl ether were extracted and mapped to Al-normalized SBAS densities utilizing published linear correlations. Despite similar average crystallite sizes, SBAS densities did not trend with total aluminum content, demonstrating that aluminum siting cannot necessarily assumed to be uniform in MFI zeolites. To extend reaction-based formalisms for quantification of SBAS densities to large-pore zeolites, self-etherification reactions were performed with 1‑pyrenemethanol, which cannot access BEA or FAU micropores. Al-normalized self-etherification rate constants trended linearly with SBAS densities in microporous MFI zeolites, thus enabling extraction of SBAS densities for MOR, BEA, and FAU frameworks. SBAS values extracted via self-etherification reactions were corroborated by temperature-programmed reactions with bulky 2,2-diphenylethylamine molecules that undergo Hofmann eliminations on SBAS. The evaluations of BAS siting presented herein can be applied to small-, medium-, and large-pore zeolites, and thus, through deconvolution of chemistries on surfaces and in micropores, inform mechanistic phenomena that govern rates and selectivities for a myriad of zeolitic catalytic systems including polyolefin (hydro)cracking and BTX methylation.
