____________________________________________________________________________ Lawal & Abdelrahman Page 1 Paired Site Characterization in Aluminosilicate Zeolites through Limited Alkylammonium Stability Ajibola Lawal † and Omar A. Abdelrahman †* † Department of Chemical Engineering, University of Massachusetts Amherst, 686 N. Pleasant Street, Amherst, MA 01003, USA *Corresponding Author: abdel@umass.edu 1. Introduction. The use of heteroatom-substituted zeolites as catalysts, 1-5 adsorbents, 6-10 and membranes 11-15 has provided a valuable platform to impact various parts of the chemical industry. In particular, aluminosilicate zeolites, resulting from the incorporation of an Aluminum atom into a tetrahedrally coordinated position within the otherwise siliceous zeolite framework, have been predominantly used. Incorporation of Aluminum into the framework leads to a charge imbalance, reflected in a negatively charged oxygen bridging the gap between Si and Al, and can be balanced by a large number of cations depending on the desired application. Monovalent metal cations like sodium (Na + ) and lithium (Li + ) afford zeolites are desirable properties for the adsorption-based separation of xylene mixtures 18 and propane/propylene, 19 respectively. A proton can also be leveraged as the balancing cation, creating a Brønsted acidic bridging hydroxyl, that serves as the catalytic backbone for multiple industrial chemistries. 20- 24 As a result, a large body of research has been devoted to the study of aluminosilicate zeolites, seeking to understand their catalytic properties through kinetic investigations, 25-29 material characterization, 30-32 and computational calculations. 33-37 Early research in the field centered on the quantification of the protonic Brønsted acid sites of aluminosilicate zeolites; temperature Abstract. The spatial distribution of tetrahedrally coordinated aluminum incorporated into a mordenite framework inverse framework zeolite was investigated through the proton site-selective probe chemistry of alkylamine Hofmann elimination. Protons associated with paired Al sites were exclusively exchanged with divalent cobalt, while isolated protons remained intact and readily quantified through Hofmann elimination of adsorbed alkylammonium species. Through a combination of temperature-programmed methods and spectroscopic characterization, the stability of the adsorbed alkylammonium was found to be crucial to the accurate estimation of isolated protons. Only alkylammonium species undergoing Hofmann elimination through a primary carbocation were found to exclusively probe isolated protons on the surface of cobalt-exchanged ZSM-5. More stable secondary and tertiary carbocations typically used in the characterization of acid sites, resulted in significant overestimates of protonic site densities. The result is contrary to purely protonic zeolites, where alkylammonium stability is inconsequential to the quantification of the acid sites they are coordinated with. Consequently, n-propylamine was found to be a suitable reactive probe molecule for the quantification of isolated protonic sites on divalent metal- cation zeolite surfaces, forming n-propylammonium upon adsorption. Ensuring accurate probing of isolated protons on the surface of metal cation exchanged zeolites was also demonstrated to require control of ion-exchange conditions and sufficient extent of zeolite exposure to the adsorbing n- propylamine. The fraction of paired Al sites in ZSM-5 was measured to systematically increase with Al content, ranging between 6 and 63 % across a Si/Al of 140 and 11.5, respectively. Experimentally measured Al pairing was in excellent quantitative agreement with a binomial distribution, suggesting a random incorporation of Al into the tetrahedral sites of ZSM-5 at next-nearest-neighboring positions.