Paired Site Characterization in Aluminosilicate Zeolites through Limited Alkylammonium Stability

Abstract

The spatial distribution of tetrahedrally coordinated aluminum incorporated into a mordenite 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 the 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 that of 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 a 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 Si/Al values of 140 and 11.5, respectively. Experimentally measured Al pairing was in excellent quantitative agreement with a binomial distribution, suggesting random incorporation of Al into the tetrahedral sites of ZSM-5 at next-nearest-neighboring positions.

Unravelling Irreversible Adsorbate Thermodynamics through Adsorption-Assisted Desorption

“…Alkene peaks corresponding to the respective alkylamines were detected. Based on our prior measurements, the Brønsted acid site density of H-ZSM-5 (Si/Al = 140) is taken to be 100 μmol H + g –1 ( n BAS ), with a 6% fraction of framework-incorporated Al that are next-nearest neighbors (paired) . Use of a high Si/Al ratio yields an H-ZSM-5 with mostly isolated Brønsted acid sites, where the opportunity to investigate the adsorption energetics of relatively isolated adsorbates was afforded.…”

“…Alkylamines have long served as powerful probe molecules for the characterization and design of porous materials, , proving especially useful for characterizing the density, , identity, location, , and spatial proximity of acid sites in heteroatom-substituted zeolites. Upon adsorption, alkylamines additionally influence the catalytic activity and selectivity of zeolites through postsynthetic modification of the active site environment in zeolite pores. , Underpinning the utility of alkylamines on zeolite surfaces is the formation of a chemisorbed alkylammonium ion upon adsorption onto a Brønsted acidic site, the stability of which is governed by both adsorbate enthalpy and entropy, with the role of entropy underscored at higher temperatures .…”

“…Alkylamine coverages were governed by relative free energies of adsorption and, by extension, their relative enthalpies and entropies of adsorption. By controlling the zeolite Al content (Si/Al = 140) to expose mostly isolated Brønsted acid sites, , differences in the entropy and enthalpy of isolated alkylammonium adsorbates in the pore of H-ZSM-5 were measured, where a fixed contribution to surface enthalpy and entropy per methylene unit was observed for C3–C5 s -alkylamines.…”