(594c) Ion Exchange and Chemical Titration for Aluminosilicate Paired Site Quantification

In designing solid acid catalysts, understanding adsorption energetics informs catalyst optimization for different chemistries. The designed catalyst influences the energetics of the transition state (ΔG^‡), the product selectivity trend is dependent on the extent of stabilization of these transition states. For Brønsted catalyzed reactions, turnover rates and selectivity of hydrocarbon reactions have since been recognized to depend on the location of framework Al, that are compensated by H⁺ sites within a given zeolite framework. With similar acid strength, the shape of the zeolite micropores influences the energies of the transition states and confined reaction intermediates through van der Waals interactions. The location and proximity of this framework Al, influence the adsorbate energetics, with varying catalytic consequence. We measure proximate sites on the H-MFI framework, by exchanging parent form (H-MFI), with divalent Co²⁺. Co-MFI residual H⁺ site count is compared with H-MFI Brønsted site count to accurately estimate proximate sites. We also establish through Py-IR that after Co²⁺ exchanges, the alkylamine used for site count accesses exclusively residual H⁺ site with no Lewis site interference. The effectiveness and extent of metal-cation (Na⁺, Co²⁺) exchange was quantitatively understood through alkylamine Hofmann elimination, which was demonstrated to be a selective probe of protonic sites on the surfaces of distinct forms of ZSM-5. By probing the residual protons of cobalt exchanged ZSM-5 through alkylamine Hofmann elimination, the fraction of Al-pairs was readily determined regardless of Al content (Figure 1). Ultimately, the fraction of Al pairs in the MFI framework was found to follow a random statistical distribution, across a wide range of Al content (Si/Al 11.5-240). We envision the applicability of this technique to measure Al-pairs in aluminosilicate samples, due to its simplicity involving the chemical titration of cation-exchanged zeolites. Establishing the effect of proximate sites for zeolite catalyzed reactions.