(228f) Spatial Proximity Influence on Adsorbate Thermodynamics over Zeolite Surfaces

In designing solid acid catalysts, understanding adsorption energetics informs catalyst optimization for different chemistries. For Brønsted catalyzed reactions the location and proximity of framework Al, influence the adsorbate energetics, with varying catalytic consequence. Using strongly bound intermediates like alkylamines adsorbed on a Brønsted acid site, representative of catalytically relevant reaction intermediates and transition states, we investigate the influence of proximate sites on multiple alkylamine energetics at equilibrium. Leveraging the concept of adsorption assisted desorption through the co-adsorption of multiple strongly bound adsorbates to attain adsorption equilibrium. We first establish the adsorption energetics of these alkylamines without the influence of proximate sites and measure the overall adsorption thermodynamics of these strongly bound species over isolated sites. Comparing a homologous family of sec-alkylamines (C3-C5), we find a fixed contribution to both the enthalpy (18 kJ mol^-1 CH₂^-1) and entropy of adsorption (25 J mol^-1 K^-1 CH₂^-1) per methylene unit on isolated sites (Si/Al=140). We use the adsorbate thermodynamics methodology, to investigate the interplay of proximate sites for these alkylamines. Differences in the relative adsorption energetics of chemisorbed alkylamines diminish with increasing Al content, which is attributed to increasing interactions between neighboring adsorbates that are sufficiently close to form Al pairs. We demonstrate this concept of lateral interactions between neighboring alkylammonium species by manipulating Al content to control the fraction of Al pairs in H-MFI, revealing a linear trend between relative adsorption energetics and the fraction of Al pairs onto which alkylamines adsorb. We also show that for adsorbates with non-existent difference in their energetics, lateral interactions have little to no influence on the adsorbate thermodynamics (Figure 1).