(641d) Understanding the Adsorption Thermodynamics of Irreversibly Bound Adsorbates on Zeolite Surfaces

In designing solid acid catalysts, adsorption energetic studies act as tools to inform the optimisation of catalyst for different chemistries, where the relative stability of surface species dictates the overall catalytic turnover frequency. While enthalpic contributions are relatively well understood, limited information exists for entropic contributions to adsorption. Particularly for chemisorbed species, representative of reaction intermediates and transition states. This is due to the fact that existing methods to measure an adsorbate’s entropy, require the adsorption/desorption process to equilibrate Strongly bound intermediates like alkylamines adsorbed on a Brønsted acid site, however, do not equilibrate on relevant timescales, limiting the applicability of existing methods. To overcome this issue, we leverage the concept of adsorption assisted desorption through the co-adsorption of multiple strongly bound adsorbates to attain adsorption equilibrium. Applying this concept to alkylamine adsorption on Brønsted acid sites in H-MFI, we demonstrate for the first time the experimental ability to measure the overall adsorption thermodynamics of strongly bound species. Comparing a homologous family of sec-alkylamines (C3-C5), we find a fixed contribution to both the enthalpy and entropy of adsorption per methylene unit on isolated sites (Si/Al=140). Terminal adsorption of n-alkylamines is favored when compared with the central adsorption of sec-alkylamines. Differences in the relative adsorption energetics of chemisorbed alkylamines diminish with increasing Al content, which we propose to be the result of increasing interactions between neighbouring adsorbates that are sufficiently close to form Al pairs. We demonstrate this concept of lateral interactions between neighbouring alkylammonium species by manipulating Al content to control the fraction of Al pairs in H-MFI, revealing a linear trend between the measured relative adsorption energetics and fraction of Al pairs onto which alkylamines adsorb (Figure 1). This affords quantitative understanding of adsorption selectivity for these transition-state like molecules, which will ultimately dictate product selectivity under catalytic conditions.