(344u) Understanding the Entropic Contributions of Irreversibly Bound Adsorbates on Zeolite Surfaces

The design of solid acid catalysts has greatly benefitted from understanding the energetics (enthalpy and entropy) of adsorbates, 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 the adsorption, particularly for strongly bound species representative of reaction intermediates and transition states. This is due to the fact that traditional methods used to measure an adsorbate’s entropy, require the adsorption/desorption process to equilibrate. Strongly bound intermediate like alkylamines adsorbed on a Bronsted acid site however do not equilibrate on relevant timescales, given their negligible rate of desorption, which limits the applicability of existing methods. To overcome this issue, we apply the concept of adsorption assisted desorption by investigating the co-adsorption of multiple strongly bound adsorbates. While one strong adsorbate does not equilibrate due to limited desorption, two or more strongly bound adsorbate can continuously displace one another from an active site and attain adsorption/desorption equilibrium. Applying this concept to alkylamine adsorption on Brønsted acid sites in H-ZSM5 (Si/Al = 140), we demonstrate for the first time the experimental ability to measure the overall adsorption thermodynamics of strongly bound species. Despite the relatively strong adsorption, we find that the Langmuir adsorption isotherm can capture the adsorption of multiple alkylamines varying in size and basicity over a wide range of conditions. By comparing a homologous family of primary alkylamines, varying only in alkyl chain length, we find a fixed contribution to both adsorption enthalpy and entropy per methylene unit (Figure 1).