The design of solid acid catalysts has greatly benefitted from understanding the energetics of adsorbates (enthalpy and entropy), 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 strongly bound 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 given their negligible rate of desorption, 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. While one strongly bound adsorbate does not equilibrate due to limited desorption, two or more strongly bound adsorbates 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-ZSM-5 (Si/Al = 140), we demonstrate for the first time the experimental ability to measure adsorption thermodynamics of strongly bound species. Despite the relatively strong adsorption, we find that the Langmuir isotherm can capture the adsorption of multiple alkylamines varying in size and basicity over a range of conditions. Comparing a homologous family of sec-alkylamines, varying only in alkyl chain length (C3-C5), we find a fixed contribution to both the enthalpy and entropy of adsorption per methylene unit. Terminal adsorption of n-alkylamines is favored on H-ZSM-5 when compared with the central adsorption of sec-alkylamines; relative to isopropylamine, n-propylamine adsorbs with an enthalpy and entropy of 21 kJ mol-1 and 45 J mol-1 K-1, respectively. Similar to the adsorption energetic trend observed for sec-alkylamines, n-alkylamines exhibit a fixed contribution to the enthalpy and entropy of adsorption per methylene unit.

Understanding the entropic contributions of irreversibly bound adsorbates on zeolite surfaces

ACS Spring Meeting 2021

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