Unravelling Irreversible Adsorbate Thermodynamics through Adsorption-Assisted Desorption

Strongly bound surface species like alkylamines adsorbed on the Brønsted acid site of aluminosilicate zeolites exhibit negligible rates of molecular desorption, preventing them from achieving an equilibrated state on experimentally relevant time scales that limit the measurement of their adsorption thermodynamics. Through adsorption-assisted desorption, whereby distinct alkylamines facilitate desorption from Brønsted acid sites, we demonstrate that equilibrated states are achieved. Breakthrough adsorption measurements reveal that while 2-butylammonium on a Brønsted acid site is irreversibly adsorbed, it readily undergoes molecular desorption when exposed to a distinct alkylamine like 2-propanamine. As a result, two-adsorbate equilibrium was achieved when the Brønsted acid sites of aluminosilicate zeolites were exposed to a binary vapor-phase alkylamine mixture. By varying relative vapor-phase partial pressures and temperatures, we demonstrate the ability to experimentally measure the adsorption enthalpy and entropy of alkylammonium adsorbates on mostly isolated Brønsted acid sites in H-ZSM-5 (Si/Al = 140). A multiadsorbate Langmuir isotherm was found to quantitatively describe the coadsorption of alkylamines varying in size and basicity over a wide range of conditions through which the relative adsorption enthalpy and entropy of alkylamines were measured. Across a homologous family of sec-alkylamines (C₃-C₅) adsorbed on isolated Brønsted acid sites, a fixed contribution to the enthalpy (19 ± 4 kJ mol CH₂^-1) and entropy (25 ± 4 J mol CH₂^-1 K^-1) of adsorption per methylene unit was found to exist, likely resulting from electrostatic interactions between the alkyl chain and the surrounding pore environment.