(349k) Reaction Calorimetry for Adsorption Thermodynamics in Zeolite

Solid acid catalyst characterization involves understanding adsorption interactions within aluminosilicate pores, which helps guide subsequent catalyst design. Different techniques exist for zeolite characterization to detail functionality and adsorbate interaction with the Brønsted acid site. Despite these techniques, information on the adsorption thermodynamics of adsorbates strongly bound to a Brønsted acid site, particularly entropic contributions, are relatively limited. The challenge arises from the requirement of establishing equilibrium between an adsorbate and Brønsted acid site to measure thermodynamic information; strongly bound adsorbates which do not desorb cannot attain an adsorption/desorption equilibrium. Relative thermodynamic information however can be obtained when exposing a Brønsted acid site to multiple strongly bound intermediates, which collectively behave as competitive adsorbates that attain an adsorption/desorption equilibrium. Here we present a newly developed method that leverages this approach of relative thermodynamics, reactive calorimetry, where we measure the adsorption thermodynamics of alkylamines on Brønsted acid sites. The method is comprised of three main steps: in-situ pre-treatment, establishing co-adsorbate equilibrium (Surface Equilibration) and temperature programmed surface reaction of the adsorbed alkylamines (Reaction Analysis, Figure 1). The entire method is carried out within a gas chromatograph, serving both as an automated reactor and a quantification tool. By measuring the relative coverages of multiple alkylamines at varying partial pressure and temperatures, we are able to establish the adsorption energetics of alkylamines over MFI zeolites as a proof of concept. Reactive calorimetry as a technique therefore affords us the ability to expand our understanding of adsorption thermodynamics, particularly for adsorbates strongly bound to Brønsted acid sites in solid acid catalysts.