Low-temperature adsorption calorimetry for zeolite characterization

The motivation behind this thesis was to use low-temperature adsorption calorimetry to characterize zeolites through the use of simple molecular probes. Three different contributing factors to adsorbate-zeolite interactions were analyzed: the geometric structure of the zeolite, the acidity of Brønsted sites, and adsorbate interactions with extra-framework alkali cations. Heats of adsorption are strong functions of the zeolite structure, even in the virtual absence of ions or defects. The increased affinity of alkanes for narrow pores can be attributed to the fact that a small molecule can interact with a much larger surface area of the zeolite in a narrow pore than in a wide one. The use of a thermodynamic cycle involving n-hexane and pyridine on H-MFI demonstrated that the cal-ad method is not accurately measuring the acidity of this material. Two weakly interacting molecular probes, CO and N2 , were used to study acidity in H-MFI by comparing calorimetric results on that material with those on silicalite. A CO-Brønsted site interaction of 10.5 kJ/mol was measured, and was well-described by a Langmuir two-site model. Nitrogen showed a much smaller interaction with Brønsted sites. CO, N2, and O2 were used to probe adsorbate interactions with a series of alkali-exchanged MFI structures. Enthalpies of interaction with individual cations were shown to be inversely related to cation size. There was evidence that multiple adsorbate molecules were able to interact with a single ion site for sodium and potassium cations. In another study, a series of sodium FAU zeolites of varying Si/Al ratio were probed with chloroform and trichloroethylene. The electrostatic interactions of these molecules with the extra-framework cations were a strong function of the Si/Al ratio. This is likely due to the occupancy of highly accessible site III positions in zeolite X, and their absence in zeolite Y. In the last investigation, adsorption on FAU zeolites was studied as a means of separating two hydrofluorocarbon isomers. The selectivity of this process indicates that it should be further investigated for use in industry. y