High temperature microcalorimetry of high silica zeolites

The adsorption of simple amines in high-silica zeolites was studied using microcalorimetry and simultaneous TPD (temperature-programmed desorption) and TGA (thermogravimetric analysis) in order to obtain a better understanding of the nature of acidity in high silica zeolites. The effect of zeolite type, aluminum content, and adsorbate basicity on adsorption enthalpies was investigated. In addition, copper exchange into high-silica zeolites was studied. The adsorption of simple amines, including methylamine, ethylamine, n-propylamine, isopropylamine, t-butylamine, and pyridine, was investigated on H-ZSM-5 using TPD-TGA and transmission infrared spectroscopy. For each adsorbate, a well-defined adsorption state corresponding to a coverage of one molecule/Al atom could be easily identified. In order to investigate the effect of aluminum content and synthesis conditions on the adsorption enthalpies for pyridine and ammonia in H-ZSM-5, we have used microcalorimetry. The results show that the heats of adsorption for both ammonia and pyridine are constant up to a coverage of one molecule/Al, independent of Si/Al ratio, with values of 145kJ/mol for ammonia and 200 $\pm$ 5kJ/mol for pyridine. These results suggest that the Bronsted-acid sites in unsteamed H-ZSM-5 are independent of sample and equal in concentration to the framework Al content. In order to investigate the effect of basicity on adsorption enthalpy of amines, differential heats of adsorption for a series of simple amines in H-ZSM-5 were measured as a function of coverage. For each molecule studied, the heats of adsorption were approximately constant to a coverage of close to one per Al. An excellent correlation is found between these heats and gas-phase, proton affinities. In contrast to the excellent correlation between heats of adsorption and gas-phase acidity scales, the correlation with solution-phase acidities is very poor. The effect of zeolite structure on the adsorption enthalpies amines was investigated using microcalorimetry. The adsorption enthalpies of ammonia, pyridine, and isopropylamine were measured as a function of coverage at 480K on several high-silica zeolites, including H-ZSM-5, H-ZSM-12, H-Mordenite, and H-Y. The results demonstrate that structure does not significantly affect the adsorption enthalpies of the bases examined. An investigation of copper-exchange in a series of Cu(H)-ZSM-5 and Cu-Y zeolites was conducted using simultaneous temperature-programmed desorption/thermogravimetric analysis (TPD-TGA) measurements of isopropylamine and $\rm N\sb2O$-decomposition measurements. The results demonstrate that the ZSM-5 structure can stabilize a catalytic site which is extremely active for $\rm N\sb2O$ decomposition. Results on Cu exchange in the Y structure indicate that the Cu may not be present at ion-exchange sites, explaining the low activity of H-(Cu)-Y