CHARACTERIZATION OF CATALYST ACIDITY BY MICROCALORIMETRY AND TEMPERATURE-PROGRAMMED DESORPTION

Microcalorimetric measurements and temperature-programmed desorption (TPD) experiments were conducted to study the acidic properties of H-mordenite and H-ZSM-5. Both zeolites possess a relatively homogeneous acid-strength distribution. The enthalpy changes of adsorption for ammonia on H-mordenite and H-ZSM-5 are - 157 and - 150 kJ mol-1, respectively. Analyses of the TPD spectra, constrained by the microcalorimetric results, indicate that the standard entropy changes of ammonia adsorption on H-mordenite and H-ZSM-5 are - 159 and - 171 J mol-1 K-1, respectively. Accordingly, ammonia retains a significant fraction of its local entropy on H -mordenite (i.e., rotation and vibration), while ammonia adsorbed on H-ZSM-5 loses approximately a third of its local entropy. Pyridine desorption from these zeolites is limited by molecular diffusion in the zeolite particles and for this reason is a poor choice as a probe molecule for assessment of acid-site strength by TPD.