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-mo rdenite and H-ZSM-5. Both zeolites possess a relatively homogeneous ac id-strength distribution. The enthalpy changes of adsorption for ammon ia on H-mordenite and H-ZSM-5 are - 157 and - 150 kJ mol-1, respective ly. Analyses of the TPD spectra, constrained by the microcalorimetric results, indicate that the standard entropy changes of ammonia adsorpt ion on H-mordenite and H-ZSM-5 are - 159 and - 171 J mol-1 K-1, respec tively. Accordingly, ammonia retains a significant fraction of its loc al entropy on H -mordenite (i.e., rotation and vibration), while ammon ia adsorbed on H-ZSM-5 loses approximately a third of its local entrop y. Pyridine desorption from these zeolites is limited by molecular dif fusion in the zeolite particles and for this reason is a poor choice a s a probe molecule for assessment of acid-site strength by TPD.