Characterization of catalyst acidity by microcalorimetry and temperature-programmed desorption

✍ Scribed by Sanjay B. Sharma; Bernard L. Meyers; Daniel T. Chen; Jeffrey Miller; James A. Dumesic

• Publisher: Elsevier Science
• Year: 1993
• Tongue: English
• Weight: 811 KB
• Volume: 102
• Category: Article
• ISSN: 0926-860X
• DOI: 10.1016/0926-860x(93)80232-f

✦ Synopsis

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-', 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-' K-', 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.