SiO2-CLD modification for ZSM-5 acidity was investigated by pyrolysis gas chromatography-mass spectrometer (PGC-MS), Py-IR, NH3-TPD. In SiO2-CLD modification, the decomposition of polysiloxane to silica is the acid-catalyzed degradation combing the thermal pyrolysis, with the following oxidation at high temperature. Though the silanol groups disappear after the modification, most of the framework bridging hydroxyls and non-framework-Al hydroxyls remain. Besides, the acidic amount of the modified ZSM-5 drops gradually with increasing extent of modification, but the acidic strength distribution of the modified ZSM-5 is similar to that of parent ZSM-5. After two-cycle modification, TIPB conversion of the modified ZSM-5 is reduced by 95%, so its external surface may be regarded to be nearly inactive. The activity for cracking of 1,3,5-triisopropylbenzene (TIPB) is close to zero over the modified ZSM-5, but the modified ZSM-5 retains high activity for cracking of cumene. So, SiO2-CLD with polysiloxane is an ideal way of modification for obtaining the shape-selective ZSM-5 catalyst.
