Chemical liquid deposition with polysiloxane of ZSM-5 and its effect on acidity and catalytic properties

ZSM-5 acidity was modified by chemical liquid deposition of silica (SiO2-CLD), and the modification mechanism was investigated by pyrolysis gas chromatography-mass spectrometer (PGC-MS), absorbed pyridine infrared spectroscopy (Py-IR), temperature programmed desorption of ammonia (NH3-TPD), thermal gravity analysis (TGA), X-ray diffraction (XRD) and physical adsorption and probe-molecule reactions. In SiO2-CLD modification, the deposition process of polysiloxane modifier to silica over ZSM-5 is the acid-catalyzed degradation companying with the thermal pyrolysis, following oxidation at high temperature. Though the silanol hydroxyls of ZSM-5 disappear after the modification, most of the framework Al bridging hydroxyls and non-framework Al hydroxyls remain. Besides, the acidic amount of the modified ZSM-5 drops gradually with increasing extent of the modification, but the acidic strength distribution of the modified ZSM-5 is almost unchanged compared to parent ZSM-5. After the four-cycle modification, the conversion of 1,3,5-triisopropylbenzene (TIPB) in cracking over the modified ZSM-5 is reduced by 92%, so the external surface of the modified ZSM-5 may be regarded to be almost non-acidic due to coverage of deposition silica. Moreover, the modified ZSM-5 shows the high activity for cracking of cumene. High selectivity of para-xylene up to 96.2% is obtained in toluene disproportionation over the modified ZSM-5, compared to 24.3% selectivity of para-xylene over parent ZSM-5. Therefore, SiO2-CLD with polysiloxane is an ideal modification for obtaining the high shape-selective ZSM-5 catalyst.