ZSM-5表面酸性的CLD改性及其对择形催化性能的影响

为了获得分子筛催化反应中所需产物的高择形催化选择性,往往必须消除分子筛外表面酸性和减少孔道开口尺寸.目前,各种改性方法已被用于提高ZSM-5的选择性,例如金属、非金属化合物的浸渍,化学气相硅沉积(SiO2-CVD)和预结焦处理等1.虽然采用大分子有机物可以消除分子筛外表面的酸性位,但这种改性在反应条件下尤其在高的反应温度下是不稳定的2.……

CLD Modification for Acidity of ZSM-5 and Its Effect on the Shape-Selective Catalysis

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.