Effects of Support Acidity on the Catalytic Performance of Mo/HZSM-5-Al2O3 Catalysts in Olefin Metathesis

Mo/HZSM-5-Al2O3 catalyst samples, where the HZSM-5 had different Si/Al ratios (12, 42, and 213), were prepared and evaluated in a fixed-bed reactor. Effects of support acidity changes on the catalytic performance of Mo/HZSM-5-Al2O3 in the metathesis of ethene and 2-butene to propene were investigated. NH3 temperature-programmed desorption and pyridine infrared spectra revealed that the differences in support acidity led to the different anchoring modes of Mo species on the composite supports. Quick deactivation phenomena were observed on Mo/HZSM-5-Al2O3 (Si/Al = 12) due to the high acidity density of the catalyst. For Mo/HZSM-5-Al2O3 (Si/Al = 213), more Mo species dispersed on the alumina in octahedral form and those locating in the zeolite channels existed in the state of (Mo2O5)(2+) clusters evidenced by ultraviolet-visible diffuse reflectance spectroscopy and H-2 temperature-programmed reduction results. Such distribution was linked with the low metathesis activity and the appearance of the induction period on Mo/HZSM-5-Al2O3 (Si/Al = 213). Mo/HZSM-5-Al2O3 (Si/Al = 42) sample demonstrated the best catalytic activity and reaction stability among the three samples. This was attributed to its suitable acidity amount and optimal distribution of Mo species on the support.