Abstract: The behavior of Sb₂O₃on HZSM-5 was investigated by employing the techniques such as XRD, N₂and n-hexane adsorption, NH₃-TPD etc.. Mechanic mixture of Sb₂O₃and HZSM-5 was calcined at 500℃ under nitrogen for two hours. XRD results showed that crystalline Sb₂O₃disappeared while HZSM-5 framework suffered a noteworthy damage.Quantitative phase analysis indicate that the crystalline HZSM-5 amount in Sb₂O₃/HZSM-5 shrank from 76% to 59% after calcination. The Sb₂O₃/HZSM-5 to HZSM-5 ratios of pore volumes (derived from N₂adsorption experiment) and n-hexane saturated adsorption amount were 0.60 and 0.62, both corresponding to that ratio of the crystalline HZSM-5 amount(0.59/0.95=0.62). The fact suggests that in calcined Sb₂O₃/HZSM-5, nothing exists in the channels of the zeolite. Moreover, it is also found that the n-hexane saturated adsorption pressure of Sb₂O₃/HZSM-5 is greater than that of HZSM-5. Based on the results above, a new mechanism of the interaction between Sb₂O₃and HZSM-5 was proposed. During calcination, the framework of HZSM-5 is partly broken by acidic Sb₂O₃, which in turn highly disperses into the silica-alumina gel yielded from framework collapse. This gel modifies the external-surface of HZSM-5 by diminishing its pore openings, subsequently enhancing the shape-selectivity of the zeolite. Toluene alkylation reaction was applied to characterize Sb₂O₃/HZSM-5, the para-selectivity attained as high as 89%.

Key words: Sb₂O₃, HZSM-5, Crystallinity, Pore opening modification