Effect of SiO₂/Al₂O₃ ratio on the conversion of methanol to olefins over molecular sieve catalysts

Abstract

A series of SAPO-34 molecular sieves with different SiO₂/Al₂O₃ ratios have been synthesized for the methanol-to-olefin (MTO) reaction. Their physico-chemical properties are characterized by various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR) and N₂ adsorption-desorption. The results are compared with those of the commercial HZSM-5, which show that the crystallinity and particle diameter of SAPO-34 as well as HZSM-5 increase with SiO₂/Al₂O₃ ratio. The variation of BET surface area of SAPO-34 is different from that of HZSM-5 and the sample with SiO₂/Al₂O₃ ratio of 0.4 exhibits the highest BET surface area. FT-IR spectra indicate that HZSM-5 has both Brǿnsted and Lewis acid sites and Brǿnsted acid sites are stronger, whereas SAPO-34 samples are dominated only by Lewis acid sites. When the SiO₂/Al₂O₃ ratio increases, propylene and butylenes become the predominant product of the MTO reaction over HZSM-5. In contrast, the main products of this reaction catalyzed by SAPO-34 are ethylene and propylene. According to the product distribution, the reaction mechanism over HZSM-5 catalysts is proposed.

Key words： HZSM-5; SAPO-34; methanol-to-olefin (MTO); SiO₂/Al₂O₃ ratio

Acknowledgments

This research was supported financially by the National High Technology Research and Development Program of China (863 Program) (Grant Nos. 2006AA06Z371 and 2008AA06Z324) and by the Innovation Project of Institute of Process Engineering, Chinese Academy of Sciences (No. 082702).

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