Abstract
A series of SAPO-34 molecular sieves with different SiO /Al O 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 N 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 SiO /Al O ratio. The variation of BET surface area of SAPO-34 is different from that of HZSM-5 and the sample with SiO /Al O ratio of 0.4 exhibits the highest BET surface area. FT-IR spectra indicate that HZSM-5 has both Br?nsted and Lewis acid sites and Br?nsted acid sites are stronger, whereas SAPO-34 samples are dominated only by Lewis acid sites. When the SiO /Al O 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.
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