Influence of phosphorous contents on Si incorporation mechanism and properties of SAPO 34
Abstract SAPO 34 molecular sieves with different phosphorus content were prepared by adjusting P 2 O 5 /Al 2 O 3 molar ratios in starting gel. The crystal patterns, crystallinity, crystal morphology, element composition, functional group and thermal stability were characterized by XRD, SEM, XRF, FT IR and TGA. It reveals that the properties and Si substitution mechanism of the framework can be changed with P 2 O 5 /Al 2 O 3 molar ratio. Sample with P 2 O 5 /Al 2 O 3 molar ratio of 1.5 possess the highest crystallinity and smooth cubic crystals. In addition, SP 1.5 has the lowest external surface area, the largest micropore surface area and the average pore diameter. With P 2 O 5 /Al 2 O 3 molar ratio of 1, Si atoms directly replace P atoms during the crystallization of SAPO 34 with the formation of Si(4Al) species. When the P 2 O 5 /Al 2 O 3 molar ratio is lower than 2, the Si substitution mechanism can be attributed to double substitution of SM2 and SM3.