Synthesis and MTO Catalytic Performance of SAPO-34

Effects of n(SiO2) : n(Al2O3), n(P2O5) : n(Al2O3) and n(H2O) : n(Al2O3) in the synthesis gel on the crystallization of SAPO-34 were systematically studied with triethylamine as a templating molecule for the purpose of improving its catalytic performance in methanol to olefin(MTO) reaction. Pure SAPO-34 phase can only be synthesized with the synthesis gel having a n(SiO2): n(Al2O3) larger than 0.25, demonstrating the structure-directing role of Si. Nevertheless, a further increase in the Si content in the synthesis gel led to not only the formation of Al2O3, but also the decrease in the crystallinity, surface area and catalytic stability of SAPO-34. The n(P2O5) : n(Al2O3) and n(H2O) : n(Al2O3) in the synthesis gel also significantly influenced the crystallization and catalytic performance of SAPO-34. When n(P2O5) : n(Al2O3) and n(H2O) : n(Al2O3) were 1.1 and 36, respectively, the synthesized sample with a n(P + Si) : n(Al) close to I showed high surface area and appropriate acidity, resulting in a catalytic lifetime of 520 min without lowering olefin selectivity. The catalytic performance of SAPO-34 is also highly dependent on its calcination conditions. The sample calcined at 550 degrees C for 8 h in flowing air showed a much higher catalytic stability(its catalytic lifetime reached 580 min) than the sample calcined for 10 h under the same conditions as a result of different hydrocarbon residuals kept in SAPO-34 after calcination.