The Synthesis Of Low-Silicon SAPO-34 Zeolite And Its MTO Catalytic Performance

SAPO-34 is the most promising molecular sieve for methanol to olefins（MTO）reaction.Low-silicon SAPO-34 molecular sieve with weaker acidity can suppress the coke formation and pore blockage,resulting in good catalytic stability.However,the synthesis of low-silicon SAPO-34 is impeded by expensive template,long crystallization time,and unexpected crystal.This paper aimed at synthesizing the low-silicon SAPO-34 molecular sieve with cheap triethylamine（TEA）as chief template and morphine moiety（Mor）,diethylamine（DEA）and tetraethyl ammonium hydrogen（TEAOH）as complementary templates by direct hydrothermal,seed and seed-induced-agent method.The effects of synthesis condition and type of templates on MTO performance were investigated.Although the highly crystallized SAPO-34 molecular sieve（SiO₂/Al₂O₃=0.1）could be synthesized by direct hydrothermal method with TEA as a single template,it showed rough surface and uneven grain with an average size up to 8.5μm.In the investigation of the composite templates（TEA/HF,TEA/TEAOH,TEA/Mor and TEAOH/Mor）,a highly crystallized SAPO-34 molecular sieve（SiO₂/Al₂O₃=0.1）with flat surface was obtained by using TEAOH/Mor as template,in spite of larger grain size（about 20μm）.The grain size can be significantly decreased to 5.5μm by increasing the ratio of SiO₂/Al₂O₃ to 0.2,indicating that a striking impact of the ratio of SiO₂/Al₂O₃ on grain size.Compared with direct synthesis method,the seed synthesis method shows advantages in short crystallization time（24 h）,uniform grain,and small size（2μm）by using TEA as template.Furtherly,when using TEA/Mor and TEA/TEAOH as template,the SAPO-34（SiO₂/Al₂O₃=0.2）with flat surface and small uniform grain（1.6²μm）was obtained.When the seed-induced-agent method and TEA was employed,the crystallization time can be furtherly shortened to 24 h.The obtained SAPO-34 showed uniform and the average particle size（3.4μm）.That is,the low-silicon pure SAPO-34 molecular sieve with small grain was successfully synthesized by cheap template and short crystallization time.When other templates were used,such as Mor,TEAOH/Mor and TEAOH/TEA,it was necessary to add the same templates in the seed-induced-agent and gel to synthesize the desired SAPO-34.In conclusion,the SAPO-34（SiO₂/Al₂O₃=0.2）with high crystallization degree and smaller grains（about 3μm）can be synthesized by using low-cost template agent Mor and composite template TEAOH/Mor.The MTO reaction performance of SAPO-34 synthesized by different methods and templates was investigated.It was found that the sample synthesized by seed-induced-agent method exhibited the better MTO performance than that obtained by seed method and direct method when the same template was used.Additionally,the SAPO-34 with low SiO₂/Al₂O₃ratio（0.2）highly yielded the olefins when the same synthesis conditions was used.Moreover,when using the seed-induced-agent method with different templates,SAPO-34 molecular sieves synthesized by TEAOH/TEA behaved the highest selectivity towards ethylene and propylene（73.80 wt%）.