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 8.5?m.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 20?m?.The grain size can be significantly decreased to 5.5?m 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?2?m?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?1.6²?m?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?3.4?m?.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 3?m?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%?.