Synthesis And Characterization Of SAPO-34

Methanol to olefins (MTO), as a potential method to replace the conventional oil route for the production of olefins, has received great attention in recent years. The key issue for the MTO reaction is the catalyst. Zeolite SAPO-34 has turned out to be a superior catalyst for the MTO reaction, due to its high methanol conversion and high selectivities to ethylene and propylene. However, coke deposition easily occurs over this catalyst surface during the MTO reaction, which blocks the channels of the zeolite, leading to fast deactivation of the catalyst. Previousworks have shown that reducing the crystal size of SAPO-34 can prolong the lifetime of the catalyst. In this work, SAPO-34 has been synthesized with the hydrothermal method, with the focus on reducing the crystal size of SAPO-34.First, tetraethyl ammonium hydroxide (TEAOH) and triethylamine (TEA) or their mixture are used as templates for the synthesis of SAPO-34 and the synthesized samples are characterized by XRD, SEM and BET techniques. The results show that TEAOH as template gives the average smallest particle sizes of 1μm, while TEA as template gives the average largest particle sizes of 15μm; substituting TEA with TEAOH can effectively reduce the particle sizes to some extent, with the average particle sizes of 1.5μm and the highest crystallinity and the highest BET surface area (533m²Â·g^-1) being obtained at TEAOH/TEA=0.15.Then, considering that TEA is much cheaper than TEAOH, pure TEA is used as the template for the synthesis of SAPO-34, but trying to reduce the particle sizes by optimazing the sythesis conditions. Under the following optimized conditions, i.e., molar ratio of n(Al₂O₃): n(P₂O₅): n(SiO₂): n(TEA): n(H₂O) = 1.0: 1.0: 0.6: 3.0: 50, crystallization temperature 200°C and crystallization time 48 hours, the molecular sieve has the highest crystallinity with the average crystal size of 7μm. To further reduce the particle sizes of SAPO-34, the sol for the synthesis is stirred for certain times at different temperatures with the above conditions. The results show that stirring the sol for 8 hours at 150°C gives the zeolite the highest degree of crystallinity and the smallest average particle size of 2.5μm.