Synthesis And Characterization Of Mesoporous SAPO-34 Molecular Sieve

Owing to the excellent thermal and hydrothermal stabilities, adjustable acidity, SAPO-34 molecular sieve has showed excellent performance in the process of Methanol-to-Olefins(MTO). As the catalyst, SAPO-34 has higher selectivity of ethane and propene and conversion of methanol for MTO reaction,which makes the SAPO-34 be the most famous catalyst.However, owing to the small pore structure limit the diffusion of reactants and products. On the one hand, reduce the availability of active site, on the other hand, the products can not diffuse to the outside of pore, which makes the secondary reaction happen even coke, seriously blocking the channel, leading to affect its catalytic selectivity and activity. At present, researchers have paid enough attention to how to improve the property of SAPO-34.Mesoporous molecular sieve not only improve the accessibility of big molecules, but also improve the rate of diffusion of big molecules. Besides,mesoporous molecular sieve not only retains the inherent properties, but alsointroduces mesopores and macropores into the molecular sieve, which makes the structure of channel be more abundant. So compared with microporous molecular sieve, the property and field application of mesoporous molecular sieve will be improved.In the article, we adopt the first method of synthesizing mesoporous SAPO-34 through the hydrothermal method and using the organosilane surfactants PHAPTMS and TPOAC as mesoporous template, which are immediately added into the crystallization system. Besides the mesoporous SAPO-34 are hydrothermally synthesized from silica source SiO2 organic functioned by PHAPTMS and TPOAC.1. Mesoporous SAPO-34 molecular sieve were hydrothermally synthesized by using organosilane surfactant as the mesoporous generating agent.(1) Using TPOAC as mesopore-generating agent, and the crystallization temperature is160 ℃ and 140 ℃. The results show that: reducing the crystallization temperature and adding TPOAC can make the size of SAPO-34 molecular sieve decrease(100~200nm) and change the morphology of the SAPO-34 molecular sieve. Besides, with the increasing of the amount of TPOAC, the external surface area of SAPO-34 molecular sieve increasing. When the amount of TPOAC is 2mL, the crystallization temperature is 140℃, the external surface area of molecular sieve is 324m2/g.(2) Using PHAPTMS as mesopore-generating agent, and the crystallization temperature is 180℃, 160℃and 140℃. The results show that reducing the crystallization temperature andadding PHAPTMS led to the formation of assemblies of nanoscale molecular sieve crystals, and the external surface area of molecular sieve increase obviously.2. Mesoporous SAPO-34 molecular sieve are hydrothermally synthesized by using silica source SiO2 organic functioned by organosilane surfactant PHAPTMS and TPOAC. The results show that using PHAPTMS silanization SiO2 as solid silicon source and crystallization in the low temperature also led to the formation of assemblies of nanoscale molecular crystals, and external surface area of molecular sieve increase obviously, and with the increasing of the silanization degree, the external surface area of molecular sieve decreasing.(2) Using TPOAC silanization SiO2 as solid silicon source and crystallization in the 160℃ and 140℃ led to the formation of assemblies of nanoscale molecular crystals, and the lower(140 ℃) is more beneficial for the synthesis of mesoporous SAPO-34 molecular sieve with higher external surface area.Besides we also find that with the increasing of the silanization degree, the external surface area of sample increase.According to the both methods, we can deduce that the low temperature is beneficial for the synthesis of mesoporous SAPO-34. In the process of MTO,mesoporous SAPO-34 has more outstanding stability, and the sum of selectivity for C2=, C3=and C4=is 78%, and the selectivity for C2=, C3=and C4=is 25%, 40%and 13% respectively(document), compare with the conventional microporous SAPO-34, the selectivity for C2=, C3=and C4=is 42%, 34% and 5% respectively(document), the sum of olefins remain unchanged, but the selectivity of C3=and C4=has increased evidently.