Synthesis Of Porous SAPO-34 Aluminosilicaphosphate Molecular Sieve And Its Application In MTO Reaction

SAPO-34 belongs to CHA structure and is composed of D6Rs basic units with 0.38 nm, whose double six rings are connected by each vertex. Multi-D6Rs are arranged as rhombic hexahedron and finally crystallize as cubic morphology with a plane spacing of 0.94 nm. Because of the special pore structure and large amount of acid sites originating form Si substitution, SAPO-34 shows its intensive application in methanol to olefins (MTO) reactions. For the industry utility, SAPO-34 is synthesized by using low-cost triethylamine or mixture of triethylamine and tetraethyl ammonium hydroxide as template, however, this method generally leads to the formation of SAPO-5 impurity, which is adverse to the selectivity of ethylene and single life time. This thesis first studied the phase transform from SAPO-5 to SAPO-34, and proposed a mechanism for the transform to provide instruction for the following chapters. Subsequently, in consideration of the weak mass diffusion ability and short single life time of conventional SAPO-34 due to the pore size limitation and the great importance for the fabrication of SAPO-34 with multiple pore-size structure, this thesis mainly concentrated on the construction of muti-porous SAPO-34 by using template and none-template method. Additionally, the relationship between morphology and the activity of MTO was also studied. Possible mechanism was proposed to illustrate the formation of special porous structures and morphologies.The second chapter of this thesis studied the reason for the formation of SAPO-5 under ethanol system and the mechanism for the phase transform from SAPO-5 to SAPO-34. It was found that single SAPO-5 crystal could form in the ethanol solvothermal system and gradually grew up to big single crystals with 10μm dimension, wich could futher transform into SAPO-34 in hydrothermal system. We deemed that in ethanol system, the volume confinement of ethanol molecular lead to the easily formation of basic unit 12T for SAPO-5, while in hydrothermal system, the volume of water molecular was nearly the same with that of basic unit D6Rs for SAPO-34, hence SAPO-5 was formed in ethanol system while SAPO-34 was formed in hydrothermal system. Moreover, the phase transform process from SAPO-5 to SAPO-34 can be devided into three steps:First, SAPO-5 crystals dissolves into basic unit 12T; Second,12T transforms to D6Rs, Finally, D6Rs units combine to grow into SAPO-34 crystals. The studies for the phase transform process from SAPO-5 to SAPO-34 gave instructions for the following chapters.The third chapter of this thesis studied the construction of SAPO-34/SAPO-5 composite microspheres by using mesoporous Al2O3 microspheres as indirect template. The result showed SAPO-5 was first formed in the beginning phase, and then more and more SAPO-34 phase appeared. Finally pure SAPO-34 phase was obtained after 72 hours. However, since SAPO-5 crystal would dissolve in aqueous phase, the microphere structure would collapse before pure SAPO-34 was formed and only the composite of SAPO-34 and SAPO-5 was obtained. We believed that the uneven spread of precursors in microspheres led to the low Si:Al ratio, which resulted in the formation of soluble SAPO-5 phase and the unstable of the Al2O3 microsphere.The fourth chapter of this thesis studied the fabrication of macroporous SAPO-34 microspheres by using polystyrene beads as hard templates. Considering of the unsteady of microspheres during the hydrothermal treatment, we adopted a secondary hydrothermal crystallization method:First, pre-crystallized 1μm SAPO-34 cubes and 2μm polystyrene beads were prepared; Then homogenous mixtures of SAPO-34 cubes, polystyrene beads, Si and Al precursors were made into 30～50μm microspheres by spray dry method and followed by second hydrothermal treatment. It was found that the sample prepared by using polystyrene beads as hard template has combined structure of micro-meso-macro pore, which could be directly used in fluid bed MTO reactions and showed high selectivity to light olefins with long single life time. We deemed that the polystyrene beads acted not only hard templates but also diffusion strengthener.The fifth chapter of this thesis prepared mesoporous SAPO-34 by using uncalcined SBA-15 mesoporous silica as silica source.20μm SAPO-34 cubic particles with decussate morphology were obtained. The sample possessed micro-meso bimodal pores, and the pore diameter of mesopores was 10～20 nm. Other silica source such as calcined SBA-15 and colloidal silica can not lead to the formation of mesoporous SAPO-34. We believed the coinfluence of silica source and P123 template led to the formation of decussate morphology. When applied in MTO reactions, decussate mesoporous SAPO-34 showed higher selectivity to light olefins and longer deactivation time, which were due to the higher crystallization and better mass diffusion ability compared to conventional SAPO-34.Finally, the sixth chapter of this thesis prepared a novel floral mesoporous SAPO-34 simply by adding sodium fluoride into the synthesis system. The optimum ratio of F:Si was 0.1. However, the addition of ammonium fluoride could not lead to the floral morphology, which could be attributed to the decomposition of ammonium fluoride at high temperature (200℃). When applied in MTO reactions, floral SAPO-34 showed higher selectivities to propylene and butylenes due to the increment of intrinsic micropore (0.9 nm) volume by fluoride ion etching.