Synthesis Of SAPO-34Catalysts And Their Application In Methanol Transforming Olefins Reaction

The increasing shortage of the crude oil and the growing need of light olefins, such as ethylene and propylene, have greatly stimulated the development of the alternative non-oil routes for light olefins production. Up to date, methanol to olefins (MTO) coupled with transformation of coal or natural gas to methanol has been considered as the most promising route with an economic advantage. However, the catalyst for MTO is still a challenge.SAPO-34, a silicoaluminophosphate molecular sieve with CHA structure, due to the moderate acid strength, relatively small pores (0.43nm) and hydrothermal stability, appears to be most attractive and many research works have been reported relating to its synthesis and crystallization mechanism, catalytic properties and catalytic reaction mechanisms for MTO reaction. However, as a microporous zeolite material, the traditional SAPO-34catalyst with the high density of strong acid sites and poor abrasive resistance is known to suffer rapid deactivation easily by carbon deposition in the strong exothermic MTO reaction, thereby leading to a rapid decrease of the catalytic performance and short lifetime.In this dissertation, the SAPO-34catalyst was prepared hydro thermally and optimizedly and the effects of dispersity of silica source, mixed templates etc on the particles size and properties of SAPO-34samples were studied in detail. SAPO-34on the calcined kaolin microspheres was in-situ hydrothermal synthesized successfully. The spherical SAPO-34catalyst used in fluidic bed reactor with high catalytic activity and long lifetime was explored. The main results obtained are as follows.The effect of the dispersity of silica source on the properties of SAPO-34was investigated. It is found that the dispersity of silica source after adding HCl as modifier and NH4F as dispersant could be obviously enhanced. When this high-dispersity silica was used as a precursor, is it in favor of preparing the sample with very low SAPO-5content and small size. As a result, the catalytic selectivity in MTO reaction can be improved and the single-pass lifetime can be prolonged. The incorporation of Ni in the preparation process of SAPO-34leads to an increase of the SAPO-5and a change of the acidic site in the samples, resulting in a decrease of the selectivity to olefins.The effect of the mixed template of TEA and TEAOH on the properties of SAPO-34molecular sieve was studied. The results show that, the chemical composition, morphology. crystal size and Si incorporation and the catalytic activity and single-pass lifetime in MTO reaction of SAPO-34can be influenced by the ratio of TEAOH/TEA. When the TEAOH/TEA ratio of0.05-0.1was used, the SAPO-34molecular sieves dominated by weak acid sites with moderate acid strength can be prepared. Using the SAPO-34catalyst prepared with the mixed complete of TEAOH/TEA=0.1, the C2--C4-selectivity and lifetime of the catalyst reached the maximum of95.9%and430min, respectively, which is obviously superior to that SAPO-34prepared by single TEA or TEAOH template.SAPO-34molecular sieve with crystalline size of160～550nm were synthesized by a two-step varying-temperature hydrothermal crystallization and H2O2solution de-template method. The obtained SAPO-34exhibits the similar CHA structure, cubic-like morphology and ratio of Si/(Al+P+Si) to traditional SAPO-34. With the reduction in the crystalline size of SAPO-34catalyst, its conversion of methanol to olefins, the catalytic activity and single-pass lifetime obviously increase. When the160nm SAPO-34prepared was used as the catalyst for MTO, the olefins selectivity and single-pass lifetime can reach over94.8%and596min, respectively.Supported SAPO-34catalyst on the fully calcined kaolin particles (CKMs) with the average size of60μm was synthesized in situ successfully, in which CKMs were pretreated by NaOH solution and the mixture of TEAOH and TEA was used as a template. The chemical composition and the morphology of the kaolin microsphere and supported SAPO-34catalyst prepared can be modulated by the concentration of NaOH of4-14%. With the increasing of the concentration of the NaOH solution, the Si2p/Al2p ratio on the surface of CKMs decreases and the surface roughness of CKMs increases. Using CKMs pretreated by4%NaOH solution as the support, SAPO-34in situ grown on CKMs exhibits the high methanol conversion (100%), longest single-pass lifetime (964min) and strongest abrasive resistance.Using silica gel and aluminium phosphate as the main adhesive and auxiliary agent, SAPO-34-based kaolin microspheres were prepared by a spray-drying method. SAPO-34/kaolin supported catalyst was prepared by in situ hydrothermal process onto the surface of the calcined kaolin microspheres pretreated by alkali solution treatment and subsequent acid exchange. In the testing with fluid-bed MTO reactor system, the selectivities of the olefins over the prepared SAPO-34-based kaolin microspheres and SAPO-34/kaolin supported catalyst can reach94.8%and93.1%, respectively. And the attrition rates of two catalysts are1.28%/h and0.72%/h, respectively.