Syntheses Of Hierarchical SAPO-34 Zeolites And The Catalytic Performance Of MTO Reaction

Olefin-based petrochemicals have been widely used in all aspects of human living.Growing global demand for oil and more generally for energy will have significant impact on the availability and pricing of traditional feedstocks for light olefin production.Methanol-to-olefin(MTO)reaction,based on non-petroleum resources,has been considered as a successfully alternative method for olefin production to balance the deficit of petroleum and natural gas.Recent advancements both in process technology via integration of the MTO process with the olefin cracking process and optimized catalyst synthesis chemistry have led to significant improvement in process performance.Zeolites,as a class of microporous crystalline catalysts have been widely used in the petrochemical industry,because of their unique molecular sieving effect,excellent catalytic activity and superior hydrothermal stability.Silicoaluminophosphate SAPO-34 zeolite with CHA topological structure has proven to be the best catalyst for the production of ethylene and propylene in the MTO reaction,due to the small three-dimensional eight-ring intersecting channels and large cha cages as well as moderate and tunable acidity.However,the conventional microporous SAPO-34 catalysts always suffer from rapid deactivation during the MTO reaction,due to the rapid coke deposition.Therefore,in order to improve the performance of catalyst,various synthetic strategies have been developed to overcome the inherent diffusion limitations and retard coke deposition.According to previous studies,the hierarchically porous SAPO-34 zeolites can significantly improve the transformation of reactants and products within the crystal,effectively reduce the rates of coke species,then prolong the lifetime of catalysts and improve the selectivity of light olefins.This thesis mainly focused on the synthesis of hierarchically porous SAPO-34 zeolites with excellent catalytic performance in MTO reaction.We studied the morphology,acidity,catalytic performance and the characterization of internal pore structure in the crystal with novel synthetic methods.We also explored the crystallization process of the synthesis of zeolitic catalysts briefly.The main research results are as follows:1.The hierarchical SAPO-34 catalysts with abundant intercrystalline mesopores have been successfully synthesized using a cheap cationic surfactant and morpholine as the templates through a facile hydrothermal strategy.Thanks to the significantly enhanced transport efficiency and decreased acidity,the hierarchical SAPO-34 zeolites exhibit excellent catalytic performance in MTO reactions,with about 5-fold prolonged catalytic lifetime and more than 12% increased selectivity of ethylene and propylene compared with microporous counterpart.This work demonstrates a facile and cost-effective method for synthesizing hierarchically morpholine-based SAPO-34 catalysts with superior MTO performance,and this synthetic method may be also expected to extend to fabricate other high-performance hierarchical SAPO zeolites.2.We demonstrate an easy post-synthesis of hierarchically porous SAPO-34 catalysts by treating in a facile and green solution system,mother liquor.Parent SAPO-34 crystals were synthesized with triethylamine as template under hydrothermal conditions and then placed in stationary environment with dipping in original mother liquor.The chemical etching led to the formation of hierarchical porous structure which enhanced mass transport inside and decreased acidity appropriately,so that the hierarchical SAPO-34 catalysts exhibit better performance in MTO reactions with longer catalytic lifetime and higher selectivity of ethylene and propylene than the parent SAPO-34 crystals synthesized in traditional way.Moreover,the morphology characteristics of hierarchical structure become more obvious as the post-treated time increasing.This change reveals a crystal growth process of the SAPO-34 zeolites templated by triethylamine.This work opens new perspective for synthesis of SAPO-34 zeolite with promoted MTO performance by post-treatment method.The utilization of mother-liquor is simple,cost-effective and environmentally friendly,which offers a new approach to prepare other type hierarchical porous zeolites with excellent catalytic properties through such post treatment.