Study On Green Synthesis Of SAPO-LTA Zeolite And Their Catalytic Performance In The Dehydration Of Ethanol To Ethylene

At present,the zeolite synthesis process still suffers from serious waste liquid pollution,low product yield,high autogenous pressure and high synthesis cost.So the development of green zeolite synthesis methods has therefore been of great interest.Small-pore zeolites have received much attention for their important applications in chemical processes such as shape-selective catalysis,adsorption,and separation.This work investigated the green synthesis of smallpore silico-aluminophosphate(SAPO)zeotype with LTA structure,developed dry-gel direct conversion and one-step calcination method,characterized the resulted zeolites for their physic-chemical properties and evaluated for their catalytic performance in the dehydration of ethanol to ethylene.The main research results are as follow.The SAPO-LTA zeolite was synthesized by dry-gel direct conversion using alkylimidazolium ionic liquid and tetramethylammonium hydroxide as templates.It was found that the alkyl chain length of the ionic liquids had significant effect on the crystalline phase of the zeolite products,and 1-butyl-3-methylimidazole cation was the most suitable template for LTA structure.The tetramethylammonium cation was also essential as co-template during the formation of LTA structure.An unknown phase was found in the initial stage of crystallization process,which may be the precursor of the LTA structure.The introduction of silica did not affect the topology of the zeolite,but large amounts of silica resulted in a decrease of the crystallinity,specific surface area and micropore volume,but an increase of acid amount.The silicon introduced without phosphorus decrease presented in the form of silicon island and the zeolite had bad catalytic activity in the ethanol dehydration to ethylene reaction.The silicon introduced with phosphorus decrease facilitated the formation of isolated silicon,and led to a significant increase of the catalytic activity of zeolite in the reaction.The one-step synthesis of SAPO-LTA zeolite was achieved by combining the crystallization and calcination processes in a single step.The rate of temperature rise in the one-step calcination process influenced the topology of the zeolite,with lower rate being more favorable.The type of silicon source did not affect the topology,but it affected the crystallinity of the zeolite,and colloidal silica was the most suitable.The amount of silicon doping did not affect the crystalline structure,but the high amount of silicon also made the zeolite less crystallized.Compared with zeolite obtained by dry-gel direct conversion,the zeolite obtained by the one-step calcination had more isolated silica and stronger acidity and exhibited better catalytic performance in the reaction of ethanol dehydration to ethylene.99% ethylene yield was achieved at 300℃,which was only 25℃ higher than the commercial SAPO-34 zeolite.