Study Of In-situ Synthesis And Modification Of Zeolite L On Kaolin Microspheres

The zeolite in-situ synthesized on Kaolin microspheres was widely used in the processes of petroleum cracking not only because of its good activity,stability, selectivity,and its good performance of wear-resistant and counterpoisoning to heavy mental,but also because of its low price and good bonding property.The zeolite L had been insitu-synthcsized on Kaolin microspheres in our lab,but the product appears fine dust with low strength and bad fluidity which restrict its use in fluid catalytic cracking process.For this reason,the study systematically investigated several preparative factors including stiring,aging time,crystallization time,the volume of zeolite L initiator and discussed their effects on the synthesis of zcolites on calcined kaolin supports.Thus,zeolite L was successfully synthesized in 1 liter autoclave,its product was modified to reducc its K₂O content and to improve its framework Si/Al ratio for better catalytic performance.With a series of modification methods including ion-exchange,thermal calcination,sodium fluosilicate modification,self-made gel modification,ethanedioic acid modification,the combined use of acid(citric acid, hydrochloric acid) treatment,ion-exchange and thermalcalcination.The phase structure,crystal morphology,packing density and surface area were characterized by XRD,SEM,BET etc.The results showed that aging time and the volume of zeolite L initiator had little effect on the product.Stiring or not in crystallization process posed great influence on the property of the product.The product with stiring in crystallization appeared fines,with its packing density 0.51 g/cm³,crystallinity 57%,surface area 205.8m²/g;while the one without stirring in crystallization appeared microspheres and had good fluidity,with packing density 0.86 g/cm³,crystallinity 44%,surface area 57.3m²/g.Its crystallinity and surface area was inferior to the product with stiring in crystallization.Also crystallization time affected the crystallinity of products dramatically.When crystallization time without stiring was 6h,10h,14h,18h,22h, the crystallinity of products was 20%,31%,37%,32%and 27%respectively.It could be concluded that with the increasing of crystallization time,the crystallinity could reach a peak then decline again.The optimal reaction condition in ion-exchange reaction is as follows:the amount of zeolite:water:ammonium sulfate:1g:10ml:1g;ion-exchange 2h at 80℃.The K₂O content of the sample which was ion-exchange three times could decreased to 0.55wt%.It could satisfy the demand of petroleum cracking process.With the increasement of calcination times,the Si/Al ratio of zeolite raised correspondingly, while the crystallinity decreasd.The Si/Al ratio of the sample which was ion-exchanged and thermalealcined 2 times reached 3.70,its relative crystallinity was 75%.Sodium fluosilicate modification combined with calcination can improve the Si/Al ratio of the sample obviously,but the structure of kaolin microspheres was destroyed severly.The three modification methods including self-made gel modification,ethanedioic acid modification,hydrochloric acid treatment couldn't improve the Si/Al ratio of zeolite L effectively.The combined use of ion-exchange, critric acid treatment and hydrothermal treatment could improve the Si/Al ratio of zeolite L dramatically while the high crystallinity was held.When the concentration of citric acid is 0.10mol/L,the Si/Al ratio of the modified zeolite which was treated respectively three times by ion exchange,citric acid dealuminzation,and hydrothermal calcination reached to 14.22,its relative crystallinity was 73%.The optimum condition to get ideal zeolite L using kaolin microspheres by insitu hydrothermal crystallization was as follows:the mole ratio of reagent: 4.2K₂O·Al₂O₃·15SiO₂·210H₂O,without aging,the volume of zeolite L initiator: 10wt%of kaolin microspheres,crystallize 14h stewingly at 120℃.The crystallinity of product was 37.5%,its yield reached 56wt%.The combined use of ion-exchange, critric acid treatment and hydrothermal treatment for zeolite L modification obtained good results.