Two-stage Crystallization Synthesis Of SAPO-34 Molecular Sieve And Recycling Use Of Its Mother Liquor

SAPO-34 zeolite has a unique three-dimensional pore structure,suitable acid property,good thermal stability and hydrothermal stability.It exhibits excellent catalytic performance in the methanol-to-olefin reaction（MTO）and is the most commonly used catalyst for MTO.Synthesis of SAPO-34 by an environmentally friendly method and synthesis of SAPO-34 zeolite with high MTO catalytic performance has been the important issues that researchers pay attention to.Traditional hydrothermal synthesis of SAPO-34 zeolite will produce crystallized mother liquor containing a large amount of unreacted raw materials.Recycling the crystallized mother liquor to synthesize SAPO-34 is an effective green synthesis method.In this work,triethylamine（TEA）was used as a template to hydrothermally synthesize SAPO-34 zeolite from gel with composition of 1.0Al₂O₃:1.0P₂O₅:0.25Si O₂:3.0TEA:50H₂O,and the crystallized mother liquor was produced and recycled for use.SAPO-34 zeolites were synthesized through recycling use of crystallization mother liquor up to five cycles.Because the small amount of microcrystals remaining in the mother liquor plays the role of seed crystal in the synthesis of SAPO-34 zeolites using recycling mother liquor,the size of zeolite synthesized with recycling mother liquor is smaller than that by the traditional hydrothermal synthesis.The SAPO-34 synthesized with recycling mother liquor showed comparable or even better catalytic performance in the MTO reactionin comparison with that synthesized through traditional hydrothermal synthesis.In comparison with the triditional synthesis method,the amount of aluminum source and phosphorus source used in the mother-liquor recycling synthesis of SAPO-34 decreased by 7.78%and 24.61%,respectively,and the amount of triethylamine decreased by 92.62%.The crystallization conditions of SAPO-34 zeolite synthesis were explored when the synthesis gel with a composition of 1.0 Al₂O₃:0.25 Si O₂:1.0 P₂O₅:0.4 TEAOH:2.6TEA:50 H₂O（TEAOH is tetraethylammonium hydroxide,used as template agent together with TEA）was used to hydrothermally synthesize SAPO-34 zeolite.Especially under the condition that the finial crystallization temperature was set as 180℃and the crystallization was fixed at 48 hours,the influence of the initial crystallization time on the physical and chemical properties and the catalytic performance of MTO reaction of SAPO-34 zeolites were investigated.The particle size、yield and the amount of acidity for the synthesized zeolite increased with increasing crystallization time of the first stage.It was found that when the crystallization of SAPO-34 in the first stage performed at 160℃for 48 h,the synthesized zeolite had the largest amount of strong acid,and also had the highest ethylene selectivity（up to 56.74%）in MTO reaction.The selectivity of ethylene and propylene on the synthesized SAPO-34 zeolite can reach83.95%and the lifetime can reach 215 min.Finally,the influence of the initial crystallization temperature（120℃、140℃、160℃）on the physical and chemical properties and the catalytic performance of MTO reaction of SAPO-34 zeolites were investigated under the condition that the finial crystallization temperature was set as 180℃and the crystallization was fixed at 48hours.It was found that with increasing crystallization temperature in the first stage,the particle size、yield and the amount of acidity of the synthesized zeolite showed increasing trend and the selectivity of ethylene and the selectivity of ethylene plus propene increased.When the crystallization of SAPO-34 in the first stage was performed at 160℃for 48 h,the synthesized zeolite had the highest ethylene selectivity（up to 56%）in MTO reaction.And the selectivity of ethylene and propylene can reach 84%.