Synthesis Of SAPO-34 By Recycling Of Mother Liquors And Its Catalytic Performance In MTO Reaction

SAPO-34 molecular sieve is considered as the best catalyst for methanol to olefins/dimethyl ether to olefins?MTO/DTO?process due to its good selectivity to light olefins,excellent hydrothermal stability and thermal stability.SAPO-34 molecular sieve is generally obtained by hydrothermal synthesis.This work aimed to investigate the type and amount of templating agent,the n?SiO₂?/n?Al₂O₃?ratio and so on in the synthesis of SAPO-34 molecular sieve,and investigate the recycling of mother liquor produced in the SAPO-34 synthesis process,for preparation of SAPO-34.The synthesis of SAPO-34 molecular sieve was conducted using a gel with molar ration of 1Al₂O₃:a SiO₂:1 P₂O₅:x TEAOH:y TEA:50 H₂O?TEAOH is tetraethylammonium hydroxide,TEA is triethylamine?.The results showed that the best Si/Al molar ratio was 0.2 when x=2 and y=0?ie,pure TEAOH template system?,and the lifetime of the as-synthesized can reach 275 min in MTO reaction and the selectivity of ethylene and propylene can reach 85.2%.While when x=0,y=3?ie,the pure TEA template system?,the optimal of n?SiO₂?/n?Al₂O₃?molar ratio is 0.25,and the catalytic lifetime of the as-synthesized molecular sieve can reach 215 min the MTO reaction,and the selectivity of ethylene and propylene can reach 84.2%.When x=0.4,y=2.6?composite template system?,the optimal of n?SiO₂?/n?Al₂O₃?molar ratio was 0.30,and the catalytic lifetime of as-synthesized molecular sieve can reach 245 min in MTO reaction,and the selectivity of ethylene and propylene can reach 84.9%.After systhesis of SAPO-34 molecular sieves,we added some amount of ingredients in mother liquor to kept the gel ingredient ratio constant as initial gel in each preparation of SAPO-34 molecular sieves by recycling of mother-liquors,and found that SAPO-34 molecular sieves were successfully synthesized this way.In this way,the cost of molecular sieve at least reduced⁵2%.It was found that TEAOH template used in the synthesis of SAPO-34 decomposed into TEA during the process of systhesis of SAPO-34 molecular sieves,making the size of synthesized SAPO-34 crystals become larger and larger.As a result,the MTO catalysic performance of the SAPO-34 deteriorated after three recycles.For the gel with TEA as templete,due to the presence of small grains in the crystallization mother liquor,the molecular sieve size obtained by the mother liquor recycling in the TEA system was smaller than that obtained by the conventional hydrothermal synthesis method,and thus had more excellent MTO catalytic performance,and the mother liquor can be recycled at least 5 times under the TEA system.When 0.4TEAOH/2.6TEA was used as template,the size of molecular sieve obtained by recycling mother liquor decreased first and then increased,and the MTO performance of the SAPO-34,prepared with four cycles of mother liquor,was better than that prepared in the initial synthesis,indicating that,when 0.4TEAOH/2.6TEA was used as template,the mother liquor could be recycled at least 4 times.The ¹³C NMR characterization results of SAPO-34 samples,prepared with different template,proved that TEAOH was easier to enter the SAPO-34 framework than TEA in the composite template system,and TEAOH template used in the synthesis of SAPO-34 decomposed into TEA during the process of systhesis of SAPO-34molecular sieves.