The Effect Of Silicon Distribution In SAPO-34 Molecular Sieves On The Catalysis Of Methanol To Olefins

As the shortage of non-renewable energy resource,light olefins can be acquired from methanol to olefins?MTO?process which has drawn attentions in the world.SAPO-34,as its unique CHA cages and moderate acidity,has shown superior catalytic performance and light olefins selectivities in MTO process.But the deactivation can be easily occurred on SAPO-34 due to its narrow 8 ring pore structure which results in the rapid formation of coke.The acidity of SAPO-34 is varied with its Si distribution.SAPO-34 with suitable acidity will enhance its catalytic performance.Thus,this paper mainly committed to control the Si distributions in SAPO-34 which enhanced its catalytic performance in MTO process.The Si content and distribution of SAPO-34 can be changed by secondary crystallization with seeding SAPO-34.The volume of the gel in secondary crystallization was 30 mL.The ratio of the seeding weight and gel volume was at a range of 1:10-15.SAPO-34 with low Si content and acidity can be obtained after secondary crystallization.In order to explore the properties of these SAPO-34 catalysts,a series of characterizations was carried out,such as XRF,²⁹Si MAS NMR,NH₃-TPD.The results indicated that the Si distribution and content of seeding SAPO-34 have changed after the secondary crystallization.The catalytic test results showed that CSSAPO-34-2 had the longest lifetime?411 min?and the highest light olefins seleceivities?87.3%?.The ratio of the seeding weight and gel volume over CSSAPO-34-2 was 1:12.CSSAPO-34-2 with much lower Si content and acidity can reduce the products further condensation and enhance the performance of the catalyst in MTO reaction.A series of SAPO-34 catalysts can be acquired by using different hydrothermal temperatures at 190,200,210 and 220 ^oC.The catalysts were characterized by XRD,SEM,XRF,²⁹Si MAS NMR,NH₃-TPD and so on.The results proved that SAPO-34-210 had the unique Si distribution due to the formation of the much larger Si islands.The results of MTO activity test indicated that SAPO-34-210 showed the longest lifetime?947 min?and the highest light olefins seleceivities?85.2%?among these SAPO-34 catalysts.This result was contributed to the much larger Si islands decreased the amount of the Si atoms at borders which can decrease the local strong acidity to inhibit the coking and increase the lifetime.