Preparation Of SAPO-34 With Seed Method And Its Application In MTO

With the increasing light olefins demand,producing light olefins via non-oil routes,especially methanol-to-olefin(MTO)technology has received great improvements.Owing to the medium acidity and unique pore structure,SAPO-34 molecular sieve usually shows excellent conversion for methanol and high selectivity to light olefins in MTO reaction,and it is one of common catalysts uesed in MTO technology,but its life span is short because of coke deposotion,and the cost of conventional hydrothermal method is high.Therefore,the application of SAPO-34 in MTO reaction is largely limited.Herein,a method to synthesize small crystal SAPO-34 with less crystal seed and low template consumption was adopted.Besides,in MTO reactions,the catalytic performance of SAPO-34 synthesized with hydroythemal method or seed method was investigated in therms of their life spans and their selectivity to light olefins.The results manifest that:(1)SAPO-34 molecular sieves synthesized with different templates showed different properties and reaction performances,and among them,the sample synthesized on the guidance of morpholine showed highest crystallinity,but it had the shortest life span.In addition,the sample on the guidance of morpholine and a little tetraethylammonium hydroxide showed higher conversion and selectivity to light olefins in MTO reaction;(2)When triethylamine was chosen as the template of the initial gel,different seeds had different structure directing effect during the crystallization.The seed synthesized with triethylamine had the weakest effect,and the sample contained SAPO-5 molecular sieves,but pure SAPO-34 molecular sieves could be got when morpholine was used to synthesize the seed.Compared with uncalcined seed,calcined seed was more beneficial to synthesize small crystal SAPO-34 molecular sieves with medium acidity.(3)With less template and seed,seed method was easier to synthesize SAPO-34 molecular sieves than conventional method,SAPO-34 molecular sieves with high crystallinity could be synthesized under crystallization time of 2 h or crystallization temperature of 140 o C.Besides,adding seed prevented forming impurity,accelerated the crystallization rate,and promote more template to enter the channel of SAPO-34.Compare with the samples synthesized by conventional method,samples synthesized by seed method had smaller and more uniform crystals.(4)In MTO reaction,the sample synthesized with no crystal seed and less template showed the lowest selectivity of light olefins and inactivated rapidly.However,samples synthesized in the presence of a little crystal seed had longer life and higher selectivity to light olefins,even exceeding the commercial SAPO-34 molecular sieves and the sample synthesized under the best condition of hydrothermal method.