Synthesis, Modification And MTO Properties Of Silicoaluminophosphate Molecular Sieves

With the development of the national economy,the demand for light olefins was getting higher.At present,the light olefins were mainly produced through the traditional oil routes,but our country was lack of oil resources,so people were required to develop new non-oil routes for light olefins.Rencently,it is the emergence of methanol to olefins(MTO)that alleviate the above contradictions,because our country was rich in coal resources,and methanol could be obtained through the coal.The key of MTO lied in the research and development of catalysts.At present,the aluminosilicophosphate molecular sieves were used most in the MTO,but the synthesis of silicoaluminophosphate molecular sieves need to be further optimized and the MTO performance need to be further improved.SAPO-18 molecular sieves with differnt Si/Al ratio were synthesized by hydrothermal method in this paper.The results showed that SAPO-18 molecular sieves could be successfully synthesized when the Si/Al ratio was between 0.2 and 0.6,then the molecular sieves were evaluated by MTO.It was found that when the Si/Al ratio was low,the catalytic life of the molecular sieves was longer,but the selectivity of ethylene and propylene was lower,with the Si/Al ratio increased,the catalytic lifetime of the molecular sieves decreased and the selectivity of ethylene and propylene increased.In the process of hydrothermal synthesis of SAPO-18 molecular sieves,the crystallization of SAPO-18 molecular sieves was accelerated by the addition of Fenton reagent or potassium persulfate in the synthetic gel.The crystal process and the properties of the molecular sieve samples were characterized by ESR,XRD,SEM,FTIR and NH3-TPD.The effects of the amount of Fenton reagent or potassium persulfate on the SAPO-18 molecular sieve samples and its MTO catalytic performance were investigated.When added Fenton reagent,because the synthesis of the gel pH = 9?10,it will affect the Fenton reagent to generate hydroxyl radicals,so the addition of Fenton could not accelerate the crystallization rate.When added appropriate amount of potassium persulfate in the synthetic gel,the amount of hydroxyl radicals(·OH)was increased,the crystallinity of SAPO-18 molecular sieves was increased,and the morphology of SAPO-18 molecular sieves was improved.In addition,it was found that the life of SAPO-18 molecular sieves was increased by adding potassium persulfate.SAPO-18 molecular sieves were modified with Ba(NO3)2,Ce(NO3)2 and MnCl2 respectively.The Mn could improve the MTO performance of SAPO-18 molecular sieves,extending the catalytic life of SAPO-18 molecular sieves from 70 min to 80 min.SAPO-34 molecular sieves were modified with Ni(NO3)2,Ba(NO3)2 and CaCl2 by ion exchange method and the selectivity of ethylene and propylene was improved from 86.2%to 87.2%after CaCl2 modification.The saturated Ca(OH)2 was used to modify SAPO-34 molecular sieves and the MTO performance test showed that the selectivity of ethylene and propylene increased from 86.2%to 86.7%.The SAPO-34 molecular sieves were impregnated with 0.2 mol/L oxalic acid at the same volume,when the molecular sieves were impregnated with oxalic acid and washed,the selectivity of ethylene and propylene increased from 86.2%to 86.6%.Using methanol as solvent,the SAPO-34 molecular sieves were modified with KH-550 coupling agent to increase the selectivity of ethylene and propylene from 86.2%to 87.6%.