Synthesis And Characterization Of SAPO-14 Molecular Sieves And Their MTO Catalytic Mechanism

Methanol-to-olefin(MTO)reaction provides an alternative way to produce light olefin from coal,natural gas or biomass via methanol,which is of great significance for ensuring the energy security of our country.SAPO-34 molecular sieve with small pore and large cage structure has been applied in MTO industrial process due to its excellent catalytic performance.However,the yield of ethylene or propylene is comparable.It is still challenging to modify the ratio of ethylene and propylene according to the market demand.SAPO-14 molecular sieve with AFN topology was recently synthesized and found to have very high propylene selectivity over 70%.In this work,the synthesis of SAPO-14 was further investigated to control its morphology and acidity with the aim to improve its MTO catalytic performance.The reaction route of MTO and the deactivation process of SAPO-14 molecular sieve were also studied.The specific research contents and results are as follows:1.SAPO-14 molecular sieve was synthesized with the assistance of various organosilanes such as octyl triethoxy silane(KH-350),[3-(trimethoxysilyl)propyl]octadecyldimethylammonium chloride(TPOAC)and 3-piperazinylpropylmethyldimethoxysilane(PZPMS).SAPO-14 with high silicon contents was synthesized by using KH-350.When PZPMS was introduced,SAPO-5 and SAPO-34 molecular sieves with special morphology were synthesized.The effects of OSD A,organosilane and silicon contents on the crystallization results and the reason for the formation of the special morphology of SAPO molecular sieve were investigated in detail.Notably,SAPO-34 samples with hexagon-pyramid morphology showed high ethylene plus propylene selectivity up to 86.95%in the MTO reaction.2.The MTO reaction routes over SAPO-14 was studied by using the self-made fixed bed reactor with seconds-order sampling function and in-situ UV Raman spectroscopy system combined with the 12C/13C methanol switching experiment.A reaction route transformation from aromatics cycle dominant to olefins cycle dominant was observed,and the reaction path was regulated by propylene co-feeding experiment.The carbon deposition and deactivation of SAPO-14 were investigated by GC-MS,UV-Vis spectroscopy and MALDI FT-ICR MS.It was confirmed that the deactivation of SAPO-14 was mainly caused by the formation of methyl-substituted aromatic hydrocarbon species,which blocked the pore channels of the catalyst.