Synthesis And Characterization Of SAPO-34 As Catalyst For Methanol To Olefins

Light olefins,especially ethylene,are very important raw materials for the chemical industry,the production of which is a standard to measure the chemical industry of a country.In the traditional chemical industry,ethylene is produced by petroleum cracking.With the decrease of the global oil reserves,ethylene produced by petroleum cracking could no longer meet the increasing demand of the world.Thus,producing light olefins from methanol?MTO?becoming more and more popular all around the world.Even though MTO process has already been applied in the chemical industry,the lifetime and the selectivity to light olefins of the MTO catalyst still need further development.The synthesis of SAPO-34,which is the catalyst in the MTO process,has been researched to prolong the lifetime in the MTO reaction.Firstly,SAPO-34 had been synthesized by the hydrothermal method with the addition of different molecular weights of polyethylene glycol?PEG?,and has been characterized with XRD,SEM,N₂ adsorption-desorption,FTIR,and NH₃ temperature programmed desorption?NH₃-TPD?,and is studied as a catalyst in the methanol-to-olefins?MTO?reaction,in a fixed bed reactor.PEG acts as inhibitor in the crystallization process.The results show that the chain length of PEG has a great influence on the particle size and morphology of SAPO-34.With the increase of the chain length of PEG,used in the synthesis,from a relative molecular weight of 400 to6000,the morphology of SAPO-34 changes gradually from cubic to nanoplate-like and then changes to cubic again.The particle size decreases markedly at first and then increases to some extent.The catalytic stability in the MTO reaction also increases first and then decreases,with all the catalysts having almost the same selectivity to olefins.When the sample is synthesized with PEG800,the particles become nanoplate-like with a thickness of 46 nm on average,and the catalytic stability is appreciably prolonged,which is attributed to the shorter diffusion paths of the reactants in the zeolite.Secondly,the effect of the aging temperature in the synthesis of SAPO-34 on the catalytic properties and the catalytic performance in MTO reaction was investigated.The results showed that the aging temperature had great effect on the particle size of the catalyst.When the aging temperature was below 120?,the particle size of the synthesized SAPO-34 was in the range of 400 nm to 500 nm,and the synthesized SAPO-34 performed well in MTO reaction.When the aging temperature was too high,for example 150? and 170?,the particle size of the synthesized SAPO-34increased from nanoscale to micron scale.And the high aging temperature made the concentration of the strong acid sites increase,thus accelerated the carbon deposition and decreased the lifetime of the catalyst.