Alkaline Earth Metal Modified SAPO-34 Used For Methanol Conversion To Light Olefins

As a potential olefin manufacture method,methanol to olefins(MTO) has received wide attention in the past years.The key issue for the MTO investigation is to develop new catalyst with the property of higher selectivity to olefins,longer lifetime,and more excellent reproducibility.SAPO-34 zeolite is a promising catalyst for the MTO reaction due to its high conversion of methanol and selectivity to olefins.However,coke deposition easily occurs over catalyst surface during the MTO reaction,which blocks the channels of the zeolite, leading to fast deactivation of the catalyst.Therefore,it is one of important topics for the researchers to improve the anti-coking ability of the catalyst and thus prolongs the lifetime of catalyst.This thesis aimed to prolong the lifetime of the MTO catalyst by modification of SAPO-34 with alkaline earth metals(Mg,Ca,Sr,and Ba).In this thesis,the catalysts prepared by modifying SAPO-34 with alkaline earth metals were investigated for the reaction of MTO and it was found that Ba modified SAPO-34 catalyst exhibited longer lifetime compared with catalysts modified with other alkaline earth metals.The lifetime of catalyst was greatly influenced by Ba content in the catalyst.The incorporation of small amount of Ba remarkably promoted the MTO reaction and significantly increased the anti-coking ability of SAPO-34.The 0.5-1%Ba modified SAPO-34 catalyst exhibited a prolonged lifetime by 27%compared with SAPO-34 under the reaction conditions of WHSV=2h^-1,450℃and atmospheric pressure.However,the increasing Ba content in the catalyst resulted in a reduced lifetime and the catalyst containing large amount of Ba(more than 10%) even behaved a little shorter lifetime than SAPO-34. The characterization of Ba/SAPO-34 catalyst by XRD and FT-IR showed that crystal of BaO has appeared over the 6%Ba/SAPO-34 and 10%Ba/SAPO-34 catalyst.It indicates that Ba²⁺ cation aggregates to form BaO outside of the channels of SAPO-34 when more than 6%of Ba is contained in the catalyst.The selectivity to methane,ethane,propane and butane was significantly depressed by incorporation of small amount of Ba.Correspondingly,the selectivity to ethylene and propylene was greatly increased.The depression of hydrogen-rich products(mainly presents as methane) can be well correlated with those of the hydrogen-deficient species deposited over catalyst surface(mainly presents as coke).It is the reason why the coke formation was remarkably depressed and the lifetime of 1%Ba/SAPO-34 catalyst was effectively prolonged.The factors influencing the reaction were also investigated and the optimized reaction condition was given for the MTO over the 1%Ba/SAPO-34 catalyst.