Effects Of Template,Silicon Source And Crystallization Process On Properties And Performances Of SAPO-34

SAPO-34 molecular sieve is one of the common used catalysts for methanol to olefin(MTO)reaction,and has good selectivity to low carbon olefins in MTO reaction.However,due to its small pore size,large scale intermediates such as multi-branched hydrocarbons,highcarbon hydrocarbons and aromatic hydrocarbons are difficult to diffuse out from cages,which resulting in rapid carbon deposition and lead to the deactivation of catalyst.Reducing the size of SAPO-34 crystal favors the diffusion of olefin,reduces the further polymerization and dehydroaromatization of the olefin,thereby slows down the deposition of coke and prolonging the lifetime of catalyst.In the previous work,our group have proposed a three-stage crystallization method for producing small size SAPO-34.Compared with one-stage and twostage crystallization methods,the SAPO-34 molecular sieve obtained by three-stage crystallization method showed smaller grain-size,higher olefin selectivity and longer catalyst life.The influences of template agents(TEAOH,TEA and MOR),silicon sources(gas phase silica,acidic silica sol,alkaline silica sol,tetraethyl silicate(TEOS))and crystallization schemes(One-stage,two-stage and three-stage crystallization methods)on the properties and performances of SAPO-34 were further studied in this work.The main research results are as follows:(1)With TEAOH template,samples obtained by three-stage method are pure SAPO-34 or with very little SAPO-5;while notable SAPO-5 existed in ones obtained by one-stage and twostage methods except the one with TEOS as the silicon source.The sample obtained by the three-stage method shows smaller particle size,higher acid amount,higher olefin selectivity,and longer life.Three-stage sample with TEOS as silicon sourceshowed a selectivity of ethylene + propylene up to 80%,and maintained a methanol conversion over 98% in 420 min.(2)When TEA is used as a template,all samples are pure SAPO-34 crystal phase.The three-stage sample is generally higher in yield than the same silicon source by one-stage and two-stage crystallization methods,the crystal grain size is small,the acid amount is high,the catalyst has high olefin selectivity and long life.When the acidic silica sol or tetraethyl silicate was used as the silicon source,the selectivity of ethylene + propylene in the three-stage sample is above 76%,and both catalysts maintain the methanol conversion rate of over 98% in 240 min.(3)When MOR was used as the template,the samples obtained by the three-stage method are pure SAPO-34 crystal phase;when the silicon source is fumed silica or acidic silica sol,SAPO-5 is obtained from the one-stage and two-stage methods.Acid and specific surface area of the samples obtained by the three-stage method is generally higher than the same silicon source by a one-stage and two-stage crystallization method.When the silicon source is alkaline silica sol and tetraethyl silicate,all samples are pure SAPO-34 crystal phase;one-stage sample is generally more acid than the same silicon source using two-stage and three-stage crystallization samples.The catalyst has higher olefin selectivity and longer life.When tetraethyl silicate is used as the silicon source,the selectivity of ethylene + propylene of onestage sample is as high as 82%,and the catalyst maintains a methanol conversion rate of over 98% in 150 min.(4)The performance of the obtained sample is best when TEAOH is used as the template between different templating agents;the second is TEA;and the performance of the sample is relatively poor when MOR is used as the template.