Performance Improvement And Coke-burning Kinetics Of SAPO-34 Catalyst For Methanol To Olefins(MTO)reaction

Using methanol as raw material to produce light olefins can greatly alleviate China's dependence on petroleum resources.SAPO-34 molecular sieve is considered as the best catalyst for methanol to olefins?MTO?process due to its good selectivity to light olefins,excellent hydrothermal stability and thermal stability.However,because the pore size of SAPO-34 molecular sieve is small,it is easy to block the catalyst pores by the macromolecular polycyclic aromatic hydrocarbons formed in the MTO reaction.The macromolecular polycyclic aromatic hydrocarbons are finally condensed into coke and deposit on molecular sieve,which causes the deactivation of the catalyst.Therefore,developing simple and efficient methods to prolong the catalyst lifetime,and exploring the kinetics of regeneration by burning the coke on the deactivated catalysts are the most important issues of the MTO reaction.First,SAPO-34 molecular sieve was synthesized using a gel with molar ratio of 0.2Si O₂:1.0 Al₂O₃:1.0 P₂O₅:2.9 TEA:0.1 TEAOH:40 H₂O?TEAOH is tetraethylammonium hydroxide,TEA is triethylamine?.Then,catalyst forming of SAPO-34 was performed and then it was hydrothermally aged at 750°C for 6 h.Comparing the catalytic performance in the MTO reaction and coke-burning kinetics of SAPO-34catalyst with the hydrothermally aged SAPO-34 catalyst,it was found that the crystal structure and pore structure of the SAPO-34 catalyst did not appear obvious change after being hydrothermally aged,but the acidity reduced.In the MTO reaction,the lifetime of the hydrothermally aged SAPO-34 catalyst was significantly shortened,and the amount of carbon deposited on SAPO-34 catalyst decreased significantly when SAPO-34 catalyst was deactivated,but the coke-burning activation energy of SAPO-34 catalyst and hydrothermally aged SAPO-34 catalyst was similar to each other,indicating that the properties of carbon deposited on the both SAPO-34 catalysts were almost the same.The effect of physical mixture of SAPO-34 molecular sieve with Mg O or Mo O₃under different packing modes on the MTO reaction was studied.When Mg O was packed in the upstream or downstream of the catalyst SAPO-34,the lifetime of the catalyst was not extended.When Mg O was physically mixed with SAPO-34,the catalyst lifetime was greatly prolonged.However,when Mo O₃ was physically mixed with SAPO-34,the catalyst lifetime was greatly shortened.In addition,the amount of formaldehyde formed in MTO under different catalyst packing modes was measured by ultraviolet spectrophotometry,and the flame ionization detector?FID?and thermal conductivity detector?TCD?were both used to analyze the yields and selectivity of all products,including CO,CO₂ and H₂.It was found that formaldehyde is the precursor of carbon deposit,and Mg O can decompose formaldehyde,thus reducing the formation rate of coke deposited on SAPO-34 and prolonging catalyst lifetime.Mo O₃ can catalyze the conversion of methanol to formaldehyde,which is further condensed into coke on SAPO-34,so physical mix Mo O₃ with SAPO-34 can accelerate the deactivation of the catalyst.Besides,the methanol-formaldehyde solution was used as feedstock of MTO reaction,further proved that the formaldehyde was the precursor of the coke deposited on SAPO-34 molecular sieve in the MTO reaction.Finally,in order to investigate the effect of Mg O and Mo O₃on the properties of carbon formed on the SAPO-34 catalyst,the FT-IR,UV-Vis and ¹³C NMR characterization were performed to analyze the coke formed on the SAPO-34 catalyst,Mg O/SAPO-34 catalyst,and Mo O₃/SAPO-34 catalyst,respectively,and the coke-burning kinetics of the deactivated catalysts was also investigated by thermogravimetric analysis.It was found that,the coke-burning activation energy of these deactivated catalysts also varied very little.Therefore,physical mixture of Mg O and Mo O₃ with SAPO-34 catalyst did not change the properties of carbon deposited on the catalysts.When methanol-formaldehyde was used as feedstock,the coke-burning activation energy of the deactivated SAPO-34 catalyst was almost similar to that when methanol was used as feedstock.Although the formaldehyde contained in the feedstock made catalyst deactivate quickly,the properties of coke were less affected.