Study On Catalytic Mechanism And Coking Behavior Of SAPO-34/t-ZrO₂ Composite Catalyst In MTO Reaction

The methanol to olefin?MTO?process effectively alleviated the current contradiction between oil supply and demand.SAPO-34 molecular sieve is the most promising MTO reaction catalyst,and its modification to effectively prolong the lifetime has been a central issue.In connection with the lifetime extension of SAPO-34 is not obvious after current modification of surface acidity and structural properties,the author's research group has proposed to construct a SAPO-34/ZrO₂composite catalyst based on micro-meso porous structure.By enriching the active center of the catalyst and constructing a porous structure coordinated with the acid center to suppress the formation of coke and prolong the lifetime.The results showed that the catalytic performance and lifetime of the composite catalyst are significantly superior than single SAPO-34.In this paper,SAPO-34 and physical mixed t-ZrO₂/SAPO-34 catalysts were used as reference.The catalytic mechanism and coking behavior of SAPO-34/t-ZrO₂ composite catalyst in MTO reaction process were studied,the structure-activity relationship of SAPO-34 and t-ZrO₂ two-phase synergistic catalysis were revealed.The structure activity relationship of SAPO-34/t-ZrO₂ composite catalyst was revealed by means of XRD,XRF,XPS NH₃/CO₂-IR.The results showed that the SAPO-34 and t-ZrO₂ two phase occurred solid solution reaction,which reduced the silicon content ratio of SAPO-34 and enriched the active centers.The in-situ infrared technique was used to investigate the catalytic conversion mechanism of methanol on above three catalysts under the conditions of reaction temperature of 380?and mass space velocity of 2h^-1.The results showed that the synergistically catalyze between SAPO-34 and t-ZrO₂ changed a part of reaction pathway and inhibited the coking formation.Specifically,during the reaction,CO and H₂ catalyzed by t-ZrO₂ were further reacted with ethylene produced by SAPO-34 to form aldehyde specie,and the aldehyde specie returned to the SAPO-34 catalytic system to form a long-chain propyl substituent,followed by continue to react to form ethylene.In addition,the effect of reaction temperature on the catalytic conversion mechanism of SAPO-34/t-ZrO₂composite catalyst was studied.The results showed that the species on the SAPO-34/t-ZrO₂ composite catalyst gradually changed from methylbenzenes to polycyclic aromatic hydrocarbons?PAHs?with the increase of reaction temperature,but different from SAPO-34,there was no obvious adamantane specie at the lower reaction temperature of 330?.The TG-IR and O₂-TPO-MS method were used to studied the change of coke amount on above three catalysts with the reaction time.The results showed that SAPO-34/t-ZrO₂ exhibitedmuchless cokeamountthanSAPO-34and t-ZrO₂/SAPO-34 with same reaction time under the the conditions of reaction temperature of 380?and mass space velocity of 2h^-1,the coke amount of SAPO-34/t-ZrO₂ in reaction at 60min,360min,720min and 1080min was11.48mg_coke/g_catalyst,22.74mg_coke/g_catalyst,23.85mg_coke/g_catalystatalyst and 25.46mg_coke/g_catalyst,and the oxygen consumption of coke was 1.15mmol/g_catalyst,0.94mmol/g_catalyst,1.33mmol/g_catalystatalyst and 1.70mmol/g_catalyst.The evolution of coke species of above three catalysts with the reaction time was studied by FT-IR and dissolution-extraction experiment.The results showed that the coke species of the three catalyst in MTO reaction were mainly methylbenzenes and PAHs,the methylbenzenes gradually transformed into PAHs as reaction time prolonged.The spatial distribution of coke was investigated the confocal fluorescence imaging technique,the result showed that SAPO-34 only produced fluorescence feedback at the edge of the grain,while SAPO-34/t-ZrO₂ also produced fluorescence feedback at the inner core,indicating that its internal core active site was also involved in the MTO reaction.The characteristics of coke cracking were studied by TG-IR and TPO-MS.The cracking coke account for a high proportion on SAPO-34/t-ZrO₂,in reaction at 60min,360min,720min and 1080min was 54.5%,47.2%,99.2%and 53.7%.the cracking products were mainly ethylene and propylene.The apparent activation energy of ethylene and propylene cracked by coke were calculated by the production rates at different heating rates.The coke of SAPO-34/t-ZrO₂ showed smaller apparent activation energy of cracking to ethylene and propylene than SAPO-34 and t-ZrO₂/SAPO-34,was 138.3kJ/mol and 123.9kJ/mol respectively,implied excellent coke stripping property and industrial application value.