MTO Catalytic Performance Of Monolithic AlPO₄-5/SAPO-34 Composite Molecular Sieve

SAPO-34 molecular sieve is recognized as the prime catalyst in MTO reaction which possess good hydrothermal stability and high ethylene selectivity.However,due to the narrow intracrystalline micropores,the slow diffusion of reactants and products restricts the mass and heat transfer,resulting in its short catalytic lifetime.The monolithic catalysts prepared by 3D printing technology not only provide macroscopic structural optimization and microscopic regulation of active components,but also enhance the mass and heat transfer in the reaction process.Exploiting the features of AlPO4-5 molecular sieve with adjustable pore structure and acidity and high DME selectivity,the AlPO4-5/SAPO-34 monolithic composite molecular sieve with core-shell structure was prepared by 3D printing technology and secondary growth method which can extend the catalytic lifetime of SAPO-34 molecular sieve while maintaining high olefins selectivity and methanol conversion.Details are as follows:Firstly,the controlled synthesis conditions of AlPO4-5 molecular sieve were systematically investigated.The experimental results showed that to regulate the crystal morphology by controlling the water content in the synthesis solution,the content of template agent,crystallization time and crystallization temperature could produce the hexagonal platelet-shaped AlPO4-5 molecular sieve with regular morphology.The synthesis conditions of the subsequent AlPO4-5 molecular sieve get determined as follows:1Al2O3:1.3P2O5:1.2TEA:100H2O,and crystallized for 6 hours at 180℃.Secondly,the prepared AlPO4-5 was mechanically mixed with SAPO-34 and then evaluated by MTO catalytic performance.When the reaction conditions are 10:1 mixing,reaction temperature of 430 ℃,reaction space velocity of 1 h-1,its olefins(C2=-C4=)selectivity can reach 84.5%and the catalytic lifetime was about 5 hours.The monolithic SAPO-34 catalyst(MS)and monolithic SAPO34@AlPO4-5 catalyst(MSA)were prepared directly by 3D printing technology.Compared with the SAPO-34 molecular sieve catalyst,the catalytic performance of the monolithic catalyst has been improved.The olefins selectivity of the catalyst MS is 83.96%,and the catalytic lifetime is about 5 hours;the olefins selectivity of catalyst MSA can reach 84.71%with a significantly longer catalytic lifetime,the catalyst has not been deactivated for 5 hours,and the conversion rate has kept stable with the increase of reaction time.Finally,using MS and MSA monolithic molecular sieves prepared by 3D printing technology as the main catalysts,the monolithic AlPO4-5/SAPO-34 composite catalysts(MA/S)with core-shell structure were synthesized by secondary growth method.The effect of crystallization time on catalysts and their MTO catalytic properties were studied systematically.It shows that the catalytic lifetime of catalysts(MA/S)get extended to 6 hours.The catalysts exhibit good MTO performance when crystallization time is 1 hour.The olefins selectivity of MA/S-4(84.07%)supported on MS is lower than that of MA/S-1(86.42%)supported on MSA.The reason is that the surface of catalysts supported by MSA has not only AlPO4-5 molecular sieve layer,but also a large number of pores communicating with SAPO-34 and AlPO4-5 molecular sieve,which makes it show a better catalytic performance and synergistic effect in MTO reaction.