Preparation Of Meta-doped SAPO-34 Molecular Sieve And Its Application In MTO Reaction

As a representative process for preparing light olefins by non-petrochemical routes,methanol-to-olefins（MTO）technology has received extensive attention.As the preferred catalyst for MTO reaction,SAPO-34 molecular sieve shows the excellent MTO reaction performance.However,due to the increasing demand for light olefins,increasing the selectivity of ethylene,light olefins（ethylene+propylene）and C₂⁼/C₃⁼ratio in the MTO reaction has great significance and industrial application value.In this paper,the SAPO-34 molecular sieves modified by metal cations,in order to improve the selectivity of ethylene and light olefins.The SAPO-34 molecular sieve doped with transition metal Ni and Co was synthesized by one-pot hydrothermal synthesis method.The incorporation of Ni can increase the selectivity of ethylene,and with the increase of Ni content,the selectivity of ethylene and C₂⁼/C₃⁼ratio gradually increases,while the selectivity of light olefins decreases;the MTO reaction performance of SAPO-34 molecular sieve after Co doping showed high diene and ethylene selectivity.The diene selectivity increased from 84.35%of the undoped sample to 87.86%,and the ethylene selectivity increased from 48.89%to 56.46%,but the propylene selectivity of the sample after the Co addition is higher.The C₂⁼/C₃⁼ratio is higher than undoped sample but is lower than Ni doped samples.Then,the Zn-doped SAPO-34 molecular sieves were synthesized by one-pot method.After MTO reaction,SAPO-34 molecular sieve doped with Zn has an excellent MTO reaction performance.Compared with undoped SAPO-34 molecular sieve,ethylene selectivity of SAPO-34 molecular sieve doped with Zn increases by about 11%to 60.02%;diene selectivity increased by about 6%to 89.61%;the ethylene/propylene ratio was up to 2.04,and the incorporation of Zn can improve the initial products selectivity,making the initial stage of light olefin selectivity increased significantly.The Zn-modified SAPO-34 molecular sieve was prepared by ion exchange method and impregnation method in different ways,and the influence of synthesis method on the performance of the catalyst was investigated.Exploring different ion exchange processes,it is found that the template agent and inorganic NH₄⁺in the template method and the two-step method protect the structure of SAPO-34 molecular sieve.Exploring the different impregnation processes found that the inorganic NH₄⁺in the two-step method has a protective effect on the structure of the SAPO-34 molecular sieve,so that the prepared samples maintain a high crystallinity and micropore surface area.