Seed Assisted Synthesis Of SAPO-34 And Their Performance In CH₃Br To Light Olefins

CH₃Br to light olefins is a process of the methane activation,using halogen to produce CH₃Br then converting CH₃Br to ethylene and propylene with high selectivities on molecular sieve catalyst,finally realizing the aim of natural gas to light olefins.SAPO-34 is commonly used to catalyze the reaction of methyl bromide to olefins due to the suitable pore structure.At present,the main problem of SAPO-34 catalyzing the reaction of CH₃Br to olefin is that the short catalyst lifetime and the conversion of CH₃Br is not high.In this thesis,the SAPO-34s with adjustable particle size were prepared by seed-assisted method,in which the seed was obtained under solvent-free condition;Nano-sized SAPO-34 was synthesized by seed-assisted method,using seeds to replace oganic template,the nano-sized SAPO-34 shown excellent catalytic performance;A process of CH₃Br to 300000 t/a of low carbon olefin was designed according to the experimental results.The main research contents and results of the thesis are as follows:Firstly,SAPO-34 seeds with different crystallization degrees were prepared by changing the crystallization conditions.On this basis,the SAPO-34s with different particle sizes were prepared by seed-assisted method.In addition,the acidity of the SAPO-34 is adjusted by changing silica content of the gel composition.Compared with the seed synthesized by conventional hydrothermal condition,the solvent-free seeds contain a large number of defects and multi-stage pores,which are favorable for dissolving to form smaller crystal nuclei,thereby producing the SAPO-34 with the particle size of 1⁷μm by the solvent-free seed-assisted method;the acidity of the SAPO-34 can be reduced by reducing the silicon content of the gel.Compared with the conventional SAPO-34,the CH₃Br conversion of the SAPO-34 synthesized by solvent-free seed assisted method can increase from 82.1%to 96.6%,and the lifetime was prolonged more than twice.Secondly,the seed could guide the growth of SAPO-34,thus the seed can partially replace the template.The nano-sized SAPO34 was thereby synthesized by reducing the amount of the template agent.Based on that,the catalytic performance of the SAPO-34 was further optimized by adjusting the gel composition to modulate the acidity of the molecular sieve.The analysis results showed that the nucleation is dissolved and the size is reduced with the decrease of the amount of templating agent,which is beneficial to the synthesis of nano-sized SAPO-34.When the amount of templating agent and silica sol in the system is reduced to 2/3,the CH₃Br conversion and the olefins selectivities are greatly improved,and it showed the best catalytic performance.Finally,a process of CH₃Br to 300000 t/a light olefins was designed.The material balance of the process was calculated according to the experimental data.The technological parameters of the fluidized bed reactor and ethylene distillation column were determined.The process is simple,showing high products efficiency and environment protection.