Preparation Of CHA Zeolite Confined Metal Catalyst And Its Performance In Propane Dehydrogenation

Propylene,as an important basic organic chemical raw material,is crucial for the production of high-value chemicals such as polymers and oxygen-containing compounds.In view of the changes in the global energy structure,the limitations of traditional propylene production capacity and the growing maturity of shale gas mining technology,propane dehydrogenation（PDH）process has received extensive attention in recent years,and is expected to become an important way to solve the continuous growth of propylene demand.Platinum based catalysts are widely used in industrial PDH processes due to their ability to activate C-H bond cleavage dehydrogenation and inhibit C-C bond cleavage.The agglomeration and sintering of Pt nanoparticles caused by high-temperature reaction is one of the most important factors affecting the activity and stability of the catalyst.The use of molecular sieves with ordered microporous structure and excellent thermal stability,through"regioselectivity encapsulation",confines metal clusters to specific positions in the molecular sieve,which helps to reduce the migration of metal nanoparticles at high temperatures,thus increasing the accessible active site and improving the thermal stability of the catalyst.Based on previous research experience,this article prepared CHA molecular sieves with different elemental compositions and utilized their confined Pt Zn metal for propane dehydrogenation reaction.The specific research content is as follows:（1）Si-Al-CHA,Si-Ti-CHA,and Si-CHA molecular sieves with CHA structures were synthesized through crystal transformation using ZSM-5（Si O₂/Al₂O₃=300）,TS-1（Si O₂/Ti O₂=40）,S-1,and MCM-41,respectively.By using characterization methods such as XRD and SEM,the effects of crystallization time,crystal seeds,and template addition on the composition,structure,and properties of Si-Al-CHA molecular sieves synthesized through conversion were investigated.Based on this,the mechanism of inter zeolite conversion was explored.The results showed that the similarity of inter zeolite structure,skeleton density,and crystallization conditions had a significant impact on the zeolite conversion process.（2）By using the in situ synthesis strategy amine ligand protection method,metal Pt and Zn species complexed with ethylenediamine were added to the Si-Ti-CHA molecular sieve prepared by TS-1（Si O₂/Ti O₂=40）crystal transformation.Pt Zn species were encapsulated in the Si Ti CHA molecular sieve,and the prepared catalyst was characterized.The results indicate that Pt Zn species loaded on the outer surface of Si-Ti-CHA molecular sieve will migrate and aggregate at high temperatures to form larger Pt-Zn alloy particles,while Pt species encapsulated in the pores and cages of Si-Ti-CHA molecular sieve avoid agglomeration and sintering at high temperatures.The propane dehydrogenation performance of the prepared catalyst was studied,and0.5Pt2Zn@Si-Ti-CHA showed good propane dehydrogenation performance,with propane conversion rate and propylene selectivity reaching 16.8%and 86.1%,respectively.（3）By using the in situ synthesis strategy amine ligand protection method,metal Pt and Zn species complexed with ethylenediamine were added during the preparation of high silicon aluminum ratio Si-Al-CHA molecular sieves using ZSM-5（Si O₂/Al₂O₃=300）.Pt Zn was successfully encapsulated on Si-Al-CHA molecular sieves and the prepared catalyst was characterized.The results indicate that Pt species are completely confined in Si-Al-CHA molecular sieves at sub nanoscale and form Pt-Zn alloys with Zn species during the reduction process.The propane dehydrogenation performance of the prepared catalysts was investigated,and the dehydrogenation performance of Pt Zn catalysts with different preparation methods and platinum zinc contents was compared.Among them,0.5Pt3Zn@Si-Al-CHA showed the best propane dehydrogenation performance,with propane conversion rate and propylene selectivity reaching 23.7%and 96.7%,respectively.After three cycles of regeneration,the catalyst still exhibits excellent propane dehydrogenation performance.