Hierarchical SAPO-34 Preparation Based On The Crystal Metastability And The Catalytic Performance Of MTO Reaction

Light olefins,including short-chain olefins C₂H₄ and C₃H₆,are basic building blocks employed in the various industrial processes.Light olefins are playing a vital role in both petrochemical and fine chemical processing.The traditional approach to obtain light olefins is mainly high-temperature naphtha cracking.However,non-renewable petroleum resources are currently facing exhaustion.The methanol-to-olefins（MTO）process is developed as an alternative non-petroleum route to produce light olefins.With methanol as intermediates,coal,natural gas,and even biomass can be effectively transformed and light olefins products can be obtained finally.The development of MTO technology,an efficient non-petroleum route to obtain light olefins,will help to get rid of dependence on oil,thereby alleviating the shortage of petroleum resources.Silicoaluminophosphate molecular sieve SAPO-34 is a member of the CHA-typed zeolites.Both SAPO-34 and ZSM-5（MFI-typed）are employed as superior catalysts with ultra-high performance in the MTO industry.Especially,the SAPO-34 molecular sieve has a unique larger cha cage and 8-membered ring three-dimensional cross pores,with pore size of 3.8A.Therefore,compared with the ZSM-5 catalyst,SAPO-34exhibits a much higher product selectivity towards light olefins.As the methanol is completely transformed,the yield of C₂H₄ and C₃H₆ can reach over 80%.Nonetheless,due to the narrow microporous channels in SAPO-34s,the diffusion of molecules inside the SAPO-34 catalyst is severely hindered,subsequently resulting in the formation of bulky coke species.These coke species,which are difficult to escape from the catalysts,will further block the pores and cha cages,as well as the outer surface of the SAPO-34catalysts.The active sites in the catalysts would be covered by the bulky coke species,which ultimately leads to the rapid deactivation of the SAPO-34 catalysts.To restrain the formation of coke species and elevate the lifespan of catalysts,many approaches have been developed.The introduction of secondary larger pores provides more space for reactants and products,allowing larger coke species to be accommodated.Besides,more active sites are exposed in the hierarchical pore structure,and the accessibility to the active sites can be increased as well.Thus,the catalytic performance of the SAPO-34s can be remarkably enhanced.In this thesis,based on the metastability of crystals,SAPO-34 catalysts with hierarchical structures were constructed by the in-situ mother-liquor chemical etching approach.The exterior morphology,interior structure,elements composition,and acidity in pores of the as-synthesized catalysts were characterized in detail,and the catalytic performance was evaluated by the MTO process.The core results of this thesis are elaborated as follows:1.An approach to prepare hierarchical zeolite based on the metastability of crystals was developed.After the crystallization is completed,extending the processing time or transferring the system to a lower temperature will make the zeolites in a non-equilibrium state,and thus the partial dissolution process starts.Therefore,based on the metastability,one of the basic characteristics of zeolite,the SAPO-34 zeolite with hierarchical structures can be obtained.We found that SAPO-34 crystals will dissolve at low temperature（room temperature to 80°C）.Conversely,at high temperature（100to 180°C）,the etched SAPO-34 crystal will regrow.The highest catalytic lifespan of hierarchical SAPO-34 catalysts is about 5.8 h,which is nearly 3-folds of that over the microporous counterpart.A simple,direct,and eco-friendly method for fabricating hierarchical zeolites is developed and investigated.This method is expected to be applied to different zeo-types.2.Using triethylamine as a template,a series of SAPO-34 catalysts with different silicon contents and crystal sizes were synthesized and then etched in the mother liquor to finally obtain a series of microporous and hierarchical SAPO-34 samples.The results indicate that the inherent factors of the parent SAPO-34 crystal,such as crystal size and silicon content,will greatly affect the etching process.Specifically,the SAPO-34crystals with lower silicon content and smaller crystal size are easier to be etched,thereby introducing a large number of hierarchical structures in the microporous SAPO-34 crystals.After systematic characterization,we found that a large number of defects and unstable crystal domains in the nanosized SAPO-34 crystals with lower silicon content will be preferentially dissolved in the mother liquor.Compared with the traditional microporous catalyst of micron size,the catalytic lifetime of the nanosized hierarchical SAPO-34 catalyst is increased by about 14 times,and its selectivity to ethylene and propylene is also increased by 5%.This work reveals the impact of the inherent factors of SAPO-34 catalysts on the etching effect,which lays the foundation for deeper and more advanced studies for the pore engineering of zeolites in the future.