Crystal Morphology Regulation Of ZSM-5 Zeolite And Its MTP Catalytic Performance

Zeolites with regular and ordered micropores,unique shape-selective effect,tunable acidity,and good thermal/hydrothermal stability,are important heterogeneous acid catalysts and widely used in petroleum refining,petrochemical industry,coal chemical industry,and fine chemical industry.Among them,ZSM-5 zeolites（MFI topology）with three-dimensional 10-membered ring cross channels play an extremely important role in catalytic reactions such as catalytic cracking,deep catalytic cracking,alkylation,isomerization,biomass conversion,and methanol-to-propylene（MTP）reactions.At present,propylene is an important basic chemical in the world,and it is mainly produced from petroleum steam cracking and fluid catalytic cracking（FCC）process.However,oil resources are becoming increasingly scarce,and economic development has also caused the demand for propylene to increase,so it is urgent to seek a non-petroleum-based propylene production technology route.Recently,MTP process has attracted much attention.With the large-scale of coal-based methanol plant,the production cost for methanol is decreasing,and the“C1”chemical industry with methanol as the source has attracted the attention of researchers and investors all over the world,among them especially the MTP reaction gaining much attention.The MTP process is mainly used to produce propylene.It has been successfully industrialized in China,achieving the goal of producing propylene with methanol,and establishing the non-petroleum route of producing propylene with high selectivity with coal as raw material,which is very suitable for the energy status of rich coal and short oil in China and is an ideal solution to meet the rapid growth of China’s propylene demand.However,this process has long been limited by the problem of coke deposition and catalyst deactivation due to the slow diffusion of molecules and other substances.This is because the narrow micropores of zeolite seriously limit the diffusion and mass transfer efficiency of related substances in its crystal,which leads to an increase of side reactions and coke deposition in the catalysis.The effective solution to this problem is mainly to optimize the crystal morphology to obtain hierarchical zeolites with mesopores and/or macropores and improve their diffusion efficiency.However,the hierarchical zeolites synthesized by current methods usually have disadvantages such as polycrystalline aggregate structure,low crystallinity,large number of defects,poor thermal/hydrothermal stability,low yield,and high synthesis cost,which are not conducive to industrial production and catalytic application.In this thesis,new synthesis strategies for hierarchical zeolites without any mesoporous templates and etching treatment,were developed for ZSM-5 zeolites,which effectively improved the mass transfer efficiency and the catalysis performance,got rid of the dependence of complex and expensive mesoporous templates for high-performance hierarchical zeolites,simplified the synthesis steps,reduced the synthesis cost,and provided a new synthesis route with advantages in both synthesis methodology and product properties.The specific research results in this thesis are as follows:1.The single-crystalline hierarchical ZSM-5 zeolite with facet-shaped mesopores was successfully synthesized without mesoporous templates by using amorphous zeolite precursor（named as protozeolite）nanoparticles as seeds,and the zeolite showed the advantages of few defects and high hydrothermal stability.By studying the crystallization mechanism,it was found that protozeolite seeds regulated crystal growth behavior by initiating the intraparticle ostwald ripening process while directing the crystallization of the nutrients,and ultimately led to the formation of faceted mesopores.Compared with the traditional hierarchical ZSM-5 zeolite,the as-synthesized hierarchical ZSM-5 zeolite exhibited better hydrothermal stability due to the advantage of few defects.Due to the unique hierarchical framework and the more accessible acid sites in the open position,the prepared zeolite catalyst exhibited a significantly increased lifetime（18 h）and a high propylene selectivity（52.7%）in the methanol-to-propylene reaction,with a propylene/ethylene molar ratio of 3.64(WHSV=12.5 h^-1).Compared with the traditional seed method,this work unravels the working mechanism of protozeolite seed in zeolite synthesis,and also makes an important innovation in the seed-assisted method,which provides an important method supplement for the synthesis of hierarchical zeolites.2.For hierarchical zeolites,the intrapenetrated mesopores that connect the external surface of the entire crystal with the interior area,can significantly improve the transfer efficiency and the accessibility of active sites.However,the hierarchical zeolites prepared by current methods usually only have isolated mesopores,and the intrapenetrated mesopores rely on expensive and complex mesoporous templates.In this work,without any mesoporous templates,the single-crystal hierarchical ZSM-5zeolite with abundant intrapenetrated mesopores(mesoporous volume:0.51 cm³g^-1)and highly complete framework,was synthesized via applying protozeolite as seed.Studies have shown that protozeolite as seed can induce abundant nuclei or crystallization sites into the nutrients,thereby promoting the consumption of local precursor species,followed by oriented assembly and coalescence of crystallites,and ultimately constructing abundant intrapenetrated mesopores.The as-synthesized zeolite,with its unique hierarchical pore texture,good diffusion efficiency,and easier access to active sites,exhibited an ultra-long lifetime(443.9 hours（conversion=100%）,WHSV:2 h^-1,reaction temperature:500°C)in the methanol-to-propylene reaction,and exhibited high and stable propylene selectivity（47.92%）through the entire catalytic process.This work reveals the effective mechanism of the synthesis of hierarchical zeolites by kinetical regulation with seed,which gets rid of the complex and expensive mesoporous templates,provides a new facile strategy for the construction of hierarchical zeolites with intrapenetrated mesopores under no mesoporous template conditions.3.For ZSM-5 zeolite,the macroscopic distribution of Al atoms in the crystal is often uneven and concentrated near the outer shell,which is very unfavorable for its catalytic performance.Especially in MTP reactions,the dense acid sites can lead to increased side reactions,reduced catalytic activity,and rapid deactivation.In this work,the growth kinetics of zeolite were regulated by finely adjusting the alkali amount,water amount and crystallization temperature in the hydrothermal synthesis system,so that more Al atoms were inserted into the formed framework in the early stage of crystallization,and combined with the weak growth blocking effect of Al,abundant finned nanosheets were formed outside the crystal particle.The macroscopic Al distribution was regulated by growth kinetics,and the morphology of the crystal surface was finely controlled,and the hierarchical ZSM-5 zeolite with relatively even macroscopic Al distribution and rich finned nanosheets was obtained.Compared with the traditional smooth regular ZSM-5 zeolite with the same size,this hierarchical ZSM-5 zeolite showed a significantly improved surface area,mesopore content and a more even macroscopic Al distribution,and showed a significantly improved catalytic stability in the MTP reaction under the same conditions,and its catalytic life was increased by more than 30%.This study provides an important synthesis method supplement and is of significance for constructing hierarchical zeolites by regulating macroscopic Al distribution and crystal morphology through growth kinetic control.