Regulating Hierarchical Structure And Acidity Of HZSM-5 And Its Catalytic Performance In Methanol Aromatization

Coal resources account for about 94%of China’s fossil energy reserves,and it is the ballast stone that guarantees national energy security and economic lifeline.The development of green,efficient and sustainable use of coal is a strategic choice in line with the basic national conditions,which is of great significance.Methanol to aromatics reaction（MTA）is a key way to produce aromatics from coal-based routes.It has important basic research and industrial application value,and has become one of the research hotspots of coal-based alternative energy sources.The main catalyst currently used in the methanol to aromatics reaction is HZSM-5 molecular sieve catalyst.The microporous network structure of HZSM-5 molecular sieve is regular and orderly,Br?nsted acid is easy to adjust and has unique shape-selective ability.It is widely used in the field of acidic catalytic reactions such as isomerization,alkylation,and aromatization.However,the narrow microporous channels in the crystals of HZSM-5 molecular sieve severely limit the effective diffusion coefficient Deff of macromolecules,reduce the utilization rate of catalyst active sitesη,and are prone to coking and deactivation,which greatly shortens the catalytic life.Therefore,the precise construction of a pore structure with good diffusibility and a suitable acidic site is the key to improving the selectivity and catalytic stability of the target product.This paper aims to improve the aromatization catalytic performance and catalytic life of HZSM-5 molecular sieve.The pore structure,acidity and morphology of HZSM-5molecular sieve are adjusted by different methods,and finally the effective improvement of aromatization yield and the significant improvement of catalyst life are realized.The main research contents and results are as follows:（1）The Multi-stage pore HZSM-5 zeolite was constructed by traditional alkali treatment design.Too fast the desiliconization rate is difficult to control,and it is easy to cause excessive etching to cause uneven distribution of mesopores and obvious damage to the framework structure.By introducing structure directing agents（PDA_S）with different chain lengths in the alkaline solution,the HZSM-5 molecular sieve-based catalyst of the micro/mesoporous system with concentrated and uniform pore size was successfully prepared.Using X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,N₂physical adsorption and desorption and ammonia temperature program desorption（NH₃-TPD）and other characterization methods the physical and chemical properties of the prepared samples were studied.The results show that the structure-directing agent can effectively control the desiliconization rate and improve the uniformity of the mesopores,and the chain length of PDAs has a positive correlation with the pore diameter of the mesopores.Taking MTA reaction as the research object,the aromatization performance of different catalysts was investigated.At the same time,thermogravimetry（TG）was used to analyze and calculate the coke deposition kinetics.The protective desiliconization catalyst showed excellent aromatic selectivity and anti-carbon accumulation ability.When the reaction temperature 400℃,the reaction pressure 0.1MPa,and the methanol mass space velocity（WHSVs）3.16 h^-1,compared with the commercial HZSM-5,the Z5-Na+TP catalyst exhibited a higher BTX yield（39.55%）and Lower carbon deposition rate(1.91 mg g^-1_cath^-1).（2）Although HZSM-5 molecular sieves have a certain degree of methanol aromatization activity,its large number of strong acid centers leads to serious side reactions such as cracking and isomerization,resulting in a large amount of alkane by-products,but the yield of aromatics is not high.A series of HZSM-5 molecular sieve-based catalysts with different Ga metal contents were successfully prepared by the equal volume impregnation method,and characterized by XRD,SEM,Mapping,N₂physical absorption and desorption,X-ray photoelectron spectroscopy（XPS）,NH₃-TPD,etc.The physicochemical properties of the catalyst were studied,and the aromatization of methanol was used as the reaction probe to investigate its corresponding catalytic performance.The results show that Ga metal can effectively adjust the acidity of the HZSM-5 molecular sieve,forming Ga O⁺active sites with the adjacent Br?nsted acid center,promoting the dehydrogenation and aromatization of alkane intermediates,thereby increasing the yield of aromatics.Although the Ga metal content is positively related to the yield of total aromatics,the selectivity of light aromatics BTX has decreased.In order to improve the selectivity of BTX,Ga metal was used to modify the micro/mesoporous HZSM-5 molecular sieve,of which5%Ga/Z5-Na+TP showed excellent total aromatics selectivity（71.14%）and high Selectivity to light aromatics BTX（40.07%）.（3）The pore structure and acidic characteristics of HZSM-5 molecular sieve have an important influence on its catalytic performance.Therefore,we have adjusted the structure and morphology of different catalysts through silanization treatment and successfully prepared a core-shell Si O₂@HZSM-5 composite materials,and through XRD,FT-IR,SEM,TEM,N₂physical adsorption and desorption,NH₃-TPD and other characterization methods to study the physical and chemical properties of the catalyst.The results show that the liquid phase deposition method（LCD）can effectively adjust the pore size of the HZSM-5 molecular sieve,enhance its selective catalytic ability,and increase the selectivity of the BTX product;At the same time,it can produce one without affecting the inherent properties of the catalyst.An inert amorphous Si O₂thin shell passivates the acidity of the outer surface,inhibits unfavorable secondary reactions and improves the ability to resist carbon deposition.By investigating the performance of this composite catalyst,it is found that the micro/mesoporous HZSM-5 molecular sieve with rich silicon on the surface and Ga modification in the core exhibits an excellent proportion of light aromatics（56.60%）and excellent resistance to carbon deposition(1.25 mg mg g_cat^-1h^-1).