Study On Channel Control Of ZSM-5 Zeolite And Catalytic Pyrolysis Of Lignite To Enhance Aromatic Products

Pyrolysis is one of the important ways to convert low rank coals such as lignite into high value chemicals only by upgrading.However,the proportion of heavy components in tar from lignite pyrolysis is not only high,but also the oxygen content is high,which seriously restricts its storage and utilization.The preparation of aromatic hydrocarbons?AHs?such as BTEXN by upgrading pyrolysis tar is one of the key technologies to be solved urgently in lignite pyrolysis process.A suitable catalyst used for lignite pyrolysis can not only reduce the reaction activation energy but also obtain high-quality tar with high conversion rate and regulate the distribution of pyrolysis products as well as finally achieve the directional conversion of pyrolysis tar.ZSM-5 zeolite has become one of the important choices for pyrolysis vapors as precursor of tar reforming because of its unique skeleton structure and catalytic performance.However,the single active center and the smaller pore size?0.5 nm?of traditional ZSM-5 zeolite limited the mass transfer of macromolecular compounds?MCs?,resulting in a sharp decline in catalytic activity and the tendency to induce the formation of carbon deposition.Therefor it is very important to design and modify the hierarchical pore zeolite with meso/microporous system.In this paper,hierarchical ZSM-5 zeolite was prepared by alkaline desilication method using the traditional ZSM-5 as support,and further synthesized bifunctional zeolite by impregnating monometal and/or bimetal to improve mass transfer and catalytic performance.The physical properties and acidity of the catalysts were characterized by X-ray diffractometer,physical/chemical adsorbent and so on.The catalytic performance of the modified zeolite on the catalytic reforming reaction of Baiyinhua lignite was studied in a drop tube reactor under mild conditions to achieve the light compounds of tar.We have carried out the following three parts of the study:In the first part,a series of catalytic material with various metal-Co loading?1,3,6 and 10 wt%?were prepared by wet impregnation method for modifying commercial ZSM-5 zeolite?SiO₂/Al₂O₃=25?.After the modification,the total acid amount and pore size of zeolite decreased.With the increase of metal-Co loading,the gas yield produced by pyrolysis vapors cracking increased first and then decreased,especially for CO₂.When the metal-Co loading was 3 wt%,the coke yield reached a minimum value of 5.2%compared with other metal-Co loading.Thus,3Co/Z5 zeolite showed a high catalytic activity and anti-carbon deposition capacity in catalytic fast pyrolysis?CFP?of lignite.In particular,the total content of benzene,toluene,ethylbenzene,xylene and naphthalene?BTEXN?increased significantly to 24.2 mg/g at 600 ^oC.Furthermore,GC-MS analysis showed that 3Co/Z5 had better catalytic cracking performance for heavy compounds in tar,resulting in the relative content of AHs increased to 77.0%?area,%?.Adding metallic cobalt not only leads to an increase in AHs,but also effectively reduces the content of organic oxygen in tar.The effect of catalytic temperature?400-700 ^oC?on catalytic reforming reaction of vapors was also studied in this work.The results showed that the composition of tar became simple,and the content of AHs was remarkable increased at 600 ^oC,especially for BTEXN.When the temperature increased to 700 ^oC,the gas yield significantly increased,instead,the tar yield reduced.In the second part,the narrow pore structure of ZSM-5 zeolite severely limits the diffusion of vapors in the pores,which leads to the decrease of the catalytic reaction rate,and the small pore channels can easily lead to the occurrence of carbon deposition.These issues can shorten the life of zeolite to a large extent.In order to overcome it,we designed and post-synthesized hierarchical ZSM-5 zeolite via alkaline desilication method and further modified via wet impregnation method using bimetallic Mo-Co and Ni-Co.The post-treated zeolites were tested in a drop tube reactor at 600 ^oC to catalyze vapors.Compared with the non-catalytic experiments,the chemical composition of pyrolysis tar is obviously simplified after using modified catalyst,which mainly contains light AHs such as BTEXN.However,Ni-Co/Z5post-treated with NaOH solution showed significant deoxygenation performance compared with that ones,meanwhile it enhanced the relative content of AHs about86.6%.The addition of bimetallic Mo-Co or Ni-Co significantly improved the selectivity of ZSM-5 to BTEXN.The metal-Ni promoted the formation of H₂ in the gaseous product and leaded to a decrease in naphthalene yield.The yield of toluene and o-xylene increased after using Mo-Co/Z5 zeolite.Hierarchical ZSM-5 zeolite facilitated the formation of AHs and phenols.Conversely,the production of naphthalene derivatives is inhibited.In addition,Mo-Co/ATZ5 and Ni-Co/ATZ5 were effective in suppressing the formation of coke.For the introduction of bimetallic metal as metal sites,we proposed a catalytic pathway to describe the diffusion process of pyrolysis vapors at catalytic active sites.In the third part,mild treatment of NaOH solution can not only cause the decrease of total acid amount but can also make a destructive damage to the skeleton structure of zeolite,thereby limiting the catalytic performance of catalyst itself to a certain extent.Therefore,we used the organic alkali assisted desilication method which adding piperidine as a protective agent in NaOH solution.The parent ZSM-5was post-treated by piperidine-assisted desilication and metallic Co and/or Zr for creating micro/mesoporous systems and metal sites to enhance the stability of catalytic material through increasing the yield of light AHs.The results showed that assisted desilication with PI concentration of 0.3 mol/L retained the morphology of ZSM-5 and avoided severe alkaline corrosion.It not only inhibited the deactivation rate of the catalyst,but also enhanced the mass transfer in the catalyst.The selectivity of BTEXN remarkably increased to 24.9%over AT_0.2-PI_0.3 in comparison to the parent ZSM-5.In addition,introducing bimetallic Zr-Co facilitated the hydrogen transfer of pyrolysis fragments at metal sites and speeded up the cracking reaction and deoxygenation step of the cascade reactions.5Zr/Co-AT_0.2-PI_0.3.3 exhibited an excellent activity for upgrading of pyrolysis vapors,and its BTEXN selectivity further increased to 30.5%.Meanwhile,the OOSs and MCs(C⁺₁₄ and C⁺₁₈)contents were decreased gradually.This work provides the theoretical basis for establishing the structure-activity relationship between the physical/chemical characteristics of herarchical ZSM-5 and the generated law of BTEXN,and revealing the mechanism of directional cracking of lignite heavy components for producing light AHs.This paper showed that mono/bimetal modified microporous or hierarchical pore ZSM-5 changes the acidity and pore size of zeolite,which is a simple and feasible method to improve the yield of high value AHs in catalytic fast pyrolysis of lignite.