Study On Ni/ZSM-5 Catalyst For In-situ Catalytic Upgrading Of Tar From Coal Pyrolysis

Catalytic upgrading of coal pyrolysis gaseous tar can increase the yield of light aromatics such as BTEXN in the tar,and significantly improve the quality of the tar.The selection of catalyst is particularly important in the catalytic pyrolysis of lignite.The metal-modified molecular sieve catalyst has attracted extensive attention due to its good catalytic effect.In order to explore the synergistic effect between metal and acid in the modified molecular sieve catalysts and provide a theoretical basis for further construction of suitable bifunctional catalysts,this study selected HZSM-5 andγ-Al₂O₃ as supports,which were modified with metals and then used catalytic pyrolysis experiments on lignite.The main experiments contents are as follows:Firstly,the catalytic pyrolysis experiment of lignite was carried out using HZSM-5（Si/Al=18,25,50,80,100）and metal-loadedγ-Al₂O₃ catalysts,the effects of the acidity and the metal components on pyrolysis tar was investigated.Then,the metal modification of HZSM-5 with different Si/Al ratios was carried out by two different metal loading methods,the incipient wetness impregnation method and the ion exchange method.The effects of metal form on the yield of light aromatics and the product distribution in the catalytic upgrading process of coal pyrolysis gaseous tar were investigated.And HZSM-5 with Si/Al=25was selected to support metallic nickel,the effect of the ratio of metal to acid in the bifunctional catalyst on the in-situ catalytic upgrading of pyrolysis tar was investigated.The main research conclusions are as follows:（1）HZSM-5 molecular sieve with low silicon-alumina ratio has higher acid content and stronger tar cracking ability,which can significantly promote the deoxygenation of oxygen-containing compounds and the aromatization of aliphatic hydrocarbon compounds,and the lightening effect of tar is better.Among them,Z-18 has the highest acid content and the best tar lightening effect.The yield of BTEXN is 4064.5 ng/mg,which is 4.5 times that of raw coal pyrolysis,and the selectivity to benzene is as high as 39.60%.（2）The increase of BTEXN in catalytic pyrolysis tar byγ-Al₂O₃ loaded with metal is limited,and the amount of BTEXN increases first and then decreases with the increase of Ni loading.The metal-loaded catalyst has little effect on the content of aliphatic hydrocarbons in the catalytic pyrolysis tar,and the selectivity to benzene increases.The presence of Ni O promotes the chain scission of aromatic side chains and increases the effect of acidic active sites on oxygenated compounds.Reactions such as dehydroxylation occurred phenolic compounds,which promoted the formation of benzene in tar.（3）The catalytic effect of the catalyst prepared by the incipient wetness impregnation method was enhanced,and the yield of BTEXN in the tar was increased.The metal exists in two forms,nickel oxide and elemental nickel.The introduction of new active sites enhances the ability of the catalyst to remove phenolic hydroxyl groups and alkyl side chains.The yield of BTEXN obtained by Ni/HZSM-5 with a low Si/Al ratio is still higher.Among them,5Ni/Z-100has a moderate distance and ratio between metal and acid centers,and the synergistic effect of the two active centers improves the selectivity of the catalyst to light aromatics,with the highest increase in the yield of BTEXN in the tar.（4）The catalytic effect of the catalyst prepared by the ion exchange method is reduced,the aromatization ability of the catalyst to hydrocarbon compounds is reduced,and the selectivity to benzene is also reduced compared with the catalyst prepared by the impregnation method.The exchange of metal ions and H ions leads to the loss of some acid active sites,especially the reduction of strong acid sites reduces the cracking ability of the catalyst.At the same time,the metal loading is small and the existing form is relatively single.So although new reactive sites are introduced,the synergistic effect between the different reactive centers of the final catalyst is poor,the hydrogen-rich radicals and the tar cracking fragments cannot be completely matched,and the yield of BTEXN in the tar decreases.（5）After the introduction of metallic nickel on the sample Z-25 by incipient wetness impregnation method,the BTEXN yield in the tar generally increased first and then decreased with the increase of the loading amount,and the selectivity to benzene first increased and then decreased.When the Ni loading is 5%,the ratio of catalyst metal to acid reaches equilibrium,the effect of catalytic is the best,and the selectivity to benzene is the highest.The effect of metal loading on aliphatic compounds is relatively small,and higher loading is beneficial to the removal of hydroxyl groups of phenolic compounds.Sample 5Ni/Z-25 shows the best ability to remove phenolic hydroxyl groups.The appropriate ratio of metal and acid in the catalyst can maximize the effect of metal and acid,promote the synergistic effect of metal and acid,and convert heavy components in tar into light aromatics.