Catalytic Conversion Of Coal Pyrolysis Gaseous Tar To Light Aromatics Over Hierarchical Zeolites

Catalytic upgrading of coal pyrolysis gaseous tar is an effective way to improve the quality of tar and the yield of light aromatics.Among many catalysts,Y-type and ZSM-5 zeolites have attracted extensive attention due to their regular pore structure,strong acidity and good hydrothermal stability.However,Y-type and ZSM-5 zeolites are typical microporous materials,severly limiting the efficiency of the mass transfer and diffusion of macromolecular reactant in the channel.Therefore,in order to provide a theoretical basis for the clean and efficient conversion of coal tar,it is essential to study the role of pore structure of catalyst in catalytic upgrading of coal pyrolysis gaseous tar,and to reveal the process mechanism of catalytic upgrading of coal tar to form light aromatics.In this study,therefore,the hierarchical Y-type and ZSM-5 zeolites with different pore structures were prepared by template method and post modification method,respectively.The effect of pore structure of catalyst on catalytic upgrading of coal pyrolysis gaseous tar was investigated.Further,in order to study the mechanism for catalytic upgrading of coal tar to produce light aromatics over Y-type and ZSM-5 zeolites,different groups and fractions of coal tar were sepaerated and characterized,and the pyrolysis behavior of each component of coal tar over zeolites were studied.In addition,model compounds,representing different group components of coal tar,were used to reveal the mechanism of light aromatics formation from coal tar by catalytic upgrading.Finally,carbon aerogels were prepared from coal tar pitch,which was difficult to catalytic conversion in coal tar.The effects of experimental conditions on carbon gel formation,and the density of carbon aerogel,were investigated.So as to provide theoretical and technical basis for realize the graded and quality utilization of coal tar.The main results are as follows:(1)The hierarchical Y-type zeolite with mesopores diameter of 4.5 nm can be quickly prepared by microwave hydrothermal method using organosilane as template.The catalytic effect of hierarchical Y-type zeolite on coal pyrolysis gaseous tar is closely related to the chemical structure of raw coal.For PSM and FCM coal rich in methylene carbon and protonated aromatic carbon,the yield of light aromatics in pyrolysis products increases with increase of the proportion of mesopores in the catalyst.The formation of light aromatics was promoted by hierarchical zeolite,because of its mesopores and larger pore volume,which is conducive to the mass transfer and diffusion of reactants in the catalyst channel and the contact with the active sites.The total amount of BTEXN in the pyrolysis gaseous tar of PSM and FCM coal can be up to 3.68 and 2.93 times of that of raw coal pyrolysis,which is 78.4%and 43.9%higher than that of microporous Y-type zeolite.HXM coal is mainly composed of bridging aromatic carbon and protonated aromatic carbon.Its pyrolysis gaseous tar contains more polycyclic aromatic hydrocarbons,so it needs not only abundant mesoporous structure but also more acid sites.(2)Mesopores can also be introduced into microporous Y-type zeolite by sequential dealumination and desilication.The mesoporous diameter of the samples increased from 2.5 nm to about 10 nm,and the mesoporous volume increased with the increase of the degree of desilication.The acidity of zeolites was adjusted through the removal of Si and Al atoms in the modification process of pore structure.Compared with microporous Y-type zeolite,pore structure modified Y-type zeolite showed a better catalytic effect due to its wide pore size,large pore volume,and suitable acidity.After catalytic upgrading of FCM coal pyrolysis gaseous tar,the yield of BTEXN increases with the increase of the number of mesopores in the catalyst,which is 67.9%higher than that of microporous Y-type zeolite.(3)The hierarchical ZSM-5 zeolite with mesoporous diameter of 4 nm and20 nm can be synthesized in a short time,by microwave hydrothermal method with organosilane and nano-carbon black as mesopores templates,respectively.The proportion of mesopores can be controlled by changing the amount of template.For the catalytic upgrading of gaseous tar from coal pyrolysis,the proportion of mesopores and the acid amount of catalysts should be suitable.The ZSM-5 zeolite with appropriate mesopores can significantly reduce the mass transfer resistance of macromolecular reactants in the catalyst channel,and increase the accessibility of reactants to the active center,thus facilitate the formation of light aromatics.After catalytic upgrading of FCM coal pyrolysis gaseous tar over two kinds of ZSM-5 zeolites with different pore size of mesoporous,the yield of BTEXN was up to 2.70 and 2.55 times of that of raw coal pyrolysis,which was 14.4%and 8.3%higher than that of microporous ZSM-5 zeolites.The catalytic performance becomes worse gradually due to the significant reduction of amount of acid sites caused by the decrease of skeleton aluminum.Hence the catalyst can not provide enough active sites for the catalytic reaction even with the plenty of mesopores.(4)Mesopores with a wide pore size distribution of 4–30 nm can be introduced into the microporous ZSM-5 zeolites through alkali desilication.The proportion of mesopores increased with the increase of alkali concentration.After catalytic upgrading of FCM coal pyrolysis gaseous tar by pore structure modified ZSM-5 zeolite,the yiled of light aromatics in the tar increased first and then decreased.This is because alkali treatment will destroy the acid sites of ZSM-5zeolites,resulting in a gradual decrease in the acidity of the catalysts.The pore structure and acid sites of the zeolite play the role of promoting the mass transfer and diffusion of the reactants as well as providing reactive sites during the catalytic upgrading of coal tar,respectively.When the concentration of alkali is0.2 mol/L,the samples show a good catalytic performance due to their abundant mesoporous structure and suitable acidity.The amount of BTEXN in the product after catalytic upgrading reaches 2.95 times of that of raw coal pyrolysis,which is 43.5%higher than that of un-modified ZSM-5 zeolite.(5)Both Y-type and ZSM-5 zeolites promote the formation of light aromatic hydrocarbons,through the aromatization of aliphatic compounds,the catalytic cracking of polycyclic aromatic hydrocarbons,the breaking of phenolic hydroxyl groups in phenolic compounds,and the removal of heteroatoms in nitrogen-containing heterocyclic compounds.Y-type zeolite has stronger phenolic hydroxyl removal ability,while ZSM-5 zeolite shows a better capability of aromatization,PAHs cracking,and removal of heteroatom.In the process of catalytic upgrading of coal tar,the hydrogen-rich radicals generated from the reactions of catalytic cracking and aromatization of aliphatic compounds can be used to stabilize the fragments of small aromatic radicals generated from the cracking of polycyclic aromatic hydrocarbons,phenolic compounds,and heterocyclic compounds,thus facilitating the formation of BTEXN.The catalytic effect of two kinds of zeolites on different fractions of coal tar upgrading varies with their peoperties.Abundant phenolic compounds were detected in fractions with boiling point less than 210?.Y-type zeolite has better catalytic effect on low boiling point fractions due to its stronger phenolic hydroxyl breaking ability.The proportion of PAHs in high boiling fractions is relatively high.ZSM-5 zeolite has excellent catalytic cracking properties of PAHs,and its catalytic effect on high boiling fractions is better than that of Y-type zeolite.(6)Carbon gels were formed through the cross-linking reaction between coal tar pitch and furfural under catalysis of sulfuric acid,following by drying and carbonization under constant pressure,carbon aerogels was prepared.The ratio of coal tar pitch to solvent,crosslinking agent and catalyst affect the gelation degree of carbon gels and the density of carbon aerogels.Distribution of pore size and the specific surface area could be affected by the activation conditions of carbon aerogels.For example,both CO₂ activation and KOH activation can significantly improve the pore structure of carbon aerogels.For CO₂ activation,the specific surface area of the sample can reach 2852 m²/g after activation 2 h at 950?.Compared with CO₂ activation,the specific surface area of the sample obtained by KOH activation is relatively low.When the activation temperature is 900? and the ratio of alkali to carbon is of 3:1,the specific surface area of the carbon aerogelis 1749 m²/g.