The Dealkylation Of Alkylphenol In Coal Tar Over Zeolite

Low temperature coal tar of lignite contains rich aromatic compounds and hydrocarbons,and is an important organic chemical material.The phenolic compounds in coal tar exist in the fraction below 300℃.The recovery and utilization of phenolic compounds was through distillation,alkali treatment and crude phenol refining in industry.After extraction of the products,the waste liquid was called phenol residue,which contained neutral oils,resinous substances,free carbon and phenolic compounds.These phenolic compounds mainly included mixed phenols such as ethylphenol and propylphenol with high boiling point,which were difficult to be separated by rectification.At present,most of the phenol residue in industry is directly burned or treated as waste.Phenol,an important chemical product,can be obtained by dealkylation of alkyl phenol in phenol residue,which can increase the added-value of products in coal chemical industry.Zeolite catalysts had unique crystalline structure,each kind of which had a certain size and shape of the channel structure,a large specific surface area and strong acid sites.These characteristics made it an excellent catalyst for dealkylation reaction.In this paper,the effects of the physical and chemical properties of different zeolite on the selectivity of dealkylation products,the stability of catalysts and the carbon deposition were studied.The molecular dynamic diameters of reactants and products were compared with the pore diameters of the catalysts,which used to explore the relationship between the selectivity of the phenol product,the pore size and the constraint index.The hierarchical ZSM-5 zeolite was further constructed to retain its shape selectivity while accelerating molecular diffusion and reducing the generation of carbon deposition.The forming process of carbon deposition over hierarchical and parent zeolite were compared,and the reason of improved stability of hierarchical zeolite was analyzed.The main results and conclusions are as follows.1.For the dealkylation of propylphenol,the conversion and selectivity of medium and large pore zeolites were better than that of the small pore zeolites,because the reactants cannot diffuse into the small pore.Among them,the stability of medium pore MCM-22 with supercage,large pore USY andβwere better than that of medium pore ZSM-5 with straight channel,but just because of this,the phenol selectivity of them were lower than that of ZSM-5.The straight channel of ZSM-5 was not conducive to the generation of carbon deposition,and the carbon would deposit on the outer surface or the intersection of the channel,which blocking the pore and making the catalyst deactivate quickly.2.It was found that adding water could improve the stability of the catalyst in this process.This was because of the competitive adsorption between water and the phenolic products in the zeolite channel,and it had the stronger adsorption effect on water,which made the phenolic products diffuse out of the pore in time,reduced the carbon deposition,and improved the catalyst activity.3.The catalytic stability of hierarchical ZSM-5 zeolite obtained after alkali treatment was greatly improved compared with that of the parent.The zeolite treated by 0.1 mol/L NaOH was the most effective one.4.The graphitization degree of the parent was higher than that of the hierarchical sample after the reaction.Carbon deposition was likely to block the pore,and the internal active sites cannot act,resulting in a short life.The mesopore in hierarchical zeolite shortened the diffusion path of the products in the channel,reduced the secondary reaction,and avoided the condensation into the more complex aromatic compounds blocking the pore.The amount of acid sites in the mesopore were small,and carbon deposition mainly occured in the micropore channel after 8 h reaction.