Study On Catalytic Upgrading Of Shengli Lignite Pyrolysis Volatiles Over HZSM-5 Supported By Solid Acid

In situ catalytic reforming of lignite pyrolysis over HZSM-5（H5）zeolite to prepare light aromatic hydrocarbons,such as BTEXN,which can alleviate the dependence of industrial production on petroleum resources to a certain extent.Due to the single active center and small pore size（0.5 nm）of the traditional H5 zeolite,the mass transfer of heavy components in the pyrolysis volatiles of lignite is limited,resulting in a decrease in anti-toxicity and formation of carbon deposits blocking mass transfer channels.The dealumination of the alkali solution can be introduced to creat mesopores to some extent.With the introduction of the mesopores,the mass transfer ability is improved,but the loss of a certain acidic site due to dealumination has a great influence based on the reaction.Therefore,the introduction of mesopores while increasing the acidity of the catalyst to increasing the yield and selectivity of the light aromatic hydrocarbons is a key problem to be solved in the preparation of light aromatic hydrocarbons by lignite pyrolysis.The molecular sieve-based solid super acid catalyst not only has the high-efficiency shape-selection of the carrier H5molecular sieve,but also has stable structure,large specific surface area and controllable acid center and acid amount,and can satisfy the acid of the pyrolysis volatile matter of lignite by secondary catalytic cracking reaction.With high catalytic cracking capacity,therefore it is widely used.In this paper,H5 molecular sieve was used as carrier,and the carrier was treated by different concentrations（0.1-0.8 mol/L）of NaOH solution to prepare multi-stage pore H5 zeolite.The physicochemical structures of the catalysts were characterized by BET,XRD and NH₃-TPD.The effects between the prepared catalysts at different NaOH concentrations on the pyrolysis with the yield of BTEXN were investigated.The results showed that with the introduction of the mesopores,the mass transfer ability was improved,but the loss of aromatics was obvious due to the loss of a certain acidic active site because of the alkaline dealumination.It was proved that the acidic active sites had a great influence on the cracking reaction.In order to improve the acidity of the molecular sieve after desilicication,a certain amount of zirconium sulfate solid acid is supported on the carrier after the reaming in the experiment to compensate for the lost acidity.As a result,it was found that the formation of the zirconium sulfate solid acid was pyrolyzed,and the amount of acid formed differed depending on the molar ratio of sulfuric acid to zirconium oxide,and the effect on the yield of light aromatic hydrocarbons was also different.When the ratio of sulfuric acid to zirconia is 6:1,the treated catalyst has the best effect in the catalytic cracking process.By analyzing the physicochemical structure of the modified molecular sieve solid acid catalyst,it is found that the super acid center B and L acting synergistically in the pyrolysis volatile catalytic cracking reaction.The carrier treated with 0.2 mol/L NaOH solution retains the basic skeleton structure of the original pores and also has a certain pore-expanding effect.Compared with H5,the yield of aromatic hydrocarbons obtained from AT_0.2/SZ_6:1H5 is 15.2 mg/g increased to 19.3 mg/g.In addition,ZrO₂after calcination has no active center by conversion from tetragonal phase to monoclinic phase.Adding excessive metal W and B can increase the crystal transformation temperature of zirconia,but the yield of light aromatic hydrocarbons drecased because metal agglomeration and loading problems.In addition,the possible mechanism of molecular sieve-based solid acid catalytic reforming of pyrolysis volatiles of lignite is proposed.