Study On Catalytic Reforming Of Shengli Lignite Pyrolysis Volatiles To Light Aromatics Over Sulfated HZSM-5 Catalyst

The clean and efficient utilization of low-rank coal is of great significance to alleviate the shortage of petroleum resources and optimize the consumption structure of resources for China,which is a major national strategic demand.Pyrolysis is an effective method for clean and efficient utilization of low-rank coal.However,the high oxygen content in coal tar seriously affects the subsequent utilization of tar.The use of catalysts can improve the pyrolysis efficiency,effectively regulate the product distribution,improve the yields of light aromatics,realize the directional preparation of chemical by lignite pyrolysis,and provide a theoretical basis for the construction of lignite oriented thermal transformation theory.In this paper,catalytic upgrading of pyrolysis volatiles from Shengli lignite was carried out in a drop tube reactor.A series of sulfated HZSM-5 were prepared by using different SO₄^2-（（NH₄）₂SO₄,（NH₄）₂S₂O₈,H₂SO₄）.The effects of SO₄^2-and calcination temperature on catalytic modification were investigated.A series of different extra-framework Al modified HZSM-5 were prepared by HCl dealuminization,realumination,（NH₄）₂SO₄ sulfation.The structural of the modified catalysts were characterized by XRD,FT-IR,BET,NH₃-TPD and other methods to investigate the effect of chemical state of extra-framework Al on catalytic reforming activity of catalyst.The effects of the pore structure and the acid sites of HZSM-5 before and after modification on the distribution of lignite products were explored.It was found that after sulfation,the complexation of SO₄^2-with the extra-framework Al formed the strong acidic site,dealuminization lead to the increase of the pore size and the reduction of the non-shape selective acid sites.Among all the modified catalysts,the total yield of light aromatics obtained by 0.5AS-550（modified with 0.5mol/L（NH₄）₂SO₄ and calcined at 550°C）was 28.8 mg/g,which was higher than that obtained by（NH₄）₂S₂O₈ and H₂SO₄ modification（26.9 mg/g and 25.4 mg/g）,which was 5 times of that obtained by non-catalytic pyrolysis.Compared with HZSM-5,the coke yield obtained by 0.5AS-550 decreased by 0.6 wt%.（NH₄）₂SO₄ modification can better maintain crystallinity while dealuminating to enlarge the pore size and reduce the external non-shape selective acid sites.In addition,the pretreatment with HCl-dealumination increased the total pore volume（especially the mesopores）and average pore sizes of HZSM-5,while reducing the acid strength,thereby providing realumination space to ensure catalytic activity.The introduced extra-framework Al species was followed by the modification of（NH₄）₂SO₄ and then combined with SO₄^2-to form superacidic surface sulfate complex on HZSM-5.The yield of light aromatics obtained over SO₄^2-/DHZ5-A（modified by Al（NO₃）₃ after HCl-dealumination and then modified with（NH₄）₂SO₄）was as high as27.3 mg/g.The different chemical states of extra-framework Al species may affect the complexation with SO₄^2-,the state of calcined Al₂O₃ were more conducive to the formation of superacidic sites by sulfation,thereby improving the catalytic activity of the catalyst,promote the production of light aromatics,and achieve the directional reforming of coal pyrolysis volatiles to light aromatics.