Preparation Of Hierarchical Zeolite And Its Catalytic Regulation Of Coal Pyrolysis Products

Coal still occupies the dominant position in China’s energy consumption due to the prominent energy structure characteristics of "rich coal,poor oil and little gas".Reducing the content of heavy components in coal pyrolysis tar vapor is one of the ways to maximize the utilization of coal resources.Developing an efficient catalytic cracking catalyst is an important method to solve this problem.ZSM5 and Y zeolites have attracted extensive attention because of their controllable acid sites,good shape-selective function,and high specific surface area.However,both of them have microporous structures,which seriously limit the diffusion of heavy components.These heavy components will coke on the surfaces of the catalysts and block the pores,resulting in the deactivation of the catalysts.Therefore,it is necessary to construct high-performance hierarchical zeolite catalyst to improve the quality of coal pyrolysis tar.In this paper,firstly,HZSM5 zeolites with high crystallinity and large specific surface area were synthesized by using different aluminum sources.Secondly,the physicochemical properties of USY zeolite were changed by acid-leached,base-leached and acid-base treatment.Finally,HZSM5 zeolite with a high specific surface area was compounded with modified USY zeolite with high mesoporous volume to obtain hierarchical zeolite.The physical and chemical properties of zeolites were characterized by several techniques,such as X-ray diffraction（XRD）,scanning electron microscope（SEM）,nitrogen adsorption and desorption.The catalytic cracking performance of zeolites in Shenfu coal in-situ pyrolysis gaseous tar was evaluated by pyrolysis-gas chromatography-mass spectrometry（Py-GC/MS）.HZSM5 zeolite was prepared by hydrothermal synthesis.Compared with the commercial ZSM5,it was suggested that the relative crystallinity of Al（NO₃）₃·9H₂O-HZSM5 zeolite was 172%,the specific surface area was 367.2 m2/g,and the average pore diameter was 3.0 nm.The contents of benzene,toluene,ethylbenzene,xylene,and naphthalene（BTEXN）in gaseous tar before and after adding Al（NO₃）₃·9H₂O-HZSM5 catalyst were investigated online by Py-GC/MS.It was found that the total amount of benzene,toluene,xylene,and naphthalene（BTXN）after catalysis was 4.3 times that before catalysis,while the chemical composition and content of pyrolysis gas products of Shenfu coal changed little.It was indicated that the relative crystallinity of USY zeoites modified by HCl with different concentrations decreased,the framework A1 atoms fell off,the lattice shrank,the unit cell volume decreased,and the framework SiO2/Al2O3 ratio increased.The mesoporous volume of 0.4 mol/L HCl modified USY zeolite（0.4HC1-USY）was 0.080 cm³/g,and the specific surface area was 340.5 m2/g.The catalytic pyrolysis of Shenfu coal results further verified that the 0.4HC1-USY zeolite had a better catalytic performance.The light oil fraction in the gaseous tar with catalysis by 0.4HC1-USY increased substantially,and the total amount of BTXN was 4.83 times that of raw coal pyrolysis.The USY zeolite modified by base-leached with NH4OH showed that the addition of tetrapropylammonium bromide（TPABr）can protect the microporous structure of zeolite from being destroyed,and the addition of cetyltrimethylammonium bromide（CTAB）can promote the recombination of non-skeleton silicon.The analysis of nitrogen adsorption-desorption at low temperature showed that the mesoporous volume of desilicated USY zeoite No.5（DSUSY5）increased from 0.049 cm³/g to 0.081 cm³/g,and the micropores also increased from 0.191 cm³/g to 0.272 cm³/g,with an average pore diameter of 9.6 nm.Pyridine infrared analysis showed that the strong B acid sites of DSUSY5 zeolite also increased.Py-GC/MS analysis showed that the total amount of BTXN from Shenfu coal catalytic pyrolysis with DSUSY5 zeolite was 5.56 times of that from non-catalytic pyrolysis.Acid-base leached USY zeolite（DADSUSY）was prepared by dealumination with 0.4 mol/L HCl and desilication with NH4OH and template agent.It was found that the ordered dealumination and desilication could further increase the mesoporous volume of USY zeolite,and the mesoporous volume of DADSUSY reached 0.082 cm³/g.Py-GC/MS analysis showed that the content of heavy components such as asphalt in the gaseous tar of Shenfu coal catalytic pyrolysis with DADSUSY zeolite decreased significantly,the total content of BTEXN was 5.01 times of that of pyrolysis,and the contents of benzene,toluene and xylene were 4.17,3.5 and 2.43 times of that of non-catalytic pyrolysis,respectively.Based on the above research,HZSM5-USY hierarchical zeolite was obtained by mixing HZSM5 and DADSUSY by ball milling.The pore structure parameters showed that the microporous and mesoporous pore volumes of HZSM5-USY were 0.116 cm³/g and 0.119 cm³/g respectively.Py-GC/MS analysis showed that the total content of BTEXN in gaseous tar obtained by HZSM5-USY catalytic pyrolysis was 5.84,1.36 and 1.17 times of that of coal pyrolysis,HZSM5 zeolite and DADSUSY zeolite catalytic pyrolysis respectively,which confirmed that HZSM5-USY hierarchical zeolite had better performance in upgrading the quality of Shenfu coal pyrolysis vapors.