Catalytic Upgrading Lignite Pyrolysis Volatiles To Light Aromatics Over Ga/HZSM-5 Zeolite

Lignite pyrolysis tar is extremely complex in composition,with high organic oxygen content,heavy components as high as 50%-60%,resulting in problems such as high viscosity,poor thermal stability and low calorific value of combustion,which seriously restrict its industrial application.The heavy component of coal tar is one of the important reasons to limit the large-scale utilization of coal tar.Upgarding of coal tar to obtain light products is a key technical problem faced by coal chemical industry.Therefore,improving the efficient and clean utilization of coal tar is a research focus of coal chemical industry at present.The selection of catalysts is very important during the process of upgrading coal tar.Suitable catalysts in the catalytic transformation of volatiles from lignite pyrolysis not only effectively reduce the activation energy required for pyrolysis products,but also regulate the distribution of pyrolysis products to obtain high-quality coal tar.HZSM-5 catalysts（H5）have been widely used to catalyze coal tar to obtain high quality tar due to its large specific surface area,regular three-dimensional,multistage pore structure,abundant acidic sites and high hydrothermal stability.However,the commercial ZSM-5 catalyst（CZ5）with single fixed active site in the pore structure and the limited mass-transfer rates by low reactant results in catalysts inactivation and shorts the life-time.Therefore,it is very important to develop catalysts whose structure and properties are more suitable for catalytic reforming of volatiles.Acid etching post-treatment,introduction of heteroatoms to fine-tune cell parameters could achieve the effect of pore expansion,improve mass transfer efficiency,fine control of pore structure.The modified H5 catalyst with the same main group as Al from the active metal could improve the catalytic performance effectively.Ga modified H5catalyst（Ga/H5）has excellent performance during dehydrogenation aromatization.The following research contents were made in order to develop Ga/H5 whose structure and performance are more suitable for catalyzing volatilization from lignite pyrolysis to obtain the light aromatics:（1）Hierarchical CZ5 catalyst was obtained after Na OH treatment which was used as the support.Ga/AT_0.2H5（loaded with Ga after 0.2 M Na OH treatment）and Sn/Ga/AT_0.2H5 were prepared by impregnation method and applied to catalyze volatiles from lignite pyrolysis.Compared with CZ5,Ga/AT_0.2H5 and Sn/Ga/AT_0.2H5could effectively improve the selectivity of benzene,toluene,ethylbenzene,xylene and naphthalene（BTEXN）,and the maximum yield of BTEXN and benzene reached to 24.1 mg/g and 14.8 mg/g over 5%Ga/AT_0.2H5 catalyst.Sn cations could promote the decarbonylation of pyrolysis volatilization to form ethylene and light olefins,which underwent hydrogen transfer and aromatization reaction（Diels-Alder reaction）at the active site of Ga-O,the BTEXN yield of Sn/Ga/AT_0.2H5 catalyst was further improved to 25.1 mg/g.The synergistic effect of Sn-Ga/AT_0.2H5 compensated the defect location of Si-OH,which could effectively improve the transfer rate of coke precursor,reducing carbon deposition and prolong the catalyst life.The2%Sn-4%Ga/AT_0.2H5 catalyst prepared by impregnation method lose the active species easily after recycling and regeneration,leading to a decrease in acid content（from 0.0025 mmol/m² NH₃ to 0.0016 mmol/m² NH₃）and a poor cycle service life.（2）Isomorphous substitution of Ga to obtain Ga/H5 catalysts can increase the yield of BTEXN（reach to 29.3 mg/g）due to the increase of active species and acid density（from 0.00062 mmol/m² NH₃ to 0.00144 mmol/m² NH₃）.Ga/H5 could increase the mesoporous volume and average pore size due to the longer Ga-O bond than Al-O bond,which reduced the diffusion resistance of volatiles and inhibited the formation of carbon deposition and prolong the catalyst lifetime.1.26 Ga/H5（A）（1.26 g Ga（NO₃）was added during crystallization）showed good stability.B/H5catalysts had high crystallinity and appropriate pore structure.With the increase of the addition amount of B,the increase of specific surface area could provide more reaction sites and promote the catalytic reaction,the highest yield of BTEXN was24.1 mg/g.But the selectivity of the products was different.Under the condition of Na-0.69B-H5（0.69 g HBO₃ under Na OH solution）,the highest yield of alkylbenzene was 11.4 mg/g due to the B-OH-Si formed by Na-x B-H5 provided more weak acid sites that could adsorb intermediate products such as ethylene and propylene to increase the methylation and alkylation rates for producing alkylbenzene.The increased mesoporous volume and average pore size improved the mass transfer efficiency and inhibited the formation of carbon deposition.In/H5 catalysts exhibited the poor selectivity for BTEXN when it was applied to catalyze volatilization from lignite pyrolysis.It was ascribed to the reduced acidity and specific surface area,leading to rapid deactivation of catalyst.（3）Compared with CZ5,the total yield of BTEXN decreased over hollow H5,but the product selectivity was different in which the B yield decreased while the yield of alkyl benzene generally increased,and the highest alkylbenzene yield of11HSZ5（11 g polyethylene glycol）reached to 9.91 mg/g.The total BTEXN yields and the selectivity of each experiment almost unchanged with the increase of polyethylene glycol addition due to the internal active site fell slightly.The results showed that the hollow H5 was conducive to the formation of alkylbenzene due to the shortened diffusion path and more mesoporous pores（0.53 cm³/g）.The hollow H5could shortened the diffusion length and enhance mass transfer efficiency.The cycle regeneration test showed the stability was poor due to the acid decreased（from0.00336 mmol/m² NH₃ to 0.00227 mmol/m² NH₃）,and the high temperature lead to dealumination.（4）Hollow Ga/H5 catalyst was prepared by after-treatment of Ga/H5 catalyst that Ga/H5 was recrystallization in tetrapropyl ammonium hydroxide（TPAOH）or sodium hydroxide/tetrapropyl ammonium hydroxide（Na OH/TPAOH）solution.The maximum BTEXN yield reached to 26.6 mg/g.The results showed that hollow Ga/H5not only provided more types of active sites,but also greatly shortened the diffusion path of volatiles and intermediates,and promoted the mass transfer efficiency.The hollow Ga/H5 with TPAOH post-treated kept most active sites due to mild conditions and had little damage for the catalyst skeleton,which generally improved the BTEXN yield,Hollow Ga/H5 catalyst had a larger specific surface area（420 m²/g）,providing more reaction sites for catalytic reactions.The results of cyclic regeneration experiment showed that the yield of BTEXN decreased seriously,which may be caused by the weakening of acidity（0.17 mmol NH₃/g）under high temperature steam and the lacking of sufficient active sites for catalytic transformation.This thesis contains 97 Figures,24 Tables,and 304 references.