Catalytic Hydroconversion Of Model Compounds And Soluble Organic Species Of Mengdong Lignite Over Hydrotalcite Derived Ni/Al₂O₃

The macromolecular structure of lignite is rich in aromatic structural units and oxygen-containing bridged bonds,therefore it has the unique advantage of directional depolymerization to obtain high value-added fuels and chemicals.In this thesis,a nickel-aluminum hydrotalcite material（Ni Al-LDH）was firstly synthesized by a coprecipitation method,and a series of Ni/Al₂O₃-T catalysts were prepared by calcination and reduction methods.Physical and chemical properties,such as crystal structure,porosity,micromorphology,thermal decomposition and reduction characteristics,distributions and morphology of elements of catalysts and their precursors were characterized systematically by modern analytical technologies including X-ray diffraction（XRD）,thermogravimetric analyzer（TGA）,Fourier transform infrared spectroscopy（FT-IR）,H₂ temperature programmed reduction（H₂-TPR）,N₂ adsorption-desorption,scanning electron microscope（SEM）,X-ray energy spectrometer（EDS）,transmission electron microscope（TEM）,laser ablation inductively coupled plasma mass spectrometry（LA-ICP-MS）and X-ray photoelectron spectroscopy（XPS）.Diphenyl ether（DPE）,benzyl phenyl ether（BPE）and dinaphthyl ether（DNE）were selected as lignite-derived model compounds（MCs）to investigate the hydrocracking activity of Ni/Al₂O₃-T on C-O bonds in lignite and reveal the structural-activity relationship and functional mechanism of catalysts on the basis of optimization of catalytic hydroconversion conditions.XRD,TGA,XPS,FT-IR and SEM were used to characterize the physical and chemical structure,elemental composition,pyrolysis reactivity and micromorphology of Mengdong lignite（ML）and its soluble portions(SP₃₂₀)and residue(TDR₃₂₀)obtained by thermal dissolution at 320 ^oC.The distribution characteristics of soluble portions in isopropanol from catalytic hydroconversion of ML and SP₃₂₀ were analyzed by gas chromatography mass spectrometry（GC-MS）.Combined with GC-MS and FT-IR analyses,the effect of reaction temperature on the composition and functional group characteristics of catalytic hydroconversion products（CHCs）of SP₃₂₀ was investigated to reveal the catalytic hydroconversion characteristics of soluble organic species.The results show that Ni/Al₂O₃-T are mesoporous materials with well-developed pore structure and their specific surface area are in range of 87.36-198.45 m²·g^-1.Ni,Al and O elements are uniformly distributed in the catalysts.The size of Ni nanoparticles ranges from 6.30 to 11.61 nm and Ni⁰ is the main active component.Calcination temperature significantly affect the morphology,structure and catalytic performance of catalysts.Among the catalysts,Ni/Al₂O₃-500 shows excellent catalytic activity,selectivity and stability in hydrocracking reactions of various MCs.The effects of reaction temperature,initial hydrogen pressure,amount of catalyst,reaction time,solvent and atmosphere on the conversion and product selectivity of MCs were investigated systematically and the reaction pathways were proposed according to product distributions.The results show that DPE,BPE and DNE could achieve complete conversion with 100%monomeric selectivity at 200,140 and 160^oC,respectively,in isopropanol over Ni/Al₂O₃-500.Under optimal reaction conditions,direct cleavage of aryl C-O bonds in MCs is the main reaction pathway and aromatic ring hydrogenation of the resulting aromatic monomeric compounds occurred meanwhile.After 6 cycles of reaction,Ni/Al₂O₃-500 still maintains high activity.The excellent catalytic performance of this catalyst is originated from well-developed porosity,small crystallite size,high Ni⁰ concentration and strong metal-support interaction.ML contains a certain graphite-like microcrystalline structure and is rich in C-C,C-O,C=O,-OH and-O-functional groups,in which C atoms mainly exist in the form of C-C and C-O.In the catalytic hydroconversion products of ML,the relative content of complex aromatics and alkanes with larger molecular weight reaches a high proportion.Effective thermal dissolution and depolymerization of ML contribute to the release of soluble organic species at 320 ^oC in isopropanol.The compounds in SP₃₂₀ are mainly made up of hydrocarbons,phenols,alcohols,ketones and ethers,which are rich in alkyl side chains,-OH and C-O bonds.After catalytic hydroconversion under the conditions of 240-300 ^oC,2.0 MPa H₂ and 4.0 h,the compounds in the CHCs mainly consist of relatively micromolecular alcohols,hydrocarbons and phenols substituted by short side chain alkyl groups including methyl,ethyl and isopropyl.During the catalytic hydroconversion process,phenols and ketones are converted into alcohols,some relatively macromolecular organic species are hydrocracked into alkanes,aromatics,olefins and ethers,and esters and acids compounds could be completely converted.These results indicate that Ni/Al₂O₃-500 could effectively catalyze the hydroconversion of SP₃₂₀,improving the quality of soluble organic species of ML.This work provides a certain theoretical foundation for the development of high-performance catalysts in catalytic hydroconversion,as well as the mild conversion and high-value utilization of soluble organic species of lignite.