Catalytic Selective Hydrogenolysis Of Lignin And Its Model Compounds Over Ni/CeO₂

Lignin is a natural aromatic polymer composed of phenylpropane structural units connected through C-O and C-C bonds.Its efficient depolymerization to produce high value-added chemicals could effectively reduce the excessive dependence on fossil resources and promote the diversified development of energy structure.In this thesis,a series of Ni/CeO₂ catalysts were synthesized by direct impregnation-calcination-reduction method.Modern analytical instruments such as X-ray photoelectron spectroscopy（XPS）,scanning electron microscopy X-ray spectroscopy（SEM-EDS）,and field emission transmission electron microscopy（TEM）were used to characterize the structure,morphology,elemental valence,and chemical properties of the catalyst in detail.Benzyl phenyl ether（BPE）,2-phenoxy-1-phenylethanol（2-PPE）,and diphenyl ether（DPE）were used as lignin model compounds to investigate the catalytic hydrocracking activity of Ni/CeO₂ on the C-O ether bonds,and the effects of reaction conditions such as reaction temperature,reaction time,and initial hydrogen pressure on the hydrogenolysis of the model compounds were investigated.The reaction mechanism of C-O ether bonds hydrogenolysis over Ni/CeO₂ was deeply analyzed through density functional theory（DFT）.The screened 15%Ni/CeO₂-500catalyst with the best activity was used for catalytic hydrocracking of lignin.The structural changes before and after the hydrolysis of lignin,as well as the composition and structural characteristics of liquid lignin products,were analyzed by gas chromatography/mass spectrometry（GC/MS）,Q exactive orbitrap mass spectra（Orbitrap MS）and 2D¹H-¹³C heteronuclear single quantum coherence（2D HSQC）.The pores of Ni/CeO₂ catalysts are mainly mesoporous.The loading of active species Ni and appropriate calcination temperature could promote the formation of oxygen vacancies,enhancing the adsorption and activation of oxygen-containing substrates on the surface of the catalysts.Ni nanoparticles are highly dispersed on the surface of CeO₂,which effectively improving the activation of H₂ and releasing more hydrogen radicals.15%Ni/CeO₂-500 could catalyze the complete conversion of BPE to toluene and phenol under mild conditions（100 ^oC）without the side reaction of aromatic ring hydrogenation.Under the optimal reaction conditions of 180 ^oC,the conversion of 2-PPE reached 100%,and the total selectivity for aromatic monomers in products reached 85.6%.The optimal reaction conditions for DPE were 180 ^oC,1.5MPa H₂,and 2 h.The main reaction path for DPE was the cleavage of the C_alk-O bonds to produce benzene and cyclohexanol.Taking BPE hydrogenation reaction as probe,the catalyst recycle experiment of 15%Ni/CeO₂-500 was investigated.The results showed that the catalyst still maintained good catalytic activity and selectivity after being recycled for five times.DFT calculations reveal that 15%Ni/CeO₂-500 can efficiently adsorb and activate reactants such as 2-PPE,and significantly decrease the dissociation energy of C-O bonds.The catalytic hydrogenolysis of lignin over 15%Ni/CeO₂-500 catalyst showed that the yield of liquid products and the relative content of phenolic compounds increased by 8.5%and 12.4%,respectively,compared to the uncatalyzed reaction.GC/MS profile showed that the components in the liquid products from hydrogenolysis were dominated by phenols,followed by aromatic ethers and ketones.The phenols mainly consist of guaiacyl phenols such as 2-methoxyphenol and 4-ethyl-2-methoxyphenol.Some guaiacyl phenols undergo deoxidation and aromatic ring hydrogenation to obtain aromatic ethers,alkylphenols,and cycloalkanes.Under reaction conditions of 300 ^oC,1.0 MPa H₂ and 4 h,the liquid yield of lignin and the relative content of phenolic compounds reached 34.3%and 70.91%,respectively.Orbitrap MS and 2D HSQC analysis showed that 15%Ni/CeO₂-500 could effectively catalyze the hydrogenolysis and cleavage of the C-O bridge bonds in lignin to generate high value-added aromatic compounds such as phenols.