Catalytic Hydroconversion Of Lignin And Its Model Compounds Over Co@C And Ni@C

In this paper,Co@C and Ni@C catalysts were prepared by one-step pyrolysis of Co-MOF and Ni-MOF,then they were used as the catalysts in the catalytic hydroconversion（CHC）of lignin and its model compounds（MCs）.The physical structures and morphologies of Co@C and Ni@C catalysts were characterized by nitrogen adsorption and desorption,X-ray diffraction（XRD）,inductively coupled plasma mass spectrometer（ICP-MS）,X-ray photoelectron spectroscopy（XPS）,field emission transmission electron microscopy（TEM）and scanning electron microscope-energy dispersive X-ray fluorescence spectrometer（SEM-EDX）.The optimal pyrolysis temperature of MOF were selected using guaiacol（GUA）as reaction probe.The catalytic hydrogenation activities of Co@C and Ni@C catalysts prepared at the optimal pyrolysis temperature（600 ^oC）on MCs including GUA and diphenyl ether（DPE）,as well as real lignin were explored,then the effects of reaction temperature,time,initial hydrogen pressure and solvent were investigated.The liquid product yield(Y_LP)and product distribution of lignin hydroconversion under catalytic conditions and non-catalytic conditions（NCHC）were compared.According to the selectivity of the products under different reaction conditions,the possible reaction pathways of lignin and its MCs are proposed.The main compositions of the liquid products and the structural changes of lignin before and after the lignin reaction were analyzed by gas chromatography-mass spectrometry（GC/MS）,Fourier transform infrared spectroscopy（FTIR）,Quadrupole/Electrostatic Field Orbitrap High Resolution Mass Spectrometer（Orbitrap MS）and Two-dimensional Nuclear Magnetism（2D-HSQC）,respectively.The characterization results of the catalyst show that the specific surface area of Co@C and Ni@C are 31.93 and 102.33 m² g^-1,and the particle size of them are 40-60nm and 7-13 nm,respectively.Both Co@C and Ni@C are regular spherical shape with good dispersion.During the pyrolysis of MOF,the decomposition of organic ligand1,4-terephthalic acid results in the partial reduction of oxidized metal.Both Co@C and Ni@C can effectively catalyze the cleavage of C-O bonds in lignin MCs.Under the conditions of 180 ^oC,2.0 MPa H₂,2 h reaction,Co@C catalyzes GUA to selectively produce 92.8%cyclohexanol;Ni@C catalyzed the reaction of DPE at 250℃and 2.0MPa H₂ for 3 h,and the selectivity of monomer compounds in the product was 93.7%.Co@C and Ni@C first catalyze the cleavage of the C-O bond in MCs to generate monomers,and then and the monomer aromatic ring further undergoes hydrogenation reaction.Liquid product yield(Y_LP)increase significantly in the presence of Co@C and Ni@C catalyst,and the Y_LP of Ni@C catalyzed lignin hydrogenation conversion is higher（29.0 wt%）.The effects of solvent,temperature and time on Y_LP and product distribution of lignin hydroconversion were investigated using Ni@C as catalyst.The results showed that phenolic compounds were the main liquid products of lignin hydroconversion.There is the highest Y_LP during the lignin hydroconversion in ethanol over Ni@C.Y_LP increases with the increase of temperature and reaction time,but the relative content of phenol compounds first increases and then decreases.Under the reaction conditions of 280 ^oC and 3 h,the relative content of phenol compounds in the products reached 91.6%.In the process of catalytic hydrogenation,theβ-O-4 and 4-O-5 bonds in lignin structure were cracked firstly to generate monomers and dimers,and then the corresponding phenolic monomers were generated through the dehydroxylation and demethoxylation of monomers and dimers,indicating that Ni@C catalyst can effectively promote the hydroconversion into phenol compounds.The paper has 44 figures,16 tables and 136 references.