Study On The Preparation Of Phenolic Compounds By Directed Depolymerization Of Lignin Under Nickel-based Catalysis

Lignin is one of the most abundant aromatic natural polymers on earth,accounting for 20 to30%of lignocellulose,and is widely used as a potentially excellent renewable raw material for bulk chemical production due to its richness in functional groups（e.g.hydroxyl,aromatic rings,aldehyde groups,etc.）.The directional conversion of lignin into phenolic compounds is a hot and difficult point in realizing efficient utilization of lignin.In this paper,focusing on the high-value utilization of lignin,two nickel-based catalytic systems were prepared for the depolymerization of lignin under mild conditions.In order to grasp the basic structure and physicochemical properties of lignin substrates,lignin was characterized and pyrolysis kinetics were calculated.The results showed that lignin was formally degraded at 170～600℃,mainly cleavage theβ-O-4,α-O-4,4-O-5,β-5,5-5,β-1 andβ-βbond.Two model methods（Kissinger-Senum Yang iteration and Ozawa-Senum Yang iteration）were used to analyze the pyrolysis kinetics of lignin,and the activation energy of lignin at different conversion rates was obtained.The pre-experimental results of lignin depolymerization showed that when the concentration of formic acid was 2.50 mmol·mL^-1,isopropanol was the solvent,the reaction temperature was 220℃,and the reaction time was 8 h,the highest bio-oil yield was 23.15wt%.ZrP support were prepared using ammonium dihydrogen phosphate（NH₄H₂PO₄）and zirconium chloride octahydrate（ZrOCl₂·8H₂O）as raw materials,and metal Ni and W were supported by chemical reduction method to prepare（Ni-W）_x/ZrP catalysts for use in depolymerization of lignin.The results showed that the catalyst（Ni-W）₂₀/ZrP with a metal loading of 20 wt%（Ni to W mass ratio of 1:1）,a formic acid concentration of 2.50 mmolmL^-1,a reaction temperature of 220℃ and a depolymerization time of 8 h resulted in the best bio-oil yield of 74.66wt%and the selectivity of the product species which phenolic products were 88.25%.The most abundant phenolic compounds were vanillin（4.09 wt%）and 2-methoxyphenol（1.21 wt%）.Fourier transform infrared spectroscopy（FTIR）analysis showed that the peaks at 1267 cm^-1（C-O bond vibrations from guaiacol）,1130 cm^-1 and 1046 cm^-1（C-O bond vibrations in fatty ether bonds）decreased gradually with increasing temperature which indicate that the lignin was effectively depolymerized into monoaromatic units and bio-oils.The catalyst cycling experiments showed that（Ni-W）_20/ZrP was used for five consecutive times without significant activity loss and could be used as a stable catalyst for lignin depolymerization.ZrO₂-MgO support were prepared using zirconium chloride octahydrate（ZrOCl₂·8H₂O）and magnesium chloride hexahydrate（Mg Cl₂·6H₂O）as raw materials,while Ni_xZn_1-x/ZrO₂-MgO catalysts were prepared by chemical reduction method loaded with Ni and Zn metal.The results show that the introduction of metal Zn can increase the specific surface area of the catalyst(from 13m²g^-1 to 26 m²g^-1)and the acidity(from 0.84 to 1.20 mmol NH₃·g^-1)and play a good synergistic catalytic effect with Ni in the lignin depolymerization process.The results of depolymerization experiments showed that the bio-oil yield reached 65.22 wt%under Ni_0.75Zn_0.25/ZrO₂-MgO catalytic conditions when the formic acid concentration was 1.50 mmolmL^-1,the reaction temperature was220℃ and the depolymerization time was 6 h.The results of thermal cracking gas mass spectrometry（GC/MS）showed that dozens of phenolic compounds were detected in the cracked bio-oil,and the main phenolic compounds with high content were guaiacol（2.53 wt%）and vanillin（3.6 wt%）.The results of catalyst cycling experiments showed that the catalytic activity remained effective and stable after five consecutive uses.