Preparation Of Phenolic Compounds By Catalytic Conversion Of Lignin With Cobalt-Based Attapulgite Materials

Due to environmental pollution and dwindling fossil fuel reserves,scientists are constantly exploring new alternative energy sources.Biomass resources have attracted the attention of researchers for their green,clean and renewable characteristics.Lignin is one of the renewable natural aromatic polymers in nature.Can be used in chemical,material,food and other fields.Especially in the context of"lignin priority"strategy,the production of high value-added chemicals and fuels using lignin as raw materials is considered to be a very potential lignin value-added strategy,and hydrogen hydrolysis is one of the more common and effective means of lignin depolymerization.In view of the complex chemical bonds of lignin structural units,easy coking in the reaction process,poor selectivity of target products and other problems,this topic selects attapulgite（ATP）,a green,economic clay material with good thermal stability as the carrier,cobalt metal（Co）as the active component to prepare a series of highly efficient and stable catalysts to achieve high efficiency depolymerization of lignin.The concrete achievements are as follows:（1）A series of Co（5,10,15 and 20 wt.%）supported acid-modified attapulgite（m-ATP）catalysts were synthesized by chemical reduction and impregnation methods,and the catalytic polymerization of lignin was tested under various parameters such as reaction temperature,reaction time and metal loading.The results showed that when the Co loading was 15 wt.%,63.1wt.%bio-oil was obtained at 220℃and 8 h.Analysis by GC-MS showed that the vanillin content in bio-oil was the highest.Fourier transform infrared spectroscopy（FT-IR）analysis of the recovered lignin showed that it still had similar structure to the original lignin,and the benzene ring and other characteristic groups（such as methoxy,phenol hydroxyl,etc.）"core"remained.The catalyst still showed high catalytic activity after 5 cycles of hydrolysis.（2）In addition to the activity of the components,the support is also one of the important factors that determine the depolymerization effect of the catalyst.Cerium-zirconia（Ce O₂-Zr O₂）has the advantages of high oxygen vacancy rate,reversible oxygen storage capacity,coking prevention,chemical and thermal stability,and is widely used as the support of lcellulosic biomass catalysts.However,both Ce O₂ and Zr O₂ have low pore volume and specific surface area.Therefore,the Co/m-ATP-Ce_0.75Zr_0.25O₂（Co/m-ATP-CZO）catalyst was prepared by cerium-zirconium modification of ATP on the basis of Co/m-ATP catalyst to improve the cyclic property,thermal stability and catalytic polymerization activity.The effects of hydrogen supplier selection,dosage of formic acid（FA）,reaction temperature and time on catalytic activity were investigated.The results showed that under the reaction conditions of ethanol/isopropyl alcohol volume ratio of 1/1,FA/lignin mass ratio of 4/1,240℃and 8 h,59.28 wt.%of bio-oil was obtained,in which the monomer yield was 30.5 wt.%,and the guaiacol yield was the highest（5.94 wt.%）.The next was 4-methyl guaiacol（5.74 wt.%）.Nuclear magnetic resonance spectroscopy（¹H-NMR）showed that the absorption peaks of hydrogen atoms in the aromatic ring of lignin decreased and separated from each other,indicating that monomers containing structural units were formed and lignin was effectively depolymerized.（3）On the basis of Co/m-ATP-CZO catalyst,the catalytic polymerization efficiency and monomer yield of lignin can be further improved by introducing other metal additives.Therefore,bimetallic catalyst Co-Mo/m-ATP-CZO was developed in this paper.The effects of Mo loading,reaction time and temperature on catalytic activity were investigated.The results show that the doping of Mo metal leads to the change of metal oxide phase,the increase of oxygen vacancy（O_V）and the decrease of acid site.Under the condition of Co/Mo ratio of 5/3,240℃and 8 h,57.44 wt.%of bio-oil was obtained,in which the monomer yield was 37.58 wt.%,and the calorific value was increased from the initial 19.62 MJ/kg to 33.20 MJ/kg.（4）The pyrolysis behavior of lignin under different catalysts was analyzed by TG technology,and the activation energy（E）,pre-exponential factor（A）and reaction mechanism function were calculated by isoconversion method and double isostep method.The results show that the introduction of the catalyst promotes the lignin decomposition process and reduces the activation energy of the pyrolysis reaction.Its plausible pyrolysis mechanism function is G（α）=[-ln（1-α）]⁴.