The Research On The Catalytic Selective Transformation Of Lignin Models Under Oxygen Atmosphere

The excessive use of fossil energy has led to increasingly serious problems of environmental pollution and energy crisis around the world,affecting global security.With the continuous development of society,the exploitation and use of biomass energy have received the attention of scientists.In the process of biomass transformation,lignin and lignin models can be selectively converted to prepare aldehydes,ketones,acids,esters and other high value-added chemical products with significant economic benefits through reduction,oxidation,hydrogenation,hydrolysis and pyrolysis processes.Therefore,the research on selective oxidative cleavage reaction of lignin models is of great scientific significance and practical value for efficient valorization of biomass.In this thesis,the iron-based heterogeneous catalysts and multi-metallic high entropy catalysts were designed and developed for the catalytic oxidative cleavage process of C-O and C-C bonds in lignin models,and the selective conversions of2-phenoxy-1-phenylethanone and 1-phenyl-1,2-ethanediol were respectively investigated under the aerobic conditions.The details are presented in the following.Firstly,a simple Fe-C-N-850 nano catalyst has been developed using the direct pyrolysis of the mixture of ferrous chloride and chitosan in argon.The effects of different preparation methods,different organic carbon sources and calcination temperatures on the performance of the iron-based catalyst（Fe-N-C）were investigated in detail.Results show that,in the presence of molecular oxygen,theβ-O-4 lignin models could be converted to phenol and methyl benzoate in the methanol solvent at 120℃for 4 h using Fe-N-C-850 as the heterogeneous catalyst,where the conversion of the substrate was 95.0%,and the yields of phenol and methyl benzoate reached 94.9%and 63.7%,respectively.In addition,the oxidative cleavage reaction of other lignin models was also achieved with the high converision and product selectivity.Further studies indicated that the presence of Fe₃C phase in the catalyst plays an important role in the aerobic oxidative cleavage processes of the C-C and C-O bonds in theβ-O-4 lignin.In addition,a series of tests such as SEM,TEM and XPS on the iron-based catalysts were carried out to clarify the correlation between the catalyst morphology,the surface composition and the catalytic performance.Finally,a possible catalytic mechanism for the aerobic selective oxidative cleavage reaction of 2-phenoxy-1-phenylethanone by Fe-C-N-850 catalysts was proposed based on the data of control experiments and fundamental theory.Furthermore,the magnetic high entropy catalysts were prepared using five metals Cu,Fe,Co,Mn and Mg to systematically study the selective aerobic oxidative cleavage process of 1-phenyl-1,2-ethanediol in the methanol solvent.After a detailed study,it is found that the catalyst obtained from the reduction of the precursor at400℃for 2 h in a hydrogen environment provided a high catalytic activity for the oxidative cleavage reaction of 1-phenyl-1,2-ethanediol,in which a 96.1%conversion of the substrate could be acquired with a 96.2%product selectivity of benzaldehyde at 120℃for 4 h under a 0.3 Mpa oxygen pressure.The characteristics of the catalyst were investigated by performing physical tests such as XRD,XPS,SEM,TEM,BET and H₂-TPR techniques.The recyclability and generalisability of the catalysts were also investigated.Results show that the multimetallic solid catalysts still had the high activity and selectivity after 11 times repeated uses,and the active components in the catalysts were investigated by several designed control experiments.Finally,a possible catalytic mechanism was proposed through radical trapping experiments as well as the corresponding reaction phenonema.