Preparation Of Copper-supported Hierarchical Zeolite Catalyst For Selectively Catalytic Oxidative Depolymerization Of Lignin

As the main component of lignocellulose,lignin is the most abundant source of renewable aromatics on Earth.The selective depolymerization of lignin to highly value-added chemicals is thus of great significance for biomass utilization.Depolymerization by oxidative techniques has received considerable attentions,as lots of highly functionalized oxygenates including ketones,aldehydes and organic acids,are produced potentially from this strategy.From the views of the product separation and catalyst recycle,lignin conversion with heterogeneous catalysts shows more tremendous advantages and potentials comparing to that with homogenous catalysts in industrial applications.However,the heterogeneous catalytic system for lignin conversion still faces some technical bottlenecks:（1）harsh reaction conditions in lignin oxidation due to the stable interunit C-C linkages and complicated structure of lignin,（2）low yield and selectivity of target product resulted from the limited mass transfer of lignin macromolecule to the active sites of catalysts.Therefore,developing novel solid catalysts with multiple levels of porosity can be possibly compatible with the larger molecular size of lignin and desirable for the lignin depolymerization with high efficiency.Industrial zeolite catalysts with large surface area,abundant active sites,outstanding thermal stability and good size discrimination,have been widely used in the catalytic degradation of biomass.However,the solo micropores in zeolite present limited mass transfer for lignin macromolecules obviously,leading to low depolymerization efficiency.To overcome this problem,we propose a series of novel catalytic systems for the selective conversion of lignin to useful biochemicals using the metal oxides supported on the hierarchical nanosheets and metal oxides encapsulated polycrystalline catalysts,respectively.Meanwhile,the key scientific problems such as the probable active sites,the relationship between the catalytic performance and properties of catalyst,and the catalytic mechanism have been investigated in details.The main contents of this study are summarized as follows:（1）A series of hierarchical nanosheets catalysts（Ce-Cu/MFI-ns）have been designed and prepared for the selectively oxidative depolymerization of organosolv lignin.By using ethanol as the solvent and oxygen as the oxidant,81.6%conversion of lignin can be achieved under150°C for 24 h over 15Ce-5Cu/MFI-ns with 29.4 wt.%yield of volatile products,including18.1 wt.%yield of diethyl maleate（DEM）.The results of density functional theory（DFT）calculation coupled with static adsorption measurements show that the unique layer structure and the hierarchical pore sizes distribution favor the mass transfer of lignin to the active sites of the catalyst;the XPS characterization method indicates that the electronic effect between CuO_x and Ce O_y species promotes the cleavage of the linkages in lignin.（2）The oxidation of β-O-4 model probes over Ce-Cu/MFI-ns catalyst is performed to explore the activation and cleavage of specific chemical bonds of lignin.The results indicate that the C-C,C-O bonds and benzene ring of lignin molecule are all cleaved in this catalytic system at the same conditions as that for lignin conversion（150°C,24 h）,yielding a similar distribution of products as lignin oxidation.The catalyst of 15Ce-5Cu/MFI-ns is also highly stable,where no significant activity loss and structural change are observed after the recycling.In addition,the catalytic mechanism for lignin depolymerization is investigated via controlling oxidative degree of model compounds under milder conditions（100°C,4 h）by the isotope-labeling experiments and electron paramagnetic resonance（EPR）characterization.It is found that the cleavage of C-C bonds occurs essentially during lignin depolyemrization to form monophenols and then the resulting monophenols undergo ring-cleavage and esterification to generate DEM.（3）To further promote the conversion of lignin,yield and selectivity of target product,a nanovesicle assembly with CuO_x and Fe O_y encapsulated polycrystalline catalyst（CuO-Fe₂O₃@S-1）is designed and prepared for the oxidative depolymerization of lignin.In the presence of Cu_2.5-Fe_2.2@S-1 catalyst,the conversion of lignin reaches to 92.0%after 24 h at150°C,giving the yield and selectivity of DEM of 35.6 wt.%and 70.7%,respectively.The static adsorption measurements demonstrate that the mass transfer of lignin macromolecules to the surface of catalyst is significantly enhanced due to the micro-meso-maroporous structure of Cu_2.5-Fe_2.2@S-1.Furthermore,the results of UV-Vis DRS and XPS indicate that the catalyst with the polycrystalline structure has increased amounts of isolated framework Fe³⁺and Cu²⁺species than that with a single crystalline structure;N₂ adsorption and desorption measurement suggests that the catalyst with polycrystalline structure exhibits a higher hierarchy factor by balancing the relative micropore volume and relative external surface area than that with a single crystalline structure.Both properties are considered to be important for the promotion of the degradation efficiency of lignin and the yields of products.Moreover,the catalyst Cu_2.5-Fe_2.2@S-1 system exhibits good feedstock adaptability for different kinds of lignins and even for the native lignin in the bagasse and the catalyst can be reused up to 6 times.（4）The reaction pathway of lignin oxidative depolymerization over CuO-Fe₂O₃@S-1catalysts is also investigated by using lignin models containingβ-O-4 linkage.It is found that the product distribution from this process is highly similar as that of organosolv lignin and native lignin in bagasse under the identical conditions（150°C,24 h）.This result indicates that a cleavage of chemical bonds between the aromatic ring of lignin occurred prior to the formation of the final product of DEM.To better understand this issue,the isotope-labeling experiments and EPR have also been performed.Combining all of the abovementioned results,we suggest that the conversion of lignin to DEM is a cascade process of:formation of lignin radical cation intermediate via single electron transfer pathway during the oxidative process;the cleavage of C-C bonds,demethoxidization and hydroxylation to generate monophenol;the ring-cleavage reaction which affords maleic acid;and（4）the esterification reaction of maleic acid with ethanol,yielding the final product of DEM.