Study On Selective Depolymerization Of Lignin Over MoO_x/MFI

Lignin is the most abundant renewable aromatic resource on the globle.Thus,it is considered to be a perfect alternative of fossil fuel on producing high-value-added fine chemicals and high-grade biofuels,and the efficient utilization of lignin is of great significance to the implementation of sustainable development strategy in China.Heterogeneously catalytic conversion of lignin is beneficial to the reuse of catalyst,but there are some obstacles such as high diffusion resistance and low yield to be solved.In view of this,it is necessary to develop heterogeneous catalytic system which is conducive to mass transfer of lignin macromolecules and has high activity for the lignin depolymerization.Zeolites generally show regular shape,adjustable pore structure,strong acidity and high hydrothermal stability and have been widely used in the petrochemical industry and biomass conversion.However,the sole micropores in traditional zeolites present limited mass transfer between lignin macromolecules and the active sites of catalyst,resulting in low lignin conversion,poor product selectivity and severe coked-deactivation.In this study,we provided novel Mo O_x supported by hierarchical zeolites for the catalytic depolymerization of lignin,which realize the effective mass transfer and high activity of lignin depolymerization.Further more,the catalytic mechanism has been investigated on the basis of the results of lignin conversion and catalyst characterization.The main contents of this dissertation are summarized as follows:(1)A series of hierarchical zeolite catalysts had been designed.Compared with the conventional microporous zeolites,the hierarchical zeolites have better compatibility with lignin macromolecules and show higher activity in the catalytic depolymerization of lignin.It is found that Mo₅/MFI has the highest catalytic activity and selectivity for the conversion of lignin to ethyl p-coumarate(EPC).Under 200?for 4 h,the yield and selectivity of EPC reach11.8 wt.%and 85.5%,respectively.Through a series of characterization,it can be seen that the hierarchical structure of Mo₅/MFI catalyst is conducive to the mass transfer and the contact between lignin and active sites.The appropriate amount of weak acid and a large amount of Mo O₃ crystal on the surface of the catalyst are in favor of catalytic activity.Meanwhile,Mo⁶⁺species plays an important role in the catalytic process.(2)The kinetic properties of lignin model compounds with?-O-4 bond and ester bond were studied.The results show that the lignin model compounds with?-O-4 bond and ester bond are both second-order reaction over Mo₅/MFI.The activation energy of the catalytic conversion of 2-phenoxy-1-phenyl ethanol and phenyl ethyl acetate are 41.2 k J mol^-1 and 72.5k J mol^-1,respectively.In combination with the results of the conversion of lignin model compounds containing other characteristic bonds over Mo₅/MFI,we can conclude safely that Mo₅/MFI can break not only the?-O-4 bond which is abundant in lignin,but also the ester bond which connects p CA unit and other lignin fractions.(3)In order to further promote the catalytic activity of Mo₅/MFI on the lignin depolymerization,Mo-Cu bimetallic oxide catalytic system has been constructed.The addition of Cu species substantially enhances the activity of catalyst.63.8%conversion of lignin can be achieved under 200?for 4 h in nitrogen atmosphere with 15.2 wt.%yield of aromatic monomer,including 13.3 wt.%yield of EPC.After the introduction of the second component of copper oxide,the pore structure of catalyst is not significantly changed;instead,an important Cu₃Mo₂O₉ crystal is formed,which favors the depolymerization of lignin.