Oxidative degradation of lignin by biomass-mediated hierarchical ZSM-5 zeolites: Cβ-O bond breaking and mechanistic analysis.

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The catalytic depolymerization of lignin has long been challenged by the limited catalytic mass transfer and the complexity of its polymer structure. In this work, a series of hierarchical MFI nanosheet catalysts (named AL-MFI, M-BKC-MFI, M(metal)-AL-MFI and M(metal)-BKC-MFI, respectively) using biomass (lignin and lignin biochar) as a template were designed to realize the oxidative depolymerization of lignin and its derivatives efficiently and stably. The Zn element in the M-AL-MFI and M-BKC-MFI compensated for the acidity, while Ce could stimulate the production of O2- through redox and trigger a free radical pyrolysis reaction. The conversion rate of lignin was as high as 80.7% and 82.5%, respectively, with acetophenone as the main product in yields as high as 42.82% and 47.13%, respectively. DFT calculations revealed that the bulk sizes of alkali lignin and its derivatives (≤6.112nm) were smaller than the average pore size of the catalysts (≥7.27nm). And this finding provided direct evidence for the critical role of the mesoporous structure of the catalysts in lignin depolymerization. Specifically, the mesoporous structure at suitable acidity contributes to the mass transfer of lignin to the active sites of the catalyst, resulting in an efficient depolymerization process. What's more, the degradation pathways and mechanisms of lignin were analyzed with the help of DFT and GC-MS using 2-phenylethyl phenyl ether (PPE) and 2'-Phenoxyacetophenone (PTE) as model compounds. β-O-4 bonds were broken at a rate of more than 80%, which is the primary mechanism of lignin cleavage. And the order of bond breaking was Cβ-O bond (253J/mol)>Cα-Cβ bond (285.1J/mol)>Caromatic-O bond (407.9 J/mol). M-MFIs promoted the cleavage of Cα-Cβ and Cα-O to a certain extent, which indicated that MFI nanosheets contributed to the cleavage of bonds with higher dissociation energies. This work not only helps to reveal the detailed process of lignin depolymerization, but also provides valuable theoretical guidance for further optimizing the catalyst design and improving the depolymerization efficiency.