Critical Techniques for Overcoming the Diffusion Limitations in Heterogeneously Catalytic Depolymerization of Lignin.

Heterogeneously catalyzed depolymerization of lignin to value-added chemicals is increasingly attractive but highly challengeable. Particularly, the diffusion limitation of lignin macromolecule to the solid catalyst surface is a big barrier, which significantly decreases the yield of monomer while increases the char formation. Therefore, for the potential industrial utilization of lignin, new knowledges focused on the size of lignin particles is of great importance to offer the guidance for promoting the lignin depolymerization and suppressing the condensation in the heterogeneously catalytic systems. In this work, the size of lignin particles and macromolecules are summarized. Previous approaches for improving the mass diffusion including enhancing the solubility of lignin, and exploitation of hierarchical and "solubilized" materials are also discussed. Based on these, a constructive perspective is proposed. Thus, this work provides a new insight on the rational design of heterogeneously catalytic techniques for efficient utilization of the aromatic polymer of lignin.