Self-Oxidation of Lignin to Aromatic Acids with High Selectivity Catalyzed by Designed Acidic Mesoporous Molecular Sieves Incorporating Heteroatoms

An economic method for the synthesis of crystalline mesoporous sieves and the self-oxidation of alkali lignin (INDULIN AT) to aromatic acids is reported herein. Crystalline mesoporous molecular sieves ILM-Al have been prepared in strongly acidic ionic liquid (IL) media and used as catalysts for the self-oxidation of alkali lignin to aromatic acids. In the preparation of the new type of mesoporous molecular sieves, the imidazolium-based acidic functional IL system served as both solvent and structure-directing agent. The ILM-Al exhibited higher catalytic performance and selectivity for aromatic acids in the self-oxidation of lignin than in that of fossil fuels, and the main products were homoveratric acid and homovanillic acid. The catalyst could be recycled and reused with negligible loss in activity over five cycles, and the IL could also be recycled for further use. Moreover, Zr and Sn were also successfully incorporated into the framework of ordered crystalline mesoporous materials by the same simple method. The obtained ILM-Zr and ILM-Sn also showed excellent catalytic activity for the self-oxidation of alkali lignin.