Integrated MOF-mesh and TEMPO-grafted carbon fiber as a sandwich-like catalytic system for selective valorization of lignin-derived compound under microwave irradiation

Abstract

Oxidation of vanillyl alcohol to vanillin represents a critical step towards sustainable valorization of lignocellulosic biomass. Although catalytic oxidation of vanillyl alcohol by Cu and 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO) is promising for vanillyl alcohol oxidation, the traditional Cu/TEMPO adopts homogeneous Cu ions and TEMPO, which are difficult for recovery and reuse. In this study, a unique sandwich-like catalytic system (SCS), which comprises HKUST-1 mesh and TEMPO-grafted on carbon cloth, is developed as a heterogeneous catalyst for vanillyl alcohol oxidation. Through the electro-chemical technique, copper (Cu) mesh is used as a source of Cu to grow HKUST-1 directly, whereas carbon cloth is functionalized by TEMPO via covalent bonds. These resultant materials are then stacked to achieve layer-by-layer contacts between HKUST-1 and TEMPO, and enable flow-through reactions of VAL oxidation. Especially, such a SCS exhibits much higher conversion of vanillyl alcohol to vanillin under microwave irradiation than conventional oven heating. SCS could achieve 100% of conversion, 100% of selectivity and 100% of yield of vanillin at 120 °C for 60 min. This full conversion of vanillyl alcohol to vanillin surpasses almost all the reported values by other processes in literature, and SCS could be also reusable and continuously implemented for vanillyl alcohol conversion to vanillin. The used HKUST-1 mesh could retain crystalline structures of HKUST-1, whereas TEMPO is also preserved on TEMPO@CC, indicating that SCS would be a stable and reusable integrated catalyst for vanillyl alcohol oxidation to vanillin.