Selective value-added conversion of lignin derivatives over heterogeneous catalysts of TEMPO-functionalized metal-organic frameworks

Abstract

Selective oxidation of vanillyl alcohol (VAL) to vanillin (VN) is a key step towards sustainable valorization of lignin biomass. The combination of Cu metal and TEMPO has been shown to be an effective catalytic system for the oxidation of alcohols to aldehydes. However, most previous studies have stayed in homogeneous catalytic systems, where the catalysts are difficult to recycle. In this study, Carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) were utilized as the substrate, and the carboxyl functional groups could provide partial ligands for MOFs (Cu3(BTC)2), thus allowing MOFs to be encapsulated by MWCNTs. And the hydroxyl group of 4-OH-TEMPO could be dehydrated and condensed with the carboxyl group to graft with carbon nanotubes to form a multiphase catalytic system. The addition of carbon nanotubes improved the stability and thermal conductivity of the catalyst, which was beneficial to the catalytic conversion of VAL. Under the conventional oven heating, the conversion of VAL and the selectivity of VN could reach 98.33% and 91.37% at 120 ℃ for 2 h, respectively. And under microwave radiation, the complete conversion of VAL to VN could be achieved at 120 ℃ for 1 h. After five cycles, the efficiency of the catalyst could still reach more than 85%, which proved to be a stable and reusable integrated catalyst for the oxidation of VAL to VN.