Selective Aerobic Upgrading of Lignin-Derived Compound Using a Recyclable Dual-Functional TPO-Loaded Cu-BTC Catalyst

Abstract

As oxidative conversion of lignin-derived compound, vanillic alcohol (VA), to vanillin aldehyde (VL) represents as a useful process for lignocellulosic biomass valorization. While catalytic aerobic oxidation by Cu and 2,2,6,6-tetramethylpiperidine 1-oxyl (TPO) with O2 as an oxidant is proposed as a promising oxidative process for VA conversion, this conventional Cu/TPO employs homogeneous Cu ions and TPO, making recovery of homogeneous Cu ion and TPO from reaction media extremely challenging. Herein, Cu species and TPO are integrated into a single heterogeneous catalyst through a co-precipitation method to embed TPO into copper benzene-1,3,5-tricarboxylate (CuBTC) metal organic frameworks (MOFs), forming a bifunctional heterogeneous catalyst. The resulting TPO-embedded CuBTC (TEC) retained the typical octahedral morphology and crystalline structure of CuBTC, while encapsulating TPO within CuBTC. The TEC exhibited higher conversion efficiencies of VA than the simple combination of pristine CuBTC and TPO. A higher TPO embedment in TEC, higher temperature and longer reaction time substantially enhanced VA oxidation, and thus TEC with 12.5 wt% of TPO embedment at 120 °C and 2-h reaction time could achieve CVA > 98%, SVL = 100% and YVL > 98%. The TEC is also proven to be reusable for continuous VA oxidation. These features indicate that TEC is a promising heterogeneous catalyst which incorporates the dual catalytic functions of Cu sites and TPO moieties into a catalytic system to continuously, effectively and selectively oxidize VA to VL.