Rapid microwave-hydrothermal conversion of lignin model compounds to value-added products via catalytic oxidation using metal organic frameworks

Microwave irradiation is an effective method for faster heating to shorten reaction time of oxidative valorization of lignin. However, studies using microwave irradiation for lignin oxidation all employ homogeneous catalysis. Thus, this study aims to investigate heterogeneous catalytic oxidation of lignin under microwave irradiation. Especially, metal organic frameworks (MOFs) are adopted as transition metal-containing heterogeneous catalysts for lignin oxidation. In particularly, MOFs (MIL-101 (Cr), MIL-101 (Fe), UiO-66, HKUST-1, and MOF-801) are also prepared using microwave irradiation and used as for oxidative conversion of a model lignin compound, vanillyl alcohol (VAL), to the valuable products, vanillin (VN) and vanillic acid (VAC), using H2O2 as an oxidant. While the tested MOFs all exhibit catalytic activities for VAL conversion to VN/VAC, MIL-101 and MOF-801 appear to be relatively effective. Through investigating the effect of temperature, VAL conversion to VN/VAC is less favorable at higher temperature possibly due to degradation of H2O2 at high temperatures. While a higher dosage of H2O2 increases VAL conversion, the additionally added H2O2 seems to further oxidize VN to VAC instead of converting more VAL to VN. Through the EPR analyses, the mechanism of VAL conversion to VN/VAC may be attributed to both the OH-based and non-OH• routes. The most effective MOF, MOF-801, also exhibited very similar catalytic activities over several cycles. The results indicate that MOFs can convert VAL to valuable products of VN and VAC within a very short time (10 min) under microwave irradiation. MOF-801 was also validated as a promising MOF for VAL conversion.