Enhanced lignin depolymerisation to produce butylated hydroxytoluene and 4-propylguaiacol on carbon-nitride supported molybdenum catalysts

Lignin valorisation is hindered by intrinsic difficulties. The breaking of the anti-degradation barrier of lignin for the transformation of lignin has become a hot topic in current research. In the present work, the carbon-nitride-supported molybdenum catalyst (Mo/CN) was synthesised by a two-step method from ammonium molybdate and dicyandiamide and used for the enhancement of lignin depolymerisation to produce high-value phenols. The SEM, TEM, XPS, XRD and FT-IR results of Mo/CN confirmed that the two-step method is advantageous for the high dispersion of MO nanoparticles on the CN carrier. The CN and various CN-supported metal catalysts were examined carefully to evaluate their catalytic activities for kraft lignin conversion, and this indicated that the loading of molybdenum on CN was a favourable method for the catalytic depolymerisation of kraft lignin. A depolymerisation residue yield of 4.33%, tetrahydrofuran-soluble products (TSPs) yield of 61.6%, and a monomer yield of 23.4% could be obtained when kraft lignin depolymerisation was performed on 2% Mo/CN in 50% ethanol at 220 °C for 2 h. High-value phenols such as butylated hydroxytoluene (BHT) and 4-propylguaiacol dominated the depolymerisation products. Moreover, the synthetic catalyst presents good applicability for different industrial lignins. To understand the role Mo/CN played in the degradation of lignin, a β-O-4 type lignin dimer 2-(2-methoxyphenoxy)-1-(4-methoxyphenyl)ethenone was used as a model, and the degradation pathways of the model based on Mo/CN are proposed.