Catalytic transfer hydrogenolysis of lignin into monophenols over platinum-rhenium supported on titanium dioxide using isopropanol as in situ hydrogen source

Abstract

Using isopropanol as an in situ hydrogen donor, catalytic transfer hydrogenolysis of lignin into monomeric phenols was studied at mild conditions. The performance of catalysts and the effects of H2, temperature, and time on depolymerization of acid extracted birch lignin (ABL) were extensively examined. Platinum-rhenium supported on titanium dioxide (PtRe/TiO2) exhibited much higher activity on disrupting CO bonds than Pd/C, HZSM-5, Pt/TiO2, and Re/TiO2. 18.71 wt% monophenols was achieved for depolymerization of ABL over PtRe/TiO2 at 240 °C for 12 h with He. 4-Propylsyringol had the highest yield of 7.48 wt%. 2D HSQC NMR analysis reveals that β-O-4 bonds have been fully disrupted during depolymerization. Addition of H2 led to less monophenols, likely due to the competitive adsorption of active sites on catalysts. Structure-reactivity analysis based on six representative lignins shows that the total yields of monophenols were highly linearly correlated with the β-O-4 contents (R2 = 0.97).