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 H-2, 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 C-O 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 degrees C for 12 h with He. 4-Propylsyringol had the highest yield of 7.48 wt%. 2D HSQC NMR analysis reveals that beta-O-4 bonds have been fully disrupted during depolymerization. Addition of H-2 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 beta-O-4 contents (R-2=0.97).