Comparing to homogeneous systems, direct alcohol to acetals conversion (DAAC) reaction through heterogeneous catalysts has obvious advantages of simple separation, less corrosion and waste. However, progress on this research is quite limited. Recently a breakthrough of Ru@MIL-101(Cr) catalyst was found with high selectivity and conversion for DAAC of aliphatic alcohol and diols towards desired products. In this study, time resolved in situ FT-IR was performed for capturing molecular-level information on the active sites of the catalysis system. Transients analysis of interaction between n-butanol/butanal and two active surface sites induced by Ru loading on MIL-101(Cr) provides key insight into the reaction, revealing that Ru@ MIL-101(Cr) confines the reaction to the ideal acidic sites between Ru nanoparticles and MIL-101(Cr) support for efficient process.