Hydrolytic Cleavage of β-O-4 Ether Bonds of Lignin Model Compounds in an Ionic Liquid with Metal Chlorides

The hydrolytic cleavage of β-O-4 ether bonds in lignin model compounds, guaiacylglycerol-β-guaiacyl ether (GG) and veratrylglycerol-β-guaiacyl ether (VG), was studied in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) with metal chlorides and water. FeCl 3, CuCl 2, and AlCl 3 were found to be effective and functioned catalytically in cleaving the β-O-4 bond of GG, although a number of other metal chlorides are considerably less active. AlCl 3 functioned more effectively in cleaving the β-O-4 bond of VG than did FeCl 3 and CuCl 2 . After 120 min at 150 °C, GG conversion reached 100%, and about 70% of the β-O-4 bonds of GG were hydrolyzed, liberating guaiacol, in the presence of FeCl 3 and CuCl 2, while about 80% of the β-O-4 bonds of GG were hydrolyzed in the presence of AlCl 3 with 100% GG conversion. About 75% of the β-O-4 bonds of VG were hydrolyzed in the presence of AlCl 3 after 240 min at 150 °C. The catalytic activity is associated with the hydrochloric acid, working as the acid catalyst, formed in situ by the hydrolysis of the metal chlorides. A dehydration product and dimer products from GG were detected and proposed as the possible intermediate products in the GG reaction. One possible acid-catalyzed pathway accounting for the guaiacol production from GG is presented.