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

The hydrolytic cleavage of beta-O-4 ether bonds in lignin model compounds, guaiacylglycerol-beta-guaiacyl ether (GO) and veratrylglycerol-beta-guaiacyl ether (VG), was studied in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) with metal chlorides and water. FeCl3, CuCl2, and AlCl3 were found to be effective and functioned catalytically in cleaving the beta-O-4 bond of GO, although a number of other metal chlorides are considerably less active. AlCl3 functioned more effectively in cleaving the beta-O-4 bond of VG than did FeCl3 and CuCl2. After 120 min at 150 degrees C, GO conversion reached 100%, and about 70% of the beta-O-4 bonds of GO were hydrolyzed, liberating guaiacol, in the presence of FeCl3 and CuCl2, while about 80% of the beta-O-4 bonds of GO were hydrolyzed in the presence of AlCl3 with 100% GO conversion. About 75% of the beta-O-4 bonds of VG were hydrolyzed in the presence of AlCl3 after 240 min at 150 degrees 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 GO were detected and proposed as the possible intermediate products in the GO reaction. One possible acid-catalyzed pathway accounting for the guaiacol production from GO is presented.