The correlation between acid property of beta zeolite and its catalytic performance for phenol oxidation has been studied. Acid properties of beta zeolites in the proton form (H-BEA) and modified beta zeolite by alkaline earth metal ion exchange treatment (AE/H-BEA, AE: Mg, Ca, Sr, and Ba) were investigated by diffuse-reflectance FT-IR (DR-FTIR) spectroscopy. It was revealed that the features of the OH group stretching region of Ca/H-BEA are different from those of H-BEA. The peak at 3782 cm(-1) observed on H-BEA disappeared and new peaks appeared at 3672-3692 cm(-1) after the ion exchange treatment. NH3 temperature programmed desorption analyses (NH3-TPD) of these zeolites were also carried out. The NH3 desorption peak at similar to873 K observed on H-BEA, which has been assigned to a very strong acid, disappeared, and a new peak at 743 K and a shoulder at the temperature lower than the peak at about 623 K appeared after the ion exchange treatment. These observations suggested that the acid properties of H-BEA were substantially modified by the ion exchange treatment. Phenol oxidation with hydrogen peroxide as oxidant over AE/H-BEA was conducted in the presence of diethylketone, and the result was compared with that over H-BEA. While the selectivity for diphenols, catechol and hydroquinone, over H-BEA based on added hydrogen peroxide was similar to89% that over AE/H-BEA was higher than 94%. The best catalyst was Ca/H-BEA, where the selectivity was as high as similar to98%. From these results, it is concluded that AE/H-BEAs are more suitable catalysts for the phenol oxidation with hydrogen peroxide than H-BEA, and it is suggested that the removal of strong acid sites by the ion exchange treatment would be the reason for their high selectivity. (C) 2004 Elsevier B.V. All rights reserved.