Palladium catalysts supported on HZSM-5 (Pd/HZSM-5-X (X = 15, 30, 75, 100, and 150)) were prepared with a variation of Si/Al molar ratio (X), and they were applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Conversion of hydrogen over Pd/HZSM-5-X catalysts showed no great difference, while selectivity for hydrogen peroxide over the catalysts exhibited a volcano-shaped curve with respect to Si/Al molar ratio. As a consequence, yield for hydrogen peroxide over the catalysts showed a volcano-shaped curve with respect to Si/Al molar ratio. Yield for hydrogen peroxide over Pd/HZSM-5-X catalysts was closely related to the Bronsted acidity/Lewis acidity (B/L) ratio of the catalysts. Yield for hydrogen peroxide increased with increasing B/L ratio of Pd/HZSM-5-X catalysts. This indicates that Bronsted acid sites of Pd/HZSM-5-X catalysts played an important role for enhancing the catalytic performance in the direct synthesis of hydrogen peroxide. Furthermore, yield for hydrogen peroxide increased with increasing Bronsted acidity of Pd/HZSM-5-X catalysts. Among the catalysts tested, Pd/HZSM-5-30 catalyst with the largest Bronsted acidity showed the highest yield for hydrogen peroxide. Therefore, it is concluded that Bronsted acidity of Pd/HZSM-5-X catalysts served as an important factor determining the catalytic performance in the direct synthesis of hydrogen peroxide. (c) 2012 Elsevier B.V. All rights reserved.