Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen over Supported Palladium Catalyst

Abstract

Direct synthesis of hydrogen peroxide from hydrogen and oxygen has attracted much attention as an economical and environmentally benign process. Acids and halides have been used as additives to enhance the selectivity for hydrogen peroxide in the direct synthesis of hydrogen peroxide. However, acid additives cause the corrosion of reactor and accelerate the dissolution of active metal component from the supported catalyst. Therefore, acidic solid supports have been employed for the direct synthesis of hydrogen peroxide, in order to minimize the amount of acid additives. In this work, palladium catalysts supported on various acidic solid supports were employed for the direct synthesis of hydrogen peroxide. The effect of acidic solid supports on the catalytic performance of the catalysts in the direct synthesis of hydrogen peroxide was investigated. A correlation between acid property and catalytic performance of the catalysts was then established. SO3H-functionalized mesoporous silica was prepared using MCM-41, MCM-48, MSU-1, SBA-15, and MCF silica. Palladium catalysts supported on SO3H-functionalized mesoporous silica (Pd-SO3H-functionalized mesoporous silica; Pd-SO3H-MCM-41, Pd-SO3H-MCM-48, Pd-SO3H-MSU-1, Pd-SO3H-SBA-15, and Pd-SO3H-MCF) were applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. For comparison, palladium catalysts supported on mesoporous silica (PdO/mesoporous silica; PdO/MCM-41, PdO/MCM-48, PdO/MSU-1, PdO/SBA-15, and PdO/MCF) were also employed for the direct synthesis of hydrogen peroxide. Yield for hydrogen peroxide over Pd-SO3H-functionalized mesoporous silica catalysts was much higher than that over PdO/mesoporous silica catalysts. This indicates that high catalytic performance of Pd-SO3H-functionalized mesoporous silica catalysts was attributed to the enhanced acid property of the catalysts. Furthermore, it was revealed that Pd-SO3H-functionalized mesoporous silica catalysts enhanced the yield for hydrogen peroxide by preventing the decomposition of hydrogen peroxide. Yield for hydrogen peroxide increased with increasing acid density of Pd-SO3H-functionalized mesoporous silica catalysts. Among the catalysts tested, Pd-SO3H-MCF catalyst with the highest acid density showed the highest yield for hydrogen peroxide. Acid density of Pd-SO3H-functionalized mesoporous silica catalysts served as a crucial factor determining the catalytic performance in the direct synthesis of hydrogen peroxide. SO3H-functionalized mesoporous silica supports efficiently served as an alternate acid source in the direct synthesis of hydrogen peroxide. A set of palladium-exchanged insoluble heteropolyacid (Pd0.15CsXH2.7-XPW12O40; denoted as Pd-CsXPW (X=2.0, 2.2, 2.5, and 2.7)) catalysts were prepared with a variation of cesium content. They were then applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Yield for hydrogen peroxide over Pd-CsXPW catalysts showed a volcano-shaped curve with respect to cesium content. Surface acidity of Pd-CsXPW catalysts also showed a volcano-shaped trend with respect to cesium content. Among the catalysts tested, Pd-Cs2.5PW catalyst with the largest surface acidity showed the highest yield for hydrogen peroxide. Therefore, surface acidity of Pd-CsXPW catalysts played an important role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. Pd-CsXPW catalysts acted as an efficient metal catalyst and served as an alternate acid source in the direct synthesis of hydrogen peroxide. In order to facilitate the separation of insoluble heteropolyacid (CsXH3-XPW12O40) from the reaction mediuim after the reaction, CsXH3-XPW12O40 was supported on MCF silica. A series of palladium catalysts supported on CsXH3-XPW12O40/MCF (Pd/CsXH3-XPW12O40/MCF; denoted as Pd/CsXPW/MCF (X=1.7, 2.0, 2.2, 2.5, and 2.7)) prepared with a variation of cesium content were applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Yield for hydrogen peroxide showed a volcano-shaped curve with respect to cesium content. Acidity of Pd/CsXPW/MCF catalysts also showed a volcano-shaped trend with respect to cesium content, because acidity of Pd/CsXPW/MCF catalysts was mainly due to surface acidity of the catalysts. Yield for hydrogen peroxide increased with increasing acidity of Pd/CsXPW/MCF catalysts. Among the catalysts tested, Pd/Cs2.5PW/MCF catalyst with the largest acidity showed the highest yield for hydrogen peroxide. Acidity of Pd/CsXPW/MCF catalysts played a crucial role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. It can be inferred that CsXH3-XPW12O40 of Pd/CsXPW/MCF catalysts efficiently served as an alternate acid source in the direct synthesis of hydrogen peroxide. Heteropolyacid (H3PW12O40) was incorporated into MCF silica to simplify the catalyst preparation step. Palladium catalysts supported on H3PW12O40-incorporated MCF silica (Pd/H3PW12O40-MCF; denoted as Pd/HPW-MCF-X (X=1.0, 4.8, 9.1, 13.0, 16.7, 20.0, 23.1, and 25.9)) were prepared with a variation of H3PW12O40 content. They were then employed for the direct synthesis of hydrogen peroxide from hydrogen and oxygen. High catalytic performance of Pd/HPW-MCF-X catalysts compared to Pd/MCF was attributed to the enhanced acid property of Pd/HPW-MCF-X catalysts. Furthermore, it was revealed that Pd/HPW-MCF-X catalysts increased the yield for hydrogen peroxide by preventing the decomposition of hydrogen peroxide. Yield for hydrogen peroxide showed a volcano-shaped curve with respect to H3PW12O40 content. Acidity of Pd/HPW-MCF-X catalysts also showed a volcano-shaped trend with respect to H3PW12O40 content, because acidity of Pd/HPW-MCF-X catalysts decreased due to the restriction of effective exposure of H3PW12O40 when an excess amount of H3PW12O40 was incorporated into MCF silica. It was revealed that yield for hydrogen peroxide increased with increasing acidity of Pd/HPW-MCF-X catalysts. Among the catalysts tested, Pd/HPW-MCF-20.0 catalyst with the largest acidity showed the highest yield for hydrogen peroxide. Acidity of Pd/HPW-MCF-X catalysts played an important role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. In order to avoid the dissolution of heteropolyacid (H3PW12O40) from the Pd/H3PW12O40-MCF catalysts into the reaction medium during the reaction, insoluble heteropolyacid (Cs2.5H0.5PW12O40) was incorporated into MCF silica. Palladium catalysts supported on Cs2.5H0.5PW12O40-incorporated MCF silica (Pd/Cs2.5H0.5PW12O40-MCF; denoted as Pd/CsPW-MCF-X (X=14.3, 21.8, 28.1, 33.4, and 38.0)) were prepared with a variation of Cs2.5H0.5PW12O40 content. The prepared catalysts were applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Yield for hydrogen peroxide over Pd/CsPW-MCF-X catalysts showed a volcano-shaped curve with respect to Cs2.5H0.5PW12O40 content. Acidity of Pd/CsPW-MCF-X catalysts also showed a volcano-shaped trend with respect to Cs2.5H0.5PW12O40 content. Yield for hydrogen peroxide increased with increasing acidity of Pd/CsPW-MCF-X catalyst. Among the catalysts tested, Pd/CsPW-MCF-21.8 catalyst with the largest acidity showed the highest yield for hydrogen peroxide. Acidity of Pd/CsPW-MCF-X catalysts played a crucial role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. In summary, palladium catalysts supported on various acidic solid supports such as SO3H-functionalized mesoporous silica and insoluble heteropolyacid supported on mesoporous silica showed high catalytic performance in the direct synthesis of hydrogen peroxide from hydrogen and oxygen. This indicates that acidic solid supports are able to replace the acid additives in the direct synthesis of hydrogen peroxide. Furthermore, it was revealed that the enhanced yield for hydrogen peroxide over the catalysts was attributed to the improved acidity on the surface of the catalysts. Therefore, palladium catalysts supported on acidic solid supports with the largest surface acidity exhibited the highest yield for hydrogen peroxide in the direct synthesis of hydrogen peroxide. It is concluded that surface acidity of the catalysts played a crucial role in determining the catalytic performance in the direct synthesis of hydrogen peroxide.