Abstract

Background: Au-catalyzed selective oxidation of benzyl alcohol to benzaldehyde was investigated over Au/γ-Al2O3 nanocatalysts under aerobic conditions. The homogeneous depositionprecipitation method was used to prepare these catalysts. In the present work, varying Au content over γ-Al2O3 support has been used as heterogeneous catalysts to investigate the vapor phase oxidation of benzyl alcohol to benzaldehyde. In this study, it was observed that an Au/γ-Al2O3 catalyst shows an optimum conversion of benzyl alcohol to benzaldehyde at Au loading of 2 wt%. The catalytic activity of Au/γ-Al2O3 was interrelated with Au particle size supported by Au on γ-Al2O3 support. The catalytic activity depends on the size of the gold nanoparticles, as well as on the interaction between gold nanoparticles and various support materials. We have optimized various parameters such as Au nanoparticles loading, reaction temperature, and reaction time for efficient conversion as well as product selectivity.

Methods: The Au nanocatalysts supported on γ-Al2O3 were prepared with by homogeneous depositionprecipitation (HDP) method by varying the loading of Au nanoparticles and using urea as the precipitating agent.

Results: A varying Au loading has been employed over γ-Al2O3 support material after their structural investigation. The vapor phase oxidation of PhCH2OH to PhCHO was investigated over Au/γ-Al2O3 catalysts under air. Only a few percent of benzyl alcohol were found to convert in the absence of assynthesized catalysts. Moreover, bare support also showed a very poor catalytic activity towards benzyl alcohol conversion. Hence, the synergistic effect of Au nanoparticles over support materials facilitated as an excellent catalyst for efficient conversion of benzyl alcohol to benzaldehyde. The 2 wt% of Au/γ- Al2O3 catalysts after 2 h of reaction time showed the maximum conversion of 33%.

Conclusion: Au/γ-Al2O3 catalyst prepared by HDP method exhibits a high catalytic activity as well as stability for the vapor phase conversion of benzyl alcohol to benzaldehyde. The optimum condition for efficient conversion of benzyl alcohol is observed to be 2 wt% Au loading, 2 hr reaction time and 320 °C of reaction time for all cases.

Keywords: Gold nanoparticles, benzyl alcohol, oxidation, benzaldehyde, aerobic condition, HDP.

Graphical Abstract