The Direct Synthesis of Hydrogen Peroxide Using Platinum-Promoted Gold–Palladium Catalysts
J. K. Edwards, J. Pritchard, L. Lu, M. Piccinini, G. Shaw, A. F. Carley, D. J. Morgan, C. J. Kiely, G. J. Hutchings., Angew. Chem, Int. Ed., 2014, 53 (9), 2381-2384.
The direct synthesis of hydrogen peroxide from hydrogen and oxygen offers a cleaner more atom efficient alternative to the current commercial production process for this important commodity chemical. Currently over three million metric tonnes of H2O2 are produced from the indirect anthraquinone process, around 80% of which was used for fine chemical synthesis and in the paper and textile bleaching industries. The indirect industrial process produces concentrated H2O2 that has to be transported to its point of use where it is diluted, the direct synthesis route would enable H2O2 production to take place at its point of use and for this reason there is significant interest in developing such a catalytic process.
We have very recently shown1 that the addition of small amounts of Pt to a 1:1 AuPd catalyst significantly enhances the activity for the direct synthesis of H2O2. Detailed spectroscopic and microscopic studies showed that the addition of Pt to the AuPd nanoparticles significantly affects the metal surface composition and this leads to the marked effects we observe on the catalytic formation of hydrogen peroxide. In addition, we have demonstrated an experimental approach that can help to identify the optimal nominal ternary alloy compositions for this reaction by studying the synthesis and hydrogenation/decomposition reactions as separate data sets and superimposing the activity values as a fourth (vertical) dimension on a ternary composition diagram.
H2O2 synthesis rates for CeO2-supported 5 wt% Au-Pd-Pt catalysts presented as a contour diagram
The benzaldehyde oxidation paradox explained by the interception of peroxy radical by benzyl alcohol
Meenakshisundaram Sankar, Ewa Nowicka, Emma Carter, Damien M. Murphy, David W. Knight, Donald Bethell & Graham J. Hutchings
In the February issue of “Nature Communications” we reported a detailed investigation into the long standing mystery behind the > 98% yield of benzaldehyde during the selective oxidation of benzyl alcohol observed by several researchers around the world.
It has long been known that benzaldehyde spontaneously undergoes autoxidation to yield benzoic acid simply upon exposure to air at ambient temperature (293 K). A common laboratory observation is the appearance of crystalline benzoic acid in bottles of benzaldehyde stored over a period of time. The presence of catalysts, such as transition-metal ions, or free radical initiators, such as benzoyl peroxide or azo-bis-isobutyronitrile, increases the rate of this oxidation. It was therefore logical to ask how benzyl alcohol can be partially oxidized to give benzaldehyde in very high yield even in the presence of oxidation catalysts that are active in promoting conversion of benzaldehyde into benzoic acid, in some cases at temperatures as high as 433 K in the presence of 10 bar O2 pressure. This situation prompted us to approach the problem from a perspective of radical studies. Herein we investigate this interesting phenomenon by employing electron paramagnetic resonance (EPR) spectroscopy combined with the spin trapping methodology and demonstrate the role of free radicals in the autoxidation of benzaldehyde.
We found that the presence of a very small amount of benzyl alcohol and other alcohols, with at least one C–H group active for hydrogen atom transfer, inhibits the oxidation of benzaldehyde to benzoic acid even under extreme conditions of high temperature and high O2 pressure. Here, with the aid of EPR spin trapping experiments, we show that benzyl alcohol (even as little as 2 wt%) quenches the oxidation of benzaldehyde to benzoic acid principally by intercepting the acylperoxy radicals, which are responsible for the oxidation of benzaldehyde to benzoic acid. Testing of several alcohols, in place of benzyl alcohol, reveals that alcohols with at least one benzylic or allylic α-H quenches the oxidation of benzaldehyde to benzoic acid. Our preliminary experiments, reveal surprisingly that 1-octanol has an analogous inhibiting effect on the autoxidation of octanal.