Abstract
AbstractThe direct synthesis of hydrogen peroxide has been for about 20 years a hot topic in “green” catalysis. Several methods, which are well established to measure the concentration of hydrogen peroxide in water are also applied to the analysis of reaction mixtures from the direct synthesis of H2O2. However, this step could not be always straightforward, because these mixtures contain almost invariably organic solvents and, sometimes, selectivity enhancers which can interfere in some, at the least, of the most popular titrimetric methods. This work presents a comparative investigation of iodometry, cerimetry, permanganometry (titrimetric methods) and spectrophotometric analysis of TiIV/H2O2 adduct, as applied to analysis of hydrogen peroxide produced by its direct synthesis. They account for more than 90 % of the competent literature since 2000. Their pros and cons are highlighted to provide a guideline for the choice of the best possible method of analysis and for the comparison of catalytic results assessed in different ways in the context of the direct synthesis of hydrogen peroxide.
Highlights
The reaction of formation of hydrogen peroxide (HP) from its elements over heterogeneous palladium-based catalysts, commonly referred to as the “direct synthesis” (DS), has been intensively investigated for at least 20 years, with contributions from several research groups.[1–7]It outstands among many other catalytic reactions in that no organic substances are practically involved with the exception of methanol, which is most often used as the cosolvent in order to increase the solubility of dihydrogen and dioxygen in the aqueous liquid phase
We carried out a systematic assessment of the differences in the amount of H2O2 measured in the same DS reaction mixture with different methods and we report our results
Organic substances, such as methanol or alkyl bromides, or even inorganic additives such as hydrogen bromide are redox active and their effect on the result of the analysis should be carefully verified. Another reason of concern is the concentration of H2O2 in the reaction mixtures, which could not match the optimal conditions for the analytical method of interest
Summary
The reaction of formation of hydrogen peroxide (HP) from its elements over heterogeneous palladium-based catalysts, commonly referred to as the “direct synthesis” (DS), has been intensively investigated for at least 20 years, with contributions from several research groups.[1–7] It outstands among many other catalytic reactions in that no organic substances are practically involved with the exception of methanol, which is most often used as the cosolvent in order to increase the solubility of dihydrogen and dioxygen in the aqueous liquid phase. Analysis of the reaction mixtures of the DS mainly relies on classical volumetric analysis (redox titration) or colorimetric analysis (spectrophotometric determination of the adduct between TiIV and H2O2) These methods fit batch-wise testing, the most common one, which requires a single final measure, but they can be exploited for experiments under semi-batch and continuous conditions. They are cheap, require a very simple work-up, if any, and are relatively easy to use for the skilled chemist
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