When high metal dispersion has a detrimental effect: Hydrogen peroxide direct synthesis under very mild and nonexplosive conditions catalyzed by Pd supported on silica
Graphical abstract
H2O2 direct synthesis was studied on different Pd-based catalysts under very mild and nonexplosive conditions. Pd/SiO2 gives the highest selectivity, and the productivity follows the trend: . A high Pd dispersion is not advisable to observe good catalytic performance. On the contrary, the presence of less energetic sites at the surface of big Pd particles, where O2 chemisorbs without dissociation, is a necessary condition to produce H2O2.
Highlights
► Pd/SiO2 catalysts were studied in H2O2 direct synthesis under mild and nonexplosive conditions. ► The optimum Pd content is 1.5 wt.% and calcination is mandatory to preserve catalytic performance. ► Pd/SiO2 gives the highest selectivity and productivity under the adopted experimental conditions . ► Less energetic Pd sites, where O2 chemisorbs without dissociation, are necessary to produce H2O2.
Introduction
Hydrogen peroxide is a clean and excellent oxidizing reagent for the production of both fine and bulk chemicals and finds application also in the area of environmental cleanup, like in water treatment, or in “green” technologies like the paper and pulp bleaching. More recent applications in the chemical industry are related to the discovery of TS-1 molecular sieves and their ability to promote large scale selective oxidation processes such as the epoxidation of olefins, the hydroxylation of aromatics, and the synthesis of cyclohexanone oxime, a key intermediate in the production of nylon-6 [1]. These and other possible smaller scale applications of hydrogen peroxide would greatly benefit from on-site, moderate-scale production facilities capable of reducing transport costs. The development of a new economic process to synthesize H2O2 is considered a key step toward the introduction of new selective oxidation processes for a sustainable production [2], [3].
Presently, the anthraquinone route [4] is almost the only process used worldwide, with more than 90% share. This process, although practised on a multi-million ton scale annually, suffers from several limitations: (i) it produces a significant amount of organic waste due to the over-reduction of anthraquinone, (ii) it needs several energy-consuming separation and concentration steps, (iii) it uses benzene as the solvent, and (iv) it is economically feasible only on large scale plants.
The direct reaction of H2 + O2 → H2O2 may potentially half the cost of H2O2 with respect to the present commercial process, and its environmental impact would be much lower. With respect to other metals, palladium, either alone or associated with platinum [5], [6], [7] or gold [8], [9], [10], is the best catalyst as far as activity and, very important for commercial applications, selectivity are concerned [11], [12], [13]. The latter, however, is still unacceptably low and must be significantly improved for a successful industrial exploitation of hydrogen peroxide direct synthesis. As reported below, there are four reactions, all catalyzed by palladium, that are involved in this process, with water as the most thermodynamically favored product:
Furthermore, the very broad explosion limits of H2–O2 gas mixtures (4–96%) impose some severe limitations to the practicability of the process under safe conditions. Therefore, in spite of several published patents [4], [14], [15], [16], [17], [18], [19] and recent literature [20], [21], [22], [23], [24], [25], [26], [27], [28], no process for the direct synthesis of hydrogen peroxide has yet been marketed. In this reaction, the nature of the support is an important factor: Hutchings and coworkers found that carbon-supported Au–Pd alloy catalysts give the highest reactivity, while silica performed better than Al2O3 and Fe2O3 under the same experimental conditions [24]. Pd/SiO2 catalysts have been extensively studied inside the explosive regime by Lunsford et al. with useful results [7], [28], [29]. The influence of different halides ions on the performance of a Pd/SiO2 sample in the direct H2O2 formation from H2 and O2 has also been investigated [22].
In the present paper, we present the results obtained in the direct synthesis of hydrogen peroxide using a series Pd/SiO2 catalysts. Catalytic tests were performed under very mild conditions (1 bar and 20 °C), outside the explosion range and without halide addition. Further catalytic tests were performed also at higher pressure (p = 10 bar) using solvents expanded with CO2 in order to increase the performance of catalysts and evaluate their application potential. Moreover, the catalysts here reported were compared with other Pd samples supported on ZrO2, sulfated , and sulfated under identical conditions in order to evaluate the effect of the support on Pd dispersion and the relevant effect on catalytic activity.
Section snippets
Catalysts preparation
SiO2 (Akzo) was used as received for samples synthesis. It was impregnated by incipient wetness with H2PdCl4 aqueous solutions to give the desired metal loading (0.5, 1.5, 3.0 wt.%) and typically calcined at 500 °C (0.5Pd500, 1.5Pd500, 3.0Pd500).
In addition, in order to optimize the preparation procedure and evaluate the effect of the calcination temperature on catalyst performance, part of the 1.5 wt.% Pd material was used directly after drying without calcination (1.5Pdnc), while other amounts
Catalytic data
A summary of the catalysts analytical and CO chemisorption properties along with their productivities in hydrogen peroxide direct synthesis is reported in Table 1.
All catalytic tests were carried out in methanol, the best solvent for this reaction according to previous work [30]. In fact, methanol has many advantages: (i) it helps H2 solubility while avoiding the formation of peroxides, at variance with higher alcohols [34]; (ii) the use of methanol as a solvent allows water titration and
Acknowledgment
We thank MIUR (Rome) for financial support.
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