Pd-Au and Pd-Pt catalysts for the direct synthesis of hydrogen peroxide in absence of selectivity enhancers
Graphical abstract
Introduction
Hydrogen peroxide is a strong and environmental friendly oxidizing agent with many applications, such as paper and pulp bleaching, water treatment, chemical synthesis and electronics [1]. More than 95% of the worldwide production of H2O2 currently comes from the two-step Riedl–Pfleiderer process. In the first step an alkyl-anthraquinone is hydrogenated over a Ni or Pd catalyst to the corresponding alkyl-anthraquinol. Hydrogen peroxide is formed in the second step upon oxidation of the anthraquinol with air to the starting alkyl-anthraquinone, which is eventually returned to the hydrogenation unit. The major drawbacks of this process are the use of extensive amounts of organic solvents, loss of the alkyl-anthraquinone with formation of large amounts of by-products, the need of several steps of separation and concentration, large energy input. Moreover it is affordable only for large scale productions, which implies high investments costs and concentration of the production in a limited number of sites, from where hydrogen peroxide must be transported as (dangerous) concentrated solutions. On the other hand it allows the safe production of H2O2 at moderate temperature (40–60 °C), with no direct contact of H2 and O2. Although it has been known since the beginning of the 20th century, the direct synthesis of hydrogen peroxide (DSR) [2], only recently has started to raise interest as an alternative to the anthraquinone route [1], [3], [4] and its appeal even for potential industrial application is demonstrated by a pretty large patent literature [5], [6], [7], [8], [9], [10]. It is a conceptually simple reaction, where H2 and O2 dissolved in a liquid react over a solid catalyst in a single step.
Palladium is the best suited metal for this reaction, but unfortunately it is also active for the decomposition, the hydrogenation of hydrogen peroxide and the combustion of hydrogen to water. This can greatly lower the selectivity towards H2O2, because of the formation of water, which is by far the most thermodynamically favoured (but undesired) product (Scheme 1).
Another important issue to be coped with in the DSR is the safety in handling mixtures of H2 and O2. They must be kept out of the explosive (4–96%, v/v, for H2/O2 mixtures) and flammability ranges [11], [12], which are very large. Moreover the problem of flammability can concern the solvent too, for instance when methanol (as it is often the case) is used [13]. Being the most expensive reagent dihydrogen is usually used in the least possible amount, so that the DSR is generally carried out in the low H2 concentration safety range. Therefore the catalysts must ensure conversion of dihydrogen and selectivity towards hydrogen peroxide high enough to obtain useful concentrations of the product in the liquid phase. For all these reasons the DSR is a very demanding reaction, and in spite of great efforts from both the Academia and the Industry no technical breakthrough has been achieved yet.
The design of the catalyst has been so far the main tool employed to attain the required level of catalytic performance and a lot of studies on this topic have been continuously appearing in the literature. Most of them are devoted to palladium or bimetallic palladium/gold catalysts supported onto a number of different solids, such as SiO2 [14], TiO2 [15], ZrO2 and CeO2 [16], [17], carbon [18] and ion-exchange resins (sulfonated polystyrene-divinylbenzene) [19], [20], [21]. The colloidal palladium described by Liu and Lunsford [22] and gold–palladium systems [23] developed by Ishihara et al. were are also highly active in the DSR.
The bimetallic supported catalysts showed particularly promising results.
Pd/Au systems were carefully investigated, showing the benefits of the gold promotion of palladium [17], [24], [25]. Although bimetallic platinum–palladium systems have been reported in the patent literature [6], [7], [8] more often than the gold–palladium ones, they received comparatively much smaller attention in the open literature [16], [26], [27]. A 0.5% (w/w) Pd/SiO2 catalyst containing a small platinum amount (Pt/Pd = 5/95, mol/mol) showed good results, with a 2.5-fold increase in the production of H2O2 and only a slight decrease of the selectivity in comparison with a monometallic catalyst [26]. However the addition of higher amounts of platinum resulted in a significant decrease of the selectivity, which dropped from ca 60% to ca 50 and 20% when the molar Pt/Pd ratio was increased from 5/95 to 50/50 and 95/5 respectively [26].
It has been already shown in our laboratories that reduced palladium catalysts (1%, w/w) supported by the macroreticular ion-exchange resin Lewatit K2621% are excellent catalysts for the DSR in absence of acids and halides, performing better than Pd/SiO2 catalysts [20]. We report herein on the performance of Au/Pd and Pt/Pd catalysts supported on the same K2621 resin in the DSR. All the reported catalysts contain 1% (w/w) palladium, but the content of either gold or platinum differs from one to another (for gold 0.25%, 0.5% and 1%, w/w; for platinum 0.1%, 0.25%, 0.5% and 1%, w/w). Under halide-free conditions the best results in terms of selectivity towards hydrogen peroxide and its top concentration achieved were obtained with small additions of either Pt and Au, with the second being more effective.
Section snippets
Materials and apparatus
Unless otherwise stated, all the reagents and the materials were used as received from the supplier. A batch of Lewatit K2621 (sulfonated polystyrene-divinylbenzene macroreticular ion-exchange resin; exchange capacity = 1.92 mmol/g) was kindly provided by Lanxess. [Au(en)2]Cl3 was synthesized according to ref. [28]; [Pd(NH3)4]SO4 and [Pt(NH3)4](NO3)2 were purchased from Alfa Aesar; sodium thiosulfate pentahydrate (99.5%), potassium iodide, starch, concentrated sulphuric acid, 37% aqueous
Results and discussion
Lewatit K2621 is a macroreticular, sulfonated polystyrene-divinylbenzene (S-PSDVB) resin. It possesses permanent meso- and macropores both in the dry and in the swollen state and is a strongly acidic ion-exchanger, with an exchange capacity of 1.92 mmol/g [20].
S-PSDVB resins have several features making them attractive supports for DSR catalysts. The introduction of the metal precursors in the support can be accomplished with a simple ion-exchange of the counter-ions of the sulfonic groups (H+
Conclusions
Two sets of bimetallic Pt-Pd (Pd: 1.0; Pt: 0.1–1.0%, w/w) and Au-Pd (Pd: 1.0; Au: 0.25–1.0%, w/w) catalysts supported on the macroreticular ion-exchange resin Lewatit K2621 were prepared by simple ion-exchange in water of suitable precursors and reduction thereof with a refluxing aqueous solution of formaldehyde. It was found that the addition of a small amount of metal for both platinum (0.1%) and gold (0.25%) makes the catalysts less active and more selective than a 1% palladium catalyst on
Acknowledgments
S.S. is grateful to the University of Padova for financial support under the “Progetti di Ateneo 2010” initiative-project number CPDA100928. P.B. is grateful to the Otto A. Malm Foundation for financial support. This work is part of the activities at the Åbo Akademi Process Chemistry Centre (PCC) within the Finnish Centre of Excellence Programmes (2000–2005 and 2006–2011) by the Academy of Finland. The TEM pictures in Fig. 10 are courtesy of J.P. Mikkola, Umeå University.
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2020, Journal of Electroanalytical ChemistryCitation Excerpt :Several studies published in the literature have sought to improve the capacity of hydrogen peroxide generation. Among the studies that deserve mentioning are those involving the modification of carbon with either organic compounds [10–14] or inorganic compounds [15–19]. Valim et al. (2013) studied the modification of Printex L6 carbon with tert-butyl-anthraquinone, which is an organic molecule that contains a quinine group [20].