Mesoporous silica as supports for Pd-catalyzed H2O2 direct synthesis: Effect of the textural properties of the support on the activity and selectivity
Graphical abstract
SBA-15 allows preparing palladium catalysts with enhanced productivity and selectivity in the direct synthesis of hydrogen peroxide by imparting Pd particles the proper size and distribution.
Introduction
Hydrogen peroxide is an excellent “green” oxidant but its relatively high cost has so far hampered a more general application of this commodity in the production of both fine and bulk chemicals. The development of a new, more economic process to synthesize H2O2 is considered to be a key step towards the introduction of new selective and sustainable oxidation processes [1]. In this respect, the direct H2 + O2 → H2O2 reaction will significantly reduce the environmental impact and may potentially halve the production cost of H2O2 in comparison with the current commercial process based on the methyl-anthraquinone route. The latter is economically profitable only when operated in large-scale plants, making the market a monopoly of a few producers. Several supported metal catalysts have been studied for the direct reaction, palladium, either alone or mixed with platinum [2], [3], [4] or gold [5], [6], [7], having emerged as the best option with respect to both activity and, very important for commercial applications, selectivity [8], [9], [10]. The latter is still unsatisfactory, and this drawback must be overcome for a possible successful industrial exploitation.
As shown in Scheme 1, there are four reactions, all catalyzed by palladium that are involved in the direct synthesis of H2O2, water formation being the most thermodynamically favored process.
Furthermore, because of the very broad explosion limits of H2/O2 gas mixtures (4–96%), the direct formation of H2O2 is regarded as potentially very hazardous, while operation under intrinsically safe conditions is very important for the viability of the process. Therefore, in spite of several published patents [11], [12], [13], [14], [15], [16], [17] and recent literature [18], [19], [20], [21], [22], [23], [24], at present no process for the direct synthesis of hydrogen peroxide has yet been marketed.
Many different supports have been investigated for this reaction, but the most common ones are silica [25] and carbon [26]. By comparing various supports, Edwards et al. have found that the nature of the support is an important factor for a successful reaction, observing, with carbon supported catalysts, high selectivity for the reaction (95%) in the absence of acid or halide promoters in the reaction medium. Carbon-supported Au–Pd alloy catalysts give the highest reactivity, while silica performed better than Al2O3 and Fe2O3 [22] under the same experimental conditions. Pd/SiO2 catalysts have been extensively studied by Lunsford et al., albeit inside the explosive regime, proving useful candidates for the synthesis of hydrogen peroxide [4], [24], [27]. The influence of different halide ions on the performance of a Pd/SiO2 sample in the direct H2O2 formation has also been reported [20]. In the present work, we have investigated the use of ordered mesoporous silica materials of the M41S family that was first successfully synthesized by the assembly of cationic surfactants with inorganic precursors in the early 1990s. Among these materials, MCM-41 and SBA-15 have received increasing scientific interest because of their ordered channel structure, narrow pore size distribution, high surface area and pore volume that make them successful catalyst supports for a variety of reactions. In particular, SBA-15 is characterized by larger uniform pore size (4.7–30 nm), thicker silica walls (3–6 nm) and higher hydrothermal stability than MCM-41 [28]. The advantages of using ordered mesoporous solids in catalysis are related to the relatively large pores which facilitate mass transfer and the very high surface area which allows a high concentration of active sites per mass of material. In fact, a good heterogeneous catalyst must possess a pore structure capable to host the reagents and to allow the formation of the desired product. Consistently, metal nanoparticles deposited in porous and ordered host materials are desired for many practical applications and could be especially useful in a triphasic reaction with selectivity problems such as the H2O2 direct synthesis. The idea of the present work is to investigate the impact of mesopore confinement effects not only for reactants [29] but also for Pd effective dispersion. In particular, we aim at verifying whether geometry-dependent contributions can influence the catalytic behavior in hydrogen peroxide direct synthesis. For this purpose, we have prepared a series of palladium catalysts supported over commercial silica (Akzo) [30], MCM-41 and SBA-15, with different morphological properties. Herein, we describe the results obtained in hydrogen peroxide direct synthesis both under very mild conditions (1 bar and 293 K) and at higher pressure (10 bar) both to increase the productivity and to investigate the influence imparted by different silica supports in a wider range of conditions.
Section snippets
Materials
All kinetic tests were performed in anhydrous methanol (SeccoSolv, Merck, [H2O] < 0.005%). Commercial standard solutions of Na2S2O3 (Fixanal [0.01], Hydranal-solvent E, and Hydranal-titrant 2E, all from Riedel-de Haen) were used for iodometric and Karl-Fischer titrations.
Catalysts preparation
The MCM-41 mesoporous support was synthesized as previously reported [31]: a given amount of cetyltrimethylammonium bromide (CTABr, ALDRICH) was added to a NaOH aqueous solution and stirred slowly at room temperature for 40 min.
Textural and chemical–physical properties of the supports
N2 physisorption analyses were carried out in order to determine the surface area and pore size distribution of the supports. This point is crucial in this study as the choice of an appropriate mesoporous material can rule out mass transfer problems and at the same time allows a good dispersion of the Pd active phase. The adsorption isotherms of the three samples, as well as their BJH pore size distributions, are shown in Fig. 1, while the corresponding values are reported in Table 1. The
Discussion
The data reported in Section 3 indicate a close correlation between textural properties of the samples and their catalytic activity, showing that surface area, pore size distribution and metal dispersion of the catalysts have a key role on directing the course of reaction. We have already discussed [42] that oxygen adsorption without dissociation is a necessary condition to hydrogen peroxide formation and it is strictly correlated with the nature of the adsorbing Pd sites that can be more or
Conclusions
Pd-based samples supported on silica were successfully tested for the direct synthesis of hydrogen peroxide outside the explosion range even under very mild conditions (1 bar and 293 K). The ability of the SBA-15 support to influence the final Pd particles dispersion makes it the best support for this reaction, even if compared with previously investigated catalysts [7], [36], [45], [46]. In SBA-15, besides the thick wall thickness that makes it stable in the reactor, the pore size is
Acknowledgments
We thank Prof. Giuseppe Cruciani for XRD data and Mrs. Tania Fantinel for technical assistance. Financial support to this work by MIUR (Rome) is gratefully acknowledged.
References (46)
- et al.
Catal. Today
(2006) - et al.
Appl. Catal. A – Gen.
(2006) - et al.
Appl. Catal. A – Gen.
(2008) - et al.
J. Catal.
(2008) - et al.
Appl. Catal. B – Environ.
(2003) - et al.
J. Catal.
(2007) - et al.
Catal. Commun.
(2007) - et al.
Catal. Today
(2007) - et al.
J. Catal.
(2006) - et al.
J. Catal.
(2005)