Direct synthesis of H2O2 in methanol at low pressures over Pd/C catalyst: Semi-continuous process
Research highlights
▶ The production of H2O2 via direct synthesis is studied in a semi-continuous reactor. ▶ Nano-Pd/C commercial catalyst and methanol as solvent are used. ▶ Low pressures (below 0.9 MPa) and low temperatures (below 40 °C) are used. ▶ H2O2 concentrations up to 1.33 wt.% obtained. ▶ Yields up to ca. 40% and selectivities up to ca. 70% achieved.
Introduction
Finding a green process for the production of hydrogen peroxide by direct synthesis as an alternative to the traditional anthraquinone route has fascinated scientists for decades [1]. Over 95% of the current industrial production of H2O2 is carried out via the anthraquinone route or Riedl–Pfleiderer process which avoids direct contact between H2 and O2 and offers continuous production at moderate temperatures [2], [3]. Nevertheless, and this is a hackneyed argument, large amounts of by-products are produced in the traditional process, and the process requires several steps of separation and concentration, consequently requiring a rather large energy input.
Analysing the chemistry, one can easily find that the direct reaction between hydrogen and oxygen would be the simplest way to produce H2O2. However, the real reaction scheme is more complex due to the occurrence of simultaneous and consecutive reactions, all of them thermodynamically favoured and highly exothermic. Among these reactions are: decomposition of hydrogen peroxide, reduction of hydrogen peroxide and direct water formation by combustion.
The well-known safety issue associated with the direct synthesis process is the explosive nature of the hydrogen–oxygen mixtures. In the traditional route this problem is avoided because H2 and O2 are not in direct contact. Under atmospheric pressure conditions, the explosive range falls between 4.0 and 95 mol% H2 at 25 °C [4]. Although the direct synthesis could be conducted over the Upper Flammability Limit (UFL), the operation under Low Flammability Limit (LFL) is preferred for economic (H2 is the limitant reagent), safety (the lower the quantity of H2, the lower is the power of an undesired explosion) and process reasons (to minimise hydrogenation of H2O2). This forces the use of large amounts of diluents, usually nitrogen (pure or from air), although CO2 has also been studied [5].
An aqueous reaction medium provides the safest conditions [6], [7], [8]; however, its main drawback is the low solubility of the reacting gases, which strongly limits mass transfer and thus the rate of peroxide production. The use of alcohols [9], [10] or alcohol–water mixtures [11], [12] increases the solubility of the gases. As an example, Degussa/Headwaters recently announced the construction of the first pilot plant for direct synthesis of H2O2 to be used as an in situ source integrated into a propylene oxide plant, disclosing that methanol will be used as solvent [13], [14].
The other main issue associated with the direct synthesis of hydrogen peroxide is the discovery of an active and highly selective catalyst. Most catalysts described in the literature are supported on carbon, zeolites, Al2O3 or SiO2. Ntainjua et al. [15] studied the role of the support, finding that supports with low isoelectric points such as carbon and silica give the highest rates of synthesis. The active metals are usually palladium [16], [17], [18], gold [8], [19] or combinations of Pd + Au [10], [11], [20], [21] or Pd + Pt [9], [21]. Unfortunately, these catalysts are also responsible for the side reactions, namely the combustion of hydrogen to water and the decomposition of hydrogen peroxide. The role of size and crystalline type of the metal particles in the selectivity to H2O2 is still uncertain, but the effect is clear. Several authors claim the beneficial effects of nanoparticle and nanocrystals use [12], [20], [22]. In addition to the catalyst selection, the use of promoters and H2O2 protective additives is crucial, especially when using monometallic Pd catalysts. Pospelova et al. [23] early reported the beneficial effect of adding a mineral acid such as HCl in order to inhibit the decomposition of the peroxide over a Pd catalyst. More recently, the role of these so-called promoters has been analyzed in detail, and it is now generally accepted that halide anions increase the selectivity towards hydrogen peroxide, while the role of protons is to facilitate the adsorption of halide anions on the catalyst and to inhibit peroxide decomposition by decreasing its adsorption on decomposition sites [6], [24], [25]. The promoters are most commonly added homogenously to the solvent, although they might also be incorporated onto the catalyst via catalyst modification with similar or slightly better results [26]. On the other hand, Hutchings and his coworkers have demonstrated that it is possible to achieve high selectivities without any promoters by combining metals, such as Au–Pd catalysts [20], [27].
Finally, the existence of three phases in the reactor, i.e. gases (H2, O2 and an inert), liquid (solvent) and solid (catalyst), usually involves important mass transfer limitations and complicates the process. Mass transfer can be enhanced by: (1) increasing mass transfer coefficient or reducing the thickness of stagnant film by stirring, (2) increasing surface area of the bubbles using low-diameter-bubble spargers or injection systems, (3) increasing the solubility of the gases by increasing the partial pressure of the gases (Henry's law); this implies the use of high pressures and subsequently the use of more expensive equipment.
In a previous work, we have demonstrated the possibility of producing H2O2 in both H2 and methanol using CO2 and N2 as inert gases at high pressures and moderate temperatures [28]. In this work we demonstrate that H2O2 can be produced semi-continuously in methanol at low pressures, up to 0.9 MPa, and moderate temperatures, near ambient conditions, using a commercial Pd/C catalyst. The effect of key variables such as O2/H2, halide/Pd and acid/Pd ratios, reaction time and pressure has been studied. The catalyst has been characterized by TEM and SEM–EDX before and after the use in order to elucidate the effect of the support and active metal.
Section snippets
Materials and methods
The catalyst used in this study was fine particles with an average of 5 wt.% Pd over carbon support purchased from Aldrich and used fresh for each experiment. It has been selected because it is well-known as an active catalyst, commercially available and monometallic. Research grade oxygen, hydrogen and nitrogen were purchased from BOC Gases (UK) and used without further modification. Methanol with a purity of 99.8% was used as solvent (Fischer Scientific). KI (Fluka), H2SO4 (Sigma) and Na2S2O3
Catalyst characterization
The bulk and surface properties of the fresh Pd/C catalyst are shown in Table 1. It is a highly microporous material, which provides a high specific surface area (1314 m2 g−1) and a significant micropore volume (0.503 cm3 g−1). SEM analysis of the catalyst showed a nanotube carbon structure (see Fig. 1). The size of the palladium particles on the fresh catalyst was found to be ca. 5 nm (see Fig. 2). The presence of the bulk metallic Pd (Pd0) phase in the catalyst was confirmed by X-ray diffraction
Conclusions
In this work we have successfully carried out the direct synthesis of hydrogen peroxide under mild pressure conditions and room temperature, using a commercial nano-Pd/C catalyst. The influence of promoters, O2/H2 ratio, reaction time and reaction pressure has been evaluated. Based on our previous experiments using the same catalyst, under high pressure and using scCO2, we have found that the interaction between Br− anions and Pd does not depend on the reaction conditions but solely on the
Acknowledgements
The authors wish to thank the Spanish Science and Innovation Ministry, Project Reference: CTQ 2006-0299/PPQ and CTQ2009-14183-C02-01 for funding and for the FPI fellowship granted, the University of Bath for their collaboration in the European PhD of Teresa Moreno, Dr. Francisco Sobrón for the numerical filtering of the X-ray diffraction pattern and the Laboratorio de Técnicas Instrumentales of the University of Valladolid for their invaluable help with the analysis of the catalyst.
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