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Catalysts 2018, 8(9), 379; doi:10.3390/catal8090379

Review
Recent Advances in the Direct Synthesis of Hydrogen Peroxide Using Chemical Catalysis—A Review
1
Chair of Chemistry of Biogenic Resources, Technical University of Munich–Campus Straubing for Biotechnology and Sustainability, Schulgasse 16, 94315 Straubing, Germany
2
Catalysis Research Center (CRC), Technical University of Munich, Ernst-Otto-Fischer Straße 1, 85748 Garching, Germany
3
Fraunhofer Institute of Interfacial Engineering and Biotechnology (IGB), Bio-, Electro-, and Chemo Catalysis BioCat Branch Straubing, Schulgasse 11a, 94315 Straubing, Germany
4
School of Chemistry and Molecular Biosciences, The University of Queensland, 68 Copper Road, St. Lucia 4072, Australia
*
Author to whom correspondence should be addressed.
Received: 1 August 2018 / Accepted: 28 August 2018 / Published: 5 September 2018

Abstract

:
Hydrogen peroxide is an important chemical of increasing demand in today’s world. Currently, the anthraquinone autoxidation process dominates the industrial production of hydrogen peroxide. Herein, hydrogen and oxygen are reacted indirectly in the presence of quinones to yield hydrogen peroxide. Owing to the complexity and multi-step nature of the process, it is advantageous to replace the process with an easier and straightforward one. The direct synthesis of hydrogen peroxide from its constituent reagents is an effective and clean route to achieve this goal. Factors such as water formation due to thermodynamics, explosion risk, and the stability of the hydrogen peroxide produced hinder the applicability of this process at an industrial level. Currently, the catalysis for the direct synthesis reaction is palladium based and the research into finding an effective and active catalyst has been ongoing for more than a century now. Palladium in its pure form, or alloyed with certain metals, are some of the new generation of catalysts that are extensively researched. Additionally, to prevent the decomposition of hydrogen peroxide to water, the process is stabilized by adding certain promoters such as mineral acids and halides. A major part of today’s research in this field focusses on the reactor and the mode of operation required for synthesizing hydrogen peroxide. The emergence of microreactor technology has helped in setting up this synthesis in a continuous mode, which could possibly replace the anthraquinone process in the near future. This review will focus on the recent findings of the scientific community in terms of reaction engineering, catalyst and reactor design in the direct synthesis of hydrogen peroxide.
Keywords:
catalyst; direct synthesis; hydrogen peroxide; Pd based catalyst; reactor engineering; microreactor

1. Introduction

Hydrogen peroxide (H2O2) is a colorless, odorless, and slightly acidic liquid used mainly as an oxidant in chemical synthesis [1,2,3]. It is an atom efficient, benign, and eco-friendly oxidant that produces water or oxygen as a degradation product, depending on the catalyst used [4,5]. Commercial H2O2 production has a concentration range of 30–70% and the areas of application depend on the concentration of H2O2 used. Usually, household, medical/dental, and cosmetic applications need diluted concentrations of Ca. 3–5% H2O2. Higher concentrations of up to 70% are needed for synthesis, wastewater treatment, mining, and bleaching applications. The electronics industry needs higher H2O2 concentrations (ranging from 70–90%) for cleaning and anti-corrosion purposes. Finally, the concentrated versions of 90–98% are used for military and aerospace purposes [6]. Given its wide range of applications in almost every aspect of human life, the method of H2O2 production in the industry is of utmost importance. The demand for H2O2 is always increasing with recent processes preferring H2O2 as an oxidant. H2O2 production can be done using chemical, electrochemical, enzymatic, or photocatalytic means. Of these routes, only the chemical processes are capable of industrial production [3,7] in an economical manner and only these will be discussed in detail in this work.

1.1. Industrial H2O2 Manufacture—A Historical Perspective

To discuss the chronological advancements in industrial H2O2 production, one has to split the progress into electrochemical and chemical methods. A detailed description of the processes has been summarized in the works of Goor [1,3] and Jones [8] (Table 1) and the reaction schemes are depicted in Scheme 1.

1.2. State of the Art in the Industrial Production of H2O2

On an industrial scale, the anthraquinone autoxidation (AO) process described by H.J. Riedl and G. Pfleiderer produces H2O2 in a cyclic manner (Scheme 2).
Scheme 2a shows the reaction mechanism of the AO process and Scheme 2b depicts the practice of H2O2 manufacture in the industry. The hydrogenation chamber is usually a slurry reactor consisting of the alkylated anthraquinone dissolved in a so-called “working solution” along with a catalyst. Hydrogen gas is initially fed through the reactor for hydrogenation to alkyl anthrahydroquinol. After this, the contents of the hydrogenation chamber are degassed in a separate chamber to remove traces of hydrogen. Following this step, the alkyl anthrahydroquinol is transferred to the oxygenation chamber. Here, oxygen or air is used to produce H2O2 and alkyl anthrahydroquninone, which is then transferred to a second degassing chamber to remove trace oxygen. The degassed solution is then extracted with water to yield H2O2 and the working solution is transferred to the hydrogenation chamber to complete the synthetic cycle. In the mid-1990s, the world capacity of 100% H2O2 was approximately 1.5 million tons with an average plant capacity of around 20 kt–40 kt per annum. By 2015, the capacity was 5.5 million tons with a plant capacity of 300 kt per annum [25]. Solvay (30%), Evonik (20%), and Arkema (13%) represented the global contributors of H2O2 [26].

1.3. Why Is There an Increased Interest in the Direct Synthesis of Hydrogen Peroxide?

Currently, the industrial manufacture of H2O2 is based on the work of Riedl and Pfleiderer, also known as the AO process [21] (Scheme 2) [24] using polynuclear hydroquinones [1]. Although this process is capable of meeting the world’s H2O2 demand, it does have certain drawbacks such as:
  • excessive use of solvents for the process
  • a negative environmental impact owing to the production of unwanted waste
  • complex and multi-step process
  • mass transfer limitations and low efficiency
  • transport limitations of reactants between reactors
  • organic contamination of H2O2 stemming from organic solvents or hydroquinones/hydroquinols during liquid-liquid extraction [27,28].
Considering environmental issues and resource conservation aspects, academia and the industry have set out to design benign and non-polluting processes. The principles of green chemistry helps in achieving this goal. The principles outlined in the 1990s clearly state that it is necessary to design and execute industrial processes that are clean, benign, non-polluting, and safe [29,30,31,32]. Two important terms, atom utilization and E-factor, are important in assessing the greenness of a process. Atom utilization, atom efficiency or atom selectivity (AE) maybe defined as the actual mass of reactants that actually end up in the final desired product, the rest of which is termed as “waste”. AE helps in assessing the amount of wastes generated by a certain process, which in turn, will determine the E-factor. The E-factor is the ratio of the amount of waste produced to the amount of desired product [33,34,35]. Considering the AO process, the amount of solvents and the alkylated hydroquinones used in the process are waste products, as the reaction is not atom efficient [1,3,7,8]. This brings up an important question: why is the AO process still practiced at an industrial level if it is not sustainable? The answer: operating the AO process is economically feasible at a scale of 1 × 105 tons per annum producing high concentrations of H2O2, which are diluted prior to use. For a majority of the applications mentioned previously in the introductory section (Section 1), diluted versions of H2O2 (typically 3–8%) is required, in small amounts, and on site. To circumvent the issues mentioned above and promote a green H2O2 production process, the direct synthesis of H2O2 from H2 and O2 was researched, but only at the laboratory scale. Theoretically, it is clear that the direct synthesis approach is the simplest way to synthesise H2O2; however, issues with respect to practicality limit the industrialization of this process [36,37]. This review will address the challenges and developments in the field of direct synthesis of H2O2, focussing on chemical catalytic methods.

2. The Direct Synthesis Approach to H2O2 Production Using Chemical Catalysis

In 1914, H. Henkel and W. Weber reported the very first process that was capable of producing H2O2 from hydrogen (H2) and oxygen (O2). The patent described the reaction of two gaseous mixtures: an oxygen species (free and bound) and hydrogen in a pressurised vessel along with water. An important aspect of the patent was the use of noble metals capable of fixing hydrogen as catalysts; e.g., palladium (Pd), platinum (Pt), nickel (Ni), etc. [18].

2.1. Pros and Cons of the Direct Synthesis Approach to H2O2 Synthesis

The direct synthesis approach has not been industrially practiced due to several technological and scientific barriers [38] and Table 2 lists the advantages and disadvantages of this approach.
Scheme 3 depicts the direct synthesis of H2O2. As seen, the reaction produces either water H2O or H2O2 depending on the reaction conditions.

2.2. Mechanism of the Direct Synthesis of H2O2

The mechanism of hydrogen peroxide synthesis using H2 and O2 is shown in Scheme 4. A possible mechanism of such a synthesis was proposed by Bianchi et al. in 1999 [46], based on the 1980 report published by Zudin et al. [47]. While Zudin et al. used palladium triphenylphosphane in a biphasic system, Bianchi et al. found out that 2,9-dimethyl-4,7-diphenyl-1,10-phenantroline ligand was the best among other ligands tested. Based on these findings, Werner published the proposed mechanism of H2O2 synthesis by the reduction of dioxygen [48]. Stahl et al. used a bathocuproine palladium complex in order to catalyse the direct synthesis of H2O2 [49]. All three processes utilised acid halides such as hydrochloric acid or hydrogen bromide to facilitate efficient catalysis [46,47,49]. However, the actual mechanism was reported in 2001 by Stahl et al. [49] Scheme 4(i), which was confirmed by Chinta and Lunsford in 2004 using tetrachloropalladate (PdCl42−) prepared prior to use (Scheme 4(ii)) by replacing nitrogen ligands with chloride [50].
The mechanism follows these steps: initially, Pd0 reduces molecular oxygen in the presence of the nitrogen ligands (Scheme 4(i))/halide ions (Scheme 4(ii)) to form the respective PdII complex that contains a peroxo-species. This is replaced by halides yielding H2O2. Subsequently, PdII is reduced by molecular hydrogen to yield Pd0, thereby completing the catalytic cycle.

2.3. Series of Elementary Steps in H2O2 Synthesis

Several accounts of the possible elementary steps during the direct synthesis of H2O2 have been described in literature by Wilson and Flaherty [40], Plauck et al. [51] and Yi et al. [52], to name a few. Scheme 5 depicts the series of steps that lead to the synthesis of H2O2 and H2O during the direct synthesis approach using Pd catalysts in liquid solvents. In each of the steps in the process, bimolecular reactions exist between the H+ and chemically adsorbed intermediates on the catalyst surface. The elementary steps are based on the following assumptions:
  • Free energies of H2 and O2 adsorption are negligible under saturation conditions
  • The adsorption and desorption of the H2O2 species is unrestricted
Based on these assumptions, the first step is that the hydrogen adsorbs dissociatively on to the catalyst surface (Step (1) in Scheme 5) yielding H* and is subsequently oxidized (Step (2) in Scheme 5). In Step (3) of Scheme 5, the molecular adsorption of O2 takes place. This initiates Step (4) in Scheme 5, wherein O2** undergoes proton-electron transfer under quasi-equilibration conditions to form OOH** (hydroperoxy radical). Alternatively, O2** cleaves the O-O bond irreversibly to form O*, also known as chemi-absorbed oxygen atoms (Step (7) Scheme 5). The OOH** is then adsorbed, which then reacts further to form either H2O2** by proton-electron transfer, as shown in Step (5) of Scheme 5, or dissociates itself (Step (8) of Scheme 5). This finally leads to the release of H2O2 (Step (6), Scheme 5) or H2O (Step (11), Scheme 5) [40,51,52].

2.4. Process Conditions for the Direct Synthesis of H2O2 from H2 and O2

The direct synthesis of H2O2 requires certain operating conditions such as the ratio of H2 and O2, reaction medium for the synthesis, the reactor used, additives and/or promoters, a catalyst, and its supporting material [53,54,55]. The following sections explain the influence of each of these parameters on the conversion and yield of the direct synthesis of H2O2.

2.4.1. Ratio of the Gaseous Mixture

During direct synthesis, one would expect the reaction of H2 and O2 on a catalytic surface to form only H2O2 as the product. However, unwanted side reactions also occur that reduce the productivity of this otherwise green process. The first side reaction is the oxidation of H2 to H2O instead of H2O2 and the second one is the reduction of the H2O2 produced to H2O (Scheme 3). One can influence the selectivity of the process by optimising the parameters mentioned previously, but it is self-explanatory that the amount of H2 and O2 in the reaction mixture directly influences the H2O2 output. Three combinations of H2/O2 are possible for this reaction excess H2, excess O2, and stoichiometric amounts. Using excess H2 would favour the reduction of H2O2, while using stoichiometric amounts would increase H2O2 concentration during synthesis. However, an excess of oxygen, up to three times compared to hydrogen, would also increase the selectivity and yield of the direct synthesis approach [56]. It is worth mentioning that the flammable and explosive nature of these two gases over a wide concentration range at 25 °C and 0.1 (MPa) (1 atmospheric pressure) is a point of great concern. The flammability limit for H2 in O2 is 4% (lower flammability limit) to 94% (upper flammability limit), while the detonation limit is at 15% to 95%, with an increased risk of explosion with increasing pressure [42,54,57]. DuPont faced frequent explosions in their pilot plant by feeding 10% H2 in O2 to their process, which led to the discontinuation of the pilot plant studies [27,54].
In order to minimise the explosion risk, it is suitable to perform the reaction at lowered feed rates of H2 and O2, diluted with inert gases [54]. The most common diluents are helium (He), argon (Ar), nitrogen (N2), or carbon dioxide (CO2) [26]. Most of the recent literature indicates the use of either CO2 [44,58,59] or N2 [39,60,61]. The work of Wilson and Flaherty described the use of N2 and CO2 as the diluents during synthesis using palladium catalysts (Pd) supported on silica (Si). By doing so, the researchers reported an overall selectivity of 31% towards H2O2 using only CO2 as the diluent. However, the combined effect on the overall selectivity was not reported [40]. Using CO2 as a diluent is advantageous as it can expand different solvents during the reaction and increase H2 solubility. Secondly, CO2 dissolves in water to form carbonic acid (HCO3), which makes the medium acidic. The acidic condition is helpful as it is the most commonly used storage condition for H2O2 [5]. The chemists and engineers in the field have agreed that increasing the solubility of H2 and O2 in the reaction medium would also lead to a better adsorption of the gases on to the catalytic surface. This, in turn, would lead to a better yield of H2O2 [62]. Selinsek et al. recently reported a process design with two separate tanks containing H2 and O2 dissolved in water with a two-fold benefit. First, an explosion is circumvented due to the separate feeding of gases. Second, the H2 to O2 ratio in the reaction cell can be easily controlled by varying the flow rate of the pump, ensuring that at any given point in time, the process operates in a safe manner [4]. The recent work of Urban et al. describes the use of an electrochemical sensor system to detect the hydrogen and oxygen amounts present in the system during direct synthesis. Additionally, the sensor is capable of monitoring the H2O2 levels in the reactor as well. This sensor is capable of performing under high analyte concentrations and high pressures. The authors also claim that the usage of this novel electrochemical sensor could minimise the risk of explosion due to high accuracy of detection [63].

2.4.2. Reaction Medium

As mentioned previously, the direct synthesis of H2O2 from H2 and O2 in the gaseous state without any reaction medium is highly dangerous. This is because the gases form an explosive mixture over a wide range of concentrations [64]. Hence, performing the synthesis of H2O2 at lowered temperatures in highly pressurised environments, in an appropriate reaction medium, prevents explosions and produces high yields [27]. Therefore, the choice of the reaction medium is crucial to the success of the process. Most of the existing literature uses water as the reaction solvent, with some exceptions where pure methanol or ethanol is used. Additionally, using water with co-solvents such as methanol or ethanol favours a higher dissolution of H2 when compared to pure water as a reaction medium [56,65]. In 2001, Hâncu and Beckmann reported the use of CO2 as a reaction medium for the direct synthesis of H2O2 using a CO2 soluble ligand-supported Pd catalyst. The researchers worked on the assumption that the H2O2 solubility in CO2 is considerably less than the conventional working solutions of the AO process, i.e. organic solvents. Furthermore, the CO2 used was liquid under the reaction conditions (298 K, 17 MPa) and the presence of a CO2-phillic catalyst would minimize the contact time of H2O2 on Pd, thereby increasing the selectivity of the process (Figure 1) [28,66].
Moreno et al. reported the use of a supercritical CO2 (scCO2) with methanol (MeOH) water mixture at an operating temperature range of 283 K–318 K and pressure of 16.7 MPa to synthesize H2O2 from H2 and O2. N2 was used as a diluent in this reaction to achieve a yield between 11.6% and 45.9% [42]. Except for these works, almost every direct synthesis of H2O2 using chemical catalysis is documented in water, alcohol, or a defined ratio of both. Landon et al. reported that at 31.1 °C, which is the critical temperature of CO2, more decomposition of H2O2 was observed than formation. The researchers suggested that the synthesis was performed at a temperature just below the critical temperature [67]. This could be one of the many reasons as to why the use of CO2 as a solvent has not been widely researched. Abate et al., on the other hand, used scCO2-expanded methanol (a solution operating below the triple point in the presence of a solvent to form a two-phase fluid system) as the solvent in the presence of a Pd catalyst supported on mesoporous silica for the direct synthesis of H2O2. On doing so, a selectivity of 40% towards H2O2 and a productivity of 0.11 molH2O2m−2·Pd·h−1 at the end of 3 h [65].

2.4.3. Additives/Promoters

Along with the reaction medium, special additives termed “promoters” are often used to stabilize H2O2 production and to increase the process yield. The most commonly used promoters are acids or halides. Edwards et al. published the effect of acids and halides on the outcome of the direct synthesis of H2O2. In this report, the researchers classified the promoters into two groups:
(i)
oxyacids such as acetic acid, perchloric acid, phosphoric acid (H3PO4), nitric acid, and sulphuric acid (H2SO4)
(ii)
halide acids such as hydrochloric acid (HCl), hydrobromic acid (HBr), and hydroiodic acid (HI).
The authors reported that the reactions could accept potassium and sodium salts of halide acids as well. The presence of halide ions in the reaction could influence the H2O2 conversion in the following order: KF > no halide ions > KCl > KBr and selectivity in the following order: KBr > KCl > no halide ions > KF. The report also claimed that the presence of iodide ions eventually poisoned the catalyst [55].
The next pioneering work in the field was that of Wilson and Flaherty, pointing out that the presence of protons (H+) from mineral acids such as HCl, H2SO4, H3PO4 and HNO3 aids in reducing molecular oxygen. They also suggested that the corresponding counter ions such as Cl, SO42−, PO42−, and NO3 modify the structure of the metal catalyst. By adding halide groups to the Pd catalyst and performing the synthesis in ethanol at an acidic pH, the selectivity would eventually increase from 60% to 80% [40]. The recent report of Gallina et al. compared the roles of sodium bromide (NaBr), H3PO4, and H2SO4 on the outcome of the direct synthesis of H2O2 by varying the concentrations of NaBr and H3PO4. The authors conducted the experiments at 288 K, 2.0 MPa, 3 h reaction time, Pd/C, with a gas mixture percent of H2/O2/CO2 at 4/20/76%. The table (Table 3) below describes the observations of Gallina et al. on testing various combinations of the promoters.
The researchers formulated that the absence of promoters (H3PO4 or NaBr) led to either (i) a much more prominent water forming reaction than the H2O2 forming one or (ii) subsequent reaction of H2O2 further to form H2O. The researchers also concluded that NaBr/H3PO4 had a combined effect on the leaching of the Pd catalyst and that there was no fixed NaBr to H3PO4 ratio to improve the direct synthesis of H2O2 [68]. The authors also suggested that a pH of 2.0 in the reaction medium would favour a better selectivity towards H2O2 production from H2 and O2 [68].
Another method of acidifying the reaction medium to suit the direct synthesis of H2O2 is to use solid acid catalysts (SAC) as reported by the publication of Lewis et al. [57]. The problem of the acidic additives and halide salts being soluble in aqueous medium making the recovery of these a tedious downstream operation was addressed in this work. By using a SAC such as caesium substituted phosphotungstic acid (HPA) in the presence of a Pd or Au-Pd alloyed catalyst, the productivity of H2O2 could be increased and the degradation to H2O could be decreased [69].

2.4.4. Reactor Design

One of the most important parameters to be discussed for the direct synthesis of H2O2 is the type of reactor used for the reaction. Until today, slurry reactors, plugged flow reactors, microreactors or trickle bed reactors are often used to synthesise H2O2 directly from H2 and O2. One of the major requirements when choosing a reactor is that the vessel should withstand high pressure. It is known that the prolonged exposure of H2O2 to H2 in the presence of a Pd catalyst would lead to the formation of water (Scheme 3). One possible way to overcome this phenomenon is to limit the exposure of H2O2 to H2 on the catalytic surface.
Microreactors present a unique way to operate this process in a continuous manner, with defined flow characteristics, large surface area to volume ratio, promising heat and mass transfer rates, and excellent process safety. A microreactor is “a device that contains micro structured features with a sub millimetre dimension, in which chemical reactions are performed in a continuous manner.” The microreactors are constructed from silicon, quartz, glass, metals, polymers, and ceramics, to name a few [70]. The work of Shang and Hessel describes the operational- and reaction-based benefits of using microreactors for the direct synthesis of H2O2 in their work [71]. One drawback of using a microreactor for the direct synthesis of H2O2 is the incorporation of the metal catalyst within the capillaries of such a reactor [72]. Kanungo et al. described a technique to incorporate an Au-Pd alloy on to the walls of a silica coated capillary microreactor. The innovation in the design lies in the fact that the catalytic particles were formed in situ on the walls of the microreactor by a layer-by-layer self-assembly creating a multi-layer catalyst. By using this approach, the researchers were able to produce 210 molH2O2·kgcat−1·h−1 with a H2 conversion of 40% and selectivity of 70% (H2/O2–1:1, 315 K, 2.0 MPa, 5 h) (Figure 2a) [72]. Paunovic et al. reported a production process using the same catalysts in a microchannel reactor. The researchers were able to obtain five mass percent H2O2 solutions at a conversion rate of 15% at 42% selectivity (315 K, 2.0 MPa, 0.05 M H2SO4, 9 parts per million (ppm) NaBr, H2/O2 ratio 20%) [73]. Voloshin et al. elucidated the mass transfer mechanism that occurs in a microreactor during the direct synthesis of H2O2 (Figure 2b). In their work, the researchers claimed that flow of the fluids through the microreactor was slug-flow like. In other words, the liquid flow pattern is interrupted by catalytic particles and the pattern resembles that of a liquid slug being broken down. Using a set of assumptions, the researchers came up with a kinetic model that would explain the behaviour of a packed bed microreactor during direct H2O2 synthesis [74]. Hirama et al. used 32 parallel microreactors made up of silica and glass to produce H2O2 directly from H2 and O2. The authors were able obtain H2O2 at 10 mass percent at a productivity of 0.5 kg h–1. Inoue et al., on the other hand, used four parallel microreactors to produce four mass percent H2O2 at 0.042 kg h−1. Ng et al. reported the use of palladium nanoparticles immobilised on to polystyrene based polymer supports in a capillary microreactor, enabling a continuous production of 1.1 mass percent H2O2 over 11 days [75]. The reviews of Kolehmainen et al. [56] and Dittmeyer et al. [53] summarize the innovations in the field of catalyst design and reactor engineering in a detailed manner.
Another technique to overcome H2O2 decomposition by reactor engineering is the use of a trickle bed reactor or a plugged flow reactor. Almost all works of Biasi et al. uses trickle bed reactors (Figure 3) to improve the selectivity towards H2O2 [68,76,77,78,79,80], with the maximum being 80% using Pd on a sulfated ceria (CeS) catalyst.

2.4.5. Influence of the Catalytic Material

A major hindrance in the direct synthesis approach is the process’ low selectivity, as the formation of H2O2 is not thermodynamically favoured. Although promoters such as H2SO4, H3PO4, NaBr, and KBr help enhance the selectivity of the process, one might consider the catalyst and its supporting material to be the most influencing parameter [81]. Furthermore, the presence of noble metals or noble metal alloys as catalysts aid in the hydrogenation and subsequent decomposition of H2O2 to water (Scheme 3) [4]. On surveying the recent literature in the direct synthesis of H2O2, it is certain that the majority of scientists in the field are dedicated towards developing new, robust and stable catalysts. Existent catalysts and the newly developed versions of noble metal catalysts are characterized based on two criteria: conversion and selectivity. Pd catalysts are almost exclusively used for such reactions either as obtained or alloyed with other metals and/or supports to enhance the selectivity of the process.
Edwards et al. published the importance of alloying Pd with other metals to increase the efficiency of H2O2 production. Their observation stemmed from the highly reactive nature (25 times more active) of an Au-Pd catalyst that was capable of oxidising alcohols better than the corresponding monometallic catalysts. They also reported that when Au-Pd alloys were attached to a titanium dioxide (TiO2) or alumina (Al2O3) support, core-shell structures were formed. With the development of the new catalyst, the researchers still faced the problem of H2O2 being reduced to water [82]. The same working group developed a new tin-based Pd alloy to stop the hydrogenation of H2O2. With the new alloyed catalyst accompanied by a heat treatment cycle, the hydrogenation reactions were prevented and selectivities of more than 95% were reported [37]. Ntainjua et al. used ruthenium (Ru) alloyed with Au and Pd to perform the direct synthesis of H2O2. The authors investigated the Ru-Au, Ru-Pd and Ru-Au-Pd catalyst for H2O2 synthesis. The amount of Ru added to the alloy, along with the calcination conditions, had an effect on the catalyst activity and reusability. So far, this report is the only one using an Ru catalyst for the direct synthesis of H2O2 [83]. Besides Au, only silver [44,84], tellurium [85], tin [37], and zinc [86] are described as possible metals for alloying with Pd. Xu et al. reported the possible increase of H2O2 production by using different metals such as tungsten (W), lead (Pd), molybdenum, etc. and validated their results with density functional theory (DFT). The researchers suggested that all these metals were superior to platinum (Pt) as a promoter. The research was a computational model and experimental evidence to substantiate this fact is needed [87].
Tian et al. reported that by increasing the amount of Pd in the system, the H2 conversion increases as well, which is self-explanatory. However, the selectivity and productivity increases with decreasing Pd content. From these results, they were able to conclude that having a Pd particle size in the range of 2.5 nm to 1.4 nm would yield a selectivity of approximately 94% with 0.5% Pd loading [88]. Most recently, Howe et al. used microwaves to prepare an Au-Pd alloy supported on TiO2 for synthesizing H2O2 from H2 and O2. The authors claim that the catalysts were capable of maintaining their activity for four reaction cycles. Compared to other alloyed Pd particles, these particles have a core-shell structure and can be prepared in 0.25 h [89]. The recent 2017 patent of Desmedt et al. used metallic catalysts supported on sulphate and phosphate to reduce the amount of inorganic acid content in the reaction medium. The inventors varied the metal content between 0.001 mass % and 10 mass % (0.62 to 2 wt % Pd) to obtain conversions ranging between 26.9% and 46% and selectivities between 19.9% and 74% [90].
Table 4 below summarises the reaction conditions and the catalysts for the direct synthesis of H2O2 along with the catalyst used.

3. Summary, Conclusions and Future Perspectives

More than a century has passed since the first documented work of Henkel and Weber in 1914 producing H2O2 directly from H2 and O2 using Pd catalysts, and the direct synthesis has only reached to pilot plant scale of production. This is because the synthesis needs to be operated beyond the explosive range of H2 and O2 ratios and the thermodynamic favoring of water formation over hydrogen peroxide formation. The conversion and selectivity of the process depends on several parameters such as the H2/O2 ratio, the diluent used in the process, the reaction medium used to prevent explosion, the catalyst and its supporting material, reactor design, and the operating temperature and pressure, to name a few. From recent literature, it is understood that it is an advantage to perform the synthesis at reduced temperatures (263 K to 283 K) and pressures (most commonly 2.0 MPa to 4.0 MPa). To synthesise H2O2 selectively from H2 and O2, oxygen is to be used in excess to avoid hydrogenation of H2O2. With the new developments in microreactor technology, great advancements are being achieved in increasing the selectivity of the process. Moreover, robust and stable catalysts have been the research focus of the scientific community ever since this reaction was reported. The use of Pd in its pure form or as an alloy presents an opportunity towards industrializing this process. Additionally, additives such as NaBr, H3PO4, H2SO4 etc. aid in stabilising the synthesised H2O2. Finally, with the scientific community focusing on green and sustainable processes, ably supported by the advancements in the field of direct synthesis, the first commercial plant producing H2O2 using direct synthesis technology is not that far in the future.

Acknowledgments

This work was funded by the “Bayerische Staatsministerium für Wirtschaft und Medien, Energie und Technologie” as a part of the “Gemeinsame Erforschung von Naturstoffen aus Blaualgen als Entwicklungsmodell der grenzüberschreitenden wissenschaftlichen Partnerschaft” (Translation: Joint research on natural products from blue algae as a developmental model for cross-border scientific cooperation), project number: 41.

Conflicts of Interest

The authors declare no conflicts of interest.

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Scheme 1. (a) L.J. Thénard method of H2O2 production from BaO2. (b) Electrolysis of sulphuric acid to produce hydrogen peroxide according H. Meidinger and M. Berthelot. (c) Münchner Process to produce H2O2 (d) Riedel-Löwenstein Process [3,8,23]. (Image taken from Berl [23], reproduced here with the kind permission of the Journal of the Electrochemical Society) (e) Hydrazobenzene oxidation to produce H2O2 (scheme modified from Walton and Filson [19], copyright 1932, American Chemical Society). (f) Georg Pfleiderer Process of producing H2O2, an adaptation of the Walton and Filson Process [20] (g) Shell 2-propanol Process to produce acetone/H2O2 [3] (Scheme taken from [3], reproduced here with the kind permission of John Wiley and Sons)
Scheme 1. (a) L.J. Thénard method of H2O2 production from BaO2. (b) Electrolysis of sulphuric acid to produce hydrogen peroxide according H. Meidinger and M. Berthelot. (c) Münchner Process to produce H2O2 (d) Riedel-Löwenstein Process [3,8,23]. (Image taken from Berl [23], reproduced here with the kind permission of the Journal of the Electrochemical Society) (e) Hydrazobenzene oxidation to produce H2O2 (scheme modified from Walton and Filson [19], copyright 1932, American Chemical Society). (f) Georg Pfleiderer Process of producing H2O2, an adaptation of the Walton and Filson Process [20] (g) Shell 2-propanol Process to produce acetone/H2O2 [3] (Scheme taken from [3], reproduced here with the kind permission of John Wiley and Sons)
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Scheme 2. (a) Mechanism of the autoxidation (AO) process developed by Riedl-Pfleiderer [21,24] (Image taken from Li et al. [24], 2017, reprinted here with the kind permission of Springer Nature) and (b) block diagram of the AO process steps to synthesise H2O2 (scheme modified from Campos-Martin et al. [7], reprinted here with the kind permission of John Wiley and Sons).
Scheme 2. (a) Mechanism of the autoxidation (AO) process developed by Riedl-Pfleiderer [21,24] (Image taken from Li et al. [24], 2017, reprinted here with the kind permission of Springer Nature) and (b) block diagram of the AO process steps to synthesise H2O2 (scheme modified from Campos-Martin et al. [7], reprinted here with the kind permission of John Wiley and Sons).
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Scheme 3. The direct synthesis approach consisting of two parallel reactions (reduction and oxidation) during the production of H2O2. Scheme taken from Gervasini et al. [43], reprinted here with the kind permission of the American Chemical Society, Copyright 2017, Khan et al. [44], and Seo et al. [45].
Scheme 3. The direct synthesis approach consisting of two parallel reactions (reduction and oxidation) during the production of H2O2. Scheme taken from Gervasini et al. [43], reprinted here with the kind permission of the American Chemical Society, Copyright 2017, Khan et al. [44], and Seo et al. [45].
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Scheme 4. (i) Mechanism of H2O2 formation in water with CO and O2 in the presence of palladium catalyst complexed with nitrogen ligands (scheme from Werner et al. [48], and reprinted here with the kind permission of John Wiley and Sons, copyright 1999). (ii) The catalytic cycle of H2O2 manufacture using palladium catalysts complexed with chloride ions (Chinta and Lunsford [50], and reprinted here with the kind permission of Elsevier, copyright 2004).
Scheme 4. (i) Mechanism of H2O2 formation in water with CO and O2 in the presence of palladium catalyst complexed with nitrogen ligands (scheme from Werner et al. [48], and reprinted here with the kind permission of John Wiley and Sons, copyright 1999). (ii) The catalytic cycle of H2O2 manufacture using palladium catalysts complexed with chloride ions (Chinta and Lunsford [50], and reprinted here with the kind permission of Elsevier, copyright 2004).
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Scheme 5. Plausible series of elementary steps for hydrogen peroxide and water formation during the direct synthesis approach on Pd catalyst (* is an empty site on the catalyst surface, X (H, O, OH, H2O)* is an adsorbate bound to a single Pd atom, X**, where X can be O2, OOH, or H2O2, is an intermediate adsorbed. All reversible arrows represent a quasi-equilibrate step. It is to be noted that each of the steps has its own rate constants) [40]. Scheme taken from Wilson & Flaherty, 2016 https://pubs.acs.org/doi/abs/10.1021/jacs.5b10669.
Scheme 5. Plausible series of elementary steps for hydrogen peroxide and water formation during the direct synthesis approach on Pd catalyst (* is an empty site on the catalyst surface, X (H, O, OH, H2O)* is an adsorbate bound to a single Pd atom, X**, where X can be O2, OOH, or H2O2, is an intermediate adsorbed. All reversible arrows represent a quasi-equilibrate step. It is to be noted that each of the steps has its own rate constants) [40]. Scheme taken from Wilson & Flaherty, 2016 https://pubs.acs.org/doi/abs/10.1021/jacs.5b10669.
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Figure 1. Comparison of direct synthesis of H2O2 in conventional (aqueous solutions) and CO2 with a modified CO2-phillic Pd catalyst (Figure taken from Hâncu and Beckmann [28]). Reprinted here with the kind permission of the Royal Society of Chemistry.
Figure 1. Comparison of direct synthesis of H2O2 in conventional (aqueous solutions) and CO2 with a modified CO2-phillic Pd catalyst (Figure taken from Hâncu and Beckmann [28]). Reprinted here with the kind permission of the Royal Society of Chemistry.
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Figure 2. (a) A general microreactor design adapted by Kanungo et al. with a magnified view of the Au-Pd packing in the capillaries of the channel (scheme adopted from Kanungo et al. [72], https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b03589). (b) Mass transfer scheme in a packed bed microreactor (G-gas, L-liquid, S-Solid) (scheme taken from Voloshin et al. [74], reprinted here with the kind permission of Elsevier publishing group).
Figure 2. (a) A general microreactor design adapted by Kanungo et al. with a magnified view of the Au-Pd packing in the capillaries of the channel (scheme adopted from Kanungo et al. [72], https://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b03589). (b) Mass transfer scheme in a packed bed microreactor (G-gas, L-liquid, S-Solid) (scheme taken from Voloshin et al. [74], reprinted here with the kind permission of Elsevier publishing group).
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Figure 3. Trickle bed reactor used by Gallina et al. [68] to produce H2O2 directly from H2 and O2 in water in the presence of a commercial Pd/C catalyst (Scheme modified from Gallina et al. [68], 2017, copyright 2017. Reprinted here with the kind permission of the American Chemical Society).
Figure 3. Trickle bed reactor used by Gallina et al. [68] to produce H2O2 directly from H2 and O2 in water in the presence of a commercial Pd/C catalyst (Scheme modified from Gallina et al. [68], 2017, copyright 2017. Reprinted here with the kind permission of the American Chemical Society).
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Table 1. Historical development in the industrial manufacture of hydrogen peroxide.
Table 1. Historical development in the industrial manufacture of hydrogen peroxide.
YearInventorCatalysis TypeDetails
1818L.J. ThenardChemical Barium peroxide reacts with hydrochloric acid to form barium chloride and H2O2. Yield of 3% H2O2 was 2000 tons/annum (t/a) (Scheme 1a) [9,10]
1853H. MeidingerElectrochemicalElectrolysis of sulphuric acid to yield H2O2 [11] (Scheme 1b)
1878M. BerthelotElectrochemicalElucidated the mechanism of sulphuric acid electrolysis. Reported the formation of peroxodisulphuric acid as an intermediate [12] (Scheme 1b)
1901W. ManchotChemicalAutoxidation of hydroquinones and hydrazobenezenes under alkaline conditions in the presence of molecular oxygen to yield H2O2 [13,14,15] (Scheme 1e)
1908Degussa-Weissenstein ProcessElectrochemicalFirst production plant set up in Wiessenstein, Austria [16,17]
1910Münchner Process or the Pietzsch-Adolph ProcessElectrochemicalDeveloped by Pietzsch and Adolph at the Elektrochemische Werke, Munich. Used potassium peroxodisulphate instead of sulphuric acid to produce H2O2 [16,17] (Scheme 1c)
1914Hugo Henkel and Walter WeberChemicalFirst account of direct synthesis of hydrogen peroxide using its constituent gases [18]
1924Reidl-Löwenstein ProcessElectrochemicalSimilar to the Pietzsch-Adolph Process; used ammonium peroxodisulphate to produce H2O2 by electrolysis (Scheme 1d). Yield of 100% H2O2 was 35 kt [16,17]
1932Walton and FilsonChemical autoxidationPublished their work on the alternate oxidation and reduction of hydrazobenzenes to produce H2O2 [19] (Scheme 1e)
1935Pfleiderer, Baden Aniline and Soda Factory (BASF)Chemical autoxidationAlkaline autoxidation of hydrazobenzenes to form sodium peroxide, later hydrolysed to form H2O2 [20] (Scheme 1f)
1935–1945Riedl and PfleidererChemical autoxidationAnthraquinone autoxidation process (AO) set up in two different cities each with 2000 t capacity [16,21] (Scheme 2a)
1953E.I. du Pont de NemorsChemical autoxidationCommercial plant setup with based on the Riedl and Pfleiderer process [1,3,8] (Scheme 2a)
1957–1980Shell processChemical autoxidationOxidation of 2-propanol to yield H2O2 at a capacity of 15 kt [22] (Scheme 1g)
Table 2. Analysis of the advantages/disadvantages of the direct synthesis approach [27,39,40,41,42].
Table 2. Analysis of the advantages/disadvantages of the direct synthesis approach [27,39,40,41,42].
AdvantagesDisadvantages
Absence of organic substrates such as anthraquinones or organic solventsUnselective reactions leading to simultaneous side products other than H2O2, namely water (H2O)
Usage of green solvents like water, methanol, or ethanolComplex process with mass transfer limitations involving three phases: gas (H2/O2), liquid (reaction medium), and solid (catalyst)
Economical because of fewer downstream operations to produce H2O2Safety: explosive nature of the H2 and O2 mixture over a wide range of concentrations (4 mol %–94 mol %)
The whole process can be accomplished with a single reactor systemPresence of chloride and/or bromide ions in the reaction medium
Table 3. Effect of different promoters on H2 conversion, H2O2 selectivity, H2O2 productivity as reported by Gallina et al. [68].
Table 3. Effect of different promoters on H2 conversion, H2O2 selectivity, H2O2 productivity as reported by Gallina et al. [68].
Serial No. (S/N)Additive AddedOutcome
NaBr (M *)H3PO4 (M)H2SO4 (M)H2 Conversion (%)H2O2 Selectivity (%)H2O2 Productivity (molH2O2·Kg(Pd)−1·h−1)
100010000
200.003 10000
30.0005009250740.1
40.00050.00308561891
50.00050.0050.0257965830
* mole (mol)/litre (L); molar.
Table 4. List of operating conditions in literary works involving the direct synthesis of H2O2 between 2010 and 2018.
Table 4. List of operating conditions in literary works involving the direct synthesis of H2O2 between 2010 and 2018.
Temperature and Pressure (K and MPa)CatalystReactor TypeSolvent(s)PromotersConversion and Selectivity (%)Literature Reference
263 and 2Palladium-CeSTrickle bed reactor with Teflon liningMethanolNoneNo data, 80Biasi et al. [76]
298 and 0.1 (ambient)Palladium on porous alumina tubingMembrane reactorWaterSulphuric acid (H2SO4) sodium bromide (NaBr), phosphoric acid (H3PO4)No data, 50Inoue et al. [91]
283–324 and 4.6–16.7Palladium on carbonStirred slurry batch reactorWater + scCO2 and methanol + scCO2H3PO4 and NaBrNo data availableMoreno et al. [42]
298 and 0.1 (ambient)Pd nanoparticles immobilized on polystyrene based polymerCapillary microreactorMethanolNo additive47 and 0.65Fei Ng et al. [75]
H2SO4, KBr3.9 and 77
301 and 1.01Insoluble heteropoly acid supported on Pd immobilized on mesostructured foam (MCF) silicaAutoclave reactorMethanolH3PO4 and NaBr85 and 35Park et al. [92]
298 and 4.5Metallic Pd deposited on ceramic tubesPorous tubular membrane reactorMethanolH2SO4, NaBrNo data and 83Pashkova et al. [93]
293 and 4.0Au-Pd on TiO2 on carbonStainless steel autoclaveMethanol/waterNo data availableNo data availablePritchard et al. [94]
315 and 2.06Pd on SiO2.MicroreactorWaterH2SO4, NaBrNo data and 85Voloshin et al. [74]
263 and 1.0Bimetallic Pd-Au catalyst on CeS and sulphated zirconiaTrickle-bed reactorMethanolNo data available90Biasi et al. [77]
293 and 0.65Pd on SiO2Teflon coated steel reactorMethanolH2SO4No data and 21Abate et al. [65]
Pd on mesoprous silica (SBA-15)No data and 58
263 & 1.0Pd-Au CeS Trickle bed reactorMethanolNo data availableNo data and 50Biasi et al. [95]
Pd-Au on ZSNo data and 60
278–313 and 2.0Pd/CBatch reactorMethanolH2SO4100 and 35Biasi et al. [78]
298 and 2.4Commercial 5% Pd/CBatch slurry reactorMethanolNone addedNo data availableGemo et al. [96]
263 and 2.0Pd on CeSPacked bed reactorMethanolH2SO4No data and 70Kilpiö et al. [97]
303 and 5Pd nanoparticles immobilized on a functionalised resinFixed bed reactorMethanolNone addedNo data and 73Kim et al. [98]
293 and 0.1Pd on SiO2Glass stirred tank reactorMethanolH2SO4No data and 60Menegazzo et al. [99]
275 and 4.0Ru-Au-Pd catalyst on TiO2 supportStainless steel autoclaveMethanol/waterNone addedNo data availableNtainjua et al. [83]
301 and 1.01Palladium on zeolite HZSM-5Autoclave reactorMethanolH3PO490 and 16Park et al. [100]
294 and 4.0AuPd/CStainless steel autoclaveMethanol/waterNone addedNo data availablePiccinini et al. [101]
298 and 2.3Pd on sulfated zirconia and Pd on aluminaBatch autoclave reactorMethanolNone addedNo data availableRossi et al. [102]
278–308 and 2.8Commercial Pd/CTrickle bed reactorWaterNaBrNo data and 90Biasi et al. [79]
298 and 2.4Commercial Pd/CStainless steel batch autoclaveMethanolNone addedNo data and 33Biasi et al. [80]
278 and no dataCommercial Pd/CTrickle bed reactorWaterH3PO4 and NaBrNo data available
278 and 1.0Au-Pd/TiO2MicroreactorWater/MethanolNone90 and 25Freakley et al. [103]
273 and 3.8Pd on activated carbon cloth (ACC)Stainless steel autoclaveMethanolNone addedNo data and 70Gudarzi et al. [104]
295 and 3.0Pd-Au on carbon nanotube (CNT)Stainless steel autoclave with Teflon coatingMethanol and sulphuric acidH2SO4No conversion values and 15–65 depending on H2/O2 flowAbate et al. [105]
295 and 3.0Pd on nanocarbonStainless steel autoclave with Teflon coatingMethanolH2SO4No data and 25Arrigo et al. [106]
293 and 0.1Pd nanocubes on silicaSimilar to the work of Lee et al. (2011)10 and 25Kim et al. [107]
273 and 2.0Pd on activated carbon cloth (ACC)Stainless steel microreactorMethanolNone addedNo data and 23 Ratchananusorn et al. [108]
278 and 2.0Au-Pd on nanostructured TiO2 nanotube supportStainless steel autoclaveMethanolHClNo data availableTorrente-Murciano et al. [109]
275 and 2.0Pd/C treated with NaBrCustom made stainless steel batch reactorMethanolNaBr95 and 1Biasi et al. [58]
293 and 2.0Pd supported metal organic framework (MOF)Autoclave reactorMethanolNone addedNo data and 26Chung et al. [60]
296 and 1.0Combination of Pd/TiO2 and Au-Pd/TiO2Microreactor with parallel packed beds (1, 8 and 16)WaterH2SO4, H3PO4, and NaBrVaried depending on number of channels and the flow rateInoue et al. [110]
303 and 2.0Supported Au, Pd and Au-PdAutoclave reactorWaterNaBr15 and 50Paunovic et al. [111]
1-pentanol20 and 80
Chloroform18 and 38
Hexane20 and 17
Methyl isobutyl ketone27 and 10
1-butanol28 and 60
2-butanol25 and 55
Isopropanol35 and 75
Methanol45 and 47
Ethanol50 and 47
Dimethyl sulphoxide (DMSO)25 and 87
Acetonitrile32 and 100
Acetone35 and 95
t-butanol40 and 70
303 and 2.0Au-Pd catalystAutoclaved slurry reactorWaterH2SO4 and NaBr5 and no dataPaunovic et al. [112]
313 and 2.010 and no data
323 and 2.030 and no data
313 and 2.0Au-Pd colloidal nanoparticlesMicrochannel-Silica capillary reactorWaterH2SO4 and NaBr80 and 85Paunovic et al. [73]
298 and 2.6Commercial Pd/CTrickle bed reactorWaterH2SO4, H3PO4, and NaBrNo data availableAbejón et al. [81]
333 and 0.1Au-Pd/TiO2Fixed bed reactorGas phase synthesis (2% H2/air)NoneNo data availableAkram et al. [113]
313 and 9.5Pd loaded on a sulfonic acid resinHigh pressure stirred reactorMethanolThree compounds tested:
2-bromo-2-methyl propane, 2-bromopropane, bromobenzene compared to NaBr
No conversion data; selectivity for NaBr was 80 and 75 for the restBlanco-Brieva et al. [114]
298 and 0.5Au-Pd/TiO2Stainless steel autoclaveMethanol/waterNone addedNo data availableCrole et al. [36]
275 and 3.7Pd-SnStainless steel autoclaveMethanol/waterNone added9 and 96Freakley et al. [37]
275 and 3.0Pd-Ag (varying amounts)Teflon coated stainless steel autoclaveMethanolH2SO4Depended on alloy ratioGu et al. [84]
303 and 2.0Porous Pd/SiO2Teflon coated stainless steel autoclaveMethanolH2SO4No data and 46Sierra-Salazar et al. [115]
273–305 and 0.1–3.0Pd/SiO2Packed-bed flow reactorMethanol/waterHCl, H2SO4, and sodium bicarbonate (NaHCO3)No data and 31Wilson et al. [40]
288 and 2.0Commercial Pd/CTrickle bed reactorWaterH3PO4, H2SO4, and NaBr77 and 72Gallina et al. [68]
278 and 5.0Pd particles on acidic niobia (Nb)-silica (Si) supportSlurry reactorMethanolNone added38 and 78Gervasini et al. [43]
278 and 10.0 Water38 and 85
293 and 0.1Pd/SiO2 (sonochemical approach)Stirred glass reactorEthanol/waterKBr and H3PO422 and 85Han et al. [116]
Pd/SiO2 (incipient wetness approach)12 and 85
Pd/TiO2 (sonochemical approach)22 and 75
Pd/TiO2 (incipient wetness approach)12 and 80
300 and 0.95Pd/TiO2Flow reactor comprising of 8, 16 and 32 parallel micro-packed bedsWaterH3PO4, H2SO4, and NaBr64 and 66 (eight glass beds)
61 and 70 (eight glass + Si beds)
Hirama et al. [117]
315 and 2.0Au-Pd nanoparticlesMicroreactorWater H2SO4, NaBr and acetonitrile (MeCN)20 and 85Kanungo et al. [72]
273 and 4.0Pd on a hexadecyl-2-hydroxyethyl-dimethyl ammonium dihydrogen phosphate (HHDMA)Stirred reactorMethanol/waterNo data availableNo data and 80Lari et al. [61]
275 and 4.0Au-Pd nanoparticles supported on cesium substituted phosphotungstic acid (HPA)Stainless steel autoclaveMethanol/waterNo data available69 and 86Lewis et al. [69]
293 and 0.1Pd/SiO2Glass stirred reactorEthanol/waterKBr30 and 27Seo et al. [45]
283 and 0.1Pd on hydroxyapatiteSlurry reactorEthanol H2SO42 and 94 Tian et al. [88]
283 and 0.1Pd-Tellerium (Te)/TiO2Micro triphase reactorEthanolH2SO46 and 100Tian et al. [85]
283 and 0.1Pd on mesoporous anatase TiO2Glass triphase reactorWaterH2SO440 and 40Tu et al. [118]
275 and 3.0Pd-zinc (Zn) on alumina (Al2O3)Stainless steel autoclaveMethanolH2SO457 and 78.5Wang et al. [86]
278 and 3.0Au-Pd supported on carbonStainless steel autoclaveMethanol/waterNone added65 and 60Yook et al. [119]
275 and 4.0Au-Pd/TiO2Stainless steel autoclaveMethanol/waterNone addedNo data availableHowe et al. [89]
No data and 4.0Ag-Pd/TiO2Stainless steel autoclaveMethanol/waterNone addedNo data availableKhan et al. [44]
275 and 4.0Au-Pd/mesoporous silica (SBA-15)AutoclaveMethanol/waterNone addedNo data availableRodrigéz-Goméz et al. [5]
298 and 1.1Pd/TiO2Teflon coated reaction cellWaterNaBrNo data availableSelinsek et al. [4]
273–337 and 0.1–3.1Au-Pd alloyPlugged flow reactorMethanol/waterNone added32 and 40Wilson et al. [39]

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