The demand for hydrogen peroxide is booming since it is considered as one of the most environmentally friendly and versatile chemical oxidants available and has a wide range of applications. The annual market, close to 3000 kt per year being produced via the auto-oxidation process (with 2-ethyl anthraquinone (traditional) or amyl anthraquinone for mega-plants), is mostly supplied by the company Solvay (30%), followed by Evonik (20%) and Arkema (13%). Nevertheless, the dream of a direct synthesis process is close to a century old and it has gained momentum in research efforts during the last decade with more than 15 groups active in the world. In this review, we focus the discussion on the targets, e.g. plant tonnage, the reactors and the most feasible industrial operational conditions, based on our experience and point of view using the chemical engineering tools available. Thus, direct synthesis can be competitive when on-site production is required and capacities less than 10 kt per year are He has led and participated in 3 EU, 6 national/regional projects and 10+ SME contracts and has published 30+ research papers. Teresa MorenoTeresa Moreno obtained her degree in Chemical Engineering from Complutense University (Madrid, Spain) and later completed her PhD in Chemical Engineering at the University of Valladolid (Spain, 2011) studying the catalytic direct synthesis of hydrogen peroxide in supercritical CO 2 and the online determination of the product using Raman spectroscopy. She is currently a Research Scientist in the Industrial Bioactive Technologies Group at Callaghan Innovation (New Zealand) working on the development of processes for adding value to natural materials toward high value products and applications, including the development of sustainable technologies using supercritical fluids. View Article OnlineView Journal | View Issue and semi-continuous modes of operation. However, at the moment, demonstrations of continuous operations as well as carefully determined kinetics are needed in order to scale up the process. Finally, operational conditions, including the catalyst composition (active metal, oxidation state and support), promoters (halides and acids-pH-isoelectric point), solvents, pressure and temperature need to be carefully analysed. In our opinion, as we try to show here, H 2 O 2 direct synthesis is a competitive process and is ready for larger scale demonstration. Also, more than a hundred patents within the area support this claim, although the barriers of technology demonstration and further licensing are still pending.

H 2 O 2 direct synthesis over a palladium catalyst based on sulfated ceria (Pd-CeS) has been studied in a trickle-bed reactor at -10°C. The combined effect of liquid and gas flow rates was studied by independent variations. The maximum productivity and selectivity was always found at flow rate combinations intermediate within the investigated range. The reactor operated under pressure and its effect was investigated (10 and 20 bar), resulting in a significant gain in selectivity. Selectivity up to 80% has been measured at the highest pressure tested (20 bars), liquid flow rate of 2 mL/min, and 6 mL/min gas flow rate. The maximum production rate measured was 0.0035 mmol/min with 0.5 mL/min liquid flow rate and 2.7 mL/min gas flow rate. Relevance of direct water formation has been isolated by independently investigating H 2 O 2 decomposition and hydrogenation. Results indicate directions of further performance improvements and the importance of reactor type and operation.

A tailor-made Pd 0 /K2621 catalyst was subjected to post synthesis modification via a wet treatment procedure. The aim was the understanding of the role of promoters and how -if any -improvements could be qualitatively related to the catalyst performance for the H 2 O 2 direct synthesis. The Catalyst Wet Pretreatment Method (CWPM) was applied in different methanolic solutions containing H 2 O 2 , NaBr and H 3 PO 4 , either as single modifiers or as a mixture. The catalyst was characterized by Transmission Electron Microscopy (TEM) and X-ray Photoelectron Spectroscopy (XPS). It was concluded that the modified catalysts give rise to higher selectivities compared to the pristine reference catalyst thus opening a possibility to exclude the addition of the undesirable selectivity enhancers in the reaction medium. This work provides original evidence on the role of promoters, especially bromide, allowing the formulation of a new reaction mechanism for one of the most ISCRE 24 -Foundations and

a b s t r a c t H 2 O 2 direct synthesis over two bimetallic, palladium-gold catalysts based on sulfated ceria (PdAu-CeS) and on sulfated zirconia (PdAu-ZS) has been studied in a continuous, trickle-bed reactor, at −10• C and 10 bar. Three different liquid flow rates and seven different gas flow rates were used. The combined effect of liquid and gas flow rates, with two different H 2 /O 2 ratio (2/18 and 4/16) in the gas feed, has been studied, by independent variations. The highest H 2 /O 2 ratio always increases production rate and selectivity. In the case of 4/16 ratio, PdAu-CeS catalyst shows an enhancement in selectivity only, while the increase in H 2 O 2 production rate is not significant compared to the 2/18 ratio case. PdAu-ZS catalyst shows a remarkable gain in both selectivity and production rate when the 4/16 ratio in gas feeding is used, outperforming the PdAu-CeS catalyst. The enhancement of the catalytic activity is more pronounced in the case of PdAu-ZS, compared to PdAu-CeS, when the highest H 2 /O 2 ratio is used. A selectivity up to 90% has been measured on PdAu-ZS with the higher H 2 /O 2 ratio and larger liquid flow rates. The maximum production rate measured is 6 mol/min (i.e. a productivity 0.18 mol H 2 O 2 g Pd −1 h −1 ) with 1 ml/min liquid flow rate and 2.7 ml/min gas flow rate again on PdAu-ZS.