Catalysis Today 248 (2015) 91–100 Contents lists available at ScienceDirect Catalysis Today j our na l ho me page: www.elsevier.com/locate/cattod Effect of low hydrogen to palladium molar ratios in the direct synthesis of H 2 O 2 in water in a trickle bed reactor I. Huerta a , P. Biasi b,c,∗ , J. García-Serna a,∗ , M.J. Cocero a , Jyri-Pekka Mikkola b,c , T. Salmi b a High Pressure Processes Group, Department of Chemical Engineering and Environmental Tech., University of Valladolid, 47011 Valladolid, Spain b Industrial Chemistry and Reaction Engineering Department, Åbo Akademi University, FI-20500 Turku/Åbo, Finland c Department of Chemistry, Chemical-Biochemical Centre (KBC), Technical Chemistry, Umeå University, SE-90187 Umeå, Sweden a r t i c l e i n f o Article history: Received 29 December 2013 Received in revised form 18 April 2014 Accepted 19 April 2014 Available online 28 May 2014 Keywords: Hydrogen peroxide Direct synthesis Heterogeneous catalysis Palladium on carbon Gas solubility Trickle bed reactor a b s t r a c t Application of a trickle bed reactor (TBR) renders a very compact solution to carry out direct synthesis of hydrogen peroxide in water over a carbon supported palladium. The laboratory scale reactor was filled with silica particles (50–70 mesh) physically mixed with 37.5 to 75 mg of 5%Pd/C particles. The reac- tion conditions applied were 15 ◦ C, 15–28 barg, 0.5 to 6 mL min -1 of liquid and 4.0–4.5 mL min -1 of gas flowrate (86.7/11/2.23 mol% of CO 2 /O 2 /H 2 ). Thus, we demonstrated that the ratio between H 2 and Pd is one of the key factors to achieve optimized, higher yields of hydrogen peroxide. Consequently, low H 2 concentrations lead to low productivities. One of the least studied parameters, addressed here, is the ratio between the bed filling (SiO 2 ) and the catalyst; i.e. the active catalytic species dilution effect. In short, it was found that when the amount of Pd was reduced below 0.094 mg Pd cm -3 SiO 2 the highest pro- ductivity of H 2 O 2 could be achieved. The selectivity obtained were between 5.3 and 38.0%, respectively, corresponding to turn-over-frequencies (TOF) ranging from 65 to 921 mmol H 2 O 2 gPd -1 h -1 . © 2014 Elsevier B.V. All rights reserved. 1. Introduction The need of products and processes that promote develop- ment of more sustainable industrial practices over the traditional approaches is today a self-evident goal. Thus, as an example, the hydrogen peroxide demand has recently increased considerably, as it is an excellent chemical oxidant in a wide range of appli- cations, such as pulp and paper bleaching, electronic and textile industries, metallurgy and chemical synthesis [1]. Solvay (30%), followed by Evonik (20%) and Arkema (13%) lead the annual pro- duction volumes whereby close to 3000 kt y -1 is being produced via the auto-oxidation process [2]. Direct synthesis process can compete with the auto-oxidation process (traditional process with more than 95% H 2 O 2 production quota, nowadays) if H 2 O 2 solutions similar to the ones produced with current technology after dilution, i.e. around 15–17 wt%, can be made in an economical way [3–5]. Therefore, direct synthesis is conceived as an on-site process for continuous production of H 2 O 2 on-demand. ∗ Corresponding authors. Tel.: +34983184934. E-mail addresses: bpierdom@abo.fi (P. Biasi), jgserna@iq.uva.es, jgserna@gmail.com (J. García-Serna). Direct synthesis of H 2 O 2 is a classic example of a three phase process. The gas phase is composed of H 2 and O 2 plus an inert gas in order to maintain the H 2 concentration below the low flamma- bility limit (LFL = 3.6–4.0 mol%). The most common inert gases are N 2 (when air is used) or CO 2 when an enhancement in mass trans- fer is pursued [3,6]. Typically, the liquid phase is water, methanol, ethanol or a mixture. The solid phase, on the other hand, consist of a mass-transfer enhancing support (e.g. ceramic or metallic rings, etc.) and the active metal supported on it or in another specific support (e.g. zeolites, micro-particulates of activated carbon or zir- conia, etc.). Trickle bed reactors (TBRs) are type of three-phase fixed bed reactors widespread in the operations of petrochemical industry and in the production of bulk chemicals. In a classical setup, the liquid phase flows in downward direction (gravity) and gas is fed either as a down-flow (co-current) or up-flow (counter-current or concurrent) over a bed of solid catalyst particles. The special feature of TBRs is that the liquid flows down intermittently like a chaotic rain wetting the solid particles in the form of droplets, films or rivulets [7]. To assure that, indeed, trickle flow conditions are achieved, the reactor must operate under restricted liquid or gas availability characterized by Reynolds numbers in the order of Re G < 10 3 and Re L < 10 3 [7,8]. The direct synthesis of H 2 O 2 can be schematized by a sys- tem of four reactions, appearing as both consecutive and parallel http://dx.doi.org/10.1016/j.cattod.2014.04.012 0920-5861/© 2014 Elsevier B.V. All rights reserved.