Direct Synthesis of Hydrogen Peroxide in a Catalytic Membrane Contactor
Advancing the chemical engineering fundamentals
Chemical Reaction Engineering: Practical Applications (T2-2c)
Keywords: H2O2 direct synthesis, catalytic membranes
Hydrogen peroxide is a widely used oxidizing agent in many industrial areas because it is environmentally harmless (the only by-product is water) and it offers higher selectivity and activity than molecular oxygen. Today, it is almost exclusively produced according to the so-called “anthraquinone method” which has the limitations to be economically viable only in large scale production units and to require an expensive and complex solvent system. Therefore, the direct synthesis of H2O2 from O2 and H2 is an attractive alternative which has been investigated for decades already [1, 2]. However, due to drawbacks related to safety and selectivity, this process as yet is not used on a commercial scale although this might change in the near future [3].
The aim of our project is to synthesize H2O2 by direct contact of H2 and O2 in a catalytic membrane contactor, which operates on the principle of the catalytic diffusor. The concept and preliminary results are described in Refs. [4, 5]. The catalytic diffusor represents an asymmetric structured tubular inorganic membrane into which a noble metal catalyst is incorporated in highly dispersed form only in the fine-porous surface layer. The reactants are supplied separately on opposite sides, i.e. gaseous oxygen on the coarse-porous support side and hydrogen dissolved in a solvent wetting the membrane on the surface-layer side. By applying a controlled overpressure on the gas side the gas/liquid contact plane is established inside the membrane close to the fine-porous layer with the active catalyst. This system allows good access of both gas phase and liquid phase to the catalyst, reduced mass transfer limitation and safe operation due to the separate supply of O2 and H2. As no complex solvents are required, it is well suited for small-scale on-site production of H2O2.
Preliminary results have been obtained with single-channel membrane elements of 10 cm length and 100 nm pore diameter in a lab scale batch–reactor with 1L capacity. The optimum Pd loading for the reaction has proved to be 4 – 5 g·m-2 with the formation of Pd-clusters with mean particle size of dp=13nm. The influence of several process parameters on the formation of H2O2 has been investigated. The experiments indicate that high liquid pressure (up to 69 bar) and pure methanol as solvent are most beneficial leading up to 16 mol·m-2·h-1 H2O2 maximum productivity with varying selectivities from 20% to 80%, depending on the mode of operation of the process.
The objectives of the ongoing research are: to perform the process in a continuous mode and to prove its safe operation; to achieve H2O2 concentrations of 3-5 wt.-% suitable for use “as produced” with a selectivity of at least 80 %, and to obtain high activity (related to the quantity of Pd) and productivity (related to the geometric membrane area) and sufficient catalyst lifetime in order to meet the economic requirements for a small-scale process for local H2O2 production.
References
[1] L. W. Gosser, J.-A. T. Schwarz (DuPont), US Patent 4,832,938, 1989
[2] J. H. Lunsford, J. Catal., 216, 2003, 455
[3] http://www.azom.com/details.asp?newsID=2689
[4] G. Centi, R. Dittmeyer, S. Perathoner, M. Reif, Catal. Today, 79-80, 2003, 139
[5] R. Dittmeyer, K. Svajda and M. Reif, Top. Catal., 29, 2004, 3
Presented Tuesday 18, 15:20 to 15:40, in session Chemical Reaction Engineering: Practical Applications (T2-2c).
