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Asymmetric porous alumina membranes with palladium nanoparticles deposited into the thin surface layer are used as a structured catalyst to perform the direct synthesis of hydrogen peroxide with separate supply of the reactants in a catalytic membrane contactor. Parameters such as mode of supply of the gaseous species, type of liquid medium used, as well as differential and liquid side pressure, have been investigated and their influence on the productivity and selectivity of the reaction is discussed. The presented concept, which targets local hydrogen peroxide production, offers improved safety of the process compared to conventional methods for direct synthesis described in the literature. By optimization of the reactant supply, high productivity (up to 16.8 mol h −1 m −2 in methanol) and high selectivity (between 80% and 90%) to hydrogen peroxide could be demonstrated in this reaction for the first time with catalytic membranes.

This paper summarises the main ideas of how to use membranes with built-in catalytic function for performing catalysed gas/liquid reactions. Mass transfer effects on catalyst performance and other decisive factors for the design of industrial gas/liquid and gas/liquid/solid reactors are briefly reviewed. Various concepts for applying catalytic membrane layers in multiphase reactors are introduced and the motivation behind their use is explained. What follows is an assessment of the present achievements in using such concepts for gas/liquid reactions. The results available in literature are critically evaluated, focussing mainly on the field of liquid-phase hydrogenations and oxidations, which constitute the major areas where catalytic membranes have been applied to date to gas/liquid systems. The aim of such treatment is to point out the targets, to elaborate on how far the whole field has developed and what challenges remain, rather than to cover a multitude of specific aspects of the many different studies reported in literature.

An automated flow injection analysis system for H(2)O(2) detection in the liquid phase, based on the photometric determination of the complex between hydrogen peroxide and a titanium (IV) reagent is presented. The system was tested for monitoring the H(2)O(2) concentration in a lab-scale experimental setup for H(2)O(2) direct synthesis. In its final configuration, the developed system allows a reliable and reproducible determination of H(2)O(2) for a broad concentration interval between 1.02x10(-3) mol l(-1) and 1.16 mol l(-1).