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Novel eco-efficient oxidation processes based on h2o2 synthesis on catalytic membranes

Exploitable results

A ceramic membrane was developed to separates the space of and a gas and a liquid phase from each other but allows a contact and in presence of a catalytic material a chemical reaction between both phases inside of the pores. By optimisation of pore size distribution and mechanical stability of the membrane layers a differential pressure of several bares between the gas and the liquid phase allows the fast transport of the gas into the pores without a bubbling through the membrane. The membrane itself is made of alpha-Al2O3 and therefor catalytic inactive and has high chemical and mechanical stability. Multichannel ceramic membrane inner coated with a ceramic and eventual carbon layer have been also developed. They show improved effectiveness/cost ratio. Scale-up of the preparation of these membranes has been realized.
Procedures have been optimised for coating a thin carbon layer on a multilayer alpha-alumina macroporous tubular-type membrane as well as to deposit a thin microporous carbon layer inside the pores of the ceramic membrane. Work in the second half of the project demonstrated that the performance of the carbon membrane layer was strongly dependent on the degree of cure of the resin precursor achieved through thermal ageing of the resin solution. Following detailed optimisation of the resin precursor the as deposited thin (~1-3micron) surface carbon layer is an excellent gas separation membrane and has shown interesting separating properties for H2/N2 with potential in a wide range of other gas separations. Activation of the carbon layer is necessary to provide sufficient porosity to support the palladium catalyst for the generation of the hdyrogen peroxide. The carbon deposition and activation procedure has been optimised to provide the required catalyst loading and activity. The carbon-modified ceramic membranes can be used as versatile starting materials for preparation of catalytic membranes with different types of active metals for various applications.
Catalytic membranes based on dispersed Palladium as the active component have been prepared. Compared to dense Palladium membranes, these catalytic membranes offer better gas transport rates, reduced costs and higher reaction rates. The catalytic membranes can be applied (besides the target reaction of hydrogen peroxide direct synthesis) to a variety of solid catalysed gas/liquid reactions, which can benefit in terms of improved safety and reduced diffusion limitation. Examples include the (selective) elimination of pollutants in water by hydrogenation or oxidation and the selective synthesis of organics by hydrogenation or oxidation. Different methods were developed to deposite Palladium uniformly into the fine-porous surface layer of inside-coated as well as outside-coated asymmetric tubular ceramic single-tube and multi channel membranes with or without modification by carbon. Metal organic chemical vapour deposition (MOCVD) was used at DECHEMA: Two tailored batch-type MOCVD variants were developed, both based on a control of the deposition range of the active component in the membrane. One uses an organic wax to trap the sublimed metal precursor prior to (mainly thermal) decomposition; the other relies on a stoichiometric reluctant introduced in the membrane surface layer before the MOCVD by impregnation which later reacts with the precursor vapour. Both methods enable a high Palladium loading; they reach an excellent uniformity of the Palladium content over the whole surface of the membrane and a good control of the deposition depth. The second method, which was developed, is based on precipitation-deposition, and the third method is based on electroless plating deposition. These methods allow a good and reproducible deposition and a fine control of the amount of Palladium, which is deposited, uniformly in a thin layer of the membrane. The membranes prepared were characterized by various techniques (AAS, EPMA, SEM, TEM, CO- chemisorption) in order to assess the amount of Palladium deposited, its distribution over the cross section and along the tube axis as well as the Palladium particle size.
Based on the optimized contacting mode of H2 and O2 (O2 through the membrane, diluted H2 in the liquid phase) a tentative process design was identified which is particularly well suited for safe small-scale production of hydrogen peroxide by direct synthesis.