Nanostructured Catalytic Films for Multiphase Microstructured Reactors

Abstract

The application of microstructured catalytic reactors for gas-liquid reactions requires the development of new techniques for the incorporation of highly active catalytic thin films onto their microstructured surfaces. These catalytic thin films may have open porosities even up to 50%. Their application limits the pressure drop over the microreactor in comparison to micro packed beds, it enhances the catalyst accessibility, and it may significantly reduce mass transfer limitations. In this PhD thesis, ordered mesoporous silica and titania films with a thickness of 100 to 400 nm were developed via an evaporation induced self assembly method on different substrates (glass, silicon, titanium). These polymer templated meso¬porous silica films have a narrow pore size distribution and were synthesized with a wide range of pore sizes (2 to 8 nm). A new calcination protocol was developed which allows the complete removal of the surfactant at mild conditions. The thermal and hydro¬thermal stability of the films that were obtained with an ionic surfactant was improved by pH adjust¬ment during hydrolysis and by Al incorporation. Microwave assisted hydro¬thermal synthesis of these ordered microporous films was also investigated in an attempt to reduce the synthesis time from several days to less than 10 hours. The obtained thin films have been loaded with polymetallic nanoparticles with a size of 1 to 3 nm to specifically activate a selected functionality of complex organic molecules. Methods for the deposition and the stabilization of bi-metallic and tri-metallic clusters by adsorption onto the mesoporous thin films have been investigated. A one pot sol-gel synthesis of the mesoporous films with embedded colloidal nano¬particles was developed which eliminates an additional impregnation step and produces a uniform distribution of the active components throughout the mesoporous films. Various experimental techniques such as ellipsometric porosimetry, XRD, 2D SAXS, XPS, SEM, and TEM have been applied to obtain insight in the physical and chemical phenomena that determine the performance as well as the stability of the thin films. The activity and the selectivity of the resulting catalytic thin films have been investigated in the batch and in the continuous mode in the hydrogenation of citral and phenylacetylene. The latter was done in a 10 m long micro capillary (i.d. 250 µm) with a catalytic thin film deposited onto its inner channel wall surface. It was shown that the selectivity towards the target product can be changed by varying the metal ratio in the bimetallic nanoparticles. The high stability of these catalytic thin films allows their further implementation in fine chemicals synthesis using microstructured reactors.