Synthesis of nano-catalysts in flow conditions using millimixers

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Abstract

Many relevant sorbents, bulk catalyst or their precursors and catalyst supports are prepared by precipitation. Precipitate properties such as specific surface area, grain size and structural homogeneity show strong sensitivity on used conditions and equipment. The frequently observed lack of repeatability in precipitation is mostly caused by the latter sensitivity and upscaling issues or a limited control in production vessels are resulting challenges in industrial process design. On molecular level the outcome of the precipitation is governed by a delicate interplay of local transport, nucleation and grain growth (solidification) rates. Individual rates depend on the nature of cation solution and precipitation agent, mixing properties and reaction conditions like temperature and concentration levels. Better control can be achieved by precipitation in a confined space with short diffusion paths and well-defined hydrodynamics. This chapter illustrates achieving superior repeatability and better product properties employing confined precipitation in millimixers. The first case study addresses the formation of a material with well-defined LDH structure requiring a comparably slow solidification rate which ideally stays below the rate of proper molecular motion to ensure the required molecular orientation (thermodynamic control of precipitation). The second case study illustrates an ultra-fast precipitation suitable to preserve in the solid the prefect molecular dispersion in the precursor fluid. The latter is achieved when increasing solidification rates well above the segregation rate (kinetic control of precipitation). Both limit cases are successfully implemented by rational design of equipment and choice of precipitation agents and conditions. Obtained products show superior properties compared to those achieved by conventional precipitation in vessel-type equipment.

A vast number of functional materials are synthesized by precipitation with prominent examples including sorbents, catalyst supports, and bulk catalysts or their direct precursors. A high sensitivity of precipitate properties like specific surface area, grain size, and structural homogeneity makes repeatable preparation by precipitation a major challenge. The outcomes of a precipitation are governed by local nucleation and grain growth rates, which themselves depend on the delicate interplay of cation solution and precipitation agent mixing and reaction conditions like temperature and concentration level. Local phenomena, for example, reaction in vortices leading to tiny and virtually isolated cells, may determine the overall process making properties observed averaged over an entire reaction device being an imprecise description of actual conditions. Frequently observed upscaling issues of precipitations or a limited control in larger conventional vessels well possibly originates from the latter strong segregation effects.

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