Cu/Zn Hydroxycarbonates obtained by co‐precipitation of Cu2+ and Zn2+ with Na2CO3 have been investigated regarding phase formation and thermal decomposition in two series with varying Cu/Zn ratios prepared according to the decreasing pH and constant pH method. Hydrozincite, aurichalcite and (zincian)‐malachite were found to form at differing Cu/Zn ratios for both series. For the constant pH preparation the Cu/Zn ratio in zincian‐malachite was close to the nominal values whereas excess values were found for the decreasing pH samples. The degree of crystallinity as well as the thermal decomposition temperatures were lower for the constant pH series. All samples containing aurichalcite revealed an unexpected decomposition step at high temperatures evolving exclusively CO2. The differences in composition and microstucture were traced back to the different pathways of solid formation for the two preparation methods. Substantial changes were observed during the post‐precipitation processes of ageing and washing. The effects were studied in detail on samples with a cation ratio of Cu/Zn 70:30 mol %. Ageing of the precipitates in their own solutions is accompanied by a spontaneous crystallization of the initially amorphous solids. The decreasing pH sample develops from a hydroxy‐rich material comprising basic copper nitrate (gerhardtite) as an intermediate. Only small changes in the chemistry of the samples were detected for the constant pH precipitation. The findings are summarised into a scheme of solid formation processes that explains the phenomenon of a “chemical memory” of the precipitates when they are converted into Cu/ZnO model catalysts.

Bulk catalysts are indispensable for the production of primary chemicals. A Cu/ZnO system, which has been tremendously improved through promotion with Al, is used for the lowpressure synthesis of methanol. [1][2][3] This material is synthesized through the coprecipitation of a hydroxycarbonate (HC) precursor followed by calcination to form CuO/ZnO. [4,5] The properties of the activated catalyst are dictated by the kinetic details of the synthesis of the solid, that is, by the properties of the precursor HC material; [3,[5][6][7] the catalyst retains a "chemical memory".The precipitation of large quantities of material is an illdefined process. Even under "constant" precipitating conditions, the chemical potential of the reagents are neither spatially nor temporally constant when a drop of the precipitating agent is added to the reactor in which precipitate and dissolved ions already coexist. During ageing, washing, and drying of the catalyst precursor, complex exchange reactions occur, particularly between the anions (carbonate/ hydroxide and hydroxide/nitrate). These exchange reactions are not controlled and thus result in variations in the properties of materials that are synthesized in a similar manner. [8] A remedy for such a situation could be a continuous precipitation process in which, in the shortest possible time, a precipitate is formed in a small volume of solution, and is then collected under inert conditions (room temperature, water) followed by conventional processing. The kinetically controlled steps of nucleation and growth are thus carried out under much more reproducible boundary conditions. Furthermore, one can hope to minimize ensuing processes such as redissolution, reprecipitation, and ion exchange.In our studies, a commercial microreactor designed for reactions in solution was used for the precipitation.[9] The reactor consists of a plate (with parallel channels approximately 100 mm long and about 200 m wide), which is sealed between two temperature-controlled plates. The reagent solutions (0.15 m metal nitrate and 0.18 m sodium carbonate; pH = 7.0) are combined in the channels at a constant throughput (5 mL min À1 ) under fully defined flow conditions (very short residence time and rapid, intense mixing in the reaction zone). Heat transfer is thus rapid and precise temperature control (328 K) is ensured. The product is collected in a cooled settling container and is processed with the conventional washing and drying steps, followed by calcination. In a future, improved version of this method, the postprecipitation steps of collection, washing, and drying could also be organized into a continuous processing system to produce an even more uniform product.The fundamental experiments described herein demonstrate that such a production scheme, which above all distinguishes itself from standard batch precipitation in that the precipitate is rapidly isolated from the reactive mother liquor, does not produce a worse catalyst. Thus, such a continuous (parallel) precipitation technique is possible.T...

For Abstract see ChemInform Abstract in Full Text.

Vollkatalysatoren ohne Träger sind unverzichtbare Materialien in der Produktion von Basischemikalien. Zur Niederdruck-Synthese von Methanol wird ein System Cu/ZnO genutzt, [1][2][3] das durch Promotoren wie Al wesentlich verbessert wird. Die Materialien werden durch Cofällung einer Hydroxy-Carbonat(HK)-Vorstufe und deren Calcinierung zu CuO/ZnO hergestellt. [4,5] Die Eigenschaften der aktivierten Katalysatoren werden von den kinetischen Details der Festkörperbildung, d. h. von den Eigenschaften der Vorstufe HK, bestimmt; [3,[5][6][7]