Continuous Coprecipitation of Catalysts in a Micromixer: Nanostructured Cu/ZnO Composite for the Synthesis of Methanol

Abstract

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...

Cu,Zn-based catalysts for methanol synthesis: On the effect of calcination conditions and the part of residual carbonates

“…Further increase of calcination temperature may cause a decrease of the surface area and segregation and sintering of the metal oxides. High temperature carbonate (HT-CO 3 ) has been considered as a possible reason for the superior catalytic activity of methanol synthesis catalysts prepared by the co-precipitation route via basic carbonate precursors, but its exact role for the catalytic activity remained unclear [10,25]. In order to gain better understanding of the HT-CO 3 we prepared two series of catalysts from AU and ZM by variation of the calcination conditions.…”

“…The intermediate is called anion modified mixed metal oxide [4] or high temperature carbonate (HT-CO 3 ) [2]. Although the occurrence of this high temperature stable carbonate in the precursor phase is said to be beneficial for the Cu dispersion in the final catalyst [10], detailed investigations about the calcination process and understanding of the role of HT-CO 3 for catalysis are still lacking.…”

Catalytic Activity of Copper Oxide/Zinc Oxide Composites Prepared by Thermolysis of Crystallographically Defined Bimetallic Coordination Compounds

“…It is important to note that the morphology (by scanning electron microscopy) and the chemical composition (CuO and ZnO) of the thermolysis products from both kinds of precursors are practically indistinguishable. This underscores the strong influence of the precursor structure on the catalytic activity of the resulting oxide, [1,2] which, despite sophisticated in situ studies, [21,22] is not yet fully understood. (b) Single-tube reactor: 100 mg of the sample (sieves fraction 250-355 µm) were placed in a fixed-bed glass-lined U-tube reactor.…”

“…[1][2][3] Although Cu/ZnO/Al 2 O 3 is a more efficient catalyst, Cu/ZnO is often examined as an easier model to understand the catalytic process (note that even ZnO alone is an active catalyst in methanol synthesis [4] ). Conventionally, such catalysts are prepared by thermolysis of coprecipitated carbonates or oxalates.…”

Preparation of Multicomponent Metal Oxides Using Nozzle Spray and Microwaves

“…According to Schur et al, [8] each and every step of the preparation method can have profound effects on the structure and performance of mixed metal oxide catalysts. The preferred preparation method for an effective mixed metal oxide catalyst includes the intimate mixing of all starting chemicals to form a precursor solution.…”