Malachite catalyst

Figure 5.3.7 The variation of CO and methanol production rates with space velocity (A) and methanol synthesis rates on promoted CuZn-based catalysts derived from hydrotalcite and Malachite precursors (B). Reaction conditions 200 mg catalyst, 30 bar, 230°C, 3 1 H2 C02.

Figure 5.3.9 (A) Simplified geometric model [46, 89] for the preparation of industrial Cu/ZnO catalysts comprising subsequent meso- and nanostructuring of the material from [56], In a first micro structure directing step (mesostructuring), the Cu,Zn coprecipitate crystallizes in the form of thin needles of the zincian malachite precursor, (Cu,Zn)2(0H)C03. In a second step, the individual needles are decomposed and demix into CuO and ZnO. The effectiveness of this nanostructuring step depends critically on a high Zn content in the precursor, which in zincian malachite is limited to Cu Zn ca. 70 30 due to solid-state chemical constraints [75]. Finally, interdispersed CuO/ZnO is reduced to yield active Cu/ZnO. (B) Chemical memory Dependence of catalytic activity in methanol synthesis on the conditions of the coprecipitation and aging steps, from [85].

Catalysts have been used to make end points detectable these have been reviewed by Mottola. In such systems the titrant reacts rapidly with the substance titrated, but excess reagent reacts only slowly with an indicator in the absence of an appropriate catalyst. For example, small amounts of complexing agents such as EDTA can be determined by titration with Mn(II). With malachite green as indicator in the presence of periodate, the excess Mn(II) catalyzes the indicator oxidation. 

The following spectrophotometric reagents were applied for determining indium 2-(2-thiazolylazo)-p-cresol (TAC), in catalysts [47] Malachite Green, in gallium metal and in ZnGeAs2 [53] and Pyrogallol Red, in zinc alloys [7]. 

Precvirsors obtained from the precipitation from both solutions of raw metal salts and alkali are composed of basic carbonates of copper and zinc such as malachite (MA) CU2CO3 (0H)2, aurichalcite (AU) (Zn,Cu)5(C03)2(OH)5, and hydrozincite (HZ) Zn5(C03)2(0H)g. For the ratio of Cu to Zn in industrial catalysts, the precursor would not exist as mono-phasic. Even if it seems to be monophasic, the atomic ratio of Cu to Zn in the double salts would be continuously variable. Since amorphous intermediates of the precursor are also known, the real phase cannot be well characterized by XRD measurements. Of course the structure and distribution of precursors are also very sensitive to the precipitation conditions (temperature, rate, pH, etc.) as well as precipitation agents. Consequently, the characteristics of precursors are not determined only by the starting composition. 

Mixed copper/zinc catalysts with high copper-to-zinc ratios are widely used as catalysts for low-pressure methanol production and for low-temperature shift reaction [2, 31], see also Chapter 15. These catalysts are commonly made by coprecipitating mixed-metal nitrate solutions by addition of alkali. Li and Inui [32] showed that apart from chemical composition, pH and temperature are key process parameters. Catalyst precursors were prepared by mixing aqueous solutions of copper, zinc, and aluminum nitrates (total concentration 1 mol/1) and a solution of sodium carbonate (1 mol/1). pH was kept at the desired level by adjusting the relative flow rate of the two liquids. After precipitation was complete, the slurry was aged for at least 0.5 h. When the precipitation was conducted at pH 7.0, the precipitate consisted of a malachite-like phase (Cu,Zn)C03(0H)2 and the resulting catalysts were very active, while at pH < 6 the formation of hydroxynitrates was favored, which led to catalysts less active than those prepared at pH 7.0 (Figure 7.8). 

Aging temperature was also an important parameter with catalysts prepared at 70 °C being more active than those prepared at 50 °C. It was concluded that where pH exerted its effect through modification of the chemical composition, temperature mainly affects the precipitation kinetics. In general, the presence of basic nitrate in the precipitate was detrimental, whereas malachite was beneficial [32]. 

The catalysis rests on the fact that W i is reduced rapidly by Ti M to Wv, which quickly reduces malachite green to the colorless leuco base. Consequently, this intermediate reaction catalysis involves two rapid partial reactions in which the catalyst (tungsten) participates. The summation of the partial reactions (1) and (2) gives the net catalysed reaction (3) in which the catalyst does not appear. 

The leuco-base of the triphenylmethane dye malachite green is oxidized by periodate to the dye-stuff very slowly at pH 4 in an acetate buffer. In the presence of manganese the oxidation proceeds rapidly, the manganese acting as a catalyst. The dye is slowly destroyed by a second reaction but after a suitable time interval extinctions at 6150 A are nearly proportional to the initial amount of manganese present in the sample. 

Fig. 6.7. Copper-based catalysts for methanol synthesis. A novel device for controlled precipitation enabled separation of blue from green products. Structural analysis (top left) revealed that the blue products are disordered nanocrystalline materials furnishing poor catalysts. The green products are mixtures of two phases, malachite (violet) and auricalcite (red). By systematically optimizing the reaction conditions it was possible to prepare phase-pure green products and thereby to improve thesynthesisofthe working catalyst based on pure malachite precursors. In the X-ray diffraction pattern (top right), the features are labeled by the Miller Indices, indicating the diffraction lattice plane of the crystal °29 is the diffraction angle.

---