CO Hydrogenation to Methanol over Cu/MgO Catalysts and Their Synthesis from Amorphous Magnesian Georgeite Precursors

Abstract

The synthesis of Mg-substituted copper hydroxycarbonates by constant-pH co-precipitation with subsequent ageing of the precipitate has been studied in detail. This allowed the retrieval of crystalline magnesian malachite samples, which showed a "radical-sign-shaped" pH drop and the blue/green color change during ageing that is well-known from the analogous Cu/Zn system. The crystallization of Cu-rich samples has been studied elaborately by means of powder diffraction, pair distribution function (PDF), and infrared spectroscopy, thus elucidating the ageing chemistry of Cu,Mg hydroxycarbonates in general. It was found that up to 17 % Mg can be incorporated into the crystalline malachite using a synthesis route comparable to that established for Cu/ZnO catalysts, but with drastically elongated precipitate ageing times. For higher Mg contents, a step-like pH drop was observed instead, and an amorphous variant of the hydroxycarbonate, referred to as "magnesian georgeite", could be isolated. From such a Mg-rich sample (Cu/Mg 70 : 30 at. %), a Cu/MgO catalyst has been prepared that shows high activity and good stability in CO hydrogenation to methanol over 800 hrs. time on stream.

Substitution of Copper by Magnesium in Malachite: Insights into the Synthesis and Structural Effects

“…This is a marked difference to the Cu,Zn case, but beyond that the similarities are obvious: in both cases, Cu,Mg at 103 °C and Cu,Zn at 65 °C, the rapid crystallization occurs after the same 60–65 min, and identical IR bands are found after 120–180 min of total aging time. As we have previously seen that the Cu,Mg crystallization kinetics at 65 °C are extremely slow compared to Cu,Zn, the observed similarity can be attributed to the accelerating effect of the increased aging temperature. The findings from in situ IR indicate that, on a time scale, the hydrothermal aging brings Cu,Mg crystallization closer to the Cu,Zn aging, which therefore opens a path to Cu,Mg hydroxycarbonates with similar Cu/ M +II ratios, as already known for Cu,Zn.…”

“…420 °C for ratios of 26 atom % Mg and higher, those samples that have been found to be outside the phase limit of Mg substitution into malachite (see Figure S13 in the SI). Also, this value is higher than the typical 330–350 °C for the preparation of CuO/ZnO from zincian malachite and of CuO/MgO from amorphous magnesian georgeite precursors . On the one hand, this is plausible by the higher crystallinity obtained after hydrothermal aging, but it also indicates a higher thermal stability of the magnesium-substituted malachites compared to that of zincian malachite.…”

“…Also, this value is higher than the typical 330−350 °C for the preparation of CuO/ZnO from zincian malachite 8 and of CuO/MgO from amorphous magnesian georgeite precursors. 21 On the one hand, this is plausible by the higher crystallinity obtained after hydrothermal aging, but it also indicates a higher thermal stability of the magnesium-substituted malachites compared to that of zincian malachite. The catalytic performance of the resulting catalyst material is greatly influenced by the specific surface area that can be measured by N 2 physisorption following the method of Brunauer, Emmett, and Teller (BET).…”

“…In a recent study, we were able to obtain Mg-substituted (“magnesian”) malachite with up to 17 atom % Mg by coprecipitation, following a known recipe for the preparation of zincian malachite, but found the crystallization kinetics of the Cu,Mg precipitates to be much slower than in the Cu,Zn case . This can pose a problem in the wet-chemical synthesis, as for high pH values that are necessary to precipitate magnesium in near-quantitative amounts, copper hydroxides can be oxolated to CuO during aging in the mother liquor. , The competition between both reactionswhether the crystallization of the hydroxycarbonate from the amorphous precipitate is completed at first or the oxolation to CuO wins the race and prevents successful preparation of phase-pure hydroxycarbonatesdepends on the amount of Mg to be incorporated.…”

Cu‐Co/ZnAl2O4 Catalysts for CO Conversion to Higher Alcohols Synthesized from Co‐Precipitated Hydrotalcite Precursors

“…These results already established the composition dependency of selectivity on a ternary catalyst component such as chromium. Addition of zinc is known to have an enormous effect on methanol synthesis on copper-based catalysts being able to tune the reactant selectivity between carbon monoxide and dioxide [18], but the presence of zinc in Cu-Co tri-metallic catalysts for HAS is not studied to an extent comparable to methanol synthesis. So far it is well understood that ZnO acts as a steric promoter preventing sintering and suppresses methane formation due to intermediate stabilization in CuCoZnAl containing catalysts.…”