Precursor to catalyst: Magnesium can substitute copper in the malachite structure to form mixed hydroxycarbonates with up to 17 at. % Mg incorporated. For higher Mg contents, amorphous “magnesian georgeites” are formed, that are well-suited precursors to highly active and stable Cu/MgO catalysts for methanol synthesis from CO/H2. Precursor chemistry and catalyst performance are investigated in this study.
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.Zum Volltext