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

Abstract

The phase width of the copper hydroxycarbonate malachite, Cu 2 CO 3 (OH) 2 , upon substitution with magnesium has been studied in detail. In extension of a previous study on amorphous precursors, the introduction of a hydrothermal aging step allowed the retrieval of crystalline hydroxycarbonate samples with up to 37 atom % Mg (metal content) that are suitable candidates as precursors to Cu/MgO catalysts for CO hydrogenation. Simultaneous refinements of X-ray powder diffraction and pair distribution function (PDF) data as well as complementary spectroscopic insight (X-ray absorption and infrared spectroscopy) revealed that samples with up to 18 atom % Mg are phase-pure magnesian malachites but the magnesium content can be increased beyond this threshold when mcguinnessite (CuMgCO 3 (OH) 2 ) is accepted as a side phase. In a complementary study, a continuous increase of the magnesium fraction was found during aging and the corresponding structural evolution was studied by means of PDF. These findings add significant insight into the aging chemistry of crystalline Cu,Mg hydroxycarbonates. Furthermore, both phase-pure magnesian malachite and mcguinnessite-containing samples with up to 37 atom % Mg have been examined by thermogravimetry, X-ray powder diffraction, and N 2 physisorption and were found to be promising candidates for use as precursors for the preparation of Cu/MgO catalysts.

Enhanced Methanol Formation over Cu/MgO Catalysts Derived from Mg-Rich Active Hydroxycarbonate Precursors

“…As the substitution level reaches 50%, the intensities in the 34–37° region become more prominent, matching well with the relative intensities observed in the naturally occurring mineral data (Figure a). These observations contrast with a previous study where Mg ion substitution beyond 18% resulted in a mixture of magnesian malachite and mcguinnessite . The PXRD patterns of the single-phase products could readily be indexed in monoclinic symmetry (S.G. P12 1 /a1 ) with the refined lattice parameters of a = 12.15157(5); b = 9.20767(3); c = 3.19374(8) Å; and β = 94.999 (5)°.…”

“…These observations contrast with a previous study where Mg ion substitution beyond 18% resulted in a mixture of magnesian malachite and mcguinnessite. 37 The PXRD patterns of the single-phase products could readily be indexed in monoclinic symmetry (S.G. P12 1 /a1) with the refined lattice parameters of a = 12.15157( 5); b = 9.20767(3); c = 3.19374(8) Å; and β = 94.999 (5)°. Moreover, the broad peaks in the PXRD pattern of CM50 indicated a small average crystallite size of 32 nm calculated using Scherrer's formula.…”

“…The gain in the thermal stability of the precursor also supports the presence of more Mg 2+ ions in the lattice, which agrees well with the literature. 37 Figure S5b shows the UV-DRS spectrum of the CM50 sample; the blue-green colored precursor showed a broad band in the visible region with an absorbance edge falling at 500 nm, exhibiting a typical semiconductor behavior. The d−d transition of Cu 2+ ions was encountered in the range between 600 and beyond 800 nm, along with an LMCT band around 200−300 nm in the UV region of the spectrum.…”

“…However, the increase in magnesium content beyond this limit resulted in mcguinnessite (CuMgCO 3 (OH) 2 ) as a secondary phase. 37 This discovery further offers the potential for future optimization of synthesis conditions to achieve even higher substitution levels while maintaining phase purity. This requires methods capable of stabilizing crystalline Cu and Mg hydroxycarbonates while incorporating higher atomic percentages of Mg 2+ , surpassing the established threshold into a single precursor crystalline phase while simultaneously preventing the oxidation of Cu to CuO.…”

“…They found that samples containing up to 18 atom % Mg exhibit phase-pure magnesian malachite. However, the increase in magnesium content beyond this limit resulted in mcguinnessite (CuMgCO 3 (OH) 2 ) as a secondary phase . This discovery further offers the potential for future optimization of synthesis conditions to achieve even higher substitution levels while maintaining phase purity.…”