Copper-manganese catalysts with high activity for methanol synthesis
Graphical abstract
Introduction
Methanol is a compound of great industrial importance because of its use as raw material in the synthesis of many important chemical products. It can also be used as an energy carrier for hydrogen storage and as a fuel [1,2]. Methanol synthesis is an already well-established process, with an annual production of approximately 90 million tons. It consists of the hydrogenation of synthesis gas (COx/H2 mixture), in a heterogeneous catalytic process [[3], [4], [5], [6]]. The catalyst has been the center of constant research in the last 40 years [7] and is the main focus for the optimization of methanol synthesis [2], but mostly around the same ternary system, Cu/ZnO/Al2O3 [2,[7], [8], [9], [10]].
Layered double hydroxides (LDHs) are minerals from the family of anionic clays. The general formula representing the LDHs is [M2+1-xM3+x(OH−)2]x+[An-x/n].mH2O, where M2+ and M3+ are divalent and trivalent cations, respectively, An− is a compensation anion, m is the interlayer water and x is the molar fraction of the trivalent cation with respect to total cations. The preparation of the materials is simple and their composition allows a wide range of element combinations, as well as of their relative proportions. Catalysts resulting from LDH decomposition are generally mixed oxides with good active phase dispersion, thermally stable, and with high specific surface area [11].
In the present work, bulk catalysts were prepared from trimetallic LDHs, containing Mn, Cu and Al, keeping constant the x value (Al3+ molar fraction) at 0.19 (nominal value) and varying the Cu2+/Mn2+ molar ratio (y value). For the material with y = 0.5 (nominal value), two different precipitation pH values were used. The catalysts were evaluated in the liquid-phase methanol synthesis reaction from synthesis gas (syngas), using a conventional commercial CuZnAl catalyst as a reference. The liquid-phase methanol synthesis process was developed in 1975. Claimed distinguishing features include usage of CO-rich syngas, superior once-through conversion from syngas, higher heat transfer efficiency, and better heat management [12]. Previous work in the literature already show some benefits of adding manganese to methanol synthesis catalysts, but mainly as additives to the conventional CuZnAl system [[13], [14], [15], [16], [17]]. Turning our attention to LDH based catalysts, CuMnAl ternary structures are reported in the literature [[18], [19], [20], [21], [22], [23]], however they were not evaluated in the methanol synthesis reaction.
This work is part of broader project, where the methanol synthesis catalyst will be used in conjunction with a methanol dehydration one, in the direct synthesis of dimethyl ether from syngas in a slurry reactor. However, as alcohol dehydration reactions require acid sites and sodium is a poison for these sites, one of the objectives of this work was also to obtain methanol synthesis catalysts from sodium-free raw materials. There are reports in the literature on LDHs made from Na-free precipitating agents using NH4OH [24,25] or gaseous NH3 + CO2 [26], Na-free methanol synthesis catalysts also exist [[27], [28], [29], [30]], however CuMnAl ternary LDHs have never been prepared from Na-free reactants using our method and they also were not evaluated in the methanol synthesis reaction, to the best of our knowledge.
Section snippets
Preparation of the catalysts
LDH precursors were prepared using the controlled pH precipitation method. Two solutions were added to a glass reactor with 200 ml of water. One solution containing metal nitrates in the appropriate proportions for the desired compositions (0.4 M in total divalent metal nitrates and 0.1 M in aluminum nitrate) and another 0.05 M in ammonium carbonate (the commercial name for a bicarbonate plus ammonium carbamate mixture) and 0.7 M in ammonium hydroxide. The pH (6.5 or 8.0) and temperature
X-ray diffraction
Fig. S1 presents the X-ray diffraction patterns of the LDH precursors. All samples, with exception of MnpH8, show intense and narrow lines at low 2θ values, while less intense and less symmetrical ones appear at high 2θ values, as typically seen in LDH diffraction patterns [11]. The diffractograms correspond to that of a CuAl LDH (PDF 46-0099). Comparison with a CuMnAl LDH standard was not possible because no XRD pattern was found for this system, either in PDF (Powder Diffraction File) or ICSD
Conclusions
Carbonate-intercalated LDHs based on Cu, Mn and Al were successfully synthesized with varying Cu/Mn ratios and pH of precipitation, although the amounts of manganese incorporated into the catalysts were significantly smaller than the ones employed in the syntheses. The precipitating agents employed were sodium free, for which there are no previous reports in the literature for this system.
The characterization of the mixed-oxides obtained by calcination of the LDH precursors showed that the
Acknowledgment
The authors acknowledge the XRD and XPS multiuser laboratories at IQ-UFRJ for sample analysis. To the Laboratório de Petróleo e Meio Ambiente (LCPMA) at UERJ for making available the equipment to perform the N2O decomposition measurements. We acknowledge LNLS (Campinas, Brazil) for provision of synchrotron radiation facilities and we would like to thank Cristiane Rodella for assistance in using XPD (project 20,170,209) beamlines. We acknowledge LNLS (Campinas, Brazil) for XAS measurements at
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