Synthesis of alcohols from carbon oxides and hydrogen: XVIII. Preparation chemistry, phase transformations and catalytic behaviour of unpromoted Mn [sbnd]Cr [sbnd]O systems in the synthesis ofalcohols from carbon monoxide and hydrogen

Abstract

Unpromoted Mn Cr O catalysts with manganese-to-chromium (Mn/Cr) atomic ratios ranging from 0.5 to 5 have been prepared and characterized by XRD, IR, UV-VIS diffuse reflectance, differential thermal analysis-thermogravimetric analysis and surface area measurements. The preparation chemistry is based on the redox reaction 3Mn II+Cr VI→3 Mn III+Cr III and the phase transformations occurring during activation depend on the Mn/Cr ratio and the annealing atmosphere. Activation in air at 400–600°C results in the formation either of α-Cr 2O 3 and a spinel phase or of α-Mn 2O 3 for low and high Mn/Cr ratios, respectively. Activation in hydrogen at similar temperatures results in the formation of MnCr 2O 4 for Mn/Cr = 0.5 and of an MnO-type phase for high Mn/Cr ratios, which is pyrophoric when exposed to air at room temperature. Activation in nitrogen is most suited for the preparation of Mn Cr O catalysts because it prevents both pyrophoric effects and the formation of oxides of Mn and Cr, which are known to catalyse the formation of undesirable reaction products during the synthesis of higher alcohols. Activation in air at higher temperature causes the disappearance of α-Cr 2O 3 and α-Mn 2O 3 and the formation of a spinel-like phase; for high manganese contents a tetragonal distortion of the cubic spinel phase is observed owing to the Jahn-Teller stabilization of octahedrally coordinated Mn III. In the synthesis of higher alcohols, (i) Mn/Cr ratios > 0.5 ensure high activity in methanol synthesis, (ii) high Mn/Cr ratios depress the productivity with respect to ethers and methane because of the lower surface acidity and (iii) irrespective of the Mn/Cr ratio, all Mn Cr O catalysts are effective systems for the water-gas shift reaction.