Consequences of the iron–aluminium exchange on the performance of hydrotalcite-derived mixed oxides for ethanol condensation

The effect of the partial and total substitution of aluminium by iron on the performance of different hydrotalcite-derived mixed oxides for the condensation of ethanol for obtaining valuable C4 products (mainly butanol and 1,3,-butadiene) has been studied in this work. Ethanol condensation reactions have been performed in a fixed bed reactor at 0.1 MPa, WHSV = 0.215 h−1 and 473–723 K. Three different hydrotalcite-derived mixed oxide with an atomic Mg2+/M3+ ratio of 3 (namely Mg6Al2O9, Mg6AlFeO9 and Mg6Fe2O9) have been prepared by a previously optimized procedure, tested as catalyst for the above mentioned reaction and characterized by TG–DTG, NH3-TPD, CO2 adsorption (calorimetric, TG–DTG and FTIR), Mossbauer Spectroscopy and TPR.The substitution of Al3+ by Fe3+ in the structure of the resulting mixed oxide leads to a slight decrease of the basic sites and a more marked decrease of the concentration of acid sites, being the concentration of these last sites negligible when the Al3+ cation is completely replaced by Fe3+. Accordingly, Mg–Fe mixed oxide is the most selective catalyst for the formation of C4 compounds, especially butanol. The almost total abatement of the acid sites of this material largely decrease the selectivity for ethanol dehydration, resulting in an increase of the formation of the dehydrogenation product (acetaldehyde), key reactant for condensation reactions.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ The substitution of Al by Fe in the MgAl hydrotalcite-derived mixed oxides only leads slight variations in crystallographic and morphological properties. ▶ This substitution leads to sharp decreases in the surface acidity of the materials, whereas the surface basicity slightly decreases. ▶ MgFe mixed oxide present higher selectivities for C4 fraction (especially 1,-butanol), largely decreasing the selectivity for ethylene and C4 olefins. ▶ Fe is present in the oxides as Fe3+, but its redox properties do not play any significant role in the reaction pathways.