Catalytic upgrading of ethanol to n-butanol over a novel Ca-Fe modified mixed oxide Mg-Al catalyst from hydrotalcite-base precursor

Abstract

n-Butanol is used in a wide range of applications and recently, the use of n-butanol as a fuel, which can be employed pure or blended with gasoline or diesel, has been proposed. So, in an attempt to deepen in the one-step catalytic process to produce n-butanol from ethanol, bifunctional catalysts in different chemical/spatial environment as MgAl- and CaFeMg-multioxide catalysts were developed, prepared and characterized. Both mixed oxides were obtained by thermal decomposition of the synthesized hydrotalcites considering the partial substitution of Mg2+ and Al3+ by Ca2+ and Fe3+, namely MgAl-Ox and Ca20Fe20MgAl-Ox. The nature of acid and basic properties of the samples were analyzed employing NH3 and CO2 as probe molecules, and TPD analysis were performed to quantify the relative contribution of acidic/basic sites. Ca20Fe20MgAl-Ox sample presented an improved basicity with minimal acidity reduction. The performances of the activated catalysts were measured in a fixed-bed reactor under transient and continuous operation modes by DRIFT-MS studies and under transient pulse experiments. It was verified that both catalysts, but specially Ca20Fe20MgAl-Ox, promote the synthesis of C4-compounds from ethanol due to the modulated acid and basic sites capable of modify the ethanol adsorption in the form of ethoxy-species and a combined higher basicity with centers to catalyze the hydrogenation using the ethanol as the hydrogen donor. Ca20Fe20MgAl-Ox reached ethanol conversion values of 36% at 280°C and 71% at 400°C with a higher selectivity to n-butanol and other C4-compounds. Two parallel routes at differentiated temperatures: direct acetaldehyde self-condensation and the intermediary crotyl alcohol transfer hydrogenation, were evidenced and prevailing in n-butanol formation.