Tuning the selectivities of Mg-Al mixed oxides for ethanol upgrading reactions through the presence of transition metals

Abstract

The effect of the presence of reduced Co and Ni (chosen as representative metals because of their good activity for dehydrogenation reactions) on the catalytic performance of basic mixed oxide (Mg-Al) for ethanol condensation is studied in this work. This effect has been studied both in absence and in presence of hydrogen, and considering the different steps of this complex reaction. Globally, best results were obtained with Co/Mg-Al, under reducing atmosphere, at mild temperature (below 600 K). At these conditons, 1-butanol production rates up to eight times higher than the obtained with Mg-Al under inert atmosphere. Co has a marked activity in the dehydrogenation step, that prevails over its less relevant activity in aldolization and hydrogenation reactions. This result indicates the relevant role of this first reaction step. DRIFT spectroscopy analyses were carried out to support the experimental results and to identify the role of hydrogen and metals on the oligomerization and permanent adsorption processes, which can produce the deactivation of the catalyst.

Enhancement of the 1-butanol productivity in the ethanol condensation catalyzed by noble metal nanoparticles supported on Mg-Al mixed oxide

“…In the cases of Mg-Al and Ru/Mg-Al, a very high selectivity to main reaction is observed, obtaining in all the cases condensation ratios higher than 90 %, for both materials and not depending on the hydrogen presence. The parallel behavior shown by both materials, independently of the type of atmosphere, is in good agreement with the general mechanism of aldol condensation, where medium-strength basic sites are the main responsible of the catalytic activity, prevailing these types of sites over the any catalytic effect of the metal nanoparticles[13,29]. Basicity analysis by temperature programmed desorption studies demonstrate the almost null effect of supporting ruthenium on Mg-Al on the medium-strength basicity, keeping constant more than 85 % of the initial concentration of these type of sites.…”

“…After introducing these metal phases, a clear improvement was obtained working under inert conditions, reaching 1-butanol productivities up to eight times the values obtained with Mg-Al. Best results were obtained with cobalt, metal with the highest dehydrogenation capacity [13]. In a similar set of experiments, a relevant role of reducing atmosphere was also suggested observing an increment in the 1-butanol productivity mainly marked by the decrease in other side reactions related to dehydration and side-dehydrogenation steps (i.e., formation of 1,3-butadiene).…”

“…In fact, both series can be hardly distinguished in the plot because most of the points are exactly overlapped. A detailed study of the behavior of this Mg-Al under reducing and inert conditions is reported in our previous work[13]. In general, considering that there is not any metal phase to activate the hydrogen molecule, reducing conditions has no noticeable effect on the behavior of Mg-Al.…”

“…Thus, we analyzed the role of cobalt and nickel supported on Mg-Al mixed oxide [13]. This support has a well-known activity for the aldol condensation, being previously proposed as catalyst for this reaction [14][15][16].…”

Oxygenate Reactions over PdCu and PdAg Catalysts: Distinguishing Electronic and Geometric Effects on Reactivity and Selectivity

“…Based on this, we posit that the selectivity trend of the butyraldehyde hydrogenation will parallel that of the selectivity of the industrially-relevant Guerbet reaction. The Guerbet reaction of alcohols requires the dehydrogenation of the alcohol to the reactive aldehyde [38,39], a reaction that can be viewed as the microscopic reverse of the hydrogenation of an aldehyde to an alcohol. Moreover, one of the main pathways to selectivity losses during the Guerbet reaction is the C-C bond cleavage of acetaldehyde via decarbonylation [40].…”