Tuning the acid-base properties of layered double hydroxides for the selective obtention of cyclohexane and cyclohexanol in the hydrodeoxygenation of guaiacol

Identifier:  imarina:9452538

Handle: https://hdl.handle.net/20.500.11797/imarina9452538

Authors:  Rueda AC; Granados-Reyes J; Delaunay J; Mora-Masià P; Cesteros Y

Abstract:
Ni and NiCu catalysts were prepared from Ni/Al, Ni/Cu/Al, Ni/MgAl, and Ni/Cu/Mg/Al hydrotalcites (HT) and tested for the hydrodeoxygenation of guaiacol, a lignin model compound, under H2 pressure at 300 °C to obtain the added-value products cyclohexane and cyclohexanol. The catalysts prepared from HT without magnesium were more selective to cyclohexane (48–65 %) than those prepared from HT with magnesium, which obtained higher selectivity to cyclohexanol (46–72 %). The presence of magnesium oxide species decreased the reducibility and the metallic area of the catalysts, decreased the Ni content in the NiCu alloy formed in the bimetallic catalyst, and, due to their higher basicity, inhibited the dehydration of cyclohexanol, a key step for obtaining cyclohexane. The obtention of cyclohexane with catalysts prepared from HT is remarkable and could be mainly explained by the reaction temperature used (300 °C) since when it was lowered to 240 °C, a significant decrease of cyclohexane accompanied by an increase of cyclohexanol was observed. When these catalysts were combined with H-βeta, the additional Brønsted acidity led to the overproduction of cyclohexane due to the parallel transformation of the solvent used, dodecane, under the reaction conditions. The detection of some amounts of benzene and toluene suggested a different reaction pathway compared with the catalysts without H-βeta. Finally, the addition of small amounts of Lewis acidity (ReOx) to the Ni/Al and NiCu/Al catalytic systems made difficult the reducibility to Ni of their catalytic precursors and promoted the binding of the oxygenated compounds, favoring the formation of cyclohexanol.