Acetilação do glicerol com butiraldeído sobre catalisadores à base de óxidos mistos contendo Ni ou Co

Abstract

Acetalization of glycerol with carbonyl compounds is a catalytic reaction to produce cyclic acetals. Due to the needs of biodiesel industry to convert the excess of impure glycerol, new heterogeneous catalysts are searched. Among them, the acid catalysts tested in the acetalization reaction exhibited good performance in the reaction. However, these materials exhibited low selectivity to the desired products due their high acidity. On the contrary, redox catalysts showed promising selectivity to the desired products in reason of the mechanism changes provided by these materials. In this sense, mixed TiO2-Al2O3 and CeO2-MnOx acid oxides, which were prepared via sol-gel method, were synthesized. After impregnating Co or Ni salts, the Co3O4/TiO2-Al2O3, NiO/TiO2-Al2O3, Co3O4/CeO2-MnOx and NiO/CeO2-MnOx catalysts were obtained. The solids were characterized by X-ray difraction (XRD), Raman spectroscopy, Scanning electron microscopy coupled to energy dispersive X-ray spectroscopy (SEM-EDS) and nitrogen adsorption–desorption isotherms. XRD and Raman spectroscopy measurements showed the presence of acid Co3O4 and NiO phases. SEM analysis showed plate-like shaped agglomerates on the surface of the prepared solids. The Ti-Al based materials did not displayed Co or Ni species over their surfaces, as showed by EDS. Nitrogen adsorption–desorption isotherms illustrated that the catalysts were mesoporous and exhibited good textural properties. The catalytic activity of the materials were evaluated in the reaction of acetalization of glycerol with butyraldehyde reaction in presence of toluene, as solvent. The tests were carried out in a bath reactor at 30, 50 and 70 °C. The conversions and selectivities were correlated with the textural and structural properties of the solids and acidity. The Co/Ti- Al, Ni/Ti-Al, Co/Ce-Mn e Ni/Ce-Mn catalysts achieved maximum conversion of glycerol of 52, 35, 44 e 61%, respectively. The low activation energy value was found over Co/Ti-Al, Ni/Ti-Al, Co/Ce-Mn e Ni/Ce-Mn catalysts Ea values of 5,0, 8,9, 7,3 and 3,3 J·mol-1·K-1 meaning that reactions pathway was changed due to the catalysts sites promoting an alternative mechanism with low energy. Also, all solids selectively produced 2-propyl-1,3- dioxan-5-ol and (2-propil-1,3-dioxolan-4-il)metanol, valuable cyclic acetals.