Abstract
Copper nanoparticles (NPs) and ZrO2-supported copper NPs (Cu NPs/ZrO2) were synthesized via a chemical reduction method applying different pH (4, 7 and 9) and evaluated in a glycerol dehydration reaction. Copper NPs were characterized with transmission electron microscopy (TEM) and UV–vis spectroscopy. Transmission electron microcopy (TEM) results revealed a homogeneous distribution of copper NPs. A hypsochromic shift was identified with UV–vis spectroscopy as the pH of the synthesis increased from pH = 4 to pH = 9. Zirconia-supported copper NPs catalysts were characterized using N2 physisorption, X-ray diffraction (XRD), TEM, X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), temperature-programmed desorption of ammonia (NH3-TPD) and N2O chemisorption. The presence of ZrO2 in the chemical reduction method confirmed the dispersion of the copper nanoparticles. X-ray diffraction indicated only the presence of tetragonal zirconia patterns in the catalysts. XPS identified the Cu/Zr surface atomic ratio of the catalysts. TPR patterns showed two main peaks for the Cu NPS/ZrO2 pH = 9 catalyst; the first peak between 125 and 180 °C (region I) was ascribed to more dispersed copper species, and the second one between 180 and 250 °C (region II) was assigned to bulk CuO. The catalysts prepared at pH = 4 and pH = 7 only revealed reduction at lower temperatures (region I). Copper dispersion was determined by N2O chemisorption. With NH3-TPD it was found that Cu NPs/ZrO2 pH = 9 exhibited the highest total quantity of acidic sites and the highest apparent kinetic constant, with a value of 0.004 min−1. The different pH applied to the synthesis media of the copper nanoparticles determined the resultant copper dispersion on the ZrO2 support, providing active domains for glycerol conversion.
Highlights
The dark spots correspond to the copper NPs and the lighter background corresponds to the carbon matrix of the sample holder
The copper nanoparticles were homogeneously dispersed in all experimental conditions
Copper NP catalysts with varying synthesized pH were deposited on ZrO2 after preparation via the chemical reduction method
Summary
Glycerol generated from biodiesel is a platform molecule with the potential to carry out transformation towards other products with higher value-addition. Several studies have been carried out on glycerol dehydration reactions on different metals and supports [15,16,17,18,19]. Three main products are formed by glycerol dehydration: acetol, 3-hydroxypropanal and its subsequent dehydration to acrolein [20,21]. The dehydration routes to acetol and acrolein are competitive. Acetol can be formed via direct dehydration of glycerol and subsequent keto–enol tautomerization. The acrolein pathway proceeds through the dehydrogenation–hydrogenation pathway of 1,3 propanediol (1,3-PDO). The acetol pathway is promoted by acidic Lewis sites and the acrolein pathway is promoted by acidic Brönsted–Lowry sites
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