Copper catalysts for the conversion of polyunsaturated hydrocarbons

This thesis covers the activity, stability, and selectivity of supported Cu catalysts for relevant gas- and liquid-phase selective hydrogenation reactions. The aim was to gain insight into the reaction mechanisms and kinetics, develop a reliable testing methodology, and understand the mechanism governing catalyst selectivity. Chapter 1 provides a general overview on Cu nanoparticles, on the basics of nanoparticle catalysis and synthesis methodologies. A brief review on particle size effects is provided. Lastly, a detailed introduction to the selective hydrogenation of 1,3-butadiene, from a molecular point of view to practical applications, is given. Chapter 2 focuses on the use of nanoparticulate Cu in the selective hydrogenation of 1,3-butadiene in the presence of a 100-fold excess of propene. The reaction is particularly relevant for the production of high-quality olefins and more complex products. We demonstrate that the use of inert supports and accurate particle size control via incipient wetness impregnation, are valuable strategies to produce highly selective to butenes (~99%) and stable Cu nanoparticulate catalysts. Chapter 3 focuses on particle size effect on activity and selectivity of Cu nanoparticles in the selective hydrogenation of 1,3-butadiene. The activities and selectivities of 2, 3, 4, 7 and 10 nm Cu nanoparticles supported on SiO2 are reported. Maximum surface-normalized activity was observed for 4 to 7 nm Cu, which were 3 to 4 times more active than 2 nm particles. This higher activity was ascribed to the larger fraction of kinks and steps sites, which are essential to activate hydrogen (rate limiting step for the selective conversion of 1,3-butadiene). Particles of 4 and 7 nm were also the ones showing highest selectivity. Kinetic analysis of propene hydrogenation, adsorption tests and calorimetric experiments, established that the selectivity is due to the preferential adsorption of 1,3-butadiene with respect to propene and butenes on the Cu surface. Chapter 4 presents a preliminary investigation of the effect of common metal/metal oxide additives for Cu-based hydrogenation catalysts, in this case Mn, Fe or Zn oxide. The experiments showed that Fe oxides mildly suppressed Cu hydrogenation activity and decreased Cu selectivity already at very low concentration of modifier, i.e., 1:99 Fe:Cu atomic ratio. The addition of Zn oxides did not have a significant effect on total hydrogenation activity but lead to a strong decrease in catalyst selectivity. Mn oxides, on the other hand, improved the catalyst activity 2 to 3-fold, while completely retaining the high selectivity to butenes of the unmodified sample. Chapter 5 moves from selective hydrogenation in gas phase, to the same type of reaction in liquid phase. Cu/SiO2 was tested for the selective (partial) hydrogenation of alkynols to alkenols, in this case 2-methyl-3-butyn-2-ol to 2-methyl-3-buten-2-ol. We show that 7 nm Cu nanoparticles are 2.5 times more active (surface normalized) than 2 nm ones. At the same time, the larger particles showed much higher selectivities, around ~100% at 50% conversion (Treaction=140 °C). The knowledge gathered in this thesis may guide both testing and Cu catalysts design for reactions where hydrogen surface availability and selectivity play a key role.