Selective Hydrogenation of Acrolein over Supported Silver and Silver Alloy Catalysts

This thesis aims to investigate multiple critical properties of catalysts which can affect the activity and selectivity greatly in heterogeneous catalysis. One reaction of interest was chosen as a model reaction for this study: the selective hydrogenation of α,β-unsaturated aldehyde to α,β-unsaturated alcohol (using acrolein as a model reactant). For the selective hydrogenation of acrolein to allyl alcohol, it is a particularly difficult reaction to obtain selectivity towards C=O bond hydrogenation as the hydrogenation of C=C bond is easier than that of C=O bond thermodynamically. The lack of substituents at C=C bond makes it especially vulnerable to hydrogenation. Thus kinetic control in this system is required. Previous research has made great progress in understanding some of the factors (choice of metal, process conditions) which may improve selectivity. However, no catalyst exists with both high activity and selectivity. Therefore we studied this reaction systematically from four aspects: 1) particle size effects; 2) support effects; 3) alloy effects (utilizing single atom alloy to improve the performance of catalysts); and 4) employing single site heterogeneous catalysts. We found that both selectivity and activity of the catalyst increased as the particle size increased for Ag/SiO2 catalysts. It is speculated that the adsorption mode of the acrolein molecule is critical to high selectivity and that the extended terraces of the larger particles allows for favorable adsorption configurations. Although this trend in selectivity was found to extend to other supports (Al2O3, TiO2), the activity of Ag/Al2O3 dropped with increasing particle size. This could be related to activation (and subsequent spillover) of H2 on the support. Further demonstrating the importance of H2 activation in this reaction, the activity of dilute alloys was assessed. In this case, very low loadings of a d-band transition metal (eg. Pd) was impregnated into the Ag nanoparticle. Even at loadings of 0.01wt%, the presence of Pd had a profound effect on the catalyst performance, greatly increasing activity. Finally, we have evaluated a novel Zn2+ single site catalyst for acrolein hydrogenation which must activate hydrogen through a completely different mechanism. Suggestions for further experiments will be discussed

Single-Atom Alloy Pd–Ag Catalyst for Selective Hydrogenation of Acrolein

Pd–Ag alloy catalysts with very dilute amounts of Pd were synthesized. EXAFS results demonstrated that when the concentration of Pd was as low as 0.01 wt %, Pd was completely dispersed as isolated single atoms in Ag nanoparticles. The activity for the hydrogenation of acrolein was improved by the presence of these isolated Pd atoms due to the creation of sites with lower activation energy for H₂ dissociation. In addition, for the same particle size, the 0.01% Pd/8% Ag alloy nanoparticles exhibited higher selectivity than their monometallic counterparts, suggesting that the Pd atom may act as a site for the favorable bonding of the acrolein molecule for facile hydrogenation of the aldehyde functionality

Direct Synthesis of Bimetallic Pd₃Ag Nanoalloys from Bulk Pd₃Ag Alloy

We report a transformative, all inorganic synthesis method of preparing supported bimetallic Pd₃Ag alloy nanoparticles. The method involves breaking down bulk Pd₃Ag alloy into the nanoparticles in liquid lithium, converting metallic Li to LiOH, and transferring Pd₃Ag nanoparticles/LiOH mixture onto non-water-soluble supports, followed by leaching off the LiOH with water under ambient conditions. The size of the resulting Pd₃Ag nanoparticles was found narrowly distributed around 2.3 nm characterized by transmission electron microscope (TEM). In addition, studies by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, and X-ray absorption near edge structure (XANES) spectroscopy showed that the resulting Pd₃Ag nanoparticles inherited similar atomic ratio and alloy structure as the starting material. The synthesized Pd₃Ag nanoparticles exhibited excellent catalytic activity toward hydrogenation of acrolein to propanal