Selective Hydrogenation of Acrolein over Supported Silver and Silver Alloy Catalysts
thesisposted on 28.06.2013 by Haojuan Wei
In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.
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.