|The current demand for mono-unsaturated hydrocarbons is large and it is expected that it will keep growing. To produce these alkenes more purely, selective hydrogenation is needed to remove small amounts of alkadiene or alkyne impurities in the stream. This is done by using supported Pd catalysts, which are very active for this reaction, but not as selective. To improve the selectivity of the catalyst, the Pd metal can be promoted or alloyed with other metals. The aim of this research is to study the effects of K, Mn, Cu, Zn and Ag promoters on Pd catalysts in the selective hydrogenation of poly-unsaturated hydrocarbons. To investigate this, the selective hydrogenation of a 1,3 butadiene impurity in a propylene feed is used as a model reaction.
A synthesis method has been established to prepare comparable promoted catalysts, namely sequential incipient wetness impregnation with a subsequent reduction. Promoted Pd catalysts are obtained with the desired mol ratio of promoter metal to Pd of 1:10. Electron Microscopy (EM) analysis showed that the metal nanoparticles have an uniform distribution over the support and the particles have a surface averaged particle size between 6 and 11 nm. With Temperature Programmed Reduction (TPR) analysis it was determined that Pd promotes the reduction of ZnO and possibly CuO, hence ZnO and potentially CuO are probably in close proximity to Pd. X ray Diffraction (XRD) characterisation showed that there was a lattice contraction for the Mn, Ag, Zn and Cu promoted catalysts of 0.46, 0.51, 0.91 and 0.97%, respectively. This suggests that the metals might be incorporated into the Pd crystal lattice.
Overall, the monometallic Pd catalyst yielded the highest activity and total butene selectivity in the selective hydrogenation of butadiene. Only at room temperature, the K-Pd/C catalyst exhibited an increase in Turnover Frequency (TOF) from 25 to 41 s-1 compared to the Pd/C catalyst. Nevertheless, all catalysts still displayed a high activity (>100 s-1 at 90 °C) for the hydrogenation of butadiene in comparison to other metals. The Ag promoted catalyst exhibited the highest total butene selectivity among the promoted catalysts, which was just a little bit lower than the monometallic Pd catalyst. The Ag promoted Pd catalyst also showed a higher 1-butene selectivity than the monometallic Pd catalyst, both at low and high conversion levels. The Zn and Cu promoted samples also exhibit a higher 1 butene selectivity, in contrast to the K and Mn promoted samples, which showed a decrease at higher conversion. This effect on the 1-butene selectivity is explained by the isomerisation activity of the catalysts. The Pd catalyst displays a high isomerisation activity, while the Ag and Zn promoted catalysts show a significant lower 1-butene TOF over the whole temperature range (22-150 °C).
The (isomerisation) activity and selectivity were compared to the relative electronegativity (ΔEN), van der Waals radius (rvdw) of the promoter metals and the XRD derived lattice contraction of each catalyst. The butadiene and 1-butene TOF did not seem to correlate with the ΔEN or the lattice contraction. However, when the promoter metal has a smaller rvdw, both the butadiene and 1-butene TOF seem to increase. The rvdw did also seem to have an influence on the selectivity to all butenes and 1-butene; the larger the promoter atom, the higher the selectivity. Using a promoter metal with a lower electronegativity seemed to increase the total and 1-butene selectivity. No distinct correlation is found between the lattice contraction and the selectivity.
Oxidation (O), reduction (R), oxidation-reduction (OR) and reduction-oxidation-reduction (ROR) pre-treatments (PTs) are performed before the catalytic test of the Mn promoted Pd catalyst. All PTs decreased the temperature that was needed to reach full conversion, compared to the catalys