Morphological and chemical influences on alumina-supported palladium catalysts active for the gas phase hydrogenation of crotonaldehyde

A series of five alumina-supported palladium catalysts have previously been prepared and characterised by a combination of CO chemisorption and infrared spectroscopy. The reactive attributes of these catalysts are examined using the hydrogenation of crotonaldehyde as a test reaction, using a modified infrared gas cell as a batch reactor. Periodic scanning of the infrared spectrum of the gaseous phase present over the Pd/Al2O3 catalysts was used to construct reaction profiles. Four of the catalysts were able to facilitate a 2-stage hydrogenation process (crotonaldehyde → butanal → butanol), whilst one catalyst was totally selective for the first stage hydrogenation process (crotonaldehyde → butanal). Rate coefficients for the first and second stage hydrogenation processes are normalised to the number of surface palladium atoms for the particular catalyst. Correlation of these kinetic parameters as a function of mean particle size indicates the first stage process to be structure insensitive, whilst the second stage hydrogenation is structure sensitive. Chlorine residues associated with the preparative process of one of the catalysts is seen to selectively poison the second stage hydrogenation process for that catalyst. Structure/activity relationships are considered to explain the observed trends