Catalyst Deactivation by Coke Formation. A Study of the Carbonaceous Layer on a Used Hydrogenation Catalyst

Abstract: The carbonaceous layer, formed on a Ni/SiO₂ catalyst, and its influence on the selectivity in a hydrogenation reaction have been studied. The model reaction system studied was the consecutive hydrogenation of 2-ethyl-hexenal:

Exposure of the catalyst to 2-ethyl-hexenal or cyclopentadiene at elevated temperatures before the hydrogenation, yielded an increased selectivity toward the formation of 2-ethyl-hexanal.

The carbonaceous layer was studied with temperature programmed analysis techniques including temperature programmed desorption (TPD), temperature programmed hydrogenation (TPH), and temperature programmed oxidation (TPO). During both TPD and TPH the following species were detected: octenes, 2-ethyl-hexanol, heptenes, 2-ethyl-hexanal, benzene, methyl benzene and ketones. Some of these species were formed during the experiment. The TPD analysis was found to be reproducible but catalysts exposed to different reaction conditions displayed different TPD/TPH curves. By using TPO the amount of carbon on the surface and the H/C ratio of the residues were determined. There were 15-20 carbon atoms per surface nickel atom indicating that a major part of the coke was on the support. The H/C-ratio was 1.5-2.0.

The active surface area of the used catalyst was measured with CO adsorption and by using the H₂/D₂ equilibration reaction as a probe. Both methods gave the same result: that the active area decreased very fast at the beginning of the reaction, to about 40% of the initial active surface area. Then there followed a very slow decrease in surface area. Using the H₂/D₂ equilibration reaction, it was also possible to follow the decrease in free surface area during the reaction.