Catalyst performance changes induced by palladium phase transformation in the hydrogenation of benzonitrile

The application of a supported palladium catalyst for the hydrogenation of aromatic nitriles

“…This arrangement would permit hydrogenation and hydrogenolysis to occur simultaneously, as is seen in Figure 7. We acknowledge that this is in contrast to what Bakker et al have previously reported [22], where their co-adsorption studies saw a drop in hydrogenation and hydrogenolysis activities, which is attributed to competitive adsorption on the same active sites of the catalyst.…”

“…Given the modest hydrogen overpressure used here (4 bar), it is assumed that the palladium is buffering hydrogen as chemisorbed hydrogen and sub-surface hydrogen [33]. A role for β-hydride formation within this reaction system, as proposed by Bakker and co-workers [22], is also possible.…”

“…Bakker et al looked at this reaction over a Pd/Al 2 O 3 catalyst and reported that toluene can be formed by direct hydrogenolysis of benzonitrile or indirectly from re-adsorbed benzylamine [22].…”

“…However, recent work from Bakker and co-workers has highlighted the importance of this reaction pathway in their study of hydrogenation of benzonitrile over a 5% γ-Al 2 O 3 supported Pd catalyst, where toluene was observed as a by-product [22]. Here, the formation of a β-PdH phase at the catalyst surface, populated at hydrogen pressures in excess of 10 bar hydrogen pressure were thought to be responsible for preventing the hydrogenolysis reaction, that is mirrored by an increase in the formation of the coupling product dibenzylamine [22].…”

“…Here, the formation of a β-PdH phase at the catalyst surface, populated at hydrogen pressures in excess of 10 bar hydrogen pressure were thought to be responsible for preventing the hydrogenolysis reaction, that is mirrored by an increase in the formation of the coupling product dibenzylamine [22]. The formation of β-palladium hydrides has previously been shown to affect the selectivity of other hydrogenation reactions, such as in the hydrogenation of ethyne, were a transition from the alpha to the beta hydride phase decreases ethane selectivity [23].…”