Hydrogenation of Benzonitrile over Supported Pd Catalysts: Kinetic and Mechanistic Insight

Abstract

The liquid phase hydrogenation of benzonitrile over a 5 wt % Pd/C catalyst using a stirred autoclave is investigated. The reaction conforms to a consecutive reaction sequence: first benzonitrile is hydrogenated to produce benzylamine, which subsequently undergoes a hydrogenolysis step to form toluene. Benzonitrile hydrogenation obeys first-order kinetics with an activation energy of 27.6 kJ mol–1. In contrast, the benzylamine hydrogenolysis stage obeys zero-order kinetics and exhibits an activation energy of 80.1 kJ mol–1. A 1 wt % Pd/Al2O3 catalyst is additionally examined, which is also seen to support hydrogenolysis activity alongside the hydrogenation pathway. Gas phase transmission infrared spectroscopic measurements of the hydrogenation of benzonitrile and benzylamine over the 1 wt % Pd/Al2O3 catalyst utilizing hydrogen and deuterium are undertaken, which enable reaction schemes incorporating adsorption geometries of intermediate adsorption complexes to be proposed.

The hydrogenation of mandelonitrile over a Pd/C catalyst: towards a mechanistic understanding

“…However, based on previous investigations by McMillan and co-workers into the hydrogenolysis of benzylamine, where the loss of ammonia was shown to be facilitated by a hydrogenolytic step, it is proposed that for the mandelonitrile system, it is a corresponding hydrogenolysis mechanism which facilitates the loss of water. 19,20 Thus, with reference to Scheme 2, two possible routes to the desired product may be considered. 25 Moreover, Scheme 2 highlights that the order in which the hydrogenation and hydrogenolysis steps take place dictates the intermediate species observed.…”

“…25 Moreover, Scheme 2 highlights that the order in which the hydrogenation and hydrogenolysis steps take place dictates the intermediate species observed. With a thorough exploration of the hydrogenation of benzonitrile already undertaken, 19,20 and as Scheme 2 suggests, a simple consecutive process from nitrile to target amine was initially envisaged.…”

“…17,18 Despite this, production of the hydrogenolysis product under mild conditions has been reported, for example, by Maschmeyer et al, 17 through the study of benzonitrile over a Pd/C catalyst. Whilst there are several instances where hydrogenolytic removal of a functional group is undesirable, such as the hydrogenolysis of benzylamine to yield toluene, 19,20 there are occasions where no hydrogenolysis of the amine functionality is observed. 21 Adding a further degree of complication, there are instances where the selective hydrogenolysis of one functional group in the presence of another is required.…”

Application of supported lanthanum catalysts in the hydrogenation of nitriles

“…Usually a Raney® nickel [17,19,20] or rhodium catalyst [18] is required to achieve high primary amine selectivity. However, more and more methods have been elaborated for the selective hydrogenation of benzonitriles using nickel [21][22][23] ruthenium [24][25][26] and palladium [27][28][29][30][31] catalysts.…”

Boosting the Performance of Nano-Ni Catalysts by Palladium Doping in Flow Hydrogenation of Sulcatone

“…Currently, liquid-phase hydrogenations are carried out on large-volume batch reactors [39] that present several disadvantages, such as low atom efficiency, complicated reaction schemes, low versatility, problems with catalyst separation, and overall product quality and specificity. It is not surprising that flow catalytic hydrogenation has received renewed interest by industry and academia in the last decade [40].…”