Toward Sustained Product Formation in the Liquid-Phase Hydrogenation of Mandelonitrile over a Pd/C Catalyst

Abstract

The liquid-phase hydrogenation of the aromatic cyanohydrin mandelonitrile (MN, C 6 H 5 CH(OH)CN) over a carbon-supported Pd catalyst to produce the primary amine phenethylamine (PEA, C 6 H 5 CH 2 CH 2 NH 2 ) is investigated with respect to the transition from operation in single-batch mode to repeat-batch mode. While a single-batch reaction returns a complete mass balance, product analysis alongside mass balance measurements for a six-addition repeat-batch procedure shows an attenuation in the rate of product formation and an incomplete mass balance from the fourth addition onward. This scenario potentially hinders possible commercial operation of the phenethylamine synthesis process, so it is investigated further. With reference to a previously reported reaction scheme, the prospects of sustained catalytic performance are examined in terms of acid concentration, stirrer agitation rate, catalyst mass, and hydrogen availability. Gas−liquid mass transfer coefficient measurements indicate efficient gas → liquid transfer kinetics within the experimental constraints of the Henry's law limitation on hydrogen solubility in the process solvent (methanol). Deviations from the optimized product selectivity are attributed to mass transport constraints, specifically the H 2 (solv) → 2H(ads) transition, which is ultimately restrained by the availability of H 2 (solv). Finally, in an attempt to better understand the deactivation pathways, inelastic neutron scattering measurements on a comparable industrial-grade catalyst operated in an analogous reaction in fed-batch mode indicate the presence of an oligomeric overlayer postreaction. This overlayer is thought to be formed via oligomerization of hydroxyimine or imine species via specific pathways that are identified within a postulated global reaction scheme.

Hydrogenation on Palladium Nanoparticles Supported by Graphene Nanoplatelets

“…Supported Pd catalysts are particularly important for industrial hydrogenation reactions, including fine chemicals synthesis. − Palladium is considered the most selective among the platinum metals, as hydrogenation is much faster than dehydrogenation to undesired carbonaceous species. High dispersion (mean Pd particle sizes below 3 nm) is a prime asset of these catalysts.…”

Best Practices for the Collection of Robust Time Course Reaction Profiles for Kinetic Studies

“…An inability to definitively identify and monitor all mass balance contributors is common in reaction monitoring. − This issue often goes hand in hand with analytical stability issues. Another reason for decreasing the mass balance over time could be precipitation of a species.…”

“…Alternatively, if the choice of analytics was insufficient for monitoring the reaction components, then an additional reaction monitoring method should ideally be found. If it is not possible to monitor all of the reaction components, a meaningful mechanistic and kinetic study can still be conducted. − Once another set of “re-optimized” reaction conditions has been found and changes to the sampling or analytical methods made, another time course reaction profile should be taken. The process of optimizing reaction conditions, sampling/analytical methods, and collecting a time course profile should be iterated until satisfactory reaction conditions are found.…”

Continuous Hydrogenation of Nitriles to Primary Amines with High Selectivity in Flow by Using Raney Cobalt

“…Nowadays, due to its simple structure and mature technology, the batch reactor is widely used in the industrial production process [10b] . Currently, the majority of catalytic hydrogenation of nitriles in the industry still adopts the batch synthesis method in a reactor, in view of its technological advancement and simple structure.…”