SOLID PHASE HYDROGENATION OF PHENYL ACETYLENES BY HYDROGEN SPILLOVER: THE IMPACT OF HYDROGENATION PRESSURE AND SPILLOVER MEDIA ON PRODUCT DISTRIBUTION

Hydrogenation in the solid phase is important in “green” synthesis, for the use of solid, recyclable catalysts as well as in the field of hydrogen scavenging, where it is necessary to prevent the risks of explosion, metal corrosion or embrittlement, due to its reaction with hydrogen.

Two solid phenyl-acetylenes with a Pd/C catalyst were hydrogenated to various degrees at different initial pressures; the partially hydrogenated samples were analyzed by gas-chromatography, to quantify the reaction products.

Comparison of the products’ distribution at different hydrogenation pressures and at different stages of the hydrogenation reaction, gives insight into the mechanism of hydrogenation in the solid phase.

We claim that the hydrogen travels from the metal catalyst to the substrate by a mechanism known as hydrogen spillover, that is, the hydrogen migrates across the catalyst’s carbon support.

The results of the study show that the reaction intermediates prefer syn-addition; and the amounts of these products are greatly affected by the hydrogenation pressure.

Addition of carbon nanotubes to the solid reaction media was previously shown to increase the reaction rate. In this study, the influence of carbon nanotubes (CNTs) on the products’ distribution shows trends similar to the hydrogenation pressure increase. We believe that the CNTs, which are known as spillover agents, facilitate the migration of hydrogen from the metal catalyst to the solid substrate and increase the distance that activated hydrogen species can travel. This phenomenon could be described as reducing the energy barrier for the diffusion of the hydrogen species.

In summary, by studying the product distribution of the solid phase hydrogenation at different pressures with the addition of spillover agents, one gains insight into the reaction mechanism.