Selective Hydrogenation in Supercritical Carbon Dioxide Using Metal Supported Heterogeneous Catalyst

Abstract

The use of supercritical carbon dioxide as a reaction medium was tested on the selective hydrogenation of different functional groups using heterogeneous catalyst. Supercritical carbon dioxide offers opportunity not only to replace conventional hazardous organic solvents but also it could improve catalytic activity and selectivity, thus, produces least waste. In this chapter, we highlighted various examples of the selective hydrogenation of different functional groups such as −C=O, −C=C, −CN and −NO2. In addition, selective hydrogenation of conjugated and isolated −C=C and aromatic ring were also presented. Most of the reactions are capitalizes on (i) miscibility of the reactant gas (hydrogen) in supercritical carbon dioxide, which could enhance the reaction rate by eliminating mass transfer limitation, generally occurred in the liquid phase reaction. (ii) Enhancement of selectivity because of the tunable solvent properties arises with potentially small changes in the operating condition. (iii) Interaction with substrate: sometimes CO2 can act as strong protective agent to prevent further hydrogenation of amine as it has tendency to interact with amine to form carbamic acid. Moreover, (iv) easy product separation and (v) longer catalyst lifetime; coke formation can be prevented. We also attempted to get an insight into the molecular level interaction between substrate−Catalyst or substrate−CO2 interaction via theoretical calculation in combination with experimental observation.