CHAPTER 6: Heterogeneous Catalysis in Supercritical Carbon Dioxide

This chapter describes the application of heterogeneous catalysts in supercritical carbon dioxide (scCO₂) for a vast array of reactions including the hydrogenation, oxidation, hydrogenolysis and oxidative dehydrogenation of different organic compounds. It also includes prospective reactions and future trends. The combination of heterogeneous catalysts and scCO₂ offers endless opportunities in organic synthesis, including accelerated reaction rates, improved product selectivity, easy product separation, enhanced catalyst lifetime and regeneration. Employment of scCO₂ can overcome the inherent difficulties associated with heterogeneous catalysts in conventional organic solvents such as low reaction rates and catalyst deactivation. The efficiency of solid catalysts is significantly increased through the effective contact between the reactant and catalyst surface, which is strongly promoted by the supercritical solvent. Herein, some examples are given, in which useful strategies have been developed to enhance the catalytic activity and selectivity, particularly in hydrogenation and oxidation, two of the most widely used reactions in scCO₂. Moreover, the applicability of scCO₂ is not restricted simply to the role of a solvent, it also acts as a ‘masking agent’ and ‘hydrogen surrogate’ in reductive amination and dehydrogenation, respectively, offering enormous opportunities in selectivity control when compared to conventional solvents. The real advantage of heterogeneous catalysis in scCO₂ is the minimisation of energy consumption in the product separation step: a facile separation process involving depressurisation and filtration that easily remove the solvent and catalyst, respectively. Additionally, scCO₂ is able to extract and transport the by-products related to coke formation because of its high diffusivity and increase the catalyst lifetime. The diversity of scCO₂ and heterogeneous catalysts must be explored to replace hazardous technologies with clean, green and sustainable processes.