Catalysis

Catalysis is central to green synthetic chemistry, presenting an opportunity for substantial savings in materials, energy, and costs. Compared to stoichiometric reagents which need to be used in comparable quantities to the reactants, catalyst molecules can perform multiple transformations and so can be used in lower quantities. Frequently the amount of catalyst needed is less than 1% of the reactants. Catalysts can also enhance product selectivity, offering control over diastereo- and regio-selectivity.

Choosing catalytic over stoichiometric methods significantly improves mass metrics, and substantially enhances both the economic and environmental aspects of chemical synthesis (Sheldon et al., 2007; Sheldon, 2012). Factors to consider when selecting a catalyst with environmentally friendly characteristics include:

Catalyst efficiency: This encompasses factors like product yield, turnover number (TON), and space-time yield, which determine how effective the catalyst is in facilitating the desired reaction (Sheldon, 2012).

Homogeneous vs. heterogeneous catalysts: Catalysts can either be dissolved in a solvent alongside the reactants, or otherwise in the same phase together (homogeneous), or present as a separate phase (heterogeneous). Homogeneous catalysis is more common in fine chemical (including pharmaceutical) synthesis because it offers higher activity and selectivity. Heterogeneous catalysis is prevalent in commodity chemical production where often gaseous reactants come into contact with a solid catalyst (Sheldon et al., 2007).

Heterogenisation: This refers to the process of converting homogeneous catalysts into heterogeneous forms by entrapping or grafting active molecules onto solid supports like silica, alumina, and ceria. These are often referred to as immobilised catalysts (Fang et al., 2023). This approach can impart some of the benefits of heterogeneous catalysts to highly active homogeneous catalysts, providing ease of operational use and facile recycling.

Chemical vs. biological catalysts (enzymes):  Chemical catalysts may rely on rare and precious heavy metals to achieve specific reactions, which can have environmental implications. In contrast, biological catalysts, such as enzymes, often work under milder conditions with higher selectivity (Su et al., 2010).