Introduction to Catalysis

Catalysis, one of the overarching principles of green chemistry, is capable of producing substantial material and energy savings as well as economic benefits.  When comparing catalytic methods to traditional stoichiometric syntheses, catalysis offers undisputable economy in reagent use. Stoichiometric reactions would require at least one mole of reagent per mole of starting material, whereas a catalyst, subject to turnover number (TON), can carry out a transformation multiple times per mole of catalyst.  Catalytic reagents can be used to improve product selectivity, by enabling diastereomeric control and site specific transformations in multi-functionalised molecules.  Catalysts also allow reactions to proceed under milder reaction conditions and catalytic methods can circumvent the need for pre-functionalisation of the starting materials with activating or directing groups, which would later need to be removed from the final molecule. Thus the use of catalytic rather than stoichiometric methods can dramatically enhance the atom economy of a synthesis, save time and energy, as well as decreasing the amount of raw materials consumed and waste generated. The development and application of catalytic methods can significantly improve both the economic and environmental profile when designing synthetic routes within process research and development (R&D).  [1].  

There are several factors to consider when selecting a catalyst in terms of its green credentials, including:

• Catalyst efficacy eg. product yield, Turnover number (TON), space-time yield etc.
• Homogeneous or heterogeneous catalysts: while homogeneous catalysis tends to afford high activity and selectivity, heterogeneous systems benefit from ease of operational use and catalyst recovery and recycling, which has obvious advantages in terms of economy and sustainability. Catalysts can be heterogenised by the entrapping or grafting of the active molecules onto the surfaces or pores of solid supports such silica, alumina and ceria among others (Also known as immobilised catalysts).
• Chemical catalysts vs biological catalysts (enzymes). Chemical catalysts can often rely on the usage of rare and precious heavy metals (see Critical Elements module) to achieve the desired chemistry whilst biological catalysts can require dilute conditions and operate under a smaller range of solvents.