Cascade engineered phase transfer catalysis: a novel concept in green chemistry

Abstract

End-of-pipe solutions as a regulatory approach have been practised in industry with a view that the technology is perfect and only the effluent should be treated. Although total waste cannot be eliminated, there are several approaches that can be easily adopted by industry to reduce waste and also to improve economics. In the case of industrial phase transfer catalysed processes, the catalyst is not recovered but treated as a waste since the quantities are very small and do not contribute much to the expensive product. Although totally waste-free chemical processes are not realizable in practice, the concept of eco-efficiency is useful for optimizing chemical processes with the aim of minimizing risk, resource depletion, and waste-generation. In this context it is necessary to make the popular phase transfer catalysis (PTC) more acceptable to industry by unusual strategies. So combining several steps in a single-pot reaction and using the same catalyst and solvent without separation or with partial replenishment of the reactants for all the steps fits elegantly into the concept of waste minimization. The use of liquid–liquid–liquid, solid–liquid–liquid PTC, including a change in nature of the phase, reactant, and product state, can be advantageously made in a cascaded reaction. Cascade engineered PTC (CEPTC) will offer the additional benefit of enhancing the selectivity of the desired product through manipulation of various parameters such as the number of phases, catalyst loading, type of catalyst, stoichiometric ratio of the starting materials and temperature. In agrochemicals and pharmaceuticals this approach can be employed where multi-step synthesis coupled with several separation stages add to the cost of the product. This paper discusses the novel concept of CEPTC, which is illustrated with the production of an agrochemical fenvalerate. Here the same catalyst is used without separation and the nature of phases is changed to achieve a reduction of waste. The concept is discussed with experimental data and has a lot of promise.