Role of Mass Transfer in Phase Transfer Catalytic Heterogeneous Reaction Systems

Abstract

Conventional techniques [1] for removing the constraints of mutual insolubility of aqueous phase with organic phase are industrially unattractive and polluting. A plausible technique now widely known as ‘‘phase transfer catalysis” (PTC) has emerged as a broadly useful tool [2–7] in solving the predicament of insolubility of aqueous phase with organic phase. In this methodology, involving a substrate (in the organic layer) and an anionic reagent or a nucleophile (in the aqueous layer), reacting anions are continuously introduced into the organic phase. Currently, PTC is an important choice in organic synthesis and is widely applied in the manufacturing processes of specialty chemicals, such as pharmaceuticals, perfumes, dyes, additives, pesticides, and monomers. Further, the recent tendency toward “green and sustainable chemistry” has again attracted strong attention to this technique [8-13]. In the last five decades, a steadily increasing number of papers and patents dealing with phase transfer topics and related to their applications have been published in the literature [14-30]. It is understood that the complicated nature of the PTC system stems from the two masstransfer steps and two reaction steps in the organic and aqueous phases. In addition, the equilibrium partitions of the catalysts between two phases also affect the reaction rate. The difficulty in realizing the mass-transfer rates of catalysts between two phases is probably due to the uneasy identification of the catalyst (or intermediate product) during reactions. Inoue et al. [31] investigated mass transfer accompanied by chemical reaction at the surface of a single droplet. They studied the mass-transfer effect for both neglecting and accounting for the mass-transfer resistance in the continuous phase. Wang and Yang [32] investigated the dynamic behavior of phase transfercatalyzed reactions by determining the parameters accounting for mass transfer and the kinetics in a two-phase system. However, the main disadvantage of PTC in the industrial application of soluble phase-transfer catalyst (PTC) applications, such as quaternary ammonium salts, is the need to separate the catalysts from the reaction mixture and its subsequent reuse or disposal. Hence, from industrial point of view, polymer-anchored catalyst is more desirable in order to simplify catalyst separation from the reaction mixture and its reuse thereby the need for complex chromatographic techniques can be avoided for product separation and isolation [33–38]. To circumvent the problem of separation of catalyst from the reaction mixture, for the first time Regen [39] reported anchoring the phase transfer catalysts to a polymer backbone and suggested the name “Triphase Catalysis”. Quaternary onium salts,