**Role of Mass Transfer in Phase Transfer Catalytic Heterogeneous Reaction Systems**

P. A. Vivekanand and Maw-Ling Wang *Department of Environmental Engineering, Safety and Health Hungkuang University; Shalu District, Taichung 43302 Taiwan, Republic of China* 

### **1. Introduction**

684 Mass Transfer - Advanced Aspects

Sakornwimon, W. & Sylvester, N.D. (1982), Effectiveness Factors for Partially Wetted

Thomas, J.M. & Thomas, W.J. (1997). *Principles and Practice of Heterogeneous Catalysis*, VCH,

*and Development*, Vol.21, No.1, (January 1982), pp. 16-25, ISSN: 0196-4305 Satterfield, C.N. (1970). *Mass Transfer in Heterogeneous Catalysis*, MIT Press (MA), ISBN 0-26-

219062-1, Cambridge

ISBN 03-527-29288-8, Weinheim, New York

Catalysts in Trickle-Bed Reactors, *Industrial and Engineering Chemistry Process Design* 

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 transfer- catalyzed 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,

Role of Mass Transfer in Phase Transfer Catalytic Heterogeneous Reaction Systems 687

In order to formulate a mathematical model to describe the dynamic behavior of the twophase reaction shown above, a two-film theory is employed to consider the mass transfer of the catalysts between two phases. Hence, those equations which model the two-phase reaction are presented below. The rate of change for ArOQ in the organic phase is the

*ArOQ aq ArOQ org org*

⎝ ⎠

The rate of change for ArOQ in the aqueous phase is the difference of aqueous-phase

*aq org ArOQ aq aq aq ArOQ org ArOK QBr ArOQ ArOQ*

Similarly, the rate of change for QBr either in the organic phase or in the aqueous phase is

( ) *org QBr org org org aq org ArOQ RBr QBr QBr QBr QBr*

*QBr org aq aq aq QBr QBr QBr QBr aq ArOK QBr*

*ArOK aq aq*

*RBr org org org ArOQ RBr*

In the above equations, "f" is defined as the ratio of the volume of organic phase (V0) to the

*ArOQ aq s*

*C*

*C*

*C*

*o a V f V*

> ( ) ( ) *org s ArOQ*

> *ArOQ*

( ) ( ) *org s QBr QBr aq s QBr*

*dC KC C*

*dC KC C*

*aq ArOK QBr*

*K C C K AC m C*

*dC C K C C K Af C dt <sup>m</sup>*

( ) *aq*

*aq*

*org*

The distribution coefficients of catalysts mArOQ and mQBr are defined as

*m*

*m*

⎛ ⎞ = −− ⎜ ⎟

*K AC KC C*

⎛ ⎞ =− − ⎜ ⎟

*ArOQ ArOQ org RBr ArOQ ArOQ*

(1)

(2)

*ArOQ*

⎝ ⎠

*dt* = − (5)

*dt* = − (6)

= (7)

= (8)

*<sup>C</sup>* <sup>=</sup> (9)

*dt* = −− (3)

*K Af C m C K C C dt* = −− (4)

difference of mass-transfer rate and organic-phase reaction rate.

reaction rate and mass transfer rate.

obtained as shown in (3) and (4).

*dC*

*dC*

The reaction rate of ArOK in the aqueous phase is

The reaction rate of RBr in the organic phase is

volume of aqueous phase (Va), *i.e.*

*org org*

*dt m*

*dC C*

crown ethers and cryptands have all been immobilized on various kinds of supports, including polymers (most commonly (methylstyrene-costyrene) resin crosslinked with divinylbenzene), alumina, silica gel, clays, and zeolites [40–51]. Kinetics of triphase phasetransfer-catalyzed reactions [52] are influenced by (i) mass transfer of reactant from bulk liquid to catalyst surface; (ii) diffusion of reactant through polymer matrix to active site; (iii) intrinsic reaction rate at active site; (iv) diffusion of product through polymer matrix and mass transfer of product to external solution; (v) rate of ion exchange at active site. In heterogeneous conditions, for a proper mass transfer to occur, both the liquid phases should be in contact with catalyst. Thus, mass transfer of reactant from bulk solution to catalyst surface and mass transfer of the product to the bulk solution are the significant steps involved. The reaction mechanism of these PTC's system is often complicated and several factors affect the conversion of reactants. With all these antecedents, in this chapter, a kinetic and mathematical model of phase-transfer catalysis concerning mass transfer with various organic reactions will be presented. An extensive detail has been made on the effects of mass transfer in the PTC reaction systems. Further, it is proposed to present the diffusion resistance of an active phase-transfer catalyst in the organic phase and mass-transfer resistance between the droplet and the bulk aqueous phase.
