**6. Conclusion**

In a gas-liquid reaction catalysed by a porous-solid catalyst, one of the most important key parameters which influence the catalyst activity and selectivity is the mass transfer between the multiphases. The reactants which are typically in the gaseous and liquid phase diffuse through the boundary layer (external diffusion), then through the catalyst pore mouths into the internal catalytic surface (internal diffusion). After adsorption on the active surface, the reaction occurs on the specific active sites. Subsequently, the formed products desorb and diffuse to the catalyst pore mouths and finally to the bulk phase.

Numerous reactions are limited by the step of external or internal diffusion. The reaction depends on the parameters involving the diffusion coefficient between gas and liquid phase (*DAB*), the size of the catalyst particle, the pore size diameter, the diffusive surface area, the physical properties (density and viscosity) of the fluids, and the flow conditions (temperature, pressure, and flow rate) of the gas and liquid reactants. The relationships of these parameters are analogous between heat and mass transfer, which can be written as in the dimensionless terms, e.g. Sherwood, Schmidt, and Reynolds numbers. However, these correlated equations are limit in explanation for the multicomponent mixtures; as a result, the Maxwell-Stefan equation is more widely used. Subsequently, the mass transfer factors, e.g. molar flux and effectiveness factors can be determined.

The rate of catalytic reaction regularly decreases with time, due to catalyst deactivation. This deactivation occurs because of three main mechanisms: sintering, fouling, and poisoning. Different deactivation mechanisms affect catalytic rate in different ways.

Different flow characteristics in different reactor types influence the mass transfer between different phases. Hence, for a selection and invention the catalytic reactor, besides the factors, i.e. ensuring a given yield include the volume, flow rate, heat-exchange surface, rate of catalyst substitution, and various structural parameters (particularly in the case of highpressure reactors), the obligatory need involves the use of experimental data on the kinetics of reactions, catalytic poisoning, and the rates of heat and mass transfer (particularly the effectiveness factor).

For reactions involving gaseous, liquid, and solid (as the catalyst) phases, a high interface area can be achieved by dispersing one of the reactants as in the TBR or bubble operation reactor. The partial catalyst wetting is a common phenomenon found in TBR, which is an important effect on the rate of catalytic reaction. In order to calculate the overall effectiveness factor, the important step is to consider the phase of reactant which limits the catalyst wetting.

#### **7. References**

Bird, R.B., Stewart, W.E. & Lightfoot, E.N. (2002). *Transport Phenomena* (2nd Edition), pp. 679-689, John Wiley & Son, ISBN 0-471-41077-2, New York

Dittmeyer, R. & Emig, G. (2008). Simultaneous Heat and Mass Transfer and Chemical Reaction. In: *Handbook of Heterogeneous Catalysis*, Vol.1, G. Ertl, H. Knözinger, F.

effectiveness factor depend on the limiting phase (gas or liquid) and completion of wetting in the catalyst pellet, which the more detail are documented elsewhere (as cited in Lemcoff

In a gas-liquid reaction catalysed by a porous-solid catalyst, one of the most important key parameters which influence the catalyst activity and selectivity is the mass transfer between the multiphases. The reactants which are typically in the gaseous and liquid phase diffuse through the boundary layer (external diffusion), then through the catalyst pore mouths into the internal catalytic surface (internal diffusion). After adsorption on the active surface, the reaction occurs on the specific active sites. Subsequently, the formed products desorb and

Numerous reactions are limited by the step of external or internal diffusion. The reaction depends on the parameters involving the diffusion coefficient between gas and liquid phase (*DAB*), the size of the catalyst particle, the pore size diameter, the diffusive surface area, the physical properties (density and viscosity) of the fluids, and the flow conditions (temperature, pressure, and flow rate) of the gas and liquid reactants. The relationships of these parameters are analogous between heat and mass transfer, which can be written as in the dimensionless terms, e.g. Sherwood, Schmidt, and Reynolds numbers. However, these correlated equations are limit in explanation for the multicomponent mixtures; as a result, the Maxwell-Stefan equation is more widely used. Subsequently, the mass transfer factors,

The rate of catalytic reaction regularly decreases with time, due to catalyst deactivation. This deactivation occurs because of three main mechanisms: sintering, fouling, and poisoning.

Different flow characteristics in different reactor types influence the mass transfer between different phases. Hence, for a selection and invention the catalytic reactor, besides the factors, i.e. ensuring a given yield include the volume, flow rate, heat-exchange surface, rate of catalyst substitution, and various structural parameters (particularly in the case of highpressure reactors), the obligatory need involves the use of experimental data on the kinetics of reactions, catalytic poisoning, and the rates of heat and mass transfer (particularly the

For reactions involving gaseous, liquid, and solid (as the catalyst) phases, a high interface area can be achieved by dispersing one of the reactants as in the TBR or bubble operation reactor. The partial catalyst wetting is a common phenomenon found in TBR, which is an important effect on the rate of catalytic reaction. In order to calculate the overall effectiveness factor, the important step is to consider the phase of reactant which limits the

Bird, R.B., Stewart, W.E. & Lightfoot, E.N. (2002). *Transport Phenomena* (2nd Edition), pp.

Dittmeyer, R. & Emig, G. (2008). Simultaneous Heat and Mass Transfer and Chemical

Reaction. In: *Handbook of Heterogeneous Catalysis*, Vol.1, G. Ertl, H. Knözinger, F.

679-689, John Wiley & Son, ISBN 0-471-41077-2, New York

et al., 1988; Sakornwimon & Sylvester, 1982).

diffuse to the catalyst pore mouths and finally to the bulk phase.

e.g. molar flux and effectiveness factors can be determined.

Different deactivation mechanisms affect catalytic rate in different ways.

**6. Conclusion** 

effectiveness factor).

catalyst wetting.

**7. References** 

Schüth, J. Weitkamp, (Eds.), pp. 1727-1784, ISBN-13: 978-3-527-31241-2, Wiley-VCH, Weinheim


**30** 

*Republic of China* 

**Role of Mass Transfer in Phase Transfer** 

P. A. Vivekanand and Maw-Ling Wang

*Department of Environmental Engineering, Safety and Health Hungkuang University; Shalu District, Taichung 43302 Taiwan,* 

**Catalytic Heterogeneous Reaction Systems** 

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

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,

for the mass-transfer resistance in the continuous phase.

**1. Introduction** 

