**3. Conclusion**

704 Mass Transfer - Advanced Aspects

attributed to low mass transfer of reactants from the bulk phase to the particle surface and hence the reaction rate is reduced. However the reaction rate is better than the uncatalyzed reaction. In presence of highly active catalyst, mass transfer from the bulk phase to the surface of the catalyst particle can be rate controlling step. They reported that the mass transfer of benzoyl chloride from the bulk organic phase to the surface of the catalysts depends on the contact between the polymer particles and organic droplets, both of which are suspended in a continuous phase of aqueous sodium phenolate. Authors interpreted the kinetic results in terms of three aspects *viz.,* i) mass transfer of benzoyl chloride to the catalyst surface, ii) diffusion of benzoyl chloride through polymer matrix, and iii) intrinsic

> Time(min) 0 20 40 60 80 100 120 140 160 180

Esterifcation of benzyl chloride with sodium acetate to form benzyl acetate and sodium

tributylmethylammonium chloride as the phase transfer catalyst [77]. The investigation focused on the determination of external mass transfer coefficient from the liquid bulk phases to the surface of the catalyst in tri-phase catalytic systems. Special emphasis was placed on the equipment (rotating disk contactor, RDC) which has been conceived and designed for this purpose. Determination of mass transfer coefficient involves an analysis of the various regimes of solid-liquid systems with the solid not soluble in the liquid phase. Esterfication was found to be mass transfer controlled at low agitation speeds and it was found to be characterized by considerable non-catalytic reaction and effects due to dispersion associated with the catalyst. Nevertheless, it was possible to determine the external mass transfer coefficient as a function of the bulk agitation speed. The mass transfer

Fig. 9. Effect of the agitation speed on the conversion of 1,7-octadiene at low alkaline concentration (30% NaOH): 10 mmol of 1,7-octadiene, 20 ml of chloroform, 0.2 mmol of tetrabutylammonium chloride (TBAC), 6 g of NaOH, 14 ml of water, 40 °C. (Adapted from

chloride were carried out under tri-phase conditions using polymer supported

200rpm 400rpm 600rpm 700rpm 800rpm 1000rpm

reactivity at the active sites.

1.0

0.8

0.6

Conversion(X)

0.4

0.2

0.0

Ref. [74], by permission)

Conventional technologies for multiphase organic reactions were largely uneconomical and polluting and hence were commercially not feasible. In recent years, several new techniques have emerged that use homogeneous or heterogeneous catalysts such as phase transfer catalysts, supported metal catalysts, biocatalysts etc. Among various types of catalysts, phase transfer catalysts have attracted more and more attention. It facilitates interphase transfer of species, making reactions between reagents in two immiscible phases possible.

Many organic synthetic applications based on PTC have shown great success. The present chapter has hence concentrated on the PTC reactions, *viz.,* alkylation reactions, esterfication reactions, dichlorocarbene addition reactions etc., using both soluble and immobilized forms of the catalyst. By its very nature, PTC involves interphase transport of species, neglecting which can grossly over predict the conversion of a PTC mediated reaction. Hence, greater emphasis has been given to present the role of mass transfer in PTC-assisted reactions. Nevertheless, there are definitely numerous catalytic reactions still waiting to be discovered and hence, opportunity for major discovery will remain vibrant for a very long time indeed. The new designs for microwaves and ultrasound assisted PTC reactions and the relevant mass transfer data is most essential to understand the design and scale-up of these emerging technologies in industries. It is hoped that this chapter will spur further research in this area, whose applications in the manufacture of organic intermediates and fine chemicals seems almost unlimited.
