Phase-transfer engineering

Macromolecular Materials and Engineering 286, No. 10, 25th Oct.2001, p.640-7 POLY(ETHYLENE TEREPHTHALATE) RECYCLING AND RECOVERY OF PURE TEREPHTHALIC ACID. KINETICS OF A PHASE TRANSFER CATALYZED ALKALINE HYDROLYSIS... 

The presence of two phases in the reaction mixture may seem to be a mass-transfer engineering problem, but even moderate stirring of the mixture produces an emulsion, which greatly facilitates the phase transfer steps of the reaction mechanism. In our fixed-bed reactor, the turbulence resulting from the flow rates used seemed to suffice to eliminate external mass transfer hmitations. At MeOH SA of 20 and identical LHSV values, similar acid conversions were observed for two linear flow velocities differing by a factor of two. 

Figure 5.5. Concept of dense phase transfer of friable materials, by intermittent injection of material and air pulses, air pressures normally 10-30psig and up to 90psig (Sturtevant Engineering Co., Boston, MA).

Naik SD, Doraiswamy LK (1998) Phase transfer catalysis chemistry and engineering. AIChE J 44(3) 612-646... 

A somewhat different approach to catalyst separation has been devised by engineering the chiral salen catalyst to have built-in phase-transfer capability, as exemplified by the Mn(III) complex 10 <02TL2665>. Thus, enantioselective epoxidation of chromene derivatives (e.g. 11) in the presence of 2 mol% catalyst 10 under phase transfer conditions (methylene chloride and aqueous sodium hypochlorite) proceeded in excellent yield and very good ee s. The catalyst loading could be reduced to about 0.4% with only marginal loss of efficiency. 

Many operations in chemical engineering require the contact of two liquid phases between which mass and heat transfer with reaction occurs. Examples are hydrometallurgical solvent extraction, nitrations and halogenations of hydrocarbons, hydrodesulfurization of crude stocks, emulsion polymerizations, hydrocarbon fermentations for single-cell proteins, glycerolysis of fats, and phase-transfer catalytic reactions. A most common method of bringing about the contact of the two phases is to disperse droplets of one within the other by mechanical agitation. 

Selectivity engineering is a new term that is coined with the engineering aspects of multiphase reactions that could be manipulated through the use of several techniques such as use of an additional immiscible liquid phase, porous inert solids, particles smaller than diffusion film thickness, etc. in order not only to intensify the rates of reaction but also to improve greatly the selectivity of the desired product. We are particularly concerned in this paper with the selectivity engineering aspects of the capsule membrane phase transfer catalysis, which has interesting attributes, for the preparation of benzyl alcohol and benzaldehyde by selective alkaline hydrolysis and oxidation, respectively, of benzyl chloride. 

Selective oxidation of benzyl chloride to benzaldehyde is another reaction of great commercial importance. There exist several methods to prepare benzaldehyde, both commercially and synthetically, but the oxidation of benzyl chloride directly to benzaldehyde in a single pot by using CM-PTC merits special attention. We have studied in our laboratory [3] the kinedcsand mechanism of the liquid-liquid and solid-liquid phase transfer oxidation by using chromate and hypochlorite s ts and it was thought desirable to study the selectivity engineering aspects of this reaction. 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

Mass transfer, an important phenomenon in science and engineering, refers to the motion of molecules driven by some form of potential. In a majority of industrial applications, an activity or concentration gradient serves to drive the mass transfer between two phases across an interface. This is of particular importance in most separation processes and phase transfer catalyzed reactions. The flux equations are analogous to Ohm s law and the ratio of the chemical potential to the flux represents a resistance. Based on the stagnant-film model. Whitman and Lewis [25,26] first proposed the two-film theory, which stated that the overall resistance was the sum of the two individual resistances on the two sides. It was assumed in this theory that there was no resistance to transport at the actual interface, i.e., within the distance corresponding to molecular mean free paths in the two phases on either side of the interface. This argument was equivalent to assuming that two phases were in equilibrium at the actual points of contact at the interface. Two individual mass transfer coefficients (Ld and L(-n) and an overall mass transfer coefficient (k. ) could be defined by the steady-state flux equations ... 

Selection or development of a phase transfer catalyst often plays the most important role in developing a new PTC system. Two main factors considered in selecting a PTC catalyst are the ability to transfer one of the reactants into the normal phase of the other reactant and the ability to activate the transferred species to facilitate the chemical reaction. In practice, other features of PTC catalysts considered by chemists or engineers in developing a PTC process include the stability, cost and availability, toxicity, recovery, recycling, and disposal of catalysts. 

In the present edition some space is dedicated to special topics such as electrocatalysis, photocatalysis, asymmetric catalysis, phase-transfer catalysis, environmental catalysis, and fine chemicals manufacture. On the basis of fundamental reaction engineering equations, examples for calculation and modeling of catalysis reactors are given with the easy-to-learn PC program POLYMATH. Well over 170 exercises help die reader to test and consolidate the gained knowledge. 

Phase-transfer catalysis (PTC) is an area that has largely been the province of the preparatory organic chemist (defined broadly to include organometallic and polymer chemists). It is only since the early eighties that the engineering aspects... 

Phase-Transfer Reaction Engineering 635 Table 19.6 Dimensionless groups and their values for Example 1.1... 

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