Phase Boundary Catalysis

Phase Boundary Catalysis (PBC) in chemistry is a type of heterogeneous catalytic system which facilitates the chemical reaction of a particular chemical component in immiscible phase react on a catalytic active site located at phase boundary. The chemical component is soluble in one phase but insoluble in the other. The catalyst for PBC has been designed in which the external part of the zeolite is hydrophobic, internally it is usually hydrophilic, notwithstanding to polar nature of some reactants.^{[1] [2] [3] [4] [5]} In this sense, the medium environment in this system is close to that of an enzyme. The major difference between this system and enzyme is lattice flexibility. The lattice of zeolite is rigid, whereas the enzyme is flexible.

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Design of Phase Boundary Catalyst

Figure 1 shows schematic representation of design of Phase Boundary Catalytic (PBC) system and its comparison with conventional catalytic system. The PBC is useful primarily for performing reaction at the interface of aqueous phase and organic substrate phases. PBC is needed because the immiscibility of aqueous phase and organic substrate. The name phase-boundary catalysis does what it says; the catalyst acts as a catalyst at the interphase between the aqueous and organic phases as shown in Figure 1. The reaction medium of phase-boundary catalysis system for the catalytic reaction of immiscible aqueous and organic phases consist of three phases; an organic liquid phase, containing most of the substrate, an aqueous liquid phase containing most of the substrate in aqueous phase and the solid catalyst. The two liquid phases are almost completely insoluble in one another.

In case of conventional catalytic system (see Figure 1);

• When the reaction mixture is vigorously stirred, an apparently homogeneous emulsion is obtained, which segregates very rapidly into two liquid phases when the agitation ceases. Segregation occurs by formation of organic bubbles in the emulsion which move downwards to form the aqueous phase, indicating that emulsion consists of dispersed particles of the aqueous phase in the organic phase.

• Due to the triphasic reactions conditions, the overall reaction between aqueous phase and organic phase substrates on solid catalyst requires different transfer processes. The following steps, which are schematically represented in Figure 2 are involved:

1. transfer of aqueous phase from organic phase to the external surface of solid catalyst; 2. transfer of aqueous phase inside the pore volume of solid catalyst; 3. transfer of the substrate from aqueous phase to the interphase between aqueous and organic phases; 4. transfer of the substrate from the interphase to the aqueous phase; 5. mixing and diffusion of the substrate in the aqueous phase; 6. transfer of the substrate from the aqueous phase to the external surface of solid catalyst; 7. transfer of the substrate inside the pore volume of the solid catalyst; and 8. catalytic reaction (adsorption, chemical reaction and desorption).

It was reported that without vigorous stirring, no reactivity of the catalyst was observed in conventional catalytic system.^{[1] [2] [3] [4] [5]} As proposed in Figure 2, it is clear that stirring and mass transfer from organic to aqueous phase and vice-versa are required for conventional catalytic system. In the PBC (see Figure 2), the stirring is not required because the mass transfer is not rate determining step in this catalytic system. It is already demonstrated that this system works for alkene epoxidation without stirring or the addition of a co-solvent to drive liquid-liquid phase transfer.^{[1] [2] [3]} The active site located on the external surface of the zeolite particle were dominantly effective for the observed phase boundary catalytic system.^{[4] [6]}

How to synthesize Phase Boundary Catalyst?

Modified zeolite on which the external surface was partly covered with alkylsilane, called “Phase boundary catalyst” was prepared in two steps (see Figure 3).^{[1] [2] [3] [4] [5]} First, titanium dioxide from titaniumisopropoxide was impregnated into NaY zeolite powder to give sample W-Ti-NaY. In the second step, alkysilane from n-octadecyltrichlorosilane (OTS) was impregnated into the W-Ti-NaY powder containing water. Due to the hydrophilicity of the w-Ti-NaY surface, addition of a small amount of water led to aggregation owing to the capillary force of water between particles. Under these conditions, it is expected that only the outer surface of aggregates, in contact with the organic phase can be modified with OTS, and indeed almost all of the particles were located at the phase boundary when added to an immiscible water–organic solvent (W/O) mixture. The partly modified sample is denoted w/o-Ti-NaY. Fully modified Ti-NaY (o-Ti-NaY), prepared without the addition of water in the above second step, is readily suspended in an organic solvent as expected.

References

1. ^ H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis: a new approach in alkene epoxidation with hydrogen peroxide by zeolite loaded with alkylsilane-covered titanium oxide, Chemical Communications, 2000, 2235 - 2236. Abstract
2. ^ H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysis of alkene epoxidation with aqueous hydrogen peroxide using amphiphilic zeolite particles loaded with titanium oxide, Journal of Catalysis, 2001, (204) 402 - 408. Abstract
3. ^ S. Ikeda, H. Nur, T. Sawadaishi, K. Ijiro, M. Shimomura, B. Ohtani, Direct observation of bimodal amphiphilic surface structures of zeolite particles for a novel liquid-liquid phase boundary catalysis, Langmuir, 2001, (17) 7976 - 7979. Abstract
4. ^ H. Nur, S. Ikeda and B. Ohtani, Phase-boundary catalysts for acid-catalyzed reactions: the role of bimodal amphiphilic structure and location of active sites, Journal of Brazilian Chemical Society, 2004, (15) 719-724 - 2236. Paper
5. ^ H. Nur, S. Ikeda, and B. Ohtani, Amphiphilic NaY zeolite particles loaded with niobic acid: Materials with applications for catalysis in immiscible liquid-liquid system, Reaction Kinetics and Catalysis Letters, 2004, (17) 255 - 261. Abstract
6. ^ S. Ikeda, H. Nur, P. Wu, T. Tatsumi and B. Ohtani, Effect of titanium active site location on activity of phase boundary catalyst particle for alkene epoxidation with aqueous hydrogen peroxide, Studies in Surface Science and Catalysis, 2003, (145) 251-254.