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Preparation of Sulfonated Ion-Exchange Polymer/Carrier Catalysts

The precipitation polymerization takes place in a batch reactor. Azoisobutyronitrile is used as the radical initiator, and styrene and divinylbenzene were chosen as monomers. A solution of the monomers in a long-chain hydrocarbon liquid is prepared [23]. After a clear solution has formed the megaporous glass Raschig rings are immersed in the solution, vacuum was used to remove the air from the pores, and the polymerization was initiated by heating. Fig. 8.7 shows the course of the precipitation polymerization in the pore volume of the carrier.

At first there is a homogeneous solution of the monomers in the solvent; heating initiates chain growth. After about 3 h the macromolecules formed are no longer soluble in the solvent (the solvent needs to be a good solvent for the monomers but a bad solvent for the polymer). First polymer particles are formed: the concentra-

T > 3h Particle formation and growth

T = 3 h Start of precipitation

T > 3h Particle formation and growth

T = 3 h Start of precipitation o Monomers O Oligoradical

Solvent -'0o 1 Particle

Fig. 8.7 Course of precipitation polymerization inside carrier materials

Carrier

Fig. 8.8 SEM image of a polymer/carrier composite

tion of the monomers is still high. Monomer molecules diffuse to the existing particles and there is further growth. When the particles have reached a certain size they touch each other. Still monomers diffuse to the particles and are added to the particles by the polymerization reaction. This leads to the polymer bridges we can see in Fig. 8.8. In Fig. 8.9 typical dimensions of the polymer/carrier composite materials are given.

Fig. 8.10 shows a comparison between the smallest available fraction of commercial resin beads (50-500 ^m) and the particles from our preparation method. The dark areas in the picture are the pores, the white area is the glass carrier. The small white dots in the dark area are the polymer particles. This SEM image shows that the size of the polymer particles is several micrometers. This is much smaller than commercial resin beads. Between the particles are large pores with a diameter in the micrometer range. This morphology is beneficial for good mass transfer

It is evident that the small size of the polymer particles will lead to better active site accessibility. The pores between the particles are megapores, enabling convec-tive flow through the catalyst at low pressure gradients.

After the polymerization step the styrene-divinylbenzene copolymer is activated by sulfonation with chlorosulfonic acid. The result is a polymer/carrier composite material that is a universal heterogeneous catalyst with the shape of a Raschig ring, well-suited for RD purposes.

Fig. 8.10 Comparison of polymer particle size of polymer/carrier composites with commercial resin beads

Table 8.5 compares technical data of the new catalyst with a well-known Amber-lyst ion-exchange resin.

Table 8.5 Comparison of amberlyst 15 with polymer/glass Raschig rings

Parameter

Amberlyst 15

Polymer/glass Raschig ring

Shape

bead

Raschig ring

Diameter [mm]

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