Fig. 8.4 Wire mesh filled with catalyst 8.3.2
Fig. 8.4 Wire mesh filled with catalyst 8.3.2
In addition to commercial packing there are several other methods described in the literature for preparing catalytic packing materials for RD purposes; the concepts can be divided into two approaches.
• Catalysts made of pure catalytic material.
• Catalysts supported on carrier materials.
184.108.40.206 Catalysts Made of Pure Catalytic Material
All catalysts used in the wrapping technique described above belong to the first group. The beads are pure polymer, completely sulfonated.
To avoid wrapping, Spes was the first to prepare monoliths of polymer with acidic properties . He mixed ion-exchange beads with inert thermoplastic polymer and sintered this mixture. The temperature control of the sintering process is critical: if it is too hot the resin beads are plugged by the molten thermoplastic polymer. To stabilize the bodies during sintering, activated carbon or other structural promoters are needed. To ensure accessibility of the active sites, sodium chloride was used as a pore-forming agent, but after removing the salt by washing, the catalyst has to be transformed to the hydrogen form. The size of the porous bodies was several centimeters. Mechanical stability was not good because of swelling.
The low mechanical stability is caused by the large dimensions of the bodies prepared by Spes. To avoid this, Fuchigami decreased the size of the sintered bodies
. He prepared 7 mm diameter tablets, but these are too small for RD processes because of the pressure drop. Both authors suggested methylacetate decomposition as a field of application.
Chaplits prepared similar products in an extruder . The main difference was that he extruded copolymers consisting of cross-linked polystyrene with a thermoplastic polymer. As a pore-forming agent water was used in the extrusion process. Acid sites are created by sulfonation of the cut extrudates. As an application, t-butanol production from water and isobutylene was suggested.
Yoshioka prepared fibers with ion-exchange properties [9, 10]. Polypropylene and polystyrene were melted and fibers produced. Then the fibers were cross-linked and functionalized by treatment with paraformaldehyde, acetic acid, and sulfuric acid. The fibers can be woven into fabrics that can be used in RD processes.
Rehfinger prepared Raschig rings by copolymerization of styrene and divinylben-zene . To create a macroporous resin he used organic solvents as the pore-forming agent. The polymerization was performed in a mold, resulting in a tube that was cut into Raschig rings. After sulfonation, acidic ion-exchange resin was obtained. The rings were used for MTBE synthesis. As for all pure polymer catalysts, they have the disadvantage that swelling forces (unavoidable in different reaction mixtures) cause a change in volume. The swelling forces can be sufficient to fracture the catalyst.
Supported catalysts were prepared by Smith . Carrier materials of various shapes were coated with solutions of monomers, which were then polymerized by UV light, creating a surface film. Mechanical stability was poor. Swelling forces led to crumbling of the film, loss of adhesion, and loss of polymer. The ion-exchange capacity was poor because only the outer surface of the carrier is available for polymer attachment.
Childress evaporated solutions of polymers on the surface of carrier materials . To avoid removal of the polymer by swelling forces, he heated the polymer above the melting point. This led to a non-porous polymer coating on the outer surface of the carrier.
Hiramatsu also prepared films on a carrier material . Halogenated polymers containing noble metals were prepared on the carrier surface. These catalysts had no acidic properties; they were used in hydrogen peroxide synthesis.
Bareis et al. and Eiceman impregnated porous powders with polymers and sulfonated them to get an acidic ion-exchange catalyst [14, 15]. The powders were used in chromatographic applications. The resins were non-porous, so pore-forming agents were not used.
Dromard impregnated porous inorganic carrier materials (4 pm-5 mm) with solutions of monomers and polymers . After evaporation of the solvent, films on the carrier surface were obtained. These catalysts can be activated with sulfonic or phosphonic acid groups. The catalysts were used for the production of silicones. The preparation procedure seems to be problematic, because during polymeriza tion the carrier material particles adhere to each other leading to difficulties in removing them from the polymerization reactor.
We tried to sulfonate polystyrene tubes, but this led to unstable products. The sulphonic acid group needs a space in the polymer resulting in breakage of the tubes. Pre-swelling in solvents did not solve this problem.
We tried coating inorganic carrier materials by gluing commercial resin beads to the carrier surface, but this was not successful. Most glues are not stable in organic solvents, so the beads leave the carrier material and glue enters the reaction mixture. Even if the glue is stable in the solvent, the swelling of the ion-exchange beads prevents them remaining attached. The swelling forces in ion-exchange resins can reach several hundred bar, making it impossible to keep the resin beads anchored to a surface by gluing.
To summarize the state of the art reported in the literature: all the methods described do not take into consideration the requirements of RD processes. The methods result in films on carrier surfaces not adhering to the support over a long time. The films are not porous, so mass transfer will be slow. The preparation procedures require the handling of highly viscous polymer mixtures or solutions, resulting in agglomeration of the carrier material particles during preparation. In principle thin films allow only small polymer load on the support, leading to low ion-exchange capacities not attractive for technical processes. With one exception  the fact that all resins for MTBE synthesis are macroporous was not implemented in the preparation procedures.
New Catalyst Concept: Porous Polymer/Carrier Composite
To develop a new catalyst for etherification reactions in RD processes we tried to use all the information available in the literature. The first aim was to develop a catalyst for the production of MTBE from isobutylene and methanol.
The general formula for the etherification reaction of olefins with alcohols is given in Fig. 8.5.
From the literature it is well known that this etherification reaction requires an acidic catalyst. In industrial processes ion-exchange resins with sulfonic acid groups are in use. The structure of this cross-linked resin is depicted in Fig. 8.6
To have a catalyst that can compete with the existing materials we decided to prepare a catalyst that also contains sulfonic acid groups. To compile the requirements for the new catalyst, kinetic data for MTBE synthesis [11, 17] and information on chemical  and mechanical  stability of ion-exchange resins under the reac-
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