Fig. 8.6 Structure of sulfonic acid catalysts based on sulfonated styrene-divinylbenzene copolymers tion conditions of RD columns were evaluated. In [11] the rate equation for the MTBE synthesis is given in activities, the first approach to do this for a process of industrial relevance.

It is known that the kinetics of the MTBE synthesis has a negative reaction order with respect to methanol. That means that at lower concentrations of methanol the reaction towards MTBE becomes faster. A faster reaction means that the concentration of the reactant methanol decreases inside the catalyst. On the other hand the concentration of isobutylene increases inside the catalyst. If this happens, the side reaction of dimerization of isobutylene occurs, lowering the selectivity towards MTBE. This effect can also be seen if the size of the catalyst particles is decreased. Smaller particles are more selective than large ones; large particles are deactivated by plugging the catalyst with dimer, starting from the center of the particle. Only an outer shell is the active part of the catalyst particle.

To produce a highly selective catalyst it is necessary to prepare the catalyst as a fine powder or to prepare a catalyst in which only the outer surface contains active sites. The decrease in size will cause problems with an RD column as reactor. Fine powders can only be handled in a slurry reactor, but this is not advantageous because we have a reaction that is equilibrium limited. For such a reaction a distillation column is superior. So the catalyst particles should be large enough to give a low pressure drop. The preparation of a shell catalyst will result in high selectivity but also in low activity because the outer surface of suitable catalyst particles is too low to make this concept attractive for an industrial application.

Commercial catalysts for MTBE synthesis are macroporous. The diffusion of methanol inside the macropores of the catalyst is the rate-determining step of the reaction to MTBE. In the gel phase of the catalyst there is no mass-transport limitation [11]. A factor influencing macroporosity is the degree of cross-linking. The higher the cross-linker concentration, the more of the polar compound enters the catalyst. So cross-linking is a factor influencing selectivity. The higher cross-linking is, the higher selectivity should be. The degree of cross-linking can only be adjusted between certain limitations. At very high cross-linking the polymer becomes brittle, and catalyst damage by mechanical forces can happen easily. A compromise is to use a medium cross-linker concentration of 5-20%.

We mentioned that MTBE synthesis has a special kinetic behavior. Depending on the concentration of methanol in the feed, the reaction rate can be very fast at low methanol concentrations or rather fast at high methanol concentrations (a range from about 2 to 160 mol/m3 is covered). The reasons for this are mass-transport effects in the pores of the catalyst. To avoid this, the pores in the new catalyst should be large: larger than in commercial macroporous resins.

Styrene-divinylbenzene cross-linked resins with sulfonic acid groups can be used up to 110-120 °C. Exceeding this temperature range will result in a loss of acid groups. Desulfonation will occur at elevated temperatures. Resins with higher cross-linking will deactivate faster than those with low cross-linking because the concentration of acidic sites is higher (lower swelling at higher cross-linking). A high concentration of acidic sites means a high concentration of that catalyst that catalyses the desulfonation. The acidic sites catalyze their own deactivation. So the degree of cross-linking should be low for good chemical stability at elevated temperatures.

From this evaluation, taking an RD column as a reactor into consideration, the resulting requirements for the new catalyst can be summarized in Table 8.1.

As a result it is clear that the new catalyst can not be prepared easily. Some requirements are contradictory. A compromise is necessary to produce a product that fulfils the technical requirements.

As mentioned in Table 8.1, the catalyst should have small particle size to enhance selectivity. The smallest size that one can imagine is a monomolecular layer of active sites on a large pore support. From the literature it is known that silanes can react with silanol groups of inorganic carrier materials [20]. Suitable carrier materials are silica or glass. So we calculated what ion-exchange capacity can be obtained if the surface of megaporous glass Raschig rings is covered with trichlorophenylsilane. Reaction of the chloro groups with the silanol groups of

Table 8.1 Requirements and reasons for the requirements for a reactive distillation catalyst


Reason for the requirement

Small catalyst particles

high selectivity for MTBE

High ion-exchange capacity

high activity per volume

Large pores

high reaction rate

Sulfonic acid groups as active sites

well known active etherification catalyst

Shape of a Raschig ring

reactor is reactive distillation column

Large size

low pressure drop

High mechanical stability

resistance to swelling forces

the glass will lead to a chemically bound aromatic ring that can be sulfonated. Taking the data from the literature for the density of silanol groups on silica and the technical data of the Raschig rings (40 m2/L), it is clear that the obtainable ion-exchange capacity is very low. Even if silica with high surface area is used only ionexchange capacities in the range of some microequivalents H+/g are obtainable [21]. This is not attractive for a technical application. In addition the pores of silica are smaller compared with macroreticular ion-exchange resins, probably causing even more problems with mass-transfer effects.

So we decided to establish a catalyst concept that makes use of the pore volume to incorporate ion-exchange resins. The information obtained from the literature revealed that it is not wise to try to anchor the catalytic polymer to the surface because swelling forces will break this linkage. Only the concept that makes use of the pore volume to attach the polymer allows high ion-exchange capacities suitable for technical processes.

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