Characteristics of Styrene-Divinylbenzene Ion Exchange Resins as Catalyst

2 Characteristics of Styrene-Divinylbenzene Ion Exchange Resins as Catalyst 

The ion exchange resins have various advantages over conventional acid or base catalysts. The use of aqueous acids has a number of drawbacks such as corrosion, side reactions, and environmental problems. These problems can be avoided by using ion-exchangers. Further advantages of ion-exchangers are that separation problems are manifestly simpler and the same catalyst can be used repeatedly. In some cases, it is even possible to distill the product directly in the presence of the cation-exchanger catalyst. 

The resins also have disadavantages. They aie less resistant to temperature and abrasion and are more expensive. 

Anderianova studied the decomposition of formic acid and esterification of acetic acid with ethyl alcohol in the vapor phase over gel-type resins of divinylbenzene content of 1 % and 20%. At lower temperatures, the resin with lower degree of crosslinking was more active for both reactions. With increasing temperature, the difference in the rates decreased. This was attributed to the change in resin sorption capacity with temperature. On the other hand, it was reported that, for the liquid-phase dehydration of t-butyl alcohol with gel-type resins, the 8%-crosslinked resin had about twice the catalytic activity of the 2% resin. 

Water present in the catalytic systems shows versatile effects on reaction rates. Heath and Gates found the induction period in the dehydration of t-butyl alcohol and also noted that water addition reduced the induction time. This effect of water was attributed to the swelling of the resin network. The swelling reduces intraparticle resistance to mass transport, and makes an increasing fraction of the catalytic sites accessible to the reactant. Though water accelerated the reaction initially, it also inhibited the reaction. The retardation with water was observed also in estrification of salicic acid with methanol and benzene propylation. The retarding effect of water was explained by a kinetic expression based on the Langmuir-Hinshelwood model, in which the competitive chemisorption of water and a reactant (alcohol or acid) is assumed. 

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