ABSTRACT
The most obvious advantage of a solid catalyst or reagent is that it can be removed from the reaction mixture by simple ltration or centrifugation. This allows quick recovery for reuse in the next run. Alternatively, the solid can be put in a column with the reaction mixture owing through it. There can be other advantages as well. If the catalyst is expensive or toxic, this provides a way to not lose it and to minimize exposure to it. If the bulk solid catalyst is expensive, less of it will be needed if it is spread over the surface of a solid support. The use of the solid material can minimize waste. Consider, for example, the use of a strong acid ion-exchange resin in place of p-toluenesulfonic acid to catalyze an esteri- cation. The resin can be recovered for reuse by ltration. The p-toluenesulfonic acid has to be removed by washing with aqueous base, after which it is usually discarded as waste. The solid catalyst can be used at very high catalyst levels because it can be recovered and used again. If the reaction is being performed on a solid support, a large excess of another reagent can be used to drive the reaction to completion. The excess reagent and any by-products can easily be washed off the solid. The vigor of some reactions can be moderated by putting a reagent on a solid. However, this may involve the use of extra solvent to put it on to the solid and then to extract the product from the solid. Some solids, such as clays and zeolites, offer size and shape selectivity, so that higher yields and fewer by-products can be obtained. Some catalysts can catalyze not only the desired reaction, but also side reactions. Isolating catalytic sites on a support can sometimes eliminate these side reactions. The support may also limit the possible conformations of the catalytic site in a way that limits side reactions. As an example, the immobilization of some enzymes increases their thermal stability. Altus Biologics, Cambridge, Massachusetts, does this by cross-linking enzyme crystals.1
