Theoretical study of nitrile hydrolysis reaction on Zn(II) ion exchanged zeolites

The hydrolysis of acetonitrile catalyzed by Zn2+ ion exchanged zeolites has been studied theoretically by DFT (Density Functional Theory). A 4T cluster (Si/Al = 1) with zinc in the cation position has been chosen to model the zeolite site. The action of the metal ion is found to be analogous to that of the hydrolyzing enzymes, especially carbonic anhydrase. The primary reaction of Zn2+ with H2O is a dissociate reaction, in which ZnOH+ species is formed together with a zeolitic proton. Many alternate reaction paths have been analyzed and their reaction energy diagrams, including transition states, are presented. The hydrolysis reaction can be divided in three parts: hydration, isomerization and product desorption. The rate-limiting step is the nucleophilic attack of water molecule during the hydration reaction, Water molecules play an important role in many elementary reaction steps. It may act as proton donor in the hydration and tautomerization steps and facilitates the desorption of reaction products that inhibit the reaction. A detailed comparison is made with related enzymatic reactions. (C) 2001 Elsevier Science B.V. All rights reserved.