Abstract
We report a theoretical DFT study of the reaction pathways for chloromethane hydrolysis over metal-exchanged zeolites (Li(+), Na(+), K(+), and Mg(2+)). A cluster of 78 T atoms (T referring to Si or Al atoms), comprising the zeolite Y super cavity coupled with the sodalite cage and three hexagonal prism units (Si((78-x))Al (x) O(129)H(54); x = 1 or 2), was used in the calculations. The study was carried out using the ONIOM method. The high layer was computed at M062X/6-31++G(d,p), whereas the low layer was computed at the PM6 level of theory. The energy profile was obtained by single-point calculations at the M062X/6-31++G(d,p) for the entire structure. The first step studied was the adsorption of chloromethane on the zeolite, which involves an ion-dipole interaction between the metal cation and the chlorine atom. Then, two mechanistic pathways were investigated: one via formation of an adsorbed methoxide and the other involving a direct, one-step, nucleophilic attack of the water molecule to the adsorbed chloromethane. In all systems studied, the direct mechanism showed a lower energy of activation than the route involving the methoxide intermediate. The same behavior was also observed for chloromethane methanolysis to afford dimethyl ether on the metal-exchanged zeolites. The theoretical results are in agreement with the experiments of chloromethane hydrolysis over metal-exchanged zeolite Y, explaining the formation of dimethyl ether upon chloromethane methanolysis on zeolites of low or no acidity. The transition state for the direct route resembles a S(N)2 process assisted by the surface, reinforcing the concept of zeolites as solid solvents, providing a polar nanoenvironment that favors and assists the formation of ionic transition states and intermediates.