Zeolites as Solid Solvents: Explaining the Chloromethane\nHydrolysis over Metal-Exchanged Zeolite Y by DFT Calculations

We report a theoretical\nDFT study of the reaction pathways for\nchloromethane hydrolysis over metal-exchanged zeolites (Li<sup>+</sup>, Na<sup>+</sup>, K<sup>+</sup>, and Mg<sup>2+</sup>). A cluster\nof 78 T atoms (T referring to Si or Al atoms), comprising the zeolite\nY super cavity coupled with the sodalite cage and three hexagonal\nprism units (Si<sub>(78–<i>x</i>)</sub>Al<sub><i>x</i></sub>O<sub>129</sub>H<sub>54</sub>; <i>x</i> = 1 or 2), was used in the calculations. The study was carried out\nusing the ONIOM method. The high layer was computed at M062X/6-31++G(d,p),\nwhereas the low layer was computed at the PM6 level of theory. The\nenergy profile was obtained by single-point calculations at the M062X/6-31++G(d,p)\nfor the entire structure. The first step studied was the adsorption\nof chloromethane on the zeolite, which involves an ion–dipole\ninteraction between the metal cation and the chlorine atom. Then,\ntwo mechanistic pathways were investigated: one via formation of an\nadsorbed methoxide and the other involving a direct, one-step, nucleophilic\nattack of the water molecule to the adsorbed chloromethane. In all\nsystems studied, the direct mechanism showed a lower energy of activation\nthan the route involving the methoxide intermediate. The same behavior\nwas also observed for chloromethane methanolysis to afford dimethyl\nether on the metal-exchanged zeolites. The theoretical results are\nin agreement with the experiments of chloromethane hydrolysis over\nmetal-exchanged zeolite Y, explaining the formation of dimethyl ether\nupon chloromethane methanolysis on zeolites of low or no acidity.\nThe transition state for the direct route resembles a S<sub>N</sub>2 process assisted by the surface, reinforcing the concept of zeolites\nas solid solvents, providing a polar nanoenvironment that favors and\nassists the formation of ionic transition states and intermediates.